Improved high temperature stability of anatase TiO₂ photocatalysts by N, F, P co-doping

Among the three commonly occurring phases (anatase, rutile, and brookite) of TiO_2, the anatase form is reported to be the best photocatalyst due to the improved charge-carrier mobility and the greater number of surface hydroxyl groups. The anatase to rutile transition in titania photocatalysts usually occurs at a temperature between 500 °C to 700 °C. Development of a high temperature stable (above 1000 °C) anatase phase is important for various environmental applications (e.g. self-cleaning ceramic tiles, anti-microbial sanitary wares, etc.). In this study, the use of ammonium hexafluorophosphate as a single source dopant (method A) and urea, trifluoroacetic acid and phosphoric acid as multiple sources (method B) was undertaken to improving its high temperature stability. Method A was seen to produce a more stable anatase phase, with 68% anatase present at 1100 °C, compared to method B which showed 100% rutile at 900 °C. Kinetic analysis shows a marked increase in the photocatalytic degradation of a model dye using materials calcined at 1100 °C for method A (0.042 min^-1) compared to that for method B (0.005 min^-1) and the commercial photocatalyst Evonik-Degussa AEROXIDE® (0.031 min^-1) at 1100 °C. XPS results showed that, the only dopant detected at high temperatures is phosphorus in its P⁵⁺ form. The incorporation of phosphorus has proved to be an effective method in stabilising the anatase phase at high temperature. The current investigation also showed that a single source precursor is more favourable to obtain high temperature stable anatase phase photocatalysts.