Analysis of Surface OH Groups on TiO₂ Single Crystal with Polarization Modulation Infrared External Reflection Spectroscopy

The surface OH groups on rutile TiO 2 single-crystal surfaces under atmospheric conditions were investigated with polarization modulation infrared external reflection spectroscopy (PM-IER). The PM technique is effective for selective detection of the oriented surface OH groups on the TiO 2 surface due to the cancellation of strong IR signals from three-dimensional isotropic water vapor and bulk water adsorbed on the surface under atmospheric conditions. The PM-IER spectrum on the TiO 2 (110) surface gave only a single peak at 3279 cm −1 . The absence of coexisting symmetric and asymmetric peaks of water molecules reveals that the peak is not derived from the water molecules adsorbed on the surface but from the OH groups directly binding to the TiO 2 surface. A remarkable decrease of the peak frequency reaching ca. 400 cm −1 compared to that observed by high-resolution electron energy loss spectroscopy (HREELS) under a vacuum condition (3690 cm −1 ) was observed, indicating that the surface OH groups under atmospheric conditions form strong hydrogen bonds with adsorbed water molecules on the surface. The PM-IER spectra of the (110), (100), and (001) single-crystal surfaces were compared to clarify the binding position of the surface OH groups, which has not been assigned yet. The appearance of the peak on the (110) and (100) surfaces and its disappearance on the (001) one reveal that the surface OH groups mainly locate at the bridging oxygen sites on the TiO 2 single-crystal surfaces. We also investigated the PM-IER spectra under UV irradiation to determine the relationship between the changes in the number of bridging OH groups and the photoinduced hydrophilicity of the TiO 2 surface under UV irradiation. According to widely held belief, the increase in the number of surface OH groups by photochemical reactions under UV irradiation induces photoinduced hydrophilicity. Unexpectedly, no remarkable change in the number of surface OH groups was observed, although the surface showed a hydrophilic nature, indicating that another mechanism that is not dependent on the increase in the number of surface OH groups is responsible for photoinduced hydrophilicity.