XANES

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

Carbon (1s) NEXAFS spectroscopy of biogeochemically relevant reference organic compounds

Abstract: Natural organic matter (NOM) is a highly active component of soils and sediments, and plays an important role in global C cycling However, NOM has defied molecular-level structural characterization, owing to variations along the decomposition continuum and its existence as highly functionalized polyelectrolytes We conducted a comprehensive systematic overview of spectral signatures and peak positions of major organic molecules that occur as part of NOM using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy The spectra of carbohydrates and amino sugars show resonances between 28910 and 28959 eV, attributed to 1s-3p/σ * transitions of O-alkyl (C-OH) moieties They also exhibited distinct peaks between 28842 and 28874 eV, representing C 1σ―π * C=O transition from COOH functionalities Amino acids produced a strong signal around 28870 eV, which can be identified as a C 1s―π * C=O transition of carboxyl/carbonyl (COOH/ COO-) structures Spectral features near 28529 eV were ascribed to C 1s―π * C=O transition of ring structure of aromatic amino acids, while spectra between 28714 and 28786 eV were attributed to C 1s―π * C-H and C 1s―π * C-H/ 3 p Rydberg-like excitations from CH and CH 2 groups Phenols and benzoquinone produced strong resonances between 28508 and 28537 eV, attributed to the π * orbital of C (C 1s―π * C=C ) atoms connected to either C or H (C―H) in the aromatic ring The next higher excitation common to both phenols and quinone appeared between 28605 and 28635 eV, and could be associated with C 1s―π * C=C transitions of aromatic C bonded to O atom in phenols, and to C 1s―π * C=O transitions from aromatic C connected to O atom (C-OH) in phenols or to a C=O in p-benzoquinone and some phenols with carbonyl structures, respectively Nucleobases exhibited complex spectral features with pronounced resonances between 28602 and 28684 eV and between 28801 and 28870 eV Molecular markers for black C (benzenecarboxylic acid and biphenyl-4,4'-dicarboxylic acid) exhibit sharp absorption bands between 28501 and at 28543 eV, possibly from C 1s―π * C=C transition characteristic of C-H sites or unsaturated C (C=C) on aromatic ring structures These aromatic carboxylic acids also exhibit broad peaks between 28835 and 28848 eV, reflecting C 1s―π * C=O transition of carboxyl functional groups bonded to unsaturated C This investigation provides a more comprehensive NEXAFS spectral library ofbiogeochemically relevant organic C compounds The spectra of these reference organic compounds reveal distinct spectral features and peak positions at the C K-edge that are characteristic of the molecular orbitals bonding C atoms Detailed structural information can be derived from these distinctive spectral features that could be used to build robust peak assignment criteria to exploit the chemical sensitivity of NEXAFS spectroscopy for in situ molecular-level spatial investigation and fingerprinting of complex organic C compounds in environmental samples

Picosecond X-ray Absorption Spectroscopy of a Photoinduced Iron(II) Spin Crossover Reaction in Solution

Abstract: In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron−nitrogen (Fe−N) bond is lengthened by 0.21 ± 0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change i...

Microfluidic Synthesis of Cobalt Nanoparticles

Abstract: Co nanoparticles with three different crystal structures were synthesized in a microfluidic reactor through manipulation of reaction times, flow rates, and quenching procedures. Cobalt nanoparticles of face-centered cubic (β) phase were obtained from a high flow rate of the reactants followed by in situ quenching of the reaction. hcp and e-cobalt nanoparticles were obtained at a low flow rate of the reactants followed by in situ quenching and delayed quenching, respectively. The crystal structures were characterized using Co K-edge X-ray absorption near edge structure (XANES) spectroscopy, X-ray diffraction (XRD), and selected area electron diffraction (SAED). In situ XANES measurements on Co nanoparticles coming out of the outlet of the microfluidic reactor at different flow rates seem to indicate that the difference in flow rate influences the nucleation process in a critical way and that particle growth occurs mainly outside the reactor. The magnetic properties of the cobalt nanoparticles, measured usi...

Nanoscale Morphological and Chemical Changes of High Voltage Lithium–Manganese Rich NMC Composite Cathodes with Cycling

Abstract: Understanding the evolution of chemical composition and morphology of battery materials during electrochemical cycling is fundamental to extending battery cycle life and ensuring safety. This is particularly true for the much debated high energy density (high voltage) lithium–manganese rich cathode material of composition Li1 + xM1 – xO2 (M = Mn, Co, Ni). In this study we combine full-field transmission X-ray microscopy (TXM) with X-ray absorption near edge structure (XANES) to spatially resolve changes in chemical phase, oxidation state, and morphology within a high voltage cathode having nominal composition Li1.2Mn0.525Ni0.175Co0.1O2. Nanoscale microscopy with chemical/elemental sensitivity provides direct quantitative visualization of the cathode, and insights into failure. Single-pixel (∼30 nm) TXM XANES revealed changes in Mn chemistry with cycling, possibly to a spinel conformation and likely including some Mn(II), starting at the particle surface and proceeding inward. Morphological analysis of the...

Hydrothermal Microwave: A New Route to Obtain Photoluminescent Crystalline BaTiO3 Nanoparticles

Abstract: Hydrothermal microwave method was used as a new route to synthesize pure BaTiO3 (BT) nanoparticles at 140°C for 10 min under rapid reacting with stoichiometric Ba/Ti ratio. The crystalline products were characterized by X-ray powder diffraction (XRD) and the structure was refined by the Rietveld method from the tetragonal structure, which was supported by the Ti K-edge X-ray absorption near-edge structure (XANES). The pre-edge of Ti in the XANES spectra indicated that titanium ions are localized in a nonregular octahedron. Typical FT-Raman spectra for tetragonal BaTiO3 nanoparticles presented well-defined peaks, indicating a substantial short-range order in the system. However, a scattering peak at 810 cm−1 was attributed to the presence of lattice OH− groups, commonly found in materials obtained by hydrothermal process. Besides, the peak at 716 cm−1 can be related to eventual Ba2+ defects in the BaTiO3 lattice. BaTiO3 (BT) nanoparticles presented spherical morphology with a non-uniform distribution of pa...