The preparation, characterization, and their photocatalytic activities of rare-earth-doped TiO2 nanoparticles

RE/TiO2 photocatalysts were prepared by the sol-gel method using rare earth (RE = La3+, Ce3+, Er3+, Pr3+, Gd3+, Nd3+, Sm3+) metal salts and tetra-n-butyl titanate as precursors, and were characterized by XRD, IR, UV-vis diffuse reflection, and transient absorption spectra. Their photocatalytic activities were evaluated using nitrite as a decomposition objective. As a result, suitable content of doping rare earth in TiO2 can efficiently extend the light absorption properties to the visible region. At the same time, it is beneficial to NO2- adsorption over the catalysts due to rare earth doping. RE/TiO2 samples can enhance the photocatalytic activity to some extent as compared with naked TiO2. The increase in photoactivity is probably due to the higher adsorption, red shifts to a longer wavelength, and the increase in the interfacial electron transfer rate. Nitrite is almost completely degraded over RE/TiO2 catalysts after longer irradiation, which is different from Degussa P-25 with a plateau of activity after ca. 20 min irradiation. Gd3+-doped TiO2 showed the highest reaction activity among all concerned RE-doped samples because of its specific characteristics. The amount of RE doping was an important factor affecting photocatalytic activity; the optimum amount of RE doping is ca. 0.5 wt%, at which each RE/TiO2 sample shows the most reactivity. The photocatalytic degradation reaction of nitrite over Gd3+-doped samples and P-25 follows apparent first order kinetics, which is different from that of Sm3+, Ce3+, Er3+, Pr3+, La3+, and Nd3+-doped TiO2 catalysts, which obey zero-order kinetics, indicating that these processes were dominated by electron-hole recombination. (C) 2002 Elsevier Science (USA).