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

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.