A simple template-free approach to TiO2 hollow spheres with enhanced photocatalytic activity

Abstract

Mesporous anatase-phase TiO₂ hollow spheres with high photocatalytic activity were prepared by hydrothermal treatment and self-transformation of amorphous TiO₂ solid spheres in an NH₄F aqueous solution. The prepared samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N₂ adsorption-desorption isotherms and UV-vis absorption spectroscopy. The photocatalytic activity was evaluated by photocatalytic oxidation decomposition of acetone in air under UV irradiation. It is found that F⁻ plays an essential role in the formation of TiO₂ hollow spheres. F⁻ not only induces the hollowing of TiO₂ solid spheres, but also promotes the crystallization of anatase TiO₂ nanocrystals. A possible formation mechanism for the TiO₂ hollow spheres by localized Ostwald ripening or chemically induced self-transformation is proposed based on the experimental observations. Furthermore, the molar ratios of NH₄F to TiO₂ (R) exhibit a significant influence on the morphology and photocatalytic activity of the TiO₂ samples. The photocatalytic activity of the samples prepared in the presence of NH₄F is higher than that of TiO₂ sample prepared in pure water and commercial Degussa P25 (P25) powders. Especially, the TiO₂ hollow spheres prepared at R = 1 show the highest photocatalytic activity and the specific rate constant S_k exceeds that of P25 by a factor of more than two.