Development of New Tools for the Chemical Analysis of the Internal Surfaces of Porous Materials using Annihilation Gamma Spectroscopy

在化学系化学测量和成像(CMI)计划的支持下，由德克萨斯大学阿灵顿分校(UTA)的Alexander Weiss、Varghese Chirayath和Ali Koymen教授领导的研究小组正在开发一种新技术，以探测原本无法接触到的多孔材料内表面的化学成分。UTA的研究小组最近表明，正电子(反物质电子)与表面电子湮灭所产生的伽马射线的能谱可以用来确定外表面最顶层的元素组成。该团队目前正在努力证明，正电子湮没伽马光谱可以用来揭示催化或反应气体暴露带来的纳米多孔金属内部隐藏表面的化学变化。该项目的目的是开发一种迄今无法获得的工具，用于对多孔材料的内表面进行化学分析。这项研究的结果对许多研究领域都有潜在的影响，包括催化、能量储存、气体分离、碳捕获和稀合金设计。该项目为本科生和研究生提供先进的表面改性和表征方法方面的培训。参与这个项目的学生来自UTA的高度多样化的学生群体，这是一个西班牙裔服务机构。研究小组还与阿灵顿独立学区(AISD)的STEM学院合作，为选定的高中生提供培训。学生参与该项目的前沿研究有助于塑造下一代先进化学仪器的领导者，并为这一关键国家需求领域的劳动力发展做出独特贡献。多孔材料的催化活性内表面对其环境的响应强烈表明，表面组成控制催化功能，表明需要能够在操作条件下以高表面选择性探测内表面的工具。显然，目前还没有实验方法可以非破坏性地提供有关内表面顶层原子层的化学信息，而不受大量信号的干扰。在化学测量和成像(CMI)计划的支持下，UTA的正电子研究团队正在开发一种操作中表征工具，用于无法接触的内表面，元素敏感性低至最顶部原子层的1%。该团队将正电子能谱卓越的表面选择性与511keV湮没伽马辐射的能力结合在一起，使其能够穿过毫米的样品或反应池，而不会丢失任何信息。该技术无与伦比的表面选择性源于在湮灭前将注入的正电子俘获在样品表面真空侧的像势井中。正如UTA团队最近证明的那样，由表面俘获正电子湮没产生的伽马辐射的能谱通过湮没伽马能量的特定于元素的多普勒展宽反映了最顶层原子层的元素组成。使用从受控外表面获得的校准光谱来作为多普勒加宽湮没光谱的基准，该团队现在正在开发一种技术来测量纳米多孔金(Au)和纳米多孔铜(Cu)内表面的化学成分，包括暴露在反应气体中或催化过程中发生的变化。研究小组正在探索这些材料中催化活性中心的性质，目的是通过在超高真空和手术条件下的受控实验，提供纳米孔Au和Cu在臭氧和一氧化碳等反应刺激作用下的内表面构型的全局视图。除了这项工作的更广泛的科学影响外，该项目还为研究生和本科生提供高级培训机会，并支持让当地高中生参与STEM研究的外联活动。该奖项反映了NSF的法定使命，并通过使用基金会的智力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 Measurement and Imaging (CMI) Program in the Division of Chemistry, the research team led by Professors Alexander Weiss, Varghese Chirayath, and Ali Koymen from the University of Texas at Arlington (UTA) are developing a new technique to probe the chemical composition of otherwise inaccessible internal surfaces of porous materials. The research team at UTA has recently shown that the energy spectrum of gamma rays resulting from the annihilation of positrons (anti-matter electrons) with electrons at the surface can be used to determine the elemental composition of the topmost layer of external surfaces. The team is now working to demonstrate that positron-annihilation gamma spectroscopy can be used to reveal chemical changes to the hidden surfaces inside of nanoporous metals brought about by catalysis or reactive gas exposure. The project is aimed at the development of a hitherto unavailable tool for the chemical analysis of porous materials' internal surfaces. The results of this research have potential impact across many areas of research, including catalysis, energy storage, gas separation, carbon capture, and dilute alloy design. The project provides training to undergraduate and graduate students in advanced surface modification and characterization methods. The students involved in this project are from the highly diverse student body of UTA, which is a Hispanic Serving Institution. The research team also collaborates with the STEM academy of the Arlington Independent School District (AISD) to provide training to select high-school students. Participation by students in the cutting-edge research of this project is serving to mold the next generation of leaders in advanced chemistry instrumentation and making a unique contribution to workforce development in this area of critical national need.The response of catalytically active internal surfaces of porous materials to their environment strongly suggests that surface composition controls catalytic function, demonstrating the need for tools capable of probing internal surfaces with high surface selectivity under in operando conditions. There is a glaring absence of experimental methods that can non-destructively provide chemical information about the top atomic layer of internal surfaces that is free of interference from bulk signals. With the support of the Chemical Measurement and Imaging (CMI) program, the positron research team at UTA is developing an in operando characterization tool for the inaccessible internal surfaces with elemental sensitives as low as 1% of the topmost atomic layer. The team is combining the superior surface selectivity of positron spectroscopy with the ability of 511 keV annihilation gamma radiation to exit through millimeters of sample or reaction cells without any loss of information. The unparalleled surface selectivity of the technique stems from the trapping of the implanted positrons in an image-potential well on the vacuum side of the sample surface before annihilation. As recently demonstrated by the UTA team, the energy spectrum of the gamma radiation resulting from the annihilation of the surface-trapped positrons reflects the elemental composition of the topmost atomic layer through the element-specific Doppler broadening of the annihilation gamma energy. Using calibration spectra obtained from controlled external surfaces to benchmark the Doppler broadened annihilation spectrum, the team is now developing the technique to measure the chemical composition at the internal surfaces of nanoporous gold (Au) and nanoporous copper (Cu), including variations that occur upon exposure to reactive gases or during catalysis. The research team is exploring the nature of catalytically active sites in these materials with the goal of providing a global view of the internal surface configuration of nanoporous Au and Cu under the action of reactive stimuli like ozone and carbon monoxide through controlled experiments in ultrahigh vacuum and in operando conditions. In addition to the scientific broader impacts of the work, the project provides advanced training opportunities for graduate and undergraduate students, and supports outreach activities to engage local high school students in STEM research.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.