Photoreduction of 4-Nitrobenzenethiol on Au by Hot Electrons Plasmonically Generated from Ag Nanoparticles: Gap-Mode Surface-Enhanced Raman Scattering Observation

Abstract

Hot electrons generated plasmonically from a Ag substrate can reduce 4-nitrobenzenethiol (4-NBT) to 4-aminobenzenethiol (4-ABT). In order to evaluate the effectiveness of hot electrons, we have carried out a surface-enhanced Raman scattering (SERS) study by forming a nanogap system composed of a planar Au substrate and an Ag-coated micrometer-sized silica bead, wherein 4-NBT was adsorbed first onto the Au substrate, and then Ag-coated silica beads, derivatized with 1-alkanethiols, were spread over the 4-NBT layer: the distance between 4-NBT and a nanostructured Ag substrate was varied by the chain length of alkanethiol molecules. Although the planar Au substrate itself was ineffective in the reduction of 4-NBT, hot electrons usable in the reduction of 4-NBT were generated from the Ag-coated silica beads. The hot electrons generated by 514.5 nm radiation were more effective in the reduction of 4-NBT to 4-ABT than those generated by 632.8 nm radiation, although the nanogap was more SERS-active with the excitation at 632.8 nm than at 514.5 nm. The photoreduction efficiency of hot electrons nonetheless decreased exponentially with the distance they traveled from the Ag surface: the reduction capability at a distance of 2 nm apart is about one-fourth of that in contact situations.