An effective method for the preparation of high temperature stable anatase TiO₂ photocatalysts

Highlights: • A rapid preparation method for the N, F codoped TiO₂ is reported. • Anatase stability was achieved by using N, F doping and a microwave process. • High temperature stable anatase up to 1200 °C is reported. • Production of ammonium oxofluorotitanates (NH₄TiOF₃) intermediate is reported. - Abstract: An efficient, rapid and straightforward method for the preparation of nitrogen and fluorine (N, F) codoped high temperature stable anatase using a microwave pre-treatment is reported. Using a single source, ammonium fluoride (NH₄F) for both nitrogen and fluorine, effective doping of the precursor titanium isopropoxide (TTIP) was possible. These samples were characterised for their structural and optical properties using X-ray diffraction (XRD), Fourier Transform IR (FTIR), Raman spectroscopy and UV–vis spectroscopy. In terms of the anatase to rutile transition enhancement using a novel microwave assisted technique, the sample prepared in a composition of 1:8 TiO₂: NH₄F at 1200 °C was seen to be most effective, having stable anatase present at 57.1% compared to undoped TiO₂ being 100% rutile from 900 °C. This method involves the production of ammonium oxofluorotitanates (NH₄TiOF₃) at low temperatures. The inclusion of these intermediates greatly reduces the particle size growth and delays the anatase to rutile transition. The photocatalytic activity of these materials was studied by analysing the degradation of an organic dye, rhodamine 6G as a model system and the rate constant was calculated by pseudo-first-order kinetics. These results showed that the doped sample (0.0225 min⁻¹) was three times more active than the undoped sample (0.0076 min⁻¹) and over seven times faster than the commercial TiO₂ photocatalyst standard Degussa P-25 calcined at 1200 °C (0.0030 min⁻¹). The formation of intermediate compounds, oxofluorotitanates, was identified as the major reason for a delay in the anatase to rutile transition.