Dielectric behaviour of adsorbed water. Part 2.—Measurement at low temperatures on TiO2

Abstract

On TiO₂ samples with controlled amounts of chemisorbed and physisorbed H₂O, the dielectric permittivity and dielectric loss have been measured at frequencies from 0.1 Hz to 5 MHz at temperatures from room temperature to 77 K. As a result, three kinds of relaxations have been discovered and the crude Cole–Cole plots have been analysed into three arcs I, II and III; relaxation I is due to interfacial polarization as reported in our previous paper. The arcs II and III are assigned to the relaxations due to surface hydroxyls and physisorbed H₂O, respectively. It is found that arc II decreases with increasing coverage θ of physisorbed H₂O and becomes extinct at θ > 1. Furthermore, both the chord length of arc III and the dielectric activation energy of the relaxation III reveal a similar variation as θ increases: first they increase linearly until θ= 2 is reached and then the slope of each curve becomes more gradual. Finally, the dielectric activation energy of adsorbed H₂O approaches that of ice. These phenomena are explained on the basis of the proposed models of H₂O adsorbed on TiO₂.