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 

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: wenlong.cheng@monash.edu.

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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/.

70

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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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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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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.