Synergistic effects of sodium fluoride (NaF) on the crystallinity and band gap of Fe-doped TiO2 developed via microwave-assisted hydrothermal treatment

Abstract

Titanium dioxide-based nanocomposite materials are the most widely used materials to enhance the photocatalytic activity of titanium dioxide (TiO2) due to their excellent light absorbing properties. However, the wide use of TiO2 is restricted by its large band gap. Doping TiO2 with iron (Fe) is one of the strategies that have been employed in order to narrow the band gap of TiO2, thus promoting its photocatalytic activity to the visible light region. In this work, new energy levels between the valence band and the conduction band were introduced by incorporating Fe in the TiO2 lattice resulting to an effective shift of the light absorption to the visible light region. On the other hand, the incorporation of NaF in the TiO2 lattice led to significant changes on its crystal phases from brookite-anatase to anatase-rutile phases. In this study, the hydrothermal synthesis route was combined with microwave heating as a smart pathway of reducing the synthesis cost by reducing synthesis temperature and time while achieving an efficient photocatalytic activity of the TiO2 nano-photocatalysts. XRD diffraction patterns demonstrated that a mixture of brookite-anatase was obtained, which was transformed to the anatase-rutile phases after doping TiO2 with Fe in the presence of NaF. Microscopic investigations further showed that the doped TiO2 nano-photocatalyts exhibited a spherical shape with an average particle size in the range of 10–20 nm. UV–Vis analysis showed that the indirect and direct band gap of Fe-doped TiO2 in the presence of NaF was significantly narrowed to 1.67–1.83 eV and 2.05–2.17 eV, respectively. The synergistic effect of NaF on the crystallinity of Fe-doped TiO2 calcined at 400 °C in air was demonstrated to form an anatase-rutile phase.