First Principle Study Of TiO₂ Surface And Interface Property

TiO₂ is an inexpensive,non-toxic metal oxide with applications in a wide range of areas,such as organic pollutants degradation,photocatalysis and self-cleaning materials.Most of the reactions associated with the application occur at their solid-liquid,solidgas interfaces,and their interfacial properties are critical to understanding the mechanism of reaction at the atomic scale.In this thesis,we use the method of density functional theory to study the adsorption of small molecules on the surface of TiO₂ and their interactions with the surface active sites,especially the surface oxygen vacancies.This thesis is divided into six chapters:Chapter 1 introduces the application of TiO₂,the crystal structure,the electronic structure and common surface defects and their effects.Chapter 2 introduces the theoretical background used in this thesis,including density functional theory and energy band theory.In chapter 3,the interaction between water molecules and oxygen vacancy on anatase?101?surface is studied.The adsorption of water molecules reduces the relative energy difference between the surface and sub-surface oxygen vacancy,improves the stability of the surface oxygen vacancy defects.The adsorbed water induces the migration of oxygen vacancies from the subsurface to the surface,which promotes the dissociation of water molecules,and ultimately the formation of surface bridge hydroxyl species.Our findings help to understand and explain the higher catalytic activity of the anatase?101?surface in the water environment and increase our understanding of the micro-scale of the metal oxide-solid interface.In chapter 4,the carboxyl group induced anatase?101?surface oxygen vacancy is studied.We selected formic acid and benzoic acid as the model system to study the interaction between the carboxyl groups and the oxygen vacancies as carboxyl group is a commonly used anchor group in dye-sensitive solar cells.For the two molecules,the relative stability of the subsurface oxygen vacancy and the surface oxygen vacancy is reversed after the adsorption on the anatase?101?surface,and the surface oxygen vacancy is energetically more stable.The migration barrier of the oxygen vacancy from the subsurface to the surface is also significantly reduced to 0.16 eV,which means that the oxygen vacancy can migrate from the subsurface to the surface site at low temperatures.By means of the first-principles molecular dynamics calculations,we found that oxygen vacancies migration process can happen in a few picoseconds simulation.This study will help to further understand the interfacial properties of dyeTiO₂ system.In chapter 5,we studied the evolution of active site of the anatase?101?surface under ambient conditions.We chose methanol and water molecules,which are common reactant in photocatalytic reaction,as well as nitrogen and hydrogen molecules,which interact very weak with the surface to study their interaction with the oxygen vacancy with joint experimental and theoretical efforts.We found that not only water can affect the relative stability of oxygen vacancies,methanol and even nitrogen,hydrogen molecules can also affect the stability of oxygen vacancy.As the surface oxygen vacancy concentration is a very important factor affecting the catalytic performance of the TiO₂,this study can help people understanding dynamic interface of TiO₂ during reaction process.In chapter 6,we studied the adsorption of methanol on rutile?110?and anatase?101?surface.It was found that when the coverage was less than 0.5 ML,the methanol adsorbed on the low-coordinated Ti site and forming hydrogen bonds with the surface.When the coverage is greater than 0.5 ML,the methanol tends to adsorb on the second layer,stabilized by the hydrogen bonds between methanol-methanol and methanolsurface.Methanol shows similar adsorption energy on both surfaces,so the difference in catalytic activity between the two surfaces may come from other factors such as the nature of the electronic structure and the distribution of defects.In chapter 7,we summarized this thesis.We found that the surface of TiO₂ can react dynamically upon external stimuli.These findings is critical in understanding the reactivity of Ti O2,and can shield lights on the study of metal oxide interface properties.