Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography
2019
Schweinar, Kevin | Nicholls, Rachel L. | Rajamathi, Catherine R. | Zeller, Patrick | Amati, M. (Matteo) | Gregoratti, Luca | Raabe, Dierk | Greiner, Mark | Gault, Baptiste | Kasian, Olga
The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir–Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir–Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir₀.₉₄Ru₀.₀₆O₂, the composition in the grain boundary region varies from Ir₀.₇₀Ru₀.₃₀O₂ to Ir₀.₄₀Ru₀.₆₀O₂ and eventually to Ir₀.₇₅Ru₀.₂₅O₂ from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond.
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