316 research outputs found
Phase transition in nanomagnetite
Recently, the application of nanosized magnetite particles became an area of growing interest for
their potential practical applications. Nanosized magnetite samples of 36 and 9 nm sizes were
synthesized. Special care was taken on the right stoichiometry of the magnetite particles. MĂśssbauer
spectroscopy measurements were made in 4.2â300 K temperature range. The temperature
dependence of the intensities of the spectral components indicated size dependent transition taking
place in a broad temperature range. For nanosized samples, the hyperfine interaction values and their
relative intensities changed above the Verwey transition temperature value of bulk megnetite. The
continuous transition indicated the formation of dendritelike granular assemblies formed during the
preparation of the samples
In Situ Detection of Active Edge Sites in Single-Layer MoS Catalysts
MoS2 nanoparticles are proven catalysts for processes such as
hydrodesulphurization and hydrogen evolution, but unravelling their
atomic-scale structure under catalytic working conditions has remained
significantly challenging. Ambient pressure X-ray Photoelectron Spectroscopy
(AP-XPS) allows us to follow in-situ the formation of the catalytically
relevant MoS2 edge sites in their active state. The XPS fingerprint is
described by independent contributions to the Mo3d core level spectrum whose
relative intensity is sensitive to the thermodynamic conditions. Density
Functional Theory (DFT) is used to model the triangular MoS2 particles on
Au(111) and identify the particular sulphidation state of the edge sites. A
consistent picture emerges in which the core level shifts for the edge Mo atoms
evolve counter-intuitively towards higher binding energies when the active
edges are reduced. The shift is explained by a surprising alteration in the
metallic character of the edge sites, which is a distinct spectroscopic
signature of the MoS2 edges under working conditions
Density functional study of the adsorption and van der Waals binding of aromatic and conjugated compounds on the basal plane of MoS2
Accurate calculations of adsorption energies of cyclic molecules are of key importance in investigations of, e.g., hydrodesulfurization (HDS) catalysis. The present density functional theory (DFT) study of a set of important reactants, products, and inhibitors in HDS catalysis demonstrates that van der Waals interactions are essential for binding energies on MoS2 surfaces and that DFT with a recently developed exchange-correlation functional (vdW-DF) accurately calculates the van der Waals energy. Values are calculated for the adsorption energies of butadiene, thiophene, benzothiophene, pyridine, quinoline, benzene, and naphthalene on the basal plane of MoS2, showing good agreement with available experimental data, and the equilibrium geometry is found as flat at a separation of about 3.5 \uc5 for all studied molecules. This adsorption is found to be due to mainly van der Waals interactions. Furthermore, the manifold of adsorption-energy values allows trend analyses to be made, and they are found to have a linear correlation with the number of main atoms. \ua9 2009 American Institute of Physics
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