Kinetic and Mechanistic Investigation of the Photocatalyzed Surface Reduction of 4-Nitrothiophenol Observed on a Silver Plasmonic Film via Surface-Enhanced Raman Scattering

Hot electrons generated by photoinduced plasmon decay from a plasmonic metal surface can reduce 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP). Compared to the reduction with a reducing agent such as sodium borohydride, surface-enhanced Raman scattering (SERS) measurements were performed here to elucidate the complex molecular mechanism of the reduction in the presence of halide ions and hydrogen ions. The SERS measurements were performed using a simply prepared silver plasmonic film (AgPF), which enables monitoring of the reaction under different conditions at a solid-liquid surface and eliminates the need for the use of a reducing agent. As the concentration of H+ and Cl- could be controlled, the observation of the reaction under a systematic set of conditions was possible. Based on the kinetic traces of the intermediates, a reaction mechanism for the 4-NTP to 4-ATP reduction is suggested. Rate constants for the individual reactions are presented that fit the measured kinetic traces, and the role of hydrogen in each reaction step is characterized. This work provides clarification on the molecular transformation directly using protons as the hydrogen source and demonstrates an effective method of applying a simple and low-cost silver surface catalyst for SERS studies. Moreover, the monitoring of Cl--concentration-dependent spectra provides insight into the hot-electron conversion process during the photoreduction and strongly supports the formation of AgCl for the activation of H+.