Azo-Dimerization Mechanisms of P-Aminothiophenol and P-Nitrothiophenol Molecules on Plasmonic Metal Surfaces Revealed by Tip-/Surface-Enhanced Raman Spectroscopy

Surface plasmon-mediated photocatalysis has attracted great interest, where the creation and transfer of hot electrons or hot holes may play important roles in the chemical reactions. One model system is that p-aminothiophenol (PATP) could be dimerized into a new molecule of p,p'-dimercaptoazobenzene on noble metal surfaces under laser illumination. The mechanism of this catalytic reaction has caused wide discussion, but it is not fully understood yet and remains a channeling task to study the role of hot carriers. Here, we performed a series of experiments for PATP and another associated molecule p-nitrothiophenol by tip-enhanced Raman spectroscopy and surface-enhanced Raman spectroscopy in a well-controlled gas environment (high vacuum, air, N-2, and O-2), and on different substrates (Au, Ag, Cu, Al, and corresponding oxides). The experimental results indicate that the electron acceptor or donor plays a decisive role in whether the reactions can occur or not. Density functional theory calculations were performed to quantitatively study the reaction barriers and thermodynamic processes, and the results agree very well with the experiments. These results help in well understanding the mechanisms of azo-dimerization reactions on plasmonic metal surfaces and designing catalytic reactions with high controllability.