Heterogeneous catalysis has grown from an art to a science. Numerous catalytic materials are being proposed every day in the literature, with surprising and exciting new reactivities being disclosed, and interesting novel concepts being proposed. The study of catalytic processes starting from better defined materials that are tuned in morphology, composition and shape is opening new perspectives for catalyst design. For example, use of nanocrystals in CO oxidation turned out to be crucially dependent on the interfacial contact of the support (ceria) with the metal nanoparticle active sites [1], while valorization of biomasses were employed as reactants for catalysis by carbon-supported, Pt and PtCo nanocrystals (NCs) with controlled size and composition [2]. A particular type of structure is represented by nanosized core-shell phases, which proved to be superior catalysts in several catalytic reactions, such as methane combustion [3] as well as photocatalytic hydrogen evolution from biomass-derived substrates [4] or electrocatalytic water electrolysis [5] when integrated with multi-walled carbon nanotubes. Every day, nanotechnology applied to heterogeneous catalysts is creating new opportunities for important breakthroughs.
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Our ability to determine crystal structure of almost any material underlies the advance of so many scientific disciplines and it has become so naturally accepted that we may be misled to believing it is an easy and always available procedure. However, even now, after more than hundred years of the development of structural crystallography, there are problems that are very hard or impossible to tackle. The analysis of nanocrystalline materials is one such example. At the same time it becomes increasingly important to be able to understand the structures from materials that are available only in nanocrystalline form. Examples of such materials are sintered ceramics, many framework materials (zeolites), many organic compounds, but also thin films grown on a substrate, with structures different from the bulk of the same composition. In these cases it is very hard or impossible to grow large crystals suitable for x-ray analysis.
The solution to the problem is the use of electrons as the probing radiation. Electrons interact with atoms about a thousand times more strongly than x-rays, and diffraction patterns can be obtained from extremely small nanocrystals. Unfortunately, the strong interaction leads to complicated diffraction phenomena, which, until recently, hampered the possibilities to use electron diffraction for accurate structure analysis. However, the developments in the field of electron diffraction over the past decade changed electron diffraction from an essentially unsuitable tool to a commonly accepted and accurate method in structure analysis of nanocrystals. The presentation will give an account on the history of this development, on the basic principles of the method and it will also demonstrate its potential on a series of examples from different fields of material science.