Synthesis of Pd Dendritic Nanowires by Electrochemical Deposition
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 505–507
You-Jung Song, Jy-Yeon Kim, and Kyung-Won Park* Department of Chemical and EnVironmental Engineering, Soongsil UniVersity, Seoul 156-743, South Korea ReceiVed July 15, 2008; ReVised Manuscript ReceiVed September 9, 2008
ABSTRACT: Pd dendritic nanowire electrodes were synthesized by the electrodeposition method with different reduction potentials and deposition times. In particular, it was found that during the electrochemical deposition process, the Pd dendritic nanowire grew along the 〈111〉 direction resulting from the favorable adsorption of the sulfuric acid anion on the Pd (111) plane. With an increased potential, the branches of the dendritic nanowires with single crystal structure became longer in length and thinner in thickness. In addition, depending on reduction potentials, the diameters of main and side branches in the Pd dendritic nanowires were almost the same without respect to the deposition time. Introduction The direct methanol fuel cell (DMFC), which uses methanol directly as fuel, has been a subject of intense study because of its numerous advantages such as high energy density, ease of handling a liquid, and low operating temperatures.1-5 High activity of methanol oxidation on platinum makes this metal a suitable electrocatalyst for the DMFC anode.6 However, if DMFCs operate in an alkaline instead of an acidic electrolyte, the kinetics will be significantly improved and Pt-free electrocatalysts can be used.7-9 Among a variety of candidates, Xu et al. have reported that the development of Pt-free electrocatalysts for alcohol oxidation has focused on the Pd nanostructure as a good electrocatalyst for alcohol oxidation in alkaline media.10-12 The catalytic and electrochemical properties of nanostructure materials are extremely different from those of bulk materials.13 Because the size and structure of nanoparticles have a significant effect on catalytic reactions, well-controlled nanostructures are essential for achieving efficient catalysts and in the preparation of catalysts for use in fuel cells. Recently, many reports about the synthesis of a dendritic material, which has a main stem from side branches, have been found because of its potential application to the catalysis and technology fields.14-21 Much recent attention has been paid to the synthesis of dendritic nanomaterials on the basis of electrochemical deposition methods because of simple operation, high purity, uniform deposits, and easy control.22-24 If the dendritic structure can be electrochemically deposited on the surface of the electrode, the modified electrode with a high surface area can be used as an electrochemical power source such as fuel cells, solar cells, sensors, and batteries. Herein, we describe the synthesis of Pd dendritic nanowires (Pd DNWs) using an electrochemical deposition method. The morphology and crystal structure of the nanowires deposited at different potentials were characterized by scanning electron microscopy and transmission electron microscopy. X-ray diffraction was used to investigate the crystal structure of the electrodes. Moreover, the formation mechanism of the Pd DNWs was discussed. Experimental Section Synthesis of Pd Dendritic Nanowires. Pd DNWs were prepared on indium tin oxide (ITO) glass by means of electrochemical deposition * Corresponding author. Tel: 82-2-820-0613. Fax: 82-2-812-5378. E-mail:
[email protected].
in a solution containing 0.2 M H3BO3 and 0.2 M PdSO4 at room temperature. Electrochemical deposition was carried out under a constant potential of -0.3, -0.6, or -0.9 V for 10 min in a threeelectrode cell consisting of a Pt wire, an Ag/AgCl, and ITO glass as a counter, reference, and working electrode, respectively. In addition, the electrochemical deposition was carried out under a constant potential of -0.9 V for 10 s, 30 s, 60 s, 300 s, and 600 s in order to investigate the growth mechanism of the DNWs. After an electrochemical deposition, the electrodes were washed with deionized water and dried at room temperature. Characterizations. The morphology and crystal structure of the electrodes deposited at different potentials were characterized by scanning electron microscopy (SEM, JEOL JSM-6360A) and transmission electron microscopy (TEM, Phillips-F20). The SEM investigation was carried out at room temperature at a voltage of 13 kV with a spot size of 40 nm. The TEM analysis was carried out at an accelerating voltage of 200 kV, and Cu grids were used as substrates. X-ray diffraction (XRD, Rigaku X-ray diffractometer equipped with a Cu KR source at 40 kV and 100 mA) was used to investigate the crystal structure of the electrodes.
Results and Discussion Figure 1 shows scanning electron microscopy (SEM) and transmission electron micrograph (TEM) images of electrodes (Pd DNW-0.3, Pd DNW-0.6, Pd DNW-0.9) electrodeposited at applied potentials of -0.3, -0.6, or -0.9 V. Although different potentials were applied during the electrochemical deposition process, as shown in Figure 1a-c, the morphology of the electrodes seems to be similar to that of dendritic nanowires (DNWs). However, as indicated in Figure 1d and 1e, the DNWs formed under different applied potentials were clearly different in the branch tips and sides of the DNWs. As the potential was increased from -0.3 to -0.9 V, the branches of the DNWs became longer and thinner. This implies that the applied potential plays an important role in the growth of the dendrites. It is likely that a lower applied reduction potential could produce thick and short branches of the DNWs due to a slow growth rate while a higher applied reduction potential could lead to thin and long branches of the DNWs caused by a fast growth rate. As shown in the high-resolution TEM images of the DNWs of Figure 1d and 1e, both Pd DNWs grew under -0.3 and -0.9 V of applied potential along the κ directions at the branch tip with a d-spacing of 0.224 nm corresponding to the distance between {111} planes of Pd crystal structure. A more detailed growth mechanism of Pd DNWs will be discussed later. To confirm the crystal structure of the Pd DNWs, the electrodes were characterized by XRD. As shown in Figure 2,
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Figure 1. Scanning electron microscopy (SEM) images of Pd DNWs electrodeposited at (a) -0.3 V, (b) -0.6 V, and (c) -0.9 V. Transmission electron microscopy (TEM) images of Pd DNWs electrodeposited at (d) -0.3 V and (e) -0.9 V.
Figure 3. Scanning electron microscopy (SEM) images of Pd DNWs electrodeposited at -0.9 V for (a) 10 s, (b) 30 s, (c) 60 s, (d) 300 s, and (e) 600 s. Transmission electron microscopy (TEM) images of Pd DNWs electrodeposited at -0.9 V for (f) 10 s and (g) 600 s. The left top and right bottom of the inset of f and g is the transmission electron diffraction (TED) pattern and high-resolution transmission electron microscopy (TEM) image of Pd DNWs, respectively.
Scheme 1. Formation Mechanism of Pd DNW Structures Electrodeposited at Different Reduction Potentials of (a) -0.3 V and (b) -0.9 V
Figure 2. X-ray diffraction (XRD) patterns of Pd DNWs electrodeposited at (a) -0.3 V, (b) -0.6 V, and (c) -0.9 V. The asterisk represents the XRD patterns of ITO.
the diffraction peak at 40.1°, 46.7°, and 68.0° corresponds to the (111), (200), and (220) plane of a typical Pd crystal structure, respectively. All the DNWs synthesized at different potentials show the face-centered-cubic (fcc) structure of a Pd metal crystal. To study the growth mechanism of Pd dendritic nanowires, as shown in Figure 3, we synthesized the Pd DNWs (Pd DNW10, Pd DNW-30, Pd DNW-60, Pd DNW-300, Pd DNW-600) by electrodeposition at -0.9 V as a function of deposition time such as 10 s, 30 s, 60 s, 300 s, and 600 s, respectively. As shown in Figure 3a and 3f, Pd DNW-10 is 120 nm in diameter and 1 µm in length and begins to branch out. Although the stem
of the Pd DNW is polycrystalline, branches of Pd DNW are single crystalline. The (111) single crystal structure of the edge in the Pd DNW is confirmed using the transmission electron diffraction (TED) pattern in the inset of the Figure 3f. In the Pd DNW-30 and Pd DNW-60, a few DNWs with a short branch of 600 nm in length are formed (Figure 3b and 3c). The Pd DNW-300 shows a longer branch up to 10 µm in length (Figure 3d). Finally, as shown in Figure 3e and 3g, the Pd DNW-600 consists of branches up to 100 nm in diameter and 800 nm in length representing the (111) single crystal structure of the branch in the Pd DNW as seen in the HR-TEM and TED images (the inset of the Figure 3g). Herein, when formed under the reduction potential of -0.9 V for the deposition time of 10 or 600 s, the diameter of the Pd DNW-600 is almost similar to that of the Pd DNW-10. On the other hand, as the potential
Synthesis of Pd Dendritic Nanowires
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in the Pd dendritic nanowires were almost the same without respect to the deposition time. Acknowledgment. This work was supported by Korea Research Foundation (KRF-2007-331-D00114).
References
Figure 4. Scanning electron microscopy (SEM) images of electrodes electrodeposited at -0.9 V for 600 s in (a) 0.2 M PdCl2 and (b) 0.2 M PdSO4 at room temperature.
was increased from -0.3 to -0.9 V, the branches of the DNWs became longer and thinner. The lower reduction potential could produce thick and short branches of the DNWs while the higher reduction potential could lead to thin and long branches of the DNWs. Therefore, it is considered that the thickness of branches in the DNWs strongly depends on a reduction potential rather than a deposition time. A schematic illustration of the formation of Pd DNW structures, based on the SEM and TEM results, is presented in Scheme 1. First, Pd nanonuclei are formed on the substrate through the reduction of PdSO4, and then they grow along the κ directions forming a nanowire structure. This is because sulfuric acid anions such as HSO4- or SO42- in solution are adsorbed on the Pd (111) surface, disturbing growth to the plane.25-27 The sulfuric acid anion is known to adsorb on the (111) surface of metal electrodes with an fcc crystal structure especially, as already observed in the adsorption of bisulfate and sulfate on a Pt (111) electrode.27 Experimentally, as shown in Figure 4, we found that Pd DNW structures were not formed on the substrate through the reduction of PdCl2 compared to the growth through the reduction of PdSO4. Especially, the adsorption of the sulfuric acid anion on Pd (111) is more favorable than that on others, thus the growth along the κ direction. However, it is likely that since the step site of the edge in the Pd nanowire hardly adsorbs sulfuric acid anions, branches could be formed at edges of the nanowire. Moreover, as reduction potentials are increased, the diameters of main and side branches in the Pd DNW are constant, irrespective of deposition time. However, as the potential is increased, the branches of the DNWs become longer and thinner. Conclusion The Pd dendritic nanowire electrodes were synthesized by means of electrochemical deposition as a function of potential or deposition time. In particular, during the electrochemical deposition process, the Pd dendritic nanowire grew along the κ direction, resulting from the favorable adsorption of the sulfuric acid anion on the Pd (111) plane. As the potential was increased, the branches of the dendritic nanowires with single crystal structure became longer and thinner. Moreover, depending on reduction potentials, the diameters of the main and side branches
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