Pine needle-like Pd micro/nano-leaves have been synthesized by a facile, template-free electrochemical method. As-synthesized Pd micro/nano-leaves were directly electrodeposited on an indium tin oxide substrate in the presence of 1.0 mM H2PdCl4 + 0.33 M H3PO4. The formation processes of Pd micro/nano-leaves were revealed by scanning electron microscope, and further characterized by X-ray diffraction and electrochemical analysis. Compared to conventional Pd nanoparticles, as-prepared Pd micro/nano-leaves exhibit superior electrocatalytic activities for the formic acid oxidation.
Energy storage devices including fuel cell, Li-batteries etc. have been developing especially today [1, 2]. Direct formic acid fuel cell has been receiving much attention as one of the most attractive energy sources . Palladium (Pd) was found to show superior catalytic activity for formic acid electrooxidation compared with Pt-based catalysts [4, 5]. Considerable efforts have currently been directed to developing novel Pd catalysts. Due to high-surface area and other unique physicochemical properties, nano-catalysts are known to have a significant effect on promoting the electro-oxidation of formic acid. Well-controlled nanostructures are thereby essential for achieving high efficient catalysts used in fuel cells. From this prospect, Pd nanoparticles with a variety of shapes have been explored, such as microspheres , polygonal nanoparticles [7, 8], nanotubes , nanothorns , nanorods , and nanowires [12–15]. Sun et al. reported the efficiency of formic acid electro-oxidation can be improved by changing the morphology of the Pd nanostructures from nanoparticle to nanowire .
Recently, much attention has been paid to the synthesis of nanomaterials on the basis of electrochemical deposition methods because of their simple operation, high purity, uniform deposits, and easy control [17–19]. In order to obtain nano-architectural Pd catalysts directly grown on substrates by electrodeposition, templates are commonly used . However, the fabrication is relatively complicated with multiple steps. Recently, a few studies on nano-architectural Pd fabrication using direct template-free electrodeposition on an indium tin oxide (ITO) electrode have been reported [21, 22]. Park et al. reported the potentiostatic electrodeposition of Pd dendritic nanowires on an ITO electrode in a solution containing 0.2 M H3BO3 and 0.2 M PdSO4, and they did not find the formation of Pd dendritic nanowires on the ITO substrate through potentiostatic reduction of PdCl2. Kwak et al. reported the electrodeposition of Pd nanoparticles on an ITO electrode by a cyclic voltammetry method in a 0.1 M H2SO4 + 0.1 mM PdCl2 + 0.2 mM HCl solution and their catalytic properties for formic acid oxidation . Clearly, the composition of electrolytes and the different electrochemical methods employed for electrodeposition are critical to the morphology of the formed metal products. The present article provides a facile, one-step, template-free electrodeposition route of Pd micro/nano-leaves. As-formed Pd micro/nano-leaves were found to show promising activity for formic acid electro-oxidation.
Materials and apparatus
PdCl2 (Shanghai Sinopharm Chemicals Reagent Co., Ltd., China) was used as received. Formic acid, H3PO4, and H2SO4 were of analytical-grade purity. Doubly distilled water was used throughout. A 1.0 mM H2PdCl4 solution was prepared by dissolving 0.1773 g of PdCl2 in 10 mL of 0.2 M HCl solution and further diluting to 1000 mL with double-distilled water . The electrochemical experiments were carried out in a conventional three-electrode cell using a CHI 660B potentiostat/galvanostat (Shanghai Chenhua Instrumental Co., Ltd., China) at room temperature. An ITO glass substrate was used as the working electrode. The counter electrode and the reference electrode were platinum wire and saturated calomel electrode (SCE), respectively. The solutions were deaerated by a dry nitrogen stream and maintained with a slight overpressure of nitrogen during the experiments. A scanning electron microscope (SEM, S-4700, Japan) and X-ray diffraction (XRD, X' Pert-Pro MPD, PANalytical Company) were used to determine the morphology and the crystal structure of the sample nanomaterials, respectively.
Preparation of the modified electrode
Before electrodeposition, ITO surface was ultrasonicated sequentially for 20 min in acetone, 10% KOH ethanol solution, and doubly distilled water. The electrodeposition process was conducted in a solution consisting of 1.0 mM H2PdCl4 and 0.33 M H3PO4 using cyclic voltammetry from -0.24 to 1.2 V with a scan rate of 50 mV s-1. The conventional Pd nanoparticles deposited on ITO were prepared by the potentiostatic method at a constant applied potential of -0.2 V in the solution as stated above. As-prepared Pd/ITO electrode was rinsed with water for three times and dried at room temperature. Before the activity test, the electrode was cycled at 50 mV s-1 between -0.3 and 0.8 V in 0.5 M H2SO4 for at least 20 scans. After that the electrode was transferred to the cell containing 0.5 M H2SO4 + 0.5 M HCOOH electrolyte solution. Subsequently, 20 scans were recorded at 50 mV s-1 in the potential range -0.3 to 0.8 V. The amount of Pd (WPd) loaded onto ITO was analyzed by an inductive coupled plasma emission spectrometer (ICP).
Results and discussion
Pine needle-like Pd micro/nano-leaves were prepared by a cyclic voltammetry method, i.e., electrodeposition in the presence of 1.0 mM H2PdCl4 + 0.33 M H3PO4 electrolyte at room temperature. To observe the growth process of Pd micro/nano-leaves, as shown in Figure 1, the Pd nanoparticles were synthesized by controlling cyclic voltammetry electrodeposition from -0.24 to 1.2 V as a function of deposition cycles such as 5 (a), 10 (b), 20 (c), 35 (d), 75 (e), 100 (f), and 200 (g) cycles. At the initial stages (Figure 1a,b), featureless Pd nanoparticles of about 70 nm were formed. Extending the electrodeposition cycles, as shown in Figure 1c, d, Pd nanorod structure of 90 nm in width and 150 nm in length began to branch out. As the deposition cycles being further increased to 75 cycles, however, many nanoleaves started to form and grow from the edges of the nanorod particles, and a few completed nanoleaves with a short branch of 500 nm in length (Figure 1e) appeared. Further increasing the deposition cycles to 100 cycles, perfect Pd micro/nano-leaves were formed on the surface of ITO (Figure 1f). After 200 cycles, as shown in Figure 1g, the Pd micro/nano-leaves consisting of branches up to 500 nm in width and 1 μ m in length were formed, as shown in the high magnification image (inset in Figure 1g).
To pin down the related factors for the formation of Pd micro/nano-leaves, two control experiments have been carried out independently. First, replacing H3PO4 with other acids, e.g., H2SO4, HCl, HNO3, while keeping the other conditions unchanged, no Pd micro/nano-leaves were observed. It is proposed that the formation of Pd micro/nano-leaves is related to the effect driven by phosphate anions. Secondly, using a potentiostatic method instead of the cyclic voltammetry method and keeping the other conditions unchanged, featureless Pd nanoparticles (Figure 1h) were formed. Based on these observations, the existence of H3PO4 and the cyclic voltammetry method are two key factors, which are beneficial to the formation of Pd micro/nano-leaves. First, phosphate anions such as the hydrogen phosphate ion (HPO42-) or the dihydrogen phosphate ion (H2PO4-) in solution are preferentially adsorbed on noble metal single crystals, which can greatly disturb the growth of the plane . The phosphate anions are known to adsorb on the (111) surface of metal electrodes with a face-centered cubic (fcc) crystal structure. Especially, they have already been observed in the adsorption of both H2PO4- and HPO42- on the Pt(111) . Secondly, compared to the potentiostatic method, cyclic voltammetry is an alternating redox process, involving both electrodeposition and dissolution processes, which are critical to the formation of Pd nanoleaf structure. At the same time, varying the experimental conditions, such as the concentration, pH of the initial solution, reaction temperature, and time, may also effect the shape evolution .
Figure 2 shows XRD patterns of Pd micro/nano-leaves prepared in the electrolyte consisting of H2PdCl4 and H3PO4 for 20 (a), 50 (b), 100 (c), and 200 (d) cycles. As seen from Figure 2, the impurity peak between 53° and 54° is attributed to the diffraction peak of SnO2 face (211), which is the main composition of the ITO glass. At the early stage, the well-defined peaks around 40° and 47° are observed and they are, respectively, attributed to the diffraction peaks of Pd crystal faces (111) and (200); as the cycles increase, the peaks around 68° and 83° appear, which could be indexed to the (220) and (311), respectively. All these demonstrate that Pd micro/nano-leaves possess an fcc structure.
Inspired by their intriguing structure, Pd nanoparticles were tested as electrocatalysts. Figure 3 shows the cyclic voltammograms (CVs) of Pd nanoparticles recorded in a 0.5 M H2SO4 solution at 50 mV s-1. The shape of the profile is similar to what reported in literature . The multiple peaks between -0.25 and 0 V are attributed to the adsorption and desorption of hydrogen. It is well known that the integrated intensity of hydrogen adsorption/desorption represents the number of available sites on catalyst . It is also observed from Figure 3 that Pd electrodes produced by cyclic voltammetry deposition deliver reduction peaks at ca. 0.41 V while by potentiostatic deposition the reduction peaks shift to ca. 0.52 V. The peaks are attributed to the reduction of the oxide formed on the Pd during the forward scan. Compared to Pd nanoparticles, Pd micro/nano-leaves have the larger area of Pd oxide and lower reduction peak in the process of CVs. It is proved that Pd micro/nano-leaves have large active surface area and good electrocatalytic performance of as-prepared catalysts for the formic acid electro-oxidation.
The inset of Figure 4 shows the CV of formic acid oxidation on the Pd electrode, which was deposited for 100 cycles. In the forward scan, formic acid oxidation produced an anodic peak; while in the reverse scan, there was also an oxidation peak, which is attributed to formic acid oxidation after the reduction of the oxidized Pd oxide and the removal of the incompletely oxidized carbonaceous species formed in the forward scan. The oxidation peak in the forward scan is usually employed to evaluate the electrocatalytic activity of the catalysts and the anodic scan allows the formation and builds up of the poisonous intermediate, we thereby focus our observations on the evolution of the anodic scans, as is presented in Figure 4. From the curves shown in Figure 4, there are a main current peak between 0.1 and 0.4 V and two small current peaks near -0.1 and 0.6 V, respectively. The peak near -0.1 V is attributed to the adsorption and desorption of hydrogen, which is similar to that in Figure 3. The main peak between 0.1 and 0.4 V corresponds to formic acid oxidation via a direct pathway, while the peak near 0.6 V could be mainly attributed to formic acid oxidation via the CO pathway [29, 30]. Moreover, the main peak is much larger than that near 0.6 V, indicating that the formic acid oxidation on Pd catalysts is mainly through the direct pathway. Especially in the curve a, b, and d, there are almost no peaks near 0.6 V. As observed from the curves a, b, c, and d in Figure 4, the onset potential of formic acid electro-oxidation locates near -0.04 V (a), -0.04 V (b), -0.07 V (c), and -0.05 V (d) vs. SCE, respectively, and the peak current density reaches 80.24 mA mg-1 (a), 112.99 mA mg-1 (b), 295.57 mA mg-1 (c), 105.47 mA mg-1 (d) for Pd catalysts, respectively. Among all the four electrodes, the Pd micro/nano-leaves exhibit the lowest onset potential and the highest current density of formic acid oxidation. This demonstrates that the electrocatalytic stability of the Pd micro/nano-leaves for formic acid oxidation is much higher than that of the Pd nanoparticles, which agrees with the literature . Additionally, the commercial catalyst (E-TEK Pd/C) shows the peak current density at 190 mA mg-1 in the same conditions (in a 0.5 M HCOOH + 0.5 M H2SO4 solution at 50 mV s-1) , which is lower than Pd micro/nano-leaves catalyst. Generally, catalytic performance of an electrode is assessed in CVs by the position and intensity of kinetically controlled process current on the potential scale. This may be attributed to the special structure that increases the electrochemically active surface area, thus greatly increases the activity for formic acid electro-oxidation.
Using a simple electrodeposition method, Pd micro/nano-leaves were loaded onto a clean ITO. The Pd micro/nano-leaves are demonstrated to have superior performance in electrocatalytic activity toward the oxidation of formic acid.
inductive coupled plasma emission spectrometer
indium tin oxide
saturated calomel electrode
scanning electron microscope
This work was supported by the National Natural Science Foundation of China (Grant nos. 20933007, 51073114, 51073074, and 50963002), the 'One Hundred Talents' program of Chinese Academy of Sciences (1029471301), the Opening Project of Xinjiang Key Laboratory of Electronic Information Materials and Devices, the Priority Academic Program Development of Jiangsu Higher Education Institutions.
College of Chemistry, Chemical Engineering and Materials Science, Soochow University
Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University
Liu J, Xue D: Hollow Nanostructured Anode Materials for Li-Ion Batteries. Nanoscale Res Lett 2010, 5: 1525. 10.1007/s11671-010-9728-5View Article
Liu J, Xia H, Xue D, Lu L: Double-Shelled Nanocapsules of V
-Based Composites as High-Performance Anode and Cathode Materials for Li Ion Batteries.J Am Chem Soc 2009, 131: 12086. 10.1021/ja9053256View Article
Rice C, Ha S, Masel RI, Waszczuk P, Wieckowski A, Barnard T: Direct formic acid fuel cells. J Power Sources 2002, 111: 83. 10.1016/S0378-7753(02)00271-9View Article
Capon A, Parsons R: The oxidation of formic acid on noble metal electrodes: II. A comparison of the behaviour of pure electrodes. J Electroanal Chem 1973, 44: 239. 10.1016/S0022-0728(73)80250-5View Article
Hoshi N, Kida K, Nakamura M, Nakada M, Osada K: Structural Effects of Electrochemical Oxidation of Formic Acid on Single Crystal Electrodes of Palladium. J Phys Chem B 2006, 110: 12480. 10.1021/jp0608372View Article
Qiu CC, Zhang JT, Ma HY: Fabrication of monometallic (Co, Pd, Pt, Au) and bimetallic (Pt/Au, Au/Pt) thin films with hierarchical architectures as electrocatalysts. Solid State Sci 2010, 12: 822. 10.1016/j.solidstatesciences.2010.02.011View Article
Lee YW, Han SB, Park KW: Electrochemical properties of Pd nanostructures in alkaline solution. Electrochem Commun 2009, 11: 1968. 10.1016/j.elecom.2009.08.030View Article
Tian N, Zhou ZY, Yu NF, Wang LY, Sun SG: Direct Electrodeposition of Tetrahexahedral Pd Nanocrystals with High-Index Facets and High Catalytic Activity for Ethanol Electrooxidation. J Am Chem Soc 2010, 132: 7580. 10.1021/ja102177rView Article
Steinhart M, Jia Z, Schaper AK, Wehrspohn RB, Gösele U, Wendorff JH: Palladium Nanotubes with Tailored Wall Morphologies. Adv Mater 2003, 15: 706. 10.1002/adma.200304502View Article
Meng H, Sun SH, Masse JP, Dodelet JP: Electrosynthesis of Pd Single-Crystal Nanothorns and Their Application in the Oxidation of Formic Acid. Chem Mater 2008, 20: 6998. 10.1021/cm8014513View Article
Wang XG, Wang WM, Qi Z, Zhao CC, Ji H, Zhang ZH: Electrochemical catalytic activities of nanoporous palladium rods for methanol electro-oxidation. J Power Sources 2010, 195: 6740. 10.1016/j.jpowsour.2010.03.098View Article
Taşaltın N, Öztürk S, Kılınç N, Yüzer H, Öztürk ZZ: Fabrication of vertically aligned Pd nanowire array in AAO template by electrodeposition using neutral electrolyte. Nanoscale Res Lett 2010, 5: 1137. 10.1007/s11671-010-9616-zView Article
Fukuoka A, Araki H, Sakamoto Y, Inagaki S, Fukushima Y, Ichikawa M: Palladium nanowires and nanoparticles in mesoporous silica templates. Inorg Chim Acta 2003, 350: 371.View Article
Ksar F, Surendran G, Ramos L, Keita B, Nadjo L, Prouzet E, Beaunier P, Hagège A, Audonnet F, Remita H: Palladium Nanowires Synthesized in Hexagonal Mesophases: Application in Ethanol Electrooxidation. Chem Mater 2009, 21: 1612. 10.1021/cm803492jView Article
Yoo Y, Seo K, Han S, Varadwaj K, Kim HY, Ryu JH, Lee HM, Ahn JP, Ihee H, Kim B: Steering Epitaxial Alignment of Au, Pd, and AuPd Nanowire Arrays by Atom Flux Change. Nano Lett 2010, 10: 432. 10.1021/nl903002xView Article
Wang JJ, Chen YG, Liu H, Li RY, Sun XL: Synthesis of Pd nanowire networks by a simple template-free and surfactant-free method and their application in formic acid electrooxidation. Electrochem Commun 2010, 12: 219. 10.1016/j.elecom.2009.11.029View Article
Tsai MC, Yeh TK, Tsai CH: An improved electrodeposition technique for preparing platinum and platinum-ruthenium nanoparticles on carbon nanotubes directly grown on carbon cloth for methanol oxidation. Electrochem Commun 2006, 8: 1445. 10.1016/j.elecom.2006.07.003View Article
Chen X, Li N, Eckhard K, Stoica L, Xia W, Assmann J, Muhler M, Schuhmann W: Pulsed electrodeposition of Pt nanoclusters on carbon nanotubes modified carbon materials using diffusion restricting viscous electrolytes. Electrochem Commun 2007, 9: 1348. 10.1016/j.elecom.2007.01.034View Article
Sun M, Zangari G, Shamsuzzoha M, Metzger RM: Electrodeposition of highly uniform magnetic nanoparticle arrays in ordered alumite. Appl Phys Lett 2001, 78: 2964. 10.1063/1.1370986View Article
Xu CW, Wang H, Shen PK, Jiang SP: Highly Ordered Pd Nanowire Arrays as Effective Electrocatalysts for Ethanol Oxidation in Direct Alcohol Fuel Cells. Adv Mater 2007, 19: 4256. 10.1002/adma.200602911View Article
Song YJ, Kim JY, Park KW: Synthesis of Pd Dendritic Nanowires by Electrochemical Deposition. Cryst Growth Des 2009, 9: 505. 10.1021/cg8007574View Article
Kim BK, Seo D, Lee JY, Song H, Kwak J: Electrochemical deposition of Pd nanoparticles on indium-tin oxide electrodes and their catalytic properties for formic acid oxidation. Electrochem Commun 2010, 12: 1442. 10.1016/j.elecom.2010.08.004View Article
Wang DW, Li T, Liu Y, Huang JS, You TY: Large-Scale and Template-Free Growth of Free-Standing Single-Crystalline Dendritic Ag/Pd Alloy Nanostructure Arrays. Cryst Growth Des 2009, 9: 4351. 10.1021/cg900217tView Article
Moraes IR, Nart FC: Vibrational study of adsorbed phosphate ions on rhodium single crystal electrodes. J Electroanal Chem 2004, 563: 41. 10.1016/j.jelechem.2003.08.018View Article
He QG, Yang XF, Chen W, Mukerjee S, Koel B, Chen SW: Influence of phosphate anion adsorption on the kinetics of oxygen electroreduction on low index Pt(hkl) single crystals. Phys Chem Chem Phys 2010, 12: 12544.View Article
Xu J, Xue D: Five branching growth patterns in the cubic crystal system: A direct observation of cuprous oxide microcrystals. Acta Mater 2007, 55: 2397. 10.1016/j.actamat.2006.11.032View Article
Zhang SX, Qing M, Zhang H, Tian YN: Electrocatalytic oxidation of formic acid on functional MWCNTs supported nanostructured Pd-Au catalyst. Electrochem Commun 2009, 11: 2249. 10.1016/j.elecom.2009.10.001View Article
Zhang HM, Zhou WQ, Du YK, Yang P, Wang CY: One-step electrodeposition of platinum nanoflowers and their high efficient catalytic activity for methanol electro-oxidation. Electrochem Commun 2010, 12: 882. 10.1016/j.elecom.2010.04.011View Article
Zhou WJ, Lee JY: Highly active core-shell Au@Pd catalyst for formic acid electrooxidation. Electrochem Commun 2007, 9: 1725. 10.1016/j.elecom.2007.03.016View Article
Liu ZL, Hong L, Tham MP, Lim TH, Jiang HX: Nanostructured Pt/C and Pd/C catalysts for direct formic acid fuel cells. J Power Sources 2006, 161: 831. 10.1016/j.jpowsour.2006.05.052View Article
Guo S, Dong S, Wang E: Pt/Pd bimetallic nanotubes with petal-like surfaces for enhanced catalytic activity and stability towards ethanol electrooxidation. Energy Environ Sci 2010, 3: 1307. 10.1039/c0ee00053aView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.