Microstructure Evolution And Photocatalytic Mechanism Of TiO₂ Based Photocatalyst

In the new era of promoting green energy conservation,environmental pollution and resource shortage are the difficult problems to be solved in the world.Photocatalysis technology can use solar energy skillfully and break the limitation of traditional thermal catalysis technology.It opens up a new way to solve the problem of global resources and environment.Titanium dioxide is one of the most important and widely used semiconductor oxide photocatalysts among the currently reported nano photocatalyst materials.However,most applications are still in the experimental stage.Limited catalytic activity and stability greatly limit its industrial application.In recent decades,researchers have been committed to improving the preparation process to continuously optimize the performance of TiO₂-based materials.However,due to the lack of understanding of the core issue of the relationship between the microstructure and charge transfer of photocatalyst materials,the research in this field has entered a bottleneck period,it is necessary to understand the mechanism of photocatalysis at micro-scale and further explore the preparation process to achieve a possible breakthrough in application.In this thesis,a series of TiO₂-based photocatalysts were prepared by systematically introducing surface oxygen vacancy doping,supported Au nanoparticles and supported Ag₂O nanoparticles,by using HRTEM,HAADF,EELS and Photo-KPFM,the surface and interface structure,chemical valence,charge distribution and the influence of light on surface potential were systematically studied with high spatial resolution,the effects of oxygen vacancy on photocatalytic activity,the coupling of Au nanoparticles with oxygen vacancy to enhance charge transfer,the size effect of Au nanoparticles in the presence of oxygen vacancy,and the microstructure evolution of the supported Ag₂O nanoparticles during photocatalytic reaction were revealed,it provides a theoretical basis and a new perspective for the development of efficient photocatalysts.（1）The effect of oxygen vacancy content on the microstructure and properties of TiO₂:a series of TiO₂ photocatalysts with different oxygen vacancy content Ti⁴⁺/Ti³⁺core-shell structure were prepared by reduction in hydrogen atmosphere at high temperature.With the increase of reduction temperature,the thickness of Ti³⁺ shell increases and the O/Ti atom ratio of outermost layer decreases.On the one hand,the introduction of oxygen vacancy reduces the space between the crystal planes,which results in certain surface strain effect and increases the carrier mobility.On the other hand,the increase of oxygen vacancy leads to the increase of Fermi energy level and the photocarrier concentration,which is beneficial to the improvement of photocatalytic performance.However,excess oxygen vacancy will increase the complex center and decrease the concentration of photocarriers.Finally,the optimal balance of oxygen vacancy and particle size was achieved in the 700℃ reduction sample,and the rate of photocatalytic hydrogen production was as high as 14.7 mmol/L.（2）Oxygen vacancy and Au nanoparticles coupling enhance Au-TiO_2-x interfacial charge transfer:TiO₂,TiO_2-x,Au-TiO₂ and Au-TiO_2-x photocatalytic systems were designed,the phenomenon of charge separation induced by localized surface plasmon resonance（LSPR）in interface defect structures is revealed at microscale.The results show that the oxygen vacancy regulates the electronic state at the interface of the plasma metal semiconductor system,and the amorphous interface layer plays a role of local charge transfer and Schottky barrier,it is helpful for the Au nanoparticles to excite more hot electrons and have a greater transition probability to the surface of TiO_2-x.A large number of electrons were transferred from TiO_2-x to Au in the dark due to the coupling between oxygen vacancy and Au,and the hot electrons excited on Au surface were transferred to TiO_2-x surface in the visible light for photocatalytic reaction.Finally,the“Pole vault”effect is proposed to explain the charge transfer mechanism at the interface,which is useful for the development and design of other catalysts.（3）The effect of the size of Au nanoparticles on the structure and properties of Au-TiO_2-x:a series of Au-TiO_2-xcatalysts with different Au particle sizes were prepared by changing the loading of Au.The Au-TiO_2-x catalysts were used for photodegradation of methylene blue.The Au-TiO_2-x with the smallest Au particle size has the best photocatalytic activity.With the increase of Au particle size,the LSPR intensity increases gradually.But the relaxation mode of LSPR is changed from non-radiation relaxation to radiation relaxation,that is,most of energy is converted into heat energy and light energy.At the same time,the absorption area,interface contact area and the number of reaction sites decreased with the increase of Au particle size.The results show that,although the size effect determines the overall photoelectric properties of the noble metal/semiconductor photocatalysis system,it is essentially controlled by the interface structure and charge transfer mechanism of the plasma metal/semiconductor system,which affects the photocatalytic activity.（4）Study on the interfacial structure and cycling performance of Ag₂O/TiO₂heterojunction photocatalyst:the Ag₂O/TiO₂heterojunction photocatalyst was prepared by precipitation and hydrothermal method,and the methylene blue dye was degraded continuously under visible light for five times.It was found that the photocatalysis performance was improved obviously at the beginning of the second round,and then remained stable,which was always higher than that of the first round.Due to the light instability of Ag₂O,the Ag₂O@Ag core-shell structure is formed when Ag₂O is irradiated by visible light.Therefore,the introduction of the light source not only promotes the redox at the interface,but also reconstructs the structure through the interface control,finally forms the relatively stable Ag₂O@Ag-TiO_2-x composite structure,enhances the photocatalytic performance greatly.