Metal-organic framework thin films as versatile ... - DiVA Portal

Preparation, functionalization and analysisof UiO-66 metal-organic framework thin

films on silicon photocathodes

Andreas Wagner

Uppsala UniversityMolecular Inorganic Chemistry - Biomimetic Chemistry

Master Thesisunder the supervision of

Dr. Sascha OttMSc Sonja Pullen

Uppsala, June 2015

Contents

Symbols and Abbreviations vi

Acknowledgement viii

Abstract ix

Popular scientific summary x

1 Introduction 11.1 Artificial Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 [FeFe]-hydrogenase and its model complexes . . . . . . . . . . . . . . . . 41.3 Metal-organic frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Theoretical background 132.1 UiO Metal-Organic Framework . . . . . . . . . . . . . . . . . . . . . . . 132.2 MOF thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.1 Electroactive MOF thin films . . . . . . . . . . . . . . . . . . . . 192.3 Silicon photoelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.3.1 Silicon-liquid interface . . . . . . . . . . . . . . . . . . . . . . . . 242.3.2 Silicon surface modification . . . . . . . . . . . . . . . . . . . . . 27

2.4 Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.4.1 Cyclic voltammetry (CV) . . . . . . . . . . . . . . . . . . . . . . 302.4.2 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . 312.4.3 X-Ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . . 322.4.4 Powder X-Ray Diffraction (PXRD) . . . . . . . . . . . . . . . . . 332.4.5 Infrared (IR) Spectroscopy . . . . . . . . . . . . . . . . . . . . . 332.4.6 Ion-Beam Analysis (IBA) . . . . . . . . . . . . . . . . . . . . . . 342.4.7 Time-of-flight secondary-ion mass spectrometry . . . . . . . . . . 352.4.8 Dynamic light scattering (DLS) . . . . . . . . . . . . . . . . . . . 352.4.9 Contact angle measurement . . . . . . . . . . . . . . . . . . . . . 362.4.10 Inductively coupled plasma atomic emission spectroscopy (ICP-AES) 36

iv Contents

3 Experimental 373.1 Chemicals and Purification . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2 Synthesis of para-ethynylbenzoic acid . . . . . . . . . . . . . . . . . . . . 383.3 Substrate cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.4 Silicon surface functionalization . . . . . . . . . . . . . . . . . . . . . . . 393.5 UiO-66 thin film synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . 403.6 Methods of Characterization . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.6.1 Cyclic voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . 433.6.2 Contact Angle Measurement . . . . . . . . . . . . . . . . . . . . . 433.6.3 Ion Beam Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 443.6.4 XPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.6.5 SEM-EDX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.6.6 Nuclear Magnetic Resonance (NMR) . . . . . . . . . . . . . . . . 453.6.7 Powder XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.6.8 FTIR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 463.6.9 TOF-SIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.6.10 DLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.6.11 ICP-AES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4 Results and Discussion 474.1 UiO-66 thin film synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.1.1 Electrophoretic deposition . . . . . . . . . . . . . . . . . . . . . . 474.1.2 Solvothermal synthesis . . . . . . . . . . . . . . . . . . . . . . . . 49

4.2 Analysis of functionalized UiO-66 . . . . . . . . . . . . . . . . . . . . . . 554.2.1 IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564.2.2 ICP-AES analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 564.2.3 SEM-EDX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574.2.4 Ion-beam analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.3 Electrochemistry of p-type silicon . . . . . . . . . . . . . . . . . . . . . . 664.4 Si surface functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5 Conclusion and Outlook 695.1 UiO-66 thin film synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . 695.2 Analysis of functionalized UiO-66 . . . . . . . . . . . . . . . . . . . . . . . 71

Contents v

5.3 Silicon electrochemistry and functionalization . . . . . . . . . . . . . . . . 71

References 75

vi Contents

Symbols and Abbreviations

Values for fundamental constants were taken from the International Committee on Datafor Science and Technology latest dataset from 2010 [1].

Å Ångstrom, 10−10 mbdc Benzene-1,4-dicarboxylic acid (=terephthalic acid)bdt benzene-1,2 dithiolcp centipoise, a common unit for viscosity, 1 cp = 1 mPasdcbdt 2,3-dithiolato-1,4-benzenedicarboxylic acidCV Cyclic VoltammetryDCM DichloromethaneDMF N,N-dimethylformamideEDX Energy Dispersive X-ray spectroscopyERDA Elastic Recoil Detection AnalysisEXAFS Extended X-ray Absorption Fine StructureFTO Fluorine-doped Tin OxideGC Glassy CarbonICP-AES Inductively Coupled Plasma Atomic Emission Spectroscopymcbdt 2,3-dithiolato-benzoic acidMEIS Medium Energy Ion ScatteringMOF Metal-Organic FrameworkPSE Post Synthetic ExchangePXRD Powder X-ray DiffractionRBS Rutherford Backscattering SpectrometryRCA Radio Cooperation of AmericaSAM Self-Assembled MonolayerSBU Secondary Building UnitSEM Scanning Electron MicroscopySIMS Secondary Ion Mass SpectrometryTBAPF6 Tetrabutylammonium hexafluorophosphateTHF TetrahydrofuranTOF Time-of-flightUiO University in Oslo

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XPS X-Ray Photoelectron Spectroscopy

α Charge transfer coefficientc Speed of light ; 299792458 ms−1

E Potential at the working electrodeEeq Potential at which cathodic and anodic reaction are in equilibriumE0 Energy of incident ionη Difference between electrode potential E and equilibrium potential EeqF Faraday constant 96485.3365 Cmol−1

h Planck constant; 4.135667516 · 10−15 eVs = 6.62606957(29) · 10−34 Jsj0 exchange current densityk Kinematic factor in ion beam analysisn Number of electrons per redox reactionR Universal gas constant 8.3144621 Jmol−1K−1

T Temperature

viii Contents

Acknowledgement

I want to thank a few people for their continuous support throughout the last eight months.I would like to start with my supervisor Sonja - she has been incredibly important in mydevelopment during my time here in Uppsala. It all started several months ago duringa lab day where she was supervising me and fellow students. It was the first time forme to get into touch with biomimetic [FeFe]-hydrogenase model complexes, their use asproton reduction catalysts and metal-organic frameworks. Sonja had to withstand 1 hourof rigorous questioning from my side. Her dedication motivated me and I started workingin the Ott lab soon after that. Even though we were not always of the same opinion, itwas always great working together with her. Thanks for all your help throughout thelast 14 months. After that I have to thank Sascha for his incredible support and all thefreedom he gave me during these projects. I would also like to thank Edgar, Ulrike, Reinerand Andreas for their help with all my practical (and not so practical) questions and allthe things they explained, showed and taught me. Dr. Daniel Primetzhofer, Dr. JensJenssen, Prof. Jean Petterson, Viktoria Stenhagen, Dr. Wei Xia and Hauke Krammare greatly acknowledged for their help with Ion-Beam-Analysis, TOF-SIMS, ICP-AES,SEM-EDX, XPS and contact angle measurements respectively. Another "merci" goesto my fume hood partner Charlene and the other lab 1 colleagues Daniel, Fabien andJohann. I wouldn′t want to forget to mention the rest of Ott crew (Max, Bis, Somnath,Ben, Yuri, Keyhan, Rosa, Carlotta, Ricardo, Michaele, Hemlata, Giovanni, Shameem,Tianfei, Valentina, Anders) and all the people from the neighbouring research groupsfor all the fun we had together. At last but not least I would like to thank my parentsfor their financial support and my friends that showed a lot of understanding that I wasalways late since I was stuck in the lab.

Contents ix

Abstract

Metal-organic frameworks (MOFs), metal centers (atoms or clusters) linked with organicmolecules, are of strong interest in materials research due to their chemical versatilityand extraordinary high surface area. The molecular nature of MOFs allows a post-synthetic functionalization and modification of the chemical environment within the pores.Earlier, it was shown that incorporation of Fe2(dcbdt)(CO)6 (dcbdt = 2,3-dithiolato-1,4-benzenedicarboxylic acid) - a molecular proton reduction catalyst - into a MOF results ina new material that exhibits increased catalytic reactivity and stability compared to thesame complex in solution. Recently, we have shown the electrochemical addressability ofthe same catalyst integrated into a MOF thin film on FTO (fluorine doped tin oxide).Within this thesis robust UiO-66 (UiO = University in Oslo) MOF thin films were preparedon FTO and p-type silicon electrodes and different parameters (water concentration,relative concentrations, time) influencing the synthesis were analyzed. It was observed thatterephthalic acid is not forming a self-assembled monolayer on silicon substrates while itdoes on FTO. Films with a preferential growth direction of the UiO-66 crystallites on siliconwere prepared and functionalized by post-synthetic ligand exchange with Fe2(mcbdt)(CO)6(mcbdt = 2,3-dithiolato-1-benzenecarboxylic acid). A variety of techniques (ATR-IR,SEM-EDX, ICP-AES) was used to analyze the catalyst loading. An increased surfaceconcentration as well as a depth gradient of Fe and S in the films was found by Rutherfordbackscattering and medium-energy ion scattering. Photoelectrocatalytic measurementswith functionalized MOF thin films on silicon were unsuccessful due to the electron-blockingbehaviour of silicon oxide. First experiments towards the covalent functionalization ofsilicon by hydrosilylation with 4-para-etynylbenzoic acid as MOF anchoring group arepresented. The stability of silicon photoelectrodes as well as their photoeffect leading to ashift of the reduction potential by ca. 0.5 V in positive direction were assessed.

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Popular scientific summary

The overall long-term goal of this project and others conducted within the research groupbiomimetic chemistry is to efficiently produce hydrogen as a renewable energy carrierdirectly from sunlight and water. Basic research is conducted to mimic biological systemsand understand the origins of their high efficiency.This master thesis deals with a new class of materials called metal-organic frameworks(MOFs) that was recently developed. MOFs are crystalline solids based on metal clustersthat are linked together with organic molecules. Several hundred combinations of differentmetals with different organic linkers have been developed durin recent years. This greatversatility and the fact that these materials offer the highest surface areas currently known(the record material has a surface area of ca. 1 football field per gram!) make them highlyinteresting to researchers in the fields of gas storage, catalysis, drug delivery and manymore. Another great advantage of MOFs is the fact that the organic linker in the crystalstructure can be chemically altered which improves the possibilities for modifications evenfurther.

In this thesis one of the most robust metal-organic frameworks called UiO-66 (UiO =University in Oslo) based on the metal Zirconium was grown as thin films on a siliconsubstrate. A special catalyst that contains no rare-earth elements and is able to producehydrogen was incorporated into the framework. This catalyst was chemically synthesizedand its structure is incorporating certain features of enzymes present in bacteria and greenalgae. It is typically very fragile when it is dissolved in water and degrades quickly undersunlight, but the incorporation into the MOF increases its stability a lot. One of theobjectives of this work is to understand this stabilization effect and the distribution of thecatalyst within the MOF. It was possible to show that the catalyst is not incorporatedhomogeneously into the MOF. An increased concentration on the surface of the entirefilm was detected.

The silicon used as substrate for the MOF film can act as light absorber similar toits use in photovoltaic cells and provide the energy (electrons) needed for the catalystto produce hydrogen according to 2H+ + 2e– −−→ H2. Experiments towards employingthe MOF-photoelectrodes for hydrogen generation did unfortunately not work within thetime span of this thesis due to the formation of an isolating silicon oxide layer betweenthe silicon and the MOF. The formation of this electron-blocking layer was analyzed by

Contents xi

electrochemical methods. It was tried to protect the silicon from oxidation by coveringit with a passivating organic layer. Further experiments to circumvent this silicon oxideformation are in progress.

Chapter 1

Introduction

The following introduction will guide through the main aspects of this work. Firstlythe fundamentals of artificial photosynthesis, [FeFe]-hydrogenase model complexes andmetal-organic frameworks (MOF) will be briefly explained. Based on that, the objectiveof this thesis will be outlined in the context of three publications that provide the basis ofthis work.

1.1 Artificial Photosynthesis

Human mankind faces tremendous challenges in the future: climate change, the depletionof fossil fuels and a worldwide increasing energy demand. A drastic change of the energyproduction and storage towards renewable energy sources is expected to happen within thenext decades [2, 3]. The most important emergent renewable technologies on the market(photovoltaic, wind) produce electricity inconsistently due their inherent dependence onweather changes, day-night cycles and seasons. Electricity is by definition an energycarrier, transmitting energy between the primary energy source and the end-user [4]. In2012 electricity only contributed 18% of the worlds energy demand, while fuels made upthe rest (82%) [5]. Many experts argue that the large-scale production of hydrogen withelectrolyzers from intermittent electricity will provide a renewable fuel source, but theprice for noble metals as electrocatalysts in electrolyzers is still limiting its application [6].This approach is considered as indirect method since the solar energy is firstly convertedto electricity and then in a second step into chemical energy. An increasing numberof scientists engages in the field of artificial photosynthesis: the direct conversion ofenergy from sunlight into fuels [7]. This approach potentially minimizes costs due to theincorporation of light absorption (photovoltaic) and catalysis (electrolyser) into one device.It generally involves the oxidation of water to oxygen and the reduction of protons tohydrogen or CO2 to other carbon based fuels. Throughout this work the focus will lie onwater splitting to form hydrogen as an energy carrier. The energy needed to split water is1.23 V per electron transferred [8] (see equation 1.1).

2 CHAPTER 1. INTRODUCTION

H2O −−→ 2H+ + 12O2 ∆E0 = 1.23V pH = 0 (1.1)

Taking into account that an overpotential will be needed to overcome kinetic limitationsof the reaction, a minimum of 1.6 V is frequently reported [9]. One could in principle usea photovoltaic cell to produce the necessary potential difference for the water splitting,but the output voltage of photovoltaic cells is strongly light dependent and therefore needselaborate electronic control during changing light intensity [10].

Applying literature values for Plank′s constant and the speed of light, 1.6 eV corre-sponds to the energy of a photon with a wavelength of 774.9 nm (see equation 1.2 and1.3).

E = h· ν = c

λ(1.2)

λ = h· ν

E(1.3)

Ideally a system would thereby consist of one single material that efficiently absorbsall photons with wavelengths shorter than 750 nm, separates the charges, catalyses bothwater oxidation and proton reduction and is on top of it all stable under these conditions.Different research groups are trying to find a material that is capable of doing all thesetasks mentioned, but the highest quantum efficiency reported so far is only 0.66 % [11].The more promising approach is to separate at least some of the different functions(absorption, charge separation, water oxidation catalysis and proton reduction) from oneanother.

There are different possibilities to realize a water splitting system and only two differentways will be illustrated here (a review on the topic was published by Walter et al. [8]). Avery general scheme of artificial photosynthesis is illustrated in figure 1.1 and is basedon a molecular photosensitizer to absorb the light. The water oxidation catalyst andhydrogen evolving catalyst might be a molecular catalysts in solution, on a solid supportor a heterogeneous catalyst.

I 2 H+ + 2 e− −−→ H2 II 2 H2O −−→ O2 + 4 H+ + 4 e− (1.4)

1.1. ARTIFICIAL PHOTOSYNTHESIS 3

Figure 1.1: Scheme of an artificial photosystem consisting of a photosensitizer (P), wateroxidation catalyst (WOC), hydrogen evolving catalyst (HEC) or CO2 reduction catalyst (CRC).Sacrificial electron donors (SED) and acceptors (SEA) can be used to study the half-reactionsseparately [12]

Another approach is the use of semiconductors as light absorbers instead of a molecularspecies. The water splitting half reactions (see equation 1.4) are taking place at thephotocathode and photoanode. This system might be the most obvious design principlebut is one of the least developed according to Sivula and Grätzel [10]. They argue that thestability and the magnitude of photocurrent of the photocathode remain limiting factors.

Figure 1.2: Semiconductor based photocathode/photoanode tandem water splitting cell design,HEC = hydrogen evolving catalyst, WOC = water oxidation catalyst

A tandem configuration with a large bandgap photoanode and small bandgap pho-tocathode in which the two semiconductors absorb complementary portions of the solarspectrum (see figure 1.2) can reach higher efficiencies than a single absorber system [8].


The work described in this thesis focuses on the incorporation of a molecular protonreduction catalyst into a metal-organic framework thin film on a silicon substrate asphotocathode material for solar water splitting. Cathodes based on p-type silicon show aphotoeffect under illumination, which means that the absorbed photon energy is used toshift a reduction potential to more positive values (oxidation potential to more negativepotential). Further details to silicon photocathodes and their electrochemistry will bediscussed in section 2.3. Metal-organic frameworks are - as explained in section 1.3 - aversatile material class with very high surface areas and form a highly interesting platformfor the incorporation of molecular catalysts. In contrast to heterogeneous catalysts, molec-ular species can be fine tuned with regard to their electronic and steric properties andthe molecular mechanism can be elucidated by spectroscopic techniques. In the followingsection, a short motivation for the research on enzyme active-site model complexes isgiven.

1.2 [FeFe]-hydrogenase and its model complexes

Platinum is often regarded as the best catalyst for proton reduction (and hydrogenoxidation), but it is an expensive metal with limited supply. Enzymes on the other handare natural catalysts enabling numerous chemical reactions with high selectivity, turnoverand low thermodynamic penalty without using noble metals [13]. Figure 1.3 showsan electrocatalytic measurement of two natural enzymes adsorbed to carbon electrodes[14]. The oxidized and reduced form of the substrate was added in solution and a cyclicvoltammogram (see section 2.4.1) was recorded. The enzymes show perfectly reversibleoxidation/reduction at the thermodynamic potential with high exchange current densitiesand no overpotential. In the case of the reversible proton reduction and hydrogen oxidation,nature was able to develop so called Hydrogenases containing only [Fe], [FeFe] or [NiFe]in the active sites of the enzymes. They play a crucial role in the anaerobic metabolismof bacteria and green algae [15].

Soon after the publication of the crystal structure of the [FeFe]-hydrogenase active sitein 1999, organometallic chemists started off to synthesize model complexes [18, 19]. Thiswas mainly driven by the high efficiency of [FeFe]-hydrogenases (shown in figure 1.4) froman energy related perspective and that the main structural motives of Fe2(CO)6(µ−SR)2have long been known [20]. The last 15 years have lead to an increased understanding of

1.2. [FEFE]-HYDROGENASE AND ITS MODEL COMPLEXES 5

Figure 1.3: Cyclic voltammograms of enzymes adsorbed on rotating-disc pyrolytic graphite edgeelectrodes under catalytic conditions with both the oxidized and reduced substrates present. (A)Reversible interconversion of H+ and H2 by hydrogenase-2 from Escherichia coli (pH 6,10%H2 in Ar, 30 °C) [16]. (B) Reversible interconversion of CO2 and CO by CODH 1 from C.hydrogenoformans (pH 7, 50% CO in CO2, 25 °C) [17]; adapted from reference [14]

the involved mechanism, but all synthesized model complexes lie far behind the efficiencyof the natural enzyme in terms of overpotential and rate. Nevertheless important factorsfor the high efficiency of the enzyme could be elucidated [13], namely a pending basein the second coordination sphere of the active site and independent proton, electronand molecular hydrogen pathways within the enzyme. It was furthermore noted that theactive site of the enzyme has strong hydrophobic properties and no water is present inthe vicinity of the active site. While enzymes often reach molecular masses of more than100000 g/mol, man-made model systems are typically limited to less than 1000 g/mol andcan thereby not provide channels for electrons and protons. Metal-organic frameworks,as applied in this work, might in the future be able to mimic the complex functionsof the protein shell of an enzyme [21]. Proton conductivity [22], electron conductivity(see section 2.2.1) and energy transfer [23, 24] are three hot topics in current MOF research.

In the context of this thesis the [FeFe]-hydrogenase model complexes Fe2(bdt)(CO)6(bdt = benzene-1,2 dithiol), Fe2(mcbdt)(CO)6 (mcbdt = 2,3-dithiolato-benzenecarboxylicacid) and Fe2(dcbdt)(CO)6 (dcbdt = 2,3-dithiolato-1,4-benzenedicarboxylic acid) wereused (see figure 1.5). If not mentioned differently, Fe2(mcbdt)(CO)6 was used for theexperiments. These catalysts are far less efficient than the enzymes, not stable overextended periods of time under irradiation in coordinating solvents, but are valuablecompounds to foster understanding about how natural enzymes work. The addition oftwo carboxyl-groups on the dithiolate bridge enables the incorporation into metal-organic


Figure 1.4: [FeFe] hydrogenase crystal structure from Desulfovibrio desulf uricans [25]. Theelectron transfer chain via iron-sulfur centers and the hydrogen pathway are shown schematically.The chemical structure of the active site with its open coordination site at the metal (arrow) isshown to the right; adapted from [26]

frameworks as elaborated in the next chapter. Furthermore, the carbonyl bands of thecatalyst offer the possibility to study reactions that occur at the catalyst with the help ofIR spectroscopy (see section 2.4.5).

Figure 1.5: [FeFe]-hydrogenase model complexes applied in this thesis: Fe2(bdt)(CO)6 (bdt= benzene-1,2 dithiol), Fe2(mcbdt)(CO)6 (mcbdt = 2,3-dithiolato-benzenecarboxylic acid) andFe2(dcbdt)(CO)6 (dcbdt = 2,3-dithiolato-1,4-benzenedicarboxylic acid)

1.3 Metal-organic frameworks

Metal-organic frameworks are a class of materials, first described by Yaghi et al. in 1995[27], consisting of metal clusters that are interconnected by organic linkers. Within thelast 20 years hundreds of different MOFs were developed; three of the most common onesare shown in figure 1.6.

MOFs generally have large pore sizes and thereby offer very high surface areas up to7000 m2/g, which exceeds the highest recorded porosities of any other kind of materialclass like active carbon or zeolite [29, 30]. This circumstance makes them highly interesting

1.3. METAL-ORGANIC FRAMEWORKS 7

Figure 1.6: Overview over the three most common MOFs: MOF-5 (Zn4O nodes with 1,4-benzenedicarboxylic acid linker), HKUST-1 (HKUST = Hong Kong University of Science andTechnology, copper nodes with 1,3,5-benzenetricarboxylic acid linker) and UiO-66 (UiO = Univer-sity in Oslo, [Zr6O4(OH)4] clusters with 1,4-benzenedicarboxylic acid linker). The core structure(secondary building unit) is shown to the left and the respective MOF structure to the right.HKUST-1 and UiO-66 are shown in a ’cutaway-view’ to remain clarity [28]


to material chemists who focus on many different applications ranging from drug delivery[31] over gas storage [32] to catalysis [33–35]. It also explains why over 3500 papers werepublished in 2013 mentioning "Metal-organic frameworks" according to Web of Science.

One particularly interesting aspect of MOFs is a phenomena called post-syntheticligand exchange (PSE). At the right conditions, MOFs are able to exchange a substan-tial amount of their linker molecules (or metal cluster) without loosing their structuralintegrity. A schematic picture of the process is presented in figure 1.7. Kim et al., from theresearch group of Prof. Cohen in San Diego, showed in 2012 [36], that a linker exchangeof more than 40% is possible in the UiO-66 framework. This can easily be achieved byplacing the MOF-powder into an aqueous solution of a modified ligand which containsthe same benzene-dicarboxylic acid unit.

Figure 1.7: The principle of post-synthetic metal and ligand exchange. The metal cluster (bluepyramid) can be exchanged with different metal ions (purple circle) to yield a chemically alteredcluster (purple pyramid). The ligand (orange rod) can be exchanged with a modified ligandcontaining the same linking unit (blue rod). The yellow sphere should demonstrate the cavityspace within the MOF [37]

In contrast to post-synthetic exchange, a process called post-synthetic modificationdeals with reactions inside the MOF to chemically alter functionalities at the linker or themetal nodes.

1.4 Objective

This master thesis is based on results and ideas from three different publications [38–40]within the research group of Dr. Ott and collaborators (Prof. Kubiak and Cohen fromUniversity California in San Diego). Pullen et al. showed an increased stability andcatalytic rate of the [FeFe]-Hydrogenase mimic Fe2(dcbdt)(CO)6 (dcbdt = 2,3-dithiolato-1,4-benzenedicarboxylic acid) incorporated into a UiO-66 MOF [38] by post-synthetic

1.4. OBJECTIVE 9

exchange (see section 1.3 and figure 1.8). While the homogeneous catalyst is completelydegraded in less than 1 hour under illumination, it stays intact within the frameworkunder the same conditions. An incorporation of ca. 15% was achieved, as shown bySEM-EDX (see section 2.4.2) and NMR.

Figure 1.8: Schematic representation of the system developed by Pullen et al. [38]. Abiomimetic [FeFe]-hydrogenase model complex Fe2(dcbdt)(CO)6 was incorporated into a UiO-66metal organic framework by post-synthetic exchange in water at room temperature for 24 hours.

Besides photochemical experiments that were conducted in the first publication, oneof the next projects was to solvothermally grow UiO-66 on a FTO electrode to show ifone is able to electrochemically address the catalyst within the framework. This workwas done within and after the author′s research training in the research group of Dr. Ottin summer 2014. The experiments showed that it was possible to synthesize UiO-66 onthe electrode and that thin films with incorporated catalyst show a distorted, but clearlydetectable, reduction/oxidation wave for the catalyst (see figure 1.9). It has to be notedat this point, that UiO-66 itself is non-conductive and the signal is presumably basedon a redox-hopping mechanism of the incorporated catalyst molecules. A more detaileddiscussion on electroactive MOFs can be found in section 2.2.1.

Blank samples of FTO without MOF that were similarly treated according to thePSE protocol did not show any kind of electrochemical response. A coordination of thecomplex to the FTO substrate can thereby be excluded. A difference between thin andthick films was found and lead to the conclusion that diffusion of the molecular catalystinto the films during the post-synthetic exchange is limited.


Figure 1.9: Cyclic voltammograms of Fe2(dcbdt)(CO)6 in 1 mM DMF solution (blue), andUiO-66 film before (green) and after PSE (red). Left: thick UiO-66 film (ca 20 µm). Right:thin UiO-66 film (ca 2− 5 µm). All CVs recorded in DMF with 0.1 M TBAPF6 as supportingelectrolyte.[39]

Cyclic voltammetric measurements are usually performed in organic solvents to mini-mize catalyst degradation. The protons necessary for catalytic measurements are typicallysupplied by an organic acid. Unfortunately it was not possible to measure electrocatalysison the FTO substrate since it undergoes reactions with acids at reductive potentials.The overall goal for this thesis was to show electrocatalysis of a molecular catalyst incor-porated into a metal-organic framework thin film electrode. Therefore at the beginningof this project a new substrate material had to be chosen. In 2012, Kumar et al. useda Si photocathode with a very similar [FeFe]-hydrogenase model complex in solution toperform photoelectrochemical measurements under illumination. Silicon electrodes show astrong photoeffect under illumination of ca. 0.5 V and shift the reduction potential of thecatalyst to more positive values (see section 2.3.1). They are furthermore stable againstacid and provide a perfect platform for MOF growth and analysis methods due to theirperfect monocrystalline and flat surface. It was therefore decided to use silicon as thesubstrate material for the UiO-66 thin films.

Several different tasks were targeted within this master thesis:

1. Analyzing the solvothermal growth of UiO-66 thin films under different conditions toget a better understanding of the influences of different parameters on the synthesis.

1.4. OBJECTIVE 11

2. Growing UiO-66 thin-films on silicon, incorporate Fe2(dcbdt)(CO)6 by post-syntheticexchange and analyze its capabilities towards photoelectrocatalysis. (see figure 1.10)

3. Motivated by the fact that thick films on FTO did not show any kind of electro-chemical response, apply different techniques to depth profile the MOF thin filmand analyze the lateral distribution of the catalyst within the film.

Figure 1.10: Schematic two-dimensional drawing of a UiO-66 thin film with incorporatedFe2(dcbdt)(CO)6 as a hydrogen evolving photoelectrode. The Zr-nodes represent the secondarybuilding unit of UiO-66 represented by 6 Zr atoms

Chapter 2

Theoretical background

This chapter contains five sections, which will in detail explain the theoretical backgroundfor this thesis, and review the literature. The only applied metal-organic framework withinthis work is the so-called UiO-66 framework (UiO = University in Oslo) and an entiresection will deal with details regarding this material. After that, a more specific theoreticalintroduction to MOF thin films and a review of synthetic methods to produce them follows.A major part of this section will also deal with a literature review of electroactive MOFthin films. As shown in the introduction, we were able to record a cyclic voltammogram ofa molecular catalyst inside a MOF thin film incorporated by post-synthetic exchange [39].This is to the best of my knowledge, the only report presenting cyclic voltammograms ofan electrocatalyst within a MOF thin film.The next section deals with silicon as a photoelectrode material, its electrochemistry andthe importance of its surface condition. At the end, short theoretical descriptions of theapplied analytical techniques in this thesis will be given.

2.1 UiO Metal-Organic Framework

A very stable and versatile MOF called UiO-66 was developed at the University in Osloin 2008 [41]. It consists of Zr-oxo-hydroxo clusters Zr6O4(OH)4 that are linked togetherwith terephthalic acid (or biphenyldicarboxylic acid in UiO-67). The structure of thecluster and the MOF are shown in figure 1.6. Each Zr-ion is 8-coordinated and uponfull activation at high temperature (250 ◦C), the cluster looses two water molecules andreduces the coordination to 7 [42]. This high coordination is attributed to be one of themain reasons for UiO-66′s exceptional thermal stability (up to 375 ◦C in air), stabilityagainst boiling water, mineral acids, a variety of organic solvents and mechanical stress[41–44]. Only the stability against base is limited: Miyamoto et al. showed a decrease ofcrystallinity (main reflection in PXRD dropped to 74 % intensity) after 6 hours immersedinto pH 11 NaOH solution [45]. The high stability of the UiO-Framework has almostdethroned MOF-5 and HKUST-1 as a benchmark MOF material [46].

14 CHAPTER 2. THEORETICAL BACKGROUND

In practice, UiO-66 always contains a large number of defect sites as shown bythermogravimetric analysis (TGA) [44] and later by neutron scattering [42] and singlecrystal X-ray diffraction [47]. The linker occupancy of the single crystal diffraction wasonly 73 % for UiO-66 (close to 100 % for UiO-67). It was noted that this figure might beinaccurate due to data fitting reasons and assumed that the defect sites are a mixtureof hydroxide, water, and DMF or other coordinating solvents. Even though the authorsof the study used benzoic acid as modulator (see next paragraph) during the synthesis,no benzoic acid was found in the crystal structure. Nevertheless, its use prevented theformation of chlorine terminated cluster defects. In the high quality neutron diffraction,the linker occupancy was ca. 90 %.

Schaate et al. was the first to report the use of a modulator (benzoic acid) forthe synthesis of UiO-MOFs [48]. The crystallinity and reproducibility was shown to beimproved by the addition of a mono-carboxylic species and larger, better separated crystalswere produced. Wu et al. also showed a strong influence of the modulator concentrationon the pore size, adsorption isotherms and colour of the samples [42].

Ragon et al. reported that the addition of water during the synthesis, accelerates theformation of the Zr-oxocluster, while more acidic conditions like the addition of HCl, acetic-or benzoic acid lead to slower kinetics, probably due to a decrease in the deprotonationrate of the carboxylic linker [49]. They furthermore report that the use of ZrOCl2 · 8H2Oas a precursor leads to better reproducibility (attributed to the hygroscopic nature ofZrCl4) and higher yields. By applying this precursor and HCl as modulator, the synthesisof UiO-66 particles was achieved in the exceptionally small size range of 100± 20 nm.

Shearer et al., from the group in Oslo around Prof. Lillerud that first synthesized theUiO-framework, published an elaborate study on how to eliminate any defects duringthe synthesis [50]. They describe that a Zr:bdc ratio of 1:2 and an increased synthesistemperature of 220 ◦C leads to an ideal UiO-66 MOF.

Wißmann et al. reported that, while the use of a modulator usually slows down thegrowth of the MOF, formic acid is accelerating it [51]. They hypothesize that this can beexplained by a shift of the well-known decomposition equilibrium of DMF to the left [52](see figure 2.1). More water is accessible for the synthesis of the Zr-cluster and therebythe growth rate is accelerated.

The role of DMF in the synthesis of MOFs is manifold. Formic acid (and diethylamine)have been shown to incorporate into certain MOFs during the solvothermal synthesis [53,

2.2. MOF THIN FILMS 15

Figure 2.1: Hydrolysis equilibrium of DMF leading to the formation of formic acid anddimethylamine

54]. Even though this is not the case for UiO-MOFs, the creation of dimethylamine andformic acid during hydrolysis certainly plays an important role, specifically with regardto reproducibility. Furthermore, DMF is known to decompose slowly to dimethylamineand CO at its boiling point (153 ◦C) [55]. The temperature for the solvothermal UiO-66synthesis is ranging from 80-220◦C, hence the thermal decomposition is another pathwayto create the basic dimethylamine, which is able to deprotonate the carboxylic acidsduring synthesis [56].

2.2 MOF thin films

In the first review on the topic of MOF thin films from 2009, Zacher et al. describesthat one challenge in MOF research is “...the deposition or growth of MOF thin films onsubstrates, ideally in a dense, homogeneous and oriented fashion...” [57]. The interest inthin films is directly related to the increasing demand in adjusting the optical, electricalor mechanical properties of surfaces and interfaces [58]. Different techniques have beendeveloped throughout the years to deposit or grow metal-organic frameworks on differentsurfaces like silica, porous alumina, graphite, gold and FTO.

• Microwave irradiationYoo et al. presented a microwave-induced method to synthesize MOF nanocrystalson carbon coated porous Al2O3 surfaces. They report a facile synthesis of a MOF-5thin film with nearly full surface coverage in 30 seconds at 500W irradiation. [59]

• Secondary solvothermal growthYoo et al. also compared their microwave irradiation method to the so calledsecondary growth method. After a first solvothermal treatment, that leads to anincomplete surface coverage, the film is washed thoroughly and then treated in anew solvothermal synthesis solution similar to the first time [60]. Gascon et al. and


Guerrero et al. have shown that dip-coating, spin-coating and thermal-coating withseed crystals before the solvothermal treatment can also be used [61, 62] to improvethe film growth and surface coverage.

• Functionalization of the substrate by an organic linkerHermes et al. [63] showed that the formation of a self-assembled monolayer of acarboxyl-group terminated silane is leading to a selective adhesion of MOF-5 on asilicon substrate. The MOF was not solvothermally synthesized on the surface. Intheir approach the MOF is synthesized in solution, filtered and then the sample isimmersed into the filtered solution (also called mother-solution) at room temperaturefor 24 hours. A different example for organic monolayer functionalization has beenreported by Huang et al. [64]. In this report a covalent amide linkage between acarboxyl-group terminated silane and the amine group of the imidazolate linkerof ZIF-90 was established. It should be noted that the functionalization of siliconwith silanes is based on the reaction with the silicon oxide surface layer. Thismethod can therefore not be used if the substrate is oxide free which is necessary forphotoelectrochemical measurements (see section 2.3.1). Based on the work of Liu etal. on Al2O3 from 2009 [65] and the work of Kung et al. [66] our group followed asimple functionalization of the FTO samples by immersion into a solution of thelinker (terephthalic acid) prior to the solvothermal treatment [67]. This non-covalentcoordination seems to promote the growth of the MOF on the substrate.Conductive substrates like e.g. glassy carbon (GC) or silicon (as shown in section2.3.2) may also be functionalized with an organic linker by electrochemical graftingof diazonium salts. This has been shown by Balakrishnan et al. [68] and Hou etal. [69] for HKUST-1 and MOF-5 respectively. The growth of continuous filmswithout the pretreatment of the flat glassy carbon electrode was unsuccessful forHou et al.. Balakrishnan et.al reported that roughening the GC electrode with aSiC paper also creates carboxyl-groups on the surface. It was possible to successfullysynthesize HKUST-1 on roughened as well as on diazonium-pretreated GC electrodesby solvothermal synthesis. Hou et al. did not use a solvothermal treatment, butrather used a mother-liquid approach.

• ALD deposition of Al2O3

In the same paper by Hermes et al. mentioned above [63], they also present the


methodology of atomic layer deposition (ALD) to coat a silicon substrate withAl2O3. It is argued (similar to Yoo et al. in 2008 [59]) that the low isoelectric pointof silicon (2-2.5) compared to the pKA values of terephthalic acid (3.52 and 4.46) isthe reason for the bad adhesion of MOF particles on the silicon (isoelectric point ofAl2O3: 9.1 [59])

• Coating of the substrate with polyanilineLu et al. reported that covering a substrate with polyaniline before thesolvothermal treatment leads to good film growth of HKUST-1, MIL-68 andZn2(BDC)2DABCO[70]. This method is irrespective of the substrate and its generalapplicability on stainless steel, copper and platinum was shown.

• Metal substrate as both the support and the metal sourceGuo et al. showed that the solvothermal synthesis of the copper-based MOF HKUST-1 on an oxidized copper grid leads to a compact and intergrown thin film [71]. Thecopper grid, as well as the added copper salt are a metal-source for the MOF growth.Zou et al. reported the growth of Zn3(BTC)2 on a Zn-slide [72] which was activated(hydroxyl-terminated) by hydrogen peroxide pretreatment without the addition ofany Zn-precursor.

• Layer-by-layerA very powerful technique for the thin film synthesis of MOFs is the so-calledlayer-by-layer growth, which is also known as liquid-phase epitaxy. The methodwas mainly developed by the groups of Wöll and Fischer and has been reviewed byLiu and Fischer in 2011 [73]. The principle is shown in figure 2.2 - the sample issequentially immersed into a solution containing the metal salt, a washing solutionto remove excess metal precursor and a solution containing the organic liker. Thegrowth is thereby controllable down to the level of single molecular layers and issolely depending on self-assembly. Liu et al. has been able to incorporate multiplefunctionalities into the MOF [74] and Tu et al. grew thin film heterostructures ofCu3btc2 on top of Cu2ndc2dabco on a quartz crystal microbalance [75] to illustratesome of the recent highlights with this method.

• Electrochemical synthesisThe electrochemical synthesis of metal-organic frameworks was pioneered by the


Figure 2.2: Graphical representation of the proposed model for layer-by-layer growth ofCu3(btc)2 on an oxide surface. The atoms shown are Cu – green, O – red, C – gray [76]

work of Mueller et al. [77] which was based on the anodic dissolution of metalions into a solution containing the organic linker. Several research groups havesynthesized a wide variety of MOFs by anodic dissolution and the literature hasbeen reviewed by Halls et al. [30]. Al-Kutubi et al. have reviewed the challengesand opportunities of electrosynthesis of MOFs in a broader perspective than anodicdissolution recently in 2015 [78]. One very interesting example of MOF-5 thin filmsynthesis based on cathodic electrodeposition rather than anodic dissolution waspublished by Li and Dincă [79].

Figure 2.3: Comparison of solvothermal methods (left) using NEt3 or Me2NH as base todeprotonate the carboxylic linker and Li and Dincă′s approach to electrochemically create ahydroxyl acting as a base close to the substrate [79]

They argue that the slow introduction of base equivalents to deprotonate the car-boxylic acid linker is the key step in the formation of MOFs. In the originalpublication by Yaghi and coworkers on the synthesis of MOF-5, triethylamine wasused as a base by vapour diffusion and a mixture of DMF and chlorobenzene wasused as a solvent instead of pure DMF [80]. Later on, all the literature appliedDMF in their solvothermal synthesis procedures. As described earlier, DMF is


slowly decomposing to form dimethylamine which acts as a base to deprotonatethe carboxylic acid linkers. It was pointed out that the decomposition of DMF isdepending on many factors including the metal ion concentration, pH and tempera-ture and thereby leads to strongly varying reaction time between different MOFs.In their paper, Li and Dincă use the elegant way of producing OH– as a base todeprotonate the linker directly at the electrode by the reduction NO–

3 (see figure 2.3).A phase-pure MOF-5 film of 20− 40 µm thickness was produced in 15 minutes atroom temperature, thereby showing a much faster film growth than by solvothermalreaction.

• Electrophoretic depositionHod et al. presented a novel method for MOF thin film preparation in 2014 [81].It is based on the fact that MOFs are negatively charged in solution (possibly tothe earlier described missing linker defects) and can thereby be directed towardsa positively charged electrode within an electric field. The MOF particles aresimply suspended in a low-polarity non-ionizing organic solvent, placed into a flaskcontaining two electrodes and a potential difference of 90 V is applied for ca. 3hours to get full surface coverage. The method was tested for four different kinds ofMOFs (HKUST-1, Al-MIL-53, UiO-66 and NU-1000). It was shown that patternedstructures could be produced with this approach by applying photolithographybefore the electrophoretic deposition. After removing the photoresist an additionallayer of a different MOF than the first layer was electrophoretically deposited.

In conclusion, there is a wide variety of methods to synthesize MOF thin films. Thenecessary time to produce samples ranges from several minutes to several days. All methodshave advantages and disadvantages and the method of choice is strongly dependent onthe application.

2.2.1 Electroactive MOF thin films

Many metal organic frameworks are inherently insulating [30], and porous high-surfacearea materials with electric conductivity are generally rare [82]. Nevertheless, researchon overcoming this problem is a very hot topic due to the incredible possibilities ofconducting MOFs. Morozan et al. [21] point out in their review: “The crystalline but


porous nature of MOFs combined with their vast diversity of chemical functionalizationand metal-cation coordination presents a huge potential for catalysis and for electrocatalysisin particular. Such materials are an excellent platform to mimic the complex structure andhigh catalytic activity of active sites found in enzymes, based on abundant transition metals.Simultaneously, MOFs may show the stability of inorganic nano- or microparticles that aretoday the working principle of industrial electrocatalysts or active electrode materials. Incontrast to pore-free nano- or microparticles where only surface metal atoms in contact withthe electrolyte can be electrochemically active for surface reactions, the metal-ion utilizationin electrochemically active MOFs can theoretically reach 100% when the framework providessufficient electron and ionic conductivity.”

Halls et al. [30] describe that electrochemical activity of a MOF must be attributed toeither the metal center, the organic linker or a guest molecule. This very general approachcan further be divided, as roughly sketched by the review of Stavila et al. [83], as follows :

• Inherently conductive MOFsThe first report on porous conducting metal-organic frameworks was by Kobayashiet al. [82]. It was based on nickel bis-dithiolate complexes connected by planarCu(pyrazine)4 units forming a framework with one-dimensional channels and a con-ductivity of approximately 10−4 Scm−1. Narayan et al. showed a different approachby columnar π-stacks of TTFTB (TTFTB = tetrathiafulvalene tetrabenzoate) linkerscoordinated to Zn2+ via carboxylate groups [84] (see figure 2.4).

Figure 2.4: Nickel bis-dithilate conductive MOF by Kobayashi et al.(left)[82] and p-stacks ofTTFTB linkers in work by Narayan et al. (right) [84]


There are reports demonstrating Cu3(BTC)2 as electrocatalyst for CO2 reduction[85] and N,N’-bis(2-hydroxyethyl)dithiooxamidatocopper(II) for ethanol oxidation[86], both showing cyclic voltammograms of the MOF on glassy carbon electrodes.

• Electroactive linkerKung et al. have reported the post-synthetic metalation of the free-base porphyrinlinkers used for the growth of MOF-525 thin film [66]. The films show clearelectrochemical response due to the electroactivity of the porphyrin linkers.

Usov et al. demonstrate in-situ spectroelectrochemical measurements of the elec-troactive MOF Zn2(NDC)2(DPNI) (NDC = 2,7-naphthalene dicarboxylate, DPNI= N,N′-di(4-pyridyl)- 1,4,5,8-naphthalenetetracarboxydiimide) and report that thetwice-reduced dianion species is stable over the timeframe of the experiment [87].

In 2012, Moa et al. reported Cu-bipy-btc (bipy=2,2′-bipyridine, btc=1,3,5-tricarboxylate) as electrocatalyst for O2 reduction [88]. The MOF shows onequasi-reversible peak in the cyclic voltammogram at −0.1 V vs. Ag/Ag+ that isslightly increased in the presence of oxygen compared to nitrogen in the solution.

Another contribution for MOFs with electroactive linkers from the group of Prof.Hupp was presented by Kung et al. [66]. In this work, NU-901, a MOF based onZr-nodes and TBAPy linker (H4TBAPy = 1,3,6,8-tetrakis(p-benzoic acid)pyrene)was electrochemically analyzed and showed electrochromism due to a one-electronoxidation of the linker.

Ahrenholtz et al. solvothermally prepared a thin film of a Co-porphyrin based MOF(5,10,15,20-(4-carboxyphenyl)porphyrin]Co(III)) on FTO glass and showed that theredox-hopping of the porphyrin linker can be explained by a non-nernstian behaviour[89].

• Covalently incorporated electroactive moietiesHalls et al. [90] presented a post-synthetic modification of different Al- and Zn-MOFscontaining an open amine site in the linker with ferrocenecarboxylic anhydride. Theyshowed a reversible electrochemical response for the ferrocene incorporated into theMOF when prepared on a basal plane pyrolytic graphite working electrode. In areview by the same author [30], the inherent complexity of ion insertion and expulsionprocesses that need to occur hand in hand with electron transfer processes to keep


charge balance is pointed out. The ferrocene-modified MOF was electrochemicallyunstable in aqueous solution since the oxidation of ferrocene deeper in the porescaused a local charge imbalance. This was compensated by fast diffusion of protonswhich in the end lead to high alkalinity in the outer pores and irreversibly broke theframework by disintegration.

Figure 2.5: Comparison of cyclic voltammograms of MOFs with covalently bound ferrocenepresented in literature (see text for further details): (1) scan rate for (i) 10, (ii) 35, and (iii)100 mVs−1 in 0.1 M NBu4PF6 in dichloroethane, MOF powder was immobilized on a basal planepyrolytic graphite working electrode [90]; (2) potentials are referred to Ag/AgCl/3M KCl referenceelectrode, electrolyte not mentioned, powdered MOF was suspended in ethanolic solution of Nafion(5%) and deposited on a graphite electrode [91]; (3) measured in 0.05 M TBAPF6 in acetonitrileon a FTO electrode [92]

Meilikhov et al. presented a covalent modification of MIL-53 ([Al(OH)−(bdc)]n)with 1,1′-ferrocenediyl-dimethylsilane [91]. The reaction takes place at an un-saturated Al−H bond between two AlO6 octahedra and thereby represents afunctionalization at the inorganic part rather than the organic linker of theMOF. Very recently, Hod et al. have installed ferrocene carboxylate at theZr6(µ3−O)4(µ3−OH)4(OH)4(OH2)4 nodes of the NU-1000 framework that itselfalready contains 1,3,6,8- tetrakis(p−benzoate)pyrene as an electroactive linker [92].A ferrocene loading of one molecule per node was reached and the ferrocene unitswere electrochemically active as observed by CV (see figure 2.5). An interestingfeature of these films was their blocking behaviour to cation penetration through thefilm when the ferrocene was oxidized to ferrocenium. Furthermore the electroactivepyrene linker got redox-silent in that case as well, implying a shift in redox potentialof the MOF linker due to the large number of positively charged ferrocenium.


• Non-covalently incorporated electroactive moietiesDragasser et al. tried to introduce ferrocene from the vapor phase into a layer-by-layer deposited HKUST-1 (Cu3(BTC)2) film on a Au electrode. The conductivityof these films was 2 · 10−9 Scm−1 and no clear ferrocene signal was observed in thecyclic voltammogram.

Chang et al. [93] introduced ferrocene in a one-step solvothermal treatment ofIn(NO3)3 · 5H2O, 4,5- imidazoledicarboxylic acid and 10 mmol Fc. It is not de-scribed how the ferrocene is bound within the MOF, but it seems to be trappedwithin the pores since no adsorption takes place when putting the bare MOF into aferrocene solution. The modified glassy carbon electrodes had to be conditioned bycyclic voltammography in 0.5 molL−1 sulfuric acid for 20 cycles, since the electro-chemical behaviour was unstable in the beginning. After this treatment the cyclicvoltammogram show a clear and stable peak for ferrocene.

• Conductive guest moleculesTalin et al. received a lot of attention with their work based on introduc-ing 7,7,8,8- tetracyanoquinododimethane (TCNQ) into the pores of HKUST-1(Cu3(BTC)2) [94]. The adsorption lead to a conductivity increase of 8 ordersof magnitude up to 7 Sm−1. Zhang et al. recently published [95] their work basedon the incorporation of macroporous carbon into a copper based MOF (Co2(4-ptz)2-(bpp)(N3)2, 4-ptz= 5-(4-pyridyl)tetrazole, bpp = 1,3-bi(4-pyridyl) propane) andelectrocatalysis for the oxidation of hydrazine and the reduction of nitrobenzenewas demonstrated. Lu et al. reported the electrochemical synthesis of polyanilinewithin the pores of HKUST-1 [70]. While the scope of this work was to synthesizemicroporous polyaniline with the help of a MOF-template (the MOF was dissolvedafter the polyaniline synthesis), this method might also be used to increase con-ductivity of the MOF itself. The polyaniline showed a conductivity of 0.125 Scm−1

after doping with I2.

To conclude, MOF structures are besides two exceptions non-conducting (or very-large bandgap semiconductors). It was shown in the literature that an incorporation ofelectroactive moieties as well as electroactive linkers or electroactive metal nodes is possible.But one has to point out that an electroactive species within a MOF is fundamentally


different to a conductive MOF. Within this thesis, the main focus lies in the reduction ofa molecular catalyst within a MOF. The reduction potential of an electroactive specieswould need to be more negative than the catalyst to gain a benefit from the electroactivityof the MOF. In our work on FTO substrates [39], the conduction comes directly from theincorporated catalysts which are present in sufficient amounts. Therefore no driving forceis needed to reduce the catalyst within the MOF and no energy is lost in this process.

2.3 Silicon photoelectrodes

Silicon is the second most abundant element in the earth′s crust and low-cost, highpurity mono-crystalline wafer material is readily available due to its use in semiconductorindustry. It is furthermore used intensively in photovoltaics due to its narrow bandgap of1.1 eV which is fairly well matched with the solar spectrum[9]. Silicon can be p-dopedwith elements from the third group of the periodic table and n-doped with elementsfrom the fifth group. In a p-type semiconductor, holes are the majority carriers, whileelectrons are so called minority carriers and only present in very small concentrations(vice versa in n-type). Since electrons are unavailable in the dark in p-type silicon,optical excitation is required to generate an excited electron in the conduction band, thatcan effect a reduction process (see fig 2.7). Thus, p-type electrodes are photocathodes,but can perform hole-mediated oxidation processes as anodes in darkness [96]. Thesephotocathodes typically show a photoeffect of ca. 0.5 V under illumination due to theenergy gained by the absorption of a photon. This means that the reduction potential ofa redox species in solution is shifted to more positive values compared that measured at astandard platinum working electrode [96, 97].

The research of silicon photoelectrochemistry with regard to water splitting is mostlyfocusing on two different topics. The morphology and nanostructure of silicon is optimisedwith regard to light absorption and charge carrier extraction. The second issue is thedeposition of a heterogeneous material to optimize the surface energies and kinetics withregard to water splitting while suppressing surface oxidation of the silicon [9].

2.3.1 Silicon-liquid interface

The band edges of the valence band EVB and the conduction band ECB at the electrode-liquid interface are generally fixed. In the bulk of the semiconductor the band levels

2.3. SILICON PHOTOELECTRODES 25

can be shifted by applying an external potential. The Fermi level E f represents thethermodynamic potential of the electrons in the silicon. If the Fermi level is more positivein the semiconductor than the redox couple in solution, electrons will be transferredto the solution, leaving behind accumulated, immobile positive charges at the interface.This so called space-charge layer leads to an electrical field that directly affects the localthermodynamic potential of the electrons. The Fermi level of the semiconductor andthe thermodynamic potential of the redox couple in solution in equilibrium are therebyalways the same under equilibrium conditions. The space-charge layer leads to a bandbending as shown in figure 2.6: the majority holes are drifted away from the interface dueto the band bending, while excited minority electrons are drifted towards the interfaceand can potentially reduce the protons in solution. However, the ECB of silicon is toohigh, resulting in only minor band bending and thus leading to a low driving force forproton reduction [9].

Figure 2.6: p-type silicon band edge positions at equilibrium with aqueous solution with theproton reduction redox level as reference. ECB = Conduction band level, EVB = Valence bandlevel, Ef = Fermi level, ϕB = band bending [9]

Besides this, a more important issue is hindering the use of silicon as a photocathodefor proton reduction - bare silicon is unstable in aqueous solution, getting readily oxidizedand forming an electron-blocking passivation layer [98].

The reaction kinetics of a Si photoelectrode depends on the charge transfer andrecombination rates, which are influenced by the illumination intensity and the appliedbias. The surface recombination velocity is exceptionally low for HF etched silicon [99],but as mentioned before strongly influenced by a possible surface oxidation. Royea etal. [100] showed that storing freshly etched silicon samples in air leads to a fast decreasein carrier lifetime from 491 µs for samples immersed in H2SO4 and 14 µs for samples


Figure 2.7: p- and n-type silicon-liquid interface under illumination showing the different bendbanding and thereby changed reactivity towards oxidation/reduction [97]

exposed to air for 30 min. High-resolution XPS measurements by Webb et al. [101] showthat no Si−O formation can be observed after 30 minutes. A typical detection limit ofsilicon oxide formation is around 0.05 monolayers [102]. This shows the strong influence ofsub-monolayer oxide layer formation on the electronic properties of silicon. It is thereforecommon practice to etch a silicon wafer with diluted hydrofluoric acid (HF) or a 40 %aqueous solution of NH4F to remove the oxide layer immediately before using them inelectrochemical applications. This procedure yiels hydrogen terminated silicon surfacesthat are stable in air up to several hours depending on the humidity and can even bequickly rinsed with water without surface oxidation [103, 104]. It should be noted thatthe issues addressed above only account for low and medium doped silicon. Highly doped,so called degenerate, silicon behaves similar to a metal, it does not depend on illuminationto perform reductions (p-type) or oxidations (n-type). These electrodes do not show aphotoeffect and electrons can tunnel through a very thin oxide layer (below 4 nm [9]) [105].One should also consider that the current that is measured for a silicon-liquid junctionscales linearly with the illumination intensity, which shows that the reaction is directlydriven by light [97].


2.3.2 Silicon surface modification

The aspect of silicon surface modification was not expected to play a very crucial rolewithin this thesis, but as explained above, the passivation of silicon by its oxide leads totrapped states, that prevent electron transfer over the silicon-electrolyte interface. Alreadyin the beginning of the 1980′s several techniques were suggested to improve the stabilityof silicon, including derivatization of the electrode surface [98].

This section will only deal with the covalent modification of hydrogen-terminatedsilicon, since the removal of the oxide layer is crucial for any kind of electrochemicalapplication. A SiO2 modification with silanes might promote MOF thin film growth asdescribed in section 2.2, but it is irrelevant for photoelectrochemical applications. Asmentioned earlier, treatment of Si with diluted HF or aqueous NH4F solution leads toa removal of SiO2 and well controlled hydrogen terminated Si. One has to differentiatebetween the two most common orientations Si(100), which is extensively used in thesemiconductor industry, and Si(111) (see figure 2.8). The covalent grafting of organicmonolayers on hydrogen terminated silicon is preferentially performed on Si(111) sinceit is atomically more flat and the mono-hydride can typically be replaced with higheryields [106]. There are several methods to covalently modify the silicon surface andseveral reviews have been published in the last years to summarize this large researchfield [107–114]. In general, reactions of 1-alkynes can typically also be carried out withthe respective 1-alkene, but Scheres et al. [115] showed that the packing density andordering of the monolayers is higher for alkynes. The maximum surface coverage ofalkyne-derived monolayers bound to Si(111) is 60-65 %, while it is only 50-55 % foralkene-derived monolayers. Furthermore Puniredd et al. showed that the nature of theSi−C−−C linkage is inhibiting surface oxidation in contrast to the case of Si−C−C andSi−C−−−C[116]. Surface modifications can also be performed with alcohols and aldehydesto presumably form Si−O bonds. These reactions will not be reviewed here since there isno direct evidence for the Si−O bond formation and there is only few examples in theliterature [107].

• Thermal hydrosilylation with 1-alkynesLinford and Chidsey were the first group that reported a covalent functionalizationof a silicon surface [117]. They used the thermal decomposition of a diacyl peroxideto produce alkyl radicals which will propagate a radical chain reaction on the silicon


Figure 2.8: Hydrogen terminated Si(100) and Si(111) [106]

surface. After further investigation by the groups of Chidsey and Sudhölter [118],it was found that homolytic Si−H cleavage at elevated temperature is the startingpoint for the radical chain mechanism.

Sieval et al. showed for the first time that using solutions containing only 2.5 %1-alkene in mesitylene can also be used instead of the neat alkene [119]. Othersolvents than mesitylene did not perform as well with regard to the ordering of themonolayer.

A very exciting example was presented by Ciampi et al. in 2009 [120]. They function-alized Si(100) by a thermal reaction with 1,8-nonadiyne and reported that this distalalkyne shows superior stability vs. 1-heptyne. Furthermore they functionalizedthe acetylene-terminated moiety with azidomethylferrocene and presented nicelyreversible cyclic voltammograms with up to 108 cycles stability.

• Photochemical hydrosilylation with 1-alkynesLangner et al. reported functionalization of Si(111) and Si(100) with UV-light [121].Sun et al. showed in 2005 that the hydrosilylation reaction with 1-alkenes and1-alkynes is possible under visible light irradiation and at room temperature [122].They propose an electron-hole pair mechanism due to the excitation of the siliconunder irradiation (see figure 2.9). It is also shown that the reaction rate is dopingdependent in the order highly doped n-type > lowly doped n-type > lowly dopedp-type > highly doped p-type.

• Catalyzed hydrosilylation with 1-alkynesLangner et al. describe in their publication that the use of Speier′s catalyst(H2[PtCl6]) leads to a preferred reaction of the solvent 2-propanol with the surface in-stead of the alkene-compound [121]. The use of Lewis-acid catalysis is an alternativeto noble metal catalysts without the mentioned side reactions and Pt contamination


Figure 2.9: Proposed radical chain mechanism for the functionalization under thermal and UVconditions and the electron/hole pair mechanism for visible light induced hydrosilylation [122]

on the surface. While the Lewis-catalyzed reaction is possible at room temperaturewith a porous-silicon substrate, it requires 100◦C and longer reaction times whenperformed on a Si(111) substrate [112].

Webb et al. showed that surfaces modified in presence of 1-hexene and 1-octenewith EtAlCl2 exhibit high charge recombination and oxidize in air [101] whileBoukherroub et al. describe that their functionalized silicon (1-decene) is stable forweeks [123].

• Two-step halogenation alkylationBansal et al. showed first in 1996 a two-step strategy applying chlorination withPCl5 (with benzoyl peroxide as radical initiator) and a subsequent alkylation with analkyl Grignard or alkyl lithium reagent [102]. The highest stability achieved by thismethod was a coverage of less than half monolayer oxide after 2 days exposure to air.Terry et al. show a similar method, but use UV expose of chlorine and bromine gasto halogenate the silicon [124]. Boukherroub et al. reports that the direct reactionwith a Grignard reagent also leads to alkylation of the surface [123]. Stability forseveral weeks is claimed, but no spectroscopic data is provided. It should be noted,that methyl termination of Si is the only method to get 100 % surface coverage,but it is limited in a further chemical functionalization [116]. To overcome thatproblem, Hurley et al. [125] modified this procedure by using Na-acetylides insteadof the Grignard reagent after chlorination of the surface to introduce an acetylenefunctionality. The films showed excellent electronic behaviour and high stability (nooxidation detectable after 20 days).


• Electrochemical grafting of a diazonium-saltOne of the main advantages of the electrochemical grafting method is its compatibilitytowards a number of substituents like Br, NO2, CN, NH2 , COOH and alkyl [107].The electrochemical functionalization of Si(111) with 4-NO2-benzenediazoniumand 4-Bromobenzenediazonium by cyclovoltammetric scanning in a HF and H2SO4

containing electrolyte was shown by de Villeneuve [126]. The substrates show no signof surface oxide by XPS measurements after the electrochemical synthesis. Similarwork performed by de Villeneuve, Allongue and co-workers [127] show oxidationafter 24 hours. It should be noted that scanning tunnelling microscopy showed verywell ordered monolayers with a few pit-holes that seem to be the starting point forsurface oxidation. Fabre states that the electrochemical grafting with a diazoniumsalt fails to produce monomolecular films in a reproducible and controllable manner[106].

2.4 Analytical Methods

Metal-organic frameworks are a complex material class that needs to be analyzed witha wide variety of methods. Within this section, all applied analytical methods will bediscussed shortly.

2.4.1 Cyclic voltammetry (CV)

Cyclic Voltammetry (CV) is one of the most powerful and most widely applied electro-analytical techniques, capable of determining extremely accurate thermodynamic proper-ties as well as kinetic data [128]. The setup typically contains a working-, counter- andreference electrode that are all immersed into one liquid. A potentiostat is used to controlthe potential difference between the working- and reference electrode (typically a Ag rod ina 0.01 M Ag+ solution). The reference electrode is used to control the potential differenceindependent of concentration changes in the solution due to the redox reactions. A highsurface area counter electrode is used to apply current and thereby achieve the intendedpotential difference between working- and reference electrode. In cyclic voltammetry thispotential difference is sweeped, leading to a current depending on the electrochemicalproperties of the electrode/solution interface. Theoretically the current flowing through asolution containing a redox active species is determined by the overpotential (η = E−Eeq),

2.4. ANALYTICAL METHODS 31

the exchange current density j0, the charge transfer coefficient α (indices a for anodicand c for cathodic) as well as some fundamental constants, according to Butler-Volmerequation (see equation 2.1). It is either limited by mass-transport of fresh reactant to theeletrode surface, resistive losses or by electron transfer kinetics.

j = j0 · exp(αanFηRT

− αcnFη

RT) (2.1)

2.4.2 Scanning Electron Microscopy (SEM)

In SEM, a highly focussed electron beam is scanned over a sample and various signalsdue to the interaction of the incident electrons with the sample surface can be collected.Due to the shorter wavelength of electrons compared to photons, it is possible to reacha drastically higher resolution in SEM, far beyond the Abbe diffraction limit of visiblelight. The resolution limit of a modern SEM is in the order of 1− 2 nm. There are threedifferent signals that are commonly analyzed in a SEM: backscattered electrons, secondaryelectrons and emitted X-rays. The amount of backscattered electrons (BSE) is directlyproportional to the atomic number of the element they are scattered from. This can e.g.be used to analyze different elemental compositions in a sample due to their differentbrightness in the BSE picture. More commonly used are the secondary electrons (SE)created by inelastic scattering of the incoming electrons with the atomic electrons inthe sample. They lead to a topographical contrast, because the efficiency of generatingsecondary electrons strongly depends on the incident angle [129]. The incident electroncan also eject core-level electrons from sample atoms. Outer-shell electrons will relax tothe core level and emit excess energy in the form of an X-ray (or an Auger electron, butthis case will not be discussed in this short introduction). The X-ray photon energy ischaracteristic for the element it was emitted from and it thereby allows elemental analysisof the sample. In standard commercial SEM systems, the X-ray signal is recorded byan energy-dispersive detector system (EDS). The incident electron beam is scatteredconsiderably within the sample until it has lost so much energy that it is not able to exciteX-ray emission. An increased acceleration voltage leads to a larger excitation volumeand thereby reduces the spatial resolution of the measurement. This was calculated forUiO-66 by Monte Carlo simulations performed with the CASINO software package and ispresented in figure 2.10.


Figure 2.10: Monte-Carlo simulations of electron trajectories within a 2000 nm thick UiO-66bulk sample (density = 1.443 g/mL) on a Si substrate performed with the software CASINOversion 2.48 [130]. The red paths show back scattered electrons that are re-emitted from thesample. All simulations were performed with 20000 electrons, a beam radius of 5 nm and allother software settings on standard. The accelerating voltage was stepwise increased from 5 keV(top left) to 10 keV (top right), 15 keV (bottom left) and 20 keV (bottom right)

2.4.3 X-Ray Photoelectron Spectroscopy (XPS)

A X-ray beam of precise energy (e.g. Kα line of Al) is focussed on the sample. In theopposite way as in the SEM-EDS, a core-level electron is ejected out of the sample dueto interaction with the incoming X-ray beam. The energy of the ejected electron can bedetermined by a hemispherical detector system and is element characteristic. The entireprocess is described by Einstein′s equation on the photoelectric effect:

Ebinding = Ephoton − Ekinetic (2.2)

The photon energy is well known and the kinetic energy of the electron can be measuredto such a high accuracy that influences on the local bonding environment can be analyzed.


XPS is furthermore a very surface sensitive technique, due to the low free path length ofthe ejected electrons in the solid. The depth resolution is typically around 10 nm [131].As soon as the electrons come from deeper inside the sample they will loose energy due toscattering and will thereby be lost within the spectral background.

2.4.4 Powder X-Ray Diffraction (PXRD)

Singe crystal X-ray diffraction is the most important analytical tool to analyze molecularstructures and many important scientific advances in the last century emanated from thistechnique [132]. Nevertheless, not every material can be prepared as a single crystal inthe appropriate size and quality and only powder diffraction can be used in this case.Large progress has been made in the field of powder X-ray diffraction (PXRD) and itis now routinely possible to determine structural properties of the sample without theneed for a single crystal. One main application of PXRD is nevertheless its applicationfor ’fingerprinting’ a solid and thereby enabling a quick qualitative characterization [133].Besides that, it is commonly used for the size determination of sub-micron particles byScherrer′s equation. Another application is the analysis of preferentially grown crystals ona substrate surface. This can be determined by the signal increase of certain diffractionpeaks compared to the other peaks. [133]

2.4.5 Infrared (IR) Spectroscopy

Infrared (IR) spectroscopy is an absorption technique relying on the interaction of polarizedchemical bonds with incident infrared radiation. The low energy infrared photon is excitingvibrational levels within the molecule. It is commonly used in chemistry for qualitativeas well as quantitative analysis of organic, inorganic or biological materials independentof their aggregate state. Nowadays Fourier-Transform infrared (FTIR) spectrometer arestate of the art, since it allows the measurement without a monochromator and therebydecreases the measurement time and increases signal to noise ratio. Within the field ofmetal-organic chemistry, the application of FTIR spectroscopy is very common to analysethe carbonyl stretching frequency of a complex, since it represents a direct probing methodfor electron density on the metal center. The CO ligand has empty π∗ orbitals that areinteracting with the electron density of the metal center by π − π∗ back bonding. The


increase of electron density in the antibonding orbital of the CO ligand, leads to a shift ofthe carbonyl stretching vibration to lower wavenumbers (lower energy).

It is also possible to measure monolayers on silicon substrates as shown for exampleby Webb et al. [134] or Salingue and Hess [135].

2.4.6 Ion-Beam Analysis (IBA)

Ion-Beam Analysis (IBA) is a family of different techniques which use energetic ions(several ten keV to several ten MeV) to probe the elemental composition and depth profileof solid materials. The underlying physics is well understood for several decades, but thetechniques are typically not used routinely due to the need for a particle accelerator. TheIBA-methods have in common that they are able to provide quantitative informationwithout matrix effects on a nano-meter scale which is hardly available through othertechniques. The three methods applied in this thesis are:

• Rutherford Backscattering Spectrometry (RBS)

• Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA)

• Time-of-Flight Medium Energy Ion Scattering (TOF-MEIS)

RBS utilizes high energy He or H ions in the energy range of 1-3.4 MeV, while MEISis using the same ions but in the energy range from 50-400 keV [129]. The principlefor both techniques is similar and based on Coulomb interaction of charged particles:the accelerated ions are losing energy along their way in the sample due to scatteringfrom electrons (inelastic energy loss) which leads to insignificant deflection from straighttrajectories. If the particles hit an atomic nucleus, they can be elastically scattered bylarger angles and may even rebound out of the sample. The energy lost in an elasticscattering event with the nucleus depends on the mass of the scattered ion and targetnucleus and is therefore typically element specific. Depth perception can be obtained fromthe total observed energy loss due to the inelastic energy loss to the electronic system onthe ion path in and out of the sample. In TOF-MEIS information on sample compositionis obtained in a similar manner. Due to the lower ion energy and most of all, the largedetector solid angle, TOF-MEIS can be used with much lower energy deposition in thesample and is thus also employed for more sensitive samples like organic matter. Thedifferent primary energies for RBS and TOF-MEIS also lead to clearly different depth


scales being accessible: RBS probes up to a few µm; TOF-MEIS with its higher resolutioncan probe approximately the first 50-100nm. TOF-ERDA uses heavier incident ions likechlorine or bromine with energies in the range of 20-60 MeV to eject light and mediumheavy elements from the sample. These recoiled nuclei are separated in mass with a timeof flight detector leading to element specific analysis. The energy of the recoiled ions is -besides their mass - also depending on its depth within the sample. Similar to RBS andMEIS the ions loose energy on their way out of the sample by inelastic electron scattering.For a more complete description of TOF-ERDA see reference [136].

2.4.7 Time-of-flight secondary-ion mass spectrometry

The application of time-of-flight secondary-ion mass spectrometry (TOF-SIMS) lies inthe analysis and depth profiling of solid materials with a nanometer resolution and sub-ppm detection limit. The impact of ions (typically Ga or Ar) with energies between1− 20 keV leads to the ejection of neutral and charged species from the surface [129].The charged secondary ions are accelerated into a time-of-flight mass spectrometer andanalyzed according to their mass to charge (m

z) ratio. To enhance the sputter rate a

second ion gun with higher acceleration voltage can be applied. The two ion guns (analysisbeam, sputter beam) are then used alternating to create a depth profile of the sample.An additional electron-flood-gun might be used to decrease charging effects in the sample.Absolute quantification is typically difficult due to matrix effects like varying sputteringprobability for elements in different chemical environment. The lateral resolution of themeasurement is typically 50− 2000 nm.

2.4.8 Dynamic light scattering (DLS)

Dynamic light scattering (DLS) is a technique to measure the size of particles in solutionbased on the time dependent analysis of Rayleigh light scattering of a monochromatic lasersource through the solution. The particle size can be in the range of ca. 5− 10000 nm.The particles are randomly moving in and out of the probed laser spot due to diffusionand lead to a fluctuating scattering signal. The diffusion of the particles is size-dependentand after analysis of the autocorrelation function of the scattering signal, the particle sizecan be determined from the Stokes-Einstein relation. Besides other factors, the refractive


index plays a crucial role for this mathematical treatment. Practical issues regarding theexperiment are typically dust and particle agglomeration.

2.4.9 Contact angle measurement

The measurement of the contact angle is a quick method to determine the hydrophobicityof a surface. The surface of the substrate has to be smooth and clean, due to the extremesensitivity towards roughness and contamination. In general values are only reproduciblein the range of ±2◦ under laboratory conditions [137]. Besides its use in the analysis ofthe substrate-liquid interaction, it can also be used to determine surface energetics ofsolids. The surface of the solid has to be smooth, rigid and homogeneous to be able toapply Young′s law, otherwise more elaborate models have to be used [137].

2.4.10 Inductively coupled plasma atomic emission spec-troscopy (ICP-AES)

A very versatile wet-chemical method for elemental analysis down to detection limitsof 0.1− 100 ngmL−1 for most elements is ICP-AES. It is based on the vaporization andthermal excitation of an aerosol solution containing the sample elements in an inductivelycoupled plasma at approximately 10000 K. It is possible to simultaneously determine upto 70 elements. Furthermore it is less susceptible to matrix interferences than comparablemethods, has a high stability leading to excellent accuracy and precision and a wide lineardynamic range of four to six orders of magnitude [138]. Within the research area of MOFsit is typically used to determine catalyst loadings by digesting (dissolving) the MOF underacidic conditions.

Chapter 3

Experimental

All experimental procedures applied in this thesis are summarized in this chapter. Thesynthesis of [FeFe](bdt)(CO)6, [FeFe](mcbdt)(CO)6 and [FeFe](dcbdt)(CO)6 were per-formed by S. Pullen and have been reported elsewhere [38, 139]. The anchoring group4′-trifluoromethylphenyl acetylene was synthesized by Dr. Ulrike Fluch in the sameresearch group and only deprotected with sodium hydroxide before use. It was extractedin pentane and the solvent was removed by gently purging air over the solution due tothe volatility of the target compound.

3.1 Chemicals and Purification

All chemicals and solvents were purchased from commercial suppliers and used withoutfurther purifcation if not listed below.

• ZrCl4 (> 99.99 %)• Methanol (> 99.9 %)• Sulfuric Acid (95-97 %)• Hydrogen Peroxide (30 %, stabilized for synthesis)• Hydrofluoric Acid (48 wt%, 99.99 % trace metal basis)• Ammonium Fluoride (98 %)• Hydrochloric Acid (37 %)• Sodium Hydroxide (98 %)• Tetrahydrofuran (> 99.8 %)• 4-Bromobenzoic Acid (> 98 %)• Etynyltrimethylsilan (98 %)• Ethanol (99.5 %)• Thionyl chloride (97 %)• Triethylamine (> 99 %)• PdCl2(PPh3)2 (> 98 %)

38 CHAPTER 3. EXPERIMENTAL

• Copper iodide (98 %)

Benzoic acid (>99.5 % starting material) was reprecipitated by dissolving in a sodiumhydroxide solution and addition of hydrochloric acid until pH 1 was reached. The solidwas filtered, washed and stirred for several hours in copious amounts of water to removeexcess acid. The pH of the washing liquid was ca. 4, which fits well to the reported pKA

value of benzoic acid (4.2) [140]. The reprecipitated benzoic acid was dried over P2O5

under vacuum.Benzene-1,4-dicarboxylic acid (=terephthalic acid, 98 % starting material) was subli-

mated at 3− 6 · 10−1 mbar and 250 ◦C in a Kugelrohr apparatus. It was dried over P2O5

under vacuum.For experiments performed in the glovebox, N,N-Dimethylformamide (DMF, > 99 %)

was dried overnight with 4 Å molsieves and distilled at 48− 50 ◦C under reduced pressure(22− 28 mbar) into Schlenk flasks with 4 Å molsieves. Afterwards it was de-oxygenatedwith 5 freeze-pump-thaw cycles.

3.2 Synthesis of para-ethynylbenzoic acid

The synthesis route is summarized in figure 3.1. 4-Bromobenzoic acid (2 g, 9.95 mmol)was dissolved in dichloromethane (7ml) at 0 ◦C. Thionylchloride (1.45 ml, 19.9 mmol)was added and the solution was stirred for 15 minutes before methanol was droppedinto it slowly. The solution was stirred for 3 days at room temperature, quenched withwater and neutralized with saturated NH4CO3 solution. The product was extracted withdichloromethane, dried over MgSO4 and the solvent was removed under vacuum. Methyl4-bromobenzoate was obtained with a yield of 95 % as a grey beige solid.Methyl 4-bromobenzoate (1 g, 4.65 mmol) was dissolved in triethylamine (13 ml, 93.3mmol) in a microwave vial and degassed by Ar-bubbling for 20 minutes. Cupper(I)-iodide(35.4 mg, 0.19 mmol) and PdCl2(PPh3)2 (0.130 g, 0.19 mmol) were added and the solutionwas degassed for further 5 minutes before addition of etynyltrimethylsilan (1.32 ml, 9.30mmol). The microwave assisted reaction was performed for 30 minutes at 70 ◦C. Thecrude product was diluted with dichloromethane and extracted with saturated NH4Cluntil the aqueous phase was colorless. The organic phase was dried over MgSO4 andpurified by flash chromatography on silica using a Ethylacetate:Pentane 1:30 as eluent.The product was obtained as a yellow solid with a yield of 94 %. [1H NMR (400 MHz,

3.3. SUBSTRATE CLEANING 39

Methanol-D3) δ 8.00 (d, 2H), 7.56 (d, 2H), 3.72 (s, 1H).]The deprotection was performed with 5 equivalents NaOH in THF:MeOH:H2O 3:2:1overnight at room temperature.

Figure 3.1: Synthesis route of para-ethynylbenzoic acid

3.3 Substrate cleaning

The silicon samples were cleaned in Piranha acid (3:1 vol. [conc. H2SO4):(30 % H2O2])for at least 30 minutes at 80 ◦C. Afterwards, the samples were rinsed with deionized waterand dried under a stream of air. These samples will be referred to as ’oxidized Si’. If notmentioned otherwise, these oxidized Si samples were used for the growth of UiO-66 thinfilms. Samples that did not undergo this cleaning procedure will be referred to as ’nativeoxide’. The silicon oxide layer was removed by etching with a buffered oxide etch solution(6:1 volume ratio of 40% NH4F in water and 48% HF) for at least 2 minutes. The sampleswere rinsed with deionized water and dried under a stream of air.

The FTO pieces were sonicated for 10 minutes in Alconox cleaning solution, ethanoland acetone sequentially. After the Alconox cleaning the samples were rinsed with water,and after the ethanol cleaning the samples were rinsed with acetone respectively. Finallythe acetone was removed by a stream of air.

3.4 Silicon surface functionalization

The silicon shreds were cleaned, as described above, by Piranha solution with a sequentialoxide stripping. Different methods were tested for the functionalization step, as listedbelow. One of the main obstacles was the poor solubility of para-ethynylbenzoic acid.


First experiments under illumination were performed in toluene, later in DCM/DMFmixtures and dry acetonitrile. None of the solvents was able to dissolve the compound ina target-concentration of 10 mM. Therefore, DMF was chosen as a reaction medium dueto the good solubility.

Functionalizations with DMF as solvent were entirely carried out in the glove boxbesides the sample cleaning in Piranha solution as described above. It was reported in theliterature that the monolayer formation is significantly improved if the microelectrodesmaintained under inert conditions at all time [141]. The wafer pieces were treated inbuffered oxide etch in the glove box, rinsed with water and THF and dried under vacuumin the antechamber. Three different procedures were tested:

• Reaction with EtAlCl2 Lewis-acid catalyst

• Reaction with white light illumination

• Reaction without catalyst or illumination

The Lewis-acid catalyst EtAlCl2 (220 µL, 400 µmol) was added to 4 ml of a 10 mMpara-ethynylbenzoic acid solution in DMF. The silicon samples were added, the vials wereclosed and kept inside the glovebox for at least 12 hours. Samples functionalized withwhite light (5cm distance to a 17 W, 5000 K lamp) were illuminated overnight outside theglovebox. This lead to a color change from orange to transparent for para-ethynylbenzoicacid solutions. The concentrations and procedure for samples without Lewis-acid catalystand for 4′-trifluoromethylphenyl acetylene were the same. There was no color changeobservable for these samples under white light illumination.

3.5 UiO-66 thin film synthesis

The original protocol of our recent publication [39] to solvothermally synthesize UiO-66thin films on FTO was the starting point for several tests and will be explained here:

The substrate was placed into a vial containing 1 mM terephthalic acid in DMF for atleast 12 hours prior to the MOF thin film synthesis to form a self-assembled monolayer(SAM). No difference was observed with SAMs that were stored for longer times (upto weeks). Benzoic acid (1.35 g, 11 mmol) was dissolved in 8 ml DMF before ZrCl4(35 mg, 0.15 mmol) was added and dissolved by sonication (ca. 15 min). The mixturewas kept at 80 ◦C in a sand bath in an oven for 2 hours. After cooling down to room

3.5. UIO-66 THIN FILM SYNTHESIS 41

temperature(20-30 minutes), terephthalic acid (25 mg, 0.15 mmol for ca. 20 nm thickfilms; 16 mg, 0.10 mmol for ca. 2− 5 nm thin films) was dissolved in the solution bysonication. The pretreated FTO substrate was placed at the bottom of the vial with theconductive site facing upwards. The vial was allowed to react solvothermally at 120 ◦Cin a sand bath in an oven for 24 hours. After cooling down to room temperature(20-30minutes), the film was rinsed with DMF and incubated in DMF for 24 hours. Then thesolution was changed to methanol for another 24 hours. This synthesis method will bereferred to as old-method. After incubating the sample in methanol, the film is ready tobe functionalized via post-synthetic ligand exchange. Due to the higher abundance ofthe mono-carboxy version of [FeFe](dcbdt)(CO)6 all experiments within this thesis wereperformed with this complex. The catalyst [FeFe](mcbdt)(CO)6 (41.3 mg, 0.1 mmol) wasdissolved in deoxygenated methanol (2 ml) by sonication for ca. 20 minutes. The insolubleresidue was removed by centrifugation. The UiO-66 thin film sample was placed insidethe vial with the film side upwards for 24-72 hours at room temperature. Afterwards thefilm was washed for three times with methanol, each time 24 hours.

Due to problems with reproducibility within a series and compared between differentseries, several improvements and changes were implemented in the procedure:

• Better control over water contentAs explained in chapter 2.1, the water content in the synthesis solution plays a crucialrole in the reaction rate of the cluster-formation. It is thereby highly important tocontrol the dryness of the starting materials. As mentioned in section 3.1, benzoicacid and terephthalic acid were purified and extensively dried. DMF was dried over4 Å molsieves. A certain amount of water is nevertheless necessary, therefore a fixedamount of water (typically 5 equivalents, 0.75 mmol) was added intentionally to thesolution prior to the solvothermal treatment.

• Use of stock solutionsThe separated weighing of benzoic acid, ZrCl4 and terephthalic acid is tedious anda source of variation between samples. A two step stock solution method wasdeveloped: Benzoic acid was dissolved in DMF, ZrCl4 was added to the solution andsonicated. It was transferred into a volumetric flask to give a concentration of 1.84M (benzoic acid) and 25 mM (ZrCl4) respectively. The solution was distributed intodifferent vials (6 ml each) depending on the number of samples in the series. After


the pre-treatment at 80 ◦C, 2 ml of a terephthalic stock solution (75 mM) was added.The final concentrations per sample are the same compared to the old-method.Nevertheless the volumes per sample were slightly larger with the old-method dueto a large volume increase when dissolving the starting materials.

• Orientation of the sample during solvothermal treatmentPlacing the sample at the bottom of the vial leads to an overlap of on-substrategrowth and precipitation of UiO-66 on the substrate. It was therefore suggested byour collaborator Wei Xia, to place the sample standing up straight into the solution.This is practically very difficult, therefore the substrate shreds were instead cutinto larger pieces (30 mm· 6 mm) and placed tilted into the vials (see figure 3.2).Samples prepared with this method will be referred to as ’upside-down’. When thesamples were lying on the bottom of the vial, it will be referred to as ’standard’.The top side in the upside-down method is very similar to the standard synthesisconditions with precipitating MOF particles deposited on the film, leading to higherroughness and thicker films. The silicon wafers used in this thesis were single-sidedpolished. This side was used as the bottom side in the upside-down method.

Figure 3.2: Scheme illustrating the upside-down synthesis method (description see text)

If not specifically mentioned, all films were synthesized with this improved protocol.Besides the improvements and changes mentioned above, several factors that may influencethe synthesis were at least partly analyzed. These include synthesis temperature, time,concentration of the modulator benzoic acid, water content, pressure (air/liquid ratio invial), pH, preforming time and temperature, oxygen content and the orientation of thesample in the substrate.

3.6. METHODS OF CHARACTERIZATION 43

Some tests were performed with electrophoretic deposition rather than solvother-mal synthesis: a few mg of dry UiO-66 were suspended in toluene (ca. 10 ml) by sonicationand placed in a three-neck round bottom flask. Two FTO electrodes were mounted inholders and placed at the left and right neck of the flask with the conductive site facingeach other. The central neck was sealed to minimize evaporation of toluene. A voltage of90 V was applied between the two electrodes for three hours and was kept while removingthe samples from the solution to prevent the loss of substrate adhesion.

3.6 Methods of Characterization

3.6.1 Cyclic voltammetry

A three or four electrode system, consisting of a glassy carbon working electrode and/ora silicon working electrode, a glassy carbon counter electrode and a Ag/Ag+ (10mMAgNO3) was used for the cyclic voltammetry experiments. The supporting electrolyte was0.1 M TBAPF6 in dry acetonitrile and was deoxygenated by Ar-bubbling. Measurementswith a silicon working electrode were conducted under illumination with a white lightsource. The light intensity was not determined due to a lack of equipment and practicalissues concerning the measurement.

The silicon photoelectrodes were contacted by scratching the top part of the sampleand immediately applying an In/Ga eutectic on it. A bare aluminum wire was fixed tothe wafer with conductive silver paste. To increase the mechanical stability, the silverpaste was air dried for ca. 20 minutes and applied 2-3 times. The entire sample, besides asmall ca. 3 mm hole, was covered with epoxy resin (Loktite Hysol 1C). The step-by-stepprocedure is illustrated in figure 3.3.

3.6.2 Contact Angle Measurement

Contact Angle measurements were performed at a dataphysics OCA 15EC goniometerat the solid state physics department. Distilled water was used to produce the dropletand supplied via a Hamilton syringe that was connected to a small motor dosing theliquid with 5 µLs−1. The droplet shape was fitted to an ellipse and the contact angle wascalculated with the help of the installed software package from dataphysics. The fittingand calculation was performed in less than 10 seconds after the droplet was applied. Thetime-dependent decrease of the drop size was not analyzed.


Figure 3.3: Step by step contacting procedure of the Si photoelectrodes

3.6.3 Ion Beam Analysis

The experiments were carried out in collaboration with Dr. Daniel Primetzhofer at theaccelerator system in Ångstrom laboratory. ERDA measurements were performed withBr79 at 32 MeV and with I127 as well as Cl37 nuclei at 36 MeV. The energy of the heliumions in RBS was 2 MeV and 80 keV in MEIS respectively.

3.6.4 XPS

The XPS measurements were performed on a Physical Electronics Quantum 2000 ScanningESCA microprobe system. The measurement ranges for each element were chosenautomatically. The resolution, pass energy and number of scans was adjusted to keep themeasurement time close to 60 minutes. Besides a surveillance scan, most attention waspaid to the fine scan of the carbon C1s peak to elucidate the characteristic COO– peak at288-290 eV.

3.6.5 SEM-EDX

The samples were sputtered with Au/Pd to increase the conductivity and reduce chargingeffects during the measurement. The sputter time was shorter for samples that wereanalyzed by EDX (typically 7-20 seconds), to minimize signals from Au/Pd in the spectrum.Pictures were usually taken at fixed magnifications of 1000, 5000 and 25000 to facilitatecomparison between different samples. Cross-sections were prepared by cutting the samplesinto halves at the glass cutting machine of Prof. Boschloo′s group. The cross-section

3.6. METHODS OF CHARACTERIZATION 45

samples on FTO were then sputtered standing straight up in the sputter chamber (notfor silicon samples).

Two different SEMs were used throughout the work: Zeiss Merlin and Zeiss 1550, bothequipped with an AZtex EDS system, a 80 mm2 Si-drift detector and a schottky fieldemission gun. The resolution is 1.4 nm for the Merlin and 3 nm for the 1550 at 1 kVacceleration voltage. The Zeiss Merlin is furthermore equipped with a charge compensationneedle which is applying nitrogen gas close to the sample surface. This mode was used forhighly charging samples and during EDX measurements.

The reliability of SEM-EDX measurements was tested on powder samples of UiO-66with an incorporated catalyst, prepared by Carlota Bozal Ginesta. The mathematical pro-cedure (ZAF-method) to calculate relative concentrations from EDX data is a complicatediterative calculation that assumes homogeneous and flat samples [142]. Since the MOF thinfilms presented in this work are very rough and consist of many non-conductive µm −sizedparticles, the accuracy and precision of the method was evaluated. The powder sampleswere immobilized on a conductive carbon stub and analyzed at different accelerationvoltages and sputtering conditions. In general a measurement area of ca. 4− 15 µm2

was selected and the measurement time per pixel was kept as short as possible (1 µs) tominimize sample charging.

3.6.6 Nuclear Magnetic Resonance (NMR)

Proton nuclear magnetic resonance spectra (1H NMR) were recorded on a Varian MercuryAS-400 NMR system with an oxford 400 MHz shielded narrow bore magnet. Chemicalshifts were quoted in parts per million (ppm) with reference to the solvent peak and thespectra were baseline and phase corrected.

3.6.7 Powder XRD

Powder X-ray diffraction measurements were conducted at a Siemens D5000 Th2Thparallel beam instrument optimized for thin film measurements. The X-ray intensity wasset to 40 kV and 40 mA. The incident angle was 0.5◦, the stepsize was 0.05◦ and thescanspeed was 0.5◦min−1. The sample holder was rotated with a speed of 60 rpm. Dataanalysis was performed with Bruker Diffrac.Eva Vers. 3.1. All diffractogramms were Kα1

and baseline corrected.


3.6.8 FTIR spectroscopy

Different spectrometers were used throughout the work, the spectra presented in the thesiswere recorded on a Thermo Electron Corporation Nicolet Avatar equipped with a 370DTGS detector. The resolution was 4 cm−1 and 64 scans were recorded. The wafer washeld within the spectrometer chamber with a self-built mask.

3.6.9 TOF-SIMS

The measurements were performed in corporation with Jens Jensen at Linköping University.Ion sputtering was performed with Ar and Cs and negative and positive analysis modewere tried respectively. An electron flood gun was used to reduce charging effects duringsputtering.

3.6.10 DLS

Dynamic light scattering was performed at a Malvern Zetasizer Nano in the lab of TomasEdvinsson at Ångstrom Laboratory. The measurement was conducted in a quartz cuvettewith the UiO-66 synthesis solution, still containing starting material like benzoic acidmodulator. There was no electrical field applied during the measurement. The parametersnecessary for the size calculation from the time-dependent light scattering were taken forpure DMF, not taking into account that the dissolved components may change the values.The temperature was 25 ◦C, the refractive index of DMF is 1.428 and the viscosity 0.8020cp.

3.6.11 ICP-AES

The analysis of films on FTO glass with incorporated [FeFe](dcbdt)(CO)6 with ICP-AESwas performed by Prof. Jean Petterson at BMC. The MOF thin film is completely washedoff the substrate by treatment with an HF solution (proven by SEM analysis of thesubstrates afterwards). Nevertheless it was not possible to dissolve the MOF completelyeven after boiling in concentrated HF or HNO3 solution overnight in teflon vessels.

Chapter 4

Results and Discussion

This chapter is divided according to the objectives described in the first chapter: UiO-66 thin film synthesis, depth profiling and catalyst quantification as well as siliconelectrochemistry and functionalization.


There are several different ways to prepare MOF thin films, outlined in section 2.2. In thismaster thesis, solvothermal synthesis on substrates modified by self-assembled monolayers(SAM) was mainly applied. Furthermore some experiments were conducted with therecently developed electrophoretic deposition method by Hod et al. [81].An optimal sample would consist of a flat and thin (1− 3 µm) monocrystalline UiO-66film that covers the entire substrate. This film would be expected to be thin enough to bepenetrated entirely by post synthetic ligand exchange. It would furthermore be optimalfor depth-profiling experiments since sputtering techniques could be applied to slowlyremove the film and probe it without having effects like preferential sputtering due tosample roughness. Another advantage would be that the quantification of catalyst loadingwith SEM-EDX is more accurate for flat homogeneous films (see section 4.2.3).

4.1.1 Electrophoretic deposition

Electrophoretic deposition decouples the MOF synthesis from the film formation. Thishas several advantages: the argumentation stated in chapter 1.4 that the post-syntheticexchange might be diffusion limited for thicker films is not valid for films prepared byelectrophoretic deposition since the PSE would be performed with powder samples insuspension before deposition. Furthermore the size of particles may be more easily tunedfor powder synthesis compared to direct on-substrate growth due to the decoupling ofon-substrate nucleation and crystal growth. This was the reason why the crystal size ofUiO-66 was analyzed after different times of solvothermal treatment by DLS (see figure 4.2).

48 CHAPTER 4. RESULTS AND DISCUSSION

The same experiment was used to extrapolate the time necessary to synthesize sub-100 nmUiO-66 particles. Due to problems with reproducibility, DLS measurement problems (mostlikely due to particle agglomeration) and the increased focus on solvothermal synthesison silicon, the experiments were stopped. Nevertheless it was shown that electrophoreticdeposition of UiO-66 works as reported by Hod et al. [81]. The surface coverage was veryirregular on the sample, showing areas with multi-layer coverage along with lower coverage(see figure 4.1). The adhesion of the film to the substrate was very weak, illustrated bythe fact that parts of the film dropped off by immersion into methanol.

Figure 4.1: SEM pictures of UiO-66 electrophoretically deposited on a FTO substrate illustratingthat the surface coverage is not complete all over the sample

Figure 4.2: Dynamic light scattering of UiO-66 synthesis solution after different times; synthesisprocedure according to ’old-method’ but only 4 ml synthesis solution per vial. Result was difficultto reproduce due to DLS measurement problems


4.1.2 Solvothermal synthesis

The influence of water on the UiO-66 thin-film synthesis is shown in figure 4.3. Theadditional water was added with an Eppendorf pipette just before the samples weretransferred into the oven. A decrease in crystallinity as well as a higher intergrowth ofthe UiO-66 crystallites can be observed. The particle size is decreasing with very highamounts of water.

Figure 4.3: Variation of water content in standard UiO-66 synthesis on FTO substrates: 0 eq.(top left), 3.7 eq. (top right), 14.7 eq. (bottom left), 59 eq. (bottom right) water compared toZrCl4 added before the solvothermal treatment.

The influence of relative concentrations of ZrCl4:benzoic acid:terephthalic acidhas different implications as illustrated by a series of samples grown on FTO with thestandard method, but with different terephthalic acid concentrations (see figure 4.4). Anincreased benzoic acid:terephthalic acid ratio leads to thinner films and smaller crystallites(see reference [39]). This can be explained by a decreased crystal growth rate related tothe so-called coordination modulation which was proposed by Umemura et al. [143] whichdescribes the coordination competition between the monocarboxylic acid (modulator)


and the dicarboxylic acid to the metal cluster. The crystallinity is decreased for lowermodulator (benzoic acid) concentrations. Films with a thickness exceeding 20 µm loose,at least partly, their adhesion to the substrate. The decreased crystallinity can be seen bythe rounded edges of the octahedral crystallites (see figure 4.4).

Figure 4.4: Comparison of crystal growth on UiO-66 films on FTO substrate prepared by thestandard method. The ratios of terephthalic acid:ZrCl4 were: 0.66 (top left), 0.8 (top right),1 (bottom left), 2 (bottom right). The amount of ZrCl4 was kept constant at 35 mg leading tobenzoic aicd:terephthalic acid ratios of 114.5, 92, 73.3 and 36.6 respectively; most of the filmin the bottom right broke off due to its high thickness but the decrease in crystallinity can beobserved by the rounded edges of the crystallites

The influence of synthesis time is illustrated in figure 4.5. A longer time undersolvothermal conditions leads to an increase in crystallite size and higher intergrowth ofthe crystallites.

A comparison between the growth of UiO-66 on the top side of a silicon wafer with thestandard procedure vs. the earlier described upside-down method is shown in figure4.6. The upside-down method leads to a lower surface coverage, but most of the crystalsshow a preferential orientation (middle). Furthermore there is no agglomeration due to


Figure 4.5: Two samples synthesized with the standard procedure on FTO substrates but withdifferent synthesis time: top (39h) and bottom (134h). The crystallites increase in size and growtogether in the lower parts of the film as can be seen inside the cracks (right)


random precipitation of particles from the solution as in the case for the standard method(left). A higher magnification picture is clearly showing the on-substrate growth with apreferred orientation of the crystal.

The preferential orientation of UiO-66 on a silicon substrate by the ’upside-down’method, shown in figure 4.6, was analyzed by powder x-ray diffraction (PXRD) (see figure4.7). The UiO-66 crystallites are growing along the [111] crystal orientation, all otherpeaks are too low in intensity to be measured, including the [200] peak at 8.4◦. Themain reflections in the diffractogram originate from the silicon substrate. A backgroundcorrection was not possible due to different peak intensities of all samples. This might beexplained by the lack of sample stage rotation during the measurement.

Figure 4.6: Comparison between standard method (left) and upside-down method (middle andright)

Samples prepared by the upside-down method showed repeatable different surfacecoverages. The formation of the self-assembled monolayer (SAM) was analyzed in detailby preparing samples with (1) no SAM pretreatment, (2) a higher terephthalic acidconcentration in the SAM solution and (3) removing the oxide layer prior to the SAMformation by HF treatment. The results are presented in figure 4.9. Films pretreatedwith Piranha acid show the highest surface coverage in this series, but it should be notedthat the coverages vary quite strongly on a sample and between different samples. Asimilar experiment was conducted on FTO substrates and is shown in figure 4.8. TheSAM formation is leading to an entire surface coverage on FTO. It is suspected thatthe coordination of terephthalic acid to surface hydroxyl-groups is promoting growth ofUiO-66 on the substrate. The MOF growth on hydrogen-terminated Si after treatment


Figure 4.7: Powder X-ray diffractogramm comparing samples prepared by the standard methodand the upside-down method on silicon substrates. The film with preferential growth along the[111] orientation by the upside-down method does not show the [200] reflection at 8.4◦

with HF is most likely related to surface oxidation during the solvothermal synthesisconditions.

Figure 4.8: SEM pictures of samples (FTO) prepared with the upside-down method: pretreat-ment overnight in 1 mM bdc solution in DMF (left) and no pretreatment (right)

A second solvothermal treatment was performed to increase the surface coverageof films prepared by the ’upside-down’ method. The crystallites formed at the secondtreatment are not coordinating to the MOF from the first synthesis, but rather fill upthe voids between the crystallites (see figure 4.10). Similar experiments were performedby Miyamoto et al. [45], leading to a nearly complete coverage of the entire sample afterthree treatments.


Figure 4.9: SEM pictures of samples (Si) prepared with the upside-down method but differentpretreatment: etched with Piranha acid, no SAM (top left); etched with degassed buffered oxideetch, no SAM (top right); pretreatment overnight in 1 mM bdc solution in DMF (bottom left)and in 30 mM bdc solution in DMF (bottom right)

Figure 4.10: Secondary growth of UiO-66 thin films prepared by the upside down method leadsto an increase of surface coverage - the smaller crystallites were formed at the second treatment

4.2. ANALYSIS OF FUNCTIONALIZED UIO-66 55

Further results in the field of UiO-66 solvothermal thin film preparation include theobservation that the oxygen content of the synthesis solution does not have an influenceon the solvothermal synthesis. Furthermore sonication of an UiO-66 film prepared by theold method on FTO does not lead to a full cleavage of the film from the substrate. It iscurrently under investigation if the time at which the water is added to the solvothermalsynthesis solution plays a crucial role in the preparation. The pretreatment of two hoursat 80 ◦C does not seem to be necessary for silicon samples grown with the upside-downmethod, but more studies are on their way.

4.2 Analysis of functionalized UiO-66

It is of utmost importance to quantify the catalyst loading within the UiO-66 frame-work to be able to understand the material′s catalytic and electrochemical properties.Different methods were applied to analyze the functionalized UiO-66 material in powderand thin film form. The accurate analysis of catalyst loading along the UiO-66 filmsturned out to be a highly challenging task. This is mainly due to limitations by most con-ceivable techniques. The ion sputtering guns used in XPS depth profiling and TOF-SIMSprofiling can hardly remove more than ca. 2 µm in a reasonable measurement time scale.SEM-EDX is able to reach a spatial resolution of less than 1 µm in the best case (seecalculations in section 2.4.2), but the use of a small acceleration voltage needed for such ahigh spatial resolution leads to overlaps of the spectral lines in the low energy range. Thehigh spatial resolution leads to very small excitation volumes, hence the sputtering layerto reduce charging effects may only be very thin to limit its effect on the measurement.This on the other hand urges to minimize the probe current to minimize sample chargingeffects, leading to low count rates and very long measurement times. XPS depth profilingand TOF-SIMS measurements did not lead to any useful data due to the before mentionedlimitations. The TOF-SIMS measurements in Linköping showed that the charging effectsduring the ion sputtering are so severe, that the m

zratios were not constant over time.

This makes a data evaluation extremely difficult. Furthermore particles were ejectedfrom the film into the measurement chamber due to electrostatic repulsion related to thecharging.


4.2.1 IR spectroscopy

The molecular integrity of the catalyst within the MOF was tested by IR spectroscopy.The measurement can not easily be used to quantify the concentration of catalyst withinthe MOF, but it is the only technique that probes the catalyst in its molecular staterather than elemental Fe or S used in this thesis (EXAFS: Extended X-ray AbsorptionFine Structure would be another option). An ATR-IR spectrum of a [FeFe](mcbdt)(CO)6functionalized UiO-66 thin film synthesized on silicon by the standard method is shown infigure 4.11. The carbonyl stretching vibrations of [FeFe](mcbdt)(CO)6 can be identifiedinside the MOF thin film and show that the structure of the molecular catalyst is stillintact after PSE.

Figure 4.11: ATR-IR spectra of a [FeFe](mcbdt)(CO)6 functionalized UiO-66 thin film synthe-sized by the standard method on silicon. Shown is the region between 2200 cm−1 and 1900 cm−1

4.2.2 ICP-AES analysis

As mentioned in the experimental section, there were issues with a complete dissolutionof the MOF thin films. The reason for that is unknown. The results from three samples


prepared in the same manner within the same batch show severe differences (see table4.1). The absolute concentration is irrelevant because it is depending on the size of theFTO substrate and the film thickness, but the very different Zr:Fe ratio is indicating apoor data quality. The measurements should be analyzed with caution due to the beforementioned incomplete dissolution of the MOF.

Table 4.1: ICP-AES results from three equal samples on FTO glass substrate

Sample Zr [µmol ] S [µmol ] Fe[µmol ] ratio S/Fe ratio Zr/Fe22 1.68 0.65 0.43 1.5 3.923 4.54 0.78 0.56 1.4 8.124 2.51 0.70 0.44 1.6 5.7

4.2.3 SEM-EDX

A SEM-EDX measurement of a UiO-66 powder with incorporated catalyst (prepared byCarlota Bozal Ginesta) is reported in table 4.2. The deviations are independent of theacceleration voltage and count rate. The catalyst loading can be determined from the sumformula Zr6O4(OH)4[(C8H4O4)(1- x) + (C14H2O10Fe2S2)x]6, were x describes the fractionof exchanged ligands. The theoretical atomic percentages for different catalyst loadingsare shown in table 4.3. Assuming the catalyst loading is 10 % in the sample, whichrepresents a Zr:Fe ratio of 5:1 shown for the sample in table 4.2, the carbon concentrationis relatively high while the oxygen concentration is relatively low.

Even though SEM-EDX showed strong deviations, it might be able to provide aqualitative analysis of a depth gradient within the functionalized MOF thin films. Thespatial resolution, as described earlier, is higher for low acceleration voltages but aminimum of 1.5− 2 times the energy of an X-ray line is necessary to excite the transition.The X-ray line of S is at 2.46 keV, leading to a minimum excitation energy of 5 keV. Thespatial resolution was assessed by scanning over one single (unfunctionalized) UiO-66crystal on a Si wafer, produced by the upside down method. The sample was sputteredwith Au/Pd for 30 seconds prior to the measurement. A so called linescan with 20 pointswas spanned over the crystal. The measurement time was 73.5 seconds and the probecurrent was 500 pA. The result is presented in figure 4.12 and clearly shows that a spatialresolution of less than 1 µm is feasible. The sample showed slight beam damage that


Table 4.2: SEM-EDX measurement of UiO-66 powder with incorporated catalyst. The 15kVmeasurement represents identical settings but a slightly different selected area. The sampleswere sputtered with Au/Pd for ca. 10 seconds for the measurements with 10 and 15 kV andadditional 30 seconds for the 30 kV measurement. Results for Pd, Au (sputter elements) and Na,Cl (impurities) not listed

Acceleration voltage 10 kV 15 kV 15 kV 30 kVSpectrum Area (counts) 286994 644632 264662 683845Spectrum Count Rate (cps) 1207 2455.6 2330.2 4881C at% 58.58 63.96 67.29 55.34O at% 24.89 26.66 25.01 29.96Zr at% 8.41 5.06 4.1 7.78Fe at% 2.02 0.96 0.78 1.6S at% 2.38 1.68 1.44 3.03Zr:Fe ratio 4.16 5.27 5.26 4.86

Table 4.3: Atomic percentages for different catalyst loadings calculated from the sum formula

Linker exchanged 0% 10% 20% 30% 40% 50%C [at %] 55.8 52.2 49.0 46.2 43.6 41.4O [at %] 37.2 38.7 40.0 41.2 42.2 43.1Zr [at %] 7.0 6.5 6.1 5.8 5.5 5.2Fe [at %] 0.0 1.3 2.4 3.5 4.4 5.2S [at %] 0.0 1.3 2.4 3.5 4.4 5.2Zr:Fe ratio - 5 2.5 1.67 1.25 1


was visible in the SEM after the measurement. A crack induced by the electron beam isillustrated in figure 4.13 after little more than 2 minutes measurement time.

Beam damage at red circles

µm

Cou

nts

Figure 4.12: Linescan with 20 points over one unfunctionalized UiO-66 crystal on a siliconsubstrate prepared by the upside-down method. The sample showed slight beam damage afterthe measurement marked by the red circles. The spatial resolution is below 1 µm and the MOFshowed fluorine impurities from unknown origin. Acceleration voltage: 5 kV, probe current: 500pA, measurement time: 73.5 seconds

Cross-sections prepared from films grown on FTO by the standard method (or oldmethod) typically show an intergrown area close to the substrate and looser bound crystalsat the top. Linescans along the thickness turned out to be difficult in practice mainlydue to the roughness and voids of the cross-section. A result from two different pointmeasurements along the thickness of the film are shown in figure 4.14. They show severelydifferent Zr:Fe ratio while maintaining the other elements rather constant. It has been


Figure 4.13: Beam damage after linescan along a functionalized UiO-66 film, Accelerationvoltage: 5 kV, probe current: 500 pA, measurement time: 123 seconds

pointed out before that the quality of quantitative EDX data has to be analyzed withcare. Nevertheless the difference between these two measurements might be significant.

4.2.4 Ion-beam analysis

Different Ion-Beam techniques were used to analyze potential catalyst gradients alongthe film thickness. Initial ERDA experiments performed with Br-ions and I-ions led todestruction of the MOF material (see figure 4.15). The quantification of elements in thefilm is undisturbed by this, the UiO-66 crystals basically melt together to a compactfilm but retain the original stoichiometry. Nevertheless, depth profiling can then not beperformed. RBS and MEIS on the other hand retain the film crystallinity and morphologyas proven by analysis with SEM after the experiments. ERDA with lighter ions like Clwas also applied without damage of the sample. The use of a light element like Si assubstrate material leads to higher depth resolution in RBS and MEIS than ERDA. Thisis the main reason why no ERDA data is presented in this thesis.

Rutherford Backscattering (RBS) spectra of a silicon sample prepared with the upside-down method and 24 hours PSE are shown in figure 4.17. As explained in section 2.4.6,the incident He ions are backscattered from atomic nuclei in the sample with differentenergies depending on its mass. Heavy elements like Pd and Au, which were used tocoat the samples for SEM analysis, take up only a small amount of energy from theHe ion (peaks at high detected ion energy in figure 4.17. The recorded datasets are


Glass

FTO

[FeFe]-UiO-66

Spectrum 1

Spectrum 2

Figure 4.14: Two different SEM-EDX point measurements on a UiO-66 thin film on FTOsubstrate prepared by the old method. Measurement at Leo 1550 SEM: no charge compensation,acceleration voltage: 5 kV, aperture: 30 µm, measurement time: 100 s. Gold and Palladium(sputter elements) not shown in quantification

simulated with a homogeneous, isotrop and thick material. The individual elements in thesample lead to traces at different onset energies (element-specific) and different intensities(concentration-dependent). The measured data is the sum of all individual elementalcontributions. A depth gradient can be deduced if the data deviated from the simulatedtraces. This is the case for the iron concentration showing a strong increase at the onsetedge, which represents the surface of the sample. The ion yield is also decreasing afterthe onset, representing a declining Fe concentration through the film. A similar trend,


Figure 4.15: ERDA measurement with Br ions on a UiO-66 thin film on FTO prepared by thestandard method lead to severe beam damage. left: area on the film not in contact with the ionbeam, right: destructed MOF crystallinity and topology

even though not as strongly pronounced, can be observed for the Zr-band. The depthpenetration of this method is in the range of 2− 3 µm.

Figure 4.16: SEM pictures of the same sample presented in figure 4.17 and 4.18 (top) showingparticle residues originating from the PSE process

A more detailed analysis of the situation close to the surface can be performed withMEIS. The lower incident beam energy leads to higher depth resolution and minimizesthe depth penetration to less than 100 nm. Figure 4.18 shows a MEIS spectrum (top)of the same sample as analyzed by RBS in figure 4.17 and another sample prepared bythe upside-down method but PSE for 72 hours. The bold lines marked with E0kSi (k:kinematic factor depending on the mass of the incoming He ion and scattered atomic nucleias well as geometrical factors, E0: energy of incident He ion) etc. mark the maximum


energy a He ion can have when deflected from a Si (S, Fe, Zr respectively) nucleus, similarto the onset of traces in the RBS spectra. In the top spectrum one can observe that theFe and S onset fit very well with theory, representing that these signals originate directlyfrom the surface and below. The onset for Zr and Si on the other hand show a shiftindicated by the dashed line that is in a first approximation of the same size as the peakwidth of the iron ’surface-band’. This suggests that a film containing iron and sulfur witha thickness of less than 15 nm is deposited on top of the silicon substrate as well as on theUiO-66 particles. Furthermore one can observe a decline in iron concentration after thissurface-peak. The spectrum shown in the bottom was measured on a film prepared by theupside-down method similar to above, but 72 hours post-synthetic exchange. The stretchof the iron peak (and sulfur, but less pronounced) on the surface again leads to a shiftof the zirconium and silicon onset. The Zr peak is overlapping with the iron signal dueto this effect. It should be noted, that the sample had very little surface coverage of theMOF itself, therefore only a small Zr peak would have been expected. The peak width ofthe iron is interestingly roughly three times as large as for the other sample and thereforedirectly proportional to the post-synthetic exchange time. This suggests a slow stackingof the catalyst molecules on the entire sample which is not removed by the standardwashing procedure (soaking in methanol). It was observed that the PSE treatment lead tosolid residues on the films that contain iron and sulfur (see figure 4.16). The amountof residue seems to relate to the age of the catalyst batch. The residues complicate theanalysis of depth gradients in the films, but they can not explain the results presentedabove due to their size of ca. 200− 300 nm. The increased surface concentration is only afew nanometers thick and can be found both on the silicon surface as well as the UiO-66.


Figure 4.17: RBS spectra of a sample on silicon with upside-down method and 24 hours PSEclearly showing an inhomogeneous iron distribution within the film, bottom: same spectra, zoomedinto the iron and zirconium edge


Figure 4.18: MEIS spectra of samples on silicon prepared with upside-down method and 24hours PSE (top) and 72 hours PSE (bottom). k: kinematic factor depending on the mass of theincoming ion and scattered atomic nuclei as well as geometrical factors, E0: energy of incidentHe ion


4.3 Electrochemistry of p-type silicon

The photoeffect of Si photoelectrodes under illumination leads to a shift of the reductionpotential to more positive values compared to glassy carbon as shown in figure 4.19.

Figure 4.19: Photoeffect of Si photoelectrode vs. glassy carbon (GC) working electrode (left)and comparison between an illuminated and dark Si electrode (right)

It was shown in the last sections that it was possible to grow UiO-66 thin films onsilicon and incorporate catalyst molecules into it. The original goal of fabricating aphotocathode for the production of hydrogen and photoelectrocatalysis measurementsseemed feasible. Unfortunately the oxide layer on the silicon prevents electron transfermore efficiently as expected. Experiments conducted on bare Si photoelectrodes showedthat even minor oxidation leads to a distortion in the cyclic voltammogram. The nativeoxide is perfectly electron-blocking but even storing a hydrogen terminated in air for fourhours lead to a strong increase in the peak-peak splitting (see figure 4.20). The peakintensity of the CVs shown in this work are in general not comparable due to differentelectroactive areas of the electrodes (the size of the handmade epoxy mask is difficult tocontrol), different light intensities (orientation of the sample in the measurement cell anddistance to the lamp) as well as different catalyst concentrations in solution (the catalystdegrades rapidly under illumination).

4.4. SI SURFACE FUNCTIONALIZATION 67

Figure 4.20: Comparison of a silicon electrode with native oxide and after treatment with HF(left) and stability of etched silicon electrodes in air, inert gas (Schlenk flask) and dry acetonitrile(right).

4.4 Si surface functionalization

The analysis of functionalized Si wafers turned out to be more difficult than expected dueto the lack of analytical tools. The XPS spectrometer as well as the IR spectrometer wereboth not working for several weeks in spring. The only XPS spectrum clearly showinga peak for a COO– group is shown in figure 4.21. It was not yet possible to measurean IR spectrum of the anchoring group by transmission IR spectroscopy. Contact anglemeasurements resulted in clearly hydrophobic (> 70◦) surfaces for the functionalizationwith 4′-trifluoromethylphenyl acetylene. The COOH functionalized samples showed contactangles between 30 and 67◦ depending on the chosen method (light, Lewis-acid catalyst,neither) and the solvent. As a reference, Piranha etched silicon has a contact angleof < 5◦, native oxide Si 32◦ and BOE etched Si 73◦. Cyclic voltammetry showed thatall functionalized silicon wafers retain electron-blocking behaviour as shown in figure4.22. The reason for the poor electron conductivity is most likely resulting from an oxideformation by reaction with water during the functionalization. The solvent used for theseexperiments (DMF) is very difficult to get extremely dry in practice.


Figure 4.21: XPS spectra of a para-ethynylbenzoic acid functionalized Si sample (immersionin DCM/DMF mixture for 7 days at room temperature)

Figure 4.22: Cyclic voltammetry of functionalized Si surfaces showing their electron-blockingbehaviour

Chapter 5

Conclusion and Outlook

This chapter will focus on the impact of the presented results and present an outlook tocurrent and future research questions.


Several parameters (water content, relative concentration ratios, time, sample orientationin vial) of the MOF thin film synthesis were studied. It was shown that terephthalic aciddoes not form a self-assembled monolayer on silicon in the same way as it does on FTO.It was not yet possible to reach full surface coverage on oxidized Si with one solvothermaltreatment. Nevertheless, the improved synthesis with the upside down procedure yieldsUiO-66 in a preferential growth direction on the Si substrate with high crystallinity.The influence of crystallinity and defects of the UiO-66 on the rate of catalyst incorporationby post-synthetic ligand exchange has long been speculated upon within the group of Dr.Ott, but has not yet been assessed. As described in the theoretical section, it was shownin literature that increasing the synthesis temperature to 220 ◦C and the bdc:ZrCl4 ratioto 2:1 leads to defect-free UiO-66 [50]. The influence of the catalyst incorporation ratewould be highly interesting with respect to the observation that thin films were preparedwith a bdc:ZrCl4 ratio of only 0.66:1 and hence might have a larger number of defects[39]. An electrochemical response can only be expected for films exceeding a minimumloading to achieve redox-hopping between the catalyst molecules.The films prepared by the upside-down method are flat and thin, hence open up thepossibility to be analyzed with techniques like TOF-SIMS and XPS depth profiling.Furthermore, the analysis of depth profiles is much easier compared to rough and inter-grown films.The growth of the films is limited to oxidized silicon substrates, hence no electrochemicalresponse for the catalyst within MOF thin films was observed due to the electron-blockingbehaviour. The covalent functionalization of hydrogen-terminated silicon electrodes with

70 CHAPTER 5. CONCLUSION AND OUTLOOK

carboxylic anchoring groups is one possibility to overcome this problem. Another optionis to coat the silicon substrate with a metal-oxide like Al2O3 or TiO2 by ALD providing apassivation layer for the hydrogen-terminated silicon. At very low thicknesses (less thanca. 2 nm) electrons can tunnel through this barrier. In cooperation with Filip Podjaskifrom the group of Prof. Lotsch at the Max-Planck-Institute for Solid State Research inStuttgart, experiments on ALD covered silicon electrodes are already going on.

The reproducibility of UiO-66 films is still unsatisfying. The reasons are unknown,but possible explanations and improvements are:

• As described by Li et al. [79], the rate of DMF decomposition is a crucial factorfor the MOF synthesis and strongly depending on metal ion concentration, solutionpH and reaction temperature. Another factor is the age of DMF due to its slowself-decomposition. Only a rigorous drying and distillation may cancel out theseeffects. It might be possible to determine the water and dimethylamine content of aUiO-66 synthesis solution by transmission IR spectroscopy. This will be pursued inthe near future.

• Kung et al. [66] mention that a temperature gradient inside the vial is necessaryfor the solvothermal growth of a MOF thin film. In their publication the vials areplaced on the bottom of a gravity convection oven. In this thesis, all vials wereplaced randomly into a sand bath in the oven. Maybe the influence of the sand bathimmersion height plays a bigger role on the synthesis than expected.

• The surface condition of the silicon wafer plays a crucial role in the synthesis. Theroughness on the atomic scale as well as surface coordinated species and inorganicimpurities need to be controlled. Cleaning the wafer with Piranha solution is a verycrude method and more elaborate procedures like the RCA (Radio Corporationof America) cleaning procedure are widely used in semiconductor industry [144].This two-step oxidizing and complexing treatment with hydrogen peroxide solutionat alkaline pH followed by acidic conditions leads to clean substrates with a welldefined oxide layer.

• It was reported that the use of ZrOCl2 as a Zr-precursor leads to higher reproducibil-ity due to its lower hydrophilicity compared to that of ZrCl4. [49]


• Another improvement on the thin film synthesis might be the change of the modulatorbenzoic acid to acetic acid as it was used by Miyamoto et al. [45]. Even thoughbenzoic acid is very soluble in DMF, even higher amounts of modulator could beused in case of liquid, ’water free’ acetic acid.

5.2 Analysis of functionalized UiO-66

In conclusion it was rather difficult to get a reliable quantification of the catalyst loading. Itwas shown that SEM-EDX analysis has a rather large range of variation due to the inherentproblem that the sample is not a homogeneous, polished and flat film. Furthermore theatomic concentrations of iron and sulfur are relatively low compared to oxygen and carbon,even for high catalyst loading. A small measurement error leads to very different calculatedcatalyst loadings. ICP-AES analysis was unfortunately not possible due to unknownsolubility reasons. The experiments will be repeated with a different dissolvation protocolas soon as possible. In principle it should be possible to analyze the concentration ofcatalyst within the film by transmission IR spectroscopy. One would need to know thethickness of the film and the extinction coefficient of the catalyst within the MOF, whichshould be roughly equal to the catalyst in solution in a first approximation. Unfortunatelythe high resolution IR spectrometer has had several technical issues in spring forcingthese measurements to be postponed. Ion beam analysis revealed that there is a veryhigh iron concentration in a thin film of less than 15 nm (for 24 hours PSE) all over thesubstrate and a declining gradient into the film (over several hundred nanometer). Thedata presented in this thesis is preliminary and needs to be confirmed by several moremeasurements. Nevertheless this result is very exciting and will lead to many furtherexperiments in this field. In the future ERDA might also be used to quantify the filmcomposition since this technique is capable of determining all elements (including H, C,O) simultaneously without disturbing matrix effects.

5.3 Silicon electrochemistry and functionalization

As mentioned before, surface oxidation on silicon electrodes creates electronic trap stateswhich shut down electron transfer across the interface. This effect is severe and hasbeen analyzed into some detail within this thesis. It has been shown that oxidation of


hydrogen-terminated silicon in air leads to the deterioration of the cyclic voltammogram ata time scale of less than four hours. Electrodes stored in dry acetonitrile did not degrade inthe same time. The current approaches to passivate the silicon surface are hydrosilylationwith alkynes and ALD coating with metal oxides as mentioned above. Experimentstowards organic functionalization of the Si-H surface have not been very successful yetand have thus far shown poor electrochemical behaviour. This is attributed to surfaceoxidation by residual water in DMF. The poor solubility of 4-para-ethynyl-benzoic acidis addressed in an improved synthesis shown in figure 5.1. The target compound is the4-ethynyl-2,2,2-trifluoroethyl benzoic acid ester which should be soluble in less polarsolvents than DMF and therefore contain less residual water after purification. Theester will be used for the surface functionalization while the acetylene function will bedeprotected. The proposed compound contains fluorine which makes an identificationon the substrate by the F1s XPS signal easier. It has been reported that free carboxylicacids tend to form bilayers by hydrogen bonding which can be prevented by using esters[108, 109]. The deprotection of the ester to yield the free carboxylic acid on the substratewill be done with potassium tert-butoxide in DMSO as reported in the literature [145].Thermal hydrosilylation in mesitylene will be performed with the target compound. Thismethod was chosen due to the great number of reports yielding high surface coverageswith repressing surface oxidation at temperatures exceeding 100 ◦C (e.g. reference [120]).Slow reaction kinetics with the functionalization by light on highly doped p-type siliconwas reported in reference [122]. There are also reports performing the Lewis-acid catalysedfunctionalization at 100 ◦C [123]. Furthermore a dilution approach with 1-pentyne or1-octyne will be used to increase the surface coverage (see figure 5.2). Similar methods toincrease the surface coverage were reported in the literature [106]. Another improvementin the surface functionalization would be the removal of physisorbed water and surfacesilanol groups after the HF treatment and water rinsing. This was applied by Roth et al.[141] who immediately placed the samples into septum-fitted vials that were thoroughlypurged with dry Ar for 5 min and heated to 250 ◦C while maintaining the purge for anadditional 5 min.

5.3. SILICON ELECTROCHEMISTRY AND FUNCTIONALIZATION 73

Figure 5.1: Intended synthesis procedure towards 4-ethynyl-2,2,2-trifluoroethyl benzoic acidester

Figure 5.2: Schematic representation of a surface functionalized with 4-ethynyl-2,2,2-trifluoroethyl benzoic acid ester and 1-pentyne before the deprotection with potassium tert-butoxide


In conclusion, the goal of a MOF-based silicon photocathode with incorporated molec-ular catalysts for photoelectrochemical hydrogen production was not reached. I want toend this thesis by critically evaluating the major obstacles and possible gains of such anenvisioned electrode. Semiconductors like silicon are good light absorbers and enableefficient charge separation due to their build-in electrical field at the solid-liquid interface(see section 2.3.1). Nevertheless, recombination due to surface trap states is reducing thelifetime of the electrons and competes with injection to a redox active species in solutionor inside a MOF. The electron transfer rate within the functionalized UiO-66 is unknownbut it is expected to be low due to an electron-hopping mechanism inside the MOF. Hence,recombination of excited electrons in silicon is a potential efficiency loss of a MOF-basedsilicon photocathode due to low electron injection kinetics. It is unknown whether it willbe possible at all to inject electrons into the MOF on a time scale that is faster than therecombination of electrons in silicon.

There are two main arguments for the overall idea to use metal-organic frameworksas molecular catalyst support materials: firstly, the local chemical environment withinthe MOF cavities can be altered to influence the second and third coordination spherearound the catalyst and thereby increase reactivity and/or stability. Secondly a largernumber of catalyst molecules can be coordinated to an electrode compared to monolayersensitization as for example in dye sensitized solar cells. The first argument was not partof this thesis but is undoubtedly a visionary and interesting approach that might leadto interesting scientific discoveries. The second point on the other hand is crucial to themotivation of a MOF-based photocathode. Silicon delivers ca. 25 mA/cm2 under 1 sunillumination. In a recent publication by Seo et al. [146] a catalyst turnover of less than300 s−1 for a proton reduction catalyst covering ca. 20 % of the silicon surface was neededto use all electrons generated by the silicon. A MOF-based electrode only pays off if thenumber of electrons delivered by the electrode is larger than the number of electrons amonolayer of catalyst can turnover. If the catalytic rate constant was very low, manycatalyst molecules are needed, while a fast catalyst only needs low monolayer coverage. AMOF layer of e.g. 1 µm with roughly 500 unit cells stacked above each other holding atleast one catalyst is therefore simply not necessary. These aspects should be kept in mindfor further designs towards molecular catalyst based photoelectrodes. To my opinion thefocus should therefore lie more on the stabilisation of molecular catalyst inside MOFs. It

5.3. SILICON ELECTROCHEMISTRY AND FUNCTIONALIZATION 75

might be of higher relevance towards solar fuel devices to tune catalysts towards verygood energetics and high stability instead of high turnover frequency. Nevertheless MOFthin films exceeding 100 nm will unlikely be needed.

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(141) Roth, K. M.; Yasseri, A. a.; Liu, Z.; Dabke, R. B.; Malinovskii, V.; Schweikart,K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F.J. Am. Chem. Soc 2003, 125, 505–517.

(142) Goldstein, J.; Newbury, D.; Joy, D.; Lyman, C.; Echlin, P.; Lifshin, E.; Sawyer, L.;Michael, J., Scanning Electron Microscopy and X-Ray Microanalysis; SpringerBerlin Heidelberg: 2003, p 690.

(143) Umemura, A.; Diring, S.; Furukawa, S.; Uehara, H.; Tsuruoka, T.; Kitagawa, S. J.Am. Chem. Soc. 2011, 133, 15506–15513.

(144) Kern, W. J. Electrochem. Soc. 1990, 137, 1887–1892.(145) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc

2000, 122, 1205–1209.(146) Seo, J.; Pekarek, R. T.; Rose, M. J. Chem. Commun. 2015, 51, 13264–13267.

List of Figures

1.1 Scheme of an artificial photosystem consisting of a photosensitizer (P),water oxidation catalyst (WOC), hydrogen evolving catalyst (HEC) or CO2

reduction catalyst (CRC). Sacrificial electron donors (SED) and acceptors(SEA) can be used to study the half-reactions separately [12] . . . . . . . 3

1.2 Semiconductor based photocathode/photoanode tandem water splittingcell design, HEC = hydrogen evolving catalyst, WOC = water oxidationcatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Cyclic voltammograms of enzymes adsorbed on rotating-disc pyrolyticgraphite edge electrodes under catalytic conditions with both the oxidizedand reduced substrates present. (A) Reversible interconversion of H+ andH2 by hydrogenase-2 from Escherichia coli (pH 6,10% H2 in Ar, 30 °C)[16]. (B) Reversible interconversion of CO2 and CO by CODH 1 fromC. hydrogenoformans (pH 7, 50% CO in CO2, 25 °C) [17]; adapted fromreference [14] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 [FeFe] hydrogenase crystal structure from Desulfovibrio desulf uricans [25].The electron transfer chain via iron-sulfur centers and the hydrogen pathwayare shown schematically. The chemical structure of the active site with itsopen coordination site at the metal (arrow) is shown to the right; adaptedfrom [26] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.5 [FeFe]-hydrogenase model complexes applied in this thesis: Fe2(bdt)(CO)6(bdt = benzene-1,2 dithiol), Fe2(mcbdt)(CO)6 (mcbdt = 2,3-dithiolato-benzenecarboxylic acid) and Fe2(dcbdt)(CO)6 (dcbdt = 2,3-dithiolato-1,4-benzenedicarboxylic acid) . . . . . . . . . . . . . . . . . . . . . . . . . . 6

86 LIST OF FIGURES

1.6 Overview over the three most common MOFs: MOF-5 (Zn4O nodes with 1,4-benzenedicarboxylic acid linker), HKUST-1 (HKUST = Hong Kong Univer-sity of Science and Technology, copper nodes with 1,3,5-benzenetricarboxylicacid linker) and UiO-66 (UiO = University in Oslo, [Zr6O4(OH)4] clusterswith 1,4-benzenedicarboxylic acid linker). The core structure (secondarybuilding unit) is shown to the left and the respective MOF structure tothe right. HKUST-1 and UiO-66 are shown in a ’cutaway-view’ to remainclarity [28] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.7 The principle of post-synthetic metal and ligand exchange. The metalcluster (blue pyramid) can be exchanged with different metal ions (purplecircle) to yield a chemically altered cluster (purple pyramid). The ligand(orange rod) can be exchanged with a modified ligand containing the samelinking unit (blue rod). The yellow sphere should demonstrate the cavityspace within the MOF [37] . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.8 Schematic representation of the system developed by Pullen et al. [38].A biomimetic [FeFe]-hydrogenase model complex Fe2(dcbdt)(CO)6 wasincorporated into a UiO-66 metal organic framework by post-syntheticexchange in water at room temperature for 24 hours. . . . . . . . . . . . 9

1.9 Cyclic voltammograms of Fe2(dcbdt)(CO)6 in 1 mM DMF solution (blue),and UiO-66 film before (green) and after PSE (red). Left: thick UiO-66film (ca 20 µm). Right: thin UiO-66 film (ca 2− 5 µm). All CVs recordedin DMF with 0.1 M TBAPF6 as supporting electrolyte.[39] . . . . . . . . 10

1.10 Schematic two-dimensional drawing of a UiO-66 thin film with incorporatedFe2(dcbdt)(CO)6 as a hydrogen evolving photoelectrode. The Zr-nodesrepresent the secondary building unit of UiO-66 represented by 6 Zr atoms 11

2.1 Hydrolysis equilibrium of DMF leading to the formation of formic acid anddimethylamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 Graphical representation of the proposed model for layer-by-layer growthof Cu3(btc)2 on an oxide surface. The atoms shown are Cu – green, O –red, C – gray [76] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

LIST OF FIGURES 87

2.3 Comparison of solvothermal methods (left) using NEt3 or Me2NH as baseto deprotonate the carboxylic linker and Li and Dincă′s approach to elec-trochemically create a hydroxyl acting as a base close to the substrate[79] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.4 Nickel bis-dithilate conductive MOF by Kobayashi et al.(left)[82] and p-stacks of TTFTB linkers in work by Narayan et al. (right) [84] . . . . . . 20

2.5 Comparison of cyclic voltammograms of MOFs with covalently boundferrocene presented in literature (see text for further details): (1) scan ratefor (i) 10, (ii) 35, and (iii) 100 mV−1s in 0.1 M NBu4PF6 in dichloroethane,MOF powder was immobilized on a basal plane pyrolytic graphite workingelectrode [90]; (2) potentials are referred to Ag/AgCl/3M KCl referenceelectrode, electrolyte not mentioned, powdered MOF was suspended inethanolic solution of Nafion (5%) and deposited on a graphite electrode[91]; (3) measured in 0.05 M TBAPF6 in acetonitrile on a FTO electrode [92] 22

2.6 p-type silicon band edge positions at equilibrium with aqueous solutionwith the proton reduction redox level as reference. ECB = Conduction bandlevel, EVB = Valence band level, E f = Fermi level, ϕB = band bending [9] 25

2.7 p- and n-type silicon-liquid interface under illumination showing the differentbend banding and thereby changed reactivity towards oxidation/reduction[97] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.8 Hydrogen terminated Si(100) and Si(111) [106] . . . . . . . . . . . . . . . 28

2.9 Proposed radical chain mechanism for the functionalization under thermaland UV conditions and the electron/hole pair mechanism for visible lightinduced hydrosilylation [122] . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.10 Monte-Carlo simulations of electron trajectories within a 2000 nm thickUiO-66 bulk sample (density = 1.443 g/mL) on a Si substrate performedwith the software CASINO version 2.48 [130]. The red paths show backscattered electrons that are re-emitted from the sample. All simulationswere performed with 20000 electrons, a beam radius of 5 nm and all othersoftware settings on standard. The accelerating voltage was stepwiseincreased from 5 keV (top left) to 10 keV (top right), 15 keV (bottom left)and 20 keV (bottom right) . . . . . . . . . . . . . . . . . . . . . . . . . . 32

88 LIST OF FIGURES

3.1 Synthesis route of para-ethynylbenzoic acid . . . . . . . . . . . . . . . . . 393.2 Scheme illustrating the upside-down synthesis method (description see text) 423.3 Step by step contacting procedure of the Si photoelectrodes . . . . . . . . 44

4.1 SEM pictures of UiO-66 electrophoretically deposited on a FTO substrateillustrating that the surface coverage is not complete all over the sample . 48

4.2 Dynamic light scattering of UiO-66 synthesis solution after different times;synthesis procedure according to ’old-method’ but only 4 ml synthesissolution per vial. Result was difficult to reproduce due to DLS measurementproblems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.3 Variation of water content in standard UiO-66 synthesis on FTO substrates:0 eq. (top left), 3.7 eq. (top right), 14.7 eq. (bottom left), 59 eq. (bottomright) water compared to ZrCl4 added before the solvothermal treatment. 49

4.4 Comparison of crystal growth on UiO-66 films on FTO substrate preparedby the standard method. The ratios of terephthalic acid:ZrCl4 were: 0.66(top left), 0.8 (top right), 1 (bottom left), 2 (bottom right). The amountof ZrCl4 was kept constant at 35 mg leading to benzoic aicd:terephthalicacid ratios of 114.5, 92, 73.3 and 36.6 respectively; most of the film inthe bottom right broke off due to its high thickness but the decrease incrystallinity can be observed by the rounded edges of the crystallites . . . 50

4.5 Two samples synthesized with the standard procedure on FTO substratesbut with different synthesis time: top (39h) and bottom (134h). Thecrystallites increase in size and grow together in the lower parts of the filmas can be seen inside the cracks (right) . . . . . . . . . . . . . . . . . . . . 51

4.6 Comparison between standard method (left) and upside-down method(middle and right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.7 Powder X-ray diffractogramm comparing samples prepared by the standardmethod and the upside-down method on silicon substrates. The film withpreferential growth along the [111] orientation by the upside-down methoddoes not show the [200] reflection at 8.4◦ . . . . . . . . . . . . . . . . . . 53

4.8 SEM pictures of samples (FTO) prepared with the upside-down method:pretreatment overnight in 1 mM bdc solution in DMF (left) and no pre-treatment (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

LIST OF FIGURES 89

4.9 SEM pictures of samples (Si) prepared with the upside-down methodbut different pretreatment: etched with Piranha acid, no SAM (top left);etched with degassed buffered oxide etch, no SAM (top right); pretreatmentovernight in 1 mM bdc solution in DMF (bottom left) and in 30 mM bdcsolution in DMF (bottom right) . . . . . . . . . . . . . . . . . . . . . . . 54

4.10 Secondary growth of UiO-66 thin films prepared by the upside down methodleads to an increase of surface coverage - the smaller crystallites were formedat the second treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.11 ATR-IR spectra of a [FeFe](mcbdt)(CO)6 functionalized UiO-66 thin filmsynthesized by the standard method on silicon. Shown is the region between2200 cm−1 and 1900 cm−1 . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.12 Linescan with 20 points over one unfunctionalized UiO-66 crystal on asilicon substrate prepared by the upside-down method. The sample showedslight beam damage after the measurement marked by the red circles. Thespatial resolution is below 1 µm and the MOF showed fluorine impuritiesfrom unknown origin. Acceleration voltage: 5 kV, probe current: 500 pA,measurement time: 73.5 seconds . . . . . . . . . . . . . . . . . . . . . . . 59

4.13 Beam damage after linescan along a functionalized UiO-66 film, Accelerationvoltage: 5 kV, probe current: 500 pA, measurement time: 123 seconds . . 60

4.14 Two different SEM-EDX point measurements on a UiO-66 thin film onFTO substrate prepared by the old method. Measurement at Leo 1550SEM: no charge compensation, acceleration voltage: 5 kV, aperture: 30 µm,measurement time: 100 s. Gold and Palladium (sputter elements) notshown in quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.15 ERDA measurement with Br ions on a UiO-66 thin film on FTO preparedby the standard method lead to severe beam damage. left: area on the filmnot in contact with the ion beam, right: destructed MOF crystallinity andtopology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.16 SEM pictures of the same sample presented in figure 4.17 and 4.18 (top)showing particle residues originating from the PSE process . . . . . . . . 62

4.17 RBS spectra of a sample on silicon with upside-down method and 24 hoursPSE clearly showing an inhomogeneous iron distribution within the film,bottom: same spectra, zoomed into the iron and zirconium edge . . . . . 64

90 LIST OF FIGURES

4.18 MEIS spectra of samples on silicon prepared with upside-down methodand 24 hours PSE (top) and 72 hours PSE (bottom). k: kinematic factordepending on the mass of the incoming ion and scattered atomic nuclei aswell as geometrical factors, E0: energy of incident He ion . . . . . . . . . 65

4.19 Photoeffect of Si photoelectrode vs. glassy carbon (GC) working electrode(left) and comparison between an illuminated and dark Si electrode (right) 66

4.20 Comparison of a silicon electrode with native oxide and after treatmentwith HF (left) and stability of etched silicon electrodes in air, inert gas(Schlenk flask) and dry acetonitrile (right). . . . . . . . . . . . . . . . . . 67

4.21 XPS spectra of a para-ethynylbenzoic acid functionalized Si sample (im-mersion in DCM/DMF mixture for 7 days at room temperature) . . . . . 68

4.22 Cyclic voltammetry of functionalized Si surfaces showing their electron-blocking behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5.1 Intended synthesis procedure towards 4-ethynyl-2,2,2-trifluoroethyl benzoicacid ester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.2 Schematic representation of a surface functionalized with 4-ethynyl-2,2,2-trifluoroethyl benzoic acid ester and 1-pentyne before the deprotection withpotassium tert-butoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

List of Tables

4.1 ICP-AES results from three equal samples on FTO glass substrate . . . . 574.2 SEM-EDX measurement of UiO-66 powder with incorporated catalyst.

The 15kV measurement represents identical settings but a slightly differentselected area. The samples were sputtered with Au/Pd for ca. 10 secondsfor the measurements with 10 and 15 kV and additional 30 seconds for the30 kV measurement. Results for Pd, Au (sputter elements) and Na, Cl(impurities) not listed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4.3 Atomic percentages for different catalyst loadings calculated from the sumformula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

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