Visible-light-driven N–F-codoped TiO2 powders derived from different ammonium oxofluorotitanate precursors

We report on the synthesis of N–F-codoped TiO2 powders with a homogenous anatase structure via a thermal decomposition of the ammonium oxofluorotitanate ((NH4)0.3TiO1.1F2.1, NH4TiOF3 and (NH4)2TiOF4) precursors at 550 °C for 1 h in air. The ammonium oxofluorotitanate precursors were synthesized by a wet chemical method in the mixture of deionized water and absolute ethanol using titanium tetrachloride (TiCl4) and ammonium fluoride (NH4F) as reactants. The as-prepared samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry–differential scanning calorimetry (TG–DSC), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV–vis spectroscopy. The results from XRD, TG–DSC and FTIR analyses confirmed a complete thermal conversion of the ammonium oxofluorotitanate precursors to N–F-codoped anatase TiO2 at a temperature of > 500 °C. The SEM and TEM observations exhibited that the morphologies of the precursors were not considerably affected by the thermal treatment at 550 °C for 1 h. The XPS data proved the presence of the nitrogen and fluorine atoms in the TiO2 structure. According to the resulting UV–vis spectra, the as-prepared samples demonstrated a strong visible-light absorbance in the wavelength range of 400–550 nm. The photocatalytic activity of the N–F-codoped anatase TiO2 was evaluated through a study of the decomposition of methyl orange under visible light irradiation. It was found that the sample derived from the (NH4)0.3TiO1.1F2.1 precursor by thermal treatment at 550 °C for 1 h showed much higher photocatalytic activity compared to the commercial undoped TiO2 (Degussa P25) due to the synergetic effects of a strong absorption in the visible-light region, red shift in absorption edge, good crystallinity, and porous structure.