SERS spectral evolution of azo-reactions mediated by plasmonic Au@Ag core-shell nanorods

The mechanism and application of localized surface plasmon resonance induced photocatalytic reactions remain an issue of interest. In this work, we used Au@Ag core-shell nanorods as a platform for plasmon-driven photocatalysis, which was in situ investigated by surface-enhanced Raman scattering (SERS) spectroscopy. The para-aminothiophenol (PATP) and para-nitrothiophenol (PNTP) adsorbed on the nanorods were irradiated with different excitation wavelengths (633 nm, 785 nm) and transformed into 4,4'-dimercaptoazobenzene (DMAB) as evidenced by the emerging Raman peaks at 1142 cm^-1, 1390 cm^-1, 1440 cm^-1, and 1477 cm^-1, corresponding to hot carrier dominated oxidation of PATP and reduction of PNTP. Preliminary azo-reaction kinetics and in situ SERS measurements were conducted by comparing the relative intensity ratio of SERS peaks at 1440 cm^-1 (DMAB stretching of N[double bond, length as m-dash]N) and 1080 cm^-1 (C-S stretching of PATP and PNTP). These results indicate that the catalytic efficiency was dominated by the excitation wavelength as well as the resonance condition between the plasmon band of the nanorods and the excitation line. As a proof of concept, the Au@Ag core-shell nanorods were used to catalyze 4-nitrophenol molecules, and 4-hydroxyazobenzene molecules as the product were confirmed by in situ SERS spectra as well theoretical predictions, showing potential in plasmon driven catalysis and degradation of organic molecules.