Understanding charge-carrier transport in semiconductors is vital to the improvement of material performance for various applications in optoelectronics and photochemistry. Here, we use hybrid density functional theory to model small hole polaron transport in the anatase, brookite, and TiO 2 -B phases of titanium dioxide and determine the rates of site-to-site hopping as well as thermal ionization into the valance band and retrapping. We find that the hole polaron mobility increases in the order TiO 2 -B < anatase < brookite and there are distinct differences in the character of hole polaron migration in each phase. As well as having fundamental interest, these results have implications for applications of TiO 2 in photocatalysis and photoelectrochemistry, which we discuss.

No abstract

Lithium dendrite formation and insufficient ionic conductivity remain primary concerns for the utilization of solid‐state batteries. Given that the interpretation of experimental results for polycrystalline solid electrolytes can be difficult, computational techniques are invaluable for providing insight at the atomic scale. Here, first‐principles calculations are carried out on representative grain boundaries in four important solid electrolytes, namely, an anti‐perovskite oxide, Li3OCl, and its hydrated counterpart, Li2OHCl, a thiophosphate, Li3PS4, and a halide, Li3InCl6, to develop the first generally applicable design principles for grain boundaries in solid electrolytes for solid‐state batteries. The significantly different impacts that grain boundaries have on electronic structure and transport, ion conductivity and correlated ion dynamics are demonstrated. The results show that even when grain boundaries do not significantly impact ionic conductivity, they can still strongly perturb the electronic structure and contribute to potential lithium dendrite propagation. It is also illustrated, for the first time, how correlated motion, including the so‐called paddle‐wheel mechanism, can vary substantially at grain boundaries. These findings reveal the dramatically different behavior of solid electrolytes at the microscale compared to the bulk and its potential consequences and benefits for the design of solid‐state batteries. These design principles are expected to aid the synthesis and engineering of solid electrolytes at the microscale for preventing dendrite propagation and accelerating ion transport.

First-principles calculations of the electronic structure and charge-trapping behavior of 3 {112} and 1 {110} twin boundaries (TBs) in anatase TiO 2 are performed using an accurate hybrid density functional theory approach. The former is characterized experimentally using transmission electron microscopy (TEM) and very good agreement on the structure is found. The {110} twin has not yet been observed but TEM and scanning tuneling microscopy (STM) image simulations are presented to aid experimental identification. Holes are found to trap in a polaronic configuration at both the twin boundaries. The {112} TB presents more favorable sites for hole polaron formation at the boundary with trapping energies 0.16-0.18eV, more favorable than the bulk. The {110} TB presents hole polaron trapping sites ranging from 0.07 eV, less favorable, to 0.14 eV, more favorable, than the bulk. Neither boundary is found to favor electron trapping, indicating they are relatively benign to the performance of anatase as an n-type conductor.

Polycrystalline anatase titanium dioxide has drawn great interest, because of its potential applications in highefficiency photovoltaics and photocatalysts. There has been speculation on the electronic properties of grain boundaries but little direct evidence, because grain boundaries in anatase are challenging to probe experimentally and to model. We present a combined experimental and theoretical study of anatase grain boundaries that have been fabricated by epitaxial growth on a bicrystalline substrate, allowing accurate atomic-scale models to be determined. The electronic structure in the vicinity of stoichiometric grain boundaries is relatively benign to device performance but segregation of oxygen vacancies introduces barriers to electron transport, because of the development of a space charge region. An intrinsically oxygen-deficient boundary exhibits charge trapping consistent with electron energy loss spectroscopy measurements. We discuss strategies for the synthesis of polycrystalline anatase in order to minimize the formation of such deleterious grain boundaries.