Harnessing Electrochemically-Injected Interstitial Atoms in Oxide Semiconductors for Doping and Purification

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

NON-TECHNICAL SUMMARY Metallic elements and oxygen form solid ceramic compounds such as titanium dioxide that can be used, not only in commercial products like sunscreens, but also in sophisticated electronics, sensors, and materials for energy production and storage. The usefulness of ceramics for such advanced purposes often depends upon tiny concentrations of atomic scale irregularities in the regular array of atoms that make up the solid. Certain kinds of these irregularities, such as extra atoms wedged into the structure ("interstitial atoms") can be introduced into the solid intentionally to benefit its properties. The work supported by this grant investigates how to use specially prepared surfaces exposed to water solutions with an electrical voltage to controllably create interstitial metal atoms. Success will permit metal atoms to be introduced in precise ways near room temperature, in contrast to the hot conditions at hundreds of degrees used by typical methods. As a result, manufacturing advanced ceramics should become cheaper, faster, and lead to solids with far superior performance. Work under this grant is also developing real-world learning modules implemented within the Worldwide Youth in Science and Engineering program at the University of Illinois for high-school students to help raise interest in science and engineering, and especially in new materials for sustainable energy applications. Several activities to promote the importance of ethics in science and engineering are being pursued. Additional outreach aimed especially at high school girls and first-generation students involve weeklong summer residential camps to encourage interest in science and engineering. The investigators are also hosting several undergraduate researchers to participate in the work. TECHNICAL SUMMARY Interstitial atoms in semiconductors are extra atoms wedged into the crystalline structure that represent atomic-scale irregularities, or "defects." Specifically prepared surfaces that are exposed to aqueous liquids act efficiently and controllably to inject interstitial atoms into the underlying bulk near room temperature, especially when subjected to electrochemical bias. Using surfaces for this purpose represents a new and versatile tool for tuning material properties after initial synthesis. The approach enables straightforward and inexpensive processing methods, and accesses a regime wherein kinetic, rather than thermodynamic, effects dominate defect behavior. It therefore becomes possible to create materials and structures with heretofore unattainable properties that circumvent thermodynamic constraints. For example, purification of isotopes beyond natural-abundance limits becomes possible. Applications are expected to span defect engineering, chemical and isotopic purification, and doping for electronics, sensors, and renewable energy production and storage – all performed post-synthesis. The approach is most suited for applications involving high surface-to-volume ratios, such as nanostructures, thin films, or porous materials. This work aims to understand the diffusion and trapping of injected metal interstitials in ceramic oxides under electrochemical bias. The work employs single-crystal rutile titanium dioxide as an exemplary oxide with important technological applications. The experimental approach relies mainly upon diffusion measurements of titanium and oxygen tracer isotopes, and of the example dopant manganese. Modeling of defect diffusion and reaction at the mesoscale (2-500 nm) links experimental diffusion profiles to atomistic calculations to unravel the complicated temporal phenomena that are expected to accompany both doping and solid-state purification. Atomistic calculations by density functional theory provide activation barriers for comparison with the outputs of mesoscale modeling, and identify possibilities for interstitial clustering that might inhibit movement of metal interstitials into the solid. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.