XANES-EXAFS spectroscopy for discerning and retrieval of atomic and coordination states of a diverse single-metal-atomic structures

Due to the outstanding accomplishments of single-metal-atom electrocatalysts in the context of nanomaterials for a broad spectrum of electrochemical energy conversions, there is a pressing need to further advance the X-ray absorption spectroscopic determination discerning and analysing tool for the purpose of confirming the coordination structure of a wide-range of single-metal-atomic materials. This range includes isolated/correlated single-metal-atom, single-metal-atom alloy, dual-metal single-atom, single-atom dimer, nano-single-atom, and interfacial single-atomic heterostructures. XANES-EXAFS spectra elucidate the exact electronic and geometric configuration of central metal by a combined package of FT-EXAFS spectra and WT-contour-EXAFS fitting. XANES-EXAFS spectroscopic determination of electronic and atomic structure also illustrates the significance of synchrotron X-ray spectroscopic techniques in classifying the electrochemical activity patterns of varied types of single-metal-atom electrode materials. This article addresses a range of questions such as: nature of EXAFS spectral peaks linked to varied coordination structure/environment around central metal-atoms including isolated/correlated single-metal-atom, single-metal-atom alloy, dual-metal single-atom, single-atom dimer, nano-single-atom, interfacial single-atomic heterostructuresand link between electrochemical performance trend and discerning coordination environment of central metal-atom of above single-atomic materials. Finally, the capability of XANES-EXAFS spectra is addressed for distinguishing the coordination and electronic environment of a wide-range single-atom systems including vivid electrochemical features linked to their coordination structure to take on future grand challenges. Moreover, while insightfully discussing distinct features of a broad range of single-metal-atom systems, maximum atom utilization, enhanced density of atoms, nano-confinement effects were linked with XANES-EXAFS spectral features which pave the way for future directions. Many review articles have focused on the engineering of atomically dispersed single atoms and their derivatives (such as single-atom-dimer, single-atom-alloy, etc) but methods of data extraction, their analysis protocols and revealing electronic structures are still unaddressed. In this review article we have elucidated principles of XAS and XAFS techniques, which are the core in identifying atomically dispersed single atoms. This article provides a base knowledge to the readers about soft and hard X-ray techniques, and which can be effective based on the material designing. This also provides a brief description of core shell electronic transitions which are very crucial in identifying the electronic states as well as coordination environments across the metal active center.