Surface-enhanced Raman scattering on semiconducting oxide nanoparticles: Oxide nature, size, solvent, and pH effects

Semiconducting oxide nanoparticles have proven to be excellent in detecting extremely low-concentrations of molecules through surface-enhanced Raman scattering (SERS) effects. While the enhancement of the Raman activities arises from a large increase in polarizability due to charge transfer from the molecule to the semiconducting nanoparticle, little is known about how the oxide composition, nanoparticle size, solvent, or pH affects the observed Raman activities. In the current study, we examine these effects by carrying out extensive computational investigations of semiconducting TiO₂, SnO₂ and Fe₂O₃ nanoparticles and their complexes with both catechol and dopamine. An increase in the size of the oxide cluster or a decrease in the pH of the system under observation leads to enhanced Raman activities; the variation of the activities in different solvents is very much dependent on the nature of the vibrational modes. The marked increase in the Raman activities of molecules adsorbed on SnO₂ or Fe₂O₃ over that of molecules adsorbed on TiO₂ seems to indicate that these oxide nanoparticles would be useful substrates for SERS sensors. Our results also indicate that the Raman activities of some of the TiO₂ modes are magnified upon adsorption of molecules, which concurs with some very recent experimental observations. All these results are consistent with a recently proposed theoretical model of SERS on semiconducting substrates. Further, this work has implications on the development of molecular sensing, dye-sensitized solar cells, and photocatalysis.