Keith Lehuta

The need for energy sources outside of fossil fuels has been a focal point of the scientific community in recent years. The most abundant energy source is the sun with approximately 3.85 × 10²⁴ J/yr reaching the earth’s surface. While there are multiple ways this can be done, this work focuses on using sunlight to drive the photocatalytic splitting of water into H₂ and O₂ gases where H₂ is then utilized as a fuel source. Wide bandgap semiconductor materials, such as TiO₂ and SrTiO₃, have been attractive candidates to split water ever since initial reports in the 1970s showing they could generate hydrogen gas from water. The main limitation to the use of these materials comes from a dependence upon UV light which is only about 4% of the solar spectrum. While it has been shown that visible-light photocatalysis using these materials can be accomplished by transition metal ion doping, my ResearchFest talk will present my recent work that focuses on the effects of doping on the potential applications of SrTiO₃ and Sr₂TiO₄. Through more extensive characterization to analyze the speciation of transition metal dopants during synthesis, we now have a thorough understanding of the structural properties and electronic structures in order to optimize the performance of doped perovskite oxides materials not only for photocatalysis applications but for other functions as well including solid-state memories and ferromagnetism.

The first portion of my talk will focus on the synthetic process of producing internally doped SrTiO₃ and Sr₂TiO₄ with Cr³⁺ and Mn⁴⁺. The most important aspect of this involves the characterization of these materials to distinguish the site in which the dopant is incorporated in order to directly correlate the functionality of the material to the doping process. Through the use of electron paramagnetic resonance (EPR) and emission spectroscopies I was able to fully describe not only the lattice site of the same dopant ion in multiple phases, but also the first characterization of transition metal dopant ions in Sr₂TiO₄. A discussion of the electronic properties of these materials will also occur in order to help posit differences in the potential applications.

The second section of the talk will focus on studying the effect of chemical reduction of the doped materials on their electronic structure. Chemical reductions of TiO₂ and SrTiO₃ have shown improved UV photocatalytic activity as well as visible light photocatalysis due to increased carrier concentrations resulting from oxygen vacancies and formation of Ti³⁺. Using chemical reductions on Cr-doped materials will obtain a significant increase in the concentration of Cr³⁺ in the samples that has been previously shown to improve photocatalytic performance of Cr-doped SrTiO₃ by reducing the undesirable Cr⁶⁺ dopants. Due to the increased charge carriers from reduction in combination with the additional states resulting from the dopant ions, SrTiO₃ and SrTiO₄ materials can be systematically studied to optimize photocatalytic performance. Due to the fact that previous work with chemical reduction to improve photocatalysis has only been conducted on undoped materials, careful correlation between dopant specific characterization and functionality will be discussed.