Selective Heteroepitaxy On TiO₂ Crystal Facet And Single Particle Study Of Interfacial Charge Transfer Mechanism

In the 21st century,with the theme of "green and low-carbon",photocatalysis is considered to be one of the most promising novel green technologies for achieving sustainable energy and environmental development,due to its advantages of high efficiency,environmental protection,and low cost.However,in the process of photochemical conversion,numerous problems,such as low efficiency of photogenerated carrier separation/transport and unclear photocatalytic reaction mechanisms,greatly restrict the further improvement of photocatalytic activity.Therefore,it is urgent to explore and seek the internal factors that affect the photocatalytic conversion efficiency.Numerous studies have demonstrated that the generation and transfer of photogenerated charges is the key to determining the efficiency of photocatalytic conversion.Exploring and studying the kinetic behavior of photogenerated carrier on surface-interface transfer is crucial for deeply understanding of the relationship between material structure and catalytic activity,optimizing material design,and developing photocatalysts with high activity and stability.In recent years,researchers have developed various strategies to explore and improve the efficiency of photocatalysis charge transfer.Among them,constructing heterojunctions is an effective method to improve carrier separation efficiency.However,the formation of heterogeneous interfaces inevitably generates numerous defect sites,which to some extent promotes the recombination of charge carriers.Selective construction of epitaxial heterojunctions on specific crystal facets can not only reduce interfacial defects,but also promote directional carrier transfer.Meanwhile,in order to in-depth understanding of the structure-activity relationship of photocatalytic materials,it is particularly important to accurately explore the carrier transfer mechanism at the epitaxial heterogeneous interface at the micro/nano scale.However,conventional research methods typically average the detection of a large number of catalyst particles as a whole,while not being able to study the interface carrier transfer mechanism at the micro/nano scale,nor to visualize the active sites,which greatly limits the understanding of the interface charge transfer mechanism.Dynamic monitoring of carrier separation behavior at the surface-interface of catalysts at the single particle level is conducive to insight the key roles of various active sites of materials in photocatalytic process,which is of important guiding significance for in-depth understanding of the structure-activity relationship of photocatalytic materials,and for designing and optimizing heterojunction materials,thereby enhancing photocatalytic activity.Therefore,in this thesis,in order to solve the problems of low efficiency of photocatalytic carrier separation and unclear mechanism of photocatalytic charge transfer,we propose to selectively construct epitaxial heterojunctions on TiO2-{001} facet by a topological epitaxy strategy.The coupling effect of crystal facet engineering and epitaxial heterojunctions is beneficial to reducing interface defects and promoting directional carrier transfer.Furthermore,the piezoelectric effect was introduced into the reaction system to study the effect of selective construction of heterojunctions on piezo-photocatalytic activity.More importantly,in these thesis,carrier transfer and recombination processes in different micro-regions of the heterojunction interface are in situ monitoring and imaging study by single particle PL technology,thereby revealing the charge transfer mechanism at the heterojunction interface.The specific content is as follows:In chapter 1,the basic principles,application fields,and photocatalytic materials of photocatalysis are briefly reviewed,and strategies for promoting photocatalytic charge separation are systematically introduced from both structural regulation and piezoelectric assistance.Subsequently,research methods for charge transfer mechanisms in both macro and micro/nano scale photocatalysis processes are described,with emphasis on the advantages and current research status of single particle PL technology in studying charge transfer mechanisms.Finally,the significance and research content of this paper are introduced.In chapter 2,a strategy of selective topological epitaxy was employed to improve the driving force of interfacial carrier separation,and the single particle fluorescence technique was used to in-situ monitor the interfacial charge transfer-recombination process.Firstly,a highly ordered SrTiO3 mesomorphic selective epitaxial growth on the TiO2-{001} crystal facet was achieved by an in situ topological chemical transformation strategy.Photocatalytic testing demonstrates that tetracycline degradation activity of SrTiO3/TiO2 heterojunction is much higher than that of TiO2.Secondly,the charge transfer and recombination behavior in specific microregions of heterojunctions were further monitored by single-particle PL spectroscopy,and the degradation process of tetracycline at single particle level was visually studied in situ.The results indicated that epitaxial heterojunctions can effectively inhibit the decay of PL lifetime during the degradation of tetracycline,further illustrating the key role of selective construction of epitaxial heterojunctions on specific crystal facet.In chapter 3,the piezoelectric effect was employed to photocatalytic system to investigate the effect of selective construction of heterojunctions on piezo-photocatalytic carrier separation.TiO2-{001}@BaTiO3 heterojunction was constructed by topological epitaxial strategy.The lattice stress on heterojunction realized the transformation of BaTiO3 from cubic to tetragonal,and maintained the spontaneous polarization with consistent orientation.At the interface of epitaxial heterojunction,the built-in piezoelectric field provided by BaTiO3,crystal facet effect of TiO2 and type Ⅱ heterojunction synergistically promote the effective separation of photogenerated electrons/holes,and achieving efficient hydrogen production by pure watersplitting and degradation of tetracycline.Moreover,single-particle PL spectroscopy further demonstrated that the coupling effect of epitaxial heterojunction and crystal facet engineering can effectively promote the directional transfer of carriers,which is conducive to improving catalytic activity.In chapter 4,the research content of this paper is summarized,and the main innovation points of this paper are analyzed.In view of the shortcomings of this paper,further study plans are proposed,and future work is prospected.