-
Matryoshka caged gold nanorods: Synthesis, plasmonic property
and catalytic activity Wei Xiong1,2,3, Debabrata Sikdar4, Lim Wei
Yap1,2, Pengzhen Guo1,2, Malin Premaratne4, Xinyong Li3, and
Wenlong Cheng1,2 ( ) Nano Res., Just Accepted Manuscript • DOI:
10.1007/s12274-015-0922-8 http://www.thenanoresearch.com on Oct.
14, 2015 © Tsinghua University Press 2015
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Nano Research
DOI 10.1007/s12274‐015‐0922‐8
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64
Matryoshka Caged Gold Nanorods: Synthesis, Plasmonic Property
and Catalytic Activity
Wei Xiong, †, ‡, ¶ Debabrata Sikdar,� Lim Wei Yap,†, ‡ Pengzhen
Guo,†, ‡ Malin Premaratne,� Xinyong Li,¶ and Wenlong Cheng*,†,‡
†Department of Chemical Engineering, Monash University, Clayton
3800, Victoria, Australia.
‡The Melbourne Centre for Nanofabrication, 151 Wellington Road,
Clayton 3168, Victoria,
Australia.
¶Key Laboratory of Industrial Ecology and Environmental
Engineering and State Key Laboratory
of Fine Chemical, School of Environmental Sciences and
Technology, Dalian University of
Technology, Dalian 116024, China.
�Advanced Computing and Simulation Laboratory (AχL), Department
of Electrical and Computer Systems Engineering, Monash
University,
Clayton 3800, Victoria, Australia.
*To whom correspondence should be addressed. E-mail:
[email protected].
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65
ABSTRACT
Matryoshka caged gold nanorods (mCGNRs) were successfully
synthesized by alternating seed-mediated silver coating and
galvanic replacement reaction (GRR). Broadening and red-shifting
plasmonics and enhanced catalytic activity towards reduction of
4-nitrophenol were observed as the number of Matryoshka layers was
increased. The kinetic constants of hexa-CGNRs as nanocatalysts in
the reduction of 4-nitrophenol are about 5.2 times and 3.7 times
higher than those of GNRs and CGNRs, respectively. The surface
plasmon absorption of light can enhance the catalytic performance
of mCGNRs. With the support of the polyurethane foam, the mCGNR can
act as recyclable heterogeneous catalysts for the reduction of the
4-nitrophenol.
KEYWORDS: Matryoshka caged gold nanorods, galvanic replacement
reaction, 4-nitrophenol, catalysis, surface plasmon resonance.
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1 INTRODUCTION
Metallic nanoparticles are key building blocks in nanoscience
and nanotechnology due to their unique optical, catalytic, and
photothermal properties.[1-3] Over the past 30 years, a number of
recipes in wet chemistry synthesis have been developed to control
sizes and shapes,[4-6] enabling the formulation of an artificial
periodic table.[7] In addition to their unique plasmonic
properties, the ‘artificial elements’ in the table have also
implications in tailored catalysis and electrocatalysis for
applications of chemical processing,[8-11] pollution
control,[12-14] fuel cells and hydrogen economy.[14-16]
Comparing with the poor activity of bulk gold, gold
nanostructure show excellent activity and selectivity for catalytic
reactions under mild and green conditions.[17, 18] In general,
catalytic activity of metallic nanoparticles is dependent on the
surface-area-to-volume ratio (RS/V) and the amounts of kinetically
active atoms on their edges, corners or defects.[19, 20] The
researches on gold catalysts are usually focused on the supported
gold nanoparticles.[21] However, benefiting from the development of
wet chemistry synthesis, the unsupported hollow gold nanostructures
such as gold nanocages,[22, 23] gold nanoshells[24, 25] and
double-shell nanostructures[26] have been demonstrated as highly
effective nanocatalysts. In particular, gold nanocages have shown
to exhibit at least 2 times higher catalytic performance than
corresponding solid gold nanoparticles at room temperature.[22]
Here, we report enhanced catalytic activities of a new type of
nanoparticles – Matryoshka caged gold nanorods (mCGNRs). Templated
by gold nanorods (GNRs), ultrathin nanocages could be constructed
layer by layer by alternating seed-mediated silver coating and
galvanic replacement reaction (GRR).[27] The number of Matryoshka
layers was controlled by tuning the cycle times of silver coating
and GRR. Stronger plasmonic resonances and enhanced catalytic
activities were observed as the number of layers for mCGNRs
increased.
2 EXPERIMENTAL SECTION
2.1 Chemicals
Gold (III) chloride trihydrate (HAuCl4),
hexadecyltrimethylammonium bromide (CTAB), silver nitrate (AgNO3),
sodium borohydride (NaBH4), L-ascorbic acid (AA),
Cetyltrimethylammonium chloride (CTAC, 25 wt.% in H2O),
Polyvinylpyrrolidone (PVP), and 4-nitrophenol (4-NTP) were obtained
from Sigma Aldrich. All glassware and Teflon-coated magnetic stir
bars were cleaned with aqua regia, followed by thorough rinsing
with deionized water before drying in an oven at 80°C.
2.2 Synthesis of bi-CGNRs and mCGNRs
Synthesis of Matryoshka Caged Gold Nanorods (mCGNRs): The CGNRs
were prepared according to our reported methods (seeing the
Supporting Information for the details).[28] The bi-CGNRs were
synthesized via the galvanic replacement reaction by using Ag
coated CGNRs as the templates. The Ag coated CGNRs were prepared by
mixing 10 mL 0.3 nM CGNRs in CTAC solution with the AgNO3 (1 mL, 10
mM) and AA (0.5 mL, 100 mM) solution and stirring for 3h at 65°C in
water bath. The Ag coated CGNRs prepared were centrifuged and
redispersed in 2.5 mL deionized water, followed by the addition of
CTAB (5 mL, 0.2 M) solution and PVP (2.5 mL, 2 wt%) solution
sequentially. The resultant solution was heated at 90 °C for 2
minutes, following by mixing with HAuCl4 (4 mL, 0.5 mM) for another
10 minutes. The bi-CGNRs were collected by centrifugation (7 500
rpm for 10 min) and washed with water twice. To investigate the
morphological evolution of the bi-CGNRs formed, the volumes of Ag
coated CGNRs, CTAB, and PVP solution decreased to 0.25, 0.5, 0.25
mL, respectively and various volumes of HAuCl4 were added in. The
mCGNRs were prepared by the similar method of recycling the silver
coating and galvanic replacement reaction for tri-, tetra-, penta-,
and hexa-CGNR, respectively.
2.3 Catalysis of 4-nitrophenol
The ice-cold NaBH4 solution (3 mL, 0.05 M) was mixed with 4-NTP
(60 μL, 5 mM), following by adding 10 μL 3 nM mCGNRs. UV-vis
spectra were test under a given period.[29] To investigate the
effect of sunlight, the concentration of bi-CGNRs was reduced to
0.5 nM in order to slow down the reaction.
2.4 Structural and optical characterization
Absorption spectra were recorded using an Agilent 8453 UV–vis
spectrometer. Transmission electron microscopy (TEM) images of the
nanostructures were taken with FEI Tecnai T20 TEM.
3. RESULTS AND DISCUSSION
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The mCGNRs were synthesized via the procedure schematically
presented in Figure 1. To prepare the bi-CGNRs, the mono-layered
caged gold nanorods were first prepared according to our reported
method.[28, 30] The mono-CGNRs had a typical length of 61.6 nm and
width of 37.5 nm (Figure 2a). The thickness of the cages is about
3.2 nm. After coating this hollow CGNR with a thin Ag shell and
reacted with HAuCl4 one more time, the deposited Ag shell was
transformed into a new Au/Ag alloyed cage, leading to the formation
of bi-CGNR with a larger dimension (72.0 nm in length and 47.6 nm
in width), as shown in Figure 2b and Figure S1.
By controlling the cycle number of Ag coating and GRR, the
number of Matryoshka layers could be controlled. Figure 2b–f shows
the typical TEM images of tri-CGNR to hexa-CGNR by repeating the
process. The average diameter of mCGNR increased significantly from
tri-CGNR (80 nm in length, 55 nm in width) to hexa-CGNR (99 nm in
length and 77 nm in width).
Figure 3 shows the optical properties of mCGNRs with different
layers of cages. From the experimental result in Figure 3a, the
absorbance peak redshifts from 667 nm to 715nm with the increase of
the number of the layers to six. The absorption spectra of the
mCGNRs were also numerically investigated using the
frequency-domain finite element method (see the supporting
information for the details of the models used in the simulation).
With the increase in the number of Matryoshka layers, the
extinction spectra in Figure 3b clearly showed the trends of
redshift, strengthening, and spectral broadening of the plasmon
resonance peak, while closely resembles the trend observed
experimentally. The strengthening and redshift can be ascribed to
the effect of increasing plasmonic coupling/interaction between the
layers of the gold cage.[31] The spectral broadening may be
attributed to the increase in electron surface scattering losses
owing to gradual reduction in the thickness of the outer cage
wall[32]
from CGNR to hexa-CGNR. The strengthening of plasmonic
resonances with increase in the number of Matryoshka layers
contributes towards improved optical property of these mCGNRs.
The mCGNR exhibited strong catalytic activities towards
reduction of 4-nitrophenol (4-NTP) by NaBH4. 4-NTP has a strong
absorption at 400 nm in NaBH4 solution, which can be utilized to
monitor the reaction progress or kinetics by spectroscopic
means.[29] Figure S3 indicated the UV-vis spectra of 4-NTP reduced
by NaBH4 with bi-CGNRs added as catalysts. For this experiment, the
initial concentrations of 4-NTP, NaBH4 and bi-CGNR catalysts were
kept at 10−4 M, 5×10−2 M, and 10−11 M, respectively. It can be seen
that the UV-vis spectra kept unchanged for the initial 3 min, which
implies that a certain period of time was required for 4-NTP to
adsorb onto the bi-CGNRs before the reduction initiated. The
intensity of absorption peak at 400 nm gradually dropped as the
reduction reaction proceeded, which indicated the decrease of
concentration of 4-NTP. At the same time, with the production of
(4-aminophenol) 4-AP, a new absorption peak appeared at 315 nm.
The amount of HAuCl4 added in the GRR reaction had an evident
effect on the catalytic activities. Taking bi-CGNRs for an example,
the increased addition of HAuCl4 led to gradual color changes
indicative of morphological evolution (Figure S4a), similar to the
case for the evolution of CGNRs reported by us previously.[28] The
morphologies of the three nanostructures corresponding to the
addition 0.1, 0.4, and 0.7 mL HAuCl4 solution (denoted as
bi-CGNR0.1, bi-CGNR0.4, and bi-CGNR0.7, respectively) in 1 mL
CGNR@Ag solution were shown in Figure S4b-d). Note that bilayered
cages were constructed as increasing HAuCl4 amount was added. With
10 μL 3 nM bi-CGNRs added as the catalysts, the reduction of 4-NTP
by the ice-cold NaBH4 started. During the process, the catalytic
performance were enhanced gradually (Figure 4a). No reduction
reaction was observed without the addition of catalysts, which
indicated the spontaneous 4-NTP reduction occurred under the
experiment condition could be negligible. In contrast, the reaction
catalyzed by bi-CGNR0.7 showed the shortest adsorption time (2 min)
and the highest reduction rate. Similar to other noble
nanoparticle-based catalysis,[22, 23, 29] the adsorption of 4-NTP
did occur before the reduction reaction could be initiated.
However, the required time for absorption in this induction stage
differed from particle to particle. The higher the surface area is,
the faster the reaction will be activated, and the shorter the
induction period will be.[22] With the increased amount of HAuCl4
added in the GRR step, more Ag elements were replaced by Au, more
catalytically active gold surfaces available for adsorption of
4-NTP molecules to activate the reaction, hence shorter induction
period observed (Figure 4). The conversion efficiency was 95.7% in
2 min after adsorption for the bi-CGNR0.7, while the efficiency was
only 47.5% in 11 min for the bi-CGNR0.1. A pseudo-first-order
kinetic model was employed to fit the degradation data by using the
linear transformation: −ln (C/C0) = kt (where k is the kinetic
coefficient), as shown in Figure 4b. The estimated kinetic
coefficients of 4-NTP reduction in the systems of bi-CGNR0.1,
bi-CGNR0.4, and bi-CGNR0.7 were 0.06, 0.20, and 1.02 min−1,
respectively. The kinetic coefficient of bi-CGNR0.7 is 17 times
higher than that of bi-CGNR0.1. This was because fully developed
caged particles had much higher available gold surfaces than
partially etched caged particles. The presences of pinholes in the
cage walls enabled their inner surfaces available for catalytic
reactions.
We further showed increased Matryoshka layers could further
enhance the catalytic activities. Figure 5a show a comparison of
reduction process catalysed by different layers of mCGNRs. With the
number of layers caged outside increasing from zero to six, the
adsorption time of 4-NTP was reduced from 8 min to 1.5 min. This
could be due to the increased surface area as a result of
enlargement of particle size. An additional possible reason was
that the fresh surface inside of mCGNRs was not well passivated by
the surfactants.[23]
In addition, it was noted that hexa-CGNRs exhibited highest
catalytic efficiency, with nearly 97% conversion in 2 min after
adsorption. Whereas, only 48% and 56% conversions were achieved
with GNRs and CGNRs as catalysts under the same
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conditions. From the corresponding kinetic constants in Figure
5b, it could be seen the kinetic constants of hexa-CGNRs are about
5.2 times and 3.7 times higher than those of GNRs and CGNRs,
respectively. We also normalized the kinetic constant according to
the mass of the mCGNG. It can be seen that the specific kinetic
constant for hexa-CGNR was 3.7×104 min−1 g−1, which was still 1.3
times and 1.2 times for those of NR, mono-CGNR, respectively.
The enhanced catalytic activities could be attributed to
enhanced surface-area-to-volume ratio (RS/V). Assuming the internal
surfaces of mCGNRs are fully exposed to reactions, the RS/V for
hexa-CGNRs, bi-CGNRs, and CGNRs are 0.52 nm–1, 0.35 nm–1 and 0.28
nm–1, respectively. On the other hand, the intermediate during the
reaction could be better kept at a high value in the confined space
of the cavity with the increase of the number of Matryoshka layers
– an additional possible cause for enhanced catalytic
properties.[23] It indicated that the construction of Matryoshka
style nanostructure offer another efficient approach to improve the
catalytic performance of the metallic nanoparticles.
Interestingly, ambient sunlight contributed to the enhanced
catalysis (Figure 6). The reaction occurred under sunlight
irradiation showed the shorter adsorption time and the higher
reduction rate than the same reaction carried out in the dark
condition. It indicated that the SPR-based absorption of light can
enhance the catalytic performance of bi-CGNRs.[33] The enhanced
catalytic activity could be attributed to the photothermal effect
caused by the bi-CGNRs.[28, 30] It is known that plasmonic
nanoparticles act as nanoscale heater by concentrating, and
converting light energy into local heat. The localized heat
surrounding the bi-CGNR raised local temperature which might
promote diffusion and adsorption of 4-NTP molecules, hence, enhance
overall catalytic reactions.[34] This is consistent with previous
observation that high temperature led to shorter catalytic
induction time and greater catalytic efficiency. The difference is
that the heating here was by virtue of plasmonic effects, in which
broadband resonance peaks contributed to efficient heating simply
by sunlight.
Moreover, mCGNRs can be employed as recyclable heterogeneous
catalysts in heterocatalysis with the support of polyurethane
foam.[35] The mCGNRs were loaded onto the surface of polyurethane
foam simply by dipping the foam into the nanoparticle solutions,
followed by drying under ambient conditions. Nanoparticles remained
firmly bound to the foam surfaces without desorption for any
subsequent catalytic reactions, due to strong physical forces
between nanoparticles and polyurethane foam (likely van der Waals
forces). There was no change of the color or decrease of the UV-vis
absorbance after the 4-NTP in NaBH4 solution flowed through the
polyurethane foam (Figure 7), which indicated the catalytic
reaction caused by the foam could be neglected. With hexa-CGNRs
loaded by the polyurethane foam as the catalysts, the solution
turned to colorless, indicating that 4-NTP had been reduced to
4-AP, while the color turn to light yellow when loading NRs. It
proved higher catalytic efficiency of mCGNR in comparison with NRs.
In order to examine the reusability of the catalysts, the catalysis
process was repeated 7 times with the same polyurethane foam loaded
hexa-CGNRs (Figure S5). After 7-cycle tests, 4-NTP could still be
reduced to 4-AP completely, which demonstrated the excellent
reusability of the polyurethane foam loaded mCGNRs.
4. CONCLUSIONS
In summary, we have demonstrated the synthesis of mCGNRs as
efficient catalysts for the reduction of 4-nitrophenol by NaBH4.
The number of Matryoshka layers caged outside the gold nanorod can
be controlled by the cycles of Ag coating and galvanic replacement
reaction performed. The surface plasmon resonance peak strengthens,
redshifts and broadens with the increase in the number of
Matryoshka layers caged. We have also obtained the relationship
between the catalytic activity and the particle morphology as well
as the number of the Matryoshka layers of the mCGNRs. The high
catalytic activity can be acquired by increasing the porosity and
the number of the Matryoshka layers. We believe that high-efficient
multiple hollow nanocatalysts like Pt and Pd nanostructures can
also be constructed by the similar process, showing broad
application potentiality of our methodology in energy and
environments.
ACKNOWLEDGMENT
The authors acknowledge use of facilities in Monash Centre for
Electron Microscopy. This
work was performed in part at the Melbourne Centre for
Nanofabrication (MCN) in the Victorian
Node of the Australian National Fabrication Facility (ANFF). The
work is financially supported
by the Australian Research Council Discovery Projects
(DP140100052, DP150103750,
DP110100713 and DP140100883) and DSDBI of the Victorian
Government.
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Electronic Supplementary Material: Supplementary material
(Method and numerical
modeling for the mCGNRs, synthesis for gold nanorod and caged
gold nanorods) is available in
the online version of this article at
http://dx.doi.org/10.1007/.
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Figure 1. Schematic of synthesis of mCGNRs.
Figure 2. TEM images for mono- (a), bi- (b), tri- (c), tetra-
(d), penta- (e), and hexa- (f) CGNRs.
Figure 3. Experimental (a) and simulated (b) absorbance spectra
of Matryoshka style CGNRs.
Figure 4. (a) Extinction at the peak position for 4-NTP (400 nm)
as a function of time with bi-CGNR0.1,
bi-CGNR0.4, and bi-CGNR0.7. (b) The kinetics of 4-NTP reduction
by bi-CGNR0.1, bi-CGNR0.4, and
bi-CGNR0.7.
Figure 5. (a) Extinction at the peak position for 4-NTP (400 nm)
as a function of time with
mCGNRs with different layers. (b) The kinetics of 4-NTP
reduction by mCGNRs.
Figure 6. (a) Extinction at the peak position for 4-NTP (400 nm)
as a function of time with
bi-CGNR under dark and sunlight conditions. (b) The kinetics of
4-NTP reduction by bi-CGNR
under dark and sunlight conditions.
Figure 7. Photograph of 4-NTP reduction by NaBH4 catalysized by
polyurethane foam and polyurethane foam
supported NRs or hexa-CGNRs.
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Figure 1. Schematic of synthesis of mCGNRs.
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Figure 2. TEM images for mono- (a), bi- (b), tri- (c), tetra-
(d), penta- (e), and hexa- (f) CGNRs.
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Figure 3. Experimental (a) and simulated (b) absorbance spectra
of Matryoshka style CGNRs.
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Figure 4. (a) Extinction at the peak position for 4-NTP (400 nm)
as a function of time with
bi-CGNR0.1, bi-CGNR0.4, and bi-CGNR0.7. (b) The kinetics of
4-NTP reduction by
bi-CGNR0.1, bi-CGNR0.4, and bi-CGNR0.7.
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77
Figure 5. (a) Extinction at the peak position for 4-NTP (400 nm)
as a function of time with
mCGNRs with different layers. (b) The kinetics of 4-NTP
reduction by mCGNRs.
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Figure 6. (a) Extinction at the peak position for 4-NTP (400 nm)
as a function of time with
bi-CGNR under dark and sunlight conditions. (b) The kinetics of
4-NTP reduction by bi-CGNR
under dark and sunlight conditions.
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Figure 7. Photograph of 4-NTP reduction by NaBH4 catalysized by
polyurethane foam and
polyurethane foam supported NRs or hexa-CGNRs.
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80
Matryoshka caged gold nanorods were successfully synthesized by
alternating seed-mediated silver
coating and galvanic replacement reaction. Enhanced catalytic
activity towards reduction of 4-nitrophenol was
observed as the number of Matryoshka layers was increased.