Photoelectron Emission at Diamond-Liquid Interfaces

非技术性摘要钻石表面，包括廉价的薄膜和工业级钻石粉末，在紫外线照射下，通常具有将电子直接发射到水和其他液体中的能力。水中的电子是一种强有力的化学还原剂，能够引起非常困难的化学转化，如将氮转化为氨和还原二氧化碳，但由于缺乏方便和有效的制备方法，对它们的研究还不是很好。使用廉价的、可重复使用的工业级钻石将电子直接发射到水中的能力为诱导传统的光化学或电化学方法无法实现的新的化学转变提供了许多机会。在材料研究部固态和材料化学计划的支持下，该项目的目标是了解影响钻石向水中发射电子的基本材料性质，利用这种了解创造在水和其他非真空环境中具有良好稳定性的光电子发射器，并确定是否可以对钻石进行修饰，使其能够利用可见光轻松发射电子。如果成功，利用可见光或近紫外光在水中轻松产生电子的能力将使目前不可能实现的新的、节能的化学转化途径成为可能。研究生正在接受最先进的电化学方法培训，并获得广泛的职业发展机会。首席研究员和学生还在指导本科生和高中生的暑期研究项目，作为增加有机会从事最先进科学研究的学生数量的更广泛努力的一部分。首席研究人员和学生正在积极参与几个专门针对增强科学劳动力多样性的项目。技术摘要这个更新项目是首席研究人员的研究成果，表明钻石表面在紫外光照射下能够向水中发射电子，从而允许廉价的钻石薄膜甚至工业粉末用作液体中的固态电子源。虽然对真空中的电子发射已经进行了研究，但对水和其他液体中的电子发射却知之甚少。IS项目的主要目标是调查控制电子从钻石进入邻近液体的光电子发射的因素，并利用这些信息创造在非真空环境中具有良好稳定性和效率的光电子发射器。该项目有三个主要组成部分。其一是通过表征表面终止层、溶液相离子和外加电势等变量如何影响电子发射，从而了解钻石-液体界面的电子结构如何影响电子向邻近液体的发射。第二个是确定金属-半导体结和等离子体结构是否能够增加钻石的光学吸收和/或提供替代路径来激发电子到导带。第三个是确定由替代氮或其他掺杂剂引入的中间能隙状态是否可以提供一条利用可见光实现电子发射的途径。目前正在采用广泛的实验方法。X射线光电子能谱和紫外光电子能谱被用来表征金刚石表面的化学成分和电子结构。电化学方法包括Mott-Schottky分析和电化学阻抗谱被用来表征水介质中的界面电子结构和能带排列。使用瞬时吸收光谱和化学探针直接表征溶剂化电子。总而言之，这些方法提供了对进入液体的电子发射的全面了解，并为如何设计最有效地使钻石在非真空环境中用作固态电子源的界面提供了见解。

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.