Synthesis and Application of Palladium Nanomaterials for ...

Synthesis and Application

of Palladium Nanomaterials for

Sensing and Catalysis

Jianli Zou

MSc

The dissertation is presented for the degree of Doctor of Philosophy at

The University of Western Australia

School of Biomedical, Biomolecular and Chemical Sciences

Discipline of Chemistry

2011

In the memory of my father

谨纪念父亲

i

Abstract

Palladium (Pd) nanomaterials have received considerable research interests in

hydrogen purification, hydrogen sensing and catalysis. For these large scale industrial

applications, the development of a feasible synthetic method is paramount. To this end, a

scalable method to synthesise Pd nanomaterials under continuously fluidic flow at room

temperature using hydrogen gas as a reducing agent has been developed. Spinning Disc

Processing (SDP) as a platform of continuous flow provided an intense micro-mixing on a

rapidly rotating disc, which facilitated hydrogen gas reducing Pd(II) to Pd(0) in seconds.

Three different Pd based nano-materials have been synthesised using SDP, namely,

surfactant free Pd bare nano-rosettes, Pd-PVP nanospheres, and Pd-Cellulose

nanocomposites.

The application of the three different aforementioned Pd nano-structures in hydrogen

gas sensing has been explored. 1. Pd bare nano-rosette structure possessed a high

surface-to-volume ratio that rendered an abundance of active surface available for hydrogen

adsorption. These bare Pd nano-rosettes were devoid of the possible lag in response time

caused by a passivated Pd surface which facilitated real time hydrogen gas sensing

compared with surfactant stabilised Pd nanomaterials. 2. Pd-PVP nanospheres were

synthesised in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). Here, PVP acted as scaffold

holding a large number of Pd nanocrystals together into a 3-dimentional sphere. The

dissociative adsorption of hydrogen in this case induced a Mott insulator to metal like

transition in a Pd nanosphere which was observed for the first time. These Pd-PVP

nanospheres showed a decrease in current at low concentration of hydrogen gas and an

increase in current when hydrogen concentration was above 2%. These findings are

important in the development of next generation nanomaterials based electronic switches

and sensors. 3. Pd- SCMC nanocomposites were synthesised in the presence of sodium

carboxymethyl cellulose (SCMC). The ratio of SCMC to Pd not only played a key role in

determining the morphology of the Pd- SCMC nanocomposites, also affected the response

to hydrogen in sensing applications.

ii

Pd-PVP nanospheres were also investigated as catalysts for the Mizoroki-Heck cross

coupling reaction. It was established that these nano-catalysts showed high catalytic activity

in the reaction between several aryl halides species with n-butyl acrylate, and could be

readily recycled ten times without a change in their catalytic activity. A further study by XPS

and TEM showed that surface oxygen played a pivotal role in the reconstruction of Pd

nanocrystals during heterogeneous catalysis. This reconstruction resulted in an increase in

the size of Pd nanocrystals, and a corresponding decrease in chemically active sites for the

model intermolecular Heck cross coupling reaction. This work is an important finding that

should be taken into consideration in the future design of recyclable Pd nano-catalysts.

Furthermore, a novel method for stabilising graphene in aqueous solution was also

developed in this project. p-Phosphonic acid calix[4]arenes rendered high stability to

exfoliated graphene in a range of pH aqueous solution. These calix[4]arene modified

graphene sheets were used as highly effective substrates to nucleate ultra-small Pd

nanoparticles, which in turn served as a galvanic reaction template for the generation of

high density 2D arrays of Pt nanoparticles. This simple process improved the processability

of graphene in water with potential to develop novel hybrid nanomaterials for application in

catalysis, fuel cells, sensor materials and nano-electronics.

Finally, capillary force lithography (CFL) was used to prepare hydrogen nano-sensors.

CFL was applied to generate large area patterns on silica wafer. A “grafting to” approach was

used on the patterns to induce linear assembly of Pd nanocubes through electrostatic

interaction. These resultant Pd nano-arrays were connected into integrated circuit and

tested as hydrogen gas sensors. The results showed that CFL could be potentially used as a

feasible method to build miniature hydrogen sensor on a large scale.

iii

Table of Contents

Abstract ........................................................................................................................................................................................... i

Table of Contents .................................................................................................................................................................... iii

List of Figures ............................................................................................................................................................................. vi

Abbreviations ............................................................................................................................................................................ ix

Acknowledgements .............................................................................................................................................................. xi

Details of Publications and Conferences .................................................................................................................xiii

Publications ....................................................................................................................................................................xiii

Patents ............................................................................................................................................................................... xv

Conferences ............................................................................................................................................................................. xv

Statement of Candidate Contribution ..................................................................................................................... xvi

1. Introduction ...................................................................................................................................................................... 1

1.1 Overview ................................................................................................................................................................................... 1

1.2 Pd in Nanoscale .................................................................................................................................................................... 2

1.2.1 Pd Nanomaterials for Hydrogen Sensing ................................................................................................ 2

1.2.2 Pd Nanomaterials in Catalysis ........................................................................................................................... 3

1.3 The Application of Pd Nanomaterials in Hydrogen Sensing ..................................................................... 4

1.3.1 Introduction to Hydrogen Sensing ................................................................................................................ 4

1.3.2 Hydrogen Sensor Types and Performance ............................................................................................... 5

1.3.3 Working Principles of Pd Resistive Sensing ............................................................................................ 13

1.3.4 Pd Nanomaterials Used in Resistive Hydrogen Sensing ................................................................ 17

1.4 The Application of Pd Nanomaterials in the Heck Cross Coupling Reaction ................................ 22

1.4.1 Introduction to the Pd Catalysed Heck Cross Coupling Reaction ............................................. 22

1.4.2 Proposed Mechanism in the Heck Cross Coupling Reaction ...................................................... 22

iv

1.4.3 Heterogeneous Catalyst: Pd in the Heck Reaction ............................................................................ 25

1.4.4 Catalytic Activity in Heterogeneous Catalytic System ..................................................................... 30

1.4.5 Change in Size of Nanoparticles in the Heck Reaction .................................................................... 31

1.5 Synthesis of Pd Colloidal and Supported Nanomaterials ......................................................................... 33

1.5.1 Summary of the Synthesis of Pd Nanomaterials ................................................................................. 33

1.5.2 Colloidal Pd Nanoparticles ............................................................................................................................... 34

1.5.3 Supported Pd Nanomaterials ........................................................................................................................ 41

1.6 Challenges in the Synthesis and Applications of Pd Nanomaterials .................................................. 44

2. Introduction to Series of Papers ......................................................................................................................... 46

3. Series of Papers ............................................................................................................................................................ 54

3.1 Bare Palladium nano-rosettes for real-time high-performance and facile hydrogen

sensing .................................................................................................................................................................................... 54

3.2 Hydrogen-induced reversible insulator–metal transition in a palladium nanosphere

sensor ...................................................................................................................................................................................... 60

3.3 Pd-sodium carboxymethyl cellulose nanocomposites display a morphology dependent

response to hydrogen gas .......................................................................................................................................... 65

3.4 Scalable synthesis of catalysts for the Mizoroki-Heck cross coupling reaction: palladium

nanoparticles assembled in a polymeric nanosphere ................................................................................ 69

3.5 Surface oxygen triggered size change of palladium nano-crystals impedes catalytic efficacy77

3.6 Pd-induced ordering of 2D Pt nanoarrays on phosphonated calix[4]arenes stabilised

graphenes ............................................................................................................................................................................. 81

3.7 Regiospecific linear assembly of Pd nanocubes for hydrogen gas sensing ................................... 85

4. Conclusions and Future Work ............................................................................................................................. 89

5. Appendices .................................................................................................................................................................... 93

5.1 Supporting information for “Bare palladium nano-rosettes for real time high performance

and facile hydrogen sensing” .................................................................................................................................... 93

5.2 Supporting information for “Hydrogen-induced reversible insulator– metal transition in a

palladium nanosphere sensor” ................................................................................................................................ 94

v

5.3 Supporting information for “Pd-sodium carboxymethyl cellulose nanocomposites display

a morphology dependent response to hydrogen gas ............................................................................... 99

5.4 Supporting information for “Scalable synthesis of catalysts for the Mizoroki-Heck cross

coupling reaction: palladium nanoparticles assembled in a polymeric nanosphere ........... 102

5.5 Supporting information for “Surface oxygen triggered size change of palladium

nano-crystals impedes catalytic efficacy” ........................................................................................................ 107

5.6 Supporting information for “Pd-induced ordering of 2D Pt nanoarrays on phosphonated

calix[4]arenes stabilised graphenes” .................................................................................................................. 110

5.7 Supporting information for “Regiospecific linear assembly of Pd nanocubes for hydrogen

gas sensing” ...................................................................................................................................................................... 113

6. References .................................................................................................................................................................... 117

vi

List of Figures

Figure 1.1 Schematic of an electrochemical type hydrogen sensor ............................................................... 6

Figure 1.2 Schematic of a metal oxide hydrogen sensor. The electrical resistance variation is

measured using a Wheatstone bridge .............................................................................................................. 7

Figure 1.3 Empirical current–voltage curve for a thermal conductivity sensor ........................................ 8

Figure 1.4 “Classical” schematic illustration of the hydrogen sensitive field effect devices, where

hydrogen atoms adsorbed at the metal–oxide interface result in a shift of the electrical

characteristics along the voltage axis in devices having catalytic metal (Pd) gates ...................... 9

Figure 1.5 a) Transmittance of a WO3:Pt nanoparticles dispersed on a filter paper and exposed to

0.5% H2/N2 mixture. b) A hydrogen colourimetric indicator showing the colour change when

the hydrogen gas on and off ................................................................................................................................ 10

Figure 1.6 a) Pd micromirror hydrogen sensor; b) Response of the hydrogen sensor to varying

concentrations of H2 in N2; c) Evanescent field fiber optic hydrogen sensor; d) Optical

transmission of a fibre optic hydrogen sensor after dosing with a 3% H2 in argon ...................... 11

Figure 1.7 Schematic illustration of relative resistance (R/R0) as a function of relative hydrogen

concentration (H/Pd) for absorption-desorption processes. The arrows indicate the directions

of absorption and desorption processes ........................................................................................................ 15

Figure 1.8 Change of resistance as a function of time in the presence of 0.1% hydrogen and air

alternately .................................................................................................................................................................... 16

Figure 1.9 Atomic force microscope (AFM) images of a Pd mesowire on a graphite surface, a)

acquired in air and b) acquired in a stream of hydrogen gas (a hydrogen-actuated break

junction is highlighted). c) Current response of the sensor to hydrogen (concentration of H2 in

percentage as shown) ............................................................................................................................................ 16

vii

Figure 1.10 Scanning electron microscopy (SEM) images of a) an AAO template and b) a

nanoporous Pd film on top of a pyrolytic, carbon-coated AAO template ........................................ 18

Figure 1.11 a) A representative individual Pd nanowire device with d = 20 nm; and b) the electrical

resistance response to H2 for single Pd nanowires with diameters d = 20 nm at 10,000 ppm

partial pressures of H2 ............................................................................................................................................. 19

Figure 1.12 Schematic diagram of Pd nanowire hydrogen fabricated in reference ............................. 19

Figure 1.13 SEM images of a) plain structure Pd nanowire with 85 nm diameter; b) grain structure

nanowires with diameter of 150 nm; and c) hairy structure Pd nanowires with diameter of

100 nm ...................................................................................................................................................................... 20

Figure 1.14 AFM images of a) an unmodified SWNT network and b) a SWNT network

electrochemically modified with Pd nanoparticles used in hydrogen sensing .............................. 21

Figure 1.15 Chemical structure of ligands for high catalytic Pd complex catalyst ............................. 30

Figure 1.16 TEM images of Pd nanomaterials before and after the Heck reaction, a) from

reference 153 with size distribution, and b) from reference 154 ........................................................... 32

Figure 1.17 Mean diameters and standard deviations of Pd nanoparticles synthesised at various

alcohol concentration ............................................................................................................................................ 35

Figure 1.18 TEM images of Pd nanoparticles obtained at 20 s reaction time with different

concentrations of NaBH4: a) 1.5 mM, b) 1 mM and c) 0.5 mM and size distributions. TEM

images of Pd nanoparticles obtained at 1 h reaction time with different concentrations of

NaBH4: d) 1.5mM, e) 1mM and f) 0.5mM, and size distributions. ......................................................... 36

Figure 1.19 TEM images of size evolutions of Pd nanoparticles by varying the volume of growth

solution into different seeding solutions. (a, e and i) represent seeding solutions with Pd

nanoparticles of 2.6, 3.4 and 4.3 nm, respectively; (b), (c) and (d) show the size evolution of Pd

nanoparticles from (a) with different volume of growth solution from 50 ml, 100 ml to 200 ml,

respectively. Similarly, (f), (g) and (h) and (j), (k) and (l) evolve from (e) and (i), respectively by

varying the volume of growth solution to control the size of Pd nanoparticles ............................ 37

viii

Figure1.20 Geometrical shapes of Pd nanoparticles. ......................................................................................... 39

Figure1.21 TEM images of a) the ultrathin Pd nanosheets. Inset: photograph of an ethanol

dispersion of the as-prepared Pd nanosheets in a curvette, b) the assembly of Pd nanosheets

perpendicular to the TEM grid. Inset: thickness distribution of the Pd nanosheets, c) highly

branched Pd nanostructures formed from the ultrafast growth of polyhedra nanoparticles, d)

the branched Pd nanostructures developed from nanorods. ............................................................... 40

Scheme1.1 Simplified catalytic mechanism in Heck reaction on the basis of mechanism proposed

by Heck in 1974 when Pd(OAc)2 and monophosphine ligands (L) were involved. ........................ 23

Table 1.1 The US Department of Energy (DOE) targets for hydrogen sensor ......................................... 4

Table 1.2 A summary of the catalytic ability of Pd nano-catalysts ........................................................... 31

Table 1.3 The summary of choice of Pd precursors, reducing agents and stabiliser ......................... 34

Table 1.4 Typical examples of supported Pd nanomaterials ....................................................................... 41

ix

Abbreviations

AAO anodic aluminum oxide

AFM atomic force microscope

CFL capillary force lithography

CMC carboxymethyl cellulose

DMF dimethylformamide

GC-MS gas chromatography-mass spectroscopy

GO graphene oxide

HEC hydroxyethyl cellulose

MC methylcellulose

MEMS micro-electro-mechanical system

MOS metal-oxide-semiconductor

MOX metal oxide

NMR nuclear magnetic resonance

PAMAM polyamidoamine

PDMS polydimethylsiloxane

PEG polyethyleneglycol

PGMA polyglycidylmethacrylate

PMMA polymethylmethacrylate

PPI polypropyleneimine

ppm part per million

PS polystyrene

PVP poly(N-vinyl-2-pyrrolidone)

x

SEM scanning electron microscopy

SCMC sodium carboxymethyl cellulose

SWNTs single wall carbon nanotubes

TEM transmission electron microscope

Tg glass transition temperature

TOP tri-n-octylphosphine

xi

Acknowledgements

This dissertation would not have been possible without the kind help and support

from my supervisors, colleagues, friends, and families. I can’t thank them all individually, but I

would like to acknowledge some of them as their contribution are invaluable.

I would like to thank my supervisors, Professor Colin Raston, Dr. Swaminathan Iyer, and

Dr. Scott Stewart for their support and guidance throughout my research. Professor Raston

provided me great support in securing the International Postgraduate Research

Scholarships for which I am very much indebted. Dr. Iyer guided me on both academic and

personal level. I am also extending my gratitude for his effort & time spending on discussing

the experiments and revising my papers. Dr. Stewart joined my supervisor panel as an

organic specialist after I decided to expand my research on catalysis. He provided me the

knowledge of organic chemistry. I am very much obliged.

I would like to thank Australia Research Council and the University of Western Australia

for the International Postgraduate Research Scholarships. I would also like to thank

staff from BBCS for their assistance, especially Dr. Alexandre Sobolev for the help with

XRD test, Mr. Oscar Del Borrello for the help with FTIR and UV-Vis test, Dr. Tony

Reeder for the help with GC-MS, Dr. Lindsay Byrne for the help with NMR, and Mr

Trevor Franklin for his kind help with electronic equipment. I would like to thank staff

from the Centre for Microscopy, Characterisation and Analysis, especially Professor Martin

Saunders, Mr Steve Parry, Mr John Murphy, Ms Lyn Kirilak and Mr Peter Duncan, for the

training of microscopy and all kinds of help with microscopy analysis.

I would like to thank Professor Igor Luzinov and Dr. Bogdan Zdyrko in Clemson

University, USA, for hosting my research stay and the guidance throughout enabled me to

develop an understanding of the project in a short time. I would like to thank all of

those who helped me in any aspect during my stay in Clemson University, especially

James Giammarco, Dr. Ruslan Burtovyy, Tugba Demir, Fehime Vatansever, Yuriy

Galabura and Marius Chyasnavichyus.

xii

I would like to thank Dr. Zhongtao Jiang from Murdoch University for his help

with XPS test, and Ms. Xiaoxue Xu from School of Mechanical Engineering, UWA for

her kind help with TGA test.

I would like to thank all of the members in Raston group and Iyer group. I had a

quite interesting time among those interesting fellows. The effort made by Cameron

Evans and Tristan Clemons in promoting a high-standard working environment will

not only stand as an example for me, I believe, also to those who come after. Special

thanks to Dr. Paul Eggers and Dr. Ela Eroglu for proofreading.

I would like to thank my family for their constant support. I have never

expressed how much I appreciate the understanding my parents gave to me when I

decided to quit my job and further my postgraduate study. Their great patience and trust

keep me up in those hard times. Very special thanks to my sisters; their dedication to taking

care of our parents truly put my mind in peace. I couldn’t imagine where I would end today

without their support.

感谢家人对我一直的支持。当我决定辞去工作重新回到学校继续学业时, 父母给了

我毫无保留的支持和理解, 我还从未表达过我的感激。他们对我一如既往的耐心和信任,

支撑我度过了那些艰难的岁月走到了今天。非常特别的感谢给我的姐姐们, 她们对父母

悉心的照顾了却我的后顾之忧, 所以我可以专心学业, 不能想像我的今天会走到哪里如

果没有她们的奉献和支持。

xiii

Details of Publications and Conferences

Publications

1. Zou, J.; Iyer, K. S. *; Raston, C. L., Pd-sodium carboxymethyl cellulose nanocomposites

display a morphology dependent response to hydrogen gas. Green Chemistry 2012, DOI:

10.1039/C2GC16456F.

Author contributions: Zou, Iyer, and Raston designed the research; Zou performed the

research; Zou, Iyer and Raston finalised the manuscript. (Overall contribution by Zou: 90 %)

2. Zou, J.; Zdyrko, B.; Luzinov*, I.; Raston, C. L.; Iyer, K. S.*, Regiospecific linear assembly of Pd

nanocubes for hydrogen gas sensing. Chemical Communications 2012, 48, (7), 1033-1035.

Author contributions: Zou, Zdyrko, Luzinov, and Iyer designed the research; Zou performed

the research; Zou, Zdyrko, Raston, and Iyer finalised the manuscript. (Overall contribution by

Zou: 85 %)

3. Zou, J.; Martin, A. D.; Zdyrko, B.; Luzinov, I.; Raston, C. L.; Iyer, K. S. *, Pd-induced ordering

of 2D Pt nanoarrays on phosphonated calix[4]arenes stabilised graphenes. Chemical

Communications 2011, 47, (18), 5193-5195.

Author contributions: Zou, and Iyer designed the research; Zou and Martin performed the

research; Zou, Zdyrko, Luzinov, Raston, and Iyer finalised the manuscript. (Overall

contribution by Zou: 80 %)

4. Zou, J.; Iyer, K. S. *; Stewart, S. G. *; Raston, C. L., Scalable synthesis of catalysts for the

Mizoroki-Heck cross coupling reaction: palladium nanoparticles assembled in a polymeric

nanosphere. New Journal of Chemistry 2011, 35, (4), 854-860.

Author contributions: Zou, Iyer, Stewart, and Raston designed the research; Zou performed

the research; Zou, Iyer, Stewart and Raston finalised the manuscript. (Overall contribution by

Zou: 90 %)

xiv

5. Zou, J.; Stewart, S. G.; Raston, C. L. *; Iyer, K. S., Surface oxygen triggered size change of

palladium nano-crystals impedes catalytic efficacy. Chemical Communication 2011, 47, (18),

5193-5195.

Author contributions: Zou, Iyer, Stewart and Raston designed the research; Zou performed

the research; Zou, Iyer, Stewart and Raston finalised the manuscript. (Overall contribution by

Zou: 85 %)

6. Zou, J.; Iyer, K. S. *; Raston, C. L., Hydrogen-induced reversible insulator–metal transition

in a palladium nanosphere sensor. Small 2010, 6, (21), 2358-2361.


research; Zou, Iyer and Raston finalised the manuscript. (Overall contribution by Zou: 85 %)

7. Zou, J.; Hubble, L. J.; Iyer, K. S. *; Raston, C. L. *, Bare Palladium nano-rosettes for real-time

high-performance and facile hydrogen sensing. Sensors and Actuators B: Chemical 2010, 150,

(1), 291-295.


research; Zou, Iyer, Hubble, and Raston finalised the manuscript. (Overall contribution by

Zou: 85 %)

8. Bradshaw, M..; Zou, J.; Byrne, L.; Iyer, K. S.*; Stewart, S. G.*; Raston, C. L., Pd(ii) conjugated

chitosan nanofibre mats for application in Heck cross-coupling reactions. Chemical

Communications 2011, 47, (45), 12292-12294.

Author contributions: Zou, Iyer, and Raston designed the research; Bradshaw performed

the research; Bradshaw, Zou, Iyer and Raston finalised the manuscript. (Not included in this

dissertation)

9. Fang, J.; Pillai, S. R.; Saunders, M.; Zou, J.; Lorenser, D.; Sampson, D. D.; Guo, Y.; Lu G.; Iyer,

K. S.*, Room temperature synthesis of upconversion fluorescent nanocrystals. Chemical

Communication 2011, 47 (36), 10043-10045.

xv

Author contributions: Fang, Pillai and Zou performed the research. (Not included in this

dissertation)

Patents

PCT/AU2010/000958: A device and methods of fabrication and use of said device, 2010

Inventors: Raston, C.; Iyer, K. S.; Hubble, L.; Zou, J.; Watling, J.

Conferences

Oral presentation

1. The WA Synthetic and Organic Group-Royal Australian Chemical Institute (RACI) one

day Symposium, 29, November, 2010, University of Murdoch, Australia

Scalable synthesis of catalysts for Heck cross coupling reaction: palladium nanoparticles

assembled in a polymeric nanosphere

2. 240th American Chemical Society National Meeting, 22-26, August, 2010, Boston, USA

Dynamic thin film fabrication of Pd nanomaterials for application in hydrogen gas sensing

3. Interface Science and Nanotechnology Forum, 5, March, 2010, University of Curtin,

Australia

Synthesis of Pd nanomaterials under continuous flow for the application in hydrogen gas sensing

Poster Presentation

4. International Conference on Nanoscience and Nanotechnology (ICONN), 22-26

February, 2010, Sydney, Australia

Real time hydrogen sensing involving bare Pd nano-rosettes

5. AMN4 (The MacDiarmid Institute for Advanced Materials and Nanotechnology), 8-12,

February, 2009, University of Otago, New Zealand

Synthesis of Pd nanoparticles under continuous flow and its application as hydrogen sensor

xvi

Statement of Candidate Contribution

This dissertation contains the results of work carried out by the author in the Discipline

of Chemistry, School of Biomedical, Biomolecular and Chemical Sciences at The University of

Western Australia during the period March 2008 to August 2011.

The work presented herein contains no materials which the author has submitted or

accepted for the award of another degree or diploma at any university and, to the best of

the author’s knowledge and belief, contains no material previously published or written by

another person, except where due reference is made in the text.

Jianli Zou

August 2011

xvii

Statement of Candidate Contribution

This dissertation contains the results of work carried out by the author in the Discipline

of Chemistry, School of Biomedical, Biomolecular and Chemical Sciences at The University of

Western Australia during the period March 2008 to August 2011.

The work presented herein contains no materials which the author has submitted or

accepted for the award of another degree or diploma at any university and, to the best of

the author’s knowledge and belief, contains no material previously published or written by

another person, except where due reference is made in the text.

Jianli Zou

August 2011

Chapter 1 Introduction

1

1. Introduction

1.1 Overview

The aims of this dissertation were to explore a feasible method to synthesise Palladium

(Pd) nanomaterials in large scale under continuous flow and apply these Pd nanomaterials in

hydrogen gas sensing and as recyclable nano-catalyst for C-C bond forming cross coupling

reactions. Accordingly, this chapter reviews the literature to date relating to Pd

nanomaterials synthesis and applications. The application of Pd nanomaterials in hydrogen

sensing will be first discussed. This will include an introduction of existing hydrogen sensing

technology, the working principles of Pd resistive sensing, and a summary of the Pd

nanomaterials used in resistive hydrogen sensing. Following this the application of Pd

nanomaterials in the Heck cross coupling reaction will be introduced. This will include a

preamble of Pd catalysed Heck cross coupling reaction, the proposed mechanism in Heck

reaction, and the catalytic activity of various Pd nano-catalyst in heterogeneous catalytic

system, followed by the phenomenon of size change of Pd nanoparticles in Heck reaction.

Finally, the synthesis of Pd colloidal and supported nanomaterials will be discussed.


2

1.2 Pd in Nanoscale

Pd materials have attracted considerable interest for their application in fields such as:

hydrogen purification, hydrogen storage, catalytic converters, electronics, and fuel cells.1 In

this PhD project the applications of Pd nanomaterials in catalysis and hydrogen gas sensing

have been studied.

1.2.1 Pd Nanomaterials for Hydrogen Sensing

Pd has been widely studied as hydrogen detection materials due to its high sensitivity

and selectivity to hydrogen gas, fast response, and operability at room temperature. Novel

gas sensors based on nanomaterials can enhance the performance of conventional devices

through nano-engineering.2

Improved sensitivity is a major attraction to develop nanotechnology enabled sensors.

At the nanoscale it is possible to detect a single molecule or atom which bulk materials

cannot.3, 4 The small size, lightweight, and high surface-to-volume ratio of nanostructures

enables them to be excellent candidates for improving the capability to detect chemical and

biological species with high sensitivity and fast response time.5, 6 Progress in the synthesis of

various metal and oxide materials with high surface area as well as implementation of newly

developed nanofabrication techniques offers tremendous opportunities for sensor

manufacturers. It has been reported that in gas sensitive materials the reduction of the grain

size to nanometre dimensions can significantly enhance gas sensor performance.7 By

engineering and fine tuning the morphology of particles sensitivity can also be increased.

The enhanced performance in electrical signal of such materials is due to several factors

including grain coalescence, porosity and grain-boundary alteration.2, 8 In addition,

nanostructures increase the surface-to-volume ratio compared to their microstructure

counterparts; this generally enhances the sensor response time.9


3

1.2.2 Pd Nanomaterials in Catalysis

The increase in the surface area of nanostructure materials can also lead to an increase

in catalytic activity. It has been shown that the activity and selectivity of a particular reaction

can be greatly enhanced by carefully choosing a specific crystal facet/type of a

nanocrystal.10-13 Pd serves as a good catalyst in hydrogenation, dehydrogenation reactions as

well as C-C bond forming cross-coupling reactions (eg. Heck, Suzuki, Sonogashira or Stille

reactions etc.).14-16 The dissociative adsorption of molecular hydrogen on transition metal

surfaces is the first elementary step in a catalytic hydrogenation reaction.17 Pd can dissociate

the hydrogen molecule at room temperature which makes it one of the most desirable

candidates for a hydrogenation catalyst.

Synthetic chemists have also been interested in Pd catalysed formation of C-C bonds.

Traditional homogeneous catalysts based on Pd complexes have been intensively studied

due to their effectiveness and reaction versatility.18 A wide array of new, highly effective,

heterogeneous catalysts have been developed in recent years. Catalyst recycling is

enormously important from the point of view of industrial applications and the

environment.19 Pd nanomaterials have shown good recyclability and, consequently,

provided an efficient and economical way to perform C-C cross-coupling reactions. Pd

nano-catalysts also do not require the use of phosphine ligands and are not sensitive to air

and moisture. These catalytic species often afford coupling products in high yields at short

reaction times, and can often be applied under mild conditions.20 Reactions can be carried

out in water, or, in a few rare cases, even in solventless operations. Such catalyst systems are

valuable alternatives to homogeneous Pd-complexes catalysts.21, 22

In summary, in recent decades high surface area Pd nanomaterials have

demonstrated excellent performance in hydrogen sensing, and have also been studied

extensively as catalysts for numerous reactions.


4

1.3 The Application of Pd Nanomaterials in Hydrogen Sensing

1.3.1 Introduction to Hydrogen Sensing

Hydrogen is a clean, renewable, and sustainable energy carrier. The concept of the

energy-delivery system using hydrogen, or the “hydrogen economy” and its long-term

development is important as a sustainable alternative to fossil fuels.23 Hydrogen is a

colourless, odourless gas that forms an explosive mixture with air in the concentration range

of 4–74% by volume. Moreover, at high concentrations, hydrogen will exclude an adequate

supply of oxygen, causing asphyxiation. One critical aspect for the safe and efficient

deployment of hydrogen is the ability of sensors to meet the required performance

specifications for the growing hydrogen infrastructure. Accordingly, the monitoring of the

potential leakage of this highly combustible and explosive gas has become critical. A

number of commercially available hydrogen safety sensors currently exist. Each sensor type

has unique operating principles that will ultimately control its performance, and thus each

technology has its advantages and limitations. No existing technology will be ideally suited

for all applications, and none have been shown to meet all of the DOE target specifications

(as shown in Table 1.1).

Table 1.1 The US Department of Energy (DOE) targets for hydrogen sensor. a

Targets

Measurement range: 0.1%-10%

Operating Temperature: -30 to 80 oC

Response Time: under one second

Accuracy: 5% of full scale

Gas environment: ambient air, 10-98% relative humidity range

Lifetime: 10 years

Interference resistant (e.g., hydrocarbons) a http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/safety.pdf


5

Some sensor platforms may be strongly affected by variations in ambient temperature

but would be unaffected by relative humidity (e.g., electrochemical sensors), and the

contrary might be true for other technology types (e.g., metal oxide sensors). Since

end-users have a broad range of sensor options, the final selection of an appropriate sensor

technology can be complicated.

1.3.2 Hydrogen Sensor Types and Performance

The most common sensor platforms for hydrogen, either commercially available or in

laboratory research stage, include electrochemical sensor, MOX (metal oxide) sensor, thermal

conductivity sensor, MOS (metal-oxide-semiconductor) sensor, optical device and resistive

Pd sensor.24, 25 The last three types of sensor usually involve Pd based materials.

1.3.2.1 Electrochemical Sensor

A conventional electrochemical sensor is mainly composed of an electrochemical cell:

(counter electrode) air, Pt | proton conductor | Pt, sample gas (sensing electrode).26 Gas that

comes in contact with the sensor first passes through a small capillary-type opening, diffuses

through a hydrophobic barrier, and eventually reaches the electrode surface. The gas that

diffuses through the barrier, in the case of hydrogen, reacts at the sensing electrode surface

through an oxidation reaction:

H2→2H++ 2e-

A corresponding reduction reaction occurs at the counter electrode:

O2 + 4H+ + 4e-→ 2H2O

These reactions cause a potential difference between the electrodes and the H2

concentration is correlated with this potential difference by a non-linear relationship. A

reference electrode is added to the cell to improve repeatability and stability of

measurements. The principle of operation is demonstrated in Figure 1.1. Recent studies

have intensively focused on solid polymer electrolytes, which eliminate fluid leakage, and

limit corrosion and volatilisation in comparison with liquid electrolytes.27-29


6

Figure 1.1 Schematic of an electrochemical type hydrogen sensor. 30

Electrochemical sensors consume very little power during operation which is

particularly convenient in automotive applications. Disadvantages include limited selectivity

(e.g., CO may also affect the sensor), a restricted temperature range due to a liquid

electrolyte, and a dependence on barometric pressure. Many electrochemical sensors

require oxygen for long-term stability, and thus should not be used in nitrogen or other inert

atmosphere without prior validation for a specific model.

1.3.2.2 MOX (metal oxide) Sensors

MOX sensors are fabricated with wide band gap semiconducting metal oxides, such as

SnO2, V2O5, and WO3 as the active part of the sensor over a thin, solid, and non-conducting

membrane.31 The metal oxide is usually embedded in a porous ceramic matrix and

traditionally is configured as a bead shape formed around an internal heater coil. Operation

at elevated temperatures is required to obtain a stable measurable conductivity. Gaseous

analytes, such as hydrogen, carbon monoxide and methane, diffuse into the porous

structure and react with the oxygen adsorbed on the metal oxide surface and decrease the

surface concentration of oxygen. This lowers the surface potential, thereby decreasing

resistance. In addition to the embedded heater, a probe wire is also embedded and is used

to measure device resistance (the ground point of the embedded heater often serves as the

second probe point for resistance measurements), Figure 1.2.


7

Figure 1.2 Schematic of a metal oxide hydrogen sensor (adapted from reference 32). The electrical resistance

variation is measured using a Wheatstone bridge.

The MOX sensor is a small, readily produced device and has sufficient sensitivity for

hydrogen safety application. As a high-temperature device the MOX sensor is not

dramatically affected by variations in ambient temperature. The MOX sensor is not

considered to be a selective device as its response is affected by other compounds such as

H2O, CO and CH4. Another major disadvantage of the classical MOX sensor is along response

recovery time.

Several advances in the recent MOX technology have been made, particularly in the

development thin-film designs.33, 34 In addition to potential cost reductions, the thin-film

geometry requires significantly less power for operation. The robustness of these thin-film

designs in relation to environmental parameters (pressure and relative humidity), long-term

stability and selectivity still need further investigation.

1.3.2.3 Thermal Conductivity Sensors

Thermal conductivity sensors rely upon a temperature-induced change of an

electrically heated sensing element following exposure to the analyte. The thermal

conductivity sensor is heated to a temperature in which the resistance of the sensing

element deviates from the linear limit of Ohm’s law (V = I × R). The resistance no longer

remains constant and a deviation from a linear I-V plot is observed, as shown in Figure 1.3.

The thermal conductivity (λ) of the surrounding gas affects the shape of the I-V curve. Hence,


8

for a given applied power the resistance of the sensing element will depend upon the

surrounding gas. Thermal conductivity is a property of the gas; each gas has the ability to

conduct heat at a specific rate. Hydrogen gas has the highest thermal conductivity of all

known gases (186.9 mWm-1K-1 compared to 26.2 mWm-1K-1 for air, both at 298 K and 101.325

kPa).30 Thermal conductivity sensors exploit this property for detection and monitoring of

hydrogen gas.

Figure 1.3 Empirical current–voltage curve for a thermal conductivity sensor (adopted from reference 24).

Operation of a microelectromechanical thermal conductivity sensor requires less than

a milliwatt (useful for battery operation). Unlike electrochemical sensor and MOX sensor

platforms, the thermal conductivity sensor does not require oxygen for long-term, stable

operation. This makes it amenable for use in process streams or for those applications which

use a nitrogen purge. However thermal transport between a solid and a gas will be

dependent on the temperature, density, and composition of the gas. These sensors are

affected by environmental parameters including temperature, pressure humidity and

contamination. Thermal conductivity hydrogen sensors also suffer from limitations on

detecting low concentrations of hydrogen and its selectivity, although they offer a dynamic

measuring range up to 100 vol-% hydrogen.

1.3.2.4 MOS (metal-oxide-semiconductor) Sensor

A MOS sensor, as another type of hydrogen sensor, was proposed by Lundström et al.

in 1970s, which is based on a metal oxide field effect transistor.35 Hydrogen MOS sensors

based on Pd sensing have been widely studied in the past three decades by numerous


9

groups.36-38 In a Pd based MOS sensor, the adsorbed and dissolved hydrogen atoms form a

dipole layer at the Pd/insulator interface (normally silicon oxide) and cause the work function

of the interface to change (as illustrated in Figure 1.4).39 The response caused by the

hydrogen induced dipole can be monitored by capacitance-voltage (C-V) measurements

even at room temperature.40, 41 The weak points of MOS sensors are premature saturation of

detectable hydrogen concentrations and low sensitivity.

MOS high electron mobility transistors 42 and MOS Schottky diodes 43 have emerged

for high performance H2 sensing. These devices have achieved improved sensitivity and

reasonable response times and detection limits.

Figure 1.4 “Classical” schematic illustration of the hydrogen sensitive field effect devices, where hydrogen

atoms adsorbed at the metal–oxide interface result in a shift of the electrical characteristics along the voltage

axis in devices having catalytic metal (Pd) gates.44

1.3.2.5 Optical Devices

Sensors for the direct optical detection of hydrogen are not readily available because

hydrogen does not adsorb in the ultraviolet-visible (UV) or infrared (IR) regions. However,

very sensitive sensor platforms which undergo changes in optical properties upon exposure

to hydrogen have been developed. The two main classes of chemically mediated optical

hydrogen sensors are colorimetric indicators and fiber optic based devices.

• Colorimetric indicators

In colorimetric indicators for hydrogen, tungsten oxide is partially reduced in the

presence of hydrogen in concentrations as low as 300 ppm, and this causes a distinct colour


10

change.45 Figure 1.5a shows a recording of transmittance vs. time for WO3:Pt nanoparticle

dispersed on a filter paper produced by Element One's patented technology (US patent

6895805). The transmittance of the sample decreases as the sample changes from light

off-white colour to blue colour (Figure 1.5b). The rate of change in the transmittance reflects

the rate of chemical reaction occurring between H2 and WO3. In each of these designs, Pt or

Pd is applied to the thin film of WO3 to increase its reaction rate with hydrogen gas.

Figure 1.5 a) Transmittance of a WO3:Pt nanoparticles dispersed on a filter paper and exposed to 0.5% H2/N2

mixture. b) A hydrogen colourimetric indicator showing the colour change when the hydrogen gas on and

off. (Graph a) adopted from paper presented by D. K. Benson, R. D. Smith, W. Hoagland, at NHA Meeting in

Long Beach, CA March 14, 2006, Novel wide-area hydrogen sensing technology. Image b) adopted from

Website of Element One at http://www.elem1.com/technology.htm)

Such systems do not require electronic circuitry for operation or detection, and

therefore can complement electronic sensor technology. Visual detection is all that is

required. A number of possible configurations for the colorimetric indicators include

hydrogen-indicating paints, tape, cautionary decals, and coatings for hydrogen storage

tanks. The indicator can also be incorporated into an electronic platform. The WO3/Pd or

WO3/Pt material is not significantly affected by temperature, pressure, or relative humidity.

The transformation rate increases at higher temperatures and, conversely, might be

impeded at low temperatures. This system is low cost and easily implemented, although a

quantitative output has not yet been reported.

• Optical fibre sensor-with Pd material

Various types of optical fibre sensors are used for the detection of hydrogen, amongst

them the micromirror sensor is probably the most developed one. The micromirror sensor’s

a) b)


11

structure is quite simple, as shown in Figure 1.6a. It consists of a thin film of Pd evaporated

at the cleaved end of a multimode fiber optic and the thickness of Pd is 10 to 50 nm.46 In the

presence of H2 the optical properties of the Pd change; consequently, the reflected wave is

modified whereas the incident wave remains the same. The changes in the reflectivity of

thin Pd films deposited is measured to detect the hydrogen (Figure 1.6b).

Figure 1.6 a) Pd micromirror hydrogen sensor; b) Response of the hydrogen sensor to varying concentrations

of H2 in N2; c) Evanescent field fiber optic hydrogen sensor; d) Optical transmission of a fibre optic hydrogen

sensor after dosing with a 3% H2 in argon. Also plotted is the flat response of the sensor upon exposure to

100% argon.46-48

Evanescent field fiber optic hydrogen sensors consist of a core-exposed fiber optic

where Pd is deposited (Figure 1.6c).47-49 The light travelling through the fibre causes

evanescent waves on the fiber core surface. If the core of the fibre is covered with a Pd layer,

the evanescent fields are altered and if hydrogen is absorbed by a Pd film, the refractive

index of the Pd coating changes. This change in refractive index modifies the absorption of

the guided light, which can be detected by monitoring the light intensity, via interferometer

techniques (Figure 1.6d).

In this method there is no need to use electric signals that may cause electric discharge

and remote sensing in hazardous environments is possible. However, light sources such as

lasers and complex detection systems are required, which makes it poor in both portability

and cost-effectiveness.50 Environmental effects from temperature, barometric pressure, and

relative humidity are still present and are disadvantages of this system.

a)

b)

c)

d)


12

1.3.2.6 Resistive Pd Sensor

Integrating Pd within a conventional circuit is emerging as a new type of hydrogen

sensor due to its high sensitivity, high H2 selectivity, low detection limit,

room-temperature-operability, and low power consumption. This type of sensor consists of

an active Pd surface in the form of a Pd film, wire, or particles. The electrical resistance

changes with adsorption of hydrogen, and this resistance can be monitored directly. Such

devices have the advantage of a very simple transduction signal.51-57

In general, the resistive Pd sensor is still at the experimental stage. The long-term

stability of the coating is not fully characterized in many real-world environments, but

promising performance has been reported. The devices tend to show slow response time

relative to other sensors though. However, some Pd sensors have been reported recently on

a micro-electro-mechanical system (MEMS) platform that have shown considerably

improved fast response time.58, 59

The hydrogen sensors developed in this dissertation are Pd nanomaterials related

resistive sensors. More details will be provided in next section.

1.3.2.7 Other Types of Hydrogen Sensor

Pd films can also be applied to coat mechanical devices such as the surface acoustic

wave sensor, quartz crystal microbalance sensor or micro-cantilevers, to achieve hydrogen

selectivity.

A surface acoustic wave sensor is built around two inter-giddied transducers placed

on the surface of a piezoelectric substrate. By inducing an alternating current across the

metallic conductors of the entrance transducer, alternating compressions and expansions

occurs which generates a surface wave. This wave moves towards the second transducer to

be converted back to an electric signal. When absorption or desorption of hydrogen takes

place, changes in both the density and elasticity of the Pd film occurs, causing the

propagation velocity of surface waves to change. This physical property in turn produces a

phase shift at the output of the sensing channel and as a consequence a signal is detected.60


13

In the microbalance sensor, the adsorption of hydrogen changes the mass of the Pd

film, and this mass change affects the resonant frequency of the mechanical sensor. Small

changes in frequency can be accurately measured, which gives the excellent detection limit

and range of these devices.61

Micro-cantilevers fabricated by MEMS processes are typical representatives for

chemi-mechanical methods to detect hydrogen. The surface of the micro-cantilevers is

functionalized with Pd thin films. Volume expansion of a Pd film produced by hydrogen

absorption bends the micro-cantilever and this mechanical change is monitored by

parameters such as deflection, resonance frequency, and capacitance.62

As briefly discussed, all H2 sensing methods have advantages and disadvantages. The

resistive Pd sensor is emerging as one of the central subjects in this area due to its overall

performance, simplicity, possibility of sensor miniaturisation, manufacturability, and

compatibility of sensor fabrication process with an integrated circuit. The development of

nanotechnology fabrication techniques has fuelled the development of Pd nanostructures

such as two-dimensional Pd thin films, one-dimensional Pd nanowires, Pd nanorods, and Pd

nanoparticles. The H2 sensing performance of the Pd nanostructures is in general better than

that of bulk Pd, due to the increased surface-to-volume ratio. The market demands for

commercial hydrogen sensor are increasing each year. Although commercial systems are

not yet available, devices based on nanomaterials and designs are being developed for

sensor applications and these can be expected to be incorporated into commercial devices

in the near future. Although gaps in sensor technology for real-world applications still exist,

research in sensor technology is an on-going process that will ultimately lead to

improvements in performance of commercial systems.63

1.3.3 Working Principles of Pd Resistive Sensing

When Pd is exposed to H2, hydrogen molecules are adsorbed onto the Pd surface and

dissociated into hydrogen atoms. Hydrogen atoms diffuse and occupy the interstitial sites of

the Pd lattice, causing a certain amount of lattice expansion.64 The diffusion generally takes


14

place through high diffusivity paths such as grain boundaries and dislocations or via a

vacancy exchange mechanism, resulting in the defect density dependence of H2 intake.65

The absorbed hydrogen atoms interact with Pd atoms to form Pd hydrides and increase the

frequency of scattering events of charge carriers, which directly leads to the increase in

resistance of the Pd.66 In this context, it is expected that the magnitude of resistance increase

is proportional to the atomic fraction of absorbed hydrogen atoms to Pd atoms, as

expressed by the Sieverts’ law (Equation 1):

where [H] and [Pd] are the respective concentrations of hydrogen atoms and Pd atoms in

the Pd-H system, KS is the Sieverts’ constant, and pH2 is the H2 partial pressure in the

environment.67 Sieverts’ law refers to the bulk Pd-H system in the isothermal state. According

to Equation (1), the resistance change with respect to a reference value, termed sensitivity, is

proportional to the relative hydrogen concentration in the solid state, which is correlated

with the square root of the hydrogen partial pressure in the gas phase. From this equation, it

is inferred that the resistance change on exposure to H2 is determined by the H2 flux

impinging the Pd surface, number of hydrogen accommodation sites, and the hydrogen

diffusion rate in the Pd.

Although the above description illustrates well the general features of the electrical

resistance change of Pd in the presence of H2, it is appropriate only for the α phase of the Pd

hydride, where a lattice expansion caused by hydrogen filling in the interstitial sites is smaller

than 0.13%: a0 = 3.889 Å for pure Pd vs. aα, max = 3.895 Å for the upper boundary of the α Pd

hydride. Pd exhibits two distinct hydride phases, denoted as α and β, corresponding to low

and high concentration, respectively, of hydrogen atoms incorporate into the Pd crystal

structure. Once the concentration of the absorbed hydrogen atoms exceeds the α-phase

boundary, nuclei of the β phase start to form in the α matrix. The β nuclei grow until the

whole original α matrix is transformed to the β phase, where the lattice expansion reaches

3.47%: a0 = 3.889 Å vs. aβ, min = 4.025 Å for the lower boundary of the β-Pd hydride. The

hydrogen concentration range for coexistence of the α- and β-phases depends on


15

temperature and it grows broader as temperature decreases below ~300 °C, indicating

smaller interstitial sites in the α phase and higher density of imperfections in the β-phase at

lower temperatures. Sakamoto et al. carefully investigated a relationship between hydrogen

gas pressure (pH2), hydrogen concentration in Pd (H/Pd), and relative resistance in the

pressure (R/R0) at a specific temperature, using a gas phase method.66 In contrast with the

previous monotonic increase in resistance with hydrogen concentration in Pd, as expressed

by Sieverts’ law, the Sakamoto group obtained three characteristic regions with different

rates of resistance increase. They correspond to the α phase, α+β mixed phase, and β phase,

respectively, as schematically shown in Figure 1.7.

Hydrogen Concentration (H/Pd)

Relativ

e Re

sistance (R

/R0)

α βα+β

Figure 1.7 Schematic illustration of relative resistance (R/R0) as a function of relative hydrogen concentration

(H/Pd) for absorption-desorption processes. The arrows indicate the directions of absorption and desorption

processes. Adapted from reference 25.

There are two types of resistance-based hydrogen gas sensor utilising the change of

resistance and volume of Pd in the presence of hydrogen. The first type of sensor is based on

the formation of Pd hydride causing the increase in resistance. Sensors operating through

this mechanism usually have Pd materials very well connected to two electrodes. In the

presence of H2 the current decreases, as shown in Figure 1.8. This type of sensor can be

very sensitive and the limit of detection could be down to several ppm (part per million).

Most resistive sensors are based on this mechanism.


16

Figure 1.8 Change of resistance as a function of time in the presence of 0.1% hydrogen and air alternately.54

The other mechanism is based on the volume expansion of Pd in the presence of

hydrogen forming a more connected structure which causes the decrease of resistance. The

first example showing this decrease in resistance in the presence of hydrogen gas was

demonstrated by Penner and co-workers in 2001.68 Pd meso-wires consisting of break

junctions were used and Pd swells in the presence of H2 to close the break junctions that

accounts for the decreased resistance of the sensor (Figure 1.9). According to the lattice

constant of Pd, α-phase and β-phase Pd hydride, (3.889Å, 3.895Å and 4.025Å, respectively)

the size change of Pd hydride mostly happens at α-phase to β-phase transition. At room

temperature, the transition from α-phase to β-phase occurs at 1~2 % H2, so this type of

sensor shows obvious response to hydrogen only at the concentration of > ~2 % (Figure

1.9c).

Figure 1.9 Atomic force microscope (AFM) images of a Pd mesowire on a graphite surface, a) acquired in air

and b) acquired in a stream of hydrogen gas (a hydrogen-actuated break junction is highlighted). c) Current

response of the sensor to hydrogen (concentration of H2 in percentage as shown).68

c)


17

1.3.4 Pd Nanomaterials Used in Resistive Hydrogen Sensing

1.3.4.1 Pd Nanoporous Films

Pd films are a basic form of low-dimensional Pd nanostructures, which can be easily

adapted into practical hydrogen sensor applications. In these structures, the thickness of the

films is usually restricted to less than several hundreds of nm, while the width is flexibly

varied to fit into the configuration of sensor. The very low aspect ratio of film thickness to

width generally facilitates hydrogen absorption and desorption processes, and makes the

Pd film/substrate interface profoundly important in the cyclic hydrogen sensing. Despite

many strong points the conventional dense Pd films have, they are generally difficult to use

for measuring H2 concentration lower than 500 ppm with precision and their response time

still needs to be improved. To address these issues, Pd thin films with nanopores have been

investigated. For example, AAO (anodic aluminum oxide) supported Pd thin films showed a

lower H2 detection limit and faster response than the dense Pd films due to significantly

increased surface area. Ding et al. fabricated AAO-supported nanoporous Pd thin films and

compared their response characteristics with those of dense Pd films (morphology of Pd

films see Figure 1.10).51, 69 These thin porous Pd films could clearly detect H2 concentrations

as low as 250 ppm. Comparing the responses of dense Pd and porous Pd films with an

identical thickness of 45 nm, in the presence of H2 the change in resistance in the porous film

was faster than that of the dense film. The faster response and lower detection limit of the

thin nanoporous Pd film originated from the contributions of a thin thickness effect and an

enlarged surface effect. In addition, the film delamination problem associated with the

dense Pd films is alleviated in the nanoporous structures owing to the potentially enhanced

adhesion.


18

Figure 1.10 Scanning electron microscopy (SEM) images of a) an AAO template and b) a nanoporous Pd film

on top of a pyrolytic, carbon-coated AAO template. 51

1.3.4.2 Pd Nanowires

Through the study of Pd films described previously (in section 1.3.4.1), it was

demonstrated that the H2 detection limit and response time could be reduced in

nanoporous structures due to their increased surface area. If this is the case, Pd nanowires

would be ideal structures for fast detection of low concentration of H2. Unfortunately,

nanowires are, in general, harder to fabricate compared to thin films. Nevertheless, Pd

nano-wires/meso-wires obtained from different methods are widely used as sensing

materials.5, 57, 70-72

Pd Nanowires fabricated by electron-beam lithography have been shown to

detect hydrogen with a fast response and a low detection limit.73 However, these Pd

nanowires face the risk of structural deformation at high H2 concentrations because

the nanowire body sticks to the substrate. This problem can be eliminated using

bottom-up grown Pd nanowires, which have no direct bonds with the substrate This

bottom-up approach has been demonstrated by growing the Pd nanowires by

electrodeposition in the nanochannels of an AAO template followed by chemically

removing the AAO template.9 The Pd nanowires were dispersed onto a thermally

oxidized Si substrate with patterned outer electrodes on it by a drop-casting method.

A combination of e-beam lithography and a lift-off process was used to pattern inner

Au electrodes on an individual Pd nanowire. The representative four terminal device

on the individual Pd nanowire is shown in Figure 1.11a. Figure 1.11b exhibits the


19

representative electrical resistance response to the presence of H2 of 10,000 ppm for

Pd nanowire with d = 20 nm.

Figure 1.11 a) A representative individual Pd nanowire device with d = 20 nm; and b) the electrical resistance

response to H2 for single Pd nanowires with diameters d = 20 nm at 10,000 ppm partial pressures of H2.9

Similarly, free standing Pd nano-wires fabricated by electro-deposition through AAO

template were used directly in hydrogen gas sensing without further reconfiguration

(Figure 1.12).74 Pd nanotubes grown from the pores of polycarbonate membranes have also

been described and these nanotubes have shown a very fast response to hydrogen gas.75

Figure 1.12 Schematic diagram of Pd nanowire hydrogen fabricated in reference. 74

Electrodeposition has the ability to alter the morphology of Pd nanowires by changing

the growth condition.58 For instance, three types of Pd nanowire structures were obtained

on polymethylmethacrylate (PMMA) channels between two Au electrodes by

electrophoresis (as shown in Figure 1.13). Interestingly these Pd nanowires showed

different response to hydrogen gas. The nanowires with plain structure (Figure 1.13a) had

an increased resistance whereas the grain structure (Figure 1.13b) nanowires exhibited the

inverse-type of resistance behaviour. The hairy structure Pd nanowires (Figure 1.13c) were

subdivided into two different internal structures, which showed either normal or inverse

a) b)


20

resistance behaviours, according to experimental parameter, such as current applied during

electrodeposition.

Figure 1.13 SEM images of a) plain structure Pd nanowire with 85 nm diameter; b) grain structure nanowires

with diameter of 150 nm; and c) hairy structure Pd nanowires with diameter of 100 nm.58

Pd meso-wire arrays prepared by electrodeposition onto highly oriented pyrolytic

graphite and then transferred to a cyanoacrylate film have been used to fabricate hydrogen

activated switches (Figure 1.9a).68, 76 In the absence of H2, the resistance of Pd meso-wire

was large due to the existing break junctions. The sensor also dissipated no power

and produced no noise. Above a threshold of approximately 2% H2, the breaks closed

due to the volume expansion and the device resistivity became measurable.

1.3.4.3 Pd Nanoparticles

Fabrication of Pd nanowires with precise control of the diameter can be very

challenging. Pd nanoparticles related materials can be synthesised by feasible methods and

have been demonstrated that their enlarged surface-to-volume ratio and high grain

boundary density cooperatively contributed to an increased amount of H2 up-take and fast

hydrogen response.56 Pd nanoparticles or Pd nanoparticle decorated carbon nanotubes

have been exploited as hydrogen sensor with high performance.

Pd nanoparticles stabilised by surfactant77 and DNA52 have been drop-cast onto

pre-made sensor chips and then used in hydrogen sensing.

Pd nanoparticle decorated SWNTs (single wall carbon nanotubes) have been used as

high-performance hydrogen sensing materials by several groups.54, 55, 78-82 In Sun’s method,

SWNTs were synthesised on the surface of SiO2/Si substrate and then Pd nanoparticles were


21

deposited onto SWNTs directly via electron beam evaporation.54 Pd nanoparticles decorated

SWNTs (Figure 1.14) have also been synthesised by electrochemical method.81

Figure 1.14 AFM images of a) an unmodified SWNT network and b) a SWNT network electrochemically

modified with Pd nanoparticles used in hydrogen sensing. 81

Ju et al. 83 synthesised Pd nanoparticles decorated SWNTs by a multi-step method.

First, the SWNTs were pre-treated to form terminal amine groups on their surfaces and then

chemically modified by polyamidoamine dendrimers. Subsequently, Pd nanoparticles were

grafted onto the dendrimer-modified SWNTs in aqueous solution involving the reduction of

PdCl42−. Pd decorated SWNTs further undergo pyrolysis (200 °C, 12 h) to remove the

dendrimers. These materials showed an increase in resistance upon exposure to H2 with fast

response time. Besides carbon nanotube, graphene with electron beam deposited Pd

nanoparticles have been used as hydrogen sensing materials and have shown high

sensitivity and fast response to hydrogen gas.84

In summary, Pd-based hydrogen sensing has been extensively investigated over the

past four decades. More recently, the performance of hydrogen sensors has been greatly

improved by the use of various Pd nanostructures, which has been stimulated by the

advancement of nanotechnology. Low-dimensional Pd nanostructures have emerged to

meet the requirement of fast, sensitive, and reliable detection of hydrogen gas. Although the

nanostructures have many advantages, primarily due to the high surface-to-volume ratio,

the manufacturability of hydrogen sensors based on them should be also considered for the

practical use of the nanostructured sensors.


22

1.4 The Application of Pd Nanomaterials in the Heck Cross Coupling

Reaction

1.4.1 Introduction to the Pd Catalysed Heck Cross Coupling Reaction

The Nobel Prize in Chemistry 2010 was awarded jointly to Richard F. Heck,85 Ei-ichi

Negishi 86 and Akira Suzuki 87 for their pioneering work in Pd-catalysed cross couplings

reactions in organic synthesis. Pd catalysis is now considered as an indispensable tool for

both common and state-of-the-art organic synthesis. Among those basic types of Pd-

catalysed reactions, the Heck cross coupling reactions occupy a special place. It has a

chemo-selectivity that underpins its potential to spawn new applications.88 In this section,

the mechanism for the Heck cross coupling reaction will be discussed and the comparison of

catalytic activity between different Pd nano-catalysts will be summarised.

1.4.2 Proposed Mechanism in the Heck Cross Coupling Reaction

In the late 1960s, Prof. Richard Heck published a series of papers on forming arylated

alkenes from the reaction between alkenes and a stoichiometric amount of [Ar-Pd-Cl] or

[Ar-Pd-OAc], generated in situ by reacting ArHgCl with PdCl2 or ArHgOAc with Pd(OAc)2,

respectively.89-91 These pioneering studies by Heck opened the discovery of a new reaction

later known as Mizoroki-Heck cross coupling reaction or simply the Heck reaction. In early

1970s, Mizoroki and Heck improved the aforementioned reaction independently. Mizoroki

et al. reported PdCl2 catalysed arylation of alkenes by iodobenzene in the presence of

potassium acetate as base in 1971.92 In 1972, Heck et al. reported using Pd(OAc)2 as catalyst

and n-Bu3N as base in the arylation of alkenes.85 Phosphine ligands were later introduced to

the catalytic system which dramatically improved the reaction efficiency and turnover

numbers in several cases.93-95 Nowadays phosphine-assisted approach is considered as the

classical and well-established method for the Heck reaction.


23

In 1974, Heck and co-worker proposed a mechanism for the reaction catalysed by

Pd(OAc)2 in the presence of phosphine ligands.93 The mechanism, or catalytic cycle,

uncovered key factors in many types of Pd based transformations from this time. A

simplified cross coupling catalytic cycle is shown in Scheme 1.1.

Pd(OAc)2 + nL

Pd0Ln

ArX

oxidative addition

Ar‐Pd‐X

‐‐

L

L

syninsertion

R

Ar‐Pd‐X

‐L

‐‐‐

R

PdXL2

RH

Ar

HH

β‐hydrideelimination

H‐Pd‐X

‐L

R

Ar

R

Ar

H‐Pd‐X

‐‐

L

L

base

base H+X‐

reductiveelimination

‐‐‐

Scheme 1.1 Simplified catalytic mechanism in Heck reaction on the basis of mechanism proposed by Heck in

1974 when Pd(OAc)2 and monophosphine ligands (L) were involved.

The mechanism starts from Pd(OAc)2, a Pd2+species being reduced to Pd(0) an active

species (Pd0Ln) which is co-ordinatively unsaturated through multiple ligand exchange

equilibria to enter into the catalytic cycle. The first step of the catalytic cycle is the oxidative

addition of the aryl halide to the Pd0Ln, forming an α-aryl-Pd(II) halide (ArPdXL2). Then

ArPdXL2 will dissociate one phosphine from the co-ordination sphere and allow

coordination to the alkene. This is then followed by the formation of a carbo-palladium

complex, which undergoes a syn-insertion into the alkene, leading to a α-alkyl-Pd(II) halide.

After internal C-C bond rotation, a α-alkyl-Pd(II) halide goes through a syn β-hydride

elimination giving a hydridoPd(II) halide also ligated to the arylated alkene. Finally, after


24

dissociation from the arylated alkene, the hydridoPd(II) halide undergoes a base induced

reductive elimination to regenerate the active Pd(0) complex.

The reaction mechanism of the heterogeneously catalysed Heck coupling reaction is

still a matter of discussion and speculation. There are different mechanisms proposed in

terms of true homogeneity or heterogeneity. In the early development of the Heck reaction,

it was widely regarded that both homogeneous and heterogeneous Pd species could

catalyse the reaction. Augustine and co-workers96, 97 proposed that possible catalytic route in

Heck reaction (mostly aryl iodides was used as starting materials) could happen on the

surface of Pd nanoparticles, and that the activity is related to the number of low-coordinate

corner atoms and defects. These assignments were made based on the observation that

there was no activity in the liquid phase after solid catalyst removal and additional

termination of the activity in the presence of Hg(0).98, 99 The later opinion is in favour of

homogeneous mechanism which the catalytic activity is due to Pd species leaching from the

nano-catalysts into the liquid phase. According to Jones,100 Shmidt and Mametova first

suggested that only the soluble, leached Pd species were active catalyst in the reaction

mixture when using heterogeneous catalysts such as Pd/C or Pd/SiO2.101 This leached species

mechanism has become more widely accepted since early 2000s. Several groups came to

the conclusion Pd nanomaterials act as the reservoir for the active Pd species and after the

completion of catalytic cycle dissociated Pd species was re-deposit to Pd nanoparticle or the

original support containing the Pd nanoparticles. Arai et al. presented a series of research to

show that the soluble Pd species were the true catalyst in the Heck reaction using Pd

cross-transfer experiment.102-104 In these experiments, Pd-free support and supported Pd

catalyst were added into the reaction solution together, and interestingly upon the

completion, Pd was found in both supports. The authors concluded that the supported Pd

leached from the support into solution during reaction. They also proposed that Pd leaching

could possibly be caused by oxidative addition of the aryl-halide bond on the Pd

nanoparticle surface.102 Djakovitch and Köhler105, 106 studied Heck reaction by filtration tests

and the results show significant activity associated with leached Pd species in the solution

phase. Although they were unable to rule out the possibility of the reaction occurring on the


25

surface of Pd catalyst, their results confirmed that soluble Pd species, at least partially,

catalysed the reaction. Further studies on the correlation between Pd concentration and the

reaction rate provide compelling evidence for the soluble, leached Pd catalysing reaction.107,

108 These kinetic data made Augustine’s assumption less convincing. However, it is important

to mention that different authors have used more or less different systems to investigate the

Heck cross coupling reaction and this might be the reason why they come to different

conclusions.

1.4.3 Heterogeneous Catalyst: Pd in the Heck Reaction

The focus of this dissertation will concentrate on heterogeneous Pd catalysis and thus

this topic will be reviewed in this section.

The discovery of catalytic transformation using Pd allows organic chemists to

functionalize olefins, alkynes and aromatics freely and easily. Yet problems arise from catalyst

recovery and the use of potentially toxic and expensive phosphine ligands for stabilising Pd,

as briefly introduced in previous section. These problems hinder the wide application of Pd

catalyst in pharmaceutical industry, in which metal and phosphine contaminants are

considered toxic. Phosphine-free Pd complexes are intrinsically unstable outside of the

catalytic cycle. Pd nanomaterials abandon the synthesis of the toxic ligands used in Pd

complexs, can be recovered from reaction mixture by simple filtration and reused several

times. Hence they are deemed as promising replacement of palladium complexes. Since

1990s’ Pd nanomaterials have been widely studied in many laboratories, with the emphasis

on fabricating novel nano-catalyst, enhancing their catalytic activity, and trying to

understand the nature of the catalytic species. Traditional supported catalysts for the Heck

reaction include Pd/C, Pd/MOx, Pd/Zeolite, and Pd/silica etc..

1.4.3.1 Pd/C

Pd/C is the most studied system and has been used in the industrial production of

octyl 4-methoxycinnamate, a common UV absorber used in the manufacture of sunscreen

lotion. Pd/C was applied to the Heck reaction in the pioneering work of Julia et al. as early as


26

in 1973 (according to Jürgen Liebscher, see reference 109) and has been the most important

heterogeneous, and commercially available catalyst. It was found that the addition of

triphenylphosphine, which is a common ligand in homogeneous Heck reactions, inhibits the

reaction rather than promoting it. Later Pd/C was commonly used alone in the case of

activated Heck reactions without the assistance of ligands. Pd/C usually could be produced

from wet impregnation.110, 111 Wet impregnation is the process where active carbon is

suspended in distilled water, and treated with a H2PdCl4 solution. After impregnation and

heating to 80 the suspension is adjusted to pH 10 by adding sodium hydroxide. Then

[PdCl4]2+ is reduced by adding formaldehyde or hydrazine. After further agitation the catalyst

is collected by filtration and washed with distilled water.

A very detailed investigation of Pd/C in Heck reaction of aryl bromides with olefins has

been conducted in Köhler group.107, 108 In this study a variety of Pd on activated carbon

catalysts differing in Pd dispersion, Pd distribution, Pd oxidation state, and water content

were tested in series of Heck reactions. High Pd dispersion, low degree of reduction,

sufficient content of water, and uniform Pd impregnation are essential criteria in the most

active cross coupling systems. The optimisation of the catalyst and reaction conditions

(temperature, solvent, base, and Pd loading) allowed Pd/C catalysts with very high activity

for Heck reactions of unactivated bromobenzenes to be developed.

Although it has been reported that dissolved Pd species from Pd/C are the real

catalyts,102 almost all of the dissolved Pd species re-deposited on to the surface of carbon

after the completion of reaction. Thus, the Pd/C can be recycled several times without loss of

activity.

1.4.3.2 Pd/Metal Oxides

Jürgen Liebscher mentioned in his review paper109 that the first report of a

heterogeneous Heck reaction using Pd supported on metal oxide was published by Kaneda

et al. in 1990.112 Augustine and O’Leary investigated the Heck reaction of n-butyl vinylether

and 4-nitrohalobenzenes catalysed by Pd/C, Pd/SiO2, Pd/γ-Al2O3, and Pd/MgO, respectively.

They found out that Pd/γ-Al2O3 was practically as active as Pd/C, and Pd/SiO2 was less


27

efficient. 96, 97 They later discovered that the support effects on the regio-selectivity depended

on the para-ring substitution. In the couplings of benzoylchloride and

4-methylbenzoylchloride the order of regio-selectivity was inverted with previous sequence.

The most basic solid support (MgO, prevalence of the α-isomer) showed less regio-selectivity

than the most acidic one (SiO2, prevalence of β-isomers). Another important piece of

research on metal oxide-based catalysts in Heck reaction was provided by Köhler and

co-workers,113 who studied the coupling of bromobenzene with styrene over a number of 5

wt-% Pd catalysts on different substrates. They found that under their standard conditions

the catalyst effectiveness decreased in the order Pd/C>Pd/TiO2>Pd/ZrO2>Pd/MgO>

Pd/ZnO> Pd/SiO2 catalytic systems. The finding that highly acidic supports like titania and

zirconia gave the best yields did not match the previous observation that Pd on MgO, a

definitely basic support, which appeared previously to be the most suitable for the coupling

of aryl halides with alkenes with electron withdrawing substituents.112, 114

1.4.3.3 Pd/Silica

Amorphous silica and ordered mesoporous siliceous materials with anchored

functional groups have been widely used to support Pd catalyst.115 Organochemically

modified silica has been used in the Heck reaction and the organophilicity of the surface

exhibited a strong effect on catalytic performance.116, 117 Ordered mesoporous silica, such as

MCM-41 and SBA-15, are amongst the best support candidates due to their periodic pore

structure and large surface area. MCM-41 (Mobil composition of matter, no. 41) synthesised

with the use of cationic surfactants has a long-range hexagonal framework with a uniform

pore structure. SBA-15, developed at the University of California, Santa Barbara, is prepared

with neutral triblock polyether templates and featured by highly ordered mesoporous

structure with larger pore size and thicker pore walls. By the introduction of an Si-H function

into the channel of SBA-15, Pd(II) precursor could be reduced in situ resulting in a highly

dispersed Pd colloid layer on the pore walls serving as catalyst for Heck reaction.118

Mercaptopropyl- and aminopropyl- modified SBA-15 are efficient reusable catalysts in the

Heck reaction of inactivated aryl bromides.119 Heterogeneity tests such as hot filtration


28

experiments and three-phase tests show that the reaction occurs predominantly via

surface-bound Pd. SBA-16, used also in coupling reaction, has 3D cubic symmetry and a

cage-like structure with open frameworks accessible to metal ions and reagents. Among

other interesting and important properties, their highly specific surfaces render them

attractive in catalysis.

1.4.3.4 Pd/Zeolites

Zeolites are aluminosilicate molecular sieves that have great potential as basic

catalysts and catalyst supports. Aminopropyl-functionalized NaY zeolite supported Pd

nanoparticles have been used to catalyse the Heck reaction with no observable change in

the nanoparticles size.120 On the contrary, other Pd catalyst on aminosilica-based supports in

Heck cross couplings all exhibit significant leaching, recycling, and low activity problems.116,

121, 122 As Jones pointed out, this phenomenon may indicate something unique about zeolite

support. Zeolite encapsulated Pd nanoparticles work best probably because the zeolite

cavities prevent coagulation of the Pd nanoparticles to Pd black.

1.4.3.5 Miscellaneous Novel Supported Pd Nano-catalyst

Besides the above mentioned traditional supported used for Pd catalyst in the Heck

cross-coupling, varies of examples of different materials act as supports for Pd have been

quietly intensively reported. For example, poly(vinylpyridine) nanospheres are employed as

polymeric supports for Pd nanoparticles in the study of the Heck reaction of aryl bromides

and olefins.123 No major changes in nanoparticle size or shape are observed. Microporous

polymers,124 hollow polymer spheres,125 carbon nanofibres,126 sepiolite,127 bacteria,128 and

chitosan flakes129 etc. are also used as supports for Pd nanoparticles in the Heck reaction.

Magnetic nanoparticles can act as supports for Pd nanoparticles, in which immobilized

auxiliaries are used as stabilizing ligands. Here the catalyst can be easily separated using a

magnet and reused at least three times with sustained activity.130


29

1.4.3.6 Pd Colloids

In 1996 Beller et al. first reported the use of preformed colloids as catalyst in the Heck

cross couplings.131 The colloid system effectively activate aryl bromides at 130-140 ,

although the colloids were found to agglomerate irreversibly, leading eventually to

deactivated Pd species. Reetz et al. reported independently in the same year that Pd

nanoparticles stabilised by propylene carbonate could be used as catalysts in the Heck

cross coupling reaction.132 With the temperature above 155 and high loading of catalyst,

this colloidal system was found to be active even for aryl chloride systems. In a later work,

Reetz et al. showed that the combination of Pd nanoparticles with tetraalkylammonium/

tetraphenylphosphonium salts yielded an excellent system for the activation of

chlorobenzene in the Heck reaction. 133, 134 No undesired Pd precipitate or Pd black was

observed even at these high temperatures.

PVP [poly(N-vinyl-2-pyrrolidone)] has been shown to stabilise Pd nanoparticles in the

Heck reaction in several research groups. For example, Joël et al. have described that PVP

stabilised Pd nanoparticles with the size from 1.7-3.7 nm were highly active in the Heck

reaction of p-bromobenzaldehyde and butyl acrylate.135

Block copolymer micelles136, 137 supported Pd nanoparticles are applied in a series of

Heck reactions between olefins and aryl bormides. Under optimised conditions, no

noticeable change in the size of the nanoparticles occurred during reaction and also Pd

black was not observed. The authors 136 also provided the data showing a strong inverse

correlation between activity and particle size.

Poly(propylene imine) dendrimers supported Pd nanoparticles have been used in

Heck reaction without external base. As proposed by the authors the amidoamine groups of

the dendrimer served as base sites , but unfortunately though, the catalyst lost its activity

upon recycling.138 Christensen et al. found that in their attempt to use dendrimer

encapsulated Pd nanoparticles as catalysts for the Heck reaction, Pd leached from polymeric

surroundings.139 A Pd cored-dendrimer has been claimed to be stable and recoverable in the


30

Heck reaction of bromides, although no recyclable data and leaching test have been

mentioned.140

1.4.4 Catalytic Activity in Heterogeneous Catalytic System

The reactivity of different aryl halides decreases from iodides to bromides and

chlorides because of the different strength and length of the aryl-X bond where

C-I<C-Br<C-Cl. Activated aryl halides have an electron-withdrawing group in the para

position, so the aromatic system is electron-deficient and can undergo oxidative addition

easily. Almost all forms of Pd catalysts work with aryl iodides, so the criterion of highly active

catalyst should at least work with non-activated aryl bromides. Generally speaking

traditional homogeneous Pd complex catalysts are more efficient than Pd nano-catalysts. A

monometallic Pd complex containing a specific electron-rich bulky phosphine (e.g.

di(tert-butylphosphino)ferrocene) 141,142 or an N-heterocyclic carbine with bulky N-aryl

substituents143, 144 has been shown as one of the best catalysts to activate aryl chlorides (see

Figure 1.15 for chemical structure of the ligands). The electron-releasing properties of the

ligands in these complexes favour the difficult oxidative addition into the aryl-chlorine bond,

whereas the bulk of the ligand improves the final reductive elimination step by improving

the geometry of the substrate monometallic Pd complex in the catalytic cycle to give the

final coupled olefinic product.145

Figure 1.15 Chemical structure of ligands for high catalytic Pd complex catalyst.

However, progress has been made in the efficiency and recyclability of the Pd

catalysts in heterogeneous system recently, which allow conversion of

bromobenzene and even of aryl chlorides. Although Köhler is in favor of the actually


31

active Pd species in all heterogeneous system from the dissolution, his group did

pioneer study on highly active heterogeneous supported Pd catalyst by carefully

optimising the reaction conditions.146-148 Some other typical catalysts which have

high catalytic activity are enlisted inTable 1.2.

Table 1.2 A summary of the catalytic ability of Pd nano-catalysts.

Pd Nano-Catalyst/loading Starting Materials T

( )

Time

(h)

Yield

(%)

Ref.

1 On Chitosan flakes in ionic

liquids/ 0.35 mol%

Br+

O

O

130 0.25 98 129

2 On Diatomite supports /3 mol% Br+

O

O 120 24 85 149

3 On siliceous mesocellular foam /

1 mol%

Br+

O

O

100 24 88 150

4 On mesoporous silica-carbon

composites /0.01 mol%

Cl+

100 24 27a 151

5 On Titania supports /0.26 wt%

Br+

O

O

Cl+

O

O

140

24

36

86

27

152

a conversion

In conclusion, the activation of aryl chlorides for C-C cross coupling reactions still

remains a key issue in developing of Pd nano-catalysts.

1.4.5 Change in Size of Nanoparticles in the Heck Reaction

The phenomenon of Pd nano-catalysts changing size/shape has been observed by

several different groups, but the origin of this dramatic effect is still under debate.

TEM (Transmission Electron Microscope) studies of Pd-PVP colloids with a diameter of

19.8 nm in [Bu4N]+Br- medium catalysed Heck reactions showed a significant reduction of Pd

nanoparticle size, the biggest shift of nanoparticle size distribution was from 19.8 nm to 7.6


32

nm (Figure 1.16a).153 In a study of 2 nm Pd nanoparticles catalysed Heck cross coupling

reaction in imidazolium ionic liquids, the authors observed that the size of the particles

increased to 6 nm after the first run (Figure 1.16b). The authors believed the Pd catalysts

acted as a reservoir of highly active Pd species and after reaction dissolved Pd species

re-deposit into the nanoparticles reservoir which cause the size of Pd particles to increase.154

Figure 1.16 TEM images of Pd nanomaterials before and after the Heck reaction, a) from reference 153 with

size distribution, and b) from reference 154.

Narayanan and El-Sayed observed that both dendrimer and PVP stabilised Pd

nanoparticles with the size of 1.3±0.1 nm and 2.1±0.1 nm, respectively, increased their size

after Suzuki C-C crossing coupling reaction.155 They also noticed that the size distribution of

these nanoparticles decreased after reaction and then ruled out Ostwald ripening as the

dominate factor for the size increase. The authors proposed that the excess Pd atoms in the

solution result in the further growth of the nanoparticle.

a)

b)


33

1.5 Synthesis of Pd Colloidal and Supported Nanomaterials

Morphology-controlled synthesis of nanostructures has attracted much attention

because the size and morphology of most nanostructures have a magnificent effect on their

chemical and physical properties; it is well recognized that properties of nanoparticles

depend largely on the size, shape, composition, structure, and crystallinity.156 Since the

catalytic efficiency highly depends on both the size and the shape of Pd nanomaterials,

much effort has been made toward size and morphology-controlled synthesis of Pd

nanostructures for optimising their catalytic activity. Various approaches have been

developed, to meet the requirements for various applications. They can be mainly classified

into: reduction of Pd salt, thermal decomposition of Pd precursors, electrochemical

deposition and metal-vaporisation methods. Due to the scope of this thesis, the summary of

synthesis of Pd nanomaterials will focus on chemical reduction in solution phase.

1.5.1 Summary of the Synthesis of Pd Nanomaterials

Solution-phase reduction of Pd salt has been the most widely used method for

preparing Pd nanomaterials, with a large of variety of reducing agents possibilities. In

order to prevent agglomeration, the salt solution is often mixed with

stabiliser/surfactants and then reduced by adding reducing agent over a period of

time for a certain temperature, depending on the given reactants and the

requirement for the products. Table 1.3 summarises the choice of Pd precursors,

reducing agents, and stabilisers which are often used in recent publications. Pd

nanoparticles have also been immobilised on almost all kinds of supports, including

metal oxide, zeolite, activated carbon, carbon nanotube, silica, magnetite, polymer

membrane, etc., to prevent aggregation of Pd (see section 1.5.3 ).


34

Table 1.3 The summary of choice of Pd precursors, reducing agents and stabilisers.

Pd precursors

H2PdCl4 157, Na2PdCl4

158,159, Pd(OAc)2 6-8, Pd(NH3)4Cl2

22, a Pd(hfac)2 160, b Pd(acac)2

8, 9,

Reducing agents

H2 161-164, boro-hydrides159, 165, L-ascorbic acid 166,167, citric acid 168, oxidisable polymers 169, c solvents

157, 170, 171, CO 172, 173, biological materials 174, 175

Surfactant/polymer stabiliser

e PVP 153, 157, 158, 176, d CTAB 159, 177, alkylamines 178,179, dendrimers 139, branched amphiphilic polymers 173,

biological materials 180, 181

a hfac=hexafluoroacetylacetonate, b acac=acetylacetone, c eg. alcohols and aldehydes,

d CTAB=cetyltrimethylammonium bromide, e PVP= poly(N-vinyl-2-pyrrolidone)

When it comes to choosing suitable methods for synthesising Pd nanomaterials, the

intended application of the materials needs to be taken into account. For instance the

applications of catalysis and sensing require a practical synthetic method which produces

the maximum accessible surface area. This means that the shape of the Pd particles is less

important than the size. In catalysis supported Pd nano-materials have intrinsic advantages

in preventing aggregation over dispersed Pd colloidal nanoparticles during solution phase

organic reactions, hence they are more desirable. In the next section more details of the

synthesis of Pd nanomaterials including size and shape-controlled Pd nanoparticle

formation and supported Pd nanomaterials will be reviewed.

1.5.2 Colloidal Pd Nanoparticles

• Size control

The alcohol-reduction process introduced by Hirai et al. (while they were trying to use

primary alcohols to reduce rhodium (III) chloride in the presence of polyvinyl alcohol to

synthesis Rh nanoparticles) is widely used for the preparation of colloidal noble metals.182

Alcohols containing an α-hydrogen are oxidized to the corresponding carbonyl compounds


35

during the metal-salt reduction. Both the particle size and the standard deviation become

smaller in the order of 1-propanol, ethanol and methanol, indicating that a faster reduction

rate of [PdCl4]2- ions is an important factor to produce the smaller particles (Figure 1.17).157

(Note: the reducing power of an alcohol decreases as its alkyl chain becomes longer.) A

variation of Hirai’s alcohol method is found in Figlarz's polyol method to prepare

monodisperse silver nanoparticles, which consists of refluxing a solution of the metal

precursor in ethylene glycol or larger polyols.183 Xia and co-workers have used this method

to prepare a plethora of anisometric Pd nanoparticles. Comprehensive examples of the

polyol method are described in the review papers by Xia.170, 171

Figure 1.17 Mean diameters and standard deviations of Pd nanoparticles synthesised at various alcohol

concentration.157

Alkylamines (Cn-NH2) with carbon chain length 6, 12, and 18 have been used as

stabilising ligands in the synthesis of Pd nanoparticles.178 The mean size of particles

decreased with the carbon chain increasing. The shape of Pd nanocrystals can be controlled

using combinations of oleylamine and alkylammonium alkylcarbamate. This technique has

produced Pd spheres, tetrahedra and multipods.179

PVP is the most commonly used steric stabiliser in the synthesis Pd nanoparticles, and

as pointed out by the Xia et al., it’s essential to use PVP to control the size and shape of

particles.157, 158, 176 According to a 13C NMR study, the ends of commercially available PVP are

terminated in hydroxyl group, therefore, it can act like a long-chain alcohol and serve as a

weak reducing agent. 184

Biological materials are also used as environmentally friendly stabilisers in the

synthesis of nano-materials. Peptides have been used in the synthesis of sub-10 nm Pd


36

nanoparticles with fine-tunable size in aqueous solution at room temperature (Figure

1.18).180 By changing the reaction time and concentration of reducing agent, NaBH4, Pd

nanoparticles with the size distribution from 2.2±0.3 nm to 4.3±0.5 nm have been

synthesised.

Figure 1.18 TEM images of Pd nanoparticles obtained at 20 s reaction time with different concentrations of

NaBH4: a) 1.5 mM, b) 1 mM and c) 0.5 mM and size distributions. TEM images of Pd nanoparticles obtained at 1

h reaction time with different concentrations of NaBH4: d) 1.5mM, e) 1mM and f) 0.5mM, and size

distributions. 180

In a recent study, Yang et al. reported a one-pot biogenic fabrication of Pd

nanoparticles by a simple procedure using broth of Cinnamomum camphora leaf. Pd

nanoparticles with the size from 3.2 to 6.0 nm were produced by varying the initial

concentration of the Pd ions. The polyols components and the heterocyclic components in

Cinnamomum camphora leaf are believed to be responsible for the reduction of Pd ions and

the stabilisation of Pd nanoparticles, respectively.185

NaBH4 is always used as a fast reducing agent in the synthesis of small Pd

nanoparticles.159 The reduction reaction is often instantaneous at room temperature.

Tetrabutylammonium borohydride is used to reduce Pd(OAc)2 in chloroform.165 It was

suggested that the existence of oleic acid enhances the reducing ability of

tetrabutylammonium borohydride which in this case is crucial to generate monodispersed

Pd nanoparticles.

Seed-mediated growth is a powerful and effective approach to control the size of Pd

nanoparticles. Figure 1.19 shows an example of finetuned size of Pd nanoparticle over a

range of 2 to 7 nm. This was achieved by exerting control over the nucleation and crystal


37

growth of Pd nanoparticles.180 Three nuclei, or seeds, solutions were used with different

concentrations of reducing agent.

Figure 1.19 TEM images of size evolutions of Pd nanoparticles by varying the volume of growth solution into

different seeding solutions. (a, e and i) represent seeding solutions with Pd nanoparticles of 2.6, 3.4 and 4.3

nm, respectively; (b), (c) and (d) show the size evolution of Pd nanoparticles from (a) with different volume of

growth solution from 50 ml, 100 ml to 200 ml, respectively. Similarly, (f), (g) and (h), and (j), (k) and (l) evolve

from (e) and (i), respectively by varying the volume of growth solution to control the size of Pd

nanoparticles.180

• Shape control

Sophisticated synthetic techniques adopted from the synthesis of Ag 186, 187 and Au 188

have been used to prepare anisotropic Pd nanostructures.189

According to thermodynamic arguments, Pd atoms should nucleate and grow in a

solution phase to form cuboctahedrons of spherical shape with their surfaces bounded by a

mix of {111} and {100} facets. Anisotropic nanostructures can only form under kinetically

controlled conditions, while the cubic symmetry is broken. In colloidal system, shape

controlled synthesis of Pd nanostructures starts from the nucleation and the growth of

nuclei to seeds. For an fcc structure the surface energy (γ) of the low-index crystallographic

facets take the sequence of γ{111} < γ{100} < γ{110} which indicates that a single crystal seed should


38

take the shape of octahedral or tetrahedral to minimize the total surface energy.189 However,

both shapes have larger surface areas than a cube of the same volume. As a result, Pd

nano-crystal seeds most commonly exist as truncated octahedron which possess the

smallest surface area and minimised total interfacial free energy. Experimentally, seeds with

defects always co-exist with single crystal seeds. Both seeds can be grown to all desirable

shapes by adding Fe3+, Br-, I- and SCN- ions, altering the concentration of Pd precursor and

stabilizer as well as varying the reducing agent accordingly.190 How the reaction conditions

alter the final shape of Pd nanoparticles is exemplified by the following example. L-Ascorbic

acid affords a fast reduction of a Pd precursor producing truncated octahedrons which are

thermodynamically favourable species. By adding KBr, the reaction turns to favour the

formation of Pd nano-bars and nano-cubes enclosed by {100} facets. Br- ions are

preferentially chemisorbing onto the {100} facets of Pd nano-crystals, thus the {111} facets of

a truncated octahedron is overgrown {100} facets. Eventually the nano-crystal are enclosed

by all {100} facets at the expense of {111} facets, existing in Pd nano-cube and/or nano-bar.

As a face-centred cubic (fcc) metal, Pd nanoparticles can exist in a variety of shapes, eg.

octahedron,168 cuboctahedron,191 cube,176 octagonal rod,158 rectangular bar,158 five-fold

twinned pentagonal rod and wire,192 right bipyramid,193 decahedron,168 icosahedron,168, 194, 195

rhombic dodecahedral,196 hexagonal plate,197, 198 and triangular plate197 etc. (Figure 1.20). In

addition, a few novel Pd nanostructures have been reported recently. Freestanding Pd

nano-sheets that are less than 10 atomic layers thick have been synthesised by using

carbon monoxide as a surface confining agent (Figure 1.21a, b).199 Highly branched Pd

nanostructure have been synthesised under high pressure through the ultrafast growth of

the nuclei (Figure 1.21c) 200 or through a seed-mediated growth from one-dimensional

nanorods in the presence of Cu(OAc)2 (Figure 1.21d).159


39

octahedron cuboctahedroncube

octagonal rod rectangular bar five‐fold twinned pentagonal rod

right bipyramid decahedron icosahedron

rhombic dodecahedral hexagonal plate triangular plate

Figure 1.20 Geometrical shapes of Pd nanoparticles.158, 168, 176, 191-198


40

Figure 1.21 TEM images of a) the ultrathin Pd nanosheets (Inset: photograph of an ethanol dispersion of the

as-prepared Pd nanosheets in a curvette), b) the assembly of Pd nanosheets perpendicular to the TEM grid.

Inset: thickness distribution of the Pd nanosheets, c) highly branched Pd nanostructures formed from the

ultrafast growth of polyhedra nanoparticles, d) the branched Pd nanostructures developed from nanorods.199,

200

The field of morphology-controlled synthesis of Pd is in a developing stage, many of

the reported methodologies are not well established in terms of the particle yield, formation

mechanism, reproducibility, etc. However, there are certain methodologies such as the

preformed-seed-mediated growth approach, high-temperature polyol synthesis,

template-based synthesis, electrochemical techniques, etc., which are emerging as popular

ways of developing anisometric nanoparticles.159, 167,196,201,202

Overall, the essence of shape and size controlled synthesis is thermodynamic and

kinetic control. Impurities or capping agents can change the order of free energies of

different facets corresponding to control the relative growth rates of different facets, which

leads to the formation of drastically different shapes.


41

1.5.3 Supported Pd Nanomaterials

Pd nanoparticles are usually supported on various silica materials, zeolites, charcoal

and oxides. In addition, polymers (including dendrimers), carbon nanotubes, and hybrid

organic-inorganic supports, etc. have also been investigated as supports for Pd

nanoparticles. Table 1.4 shows some typical supported Pd nanomaterials from recently

published literatures.

Table 1.4 Typical examples of supported Pd nanomaterials.

Classification of

supports

Composition (size of the Pd nanoparticle) Ref.

Colloid stabilised

by

Polymer/surfactant

1-n-Butyl-3-methylimidazolium hexafluorophosphate ( 2 nm)

PVP(~19.8 nm)

Polyethyleneglycol(~5 nm)

Polystyrene-co-poly(ethylene oxide) and cetylpyridinium

chloride micelles

Poly(styrene)-co(vinylpyridine) micelles

154

153

203

137

16, 21,

136, 204

Polymeric

nanospheres

On the surface of Polyvinylpyridine/Latex nanospheres through

polypyrrole overlayers (1-4 nm)

Attached to hollow Latex nanospheres (5 nm)

Embedded in polypyrrole nanospheres formed in situ

Cage like copolymer microspheres

123, 205

125

22

206

Dendrimer

Hydroxyl-terminated G4-PAMAM dendrimer encapsulated Pd60

Hydroxyl-terminated G2-G4 PAMAM dendrimers (1.2-3.6 nm)

PPI dendrimer (2−3 nm)

Surrounded by 17 Fréchet-type dendritic G3-polyaryl ether

disulfide (2.0 nm)

139

207

138

140

Membrane On Chitosan membrane

On organically modified thin hybrid films

129

208


42

Forming Pd-polyimide nanocomposite membrane 209

Zeolite 3-Aminopropyltrimethoxysilane -functionalized Na-Y zeolites 120

Carbon materials

Functionalized multiwall carbon nanotubes (2-10 nm)

Mesoporous silica and/or carbon nanocomposite (~3 nm)

Carbon nanofibre

Graphene

210

151

211

212

Silica materials

Natural diatomite (20-100 nm)

Fluorous silica gel

Siliceous foam

149

213, 214

150, 215

Magnetic particles

On the surface of dopamine-modified NiFe2O4nanoparticles (~10

nm)

On the surface of iron oxides through phosphate functional

groups (<1 nm)

216

130

Metal oxides TiO2, Al2O3, MgO, ZrO2 and ZnO 152,

217-220

Natural/

biological

materials

Bentonite

Bacteria

Sepiolite

Tobacco

Mosaic virus

221

128, 222

127

223,224

Hybrid framework Metal-organic framework, MIL-100(Al)a

Micro-porous Polymers framework

225

124

a for details of synthesis, see Volkringer, C.; Popov, D.; Loiseau, T.; Férey, G.; Burghammer, M.; Riekel, C.; Haouas, M.; Taulelle,

F. Chem. Mater. 2009, 21, 5695–5697.

Supported Pd nanomaterials can be synthesised by reducing Pd precursor in the

presence of substrates.206, 225-227 For example, chlorohydrosilanes modified porous silica was

treated with a saturated solution of PdCl2 in methanol to produce Pd(0) on the silica

surface.116 By the introduction of an Si–H function into the channel of SBA-15, Pd(II) precursor

can be reduced in situ resulting in a highly dispersed Pd colloid layer on the pore walls.118


43

Recently, graphene oxide (GO) supported Pd nanoparticles have been synthesised by simply

mixing [PdCl4]2- and GO together in an ice bath for 30 min with vigorous stirring.228 The

driving force for Pd deposition on GO, as the authors suggested, could be caused by the

difference between the reduction potential of PdCl42− (0.83 V vs Saturated calomel electrode)

and the oxidation potential of GO (0.48 V vs Saturated calomel electrode).

Some kind of supports also can be synthesised together with Pd nanoparticles.124 For

example, a new strategy based on polymerisation-induced phase separation (PIPS)

techniques was proposed for fabricating Pd nanoparticles captured in a microporous

network polymer. Pd(OAc)2 was premixed with a monomer having a

poly(amidoamine)-based dendrimer ligand, and subsequently the monomer was thermally

polymerized with an excess amount of ethylene glycol dimethacrylate under PIPS

conditions. Here the formation of Pd nanoparticles occurred concurrently with the polymer

synthesis in a one-pot process, with no additional reducing reagent.124

Supported Pd nanomaterials can also be synthesised by post-immobilisation, where

pre-prepared Pd nanoparticles are immobilised on the substrates. This method keeps the

morphology of nanoparticles intact, but it involves a multi-step process. For example, for the

synthesis of zeolite supported Pd nanoparticles, Pd(NO3)2 was reduced using NaBH4 at room

temperature, and then the resulted colloidal Pd solution was mixed with

amine-functionalized NaY zeolite for 12 h to allow particle immobilisation. These NaY zeolite

supported Pd nanoparticles can be used as catalyse in Heck reaction, with high stability of

the attachments.120

A variety of Pd nanomaterials have been prepared and most of them have been

studied as catalysts in the C-C cross coupling reaction. However, only a few Pd catalysts are

used in industrial production, even though Pd catalysts are very universal and convenient in

the laboratory. Thus the synthesis of highly active and reusable catalysts which satisfies the

standards of pharmaceutical and industrial applications is still a major research initiative.


44

1.6 Challenges in the Synthesis and Applications of Pd Nanomaterials

As discussed in previous sections, the synthesis and applications of Pd nanomaterials

have been studied for decades, but there are still challenges remaining in these areas.

In the synthesis of Pd nanomaterials, most of the methods are carried out in the

presence of toxic organic solvents or reagents, and the conditions involve high temperature,

long reaction time, and complicated process; only a few reports on the synthesis of uniform

Pd nanostructures in aqueous solution have been reported. These approaches are often

limited in laboratory scale, and require delicate controlled reaction conditions. Therefore, it is

important to develop a simple and environmentally friendly method for the preparation of

Pd nanostructures for large-scale applications.

In general, Pd nano-catalysts are less active than homogeneous Pd complexes

catalysts; however Pd nano-catalysts have practical advantages, such as ease of separation

and reusability. Driven by the desire to extend the Heck cross coupling reactions to

pharmaceutical and industrial applications a major focus has been screening highly active

and recyclable Pd nanocatalysts and avoiding heavy metal contamination/leaching. Despite

the Heck cross coupling reaction has been used to build complex organic molecules since its

discovery in early 1970s. However, the active species in Pd nano-catalyst still remains

unknown. Understanding the mechanism of the Heck cross coupling reaction is crucial for

designing new generation catalyst.

Resistive Pd hydrogen sensor technologies based on the conventional circuit platform

are considered as an idea candidate for daily use. The development of this type of sensor

and sensing materials that provide a reliable, fast-response, and high-performance platform

is under intensive investigation. Micro/nano-fabricated hydrogen sensors are thought of as

low-cost, compatible, durable, and easy to maintain relative to conventional gas detecting

instruments. There is little knowledge about the intrinsic finite size effects of Pd

nanomaterials in hydrogen sensing and the impact of their properties on the performance

of hydrogen sensors.


45

These issues addressed in the research undertaken and reported in this PhD

dissertation under the specific aims:

1. To develop a feasible method to synthesise Pd nanomaterials in aqueous solution

on a large scale;

2. To explore the applications of these Pd nanomaterials as catalysts in Heck cross

coupling reactions;

3. To investigate different Pd nanostructures in hydrogen sensing.

Results for each aim are reported in published articles, which form the basis of the

following chapters.

Chapter 2 Introduction to series of Papers

46

2. Introduction to Series of Papers

The synthesis of three different Pd nanomaterials and the applications of these

nanomaterials in hydrogen gas sensing will be introduced in paper 1-3. First, bare Pd

nano-rosettes were synthesised in the absence of any surfactant and stabiliser, and this

structure possessed a high surface-to-volume ratio which rendered an abundance of active

surface available for hydrogen adsorption. Indeed, these bare Pd nano-rosettes showed a

fast response in hydrogen sensing. However, this method showed less control to the

morphology of Pd nanomaterials. PVP and SCMC (sodium carboxymethyl cellulose) were

introduced into the system to give more control in the morphology. The reasons why PVP

and SCMC were chosen are the commercial availability and high solubility of both polymers

in water. As expected, a variety of Pd nanomaterials in different morphology were

synthesised and the application of these materials in hydrogen sensing will be presented in

paper 2 and 3. The further application of Pd-PVP nanospheres in Heck reaction and the role

of surface oxygen in heterogeneous Heck reaction were investigated and the results will be

introduced in paper 4 and 5. The high surface area and chemical inertness make graphene a

highly sought-after substrate for catalyst. A feasible method to fabricate Pd–graphene and

Pt–graphene 2D nano-arrays will be introduced in paper 6. As followed by CFL (capillary

force lithography) associated with electrostatic interaction to induce linear assembly of Pd

nanocubes in paper 7. These Pd nanoarrays were tested in hydrogen gas sensing and results

showed CFL could be a feasible method to build miniature hydrogen sensor on a large scale.

In the following chapter 3, these papers will be presented separately.


47

1. Bare Palladium nano-rosettes for real time high performance and facile hydrogen sensing

Reduction of H2PdCl4 using hydrogen gas at ambient pressure under continuous flow

resulted in the formation of Pd nano-rosettes built of 6 nm nanoparticles. These

nano-rosettes were effective in rapid real time hydrogen gas sensing.

The fabrication of surfactant free “bare” Pd nano-rosettes using a spinning disc

processor was reported. The technique allowed a scalable synthesis of Pd nanoparticles with

minimum downstream purification, and eliminated any interference of the capping agent

during the sensing of hydrogen gas. We demonstrated the effectiveness of using these

high-surface-area Pd nano-rosettes in hydrogen sensing down to 0.1% by volume, with fast

response time and real time sensing capability.

Result is presented in:

Zou, J.; Hubble, L. J.; Iyer, K. S. *; Raston, C. L. *, Sensors and Actuators B: Chemical 2010,

150, (1), 291-295.

Graphical abstract:


48

2. Hydrogen induced reversible insulator-metal transition in a palladium nanosphere sensor

A reversible insulator-metal like transition occurred in a self-organized 3D Pd

nanosphere sensor involving the absorption and desorption of hydrogen at ambient

pressure, which is a new paradigm in hydrogen sensing.

Self-organized 3 dimensional Pd nanospheres have been synthesised using a facile

approach involving hydrogen gas as the reducing agent within a dynamic microfluidic

platform. Even more significantly we observed for the first time that the dissociative

adsorption of hydrogen induced a Mott insulator to metal like transition in a Pd nanosphere.

For hydrogen concentrations less than 2% an increase in the resistance was observed while

concentrations greater than 2% resulted in an increase in conductance, indicating that an

insulator-to-metal transition occurred. These results could be an important development in

potentially miniaturizing 3 dimensions for applications in next generation nanoparticle

based electronic switches and sensors.

Results are presented in:

Zou, J.; Iyer, K. S. *; Raston, C. L., Small 2010, 6, (21), 2358-2361.

Graphical abstract:


49

3. Pd-sodium carboxymethyl cellulose nanocomposites display a morphology dependent

response to hydrogen gas

Hydrogen reduction of H2PdCl4 in the presence of sodium carboxymethyl cellulose

(SCMC) in dynamic thin films on a microfluidic spinning disc processor (SDP) afforded Pd

nano-structures, from rosettes to disconnected agglomerates with increasing of CMC

concentration. These nano-composites were drop cast on interdigitated electrodes (IDEs) to

afford sensors for hydrogen gas, with a decrease and increase response in current for

rosettes and agglomerates, respectively.

We reported a simple, yet novel approach to alter the morphology of Pd

nanostructures by varying the ratio of sodium carboxymethyl cellulose to Pd precursor,

involving the use of a continuous flow microfluidic platform. Hydrogen sensor developed

using different Pd–SCMC nanocomposites showed opposing responses in current to

hydrogen gas, which indicated that the ratio of SCMC to Pd not only played a key role in

determining the morphology of obtained Pd nanocomposites, it also affected the

connections between Pd nanocomposites. Importantly, the devices were robust in having

good stability during testing.


Zou, J.; Iyer, K. S. *; Raston, C. L., Green Chemistry 2012, DOI: 10.1039/C2GC16456F.

Graphical abstract:


50

4. Scalable synthesis of catalysts for the Mizoroki–Heck cross coupling reaction: palladium

nanoparticles assembled in a polymeric nanospheres

Pd-PVP nano-spheres, as an assembly of uniform 5 nm nanoparticles, were accessible

using a facile one step method under continuous flow on a spinning disc with hydrogen gas

as the reducing agent. The stable colloidal system can be used as an effective catalyst for the

Heck cross coupling reaction, and can be readily recycled without a change in its catalytic

activity.

It has been established that Pd–PVP nano-spheres can be prepared using a

microfluidic spinning disc platform under an atmosphere of hydrogen. The turbulent mixing

within the dynamic thin films arising from the high centrifugal forces resulted in the long

chains of PVP forming a compact scaffold which entangles and traps a large number of 5 nm

Pd nano-particles within the composite. It also has been demonstrated that these

nano-spheres were effective recyclable colloidal catalysts for Heck cross coupling reactions

between several aryl halides and n-butyl acrylate. The polymeric scaffold maintained the

morphology and integrity by minimising catalytic loss caused by leaching.


Zou, J.; Iyer, K. S. *; Stewart, S. G. *; Raston, C. L., New Journal of Chemistry 2011, 35, (4),

854-860.

Graphical abstract:


51

5. Surface oxygen triggered size change of palladium nano-crystals impedes catalytic

efficacy

The size of Pd nano-crystals increased during the initial recycles in Heck cross coupling

reactions. We demonstrated that oxygen adsorbed on the surface of Pd nano-crystals played

a pivotal role in driving the size increase of the nanocrystals. The increase in size was in turn

associated with a loss in catalytic activity.

It has been demonstrated for the first time that surface oxygen played a pivotal role in

the reconstruction of Pd nano-crystals bound within a polymer matrix druing

heterogeneous catalysis. This reconstruction resulted in an increase in the size of the

nano-crystals, with defaceting and a decrease in chemically active sites for the model Heck

cross coupling reaction. This is an important phenomenon in determining the chemical

outcome of Pd catalysed reactions in general, and an important finding that should be taken

into consideration in the design of recyclable Pd nano-particle based catalysts.


Zou, J.; Stewart, S. G.; Raston, C. L. *; Iyer, K. S., Chemical Communication 2011, 47, (18),

5193-5195.

Graphical abstract:


52

6. Pd-induced ordering of 2D Pt nanoarrays on phosphonated calix[4]arenes stabilised

graphenes

p-Phosphonic acid calix[4]arenes rendered high stability to exfoliated graphene in

water. This calix[4]arene modified graphene was used as a highly effective substrate to

nucleate ultra-small Pd nanoparticles, which in turn served as galvanic reaction templates for

the generation of high density 2D arrays of Pt nanoparticles.

It was demonstrated that p-phosphonic acid calix[4]arene was effective at rendering

high stability of graphene in solution. Furthermore, this p-phosphonated calix[4]arene

modified graphene was used as a highly effective template to nucleate ultra-small Pd

nanoparticles by in-situ reduction of H2PdCl4 in water using hydrogen gas. In addition, these

Pd-graphene hybrids acted as galvanic reaction templates for the generation of high density

2D arrays of Pt nanoparticles. The reaction rate of the replacement and density of Pt 2D

structure could be controlled by the introduction of FeII/FeIII species. The simple process

reported here improved the processability of graphene in water with potential to develop

novel hybrids for application in catalysis, fuel cells, sensor materials and nano-electronics.


Zou, J.; Martin, A. D.; Zdyrko, B.; Luzinov, I.; Raston, C. L.; Iyer, K. S. *, Chemical

Communications 2011, 47, (18), 5193-5195.

Graphical abstract:


53

7. Regiospecific linear assembly of Pd nanocubes for hydrogen gas sensing

Capillary force lithography (CFL) was applied in combination with a “grafting to”

approach to eletcrostatically assemble Pd nanocubes into linear arrays as a platform for

creating large area prints for developing sensors.

Large area patterns with features down to nanometre were generated using CFL. A

“grafting to” approach was used on the patterns to induce polymer brush for linear

assembly of Pd nanocubes through electrostatic interaction. Pd nanoarrays with high

density were subject to hydrogen gas sensing test. This platform could be further fine tuned

for a wide range of Pd nanoparticles of various shapes using appropriate surfactants and

complementary polymer patterns to optimise sensing response, and this platform could be

easily extended to other materials. The results also showed that CFL could be used as a

feasible method to build up a miniature hydrogen sensor.


Zou, J.; Zdyrko, B.; Luzinov*, I.; Raston, C. L.; Iyer, K. S.*, Chemical Communications 2012,

48, (7), 1033-1035.

Graphical abstract:

20 40 60 80 10022.5

23.0

23.5

24.0

N2

H2

I /μ

A

Time /min

Chapter 3 Series of Papers

54

3. Series of Papers

3.1 Bare Palladium nano-rosettes for real-time high-performance and facile

hydrogen sensing

Sensors and Actuators B 150 (2010) 291–295

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

Bare palladium nano-rosettes for real-time high-performance and facile

hydrogen sensing

Jianli Zou, Lee J. Hubble1, K. Swaminathan Iyer ∗, Colin L. Raston ∗∗

Centre for Strategic Nano-Fabrication, School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, WA 6009, Australia

a r t i c l e i n f o

Article history:

Received 24 March 2010

Received in revised form 19 June 2010

Accepted 30 June 2010

Available online 3 August 2010

Keywords:

Palladium nanoparticles

Hydrogen sensing

Microfluidics

Process intensification

a b s t r a c t

Surfactant free reduction of palladium using hydrogen gas at ambient pressure under continuous flow

results in nano-rosettes of palladium built of 6 nm particles. These palladium nano-rosettes are effective

real-time sensors of hydrogen gas via a simple drop cast technique.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The ever-growing relevance of hydrogen gas in modern tech-

nologies has initiated research towards the development of

high-performance sensors with fast response times. Indeed, detect-

ing and monitoring hydrogen gas with precision and accuracy

is becoming an increasingly important issue for applications

in chemical and petroleum refining, fuel cells and rocket fuels

for spacecrafts. Furthermore, hydrogen in breath has also been

reported as an important indicator in disease monitoring such as

fructose malabsorption [1], fibromyalgia [2] and neonatal necro-

tizing enterocolitis [3,4]. Palladium has emerged as an important

candidate for hydrogen gas sensing because of its ability to absorb

high quantities of hydrogen and its highly selective response. In

particular, palladium nanomaterials based chemical sensors have

gained momentum recently due to their miniaturization potential

and real-time monitoring capabilities. Additionally, rapid response

times are observed due to the low energy barriers associated with

∗ Corresponding author at: Centre for Strategic Nano-Fabrication, School

of Biomedical, Biomolecular and Chemical Sciences, The University of

Western Australia, M313, 35 Stirling Highway, Crawley, WA 6009, Australia.

Tel.: +61 8 6488 4470; fax: +61 8 6488 1005.∗∗ Corresponding author at: Centre for Strategic Nano-Fabrication, School

of Biomedical, Biomolecular and Chemical Sciences, The University of

Western Australia, M313, 35 Stirling Highway, Crawley, WA 6009, Australia.

Tel.: +61 8 6488 3045; fax: +61 8 6488 8683.

E-mail addresses: [email protected] (K.S. Iyer),

[email protected] (C.L. Raston).1 CSIRO Materials Science and Engineering, Lindfield, NSW 2070, Australia.

the diffusion of hydrogen into the high surface area to volume ratio

nanomaterials. Sensing herein is based on the well-established

principle that palladium spontaneously absorbs H2 gas as atomic

hydrogen which diffuses into the lattice to form palladium hydride,

PdHx, resulting in a � to � phase transition and a correspond-

ing change in the lattice spacing. The change in phase and lattice

spacing leads to a measurable resistance change of palladium

material. The range of palladium nanomaterials reported for hydro-

gen sensing by monitoring the aforementioned change include Pd

nanowires [5], Pd meso-wires [6], Pd films [7,8], Pd/carbon nan-

otube hybrids [9,10] and Pd/titania nanohybrids [11]. While a few

of the recently reported Pd-based sensors demonstrate impressive

response times in milliseconds, they suffer drawbacks due to the

limitations in scalability, reproducibility and multi-step assembly

processes. A facile method which can overcome these issues, at

the same time incorporating high-performance and fast response,

is important for this technology to become viable. To this end we

have developed a scalable method to fabricate surfactant free pal-

ladium nano-rosettes under continuous flow using a microfluidic

platform and hydrogen gas as a reducing agent. We demonstrate

that the high surface area of these unprotected, bare palladium

nano-rosettes facilitate sensing hydrogen gas with high efficiency

in real-time. Also of significance is that this involves a simple and

inexpensive drop casting approach.

There are a few reports detailing the synthesis of Pd nanoparti-

cles in aqueous solution with size and shape control in the presence

of a surfactant or a capping agent [12,13]. However, for applica-

tion in a hydrogen sensor a passivated Pd surface in the presence

of a surfactant or a capping agent would result in a lag in response

time and would lower the sensor the detection limits because of the

0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.snb.2010.06.071

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292 J. Zou et al. / Sensors and Actuators B 150 (2010) 291–295

Fig. 1. Schematic representation of (a) the hydrodynamics of the fluid flow over a

spinning surface and (b) a spinning disc processor (SDP).

reduction in active surface available for hydrogen adsorption [7,14].

A scalable fabrication technique devoid of surfactants will gener-

ate bare palladium nanoparticles with high surface area to volume

ratio that would be poised for applications in sensing hydrogen in

real-time. Hydrogen gas has been used as a reducing agent to pro-

duce Pd nanomaterials, from palladium(II) salts, albeit with long

reaction times, and involving the use of surfactants and operat-

ing under high pressure [15–17]. Bubbling a solution of H2PdCl4with hydrogen in a round bottom flask results in precipitation of

large micron sized Pd particles that are colloidally unstable in water

in the absence of a surfactant (see Supporting Information Fig.

S1). “Process Intensification” offers alternative routes to preparing

nanoparticles alleviating the obstacles of the relaxed fluid dynamic

regime associated with conventional batch processes. The demand

for intensified processing, of which spinning disc processing (SDP)

is a subset, has led to the design and development of a range of

reactors that offer operating conditions with rapid heat and mass

transfer under continuous flow conditions with residence times

reduced to seconds rather than minutes or hours. SDP offers a novel

avenue for intensified nano-fabrication via exploiting the high cen-

trifugal acceleration to generate thin films which provide rapid heat

and mass transfers, Fig. 1a and b. The geometry and key elements of

a SDP are illustrated in Fig. 1. The key components of SDP include:

(i) a 100 mm rotating disc with controllable speed (up to 3000 rpm)

and (ii) feed jets located at a radial distance of 5 mm from the cen-

tre of the disc. SDP generates a very thin fluid film (1–200 �m) on

a rapidly rotating disc surface, within which nanoparticle forma-

tion occurs. Following injection, where the reagents contact close

to the centre of the spinning disc, the fluid film initially experiences

an increase in radial flow velocity whereupon the liquid is moving

close to the disc velocity. The flow here becomes similar to the

Nusselt model [18]. The shear forces and viscous drag between the

moving fluid layer and the disc surface create turbulence and rip-

ples which give rise to highly efficient turbulent mixing within the

thin fluid layer. The turbulent waves thus generated can be a com-

bination of circumferential waves moving from the disc centre to

the disc periphery, and helical waves, depending on the operating

parameters. The wavy thin film generated on a rotating disc sur-

face in the presence of a gas, notably H2, offers the ability to control

the size of the ensuing particles by controlling the delivery of H2 to

the thin film. Recent reports highlight that waves generated in the

fluid film over a moderately spinning disc speed clearly enhance

the gas adsorption into the liquid [19–21]. The flow is accompa-

nied by non-linear waves, which strongly influence the diffusion

boundary that develops beneath the surface of the film. The pro-

gressive waves are generated in the region of active micromixing

and hence offer the proposed control necessary to synthesize bare

Pd nanoparticles, Fig. 1a.

To the best of our knowledge, this is the first report detailing

the use of H2 gas as a reducing agent to make colloidally stable Pd

nanoparticles in an aqueous thin fluid film. The technology offers a

realistic route towards large-scale synthesis of bare Pd nanoparti-

cles for applications in hydrogen sensing, in the present case.

2. Experimental

2.1. The synthesis and characterization of palladium

nano-rosettes

In a typical synthesis of palladium nano-rosettes, the H2PdCl4solution (0.6 mmol L−1) was bubbled with argon 30 min and then

was fed from one jet at the feed rate of 0.7 mL s−1. Hydrogen gas

was fed from another jet to reduce palladium(II) to palladium

nanoparticles. The speed of the spinning disc was set at 1500 rpm.

The as-synthesized nano-rosettes were washed using MilliQ water

(>18 M�-cm) three times and re-dispersed in water before any

further test. The size and morphology of the samples were deter-

mined using transmission electron microscopy (TEM, JEOL 3000F

and JEOL 2000 FX II) operating at 300 and 80 kV, respectively. The

powder XRD pattern of the palladium nano-rosettes was measured

using an Oxford Diffraction Gemini-R CCD diffractometer (using Cu

K� = 1.54178 a radiation).

2.2. The set-up and characterization of hydrogen gas sensor

In a 300–350 �m thick Si <1 0 0> n-doped wafer used in the sen-

sor device was covered by a 300 nm insulating Si3N4 layer deposited

using Plasma Enhanced Chemical Vapour Deposition. The interdigi-

tated electrode pattern was transferred by using photolithography,

which was followed by depositing a 5 nm chromium binding layer

and a 50 nm gold layer using an in-house built metal evaporator

system (more details about IDE see Supporting Information). The

palladium nanoparticle solution was drop cast onto the surface of

each IDE using aliquots of 0.02 �L from a 0.5 �L glass syringe which

were subsequently air-dried. The test procedure involved alternat-

ing nitrogen gas (20 min) and varying concentrations of hydrogen

gas (4 min). The change of the current was monitored at the same

time. The total flow rate of gas was 1000 mL min−1. The voltage

applied between electrodes was 100 mV dc. The images of the sen-

sors were recorded using a scanning electron microscope (SEM,

Zeiss 1555 VPSEM) operating at an accelerating voltage of 10 kV.

3. Results and discussions

3.1. Characterization of palladium nano-rosettes

A typical representation of the Pd nanoparticles obtained using

the above technique can be seen from Fig. 2a. Interestingly, the Pd

nanoparticles show a rosette structure which is comprised of many

smaller nanoparticles in an ordered agglomeration pattern. The

size of the individual Pd nanoparticles which construct the rosette

is about 6 nm, Fig. 2b. The crystallinity of the nano-rosettes was

confirmed using high-resolution transmission electron microscopy

(TEM), Fig. 2c. Additionally, the XRD pattern of the palladium

nano-rosettes measured with a CCD diffractometer (using Cu

K� = 1.54178 a radiation) agreed with the Pd Card (JCPDS card No.

05-0681), Fig. 2d, which indicated a high crystallinity of the pal-

ladium nanoparticles synthesized using SDP. We believe that in

the absence of a capping agent, the high surface free energy pal-

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J. Zou et al. / Sensors and Actuators B 150 (2010) 291–295 293

Fig. 2. (a and b) TEM images of Pd nano-rosettes demonstrating the unique morphology made of individual particles built from 6 nm Pd nanoparticles, (c) high-resolution TEM

image of Pd nano-rosettes demonstrating high crystallinity (inset: FFT pattern corresponding to the region in the box), and (d) powder XRD pattern of the Pd nano-rosettes.

ladium nanoparticles spontaneously assemble into more stable

rosette structures. We hypothesize, the morphology of these “bare”

nano-rosettes will indeed enable a fast response time for hydro-

gen sensing. Furthermore, the morphology will also result in a

lower area of contact to the sensor substrate and minimise the

interfacial strain due to the lattice expansion following hydrogen

incorporation.

3.2. Hydrogen gas sensing

The hydrogen gas sensing set-up to test the hydrogen response

of the nano-rosettes is shown in Fig. 3a. It basically consists

of a carrier gas (ultra high purity nitrogen), two gas mass flow

controllers, a glass cyclonic mixer, a sensor test chamber, and

a potentiostat and an electronic recorder. The mass flow con-

trollers allow different flow rates of gas passing through, from 1

to 1000 mL min−1. The palladium nanoparticle solution was drop

cast onto the surface of each interdigitated electrode (IDE) using

aliquots of 0.02 �L from a 0.5 �L glass syringe and subsequently

air-dried. Current–voltage (I–V) sweeps were carried out until an

ohmic response was achieved with the palladium nanoparticles

bridging the electrode gaps. This was further supported through

SEM, Fig. 3b and c. The drop cast nanoparticles had an ohmic

behaviour with a resistance of about 1300 � at room temper-

ature, as calculated from the I–V characterization. This value is

much larger than 1–5 � which was expected for bulk palladium.

It is evident that the Pd nano-rosettes used herein, which are an

ordered aggregation of smaller nanoparticles, have a high den-

sity of grain boundaries. Indeed, it has been previously reported

that in the case of electrodeposited Pd nanowires the grain bound-

ary scattering effect leads to a relatively high resistance of 875 �[22].

The sensing of different hydrogen concentrations (between 0.1

and 10% in N2 gas) was tested for the drop cast Pd nano-rosettes

system. Fig. 4a and c shows an increase in resistance with hydrogen

gas-flow and a return to the original state when no hydrogen gas

was present for concentrations of 0.1–1%, and 1–10%, respectively.

In the present case, we were not able to obtain hydrogen gas con-

centrations lower than 0.1% due to the limitations of the hydrogen

gas-flow controller in our current experimental set-up. The change

in resistance herein can be explained by the diffusion of atomic

hydrogen into the lattice to form PdHx, resulting in a � to � phase

transition which in turn results in an increase in resistance. At

lower hydrogen concentrations the phase transition is primarily

on the surface of the nano-rosettes (minimum volume change of

Pd), while at higher concentration the phase transition is a bulk

phenomenon in the nano-rosettes (maximum volume change).

Fig. 4b shows the current change �I (I/I0) in the presence of

0.1–1% hydrogen gas. A linear relationship was observed between

the change of current and the hydrogen gas in this range, with

minimal drift in the background current. However, above the

concentration of 2% the signals drift slightly from the original

baseline. The � to � phase transition at higher concentrations

can result in about 11% increase in volume [23], which in turn

restructures the contacts between the nano-rosettes. The response

time, �90, for 0.1% hydrogen is 183 s, 1% hydrogen is 181 s, and at

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294 J. Zou et al. / Sensors and Actuators B 150 (2010) 291–295

Fig. 3. (a) Schematic representation of the experimental set-up for hydrogen sensing, (b) SEM image of an interdigitated electrode (IDE) with drop cast Pd nano-rosettes,

and (c) a high magnification SEM image of the IDE with the Pd-nano-rosettes connecting the electrodes.

Fig. 4. (a) Response of the sensor to hydrogen gas from the concentration of 0.1 to 1%, (b) calibration curves for the current response with the concentration of hydrogen gas

varied from 0.1 to 1%, and (c) response of the sensor to hydrogen gas from the concentration of 1 to 10%.

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J. Zou et al. / Sensors and Actuators B 150 (2010) 291–295 295

sensor response saturation observed at ∼4% hydrogen is 127 s. The

sensing is highly reproducible and scalable.

4. Conclusion

In summary, we report the fabrication of surfactant free “bare”

Pd nanoparticles using dynamic thin films on a spinning disc

processor. The technique allows for a scalable synthesis of Pd

nanoparticles with minimum downstream purification, and elim-

inates any interference of the capping agent during the sensing of

hydrogen gas. In addition, we also demonstrate the effectiveness

of utilising the high surface area to volume ratio Pd nano-rosettes

in hydrogen sensing using an attractive, simple drop casting fab-

rication method. Importantly, the sensing of hydrogen is effective

down to 0.1% by volume, with fast response times and real-time

sensing capability.

Acknowledgements

The authors are grateful for the financial support for this work by

the Australian Research Council and The University of Western Aus-

tralia, and for the palladium precursors provided by AGR Matthey.

The microscopy analysis was carried out using facilities in the Cen-

tre for Microscopy, Characterization and Analysis, The University

of Western Australia, which are supported by University, State and

Federal Government funding.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.snb.2010.06.071.

References

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[3] W.I.H. Garstin, V.E. Boston, Sequential assay of expired breath hydrogen as ameans of predicting necrotizing enterocolitis in susceptible infants, J. Pediatr.Surg. 22 (1987) 208–210.

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[5] H.Y. Lin, H.A. Chen, H.N. Lin, Fabrication of a single metal nanowire connectedwith dissimilar metal electrodes and its application to chemical sensing, Anal.Chem. 80 (2008) 1937–1941.

[6] F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Hydrogen sensors andswitches from electrodeposited palladium mesowire arrays, Science 293 (2001)2227–2231.

[7] F.J. Ibanez, F.P. Zamborini, Reactivity of hydrogen with solid-state films ofalkylamine- and tetraoctylammonium bromide-stabilized Pd, PdAg, and PdAunanoparticles for sensing and catalysis applications, J. Am. Chem. Soc. 130(2008) 622–633.

[8] D.Y. Ding, Z. Chen, A pyrolytic, carbon-stabilized, nanoporous Pd film for wide-range H2 sensing, Adv. Mater. 19 (2007) 1996–1999.

[9] J. Kong, M.G. Chapline, H.J. Dai, Functionalized carbon nanotubes for molecularhydrogen sensors, Adv. Mater. 13 (2001) 1384–1386.

[10] Y.G. Sun, H.H. Wang, High-performance, flexible hydrogen sensors that use car-bon nanotubes decorated with palladium nanoparticles, Adv. Mater. 19 (2007)2818–2823.

[11] M.K. Kumar, L.K. Tan, N.N. Gosvami, H. Gao, Titania nanofilm with electricalswitching effects upon hydrogen/air exposure at room temperature, J. Phys.Chem. C 113 (2009) 6381–6389.

[12] Y.Z. Piao, Y.J. Jang, M. Shokouhimehr, I.S. Lee, T. Hyeon, Facile aqueous-phasesynthesis of uniform palladium nanoparticles of various shapes and sizes, Small3 (2007) 255–260.

[13] B. Lim, Y.J. Xiong, Y.N. Xia, A water-based synthesis of octahedral, decahedral,and icosahedral Pd nanocrystals, Angew. Chem. Int. Ed. 46 (2007) 9279–9282.

[14] F.J. Ibanez, F.P. Zamborini, Ozone- and thermally activated films of palladiummonolayer-protected clusters for chemiresistive hydrogen sensing, Langmuir22 (2006) 9789–9796.

[15] V.G. Pol, H. Grisaru, A. Gedanken, Coating noble metal nanocrystals (Ag, Au, Pd,and Pt) on polystyrene spheres via ultrasound irradiation, Langmuir 21 (2005)3635–3640.

[16] Y.S. Chun, J.Y. Shin, C.E. Song, S.G. Lee, Palladium nanoparticles supported ontoionic carbon nanotubes as robust recyclable catalysts in an ionic liquid, Chem.Commun. (2008) 942–944.

[17] H. Ohde, C.M. Wai, H. Kim, J. Kim, M. Ohde, Hydrogenation of olefins insupercritical CO2 catalyzed by palladium nanoparticles in a water-in-CO2

microemulsion, J. Am. Chem. Soc. 124 (2002) 4540–4541.[18] J.R. Burns, C. Ramshaw, R.J. Jachuck, Measurement of liquid film thickness and

the determination of spin-up radius on a rotating disc using an electrical resis-tance technique, Chem. Eng. Sci. 58 (2003) 2245–2253.

[19] G.M. Sisoev, O.K. Matar, C.J. Lawrence, Gas absorption into a wavy film flowingover a spinning disc, Chem. Eng. Sci. 60 (2005) 2051–2060.

[20] S.F. Chin, K.S. Iyer, C.L. Raston, M. Saunders, Size selective synthesis of super-paramagnetic nanoparticles in thin fluids under continuous flow conditions,Adv. Funct. Mater. 18 (2008) 922–927.

[21] K.S. Iyer, M. Norret, S.J. Dalgarno, J.L. Atwood, C.L. Raston, Loading molecu-lar hydrogen cargo within viruslike nanocontainers, Angew. Chem. Int. Ed. 47(2008) 6362–6366.

[22] Y. Im, C. Lee, R.P. Vasquez, M.A. Bangar, N.V. Myung, E.J. Menke, R.M. Penner, M.Yun, Investigation of a single Pd nanowire for use as a hydrogen sensor, Small2 (2006) 356–358.

[23] F.A. Lewis, The palladium hydrogen system, Academic Press, London and NewYork, 1967, pp. 43–49.

Biographies

Jianli Zou is a Ph.D. candidate in Centre for Strategic Nano-Fabrication, The Uni-versity of Western Australia. Her research focuses on synthesis of palladiumnanomaterials, application of palladium nanomaterials in hydrogen gas sensing andapplication of palladium nanomaterials as catalyst in C–C coupling Heck reaction.

Lee J. Hubble graduated with a Ph.D. in Chemistry from The University of WesternAustralia under the guidance of Prof. C. L. Raston. He is currently an OCE PostdoctoralFellow at Australia’s Commonwealth Scientific and Industrial Research Organisation(CSIRO). His research interests include supramolecular chemistries of carbon nano-materials and developing hybrid nano-materials for chemical sensing.

K. Swaminathan Iyer is an Australian Research Council Research Fellow and DeputyDirector of the Centre for Strategic Nano-Fabrication, The University of WesternAustralia. He graduated with a Ph.D. in materials science and engineering under theguidance of professor Igor Luzinov from Clemson University, SC, USA, followed bypostdoctoral research in Clarkson University, NY, USA with professor Igor Sokolov.His research focuses in the use of novel nanomaterials for biomedical applicationsand for alternative green-energy solutions.

Colin L. Raston is an Australian Research Council Professorial Fellow at The Univer-sity of Western Australia, being appointed to the University in 2003. He completed aPh.D. under the guidance of professor Allan White, in The University of Western Aus-tralia and after postdoctoral studies with Professor Michael Lappert at the Universityof Sussex, he was appointed a Lecturer at The University of Western Australia (1981)then to the Chair of Chemistry at Griffith University (1988), being awarded a DScthere in 1993, Monash University (1995) and Leeds (2001). His research interestscover aspects of nanochemistry and green chemistry.

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3.2 Hydrogen-induced reversible insulator–metal transition in a palladium

nanosphere sensor

2358 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2010, 6, No. 21, 2358–2361wileyonlinelibrary.com

communications

Sensors

Fabricating ordered superlattices from metals and semi-

conductor nanocrystals have generated enormous interest due

to the possibility of forming assemblies that have electrical

and optical properties that can be tuned through control over

particle size, stoichiometry, and interparticle separation. [ 1–8 ]

Tuning the interparticle separation of a superlattice is of par-

ticular interest in regulating the electrical properties of self

assembled architectures. [ 6–9 ] In particular, in the case of metal

quantum dots at large interparticle separations, the superlat-

tice is described as a Mott insulator, with a Coulomb band gap

described by the charging energies of the individual nanopar-

ticle lattice sites. [ 10–12 ] Following compression, the Coulomb

gap disappears resulting in the density of states becoming

temperature-independent with a fi nite-value at the Fermi

level, resulting in a transition to a conducting metal state. [ 13 ]

Indeed, hydrostatic pressure has successfully been applied to

Langmuir fi lms of hexagonally packed 3-nm-diameter silver

quantum dots [ 6 , 14 ] and photoconducting organic liquid crys-

tals [ 15 ] to induce a metal-insulator transition which is revers-

ible. We report that such reversible transitions occur in self

organized three-dimensional arrays of palladium particles in

a nanosphere without the need for an external hydrostatic

pressure, simply by the absorption of molecular hydrogen at

ambient pressure.

The dissociative absorption of hydrogen on palladium

results in a detectable change in mass, volume, electrical

resistivity and optical constants. These changes are based

on the well established principle that hydrogen gas sponta-

neously diffuses into the metal lattice affording palladium

hydride, PdHx. This is associated with an α to β phase transi-

tion resulting in a change in the lattice spacing and concomi-

tant change in the properties of the material. This principle

features in the development of advanced materials as highly

effi cient sensors for detecting molecular hydrogen with fast

response times, notably Pd nanowires, [ 16 , 17 ] Pd meso-wires, [ 18 ]

Pd fi lms formed by monolayer-protected clusters, [ 19 ] nano-

porous Pd fi lm deposited on anodic aluminium oxide, [ 20 ] Pd/

carbon nanotube hybrids, [ 21 , 22 ] and Pd/Titania nanohyrbids. [ 23 ]

Hydrogen-Induced Reversible Insulator–Metal Transition in a Palladium Nanosphere Sensor

Jianli Zou, K. Swaminathan Iyer,* Colin L. Raston

The detection of hydrogen in the sensors is mainly based on

the change in the electrical properties associated with the

hydrogen adsorption.

The most common hydrogen gas sensing mechanism

involves the increase in resistance of palladium arising from

the formation of palladium hydride as the concentration of

hydrogen increases. [ 20–22 ] The other mechanism relates to the

volume expansion, usually on relatively high concentration of

hydrogen uptake, resulting in a decrease in resistance. [ 17 , 18 , 24 ]

In the case of disconnected palladium mesowires, [ 18 ] a high

resistance arises from breaks in the palladium mesowires, the

resistance drops following hydrogen uptake, affording more

connected and lower resistance structure.

More recently it was demonstrated in the case of monolayer-

protected Pd clusters that depending on type of monolayer

coating the fi lms could be placed into two categories: those

that decrease in current in the presence of H 2 and those that

increase in current in the presence of H 2 . [ 19 ] In the present

study we demonstrate for the fi rst time that a single self-

assembled palladium nanosphere system held together by a

polymeric dielectric network (polyvinylpyrrolidone (PVP))

can switch from an increases in resistance (sensing mecha-

nism 1) to an increase in current (sensing mechanism 2) type

sensor reversibly depending on the concentration of H 2 .

The palladium nanospheres used in the current study

were synthesized in a dynamic thin fi lm platform on a

rotating disc using hydrogen gas as a reducing agent from

an aqueous solution of H 2 PdCl 4 , in the presence of PVP

(detailed synthesis procedure and characterizations in sup-

porting information). In essence the reaction occurs in an

atmosphere of hydrogen gas on a rotating surface. The

H 2 PdCl 4 solution experiences active shear forces and viscous

drag between the moving fl uid layer and the disc surface

resulting in turbulence and ripples which give rise to highly

effi cient micromixing within the dynamic thin fl uid layer.

The fl ow is accompanied by non-linear waves, which strongly

infl uence the diffusion boundary that develops beneath the

surface of the fi lm over a moderately spinning disc speed

which enhances the gas adsorption into the liquid. [ 25 ] These

progressive waves generated in the region of active micro-

mixing offer the important control necessary to synthesize

palladium nanospheres of uniform composition and overall

structure, which are shown in Figure 1a . Interestingly the

cross-section TEM image and high resolution TEM image

show that the self assembled structure in each nanosphere is

a three-dimensional composite comprised of 5 nm palladium

nanocrystals. As evident in the images, not all the nanocrys-

tals are tightly packed and a majority of them have prominent DOI: 10.1002/smll.201001003

J. Zou , Dr. K. S. Iyer , Prof. C. L. Raston Centre for Strategic Nano-FabricationSchool of BiomedicalBiomolecular and Chemical SciencesThe University of Western AustraliaCrawley, WA 6009, Australia E-mail: [email protected]

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Insulator–Metal Transition in a Palladium Nanosphere Sensor

2359© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall 2010, 6, No. 21, 2358–2361

interparticle spacing in projection, Figure 1 b, c. The PVP

acts as a scaffold holding the nanocrystals together in a

three-dimensional dielectric environment as represented in

Figure 1 d. We believe that the presence of both connected

and disconnected junctions in a single ensemble will enable

switching between sensing mechanism 1 and 2 described

above.

The palladium nanospheres were assembled by drop

casting a suspension of the nanospheres on an interdigitated

electrode (IDE) surface using aliquots of 0.02 μ L from a

0.5 μ L glass syringe and were subsequently air-dried. Cur-

rent-voltage ( I–V ) sweeps were carried out until an ohmic

response was achieved with the Pd nanospheres bridging the

electrode gaps. This was further supported through scanning

electron microscopy (SEM), Figure 2a (see supporting infor-

mation for low magnifi cation SEM images, Figure S4). When

a constant voltage of 100 mV was applied between the elec-

trodes, the currents were measured during a cyclic exchange

of 20 min of nitrogen gas and 4 min for different concentra-

tions of hydrogen gas (from 0.1% to 10%) in nitrogen gas at

atmospheric pressure and room temperature. For hydrogen

concentrations of less than 2%, the sensor has an inherent

increase in resistance, Figure 2 b. This is in agreement with

previously reported increase in resistance at lower hydrogen

concentrations due to the α to β phase transition primarily

on the surface (minimum volume change

of Pd). Indeed the sensing here is domi-

nated by the connected junctions of Pd

within the PVP scaffold. For concentra-

tions higher than 2%, there is an unusual

switching, with the sensor now having an

increase in conductance, Figure 2 c. It is

believed that the sensing herein is domi-

nated by the disconnected Pd junctions.

It is noteworthy that this previously unre-

ported switching mechanism is reversible

and readily reproducible.

The Mott insulator or Anderson con-

ductor behavior has been reported for

compressed superlattice monolayers of

metal quantum dots. [ 6 ] In the case of

monolayers, when the quantum dots are

far apart, the electronic response is domi-

nated by the localized Coulombic repul-

sion of electrons on a given dot. [ 6 ] This

gives rise to a Mott-like insulator behavior

with a Coulomb band gap described by

the charging energies of the individual

nanoparticle lattice sites. [ 6 ] Following com-

pression, their strong proximity coupling

results in the Coulomb gap disappearing,

allowing for a facile electron transfer from

one dot to the other. This proximity effect

leads to band delocalized states as seen

in the metal, leading to what is famously

been termed an Anderson conductor. An

Anderson transition to a delocalized elec-

tronic phase occurs at D/2R < 1.4 and a

Mott transition at D /2 R > 1.3, where D is

the distance between the centres of two adjacent dots and R

is the radius of the dot. [ 26 ] In the present case, we predicted

a similar behavior in a confi ned three-dimensional dielectric

sphere for the disconnected junctions. At room tempera-

ture, bulk PdHx undergoes an α -phase to β -phase transition,

leading to a change in the lattice constant from 3.895 Å to

4.025 Å. [ 27 ] In the present case we can assume that for dis-

connected junctions the average diameter of the palladium

nanoparticles is 5 nm and these are separated by a distance of

2 nm (represented in Figure 1 c). A change due to the α -phase

to β -phase transition would refl ect an increase in diameter

of the nanoparticles to 5.175 nm, the corresponding value of

D /2 R being 1.35.

Herein, for H 2 gas concentrations < 2%, the Pd to PdHx

phase transition is primarily on the surface of the individual

Pd nanoparticles within the three-dimensional ensembles.

This results in minimal change in the D /2 R ratio, with the

electrical response dominated by the localized Coulombic

repulsion of electrons for the disconnected junctions and the

classic increase in resistance for the connected junctions. For

concentrations > 2%, the Pd to PdHx phase transition pre-

vails through individual nanoparticles of the metal within

each nanosphere, and the D /2 R ratio undergoes a dramatic

change associated with the volumetric expansion. The prox-

imity of individual disconnected Pd nanoparticles ensures

Figure 1 . a) SEM image of the palladium nanospheres in a dielectric scaffold of PVP (scale bar: 200 nm), b) cross-section TEM image of a single palladium nanosphere, c) HRTEM image of the palladium nanosphere showing interparticle separation and d) a schematic representation of the three-dimensional palladium nanosphere.

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J. Zou et al.

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communications

strong coupling resulting in the Coulomb gap disappearing,

allowing for a facile electron transfer from one nanoparticle

to the other, which translates to a measurable increase in

conductance, Figure 3 . It is noteworthy that in the absence

of PVP, palladium nanoparticles form an agglomerate rosette,

with complete bridging of each nanoparticle, but these

nanoparticles do not show such a transition, only resulting in

an increase in resistance on adsorption of the gas (see Sup-

porting Information).

In summary, self organized three-dimensional palladium

nanospheres have been synthesized using a facile approach

involving hydrogen gas as the reducing agent within a

dynamic microfl uidic platform. Even more signifi cant is dem-

onstrating for the fi rst time that the dissociative adsorption

of hydrogen induces a Mott insulator to metal transition in

a palladium nanosphere. Moreover, this quantum mechanical

coupling of the metal insulator transition to sense hydrogen

is without precedent. Overall, these results are an important

development in potentially miniaturizing three dimensions

for applications in next generation nanoparticle based elec-

tronic switches and sensors.

Experimental Section

Synthesis of Palladium Nanospheres : The palladium nano-spheres were synthesized by reducing H 2 PdCl 4 in the presence of PVP through spinning dick processor (SDP), (See supporting infor-mation for more details). In a typical synthesis of palladium nano-spheres, the H 2 PdCl 4 aqueous solution (0.6 mmol L − 1 ) was mixed with PVP (Polyvinylpyrrolidone, MW = 40 000) and then the mix-ture was fed from one jet at the feed rate of 0.7 mL s − 1 . Hydrogen gas was fed from another jet to reduce palladium (II) to palladium nanoparticles. The speed of the spinning disc was set at 2000 rpm. The as synthesized nanospheres were washed using MilliQ water ( > 18 M Ω ) three times and re-dispersed in water before any further test. The size and morphology of the samples were determined using transmission electron microscopy (TEM, JEOL 3000F) oper-ating at 300 kV. Powder XRD pattern of the palladium nanospheres was measured using an Oxford Diffraction Gemini-R CCD diffrac-tometer (using Cu K α = 1.54178 Å radiation).

Setup of the Hydrogen Sensor : The palladium nanospheres solution was drop-cast onto the surface of each IDE using aliquots of 0.02 μ L from a 0.5 μ L glass syringe which were subsequently air-dried. The test procedure involved alternating nitrogen gas

Figure 2 . a) SEM image of the self-assembled palladium nanospheres on an interdigitated electrode (IDE), and change in current measured as a response for different hydrogen gas concentrations: b) 0.1 to 1% and c) 1 to 10%.

50 100 150 200 250

18

20

22

24

26

onoff

% H2

10.90.80.70.60.50.4

onoff

H2

0.3I /µA

Time /min

50 100 150 200 25020

25

30

35

40

108 9

onoff

%H2

76

5

1

4

onoff H

2

3

2

I /µA

Time /min

(a)

(b)

(c)

Figure 3 . The measured change in current plotted as a function of hydrogen gas concentration showing an insulator to metal transition at 2% hydrogen concentration. A schematic representation of the change in volume in each nanosphere due to the dissociative adsorption of hydrogen resulting in a well connected PdHx structure affording a facile electron transfer from one nanoparticle to the other.

Pd/PVP

H2

PdHx/PVP

(D/2R)Pd/PVP<(D/2R)PdHx/PVP

0111.0

-2

0

2

4

6

8

10

12

( I0

-I) µ

A

H2%

Insulator Conductor

Transitio

n zo

ne

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Insulator–Metal Transition in a Palladium Nanosphere Sensor

2361© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall 2010, 6, No. 21, 2358–2361

(20 min) and varying concentrations of hydrogen gas (4 min). The change of the current was monitored at the same time. The total fl ow rate of gas was 1000 mL min − 1 . The voltage applied between two electrodes was 100 mV dc. The images of sensor were recorded with scanning electron microscope (SEM, Zeiss 1555 VPSEM) oper-ating at an accelerating voltage of 8 kV.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author .

Acknowledgements

The authors are grateful for the fi nancial support for this work by the Australian Research Council and The University of Western Aus-tralia, and for the palladium precursors provided by AGR Matthey. The microscopy analysis was carried out using facilities in the Centre for Microscopy, Characterization and Analysis, The Univer-sity of Western Australia, which are supported by University, State and Federal Government funding.

[ 5 ] J. M. Wessels , H. G. Nothofer , W. E. Ford , F. von Wrochem , F. Scholz , T. Vossmeyer , A. Schroedter , H. Weller , A. Yasuda , J. Am. Chem. Soc. 2004 , 126 , 3349 .

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Phys. Rev. B 1999 , 59 , 1633 . [ 14 ] S. Henrichs , C. P. Collier , R. J. Saykally , Y. R. Shen , J. R. Heath ,

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R. M. Penner , M. Yun , Small 2006 , 2 , 356 . [ 17 ] F. Yang , D. K. Taggart , R. M. Penner , Nano Lett. 2009 , 9 , 2177 . [ 18 ] F. Favier , E. C. Walter , M. P. Zach , T. Benter , R. M. Penner , Science

2001 , 293 , 2227 . [ 19 ] F. J. Ibañez , F. P. Zamborini , J. Am. Chem. Soc. 2008 , 130 , 622 . [ 20 ] D. Y. Ding , Z. Chen , Adv. Mater. 2007 , 19 , 1996 . [ 21 ] J. Kong , M. G. Chapline , H. J. Dai , Adv. Mater. 2001 , 13 , 1384 . [ 22 ] Y. G. Sun , H. H. Wang , Adv. Mater. 2007 , 19 , 2818 . [ 23 ] M. K. Kumar , L. K. Tan , N. N. Gosvami , H. Gao , J. Phys. Chem. C

2009 , 113 , 6381 . [ 24 ] Y. Hatakeyama , M. Umetsu , S. Ohara , F. Kawadai , S. Takami ,

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60 , 2051 . [ 26 ] F. Remacle , R. D. Levine , J. Am. Chem. Soc. 2000 , 122 , 4084 . [ 27 ] T. Xu , M. P. Zach , Z. L. Xiao , D. Rosenmann , U. Welp , W. K. Kwok ,

G. W. Crabtree , Appl. Phys. Lett. 2005 , 86 , 203104 .

Received: June 12, 2010 Published online: September 27, 2010

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3.3 Pd-sodium carboxymethyl cellulose nanocomposites display a

morphology dependent response to hydrogen gas

Green Chemistry Dynamic Article Links

Cite this: DOI: 10.1039/c2gc16456f

www.rsc.org/greenchem COMMUNICATION

Pd–sodium carboxymethyl cellulose nanocomposites display a morphologydependent response to hydrogen gas†

Jianli Zou, K. Swaminathan Iyer* and Colin L. Raston

Received 14th November 2011, Accepted 18th January 2012DOI: 10.1039/c2gc16456f

Hydrogen reduction of H2PdCl4 in the presence of sodiumcarboxymethyl cellulose (SCMC) in dynamic thin films on amicrofluidic spinning disc platform affords Pd–SCMC nano-structures. The morphology of Pd nanocomposites changedfrom well-connected nano-rosettes to disconnected agglomer-ates with an increase in SCMC concentration. These nano-composites were drop cast on interdigitated electrodes (IDEs)to afford sensors for hydrogen gas, with a decrease andincrease response in current for nano-rosettes and agglomer-ates structures, respectively.

Polymer and organic ligand-stabilised palladium (Pd) nanoparti-cles are readily prepared by chemical and electrochemicalreduction of metal salts, and thermal decomposition. These Pdnanomaterials have been widely investigated for applications inheterogeneous catalysis, hydrogen purification, storage, andhydrogen sensing.1–11 The efficacy of Pd for hydrogen sensingarises from the ability of Pd selectively dissociating molecularhydrogen into atomic hydrogen at room temperature.12,13 Fol-lowing dissociation, hydrogen atoms adsorb on to the surface ordiffuse into the Pd crystal affording Pd hydride, which results ina change in conductivity, volume and optical constants. Thereare two distinct Pd hydride phases, denoted as α- and β-phases,with hydrogen atoms occupying interstitial sites or octahedrallattice sites of fcc (face-centred-cubic) Pd metal, respectively.14

Both Pd hydride phases possess higher resistance than Pd, andthis change is the basis for sensor devices which contain verywell connected Pd nanostructures between two electrodes todetect hydrogen by monitoring the decrease in current orincrease in resistance.15–18 Alternatively, hydrogen sensors canbe based on volume expansion associated with Pd hydride for-mation. Here Pd nanostructures containing disconnected junc-tions between two electrodes undergo an expansion in volume toform a well-connected structure, in turn resulting in an increasein current in the presence of hydrogen gas.18–20 Hydrogen-actu-ated switches and sensor devices based on this mechanism have

been reported starting with the pioneering work by Penneret al.21

We show that the aforementioned changes in resistance andvolume can be easily exploited in hydrogen sensor devices usingdifferent Pd nano-structures and a simple drop casting technique.Pd nanocomposites were fabricated within dynamic thin filmsgenerated on a spinning disc processing (SDP) platform. Thisinvolved the reduction of dihydrogentetrachloropalladate(II)(H2PdCl4) using hydrogen gas at ambient pressure in the pres-ence of sodium carboxymethyl cellulose (SCMC). SDP providesa continuous-flow, microfluidic platform with precise control inthe fluid dynamics of the system (e.g. feed rate, micromixing,fluid film thickness and flow conditions). The key componentsof a SDP platform include a rotating disc with controllable speedup to 3000 rpm, and feed jets located at a radial distance of5 mm from the centre of the disc (Fig. S1†). SCMC is a com-mercially available cellulose ether which is produced on anindustrial scale and widely used in pharmaceuticals, cosmeticsand food industry. SCMC has good solubility, high chemicalstability, as well as non-toxicity. It is processed in a highlyswollen state which gives rise to very thorough consistency inaqueous medium. In a typical procedure, a solution containingH2PdCl4 and SCMC was delivered close to the centre of therapidly rotating disc through one feed jet. Hydrogen gas as redu-cing agent was fed from another feed jet. The solution formed avery thin fluid film on the disc surface with progressive waves,which enhances hydrogen adsorption and facilitates rapidreduction of Pd(II), in seconds.22

Typically, the molar equivalents of SCMC to Pd 2 : 1 resultedin the formation of numerous Pd nanoparticles held together toform a rosette structure (Fig. 1a and Fig. S2†). Increasing themolar equivalents to 5 : 1 resulted in the formation of agglomer-ates structure (Fig. 1b and Fig. S2†). It is noteworthy that redu-cing the molar equivalents of SCMC to Pd to 1 : 1 resulted in theformation of non-uniform aggregates, and an increase in equiva-lents to 30 : 1 resulted in the formation of separate nanoparticles(Fig. S3†). Thus the nature of the nanocomposites produced dra-matically depends on the molar equivalent of SCMC relative toPd. The content of SCMC in the Pd–SCMC nanocomposites forthe equivalents of SCMC to Pd at 2 : 1 and 5 : 1 is 7.54 wt% and11.05 wt%, respectively, as established using thermogravimetricanalysis.

The two different Pd nanocomposites shown in Fig. 1a and 1bwere used as hydrogen sensing materials in the following exper-iments, designated sensor 1 and sensor 2, respectively. First, a

†Electronic supplementary information (ESI) available: Details of syn-thesis of Pd nanocomposites and setup of hydrogen gas sensing. SeeDOI: 10.1039/c2gc16456f

Centre for Strategic Nano-Fabrication, School of Biomedical,Biomolecular and Chemical Sciences, The University of WesternAustralia, Crawley, WA6009, Australia. E-mail: [email protected]; Fax: +61 8 6488 1005; Tel: +61 8 6488 4470

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colloidal suspension of a Pd nanocomposites solution was dropcast onto the surface of premade sensor chip with interdigitatedelectrode (IDE) and subsequently air-dried. These Pd nanomater-ials bridged the electrode gaps, as can be seen from SEM images(Fig. S4†). Alternating nitrogen gas and varying concentrations

of hydrogen gas were delivered to the IDE, and the change ofthe current was monitored at the same time to test the perform-ance of the sensors.

Fig. 2a and 2b shows a decrease in current with hydrogen gasflow and a return to the original state when no hydrogen gas waspresent for concentrations of 0.2% to 1% and 1% to 3% forsensor 1. The change of current in sensor 1 can be explained bythe diffusion of atomic hydrogen into the lattice to form PdHx,which is more resistive than Pd, in turn resulting in a decrease incurrent. As can be seen from Fig. 2a and 2b, the sensor is stabletowards hydrogen and nitrogen recycling with minimum shift ofthe baseline.

In contrast, for sensor 2, the current dramatically increasedwith 2% to 10% of hydrogen gas, and the sensor showed noobvious response to 1% hydrogen gas (Fig. 3a). Low hydrogenconcentration (<1%) only results in the minor volume expansionof Pd (α-phase), while at higher concentration (>2%), hydrogenatoms occupy the octahedral lattice sites of the Pd crystal struc-ture (β-phase) resulting in a more significant volume change.The α-to-β phase transition at higher concentrations can result inabout 11% increase in volume. This volume expansion resultsin the formation of well-connected structures in sensor 2, asevident in the increase in current. In this case, after severalcycles (up to 4 hours sensing for each cycle), the base lineslightly increased.

The molar equivalent of SCMC to Pd not only plays a keyrole in determining the morphology of the resulting Pd nano-composites, but also affects the connections between them. Athigh concentration of SCMC, the increased viscosity of thesolution facilitated the formation of small Pd nanocomposites.During the hydrogen sensing test, with less SCMC, Pd

Fig. 2 The response of sensor 1 to hydrogen gas from concentration of0.1–1% (a) and 1–3% (b).

Fig. 1 (a) and (b) TEM images of as-prepared Pd nanocompositesusing spinning disc processing: the molar equivalents of SCMC to Pdwere 2 and 5, respectively. (c) High resolution TEM image and (d) XRDpattern of Pd nanocomposites shown in Fig. 1b. Fig. 3 The response of sensor 2 to hydrogen gas with the concen-

tration from 1% to 10% (a) and 3% (b), with the applied voltage100 mVand 10 mV, respectively.

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nanorosettes formed a continuously connected structure insensor 1, while with higher SCMC concentrations, Pd nanoag-glomerates formed a disconnected structure in sensor 2.

Conclusions

In summary, we report a simple, yet novel, approach to alter themorphology of Pd nanocomposites by varying the equivalents ofsodium carboxymethyl cellulose to Pd precursor, involving theuse of a continuous-flow microfluidic platform. Hydrogensensors using different Pd nanostructures showed decreased orincreased current responses, depending on the nature of the Pdnanocomposites. Importantly, the devices are robust in havinggood stability during testing.

Acknowledgements

The authors are grateful for the financial support for this workby the Australian Research Council and The University ofWestern Australia, and for the Pd precursors provided by ThePerth Mint. The microscopy analysis was carried out using facili-ties in the Centre for Microscopy, Characterization and Analysis,The University of Western Australia, which are supported byUniversity, State and Federal Government funding.

Notes and references

1 B. Lim, M. Jiang, J. Tao, P. H. C. Camargo, Y. Zhu and Y. Xia, Adv.Funct. Mater., 2009, 19, 189.

2 Y. J. Xiong and Y. N. Xia, Adv. Mater., 2007, 19, 3385.3 K. Esumi, T. Tano and K. Meguro, Langmuir, 1989, 5, 268.4 S.-W. Kim, J. Park, Y. Jang, Y. Chung, S. Hwang, T. Hyeon andY. W. Kim, Nano Lett., 2003, 3, 1289.

5 M. T. Reetz and G. Lohmer, Chem. Commun., 1996, 1921.6 J. Watt, S. Cheong, M. F. Toney, B. Ingham, J. Cookson, P. T. Bishop andR. D. Tilley, ACS Nano, 2010, 4, 396.

7 M. T. Reetz and W. Helbig, J. Am. Chem. Soc., 1994, 116, 7401.8 W. Pan, X. Zhang, H. Ma and J. Zhang, J. Phys. Chem. C, 2008, 112,2456.

9 S. Harish, J. Mathiyarasu, K. Phani and V. Yegnaraman, Catal. Lett.,2008, 128, 197.

10 S. Fujii, S. Matsuzawa, Y. Nakamura, A. Ohtaka, T. Teratani,K. Akamatsu, T. Tsuruoka and H. Nawafune, Langmuir, 2010, 26, 6230.

11 S. Yoda, A. Hasegawa, H. Suda, Y. Uchimaru, K. Haraya, T. Tsuji andK. Otake, Chem. Mater., 2004, 16, 2363.

12 I. Lundstroem, S. Shivaraman, C. Svensson and L. Lundkvist, Appl.Phys. Lett., 1975, 26, 55.

13 K. I. Lundstrom, M. S. Shivaraman and C. M. Svensson, J. Appl. Phys.,1975, 46, 3876.

14 B. Ingham, M. F. Toney, S. C. Hendy, T. Cox, D. D. Fong,J. A. Eastman, P. H. Fuoss, K. J. Stevens, A. Lassesson, S. A. Brownand M. P. Ryan, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78,245408.

15 D. Y. Ding and Z. Chen, Adv. Mater., 2007, 19, 1996.16 X. Q. Zeng, M. L. Latimer, Z. L. Xiao, S. Panuganti, U. Welp, W.

K. Kwok and T. Xu, Nano Lett., 2011, 11, 262.17 S. Cherevko, N. Kulyk, J. Fu and C. H. Chung, Sens. Actuators, B, 2009,

136, 388.18 R. Dasari and F. P. Zamborini, J. Am. Chem. Soc., 2008, 130, 16138.19 J. Lee, W. Shim, E. Lee, J.-S. Noh and W. Lee, Angew. Chem., Int. Ed.,

2011, 50, 5301.20 K. T. Kim, J. Sim and S. M. Cho, IEEE Sens. J., 2006, 6, 509.21 F. Favier, E. C. Walter, M. P. Zach, T. Benter and R. M. Penner, Science,

2001, 293, 2227.22 G. M. Sisoev, O. K. Matar and C. J. Lawrence, Chem. Eng. Sci., 2005,

60, 2051.

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3.4 Scalable synthesis of catalysts for the Mizoroki-Heck cross coupling

reaction: palladium nanoparticles assembled in a polymeric

nanosphere

854 New J. Chem., 2011, 35, 854–860 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

Cite this: New J. Chem., 2011, 35, 854–860

Scalable synthesis of catalysts for the Mizoroki–Heck cross coupling

reaction: palladium nanoparticles assembled in a polymeric nanospherew

Jianli Zou,aK. Swaminathan Iyer,*

aScott G. Stewart*

band Colin L. Raston

a

Received (in Victoria, Australia) 15th November 2010, Accepted 6th January 2011

DOI: 10.1039/c0nj00898b

Palladium nano-spheres 160 nm in diameter, as an assembly of uniform 5 nm nanoparticles,

are accessible using a facile one step method under continuous flow on a spinning disc with

hydrogen gas as the reducing agent. The stable colloidal system is an effective catalyst for the

Mizoroki–Heck reaction, as established for the reaction between several aryl halides and n-butyl

acrylate, and can be readily recycled without a change in their catalytic activity.

Introduction

Heterogeneous catalytic systems have an advantageous

practical convenience over homogeneous systems because of

their ease of separation which is usually through simple

filtration.1–3 The recovery and reusability of catalysts is a very

important factor especially in the case of noble metal (Pd)

catalysed C–C coupling reactions like the Mizoroki–Heck and

Suzuki cross coupling reactions, and heterogeneous systems

also dispense with the need for the design and synthesis of

often expensive ligands that feature in homogeneous

systems.4–12 The interest and study of supported palladium

catalysts for applications in C–C cross coupling reactions had

its origins in 1990’s, where initially palladium was often

embedded on or in to supporting materials such as zeolite

and inorganic oxides, including graphite oxide, MgO, Al2O3

and SiO2.13–18 With nano-materials exhibiting unique physical

and chemical properties relative to their larger counterparts,

the synthesis and application of these supported palladium

catalysts is attracting ever increasing interest.19–22 Bradley

et al.19 have recently developed amino modified resins as

supported materials for in situ reduction of Pd(OAc)2 to

7 nm Pd nano-particles, which are stable and can be reused

in a variety of Suzuki reactions. More recently, Wan et al.

reported 3 nm palladium nanoparticles supported on ordered

mesoporous silica–carbon nano-composites exhibiting high

catalytic activity for coupling reactions.22 However, the

problem of catalytic recovery is not completely resolved using

the aforementioned supported-heterogeneous systems. An

inherent challenge in using supported palladium nano-

particles is their disassociation from the substrate support,

resulting in catalyst leaching from the substrate over time. This

leaching is often associated with a drop in catalytic activity

and reusability.

The use of palladium nanoparticles as heterogeneous

catalysts in C–C coupling reactions is an area of current

interest due to the possibility of fine tuning the shape and size

of the colloidal system to control the catalytic efficacy. The

first well-known case for the use of colloidal Pd catalysts was

independently reported by Beller et al. and Reetz et al. in 1996.

In the former case palladium nano-particles were stabilized by

tetra-octyl ammonium chloride,23 whereas in the latter they

were stabilized in propylene carbonate.24 More recently

palladium clusters stabilized in polymer micelles, dendrimers

and ionic liquids have been widely studied as recyclable

catalysts.25–27 The synthesis of colloidal palladium systems

usually avoids the multi-step method for generating supported

systems with their associated leaching problem, but they

are thermodynamically unstable leading to aggregations,

especially at high temperatures. Such aggregations can result

in the loss of active surface area for effective catalysis, thereby

diminishing their potential for recycling.

Poly(vinylpyrrolidone) (PVP) has been shown to be an

effective matrix in stabilising palladium nanoparticles as

catalyst for cross coupling reactions.28–31 Here we report the

synthesis of novel palladium composite nano-spheres using

spinning disc processing (SDP) as a facile one step process

with hydrogen gas as the reducing agent for scalable size

controlled synthesis of heterogeneous catalysts. The nano-

spheres herein are held together by a PVP scaffold. We

a Centre for Strategic Nano-Fabrication, School of Biomedical,Biomolecular and Chemical Sciences, The University of WesternAustralia M313, 35 Stirling Highway, Crawley, WA 6009, Australia.E-mail: [email protected]; Fax: +61 8 6488 1005;Tel: +61 8 6488 4470

b School of Biomedical, Biomolecular and Chemical Sciences,The University of Western Australia M313, 35 Stirling Highway,Crawley, WA 6009, Australia. E-mail: sgs@ uwa.edu.au;Tel: +61 8 6488 3180

w Electronic supplementary information (ESI) available: Schematic ofspinning disc processor, TEM image of as-prepared palladium nano-particles using hydrazine as the reducing reagent, TEM image of atypical palladium nanomaterials synthesized using PVP with a mole-cular weight of 360 000, TEM images of palladium–PVP spheressynthesized via mechanical stirring, EDS data of palladium nano-spheres, 1H NMR data and GC-MS data of the product ofMizoroki–Heck reactions in Table 1–5. See DOI: 10.1039/c0nj00898b

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 854–860 855

anticipated that the PVP scaffold is likely to prevent catalytic

leaching with time, with the prospect of maintaining high

colloidal stability, structural integrity and reusability.

Results and discussion

Synthesis and characterization of palladium composite

nano-spheres

SDP is a microfluidic process intensification strategy with a

continuous flowing film (1 to 200 mm) on a rapidly rotating

disc surface (usually up to 3000 rpm) (Fig. S1, ESIw). It

facilitates controlled nucleation and growth of nanoparticles

for the fabrication of different nano-materials,32,33 and has

potential for the large scale synthesis of a range of nano-

materials/nanoparticles. Such a microfluidic platform offers an

alternative approach in alleviating the obstacles and limita-

tions of the relaxed fluid dynamic regimes associated with

conventional batch processing. In essence SDP offers

improved micromixing environments on a rapidly rotating

surface which ensures that there is a very high surface area

to volume ratio for the reacting fluid, resulting in supersatura-

tion of the gas phase reagents and nanocrystal nucleation and

growth at room temperature. The mixing for SDP is achieved

by feeding the reagents close to the centre of a rapidly rotating

disc. The behavior of a thin film on a rapidly rotating disc can

be associated with two zones: (i) The injection zone where the

reagents hit the surface close to the centre of the spinning disc

forming a spin-up pool with the flow of liquid from the pool

controlled by viscous drag associated with centrifugal forces

where rapid nucleation and instantaneous growth initiation

occur. (ii) The acceleration and synchronized flow zone where

the fluid film initially experiences an increase in the radial flow

velocity with the liquid then rotating close to the disc velocity

with the flow becoming similar to the Nusselt model.34 The

shear forces and viscous drag between the moving fluid layer

and the disc surface create turbulence and ripples which gives

rise to highly efficient mixing within the thin fluid layer. The

turbulent waves thereby provide a high degree of growth

control in zone (ii).

In a typical experiment aqueous solutions of H2PdCl4 were

mixed with PVP at various molecular ratios of PVP relative to

palladium, and then fed through a jet feed onto the spinning

disc. Simultaneously the hydrogen gas reducing agent was fed

through another jet feed slightly above ambient pressure,

which resulted in the formation of palladium nano-spheres

of uniform size and shape (Fig. 1). The nano-spheres result

from the spontaneous assembly of a large number of 5 nm

palladium particles within the dynamic thin films on the

surface of the disc in the presence of PVP, rather than discrete

individual palladium nanoparticles. The role of the PVP is

multifunctional, in acting as a scaffold preventing leaching of

palladium during catalysis from the confines of the nano-

spheres into the solution and in acting as a stabiliser main-

taining high colloidal stability thereby preventing loss of

activity due to agglomeration. TEM images (Fig. 1a–c) reveal

an assembly of palladium nano-particles with a mean diameter

of 5 nm. The XRD pattern (Fig. 1d) shows that the palladium

nanoparticles are highly crystalline.

The use of hydrogen gas as the reducing agent is noteworthy

for the following reasons: (1) high purity palladium catalysts

are accessible void of reaction by-products associated with

traditional reducing agents and (2) it presumably influences

the growth of this unique nanostructure of palladium nano-

spheres in the present case, noting that other reducing agents

such as hydrazine result in individual nanoparticles (Fig. S2,

ESIw) rather than nano-spheres. It is noteworthy that the wavy

thin film generated on the spinning disc surface has been

demonstrated to enhance the hydrogen gas uptake,35 which

in turn is important for effective and rapid reduction of

H2PdCl4.

The molar ratio of PVP to palladium was varied in order to

optimise the synthesis of the palladium nano-spheres. SEM

images show that a molar ratio (PVP : H2PdCl4) of 20 : 1 gave

a more uniform shape than for a ratio of 10 : 1, for a fixed disc

speed of 2000 rpm. For a molar ratio of 30 : 1, the mono-

dispersity of the nano-spheres decreased compared to the ratio

of 20, due to coalescence in the presence of excess of polymer

(Fig. 2a–c). Varying the disc spinning speed also affected the

size and distribution of the nano-spheres when investigating

the fixed molar ratio (Fig. 2d and e). A disc speed of 1500 rpm

with a molar ratio of 10 resulted in a more uniform and

separated nano-spheres than for disc speeds of 2000 rpm and

2500 rpm. This phenomenon is attributed to shorter residence

times on the disc with increasing speed, resulting in a decrease

in the time for the reacting mixture to equilibrate.

The effect of varying the average molecular weight of the

PVP polymer was also investigated. Using a lower average

molecular weight of PVP (10 000) resulted in smaller nano-

spheres, as expected with a shorter scaffolding structure, with

the average size of 120 nm (Fig. 3), in contrast to 160 nm

(Fig. 2d) for an average molecular weight of 40 000 for the

PVP. Increasing the speed resulted in a decrease in size

Fig. 1 (a) TEM image, (b) cross-section TEM image, (c) high

resolution TEM image, (d) X-ray diffraction pattern of the typical

palladium nano-spheres used in Mizoroki–Heck cross coupling reactions.

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distribution, with uniform nano-spheres obtained at 2500 rpm.

The time needed for forming uniform nano-spheres decreased

(higher speed) for shorter chains (2500 rpm for PVP 10 000

and 1500 rpm for PVP 40 000). This establishes that the time

needed to equilibrate in forming uniform nano-spheres using

SDP is a function of the polymer chain length. Indeed, using a

much higher molecular weight, PVP360 (MW 360 000),

resulted in non-uniform and poorly-dispersable nano-spheres

for all accessible disc speeds,r2500 rpm (Fig. S3, ESIw). High

colloidal stability and uniformity in size is an important

parameter to test catalytic efficacy. The 160 nm nano-spheres

(Fig. 2d, PVP to palladium 10, disc speed 1500 rpm,

40 000 MW PVP) were used in Mizoroki–Heck cross coupling

reactions.

In further studying the effect of the use of SDP in fabricat-

ing the nano-spheres, a control batch experiment was under-

taken whereby hydrogen gas was bubbled into a rapidly stirred

aqueous mixture of H2PdCl4 and PVP 40 000 for 30 to

60 seconds. During this time the solution turned from yellow

to black which is indicative of reduction of H2PdCl4, with the

resulting palladium/polymer composite spheres varying in

size from several hundred nano-metres to micro-metres

(Fig. S4, ESIw). Thus rapid stirring in batch processing does

not provide the intense micro-mixing associated with SPD,

and moreover the reaction time using SDP is much shorter

which relates to the high mass transfer of hydrogen gas

associated with the breakdown of surface tension arising from

waves and ripples on the dynamic thin films.

Catalytic activity

The catalytic activity of the optimised 160 nm palladium nano-

spheres was first trialled for the Mizoroki–Heck cross coupling

reaction between iodobenzene and n-butyl acrylate. All

reactions were carried out at 60 1C overnight with a range of

catalyst loading, as indicated in mol%.36 In each case the

isolated yield of 1,2-disubstituted olefinic product 1 is shown

in Table 1.

Given the success of using one mole percent of the catalyst

for iodobenzene and n-butyl acrylate, this loading was chosen

in studying the Mizoroki–Heck cross coupling reaction with a

range of iodobenzenes (Table 2). Excellent catalytic activity

was established for both activated (containing a para-electron

withdrawing group) and deactivated iodobenzene derivatives

Fig. 2 SEM images of palladium nano-sphere hybrids: (a)–(c) molar

ratio of PVP to palladium 10, 20 and 30, respectively (disc speed

2000 rpm; 40 000 MW PVP), (d) and (e) disc speeds of 1500 rpm and

2500 rpm, respectively (molar ratio of PVP to palladium 10;

40 000 MW PVP). Scale bar indicated is 200 nm.

Fig. 3 SEM images of palladium nano-spheres: (a)–(c) disc speed

1500, 2000 and 2500 rpm, respectively (10 : 1 molar ratio of PVP to

Pd; 10 000 MW PVP). Scale bar indicated is 200 nm.

Table 1 Isolated yield of the Mizoroki–Heck reaction involvingiodobenzene and butyl acrylate as the mole percent of the catalyst isvaried

Entry Mol% of Pd catalyst Yield (%)

1 3.3 972 1 963 0.67 964 0.33 935 0.1 89

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(2–6). In all cases the products (7–11) were characterized by 1H

NMR and GC-MS, and the isolated yield was reported. As

expected none of the 1,1-disubstituted product was observed.

The palladium/PVP nano-sphere catalyst showed a lower

catalytic activity in the bromobenzene system, as normally

observed in the Mizoroki–Heck reactions,37–41 using the

identical conditions to the study of iodobenzene. Additionally,

there was a little effect on increasing the reaction temperature

from 60 1C to 135 1C. However, greater activity was evident

when switching the base from triethylamine to K2CO3 in the

presence of a phase transfer agent, namely tetra-n-butyl-

ammonium chloride (TBAC) (entry 5, Table 3). The use of

tetraalkylammonium salts in general as phase transfer agents

in increasing the reaction rate in the Mizoroki–Heck type

reaction has been thoroughly studied by Jeffery et al. in

phosphine free systems.42–44 The improved reaction efficiency

of TBAC relative to the bromide analogue (TBAB) was also

observed in our system. This effect of the quaternary ammonium

salt is consistent with previous work, which depends both on

the nature of the anion and the cation. Using sodium acetate

or sodium carbonate as the base failed to improve the reaction

conditions even in the presence of TBAC.

After optimizing the reaction conditions for bromobenzene,

several bromobenzene and chlorobenzene derivatives (12–16)

were investigated, Table 4. As expected, for electron poor aryl

bromides (13) the yields were very high, and conversely for

electron rich bromobenzenes (12, 14–16) the yields were

relatively low. There was no activity detected in the case of

chlorobenzenes.

Kinetic and recycling studies

Recycling the palladium nano-sphere catalyst was investi-

gated, along with the kinetics of the reaction of iodobenzene

with n-butyl acrylate. After each reaction, the catalyst was

recovered by centrifugation, washed by DMF three times,

stored under argon and used for the next reaction without

further treatment. We have established that the Pd nano-

sphere catalyst can be recycled five times involving 16 hour

reactions, without losing any catalytic activity, Table 5, and

can be easily re-dispersed in DMF or water after each reaction.

The size of palladium nanoparticle within the nano-spheres

slightly increases after the first run, and remains the same size

from the second to fifth recycle, Fig. 4. The reason for the

particle growth is widely postulated that the Ostwald ripening

effect at elevated temperatures may be a dominant force

resulting in a slight increase in the particle size in the initial

catalytic cycles.45,46

Table 2 Mizoroki–Heck reaction of iodobenzene derivatives withn-butyl acrylatea

Entry Aryl halide ProductIsolatedyield (%)

1 94

2 95

3 98

4 95

5 95

a Reaction conditions: Iodobenzene derivatives (0.98 mmol, 1 eq.),

n-butyl acrylate (1.18 mmol, 1.2 eq.), Et3N (2.45 mmol, 2.5 eq.), and

1 mol% Pd nano-sphere catalyst in DMF (2 mL), 60 1C, overnight.

Table 3 Outcome of the Mizoroki–Heck reaction involving bromo-benzene and n-butyl acrylate, and the effect of various bases and phasetransfer agentsa

Entry Base Phase transfer agent Time/h Yield (%)

1 (Et)3N 24 Trace2 K2CO3 24 203 K2CO3 TBAB 24 254b K2CO3 TBAB 24 265 K2CO3 TBAC 24 76c

6 K2CO3 TBAAe 20 347b,d K2CO3 TBAC 24 388b NaOAc TBAC 24 4c

9b NaOAc TBAC 48 5c

10b NaHCO3 TBAC 24 9c

11b NaHCO3 TBAC 48 15c

a 0.98 mmol bromobenzene, 1.47 mmol n-butyl acrylate (1.5 eq.), 2 mL

DMF, base 2.45 mmol (2.5 eq.), tetra-n-butylammonium bromide

(TBAB) or tetra-n-butyl ammonium chloride (TBAC) 0.98 mmol (1 eq.),

135 1C. b 2.0 eq. n-butyl acrylate. c GC-MS yield. d Recycled catalyst

from entry 5 was used. e Tetra-n-butyl ammonium acetate (TBAA).

Table 4 Mizoroki–Heck reaction of bromobenzene derivativesa

Entry Aryl halide Time/h Conversion (%)

1 24/48 25/30

2 4 100b

3 24 11

4 24/48 9/40

5 24 42c

a 0.98 mmol bromobenzene, 1.47 mmol n-butyl acrylate (1.5 eq.), 2 mL

DMF, K2CO3 2.45 mmol (2.5 eq.), tetra-n-butyl ammonium chloride

TBAC 0.98 mmol (1 eq.), 135 1C. b Methyl acrylate was used.c Isolated yield.

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Tracking the conversions for each recycling provides an

insight into the catalytic system. Although the final conver-

sions of recycling is at the same level, there is a significant

change in the catalytic activity between the 1st run and

subsequent runs during the first several hours, as can be seen

from GC-MS analysis, Fig. 5. Energy dispersive spectroscopy

(EDS) shows the presence of iodine in the recycled catalyst

(Fig. S5, ESIw), and iodine is retained even after vigorous

washing. It is assumed this hydrido-palladium iodide species47

does not react with the base, triethylamine, so still remains on

the surface after reaction completion. Therefore, it’s likely that

the residual iodine hinders the oxidative addition of iodo-

benzene in the next recycling. XPS studies were undertaken on

the pristine catalyst and on the catalyst recovered from the

reactions after recycling. The as-prepared material shows a

dominant Pd(0) peak (BE = 335.5 � 0.1 eV) in the Pd 3d5/2region, and an additional Pd(II) peak (BE = 336.7 � 0.1 eV),

Fig. 6. The latter presumably corresponds to PdO,30,48 noting

that the palladium nano-spheres were exposed to an oxygen-

containing atmosphere before the XPS test. The peak at

338.0 � 0.1 eV only appears in the recycled catalyst, and

according to the literature data, it can be assigned to the peak

of PdI2,49 which is consistent with the formation of palladium

iodide species in the catalytic cycle, including hydrido-

palladium iodide.

We have established that the catalytic activity of the nano-

spheres does not change after the first cycle for at least another

four cycles. The small palladium nano-particles in the nano-

spheres retain their high volume-to-surface ratio, with no

apparent agglomeration of the nano-particles and formation

of palladium black, and with no significant leaching of the

metal in any form (determined by using ICP-AAS). For

reactions involving 1 mol% palladium, 2.5 ppm palladium

was found in the undiluted mixtures for the first cycle, and less

than 2.0 ppm for subsequent cycles, the overall leaching being

less than 0.05% of palladium.

Conclusions

We have established a simple method for preparing novel

palladium-polyvinylpyrrolidone (Pd–PVP) composite nano-

spheres of uniform size using a microfluidic spinning disc

platform under an atmosphere of hydrogen. This processing

platform is ideal for large scale synthesis of uniform Pd nano-

catalysts under continuous flow conditions. The turbulent

mixing within the dynamic thin films arising from the high

centrifugal forces results in the long chains of PVP forming a

compact scaffold which entangles and traps a large number

of 5 nm palladium nano-particles within the composite. We

have established that these nano-spheres are effective recyclable

colloidal catalysts for Mizoroki–Heck cross coupling

Table 5 Recycling of palladium nano-spheres for the Mizoroki–Heckcross coupling reaction of iodobenzene with n-butyl acrylatea

Reaction number Time/hConversion determinedby GC-MS (%)

1st 16 1002nd 16 1003rd 16 994th 16 965th 16 100

a Reaction conditions: Iodobenzene (0.98 mmol, 1 eq.), n-butyl acrylate

(1.18 mmol, 1.2 eq.), Et3N (2.45 mmol, 2.5 eq.), and 1 mol% Pd

nano-sphere catalyst in DMF (2 mL), 60 1C.

Fig. 4 TEM images of reused palladium nano-spheres (a and b, after

first run; c and d, after fifth run).

Fig. 5 Kinetic and recycling studies for the Mizoroki–Heck cross

coupling reaction involving iodobenzene and n-butyl acrylate

(reaction conditions are defined in Table 2).

Fig. 6 Curve-fitting of the Pd 3d spectra obtained for pristine nano-

spheres (a), and after being recycled five times (b).

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reactions. The polymeric scaffold maintains the morphology

and integrity by minimising catalytic loss caused by leaching of

the precious metal.

Experimental section

Reagents and conditions

Polyvinylpyrrolidone (PVP, average molecular weight 10 000,

40 000, and 360 000), 4-iodotoluene, methyl 4-iodobenzoate,

40-iodoacetophenone, 4-iodoanisole, bromobenzene, methyl

4-bromobenzoate, 4-bromoanisole, 40-bromoacetophenone,

4-bromo-N,N-dimethylaniline, chlorobenzene, and 40-chloro-

acetophenone, tetrabutylammonium bromide (TBAB),

tetrabutylammonium acetate (TBAA), butyl acrylate, and

iodobenzene were purchased from Sigma-Aldrich. Triethyl-

amine (Et3N), 4-bromphenol, 4-bromotoluene, and tetrabutyl-

ammonium chloride (TBAC) were purchased from Fluka.

4-Iodophenol is from Alfa Aesar. Potassium carbonate

(K2CO3), sodium acetate (NaOAc), sodium bicarbonate

(NaHCO3), and magnesium sulfate (dried) are from Ajax

Finechem Pty Ltd. Palladium precursor solution (palladium

dissolved in aqua regia) was provided by AGR Matthey. All

chemicals were used without further purification except

N,N-dimethylformamide (DMF) which was purified by

approved procedures.50

Synthesis of palladium composite nano-spheres

In a typical experiment H2PdCl4 solution (50 mL,

0.6 mmol L�1) was mixed with PVP40 (34 mg, molecular

weight is 40 000) (molar ratio of PVP monomer to palladium is

10 : 1) and then fed into a jet feed at a flow rate of 0.7 mL s�1

onto the spinning disc for which the speed was set at

1500 rpm, with hydrogen gas fed into another jet feed,

affording a colloidal suspension of composite nano-materials.

Products were collected from beneath the disc through an

exit port. The as-synthesised palladium composite nano-

spheres were washed using MilliQ water (>18 MO) three

times and then freeze dried. The nanoparticles were highly

stable for several months and could be used as catalyst

directly.

Characterization of the composite nano-materials

The freeze dried materials were re-dispersed in water and the

size and morphology of the samples were determined using

transmission electron microscopy (TEM, JEOL 3000F)

operating at 300 kV and scanning electron microscopy

(SEM, Zeiss 1555) applying with an acceleration voltage of

2 kV, respectively. Powder XRD patterns were measured using

an Oxford Diffraction Gemini-R CCD diffractometer (using

Cu Ka = 1.54178 A radiation).

General procedure for Mizoroki–Heck reaction

(a) Aryl iodides: Iodobenzene or an iodobenzene derivative

(0.98 mmol, 1 eq.), butyl acrylate (1.18 mmol, 1.2 eq.), Et3N

(2.45 mmol, 2.5 eq.) in DMF (2 mL) were treated in one

portion with the palladium nano-sphere catalyst (varying mol%).

The reaction mixture was degassed (freeze–pump–thaw

method) before heating to 60 1C for the designated time.

The ensuing reaction mixture was centrifuged and the palla-

dium nano-sphere catalyst washed three times with DMF

(3 � 5 mL). The combined DMF were washed by HCl (0.1 M)

and ethyl acetate (V : V = 1 : 4) once and water and

ethyl acetate (V : V = 1 : 4) twice. The ethyl acetate layers

were collected and concentrated under reduced pressure

and the resulting mixture subjected to column chromato-

graphy (2%, 10%, 5%, 3%, 6%, and 4%, ethyl acetate in

hexane for iodobenzene, 4-iodotoluene (2), 4-iodophenol (3),

methyl 4-iodobenzoate (4), 40-iodoacetophenone (5), and

4-iodoanisole (6), respectively). The products (7–11) were

characterized by 1H NMR (for data see ESIw) and GC-MS

(for data see ESIw), and matched authentic samples found in

the literature.51–55 The isolated yield is reported. For recycling

studies, the palladium nano-sphere catalyst was separated by

centrifugation, washed by DMF three times, and stored in

DMF under argon prior to the next catalytic run. (b) Aryl

bromides: The same procedure was carried out for the reaction

between aryl bromides (bromobenzene and 12–16) and

acrylates except the reaction temperature and the reaction

time varied (135 1C for the designated time). For 4-bromo-

N,N-dimethylaniline (16), the mobile phase of 5% ethyl

acetate in hexane was used.

Kinetic studies

To monitor the reaction, 10 mL samples were taken at regular

intervals, washed with 0.1 M HCl, diluted with ethyl acetate,

centrifuged to remove the palladium nano-sphere catalyst,

and analyzed by gas chromatography and mass spectrometry

(GC-MS).

X-Ray photoelectron spectra (XPS) measurements

High resolution XPS measurements were preformed on a

Kratos Axis-Ultra spectrometer using a Dual anode Mg KaX-ray source (1253.6 eV) with 12 kV operational voltage and

12 mA emission current. The working pressure in the analysing

chamber was better than 10�10 Torr. The pass energies were

80 eV for the survey scan and 20 eV for the Pd 3p, Pd 3d, O 1s

and C 1s in the high resolution scans. A hybrid lens mode

with a top lens set at 01 was utilised. Samples were mounted

horizontally by the double side sticky carbon tape and the

external surfaces were examined as introduced without

any further treatment. All spectra were processed by using a

Powell peak-fitting algorithm provided by the spectrometer

software.

Acknowledgements

The authors are grateful for the financial support for this work

by the Australian Research Council and The University of

Western Australia, and for the palladium precursors provided

by AGR Matthey. The microscopy analysis was carried out

using facilities in the Centre for Microscopy, Characterization

and Analysis, The University of Western Australia, which are

supported by University, State and Federal Government

funding. The authors would also like to thank Dr L. Byrne

for NMR spectra acquisition.

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3.5 Surface oxygen triggered size change of palladium nano-crystals

impedes catalytic efficacy

This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 1803–1805 1803

Surface oxygen triggered size change of palladium nano-crystals impedes

catalytic efficacyw

Jianli Zou,a Scott G. Stewart,b Colin L. Raston*a and K. Swaminathan Iyera

Received 11th August 2010, Accepted 17th November 2010

DOI: 10.1039/c0cc03182h

Palladium nano-crystals increase in size during the initial

recycling in Heck cross coupling reactions. We demonstrate

that oxygen adsorbed on the surface of palladium nano-crystals

plays a pivotal role in driving the ripening. This in turn is

associated with a loss in catalytic activity.

Polymer stabilized nano-particles of crystalline palladium have

recently attracted much attention for a variety of catalytic

reactions.1–8 The polymer matrices in these composites serve

as scaffolds for keeping the nano-particles from aggregating

and stabilising the interface between the metal and the reaction

media. Moreover, the use of polymer matrices facilitates

recycling of the catalyst, which is an important consideration

when they are based on precious metals. Nano-crystals of

palladium have multiple shape and size dependent facets with

different fractions of atoms located at different corners, edges

and defects.9 This spatial arrangement of atoms can dictate the

stability of the nano-crystals and their catalytic activity.10

Controlling the shape and size of such particles is therefore

paramount in retaining their catalytic efficiency and reusability.

Palladium nano-particles have been reported to show a small

increase in size during the initial phase for the Heck and

Suzuki cross coupling reactions.3,11,12 The origin of this

effect is unknown, although it is widely postulated that it

relates to Ostwald ripening associated with re-entry of

palladium species which depart from the surface of the

particles during the catalytic process.13,14 In this report we

establish that oxygen adsorbed on the surface of palladium

nano-crystals plays a pivotal role in driving this ripening, with

a concomitant change in the size and shape of the reused

catalyst. This change in turn results in a drop in the catalytic

efficacy impeding their reusability.

Palladium hybrid nano-spheres approximately 160 nm in

diameter containing 5 nm palladium nano-particles embedded

in a polyvinylpyrrolidone (PVP) scaffold were used in the

study (see ESIw for a detailed description), Fig. 1. The use of

these nano-spheres (denoted here as Pd–PVP nano-spheres)

was deemed an attractive strategy in being able to use TEM to

readily track any physical change in the palladium particles

arising from ripening in the confined nano-environment.

Furthermore, the metal binding affinity of the polymer scaffold

can circumvent leaching of the metal into the bulk solution.

Thus the nano-spheres can provide an accurate 3D representa-

tion of any increase in size and change in shape of the catalyst

nano-particles prior to each recycling of the catalyst.

The model Heck cross coupling reaction used herein involved

the reaction of iodobenzene (1.0 equivalent) and butyl acrylate

(1.2 equivalents) with Et3N (2.5 equivalents) and Pd–PVP

nano-spheres (1 mol%) in DMF, Fig. 1.z In the first set of

experiments the reaction mixture was degassed before heating

at 60 1C with the reaction monitored using GC-MS analysis.

Following the completion of the reaction the catalyst was

isolated by centrifugation, washed with DMF three times

while being kept under a flow of argon before being used in

the next catalytic recycle. The TEM images, Fig. 2, show that

the size of palladium nano-particles increases only slightly

after the first recycling. For ten successive recycles thereafter

there is no apparent change in the size of the particles. Thus,

any growth within the 3D scaffold is suppressed for the second

catalytic recycle and beyond, implying that the ripening

process is driven by factors other than the standard chemical

environment of the reaction.

The pristine Pd–PVP nano-spheres consist of 5 nm

quasi-spheroidal palladium nano-crystals (see ESIw for high

resolution TEM). Wulff construction has the quasi-spheroidal

particles consisting mainly of {111} and {100} facets, with the

{111} facets dominating the structure in minimising the surface

energy which is given by the inequality {111}o {100}o {110}.9

Fig. 1 Schematic representation of the Pd–PVP nano-spheres and the

associated catalytic cross coupling reaction between iodobenzene and

butyl acrylate.

a Centre for Strategic Nano-Fabrication,School of Biomedical, Biomolecular and Chemical Sciences,The University of Western Australia, Crawley, WA 6009, Australia.E-mail: [email protected]; Fax: +61 8 6488 8683;Tel: +61 8 6488 3045

b School of Biomedical, Biomolecular and Chemical Sciences,The University of Western Australia, Crawley, WA 6009, Australia

w Electronic supplementary information (ESI) available: Synthesisand characterization of Pd–PVP nano-spheres, schematic representa-tion of a spinning disc processor (SDP), TEM image and highresolution TEM image of Pd–PVP nano-spheres, XRD patterns ofthe nano-catalyst, prior to the first cycle and after the 10th recycling,and energy dispersive spectra (EDS) of palladium nano-spheres. SeeDOI: 10.1039/c0cc03182h

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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1804 Chem. Commun., 2011, 47, 1803–1805 This journal is c The Royal Society of Chemistry 2011

Bryl et al. have established that adsorption of oxygen on

palladium nano-particles 10 to 15 nm in diameter results in

the formation of {011} facets, and expansion of {001} facets,

at the expense of {111} planes.15 Furthermore, when the

amount of adsorbed oxygen increases the {011} planes enlarge

at the expense of {133} planes, resulting in rounding of crystal

edges. They also showed that at T > 230 K only atomic

oxygen is present on the crystal surface, with faceting induced

above 500 K, indicating that a sub-monolayer of oxygen is

sufficient for the reconstruction of the facets. We predict that

in the present study, it is this oxygen induced reconstruction of

single palladium crystal surfaces which results in the size

transformation, with the ‘‘dissolution’’ of certain planes and

the growth of others.

It should be noted that the pristine catalysts stored in air

prior to the first catalytic cycle always have a sub-monolayer

of oxygen present at room temperature. This explains why the

size changes are observed in the initial experiment, Fig. 2,

and none thereafter when the catalysts were stored under an

atmosphere of argon prior to their use in the next catalytic

reaction. To validate the above hypothesis the recovered

catalyst was exposed to air overnight in between recycles

instead of being stored under argon. The TEM images,

Fig. 3(a–e), show that the size of palladium nanoparticles

dramatically increased after successive recyclings. Palladium

nanoparticles were approximately 25 nm in diameter after

8 recycling, whereas they are 5 nm in the original pristine

nano-spheres.

Note that the overall size of the Pd–PVP nano-spheres

appears to be unchanged, indicating that the polymer scaffold

is effective in preventing nano-particles leaching outside the

3D nano-structure. Consistent with this is the lack of any

significant leaching during catalysis of palladium, as established

using ICP-MS. High resolution imaging (Fig. 3(f)) on a single

palladium nano-particle shows that the palladium nano-

particles remain crystalline after each catalytic recycling, up

to ten recycling. This was further corroborated using X-ray

powder diffraction (Fig. S2, ESIw). This growth had a dramatic

impact on the catalytic efficiency with the yield subsequently

dropping from 88% for the 8th recycling down to 33% for the

10th recycling (Fig. 3(g)), while the yield of the reaction

without oxygen contact kept constantly above 95% for ten

recycles. This drop in yield is associated with a dramatic

reduction in the number of corner and edge atoms that have

been previously reported to be very catalytically active.10

XPS studies were undertaken to ascertain the nature of the

nano-particles surface species upon recycling. XPS spectra of

freshly prepared and recycled catalyst in the Pd 3d5/2 region

show a dominating Pd(0) line (binding energy of 335.5� 0.1 eV)

and an additional Pd(II) line (binding energy of 336.7� 0.1 eV),

Fig. 4. The signal at 336.7 eV is assigned to Pd–O species,16,17

noting the material was stored in air. There is an additional

peak at 338.0 � 0.1 eV which only appears in the recycled

catalyst, and is reasonably assigned to the presence of Pd–I

species,18 being consistent with EDS data (Fig. S3, ESIw).The presence of such species on the surface of the palladium

nano-particles can arise from residual iodide associated with

the oxidative addition step of the catalytic cycle remaining on

the surface of the particles, Fig. 1. The relative content of Pd–I

species is significantly more when the catalyst is not exposed to

air. Indeed, the oxygen induced shape transformation results

in a decrease in the number of corner and edge atoms, which

are known to be catalytically active for forming I–Pd–aryl

intermediates.10

In summary, we have demonstrated for the first time that

surface oxygen plays a pivotal role in the reconstruction of

palladium nano-crystals bound within a polymer matrix, as a

Fig. 2 SEM image of the pristine Pd–PVP nano-spheres (a), and

TEM images of an individual Pd–PVP nano-sphere, pristine (b), after

the first recycling (c), after the fifth catalytic recycling (d), and after the

tenth catalytic recycling (e).

Fig. 3 TEM images of Pd–PVP nano-spheres after exposure to air

prior to catalytic studies, (a) as prepared and (b)–(e) after two, five,

eight and ten recyclings, respectively. (f) High resolution TEM of the

nano-crystals and their corresponding FFT pattern after the tenth

recycle. (g) Variation in the yield of the Heck reaction and size of the

palladium nano-particles versus the number of recycles.

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heterogeneous catalysis. This reconstruction results in an

increase in the size of the nano-crystals, with defaceting and

a decrease in chemically active sites for the model Heck

cross coupling reaction. This is an important phenomenon in

determining the chemical outcome of palladium catalysed

reactions in general, and an important finding that should be

taken into consideration in the design of recyclable palladium

nano-particle based catalysts.

The authors are grateful for the financial support for this

work by the Australian Research Council and The University

of Western Australia, and for the palladium precursors

provided by AGR Matthey. The microscopy analysis was

carried out using facilities in the Centre for Microscopy,

Characterization and Analysis, The University of Western

Australia, which are supported by University, State and



z Experimental methods: synthesis of palladium composite nano-spheres: an aqueous solution of H2PdCl4 (0.6 mmol L�1) andpolyvinylpyrrolidone (molecular weight 40 000) (the molecular ratioof PVP monomer to palladium is 10 : 1) were fed into a jet feed at aflow rate of 0.7 mL s�1 onto a spinning disc operating at 1500 rpmunder a hydrogen atmosphere,19,20 with the isolated material beingwashed three times with MilliQ water (>18 MO) and freeze dried.General procedure for Heck reaction: iodobenzene (0.98 mmol, 1 eq.),butyl acrylate (1.18 mmol, 1.2 eq.), Et3N (2.45 mmol, 2.5 eq.) in DMF(2 mL) was treated in one portion with the Pd–PVP catalyst (1 mol%).The reaction mixture was degassed (freeze–pump–thaw method)before heating to 60 1C for the designated time. For recyclingstudies, the Pd–PVP catalyst was separated by centrifugation,washed with DMF three times, and stored in DMF under argon orfreeze-dried, and then exposed to the air prior to the next catalyticrecycling.

1 M. A. R. Meier, M. Filali, J. F. Gohy and U. S. Schubert,J. Mater. Chem., 2006, 16, 3001.

2 S. D. Miao, C. L. Zhang, Z. M. Liu, B. X. Han, Y. Xie, S. F. Dingand Z. Z. Yang, J. Phys. Chem. C, 2008, 112, 774.

3 R. Narayanan and M. A. El-Sayed, J. Am. Chem. Soc., 2003, 125,8340.

4 V. Calo, A. Nacci, A. Monopoli, A. Fornaro, L. Sabbatini,N. Cioffi and N. Ditaranto, Organometallics, 2004, 23, 5154.

5 K. Qiao, R. Sugimura, Q. X. Bao, D. Tomida and C. Yokoyamal,Catal. Commun., 2008, 9, 2470.

6 C. Evangelisti, N. Panziera, P. Pertici, G. Vitulli, P. Salvadori,C. Battocchio and G. Polzonetti, J. Catal., 2009, 262, 287.

7 X. Yang, Z. F. Fei, D. B. Zhao, W. H. Ang, Y. D. Li andP. J. Dyson, Inorg. Chem., 2008, 47, 3292.

8 S. Klingelhofer, W. Heitz, A. Greiner, S. Oestreich, S. Forster andM. Antonietti, J. Am. Chem. Soc., 1997, 119, 10116.

9 Y. J. Xiong and Y. N. Xia, Adv. Mater., 2007, 19, 3385.10 R. L. Augustine and S. T. Oleary, J. Mol. Catal. A: Chem., 1995,

95, 277.11 J. Hu and Y. B. Liu, Langmuir, 2005, 21, 2121.12 C. C. Cassol, A. P. Umpierre, G. Machado, S. I. Wolke and

J. Dupont, J. Am. Chem. Soc., 2005, 127, 3298.13 A. Biffis, M. Zecca and M. Basato, J. Mol. Catal. A: Chem., 2001,

173, 249.14 A. M. Trzeciak and J. J. Ziolkowski, Coord. Chem. Rev., 2007, 251,

1281.15 R. Bryl, T. Olewicz, T. V. de Bocarme and N. Kruse, J. Phys.

Chem. C, 2010, 114, 2220.16 A. Gniewek, A. M. Trzeciak, J. J. Ziolkowski, L. Kepinski,

J. Wrzyszcz and W. Tylus, J. Catal., 2005, 229, 332.17 E. H. Voogt, A. J. M. Mens, O. L. J. Gijzeman and J. W. Geus,

Surf. Sci., 1996, 350, 21.18 S. MacQuarrie, J. H. Horton, J. Barnes, K. McEleney, H. P.

Loock and C. M. Crudden, Angew. Chem., Int. Ed., 2008,47, 3279.

19 K. S. Iyer, C. L. Raston and M. Saunders, Lab Chip, 2007,7, 1800.

20 K. S. Iyer, M. Norret, S. J. Dalgarno, J. L. Atwood andC. L. Raston, Angew. Chem., Int. Ed., 2008, 47, 6362.

Fig. 4 Curve-fitting of the Pd 3d spectra obtained for pristine nano-spheres (a), and after being recycled five times without exposure to oxygen (b),

and ten times with exposure to oxygen in between each recycle (c), measured under ultrahigh vacuum conditions with a Dual Anode MgKaradiation source.

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3.6 Pd-induced ordering of 2D Pt nanoarrays on phosphonated

calix[4]arenes stabilised graphenes


Cite this: Chem. Commun., 2011, 47, 5193–5195

Pd-induced ordering of 2D Pt nanoarrays on phosphonated calix[4]arenes

stabilised graphenesw

Jianli Zou,aAdam D. Martin,

aBogdan Zdyrko,

bIgor Luzinov,

bColin L. Raston

aand

K. Swaminathan Iyer*a

Received 21st January 2011, Accepted 15th March 2011

DOI: 10.1039/c1cc10408j

p-Phosphonic acid calix[4]arenes render high stability to exfoliated

graphenes in water. These calix[4]arenes modified graphenes can

be used as highly effective substrates to nucleate ultra-small Pd

nanoparticles, which in turn serve as galvanic reaction templates

for the generation of high density 2D arrays of Pt nanoparticles.

Two-dimensional (2D) nanosheets made of sp2 carbon or

n-layer graphenes have extraordinary physical, chemical,

mechanical, electronic and optical properties which are

dependent on the number of layers (n).1–4 With the advent

of pioneering exfoliating techniques5–8 to prepare a large scale

of atomically thin layer of graphene, a wide range of applications

are currently being investigated in nanoelectronics, catalysis

and biosensing. Graphene sheets have theoretical surface areas

of B2600 m2 g�1 and are stronger than diamond, making

them attractive candidates as 2D supports. Hybridisation of

graphene is an important step towards the development of

multifunctional materials with synergistic properties. Indeed

noble metal nanoparticles modified carbon materials (carbon

nanotubes, graphene oxide, etc.) have been a subject of

numerous applications in fuel cell engineering, electrochemical

sensing and catalysis.9–14 Coupling noble metals with graphenes

is deemed important to realise the full potential of these

atomically thin carbon substrates. However, the inherent

stability problem associated with processability of graphenes

hinders their applicability as substrates to generate these novel

hybrids. Unless well separated from each other, graphene

sheets tend to form irreversible agglomerates via p–p stacking

interactions which even restack to form graphite through van

der Waals interactions. Surface modification using polymers,

surfactants/stabilisers and biomacromolecules can impart

stability and compatibility to graphene substrates in solution.15–18

Recently, various noncovalent chemical strategies have been

reported to stabilise chemically exfoliated graphenes through

p–p interaction with water soluble p-rich molecules such as

pyrene derivatives15,19 and poly(sodium-4-styrenesulfonate).20

However, such surface modified graphenes are colloidally

unstable over a range of pH. This problem has been addressed

herein using p-phosphonic acid calix[4]arene, an amphiphilic

supramolecule with the hydroxide group and phosphonate

group on the lower and upper rim, respectively, as a surfactant/

stabiliser agent. p-Phosphonic acid calix[n]arene has been

reported as a versatile macrocyclic polyphosphate surfactant

to effectively stabilise single wall carbon nanotubes in water.21

In this report, we demonstrate that high stability can be

rendered to exfoliated graphenes in aqueous solution using

phosphonated calix[4]arenes. Furthermore, we demonstrate

that these p-phosphonated calix[4]arenes modified graphenes

can be used as highly effective templates to nucleate ultra-

small palladium nanoparticles by in situ reduction of H2PdCl4in water using hydrogen gas at ambient pressure.

Finally, we demonstrate that the Pd–graphene hybrids act

as galvanic reaction templates for the generation of high

density 2D arrays of Pt nanoparticles. In addition, the reaction

rate of the replacement and density of the Pt 2D structure can

be controlled by the introduction of FeII/FeIII species.

Graphene used in the present study was synthesized by a

chemical exfoliation technique as previously reported.5,22,23

AFM analysis further confirmed that atomically flat single

layer graphene sheets were successfully synthesised (Fig. 1a).

Given that the carboxylic acid groups around graphene sheets

still remain after reduction, the resultant graphene can readily

be dispersed in aqueous solution at pH > 6 to form stable

colloids due to the electrostatic repulsion arising from

deprotonated carboxylic acid groups.5 However, effective

processability with precious noble metals in most cases

warrants stability at lower pH. In the present case, the

presence of p-phosphonic acid calix[4]arene (Fig. 1b) resulted

in a stabilised graphene (CSG) colloidal solution even at pH 2

without aggregation (Fig. 1c). The interaction of p-phosphonic

acid calix[4]arene and graphene is a balance of intermolecular

p–p interactions coupled with the epitaxial effects of the

underlying substrate as previously described for calix[8]arene

on a graphitic substrate.24 Furthermore, it has been reported

that these water soluble calix[4]arenes form bilayers/aggregates in

the solid state, in solution and in the gas phase, held together

a Centre for Strategic Nano-Fabrication, School of Biomedical,Biomolecular and Chemical Sciences, The University of WesternAustralia, Crawley, WA 6009, Australia.E-mail: [email protected]; Fax: +61 8 6488 1005;Tel: +61 8 6488 4470

b School of Materials Science and Engineering Clemson University,Clemson, South Carolina 29634-0971, USA

w Electronic supplementary information (ESI) available: Experimentalprocedures, a TEM image of graphene–Pd without phosphonatedcalix[4]arene, and high resolution TEM images of CSG–Pd and Ptnanoparticles on the CSG. See DOI: 10.1039/c1cc10408j

ChemComm Dynamic Article Links

www.rsc.org/chemcomm COMMUNICATION

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by strong hydrogen bonding between the diprotic phosphonic

acid groups and inter p-stacking.25 Herein, we suggest that the

phosphonated calix[4]arenes form a stable layer on the surface

of graphene imparting the abovementioned stability at lower

pH. Indeed, the x-potential over a range of pH shows a highly

charged property of stabilised graphenes owing to the

deprotonation of phosphonic acid which provides sufficient

repulsion to prevent restacking (Fig. 1d).

It has been recently reported that p-phosphonic acid

calix[4]arenes play a key role in accelerating the rate of

synthesis of ultra small silver nanoparticles using hydrogen

gas as the reducing agent in solution in contrast to a linear

polyphosphates.26 A similar strategy was adopted in the

current study to synthesize a uniform Pd nanoparticles coating

by exploiting the free phosphonate groups on the surface. This

was accomplished by the reduction of Pd(II) precursors to

Pd(0) on the CSG substrate using hydrogen gas (see Experi-

mental section in ESIw). TEM analysis showed Pd nano-

particles uniformly decorate graphene sheets (Fig. 2) with a

loading of 1 particle per 100 nm2 (15% coverage). It is

noteworthy that in the absence of p-phosphonated

calix[4]arenes reducing Pd(II) resulted in a sporadic nucleation

of Pd nanoparticles (Fig. S1, ESIw). The crystalline nature of

the Pd nanoparticles was confirmed using high resolution

TEM analysis (Fig. S2, ESIw). These Pd nanoparticles are

4–5 nm in size. The phosphorous signal in the EDS spectra

further confirmed the presence of p-phosphonated calix[4]arene

on the surface of graphene (Fig. 2, inset).

We further investigated the use of the Pd nanoparticle

coated CSG substrate as a template to form Pt nanoparticle

arrays on the graphene substrate (see Experimental section in

ESIw). The difference in the standard reduction potentials (E0)

for PdII/Pd and PtII/Pt has been successfully used to form Pt

nanostructures.27 Herein, the galvanic replacement reaction

occurs following the reduction of Pt(IV) to Pt(II) by ethylene

glycol at elevated temperature. However, the reduction of

Pt(II) to Pt resulted in the formation of aggregated structures

rather than well dispersed arrays of Pt nanoparticles on the

graphene substrates (Fig. 3a). It is noteworthy that the

presence of a trace amount of iron species (Fe3+ or Fe2+)

can significantly reduce the net reduction rate of the Pt

precursors as iron species greatly reduce the amount of super-

saturated Pt(0) atoms in solutions.28,29 In the present case, the

presence of trace amounts of iron can effectively control the

replacement reaction rate and in turn the growth of Pt arrays

on graphene. This strategy is based on the fact that the

standard reduction potential of [PtCl4]2�/Pt redox pair

(0.73 V vs. the standard hydrogen electrode (SHE)) is lower

than that of the Fe3+/Fe2+ redox pair (0.77 V vs. SHE). In the

presence of Fe3+ ions Pt(0) is oxidized to Pt(II) in turn

reducing the supersaturation of Pt atoms and significantly

decreasing the rate of the growth process. Additionally, the

resultant Fe2+ can be easily oxidised to Fe3+ again by air. We

found that the introduction of Fe3+ resulted in a controlled

reductive reaction. Well ordered 2D arrays of Pt nanoparticles

were formed on the graphene substrates. Furthermore, the

density of the Pt nanoparticles could also be controlled by

varying the concentrations of Fe3+ (Fig. 3b–d). The coverage

of Pt nanoparticles on the graphene sheet varies from 24% to

90% (Fig. 3b–d). The formation of Pt was further confirmed

Fig. 1 (a) AFM image of graphene with the line profile, showing the

height of graphene on silica wafer is around 0.9 nm. (b) Structure of

p-phosphonic acid calix[4]arene (carbon in grey, oxygen in red,

hydrogen in white and phosphorus in green). (c) A photograph of

CSG (calix[4]arene stabilised graphene) solution at pH 2 (left),

CSG solution at pH 3 (middle) and graphene solution at pH 3.

(d) x-potential of CSG and graphene as a function of pH.

Fig. 2 TEM image of Pd nanoparticles coated CSG, and energy

dispersive X-ray spectra (EDS) of CSG and Pd nanoparticles coated

CSG (inset).

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by high resolution TEM analysis (Fig. S3, ESIw) and EDS

(Fig. 3a, inset). It is noteworthy that a control experiment was

performed in the absence of Pd nanoparticles on the surface of

graphene. This resulted in no Pt nanoparticle attachment on

the graphene surface.

In conclusion, we have demonstrated that a macrocycle

molecule, p-phosphonic acid calix[4]arene, is effective in

rendering high stability of graphene in solution and for

anchoring Pd nanoparticles. In addition, we have established

a feasible method to fabricate Pt–graphene 2D nano-arrays

through a controllable galvanic replacement. Overall, the

simple process reported here improves the processability of

graphenes in water with potential to develop novel hybrids

for application in catalysis, fuel cells, sensor materials and

nano-electronics.

The authors are grateful for the financial support for this

work by the Australian Research Council and The University

of Western Australia, and for the palladium precursors

provided by The Perth Mint. The microscopy analysis was

carried out using facilities in the Centre for Microscopy,

Characterization and Analysis, The University of Western

Australia, which are supported by University, State and



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Fig. 3 (a) TEM image and EDS spectra (inset) of Pt nanoparticles

coated CSG (without Fe3+). (b–d) TEM images of Pt nanoparticles

coated CSG synthesised in the presence of 100 mL, 20 mL and 10 mL of

FeCl3 (12 mM), respectively.

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3.7 Regiospecific linear assembly of Pd nanocubes for hydrogen gas

sensing


Cite this: Chem. Commun., 2012, 48, 1033–1035

Regiospecific linear assembly of Pd nanocubes for hydrogen gas sensingw

Jianli Zou,aBogdan Zdyrko,

bIgor Luzinov,*

bColin L. Raston

aand K. Swaminathan Iyer*

a

Received 7th September 2011, Accepted 17th November 2011

DOI: 10.1039/c1cc15522a

Capillary force lithography was applied to generate large area

polymer patterns. A ‘‘grafting to’’ approach was used on the

patterns to induce linear assembly of Pd nanocubes through

electrostatic interaction. Pd nanoarrays with high density were

subjected to a hydrogen gas sensing test. We demonstrated a

feasible method to build up a miniature hydrogen sensor using

self-assembly with micrometre Pd nanoarrays.

Hydrogen gas is considered as one of the most promising clean

energy alternatives. For this purpose it has been extensively

investigated, and consequently the ability to detect hydrogen

gas has been widely studied.1–4 Relevant to this is the dissociative

absorption of hydrogen on palladium which results in changes in

mass, volume, electrical resistivity and optical constants.5–10 These

changes arise from the well established spontaneous diffusion of

hydrogen into the lattice of palladium metal affording palladium

hydride, PdHx. This diffusion is associated with an a to b-phasetransition resulting in a change in the lattice spacing and a

concomitant change in the properties of the material. This

diffusion features in the development of highly efficient sensors

for detecting molecular hydrogen with fast response times,

notably involving Pd nanowires,11–13 Pd meso-wires,9,14

Pd films,15 Pd/carbon nanotube hybrids,2,16,17 and large area

Pd nanowires based membranes.18 The detection of hydrogen in

these sensors is mainly based on the change in the electrical

properties associated with the hydrogen adsorption. Among

them, sensors based on Pd nanowires have demonstrated the best

responses in terms of speed of detection, sensitivity, and ultralow

power consumption.19 A major obstacle in the development of

most of the aforementioned technologies is an effective method

for nanofabrication and manipulation over a large area for

scalable commercial sensor production.

Indeed, fabrication of large area nanostructures from poly-

mers and nanoparticles is vital for technological applications in

miniaturized sensors, biochips, information storage devices, and

optical devices. Serial writing lithography methods have been a

mainstay in patterning in the sub-100 nm, such as electron-beam

lithography,20,21 ion-beam lithography,22 dip-pen lithography.23

However these techniques are expensive and suffer from low-

throughput. An ideal nanotechnology enabled sensing platform

has to be affordable, and based on a large scale production

system. Capillary force lithography (CFL) is an emerging high-

throughput lithography technique for providing well-ordered

microarray structures over a large area in a facile and cost-

efficient way.24–28 Herein we demonstrate using CFL in combi-

nation with a polymer ‘‘grafting to’’ approach that Pd nanocubes

can be electrostatically directed to self-assemble on the linear

polymer patterns with complementary charge from solution. This

technique can render a facile self-assembly platform that is not

energy intensive to fabricate large area micropatterns, as an

efficient sensing platform for hydrogen gas.

Capillarity allows the polymer melt to fill up the void space

between the polymer and the applied mould when the temperature

is above the glass-transition temperature (Tg), thereby generating a

large-area pattern which depends on the size of stamp. We use the

ubiquitous polycarbonate disk of optical data storage discs like

compact discs (CD) or digital video discs (DVD) as masters for the

PDMS stamp. An optical data storage disc is typically made of a

polymer (polycarbonate) disc, on which a single spiral track is

drilled. The typical width and depth of each line in the spiral track

are 800 nm and 130 nm, respectively, and the periodicity of the

track is B1.5 mm. The typical procedure of fabricating palladium

nano-arrays is illustrated in Fig. 1. First, a silica wafer wasmodified

with a reactive polymer containing epoxy functionalities, PGMA

[poly(glycidylmethacrylate)] as described earlier.29,30 The glycidyl

methacrylate units of the PGMA chain not only serve to anchor

the polymer to the silica substrate but the free groups also serve as

reactive sites for the subsequent attachment of additional polymers

with complementary functional groups. Following this 0.6–1%w/v

PS was dip-coated as the second layer. The PS layer here provides a

chemical resist to selectively graft polymers to the epoxy groups of

PGMA following patterning. The PS/PGMA bilayer was annealed

with the PDMS mask at 130 1C (T > Tg of PS) to induce

patterning via capillary flow. The reusable PDMS stamp was

peeled off following heat treatment to obtain a patterned surface

resulting in alternating PGMA and PS stripes. Polyanionic PAA

was then grafted to PGMA. CTAB coated Pd nanocubes (cationic,

z-potential = 29.2� 2.5 mV) were then electrostatically assembled

onto the patterned surface, followed by washing steps to remove

PS to obtain linear arrays of assembled Pd nanocubes.

The thickness of the initial PS layer used is an important

factor to be considered while generating patterns using CFL.

a Centre for Strategic Nano-Fabrication, School of Biomedical,Biomolecular and Chemical Sciences, The University of WesternAustralia, Crawley, WA 6009, Australia.E-mail: [email protected]; Fax: +61 8 6488 1005;Tel: +61 8 6488 4470

b School of Materials Science and Engineering, Clemson University,Clemson, South Carolina 29634-0971, USA.E-mail: [email protected]; Fax: +1 864 6565973;Tel: +1 864 656 5958

w Electronic supplementary information (ESI) available: Materialsand experimental details. See DOI: 10.1039/c1cc15522a

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The kinetics of the polymer flow in CFL is strongly dependent

on the viscosity of liquids. This in turn is related to the

significant extension of the long viscoelastic polymer chains

along the flow direction in the narrow gap between the

stamp–substrate and also the resulting elastic resistance to

the flow of polymeric liquids. In addition, this is coupled with

the friction between the extended polymer chains and the

rubbery walls of the stamp. The capillary pressure in CFL

has been known to be insufficient to completely pull out the

polymer chains from the gap for polymer films below a critical

thickness. This results in a residual layer under the stamp, leading

to the formation of dual line patterns. Indeed this is purely

dependent on the thickness of the composite films, as long as the

other conditions are fixed. In the present case 63.6 � 0.6 nm and

45.0 � 1.1 nm PS films resulted in single and dual lines in each

trench, as seen in Fig. 2a and b (also see Fig. S2, ESIw). TheCTAB surface renders a highly positively charged surface on the

Pd nanocubes synthesized in the current study (see ESIw).To induce electrostatic assembly and to test our hypothesis a

neutral polymer, P2VP (poly(2-vinylpyridine), 13 � 1 nm), a

negative changed polymer, PAA (poly(acrylic acid), 10� 1 nm),

and a positive changed polymer, PEI (polyethylenimine,

65 � 2 nm), were grafted to the PGMA layers. Indeed when

the substrates were exposed to Pd nanocubes only a PAA

grafted substrate resulted in attachment (Fig. S3, ESIw). Thisindicated that the assembly in the present case was dominated

by electrostatic interaction. It is noteworthy that the density of

patterned Pd nanocubes can be easily altered by changing the

concentration of Pd solution. Furthermore, the concentration

of CTAB affects the assembly of Pd nanocubes. At high

concentration, the excess of CTAB will interfere with the assembly

while low concentration of CTAB will cause aggregation of

Pd nanocubes. The optimum concentration of CTAB was found

to be around 1 mM, for stabilizing the nanocubes and inducing

self-assembly of packed structures into linear arrays. Depending on

the thickness of the initial PS layer, the final Pd nanocube assembly

can be directed to be either a single line or a dual line (Fig. 2c–f).

The Pd nanoarrays with high density were chosen to test the

hydrogen response. Two silver epoxy electrodes were painted

at the two ends of the linear assembly (Fig. 3a and b), and the

whole integrated sensor platform was mounted into a hydrogen

gas sensing chamber subject to current–voltage (I–V) sweeps.

The self-assembled Pd nanocubes have a resistance of about

5000 O at room temperature, as calculated from the I–V

characteristics. This very high resistance is due to the grain

boundaries junction effect between nanocubes. Furthermore,

the CTAB on the Pd nanocubes may hinder charge transport

causing an increase in resistance. The sensor was initially

subjected to a flow of hydrogen and nitrogen gas for 5 minutes

Fig. 1 Schematic of (a) capillary force lithography (CFL): silica

wafer was modified with a thin layer of poly(glycidylmethacrylate)

(PGMA) followed by another thin film of polystyrene (PS); a PDMS

(polydimethylsiloxane) stamp was placed over the PS film and CFL

was conducted at 130 1C; PDMS stamp was peeled off after cooling;

polyacrylic acid (PAA) was dip-coated and grafted to PGMA at 38 1C; Pd

nanocube solution was drop-casted followed by controlled evaporation to

allow nanocubes to attach to the PAA area evenly; PS mask was removed

with a solvent, leaving the patterned Pd nanocubes on the surface.

(b) Chemical reaction of grafting PAA to PGMA.

Fig. 2 AFM images of (a) PS + PAA pattern using a 63.6 � 0.6 nm

PS film, and (b) CFL generated PS strips using a 45.0 � 1.1 nm PS film

and the profile showing a second strip formed periodically. (c) and (d)

AFM phase images of PAA brush after removing PS by a solvent

generated from the PS pattern shown in (a) and (b), respectively.

(e) and (f) AFM images of Pd nanoarrays generated using patterned

PAA shown in (c) and (d), respectively, via electrostatic interaction. In

the case of (d) and (f), the height of stripes increased from 10 nm to

around 30 nm, indicating that single Pd nanoparticles are attracted

given that the average size of Pd nanocubes is 16.5 nm.

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and 10 minutes, respectively, at a total gas flow rate of

1000 mL min�1. The response time, t90, for 2% hydrogen

was determined to be 75 s, and for 3% hydrogen 70 s (Fig. S4,

ESIw). Accordingly, a 2 min hydrogen and 10 min nitrogen

alternative flow was chosen to test sensor performance. Fig. 3c

shows the response to hydrogen with an increase in resistance

when hydrogen gas flows over and a return to the original state

when no hydrogen gas is present, for concentrations of 0.3% to

3%. We were unable to obtain clear response for hydrogen gas

concentrations lower than 0.3% possibly due to the CTAB

coating hindering hydrogen contact at low concentrations.

In summary, we demonstrate that by using capillary force

lithography in combination with a ‘‘grafting to’’ approach it

is possible to electrostatically assemble Pd nanocubes into

linear arrays as a platform for creating large area prints for

developing sensors. Indeed this platform can be further fine

tuned for a wide range of Pd nanoparticles of various shapes

using appropriate surfactants and complementary polymer

patterns to optimise sensing response. This platform can be

easily extended to pattern and assemble other materials, such

as peptide, polymer, silica nanoparticles, metal and metal

oxide nanoparticles, for applications in bio-sensing, cell

growth, tissue engineering and nanoelectronics.

The authors are grateful for the financial support for this work

by the Australian Research Council and The University of

Western Australia, and for the palladium precursors provided

by The Perth Mint. The microscopy analysis was carried out

using facilities in the Centre for Microscopy, Characterization

and Analysis, The University of Western Australia, which are

supported by University, State and Federal Government funding.


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J. Phys. Chem. C, 2007, 111, 6321–6327.17 Y. G. Sun and H. H. Wang, Adv. Mater., 2007, 19, 2818–2823.18 X. Q. Zeng, M. L. Latimer, Z. L. Xiao, S. Panuganti, U. Welp,

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Fig. 3 (a) SEM image of Pd nanoarrays used in sensing experiment,

and (b) high resolution SEM image of the area indicated in the red box

in (a). (c) Current response of sensor to 0.3–3% hydrogen gas, with

alternating 2 min hydrogen and 10 min nitrogen flow.

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Chapter 4 Conclusions and Future Work

89

4. Conclusions and Future Work

A green, facile, scalable synthetic method was developed to synthesise several

different types of Pd nanomaterials in aqueous solution using spinning disc processing

(SDP). The method developed in this project did not involve any organic solution, heating, or

the use of toxic reducing agents. The Pd nanocrystals synthesised using SDP were

remarkably uniform, with the size of 5-6 nm, thus can be ideally used as seeds for further size

and shape controllable growth.

However, the inherent short residence time in SDP restricts the morphological control

of Pd nanomaterials. In future work, a rotating tube processor (RTP), which is another form of

process intensification with a longer residence time, can be explored for the synthesis of Pd

nanomaterials with different size and shape. Furthermore, the ability to selectively introduce

reagents along the long axis of the RTP would enable a higher degree of control in the

nucleation and growth processes of the nanoparticles, as well as potentially coating

nanoparticles in situ. This would be an important advancement in single step processing to

generate composite nanomaterials.

The catalytic activity of Pd-PVP nanospheres has been studied in the reaction between

several aryl halides and butyl acrylate. Pd-PVP nanospheres showed the relatively high

catalytic ability relative reported heterogeneous catalysis systems. However, they did not

show catalytic activity towards chlorobenzene and its derivatives, which are more appealing

as industrial starting materials due to the lower cost in comparison with

bromo-/iodo-benzene. In the highly active homogeneous system, the use of different

ligands to stabilise aryl-Pd(II)-Cl after oxidative addition step is considered as crucial to

activate chlorobenzene and derivatives thereof. If this is the case, the use of functionalised

supports with ligands to stabilise Pd(II) intermediates during reaction of Pd nanomaterials

should be considered in future work to enhance the catalytic activity. In the work on the

synthesis Pd decorated graphene, phosphonated calix[4]arene showed extraordinary

affinity to Pd nanomaterials. Phosphonated calix[4]arene stabilised graphene potentially


90

could be used as a support for Pd nanocatalyst to enhance the catalytic activity in the Heck

cross coupling reaction. The size of Pd nanocrystals synthesised on calix[4]arene stabilised

graphene was 2-3 nm and the ultra-small size would also make them ideal for catalysis.229

In the kinetics and recycling study of Pd-PVP nanosphere catalysts, it was found that

the surface absorbed oxygen triggered changes in size of Pd nanoparticles during Heck

cross coupling reaction. Although several different groups have observed this phenomenon

before, the reason for the change was never experimentally determined.153-155 Two parallel

Heck reactions were carried out in which the only difference was the oxygen content during

recycling. The size of the Pd nanoparticles dramatically increased in the batch which was

exposed to air during recycling; in contrast the size of the Pd nanoparticles didn't show any

magnificent change when there was no oxygen involved. These results shed light on the

synthesis of novel catalysts and how to optimise reaction conditions for Pd nanomaterials in

heterogeneous catalysis. However, elucidating the mechanism of the heterogeneous Heck

reaction is far more complicated. For example, the heterogeneous catalysis has been shown

to be controlled not only by the chemical composition and size of the catalyst but also by

the surface structure of the catalyst.12, 97, 135, 230 Reaction conditions have a dramatic influence

on the yield of the reaction as well. For instance, the choice of solvent was found to be critical

in preventing the homo-coupling reaction. Polar aprotic solvents, such as NMP

(1-methyl-2-pyrrolidone) and DMF afforded relatively high yields, but non-polar solvents,

such as xylenes or toluene, have suppressed the leaching of Pd from a SWNTs support.231

Furthermore, the reaction rates depend strongly on the based used which is a common

observation in many coupling reactions.232 Thus monitoring reaction at atomic level, such as

using STM (scanning tunnelling microscope), and ultrafast spectroscopy would be highly

useful to provide insights into the mechanism and can be carried out in next step to reveal

the pathway of heterogeneous Heck cross coupling reaction. QEXAFS (quick scanning

extended X-ray absorption fine structure) study, operando EXAFS (Extended X-ray

Absorption Fine Structure) study, and high pressure TEM or SEM in situ studies should also

be considered to identify the active species in Heck cross coupling reactions.


91

The studies on hydrogen sensors in this project provided a practical and low-cost

method to fabricate a resistive hydrogen sensor. Also the studies established that the signal

and mechanism of resistive Pd sensor can be controlled by altering the polymer surrounding

Pd nanoparticles. However, in terms of building a hydrogen sensor for daily use in the future,

in situ tests should be carried out. In general the H2 sensing performance of Pd

nanostructures is better than that of bulk Pd due to the increased surface-to-volume ratio.

However, the correlation of sensing performance and the finite size effect of nanostructure

hasn’t been understood. In the future, the effect of different Pd nanostructures on the sensor

performance should be studied. A fundamental understanding of this relationship is of

paramount importance for the practical application of hydrogen sensor.

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms. As a new

allotrope of carbon, after fullerenes (1985) and carbon nanotubes (1991), graphene has been

intensively studied since 2004. Graphene has a theoretically calculated surface area of 2630

m2/g. Such a high surface area makes graphene a potentially excellent catalyst support.

Unlike carbon nanotubes, the modification of graphene is still under-developed and

therefore hindering further applications. A practical and novel method was developed to

stabilise graphene in a wide range of pH by using phosphonated calix[4]arene. Calix[4]arene

not only acts as stabiliser but also facilitates the attachment of Pd(II) complexes, which can

be reduced by hydrogen afterwards. Moreover, Pd/graphene can be used as a template to

form a Pt 2D nano-arrays on graphene sheets. This finding provides a general way to modify

graphene with nanoparticles. The high stability of these composites makes them promising

candidates in a wide range of applications, such as sensing, catalysis, energy conversion and

hydrogen storage.

Capillary force lithography (CFL) is an emerging lithography for large-area patterning.

However, applying CFL to assembly nanoparticles has not been widely studied. CFL

associated with electrostatic attraction to assembly Pd nano-cubes was studied in this

project. The use of these nano-arrays in a hydrogen gas sensor has also been demonstrated.

Traditional hydrogen sensors are fabricated, in general, on inorganic substrates (e.g., glass,

quartz, silicon wafers, etc.). The rigidity of these sensors associated with the use of rigid


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substrates may limit their application in various new areas (e.g., portable devices, aerospace

science and civil engineering) that require flexible, lightweight, and mechanically

shock-resistant sensing components. CFL has been used to fabricate large-scale pattern on

flexible substrates.233 In the future, CFL can be used to pattern Pd nano-arrays on thin plastic

sheets for fabricating flexible hydrogen sensors, which can be used in applications

complementary to those of the conventional sensors.

In conclusion, this PhD dissertation presents the results which mainly focused on the

synthesis of Pd nanomaterials in aqueous solution, and the applications of these

nanomaterials in heterogeneous catalysis and hydrogen sensing. Most importantly, it has

demonstrated for the first time that surface oxygen triggered size changes in Pd

nanoparticles during the Heck cross coupling reactions. This work also provides a practical

technique to control the mechanism of a Pd resistive sensor which is paramount

fundamentals in hydrogen sensing technology for the daily use. In addition, the accessibility

of graphene has been improved using a novel surfactant, and the stabilised graphene has

been shown as an excellent template for immobilising Pd and Pt nanoparticles. Finally the

application of capillary force lithography in assembly of Pd nanoparticles has been reported

for the first time, which provides a low cost and practical method to fabricate large area

miniatures of hydrogen sensor. The findings presented in the above research will provide a

strong fundamental foundation in the development of novel heterogeneous catalyst and

resistive Pd sensor.

Chapter 5 Appendices

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5. Appendices

Online supporting information for the papers presented in Chapter 3, are organised in

a sequence accordingly.

5.1 Supporting information for “Bare palladium nano-rosettes for real

time high performance and facile hydrogen sensing”

The control experiment was carried out by using magnetic stirring instead of SDP in a

round bottom flask and bubbling hydrogen gas. As can be seen from SEM image (Figure S1),

the as synthesized palladium materials aggregated together forming continuous

sponge-like material. In contrast, the palladium nano-resettes synthesized by using SDP

are finely dispersed.

Figure S1 SEM images of palladium materials synthesized by bench experiment (left) and SDP (right).

Details about IDE:

Each individual set of IDEs consists of 15 fingers, each 15 μm in width and 550 μm in

length, with a finger gap of 10 μm. Each individual electrode is connected to a boding pad

(200 μm×250 μm) to provide a sufficient area for subsequent wire bonding.


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5.2 Supporting information for “Hydrogen-induced reversible insulator–

metal transition in a palladium nanosphere sensor”

S1: The synthesis and characterization of palladium nanospheres

The palladium nanospheres were synthesized by reducing H2PdCl4 in the presence of

PVP through spinning dick processor (SDP). Hydrogen gas was used as a reducing agent.

SDP offers a novel avenue for intensified nano-fabrication via exploiting the high centrifugal

acceleration to generate thin films which provide rapid heat and mass transfers. The

geometry and key elements of a SDP are illustrated in Figure S1. The key components of SDP

include: (i) a 100 mm rotating disc with controllable speed (up to 3000 rpm), and (ii) feed jets

located at a radial distance of 5 mm from the centre of the disc. SDP generates a very thin

fluid film (1 to 200 μm) on a rapidly rotating disc surface, within which nanoparticle

formation occurs. The shear forces and viscous drag between the moving fluid film and the

disc surface create turbulence and ripples which give rise to highly efficient turbulent mixing

within the thin fluid layer. The turbulent waves thus generated can be a combination of

circumferential waves moving from the disc centre to the disc periphery, and helical waves,

depending on the operating parameters. The wavy thin film generated on a rotating disc

surface in the presence of a gas, notably H2, offers the ability to control the size of the ensuing

particles by controlling the delivery of H2 to the thin film.

In a typical synthesis of palladium nanospheres, the H2PdCl4 aqueous solution (0.6

mmol L-1) was mixed with PVP (Polyvinylpyrrolidone, MW=40,000) and then the mixture was

fed from one jet at the feed rate of 0.7 mL s-1. Hydrogen gas was fed from another jet to

reduce palladium (II) to palladium nanoparticles. The speed of the spinning disc was set at

2000 RPM. The as synthesised nanospheres were washed using MilliQ water (>18 MΩ) three

times and re-dispersed in water before any further test. The size and morphology of the

samples were determined using transmission


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Figure S1 Schematic representation of a spinning disc processor (SDP).

transmission electron microscopy (TEM, JEOL 3000F) operating at 300 kV. Powder XRD

pattern of the palladium nanospheres was measured using an Oxford Diffraction Gemini-R

CCD diffractometer (using Cu Kα= 1.54178 Å radiation).

A typical representation of the Pd nanospheres obtained using the above technique

can be seen from Figure S2. The High Resolution TEM image (Figure S2) show the size of the

individual Pd nanoparticles in the spheres is about 5 nm. It also can be seen that the

nanocrystals are not tightly packed, but have an average interparticle spacing of 1-2 nm in

projection. The crystallinity of the nanospheres was also confirmed by high resolution TEM

image. Additionally, the XRD pattern of the palladium nanospheres agreed with the Pd Card

(JCPDS card No.05-0681), Figure S3, which indicated a high crystallinity of the palladium

nanoparticles synthesized using SDP.


96

Figure S2 (A) TEM of the palladium nanospheres and (B) High Resolution TEM image of palladium nanospheres with inset:

FFT pattern, showing crystallinity.

Figure S3 X-ray diffraction pattern of palladium nanospheres.

S2: The setup and characterization of hydrogen gas sensor

A 300-350 μm thick Si <100> n-doped wafer used in the sensor device was covered by

a 300 nm insulating Si3N4 layer deposited using Plasma Enhanced Chemical Vapour

Deposition. The interdigitated electrode pattern was transferred by using photolithography,

which was followed by depositing a 5 nm chromium binding layer and a 50 nm gold layer

using an in-house built metal evaporator system. The palladium nanospheres solution was

drop-cast onto the surface of each IDE using aliquots of 0.02 μL from a 0.5 μL glass syringe

which were subsequently air-dried. The test procedure involved alternating nitrogen gas (20

min) and varying concentrations of hydrogen gas (4 min). The change of the current was


97

monitored at the same time. The total flow rate of gas was 1000 mL min-1. The voltage

applied between two electrodes was 100 mV dc. The images of sensor were recorded with

scanning electron microscope (SEM, Zeiss 1555 VPSEM) operating at an accelerating voltage

of 8 kV.

Figure S4 (A,B) Low magnification SEM images of the self-assembled palladium nanospheres on an interdigitated

electrode (IDE).

S3: Control experiment to sense hydrogen gas in the absence of PVP

The control palladium nanomaterials were synthesized by using the same procedure

as palladium nanospheres in the absence of PVP. The nanoparticles thus formed adopted an

agglomerated rosette structure with no gap in between them as seen in Figure S4.

And the resultant control palladium nanomaterials were used as hydrogen gas

sensing materials according to the same procedure as palladium nanospheres. The results

of sensing of different hydrogen concentrations (between 0.1% to 10 % in N2 gas) are shown

in Figure S5. There is an increase in resistance with hydrogen gas flow


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Figure S5 (A) TEM of palladium nanoparticles synthesized in the absence of PVP and (B) High resolution image showing

aggregation with no gap between individual nanoparticles.

and a return to the original state when no hydrogen gas was present for concentrations of

0.1% to 10%. The change in resistance due to the diffusion of atomic hydrogen into the

lattice to form PdHx, results in a α to β phase transition which in turn results in an increase in

resistance. No reversible switching between increase and decrease resistance was observed

in this case.

Figure S6 Response of the palladium nanoparticles (without PVP) sensor to hydrogen gas from the concentration of 0.1 to

1%, and response of the sensor to hydrogen gas from the concentration of 1 to 10%.


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5.3 Supporting information for “Pd-sodium carboxymethyl cellulose

nanocomposites display a morphology dependent response to

hydrogen gas

S1. The synthesis of Pd- Sodium Carboxymethyl Cellulose nanocomposites:

The Pd-Sodium Carboxymethyl Cellulose (Pd-SCMC) nanocomposites were

synthesized by reducing H2PdCl4 in the presence of sodium carboxymethyl cellulose

(MW~90,000) using a spinning disc processing (SDP) platform. In a typical synthesis, the

H2PdCl4 aqueous solution (0.6 mmol L-1) was mixed with SCMC and then the mixture was fed

from one feed jet at a feed rate of 0.7 mLs-1. Hydrogen gas was fed through another feed jet

to reduce Pd (II) to Pd (0). The speed of the spinning disc was set at 2000 rpm. For Pd

nano-rosettes shown in Figure 1a, a 10 cm grooved disc was used. For Pd aggregates shown

in Figure 1b, a 20 cm smooth disc was used. The SCMC to Pd equivalents was 2 and 5,

respectively. The as synthesized nanocomposites were washed using MilliQ water (>18 MΩ)

three times and re-dispersed in water before any further test. The size and morphology of

the samples were determined using transmission electron microscopy (TEM, JEOL 3000F)

operating at 300 kV. Powder XRD pattern of the Pd aggregates was measured using an

Oxford Diffraction Gemini-R CCD diffractometer (using Cu Kα= 1.54178 Å radiation).

S2. The setup of hydrogen sensor:

The Pd- SCMC nanocomposites solution were drop cast onto the surface of each IDE

(interdigitated electrode) using aliquots of 0.02 μL from a 0.5 μL glass syringe which were

subsequently air-dried. The procedure involved alternating nitrogen gas (20 min) and

varying concentrations of hydrogen gas (4 min) for tests in Figure 2a and 3a. Alternating

nitrogen gas (10 min) and varying concentrations of hydrogen gas (2 min) feature in Figures

2b and 3b. The change of the current was monitored at the same time. The total flow rate of

gas was 1000 mLmin-1. The voltage applied between two electrodes was 100 mV dc or 10

mV dc. The images of sensor were recorded with scanning electron microscope (SEM, Zeiss

1555 VPSEM) operating at an accelerating voltage of 8 kV.


100

Figure S1 Schematic illustration of spinning disc processing (SDP).

Figure S2 Low magnification TEM images of as-prepared Pd nanocomposites from spinning disc processing: the molar

equivalents of SCMC to Pd were 2 and 5, respectively.


101

Figure S3 TEM images of as-prepared Pd-SCMC nanocomposites formed using SDP, a) the molar equivalents of SCMC to

Pd of 1:1, and b) the molar equivalents of SCMC to Pd of 30:1.

Figure S4 SEM images of Pd-SCMC nanocomposites bridging two electrodes, sensor 1 (a) and sensor 2 (b).


102

5.4 Supporting information for “Scalable synthesis of catalysts for the

Mizoroki-Heck cross coupling reaction: palladium nanoparticles

assembled in a polymeric nanosphere

Figure S1 Schematic of Spinning Disc Processor (SDP)

Figure S2 TEM image of as-prepared palladium nanoparticles using hydrazine as reducing reagent (other parameters are

same as nano-spheres showing in Figure 1(a).


103

Figure S3 TEM image of a typical palladium nanomaterials synthesized using PVP with a molecular weight of 360,000 with

a disc speed of 2500 rpm.

Figure S4 TEM images of palladium-PVP spheres synthesized via mechanical stirring by using (a) PVP10 (MW 10,000); (b)

PVP40 (MW 40,000) and (c) PVP360 (MW 360,000).


104

Figure S5 EDS data of palladium nanospheres. (a) pristine; (b) after 1st run; (c) after 5th run.

1H NMR data and GC-MS data

O

O

1

1H NMR (400 MHz, CDCl3, ppm): δ 7.68 (d, 1H, J=16.0 Hz), 7.54–7.52 (m, 2H), 7.39-7.37

(m, 3H), 6.44 (d, 1H, J=16.0 Hz), 4.21 (t, 2H, J=6.6 Hz), 1.69 (quint, 2H, J=7.2 Hz), 1.44 (sextet,

2H, J=7.4 Hz), 0.97 (t, 3H, J=7.6 Hz); GC-MS m/z (relative intensity): 204 (M+, 18), 148 (74), 131

(100), 103 (57), 77 (38).


105

O

O

7

1H NMR (400 MHz, CDCl3, ppm): δ 7.63 (d, 1H, J=16.0 Hz), 7.41 (d, 2H, J=7.8 Hz), 7.19 (d,

2H, J=7.8 Hz), 6.40 (d, 1H, J=16.0 Hz), 4.20 (t, 2 H, J=6.4 Hz), 2.38 (s, 3H), 1.64 (quint, 2H, J=7.2

Hz), 1.42 (sextet, 2H, J=7.4 Hz), 0.98 (t, 3H, J=7.6 Hz); GC-MS m/z (relative intensity): 218 (M+,

29),162 (95), 145 (100), 115 (53), 91 (26).

HO

O

O

8

1H NMR (400 MHz, CDCl3, ppm): δ 7.63 (d, 1H, J=16.0Hz), 7.41 (d, 2H, J=8.4Hz), 6.87 (d,

2H, J =8.4Hz), 6.30 (d, 1H, J =16.0Hz), 5.15 (s, 1H), 4.22 (t, 2H, J =6.4Hz), 1.68 (quint, 2H,

J=5.6Hz), 1.43 (sextet, 2H, J=7.6 Hz), 0.97 (t, 3H, J=7.2Hz); GC-MS m/z (relative intensity): 220

(M+, 21),164 (100), 147 (50), 119 (24), 107 (22).

O

O

9O

O

1H NMR (400 MHz, CDCl3, ppm): δ 8.03 (d, 2H, J = 4.0 Hz), 7.67 (d, 1H, J=8.0Hz), 7.56 (d,

2H, J=4.0Hz), 6.50 (d, 1H, J=8.0 Hz), 4.21 (d, 2H, J=6.4 Hz), 3.91 (s, 3H), 1.68 (quint, 2H, J =7.2Hz),

1.42 (sextet, 2H, J=7.6Hz), 0.95 (t, 3H, J=7.2 Hz); GC-MS m/z (relative intensity): 262 (M+, 28), 206

(100), 189 (46), 175 (75), 145 (43).

O

O10

O

1H NMR (400MHz, CDCl3, ppm): δ 7.97 (d, 2H, J =4.4Hz), 7.69 (d, 1H, J =8.0Hz), 7.61 (d,

2H, J =4.4Hz), 6.53 (d, 1H, J =8.0Hz), 4.23 (t, 2H, J=6.8Hz), 2.62 (s, 3H), 1.70 (quint, 2H, J =7.2Hz),


106

1.44 (sextet, 2H, J =7.6Hz), 0.97 (t, 3H, J =7.6Hz); GC-MS m/z (relative intensity): 246 (M+, 18),

231(46), 190 (39), 175 (100), 131 (32).

O

O

O11

1H NMR (400 MHz, CDCl3, ppm): δ 7.67 (d, 1H, J = 8.0 Hz), 7.48 (d, 2H, J = 4.4Hz), 6.90 (d,

2H, J = 4.4 Hz), 6.31 (d, 1H, J = 8.0Hz), 4.20 (t, 2H, J = 6.4 Hz), 3.84 (s, 3H),1.69 (quint, 2H,

J=7.2Hz), 1.47(sextet, 2H, J =7.2Hz), 0.96 (t, 3H, J = 7.2 Hz); GC-MS m/z (relative intensity): 234

(M+, 42), 178 (100), 161 (98), 134 (41), 121(31).

N

O

O

1H NMR (400 MHz, CDCl3, ppm): δ 7.61 (d, 1H, J=16.0 Hz), 7.41 (d, 2H, J=6.8 Hz), 6.63 (d,

2H, J=6.8 Hz), 6.22 (d, 1H, J=16.0 Hz), 4.18 (t, 2H, J=6.8 Hz), 3.00 (s, 6H), 1.70-1.64 (m, 2H),

1.46-1.41 (m, 2H), 0.96 (t, 3H, J=7.2 Hz); GC-MS m/z (relative intensity): 247 (M+, 94), 191 (41),

174 (62), 147 (100).


107

5.5 Supporting information for “Surface oxygen triggered size change of

palladium nano-crystals impedes catalytic efficacy”

Synthesis and characterization of Pd-PVP nanospheres:

Pd-PVP nanospheres were synthesized by using spinning disc processing (SDP) as a

facile one step method with hydrogen gas as the reducing agent at room temperature. SDP

(Scheme S1) is a process intensification strategy which offers continuous flowing film (1 to

200 μm) on a rapidly rotating disc surface (usually up to 3000 rpm). The wavy thin film

generated on the spinning disc surface has been reported to enhance hydrogen gas uptake

in the solution. 1 In a typical experiment, the aqueous solutions of H2PdCl4 (0.6mM) were

mixed with PVP (polyvinylpyrrolidone) with the molecular ratio of PVP to palladium10, and

then fed through a jet feed onto the spinning disc, and hydrogen gas as reducing agent was

fed through another jet feed, which resulted in the formation of palladium nano-spheres of

uniform size and shape (Figure S1). The resulting mixture was collected from the outlet and

washed three times with MilliQ water, then freeze-dried before use in the Heck reaction. A

large number of 5 nm palladium particles (Figure S1) spontaneous assembly into

nano-spheres within the dynamic thin films on the surface of disc in the presence of PVP,

rather than discrete individual palladium nanoparticles. PVP acts as a scaffold entangled with

the small palladium nano-particles within the dynamic thin films under intense shearing in

the dynamic thin films, as well as stabilising the Pd-PVP nano-spheres in solution.

Scheme S1 Schematic representation of a spinning disc processor (SDP).


108

Figure S1 TEM image and high resolution TEM image of Pd-PVP nano-spheres. Quasi-spheroidal 5 nm palladium

nano-crystals indicated in dotted box.

Figure S2 XRD patterns of the nano-catalyst, prior to the first cycle, (a), and after the 10th recycling, (b).


109

Figure S3 Energy dispersive spectra (EDS) of palladium nanospheres. a) pristine; b) after the 5th recycling in the absence of

oxygen; c) after the 10th recycling in the presence of oxygen.

1． Sisoev, G. M.; Matar, O. K.; Lawrence,C. J.; Chem. Eng. Sci. 2005, 60, 2051-2060.


110

5.6 Supporting information for “Pd-induced ordering of 2D Pt nanoarrays

on phosphonated calix[4]arenes stabilised graphenes”

Experimental procedure:

p-Phosphonic acid calix[4]arene was synthesised according to literature using a five

step procedure.1 Briefly, the parent calix[4]arene was brominated at the upper rim, followed

by protection of the lower rim with an acetyl group. A nickel-catalysed Arbuzov reaction was

then employed to attach a phosphoryl group to the upper rim, and this was followed by

subsequent deacetylation and deesterification to give the desired product.

Graphene was synthesized according to the recently reported procedure. 2-4

Graphite was first oxidised by H2SO4 and KMnO4. And the resulting graphite oxide

underwent exfoliation to afford graphene oxide. A mixture of hydrazine and ammonia were

subsequently used to reduce graphene oxide to graphene.

The synthesis palladium nanoparticles coated graphene sheet involved in

incubating 1 mg/mL p-phosphonic acid calix[4]arene solution with graphene solution

overnight. Calix[4]arene stabilised graphene (CSG) was vacuum-filtrated using a membrane

(pore size 0.2 μm, Sigma) and then washed several times using MilliQ water. CSG was

re-dispersed in MilliQ water, and incubated with H2PdCl4 overnight. The resulting mixture

was filtrated through a filter membrane and redispersed in MilliQ water to remove excess Pd

precursors. The solution was purged with hydrogen gas 2-3 min under magnetic stirring.

Following this the solution was centrifuged for 30 min at 4000 rpm, and re-dispersed in

MilliQ water.

For a typical platinum nanoparticles growth we used a modified experimental

protocol previously described by Xia et al.5-7 Briefly, CSG-Pd(0) solution 100 μL was heated in

air to remove water and then dissolved with 4 mL EG (ethylene glycol). 22.5 mg PVP (MW

55,000) was dissolved in 1 mL EG and 0.2 mL of 0.1 molL-1 H2PtCl6 was dissolve in 1 mL EG

(heated to remove water before use). PVP and H2PtCl6 solution were added simultaneously


111

to the CSG-Pd(0) mixture dropwise at 110 °C and the reaction was allowed to continue for 2

h before adding different amount of FeCl3 solution (0, 10, 20 and 200μL of 12 mM FeCl3). At

110 °C some of the EG decomposed to aldehyde which in turn reduces the H2PtCl6 to PtII

intermediate. The PtII undergoes replacement reaction with Pd to form Pt nanoparticles.

Figure S1 TEM image of graphene-Pd without phosphonated calix[4]arene.

Figure S2 High resolution TEM image of CSG-Pd.


112

Figure S3 High resolution TEM image of Pt nanoparticles on the CSG.

1. T. E. Clark, M. Makha, A. N. Sobolev, D. Su, H. Rohrs, M. L. Gross, J. L. Atwood and C. L.

Raston, New J. Chem., 2008, 32, 1478.

2. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.

3. N. I. Kovtyukhova, P. J. Ollivier, B. R. Martin, T. E. Mallouk, S. A. Chizhik, E. V. Buzaneva

and A. D. Gorchinskiy, Chem. Mater., 1999, 11, 771.

4. D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3,

101.

5. J. Chen, T. Herricks, M. Geissler and Y. Xia, J. Am. Chem. Soc., 2004, 126, 10854.

6. J. Chen, T. Herricks and Y. Xia, Angew. Chem. Int. Ed., 2005, 44, 2589.

7. E. P. Lee, J. Y. Chen, Y. D. Yin, C. T. Campbell and Y. N. Xia, Adv. Mater., 2006, 18, 3271.


113

5.7 Supporting information for “Regiospecific linear assembly of Pd

nanocubes for hydrogen gas sensing”

S1. Materials and methods

H2PdCl4 solution was made from a 1:1 molar ratio of HCl and PdCl2 (Aldrich). PS

(polystyrene, MW=280K Da), PEI (polyethyleneimine, MN=60 kDa, MW=750 kDa) and PAA

(poly acrylic acid, MW=100 kDa) were purchased from Sigma-Aldrich. PGMA

[poly(glycidylmethacrylate)], (MN=304 kDa, PDI=2.14) was synthesised using previosuly

descriped procedure.1 All ACS grade solvents were used in the current study.

Highly polished single-crystal silicon wafers of 100 orientation (Semiconductor

Processing Co.) were used as a substrate. The wafers were cleaned in a piranha solution (3:1

concentrated sulfuric acid/30% hydrogen peroxide) for 1 h, and then rinsed several times

with MilliQ water.

S2. Fabrication of PDMS stamp

First, the metal layer in a blank CD was peeled off and the CD was washed with ethanol.

The polycarbonate support (minus the metal layer) was used as master. Polymer base and

curing agent from Sylgard® 184 (Dow Corning) silicone elastomer kit were thoroughly mixed

together at ratio 10:1 by weight in a glass vial. In order to remove the trapped bubbles from

mixing of the components, the vial was placed in a vacuum desiccator. Following vacuum

treatment the elastomer was restored at atmospheric pressure slowly several times until it

was free of bubbles. Finally PDMS mixture was cast onto the surface of the grooved side of CD

and cured at 80 ºC for 2 hours.

S3. Synthesis of Pd nanocubes 2

0.5 mL of 10 mM H2PdCl4 solution was added to 12.5 mL of 10 mM CTAB

(Cetyltrimethylammonium bromide) solution under stirring, the solution was stirred for at

least 15 mins before heated to 90 ºC for 5 min prior to the addition of 80 μL of a freshly

prepared 100 mM ascorbic acid solution. The reaction was allowed to proceed for 30 min.


114

The whole reaction mixture was then left at room temperature for 3 h before washing.

Figure S1 shows the morphology of as-prepared Pd nanocubes.

S4. The typical procedure of fabricating palladium nano-array

First, silica wafer was modified with a monolayer of PGMA which contains epoxy

functional groups. Silica wafer was dip-coated in PGMA (0.07% w/v in CHCl3) and put into

oven under vacuum at 120 °C for 20 min to anneal PGMA. The unreacted PGMA was

removed using CHCl3. Next, PS film was deposited by dip-coating to cover the PGMA layer.

The PDMS (polydimethylsiloxane) stamp was placed over the PS film followed by heat

treatment in an oven at 130ºC. The assembly was left aside to cool down to room

temperature before the PDMS stamp was peeled off. The patterns of the PS can be tuned

according to the initial concentration of PS: negative replica of stamp and doubled strips of

stamp can be obtained by using 1% w/v and 0.6% w/v PS in toluene, respectively. The whole

complex was dip-coated with PAA (1% w/v in methanol) and left at 38 ºC for at least for two

hours to allow PAA to graft onto PGMA. After the grafting was complete, excess PAA was

removed by washing with an ethanol/water mixture. Pd nanocube solution was drop-casted

onto the patterned silica wafer, followed by controllable evaporation to allow nanocubes to

attach to PAA evenly. Finally the PS mask was removed using methyl ethyl ketone, leaving

the patterned Pd nanocubes on the surface.

S5. The procedure of hydrogen sensing test

Two silver epoxy electrodes were painted into two ends of the as-prepared Pd

nano-arrays, and the whole integration was mounted into hydrogen gas sensing chamber

subject to current-voltage (I-V) sweeps. The test procedure involved alternating nitrogen gas

(10 min) and varying concentrations of hydrogen gas (5 or 2 min). The change of the current

was monitored at the same time. The total flow rate of gas was 1000 mL min-1. The voltage

applied between electrodes was 100 mV dc.

S6. Characterisation by Ellipsometry, AFM, SEM and TEM


115

Ellipsometry was performed with a COMPEL automatic ellipsometer (InOmTech, Inc.)

at an angle of incidence of 70°. For testing the thickness of PS film, a four-layer model (silicon

substrate + silicon oxide layer + PGMA anchoring layer + PS layer) was used to simulate

experimental data. The refractive indices used to calculate the thickness of silicon oxide,

PGMA and PS layers were 1.457, 1.5 and 1.5, respectively. Topographical and phase images

were obtained using a VEECO Dimension 3100 AFM with Nanoscope IIIa controller and

Ver5.30r3sr3 software in ambient air. The height (topography) and phase images were both

captured using a frequency of 1.0 Hz and 256 scan lines per image. The size and morphology

of the Pd nanocubes were determined using transmission electron microscopy (TEM, JEOL

3000F) at 300 kV. Scanning electron microscope (SEM) images were recorded at a Zeiss 1555

VPSEM operating at an accelerating voltage of 10 kV.

Figure S1 TEM images of Pd nanocubes.

Figure S2 AFM image and line profile of PAA brush after removing PS by solvent, generated from the PS pattern shown in

Figure 2b.


116

Figure S3 AFM images of a) Pd attached to P2VP, b) Pd attached to PEI, and c) Pd attached to PAA.

Figure S4. The responses to 2% and 3% hydrogen gas (5min hydrogen gas followed by 10 min nitrogen gas).

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