Verification of the photoadsorption of H2O molecules on TiO2 semiconductor surfaces by vibrational absorption spectroscopy

The interactions between H2O molecules and photoreduced TiO2 surfaces were investigated by spectroscopic methods such as Fourier transform infrared (FT-IR) and UV-visible-near-infrared (NIR) spectroscopic absorption measurements. When TiO2 powders were irradiated with UV light in the absence of O-2, the white color of the TiO2 powders changed to blue-gray and the H2O molecules simultaneously desorbed from the TiO2 surfaces due to the heating effect from the light source. The H2O molecules could hardly readsorb on such photoreduced TiO2 surfaces, which could remain stable in the absence of O-2 for a long time. The photoformed holes trapped on the TiO2 surfaces are immediately consumed to oxidize the lattice oxygen and/or surface hydroxyl groups, resulting in the formation of oxygen vacancies, while the photoformed electrons are trapped on the Ti4+ sites to produce Ti3+ sites in the absence of O-2 as electron scavengers. Hence, such photoreduced TiO2 surfaces, on which the photoformed electrons are trapped, can be represented as "negatively charged surfaces" or "electron-rich surfaces". H2O molecules, which are strongly polarized due to the high electronegativity of the O atoms, are hardly able to interact with such electron-rich surfaces due to repulsion. Moreover, when the TiO2 surfaces are irradiated with UV light in the presence of O-2, the oxygen vacancies are quickly oxidized and the electrons trapped on the Ti3+ sites are immediately scavenged by O-2 molecules. Such a consuming process of the negative charges for the oxygen vacancies and trapped electrons may be the driving force for the photoadsorption of O-2 molecules on TiO2 semiconducting photocatalysts. The desorption of H2O and the simultaneous adsorption of O-2 during UV light irradiation on the TiO2 surfaces were also confirmed by Q-mass analysis.