Additive-Mediated Electrochemical Synthesis of Platelike Copper

Letter pubs.acs.org/Langmuir

Additive-Mediated Electrochemical Synthesis of Platelike Copper Crystals for Methanol Electrooxidation Rajesh Venkatasubramanian,† Jibao He,‡ Michael W. Johnson,§ Ilan Stern,∥ Dae Ho Kim,∥ and Noshir S. Pesika*,† †

Department of Chemical and Biomolecular Engineering, ‡Coordinated Instrumentation Facility, §Department of Chemistry, and Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, United States

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S Supporting Information *

ABSTRACT: A room-temperature electrochemical approach to synthesizing anisotropic platelike copper microcrystals and nanocrystals in the presence of potassium bromide is presented. Morphological and elemental characterization was performed using SEM, TEM, and XRD to confirm the anisotropic morphology and crystal structure of the synthesized copper particles. A possible mechanism for explaining the anisotropic crystal growth is proposed on the basis of the preferential adsorption of bromide ions to selective crystal faces. The shape-dependent electrocatalytic property of copper particles is demonstrated by its enhanced catalytic activity for methanol oxidation. Further development of such anisotropic copper particles localized on an electrode surface will lead us to find a suitable alternative for noble metal-based electrocatalysts for the methanol oxidation reaction relevant to fuel cells.

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INTRODUCTION The synthesis and characterization of anisotropic microparticles and nanoparticles, either in suspension or localized on a surface, are current areas of intense scientific interest because of their shape-tunable material properties with potential applications in catalysis, data storage, photonics, quantum computing, biosensors, and pharmaceuticals.1−7 Copper, being a nonnoble face-centered cubic (fcc) metal with high thermal and electrical conductivities and relatively low cost, is quickly emerging as a promising alternative to noble metals (such as Pt, Au, and Ag) in several applications.8−11 A variety of synthesis methodologies such as wet chemical reduction, hydrothermal processing, ultrasonic irradiation, and electrochemical deposition have been developed to make anisotropic copper microparticles and nanoparticles.12−16 In general, solutionbased synthesis techniques are common for nanoparticles synthesis, but they are often time-consuming and involve multiple steps. Electrochemical deposition represents a simple and versatile method of nanoparticle synthesis and shape control at room temperature. It offers a high degree of freedom in monitoring and manipulating particle growth processes by simply adjusting the deposition potential or current and controlling the extent of growth by the charge applied to the system. Such substrate-immobilized anisotropic nanostructures can be applied directly in metal electrode-based nanodevices such as sensors, electronics, and solar cells.16,17 Moreover, they allow easy handling during storage and characterization. Various anisotropic nanoparticle shapes ranging from cubes, octahedrons, and rods to dumbbells have been obtained using electrochemical deposition.18−20 Surfactants such as cetyltrimethylammonium bromide (CTAB) and sodium dodecyl © 2013 American Chemical Society

sulfate (SDS) were used as structure-directing agents in these studies. The surfactants are believed to bind preferentially to select crystal planes of the particles, thereby altering the relative growth kinetics of particular planes. Unlike anisotropic noble metal synthesis, only a few reports have been published on the anisotropic shape modification of copper particles using electrodeposition.21,22 Anisotropic copper particles were obtained in these reports by exploiting crystal growth kinetics on different faces with deposition parameters including time, potential, and solution concentrations. Recently, Ko et al. synthesized anisotropic copper nanoparticles electrochemically using surface capping agents such as dodecyl benzenesulfonic acid and poly(vinylpyrrolidone).23 In this letter, we present a simple and versatile additivemediated room-temperature electrochemical approach to synthesizing anisotropic copper microparticles and nanoparticles localized on electrode substrates. Structural and crystallographic characterizations are performed to understand the anisotropic growth mechanism of copper particles in the presence of potassium bromide as an additive to the electrolyte. Furthermore, the platelike copper particles are used as electrocatalysts for methanol electrooxidation to characterize their shape-dependent catalytic performance for use in direct methanol fuel cell (DMFC) applications as suitable anode catalysts. Received: July 16, 2013 Revised: September 19, 2013 Published: October 15, 2013 13135

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Letter

EXPERIMENTAL SECTION

Electrochemical Deposition. A three-electrode cell was used to deposit copper electrochemically from an acidic solution of copper sulfate (0.4 M CuSO4·5H2O and 0.2 M H2SO4) in deionized water. An aqueous KBr stock solution (100 μL, 1 mM) was added to 10 mL of the electrolyte solution before starting the experiment (i.e., yielding a final KBr concentration of 0.01 mM). Ag/AgCl (3 M NaCl, 0.22 V vs SHE) and a platinum wire mesh were used as reference and counter electrodes, respectively. A copper wire (in saturated CuSO4 solution, 0.32 V vs SHE) was also used as a reference for copper nanoparticle deposition in certain experiments. A 300-nm-thick layer of a sputtered gold film (deposition rate = 1.8 Å/s) on freshly cleaved mica substrates was used as the working electrode surface. The substrates were annealed at 300 °C for 3 h under a nitrogen atmosphere in a tube furnace (GSL-1500X, MTI Corporation). A 250-nm-thick layer of a gold film (with a 2.5 nm thick chromium adhesion layer) on borosilicate glass (0.7 mm thick) purchased from Arrandee (Germany) was used as a working electrode for characterization purposes. The working electrode surfaces were cleaned by rinsing with ethanol and deionized water. Potentiostatic electrodeposition was carried out using a potentiostat (263A, Princeton Applied Research) with a working electrode area of 0.7 cm2. All electrochemical experiments were performed at room temperature. After deposition, the electrode surface was rinsed thoroughly with deionized water, dried with nitrogen gas, and stored for further characterization. Characterization. The morphology and size of electrochemically grown copper particles were characterized using a Hitachi S4800 fieldemission scanning electron microscope (SEM). The particle composition and crystal structure were analyzed by four-circle highresolution X-ray diffraction (HR-XRD) and a JEOL transmission electron microscope (TEM, at 200 kV). For TEM sample preparation, fresh copper nanoparticles were scrapped off of the electrode surface into water, and a few drops of the liquid suspension was placed on a carbon-coated TEM grid and allowed to dry in air. The CV experiments were performed using the same electrochemical setup described in a previous paragraph. The working electrode potentials were cycled between 0 V and +1 V versus the Ag/AgCl reference electrode (3 M NaCl sat.) in an alkaline electrolyte solution containing methanol (0.25 M) and sodium hydroxide (0.1 M) in water. The scan rate was 10 mV s−1. All current density values were normalized to a working electrode surface area of 0.71 cm2.

Figure 1. SEM images of electrochemically grown copper particles on a gold-coated mica substrate under potentiostatic conditions in (A) the absence and (B) the presence of potassium bromide (0.01 mM) (scale bar = 10 μm). The electrolyte consists of an acidic copper sulfate solution (0.4 M). The insets correspond to highermagnification images of single copper particles with faceted morphology.

platelike-shaped particles with time. The inherent vertical alignment of the particles (i.e., with the long axis normal to the substrate surface) is presumably a result of the crystalline orientation of the gold coating and will be explored in future studies as a potential means of dictating the orientation of the anisotropic particles supported on electrodes. Figure 2 shows a transmission electron microscope (TEM) image of a single anisotropic copper particle suspended on a TEM grid. The particle has nearly triangular morphology with the base plane parallel to the grid surface and a planar angle of 60°. The selected-area electron diffraction (SAED) pattern recorded for one of the anisotropic copper particles is shown in the inset. The third set of diffraction spots is indexed to {220} Bragg reflections, in agreement with the standard fcc crystal lattice arrangement in the [111] zone axis. The first and second sets of diffraction spots are indexed to 1/6 {220} and 1/3 {220} Bragg reflections, respectively. The SAED pattern indicates that the base plane of the anisotropic platelike copper particles has a (111) crystal lattice structure. X-ray diffraction (XRD) patterns are obtained at the electrode surface containing copper particles on Au-coated glass substrate (see Supporting Information, Figure S2). A schematic illustration of the anisotropic growth mechanism of copper particles in the presence of bromide ions is shown in Scheme 1. Copper (face-centered cubic crystal structure) has a cubic unit cell, and on the basis of truncated octahedron-like

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RESULTS AND DISCUSSION Figure 1A,B shows the scanning electron microscope (SEM) images of electrochemically grown copper particles on a goldcoated mica substrate. The copper particles grown without any additives (Figure 1A) in the medium were evenly dispersed on the electrode surface with a well-defined crystal structure. These particles were grown at an applied overpotential of −0.04 V (vs Cu/CuSO4) for 5 min from an acidic copper sulfate solution. The inset image in Figure 1A shows a highmagnification view of a single copper particle having a truncated octahedron shape with the {111} face parallel to the substrate. Figure 1B shows platelike anisotropic copper particles grown in the presence of potassium bromide (KBr) as an additive. The deposition parameters were similar to those in the control experiment (Figure 1A) except for the addition of 0.01 mM KBr to the electrolyte solution before starting the electrochemical deposition. The inset of Figure 1B shows a close-up view of a single anisotropic copper particle (∼150 nm thick) with platelike morphology oriented vertically relative the electrode surface. A time-based study was carried out to understand the growth pattern of anisotropic copper particles in the presence of KBr (refer to Supporting Information, Figure S1). The particles tend to acquire anisotropic platelike morphology soon after nucleation and continue to grow as 13136

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perpendicular to the {111} plane (i.e., through adsorption, the bromide ions decrease the surface energy of the {111} plane). To maintain charge neutrality, potassium and hydronium ions may be incorporated into the diffuse double layer. The relative crystal growth kinetics is altered with relatively higher growth rates in the directions perpendicular to the {100} and {110} crystal planes than in the direction perpendicular to the {111} crystal plane as a result of the stabilization on the {111} plane by the Br− ions. As a result, the (111) basal plane continues to increase in area in an anisotropic manner, thereby leading to a triangular platelike morphology. To confirm further that the bromide ions are responsible for the morphological change (i.e., not the potassium ions), potassium sulfate (K2SO4) was used instead as the electrolyte additive. The SEM images (Supporting Information Figure S3) showed no significant change in the morphology of the copper particles compared to those obtained without additives in the growth medium, thereby eliminating the possibility of potassium ion involvement in the anisotropic growth. The significance of additive ions (particularly anions) as shape-directing agents has been well documented for the seed-mediated synthesis of anisotropic gold nanowires.25 Similar preferential adsorption behavior for Br− ions has been observed by other groups in shape-controlled metal nanocrystal synthesis.19,26 In recent work on gold nanoparticles, it is suggested that the degree of specific adsorption of halide ions on low-index Au surfaces varies as follows: I− > Br− > Cl− > F−.27 Chloride ions exhibit a lower degree of specific adsorptive ability compared to Br− ions. In other words, chloride ions would adsorb evenly to all crystal facets in particle morphology. Hence, it may not be possible to obtain the anisotropic growth of copper particles with Cl− ions. I− ions would be able to induce anisotropic growth in copper because it has a higher degree of preferential adsorption to select crystal facets. The adsorption strength of I− ions is higher than that of Br− ions, which may alter the aspect ratio of resulting anisotropic shapes with adsorbed I− ions. Because copper particles also exhibit an fcc crystal structure like that of gold, similar adsorption behavior of other halide ions could be expected, which will be inspected as part of our future work. To characterize the effect of shape anisotropy on the catalytic activity of the crystals, cyclic voltammetry (CV) was performed as shown in Figure 3 to measure the oxidation current density as a function of electrode potential for the methanol oxidation reaction. An alkaline solution containing NaOH (0.1 M) and methanol (0.25 M) was used as the electrolyte. Both the truncated octahedron and platelike copper particles show catalytic activity toward methanol oxidation, which is confirmed by the presence of distinct oxidation peaks in the CV curve at ∼0.9 V (vs Ag/AgCl in 3 M NaCl), in agreement with literature results.28 The CV curve for the bare gold film does not show any oxidation peaks for methanol at the respective oxidation potentials. The CV curve obtained for octahedron-like Cu particles in electrolyte containing only NaOH (0.1 M) did not show any additional peaks (Supporting Information Figure S4). Both of these experiments further emphasize the catalytic nature of copper particles for methanol oxidation. A recent report suggests that the catalytic activity of copper toward methanol oxidation is through a mediated electron-transfer mechanism involving Cu(III) species that are present at the electrode interface at higher anodic potentials.28 The elemental composition information obtained using EDS detects characteristic peaks for oxygen on the copper catalyst surface after methanol oxidation for 60 min. The EDS results are shown in

Figure 2. TEM image of a single anisotropic copper particle suspended on the TEM grid. The SAED pattern (inset) indicates a (111) lattice arrangement on the base plane of anisotropic copper particles.

Scheme 1. Schematic Illustration Showing the Proposed Mechanism for the Additive-Mediated Anisotropic Crystal Growth of Copper Particles in which Bromide Ions Preferentially Bind to the (111) Crystal Face

crystals seen from SEM images (Figure 1A), individual crystal faces can be assigned with their respective Miller indices{110}, {100} and {111}as shown in Scheme 1. Under equilibrium or near-equilibrium conditions, the crystal morphology is dictated by the relative order of interfacial energies of different crystal faces such that the total surface free energy of the solid is driven toward a minimum at a specific concentration and temperature.24 Faster growth rates occur perpendicular to the faces with higher surface free energies, thereby eliminating (or diminishing) them whereas lower-surface-energy faces continue to increase in area (i.e., the final crystal structure predominantly consists of lower-energy surfaces). However, anisotropic crystal growth can be achieved by introducing additive species into the growth medium that have the ability to bind preferentially only to certain crystal faces, thereby altering their surface energies and hence influencing the relative crystal growth kinetics and thermodynamics.16 On the basis of the TEM results, we infer that Br− ions preferentially bind to {111} crystal planes of copper, thereby stabilizing the crystal growth in the direction 13137

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{111} and {110} crystal planes of copper showed enhanced catalytic activity for methanol oxidation in comparison to the {100} crystal plane of copper. Therefore, an ideal copper catalyst for methanol oxidation should expose either {111} or {110} more than the {100} plane to obtain a higher catalytic activity. In platelike copper particles, the exposed surface area of the {111} crystal plane was relatively higher than that of {110} and {100} Cu planes whereas in truncated octahedron-shaped copper particles all three crystal facets are exposed almost equally. This explains the greater catalytic activity observed in platelike copper particles with anisotropic crystal morphology. We plan to investigate further and differentiate the effect of enhanced surface area versus enhanced facet-dependent electrocatalytic activity for these copper catalysts.

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OUTLOOK In the present study, we demonstrate how additive-mediated electrochemical deposition can be used as a simple and versatile approach to fabricating supported anisotropic platelike copper particles at room temperature. The latter show electrocatalytic activity for the methanol oxidation reaction that is relevant to DMFC applications. Further investigation of such anisotropic nanostructured particles (including other non-noble materials) inherently supported on a substrate surface will enable us to explore novel shape-dependent material properties in applications such as sensors, optoelectronics, and catalysis.

Figure 3. CV curves for copper particles in an alkaline electrolyte medium containing CH3OH (0.25 M) and NaOH (0.1 M) at a scan rate of 10 mV s−1. The insets show SEM images of individual copper particles involved in the methanol oxidation reaction. A CV scan of a bare Au film was performed as a control experiment (scan rate: 10 mV s−1). The current density is normalized to the working electrode surface area of 0.7 cm2.

the Supporting Information (Figure S5). The peak oxidation current densities obtained for octahedron- and platelike copper particles are 4.3 and 8.3 mA cm−2, respectively. The enhanced catalytic performance of anisotropic platelike copper particles in comparison to that of octahedron-like copper particles can be attributed to the direct dependence of particle shape and morphology on its catalytic properties. An important parameter that is attributed to this enhanced performance of anisotropic copper particles is the electrochemically active surface area. However, the difficulties encountered in measuring such an electrochemically active surface area for transition metals such as copper have been well documented.29 Most established in situ methods such as hydrogen or oxygen adsorption from solution can be applied only to the metals that allow considerable accuracy in identifying the point at which the surface has complete coverage before the evolution of adsorbed species is significant (e.g., Pt, Pd, or Au). Copper, being an easily oxidizable transition metal, does not allow enough time to mark the distinction between different characteristic regions of hydrogen and oxygen adsorption, making it unsuitable for such analytical techniques. Other probing methods such as underpotential deposition (UPD) have unique disadvantages toward application to the copper system.30 The UPD of foreign metals on a crystalline copper surface result in close-packed surface structure in preference to an epitaxial arrangement, making it more complex for the calculation of an electrochemically active surface as desired. Hence, to facilitate the comparison of fabricated electrode surfaces with two different copper particle morphologies, the mass of copper being deposited was kept the same by simply controlling the deposition charge. The exposed geometric area on the working electrode was kept constant for both electrodes. We also investigated the activity of specific crystal planes toward methanol oxidation using single-crystal Cu surfaces exposing {111}, {100}, and {110} planes (Supporting Information Figure S6). On the basis of the cyclic voltammetry (CV) measurements, it can be seen that the current density response at the characteristic methanol oxidation potential for copper varies depending on the exposed crystal plane. The

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of anisotropic copper particle morphology over time. XRD patterns on a copper electrode surface. SEM images of copper particles in the presence of potassium sulfate. CV curve for truncated octahedron-like copper particles in the absence of methanol in an alkaline electrolyte medium. EDS pattern on an electrode surface after methanol oxidation. CV curves for single-crystal surfaces of copper in an alkaline electrolyte medium. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 504-865-5771. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF EPSCoR LA-SiGMA project under award no. EPS-1003897 and partially by NSF CBET-1034175. We thank Dr. Marc Donohue (Chemical and Biomolecular Engineering Department, Johns Hopkins University) for helpful discussions related to this project.

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REFERENCES

(1) Polarz, S. Shape Matters: Anisotropy of the Morphology of Inorganic Colloidal Particles - Synthesis and Function. Adv. Funct. Mater. 2011, 21, 3214−3230. (2) Narayanan, R.; El-Sayed, M. A. Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability. J. Phys. Chem. B 2005, 109, 12663−12676. (3) Darques, M.; Piraux, L.; Encinas, A. Influence of the Diameter and Growth Conditions on the Magnetic Properties of Cobalt Nanowires. IEEE Trans. Magn. 2005, 41, 3415−3417.

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(4) McLellan, J. M.; Xiong, Y. J.; Hu, M.; Xia, Y. N. SurfaceEnhanced Raman Scattering of 4-Mercaptopyridine on Thin Films of Nanoscale Pd Cubes, Boxes, and Cages. Chem. Phys. Lett. 2006, 417, 230−234. (5) Hu, J. T.; Li, L. S.; Yang, W. D.; Manna, L.; Wang, L. W.; Alivisatos, A. P. Linearly Polarized Emission from Colloidal Semiconductor Quantum Rods. Science 2001, 292, 2060−2063. (6) Lakowicz, J. R. Plasmonics in Biology and Plasmon-Controlled Fluorescence. Plasmonics 2006, 1, 5−33. (7) Variankaval, N.; Cote, A. S.; Doherty, M. F. From Form to Function: Crystallization of Active Pharmaceutical Ingredients. AIChE J. 2008, 54, 1682−1688. (8) Ojani, R.; Raoof, J. B.; Ahmady-Khanghah, Y. Copper-poly(2aminodiphenylamine) as a Novel and Low Cost Electrocatalyst for Electrocatalytic Oxidation of Methanol in Alkaline Solution. Electrochim. Acta 2011, 56, 3380−3386. (9) Ikari, S.; Kashiwade, H.; Matsuoka, T.; Hirayama, T.; Ishida, S.; Kato, K. Improvement of Copper Plating Adhesion of PPE Printed Wiring Board by Plasma Treatment. Surf. Coat. Technol. 2008, 202, 5583−5585. (10) Xu, R.; Xie, T.; Zhao, Y. G.; Li, Y. D. Single-Crystal Metal Nanoplatelets: Cobalt, Nickel, Copper, and Silver. Cryst. Growth Des. 2007, 7, 1904−1911. (11) Eastman, J. A.; Choi, S. U. S.; Li, S.; Yu, W.; Thompson, L. J. Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol-Based Nanofluids Containing Copper Nanoparticles. Appl. Phys. Lett. 2001, 78, 718−720. (12) Tanori, J.; Pileni, M. P. Control of the Shape of Copper Metallic Particles by Using a Colloidal System as Template. Langmuir 1997, 13, 639−646. (13) Bicer, M.; Sisman, I. Controlled Synthesis of Copper Nano/ Microstructures Using Ascorbic Acid in Aqueous CTAB Solution. Powder Technol. 2010, 198, 279−284. (14) Xu, S. L.; Sun, X.; Ye, H.; You, T.; Song, X. Y.; Sun, S. X. Selective Synthesis of Copper Nanoplates and Nanowires via a Surfactant-Assisted Hydrothermal Process. Mater. Chem. Phys. 2010, 120, 1−5. (15) Tao, X. J.; Sun, L.; Zhao, Y. B. Sonochemical Synthesis and Characterization of Disk-like Copper Microcrystals. Mater. Chem. Phys. 2011, 125, 219−223. (16) Choi, K. S. Shape Control of Inorganic Materials via Electrodeposition. Dalton Trans. 2008, 40, 5432−5438. (17) Choi, K. S.; Jang, H. S.; McShane, C. M.; Read, C. G.; Seabold, J. A. Electrochemical Synthesis of Inorganic Polycrystalline Electrodes with Controlled Architectures. MRS Bull. 2010, 35, 753−760. (18) Siegfried, M. J.; Choi, K. S. Electrochemical Crystallization of Cuprous Oxide with Systematic Shape Evolution. Adv. Mater. 2004, 16, 1743−1746. (19) Yu, R.; Ren, T.; Sun, K. J.; Feng, Z. C.; Li, G. N.; Li, C. ShapeControlled Copper Selenide Nanocubes Synthesized by an Electrochemical Crystallization Method. J. Phys. Chem. C 2009, 113, 10833− 10837. (20) Huang, C. J.; Chiu, P. H.; Wang, Y. H.; Yang, C. F. Synthesis of the Gold Nanodumbbells by Electrochemical Method. J. Colloid Interface Sci. 2006, 303, 430−436. (21) Radi, A.; Pradhan, D.; Sohn, Y.; Leung, K. T. Nanoscale Shape and Size Control of Cubic, Cuboctahedral, and Octahedral Cu-Cu(2) O Core-Shell Nanoparticles on Si(100) by One-Step, Templateless, Capping-Agent-Free Electrodeposition. ACS Nano 2010, 4, 1553− 1560. (22) Tang, S. C.; Meng, X. K.; Vongehr, S. An Additive-Free Electrochemical Route to Rapid Synthesis of Large-Area Copper Nano-Octahedra on Gold Film Substrates. Electrochem. Commun. 2009, 11, 867−870. (23) Ko, W. Y.; Chen, W. H.; Cheng, C. Y.; Lin, K. J. Architectural Growth of Cu Nanoparticles Through Electrodeposition. Nanoscale Res. Lett. 2009, 4, 1481−1485. (24) Mullin, J. W. Crystallization; Butterworth-Heinemann: Oxford, U.K., 2001.

(25) Sau, T. K.; Murphy, C. J. Role of Ions in the Colloidal Synthesis of Gold Nanowires. Philos. Mag. 2007, 87, 2143−2158. (26) Filankembo, A.; Giorgio, S.; Lisiecki, I.; Pileni, M. P. Is the Anion the Major Parameter in the Shape Control of Nanocrystals? J. Phys. Chem. B 2003, 107, 7492−7500. (27) DuChene, J. S.; Niu, W. X.; Abendroth, J. M.; Sun, Q.; Zhao, W. B.; Huo, F. W.; Wei, W. D. Halide Anions as Shape-Directing Agents for Obtaining High-Quality Anisotropic Gold Nanostructures. Chem. Mater. 2013, 25, 1392−1399. (28) Heli, H.; Jafarian, M.; Mahjani, M. G.; Gobal, F. Electrooxidation of Methanol on Copper in Alkaline Solution. Electrochim. Acta 2004, 49, 4999−5006. (29) Trasatti, S.; Petrii, O. A. Real Surface-Area Measurements in Electrochemistry. J. Electroanal. Chem. 1992, 327, 353−376. (30) Siegenthaler, H.; Juttner, K. Voltammetric Investigation of Lead Adsorption on Cu(111) Single-Crystal Substrates. J. Electroanal. Chem. 1984, 163, 327−343.

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