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Nanobrush superlattice interfaces between oxides drive colossal formation of oxygen vacancies

By Boris Dyatkin July 22, 2020
Dyatkin nanobrush
Precision synthesis of a CeO2/Y2O3 superlattice nanobrush. The chevron pattern seen from a cross-sectional scanning transmission electron microscope image of 1.4-μm-tall bristles comes from alternating stacks of nanometer-thick CeO2 and Y2O3 layers, forming atomically well defined (111) interfaces. Credit: Oak Ridge National Laboratory, US Department of Energy.

Researchers are growing ever more curious about the capabilities of metal oxides, such as ceria (CeO2), in a broad array of applications encompassing battery cathodes, catalytic converters, and gas sensors. This ceramic, which exhibits a fluorite crystal structure that coordinates eight cerium atoms for each oxygen, offers high ionic conductivity that enables its promising performance in this diverse lineup of devices. However, facile ion transport hinges on the presence of a high concentration of oxygen vacancies in its structure. Although previous partially successful efforts had doped ceria with secondary metals, strained their surfaces, and, subsequently, increased oxygen vacancy concentrations, these studies did not provide sufficient fundamental understanding of the process by which the vacancies form at the atomic level.

Interfaces between ceria and yttria (Y2O3), which is a bixbyite oxide that features preset vacant sites at every fourth oxygen position in its lattice, off er a promising route to enable high vacancy concentrations in CeO2 structures sufficient to facilitate high ionic conductivities. In particular, the (111) interface between these two oxides is most energetically favorable and enables conditions that facilitate charge transfer across junctions such as those found in semiconductors and perovskite devices, enabling rapid ion transport along the plane of this interface. Researchers from Oak Ridge National Laboratory have developed a specific nanobrush architecture that can establish heterointerfaces between ceria and yttria that maximize ion conduction. This collaborative effort with scientists from the Massachusetts Institute of Technology, the University of South Carolina, Argonne National Laboratory, and the University of Tennessee-Knoxville structured a chemical valence mismatch between the yttrium and cerium cations and created a record high oxygen vacancy concentration of 10%, while maintaining stable lattices and nanostructures. The research team reported their breakthrough in a recent issue of Nature Communications.

Ho Nyung Lee, the principal investigator behind this effort, says, “Deliberate growth control allowed us to build the architecture of the crystalline nanobrush, which has unprecedented interfaces inside each bristle. I am grateful for the research team’s success not only in predictively designing interfacial materials, but also in experimentally realizing them to create atomically well-controlled interfaces through precision synthesis.”

Researchers used pulsed laser epitaxy on ceria and yttria targets to grow single- crystalline nanobrush structures. Each bristle of the nanobrush was approximately 50 nm in diameter. These vertically oriented micron-tall rods, with jagged edges, were comprised of alternating layers of CeO2 and YO2 with stable (111) interfaces between the fluorite and bixbyite structures. Characterization of the material provided insights into the valence states and found that the mismatch between the cerium and yttrium cations at the interface modulates the valence on the former between +3.3 and +3.6.

In order to balance the charge, the stoichiometry of oxygen decreases from 2 to 1.65–1.8 for each cerium ion; this subsequently facilitates formation of oxygen vacancies accounting for more than 10% of voids in the lattice without significantly distorting the overall ceria structure. Researchers corroborated scanning transmission electron microscopy and electron energy-loss spectroscopy measurements with density functional theory calculations, molecular dynamics, and Monte Carlo simulations. Small-angle neutron scattering measurements determined that the metal oxide nanobrushes were over 49% porous, with these structures exhibiting a surface area over 200 times greater than that of conventionally grown thin films. This may enable superior performance in applications such as catalytic converters, fuel-cell electrodes, and other systems that require materials with large contact areas.

Prior efforts to maximize oxygen vacancy concentrations had focused on developing space-charge regions in heterostructured ionic materials. However, this approach is bound by the diffusion (Debye) length in the material and loses efficacy past a few nanometers. In contrast, the charge modulation approach used by Lee and his colleagues delivers much higher vacancy concentrations, focuses them in the CeO2 layer of interest, and retains its intact lattice structure.

According to the researchers, this novel fundamental approach offers promising capabilities in a large range of applications that require high ionic conductivities, and the novel nanobrush architecture brings forth a viable route for novel advanced energy storage and computing breakthroughs.

Read the article in Nature Communications.