Orientation-Dependent Oxygen Evolution Catalytic Performance and Mechanistic Insights of Epitaxial Co₉S₈ Thin Films

Cobalt sulfide (Co₉S₈) nanomaterials exhibit an efficient electrochemical catalytic performance due to their unique properties and electronic structure. The preparation of epitaxial Co₉S₈ thin films with varying crystal orientations and the study of their catalytic kinetics and mechanisms remain significant gaps. This study addresses the preparation of epitaxial Co₉S₈ thin films with orientations of (100), (110), and (111) on yttrium-doped zirconia (YSZ) substrates using pulsed laser deposition. Characterization confirmed their single-crystalline nature and consistent thickness. Electrochemical measurements revealed a similar hydrogen evolution reaction (HER) performance across all films but significant differences in the oxygen evolution reaction (OER) performance. The (111) orientation showed the best OER activity, with a current density of 24.2 mA cm^-2 at 1.8 V vs RHE, outperforming the (100) and (110) orientations, which achieved 14.5 and 6.7 mA cm^-2, respectively. Density functional theory (DFT) calculations indicated that the (100) orientation favored the traditional four-electron transfer mechanism, with a lower theoretical overpotential (0.37 V). In contrast, the (110) and (111) orientations demonstrated more complex adsorption behaviors, resulting in a higher overpotential of 0.49 V and a lower overpotential of 0.29 V, respectively. These results highlight the unique reactivity of different Co₉S₈ crystal orientations and provide valuable insights for optimizing the catalyst design to enhance the OER performance.