Synthesis of palladium nanowire arrays with controlled diameter and length

Ordered palladium nanowire (NW) arrays with high aspect ratio have been synthesized using porous alumina templates and direct-current electrodeposition. The porous alumina films were prepared by double anodizing high purity aluminium foils in oxalic and sulphuric acids at 40 and 25 V, respectively. Final pore size of the processed (free-standing) alumina templates were determined to be ∼65 and 35 nm. The electrolyte consisted of 70 mM K2PdCl4 in 20 mM H2SO4 and the deposition voltage was 0.3 V (versus Ag/AgCl reference electrode). Under the employed conditions, a high filling rate (>90%) was obtained using 65 μm thick templates. The NWs synthesized in 65 nm pores were polycrystalline and textured, but those in 35 nm pores were single-crystalline. It is suggested that deposition in smaller pores proceeds under smaller overpotential due to diffusion-limitation which favors single-crystalline growth. In order to obtain self-standing Pd NWs, which may be interesting for realizing NW-array based gas sensor structures, the alumina template was dissolved away, leaving self-standing NWs supported on a conductive thin film. Such NWs were observed to form agglomerates and chemisorb O and C as surface impurities.