Spectral Identification of Methanol on TiO₂(110) Surfaces with Sum Frequency Generation in the C–H Stretching Region

In order to understand the adsorption behavior and photocatalytic reactions of organic molecules on titanium dioxide, TiO₂, at the molecular level, we have studied the vibrational spectra of methanol on TiO₂(110) by using surface-specific broadband infrared (IR) sum frequency generation (SFG) in combination with ultrahigh vacuum technique. As compared to vapor/liquid methanol interface, methanol on TiO₂(110) exhibits a much more complicated spectrum due to its molecular and dissociative adsorption (methoxy) on TiO₂(110). Furthermore, different sites are available for the methanol adsorption, including five-coordinated titanium (Ti_5c) and bridge-bonded oxygen (O_br). The methyl group of these species has symmetric stretching and Fermi resonance modes similar to those at the air/liquid methanol interface, and also presents intense antisymmetric stretching modes. These resonances of both molecular and dissociative methanol have been well resolved and assigned by means of polarization-dependent and methanol coverage-dependent SFG measurements, as well as ultraviolet (UV) irradiation treatments. This work lays the foundation for further in situ studies of the surface chemistry of methanol on TiO₂(110). The results also demonstrate the ability of SFG vibrational spectroscopy for investigating complicated structures of adsorbates on single crystal oxides