Microemulsion Preparation Of La, Ce Doping Of Tio <sub> 2 </ Sub> Nano-materials Research

Heterogeneous photocatalysis based on semiconductor nano-oxides, has become one of the most active research fields in recent 30 years. Titanium dioxide (TiO₂) has been widely used as photocatalyst due to its lost cost, nontoxic, good chemical stability and high photoactivity. However, TiO₂ with band gap energy of 3.2 eV, can only absorb ultraviolet light which accounts for 3-5% of the sun light. And the recombination of electron and the hole leads to poor photocatalytic effect. So how to expand the the optical response range of TiO₂ and improve the quantum efficiency is a hot topic in the field of heterogeneous photocatalysis.One efficient way to improve its photocatalytic ability and utilize solar energy is to dope TiO₂ with rare element which embodies unique electronic configuration. In present, the synthesis method of TiO₂ doped with rare element is mainly sol-gel, so it is very important for TiO₂ doped with rare element to try a new method and research its performance. In this report, TiO₂ doped with lanthanum (La) and cerium (Ce) was prepared in Triton X-100/n-hexanol/cyclohexane/H₂O reverse microemulsion. The microstructure, morphology and absorption light performance of the doped samples were characterized by using Thermogravimetry and differential Scanning Calorimetric analysis (TG-DSC) and X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Transmission Electron Microscopy (TEM), UV-vis Absorption Spectrum (UV-Vis) and Microscopic-Electrophorety Instrument. The photocatalytic activity was evaluated by photocatalytic degradation of methyl orange under UV and visable-light irradiation. The main experimental results are as follows.(1) The nanoparticles synthesized in Triton X-100/n-hexanol/cyclohexaneane inverse micro-emulsion system had a narrow size distribution and a good dispersion.(2) La and Ce dopping lead to lattice distortion of nanocrystalline TiO₂ and oxygen vacancies, which decreased the electron-hole pair recombination propability of TiO₂. La and Ce dopping was benificial to inhibiting phase transformation and stabilizing anatase phase, for example, rutile appeared in pure TiO₂ calcined at more than 400℃and rutile content of 0.1%La-TiO₂ and 0.5%Ce-TiO₂ calcined at 600℃was only 20.3% and 18.8%. In addition, La and Ce doping can effectively reduce the size of crystal. The more the doping content is, the smaller the size of crystal is.(3) Blue shift of absorbing peak was observed as the doping content of La increased when La-doped TiO₂ was calcined at same temperature. However, red shift of absorbing peak of Ce-doped TiO₂ was observed, La and Ce doping enhance UV and visible-light absorption of TiO₂, but visible-light absorption property of Ce-doped TiO₂ was better than La-doped TiO₂.(4) The stretch vibration peak of O-H bond became stronger with the increase of La or Ce doping content and -OH number of catalyst surface increased, so the Generation of more hydroxyl radical lead to higher photocatalytic activity.(5) The effect of La doping on isoelectric point (IEP) of TiO₂ was greater than that of Ce-doping. Ce-doping had little effect on IEP of TiO₂. In this study, IEP of pure TiO₂ was 3.95, but IEP of Ce-TiO2 and La-TiO₂ decreased to 3.84 and 2.48, respectively.(6) Result of photocatalysis experiments showed, whether under UV-light or under Visible-light irradiation, photocatalytic activity of La or Ce-doped TiO₂ was higher than pure TiO₂. Photocatalytic activity increased first and then decreased as La or Ce doping content increased. And the Photocatalytic efficiency was best at 0.1% La or 0.5% Ce doping content, espectively.(7) Calcination temperature had notable effect on photocatalytic activity. For example, TiO₂ calcined at 400℃showed the highest photocatalytic activity, but so did 0.1%La-TiO₂ or 0.5%Ce-TiO₂ calcined at 600℃. The high calcination temperature benefited enhancing crystallinity of anatase-phase and order character of internal structure, so the electron—hole pair recombination propability was decreased and photocatalytic activity was enhanced. However, higher temperature could promote the phase transformation from anatase to rutile, which was disadvantage to photocatalytic activity.