Tailoring the OER performance of Ruddlesden-Popper phase La₂CoO₄ thin films by thickness and RuO₂ 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 La₂CoO₄ thin films exhibit superior catalytic activity compared to its perovskite analogue LaCoO₃ [Z. Meng, Y. Qi, P. Qin, X. Zhou, X. Wang, H. Chen, L. Liu, G. Zhao, Z. Duan, J. Liu, Q. Zhang, P. Tang and Z. Liu, Nano Lett. 25 (2025) 4887]. Here, we further investigate the OER performance of La₂CoO₄ thin films by tuning thickness and introducing a RuO₂ buffer layer. The intrinsic activity of La₂CoO₄ 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 RuO₂ 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 La₂CoO₄ 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.