Atomically Visualizing Elemental Segregation-Induced Surface Alloying and Restructuring

In situandoperandoelectron microscopy in heterogeneous catalysis-insights into multi-scale chemical dynamics

This review features state-of-the-artin situandoperandoelectron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolvedoperandomeasurements and the available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field's future directions will also be discussed.

Atomic-scale phase separation induced clustering of solute atoms

Dealloying typically occurs via the chemical dissolution of an alloy component through a corrosion process. In contrast, here we report an atomic-scale nonchemical dealloying process that results in the clustering of solute atoms. We show that the disparity in the adatom-substrate exchange barriers separate Cu adatoms from a Cu-Au mixture, leaving behind a fluid phase enriched with Au adatoms that subsequently aggregate into supported clusters. Using dynamic, atomic-scale electron microscopy observations and theoretical modeling, we delineate the atomic-scale mechanisms associated with the nucleation, rotation and amorphization-crystallization oscillations of the Au clusters. We expect broader applicability of the results because the phase separation process is dictated by the inherent asymmetric adatom-substrate exchange barriers for separating dissimilar atoms in multicomponent materials.

Tuning the surface composition of Cu3Au binary alloy

Using ambient-pressure X-ray photoelectron spectroscopy, here we report the real-time monitoring of dynamic surface composition evolution of Cu₃Au(100) in response to the imposed environmental stimuli. Segregation of Au to the pristine surface under ultrahigh vacuum annealing leads to the phase separation with pure Au at the surface and alloyed Au in the subsurface. Upon switching to an oxidizing atmosphere, oxygen adsorption drives the surface segregation of Cu along with inward migration of pure Au to the subsurface. Switching to a H₂ atmosphere results in oxygen loss from the oxygenated surface, thereby promoting Au surface segregation and reverting the surface to the pristine state with the Au termination. These measurements demonstrated the tunability of the surface composition of the binary alloy by utilizing the interplay between the tendency of segregating a more noble constituent to the surface and the tendency to segregate the more reactive one with the chemical stimuli.

Surface-reaction induced structural oscillations in the subsurface

Surface and subsurface are commonly considered as separate entities because of the difference in the bonding environment and are often investigated separately due to the experimental challenges in differentiating the surface and subsurface effects. Using in-situ atomic-scale transmission electron microscopy to resolve the surface and subsurface at the same time, we show that the hydrogen-CuO surface reaction results in structural oscillations in deeper atomic layers via the cycles of ordering and disordering of oxygen vacancies in the subsurface. Together with atomistic calculations, we show that the structural oscillations in the subsurface are induced by the hydrogen oxidation-induced cyclic loss of oxygen from the oxide surface. These results demonstrate the propagation of the surface reaction dynamics into the deeper layers in inducing nonstoichiometry in the subsurface and have significant implications in modulating various chemical processes involving surface-subsurface mass transport such as heterogeneous catalysis, oxidation, corrosion and carburization.