Theoretical insights into the surface physics and chemistry of redox-active oxides

Redox-active oxides find use in many applications, including catalysts, photovoltaic devices, self-cleaning glasses, chemical sensors and electronic components. Their utility derives from their unique ability to access multiple metal-charge states within a finite energy window. However, this property also confounds our ability to study reducible oxides, because it leads to structural, compositional and electronic complexities that elude simplistic models of materials structure and function. Oxygen vacancies play a critical role in shaping the functional properties of such oxides; most notably, they lead to mobile-charge imbalances that impact surface processes at substantial distances from the originating defect. Atomistic simulations are inherently equipped to illuminate these phenomena at a fundamental level; however, reducible oxides pose great challenges, owing to the high level of electron correlation needed to correctly describe them. Understanding how defects form, couple, propagate, agglomerate or repel each other and influence the surface properties of reducible oxides is only now coming into the grasp of modern theory and simulation capabilities. This knowledge is also key to discovering and controlling emergent materials properties with tunable multifunctionalities at the nanometre scale and beyond. Reducible oxides are tunable, multifunctional materials used in many applications, particularly in catalysis; their attractive properties arise from their interacting charge carriers, complex electronic structure and propensity to form mobile defects. This Review surveys theoretical methods to model and understand reducible oxides, using TiO2 as a prototypical example.