Synthesis, Characterization, and Activity of Visible-Light-Driven Nitrogen-Doped TiO2-SiO2 Mixed Oxide Photocatalysts

A series of nitrogen-doped TiO2-Sio(2) mixed oxide catalysts were prepared by a sol-gel method through varying the TiO2/SiO2 molar ratio. The catalyst structure has been extensively characterized by using UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR). UV-visible spectroscopy revealed that the absorption edge of the catalysts composed of only the anatase phase was red-shifted to the visible light region. In addition, the presence of SiO2 stabilized the anatase phase even at high temperatures. The results of XRD and TEM revealed a decrease in the size of TiO2 particles with increasing the SiO2 content, but the rise of SiO2/TiO2 ratio in the mixed oxides led to a decrease in the amount of surface sulfate species as identified by the FT-IR, XPS, and TGA. Moreover, XPS results also suggested that the TiO2 lattice was interstitially doped by nitrogen species. The catalysts as prepared exhibited better photocatalytic activity in degradation of methylene blue under visible light compared to the doped TiO2 catalysts. The addition of SiO2 results in (i) the improvement of thermal stability of the catalyst, (ii) the decrease of TiO2 particle size, and (iii) the formation of Bronsted acid sites. All these factors are responsible for an enhancement in photogenerated electron-hole separation and thus better photocatalytic activity.