Three-dimensional Imaging of Surface Structural Transformations on Electrocatalyst Nanoparticles Using Multi-slice Electron Ptychography (2023)

Surface structural transformations are important in electrocatalysis as reactions take place at surfaces [1,2].Many electrocatalysts such as metal alloy, metal oxide, and metal nitride nanoparticles undergo surface transformations and generate a different phase on the surface, forming a core-shell structure [3][4][5] altering the expected surface chemistry [6].Measuring the structure, coverage and continuity of the shell, and how it coupled to the core requires a method that can provide atomic-resolution information with a depth-sensitivity beyond that of conventional imaging modes, and at a lower dose than tilt-series tomography.Multi-slice electron ptychography can in principle meet this challenge by providing a dose-efficient, high-spatial-resolution reconstruction of the scattering potential with depth information [7].Here we studied MnN nanoparticles that are of interest as cathode electrode catalysts for anion-exchange membrane fuel cells (AEMFCs) [8].As with many other metal nitrides, MnN nanoparticles generate a layer of manganese oxide (e.g., Mn 3 O 4 ) on their surface in the presence of air or during alkaline oxygen reduction reaction (ORR) electrochemical testing.Here, the combination of high angle annular dark field (HAADF) images along the [110] zone axis of MnN nanoparticles and electron energy-loss spectroscopy (EELS) demonstrated island-like growth of Mn 3 O 4 layers with different thicknesses after 10k ORR cycles (Figure 1 a-b).With less electron dose, the reconstructed ptychography stack provides the position of Mn, O, and N atoms, with an improved resolution compared to HAADF (Figure 1c).Moreover, without dose-intensive tomography, HAADF images lack the depth sensitivity to resolve the complex surface structures with overlapping nanoparticles along the Z axis and tilting often occurring in nano-domains.By changing the defocus value in HAADF, we get some sense that the some overlapped trapezoidal Mn 3 O 4 nano-islands nucleated heterogeneously on the MnN nanoparticle surface (Figure 2 a).Nonetheless, the overlap greatly complicates prospects for obtaining physically meaningful strain maps from the HAADF images.Here we apply multi-slice electron ptychography [7] to characterize the electrocatalyst surface in 3D, imaging with both heavy and light elements at sub-Angstrom lateral resolution.A single ptychographic reconstruction shows details of different MnN/ Mn 3 O 4 interfaces at different depths of view (Figure 2b).Within a single ptychography stack, we separate the structural information from two different Mn 3 O 4 nano-islands at different depths of view even when overlapped with sufficient details to allow us to study lattice displacements and strain separately from each grain.The heterogenous nucleation of the Mn 3 O 4 nano-islands is visible in the orthogonal XZ slices.The misregistration between the two Mn 3 O 4 grains can be seen in slices 2-5, and is a consequence of the degeneracy of possible registrations between the Mn-O unit and O-Mn-O unit in Mn 3 O 4 unit cells.These quantitative insights into three-dimensional atomic structures of electrocatalyst surface layers, especially continuity, uniformity and strain help us better understand reaction mechanisms and durability of the catalyst nanoparticles.[9]