| Energy and environment are two of the biggest crises of modern society, solar energy can be converted into chemical energy via photocatalytic reaction(for example, photosplit H2O into H2and O2), and photocatalytic decomposition of pollutants can purify the environment, using proper photocatalysts to take advantage of solar energy is one of the important solutions to the two problems. TiO2is the most extensively studied and applied photocatalysis nowadays, but most of the studies are experimental and mainly focused on macro-mechanism of the reaction. To achieve a fundamental understanding of the reaction mechanism and improving the performance of the photocatalyst, it is essential to understand the basic processes of photocatalytic reactions at molecular level. In the work of this thesis, we combine the scanning tunneling microscopy and first-principle calculations to explore the mechanisms of some basic photocatalytic processes and reactions on rutile TiO2(110) surface. We have studied the adsorption of O2on the surface, the mechanism of CO photooxidation and H2O photodecomposition. These reactions are very important from the energy and environmental point of view, and the basic processes involved are also vital for the understanding of other photocatalytic reactions.In chapter1, we will give a brief introduction about the basic properties of TiO2, including the geometry and electronic structure of it; then we give a short historical review of TiO2as photocatalyst and its basic principles, some general issues, from the generation of charge carries (photogenerated electron and hole) to the reaction of molecules, are discussed especially the trapping of charge carriers. Finally, we shortly introduce the scanning tunneling microscope (STM) and density functional theory.In chapter2, the adsorption of O2at the oxygen vacancy on the surface is observed directly from in situ STM experiment. From the calculation and simulation of STM image, we confirm the adsorption geometry of the molecule. It is found that the molecule adsorbed at vacancy will undergo dissociation induced by the inelastic electron tunneling, whereas the molecule adsorbed at bridge hydroxyl is much more stable. From the calculation of potential energy surface, we find that the adsorption and dissociation of O2molecule will consume the excess electrons of the system, this can explain the behavior of O2molecules at high coverage, for example, the number of O2molecule that could dissociate is at most half the number of the vacancy.In chapter3, the mechanism for the CO photooxidation is studied. The CO photooxiation is found to be a two-step process. The O2molecule first captures a hole and transforms itself to a near perpendicular geometry, and then captures a electron and connects to the CO molecule, forming a O-O-CO complex which will convert to CO2easily. This molecular mechanism is applicable to both the low and high O2coverage cases and agrees well with the desorption behavior of CO2in recent experiments.In chapter4, the nature of trapped hole on TiO2(110) surface is explored. It is found that the bridge oxygen on is the most favorite trapping site of hole and the trapped hole orbital has the feature of the p orbital of O atom. The reaction of physisorbed H2O molecule with the trapped hole is also studied, when H2O molecule approaches the trapped hole, its highest occupied molecular orbital(HOMO) will hybridize with the trapped hole orbital, and the hole is transferred to H2O molecule via the anti-bonding orbital, leading to the oxidation and decomposition of the molecule.In chapter5, the photodissociation of H2O molecule adsorbed on the surface is observed from in situ STM experiments under UV light illumination. By comparing with the dissociation process of H2O molecule reduced via the injection of electron, we conclude that the photodecomposition of the molecule is a oxidizing process. To understand the oxidation reaction, we perform first-principle calculations on the reaction of H2O molecule with both free and trapped hole. The results indicate that it is most probably mediated by free hole.Chapter6is a summary of the work in this thesis and perspective on the future work. In a word, our study have involved many basic processes and reactions on TiO2(110) surface. Most importantly, we have set up a systemic method for the photocatalytic reactions. It could be used to investigate other photocatalytic processes and reactions and could also be generalized for the study on other transition metal oxide surfaces... |
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