High Catalytic Performance of Raspberry-Like Gold Nanoparticles (Au RLNPs) and Enhancement of Stability by Silica Coating
Abstract
The raspberry-like gold nanoparticles (Au RLNPs) synthesized through the reduction of HAuCl4 by the use of NaOH and Brij35 surfactant show high catalytic activities in the reduction of 4-nitrophenol and ethanol electrooxidation. The enhanced catalytic activities of Au RLNPs are mainly due to their high surface area. However, Au RLNPs easily change to the spherical or aggregated nanoparticles in a treatment with acids, thiols, and cationic surfactants (ex, CTAB)), and are difficult to sustain the catalytic activities. To improve the stability and applicability, Au RLNPs core–silica shell nanoparticles (Au RLNPs@SiO2 NPs) were successfully synthesized in solution without losing their original morphologies through a simple solution-phase sol-gel process with the assistance of surface-stabilizing polymeric agent (Polyvinylpyrrolidone (PVP)). In comparison with Au RLNPs and other Au nanoparticles, Au RLNPs@SiO2 NPs could be more easily recovered and recycled in the repeated catalytic reactions.
Keywords
Raspberry-like gold nanoparticles (Au RLNPs), Silica coating, Catalytic reduction, Ethanol electrooxidation, Polyvinylpyrrolidone (PVP)
1. INTRODUCTION
Noble metals have gained much attention over the past two decades due to their potentials in a wide variety of applications including energy conversion[1][2], chemical and biological sensing[3], and bioengineering[4]. Tremendous research efforts have been devoted towards the exploration of how to design nanomaterials with varied topographies that has led to the discovery of their fundamental size-, shape-, and component-dependent properties and the development of new applications[5][6][7]. Moreover, it has theoretically and experimentally found that arrays of asymmetric surface features, particularly deviations from spherical geometry, mainly impart unique anisotropy in material properties7. Apparently, to achieve such desired anisotropic topographies strict control is required. Conversely, this leads to a generation of particles with novel properties from the same materials by simply tuning the particle morphology. Furthermore, anisotropic geometry offers numerous unique features and functionalities that are either difficult to obtain or even hardly obtained by simple size-tuning in spherical counterparts. Morphology of nanoparticles also strongly affects the catalytic performance. This is due to the surface anisotropy possessing a high density of low-coordinated atoms such as steps, edges, and defects serving as catalytically active sites which can markedly affect chemical and physical properties of nanoparticles[8].

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Among those colloidal gold (Au) nanoparticles exhibit not only highly tunable architecture-dependent optical properties but also show excellent performance and high selectivity in a variety of heterogeneous green catalytic processes [ref][9][10]. For a better stability, catalytic performance, and reusability of Au nanoparticles, engineering new nanocatalyst system is thus considered one of the most critical tasks. Recently, in our group, we successfully synthesized raspberry-like gold nanoparticles (Au RLNPs) with rich edges and high surface areas through the reduction of HAuCl4 by Brij35 surfactant under basic condition in a controllable fashion [ref]. The synthesized Au RLNPs possess high surface areas and show the unique, highly-red shifted surface plasmon resonances (SPRs) due to the rough, raspberry-like surface of Au RLNPs. These structures also have high surface energies due to their plenty of tips and edges. These nanoparticles are stable and retain their raspberry-like geometry in basic or neutral conditions; however gradually reshape to the spherical geometry under a specific circumstance such as acidic condition. In order to exploit the unique shape-dependent properties of the Au RLNPs in a variety of catalytic applications, further modifications of nanoparticles such as endowing core-shell structure are thus required.
Metallic nanostrutures of several different shapes have been coated with silica since the silica shells used as the coating material show substantial enhancement in the stability of the metal cores, particularly in aqueous solvents. Moreover, metallic nanostructured surface can readily functionalized by subsequently coating with silica and using silane-coupling reactions [ref]. Additionally, silica shells are chemically inert, transparent in the visible and IR regions of the spectrum, and readily converted to mesoporous layer [11]. For the direct encapsulation of Au nanoparticles within silica shells, the conventional techniques is employing coupling agents with silane group for the growth of silica shells on the surfaces of as-synthesized Au nanoparticles via the Stöber method [ref]. However, we experimentally found that directly applying this method to coat Au RLNPs brought challenges since the unusual size changes of Au RLNPs without disturbing the rough surface occurred.
Herein, we report the synthesis of Au RLNPs@SiO2 NPs in solution through a simple solution-phase sol-gel process. To protect the high-energy surface of Au RLNPs, Polyvinylpyrrolidone (PVP) was used prior to the condensation of TEOS as a polymeric stabilizer. Au RLNPs@SiO2 NPs showed great enhancement in stability under the strongly acidic condition. The catalytic performance, recovery, and reusability of both Au RLNPs@SiO2 NPs and Au RLNPs were investigated using the reduction reaction of 4-nitrophenol (4-NP) as a reaction model. We also found that and Au RLNPs were capable of electrocatalyzing alcohol oxidation reactions in alkaline media.
2. EXPERIMENTAL DETAILS
2.1. Reagents
Polyoxyethylene glycol dodecyl ether ((C2H4O)23C12H25OH, Brij35, Acros Organics), hydrogen tetrachloroauratetrihydrate (HAuCl4•3H2O, 99.9%, Sigma–Aldrich), polyvinylpyrolidone ((C6H9NO)n, PVP10, average mol wt 10,000, Sigma–Aldrich), 4-nitrophenol (O2NC6H4OH, 99%, Sigma–Aldrich), sodium hydroxide (NaOH, 97%, Sigma–Aldrich), ammonium hydroxide (NH4OH, 28-30 wt % ammonia, Sigma–Aldrich), tetraethyl orthosilicate (Si(OC2H5)4 98%, Sigma–Aldrich), hexadecyltrimethylammonium bromide ((C16H33)N(CH3)3Br, 99%, Acros Organics), (3-mercaptopropyl)methyldimethoxysilane (CH3Si(OCH3)2CH2CH2CH2SH, 95%, Alfa Aesor), HCl, HNO3, and ethyl alcohol were used as received. All stock solutions were freshly prepared before each reaction. Prior to use, all glassware was washed with Aqua Regia (volume ration of 3:1 of concentrated HCl and HNO3; Caution: Aqua Regia is highly toxic and corrosive and must be handled in fume hoods with proper personal protection equipment) and rinsed thoroughly with deionized water.
2.2. Synthesis of raspberry-like gold nanoparticles (Au RLNPs)
Au RLNPs with the mean size of approximately 60-70 nm were prepared according to our previous literature [ref]. Briefly, an aqueous Brij35 solution (1 mL; 19.3 wt%) was well mixed with NaOH (aq) (100 µL; 100mM) by shaking for 30 seconds. To this mixture, HAuCl4 (aq) (50 µL; 10 mM) was added, and shaken vigorously for 1 minute. The pale yellow reaction mixture then turned to blue within 5 minutes at room temperature. To make sure a complete reaction, this mixture was allowed to react for over 20 minutes before being collected by centrifugation (5 min; 13500 rpm), and redispersed in deionized water.
2.3. Synthesis of Au RLNPs@SiO2 NPs
The preparation of Au RLNPs@SiO2 NPs was as follows: firstly, the as-synthesized Au RLNPs were dispersed in 1 mL of deionized water. Next, 0.235 mL polyvinylpyrrolidone (PVP10) aqueous solution (128 mg of PVP10 in 10 mL of deionized water) was added to the Au RLNPs solution. The resulting mixture was then stirred at room temperature for 12 hours to ensure complete adsorption of PVP on Au RLNPs. Afterward, the PVP-capped RLNPs were purified by centrifugation (5 min; 13500 rpm), and redispersed in solvent mixture containing 1 mL deionized water and 7 mL ethyl alcohol. In the next step, tetraethylorthosilicate (TEOS, 0.03 mL) and ammonium hydroxide (0.2 mL of 14.8 M NH4OH (aq.)) were sequentially added to the PVP-capped Au RLNPs aqueous solution and the reaction mixture was further stirred at room temperature for 4 h. After the completion of the reaction, the resultant Au RLNPs@SiO2 NPs were centrifuged, and purified by repeatedly washing in ethanol and centrifugation.
2.4. Catalytic reduction of 4-nitrophenol
The catalytic reduction of 4-nitrophenol (4-NP) over nanoparticles in the presence of NaBH4 was carried out to assess the catalytic activity. In a typical experiment, 2 mL of deionized water, 1.7 mL of 0.2 mM 4-NP, and 1 mL of 15 mM NaBH4 solutions were mixed in a quartz cuvette followed by the addition of 1 mL of Au RLNPs@SiO2 NPs solution. The color of solution changed gradually from yellowish to clear as the reaction proceeded. UV-Vis spectra were recorded at a 5-minute intervals to monitor the progress of the reaction.
2.5. Ethanol electrocatalytic oxidation
All electrochemical measurements were carried out in a conventional three-electrode cell at ambient temperature (~25ËšC) using WPG 100e Potentiostat (WonAtech Inc.). The fabrication of working electrode is as follow: Prior to electrochemical experiments, glassy carbon (GC) electrode was sonicated in ethanol and deionized water successively. 10 µL of RLNP suspension was dropped onto carbon disk and the solution is dried at room temperature. Platinum and Ag/AgCl were employed as counter and reference electrodes, respectively. With an aqueous mixture of 0.5 M KOH and 1.0 M ethanol as electrolytes, at least 10 cycles of cyclic voltammetry were carried out before recyclable voltammograms were recorded. Throughout the cyclic voltammetry experiments, the potential window was between -0.2 V and 0.8 V. Prior to experiments, the electrolytes were degased by bubbling with nitrogen for 30 min.
2.6. Characterization
The nanoparticles were imaged using a Hitachi S-4800 scanning electron microscope (SEM), and a JEOL JEM-2010 Luminography (Fuji FDL-5000) Ultramicrotome (CRX) transmission electron microscope (TEM). Samples were prepared for TEM by concentrating the nanoparticle mixture by centrifuging twice for 5 min at 13500 rpm with resuspension in 100 μL nanopure water and immobilizing 10 μL portions of the solution on Formvar-coated Cu grids. Extinction spectra were recorded with a UV-vis spectra spectrometer (UVIKON XS). Solution pH was measured using an Orion 420 A+ pH meter.
3. RESULTS AND DISCUSSION
Initially, the highly monodisperse Au RLNPs with controlled diameters ranging from 60 to 70 nm (Fig. 1a and S1) [Images and size distribution of RLNP] were prepared according to protocols developed previously.ref Polyvinylpyrrolidone (PVP) was then employed as a primer and a direct growth of silica onto the PVP-capped Au RLNPs to obtain Au RLNPs@SiO2 NPs was carried out using solution-phase sol-gel method with TEOS as a precursor. PVP, which have been often used as a surface-stabilizing polymeric agent to prepare spherical Au core – SiO2 shell nanoparticles,ref was used to protect the high-energy surface of Au RLNPs. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Au RLNPs@SiO2 NPs (Fig. 1b-d) show the well-established core-shell structure in which the as-synthesized Au RLNPs were uniformly and individually encompassed within silica shells whereas still sustaining their rough and edge-rich surfaces advantageous to catalytic performance. The average diameter of individual Au RLNPS@SiO2 NPs was xxxxx (xxx particles was evaluated, Fig 1e). With respect to RLNPs cores, the average size was nearly similar to that of RLNPs before silica-coated (Fig S1). The UV-Vis spectra plotted in Fig. 1f, a noticeable broadeness in the corresponding surface plasmon bands in Au RLNPs@SiO2 NPs compared to that of Au RLNP were observed. It is well know that the weaker and broader surface plasmons are observed,, due to the change of refractive index of surrounding environment after silica coating step[b1].ref
[Fig 1]
High catalytic efficiency of Au nanocatalysts were mainly due to their high roughness and plethora of edge-rich surfaces and corners.ref Thus it necessitates assessing whether catalytically active surfaces of the synthesized nanoparticles are stable in various environments. HCl, CTAB, and MPTS were introduced into the colloidal solution of the as-synthesized RLNPs in order to understand the stability of Au RLNPs in different ambiences. Fig. 2 shows typical SEM images displaying changes in geometries of Au RLNPs as adding diverse reagents. It is experimentally observed that Au RLNPs collectively changed to spherical nanoparticles with the smooth surface as adding HCl (Fig S2)ref corresponding to a blue-shift in UV-Vis spectrum toward approx. 520 nm which can be assigned to the surface plasmon resonance (SPR) of gold nanospheres. This phenomenon could be attributed to the oxidative etching effect which has been employed to control the size of other noble nanostructures in recent papers[12] [13] [14]. Meanwhile, as shown in Fig. 2a, much agglomeration occurred as adding CTAB. However, it is interesting to note that the aggregation also occurred without disturbing their raspberry-like motifs, as adding MPTS (Fig. 2b). The SPR changes shown in UV-Vis spectra (Fig. 2e) further confirmed this aggregation. The introduction of such reagents, which might affect the hydrodynamic layer thickness of Au RLNPs, accounts for this unusual alteration in particles size. The RLNPs@SiO2 NPs however exhibited no geometrical change when HCl was added. As shown in Fig. 3a, SEM images show that the RLNPs@SiO2 NPs still retained their original raspberry-like morphology without any observable agglomeration. This observation is also consistent with results obtained from UV-Vis spectra (Fig. 3b) that there was no detectable shift in SPR peak of the core-shell nanoparticles after HCl had been added.
Ethanol comprises a lower toxicity, a higher theoretical energy density (8.01 kW.h kg-1) than methanol (6.09 kW.h kg-1) and formic acid (1.74 kW.h kg-1), and fewest environmental issues[15] [16]. Moreover, ethanol is a renewable source that can be easily produced massively from the chemical industry or fermentation of biomass. In this study, electrooxidation of ethanol [HM2]in KOH solution was performed to probe relative electrocatalytic activities of the synthesized nanoparticles. Fig. 4 shows the cyclic voltammograms of RLNPs, HCl-etched RLNPs[HM3], and RLNPs@SiO2 NPs for ethanol electrooxidation. It is clear that the RLNPs exhibit almost substantially higher electrocatalytic performance with a forward oxidation current (iF) value of 0.56 mA compared to that for HCl-etched RLNPs (iF, 0.07 mA). The high electrocatalytic activity of the RLNPs is attributed to the existence of high energetic surfaces in raspberry-like morphologies. However, RLNPs@SiO2 NPs did not show any electrocatalytic activities over the entire potential window. This is explained that silica shells hindered the electron transfer between gold cores and electrode due to silica shells are insulating.
The catalytic reduction of 4-NP to their corresponding derivatives, 4-aminophenol, in the presence of NaBH4 was chosen as a model reaction in order to evaluate the catalytic activity of Au RLNPs@SiO2 NPs. It is well established that the reduction of 4-NP by NaBH4 is thermodynamically feasible but kinetically restricted without a catalyst. The reduction progress was monitored by UV-Vis absorption spectra after the addition of catalysts. The characteristic absorption peak of 4-NP aqueous solution was located at 400 nm after NaBH4 had been added. First of all, in the absence of catalysts the reduction reaction of 4-NP did not proceed even with a large excess of NaBH4. However, when catalysts were introduced, the reduction of 4-NP was clearly observed. The absorbance of the reaction mixture at 400 nm gradually decreased as the reaction proceeded, along with the concomitant increase of 300 nm peak, corresponding to 4-aminophenol. Fig. 4 illustrates the UV-Vis spectra changes of 4-NP as a function of reaction time in the presence of Au RLNPs (Fig. 4a) and Au RLNPs@SiO2 NPs (Fig. 4c). Fig. 4e shows the change in concentration of 4-NP was plotted versus time, providing a general view to compare catalytic activities of Au RLNPs and Au RLNPs@SiO­2 NPs (Ct: absorbance of 4-NP at specific reaction time, t; C0: initial absorbance of 4-NP as catalysis starts). The Ct/C0 is measured from the relative intensity of absorbance (At/A0). As can be seen, Au RLNPs exhibited comparatively higher catalytic activity than their core-shell counterparts, possibly owning to silica shell hindering the diffusion of reactants onto inner gold active sites. Interestingly, in the presence of HCl, the catalytic activity of Au RLNPs@SiO2 NPs however was not only improved, but also dramatically higher than that of Au RLNPs which was suffering from the morphological change, leading to severe degradation of active sites (Fig. 4e). In addition, we also investigated the degree of reusability of the two catalysts. As shown in Fig. 5, the catalytic efficiencies of Au RLNPs decreased remarkably after reused 3 times whilst Au RLNPs@SiO2 still retained good catalytic performance for as far as 7 cycles. It is apparent that the stability and reusability of Au RLNPs were improved significantly after encapsulated into silica shell, resulting in maintenance in their catalytic activity.
4. CONCLUSIONS
Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0008968). This research was also supported by Hallym University Research Fund 2012 (HRF-G-2012-3)
References and Notes

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[b1]When gold NPs’ re encapsulated within silica, there’s generally red-shift in the absorbant peak of SPR. Please check this data once again.
[HM2]In Fig. 4, I think it better to insert the 2 images of RLNP and Hcl-etched RLNPs to say that the latter surface is not as edge-rich as the former.
[HM3]Considering how to name RLNPs whose morphology was change to sphere as adding HCl