Unpredictable photocatalytic ability of H2-reduced rutile-TiO2 xerogel in the degradation of dye-pollutants under UV and visible light irradiation

Photocatalytic degradation of organic and inorganic pollutants on the TiO2 semiconductor has been extensively studied as a way to solve environmental problems relating to wastewater and polluted air. Anatase and rutile are the most commonly used crystalline structures of TiO2, with anatase showing a higher photocatalytic activity attributed to its higher specific surface area and its favourable band gap energy (Eg). However, its high band gap (Eg = 3.2 eV) implies the use of UV light (lambda ≤ 380 nm) to inject electrons into the conduction band (TiO2(e-CB)) and to leave holes in the valence band (TiO2(h+VB)). Although the low band gap energy of rutile-TiO2 (Eg = 3.02 eV) allows rutile to potentially absorb more solar energy than anatase, the anatase-to-rutile phase transition leads to the collapse of the TiO2 specific surface area, which may result in a decrease in the photocatalytic activity of rutile. Low specific surface area and therefore poor absorption properties lead to strong limitations in exploring the photo-efficiency of rutile. Nevertheless, rutile has been proved to be comparable to anatase in its photoelectrochemical properties when used in dye-sensitized solar cells.

In the present study, a new process for the reduction of rutile-TiO2 xerogel under hydrogen flow was developed to enhance the photocatalytic activity of TiO2 materials synthesized by the sol-gel process. So a series of H2-reduced TiO2 xerogels of low specific surface area was prepared by hydrolysis and condensation of tetraisopropoxy titanium(IV) in 2-methoxyethanol. The gels were dried under vacuum, calcined in air at different temperatures (400°C, 500°C and 700°C) and finally reduced in H2 at 400 °C. The materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), FT-IR spectroscopy and UV/Visible diffuse reflectance spectroscopy. The texture was determined by nitrogen adsorption-desorption measurements. The effects of the calcination/reduction treatments on the adsorption of methylene blue (MB) in aqueous solution and on the photocatalytic degradation of MB and crystal violet (CV) under UV and visible light irradiation were also evaluated.

Results showed predictable modifications in the physico-chemical properties caused by the annealing of TiO2 xerogel at high calcination temperature (700 °C), such as a total anatase-to-rutile phase transition and a considerable loss of specific surface area from 260 to 2 m2 g-1. However, the higher degree of reduction exhibited by the rutile-TiO2 lattice led to unpredictable photocatalytic activity for the dye conversion under UV and visible light irradiation: the loss of specific surface area of the rutile-TiO2 sample was compensated by the increase in the affinity of this sample for the dye. Under UV light irradiation, the rutile-TiO2 xerogel obtained after a calcination at 700 °C showed a similar level of photoactivity as the one obtained with anatase-TiO2 xerogels obtained by calcination at 400 °C and 500 °C. Under visible light, unlike anatase-TiO2 xerogels, the rutile-TiO2 xerogel showed a higher dye photoconversion rate per external surface area (40 times higher) than the commercial TiO2 Degussa P25.