Local coordination geometry within cobalt spinel oxides mediates photoinduced polaron formation

Understanding the photophysics of transition metal oxides is crucial for these materials to realize their considerable potential in applications such as photocatalysis and optoelectronics. Recent studies suggest that formation of localized excited states consisting of polarons (quasi-particles comprising a charge carrier strongly coupled to a proximal lattice distortion) plays a crucial role in the photophysics of these materials. Cobalt-containing spinel oxides (Co₃O₄ and ZnCo₂O₄) offer a unique opportunity to investigate the influence of local geometry, and cation inversion on photoinduced polaron formation. Here, we use Hubbard-corrected density functional theory (DFT + <i>U</i>) paired with resonance Raman and temperature-dependent optical spectroscopies to demonstrate that low-energy transitions observed in Co₃O₄ are associated with d-d transitions involving cobalt ions occupying tetrahedral sites within the spinel lattice. These low-energy optical transitions exhibit strong coupling to phonon modes associated with tetrahedral sites. Replacing most tetrahedral cobalt ions with zinc produces the slightly inverted ternary spinel material, ZnCo₂O₄, in which we observe a phonon-coupled optical transition that occurs at the same energy as observed in Co₃O₄. We propose that these phonon-coupled optical transitions enable direct access to a polaronic state upon photoexcitation; however, the intensity of this optical transition depends on temperature in Co₃O₄, whereas no significant temperature dependence is observed in ZnCo₂O₄. We therefore hypothesize that in Co₃O₄ the mechanism of polaron formation is coupling of the optical transition to dynamic, thermally-gated lattice distortions, whereas, in ZnCo₂O₄, the transition couples to static lattice defects that arise from the presence of a small population of tetrahedrally-coordinated cobalt ions.