Synchrotron-based x-ray absorption spectroscopy for energy materials
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Introduction The rapid growth in global fossil-fuel consumption, along with the consequent threat of CO2 emission and pollution, impose urgent needs for alternative renewable and sustainable energy resources, as well as devices to convert them to electricity. In addition, the intermittent nature of some resources, such as wind and solar energy, motivates the development of effective energy-storage systems. Energy conversion and energystorage devices transform energies between different formats through a series of chemical, photochemical, electrochemical, or thermoelectrical reactions. These reactions usually involve complex, dynamic, and interrelated processes, such as charge transport, chemical bond forming and breaking, molecular adsorption, surface reconstruction, and phase transformation. The development of characterization tools to observe and capture these phenomena is a prerequisite to gain insights into the mechanisms of chemical and physical reactions and to ultimately make the necessary breakthroughs in energy technologies. X-ray absorption spectroscopy (XAS) is a powerful technique for studying materials with atomic precision as it directly probes the probability of the transition between atomic energy levels. The x-ray attenuation length depends on the photon energy and the associated x-ray cross section of the material; for 3d transition metals, this is a few hundreds of nanometers
at L-edges and a few micrometers at K-edges. This probingdepth disparity allows one to probe the surface of a sample with soft x-rays and the bulk of the specimen with hard x-rays. The bulk-probing capability is a useful asset in research on energy materials, allowing direct study of samples under various environments and conditions (such as gas, liquid, solid phases and the interfaces between them, cryogenic/elevated temperatures, vacuum, high pressure, in situ/ex situ).1 This article introduces the principle of XAS and illustrates how XAS has been utilized in studies on energy storage, such as in Li-ion and Li-S batteries, and for energy conversion, including CO2 reduction and water oxidation. The latter topic usually involves catalysts and the reactions taking place at gas–solid or liquid–solid interfaces. Examples were chosen to illustrate the capability and importance of in situ XAS.
Principle of XAS XAS2,3 measures the absorption of x-ray photons by a material as a function of x-ray energy, E. As depicted in Figure 1a, the absorption of x-rays results in the excitation of electrons from a core level of a particular atom to the unoccupied states, leaving behind a core hole. An XAS spectrum, which is the plot of absorption coefficient as a function of E, such as the
Xiaosong Liu, State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, China; [email protected] Tsu-Chien Weng, Center for High Pressure Science & Technology Advanced Research, China; [email protected] doi:10.1557/mrs.2016.113
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