Morphology change and band gap narrowing of hierarchical TiO2 nanostructures induced by fluorine doping

Abstract

A series of TiO2 hierarchical nanostructures were synthesized by a facile template-free hydrothermal method in the presence of sodium fluoride (NaF) using titanium tetrachloride (TiCl4) as precursor. The pompon-like and football-like TiO2 hierarchical microspheres are composed of aligned rutile and anatase nanoparticles. The TiO2 nanostructures are comprised of oriented anatase nanoparticles. It is found that the doping concentration of NaF plays an important role in controlling the morphology and structure of the hierarchical nanoarchitectures. A sequential phase transition is observed with increasing dopant concentration: rutile → rutile + anatase → anatase. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) results indicated that F ions physically absorbed on the surface of TiO2 and incorporated into TiO2 crystal lattice. For the first time, a sequential red shift of absorption band edge of F-doped TiO2 is observed with increasing the concentration of NaF as compared to pure TiO2. The band gap could be tuned from 3.0 eV to 2.2 eV, which depends on the fluorine doping concentration. It is found that F-doped TiO2 nanostructures exhibit higher photocatalytic activity than that of pure TiO2 under visible light irradiation.