Tailoring the OER performance of Ruddlesden-Popper-phase La ₂ CoO ₄ thin films by thickness and RuO ₂ buffer layers

The modulation of oxygen evolution reaction (OER) performance in oxide thin films is essential for understanding structure–activity relationships. Recently, we found that ferromagnetic Ruddlesden–Popper–phase La 2 CoO 4 thin films exhibit superior catalytic activity compared to its perovskite analogue LaCoO 3 [Nano Lett. 25, 4887 (2025)]. Here, we further investigate the OER performance of La 2 CoO 4 thin films by tuning thickness and introducing a RuO 2 buffer layer. The intrinsic activity of La 2 CoO 4 is found to be independent of thickness, while its stability strongly depends on thickness, with thinner films showing rapid degradation, indicating that thickness alone cannot enhance activity. In contrast, the introduction of a RuO 2 layer significantly improves OER performance, particularly at low overpotentials, and introduces a new thickness dependence. Further analysis suggests that interfacial intermixing during PLD growth may occur, leading to compositionally modified regions or new active phases. By tuning the La 2 CoO 4 overlayer thickness, a trade-off between activity and stability is revealed, governed by the depth distribution of active regions. These results highlight the distinct roles of thickness and intermixing in determining catalytic performance.