Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography

Abstract

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 Ir 0.94 Ru 0.06 O 2 , the composition in the grain boundary region varies from Ir 0.70 Ru 0.30 O 2 to Ir 0.40 Ru 0.60 O 2 and eventually to Ir 0.75 Ru 0.25 O 2 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.

Lattice Oxygen Exchange in Rutile IrO2 during the Oxygen Evolution Reaction

“…This protection layer enables the APT investigation of the topmost, catalytically relevant atomic layers, as detailed elsewhere. 14,21,33 The three-dimensional (3D) APT reconstruction, in Figure 2a, reveals a significant increase in the level of 16 O-containing species toward the surface (red spheres), while the majority of the bulk consists of 18 O-containing species (blue spheres). This is further illustrated in the cross-section views in panels b and c of Figure 2, locally reaching 2.5 atom % 16 O.…”

Direct Detection of Surface Species Formed on Iridium Electrocatalysts during the Oxygen Evolution Reaction

“…Following these electrochemical measurements, the surface state of the APT specimen was measured by XPS (Figure 1 f). Although XPS usually requires a flat specimen, we proved the feasibility of using, instead, a needle‐shaped specimen [16] . The results of this feasibility test are shown in Figure S3, where a circular region with a diameter of 20 μm from an APT tip was analyzed (Figure S3a,b) after OER for 60 and 300 seconds, respectively.…”

“…Although XPS usually requires a flat specimen, we proved the feasibility of using, instead, a needle‐shaped specimen. [16] The results of this feasibility test are shown in Figure S3, where a circular region with a diameter of 20 μm from an APT tip was analyzed (Figure S3a,b) after OER for 60 and 300 seconds, respectively. Despite the comparably low signal‐to‐noise levels of this experiment, the XPS data in Figure S3c,d clearly show Ir 4f spectra, at 60.9 and 63.9 eV, [17] demonstrating the feasibility of analyzing needle‐shaped APT specimens by XPS.…”

“…Although XPS usually requires af lat specimen, we proved the feasibility of using, instead, an eedle-shaped specimen. [16] Ther esults of this feasibility test are shown in Figure S3, where ac ircular region with ad iameter of 20 mm from an APT tip was analyzed (Figure S3a,b) after OER for 60 and 300 seconds,respectively.Despite the comparably low signal-to-noise levels of this experiment, the XPS data in Figure S3c,d clearly show Ir 4f spectra, at 60.9 and 63.9 eV, [17] demonstrating the feasibility of analyzing needle-shaped APT specimens by XPS.The peaks at 61.9 and 64.9 eV (Figure S3c) can be assigned to Ir(+ IV) species, [15a, 18] and those at 62.5 and 65.5 eV to Ir(+ III) and/or Ir(+ V) species [9,19] (the strong overlap of signals of those two species prohibits further distinguishing them). Fors amples catalyzing OER for 60 s (Figure S3c), more Ir(+ III) species (approx.…”

Direct Detection of Surface Species Formed on Iridium Electrocatalysts during the Oxygen Evolution Reaction

“…Although XPS usually requires af lat specimen, we proved the feasibility of using, instead, an eedle-shaped specimen. [16] Ther esults of this feasibility test are shown in Figure S3, where ac ircular region with ad iameter of 20 mm from an APT tip was analyzed (Figure S3a,b) after OER for 60 and 300 seconds,respectively.Despite the comparably low signal-to-noise levels of this experiment, the XPS data in Figure S3c,d clearly show Ir 4f spectra, at 60.9 and 63.9 eV, [17] demonstrating the feasibility of analyzing needle-shaped APT specimens by XPS.The peaks at 61.9 and 64.9 eV (Figure S3c) can be assigned to Ir(+ IV) species, [15a, 18] and those at 62.5 and 65.5 eV to Ir(+ III) and/or Ir(+ V) species [9,19] (the strong overlap of signals of those two species prohibits further distinguishing them). Fors amples catalyzing OER for 60 s (Figure S3c), more Ir(+ III) species (approx.…”