Surface-enhanced Raman scattering (SERS) of nitrothiophenol isomers chemisorbed on TiO2

Surface-enhanced Raman scattering (SERS) spectroscopy and density functional theory (DFT) calculations were used to investigate the nature of the charge-transfer (CT) process between nitrothiophenol (NTP) isomers and the n-type semiconductor, TiO(2). The Raman signals of p-NTP and m-NTP that were chemisorbed onto TiO(2) were significantly enhanced with respect to their corresponding neat compounds. In particular, an enhancement factor (EF) of 10(2)-10(3) was observed for both p-NTP and m-NTP, with m-NTP displaying a larger EF compared to p-NTP. The Raman signal of o-NTP on TiO(2) was not detectable, owing to interference from fluorescence emissions. A molecule-to-TiO(2) charge-transfer mechanism was responsible for the enhanced Raman signals observed in p-NTP and m-NTP. This transfer was due to a strong coupling between the adsorbate and the metal oxide, which led to an optically driven CT transition from the HOMO of NTP into the conduction band of TiO(2). Based on the mesomeric effect, the NO(2) group para to the thiol had a stronger electron-withdrawing ability than the NO(2) group at the meta position. A less-efficient CT transition from p-NTP to TiO(2) in the surface complex resulted in a weaker Raman-signal enhancement for p-NTP compared to m-NTP. The DFT calculation determined that the HOMO and the LUMO of NTP bound to TiO(2) were located entirely on the adsorbate and the semiconductor, respectively, thereby supporting the experimental findings that a molecule-to-TiO(2) mechanism was the driving force behind the observed SERS effect.