Electrodeposition of Vertically Aligned Palladium Nanoneedles and Their Application as Active Substrates for Surface-Enhanced Raman Scattering

Abstract

Vertically aligned Pd nanoneedles (NNs) with high aspect ratio and uniform growth density were fabricated on indium tin oxide (ITO) substrates by a facile electrodeposition method, which consisted of a short nanoseed-generating step followed by a growth step. The obtained Pd NNs have a full width at half-maximum (fwhm) of 22 ± 8 nm and taper angle of 15 ± 4°. X-ray diffraction (XRD) and high-resolution transmission electron microscopic (HRTEM) results imply that the NNs were built through the stacking of {111} facets. The factors influencing the electrochemical nucleation and growth of Pd NNs were systematically studied. It was found that the transient nucleation step at a highly negative overpotential is prerequisite to the formation of the uniformly dispersed and vertically aligned Pd NNs. The morphology, size, and aspect ratio of Pd nanocrystals (NCs) were extremely sensitive to the potential applied in the growth step. High aspect ratio and well-aligned Pd NNs could only be obtained in a narrow potential range from 0.100 to 0.200 V. Bromide ions played a dominant role in shaping the morphology of Pd NCs, while citrate ions acted as an assistant shape-controlling agent. A concentration-gradient-induced growth mechanism of Pd NNs was proposed on the basis of the time-dependent scanning electron microscopic (SEM) results. The formation of a uniform diffusion layer at the Pd NNs/solution interface and the concentration gradient of the precursor within the diffusion layer were responsible for the vertically oriented growth of Pd NNs. The Pd NNs exhibited good surface-enhanced Raman scattering (SERS) activity under the excitation of 488 nm laser, implying its potential application as a SERS substrate. The SERS activity of Pd NNs was attributed to the unique morphology and the perpendicular alignment of Pd NNs.