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Surface‐Enhanced Raman Scattering (SERS) of Nitrothiophenol Isomers Chemisorbed on TiO 2
Author(s) -
Teguh Jefri S.,
Liu Fang,
Xing Bengang,
Yeow Edwin K. L.
Publication year - 2012
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201100934
Subject(s) - raman spectroscopy , raman scattering , homo/lumo , chemistry , molecule , photochemistry , density functional theory , analytical chemistry (journal) , computational chemistry , organic chemistry , optics , physics
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.