Markedly Enhanced Surface Hydroxyl Groups of TiO₂ Nanoparticles with Superior Water-Dispersibility for Photocatalysis

The benefits of increasing the number of surface hydroxyls on TiO₂ nanoparticles (NPs) are known for environmental and energy applications; however, the roles of the hydroxyl groups have not been characterized and distinguished. Herein, TiO₂ NPs with abundant surface hydroxyl groups were prepared using commercial titanium dioxide (ST-01) powder pretreated with alkaline hydrogen peroxide. Through this simple treatment, the pure anatase phase was retained with an average crystallite size of 5 nm and the surface hydroxyl group density was enhanced to 12.0 OH/nm², estimated by thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Especially, this treatment increased the amounts of terminal hydroxyls five- to six-fold, which could raise the isoelectric point and the positive charges on the TiO₂ surface in water. The photocatalytic efficiency of the obtained TiO₂ NPs was investigated by the photodegradation of sulforhodamine B under visible light irradiation as a function of TiO₂ content, pH of solution, and initial dye concentration. The high surface hydroxyl group density of TiO₂ NPs can not only enhance water-dispersibility but also promote dye sensitization by generating more hydroxyl radicals.