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

A series of nitrogen-doped TiO 2 −SiO 2 mixed oxide catalysts were prepared by a sol−gel method through varying the TiO 2 /SiO 2 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 SiO 2 stabilized the anatase phase even at high temperatures. The results of XRD and TEM revealed a decrease in the size of TiO 2 particles with increasing the SiO 2 content, but the rise of SiO 2 /TiO 2 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 TiO 2 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 TiO 2 catalysts. The addition of SiO 2 results in (i) the improvement of thermal stability of the catalyst, (ii) the decrease of TiO 2 particle size, and (iii) the formation of Brønsted acid sites. All these factors are responsible for an enhancement in photogenerated electron−hole separation and thus better photocatalytic activity.