In the electron microscopy group, we investigate materials at the nanoscale with atomic resolution. Transmission electron microscopy (TEM) is a very versatile method as it allows the use of a wide variety of different techniques in a single instrument. Not only can we image the investigated samples at high resolution but also perform electron diffraction experiments. These techniques can be combined with analytical investigations using energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectrometry (EELS). For EDS analysis, the characteristic X-rays emitted from the sample due to the interaction with the electron beam are collected. Thus, we can analyse the elemental composition of the investigated materials with a high spatial resolution. In EELS we record the energy transferred from the beam electrons to the sample. This can be used to investigate the local electronic structure of the particles/material.
Analytical TEM has become an important and widely implemented part of the workflow in a modern catalysis research institute. It is a key technique for the investigation of local elemental composition, surfaces, core-shell structures and atomic interfaces, all crucial to the functionality of the catalyst. However, usually TEM specialists perform the necessary tasks for a thorough study of the materials of interest. The research efficiency can be tremendously improved by enabling non-TEM-experts in chemical laboratories to perform standardised investigations such as material screening or qualitative compositional analysis. In our group we apply the standardised procedures and workflows initially developed in the ChemiTEM project at the Fritz Haber Institute of the Max Planck Society in collaboration with Thermo Fisher Scientific. Furthermore, we will extend the workflows to the techniques available in our electron microscope (Fig. 1). This includes EELS measurements and a new way to investigate samples under inert conditions.
EELS is used to explore the local electronic structure, oxidation states and the bonding properties of the investigated materials (Fig. 2).1,2 This is necessary, for example, to gain more knowledge about the structure-function relationship in catalytic systems. Performing EELS experiments can be a challenging and complex task, as is the interpretation of the results. Comparison to reference spectra and simulations are necessary for a thorough analysis of the data. We therefore perform calculations of energy-loss spectra using a combination of software packages employing density functional theory (DFT) and a Bloch-wave formalism.3,4 Furthermore, additional software packages using different underlying computational methods are tested with regards to their applicability to different sample systems.
One challenge of TEM investigations is carbon contamination deposited on the surface of the investigated samples, which reduces the image quality and limits spectroscopic analysis (Fig. 3).5 We are investigating different approaches of individual sample treatment in order to keep carbon contamination as low as possible and thereby improve the data obtained by our TEM analysis. Current research focuses on the chemical investigation of the composition of contaminants and aims for mitigation strategies based on the individual contaminant profile of a sample that allows for a gentle removal of carbonaceous molecules without altering the specimen itself.
Understanding the structure-function relationships of catalytic systems can aid with the rational design of catalysts. Investigating the real and electronic structure of the utilized heterogeneous catalysts using chemical electron microscopy is a contribution of paramount importance to this understanding. In this project, which is part of the UniSysCat cluster of excellence, we study a series of bimetallic nanocatalyst systems to investigate the effect of nanoparticle morphology (i.e. size, composition, arrangement, etc.) on product selectivity and conversion in certain reactions (Fig. 4). Changes in catalytic behaviour when changing the synthesis and pre-treatment of the catalyst will be related to the corresponding changes in real and electronic structure as investigated by advanced chemical electron microscopy.