The Study Of The Ability Of Improving The Light Absorption And Facilitating The Photogenerated Carrier Separation In TiO₂ And Bi₂WO₆

Photocatalytic technology exhibits a great potential in solution of current energy crisis and environmental pollution. It can not only split water into hydrogen and but also degrade pollutants in the air and water by havesting the solar energy. In recent years, a great effort has been devoted in developing photocatalytic materials. However, current photocatalytic materials can not be widely used in applications due to their narrow solar spectrum response and low quantum efficiency. Therefore, it is necessary to enhance the absorption in the visible spectral range and suppressing recombination of photogenerated carriers for improving quantum production efficiency, which plays a key role in realizing social sustainable development.In this doctoral dissertation, doping and forming heterojunctions have been used to improve the photocatalytic performance of conventional TiO₂ and novel Bi₂WO₆, and we try to reveal the mechanism of extending the visible light absorption, facilitating the photogenerated carrier separation, and improving photocatalytic activity induced by modifying. This dissertation includes six chapters:In Chapter 1, we firstly introduced the research background of the semiconductor photocatslytic technology. Then, recent advances in TiO₂ and visible light Bi-based photocatalysts were reviewed. Lastly, we summarize the research contents of this doctorial dissertation.In Chapter 2, the methods used in this doctorial dissertation were briefly introduced:（i） Preparation and characterization in the experiment, （ii） The first principles calculations method based on the density functional theory （DFT） and the programs employed in this work.In Chapter 3, we studied on the synergistic effect of V and single-electron-trapped oxygen vacancy （Vo） enhanced photocatalytic oxidateion of propylene in V-doped TiO₂-Vo. Using nanotube titanic acid （H2Ti2O4（OH）2, NTA） as titanium precursor and ammonia metavanadate （NH4VO3） as V source, the novel V-doped TiO₂-Vo were prepared via a facile solid state sintering method. The stability and electronic structure of V-doped TiO₂-Vo’were also studied based on the DFT calculatied results. The crystal structure, crystallization, valence and composition, specificsurface area, optical absorption and photocatalytic activity of V-doped TiO₂-Vo’by experimental characterization techniques including Raman spectra, TEM, XPS, ESR, UV-vis DRS, and so on. The result showed that some V5+ions were reduced to V4+by Vo· and then were introduced into TiO₂ lattice, and the others exist on the surface in the form of V2O5. It is found that 1%V-TiO₂-Vo’at 500 ℃ displays remarkable photocatalytic activity for degradation of propylene under visible-light rradiation, which is much better than those of as-prepared NTA, P25-TiO₂, N-TiO₂-Vo·, and V-doped P25-TiO₂. The remarkable improved photocatalytic activity of the V-TiO₂-Vo may come from the synergistic effect:（ⅰ） the increased visible light absorption by the narrowing of band gap originated from the appearance of new states （around the top of the valence bands, due to O 2p and V 3d orbitals; around the bottom of the conduction band, due to Ti 3d, O 2p, and V 3d orbitals）. （ⅱ） the efficient separation and transfer of the photogenerated e-/h+pairs due to the presence of V5+/V4+ redox couple.In Chapter 4, the effects on the photocatalytic activity were studied by the introducing Zn to replacing the Bi lattice sites in Bi₂WO₆. The geometry structure, stability, electronic structure, band edge positions, and photocatalytic reaction rate (R, R （R ∝ QE · α） of Zn-doped Bi₂WO₆ were studied by a first-principles calculation. The results showed that Bi1.75Zn0.25WO6 is the most stable. In extending the visible light absorption, facilitating the separation of photogenerated carriers, and improving R for Bi1.75Zn0.25WO6, the possible advantages induce:Firstly, Bi1.75Zn0.25WO6 possesses a unique layered structure with large different carrier mobility along different orientations, including its high QE. Secondly, the stereochemically active Bi lone pair effect at the top of valence bands is enhanced, which is responsible for decreasing in the band gap and the large increasing the state density of electrons in the valence band maximum. Thus, Bi1.75Zn0.25WO6 can absorb a large amount of photons and exhibit enhanced visible-light α. Thirdly, the valence band and conduction band band edges of Bi₂WO₆ are slightly shifted upwards due to Zn doping, which induces that photogenerated electrons are energetic enough to react with molecular oxygen to form active radicals （ · O2- and · OH）, and inhibit carrier recombination.In Chapter 5, the first-principles calculations were carried out to explore the interfacial properties and the mechanism of enhanced photocatalytic activity for graphene/Bi₂WO₆ （GR/BWO） heterojunction. We studied that the stability of the BWO （010） surface with different terminal slab and of the GR/BWO （010） interface. The electronic properties, charge transfer, and visible-light response were further investigated in detail on the BWO （010） surface coupled with GR. An analysis of charge distribution and Bader charge shows that there is a strong covalent bonding between GR and the BWO （010） surface. The covalent interaction induces a small bandgap in GR. The interband transition of graphene and the surface states of the BWO （010） surface would cause the absorption spectrum of GR/BWO （010） to cover the entire visible-light region and even the infrared-light region. The photogenerated electrons flow to GR from the conduction band of BWO under the built-in electric field and band edge potential well. Thus, GR serves as a photogenerated electron collector and transporter which significantly reduces the probability of electron-hole recombination and increases catalytic reaction sites not only on the surface of GR but on also the surface of BWO. The decrease of charge recombination is possibly responsible for the enhancement of the visible-light photocatalytic activity of the GR/BWO （010） nanocomposite. These results shed light on the benefits for the utilization of low-cost GR as a substitute for noble metal deposited on photocatalyst to develop high-performance photcatalyst.In Chapter 6, we summarized that the main conclusions of this dissertation and proposed some possible research for achieving high photocatalytic performance.