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

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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.

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