Characteristics of water adsorbed on TiO2 photocatalytic systems with increasing temperature as studied by solid-state 1H NMR spectroscopy

The time profiles of the intensities, line widths, and chemical shifts of H-1 NMR signals of adsorbed water at 358 K were investigated for six different TiO2 photocatalysts with different properties by solid-state H-1 NMR spectroscopy. The apparent single water peak was found to comprise the signals with different line widths and chemical shifts, which correspond to those of the water molecules with different structures and mobility. The adsorbed water region observed with NMR is characterized by three layers: (I) the innermost layer with rigid water molecules with highly restricted mobility and/or hydroxyl groups on the solid surface; (II) the intermediate layer with relatively mobile water molecules; and (III) the outermost layer with highly mobile water molecules. The thickness of the water layer is almost the same for the six photocatalysts. The ratio of the number of the water molecules contained in each layer could be roughly estimated to be 3(I):2(II):5(III). The surface properties of the individual photocatalysts are distinctly reflected in the water molecules in layer I but not in those in layer III.