Metal Oxide Semiconductor-based Photocatalytic Electron/Proton Transfer Mechanism And Reaction Behavior

The environmental and energy crises are two key problems in the development of human society. How to achieve the sustainable development and to minimize the impact of economy development on the ecological environment has become the focus of modern scientific research. The photocatalysis technique, using solar energy to induce variety of photochemical redox reactions in applications of green energy transformation, selective organic synthesis and environmental remediation, is a clean,pollution-free environmental protection technology, which meets the needs of sustainable development. Due to the low cost, high-efficiency, stability and the environmental friendly character, metal oxide semiconductor （e.g. TiO₂） has become one of the most widely used semiconductor photocatalyst for now. Although the efficiency of metal oxide semiconductor photocatalysts in solar energy transformation and environmental remediation has been improved through constant attempts, the very nature of specific photochemical reactions and photo-induced charge transfer mechanism still remain unclear, which is the bottleneck to develop the stable high-efficient metal oxide for practical application.In this thesis, we focused on the profound investigation of the proton/electron transfer （PT/ET） mechanism and the subsequent chemical reaction behavior of metal oxide semiconductor photocatalysts （e.g. InVO₄ and TiO₂） induced photochemical reactions, in order to explore the very nature of photochemical reactions, the rate-determining steps and key factors that affects the reaction efficiency. By doing this, we tentatively propose a possible strategy to develop the stable high-efficient metal oxide semiconductor photocatalysts. The main work includes three aspects:（1） Preparation of InVO₄ micro-/nano- photocatalysts and their photocatalytic activitiesa. Through the organic additives-free hydrothermal synthetic method, we successfully prepared 3D InVO₄ microsphere superstructure photocatalyst. The size of 3D InVO₄ microsphere is 3～5μm,which is composed of 50 nm sized nanocrystals as building blocks. During the hydrothermal synthetic process, both the pH value of precursor and reaction time are very important to the crystallization and morphology evolution of 3D InVO₄ microsphere superstructure. After loaded with Pt nanocrystals,the 3D InVO₄ microsphere superstructure photocatalyst exhibited a CIP degradation ratio of 98.2% in 1 hr and the hydrogen production rate of 9.76 μmol·h-1·g-1 under visible-light irradiation.b. Through a facile microwave-assisted method, we prepared the 20 nm-sized InVO₄ nannocrystal photocatalyst, which is the smallest InVO₄ crystal photocatalyst has ever been reported. Due to the influence of quantum size effect, the light absorption edge of InVO₄ nannocrystal photocatalyst was blue-shifted compared to the large-sized InVO₄ microspheres （3～5μm）, the band gap energy was changed to 3.0 eV, increased about 0.5 eV. As-prepared InVO₄ nannocrystal photocatalyst exhibited higher photocatalytic hydrogen production rate （14.16 μmol·h-1·g-1） than that of InVO₄ microspheres （9 μmol·h-1·g-1） under visible-light.c. Through one-pot microwave-assisted method, we prepared Ni²⁺-doped InVO₄ nanocrystal photocatalyst with the size of 20 nm and the large surface area of 40～45 m2/g. By adjusting the Ni-doping amount （0～20%）, we achieved the subtle control over the band gap energy of as-perpared Ni²⁺-doped InVO₄ nanocrystal photocatalyst（from 3.4 eV to 2.5 eV） without the change of crystalline structure and morphology.Correspondingly, the visible-light-driven photocatalytic O₂ production activity of Ni-InVO₄ photocatalyst was increased. 1% Ni doped sample exhibited the highest activity with the O₂ production rate of 394μmmol·h-1·g-1, which is 66% higher than the pristine sample.（2） Anatase/ graphene/rutile TiO₂ heterojunction photocatalytic system and the enhanced photocatalytic H2 production activityBy using RGO as the interfacial ET media, we construct a tunable Anatase/RGO/Rutile surface phase junction photocatalytic system. Through EIS analysis, we confirm that the introducing of RGO as interfacial ET media has greatly improved the ET efficiency between the Anatase and Rutile junction. By evaluating the efficiency of H2 production from water splitting under UV of samples with different A/R ratios and RGO amount, we confirm that A/R=7:3 and 2% RGO are the optimal A/R ratio and RGO amount in our system. Thus, we make clear the promotion effect of RGO as the interfacial ET media on the photocatalytic HZ production efficiency.（3） ET/PT mechanism in liquid/TiO₂ photocatalytic systemsa. We investigate the effect of Ti-H formation on the efficiency of TiO₂ based dye-sensitized devices. By using isotopically labeled MAS-1H NMR,ATR-FTIR spectroscopy （separate H/D and 48Ti/49Ti experiments） and ESR, we revealed that the accumulative formation of Ti-H species on the TiO₂ surface is the intrinsic cause of the PCE failure of TiO₂-based dye-sensitization devices （i.e. N719-DSSC）. Such a Ti-H species is generated from the reduction of hydrogen ions （mostly released from dye carboxyl groups or organic electrolyte） accompanied with electron injection on the surface of TiO₂, which deteriorates the PCE mainly by reducing the electrical conductivity of the TiO₂ and the hydrophilic nature of the TiO₂ surface.b. We investigate the effect of reaction chemical environment on the electron trap states on TiO₂. Through in-situ UV-vis, ATR-FTIR, liquid He （4K） ESR and SR-UPS,we confirm that the surface proton presence has important influence on the photo-induced electron trap Ti-sites and the subsequent ET reaction on TiO₂. In the protic system （methanol as sacrifice reagent）, photo-induced electrons were selectively trapped at the four-coordinated tetrahedral Ti4c defect sites （e-tetra,Ti3d binding energy 1.3 eV）; conversely, in the proton-free case （I- as electron donor）,photo-induced electrons were trapped at the anatase surface octahedral Ti6c sites （e’octa,Ti3d binding energy 1.8 eV）. By adjusting the proton concentration （pH from 1 to 13）of the suspending system, we achieved the mutual conversion between these two types of electron trap Ti-sites on TiO₂ from one to the other （e-octa/e-tetra≈3.4: 1～8.7:1）. In comparison between these two types of trapped electrons （e-octa and e-tetra） in the ET/PCET reactions in dark, we found e-tetra is favorable to both ET (Fe3+ + e-→Fe²⁺)and PCET （tBuArO·e-/H+ →tBuArOH） reactions, and e-octa can only induce ET reaction. This work is the first to clarify that the chemical environment of the solution can control over the photo-induced electron trap sites and states on the n-type semiconductor TiO₂, which inspires approaches to achieve high efficient TiO₂-based solar energy conversion devices by constructing different tailored Ti sites （defect or refined crystal surface sites） as electron trap hot spots for different target ET/PCET reactions.