Photocatalytic Properties Of Rutile TiO₂ And LiNbO₃ Low-index Surfaces

Environmental pollutions and energy crisis are the two problems confronted with the world nowadays. The direct conversion of abundant and widespread solar energy into clean and efficient hydrogen energy via photocatalytic water splitting represents an attractive way to solve the ever-increasingly serious energy crisis and environmental problems. Many applications, such as heterogeneous catalysis, energy conversion and storage, are very sensitive to surface atomic structures. Therefore, understanding the mechanism of reactions on crystal surfaces is important for improving catalysts’physicochemical properties and thus synthesizing ideal photocatalysts.Recently, intensive experimental studies have centered on the reactivity of different faces of metal oxides. Most of them are based on the film or powder systems and photochemical reductions of metal ions to metal particles from an aqueous solution. However, there are two fundamental problems in these studies. Firstly, varieties of crystal faces are concurrently existed in the film or powder system. And distinct reaction condition must be found on each surface, which makes it impossible to exclude influences brought by inconsistent reaction conditions, synergistic effect, etc. Secondly, the measurement of metal ion reduction rate on a surface often involves a large variability of reported thicknesses, and such measurement should be considered as qualitative indicators that may vary drastically with experimental conditions.To address the aforementioned issues and simulate real photocatalytic reaction conditions, single crystal wafers were employed in present study. Single crystal wafers have well-ordered structures and known surface area, which provide model systems for surface studies. Photocatalytic hydrogen evolution and photoelectrochemical current density, two quantitative results, are used to determine the photocatalytic capability. Additionally, we adopted the method of density functional theory calculations to systematically study the surface properties of photocatalysts, and their interaction with water. Based on the calculated results, we provide reasonable explanations and underlying mechanism for experimental phenomena.In this dissertation, we aim to study the anisotropy in photocatalytic capabilities of rutile TiO₂ and LiNbO₃ low-index surfaces, by studying the reaction mechanism of photocatalytic and photoelectrochemical water splitting. We also provide a new method to studying the effect of crystal orientation and ferroelectric polarization on photocatalysis, in order to provide experimental results and theoretical explanation for the future development of solar energy conversion. The main research contents are as follows:Crystal orientation effect on photocatalytic properties of rutile TiO₂ surfaces and its underlying mechanism.In present study, we studied photocatalytic properties of rutile TiO₂ （100）, （110） and （001） faces. A huge anisotropy in photocatalytic capability is observed. Consideration of both photocatalytic hydrogen evolution and photoelectrochemical current density, the photocatalytic ability order of rutile TiO₂ （100）, （110）, and （001） faces is （001）> （100）> （110）. It is found that the light absorption, band energy position and bulk diffusion of differently oriented rutile TiO₂ surfaces are identical. We calculated the adsorption behavior and decomposing pathway of water on three low-index rutile TiO₂ surfaces. According to the analysis of surface atom structure and the composing of activity energy, we found some factors that determine the water-splitting process on rutile TiO₂ surfaces, and relationship between surface properties and surface activity.Effect of ferroelectric polarization on Lithium Niobate photocatalytic properties.In present study, we studied photocatalytic properties of LiNbO₃ c+ and c- faces. According to the convention used in previous studies, the c+ face is the one with positive ferroelectric polarization, the c-face is the one with negative ferroelectric polarization. A huge anisotropy in photocatalytic capability is observed. The photocatalytic H2 evolution of c+face was triple that of c-face. The underlying mechanism is that the downward bent band at c+face facilitate photoexcited electrons to transport towards cocatalyst Pt to reduce water.