Controlling Charges on Oxide Surfaces for Enhanced Photochemical Reactivity

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: The economically feasible separation of hydrogen fuel from water using light remains an important technical goal for the scientific community. Hydrogen is a valuable fuel because it has a high energy density and its combustion does not generate greenhouse gases. One factor that limits efficient hydrogen synthesis by water splitting is the performance of the available catalysts. For the surface of the catalyst to split water, it must perform two functions: it must transfer both negatively-charged electrons and positively-charged holes to water molecules on the surface. Whichever of these functions occurs more slowly limits the overall reaction. In this project, catalysts are created that have two different types of surfaces. Some areas of the surface promote the transfer of negative charge and the other areas promote the transfer of positive charge. The relative areas are adjusted to optimize the overall reaction rate. This approach provides a valuable tool for the design of water splitting catalysts for economic solar hydrogen production. The project will have broader impact through the training of undergraduate and graduate students, web-based applications for education and integration into a course on sustainability, the dissemination of the research results beyond the campus, and broadening participation, particularly of women in engineering/science. TECHNICAL DETAILS: This project is based on the hypothesis that the relative areas of reducing (positive) and oxidizing (negative) domains on an oxide surface can be controlled and that this ratio influences the overall photochemical reaction rate of catalysts that can be used to produce solar fuel or degrade environmental pollutants. Controlled-atmosphere high-temperature annealing is used to tailor the surface termination. The surface charge distribution is measured by scanning potential microscopy and evaluated using photo-reduction and oxidation reactions that leave insoluble products at the site of the reaction. The novelty and merit of this project lie in the efforts to control the termination chemistry and correlate the types and relative areas of polar domains to the chemical properties of the surface. Because oppositely charged surface terraces promote separately the reduction and oxidation half reactions, surfaces with a combination of oppositely charged domains provide a nearly ideal environment where photo-generated charge carriers are separated, reaction products are separated, and the relative rates of the two reactions are controlled by the relative areas of the charged domains. Each of these factors improves photocatalytic efficiency by mitigating losses associated with recombination, back reaction, and an imbalance in the number of reactive sites for the two half reactions. This project demonstrates how to control polar surface domains and create more efficient photocatalysts needed for the practical production of solar hydrogen.