HAADF-STEM Electron Tomography in Catalysis Research

Electron tomography (ET) has rapidly developed into a powerful technique to characterize the three-dimensional (3D) structure of complex materials, with nanometer resolution, for a wide range of applications including heterogeneous catalysis. In these 3D studies, scanning transmission electron microscopy (STEM) has been the most widely used imaging mode, due to both its incoherent nature, which avoids the appearance of diffraction-related artefacts, and its sensitivity to composition, an important question for most catalytic systems. Though initially used as a qualitative tool to visualize 3D nanostructures, more recently STEM tomography is being also exploited to retrieve quantitative information in 3D. In this article, we review recent developments of STEM-based tomography for the understanding of heterogeneous catalysts at the nanometer level. The new possibilities opened by quantitative analysis will be illustrated by selected cases in which different parameters are calculated from 3D reconstructions and compared to their macroscopic measurements. The key influence of the latest developments in reconstruction and segmentation methods to assess reliability and accuracy in the quantification process is highlighted.