Vibrational features of confined water in nanoporous TiO_2 by Raman spectra

Raman spectra of confined water adsorbed in nanoporous TiO_2 are obtained in experiment. TiO_2 samples with different pore diameters under different humidity conditions are investigated. The results indicate that the symmetric vibrational mode of water molecule is destroyed when relative humidity decreases. This indicates that the interaction between water molecules and surface of TiO_2 becomes stronger when the distance between water molecules and surface turns smaller, and the interaction plays a major role in depressing the symmetric vibrational peak. The spectra of confined water in TiO_2 and Vycor are compared. When filling fractions are the same, their spectra show distinctions no matter whether they are in partial filling condition or in full filling condition. The spectra of HDO confined in TiO_2 with different filling fractions are compared with each other. There is no clear distinction among their vibrational peaks, and the peaks mainly relate to asymmetric vibration. Therefore, the interaction between water molecules and the wall of pore decouples the symmetric vibrational mode only, and the influences on asymmetric vibrational mode show little differences among different filling fractions.