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Trapping a Ru₂O₃ Corundum-like Structure at Ultrathin, Disordered RuO₂ Nanoskins Expressed in 3D

Donakowski, Martin D., Mansour, Azzam N., Pala, Irina R., Chervin, Christopher N., DeSario, Paul A., Long, Jeffrey W., Rolison, Debra R.
Journal of physical chemistry 2018 v.122 no.50 pp. 28895-28900
X-radiation, X-ray absorption spectroscopy, aerogels, capacitance, carbon, electrical conductivity, heat treatment, nanocrystals, physical chemistry, ruthenium, silica, temperature
Protocols that express functional materials in a way that amplifies their surface-to-volume ratio offer a means to probe the structural ambiguity and surface-mediated reactivity of technologically important materials. We previously reported that three-dimensional (3D) ultraporous scaffolds, such as silica aerogels, silica fiber paper, and carbon nanofoam paper (CNF), provide a form factor that expresses energy-storing, catalytic ruthenium oxide (RuOx) as essentially all-surface—and a highly disordered one at that. To track the chemical state and solid-state structure of the 3D-expressed RuOx nanoskins as a function of thermal processing, we use X-ray near-edge structure (XANES), extended X-ray fine structure (EXAFS), and differential pair-distribution function (DPDF) analyses. We find that a Ru-centered ∼2.4 Å correlation present in the as-deposited oxide, also observed in PDF analysis of RuO₂·nH₂O but previously unassigned, fits the metastable corundum-like Ru₂O₃ structure. This corundum-like feature diminishes in concentration with increasing treatment temperature (25–200 °C), commensurate with an increase in relative rutile RuO₂ content, electrical conductivity, and charge-storing capacitance of the oxide. Yet disorder persists beyond 8 Å, and a rutile nanocrystalline structure is not attained until >200 °C. The combination of synthetic amplification and total scattering analyses offers a viable approach to elucidate the structural ambiguity of practical, disordered nanomaterials.