| The excessive utilization of fossil fuels has brought two major issues:energy crisis and environmental pollution. Looking for a new energy, which is clean and renewable, is an effective method to solve the two issues. Photocatalysis technique is to use the photogenerated electrons and holes after absorbing the sunlight of the semiconductor-based photocatalyst to split water into H2and O2. Hydrogen as the carrier of energy can avoid the environmental problem brought by utilization of traditional fossil fuels, because the production is water after hydrogen releasing the chemical energy. Thus photocatalytic water splitting is a new technology for the conversion and utilization of solar energy and has a potential prospect.The prerequisite for practical application of photocatalytic water splitting is to construct a photocatalytic reaction system which is highly efficient for converting solar energy into hydrogen energy. To explore the efficient photocatalytic materials is the key goal for achieving efficient solar conversion by photocatalysis. SrTiO3as one photocatalyst has attracted many attentions, because it has better character in the separation and transfer of photoinduced electrons and holes. The major shortcoming for SrTiO3is that it only responds to UV light (5%of natural sunlight), due to its large band gap (3.2eV). Such large band gap means the low utilization of solar energy. To broaden the region of light absorption of SrTiO3is the most important task for developing application-oriented SrTiO3-based photocatalysts. SrTiO3has excellent adjustable function, owing to that it has two cationic sites (Sr and Ti sites) and one anionic site which can be replaced by doping elements. This dissertation was mainly centered on visible-light-response SrTiO3based photocatalysts for water splitting: synthesizing element doped SrTiO3with better crystal quality by a sol-gel hydrothermal method, exploring the effect of element doping on crystal defects and the thermodynamic basis of elemental doping, broadening the region of optical absorption by using the interaction between atomic orbitals. The main conclusions are as follows:Sol-gel hydrothermal method is a good method to synthesize Cr-doped SrTiO3with high photocatalytic activity. Cr3+, which contains3d orbital electrons, can effectively broaden the optical absorption of SrTiO3into visible light region by forming a impurity level in the band gap. Cr3+-doped SrTiO3was usually synthesized by solid state reaction (SSR) at high temperatures. Samples synthesized by SSR had smaller specific surface areas and more crystal defects, thus their photocatalytic activities are low. Developing a method, which can synthesize samples with good crystallinity and large specific surface area, is benefit for obtaining high photocatalytic activities. In our dissertation, Cr-doped SrTiO3with good crystallinity and large specific surface area was obtained by a sol-gel hydrothermal method. The synthetic process is as follows:tetrabutyl titanate, strontium and chromium salts were dissolved in ethylene glycol, then the solution became gel at the suitable temperature. Under basic condition, the gel was transferred into Cr-doped SrTiO3with good crystallinity and larger surface areas by hydrothermal reaction. This method combines the advantages of sol-gel and hydrothermal method:1) Precursors were mixed atomically during forming the sol and gel, which is benefit to achieve the uniformly doping.2) Under basic condition, Sr2+,Cr3+and Ti4+exist as metal hydroxide. This is benefit to decrease the difference in reactivity between metal containing precursors, thus obtaining the purity-phase, high crystalline Cr-doped SrTiO3after hydrothermal treatment. High crystallinity can decrease the amount of crystal defects, thus suppressing the recombination of electrons with holes. Large specific surface area enhances the photocatalytic activity through providing more active sites. The optimized hydrogen evolution rate over Cr-doped SrTiO3synthesized by sol-gel hydrothermal method is82.6μmol/h.Codoping SrTiO3with a cantion and an anion can enhance the photocatalytic activities by decreasing the amount of vacancy defects. For Cr-doped SrTiO3, in order to keep the charge balance in SrTiO3, Sr vacancies were formed in the lattice of SrTiO3due to the difference in valance state of Cr3+and Sr2+. Sr vacancies as recombination sites can enhance the recombination of photogenerated electrons and holes, thus decreasing the photocatalytic activities. In our dissertation, element codoping not only can keep the charge balance to reduce the Sr vacancies, but also broaden the region of optical absorption. Cr3+,N3"-codoped SrTiO3was successfully synthesized by the sol-gel hydrothermal method using urea as the N source. The experimental analysis and theoretical calculation showed that the doping of Cr into crystal lattice of SrTiO3can decrease the reaction barrier of doping N into crystal lattice of SrTiO3. Substituting N3-and Cr3+for O2-and Sr2+in SrTiO3can reduce the amount of Sr vacancies due to the charge balance, thus decreasing the chance of recombination between photogenerated electrons and holes. The p-d repulsion between N2p and Cr3d can decrease the band gap by enhancing position of Cr3d level and broaden the region of optical absorption for SrTiO3. Cr,N-codoped SrTiO3has higher photocatalytic activity than Cr-doped SrTiO3, due to the reduction of Sr vacancies and the enhancement of visible light absorption. The optimized hydrogen evolution rate over Cr,N-codoped SrTiO3is106.7μmol/h. The IPCE is3.1%at420nm.The p-d repulsion between p orbits of anion and d orbits of cation can broaden the optical absorption region by enhancing the position of impurity level. The intensity of p-d repulsion is related to the difference of orbital energy between p orbits and d orbits. The smaller difference between orbital energy means the stronger p-d repulsion. The strong p-d repulsion will induce the enhancement of impurity level Comparing the difference of orbital energy for N2p with Cr3d, the difference of orbital energy for B2p with Cr3d is smaller, meaning that the interaction between B2p and Cr3d will be stronger. In our dissertation, Cr,B-codoped SrTiO3was synthesized by a hydrothermal method using TiB2as the precursor of B. For Cr,B-codoped SrTiO3, to keep chare balance, the Cr3+and B-was incorporated into the Ti4+and O2-site of SrTiO3, respectively. UV-Vis spectra show that the absorption edge for Cr,B-codoped SrTiO3was shifted to600nm, corresponding to the band gap of2.07eV. Theoretical calculation indicates that the strong p-d repulsion of B2p and Cr3d levels was responsible for the red shift of absorption edge by enhancing the position of Cr3d level. Cr,B-codoped SrTiO3photocatalyst exhibits higher photocatalytic activities (15.4μmol/h) for hydrogen production than Cr-doped SrTiO3(9.3μmol/h), owing to the enhancement of visible light absorption. |
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