Recent developments in X-ray absorption spectroscopy

The capacity of X-ray absorption spectroscopy (XAS) to determine the electronic and local geometric structures of materials in all aggregation states in an element-specific manner has made it a tool of choice for characterization in materials science in general. Because of the extent of penetration depth of hard X-rays, measurements can be performed under extreme conditions, such as high pressure, relevant to geologic samples, and under conditions relevant to sensors and catalysis. In situ or operando measurements are routinely performed, often in combination with complementary techniques such as infrared, Raman, UV–vis spectroscopy, and X-ray diffraction. Thus, XAS has contributed tremendously to the determination of structure and the relation of structure to its function.

Despite its age, new developments continue to be made, providing exciting new opportunities. Technical developments enable the production of smaller and smaller beams and faster and faster gathering of data. Moreover, the high fluxes at third-generation beam lines have enabled us to take full advantage of the opportunities that secondary X-ray emission spectroscopy (XES) offers. Furthermore, theoretical understanding of the pre-edge and near-edge regions has improved significantly; this knowledge enables the maximum information from experimental data. This issue of PCCP illustrates how the application of the most advanced XAS methods leads to solving questions in a variety of scientific fields.

Time- and space-resolved measurements are becoming available on a routine basis. The ability to gather high quality EXAFS data of elements of relatively low loading on a (sub-) micrometer length scale and sub-second timescales provides important opportunities to follow changing structures on these length and time scales. This is employed in the field of catalysis, where the frequent aim is to determine the structure of the catalytically active site, as addressed in several papers in this issue. The possibility for fast measurements also enabled the introduction of modulation excitation as described by Ferri et al. (DOI: 10.1039/B926886C). With this method it is possible to detect the periodic spectral changes upon a cyclic change in, for example, gas concentration, temperature, or pressure. Detection of a phase-sensitive signal dramatically enhances the sensitivity of XAS. I expect that time-resolved spectroscopy will also be used more extensively to determine kinetic data of catalytic processes as illustrated by Tada et al. (DOI: 10.1039/C000843P).

In very different, ultra-fast measurements, it is becoming possible to detect the electronic and geometric structures of short-lived reaction intermediates. The paper by van der Veen et al. (DOI: 10.1039/B927033G) describes ultra-fast measurements to identify geometric and electronic changes in a photocatalytic diplatinum molecule upon photo excitation. Observing the ultra-fast dance of atoms after excitation will become possible with free electron lasers, which will be available in years to come. The paper by Patterson and Abela (DOI: 10.1039/C003406A) identifies several new opportunities relevant to XAS, as provided by X-ray free electron lasers. They emphasize ultrafast time-dependent measurements, which will enable the measurement of chemical reaction dynamics. I hope to be proven right in my assumption that these ultra-high flux instruments will provide new opportunities that we cannot even imagine.

The improved theoretical understanding of absorption pre-edges and near-edges has led to in-depth knowledge of spectra. Such developments were at the origin of the so-called ΔXANES method. This method reveals the structure and interaction of the adsorption site with an adsorbate on the surface of nano-sized particles, which is relevant to catalysis, sensors, and fuel cells. Various papers in this issue show how this method is used to determine the surface structure of nano-sized particles and even the quantitative analysis of adsorbates on particle surfaces.

As mentioned above, new beam lines that provide very high photon flux and the development of single- and multi-crystal spectrometers has led to a large increase in the number of XES experiments. The energy dispersive detection of fluorescence radiation with high resolution enables the measurement of spectra that show less life-time broadening. Such high-energy resolution fluorescence detected (HERFD) spectra provide more detailed geometric and electronic information, as illustrated by the paper by Singh et al. (DOI: 10.1039/C000403K). While XAS provides information about the unfilled density of states, resonant inelastic X-ray scattering (RIXS) provides the electronic structure of the filled density of states. Because of the exclusive use of hard X-rays, these methods can be performed in situ and under operando conditions, shown by Singh et al. (DOI: 10.1039/C000403K) and Bauer and Gastl (DOI: 10.1039/B926385C).

Overall, this issue shows that the field of XAS, although highly developed, still provides new and exciting opportunities. The continued collaboration among physicists, spectroscopists, chemists, materials scientists, theoreticians, and technicians will, without a doubt, provide even more opportunities, some which are based on existing knowledge and others that are as yet unknown and which will broaden our understanding of structure and its function. The future is bright!

 

Thank you for your attention.

 

Jeroen van Bokhoven, ETH Zurich and Paul Scherrer Institute, Switzerland

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