The underlying theme of our research is the unique combination of optical irradiation with solid-state NMR. Broadly, our research can be classified into four primary areas:
- Optically-enhanced NMR of semiconductors in bulk and low-dimensional heterostructures;
- Photoinduced structural changes (i.e., [2+2] cycloadditions) in materials which are candidates for optical memory and/or optical switches;
- Investigations into CO2 sequestration in various mineral structures;
- Characterization of inorganic nanostructures, including Al and Ga nanoclusters.
Our group possesses unique hardware that permits optical access at the NMR sample space, thereby allowing us to irradiate samples in situ. We are looking at a variety of materials via these methods, including:
- III-V and II-VI bulk semiconductors; quantum wells, quantum wires, and quantum dots of the II-VI systems;
- Crystalline organic species, including some supramolecular complexes;
- Chalcogens (77Se, 125Te) in various forms, including clusters, II-VI nanoclusters, and some III-V passivating agents.
Our group, therefore, is developing expertise along several lines, including:
- Characterization of low-dimensional semiconductors;
- Radio-frequency probe and hardware construction;
- Simulation of NMR spectra.
Solid-State NMR Characterization of CSMC Materials
Inorganic metal oxides with group 13 metals offer good targets for solid-state NMR analyses. Our group focuses on the characterization of group 13 cluster precursors, and the transformation of the precursors to thin films. Specifically, the group has been investigating and characterizing [Ga13(μ3-OH)6(μ2-OH)18(H2O)24](NO3)15 cluster precursors and the corresponding mixed gallium and indium heterometallic clusters [Ga13-xInx(μ3-OH)6(μ2-OH)18(H2O)(NO3>15 (x = 1-6) using solid-state NMR.
For more information about the CSMC, please visit their website at www.sustainablematerialschemistry.org.
These projects are funded through DOE NETL, DOE EFRC, and NSF CBET grants. The large encompassing goal of these projects is to study CO2 and other gases for sequestration and capture.
Carbon capture and sequestration (CCS) is currently being pursued as a means of reducing net carbon dioxide (CO2) output from power plant sources by capturing the CO2 then utilizing it or sequestering it. Geological sequestration and chemical utilization of CO2 as a feedstock chemical are actively being explored as possible mechanisms for reducing net anthropogenic CO2 release.
CCS has a number of technical and scientific challenges involving CO2, which our research aims to address. These questions span a broad range of topics, including:
1. Geological Sequestration
• How is CO2 stored, and what happens to it after injection?
• What geological conditions are favorable for geochemical trapping (i.e., mineralization) versus physical trapping (i.e., containment in underground reservoirs)?
• How is the permeability of geologic reservoirs affected by carbon sequestration?
• Under what conditions will a physical trapping site release CO2 and at what rate?
• Are there specific reaction conditions that favor conversion of CO2 into a chemical endproduct?
• What are the mechanisms and kinetics governing such reactions?
• Can conditions be optimized to favor product yields?
• Can the endproducts be adequately characterized to enhance the reaction condition?
We are currently developing a new and unique set of in situ spectroscopic tools which will be able to study these different mitigation systems using nuclear magnetic resonance (NMR) measurements. The images below describe the conditions in which our apparata are designed to take measurements as well as some of our proof-of-concept experimentation and schema for instrument design.
The Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME) is funded by the U.S. Department of Energy. The focus of the EFRC is to advance understanding of how acid gases interact with energy-related materials.
UNCAGE-ME seeks to provide a fundamental understanding of acid gas interactions with solid materials through integrated studies of the interaction of key acid gases (CO2, NO2, NO, SO2, H2S) with a broad range of materials. We combine the application of in situ molecular spectroscopic studies of both the surface functionalities and bulk structures of materials relevant to catalysis and separations under relevant environmental conditions with complimentary multiscale computational and theoretical modeling of acid gas interactions with solid matter. Insights gained by the multi-investigator, multidisciplinary teams will allow us to achieve the following long-term goals:
1. Develop a deep knowledge base characterizing acid gas interactions applicable to a broad class of materials.
2. Develop fundamental knowledge allowing practical predictions of materials interacting with complex gas environments on long time scales.
3. Advance fundamental understanding of the characterization and control of defects in porous sorbents.
4. Accelerate materials discovery for large-scale energy applications by establishing broadly applicable strategies to extend material stability and lifetime in the presence of acid gases.
Some of the NMR work done at Washington University in Saint Louis involves variable temperature 13C NMR of 13CO2 loaded into a metal-organic framework (MOF) called Mg-MOF-74. The variable temperature work provides insight to the molecular dynamics of CO2 inside the 1-dimensional channels of the MOF. The 13C NMR lineshape changes as a function of CO2 loading and temperature, and probes the dynamics of the CO2 molecules adsorbed in the MOF.
27Al NMR measurements of carbide-derived carbons (CDCs) from Al4C3 have been performed at Washington University in Saint Louis. 27Al is a quadrupolar nucleus (I = 5/2), which means many times high magnetic fields and fast spinning speeds are needed for characterization. We characterized the residual aluminum content of these materials using 27Al MAS NMR with a magnetic field of 13.9 T (600 MHz) and a 2.5 mm Bruker MAS probe capable of high spinning speeds (~35 kHz).
Data Infrastructure Building Blocks (DIBBs) is a part of NSF’s Cyberinfrastructure Framework for 21st Century Science and Engineering (CIF21), focusing on construction of a cyberinfrastructure to provide assistant to theoretical, experimental and simulation-oriented efforts in science and engineering.
We are currently working with our collaborators trying to build a Nuclear magnetic resonance (NMR) data infrastructure containing existing parameters, spectrum and also computational data of inorganic solid state materials so that researchers can access via internet to identify, compare and analyze their own experimental data and even predict the NMR spectrum before the experiment. The interface of the database will be Materials Project, a website dedicated to collection and computation effort of inorganic solid state materials’ properties. Up till now, we have been focusing on Al27 NMR. With the crystal structure data collected, we are trying to get computational spectrum with The Vienna Ab initio Simulation Package (VASP) and compare with experimental data.
Optically-pumped NMR (OPNMR) combines laser excitation with NMR detection. Electrons are excited into the conduction band and spin polarized by the incident photons. Nuclear spins in the lattice become oriented by the photoexcited electrons through the hyperfine interaction. This nuclear orientation is subsequently detected by NMR (radio-frequency detection).
We are applying optical pumping techniques to systems of semiconductors to better understand their optoelectronic properties. Furthermore, we are developing on methods to examine low-dimensional III-V and II-VI semiconductors with OPNMR.
A schematic of the experimental setup and optical pumping mechanism are shown below:
Observing Structural Changes of Optical-Memory Materials with Solid-State NMR
Optical switch materials are gaining interest in the scientific community because of their use in photolithography, data storage, photoresists, and solar energy storage. Because of their applicability, an understanding is needed of the mechanisms by which solids change their structure when irradiated with light. Our work focuses on using solid-state nuclear magnetic resonance (SS-NMR) spectroscopy to elucidate structural changes in optical-switching solids when irradiated with light.
We are currently investigating crystals of cinnamic acid, a compound which undergoes the known [2 + 2] photocycloaddition reaction to form truxillic acid. We use cinnamic acid as a model compound in order to understand the kinetics and mechanism of photoreactive solids in general. We try to address issues such as the rate/reversibility of the photoreaction, the dimensionality of the growth of the photoproduct phase, and intermediates that may be key in the fate of the photo reaction.