Synthesis of Pd Dendritic Nanowires by Electrochemical Deposition

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 potentialsand deposition times. In particular, it was found that during the electrochemical deposition process, the Pd dendritic nanowire grewalong the ⟨111⟩ direction resulting from the favorable adsorption of the sulfuric acid anion on the Pd (111) plane. With an increasedpotential, the branches of the dendritic nanowires with single crystal structure became longer in length and thinner in thickness. Inaddition, depending on reduction potentials, the diameters of main and side branches in the Pd dendritic nanowires were almost thesame without respect to the deposition time.

Introduction

The direct methanol fuel cell (DMFC), which uses methanoldirectly as fuel, has been a subject of intense study because ofits numerous advantages such as high energy density, ease ofhandling a liquid, and low operating temperatures.1-5 Highactivity of methanol oxidation on platinum makes this metal asuitable electrocatalyst for the DMFC anode.6 However, ifDMFCs operate in an alkaline instead of an acidic electrolyte,the kinetics will be significantly improved and Pt-free electro-catalysts can be used.7-9 Among a variety of candidates, Xu etal. have reported that the development of Pt-free electrocatalystsfor alcohol oxidation has focused on the Pd nanostructure as agood electrocatalyst for alcohol oxidation in alkaline media.10-12

The catalytic and electrochemical properties of nanostructurematerials are extremely different from those of bulk materials.13

Because the size and structure of nanoparticles have a significanteffect on catalytic reactions, well-controlled nanostructures areessential for achieving efficient catalysts and in the preparationof catalysts for use in fuel cells. Recently, many reports aboutthe synthesis of a dendritic material, which has a main stemfrom side branches, have been found because of its potentialapplication to the catalysis and technology fields.14-21 Muchrecent attention has been paid to the synthesis of dendriticnanomaterials on the basis of electrochemical depositionmethods because of simple operation, high purity, uniformdeposits, and easy control.22-24 If the dendritic structure canbe electrochemically deposited on the surface of the electrode,the modified electrode with a high surface area can be used asan 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. Themorphology and crystal structure of the nanowires deposited atdifferent potentials were characterized by scanning electronmicroscopy and transmission electron microscopy. X-ray dif-fraction was used to investigate the crystal structure of theelectrodes. Moreover, the formation mechanism of the Pd DNWswas discussed.

Experimental Section

Synthesis of Pd Dendritic Nanowires. Pd DNWs were preparedon indium tin oxide (ITO) glass by means of electrochemical deposition

in a solution containing 0.2 M H3BO3 and 0.2 M PdSO4 at roomtemperature. Electrochemical deposition was carried out under aconstant potential of -0.3, -0.6, or -0.9 V for 10 min in a three-electrode cell consisting of a Pt wire, an Ag/AgCl, and ITO glass as acounter, reference, and working electrode, respectively. In addition,the electrochemical deposition was carried out under a constant potentialof -0.9 V for 10 s, 30 s, 60 s, 300 s, and 600 s in order to investigatethe growth mechanism of the DNWs. After an electrochemicaldeposition, the electrodes were washed with deionized water and driedat room temperature.

Characterizations. The morphology and crystal structure of theelectrodes deposited at different potentials were characterized byscanning electron microscopy (SEM, JEOL JSM-6360A) and transmis-sion electron microscopy (TEM, Phillips-F20). The SEM investigationwas carried out at room temperature at a voltage of 13 kV with a spotsize of 40 nm. The TEM analysis was carried out at an acceleratingvoltage of 200 kV, and Cu grids were used as substrates. X-raydiffraction (XRD, Rigaku X-ray diffractometer equipped with a CuKR source at 40 kV and 100 mA) was used to investigate the crystalstructure of the electrodes.

Results and Discussion

Figure 1 shows scanning electron microscopy (SEM) andtransmission electron micrograph (TEM) images of electrodes(Pd DNW-0.3, Pd DNW-0.6, Pd DNW-0.9) electrodepositedat applied potentials of -0.3, -0.6, or -0.9 V. Althoughdifferent potentials were applied during the electrochemicaldeposition process, as shown in Figure 1a-c, the morphologyof the electrodes seems to be similar to that of dendriticnanowires (DNWs). However, as indicated in Figure 1d and1e, the DNWs formed under different applied potentials wereclearly different in the branch tips and sides of the DNWs. Asthe potential was increased from -0.3 to -0.9 V, the branchesof the DNWs became longer and thinner. This implies that theapplied potential plays an important role in the growth of thedendrites. It is likely that a lower applied reduction potentialcould produce thick and short branches of the DNWs due to aslow growth rate while a higher applied reduction potential couldlead to thin and long branches of the DNWs caused by a fastgrowth rate. As shown in the high-resolution TEM images ofthe DNWs of Figure 1d and 1e, both Pd DNWs grew under-0.3 and -0.9 V of applied potential along the κ directions atthe branch tip with a d-spacing of 0.224 nm corresponding tothe distance between {111} planes of Pd crystal structure. Amore detailed growth mechanism of Pd DNWs will be discussedlater. To confirm the crystal structure of the Pd DNWs, theelectrodes were characterized by XRD. As shown in Figure 2,

* Corresponding author. Tel: 82-2-820-0613. Fax: 82-2-812-5378. E-mail:kwpark@ssu.ac.kr.

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the diffraction peak at 40.1°, 46.7°, and 68.0° corresponds tothe (111), (200), and (220) plane of a typical Pd crystal structure,respectively. All the DNWs synthesized at different potentialsshow the face-centered-cubic (fcc) structure of a Pd metalcrystal.

To study the growth mechanism of Pd dendritic nanowires,as shown in Figure 3, we synthesized the Pd DNWs (Pd DNW-10, Pd DNW-30, Pd DNW-60, Pd DNW-300, Pd DNW-600)by electrodeposition at -0.9 V as a function of deposition timesuch as 10 s, 30 s, 60 s, 300 s, and 600 s, respectively. Asshown in Figure 3a and 3f, Pd DNW-10 is 120 nm in diameterand 1 µm in length and begins to branch out. Although the stem

of the Pd DNW is polycrystalline, branches of Pd DNW aresingle crystalline. The (111) single crystal structure of the edgein the Pd DNW is confirmed using the transmission electrondiffraction (TED) pattern in the inset of the Figure 3f. In thePd DNW-30 and Pd DNW-60, a few DNWs with a short branchof 600 nm in length are formed (Figure 3b and 3c). The PdDNW-300 shows a longer branch up to 10 µm in length (Figure3d). Finally, as shown in Figure 3e and 3g, the Pd DNW-600consists of branches up to 100 nm in diameter and 800 nm inlength representing the (111) single crystal structure of thebranch in the Pd DNW as seen in the HR-TEM and TED images(the inset of the Figure 3g). Herein, when formed under thereduction potential of -0.9 V for the deposition time of 10 or600 s, the diameter of the Pd DNW-600 is almost similar tothat of the Pd DNW-10. On the other hand, as the potential

Figure 1. Scanning electron microscopy (SEM) images of Pd DNWselectrodeposited at (a) -0.3 V, (b) -0.6 V, and (c) -0.9 V.Transmission electron microscopy (TEM) images of Pd DNWs elec-trodeposited at (d) -0.3 V and (e) -0.9 V.

Figure 2. X-ray diffraction (XRD) patterns of Pd DNWs electrode-posited at (a) -0.3 V, (b) -0.6 V, and (c) -0.9 V. The asteriskrepresents the XRD patterns of ITO.

Figure 3. Scanning electron microscopy (SEM) images of Pd DNWselectrodeposited 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 PdDNWs electrodeposited at -0.9 V for (f) 10 s and (g) 600 s. The lefttop and right bottom of the inset of f and g is the transmission electrondiffraction (TED) pattern and high-resolution transmission electronmicroscopy (TEM) image of Pd DNWs, respectively.

Scheme 1. Formation Mechanism of Pd DNW StructuresElectrodeposited at Different Reduction Potentials of (a)

-0.3 V and (b) -0.9 V

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was increased from -0.3 to -0.9 V, the branches of the DNWsbecame longer and thinner. The lower reduction potential couldproduce thick and short branches of the DNWs while the higherreduction potential could lead to thin and long branches of theDNWs. Therefore, it is considered that the thickness of branchesin the DNWs strongly depends on a reduction potential ratherthan a deposition time.

A schematic illustration of the formation of Pd DNWstructures, based on the SEM and TEM results, is presented inScheme 1. First, Pd nanonuclei are formed on the substratethrough the reduction of PdSO4, and then they grow alongthe κ directions forming a nanowire structure. This is becausesulfuric acid anions such as HSO4

- or SO42- in solution are

adsorbed on the Pd (111) surface, disturbing growth to theplane.25-27 The sulfuric acid anion is known to adsorb on the(111) surface of metal electrodes with an fcc crystal struc-ture especially, as already observed in the adsorption of bisulfateand sulfate on a Pt (111) electrode.27 Experimentally, as shownin Figure 4, we found that Pd DNW structures were not formedon the substrate through the reduction of PdCl2 compared tothe growth through the reduction of PdSO4. Especially, theadsorption of the sulfuric acid anion on Pd (111) is morefavorable than that on others, thus the growth along the κ

direction. However, it is likely that since the step site of theedge 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 andside branches in the Pd DNW are constant, irrespective ofdeposition time. However, as the potential is increased, thebranches of the DNWs become longer and thinner.

Conclusion

The Pd dendritic nanowire electrodes were synthesized bymeans of electrochemical deposition as a function of potentialor deposition time. In particular, during the electrochemicaldeposition process, the Pd dendritic nanowire grew along the κ

direction, resulting from the favorable adsorption of the sulfuricacid anion on the Pd (111) plane. As the potential was increased,the branches of the dendritic nanowires with single crystalstructure became longer and thinner. Moreover, depending onreduction potentials, the diameters of the main and side branches

in the Pd dendritic nanowires were almost the same withoutrespect to the deposition time.

Acknowledgment. This work was supported by KoreaResearch Foundation (KRF-2007-331-D00114).

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CG8007574

Figure 4. Scanning electron microscopy (SEM) images of electrodeselectrodeposited at -0.9 V for 600 s in (a) 0.2 M PdCl2 and (b) 0.2 MPdSO4 at room temperature.

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