Understanding the fundamentals of TiO 2 surfaces. Part I. The influence of defect states on the correlation between crystallographic structure, electronic structure and physical properties of single-crystal surfaces

TiO2 is an important material for reasons that are related to fundamental research, as well as to existing and potential technological applications. There is no aspect of the chemistry and physics of TiO2, which is not decisively affected by the presence of defects. An increase in the present state of understanding of the science of TiO2-based material interfaces is essential for progress in this area; such progress is possible thanks to high purity TiO2 single crystals which are ideal for basic investigation of and on titanium dioxide surfaces under carefully controlled laboratory conditions. Surface functionalization through precise control of the nature of the defects, their concentration and arrangement, allows the modification and optimization of the material, or even the generation of completely new targeted properties of the surfaces independent from the bulk. A new generation of TiO2 surfaces with enhanced functionality and performance limits has been successfully created by the use of surface engineering. In Part I of this review, a brief description of the results is given of the experimental and theoretical investigations on the influence of the defect states on the correlation between crystallographic, electronic and atomic geometrical structures of the most technically important single low Miller index titania surfaces and their physical properties. In addition, the formation and detection techniques for surface oxygen vacancies are also considered. Special attention in this area is given to the behaviour of the electrons remaining in the system upon vacancy formation and the ability of current quantum-mechanical modelling methods to provide a precise description of the electronic structure of non-stoichiometric TiO2. These fundamental studies allow scientists to create real functional TiO2 surfaces, such as the targeted nanostructured thin films with predefined structure and properties required for the development of high-performance devices in the fields of energy, environment and health.