Sulfur Doped Nanocrystalline Titanium Didxide Photocatalyst: In-situ Synthesis And Photocatalysis Under Visible Light

The approach to extend the photoresponse to the low energy(visible) region and improve the photocatalytic activity of TiO₂ by doping non-metal ions such as carbon, nitrogen, sulfur etc, is a hot issue of semiconductor photocatalytic area at present. In this paper, the progress of the research on semiconductor photocatalysis was summarized and reviewed, the sulfur doped TiO₂ system was selected as research subject, new synthesis method and material was used to control the microstructure (crystal degree, crystallite size) of the TiO₂, and the photocatalytic degradation mechanism was discussed, which will contribute to the aspects both for theory research and practical application of sulfur doped TiO₂ with high photocatalytic activity using visible light radiation.Sulfur doped nanocrystalline TiO₂ photocatalyst was synthesized employing Sol-Gel or hydrothermal method respectively, with titanium tetrachloride or tetrabutyl titanate as titanium source, triethanolamine as surfactant and thiourea as doping agent. The as-prepared samples were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the products was evaluated by the photocatalytic decomposition of methyl orange(MO) or 4-cholophenol(4-CP) under visible light (λ>390nm).Sulfur doped TiO₂ prepared by Sol-Gel method were anatase structured and the crystallite sizes were in the range of 20-35nm. As the heat treatment temperatures increased from 450℃ to 500℃, the phtocatalytic activity also increased; but when the heat treatment temperatures increased from 500℃ to 650℃, the phtocatalytic activity decreased. When the mole ratio of thiourea and titanium tetrachloride (S/Ti) was between 0.35 and 2.10, the absorption edges of the sulfur doped TiO₂ all shifted to the visible light area, and the phtocatalytic activity was bestwhen the S/Ti was 0.70. XPS analysis showed that the trance of sulfur ions(S⁴⁺) entered into the crystal lattice of TiO₂, which resulted in the localized crystal deformation, and this was the reason why the absorption edges of the photocatalyst shifted to a longer wavelength. The experiment on photocatalytic stability indicated that sulfur doped TiO₂ photocatalyst was stable. At last, photocatalytic degradation process of MO with sulfur doped TiO₂ as photocatalyst under visible light irradiation was investigated by UV-Vis spectra.Sulfur doped TiO₂ prepared by hydrothermal method were anatase structured and the crystallite sizes were larger than 10nm. As the hydrothermal temperatures increased from 150℃ to 210 ℃ , the absorption edges of the sulfur doped TiO₂ also shifted to the longer wavelength, and the photocatalytic activity increased too. Heat treatment might change the exterior absorbed substances and the crystal degree of the samples, so affected their photocatalytic activity. When the mole ratio of thiourea and tetrabuty ltitanate (S/Ti) was between 0.35 and 1.75, the photocatalytic activity of the sulfur doped TiO₂ were better than pure TiO₂. The phtocatalytic activity was best when the S/Ti was 0.70. Furthermore, the phtocatalytic activity of the samples synthesized by hydrothermal method were better than samples prepared by Sol-Gel method. At last, photocatalytic degradation process of 4-CP with sulfur doped TiO₂ as photocatalyst under visible light irradiation was investigated by UV-Vis spectra.