Electrostatics and Structure at Electrode:Electrolyte Interfaces from Nonlinear Optics

在化学系化学结构、动力学和机制A （CSDM-A）项目的支持下，西北大学的Franz Geiger教授正在使用先进的激光方法研究带电物质（离子）和水分子如何在金属电极附近的溶液中排列。描述这种排列的教科书模型可以追溯到19世纪，但它只描述了离子的位置，而没有解释它们的运动。实际情况要复杂得多，因为水分子在金属表面附近的排列方式与在液体中的排列方式不同，离子在电极附近的这个界面层（称为双电层，EDL）中进出。然而，研究EDL是具有挑战性的，因为它只有几纳米厚，大约比一张纸薄10万倍。盖格教授和他的学生正在使用激光技术直接观察金属表面附近的水分子和离子的结构。他们的发现可能为从清洁能源生产、能源储存到环境修复等一系列技术的进步提供根本性的见解。该项目还通过为学生创造研究机会，为国家科学、技术、工程和数学（STEM）劳动力的发展做出了贡献，该团队正在将他们的研究纳入埃文斯顿高中化学课程，通过科学大使计划增加STEM的参与。Geiger教授和他的学生通过测量从电极-电解质界面外差检测到的二次谐波（HD-SHG）信号的振幅和相位，可以直接探测斯特恩层和扩散层的结构和静电特性。利用这种技术，研究小组正在研究这两种双电层组分之间离子的动态交换，这种交换是对外部刺激（外加电位、pH值、电解质浓度）的响应，并测量EDL上的总界面电位下降。HD-SHG方法也被应用于宽视场显微镜模式，能够在电化学反应过程中对电极表面EDL的空间变化进行成像。通过量化库仑和非库仑对界面电位下降的贡献，以及通过了解界面水分子和离子的结构，他们的目标是改进描述电极过程的百年模型。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professor Franz Geiger of Northwestern University is using advanced laser methods to study how charged species (ions) and water molecules are arranged in solution near a metal electrode. The textbook model describing this arrangement goes back to the 19th century but describes only the location of the ions and it does not account for their motion. The actual situation is more complicated because the water molecules are arranged differently near the metal surface than they are in the liquid, and the ions move in and out of this interfacial layer near the electrode, called the electrical double layer, or EDL. However, studying the EDL is challenging, as it is only a few nanometers thick, or about 100,000 times thinner than a sheet of paper. Professor Geiger and his students are using laser techniques to directly observe the structure of the water molecules and the ions near the metal surface. Their discoveries could provide fundamental insight that advances technologies ranging from clean energy generation and energy storage to environmental remediation. The project is also contributing to the development of the Nation's science, technology, engineering, and mathematics (STEM) workforce by creating research opportunities for students, and the team is integrating their research into the Evanston high school chemistry curriculum to increase participation in STEM through a science ambassadorship program. By measuring the amplitude and phase of heterodyne-detected second harmonic generation (HD-SHG) signals from electrode:electrolyte interfaces, Professor Geiger and his students can directly probe the structural and electrostatic properties of the Stern and diffuse layers . Using this technique, the team is studying the dynamic exchange of ions between these two components of the electrical double layer that occurs in response to external stimuli (applied potential, pH, electrolyte concentration) and measuring the total interfacial potential drop across the EDL. The HD-SHG methodology is also being implemented in a wide-field microscopy mode capable of imaging spatial variations in the EDL across an electrode surface during electrochemical reactions. By quantifying the Coulombic and non-Coulombic contributions to the interfacial potential drop, and by learning about the structure of interfacial water molecules and ions, they are aiming to improve century-old models for describing electrode processes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.