Observing Long-Lived Photogenerated Holes in Cobalt Oxyhydroxide Oxygen Evolution Catalysts

Steady-state and time-resolved spectroelectrochemical optical absorption techniques were used to investigate photoexcited states of amorphous cobalt-phosphate oxyhydroxide (CoPi) and cobalt-borate oxyhydroxide (CoBi) oxygen evolution catalysts. These materials revealed concurrent spectroelectrochemical intensity changes in their ground-state and photoexcited visible spectra, providing insights into the dynamics of defect states attributable to trapped holes. Notably, long-lived photoexcited states, assignable to hole-based defects persisting beyond 10 ms in H2O, were observed in CoPi and CoBi for the first time. With 380 nm band gap excitation and delays shorter than 10 ps, excited-state decay for CoPi was markedly faster than for CoBi, despite the ultimately longer lived signal of CoPi beyond the ms time scale. The distinct kinetic profiles highlight the films' differences in structural and electronic properties despite strong similarities in absorption spectral profiles. These results provide further insights into the differences between the electronic properties and dynamics of CoPi and CoBi, which have been challenging to structurally and electronically characterize due to their amorphous nature.