The discovery of new materials with tailored chemical properties is vital for advancing key technologies related to catalysis and energy conversion. Transition metal oxides (TMOs) catalyze a number of these key technologies, including water electrolysis, fuel cells, and photocatalysis. Compared to tuning the composition of TMOs, the search for metastable structures for usage in catalytic applications is relatively unexplored. One reason for this is the difficulty of synthesizing such structures. However, researchers have recently shown novel structures can be synthesized via thin film technologies, and these thin films have shown to be active for select catalytic and photocatalytic technologies. Given both the difficulty of synthesis and wide spectrum of both existent and undiscovered atomic structures, information on the potential stability and activity of possible structural polymorphs is vital.
In this contribution, we assess the potential reactivity and stability of four potential oxide polymorphs (anatase, brookite, columbite, pyrite) of MO2 (M=Ru, Rh, Pt, Ir) transition metal oxides (TMOs), which all form in a rutile-like structure at typical reactive conditions. The similar coordination and local geometry of both cations and anions in all structures lead us to hypothesize that strain alone could describe trends in chemical properties of metastable polymorphs. Our results suggest this is not the case. In addition, we observe polymorphic structures provide more tunable reactivity and increased stability with respect to strained rutile structures. Our prediction that columbite IrO2 will be a better oxygen evolution catalyst than rutile IrO2 underscores the potential activity benefits of polymorphic structure. The origin of the unique reactivity of polymorphic structures is unearthed through analysis of the electronic structure. In contrast to simple strain, distortions to the octahedral symmetry of the metal cation in polymorphic structures lead to significant changes in both the shape of the t2g-band and adsorption energy.