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Monodisperse Raspberry-Like Gold Submicrometer Spheres: Large-Scale Synthesis and Interface Assembling for Colloid Sphere Array Shaojun Guo, Shaojun Dong, and Erkang Wang*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3581–3585

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ReceiVed January 8, 2008; ReVised Manuscript ReceiVed July 8, 2008

ABSTRACT: We first suggested a one-pot method to synthesize monodisperse raspberry-like submicrometer gold spheres (MRSGS) with high yield. The resulting gold spheres were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersed X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical technology. It was found that the rough structure provided by raspberry-like gold spheres led to a tremendous electrochemical active area, which was very important because these novel hierarchical gold spheres will probably find important applications in biosensors, electrocatalysis, and others. The formation mechanism of the raspberry-like gold spheres was also discussed. Furthermore, we expanded a liquid-liquid interface assembling strategy to MRSGS systems and first constructed uniform raspberry-like gold sphere arrays on a solid substrate. Introduction Noble-metal nanostructures with specific size and morphology have attracted increasing interest because of their shape- and size-dependent physicochemical properties and potential uses in catalysis, electronics, and biology.1 In particular, gold nanostructures such as nanoparticles,2 polyhedrons,3 nanowires,4 nanotubes,5 nanopyramids,6 nanoplates,7 nanoporous,8 and nanobelts9 have attracted more attention on account of their numerous potential applications in science and engineering. For instance, Yang et al.3b reported the shape and size control of polyhedral gold nanocrystals by a modified polyol process. Ravishankar and co-workers4a have shown that Ultrafine single-crystalline gold nanowires can be produced in large quantities by an oriented attachment of gold nanoparticles. Han et al.9 developed an ultrasonic method for synthesizing single-crystalline gold nanobelts through the combination of ultrasound irradiation and a biological directing agent. Although a lot of progress has been achieved concerning the synthesis of gold nanostructrues with specific morphologies, there has only been limited success in the preparation of hierarchical gold assembly architecture.10 Prominent examples involve photochemical preparation of twodimensional (2D) gold spherical pores and hollow sphere arrays on a solution surface;10d templateless, surfactantless, and simple electrochemical routes to rapid synthesis of three-dimensional (3D) flowerlike gold microstructures;10e and straightforward methods to synthesize raspberry-like gold microspheres from a commercial gold(I) plating solution.10f However, one-pot, largescale synthesis of monodisperse raspberry-like submicrometer gold spheres (MRSGS) is still a great challenge. On the other hand, there is a large effort in the preparation of novel materials based on 2D or 3D arrays of nanoparticles due to the exciting optical,11 electrical,12 and magnetic13 properties of such systems. Particularly, the integration of nanoparticles into 2D or 3D structures can bring more novel collective properties that can be manipulated by control of the size and morphologies of the nanoparticles and the cooperative interactions between the nanoparticles.14 Inspired by this, several groups have developed advanced assembling technologies for * To whom correspondence should be addressed. E-mail: [email protected].

building 2D or 3D arrays of nanoparticles. For instance, hydrophobic or hydrophilic inorganic nanoparticles can be facilely assembled into 2D or 3D arrays by controlled evaporation of solvents15 or at the water/air interface by using the Langmiur-Blodgett technique.16 More recently, it has been demonstrated that hydrophobic or hydrophilic nanoparticles could also be trapped at liquid-liquid interfaces and assembled into intact 2D arrays, which was induced by the destabilization of nanoparticles.17 Although 2D or 3D arrays of nanoparticles have been well-developed, there are no papers reported on assembling hierarchical raspberry-like gold submicrometer sphere into 2D arrays, which is very important because controlled self-assembly of hierarchical micro/nanostructured materials might bring more novel properties caused by the particular morphology of building blocks. In the present work, we first suggest a one-pot method to synthesize MRSGS with the size controlled to be much smaller than that reported in the previous literature.10f The resulting gold spheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energydispersed X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical technology. It is found that the rough structure provided by raspberry-like gold spheres leads to a tremendous electrochemical active area, which is very important because these novel hierarchical gold spheres will probably find important applications in biosensors, electrocatalysis, and others. The formation mechanism of the raspberry-like gold spheres is also discussed. Furthermore, we expand a liquid-liquid interface assembling strategy to MRSGS systems and first construct uniform raspberry-like gold sphere arrays on a solid substrate. Experimental Section Chemicals. Sodium acetate, aniline, and HAuCl4 · 4H2O were purchased from Beijing Chemical Factory (Beijing, China) and used as received without further purification. TritonX-100 was purchased from Aldrich and used as received. Water used throughout all experiments was purified with a Millipore system. Apparatus. A XL30 ESEM SEM was used to determine the morphology and composition of products. TEM measurements were made on a Hitachi H-8100 EM with an accelerating voltage of 200

10.1021/cg800023d CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

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kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on a carbon-coated copper grid and dried at room temperature. XPS measurement was performed on an ESCALABMKII spectrometer (VG Co., United Kingdom) with Al KR X-ray radiation as the X-ray source for excitation. The sample for XPS characterization was dropped on a glass plate. XRD analysis was carried out on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 30 mA) radiation. Cyclic voltammetric (CV) experiments were performed with a CHI 832 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). A conventional three-electrode cell was used, including a Ag/AgCl (saturated KCl) electrode as a reference electrode, a platinum wire as a counter electrode, and bare or modified gold (2 mm) as a working electrode. Synthesis of Hierarchical Gold Spheres. In a typical synthesis process for the raspberry-like gold spheres, 0.82 g of sodium acetate dissolved in 17 mL of deionized water was mixed with 100 µL of aniline and 300 µL of TritonX-100 under stirring to form a uniform emulsion of aniline-TritonX-100. After the mixture was stirred for 10 min, 2 mL of an aqueous solution of HAuCl4 (1%) was added. The resulting solution was stirred for another 0.5 min to ensure complete mixing, and then, the reaction was allowed to proceed without agitation for more than 24 h at 4 °C. The resulting precipitate was centrifuged and washed with water several times. Thus, the MRSGS with the diameter of about 170 nm were obtained (2 mL). Preparation of MRSGS Modified Electrode. The gold electrode was loaded with 3 µL of raspberry-like gold sphere solution. CV measurements were carried out in a 0.5 M H2SO4 solution at the scan rate of 100 mV/s. Self-Assembly of Hierarchical Gold Spheres. A 8 mL sample of the hierarchical gold spheres (about 170 nm) diluted aqueous solution was transferred to a surface of plate, and 6 mL of toluene was added to the top of a colloid solution surface to form an immiscible water/ toluene interface. Then, 2 mL of ethanol was added dropwise to the surface of the water/toluene layers, leading to gold spheres trapped at the interface.

Results and Discussion The morphology of the raspberry-like gold spheres was investigated by SEM and TEM. Parts A-C of Figure 1 show the typical SEM images of the as-prepared gold spheres coated on the silicon substrate at different magnifications. As is shown, the silicon substrate is covered with a great deal of gold spheres with narrow size distributions. From the magnified image (Figure 1C), it can be seen that monodisperse gold spheres are an average diameter of about 170 nm. Structural details are revealed by TEM images. Figure 1D,E shows the typical TEM images of gold spheres. Interestingly, the surface of gold sphere is covered with a great number of tubercles. It should be noted that the rough surface of raspberry-like gold spheres endows them with a higher electrochemical active area as compared to bare gold electrode, which can result in a higher turnover for heterogeneous catalytic reactions or electrochemical reactions. To verify this statement, we have deposited MRSGS onto the surface of gold electrode (the corresponding SEM image is similar to that shown in Figure 1A-C). It can be seen that the peak currents associated with oxide formation/reduction events provided by MRSGS (Figure 2a) are 126 times larger than those of the bare gold electrode (Figure 2b). It is thus clear that the rough structure provided by raspberry-like gold spheres contributes to the increase in their active surface area. The chemical composition of MRSGS was determined by EDX. The EDX spectrum (Figure 1F) with only one main peak corresponding to Au (other peaks originated from substrate and ligands) revealed that these raspberry-like spheres were metallic Au. XRD analysis was used to characterize the chemical composition and crystal structure of raspberry-like gold spheres. A typical powder XRD pattern is shown in Figure 1G. Four peaks associated with the 111, 200, 220, and 311 diffractions of the face-centered cubic (fcc) gold structure [Joint Committee

Figure 1. SEM images (A-C) of the raspberry-like gold submicrometer spheres coated on the silicon substrate at different magnifications; TEM images (D and E) of the raspberry-like gold submicrometer spheres; and EDX (F), XRD (G), and XPS (H) patterns of the raspberry-like gold submicrometer spheres.

Figure 2. CVs of the raspberry-like gold sphere (a) modified gold electrode and bare gold electrode (b) in 0.5 M H2SO4 aqueous solution. The inset corresponds to curve b in the figure. Scan rate, 100 mV/s.

on Powder Diffraction Standards (JCPDS) file: 04-0784] can be seen, indicating that the gold spheres are made of pure crystalline gold. XPS was used to further characterize the MRSGS. The XPS pattern (Figure 1H) of the as-prepared product shows significant Au4f signals characteristic of metallic Au. Also, a typical UV-vis spectrum of raspberry-like gold

Monodisperse Raspberry-Like Gold Submicrometer Spheres

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Figure 3. SEM (A) and TEM (B) images of raspberry-like gold spheres obtained by increasing aniline to 200 µL and SEM (C) and TEM (D) images of raspberry-like gold spheres obtained by increasing aniline to 300 µL.

spheres is shown in Figure S1 of the Supporting Information. A wide absorption at >600 nm could probably be ascribed to gold nanoparticles with big sizes (submicrometer).18 We also examined the influence of the concentration of aniline on the morphologies of the structures thus formed. We prepared two samples, under identical conditions used for preparing sample 1 (see the above part), by increasing aniline to 200 (sample 2) and 300 µL (sample 3), respectively. To our surprise, the diameter of gold spheres could be controlled by changing the concentration of aniline. When 200 µL was employed in this synthesis system, the diameter of raspberry-like gold spheres was about 210 nm (Figure 3). Figure 3A shows a typical SEM image of the obtained precipitate. TEM images (the inset of Figure 3A,B) indicate that the gold spheres still have raspberrylike structures. By continuously increasing the concentration of aniline (300 µL), the diameter of raspberry-like gold spheres is further increased. Figure 3C shows the SEM image of the obtained product. A great deal of gold spheres can be observed. The corresponding TEM images (the inset of Figure 3C,D) indicate that the diameter of gold spheres is about 500 nm. The concentrations of sodium acetate on the morphologies of product were also investigated. We prepared two samples, under identical conditions used for preparing sample 2, by decreasing sodium acetate to 0.164 and 0 g, respectively. When sodium acetate was not used in the synthesis solution, the gold spheres with different sizes and relatively smooth surface were obtained, as shown in Figure S2B,D of the Supporting Information. When the concentration of sodium acetate was further increased (0.164 g), the gold spheres with rougher surfaces than those obtained without sodium acetate are observed (Figure S2A,C of the Supporting Information). Therefore, the high concentration of sodium acetate is of vital importance to obtain the raspberry-like gold spheres. It is found that stirring when synthesizing also plays an important role in the morphology and size of gold spheres. A number of rough gold spheres with bigger sizes were obtained when stirring than without stirring (Figure S3A,C of the Supporting Information). We further prepared one sample, under identical conditions used for preparing sample 2, by increasing TritonX-100 to 500 µL. It is found that the gold spheres (Figure S3B,D of the Supporting Information) with a diameter of about 500 nm and relatively

Figure 4. TEM images of raspberry-like gold spheres obtained by increasing aniline to 200 µL at different reaction times: 30 s (A and B), 30 min (C and D), and 6 h (E and F).

smoother surface than sample 2, indicating the high concentration of TritonX-100, played a negative effect for obtaining raspberry-like gold spheres. To understand the formation mechanism of raspberry-like gold spheres, the morphologies of the as-synthesized gold spheres at different reaction times were measured by TEM. It is found that several big gold spheres and some small gold nanoparticles (Figure 4A) were obtained after the reaction of 30 s (please note that the synthesis condition is the same as sample 2). Figure 4B shows the magnified TEM image of a big gold sphere with a diameter of about 90 nm. Interestingly, many 1D irregular short gold wires were extruding from raspberry-like gold nanospheres. When the reaction time is extended to 30 min, the big gold spheres with urchinlike structures further grew to about 200 nm. Gold nanoparticles and some short irregular gold nanowires were also observed (Figure 4C,D). By continuously increasing the reaction time to 6 h, the amount of short irregular gold nanowires further increased besides the big spheres that gradually grew (Figure 4E,F). Please also note the following experiment facts: (i) At different reaction times, the quantity of the precipitation is different. For instance, only very few precipitations were observed at the bottom of beaker after reacting for 30 s. When the reaction time was increased to 6 h, more precipitations were obtained. However, the solution was still dark and not clear. After reacting for more than 24 h, the entire product was deposited at the bottom of the beaker. (ii) The color of the solution gradually changed from pink, dark pink, dark blue, to dark yellow (the color of precipitation is dark blue) after reacting. These observations indicate that self-assembly of gold building block (irregular short gold wire) probably plays an important role in the morphology of submicrometer gold spheres. A plausible formation process is briefly presented as follows: Because TritonX-100 is an amphiphilic molecule, micelles

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Figure 5. Photograph (A) of the self-assembled raspberry-like gold sphere array film at the water/toluene interface. SEM image (B) of raspberrylike gold sphere array film. The insets are higher magnification images of selected area of panel B.

would be formed in the bulk solution, and ethylene oxide (EO) chains of TritonX-100 would locate in the outer parts of the micelles due to their hydrophilic features. When aniline is added, the hydrogen bonds between anilines and the EO groups of TritonX-100 drive the aniline molecules to be adsorbed on the EO chains. In the meantime, the aniline monomer could also diffuse to the inside of the spherical Tritox-100 micelles to form spherical micelles filled with aniline monomer because of the hydrophobic nature of aniline.19 These aniline-filled spherical micelles can probably serve as a “microreactors” for gold spheres. Moreover, according to the corresponding literatures, an aniline-TritonX-100 conjunction could probably serve as the “soft template” to form 1D nanostructures.20 Once HAuCl4 is added as the oxidant, the reduction reaction only takes place at the water/micelle interface on the surface of the spherical micelles because HAuCl4 is hydrophilic. At short times, HAuCl4 is rapidly reduced due to its high concentration. Several big gold spheres with irregular short gold wires extruding and some small gold nanoparticles (Figure 4A) will occur. This indicates that irregular short gold wires have a high surface energy and easily adsorb on the gold spheres to reduce the surface energy of the total system. As the reaction proceeds, the more irregular short gold wires would produce, and they would spontaneously pack with each other to reduce the surface energy, due to the large surface energy of such individual nanostructures. Finally, the MRSGS could be obtained. It should be noted that sodium acetate also plays an important role on this synthesis system. Several literatures21 have reported that sodium acetate could provide large electrostatic attraction to prevent the aggregation of small inorganic nanoparticles. Herein, sodium acetate probably played a similar role. When synthesizing, it could probably provide large electrostatic attraction to obtain irregular short gold nanowires in the reaction process. It would be interesting to explore if the as-prepared hierarchical raspberry-like gold spheres could be used for fabricating 2D nanosphere arrays. Herein, a liquid-liquid interface assembling strategy has been employed to obtain well-defined hierarchical gold sphere arrays on a solid surface. Figure 5A shows a typical optical image of as-prepared assembling film. It is found that a uniform film has been obtained over a large area. Note that the area of the assembling film could be controlled simply by adjusting the concentration of gold spheres in solution and the interfacial area of the two phases. It is also found that the color (dark) of the assembling film was different from that obtained from the solution (dark blue), which was probably caused by the located surface plasmon coupling of assembled gold nanospheres (the distance between the nano-

particles was very short). SEM was used to obtain more indepth information regarding the film profile of the transferred films (Figure 5B). The SEM image of hierarchical gold sphere array film shows a homogeneous morphology over a large area, containing a close-packed nanosphere architecture with small voids. This method is simple and convenient, and the uniform array film can easily be transferred onto any solid substrates from the interface and therefore probably applied in different fields such as electrocatalysis, biosensors, and surface-enhanced Raman scattering (SERS).17d For instance, the uniform array film modified electrode behaves as a nanoelectrode ensemble. In principle, the electroanalytical detection limit at a nanoelectrode ensemble can be much lower than that at an analogous macrosized electrode because the ratio between the faradaic and the capacitive currents is higher.22 Thus, the raspberry-like gold sphere array modified electrode will probably find potential applications in biosensors and electrocatalysis. Conclusions In summary, we suggest in this contribution a one-pot method to synthesize the MRSGS. The size and the surface roughness of the raspberry-like gold spheres can be easily controlled by varying the parameters such as the concentration of aniline. Particularly, the MRSGS exhibit a high electrochemical active area, which is of great importance to the development of nanoelectrochemistry. Furthermore, an assembling method at a liquid-liquid interface has been developed for building novel hierarchical raspberry-like gold sphere arrays over a large area. This method is simple and convenient, and the assembling film can easily be transferred onto solid substrates from the interface. This result is very important because some more novel collective properties caused by the particular morphology of raspberrylike gold spheres can probably be produced. Most importantly, the assembling film prepared by this method could be used as catalysts, electrodes, and the template layer for fabricating other raspberry-like bimetallic sphere arrays. Acknowledgment. This work was supported by the National Science Foundation of China (Nos. 20575063, 20575064, and 20675076) and 973 Project 2007CB714500. Supporting Information Available: Typical UV-vis spectrum of respberry-like gold spheres and SEM and TEM images of gold spheres. This material is available free of charge via the Internet at http:// pubs.acs.org.

Monodisperse Raspberry-Like Gold Submicrometer Spheres

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