69 research outputs found
Unraveling the interlayer and intralayer coupling in two-dimensional layered MoS by X-ray absorption spectroscopy and ab initio molecular dynamics simulations
Understanding interlayer and intralayer coupling in two-dimensional layered
materials (2DLMs) has fundamental and technological importance for their
large-scale production, engineering heterostructures, and development of
flexible and transparent electronics. At the same time, the quantification of
weak interlayer interactions in 2DMLs is a challenging task, especially, from
the experimental point of view. Herein, we demonstrate that the use of X-ray
absorption spectroscopy in combination with reverse Monte Carlo (RMC) and ab
initio molecular dynamics (AIMD) simulations can provide useful information on
both interlayer and intralayer coupling in 2DLM 2H-MoS. The analysis of
the low-temperature (10-300 K) Mo K-edge extended X-ray absorption fine
structure (EXAFS) using RMC simulations allows for obtaining information on the
means-squared relative displacements for nearest and distant Mo-S
and Mo-Mo atom pairs. This information allowed us further to determine the
strength of the interlayer and intralayer interactions in terms of the
characteristic Einstein frequencies and the effective force
constants for the nearest ten coordination shells around molybdenum.
The studied temperature range was extended up to 1200 K employing AIMD
simulations which were validated at 300 K using the EXAFS data. Both RMC and
AIMD results provide evidence of the reduction of correlation in thermal motion
between distant atoms and suggest strong anisotropy of atom thermal vibrations
within the plane of the layers and in the orthogonal direction
Atomic scale structure and bond stretching force constants in stoichiometric and off-stoichiometric kesterites
The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu–Se and Zn–Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells
Atomic level structure of Ge-Sb-S glasses: chemical short range order and long Sb-S bonds
The structure of GeSbS, GeSbS and
GeSbS glasses was investigated by neutron diffraction
(ND), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS)
measurements at the Ge and Sb K-edges as well as Raman scattering. For each
composition, large scale structural models were obtained by fitting
simultaneously diffraction and EXAFS data sets in the framework of the reverse
Monte Carlo (RMC) simulation technique. Ge and S atoms have 4 and 2 nearest
neighbors, respectively. The structure of these glasses can be described by the
chemically ordered network model: Ge-S and Sb-S bonds are always preferred.
These two bond types adequately describe the structure of the stoichiometric
glass while S-S bonds can also be found in the S-rich composition. Raman
scattering data show the presence of Ge-Ge, Ge-Sb and Sb-Sb bonds in the
S-deficient glass but only Ge-Sb bonds are needed to fit diffraction and EXAFS
datasets. A significant part of the Sb-S pairs has 0.3-0.4 {\AA} longer bond
distance than the usually accepted covalent bond length (~2.45 {\AA}). From
this observation it was inferred that a part of Sb atoms have more than 3 S
neighbors.Comment: 23 pages, 6 figures, submitted to Journal of Alloys and Compound
In operando study of orthorhombic Vâ‚‚Oâ‚… as positive electrode materials for K-ion batteries
Understanding the Li-ion storage mechanism in a carbon composited zinc sulfide electrode
Sulfide compounds are interesting conversion electrode materials for Li-ion batteries, due to their high theoretical capacity. However, they suffer from large volumetric changes and fast capacity fading. To overcome these issues, nanosized zinc sulfide (ZnS) modified with polyelectrolytes and graphene (ZnS-C/G) has been synthesized and investigated as an enhanced conversion-alloying anode material. In situ synchrotron X-ray diffraction and X-ray absorption spectroscopy are used to elucidate the Li storage process during the 1st cycle. In addition, the evolution of internal resistance and the corresponding solid electrolyte interphase (SEI) formation during the 1st cycle are discussed based on electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The results reveal that the formation of lithiated products and the SEI layer at different voltages can influence Li+ diffusion into the electrode. Moreover, an artificial carbon layer can not only facilitate Li+ transport but also avoid the direct formation of the SEI layer on the surface of active particles. Compared to bare ZnS, the ZnS-C/G electrode shows outstanding rate capability and cycling capacity (571 mA h g−1 after 120 cycles at a specific current of 1.0 A g−1 with a retention rate of 94.4%). The high capacity at elevated current density is ascribed to the contribution of capacitive charge storage
Electrochemical performance and reaction mechanism investigation of Vâ‚‚Oâ‚… positive electrode material for aqueous rechargeable zinc batteries
The electrochemical performance and reaction mechanism of orthorhombic VO in 1 M ZnSO aqueous electrolyte are investigated. VO nanowires exhibit an initial discharge and charge capacity of 277 and 432 mA h g, respectively, at a current density of 50 mA g. The material undergoes quick capacity fading during cycling under both low (50 mA g) and high (200 mA g) currents. VO can deliver a higher discharge capacity at 200 mA g than that at 50 mA g after 10 cycles, which could be attributed to a different type of activation process under both current densities and distinct degrees of side reactions (parasitic reactions). Cyclic voltammetry shows several successive redox peaks during Zn ion insertion and deinsertion. In operando synchrotron diffraction reveals that VO undergoes a solid solution and two-phase reaction during the 1st cycle, accompanied by the formation/decomposition of byproducts Zn(OH)VO·2(HO) and ZnSOZn(OH)·5HO. In the 2nd insertion process, VO goes through the same two-phase reaction as that in the 1st cycle, with the formation of the byproduct ZnSOZn(OH)·5HO. The reduction/oxidation of vanadium is confirmed by in operando X-ray absorption spectroscopy. Furthermore, Raman, TEM, and X-ray photoelectron spectroscopy (XPS) confirm the byproduct formation and the reversible Zn ion insertion/deinsertion in the VO
Guest Ion-Dependent Reaction Mechanisms of New Pseudocapacitive MgV(PO)/Carbon Composite as Negative Electrode for Monovalent-Ion Batteries
The influence of Sb doping on the local structure and disorder in thermoelectric ZnO:Sb thin films
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