Ab Initio Study of Electron and Hole Polaron Transport in Cobalt(II,III) Oxide Using Oxidation-State Constrained Density Functional Theory

Abstract

The nature of electron and hole polaron transport in the mixed valence cobalt(II,III) oxide Co 3 O 4 in pristine conditions and with an oxygen vacancy are studied using Oxidation-State Constrained DFT (OS-CDFT). We find that the tetrahedral site repels electron polarons as hopping from a neighboring octahedral site toward the tetrahedral site is highly endothermic while hopping away toward a neighboring octahedral site is nearly instantaneous. Hence, electron polarons travel through bulk Co 3 O 4 using pathways through only octahedral Co ions. In contrast, while the purely octahedral pathways remain the dominant pathway for hole polaron transport, the pathways involving both the octahedral and tetrahedral sites are still viable with about two thirds the rate of the octahedral pathways. Moreover, the purely octahedral pathway is enhanced with an oxygen vacancy for both the electron and hole polarons. On the other hand, pathways involving both the octahedral and tetrahedral sites are impeded in the presence of an oxygen vacancy for both the electron and hole polarons due to increased magnitude of driving forces.