Effect of thermal oxidation on titanium oxides' characteristics

Nanocrystalline TiO2, in the form of anatase or rutile, is one of the most important and used photocatalysts because of its excellent conversion efficiencies and its chemical stability. TiO2 layers can be grown on titanium by means of anodising processes: the so-obtained oxides present either amorphous or semicrystalline structure, depending on process parameters. When a homogeneous oxide with maximum thickness of the order of hundreds of nanometres is formed, a wide range of interference colours appear on the oxide surface, showing different hues and saturations. Similar effects are obtained when the oxidation process consists of a thermal treatment. This work is aimed at the exploration of the parameters involved in thermal oxidation and of their influence on the growing oxide characteristics, with particular reference to its thickness and the contingent generation of crystal phases (anatase and rutile) responsible for photoinduced properties. Oxide characteristics are investigated by means of reflectometry and X-ray diffraction. Thermal treatments proved to be an efficient way to tune oxide thickness and structure, in order to achieve the formation of oxides showing photoactivated properties both in the UV and in the visible light range, as well as to tune their chromatic properties.