Preparation And Properties Of Nano-TiO₂ Composite Films

The most serious problem we human beings are facing after we entered 21th century is environmental pollution. The pollutants can hardly be treated by current environmental technology; using photo-catalytic method to decompose these organic contaminations has become one of the most potential methods in application and the most active fields international.Photo-catalytic degradation means the organic pollutants are decomposed by photo-catalyst with illumination. Photo-catalyst is the key part in the photo-degradation process, its photo-catalytic activity and fixing method are the two decisive factors determining whether this new technique is feasible. At present the photo-catalysts applied in the degradation of environmental pollutants are N type semiconductor mainly, for example TiO₂,ZnO,CdS,SnO₂,Fe2O3 etc. TiO₂ focused much more attention for its low-price, nontoxic, better chemical stability and higher photo-activity under UV illumination. The mechanism for TiO₂ as photocatalyst is as follows: under the irradiation of UV lights withλ< 387 nm, electron on VB migrate to CB by getting energy and form photo-electron and hole. After a series reaction·OH is formed and the organic pollutants were oxidized to CO₂ and H2O. The problem is: electron and hole is easy to recombine, while the UV lights occupy only 5% of the total sunlight. It has low sunlight utilization as well as photo-catalytic properties.To avoid the drawbacks above, and widen the absorption spectrum, surface modification is needed. Doped with metal ions is a mature method, while transition metal dopant attract a large number of attention. Usually the metal is doped in TiO₂ colloid, which is called inner method. The drawback is the low utilization ratio, a proliferation process is need. In this process, a recombination may occur. Based on these problems, two methods were adopted to improve photo-catalytic degradation, which is outer method and co-doped with transant metal and rare earth metal.It is proved by experiments that by the modification of the above, the photocatalytic properties are improved greatly. This offer a new way for the application of TiO₂ films. Main contents contain the following:(1) the preparation of colloid TiO₂ and its impact factorsIn our experiments, tetrabutyl titanate was chose as raw material to prepared colloid titanium dioxide in normal temperature. The best conditions are: ageing time three days; sintering temperature 450℃and sintering time one hour; illumination time 45 minutes.(2) Property study of nano-TiO₂ filmsThere are not many reports about TiO₂ films co-doped by transition metal ions and rare earth metal ions yet, so is the spectrum study of co-doped TiO₂ films. In the experiments Acid Naphthol Red (ANR) and Methyl Green (MG) dyes that are easier to be decomposed together with Naphthol Green B (NGB) dye, which is harder to be decomposed were chose as degradation aim. Transition metal ions Fe3+,Ag+ and rear earth metal ions La3+,Ce3+ on their degradation of the given dyes with certain concentration were discussed respectively, to find the best doping concentration. Based on this, we studied on the degradation of transition metal ions co-doped with rare earth metal ions by the organic dyes. Experimental results show: there exist a best doping concentration for Fe3+, Ag+, La3+ and Ce3+, the co-doped films have higher degradation rate than any single doped film. The kinetic study show: co-doped films can improve greatly the degradation rate under the same time, with the biggest slope. It is evident from absorption spectrum that the modified composite films can broaden the spectrum respond region, which may offer a new way for the application of nano-TiO₂ composite films.(3)Characteristics of TiO₂ composite filmsWe adopt FE-SEM,XRD,EDS,BET and other methods to characterize the prepared composite films. It is shown from results that the nano-films have a smooth surface with a little reunite; particle size is beyond 10nm, with porous structure, Anatase as main crystal type. Modified by doping bring a better surface with little reunite; less Rutile type; bigger surface areas and concentrated pore size.(4) The mechanism analyse for photocatalystCo-doped TiO₂ films can improve greatly the degradation rate, possible mechanism is: transant metal ions can attract electron, while rare earth metal ions lead to oxygen defect so as to attract hole. The electron-hole was separated, and photo-catalytic properties were improved.