Theoretical And Experimental Study Of Metal And Nonmetal Codoped TiO₂

Titanium dioxide (TiO2) has received much attention as a important material for photocatalysis and photoelectricity (fields of Solar Cell), which is relatively appropriate conduction band and valance band potential, wide in band gap, chemically stable, strongly reductive and oxidative, non-poisonous, and environmentally friendly. Therefore, TiO2 is wide application in the photocatalystic reaction and the Solar Cell.However, the wide band gap requiring ultraviolet irradiation limits the practical application; Increase TiO2's light absorption region and enhance the photocatalystic active have became one of the hot spot problem.Aim at the problem, the paper adopted the theoretical and experiment method to study TiO2.Firstly, study the electron structure property, intrinsic point defects and doping et al important problems by first principles calculation in this dissertation, on these base study the metal and nonmetal codoped TiO2, and found the metal and nonmetal codoped TiO2 are not only can broaden the visible light absorption region of TiO2, but also reduce the electrons and holes recombination ratio. Secondly, according to the result of theory, N-doped and V-doped and N/V codoped TiO2 samples were prepared by a sol-gel technology, measure and analyse these samples properties, the experimental results are in agreement with the theoretical results. The conclusions have outlined as follows:1,Seven known TiO2 polymorphs crystal structure are established and the structural optimizations are performed to obtain the equilibrium structural parameters, total energy, and electron energy band structure, et al, and further analyze their energy band, density of states and difference charge density. We have carried out the first-principles calculations for TiO2 polymorphs crystal structure, and find the PW91 project of GGA is excelled other projects. In these calculations, the energy band structure shows that fluorite-type TiO2 has a narrow band gap (only is 1.13eV) and high electrical conductivity for potential photocatalytic applications under visible light.2,The geometry structure and stability of intrinsic point defects for TiO2 are studied by first principles calculations. The results show the defect types and defect concentrations are related to the nonequilibrium growth condition.In general, under the O-rich condition, VTi would form spontaneously, and under the Ti-rich condition, Tij4+ and Vo easily appear in Schottky defects. To obtain highly effective photocatalysts, the incorporation of impurities in TiO2 has been made to modify the energy band structure. It is found that the formation energy of N-or C-doped TiO2 is high, resulting in the lower p-type dopant concentration,Easily appear a hight impurity band and male it become the center of electrons and holes recombination.3,In order to solve the electrons and holes recombination of doping TiO2, the codoped TiO2 with nonmetals and transition metals is more studied. We find that one codoping have great influence on the original geometry, namely, TM/NM defect pairs tend to bind to each, the other the energy band structures of codoped-TiO2 show that N/V and C/Cr codoping is superior to other codoped systems, due to the passivated donor-acceptor codoping on anatase TiO2 photocatalysts. Our results may help understanding synergistic effect of codoping approach for improving photoelectrochemical activity of anatase TiO2. The synergistic effect is come from as follows:first, metal ion doped TiO2 is advantaged to enhancing the concentration of nonmetal ion. Second, for the N/V and C/Cr codoping systems, the electrons on the impurity bands below the conduction band bottom are right compensate the holes above the valence band, which can availably reduce the electrons and holes recombination ratio. Third, the codoping make the impurity band to become shallow, this is also reduce the recombination ratio.4,N-doped TiO2 samples have prepared, we find that pure TiO2 normally undergoes anatase-to-rutile phase transformation in the temperature range of 600℃, but the rutile phase appeared is 700℃when N-doped TiO2, compared with the pure anatase TiO2, The absorbency of N-doped TiO2 in the visible range is more better, it's absorption band red shift is obviously. The visible light absorbency of the N-doped TiO2 will reduce form 400℃to 700℃by increasing the calcined temperature, the optimum temperature in our experiments is 400℃.The results indicate that the optical absorption edge obviously shifts to the visible light range with increasing N content. The N-doped TiO2 showed a significant absorption from 380 to 450nm compared with the non-doped TiO2. However, with further increasing N content, when the N content is larger than 3%,the absorption edges become blue shift,so the optimum N content is 3%.5,TiO2 film with different V contents were obtained by changing the addition of th Triethylamine, we find the absorb peak value is about 300nm for doping or non-doping TiO2 by testing with UV-Vis. The spectral responses of TiO2 powders shift to the visible light region after doping, and the absorption range increased, which confirm the theoretical results. The optimum doping concentration of vanadium is 1%.6,For the sake of comparing, N-, V-doped and N-V codoped nanosized TiO2 powder were prepared by a sol-gel technology in experiment. Further study their spectral absorption characteristics,we find that the spectral responses of TiO2 powders shift to the visible light region after doping, and the absorption region of N-V codoped TiO2 enlarge mostly,which confirm the theoretical results.7,Finally, we study a film structure Solar Cell.We measure its property of U-I, and find the cell have a good current property without light intensity, the Cell can produce a litte current and pressure with light.The innovative conclusions have outlined as follows:(1)Adopted the theoretical and experiment method to study TiO2 is very quickly confirm the aim and mean of doping TiO2; (2) Found the metal and nonmetal codoped can availably reduce the electrons and holes recombination ratio, advanced synergistic effect of codoping approach for improving photoelectrochemical activity of anatase TiO2 is great; (3)Prepared some TiO2's powders and films that they can absorb by the visible light, and explored the law that some parameter how to affect the light absorption region; (4) Prepared an double deck film structure Solar Cell, studied how to enhance the photoelectric conversion efficiency.