Defect Design, Chemical Synthesis and Associated Properties of Multifunctional TiO2-Based Nanocrystals
Creators: name: Sun, Qingbo
Publisher: name: Canberra, ACT : The Australian National University
Year: 2017
Subjects: solvothermal
titanium dioxide
co-doping
photocatalysts
colossal permittivity materials

description: Local defect structures are significant to determine material properties since defects introduced into host materials would affect the local/average crystal environments and thus lead to a change of macroscopic physicochemical performances. The intentional design of specific local defects not only depends on the selected synthesis method and preparation process but also relies on the selected dopant or co-dopant ions. A deep understanding of the intrinsic relationships between local defect structures, chemical synthesis and associated properties is thought as one major framework of material genome plan. It also pushes the design, development and application of novel multifunctional materials. Based on local defect structural design coupled with new synthesis strategies, indium and niobium co-doped anatase titanium oxide nanocrystals are synthesized. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergistic incorporation of indium difficult-dopants and niobium easy-dopants, and theoretically evidenced that the local defect structures created by indium, niobium co-dopants, reduced titanium and oxygen vacancies are composed of defect clusters and defect pairs. These introduced local defect structures act as nucleation centres of baddeleyite- and lead oxide-like metastable polymorphic phases and induce an abnormal trans-regime structural transition of co-doped anatase titanium oxide nanocrystals under high pressure. Furthermore, these small co-doped nanocrystals can be used as raw materials to manufacture titania-based ceramic capacitors designed in terms of electron-pinned defect dipole mechanism. The sintering temperature is thus lowered to 1200 °C, which conquers the technological bottleneck using this material. To develop the third generation of high-efficient visible light catalysts, nitrogen and niobium co-doped anatase titania nanocrystals are synthesized. Experimental and theoretical investigations demonstrate that the formation of highly concentrated defect-pairs is key to significantly enhance visible light catalytic efficiency. In further combination of local defect structural design and the exploration of new synthesis strategies, anatase nanocrystals containing nitrogen and reduced titanium ions are synthesized. The formation of local defect clusters is demonstrated to play an important role on the obvious enhancement of Rhodamine B degradation efficiency under only visible light illumination. It is thus unveiled that a fundamental understanding of the functions of local defect structures and a well-controlled synthetic strategy are critical to develop highly efficient visible light catalysts with unprecedented photocatalytic performances. Through these systematic investigations, it is concluded that local defect structures generated by introduced co-dopants are complicated in strong-correlated titania systems and differ from case to case. A major difficulty to efficiently introduce difficult-dopant ions such as nitrogen and indium at high concentrations is solved. Two high-efficient visible light catalysts are achieved for environmental remediation by using the clean and renewable solar energy; and one raw material for manufacturing new ceramic capacitors and new metastable polymorphic phases is provided. The discussion on the doping mechanisms, the defect formation and their associated impacts on material performances will not only benefit the future development of physical chemistry, material science and defect chemistry, but also opens a new route to design novel multifunctional materials based on local defect structure design.