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2006, 110, 22306-22309 Published on Web 10/17/2006
Edge Plane Sites on Highly Ordered Pyrolytic Graphite as Templates for Making Palladium Nanowires via Electrochemical Decoration Xiaobo Ji, Craig E. Banks,† Wang Xi, Shelley J. Wilkins,‡ and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom ReceiVed: September 5, 2006; In Final Form: September 30, 2006
An approach for the fabrication of metal nanowires is presented. Palladium wires with diameters less than 50 nm were produced by electrochemical decoration of step edge sites on the surface of highly ordered pyrolytic graphite via the following three steps. First an electrochemical activation step was used to oxidize the edge plane sites on highly ordered pyrolytic graphite surfaces in 0.5 M Na2SO4. Second, a potential cycling step in a 1 mM PdCl2 solution in 0.1 M H2SO4 was used to form palladium oxide (s) and/or complexes of Pd on the step edges. Third, Pd nanowires were formed by electroreduction after transfer of the graphite to 0.1 M H2SO4. The resulting wires showed a high degree of uniformity. A merit of this approach is that it allowed metal nanowires to be fabricated without the simultaneous formation of nanoparticles on the basal plane terraces, in contrast to other studies of this type. The mesoscopic palladium wires are shown to be useful for the electrochemical sensing of hydrazine.
Nanosized materials have become the focus of intensive research due to their fascinating electronic, mechanical, and chemical properties. Recognition of their wide potential applications in areas ranging from semiconductors, molecular computing, sensors, nanoelectronics, and energy storage have rapidly promoted research in these areas.1,2 Mesoscopic materials such as nanoparticles, nanowires, and nanotubes are generally defined as nanostructures with one dimension between 1 and 100 nm.3-6 They have a high surface area-to-volume ratio and offer novel electron-transfer properties.1,7-10 So as to implement nanomaterials in real-world applications, it is desirable to develop new techniques to produce nanowires and nanoparticles particularly with more controllable and uniform sizes. Hitherto, as regards nanowires, several fabrication approaches including template synthesis, electrochemical deposition, laser assisted catalytic growth, and solution-phase reactions have been developed.11-17 All have various merits, but in this report we concern ourselves with electrochemical deposition. This is a powerful technique for the deposition of many metals since it is rapid and facile, allowing easy control of the nucleation and growth of metal nanoparticles and nanowires.18 It can provide an attractive route for producing metal nanowires in the pores of suitable templates imparting size control.19,20 Thurn-Albrecht et al. have obtained vertical arrays of cobalt nanowires by direct current electrodeposition using arrays of nanopores as templates.21 Additionally, note that varying the electrochemical potential can switch the balance of the rates of atom deposition and dissolution so careful control of the * To whom correspondence should be addressed. Direct Tel: 00441865 275413, Fax: 00441865 275410, Email:
[email protected]. † Present address: Chemistry, School of Biomedical and Natural Sciences, Nottingham Trent University, Nottingham NG11 8NS, UK. ‡ Present address: Asylum Research UK Ltd. Oxford Centre For Innovation, Mill Street, Oxford, OX2 0JX, UK.
10.1021/jp065776m CCC: $33.50
electrochemical parameters are required in order to create nanowires rather than nanoparticles.22,23 Penner and co-workers23 have used direct electrodeposition to grow palladium mesowire arrays with diameters ranging from 50 to 300 nm on highly ordered pyrolytic graphite (HOPG) surfaces and successfully applied them for hydrogen analysis. In their subsequent studies, they found that the size-selective electrodeposition of metal nanoparticles and nanowires could be obtained using step edge decoration on HOPG. However, it was impossible to prepare narrower noble metal nanowires without producing nanoparticles on the basal plane sites.24 In this letter we report that we have electrochemically exclusively deposited palladium nanowires on the step edge sites on the highly ordered pyrolytic graphite surface without nanoparticle formation and explored these mesoscopic palladium wires for the electrochemical oxidation of hydrazine. By a threestep procedure designed to activate the step edges, palladium nanowire growth was accomplished in the following way. First, an electrochemical activation step was used to oxidize the edge plane sites and defects on highly ordered pyrolytic graphite surfaces. Second, a potential cycling step was applied to form palladium oxide (s) and/or complexes of Pd on the step edges. Third Pd nanowires were formed by electroreduction. An increased height of step edge sites was observed following electrochemical oxidation (step 1). This is likely due to surface functional groups such as quinoid, carbonyl, and carboxyl being introduced.25 The diameters of the continuous Pd nanowires produced on these activated graphite sites, as estimated from atomic force microscopy (AFM) images, ranged from 41 to 50 nm with a high degree of uniformity along the step edges. This approach could allow various metal nanowires to be fabricated without the simultaneous formation of nanoparticles on the basal plane terraces. © 2006 American Chemical Society
Letters The strategy of decorating HOPG with palladium wires is adapted from the methodology initially reported by Guo and Li26 for single-walled carbon nanotubes. This involves the following three steps. (i) Electrochemical oxidation activation of the edge plane sites and defects on HOPG via potential cycling from +1.8 to -0.4 V (all vs SCE) at 200 mV s-1 for 10 cycles in 0.5 M Na2SO4. This step probably introduces oxygen containing functional groups on the surface of the HOPG (see below). (ii) The electrode is transferred into a 1 mM PdCl2 solution in 0.1 M H2SO4 with potential cycling from +0.4 to +1.5 V for 5 cycles at 200 mV s-1. This is thought to produce palladium oxide (s) and/or complexes of Pd on the step edge and defect sites of HOPG at various oxygen containing functional groups. (iii) Formation of Pd wires is brought along by transferring the modified electrode into 0.1 M H2SO4 and potential cycling from +1.1 to -0.5 V at a scan rate of 300 mV s-1 for 5 scans. Figure 1 shows schematically the growth of Pd nanowires using the three-step electrochemical decoration sequence. The Pd mesoscopic wires were investigated using a Digital Instruments MultiMode SPM operating in contact mode. An overview AFM image is presented in Figure 2a, showing that these wires are exclusively formed along the step edge sites on HOPG and ranged from 41 to 52 nm in diameter. In the following high resolution of AFM micrographs (Figure 1b-d), it can be clearly seen that Pd wires formed exclusively on the step edges while no Pd nanoparticles or wires were found on the basal plane sites. Figure 3 shows a three-dimensional AFM micrograph of one Pd nanowire. Penner and co-workers have demonstrated that direct metal electrodeposition at large overpotentials results in instantaneous nucleation, and these nonselective metal nuclei are formed at the basal plane and edge plane sites on the graphite surfaces.27,28 In previous works, metal nanowire growth was always accompanied with the formation of particles on the graphite surface using the electrodeposition method. It was found that the formation of metal particles could be reduced but not eliminated by the application of a nucleation pulse, which promoted nanowire formation on edge plane sites.24 In our experiments, no Pd nanoparticles were observed on the basal plane. The electrochemical oxidation of the edge plane sites is necessary for the ready growth of nanowires at the edge sites.29 For electrochemistry, the edge plane on the graphite typically shows faster electron-transfer kinetics than the basal plane for a range of redox couples, shown by Figure 4, detailing the voltammetric profiles recorded using the ferricyanide redox couple where the edge plane and basal plane pyrolytic graphite electrodes exhibit significant differences of electrochemical responses.30 According to previous work,26,29 the first step, the electrochemical decoration of the graphite surface is likely to produce oxygen containing functional groups such as carbonyls and hydroxyls at the step and defect sites, where the affinity at the edge plane sites of Pd atoms could be increased and subsequently facilitate nucleation processes. Thus the nucleation of Pd likely occurs selectively at step edges present on the HOPG surface. Next, we provide direct evidence of the oxidation of edgeplane-like defect sites by observing this ex-situ process using scanning tunneling microscopy (STM). Topographical STM images of the surface before and after the electrochemical modification procedure (step 1) are presented (see Supporting Information Figure i). It can be clearly seen from these images that the step edge is no longer rectangular and on the contrary becomes more circular. The increase of step edge height was achieved with corresponding cross-section analysis using scan-
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22307
Figure 1. Schematic diagram of three electrochemical decoration steps to produce padllium nanowires.
ning probe image processor version 4.3.2 (Image Metrology). A line scan analysis of step height on HOPG surface before any experiment has an order of ca. 3.33 Å (see in Supporting Information Figure ii a), in good agreement with the reported value of the interplanar distance between the graphite sheets in pyrolytic graphite, 3.35 Å.30 However, after the electrochemical oxidation activation step in 0.5 M Na2SO4 the step height has increased to 5.15 Å, shown from a line scan analysis across the step (in Supporting Information Figure ii b). The distance between the graphite layers has lifted by ca. 1.8 Å, resulting in a “round”-like shape at the edge. Note that the width of this edge, measured over step defects, also increases. Since analysis of the step height and width both showed an obvious increase,
22308 J. Phys. Chem. B, Vol. 110, No. 45, 2006
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Figure 3. Three-dimensional AFM image of one Pd mesoscopic wire on the graphite surface.
Figure 4. Schematic diagram (a) of a six-layer step edge and cyclic voltammograms (b) obtained from bppg and eppg electrodes (4.9 mm diameter) for the reduction of 1 mM ferricyanide (in 0.1M KCl). All are recorded at a scan rate of 100 mV s-1.
Figure 2. AFM micrographs of Pd nanowires grown along the edge plane on the graphite surfaces; (a-d) in different ranges.
this suggests that functional groups corresponding to quinoid, carbonyl, and/or carboxyl groups resulting from the electrochemical oxidation are introduced. These likely modify graphitic surfaces by changing the surface structures of these edge-plane-
like defect sites and thus increase the nucleation density of metal along the step edge. Then the influence of Pd concentration for producing nanowires using the electrochemical decoration was briefly explored. The concentration of PdCl2 in the second step was changed to 0.1 mM, 10 times less in comparison with that of original solution (1 mM). We continue to employ the other experimental conditions as mentioned above. Figure 5 details the AFM image of Pd “nanowires” grown along the decorated step edge on the graphite surfaces. It is interesting to note that after the decreased concentration of Pd the nanowires produced from higher concentration have been replaced by almost continuous lines of nanoparticles. It is reasonable to assume that the number of metal nuclei along step edges decreases with the dilution of Pd(II) solution, leading to the formation of the “beaded nanowires”. The electroanalytical use of the palladium nanowires for hydrazine sensing was next investigated. Figure 6 shows the linear sweep voltammetric response obtained at a Pd nanowire modified HOPG electrode in a solution of 1 mM hydrazine in 0.1 M phosphate buffer (pH 7). The overlaid line is the response
Letters
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22309 Pd wires is shifted electrochemically more irreversible, resulting from the nanowires, which will have a faster mass transport regime than a Pd macroelectrode.32 In summary, we here described a method for preparing arrays of metal nanowires. Pd nanowires were obtained by selectively using a three-step electrochemical decoration procedure of the step edges on highly ordered pyrolytic graphite surfaces. The merit of this method is that it allowed the fabrication of nanowires without the simultaneous formation of nanoparticles on the basal plane terraces. Finally, the electrochemical oxidation of hydrazine was explored on these mesoscopic palladium wires, showing the analytical potential of nanowire arrays. Acknowledgment. X.J. thanks the Clarendon Fund for partial funding. Supporting Information Available: A continuation of the experimental methods is described and STM images are included. The material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 5. AFM micrographs of Pd nanoparticles grown along step edges.
Figure 6. Linear sweep voltammograms of 1 mM hydrazine recorded at a palladium nanowires modified HOPG electrode in a pH 7 buffer solution at a scan rate of 100 mV s-1. The overlaid voltammogram corresponds to the response of bare HOPG electrode.
obtained from the bare HOPG electrode. A large characteristic voltammetric profile is observed at ca. +0.04 V (vs SCE), which is in good agreement with literature reports,31 showing the possible analytical uses. The electrochemical oxidation of hydrazine at metallic surfaces can be described by the following31
N2H5+ - 4e- f N2 + 5H+
(1)
Note that hydrazine oxidation potential on Pd nanowires in comparison with that obtained at a Pd macroelectrode is shifted more positive,31 indicating that the electrochemical process on
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