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.