XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

Low-lying unoccupied electronic states in 3d transition-metal fluorides probed by NEXAFS at the F 1s threshold

Abstract: The near-edge x-ray absorption fine structure (NEXAFS) at the F 1s threshold has been studied with high-energy resolution for a series of binary fluorides, including KF, TiF4, VF4, VF3, CrF3, CrF2, MnF3, MnF2, FeF3, FeF2, CoF2, NiF2, CuF2, and ZnF2 as well as for SF6 in the gas phase, and for the PF6- and TiF62- molecular anions of the solid compounds KPF6 and K2TiF6. Most of these spectra were measured at the Russian-German beamline at BESSY II, while the spectra of KF and CuF2 were taken under comparable experimental conditions at the D1011 beamline at MAX-lab. The spectra of the solid samples were recorded via the total electron yield. The NEXAFS spectra were taken with the aim to elucidate the role of covalent bonding and its manifestation in x-ray absorption spectra as well as to gain information on the electronic structure of the conduction band along the whole series of 3d transition-metal (TM) fluorides. The spectra of these most ionic compounds of the 3d TM's have been analyzed in a comparative way considering also the F 1s NEXAFS spectrum of the molecular TiF62- anion in solid K2TiF6. In its turn, the latter spectrum has been interpreted by comparing with the F 1s NEXAFS spectrum of the molecular PF6- anion in KPF6 and that of SF6 in the gas phase. In this way, the low-lying empty electronic states of the 3d TM fluorides are shown to be formed by covalent mixing of the TM 3d with the fluorine 2p electronic states. It is further found that the number of low-lying empty electronic states with TM 3d-fluorine 2p hybridized character decreases gradually along the series of 3d TM fluorides, and is essentially zero in the case of ZnF2. (Less)

Immobilization of RuO2 on Carbon Nanotube: An X-ray Absorption Near-Edge Structure Study

Abstract: The electronic structures of carbon nanotube/RuO2 core/shell nanocomposite (RuO2 thin layer coated multiwalled carbon nanotubes (MWNTs)) have been studied by X-ray absorption near-edge structures (XANES) at C K-edge, O K-edge, and Ru M5,4- and L3-edges. The variation in white-line features of the XANES at these edges supports strongly that RuO2 interacts with MWNTs through Ru−O−C bonding, which also results in charge redistribution between C 2p-derived states in MWNT and the conduction band in RuO2. Such chemical bonding is necessary to immobilize RuO2 on MWNT and ensures good conductivity of MWNT/RuO2 core/shell nanocomposite.

Complementary Operando Spectroscopy identification of in-situ generated metastable charge-asymmetry Cu2-CuN3 clusters for CO2 reduction to ethanol

Abstract: Copper-based materials can reliably convert carbon dioxide into multi-carbon products but they suffer from poor activity and product selectivity. The atomic structure-activity relationship of electrocatalysts for the selectivity is controversial due to the lacking of systemic multiple dimensions for operando condition study. Herein, we synthesized high-performance CO2RR catalyst comprising of CuO clusters supported on N-doped carbon nanosheets, which exhibited high C2+ products Faradaic efficiency of 73% including decent ethanol selectivity of 51% with a partial current density of 14.4 mA/cm-2 at -1.1 V vs. RHE. We evidenced catalyst restructuring and tracked the variation of the active states under reaction conditions, presenting the atomic structure-activity relationship of this catalyst. Operando XAS, XANES simulations and Quasi-in-situ XPS analyses identified a reversible potential-dependent transformation from dispersed CuO clusters to Cu2-CuN3 clusters which are the optimal sites. This cluster can't exist without the applied potential. The N-doping dispersed the reduced Cun clusters uniformly and maintained excellent stability and high activity with adjusting the charge distribution between the Cu atoms and N-doped carbon interface. By combining Operando FTIR and DFT calculations, it was recognized that the Cu2-CuN3 clusters displayed charge-asymmetric sites which were intensified by CH3* adsorbing, beneficial to the formation of the high-efficiency asymmetric ethanol.