Plasmon-driven catalysis of adsorbed p-nitroaniline (PNA) by surface-enhanced Raman scattering (SERS): Platinum versus silver

•A study of PNA adsorption on Pt nanoparticles is performed by SERS for the first time.•Results are discussed and compared with those published and obtained with Ag.•Similar SERS spectra were obtained on both metals when O2 is present.•However, different SERS spectra were obtained on both metals if O2 is absent.•Without O2 no plasmonic catalytic reductive coupling of NO2 group on Pt fully occurs. The adsorption of p-nitroaniline (PNA) on Pt nanoparticles, both in the absence (in aqueous solution) and in the presence of oxygen (in air environment), is studied by SERS for the first time. Differences between the plasmon-driven catalysis of adsorbed PNA on Pt and Ag nanoparticles have been found. In the presence of oxygen, the oxidative coupling of the NH2 group in PNA to yield dinitroazobenzene (DNAB) occurs on both Pt and Ag nanoparticles. However, in the absence of oxygen, PNA behaves clearly differently on Ag than on Pt. Thus, whereas diaminoazobenzene (DAAB) is catalytically produced on Ag nanoparticles by plasmon-driven reduction reactions of the NO2 group, on Pt nanoparticles this reaction does not fully take place as indicated by the presence of NO2 groups on the surface of the nanoparticles. A mechanism for this distinctive behavior is tentatively proposed in which water acts as a sacrificial agent, being reduced to hydrogen by hot electrons coming from the Pt surface, while the hot holes on Pt are proposed to attach to an occupied molecular state of adsorbed PNA. The overall photocatalytic reaction of adsorbed PNA on Pt nanoparticles, in an O2 free solution, would actually be consistent with a dehydrogenation process of water. [Display omitted]