Surface-Enhanced Raman Scattering Study of the Product-Selectivity of Plasmon-Driven Reactions of p-Nitrothiophenol in Silver Nanowires

Nanoplasmonic photocatalysis shows a huge potential for efficient light energy harvesting by leveraging the unique and peculiar properties of plasmonic metallic nanostructures (usually Ag or Au) to drive light-induced chemical reactions. Plasmonic catalysis of the coupling reaction of p-nitrothiophenol (PNTP) over silver nanowires (AgNWs) and -quasi-spherical silver nanoparticles (AgNPs) has been intensively investigated by in situ surface-enhanced Raman scattering (SERS). The reduction of PNTP is often used as a model system in plasmonic photocatalysis basic science since the conversion of the nitro group into an amino (p-aminothiophenol, PATP) or an azo (4,4'-dimercaptoazobenzene, DMAB) group is a well-documented reaction that can occur through a wide variety of chemical processes, allowing for the study of fundamental reaction mechanisms. However, the reduction of PNTP to PATP rarely occurs in the absence of a strong reducing species, such as H₂ or NaBH₄. The control and understanding of the precise molecular mechanisms of plasmonic catalysis in different chemical reactions involving different metallic nanostructures are an object of intense interest for possible applications. In this context, SERS is a powerful tool since it enables in situ tracking of the catalytic reactions because it combines the advantages of high chemical specificity (vibrational Raman scattering), high sensitivity, and surface selectivity. Our study demonstrated selectivity in the surface reactions of PNTP on AgNWs (PATP and DMAB) and AgNPs (DMAB). For the first time, the reduction of PNTP to PATP in the presence of AgNWs in air without the use of any strong reducing agent was observed. In addition, the dependence of the reaction on exciting radiation and on laser power density was investigated to control the selectivity of the reaction mechanism on AgNWs. Finally, the crucial function of the nanoparticle shape and anisotropy in the reaction mechanism is highlighted.