Electronic structure and luminescence assets in white-light emitting Ca2V2O7, Sr2V2O7 and Ba2V2O7 pyro-vanadates: X-ray absorption spectroscopy investigations.

Ca2V2O7, Sr2V2O7, and Ba2V2O7 pyro-vanadates were synthesized using a modified chemical precipitation method and annealing. Detailed crystal structure, morphology, electronic structure and optical properties were investigated by XRD, UV-visible absorption, FTIR, Raman, FE-SEM, XANES, and photoluminescence spectroscopy. Rietveld refinement on the XRD patterns of Ca2V2O7, Sr2V2O7, and Ba2V2O7 has confirmed the triclinic structure (space group; P1̄(2)) of the pyro-vanadates. The band gap energy of Ca2V2O7, Sr2V2O7, and Ba2V2O7 is estimated to be ∼2.67 eV, ∼2.97 eV and ∼3.09 eV, respectively. XANES spectra at the Ca L-edge, Sr K-edge and Ba L-edge have confirmed the Ca2+, Sr2+ and Ba2+ ions in the Ca2V2O7, Sr2V2O7 and Ba2V2O7 compounds, respectively. V K-edge XANES spectra have strengthened the presence of sub-pentavalent V ions in all of the pyro-vanadates. O K-edge XANES spectra of Ca2V2O7, Sr2V2O7 and Ba2V2O7 have shown dominating tetrahedral symmetry of the V ions which is also corroborated with the V K-edge XANES. Broad-band emission spectra, ranging from 400 nm to 700 nm, have been observed from the charge-transfer transitions of VO4 tetrahedra. 3T1 → 1A1 and 3T2 → 1A1 transitions, from the VO4 tetrahedra, have provided two distinct emission peaks from the compounds which exhibit a red-shift with the decreasing ionic-radii of alkali-earth metal ions. The mixed compounds, with equal weight proportions, have shown remarkable emission characteristics towards the realization of rare-earth element free white-light-emitting devices.

XANES, EXAFS and photoluminescence investigations on the amorphous Eu:HfO2.

We report detailed investigations on the local electronic/atomic structure and photoluminescence properties of chemically synthesized Eu:HfO2 powders. X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) measurements were performed to analyze the crystal structure, local atomic/electronic structure and luminescence properties of the samples. No crystalline phases were detected with Cu Kα (λ=1.5418Å) based XRD; however, local monoclinic structure was confirmed by the Hf L-edge XANES and EXAFS. O K-edge XANES spectral features could be deconvoluted with doublets and triplets in eg and t2g orbitals, respectively, which ascribed to the local monoclinic structure for all of the samples. Eu M5,4-edge XANES confirmed the pre-dominancy of Eu3+ ions in the HfO2 samples with a fractional amount of Eu2+ ions. PL spectra revealed the electric dipole allowed (5D0-7F0,2,4) emission properties of Eu:HfO2 samples. The orange-red emission is ascribed to the Eu interstitial/surface segregation induced defects.

Contributors to Enhanced CO2 Electroreduction Activity and Stability in a Nanostructured Au Electrocatalyst.

The formation of a nanostructure is a popular strategy for catalyst applications because it can generate new surfaces that can significantly improve the catalytic activity and durability of the catalysts. However, the increase in the surface area resulting from nanostructuring does not fully explain the substantial improvement in the catalytic properties of the CO2 electroreduction reaction, and the underlying mechanisms have not yet been fully understood. Here, based on a combination of extended X-ray absorption fine structure analysis, X-ray photoelectron spectroscopy, and Kelvin probe force microscopy, we observed a contracted Au-Au bond length and low work function with the nanostructured Au surface that had enhanced catalytic activity for electrochemical CO2 reduction. The results may improve the understanding of the enhanced stability of the nanostructured Au electrode based on the resistance of cation adhesion during the CO2 reduction reaction.

The effects of nitrogen bonding on hardness of AIN/CrN multilayer hard coatings.

AIN/CrN multilayer hard coatings with various bilayer thicknesses were fabricated by a reactive sputtering process. The microstructural and mechanical characterizations of multilayer coatings were investigated through transmission electron microscope (TEM) observations and the hardness measurements by nano indentation. In particular, the variation of chemical bonding states of the bilayer nitrides was elucidated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Many broken nitrogen bonds were formed by decreasing the bilayer thickness of AIN/CrN multilayer coatings. Existence of optimum AIN/CrN multilayer coatings thickness for maximum hardness could be explained by the competition of softening by the formation of broken nitrogen bonds and strengthening induced by decreasing bilayer thickness.

Hardness and nitrogen bonding structure of AlxTi1-xN/CrN multilayer hard coating.

AlxTi1-xN/CrN multilayer coatings were fabricated by magnetron sputtering and those hardness variations were studied by observing the crack propagation and measuring the chemical bonding state of nitrides by Ti addition. While AlN/CrN multilayer shown stair-like crack propagation, AlxTi1-xN/CrN multilayer illustrated straight crack propagation. Most interestingly, Ti addition induced more broken nitrogen bonds in the nitride multilayers, leading to the reduction of hardness. However, the hardness of Al0.25Ti0.75N/CrN multilayer, having high Ti contents, increased by the formation of many Ti-N bond again instead of Al-N bond. From these results, we found that linear crack propagation behavior was dominated by broken nitrogen bonds in the AlxTi1-xN/CrN multilayer coatings.