Contribution of the Sub‐Surface to Electrocatalytic Activity in Atomically Precise La_0.7Sr_0.3MnO₃ Heterostructures

Abstract

the redox of these metals during cycling, such as in RuO 2 [14] and Ni-based oxides, [15] can be used to estimate their surface concentration. The density of active sites is often simply approximated by the exposed geometric surface area (measured by, e.g., microscopy or N 2 adsorption [16] ) or the electrochemical surface area (ECSA) by assumptions regarding intrinsic capacitance. [15,[17][18][19] However, all of these normalization approaches imply that electrochemical activity is governed exclusively by the terminal surface layer of the oxide, without serious consideration of sub-surface layers that may influence activity as well.Size effects in the OER activity of nanoparticles, [20,21] thickness-dependent effects in thin films, [22] and substrate-dependent effects in 2D materials [23] imply that while a catalytic reaction occurs at the catalyst/electrolyte interface, the supporting layers of a catalyst (and/or its support) can influence activity as well. For oxides, these size effects can include manipulating the ability of transition metal sites to oxidize prior to the onset of OER. [20,21] Thickness effects can additionally include charge transfer to/from a support, [24][25][26] quantum tunneling through ultra-thin layers, [27,28] and formation of a Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomicscale thickness control of the La 0.7 Sr 0.3 MnO 3 (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO 3 heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202103632.

Symmetry Engineering of Epitaxial Hf0.5Zr0.5O2 Ultrathin Films

“…The high-quality of the epitaxial LSMO single layer (thickness of 28 nm) was confirmed by a separate growth (Figure S3). The main Bragg peak of the HZO thin film with t = 9.5 nm appeared at 2θ = 30.1°, closely aligning with the position of the O (or R) phase of HZO (111) reflection typically observed in polycrystalline films. , In polycrystalline films, the O(111) peak is generally observed at 2θ = 30.5°. This indicates that a lattice expansion of approximately 1.2% along the out-of-plane direction (larger d (111) spacing) occurred in the epitaxial thin film owing to the compressive strain.…”

Design Strategies of Active and Stable Oxygen Evolution Catalysts for Proton Exchange Membrane Water Electrolysis

“…The extrinsic activity of an electrocatalyst is controlled by various factors, including the number of accessible active sites, electrical conductivity, and porous structure. ,− The surface morphology of the catalyst plays a critical role in influencing these factors, thereby determining the extrinsic activity. Nanostructuring and the formation of porous structures are two widely used strategies to engineer the catalyst surface morphology for enhancing the extrinsic activity.…”

Polar Perturbations in Functional Oxide Heterostructures

“…[207] The polar heterostructure can also be useful in chemical functionalities, such as electrocatalytic activity of which the studies on transition metal oxide thin films are ever more increasing. [208,209] Kim et al showed that the watersplitting performance of WO 3 could be significantly enhanced by the interfacial band offset from the atomically thin polar LAO layer. [210]…”