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, TiO₂. The Raman signals of p-NTP and m-NTP that were chemisorbed onto TiO₂ were significantly enhanced with respect to their corresponding neat compounds. In particular, an enhancement factor (EF) of 10²–10³ 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 TiO₂ was not detectable, owing to interference from fluorescence emissions. A molecule-to-TiO₂ 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 TiO₂. Based on the mesomeric effect, the NO₂ group para to the thiol had a stronger electron-withdrawing ability than the NO₂ group at the meta position. A less-efficient CT transition from p-NTP to TiO₂ 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 TiO₂ were located entirely on the adsorbate and the semiconductor, respectively, thereby supporting the experimental findings that a molecule-to-TiO₂ mechanism was the driving force behind the observed SERS effect.