Modelling of Natural-Like Grain Generation of TiO2 and its effect on Band Structures

TiO2 is widely used in various electrochemical technologies including photosynthesis catalyst and optical applications such as dye sensitized solar cell, perovskite solar cells and batteries. Its polycrystal structure affects electronic properties, for example, introducing trap states into the band structure both in the bulk and at the interfaces and affects the ion and electron transport. It is difficult to understand these effects experimentally especially in bulk, therefore computational studies of effects of microstructural features are desired. These kinds of calculations have been done using models with a specific, postulated grain boundary. However, real materials have a distribution of grain boundaries. In monoelemental materials like Fe and Si, generation of grain structures have been reported using Molecular Dynamics (MD), but for ceramics containing several kinds of atoms, it is more difficult.<br/>In this study, natural-like generation of grain boundaries is explored using MD at a million-atom scale. Different types of temperature schedules are explored, and different structures are obtained by different heat treatments. To facilitate the formation of grains, spherical seeds that are randomly located and oriented are introduced. Temperature dependence of the grain sizes of the obtained structures is investigated. Furthermore, the effects of the microstructural features on the band structure are computed using large-scale Density-Functional Tight-Binding calculations (DFTB). Differences in the electronic properties between ideal crystal and different grainy structures are compared.