Today's synthesis methods enable us to produce increasingly complex nano-objects and nanomaterials, with sometimes unexpected properties. Understanding the origin of these new properties, which is interesting from a fundamental point of view, is essential to the search for new applications. A precise correlation must therefore be established between the synthesis method and the properties of interest, which are imposed by different types of physico-chemical parameters. If we take the field of catalysis as an example, the behavior of a catalyst made up of an active phase and its support depends on their structures, their chemical compositions, the spatial distribution of their components and the support-catalyst interaction. Nanoscale characterization of single particles is essential to overcome the averaging effects of particle ensemble studies. However, most of the imaging and spectroscopy techniques used until recently rely on the analysis of a projection of the object on a plane, where all this information is integrated in thickness.
To resolve the characteristics of nano-objects in three dimensions of space, we need to be able to reconstruct their volumes from the projected observations. The solution is to use electron tomography, which consists in reconstructing the volume of an object from a series of its projections recorded by electron microscopy at various angles. Today, it has become an indispensable tool for the study of nanomaterials, thanks in part to the large number of modes compatible with a tomographic approach: bright field in TEM, used above all for amorphous or weakly crystallized materials; annular dark field in STEM, an incoherent mode suitable for the study of crystalline materials and in which the intensity is proportional to the average atomic number. Very recently, the latest advances in instrumentation have enabled the introduction of analytical TEM tomography, which combines the tomographic approach with energy-filtered imaging. This technique is doubly selective (in terms of the 3D character of the object and its chemical composition), enabling 3D mapping of elements using a series of projections containing chemical information.
To illustrate the study potential of 3D imaging by electron tomography, different types of study were presented. For example, in bright field mode, a quantitative study of the spatial distribution of metal nanoparticles on a carbon nanotube support. In STEM-HAADF mode, the determination of the surface crystallography of Pd particles (size 5 nm), and the study of their arrangement in superlattices were presented. Finally, for analytical tomography, which we have implemented with a resolution approaching the nanometer, the study of the spatial localization of nitrogen in doped carbon nanotubes, or the determination of the spatial distribution of silica and alumina in a mixed catalyst support, were detailed by way of illustration. Prospects for the development of various nanometric 3D imaging techniques and their contribution to the study of nanomaterials were also presented.
