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A Novel Photochemical Method for the Synthesis of Au Triangular Nanoplates Inside Nanocavity of Mesoporous Silica Shells Firdoz Shaik, Weiqing Zhang, and Wenxin Niu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01365 • Publication Date (Web): 19 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017

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The Journal of Physical Chemistry

A Novel Photochemical Method for the Synthesis of Au Triangular Nanoplates inside Nanocavity of Mesoporous Silica Shells Firdoz Shaik*, Weiqing Zhang, Wenxin Niu Department of Chemical and Biomolecular Engineering, National University of Singapore Singapore 117576 *

E-mail: [email protected]

Abstract We report a new photochemical method for tuning the shapes of gold nanoparticles inside nanocavity of mesoporous silica shells (mHSS) using Ag+ ions without using any convential organic capping ligands. mHSS acts as a nano-container and used for entrapping the chloroauric acid molecules along with Ag+ ions. The shape of gold nanoparticles is tuned from spheres to triangular nanoplates inside mHSS by varying the molar ratio of [AuCl4]:[Ag+] in the reaction. The results confirm that the presence of Ag+ ions promotes the growth of Au triangular nanoplates inside mHSS. The Au triangular nanoplates demonstrate superior catalytic activity than spherical Au nanoparticles.

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Introduction In recent years, much attention has been paid towards the synthesis of Au triangular nanoplates owing to their unique optical properties,1-2 enhanced catalytic activity,3-4 anisotropic electrical conductivity,5 and enhanced electric field

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compared to spherical Au

nanoparticles. Various applications have been demonstrated for Au nanoplates in sensing,7 bioimaging,8-9 photothermal therapy,10-11 catalysis,12-13 and biological diagnostics.1,

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To

date, a number of solution-based synthetic methods have been developed for noble metal triangular nanoplates and spherical nanoparticles including seed-mediated growth,1, 15 thermal method,16 photocatalytic approach,17-19 and biological

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method. For example, Mirkin and

coworkers used seed-mediated method for the synthesis of Au triangular nanoplates.15 Periara et al. employed photocatalytic approach by using tin (IV) porphyrin as a photocatalyst for the synthesis of Au triangular nanoplates in solution.19 Current synthetic methods to produce Au triangular nanoplates inevitably involve the use organic molecules or polymers such as polyvinylpyrrolidone (PVP),19-22 cetyltrimethylammonium bromide (CTAB),23-24 and other surfactants

25-26

as shape-control agents. Although it is typical to employ organic capping

ligands during the growth of Au nanocrystals to achieve different morphologies, the presence of these organic species on the surface of the resulting Au nanocrystals may drastically affect their catalytic activity,27-29 cause instability in harsh conditions,35 and also limit their capability for biological applications.21 Oftentimes, removal or replacement of the capping agents from nanocrystal surface has been a tedious step. It is still a great challenge to grow capping agent-free Au nanocrystals with controlled shape, size, and exposed facets. In an effort to make ligand-free noble metal nanocrystals, Tang and coworkers used hydrothermal method to synthesize icosahedral Au-Pt alloy nanocrystals inside the nanocavity of hybrid porous silica shells.27 Several methods are also reported for the synthesis of spherical metal nanoparticles without using capping ligands in the reaction.30-34 Most recently, we also developed a volume-confined method to tune the size of naked Au nanoparticles by soaking mesoporous hollow silica shells (mHSS) in HAuCl4 solutions of different concentrations followed by a simple heating process.29 The resulting ligand-free Au nanoparticles showed much enhanced catalytic activity compared to Au nanoparticles capped with citric acid.29 However, due to the absence of morphology-controlling agent, only spherical Au nanocrystals were produced with this method.29 In this report, we demonstrate for the first time that using a photochemical method, ligand-free Au triangular nanoplates can be synthesized in the presence of Ag+ ions without using any organic capping agents. The 2 ACS Paragon Plus Environment

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synthesis route for tuning the shape of Au nanoparticles is schematically shown in Figure 1a. Briefly, mesoporous hollow silica shells were soaked in a mixture of HAuCl4 dissolved in saturated NaCl aqueous solution, AgNO3 aqueous solution, and ethanol. The impregnated mHSS were isolated and then exposed to UV light to form Au triangular nanoplates free of any organic capping ligands. For the same synthesis but without using AgNO3, Au nanospheres instead of nanoplates were obtained, indicating the shape-selective effect of Ag+ ions in the synthesis.

Figure 1. (a) Schematic illustration for the growth of ligand-free Au triangular nanoplates and nanoparticles inside silica shells under UV irradiation. (b-d) TEM images of Au nanocrystals synthesized under UV irradiation for 24 hours at different molar ratio [AuCl4]:[Ag+] -- (b) 30:1; (c) 50:1; and (d) without Ag+. (e) UV-vis absorption spectra of the Au nanocrystals. 3 ACS Paragon Plus Environment


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Experimental Section Chemicals The chemicals vinyltrimethoxysilane (VTMS, 98%, Sigma–Aldrich), ammonia solution (NH3.H2O, 28-30%, Merck), chloroauric acid (HAuCl4.3H2O, 99.9%, Sigma-Aldrich), sodium chloride (NaCl, Sigma-Aldrich), silver nitrate (AgNO3, Merck), Hydrofluoric acid (HF, 48%, Merck), Polystyrene (PS) particles with a diameter of 124 ± 5 nm (5 wt.% aqueous suspension, of batch PS-R-B1161) were purchased from Microparticles GmbH Berlin). All chemicals were used as received without any further purification. Synthesis of vinyl-silica coated 124 nm PS beads (PS-124@vinyl-SiO2) A modified Stober’s method reported in the literature was employed for the coating of vinyl functionalized silica on the commercial PS beads of size 124-nm. For the preparation of core shell particles, 0.35 mL of VTMS was added in 6.65 mL of H2O under vigorous magnetic stirring at 9000 rpm for a period of 30 min. During the stirring process, the organic droplets were completely dissolved and a transparent solution was obtained. Simultaneously, 15 mL of 0.5 wt % PS particles were mixed with 0.88 mL of ammonia under magnetic stirring for 15 min. Then, the VTMS solution was added to the PS beads, ammonia and water mixture solution drop-wise, using micropipette and the reaction was further stirred at 900 rpm for a period of 6 hours at room temperature. After the completion of the reaction, the resulting PS-124@vinyl-SiO2 spheres were separated from the reaction by centrifugation (9000 rpm) for a period of 15 min and washed repeatedly with ethanol and water for three times to remove excess ammonia, water and unreacted VTMS. Finally, these particles were dispersed in 10 mL of H2O for future applications. Synthesis of mesoporous hollow silica shells by calcination at 450 ºC (mHSS) The hollow mesoporous spherical silica shells with inner diameter of 100 nm were fabricated from PS-124@vinyl-SiO2 by calcination method. 5 mL PS-124@vinyl-silica shells were taken in a glass petridish and calcination was done in a muffle furnace. The core-shell particles were heated to 450 ºC at a rate of 2 ºC / min for a period of 10 hours in air, and then slowly cooled down to room temperature. The as-obtained mesoporous HSS were dispersed in required amount of H2O under sonication and stored for future applications. As obtained mHSS possess average pore size of 3.8 nm with BET surface area 332.7 m2/g.


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Synthesis of spherical gold nanoparticles in absence of Ag+ ions 250 uL of mHSS (0.3 mg/mL) were centrifuged at 10,000 rpm for a period of 10 min. After removing the supernatant, the pellet was redispersed in a mixture of 250 uL of 0.1 M HAuCl4 + 5 uL ethanol solution using sonication and vortex for a period of 1min. The solution is allowed to stand at room temperature for a period of 12 hours to ensure complete diffusion of gold precursor inside the mesoporous silica shells. The mHSS impregnated with gold precursor and ethanol was isolated by centrifugation at 10,000 rpm for 10 min. The supernatant was removed completely and the pellet was collected in a 1.5 mL microcentrifuge tubes and irradiated under UV light of λ = 254 nm in a black box container. The samples were irradiated under UV light for different time periods such as 1, 6, 12 and 24 hr to study the growth of Au core inside mHSS. After irradiation with UV light for a particular time-period, Au@mHSS yolk-shell nanoparticles were washed with 1 mL of water for one time and redispersed in 0.1 mL water. Synthesis of gold triangular nanoplates in presence of Ag+ ions The synthesis process is similar to the method used for the synthesis of gold spherical nanoparticles in absence of Ag+ ions except that the pellet was redispersed in a mixture of 250 uL of 0.1 M HAuCl4 (prepared in saturated NaCl solution) + 5 uL ethanol + 50 uL AgNO3 aqueous solution of different concentrations such as 0.01 M and 0.0166 M to maintain molar ratio of [AuCl4-]:[Ag+] as 50:1 and 30:1 respectively. The samples were irradiated under UV light of λ = 254 nm for different periods such as 1, 6, 12 and 24 hours to study the growth of Au nanoplates inside mHSS. After irradiation, triangular Au nanoplates were washed with 1 mL of water for one time and redispersed in 0.1 mL water. Etching of gold triangular nanoplates with HF The as-synthesized Au triangular nanoplates were isolated by centrifugation and soaked in 500 uL of 0.1 M PVP for a period of 12 hours to ensure that PVP was completely diffused inside the cavity of mHSS and adsorbed on the Au triangular nanoplates. After soaking for a period of 12 hours, 2 mL of HF (48%) was then added to the sample. The mixture was incubated at 60 °C in a water bath for 30 minutes. After etching, the sample was washed with 1 mL of water for two times and redispersed in 0.1 mL water.



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Catalytic reduction of 4-nitrophenol by Au@SiO2 nanoprisms and spherical nanorattles The catalytic reduction reactions were carried out using a standard quartz cuvette with a 1cm path length. An excess amount of NaBH4 solution (1 mL, 50 mM) was mixed with 50 uL of 2 mM 4-nitrophenol solution in a quartz cell. To this mixture, 50 uL of Au@SiO2 nanoprisms (synthesized under UV irradiation for a period of 24 hours for a molar ratio of Au:Ag (30:1)) and Au@SiO2 spherical nanorattles (synthesized in the absence of Ag+ ions under UV irradiation for a period of 24 hours) were added to study the catalytic activity of these particles towards the reduction of 4-nitrophenol as model system. The UV-vis absorption spectra of the solution were recorded at different periodic intervals. Characterizations Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, Selective area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX) spectra were acquired using a JEOL JEM-2100F operating at 200 kV. UV-visible (UV-vis) spectra were recorded using a Shimadzu UV-1601 spectrometer with quartz cuvettes of 1-cm path length at room temperature. The samples were irradiated under UV light produced by a 6 W Hg lamp at shorter wavelength λ ≥ 254 nm (Spectroline, Model ENF-260C/FBE at 50 Hz frequency). Results and Discussion Porous mesoporous hollow silica shells (mHSS) were used as nano-containers for the synthesis of ligand-free Au triangular nanoplates. Properties of the mHSS are as follows:29 inner diameter: 100 nm; shell thickness: 16 nm, BET surface area: 332.7 m2/g; average pore size: 3.83 nm. The Au nanoplates were formed using a UV-light driven photochemical reduction method with a mixture of HAuCl4 dissolved in saturated NaCl, aqueous AgNO3 solution, and ethanol inside the mHSS (details of the synthesis can be found in Experimental Section). TEM images of the samples show that at an atomic ratio of Au:Ag = 30:1, the resulting Au nanostructures mainly consist of Au triangular nanoplates with an average edge length of 30 ± 4.4 nm (Figure 1b). For reactions at a higher Au to Ag ratio or without Ag+, the samples are mainly spherical Au particles with an average size of 28 ± 4.3 nm (Figures 1c-d). The different shapes of the Au nanocrystals can also be confirmed from the UV-vis absorption spectrum as shown in Figure 1e. For Au triangular plates obtained at a Au:Ag ratio of 30:1, a broad peak centered at 842 nm along with a sharp peak at 537 nm can be 6 ACS Paragon Plus Environment

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observed, consistent with the typical SPR of Au triangular nanoplates.16 For Au nanoparticles prepared without the use of Ag+ or at a higher ratio of Au:Ag, the UV-vis spectra only show the 537-nm peak, indicating their spherical shape. Elemental analysis of the Au triangular nanoplates reveals both Au and Ag with atomic ratio of Au:Ag = 33:1 (Figure S1), close to the molar ratio of HAuCl4 and AgNO3 added in the reaction (30:1). To further investigate the morphology of the Au triangular nanoplates formed inside mHSS, we etched the silica shells with HF in the presence of PVP. PVP was added in the etching process to ensure that Au nanocrystals can remain dispersed. TEM images of the Au nanostructures after the removal of the silica shell show that the majority of the particles have a triangular shape, although some other shapes including decahedron and octahedron can be also found (Figure 2a). The selected area electron diffraction (SAED) pattern of a single Au triangular nanoplate (Figure 2b). The indicated spots can be indexes as 1/3{422}, {220}, and 2/3{422} diffractions of Au, respectively.36 HRTEM image of a Au triangular nanoplate shows measured lattice fringes with a d-spacing of 0.234 nm corresponds to (111) plane of Au crystal.29 Figure 2d shows TEM image of a side-view of a Au triangular nanoplate and calculated thickness is ~ 10 nm.



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Figure 2. (a) TEM image of the Au triangular nanoplates after removal of the silica shells. (b) SAED pattern of a Au triangular nanoplate. (c) HRTEM image of a Au triangular nanoplate. (d) TEM image of a side view of a Au triangular nanoplate. The growth of Au nanoplates in the presence of Ag+ inside mHSS was monitored with the reaction time. Figure 3 shows the TEM images of Au triangular nanoplates obtained at 1, 6, 12, and 24 hrs, respectively. For a period of 1 hour, most of mHSS are empty without Au nanoparticles but for 6 hour duration, mHSS contain Au nanoparticle with an average size of 11 ± 1.6 nm (Figure 3a-b). As the time period of the reaction increases to 12 and 24 hours, most of mHSS contain Au triangular nanoplates with an average edge length 23 ± 3.1 and 30 ± 4.4 nm respectively (Figure 3c-d).

a

b

c

d

Figure 3. TEM images of Au triangular nanoplates synthesized in the presence of Ag+ ions under UV irradiation at different time periods: (a) 1, (b) 6, (c) 12 and (d) 24 hrs (Scale bars: 50 nm). The insets are the corresponding HRTEM images (Scale bars: 5 nm).


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Figure 4 shows the TEM images of growth of Au nanoparticles inside mHSS in absence of Ag+ ions irradiated under UV light for different time periods. The average size of Au nanoparticle inside mHSS was 8 ± 1, 20 ± 2.1, 28 ± 4.3 and 30 ± 3.01 nm for different time period of UV irradiation such as 1, 6, 12, and 24 hours respectively. The average size of Au nanoparticle increased from 8 ± 1 to 30 ± 3.01 nm inside mHSS as the duration of UV irradiation increased from 1 hour to 24 hours, which further confirms that growth of Au nanoparticle inside mHSS is time-dependent effect of UV irradiation. In addition to this, all the mHSS contain a single movable spherical gold nanoparticle inside mHSS, which further confirms that Ag+ ions play a key role in assisting the anisotropic growth of Au nanoparticles inside mHSS.38-39 The Ag+ ions are introduced in the reaction for tuning the shape of Au nanoparticles inside mHSS using AgNO3 aqueous solution. The silver nitrate aqueous solution may react with HAuCl4 solution to form AgCl precipitate. The AgCl precipitate can soluble in saturated NaCl solution and in order to avoid the formation of AgCl precipitate, HAuCl4 is dissolved in saturated NaCl solution. To further confirm the effect of NaCl solution in formation of Au triangular nanoplates, a control experiment conducted in presence of NaCl solution and ethanol without Ag+ ions gives only spherical Au nanoparticles (Figure S2c). This further confirms that NaCl would not change the growth mode of Au nanoparticles inside mHSS.



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Figure 4. TEM images of Au@mHSS synthesized in absence of Ag+ ions under irradiation of UV light for different time periods: (a) 1 hour, (b) 6 hour, (c) 12 hour and (d) 24 hour. (All scale bars are 50 nm). The plausible mechanism for the formation of gold nanoparticle inside mHSS may be that the AuCl4- ions and ethanol molecules can freely diffuse through the mesopores of mHSS and entrapped inside the cavity of mHSS. The ethanol molecules generate superoxide and ethoxy radicals40 in the presence of UV light (254 nm) which can act as strong reducing agents for the reduction of metal ions and convert AuCl4- ions to Au monomeric atoms. The concentration of Au monomeric atoms gradually increases and easily cross the supersaturation limit and initiates the nucleation for the growth of Au nanoparticle inside mHSS. It is noteworthy to state that no Au nanoparticle was seen in mHSS in the experiments conducted in the absence of ethanol (Figure S2b), thus indicating the key role of ethanol in the reduction of AuCl4- ions, since UV light alone does not cause initiation for Au nanoparticle nucleation38 inside mHSS. In addition to this, the control experiment conducted in the presence of ethanol without irradiation of UV light gives no metal nanoparticle inside mHSS (Figure S2a) which further confirms that ethanol alone cannot act as a reducing agent for the reduction of AuCl4- ions in the absence of UV light. A similar mechanism of UV-light driven generation of radicals for the reduction of AuCl4- ions has been reported in previous studies.38-39 It has been demonstrated that addition of Ag+ ions in the growth reaction are critical to promote anisotropic growth of Au nanoparticle. For example, Placido et al.38 investigated the key role of Ag+ ions in promoting the growth of Au nanorods in presence of CTAB surfactant using photochemical method. The authors claimed that Ag (0) adsorbs preferably onto {100} and {110} facets promoting the crystal growth along [010] direction of the fcc crystal lattice which leads to the formation of Au nanorods. Yang and coworkers39 also observed similar results but they claimed that Ag (0) is present only in the initial growth of Au nanorods and its absence in final growth of Au nanorods may be due to re-oxidation of Ag (0) to AgBr in the presence of AuCl4- ions. However, we observed majorly Au triangular nanoplates instead of Au nanorods in mHSS. The plausible reason could be that twinned Au seeds may develop inside mHSS in the presence of Ag+ ions. The HRTEM images obtained for initial Au seeds in the presence and absence of Ag+ ions in the reaction confirm the formation of twinned and single crystalline seeds respectively (Figure S3). The addition of Ag+ ions can lower the reduction kinetics41-42 of AuCl4- and promote the formation of twinned 10 ACS Paragon Plus Environment

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Au seeds in UV light driven reactions,42 since slower growth of metal atoms preferably forms twinned seeds.27 In addition, the formation of twin plane metal seeds can be enhanced in silver halide system.43-44 The crystallographic structure of initial Au seeds may influence the shape of the final nanocrystal.42-43 The presence of twin planes in Au seed may be responsible for plate-like growth of Au nanoplates.42, 44 The presence of 1/3 {422} reflections in SAED (Figure 2b) which are forbidden in a single crystal fcc metal may be attributed to parallel twin planes and are typically observed in Au and Ag nanoplates.41, 46 The TEM image of a single Au triangular nanoplate in side view shows the presence of a characteristic bright/dark contrast adjacent to the twin domains (Figure 2d) which further confirms the presence of a twin plane in Au nanoplate.19 The results are consistent with previous results on formation of Au or Ag nanoplates.6, 19, 41-42 A further detailed investigation is needed to better understand the growth mechanism of a Au triangular nanoplate inside the cavity of mHSS. Though we demonstrate the formation of Au triangular nanoplates using mHSS of cavity size 100 nm, but different sizes of mHSS can also be used to control the size of Au triangular nanoplates.29 The size of Au triangular nanoplates may be controlled either by tuning the cavity volume of mHSS with same gold concentration or by changing the gold concentration but with same cavity of mHSS.29

Figure 5. Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol catalyzed by (a) Au triangualr nanopaltes@mHSS and (c) Au@mHSS spherical 11 ACS Paragon Plus Environment


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nanoparticles, respectively. (b) ln(Ct/C0) vs. time for the reduction of 4-nitrophenol catalyzed with Au triangualr nanopaltes and spherical nanoparticles in the presence of excess NaBH4. (d) Conversion of 4-nitrophenol vs. time. The Au triangular nanoplates and spherical nanoparticles formed inside mHSS are bare surface particles without any capping ligands and can show enhanced catalytic activity. We have investigated the catalytic properties of these particles towards the reduction of 4nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess NaBH4 as a model reaction.29 The reduction kinetics of 4-NP to 4-AP can be monitored easily by UV-vis spectroscopy.47,

48

The aqueous solution of a mixture of 4-NP and NaBH4 showed an

absorption peak at 400 nm, which can be attributed to 4-nitrophenolate ions in alkaline solution.47, 48 In the absence of Au triangular nanoplates the peak at 400 nm unaltered for more than a day, confirming that the reduction of 4-NP will not take place in absence of Au triangular nanoplates. After addition of Au triangular nanoplates, the absorbance at 400 nm reduced and a new peak at 300 nm started to appear due to the conversion from 4-NP to 4-AP (Figures 5a, c).47-49 The concentration of NaBH4 in all the reactions was nearly 500-times higher than that of 4-NP. Therefore, the concentration of NaBH4 can be considered as a constant during the reaction.50 In this case, the reduction rate constant can be calculated based on pseudo-first-order kinetics.47-49, 51 Figure 5b shows the linear plots of ln(Ct/C0) vs. t (Ct and C0 correspond to the concentration of 4-NP at time t and its initial concentration, respectively), which fit well with the first-order reaction kinetics. The apparent rate constants (kapp) were calculated based on the slopes of the lines and found to be 2.6 × 10-3 and 1.03 × 10-3 sec-1 for Au triangular nanoplates and spherical nanoparticles respectively. The kapp of Au triangular nanoplates is nearly 2.5 times higher than that of spherical Au nanoparticles. Au triangular nanoplates showed enhanced catalytic activity compared to spherical Au nanoparticles. Figure 5d plots the conversion vs. time for the reaction catalyzed by Au triangular nanoplates and spherical nanoparticles. Au triangular nanoplates showed a conversion of nearly 90% for 10 minutes, much faster compared to spherical nanoparticles particles (50% in 10 minutes). It is worth noting that the kapp of Au triangular nanoplates and spherical nanoparticles is higher or similar than that of other Au@SiO2 nanorattles reported in the literature.29, 38, 47, 50-51 The enhanced catalytic activity of Au triangular nanoplates and spherical nanoparticles can be attributed to the absence of capping ligands on their surface. In general, the presence of capping ligands on the surface of Au nanocatalyst may reduce the


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catalytic activity by blocking the active sites and hindering the access of reactants to the nanoparticle surface.52-53

Conclusions In summary, we successfully tune spherical gold nanoparticles to Au triangular nanoplates inside the nanocavity of mesoporous silica shells. The amount of added Ag+ ions in the reaction may indicate the final shape of Au nanoparticles. The yield of Au nanoplates inside the nanocavity of mHSS increases with the periodical irradiation time of UV light. The Au triangular nanoplates exhibited better catalytic activity for the reduction of 4-nitrophenol compared to spherical nanoparticles. Though this method is demonstrated for tuning the shape of Au nanoparticles but this method can be extended for tuning the shape of other noble metal nanoparticles such as Pt and Pd. We believe this research may be helpful not only in understanding the mechanism for the growth of ligand-free anisotropic metal nanoparticles in a confined space and on the development of new synthetic strategies for ligand-free complex anisotropic metal core yolk-shell nanoparticles. Supporting Information EDX and elemental mapping of Au nanoplates, TEM images of Au@SiO2 nanorattles prepared in presence and absence of ethanol, and saturated NaCl, and HRTEM images of initial Au seeds obtained with presence and absence of Ag+ ions in the reaction. This information is available free of charge via the Internet at http://pubs.acs.org Author Information Corresponding Author *Email: [email protected].

Notes The authors declare no competing financial interests. Acknowledgments The authors thank Prof. Lu Xianmao for his valuable suggestions and for NUS awarding a research scholarship.



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