See What Lies Beneath

The Science

For the first time, microscopy was used to observe the structural evolution of a buried interface of copper oxide and copper at the atomic scale. Monitored in real time, a reaction with hydrogen gas (called reduction) induced missing oxygen atoms at the interface. The collection of vacancies at the interface caused the oxide island to rotate. This mechanism supported by theory is fundamentally different from the processes associated with the reduction of bulk or isolated oxides. 

The Impact

Despite the difficulty in seeing buried interfaces under real conditions, this research achieved just that and revealed insights into the dynamics. The unmatched microscopic detail obtained may enable control of buried interfaces to enhance gas-surface reactions, such as turning carbon monoxide into carbon dioxide, in catalysis.

Summary

Copper oxide is an important catalyst and can convert poisonous carbon monoxide to carbon dioxide. In this research, the atomic-scale reactions at an interface between copper oxide (Cu₂O) and copper (Cu) metal were viewed in real time in a powerful electron microscope. Inside the microscope, the oxide/metal interface was transformed via a reaction with hydrogen. Hydrogen reacts with oxygen in the oxide island to produce water. The water desorbs from the surface, leaving behind oxygen vacancies. The vacancies moved through the oxide and accumulated at the interface. The real-time, atomic-scale imaging of the dynamics demonstrated that the oxide reduction occurs initially by the layer-by-layer retraction of the oxide lattice. This retraction is coordinated with the growth of the metal lattice. The metal growth is controlled by the flow of crystalline steps along the interface. Continued accumulation of oxygen vacancies at the interface region results in the abrupt collapse of the oxide lattice. This is associated with the formation of a tilted interface, which then drives the rotation of the oxide islands. These results demonstrate that atomic processes occurring at the Cu₂O/Cu interface govern the reduction dynamics of Cu₂O islands. This is fundamentally different from the mechanisms known to be associated with the reduction of bulk or isolated oxides. These results reveal the unique role of the oxide/metal interfaces in controlling the reaction mechanism. This research has broader implications for understanding and manipulating the way that buried interfaces can affect gas-surface reactions important to catalysis.

Funding

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, including research at the Center for Functional Nanomaterials (microscopy), a DOE Office of Science user facility; and the National Science Foundation (computational work).

Publication

L. Zou, J. Li, Z. Zakharov, E. Stach, and G. Zhou, “In situ atomic-scale imaging of the metal/oxide interfacial transformation.” Nature Communications 8, 307 (2017). [DOI: 10.1038/s41467-017-00371-4]

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