Extended X-ray Absorption Fine Structure (EXAFS)

Introduction

EXAFS is a part of X-ray Absorption Spectroscopy (XAS), which is usually seen in the energy range of 50–1000 eV above a particular element’s Absorption edge. It provides detailed information on the neighboring atoms, including the number of atoms, their distance, and the degree of disorder in the adjacent atoms. Because EXAFS is the element after the sample type of spectroscopy and is extremely sensitive to short-range order, it is applied to material science, chemistry, catalysis, biology, and environmental science. 

Principle and Methodology of EXAFS

When X-rays collide with an atom, they may be absorbed when the energy is large enough to remove a tightly bound core electron, which results in a photoelectron. The expelled photoelectron is a wave and can scatter off other atoms. Oscillations in the X-ray absorption coefficient as a function of energy arise from the interference between outgoing and scattered waves. These waves represent the EXAFS spectrum.

The EXAFS function χ(k) is obtained by normalization and subtracting out the smooth atomic background from the absorption spectrum. It is then converted from energy space to momentum space by the photon wave vector (k). A Fourier Transform of χ(k) yields the radial distribution function, which indicates the distances and identities of neighboring atoms. 

EXAFS Instrumentation

EXAFS experiments are generally performed at synchrotron radiation sources, owing to the requirement of high-intensity X-ray beams with tunable energy. The apparatus comprises an X-ray source (commonly synchrotron radiation) and a monochromator to focus the energy range. This sample holder can accept diverse physical states (solid, liquid, gas), and detectors for detecting either transmission or fluorescence signals. An absorptance spectrum is recorded by a data acquisition system for analysis. 

Applications of EXAFS

The technique of EXAFS has numerous applications in a wide range of scientific fields, as it provides element-specific, local structural information. In catalysis, it is applied to investigating the local environment around the active sites in the heterogeneous catalysts to help design more efficient catalytic systems. In biochemistry, EXAFS characterizes the metal coordination in metalloproteins and enzymes, providing insight into their function and reactivity. In the field of Nanotechnology, it permits the study of the nuclear configuration of nanoparticle bits at the atomic scale, which is essential for understanding the specific properties of the material. EXAFS is applicable in environmental science as a means of determining metal speciation and binding environments in complex systems, such as soils and aquatic environments. In materials science, the technique is used to investigate defects, dopants, and the local structure of amorphous and crystalline inorganic materials, particularly in the physics of materials and the science and technology of new material development. 

Advantages of EXAFS

Several essential advantages of EXAFS make it a valuable tool for structure determination. It yields element-specific data with high sensitivity to the local atomic environment, as well as throughout the periodic system. The technique is suited for both crystalline and disordered materials. In addition, EXAFS is a non-destructive technique that can be used in situ, i.e., under real-time measurement of various experimental conditions, such as temperature, pressure, or chemical environments. It provides quantitative structural characteristics, such as bond lengths, coordination numbers, and structural disorder. 

Disadvantages of EXAFS

However, there are some disadvantages of EXAFS. The approach depends on synchrotron radiation, limiting its applicability to synchrotron facilities only. EXAFS is not an appropriate technique for studying long-range order, so it does not indicate the balance of the long-range positions of the elements. The detection limit is also relatively high compared to the lighter C, N, and O elements, particularly when they are present in a complex sample matrix.