Shape-Controlled Gold Nanoarchitectures: Synthesis

The films obtained in pH 2 and 3 baths exhibited distinctive yellow, and the films .... It was noted that, for gold film obtained at 60 °C for 20 min...
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J. Phys. Chem. C 2008, 112, 13886–13892

Shape-Controlled Gold Nanoarchitectures: Synthesis, Superhydrophobicity, and Electrocatalytic Properties Hui Zhang, Jing-Juan Xu,* and Hong-Yuan Chen* Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: March 7, 2008; ReVised Manuscript ReceiVed: July 3, 2008

A simple one-step electrodeposition method was used to fabricate various gold nanostructures on glassy carbon electrodes in a low concentration of HAuCl4 solution (5 mM). The morphologies of final gold nanostructures can be easily controlled by varying the pH of the precursors or the deposition temperature. X-ray powder diffraction, scanning electronic microscopy, transmission electron microscopy, contact angle measurements, and electrochemical methods were used to characterize them. Hierarchical waxberry-like gold nanostructures with high active surface areas were obtained in pH 4 bath, and they had a higher catalytic performance for the reduction of oxygen than the other nanogold. These gold structures also displayed an extraordinary superhydrophobicity and the contact angle increased with the increase of deposition temperature and time. Their electrocatalytic response to the oxidation of glucose was also investigated. A sensitive enzyme-free sensor can be easily developed for the detection of glucose in pH 7.4 phosphate buffer solution. 1. Introduction Because of their unique physical and chemical properties, gold nanomaterials, as one of the most important metal nanomaterials, have been widely applied in optical, electronic, magnetic, catalytic, and biomedical fields.1-10 Since their physical and chemical properties can be finely tuned by the size and shape, many efforts have been devoted to develop reliable methods for the fabrication of gold nanostructures with defined size and morphology. Among the strategies for shape-controlled synthesis of gold nanostructures, electrodeposition method is one of the mostly useful approaches to prepare gold nanostructures on conducting surfaces with the advantages of convenience and the wide application in electrocatalysis and electroanalysis.11-13 Templateassisted electrodeposition method can direct the gold structure with controlled morphologies, which are limited for the restriction space of templates. This method is relatively complicated, since the appropriate templates are needed, which increase both the process of the synthesis and the production cost. Another commonly used method to manipulate the shape of gold nanostructures is to employ a growth medium containing additives, which can preferentially adsorb on specific crystallographic planes in the electrodeposition procedure. The adding of these additives changes the direction and rate of crystal growth and results in different morphologies of the final crystals. Various morphologies including pinlike,14 spherical,13,15,16 flowerlike17 gold nanostructures, and porous gold film18 have been electrosynthesized in the presence of additives such as cysteine,13-15 I-,13,15 Pb4+ ,18,19 and PVP.16,17 In addition, the nature of substrate is a key factor to the shape control of the deposited gold nanostructures. Zhang et al. used a polyelectrolyte multilayer as a preformed matrix in electrochemical deposition leading to the fabrication of gold clusters with dendritic structure.20 Li and Shi fabricated two-dimensional gold nano* To whom correspondence should be addressed. Telephone/Fax: +86 25 8359 7294; +86 25 8359 4862. E-mail: [email protected]; hychen@ nju.edu.cn.

structures with dendritic rod, nanosheet, flowerlike, and pine conelike nanostructures on the basis of the electrochemical deposition of gold nanocrystals onto indium tin oxide (ITO) glass substrate modified with thin polypyrrole film.11 Nanopyramidal, nanorodlike, and spherical gold nanostructures were also fabricated on a sputtered polycrystalline gold-modified ITO electrode via electrochemical overpotential deposition.12 Recently, the electrochemical deposition of gold onto ITO electrode using HAuCl4 solutions without introducing any template or surfactant was studied by Wang and co-workers.21 Hierarchical flowerlike gold microstructures with gold nanoplates or nanopricks as building blocks were obtained in a HAuCl4 solution with high concentration (24.3 mM), and only some irregular nanoparticle aggregates were observed in low concentration of HAuCl4 (5 mM). In this article, we extend this templateless, surfactantless electrochemical approach to the preparation of gold hierarchical nanoarchitectures in a HAuCl4 solution with lower concentration (5 mM). We can easily control the morphologies of the final gold nanostructures by simply changing the solution pH of gold precursors or the deposition temperature. The experimental results showed that the waxberry-like gold nanocrystals obtained in pH 4 bath had high surface area and exhibited higher electrochemical activities for the reduction of oxygen than the other nanogold. Superhydrophobic micro/nanostructures were obtained in pH 4 bath when the deposition temperature was more than 40 °C, which exhibited extraordinary electrochemical catalytic behavior to the oxidation of glucose. A sensitive enzyme-free sensor was further developed for the detection of glucose in a neutral solution. Experimental conditions related to the preparation of gold nanostructures with different morphologies and characteristics and their dewetting property and electrocatalytic property were investigated in detail. 2. Experimental Section 2.1. Reagents. Hydrogen tetrachloroaurate hydrate (HAuCl4 · 4H2O) and β-D-glucose were purchased from Shanghai Chemical Reagent Co. Ltd. Different pH values were adjusted with

10.1021/jp802012h CCC: $40.75  2008 American Chemical Society Published on Web 08/19/2008

Shape-Controlled Gold Nanoarchitectures

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Figure 1. SEM images of the gold nanostructures electrodeposited on glassy carbon slices in different pH baths (a) 2, (b) 3, (c) 3.5, (d) 4, (e) 5, (f) 5.5, and (g) 6, and on graphite slice in pH 4 bath (h). The scale bar in each case is 2 µm.

NaOH and HCl. Other chemicals were of analytical grade. All solutions were prepared with Milli-Q water. 2.2. Instruments. Scanning electron micrographs (SEM) were performed with a FEI Sirion 200 field-emission scanning electron microscope. Transmission electron microscopic (TEM) measurements were carried out on a JEOL 200CX transmission electron microscope using an accelerating voltage of 200 kV. Energy-dispersive X-ray (EDX) studies were carried out using Tecnai 20. X-ray diffraction (XRD) analysis of the resulting products was carried out on a Philips X’ pert Pro X-ray diffractometer (Cu K radiation, λ ) 0.15418 nm). A CAM 200 contact angle goniometer (KSV

Instruments Ltd.) was used for determination of contact angle (CA), and the size of the water droplet was 5 µL. Rolloff angles were determined from the tilt angles required to roll initially stationary droplets off the surface in opposite directions and were measured on a homemade device. Electrochemical measurements were performed on an Autolab PGSTAT-30 potentiostat/galvanostat (Eco Chemie BV). We used a conventional three-electrode system with a modified glassy carbon electrode (GCE, d ) 3 mm) as a working electrode, a platinum wire as an auxiliary electrode, and a saturated calomel electrode (SCE) as a reference electrode against which all potentials were measured.

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Figure 2. (a) TEM image of the electrodeposited WGNs in pH 4 bath. (b) EDX spectrum of the WGNs.

2.3. Preparation of Gold Nanostructures. The bare GCEs were polished with 1.0-, 0.3-, and 0.05-µm alumina slurry followed by rinsing thoroughly with Milli-Q water and then allowed to dry at room temperature. The stock solutions of 1.0% HAuCl4 were diluted to 5.0 mM with Milli-Q water. Their pH values were adjusted to a fixed value with NaOH and HCl solution and were aged above 12 h. Gold nanostructures were electrodeposited on GCE at a constant potential of 0.5 V at a certain temperature. 3. Results and Discussion Figure 1 shows the SEM images of the gold nanostructures electrodeposited from gold baths at 20 °C with different pH on glassy carbon slices and on a graphite slice. As can be seen, popcornlike gold nanostructures were synthesized from the pH 2 and 3 baths (Figure 1a,b). The insets in Figure 1a,b are highmagnification SEM images of these popcornlike gold nanostructures, which illustrate clearly that the gold nanostructures were composed with aggregated nanocrystals. With the increase in pH value, the gold nanostructures became more roundish and rougher (Figure 1c). Under pH 4 condition, hierarchical waxberry-like gold nanostructures (WGNs) with diameter of about 600 nm were homogeneously dispersed on the glassy carbon slice (Figure 1d). As can be seen from the magnified image of a single nanostructure (the inset in Figure 1d), the WGNs were built by many gold nanoparticles with average diameter of 30 nm as building blocks, as was indicated by the TEM image of the WGNs (Figure 2a). The loose structure of WGNs indicated that they had high surface area to volume ratios. Similar nanostructures could also be obtained by increasing pH value to 5, but the gold nanostructure became much looser and the average size decreased (Figure 1e). This trend became more remarkable with further increase in the pH value (Figure 1f, pH 5.5). When the pH reached 6, irregular loose nanostructures were obtained (Figure 1g). It is also worth mentioning that, with increasing pH value from 2 to 6, gold nanostructure density (number of nanostructures per square unit area) decreased gradually. In addition, from the SEM images (Figure 1h), it was clearly found that WGNs could also be electrodeposited on other carbon substrates, such as graphite substrate, but the uniformity of the nanostructures was less than that deposited on GCE. The chemical composition of the obtained WGNs was determined using EDX analysis. In the EDX spectrum (Figure 2b), except for the copper and carbon signals from the TEM grid, only peaks of gold were observed, which indicated that the obtained WGNs were composed of gold exclusively. Figure 3 shows the XRD patterns of the gold nanostructures electrodeposited on glassy carbon slice from different pH baths. With

Figure 3. XRD patterns of the gold nanostructures electrodeposited on glassy carbon slices in different pH baths (a) 2, (b) 3, (c) 4, (d) 5, (e) 6, and (f) XRD patterns of GC.

the increase in pH value, the loading amount of gold decreased and resulted in the decrease of the XRD peak intensity. The diffraction peaks corresponding to the (111), (200), (220), (311), and (222) facets demonstrated that all the electrodeposited gold nanostructures were composed of pure crystalline gold with the face-centered cubic structure (JCPDS card No. 04-0784). The higher intensity of the (111) diffraction peaks indicated that the deposited gold structure has a tendency to grow with the surfaces dominated by the lowest energy (111) facets. As we adjusted the pH of the HAuCl4 solution, with the increase of solution pH by adding NaOH, the color of the HAuCl4 solution turned from yellow to nearly colorless, and this change was reversible by lowering the pH again by adding HCl. It is commonly believed that, with the pH increasing, the Cl- from a complex anion will be displaced by water, and further hydrolysis and loss of proton from neutral hydrated ion will occur. These processes can be represented by eqs 1-6,22-24

[AuCl4]- + H2O T AuCl3(H2O) + Cl-

(1)

AuCl3(H2O) T [AuCl3(OH)]- + H+

(2)

-

-

[AuCl3(OH)] + H2O T AuCl2(H2O)(OH) + Cl -

+

AuCl2(H2O)(OH) T [AuCl2(OH)2] + H

(3) (4)

[AuCl2(OH)2]- + H2O T [AuCl(OH)3]- + H+ + Cl- (5) [AuCl(OH)3]- + H2O T [Au(OH)4]- + H+ + Cl- (6) The different pH of gold precursor baths results in different existent forms of gold complex and, therefore, leads to the different redox potential and the reduction rate of gold.25-27 It should be noticed that pH value of solutions also has a large

Shape-Controlled Gold Nanoarchitectures

Figure 4. Current-time curves for the electrodeposition of gold onto glassy carbon slices in different pH baths (a) 2, (b) 3, (c) 4, (d) 5, and (e) 6.

effect on the color of the deposited gold films. The films obtained in pH 2 and 3 baths exhibited distinctive yellow, and the films electrodeposited in higher pH baths appear blue. It is well-known that the standard reduction potential of AuCl4-/Au is 0.760 V vs SCE, which is slightly higher than the potential we employed. Thus, in this case, the electrochemical preparation of gold nanostructures was mainly affected by kinetic control.21 Figure 4 illustrated the typical current-time (I-t) curves recorded during the electrochemical deposition process. It was clear from this figure that the currents decayed in the initial period, which is related to the double-layer charging process,27 followed by a rising section, and gradually they reached a stabilized value, which corresponds to the nucleation and growth process.27-29 At lower pH, such as pH 2 and 3, AuCl4- is prevalent, which reduces to gold atoms at a comparatively high rate, and thus the current exhibits a transient sharp drop and the stable current values are much higher than others. With the pH increasing, the values of the initial decay and the stabilized currents become smaller, indicating that the reduction rate of gold decreases with the increase of pH because of the generation of gold hydroxide complexes (their reduction potentials are lower than that of AuCl4-). Higher reduction rate leads to generation of a larger number of nuclei, the final particles are easily agglomerated, and the aggregate size increases.26 In addition, the stabilized currents and absence of maxima in those curves may reflect a lack of tight packing of nuclei.29 SEM images of the various gold nanostructures at the early stage are provided in Supporting Information (Figure S1). Comparatively high reduction rate (in pH 3 bath) results in the formation of the uneven spherical aggregates (10 s, Figure S1a). With the time increasing (30 s), the spherical aggregates grew and formed popcornlike morphology gold nanostructures (Figure S1b). As the pH increased to 4, the concentration of [AuCl4]decreased and the growth rate slowed, and the loosely packing gold nanoparticles were obtained (10 s, Figure S1c) and finally developed to WGNs (60 s, Figure S1d). As the pH value increased further (pH ) 6), the influence of OH- enhanced even more, and the reduction of Au(III) was inhibited intensively. The gold nanoparticles formed at the early stage were in a low yield (10 s, Figure S1e). These nanoparticles assembled and formed more loosely aggregated structures (240 s, Figure S1f). In short, the final morphologies were controlled by the rates of nucleation and growth, which were determined by the pH value of the precursors. During the electrodeposition process, the electrodepositon temperature also influenced the morphologies. The increase in the temperature can improve the reaction rates. Figure 5 shows SEM images of gold nanostructures grown from pH 4 bath with different temperatures (40, 60, and 80 °C). As can be seen from

J. Phys. Chem. C, Vol. 112, No. 36, 2008 13889 this figure, when the temperature increased to 40 °C, WGNs with diameter of 1 µm were obtained and some of them were linked by irregular nanoparticles. It was noted that this surface texture had both micro- and nanoscale surface roughness (Figure 5a,a′). After being immersed in an ethanol solution of ndodecanethiol (1 × 10-3 M) overnight, the CA of the film was measured to be as high as 154.3°, as is shown in Figure 5a. It is known that, for a solid substrate, when the CA of water on it is larger than 150°, it is called superhydrophobic.30 Therefore, this result indicated that our method leads to the formation of a superhydrophobic surface. Further increasing the deposition temperature to 60 °C, we found that the density of gold microcrystals increased with an augmentation of the size to 1.1 µm (Figure 5b) and high-resolution SEM image indicated that these microcrystals were not separated but linked together to form a porous 3D network structures that brings a large extent of air trapping ability (Figure 5b′). The CA of the film was measured to be as high as 163.9° (Figure 5b). Obviously, the apparent CA primarily originates from the contribution of the air trapped in the interspace of rough surface and the strong water repellency of alkyl chain-terminated domains.20 To further investigate the wetting behavior of these surfaces, the rolloff angles were determined. Interestingly, the rolling of water drops on these surfaces was inhibited. As shown in the insets of panels a′ and b′, even when the films were flipped upside down, the water droplet remained immovably on the surfaces, indicating these superhydrophobic surfaces have highly adhesive forces to water droplet.30 When the deposition temperature increased to 80 °C, porous film with micro- and nanohierarchical structure covered the entire substrate (Figure 5c,c′), which was similar to that found on lotus leaves.30 As a result, the film showed superhydrophobicity with a large CA of about 175.5° and the rolloff angle of the surface is around 3°, indicating that water droplets roll off easily. According to Wenzel’s equation,31

cos θ ′ ) r cos θ

(7)

where r is the ratio of the actual area to the project area, θ′ is the measured contact angle on a rough surface, and θ is the intrinsic CA on a flat surface. According to the modified Cassie’s equation,32

cos θ ′ ) fcos θ - (1 - f)

(8)

in which f is the fraction of the solid/water interface, and (1 f) is the fraction of the air/water interface. It can be concluded that the surface wettability is dependent on the surface roughness and porosity.33-35 Therefore, CA can be used to qualitatively compare the roughness of the different gold surface. To further study the roughness and porosity of the microand nanohierarchical structures, we investigated the change of CA with the increase of deposition time. It was found that the CA increased with the increase in the deposition time (Figure 6). It was noted that, for gold film obtained at 60 °C for 20 min, water droplets were unstable, antiadhesive, and immeasurable (CA ≈ 180°). From the experimental results, we can conclude that the roughness and porosity increased with the increase of deposition temperature and deposition time. Cyclic voltammetry (CV) was employed to characterize the different gold nanostructures by estimating the real surface area via scanning in 0.5 M H2SO4, as typically shown in Figure S2. Integration of the charge consumed during the formation of the surface oxide monolayer enabled estimation of the real surface area of the different gold nanostructures using a reported value of 386 µC/cm2.36 In addition, the loading amount of gold was determined from the charge obtained from I-t curves measured

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Figure 5. SEM images of the gold nanostructures electrodeposited on glassy carbon slices in pH 4 bath at (a) 40, (b) 60, and (c) 80 °C for 10 min. The scale bar in each case is 5 µm. The insets of panels a, b, and c are the shape of water droplets on a corresponding surface modified with n-dodecanethiol. Panels a′, b′, and c′ are the magnified views of panels a, b, and c, respectively. The scale bar in each case is 1 µm. The insets of panels a′ and b′ are digital camera images of water droplets on a corresponding surface modified with n-dodecanethiol at the tilt angle of 180° at room temperature (drop weight 10 mg).

TABLE 1: Parameters of Gold Electrodeposited on the GCE deposition bath pH (temperature, time)

Figure 6. Water contact angle measurements on the surface of gold films obtained at 60 °C (2) and 40 °C (b).

during the electrodeposition process provided that the Coulombic efficiency was 100%. These parameter data were summarized in Table 1. It can be seen from Table 1 that, as the pH increased from 2 to 6, the loading amount decreased, and the real surface area increased in the following order: pH 4 > pH 3 > pH 2 > pH 5 > pH 6. At fixed pH value (pH 4), the loading amount and the real surface area of gold increased with the increase in

2 3 4 5 6 4 4 4 4

(20 (20 (20 (20 (20 (40 (60 (80 (60

°C, °C, °C, °C, °C, °C, °C, °C, °C,

10 10 10 10 10 10 10 10 20

min) min) min) min) min) min) min) min) min)

gold loading (µg)

real surface (cm2)

specific surface area (m2/g)

125 120 58.6 14.0 3.27 109 250 442 430

0.117 0.144 0.187 0.0702 0.0385 0.696 2.55 4.89 6.49

0.0936 0.120 0.319 0.501 1.18 0.639 1.02 1.11 1.51

temperature and deposition time. As for the specific surface area (real surface/gold loading), it increased with the increase in pH, which can be reflected in the SEM analysis. At pH 4, it increased with the increase in deposition temperature and time and corresponded with the changes of CA. It is known that gold nanomaterials have catalytic activity on electrochemical reduction of oxygen.11-13,37-39 It is a reaction of prime importance in view of its practical applications in

Shape-Controlled Gold Nanoarchitectures

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Figure 7. Cyclic voltammograms of the porous gold/GCE (prepared in pH 4 baths at 60 °C for 20 min) in 0.1 M pH 7.4 PBS containing 50 mM glucose at 50 mV/s.

TABLE 2: Parameters of Different Nanogold/GCE Electrocatalytic Activities toward Oxygen Reduction deposition bath pH

peak potential (V)

peak current (µA)

2 3 4 5 6

-0.157 -0.108 -0.078 -0.147 -0.181

-5.119 -5.911 -6.307 -4.733 -3.399

chemical processing, energy conversion, and chemical sensing.12 Here, we used nanogold/GCE obtained in different pH baths as a test. When bare GCE was dipped in 0.5 M H2SO4 solution without deaerating oxygen, there was no obvious oxidation peak, while for the different nanogold/GCE a reduction peak was observed (Figure S3). The catalytic activity of the different nanogold/GCE could be probed by comparing the peak potential and currents, and the enhancement of the electrocatalytic performance was demonstrated by the positive shift of the peak potential associated with an increase of the peak current.11-13,37-40 It can be seen clearly that the WGNs had a higher catalytic performance than the other nanogold (Table 2). This is attributed to the high active surface area of WGNs. It is known that gold shows high electrocatalytic activity for nonenzyme detection of glucose.41-43 Recently, more attention was focused on the synthesis of gold film with 3D porous structure for its large surface-to-volume ratio, and the structure provides a good electron-conducting tunnel and allows the electron transfer to take place easily.43 In this case, we investigated the electrocatalytic activity of the porous gold/GCE prepared in pH 4 bath toward the oxidation of glucose under physiological condition. Figure 7 showed the CV of the oxidation of glucose at porous gold/GCE in a pH 7.4 PBS solution containing 50 mM glucose at 50 mV/s. The CV in the positive potential scan process showed two anodic current peaks located at 0.09 and 0.31 V. The first peak should be attributed to the electrosorption of glucose to form adsorbed intermediate, releasing one proton per glucose molecule. Along with the electrosorption of glucose, the intermediates accumulate and occupy the active sites of the gold electrode surface. As a result, the current decreased, with the potential moving to more positive values, and AuOH start to formed, which can catalyze the oxidation of the poisoning intermediates, releasing free gold active sites for the direct oxidation of glucose. Therefore, the second current peak at 0.31 V for this direct oxidation appears. The decrease in current after the second peak should be due to the formation of gold oxide, which competes for adsorption sites with glucose, inhibiting the direct electrochemical oxidation of glucose as well. In the negative potential scan process, the surface gold oxide was reduced and enough surface active sites

Figure 8. Typical amperometric response of the sensor for glucose to successive addition of 2 mM glucose or 0.02 mM UA and 0.1 mM AA into a stirring PBS (0.1 M, pH 7.4). The applied potential was +0.36 V (vs SCE). Inset: the calibration curve of the sensor.

will be available for the direct oxidation of glucose, leading to an anodic current peak at 0.34 V. Along with the potential moving to more negative values, an anodic current peak corresponding to the first peak in the positive potential scan process appeared at 0.08 V. These phenomena were in accordance with those reported in previous literature.43 The glucose oxidation currents would be enhanced as the gold real surface area increased. Therefore, the oxidation peak currents of glucose at porous gold-modified electrode increased with the increase in deposition temperature and the increase in the deposition time. However, too high temperature and too long time would reduce the stability of the film and result in the brittleness and weak adhesion to GCE. Here an optimal deposition condition of 60 °C and time of 20 min was selected. Figure 8 showed the typical steady-state amperometric response of glucose in 0.1 M pH 7.4 PBS, at detection potential of +0.36 V. The corresponding calibration curve was shown in the inset of Figure 7. The linear responses were in the range from 2.0 to 38 mM. The upper limit of linear range is 38 mM to glucose far beyond the physiological level (3-8 mM), and the detection limit was down to 4 µM at a signal-to-noise ratio of 3. The response reached 95% steady-state value within 2 s. It should also be pointed out that gold-modified GCE displayed a nice reproducibility with a relative standard deviation of 3.0% by monitoring the current response for five replicate injections of 5.0 mM glucose. Ascorbic acid (AA) and uric acid (UA) in biological sample could be easily oxidized under a relatively positive potential and often interfere with the detection of glucose. The normal physiological level of glucose (3-8 mM) is much higher than that of AA (0.1 mM) and UA (0.02 mM). The selectivity of the porous gold/GCE to glucose at 0.36 V was evaluated in the presence of 0.1 mM AA and 0.02 mM UA in pH 7.4 PBS. The results are shown in Figure 8. It was clear that the electrochemical detection of glucose on the porous gold/GCE could be performed with negligible interferences from AA and UA in its normal physiological level. The high selectivity was attributed to the high roughness of the films with higher real surface area (calculated as 6.49 cm2), which favors kinetically controlled sluggish reaction (the electrochemical oxidation for glucose), and the oxidation of UA and AA was a diffusion-controlled electrochemical process, which does not depend significantly on the electrode roughness.44,45 The results demonstrated that the present porous gold-modified electrode is promising for fabrication of nonenzymatic glucose biosensors. 4. Conclusions In this article, we extend the straightforward, low-cost, templateless, surfactantless electrodeposition method to syn-

13892 J. Phys. Chem. C, Vol. 112, No. 36, 2008 thesize various gold nanostructures. The pH of deposition solution and the deposition temperature play important roles in controlling the morphologies. WGNs using nanoparticles as building blocks were fabricated in pH 4 bath, which showed high electrochemical activities for oxygen reduction. The diameter of the WGNs can be easily controlled by simply changing the deposition temperature, and 3D porous gold films with micronanostructure could be obtained by increasing the deposition temperature, which showed superhydrophobicity and high electrocatalytic activity toward glucose oxidation. The most distinguished features of this method include ease of operation, absence of undesired byproducts, uniformity of nanostructure, and shape controlling. More importantly, such hierarchical gold structures have great significance for their high specific surface area and potential applications in some fields, such as catalysis, superhydrophobicity, and surface-enhanced Raman scattering. Acknowledgment. Financial support from the National Natural Science Foundation (20675037, 20435010, 20635002, 20775033), the National Natural Science Funds for Creative Research Groups (20521503), the 973 Program (2007CB936404, 2006CB933201), and the program for New Century Excellent Talents in University (NCET) of China is gratefully acknowledged. Supporting Information Available: SEM images of the gold nanostructures electrodeposited on glassy carbon slices in different pH baths for different times, CV response of different nanogold/GCE in 0.5 M H2SO4, and electrochemical reduction of oxygen at different nanogold/GCE. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (3) Peto, G.; Molnar, G. L.; Paszti, Z.; Geszti, O.; Beck, A.; Guczi, L. Mater. Sci. Eng., C 2002, 19, 95. (4) Demaille, C.; Brust, M.; Tsionsky, M.; Bard, A. J. Anal. Chem. 1997, 69, 2323. (5) Chandrasekharan, N.; Kamat, P. V. J. Phys. Chem. B 2000, 104, 10851. (6) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (7) Pendry, J. Science 1999, 285, 1687. (8) Tian, Y.; Tatsuma, T. J. Am. Chem. Soc. 2005, 127, 7632. (9) Naoi, K.; Ohko, Y.; Tatsuma, T. J. Am. Chem. Soc. 2004, 126, 3664. (10) Du, Y.; Luo, X. L.; Xu, J. J.; Chen, H. Y. Bioelectrochemistry 2007, 70, 342. (11) Li, Y.; Shi, G. Q. J. Phys. Chem. B 2005, 109, 23787. (12) Tian, Y.; Liu, H. Q.; Zhao, G. H.; Tatsuma, T. J. Phys. Chem. B 2006, 110, 23478.

Zhang et al. (13) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. Electrochim. Acta 2006, 52, 1792. (14) Gao, F.; El-Deab, M. S.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2005, 152, A1226. (15) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. J. Electrochem. Soc. 2005, 152, C730. (16) Huang, S. X.; Ma, H. Y.; Zhang, X. K.; Yong, F. F.; Feng, X. L.; Pan, W.; Wang, X. N.; Wang, Y.; Chen, S. H. J. Phys. Chem. B 2005, 109, 19823. (17) Duan, G. T.; Cai, W. P.; Luo, Y. Y.; Li, Z. G.; Li, Y. Appl. Phys. Lett. 2006, 89, 211905. (18) Bonroy, K.; Friedt, J. M.; Frederix, F.; Laureyn, W.; Langerock, S.; Campitelli, A.; Sara, M.; Borghs, G.; Goddeeris, B.; Declerck, P. Anal. Chem. 2004, 76, 4299. (19) Toyama, S.; Takei, O.; Tsugue, M.; Usami, R.; Horikoshi, K.; Kato, S. Electrochem. Commun. 2002, 4, 540. (20) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (21) Guo, S. J.; Wang, L.; Wang, E. K. Chem. Commun. 2007, 3163. (22) Guo, Z. R.; Zhang, Y.; Huang, L.; Wang, M.; Wang, J.; Sun, J. F.; Xu, L. N.; Gu, N. J. Colloid Interface Sci. 2007, 309, 518. (23) Moreau, F.; Bond, G. C.; Taylor, A. O. J. Catal. 2005, 231, 105. (24) Nechayev, Y. A.; Nikolenko, N. V. Geochem. Int. 1986, 32, 23. (25) Guo, Z. R.; Zhang, Y.; Huang, L.; Wang, M.; Wang, J.; Sun, J. F.; Xu, L. N.; Gu, N. J. Colloid Interface Sci. 2007, 309, 518. (26) Lee, J. H.; Kamada, K.; Enomoto, N.; Hojo, J. J. Colloid Interface Sci. 2007, 316, 887. (27) Martín, H.; Carro, P.; Herna´ndez Creus, A.; Gonza´lez, S.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1997, 13, 100. (28) Liu, H.; Favier, F.; Ng, K.; Zach, M. P.; Penner, R. M. Electrochim. Acta 2001, 47, 671. (29) Floate, S.; Hyde, M.; Compton, R. G. J. Electroanal. Chem. 2002, 523, 49. (30) Feng, X. J.; Jiang, L. AdV. Mater. 2006, 18, 3063. (31) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (32) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (33) Adamson, A. W. Physical Chemistry of Surfaces; Wiley & Sons: New York, 1990. (34) Extrand, C. W. Langmuir 2002, 18, 7991. (35) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (36) Wang, J. Z.; Huang, H. Z.; Mao, X. A. Nanoparticle Analyses; Chemical Engineering Press: Beijing, 2003; Chapter 9. (37) Zhang, Y. R.; Asahina, S.; Yoshihara, S.; Shirakashi, T. Electrochim. Acta 2003, 48, 741. (38) El-Deab, M. S.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2003, 150, A851. (39) Li, Z. H.; Ravaine, V.; Ravaine, S.; Garrigue, P.; Kuhn, A. AdV. Funct. Mater. 2007, 17, 618. (40) El-Deab, M. S.; Ohsaka, T. Electrochem. Commun. 2002, 4, 288. (41) Sun, Y. P.; Buck, H.; Mallouk, T. E. Anal. Chem. 2001, 73, 1599. (42) Zhao, W.; Xu, J. J.; Shi, C. G.; Chen, H. Y. Electrochem. Commun. 2006, 8, 773. (43) Li, Y.; Song, Y. Y.; Yang, C.; Xia, X. H. Electrochem. Commun. 2007, 9, 981. (44) Matsumoto, F.; Harada, M.; Koura, N.; Idemoto, Y.; Ui, K. Electrochemistry 2004, 72, 103. (45) Yuan, J. H.; Wang, K.; Xia, X. H. AdV. Funct. Mater. 2005, 15, 803.

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