Photoelectron Emission at Diamond-Liquid Interfaces

金刚石表面，包括廉价的薄膜和工业级金刚石粉末，在紫外光照射下具有通常的直接向水和其他液体发射电子的能力。水中的电子是一种有效的化学还原剂，能够诱导非常困难的化学转化，如将二氮转化为氨和还原二氧化碳，但由于缺乏方便有效的制备方法，它们尚未得到充分研究。使用廉价的、可重复使用的工业级金刚石将电子直接发射到水中的能力为诱导传统光化学或电化学方法无法获得的新型化学转化提供了许多机会。在材料研究部门的固态和材料化学计划的支持下，该项目的目标是了解影响金刚石向水中发射电子的基本材料特性，利用这种理解来创建在水中和其他非真空环境中具有良好稳定性的光电子发射器，并确定是否可以对金刚石进行改性以使其能够利用可见光容易地发射电子。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的新的节能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的培训，并获得广泛的专业发展机会。 首席研究员和学生还指导本科生和高中生的暑期研究项目，作为更广泛的努力的一部分，以增加谁有机会从事国家的最先进的科学研究的学生人数。 主要研究者和学生积极参与几个专门针对提高科学劳动力的多样性的计划。技术摘要这个更新项目是主要研究者的研究成果，证明了金刚石表面在紫外光照射下能够向水中发射电子，从而使廉价的金刚石薄膜甚至工业粉末可以用作液体中的固态电子源。 虽然以前已经研究了电子发射到真空中，但对电子发射到水和其他液体中知之甚少。该项目的主要目标是研究控制电子从金刚石到邻近液体的光发射的因素，并利用这些信息来创建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。一个是了解金刚石-液体界面的电子结构如何影响电子发射到相邻的液体中，通过表征诸如表面终止层，溶液相离子和外部施加的电势等变量如何影响电子发射。 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金刚石的光学吸收和/或提供将电子激发到导带的替代路径。 第三个是确定由替代氮或其他掺杂剂引入的中间带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。 正在采用各种实验方法。X射线光电子能谱和紫外光电子能谱正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正在被用来表征界面的电子结构和能带排列在水介质中。 溶剂化的电子被直接使用瞬态吸收光谱和通过化学探针的特点。 总之，这些方法提供了对电子发射到液体中的全面理解，并提供了如何设计最有效地使金刚石在非真空环境中用作固态电子源的界面的见解。

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.