Facile Fabrication of Multifunctional Three-Dimensional Hierarchical

Jan 14, 2009 - Corresponding author: tel, +86-731-8871533; fax, +86-579-82282595; e-mail, [email protected] or [email protected]., †. Key Laboratory...
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J. Phys. Chem. C 2009, 113, 1800–1805

Facile Fabrication of Multifunctional Three-Dimensional Hierarchical Porous Gold Films via Surface Rebuilding Wei Huang,† Minghua Wang,‡ Jufang Zheng,‡ and Zelin Li*,† Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal UniVersity, Changsha 410081 China, and Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua 321004 China ReceiVed: October 29, 2008; ReVised Manuscript ReceiVed: December 05, 2008

We report here a novel one-step electrochemical method to fabricate three-dimensional (3D) micro-nano hierarchical porous gold films (PGFs) by the surface rebuilding of smooth gold substrates in a blank solution of NaOH with square wave potential pulse. The potential is controlled such that it involves repeated gold oxidation-reduction and intensive hydrogen evolution, where the hydrogen gas bubbles function as a dynamic template to shape the assembly of the gold nanoparticles from the oxidation-reduction. Particularly, this method is green, convenient, and economical, which enables us to fabricate the 3D porous structure from the metal itself requiring neither Au(III) species and additives in solution nor post-treatment of template removal. The pore formation and evolution have been characterized by scanning electron microscopy. The as-prepared 3D PGF is multifunctional, for example, (i) high electrocatalytic activity toward the oxidation of some fuel/ biomolecules like ethanol, glucose, and ascorbic acid; (ii) strong surface-enhanced Raman scattering effect with the merits of being stable and easily renewed; and (iii) interesting transition from superhydrophilicity to superhydrophobicity by decorating with a self-assembled thiol monolayer. 1. Introduction Three-dimensional (3D) porous gold materials have been attracting wide attention due to their broad applications in such areas as catalysis, biosensors, fuel cells, surface-enhanced Raman scattering (SERS), self-cleaning, and so on.1 The most popular approach to prepare porous metals is the template-directed synthesis2 by means of electrodeposition,3 electroless deposition,4 filtration,5 and chemical vapor deposition (CVD).6 Varieties of templates, classified as the soft (e.g., emulsions,7 lyotropic liquid crystallines8), the hard (like colloidal crystals1a,b,f,g,2b,d,3b-d,4a,b,5,6) and the natural (such as echinoid skeletons9), have been employed in fabricating porous metals. The template is then removed by post-treatment of dissolution, sintering, or etching. So multisteps are involved in this way to get the porous structures. Up to the present, a number of template-free synthetic strategies have also been developed for fabricating porous structures.10 For example, porous gold films (PGFs) were prepared by dealloying Au-M (M ) Ag, Cu, etc.) alloys, where the active components were selectively removed.10a,b Hollow and porous Au-Ag alloys were synthesized by galvanic replacement reactions.10c More recently, a 3D nano-PGF was made from a pure gold substrate in an HCl medium by applying a potential step, involving electrodissolution, disproportion, and deposition processes.10d Of late hydrogen bubbles have been utilized as a dynamic template in electrodeposition to produce self-supported 3D * Corresponding author: tel, +86-731-8871533; fax, +86-579-82282595; e-mail, [email protected] or [email protected]. † Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University. ‡ Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University.

micro-nano porous metals11 under highly cathodic polarization. This unique 3D micro-nano porous structure has promising applications.11 The in situ formed gas bubble template has obvious advantages such as low cost and convenience. Although several such porous metals (Cu,11a-c Sn,11a Ni,11e Cu6Sn511f) have been prepared using this template, the direct synthesis of porous gold with it is still challenging.11d Therefore, an indirect route has been proposed recently including three steps:11d (1) prepare a 3D porous Cu film first by electrodeposition using the hydrogen bubble template,11b (2) then immerse the porous Cu film in an aqueous solution of KAu(CN)2 for 2 h to replace Cu with Au using the porous Cu as a 3D template, and (3) completely remove the alloyed Cu from the replacement process by dealloying with potential cycling for ca. 8 h. Apparently, more convenient and economical methods are still needed. In this paper, we develop a novel and facile one-step method to fabricate 3D micro-nano hierarchical PGFs with pure gold substrates in a blank solution of NaOH by applying a square wave potential pulse (SWPP), which involves repeated gold oxidation-reduction and intensive hydrogen evolution. By this means, we are able to realize the surface rebuilding of smooth gold into the 3D PGFs without Au(III) species and additives in solution or post-treatments. Some promising applications of the as-prepared 3D PGFs have been demonstrated in the electrocatalysis, SERS, and transition of wettability. 2. Experimental Section Electrochemical Preparation and Characterizations of PGFs. Most electrochemical experiments were carried out in a spectroelectrochemical cell with a CHI 440A electrochemical station (Chenhua Instruments, Shanghai, China). A gold disk (1 mm diameter, purity g 99.99%), a large platinum ring (12 cm at length and 1 mm in diameter), and a saturated mercurous

10.1021/jp8095693 CCC: $40.75  2009 American Chemical Society Published on Web 01/14/2009

Porous Gold Films

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SCHEME 1: Schematic Illustration of the Pore Formation by the Surface Rebuilding of Gold Substrate

sulfate electrode (SMSE) were employed as the working, counter, and reference electrodes, respectively.12 Prior to use, the working electrode was polished with 2000 grit carbimet paper and then rinsed in Millipore water under ultrasonic waves. Afterward, the gold disk was treated in an alkaline solution (typically 2 mol dm-3 NaOH) by square wave potential pulse (SWPP) between 0.8 and -5.0 V at 50 Hz for a given period of time to fabricate the PGFs. The roughness factor (R) for the PGFs was characterized by cyclic voltammetry (CV)13 in 1 mol dm-3 HClO4. The as-prepared PGFs from 1 mm diameter gold electrodes were used for the tests in scanning electron microscopy (SEM), electrocatalytic oxidation, and SERS. While the preparation of a large piece of PGF used for the experiments of X-ray diffraction (XRD) and wetting was carried out under the same conditions as described above but in a 50 cm3 beaker, where a gold slice (8 mm × 6 mm, purity g99.99%), a carbon rod (6.4 mm diameter and 3.6 cm length), and a SMSE served as the work, counter, and reference electrodes, respectively. A PGF prepared with the SWPP for 6000 s was used to test the electrocatalytical activity toward the oxidation of 1 mol dm-3 ethanol, 0.1 mol dm-3 glucose, or 0.1 mol dm-3 ascorbic acid in 1 mol dm-3 NaOH. SEM and XRD Characterizations. The surface morphology of PGFs was characterized by a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM), and an electron beam voltage of 10 kV was used to obtain good contrast and high resolution. XRD patterns of PGFs were investigated with an X-ray diffractometry (XRD, X’Pert PW3040/60, Philips, Netherlands) equipped with a Philips Analytical X‘Celerator, using Cu KR radiation in a 2θ range from 10 to 90° with a scan rate of 0.2° s-1. The working voltage of the instrument was 40 kV, and the current was 40 mA. SERS Measurements. PGFs prepared with the SWPP treatment for 50 and 6000 s were chosen for the in situ potentialdependent SERS measurements. An oxidation-dissolution pretreatment was performed to improve or to recover the SERS activity. The PGFs were oxidized at 0 V for 100 s in 2 mol dm-3 NaOH and then immersed in 2 mol dm-3 HCl for several seconds. The in situ SERS test was taken in a solution including 0.1 mol dm-3 KClO4 + 0.01 mol dm-3 pyridine with a Renishaw RM1000 microscopy confocal Raman spectrometer (Gloucestershire, U.K.). A detailed description of the Raman measurements with a spectroelectrochemical cell can be found elsewhere.12 The exciting wavelength was 785 nm laser with a power ca. 2.72 and 0.44 mW on the surfaces for the newly prepared and pretreated PGFs, respectively. Wettability Tests. The wetting tests were preformed on the surfaces (8 mm × 6 mm) of smooth gold and PGFs with and without assembled thiol, respectively, by placing a 3.6 µL water drop and taking pictures with a camera (8 × 106 pixels). Prior

touse,thesmoothgoldslicewaspretreatedwithahydrogen-oxygen flame and then cleaned by CV (-0.7 to 1 V) in 1 mol dm-3 H2SO4. A self-assembled thiol monolayer on the surfaces of the smooth gold and the PGF was formed by immersing them overnight in the ethanol solution of n-dodecanethiol (5 mmol dm-3). Other Experimental Conditions. All solutions were freshly prepared with Millipore water and analytical grade chemicals. All experiments were performed at room temperature. 3. Results and Discussion 3.1. Electrochemical Fabrication of PGFs. 3.1.1. The Fabrication Principle of PGFs. The fabrication principle of PGFs with pure gold substrates in NaOH solution by applying SWPP is illustrated in Scheme 1. Note that the fundamental electrochemistry of gold in alkaline solutions has been systematically investigated by Burke et al.14 and described in Figure S1 (Supporting Information). When the applied pulse potential remains at 0.8 V, gold oxides form accompanying weak release of oxygen gas. As the potential switches to -5 V, gold oxides are rapidly reduced into gold atoms/clusters/nanoparticles, which are movable and are assembled under the action of intense hydrogen bubbles. The cycles repeat by the SWPP, and 2D to 3D porous structures gradually develop with a final morphology similar to that of a 3D foam Cu film that was obtained by electrodeposition of a copper salt.11a To the best of our knowledge, this is a new convenient means to realize the surface rebuilding with the help of intensive hydrogen bubbles, which enables us to fabricate a 3D micro/nanostructured PGF from a smooth gold substrate without Au(III) species and additives in the solution or post-treatments. This surface-rebuilding method is green, convenient, and economical, which apparently differs from the electrodeposition of metal salts/complexes with concurrent hydrogen release.11 3.1.2. EWolution of Porous Structures with Time. Figure 1 shows three typical porous structures developed in sequence by the SWPP: 3D nanopores only (a1, a2) and also those with 2D micropores (b1, b2) and 3D micropores (c1, c2), which are all made out of irregular gold nanopolyhedrons (a3-c3). The nanopores (Figure 1a) formed in 100 s. Then 2D micropores came forth around 750 s sizing in diameter from about 1 µm for 750 s to 2 µm for 3000 s (Figure 1b), and the irregular gold nanopolyhedrons got longer. On this pore development, hydrogen bubbles began to play the role as template to guide the formation of micropores while the layer of gold nanopolyhedrons reached a given thickness. Finally, interconnected 3D hierarchical porous structures (Figure 1c) were obtained as the irregular nanopolyhedron layer kept on growing by the repeated oxidation-reduction cycling up to 6000 s or more, and the electrode surface became dark brown.

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Figure 1. SEM images with different enlargement scales for the PGF samples prepared in a solution of 2 mol dm-3 NaOH by the SWPP (between 0.8 and - 5 V, 50 Hz) treatment for different times: (a1-a3) 100 s, (b1-b3) 3000 s, (c1-c3) 12000 s.

Figure 2. SEM images with different enlargement scales for the 3D PGF prepared by the SWPP for 6000 s in 5 mol dm-3 NaOH.

Clearly, the resulting self-supported porous films comprise micropores in the frame and nanopores in the wall (Figure 1c). Such an integration of micro-nano porous structures can facilitate fast mass transfer11b in addition to good electrocatalytic activity. More time-dependent pore structures are demonstrated in Figure S2 (Supporting Information). Besides, no obvious 2D or 3D microporous structures could be produced by the potential pulse in the regions where only anodic (0.2-0.8 V) or cathodic (-1 to -5 V) processes were present, or without hydrogen evolution (0.8 to -1.8 V). However, similar micronanoporous gold can be also obtained by extending the applied pulse potential region, say from 0.8 to -2.3 V to 5 to -5 V. These facts provide further evidence that the formation of the 3D micro/nano structured porous gold films must be from the joint processes of gold oxidation-reduction and hydrogen evolution. Note also that only nanoporous films form for the gold oxidation-reduction without hydrogen evolution.10d-f 3.1.3. The Influence of NaOH Concentration on the Porous Structures. Shown in Figure 2 is the 3D structure of a PGF made in 5 mol dm-3 NaOH with the potential pulse (0.8 to -5 V) for 6000 s. The average micropore size somewhat decreases comparing with that using 2 mol dm-3 NaOH (Figure S2-c). One possible reason is ascribed to the average size of hydrogen bubbles becoming smaller due to the increase of NaOH concentration, which increases the solution’s density, viscosity, and surface tension and thus prevents the smaller bubbles from coalescence.

3.1.4. The Roughness and XRD Patterns of the PGFs. The roughness factor (R) of the PGFs, defined as the ratio of the real surface area over the geometrical area, can be calculated through their CVs13 (the insets of Figure S2). The R increases fast at first and reaches about 35 at last when the duration of applied potential pulse varies from 0 s (smooth) to 12000 s (3D porous film) (Figure S2-a) or the concentration of NaOH from 0.1 mol dm-3 to 5 mol dm-3 (Figure S2-b). On the basis of the data in Figure S2, an optimistic condition can be easily chosen for fabricating a high-specific-surface 3D PGF, which reads 6000 s and 2 mol dm-3 for the duration of potential pulse and the concentration of NaOH, respectively. XRD patterns in Figure 3 show that the Au(200) and Au(311) facets orient preferably for the as-prepared PGF (Figure 3b), noting that they have unusual relative diffraction intensity compared with those for the smooth polycrystalline gold slice (Figure 3a). 3.2. Electrocatalytic Activity of the 3D PGF Electrode for the Oxidation of Fuel/Biomolecules. Much better electrocatalytical activities toward the oxidation of some fuel/biomolecules have been observed for the 3D PGF electrode than for the smooth one. A few examples are given in Figure 4. Figure 4a compares the CVs of the 3D PGF (the solid line) with the smooth electrode (the dotted line) in a blank solution of NaOH. Besides the much larger current due to the higher surface area of the 3D PGF, the peaks for the redox pair (-0.15/ -0.42 V) of gold oxide monolayer appear at more negative potentials, which is a typical redox characteristic of gold

Porous Gold Films

Figure 3. XRD patterns of the smooth gold slice (0.8 cm × 0.6 cm) (a) before and (b) after the SWPP treatment for 6000 s.

nanomaterials. In particular, there is a pair of broad “pre-redox peaks” between -1.1 and -0.3 V. They are related to the electrochemical transition between the gold adatoms and the incipient hydrous gold oxide (AuOHads) premonolayer. The incipient hydrous gold oxides, produced at quite low potentials, are highly reactive and may trigger and mediate the electrooxidation of some biomolecules.15a As shown in Figure 4b-d, the as-prepared 3D PGFs perform much better in the electrocatalytic oxidation of ethanol (b), glucose (c), and ascorbic acid (d). A more detailed numerical comparison on the onset potentials and the oxidative current at -0.25 V for the 3D PGF and the smooth gold electrode is listed in Table S1 within the Supporting Information. The enhanced peak current is due to the larger surface area of the 3D PGF, and the negative shift of the onset potentials is attributed to the higher reactivity of the gold nanopolyhedrons. The chronoamperometric measurements provide further information about the higher electrocatalytic activity and good stability of the 3D PGFs. The insets in Figure 4 present the current-time curves that were measured on the 3D PGFs and on the smooth gold electrodes at -0.25 V for the electrocatalytic oxidation of the three kinds of molecules. For all the reaction systems, the potentiostatic currents decrease rapidly at first and then decay slightly with time. The fact that the larger oxidation currents on the 3D PGFs remain long indicates that it is stable during the electrocatalytic reactions. There is another test result showing the good stability of the 3D-PGF, noting that the two solid lines in Figure 4a almost completely superpose each other before and after long electrocatalytic performance. The 3D micro-nano PGFs are obviously superior to the nanoporous gold films obtained by dealloying where the ligaments quickly coarsened with potential cycling.15b 3.3. SERS Effect of the PGFs. SERS spectroscopy has gained a reputation as an ultrasensitive spectroscopic tool for single molecular detections.16 Up to now, much effort has been made to exploit nanostructured metals as SERS substrates.17 The 3D-PGF as shown in Figure 1 possesses typical structures effective for SERS activity. Very interestingly, we found that the SERS activity could be further improved and repeatedly recovered by a simple “oxidation-dissolution” pretreatment of the structure-stable PGF: that is, oxidizing the PGF at 0 V for 100 s in 2 mol dm-3 NaOH at first and then dipping it into 2 mol dm-3 HCl for several seconds. The resulting SERS intensity is comparable to that by the conventional ORC method.17a It can be seen that the longer nanopolyhedrons in the wall of micropores (Figure 5a) become nanospindles (Figure 5b) with

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1803 large aspect ratio while the pore framework remains unchanged after the oxidation-dissolution pretreatment. Higher SERS activity from the electromagnetic (EM) enhancement is proposed because the pretreatment modifies the nanostructures of the nanopolyhedrons and thus the localized surface plasmon resonance as well as their coupling.18 Pyridine was chosen as the probe molecule to check the SERS activity of the newly prepared (Figure 5c) and pretreated (Figure 5d) PGFs corresponding to those substrates in Figure 5a and 5b. It can be seen from Figure 5c that both the newly prepared hierarchical 3D-PGFs exhibit considerable SERS activity as evidenced by the characteristic bands at 1011 and 1034 cm-1 for the adsorbed pyridine,19 where the SERS intensity of the strongest peak (1011 cm-1) is about 150 counts per second (cps). However, after being subjected to the oxidation-dissolution pretreatment, the SERS signals of pyridine on the PGF intensify greatly and the peak intensity at 1011 cm-1 exceeds 2200 cps (Figure 5d). Note also that the incident laser power on the substrates before and after the pretreatment is 2.72 and 0.44 mW, respectively. The merit of such a new porous substrate is that it exhibits longer SERS stability at least overnight and the SERS activity can be easily recovered via the oxidationdissolution pretreatment once it is lost. Similar results are also observed on the nanoporous surface (Figure S4 in Supporting Information). 3.4. Wettability of the PGFs. Superhydrophilicity of porous metallic surfaces has attracted a great deal of interest because of their scientific and technological importance and easy control of morphology with a variety of fabrication methods.1f,11c,20 Two factors should be taken into account to construct such a surface, i.e., surface micro-nano structures and surface coating of lowsurface-energy materials.20a Apparently, the 3D micro-nano hierarchical PGF demonstrates superhydrophilicity with a contact angle less than 10° (see the flat water film with large area in Figure 6b compared with that on the smooth gold in Figure 6a), which should be due to a 3D capillary effect of the surface. However, after being assembled with n-dodecanethiol monolayer the surface exhibits not only superhydrophobicity with a large contact angle over 150° (Figure 6d) but also excellent antiadhesion capability for the water droplet can hardly be transferred from the pipet tip to the surface and easily slide off the substrate by even very slight perturbation. Nevertheless, it is still adhering for the n-dodecanethiol monolayer decorated smooth surface with a contact angle about 95° (Figure 6c). The superhydrophobic property of a PGF surface over that of a smooth Au surface is due to its large fraction of air in the interspaces.20c 4. Conclusions. We have demonstrated that we are able to fabricate self-supported 3D micro-nano hierarchical porous gold films (PGFs) simply through the surface rebuilding of smooth gold substrates in NaOH solutions with a square wave potential pulse. The surface rebuilding involves repeated oxidation-reduction of gold and intensive hydrogen evolution, and the gold nanoparticles from the oxidation-reduction are automatically assembled under the action of hydrogen bubbles. This surface-rebuilding method is novel, facile, and green. The as-prepared multifunctional 3D PGFs exhibited excellent stability, high electrocatalytic activity toward the oxidation of ethanol, glucose, and ascorbic acid in alkaline medium, strong SERS effect renewable simply by an oxidation-dissolution pretreatment, and interesting transition from superhydrophilicity to superhydrophobicity after being decorated with n-dodecanethiol monolayer.

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Figure 4. CVs for the electrodes (1 mm diameter) of smooth gold (dotted line) and 3D PGF (solid lines) in the solutions: (a) background solution (1 mol dm-3 NaOH) only, and with reactants of (b) 1 mol dm-3 ethanol, (c) 0.1 mol dm-3 glucose, or (d) 0.1 mol dm-3 ascorbic acid. The insets in (b-d) are i-t curves obtained at -0.25 V for the smooth gold (dotted line) and 3D PGFs (solid lines). The 3D PGFs used in (a-d) were prepared by the SWPP treatment for 6000 s.. The two nearly superposed solid lines in (a) represent the CVs for the PGF before and after experiencing the electrocatalytic reactions in (b-d). Note also that the curves in (a) are 10 times magnified.

Figure 6. Photos of a 3.6 µL drop of water resting on the surfaces of (a, c) smooth gold and (b, d) 3D micro-nano PGF (a, b) without and (c, d) with the assembled n-dodecanethiol. Figure 5. (a, b) SEM images and (c, d) SERS spectra of pyridine (Py) in a solution of 0.01 mol dm-3 Py + 0.1 mol dm-3 KClO4 from the hierarchical 3D PGF (a, c) before and (b, d) after the oxidationdissolution pretreatment as described in the text. The PGFs were prepared by the SWPP treatment for 6000 s. The scale bars represent 500 nm. Laser: 785 nm; power on the sample: (c) 2.72 mW and (d) 0.44 mW; collection time: (c) 20 s and (d) 10 s.

Acknowledgment. We are grateful for the financial support of this research though National Natural Science Foundation of China (20673103; 20373063). Supporting Information Available: The CV behavior of gold in 2 mol dm-3, the porous structures obtained by the SWPP for different times, the dependence of roughness factor with pulse time or NaOH concentration, the nanoporous structures

before and after pretreatment and the corresponding SERS, and the table of numeral results from Figure 4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Lu, L. H.; Eychmu¨ller, A. Acc. Chem. Res. 2008, 41, 244. (b) Szamocki, R.; Reculusa, S.; Ravaine, S.; Bartlett, P. N.; Kuhn, A.; Hempelmann, R. Angew. Chem., Int. Ed. 2006, 45, 1317. (c) Ding, Y.; Chen, M. W.; Erlebacher, J. J. Am. Chem. Soc. 2004, 126, 6876. (d) Biener, J.; Nyce, G. W.; Hodge, A. M.; Biener, M. M.; Hamza, A. V.; Maier, S. A. AdV. Mater. 2008, 20, 1211. (e) Dixon, M. C.; Daniel, T. A.; Hieda, M.; Smilgies, D. M.; Chan, M. H. W.; Allara, D. L. Langmuir 2007, 23, 2414. (f) Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Langmuir 2005, 21, 1753. (2) (a) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (b) Velev, O. D.; Lenhoff,

Porous Gold Films A. M. Curr. Opin. Colloid Interface Sci. 2000, 5, 56. (c) He, X.; Antonelli, D. Angew. Chem., Int. Ed. 2002, 41, 214. (d) Stein, A.; Li, F.; Denny, N. R. Chem. Mater. 2008, 20, 649. (3) (a) Wang, D. H.; Luo, H. M.; Kou, R.; Gil, M. P.; Xiao, S. G.; Golub, V. O.; Yang, Z. Z.; Brinker, C. J.; Lu, Y. F. Angew. Chem., Int. Ed. 2004, 43, 6169. (b) Bartlett, P. N.; Baumberg, J. J.; Birkin, P. R.; Ghanem, M. A.; Netti, M. C. Chem. Mater. 2002, 14, 2199. (c) Chung, Y. W.; Leu, I. C.; Lee, J. H.; Yen, J. H.; Hon, M. H. J. Electrochem. Soc. 2007, 154, E77. (d) Jua´rez, B. H.; Lo´pez, C.; Alonso, C. J. Phys. Chem. B 2004, 108, 16708. (4) (a) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (b) Lu, L. H.; Capek, R.; Kornowski, A.; Gaponik, N.; Eychmu¨ller, A. Angew. Chem., Int. Ed. 2005, 44, 5997. (5) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (6) Au, R. H. W.; Puddephatt, R. J. Chem. Vap. Deposition 2007, 13, 20. (7) (a) Zhang, H. F.; Hussain, I.; Brust, M.; Cooper, A. I. AdV. Mater. 2004, 16, 27. (b) Zhang, H. F.; Cooper, A. I. J. Mater. Chem. 2005, 15, 2157. (8) Elliott, J. M.; Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Merckel, D. A. S.; Owen, J. R. Chem. Mater. 1999, 11, 3602. (9) Meldrum, F. C.; Seshadri, R. Chem. Commun. 2000, 29. (10) (a) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (b) Pickering, H. W. Corros. Sci. 1983, 23, 1107. (c) Kim, M. H.; Lu, X. M.; Wiley, B.; Lee, E. P.; Xia, Y. N. J. Phys. Chem. C 2008, 112, 7872. (d) Deng, Y. P.; Huang, W.; Chen, X.; Li, Z. L. Electrochem. Commun. 2008, 10, 810. (e) Zhao, W.; Xu, J. J.; Shi, C. G.; Chen, H. Y. Electrochem. Commun. 2006, 8, 773. (f) Prokopec, V.; Cejkova, J.; Mate´jka, P.; Hasal, P. Surf. Interface Anal. 2008, 40, 601. (11) (a) Shin, H. C.; Dong, J.; Liu, M. L. AdV. Mater. 2003, 15, 1610. (b) Shin, H. C.; Liu, M. L. Chem. Mater. 2004, 16, 5460. (c) Li, Y.; Jia, W. Z.; Song, Y. Y.; Xia, X. H. Chem. Mater. 2007, 19, 5758. (d) Li, Y.; Song, Y. Y.; Yang, C.; Xia, X. H. Electrochem. Commun. 2007, 9, 981. (e) Marozzi, C. A.; Chialvo, A. C. Electrochim. Acta 2000, 45, 2111. (f) Shin, H. C.; Liu, M. L. AdV. Funct. Mater. 2005, 15, 582. (12) Peng, Y. D.; Niu, Z. J.; Huang, W.; Chen, S.; Li, Z. L. J. Phys. Chem. B 2005, 109, 10880.

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1805 (13) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63, 711. (14) (a) Burke, L. D.; Cunnane, V. J.; Lee, B. H. J. Electrochem. Soc. 1992, 139, 399. (b) Burke, L. D.; Nugent, P. F. Gold Bull. 1997, 30, 43. (c) White, R. E.; Bockris, J. O’.M.; Conway, B. E. Modern Aspects of Electrochemistry; Plenum Press: New York, 1992. (d) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331. (e) Burke, L. D.; McRann, M. J. Electroanal. Chem. 1981, 125, 387. (15) (a) Burke, L. D.; Ryan, T. G. Electrochim. Acta 1992, 37, 1363. (b) Zhang, J. T.; Liu, P. P.; Ma, H. Y.; Ding, Y. J. Phys. Chem. C 2007, 111, 10382. (16) (a) Mulvaney, S. P.; Keating, C. D. Anal. Chem. 2000, 72, 145R. (b) Tian, Z. Q.; Ren, B. Annu. ReV. Phys. Chem. 2004, 55, 197. (17) (a) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463. (b) Zou, S.; Weaver, M. J.; Li, X. Q.; Ren, B.; Tian, Z. Q. J. Phys. Chem. B 1999, 103, 4218. (c) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. ACS Nano 2008, 2, 707. (18) (a) Tong, L. M.; Zhu, T.; Liu, Z. F. Appl. Phys. Lett. 2008, 92, 023109-1. (b) Qin, L. D.; Zou, S. L.; Xue, C.; Atkinson, A.; Schatz, G. C.; Mirkin, C. A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 13300. (c) Tian, Z. Q.; Yang, Z. L.; Ren, B.; Li, J. F.; Zhang, Y.; Lin, X. F.; Hu, J. W.; Wu, D. Y. Faraday Discuss. 2006, 132, 159. (d) Kucheyev, S. O.; Hayes, J. R.; Biener, J.; Huser, T.; Talley, C. E.; Hamza, A. V. Appl. Phys. Lett. 2006, 89, 053102-1. (e) Qian, L. H.; Yan, X. Q.; Fujita, T.; Inoue, A.; Chen, M. W. Appl. Phys. Lett. 2007, 90, 153120-1. (f) Qian, L. H.; Inoue, A.; Chen, M. W. Appl. Phys. Lett. 2008, 92, 093113-1. (19) (a) Zuo, C.; Jagodzinski, P. W. J. Phys. Chem. B 2005, 109, 1788. (b) Wiberg, K. B.; Walters, V. A.; Wong, K. N.; Colson, S. D. J. Phys. Chem. 1984, 88, 6067. (c) Wu, D. Y.; Hayashi, M.; Lin, S. H.; Tian, Z. Q. Spectrochim. Acta, Part A 2004, 60, 137. (d) Sun, S. G.; Christensen, P. A.; Wieckowski, A. In-situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis; Elsevier: Amsterdam, 2007. (20) (a) Ma, M. L.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193. (b) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (c) Cassie, A. B. D. Trans. Faraday Soc. 1944, 40, 546.

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