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.

Direct and Efficient Utilization of Solid-phase Iron by Diatoms

July 31, 2017

Diatoms, single-celled marine algae that create beautiful, symmetric cell walls composed of silica, are critical to ocean ecosystems. Responsible for up to 20% of photosynthesis in oceans, these phytoplankton are also an important part of Earth’s carbon cycles. The potential of diatoms and other phytoplankton to sequester atmospheric CO2 has led to geoengineering ideas like “iron fertilization” of oceans.

Biogenic Non-crystalline Uranium Identified as the Major Component of Uranium Roll-fronts

June 30, 2017

The radioactive element uranium is well-known for its role in nuclear energy. People mine naturally occurring uranium from deep sandstone deposits called roll fronts. Scientists have long thought that abiotic chemical reactions that occur over millions of years resulted in formation of crystalline uranium. An international team of scientists has challenged this basic theory, finding evidence for a different genesis for uranium in roll front deposits. 

Thermodynamic Preservation of Carbon in Anoxic Environments

May 31, 2017

While scientists recognize that oxygen-free soil stores large amounts of carbon, knowledge about the processes that protect and preserve carbon-rich molecules in these environments is lacking. In oxygen-rich soil, microbes break down organic molecules through aerobic respiration, allowing carbon to escape the ground as carbon dioxide gas.

Measuring Real-time Biological and Abiotic Manganese Oxide Reduction

May 31, 2016

The element manganese can have complex interactions with the environment, depending on the prevailing conditions. Manganese(IV) is a strong oxidant but can also bind to environmental toxins and heavy metals, rendering them less harmful. Both geochemical and microbial processes affect the reactions of manganese(IV) in the environment. A team of researchers were interested in following the complicated reactions and mineral products produced during the reduction of manganese(IV) under different environmental conditions.

Atomically Precise Electrocatalyst for Oxygen Evolution Reaction

March 31, 2016

The electrocatalytic conversion of carbon dioxide and water into useful chemicals and fuels is a promising way of mitigating greenhouse gas emissions and of providing sources for renewable energy. Part of these processes is the oxidation of water into molecular oxygen, a reaction that requires a catalyst. Previously, heterogeneous catalysts have been used, but adoption of homogeneous catalysts allows more understanding and fine-tuning of the atomic-level processes.

Geochemical Triggers of Arsenic Mobilization during Managed Aquifer Recharge

February 29, 2016

The practice of storing reclaimed or storm water by refilling an aquifer is called managed aquifer recharge (MAR). Advantages of MAR to regions vulnerable to drought or which have depleted aquifers include water storage for future use, reduced water loss of stored water from evaporation, and stabilization of the aquifers. However, refilling aquifers can change the chemistry, allowing naturally occurring toxins in aquifer sediments to dissolve into the water. Arsenic, a potential poison, is of particular concern, since use of MAR has led to arsenic-contaminated water.

Role of an Oxygen Vacancy Nanostructure on the Switchable Photovoltaic Effect in BiFeO3

February 29, 2016

The list of mechanical and electronic uses for oxide materials is continuously growing, piquing researchers’ interest in how the microscopic properties of these materials affect their functionalities. Oxygen vacancies, which affect electron hopping, have long been identified as a defect in oxide compounds, but researchers now view them as a way to create new, potentially useful, behaviors.

Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting

April 30, 2015

The sun provides more energy than what could ever possibly be consumed. However, switching to solar energy to end our dependence on fossil energy resources is made difficult not merely by how much is consumed, but rather by the pattern of how energy is used: significant amounts are consumed by road and air transportation and must be provided “on board” in the form of fuels. This problem could be solved with new devices that convert sunlight into renewable fuels, for example, by driving a light-induced current between two electrodes that split water by electrolysis into hydrogen and oxygen. Currently, the limiting step for the viability of this process is the oxygen evolution reaction (OER) that takes place at the anode. 

Unconventional Switching Behavior in La0.7Sr0.3MnO3/La0.7Sr0.3CoO3 Exchange-spring Bilayer

March 30, 2015

Advanced permanent magnets with low cost and high energy density are important for next-generation technologies, and one promising type of advanced magnet is the exchange spring magnet. This type of nanocomposite comprises two phases of magnetic materials: “hard” magnets, which can remain uniformly magnetized under large fluctuations in magnetic fields, and are often made of rare earth elements, and “soft” magnets, which have a high energy density but their magnetic states can easily be disturbed by small magnetic fields.

New Method Tracks Metal-ion Movement in Periplasmic Proteins

October 31, 2014

Copper is an essential element for many organisms, however, it becomes toxic to cells at high concentrations. Therefore, organisms have developed ways to tightly regulate cellular copper levels. An example of such a regulatory mechanism is the CusCBFA efflux pump in the bacterium Escherichia coli – a multi-protein system that removes toxic copper (Cu+) and silver (Ag+) ions from the space between the bacterium’s inner and outer cell membranes known as the periplasm. Researchers have recently obtained new insights into the mechanism of this system. This information may prove beneficial for the future development of antimicrobial drugs that shut down bacterial efflux pumps.

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