Photoelectron Emission at Diamond-Liquid Interfaces

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Non-technical AbstractDiamond surfaces, including inexpensive thin-films and industrial-grade diamond powders, have the usual ability to emit electrons directly in to water and other liquids when illuminated with ultraviolet light. Electrons in water are a potent chemical reducing agent, able to induce very difficult chemical transformations such as the conversion of dinitrogen to ammonia and the reduction of carbon dioxide, but they have not been well studied because of the absence of convenient and efficient methods of preparation. The ability to directly emit electrons into water using inexpensive, reusable industrial-grade diamond provides a number of opportunities for inducing novel chemical transformations not accessible with conventional photochemical or electrochemical methods. With support of the Solid State and Materials Chemistry program in the Division of Materials Research, the objectives of this project are to understand the fundamental materials properties that influence electron emission from diamond into water, to use this understanding to create photoelectron emitters that function with good stability in water and other non-vacuum environments, and to identify whether diamond can be modified to allow it to easily emit electrons using visible light. If successful, the ability to easily produce electrons in water using visible or near-ultraviolet light would enable new, energy-efficient chemical transformation pathways not currently possible. Graduate students are being trained in state-of-the-art electrochemical methods and receive extensive professional development opportunities. The principal investigator and students are also mentoring undergraduate students and high school students on summer research projects as part of a broader effort to increase the number of students who have the opportunity to engage in state-of-the-art scientific research. The principal investigator and students are actively participating in several programs targeted specifically toward enhancing the diversity of the scientific workforce. Technical AbstractThis renewal project is an outgrowth of research from the principal investigator demonstrating that diamond surfaces are able to emit electrons into water when illuminated with ultraviolet light, thereby allowing inexpensive diamond thin films and even industrial powders to be used as solid-state sources of electrons in liquids. While electron emission into vacuum has been studied previously, little is known about electron emission into water and other liquids. The primary goal of is project is to investigate the factors that control the photoemission of electrons from diamond into adjacent liquids, and to use this information to create photoelectron emitters that function with good stability and efficiency in non-vacuum environments. The project has three primary components. One is to understand how the electronic structure of the diamond-liquid interface influences the emission of electrons into adjacent liquids, by characterizing how variables such as the surface terminating layers, solution-phase ions, and externally applied potentials influence electron emission. A second is to determine whether metal-semiconductor junctions and plasmonic structures are able to increase the optical absorption of diamond and/or provide alternative pathways to exciting electrons to the conduction band. A third is to determine whether mid-gap states introduced by substitutional nitrogen or other dopants can provide a pathway to achieving electron emission using visible light. A wide range of experimental methods are being employed. X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopies are being used to characterize the chemical composition and electronic structure of diamond surfaces. Electrochemical methods including Mott-Schottky Analysis and Electrochemical Impedance Spectroscopy are being used to characterize the interfacial electronic structure and band alignments in aqueous media. Solvated electrons are being directly characterized using transient absorption spectroscopy and via chemical probes. Together, these methods are providing a comprehensive understanding of electron emission into liquids and providing insights into how to design interfaces that are most effective in enabling diamond to be used as a solid-state source of electrons in non-vacuum environments.