Se Edges Research Articles

PdO@CoSe2 composites: efficient electrocatalysts for water oxidation in alkaline media.

In this study, we have prepared cobalt selenide (CoSe2) due to its useful aspects from a catalysis point of view such as abundant active sites from Se edges, and significant stability in alkaline conditions. CoSe2, however, has yet to prove its functionality, so we doped palladium oxide (PdO) onto CoSe2 nanostructures using ultraviolet (UV) light, resulting in an efficient and stable water oxidation composite. The crystal arrays, morphology, and chemical composition of the surface were studied using a variety of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. It was also demonstrated that the composite systems were heterogeneous in their morphology, undergoing a shift in their diffraction patterns, suffering from a variety of metal oxidation states and surface defects. The water oxidation was verified by a low overpotential of 260 mV at a current density of 20 mA cm-2 with a Tafel Slope value of 57 mV dec-1. The presence of multi metal oxidation states, rich surface edges of Se and favorable charge transport played a leading role towards water oxidation with a low energy demand. Furthermore, 48 h of durability is associated with the composite system. With the use of PdO and CoSe2, new, low efficiency, simple electrocatalysts for water catalysis have been developed, enabling the development of practical energy conversion and storage systems. This is an excellent alternative approach for fostering growth in the field.

Tomonaga-Luttinger Liquid in the Topological Edge Channel of Multilayer FeSe.

A two-dimensional topological insulator exhibits helical edge states topologically protected against single-particle backscattering. Such protection breaks down, however, when electron-electron interactions are significant or when edge reconstruction occurs, leading to a suppressed density of states (DOS) at the Fermi level that follows universal scaling with temperature and energy, characteristic of Tomonaga-Luttinger liquid (TLL). Here, we grow multilayer FeSe on SrTiO3 by molecular beam epitaxy and observe robust edge states at both the {100}Se and the {110}Se steps using scanning tunneling microscopy/spectroscopy. We determine the DOS follows a power law, resulting in the Luttinger parameter K of 0.26 ± 0.02 and 0.43 ± 0.07 for the {100}Se and {110}Se edges, respectively. The smaller K for the {100}Se edge also indicates strong correlations, attributed to ferromagnetic ordering likely present due to checkerboard antiferromagnetic fluctuations in FeSe. These results demonstrate TLL in FeSe helical edge channels, providing an exciting model system for novel topological excitations arising from superconductivity and interacting helical edge states.

Aptian\u2013Albian shale oil unconventional system as registration of Cretaceous oceanic anoxic sub-events in the southern Tethys (Bir M\u2019Cherga basin, Tunisia)

The Bir M’Cherga basin (North-east Tunisia), with about 600 km2 area, had recorded four Middle Cretaceous source rocks well stratigraphically correlated with the four known oceanic anoxic sub-events: OAE1a, OAE1b, OAE1c, and OAE1d. Variety of lithology, thickness and organic richness had characterized these source rocks. The sedimentary tectonic analysis, the petrology and geochemistry study established the petroleum system of these source rocks. Basin formation began early in the Barremian–Aptian interval by synsedimentary tectonics reactivating old basement faults. During the Aptian–Albian, the formed basin had a depocenter that recorded thicker black shales while its NW and SE edges remained raised under the Triassic halokinetic activities. The evolution of the sedimentary filling during this period generated two sedimentary cycles corresponding to two filling second-order fining and thickening upwards sequences. The black shales that constitute these source rocks are formed between subtidal and external platform environment and are interbedded with juxtaposed high organic rich layers and poor ones. The rich organic facies consists of dark shale that constitutes the source rock. The poor organic beds formed by light grey and nodular limestones constitute an intra host reservoir. Thereby, petroleum system consists in an “unconventional oil shale hybrid systems with a combination of juxtaposed organic-rich and organic-lean intervals associated to open fractures”. The kerogen is essentially amorphous, with marine planktonic origin and low ligneous organic matter contribution. This organic material of dark facies had been well preserved in an anoxic environment with little or no energy. Light grey limestones were of oxic-to-sub-oxic environment. The stage of the thermal evolution for these source rocks provided by IH/Tmax diagram is of the “oil window”. The average transformation ratio (TR) is estimated as 45% suggesting thus black shales are oil shale resources which still close about untransformed 55% of its hydrocarbon generating potential. The expulsion and release of oil into these source rocks are proven by the observation of hydrocarbons filling micro-cracks and by the variable values of the oil saturation index OSI ranging from 0 to 138%. The latter exceeds 100% near the paleo-high reliefs indicating two “oil crossover” areas attributed to the high degree of oil source rock saturation and accumulation of hydrocarbons considered ideal for hydraulic fracturing. This oil crossover is a consequence of secondary migration into black shale source rock, achieved by various faults created during the distensive phase that were reactivated again several times.

Trapping intermediates in metal transfer reactions of the CusCBAF export pump of Escherichia coli

Escherichia coli CusCBAF represents an important class of bacterial efflux pump exhibiting selectivity towards Cu(I) and Ag(I). The complex is comprised of three proteins: the CusA transmembrane pump, the CusB soluble adaptor protein, and the CusC outer-membrane pore, and additionally requires the periplasmic metallochaperone CusF. Here we used spectroscopic and kinetic tools to probe the mechanism of copper transfer between CusF and CusB using selenomethionine labeling of the metal-binding Met residues coupled to RFQ-XAS at the Se and Cu edges. The results indicate fast formation of a protein−protein complex followed by slower intra-complex metal transfer. An intermediate coordinated by ligands from each protein forms in 100 ms. Stopped-flow fluorescence of the capping CusF-W44 tryptophan that is quenched by metal transfer also supports this mechanism. The rate constants validate a process in which shared-ligand complex formation assists protein association, providing a driving force that raises the rate into the diffusion-limited regime.

Powder diffraction study of Pd2HgSe3

The Pd2HgSe3 phase was synthetized from individual elements by the silica glass tube technique and its crystal structure has been refined by the Rietveld method. The Pd2HgSe3 phase crystalizes in P$\bar 3$m1 space group with the unit-cell parameters a = 7.3096(2) Å, c = 5.2829(1) Å, V = 244.45(1) Å3, Dc = 8.84 g/cm3, and Z = 2. In its layered crystal structure, the [PdSe6] octahedra share opposing Se–Se edges with adjacent [PdSe4] squares forming layers parallel with the (001) plane. The layers show AA type stacking along the c-axis. Hg atoms occupy the anti-cubooctahedral voids between two consecutive layers. Pd2HgSe3 is isostructural with Pt2HgSe3 and Pt4Tl2X6 (X = S, Se, or Te) phases. The structure can be viewed as a 2a.2a.c superstructure of PtSe2.

Synthesis and crystal structure of tischendorfite, Pd8Hg3Se9

The synthetic analogue of the mineral tischendorfite, Pd 8 Hg 3 Se 9 , was prepared using the silica glass tube technique and its crystal structure was solved and refined from powder X-ray diffraction data. The structure is orthorhombic, space group Pmmn , with a = 7.1886(2), b = 16.8083(5), c = 6.4762(2) A, V = 782.51(4) A 3 and Z = 2. There are three Pd, two Hg and four Se independent positions. Tischendorfite crystallizes in a framework structure, where Pd atoms show two types of coordination by Se atoms: [PdSe 4 ] squares and [PdSe 5 ] pyramids. The [PdSe 5 ] pyramid shares two opposite Se–Se edges with adjacent pyramids forming linear isolated chains running along the a -axis, whereas [PdSe 4 ] squares are paired via one common Se–Se edge. The paired squares and chains of pyramids form characteristic slabs parallel to (010). Both types of slabs alternate along the b -axis. The Hg atoms occupy the anti-cubooctahedral voids formed by Se atoms. The structure is stabilized by a system of Pd–Hg and Pd–Pd metallic bonds.

Stable Cu(II) and Cu(I) Mononuclear Intermediates in the Assembly of the CuA Center of Thermus thermophilus Cytochrome Oxidase

CuA is a dinuclear mixed-valence center located in subunit 2 of the ba(3)-type cytochrome oxidase from Thermus thermophilus. The assembly of this site within the periplasmic membrane is believed to be mediated by the copper chaperones Sco and/or PCuAC, but the biological mechanisms are still poorly understood, thereby stimulating interest in the mechanisms of CuA formation from inorganic ions. The formulation of the CuA center as an electron-delocalized Cu(1.5)-Cu(1.5) system implicates both Cu(II) and Cu(I) states in the metalation process. In earlier work we showed that selenomethionine (SeM) substitution of the coordinated M160 residue provided a ligand-directed probe for studying the copper coordination environment via the Se XAS signal, which was particularly useful for interrogating the Cu(I) states where other spectroscopic probes are absent. In the present study we have investigated the formation of mixed-valence CuA and its M160SeM derivative by stopped-flow UV-vis, EPR, and XAS at both Cu and Se edges, while the formation of fully reduced di-Cu(I) CuA has been studied by XAS alone. Our results establish the presence of previously undetected mononuclear intermediates and show important differences from the metalation reactions of purple CuA azurin. XAS spectroscopy at Cu and Se edges has allowed us to extend mechanistic inferences to formation of the di-Cu(I) state which may be more relevant to biological CuA assembly. In particular, we find that T. thermophilus CuA assembles more rapidly than reported for other CuA systems and that the dominant intermediate along the pathway to mixed-valence is a new green species with λ(max) = 460 nm. This intermediate has been isolated in a homogeneous state and shown to be a mononuclear Cu(II)-(His)(Cys)(2) species with no observable Cu(II)-(Met) interaction. Reduction with dithionite generates its Cu(I) homologue which is again mononuclear but now shows a strong interaction with the Met160 thioether. The results are discussed within the framework of the "coupled distortion" model for Cu(II) thiolates and their relevance to biological metalation reactions of the CuA center.

The essential role of the Cu(II) state of Sco in the maturation of the CuA center of cytochrome oxidase: evidence from H135Met and H135SeM variants of the Bacillus subtilis Sco

Sco is a red copper protein that plays an essential yet poorly understood role in the metalation of the Cu(A) center of cytochrome oxidase, and is stable in both the Cu(I) and Cu(II) forms. To determine which oxidation state is important for function, we constructed His135 to Met or selenomethionine (SeM) variants that were designed to stabilize the Cu(I) over the Cu(II) state. H135M was unable to complement a scoΔ strain of Bacillus subtilis, indicating that the His to Met substitution abrogated cytochrome oxidase maturation. The Cu(I) binding affinities of H135M and H135SeM were comparable to that of the WT and 100-fold tighter than that of the H135A variant. The coordination chemistry of the H135M and H135SeM variants was studied by UV/vis, EPR, and XAS spectroscopy in both the Cu(I) and the Cu(II) forms. Both oxidation states bound copper via the S atoms of C45, C49 and M135. In particular, EXAFS data collected at both the Cu and the Se edges of the H135SeM derivative provided unambiguous evidence for selenomethionine coordination. Whereas the coordination chemistry and copper binding affinity of the Cu(I) state closely resembled that of the WT protein, the Cu(II) state was unstable, undergoing autoreduction to Cu(I). H135M also reacted faster with H(2)O(2) than WT Sco. These data, when coupled with the complete elimination of function in the H135M variant, imply that the Cu(I) state cannot be the sole determinant of function; the Cu(II) state must be involved in function at some stage of the reaction cycle.

Assessment of groundwater contamination by landfill leachate: A case in México

In México, uncontrolled landfills or open-dumps are regularly used as "sanitary landfills". Interactions between landfills/open-dumps and shallow unconfined aquifers have been widely documented. Therefore, evidence showing the occurrence of aquifer contamination may encourage Mexican decision makers to enforce environmental regulations. Traditional methods such as chemical analysis of groundwater, hydrological descriptions, and geophysical studies including vertical electrical sounding (VES) and ground penetrating radar (GPR) were used for the identification and delineation of a contaminant plume in a shallow aquifer. The Guadalupe Victoria landfill located in Mexicali is used as a model study site. This landfill has a shallow aquifer of approximately 1m deep and constituted by silty sandy soil that may favor the transport of landfill leachate. Geophysical studies show a landfill leachate contaminant plume that extends for 20 and 40 m from the SE and NW edges of the landfill, respectively. However, the zone of the leachate's influence stretches for approximately 80 m on both sides of the landfill. Geochemical data corroborates the effects of landfill leachate on groundwater.

XANES analysis of ZnSe ternary compounds with transition metals (TM): experimental and theoretical LMTO studies

Experimental studies of X-ray absorption near edge structure (XANES) in ternary compounds of ZnSe with transition metals (TM) (where TM=Ti, V, Cr, Co and Ni) for Se and Zn K edges have been carried out and compared with the absorption edge of pure ZnSe. This comparison for Se and Zn K edges shows similar behaviour of these edges, respectively, versus number of 3d electrons in the TM. These results were compared with the electron densities of states (DOS) in the conduction band obtained from theoretical LMTO calculations for an ordered ferromagnetic phase of ZnTMSe (25% TM content). In particular the p-like DOS for ZnMnSe, ZnFeSe and other ZnTMSe around Zn and Se ions were compared in an energy range close to the Fermi level. For the Se edge, very good agreement between theoretical estimates and the experimental analysis was obtained. It has been shown that for both Se and Zn absorption edges the addition of TM to the pure ZnSe host crystal causes additional features, apart from the interaction with the valence band, in the forbidden gap of ZnSe (deep donors and acceptor states), as well as an additional contribution in the conduction band. The energy positions of these features depend on the number of TM 3d electrons and can be composed in two sequences Ti, V, Cr, and Ni, Co, Fe, respectively. The hybridized contribution of Mn 3d states around Se and Zn ions are immersed deeper in the conduction band than for other TMs.

X-ray-absorption near-edge structure ofCuGaSe2and ZnSe: Experiment and theory

X-ray-absorption near-edge structure (XANES) spectra of a ternary semiconductor ${\mathrm{CuGaSe}}_{2}$ at the Cu, Ga, and Se edges were measured and compared with Zn and Se spectra of ZnSe, taken from the literature. Having all five absorbing atoms in nearly identical coordination environments, we investigate the influence of the electronic structure on the XANES spectra. The spectra of ${\mathrm{CuGaSe}}_{2}$ and of ZnSe were calculated using a real-space multiple-scattering approach and using a pseudopotential band-structure technique. Both computational methods yield spectra that are in a good agreement with experiment. The effect of the size of the cluster involved in the real-space calculation on the calculated XANES spectra is investigated. Using self-consistent muffin-tin potentials does not lead to significantly different ${\mathrm{CuGaSe}}_{2}$ spectra than using non-self-consistent potentials. Real-space multiple-scattering spectra calculated without core holes exhibit only minor differences with respect to those obtained for relaxed screened core holes, the largest effect being found for Zn spectrum of ZnSe. Employing unrelaxed or unscreened core hole potentials resulted in spectra that did not agree with experiment. Contrary to earlier reports, no effect of charge transfer on the calculated XANES spectra of ZnSe was found.

Polarization dependent X-ray absorption study of Cu xNbS 2 ( x=0.00, 0.16) and Cu x NbSe 2 ( x=0.00, 0.38)

The X-ray absorption fine structure (XAFS) of single crystals of NbS 2, Cu 0.16NbS 2, NbSe 2 and Cu 0.38NbSe 2 is investigated at the Nb, Se and Cu edges. Measurements are made for different angles between the crystallographic c-axis and the X-ray beam. Nearest-neighbor and next-nearest-neighbor distances, as well as their variances, are extracted from the extended XAFS (EXAFS), indicating that intercalation of copper does not affect the average distances within the Nb-chalcogen layers while, at the same time, a structural disorder is introduced. The X-ray absorption near edge structure (XANES) of Cu x NbSe 2 is sensitive to the direction of empty electronic states and indicates that empty states with z-symmetry are separated from empty states with x- and y-symmetry by about 2.5 eV. It also shows that intercalation is accompanied by charge transfer from the Cu to the NbSe 2 host lattice.

RDF studies of Ge xSe 1−x glasses employing anomalous scattering

Structural studies of GexSe1−x (x=0.15 and 0.25) glasses have been done by RDFGe(Se) employing anomalous scattering at Ge and Se edges. A peak at about 1 A− in interference function may probably due to local arrangements of Selenium. Glasses of Selenium-rich compositions are consistent basically with four-fold coordination around Ge atom and two-fold coordination around Se atom. A contribution of Ge-Ge pair to the first peak at the RDF is rather small.

The crystal structure of arsenic selenide, As2Se3

Single crystals of As2Se3 were prepared hydrothermally. The crystal structure was determined by using the intensities of 1014 in- dependent reflections, measured on a computer-controlled single-crystal diffractometer. The cell constants, refined by least-squares, are: a = 12.0774(4), b = 9.9037(6), c = 4.2835(6) A, f3 = 90.458 °(9); Z = 4; space group P2dn. The structure was solved by using MULTAN combined with successive Fourier syntheses. The final R value was 0.038 and Rw = 0.037. Each As atom is surrounded by a composite polyhedron of seven Se atoms, consisting of a tetragonal pyramid and a triangular prism, one face of which coincides with the base of the pyramid. Two centro symmetrically equivalent composite polyhedra are linked by a common prismatic face, forming thus a double composite polyhedron, which is further linked on either side to two centro symmetrically equivalent composite polyhedra through common prismatic faces. Thus a block offour composite polyhedra is formed. This combination of the polyhedra is repeated along the c axis, forming infinite chains. These chains are further linked to four similar adjacent chains by common Se, Se edges.

EXAFS study of mixed crystals of the A IIB III2X VI4 family

In this work we studied the nature of the near-neighbour environment in ternary and pseudoternary defect chalcopyrites of the AB 2X 4 family by using the extended X-ray absorption fine structure (EXAFS) technique. The EXAFS was measured at the K-edge of Zn, Ga and Se elements in ZnGa 2 (S xSe 1-x) 4 and Ga and Se edges in CdGa 2 (S xSe 1-x) 4 mixed crystals.NN distance were determined for all the compositions by a linear fitting of differences of phases and by a fitting procedure in k-space. We found bond lenghts nearly constant and close to the respective distances in the end member crystals over all the composition range, whereas the unit cell parameter a(x) changes accordingly to the virtual crystal approximation (Végard's law).