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Register a new userVisualizing the Cu and Cu2O Interface Transition in Nanoparticles with Environmental Scanning Transmission Electron Microscopy
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Understanding the oxidation and reduction mechanisms of catalytically active transition metal nanoparticles is important to improve their application in a variety of chemical processes. In nanocatalysis the nanoparticles can undergo oxidation or reduction in situ, and thus the redox species are not what are observed before and after reactions. We have used the novel environmental scanning transmission electron microscope (ESTEM) with 0.1 nm resolution in systematic studies of complex dynamic oxidation and reduction mechanisms of copper nanoparticles. The oxidation of copper has previously been reported to be dependent on its crystallography and its interaction with the substrate. By following the dynamic oxidation process in situ, in real time, with high-angle annular dark-field imaging in the ESTEM, we use conditions ideal to track the oxidation front as it progresses across a copper nanoparticle by following the changes in the atomic number (z) contrast with time. The oxidation occurs via the nucleation of the oxide phase (Cu2O) from one area of the nanoparticle which then progresses unidirectionally across the particle, with the Cu-to-Cu2O interface having a relationship of Cu{111} parallel to Cu2O{111}. The oxidation kinetics are related to the temperature and oxygen pressure. When the process is reversed in hydrogen, the reduction process is observed to be similar to the oxidation, with the same crystallographic relationship between the two phases. The dynamic observations provide unique insights into redox mechanisms which are important to understanding and controlling the oxidation and reduction of copper-based nanoparticles.
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Advances in atomic resolution in situ environmental transmission electron microscopy for direct probing of gas-solid reactions, including at very high temperatures are described. In addition, recent developments of dynamic real time in situ studies at the Angstrom level using a hot stage in an aberration corrected environment are presented. In situ data from Pt and Pd nanoparticles on carbon with the corresponding FFT (optical diffractogram) illustrate an achieved resolution of 0.11 nm at 500 C and higher in a double aberration corrected TEM and STEM instrument employing a wider gap objective pole piece. The new results open up opportunities for dynamic studies of materials in an aberration corrected environment.
In this study, we have used a Zr-Nb alloy containing well-defined nano-precipitates as a model material in which to study imaging contrast
Complex oxide interfaces have been one of the central focuses in condensed matter physics and material science. Over the past decade, aberration corrected scanning transmission electron microscopy and spectroscopy has proven to be invaluable to visualize and understand the emerging quantum phenomena at an interface. In this paper, we briefly review some recent progress in the utilization of electron microscopy to probe interfaces. Specifically, we discuss several important challenges for electron microscopy to advance our understanding on interface phenomena, from the perspective of variable temperature, magnetism, electron energy loss spectroscopy analysis, electronic symmetry, and defects probing.
Thin film oxides are a source of endless fascination for the materials scientist. These materials are highly flexible, can be integrated into almost limitless combinations, and exhibit many useful functionalities for device applications. While precision synthesis techniques, such as molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), provide a high degree of control over these systems, there remains a disconnect between ideal and realized materials. Because thin films adopt structures and chemistries distinct from their bulk counterparts, it is often difficult to predict what properties will emerge. The complex energy landscape of the synthesis process is also strongly influenced by non-equilibrium growth conditions imposed by the substrate, as well as the kinetics of thin film crystallization and fluctuations in process variables, all of which can lead to significant deviations from targeted outcomes. High-resolution structural and chemical characterization techniques, as described in this volume, are needed to verify growth models, bound theoretical calculations, and guide materials design. While many characterization options exist, most are spatially-averaged or indirect, providing only partial insight into the complex behavior of these systems. Over the past several decades, scanning transmission electron microscopy (STEM) has become a cornerstone of oxide heterostructure characterization owing to its ability to simultaneously resolve structure, chemistry, and defects at the highest spatial resolution. STEM methods are an essential complement to averaged scattering techniques, offering a direct picture of resulting materials that can inform and refine the growth process to achieve targeted properties. There is arguably no other technique that can provide such a broad array of information at the atomic-scale, all within a single experimental session.
An environmental cell high resolution electron microscope (EHREM) has been developed for in situ studies of dynamic chemical reactions on the atomic scale. It allows access to metastable intermediate phases of catalysts and to sequences of reversible microstructural and chemical development associated with the activation, deactivation and poisoning of a catalyst. Materials transported through air can be restored or recreated and samples damaged, e.g. by dehydration, by the usual vacuum environment in a conventional electron microscope can be preserved. A Philips C M30 HRTEM and STEM system has been extensively modified in our laboratory to add facilities for in situ gas-solid reaction studies in controlled atmospheres of gas or vapor at pressures of 0 to 50 mbar, instead of the regular TEM high vacuum environment. The integrated new environmental cell capability is combined with the original 0.23 nm TEM resolution, STEM imaging (bright field and annular dark field) and chemical and crystallographic microanalyses. Regular sample holders are used and include hot stages to higher than 1000 C. Examples of applications include direct studies of dynamic reactions with supported metal particle catalysts, the generation of defects and structural changes in practical complex oxide catalyst systems under operating conditions and carbon microstructures.
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