Low Temperature Soft Chemical Methods To Nanostructured TiO₂ And Their Photocatalytic Activity

Nanostructured TiO2 is one of the most efficient semiconductor photocatalysts, because of its strong oxidizing power, nontoxicity, high photochemical corrosive resistance, and low cost. However, anatase TiO2 with large band gap of about 3.2 eV can be activated only in the low-UV region (< 400 nm), about 4% of the incoming solar energy. To effectively utilize visible light (43% of the energy of the solar spectrum), many attempts have been made to extend its absorption and conversion capacity into the visible portion of the solar spectrum.We first reported a one-pot facile hydrothermal method to synthesize mixed-phase TiO2 nanocrystals with tunable brookite to rutile ratios. This method utilized titanium tetrachloride as the titanium source and triethylamine as the "adjusting reagent" to tune the ratio of brookite and rutile in mixed-phase TiO2 nanocrystals. The resulting samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectroscopy (DRS), and nitrogen sorption measurements. The photocatalytic activities of the resulting TiO2 nanocrystals were examined by the degradation of RhB under artificial solar light. We found the TiO2 sample with 38% brookite and 62% rutile, obtained in the solution of 3 mL of triethylamine and 10 mL of water, exhibited the highest photocatalytic activity. Its activity was about six times of that of the Degussa TiO2 P25. The possible mechanism of brookite-rutile ratio tuning was proposed on the basis of characterization.We then demonstrated the controlled low temperature synthesis of C-Cl-codoped anatase TiO2 nanocrystals with different dopant concentrations via one-pot solvothermal treatment of titanium isopropoxide (TTIP) in chloroform containing trace amounts of water at a low temperature of 150℃without subsequent thermal procedure. The C and Cl dopants were resulted from the solvothermal decomposition of chloroform induced by water and the adjusting of molar ratios of water to TTIP was able to tune the crystal sizes and the concentration of the dopants. The as-prepared powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy (DRS). All the C-Cl-codoped TiO2 possessed mesoporous structure and high surface areas between 158.1 and 469.6 m2/g. The photocatalytic activities of the resulting C-Cl-codoped TiO2 nanocrystals were evaluated by the photodegradation of RhB under visible light irradiation. The sample synthesized with a molar ratio of water to TTIP equaling to 2 exhibited the highest photoactivity among all the synthesized C-Cl-codoped TiO2. Its activity was even much higher than that of the C-TiO2 and Cl-TiO2 reported in our previous work. The enhanced visible light photocatalyitc activity of that C-Cl-codoped TiO2 nanocrystal could be primarily attributed to the large surface area and the synergetic effect of the codoping.