Relationship between Water Desorption and Low‐Temperature Densification of Colloidal Anatase Thin Films

Films consisting of discrete anatase crystals with an average diameter of 8 nm were deposited from a colloidal sol synthesized from titanium(IV) isopropoxide. From ion-beam analysis (FRES and RBS) and ellipsometry, we found that desorption of Type I water from the films is accompanied by a decrease in the refractive index but no densification. Conversely, desorption of Type II water or surface hydroxyls at higher temperatures leads to film densification but has no apparent effect on the refractive index. The densification involves progressive condensation of hydrogen-bonded hydroxyls and finally isolated hydroxyls. The refractive index for films exposed to air for 6 months increases regardless of heat treatment because of surface rehydration which nearly recreates the original surface.