Non-monotonic thickness-dependent electrocatalytic activity in epitaxial La_0.7Sr_0.3MnO₃ thin films

The thickness in nanometer-scale has a direct correlation to the electrocatalytic activity of a transition metal oxide thin film. As the thickness increases, the active sites for the electrocatalyst increases as well, until a saturation thickness, defined as the “active depth”. This concept implies the reaction occurs at a depth deeper than conventional “surface” of the thin film. Here, we propose a way to understanding the “active depth” with the atomic-scale precision thickness (t) control of epitaxial La_0.7Sr_0.3MnO₃ (LSMO) thin films using pulsed laser epitaxy. The activity for the oxygen evolution reaction showed a non-monotonous t-dependence. It increased up to t = 23 u.c., and then decreased again. The non-monotonic trend let us identify three t-regions: (1) resistivity-, (2) active depth-, and (3) surface recombination time-dominant regions. While the parameters of (1) and (3) regions were previously reported and well-defined, here we identified the role of active depth for the first time. Using the LSMO t (u.c.)/SrRuO₃//Nb:SrTiO₃ heterostructures, we could further scrutinize the active depth and electrocatalytic surface of the LSMO layer to be t = 23 u.c. (~ 10 nm). Our study provides an understanding of the fundamental mechanism of electrocatalytic activities.