Defect disorder, transport and photoelectrochemical properties of TiO₂

Abstract

While the technology of Yttria-Stabilized Zirconia (YSZ)-based electrochemical devices is relatively well-established, there is growing interest in the development of photoelectrochemical devices aiming at the conversion of solar energy into electrical and chemical energies. Similarly, semiconducting oxides, especially TiO2, have great potential in the development of these devices. The potential of TiO2 as the leading candidate for these applications has generated enormous interest by many researchers in the modification of its properties to impose the properties necessary for its use as a photoelectrode. The properties of metal oxides are determined by the defect disorder; this chapter discusses the defect chemistry and defect-related properties of TiO2, including the photoelectochemical properties. Defect chemistry has been used as a framework to explain the functional properties and to modify these properties to achieve desired properties, which are required for specific applications. Consequently, the processing of TiO2 with desirable properties requires an increase in the present state of understanding of its defect chemistry and defect-related properties. The chapter highlights the basic defect equilibria for TiO2. It also discusses the gas/solid equilibria and the impact of the imposition of a well defined oxygen activity for the TiO2- O2 system. These equilibria are then used to derive defect diagrams for undoped TiO2 and its solid solutions with donors and acceptors. The chapter discusses the use of electrical characterization methods to verify these diagrams and the importance of the defect disorder of the surface layer.