X-ray Absorption Spectroscopy

XAS is a core-level spectroscopy technique, using a photo-excited electron from a core level (e.g. 1s or 2p) to probe unoccupied valence levels as well as the neighboring atomic structure. The ionization of core levels requires photons in the energy in the X-ray range, and spectroscopy requires an intensive continuous energy-spectrum, hence XAS is carried out at synchrotron radiation sources that provide both.

The measurement is conducted by scanning the incident photon energy using a monochromator. Once a sufficient energy is reached to ionize the atom at its core level, the absorption steeply increases at what is known as an absorption edge. Every element in the periodic table has a unique absorption edge, making the technique conveniently element-specific. The portion of the spectrum around the edge, known as the X-ray absorption near-edge structure (XANES) is a rich probe for the electronic structure of the unoccupied states as the low-energy photoelectron occupies these states. Chemical information about the oxidation state and local geometry is obtained from the XANES. As the incident energy is increased, more energy is transferred to the photoelectron, exciting it to the continuum of states and enabling it to back-scatter from neighboring atoms within ca. 10 Å. The back-scattering of the photo-electron causes a quantum-mechanical overlap between its initial and final state, causing an oscillatory modulation of absorption, or the extended X-ray absorption fine-structure (EXAFS). The Fourier-transform of the EXAFS is a radial distribution function, from which bond distances, number and speciation of neighboring atoms can be extracted.

XAS is the core technique of our group, since it is powerful in observing the chemical state and atomic structure in catalysts, especially under reaction conditions.

Reversible Cation and Anion Redox in Lithium-rich Sulfide Battery Cathodes

May 31, 2020

While steady improvement of lithium-ion batteries has allowed electronic technologies to perform better, researchers are nearing a theoretical limit to lithium-ion battery capacity. One way to overcome this limit is to change the chemistry of materials to allow more electrons to exchange between anode and cathode per unit of material. Currently, LiCoO2 lithium-ion batteries transfer one electron per unit of cathode, but other lithium-based materials may allow for higher capacity. A team of researchers has investigated the Li-rich layered sulfide Li2FeS2 and a novel analog LiNaFeS2 as potential higher capacity alternatives since they can store 1.5 or more electrons per unit.

Quantification of Heterogeneous Degradation in Li-ion Batteries

June 30, 2019

The development of better rechargeable batteries for consumer electronics and electric vehicles is difficult due to the complex interplay of many chemical, spatial, and temporal factors. Taken together, these factors are called the chemomechanical interplay, which includes chemical degradation, chemical heterogeneity, and mechanical stress that cause the battery to lose functionality over many charging and discharging cycles. A team of researchers has developed a combined methods approach that allows quantification of the processes of chemomechanical interplay over diverse length and time scales.

Large-Scale Production of 119mTe and 119Sb for Radiopharmaceutical Applications

May 31, 2019

Radioisotope therapies improve on traditional chemotherapies by being finely targeted to only the diseased cells and leaving surrounding healthy cells unharmed. A promising radioisotope for therapeutic uses is 119Sb, which releases low energy Auger electrons that can kill cancer cells. A problem with widespread use of drugs using 119Sb is its short half-life of around 38 hours. A team of scientists from Los Alamos National Laboratory have developed a novel strategy for utilizing 119Sb.

Shared-Ligand Intermediates of Metal Exchange Visualized by Rapid Freeze Quench and Selenium EXAFS of Se-Labeled Metallochaperones. A Paradigm for Studying Copper-Mediated Host-Pathogen Interactions

January 30, 2019

To defend against infections, our phagocyte cells form a vesicle called a phagosome around pathogens, which then merges with a lysosome to form a phagolysosome. To terminate the threat, the phagolysosome gives the invading cell toxic doses of copper. However, some bacteria have evolved mechanisms for pumping the copper back out of the cell, avoiding toxicity. Understanding the enzymes involved in these complicated processes is important to our understanding of disease.

Copper Mobilization and Immobilization along an Organic Matter and Redox Gradient – Insights from a Mofette Site

January 31, 2019

While a small amount of copper is essential for living organisms, too much copper contaminating our soils can be toxic and pose a serious problem. Copper has an affinity for organic matter in soils, where it mainly exists in the two redox states Cu(I) and Cu(II). In soils that fluctuate in redox conditions, the mobility of copper through the environment can be hard to predict. Mofette sites, produced by CO2 degassing usually found in seismically active areas, are good natural laboratories due to their wide range of soil redox conditions and of soil organic matter composition within a small area. Near the sites of CO2 degassing, the soil is anoxic and organic matter does not decompose well. The soils transition to oxic conditions just a few meters away. A team of researchers studied the behavior of copper in the natural gradient of a mofette site in the Czech Republic.

Gold Nanoparticle Biodissolution by a Freshwater Macrophyte and Its Associated Microbiome

September 30, 2018

Nanotechnology, which focuses on materials that measure between 1 and 100 nanometers in at least one dimension, is being applied to diverse areas of research including medicine, electronics, and biology. Yet it is unclear how these engineered nanomaterials might interact with and affect environments and ecosystems.

Direct Observation of the Kinetics of Gas–Solid Reactions Using in-Situ Kinetic and Spectroscopic Techniques

August 31, 2018

Hydrogen sulfide (H2S) is a poisonous and corrosive gas created in industrial and natural systems. Copper oxide (CuO), a crystalline solid, can be used to clean H2S from emissions by forming various copper sulfide species, a reaction that is thermodynamically favorable but often does not go to completion in industrial applications.

Activation of MnO2 Catalysts by Mn3+ Ions

July 31, 2018

The more widespread use of solar electricity is not currently limited by the technology for generating energy from sunlight but by the storage of that energy, so that it can be used when needed.  Converting water to O2 and H2 via the oxygen evolution reaction (OER) is a fossil fuel free way to store energy for later use; catalysts that improve the efficiency of OER are being sought. Manganese oxide (MnO2) films are good catalysts of OER, with additional benefits of being acid-stable and earth abundant.

Redox-transformation Kinetics of Aqueous Thio-arsenic Species Determining Arsenic Sequestration by Organic Thiol Groups of Peat

June 30, 2018

Arsenic is a well-known toxin that can contaminate our drinking supplies. Understanding how arsenic finds its way into drinking water requires research into its interaction with environmental conditions that affect redox reactions, including interactions with iron, sulfur, and carbon.

Operando Spectroscopic Microscopy of LiCoO2 Cathodes Outside Standard Operating Potentials

September 30, 2017

Given our increasing dependence of rechargeable battery containing electronic devices, including electric cars, it is important to engineer these systems to mitigate potential for catastrophic battery failure. One possible source of lithium ion battery failure is over-discharge (over-lithiation) of the cathode, which can permanently damage the battery. Electronic battery management systems are programmed to prevent and identify such failures, but sometimes do not catch problems of over-lithiation when they occur. To better understand the characteristics of battery failure from over-discharging, a team of scientists studied the chemical and morphological changes that occur from over-lithiation of a lithium battery cathode.


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