Metal/Si Nanowire Arrays for Hot Electron Catalysis

The application of high-refractive index substrates to boost near-field enhancements in plasmonic nanostructures for photocatalysis remains limited. Here, we combine hexagonally-arranged vertically-aligned silicon nanowire (VA-SiNW) arrays with Au nanoparticles and Au/Ag nanoshells to study the plasmon-driven dimerization of p-nitrothiophenol (p-NTP) into 4,4'-dimercaptoazobenzene (DMAB). In situ Raman spectroscopy reveals that VA-SiNWs can enhance local electric fields at the metal nanostructures, which we use for SERS and hot electron catalysis. Despite producing the strongest near-field enhancement and the highest SERS response, AuNPs/SiNW arrays do not deliver the highest reaction rates, indicating that near-field intensity alone does not govern catalytic performance. This discrepancy is attributed to steric hindrance within highly confined hotspots at the Au/Si interface. Compared with the Au-based substrates, Ag/SiNW arrays show higher photocatalytic activity, with 532 nm excitation dramatically accelerating p-NTP photoreduction. Ag/SiNW arrays also photocatalyze 4-mercaptobenzoic acid decarboxylation, demonstrating their ability to drive oxidation reactions via the generation of reactive oxygen radicals. Overall, our results indicate that VA-SiNW arrays can serve as high-surface-area photonic supports for plasmon-driven chemistry and highlight the critical interplay between field enhancement, hotspot accessibility, and metal choice in optimizing hot electron-induced photocatalytic performance.