Solar Fuel from Rust: Designing the Properties of Corundum-structure Oxides

There is widespread interest in discovering materials that can effectively harvest sunlight in the visible region of the electromagnetic spectrum in order to drive chemical processes on surfaces. Hematite, Fe₂O₃, is in many ways an ideal photocatalyst to split water as a source of H₂ fuel because it is non-toxic, Earth-abundant, stable in aqueous environments, and possesses a bandgap in the visible wavelength range (~2.1 eV). However, a major drawback to hematite is the fast rate of photogenerated electron-hole recombination, which severely limits the number of holes that reach the hematite surface to react with water. This has been a major obstacle in the utilization of hematite photocatalysts. Here, we engineer the properties of hematite and the related corundum-structure oxide eskolaite, Cr₂O₃, to overcome these challenges. High quality samples are fabricated as single-crystalline epitaxial thin films deposited by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). These well-defined thin films allow us to perform detailed characterization of their structural and electronic properties with techniques such as x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), synchrotronbased x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS), optical absorption spectroscopy, and electronic transport measurements. Ti doping can improve the electronic transport properties of hematite, but not eskolaite. Doping with Cr or V lowers the bandgap of hematite, which increases its visible-light photoabsorption and photoconductivity, both desirable properties for solar photocatalysts. Finally, we exploited the electronic band offset properties of hematite/eskolaite interfaces to engineer a built-in potential in Fe₂O₃/Cr₂O₃ superlattices that can electrostatically separate photogenerated electron/hole pairs.

Biography: Dr. Kaspar joined PNNL as a University of Washington graduate student to work with Dr. Scott Chambers. After receiving her PhD in 2004, she continued at PNNL as a post-doctoral researcher, and was hired as a fulltime staff scientist in 2007. Dr. Kaspar's research interests include the epitaxial growth (via oxygen-plasma-assisted molecular beam epitaxy or pulsed laser deposition) and characterization of metallic and metal oxide films and interfaces. She develops fundamental relationships between film composition and the resulting electronic, magnetic, phototactive, and structural properties for applications such as spintronics, photocatalysis, radiation-resistant materials, and advanced sensors.