The effect of the valence state of titanium ions on the hydrophilicity of ceramics in the titanium–oxygen system

Ceramics from the titanium–oxygen system were prepared by sintering TiO2 and Ti2O3 powders at 1400 °C in air or in Ar/H2 (7%). The surface compositions of the TiO2, the non-stoichiometric TiO2−x and the Ti2O3 ceramics, studied with X-ray photoelectron spectroscopy, are discussed in terms of their chemical composition, with particular emphasis on the valence state of the titanium ions. The correlation between the valence state of the titanium ions at the surface and the water-contact angle was examined. The presence of Ti3+, which can be introduced at the titanium–oxygen ceramic surface by sintering the ceramic in lower oxygen partial pressure, is related to a significant decrease in the water-contact angle. TiO2 ceramics with just Ti4+ at the interface had a water-contact angle of 67°; however, the non-stoichiometric TiO2−x and Ti2O3/Ti3O5 with Ti3+ at the interface had lower water-contact angles, i.e., 26° and 18°, respectively. The wettability of ceramics in the Ti–O system can be improved by introducing Ti3+ at the ceramic interface.