Valence Electron Distribution Research Articles

Identification of the anti-triangular etched MoS2 with comparative activity with commercial Pt for hydrogen evolution reaction

The introduction of anti-triangular dots by etching MoS2 has been recently reported as an efficient way to produce active internal edges. However, it is still ambiguous to identify the configuration of etched MoS2 with comparative hydrogen evolution reaction (HER) activity to commercial Pt. In this work, the HER activity on etched anti-triangular MoS2 are investigated by density functional theory calculations. We found the valence electron distribution of inner edge atoms can be affected by the corner, and accordingly determine their activity. The second-nearest atom to anti-triangular corner possesses the highest activity. The simulated polarization curves show etched anti-triangular Mo-edge MoS2 with moderate size (around 12 Å) maximize the HER performance. Further increasing size of etched MoS2 deactivate their HER activity to a certain degree, validated by available experimental data. This work suggests etched MoS2 catalysts with rational design may be a candidate to substitute Pt electrodes as HER electrocatalyst.

Hydrogen storage in TiZrNbFeNi high entropy alloys, designed by thermodynamic calculations

The hydrogen storage properties of the novel equiatomic TiZrNbFeNi and non-equiatomic Ti20Zr20Nb5Fe40Ni15 high entropy alloys (HEAs) were studied. These alloys were designed with the aid of thermodynamic calculations using the CALPHAD method due to their tendency to form single C14 Laves phase, a phase desirable for room-temperature hydrogen storage. The alloys, which were synthesized by arc melting, showed a dominant presence of C14 Laves phases with the (Zr, Ti)1(Fe, Ni, Nb, Ti)2 constitution and small amounts of cubic phases (<1.4 wt%), in good agreement with the thermodynamic predictions. Hydrogen storage properties, examined at room temperature without any activation procedure, revealed that a maximum hydrogen storage capacity was reached for the equiatomic alloy in comparison to the non-equiatomic alloy (1.64 wt% vs 1.38 wt%) in the first cycle; however, the non-equiatomic alloy presented superior reversibility of 1.14 wt% of hydrogen. Such differences on reversibility and capacity among the two alloys were discussed based on the chemical fluctuations of hydride-forming and non-hydride-forming elements, the volume per unit cell of the C14 Laves phases and the distribution of valence electrons.

Mapping valence electron distributions with multipole density formalism using 4D-STEM

Recent advancement in aberration correction and detector technology opened a door to various applications using 4D-STEM, which yields a diffraction pattern for each scanning position within a crystal unit-cell in scanning transmission electron microscopy (STEM) and generates incredible amounts of data in momentum space. Currently 4D-STEM analysis relies on the center-of-mass of the diffraction patterns in electric field and charge density mapping. It only derives the total projected charge density and is limited to phase objects, e.g. extremely thin samples. Here, we propose a new analytical method to accurately map aspherical valence electron distributions with atom-centered multipolar functions formalism using the whole 4D-STEM dataset. We demonstrate that, with the full dynamical calculations for various sample thicknesses, the method is sensitive not only to the miniscule charge transfer, but also to the atomic site symmetry and aspherical electron orbitals. The process of the refinement is much more robust and reliable than quantitative convergent beam electron diffraction.

Sensitivity of 4D-STEM to Valence Electron Distribution Based on Multipole Density Formalism

Sensitivity of 4D-STEM to Valence Electron Distribution Based on Multipole Density Formalism

Probing the Validity of the Zintl-Klemm Concept for Alkaline-Metal Copper Tellurides by Means of Quantum-Chemical Techniques.

Understanding the nature of bonding in solid-state materials is of great interest for their designs, because the bonding nature influences the structural preferences and chemical as well as physical properties of solids. In the cases of tellurides, the distributions of valence-electrons are typically described by applying the Zintl−Klemm concept. Yet, do these Zintl−Klemm treatments provide adequate pictures that help us understanding the bonding nature in tellurides? To answer this question, we followed up with quantum-chemical examinations on the electronic structures and the bonding nature of three alkaline-metal copper tellurides, i.e., NaCu3Te2, K2Cu2Te5, and K2Cu5Te5. In doing so, we accordingly probed the validity of the Zintl−Klemm concept for these ternary tellurides, based on analyses of the respective projected crystal orbital Hamilton populations (−pCOHP) and Mulliken as well as Löwdin charges. Since all of the inspected tellurides are expected to comprise Cu−Cu interactions, we also paid particular attention to the possible presence of closed-shell interactions.

A First-Principles Simulation of Electronic Structure of MCN2 Crystals (M = Be, Mg, Ca, Zn, Cd, Hg)

The local-density approximation of the density functional theory is used to study MCN2 crystals (M = Be, Mg, Ca, Zn, Cd, Hg) simulated in the chalcopyrite structure; their equilibrium lattice parameters are determined, the band structure and the density of states are calculated, the maps of deformation charge density distribution of valence electrons are obtained. The role of cations M in the formation of chemical bonds and the structure of valence bands of MCN2 crystals is revealed.

Электронная структура кристаллов Be-IV-P-=SUB=-2-=/SUB=- с решеткой халькопирита

A group of crystalline compounds of the type Be-IV-P2, (IV = C, Si, Ge, Sn) with a chalcopyrite structure has been studied using the density functional theory methods. The equilibrium parameters of the crystal lattice, band structures, spectra of total and partial densities of states, maps of the charge distribution of valence electrons, tensors of dielectric constant and effective charges, and optical absorption spectra are calculated and obtained.

Tuning of the phase transition between site selective SCO and intermetallic ET in trimetallic magnetic cyanido-bridged clusters.

A series of crystalline phases composed of trimetallic 3d-5d-5d' {Fe9[Re(CN)8]6-x[W(CN)8]x(MeOH)24}·yMeOH (x = 1 (1), 2 (2), 3 (3), 4 (4) and 5 (5); y = 10-15) clusters were obtained by altering the octacyanidometalate composition. The temperature dependent studies involving SC XRD, SQUID magnetic measurements, IR spectroscopy and 57Fe Mössbauer spectroscopy revealed reversible phase transition with the retention of single crystal character in each congener. The transition was assisted by reversible spin-crossover (SCO) HSFeII↔LSFeII transition at the central Fe1(ii) site for Fe9Re5W1 (1), Fe9Re4W2 (2), Fe9Re3W3 (3) and Fe9Re2W4 (4). In contrast, the tungsten-rich congener Fe9Re1W5 (5) exhibited nontrivial behavior with the SCO transition being stopped halfway through the cooling process, to be completed with single electron transfer (ET) from the external Fe2(ii) center towards one of the neighboring W(v) sites. The critical temperature Tc of SCO has been systematically increased from 193 K (1) to 247 K (4). All experimental data indicate the domination of the Fe(ii)-W(v) valence states in all crystals 1-5, however, with increasing quantity of [W(CN)8]3- (and decreasing quantity of [Re(CN)8]3-), the valence equilibrium Fe(ii)-W(v) ↔ Fe(iii)-W(iv) was systematically shifted to the right, starting from congener 3. The overall electronic configuration at low temperatures and variable amounts and location of spin carriers along the whole series suggest the remarkable competition between magnetic super-exchange Fe(ii)-CN-W(v) interactions and intermolecular interactions. The observed behavior is in line with the information collected previously for the bimetallic congeners Fe9Re6 and Fe9W6, to shed light on the role of the mixed tri-metallic composition in changing the properties observed for the relevant bimetallic cyanido-bridged skeletons.

Effect of mechanical treatment on the distribution of valence electrons and characteristics of nanocomposite (SiO2)x(Al2O3)1-x (x = 0.8, x = 0.7) electrodes in lithium power sources

Ultra-soft X-ray emission spectroscopy was used to study the distribution of Op-, Sisd- and Alsd- valence electrons in (SiO2)x(Al2O3)1-x (x = 0.8, x = 0.7) powder mixtures after mechanical treatment. An increase in atomic charges has been measured and can be explained by the transfer of electrons from Si/Al to O atoms in split Opπ-binding states and the formation of the weak long (OO)π bonds between the surface atoms of the contacted powder nanoparticles. Scanning and transmission electron microscopy images show an enhanced agglomeration of the nanoparticles of both SiO2 and Al2O3 oxides, but no changes in the crystalline parameters have been measured using X-ray diffraction. An increase in charge capacities of lithium ion power sources with 0.8SiO2–0.2Al2O3 electrode has been observed during cycling. At the same time, a decrease of the charge capacities with the 0.7SiO2–0.3Al2O3 electrode has been measured. The results are discussed in terms of an increase in the binding energy of electrons in the Op-states, which prevents the recombination and irreversible reactions of lithium with electrode atoms. Otherwise, due to cycling, electron population increase of in non-binding states near the valence band top contributes to the recombination ability of Li+ ions and leads to a decrease in the charge capacity.

Revealing the Nature of Chemical Bonding in an ALn2Ag3Te5-Type Alkaline-Metal (A) Lanthanide (Ln) Silver Telluride

Although the electronic structures of several tellurides have been recognized by applying the Zintl-Klemm concept, there are also tellurides whose electronic structures cannot be understood by applications of the aforementioned idea. To probe the appropriateness of the valence-electron transfers as implied by Zintl-Klemm treatments of ALn2Ag3Te5-type tellurides (A = alkaline-metal; Ln = lanthanide), the electronic structure and, furthermore, the bonding situation was prototypically explored for RbPr2Ag3Te5. The crystal structure of that type of telluride is discussed for the examples of RbLn2Ag3Te5 (Ln = Pr, Nd), and it is composed of tunnels which are assembled by the tellurium atoms and enclose the rubidium, lanthanide, and silver atoms, respectively. Even though a Zintl-Klemm treatment of RbPr2Ag3Te5 results in an (electron-precise) valence-electron distribution of (Rb+)(Pr3+)2(Ag+)3(Te2−)5, the bonding analysis based on quantum-chemical means indicates that a full electron transfer as suggested by the Zintl-Klemm approach should be considered with concern.

Combined quantum mechanics/molecular mechanics (QM/MM) methods to understand the charge density distribution of estrogens in the active site of estrogen receptors.

The ligand binding to protein and host–guest interactions are ubiquitous for molecular recognition. In drug design, the ligand binding to the active site of proteins is influenced by the charge density distribution and the electrostatic interactions of ligands and the nearby amino acids of the protein. The charge density analyses of ligand–protein complexes need accurate positions of hydrogen atoms and their valence electron distribution and the fine structure of proteins. Such information cannot be obtained from the conventional protein X-ray crystallography analysis in the resolution range of 1.5 to 3.5 Å. This can be realized from QM/MM based structure and charge density analysis of estrogens with the estrogen receptor. The charge density properties such as electron density, Laplacian of electron density and electrostatic properties of estrogens in the presence of active site amino acid residues have been determined and compared with the isolated estrogen molecules from theory and experimental. The present study reveals the chemical bonding nature of estrogen molecules and the strength of the intermolecular interactions in the active site of estrogen receptor, and also the importance of π⋯π interactions between the estrogens and Phe404 amino acid residue and protonation state of His524 amino acid residue have been identified using electrostatic potential maps. The difference in the electrostatic potential map of estrogens displays the hormone dependent actions of estrogen receptor. This method is very helpful to derive the charge density distribution of macromolecules to understand their biological recognition and interactions.

Electronic and mechanical properties of C/Si phases with sp2 and sp3 hybridization: A first-principles study

A first-principles approach is utilized to systematically investigate the electronic and mechanical properties of SiC3/Si3C phases with sp2 and sp3 hybridization. In the SiC3 phases, electronic states around the Fermi level mainly originate from the C-2p orbitals, whereas in the case of Si3C phases, it is the C-2p and Si-3p orbitals. Cm-SiC3 and Cmc21-SiC3 show metallic properties arising from sp2-hybridized components. P4¯m2-Si3C exhibits good ductility and metallic properties due to the formation of conductive sublattices as a result of the distribution of valence electrons in three-dimensional C and Si frameworks. Furthermore, the semiconducting P4¯m2-SiC3 phase is a superhard material with a remarkable hardness of 47.14 GPa. In general, SiC3 phases exhibit higher brittleness due to sp3-hybridized C atoms while Si3C phases are more ductile.

Study of average valence and valence electron distribution of several oxides using X-ray photoelectron spectra

Abstract X-ray photoelectron spectra of the O 1s electrons of MnFe2O4, ZnFe2O4, ZnO, and CaO were used to estimate the average valence, ValO, of the oxygen anions in these samples. The absolute values of ValO for these samples were found to be distinctly lower than the traditional value of 2.0, suggesting that the total average valences of the cations are also lower than the conventionally accepted values owing to valence balance in the compounds. In addition, we analyzed the valence band spectra of the samples and investigated the distribution characteristics of the valence electrons.

Direct Visualization of Orbital Flipping in Volborthite by Charge Density Analysis Using Detwinned Data

The distribution of d-orbital valence electrons in volborthite [Cu3V2O7(OH)2 ∙ 2H2O] was investigated by charge density analysis of the multipole model refinement. Diffraction data were obtained by synchrotron radiation single-crystal X-ray diffraction experiments. Data reduction by detwinning of the multiple structural domains was performed using our developed software. In this study, using high-quality data, we demonstrated that the water molecules in volborthite can be located by the hydrogen bonding in cavities that consist of Kagome lattice layers of CuO4(OH)2 and pillars of V2O7. Final multipole refinements before and after the structural phase transition directly visualized the deformation electron density of the valence electrons. We successfully directly visualized the orbital flipping of the d-orbital dx2–y2, which is the highest level of 3d orbitals occupied by d9 electrons in volborthite. The developed techniques and software can be employed for investigations of structural properties of syste...

Influence of Mechanical Treatment on Structural and Morphological Characteristics and Distribution of Valence Electrons of Aluminum, Silicon, Iron and Titanium Oxides

Influence of Mechanical Treatment on Structural and Morphological Characteristics and Distribution of Valence Electrons of Aluminum, Silicon, Iron and Titanium Oxides

Experimental charge-density studies: data reduction and model quality: the more the better?

In this review, recent developments concerning data and model quality in experimental charge-density investigations from a personal view-point are described. Data quality is not only achieved by the high resolution, high I/σ(I) values, low merging R values and high multiplicity. The quality of the innermost reflections especially is crucial for mapping the density distribution of the outermost valence electrons and can be monitored by (I/σ)asymptotic. New detector technologies seem to be promising improvements. Empirical corrections to correct for low-energy contamination of mirror-focused X-ray data and for resolution- and temperature-dependent errors caused by factors such as thermal diffuse scattering are described. Shashlik-like residual density patterns can indicate the need for an anharmonic description of the thermal motion of individual atoms. The physical reliability of the derived model must be thoroughly analysed. The derived probability density functions for the mean-squared atomic vibrational displacements especially should have only small negative values. The treatment of H atoms has been improved by methods to estimate anisotropic thermal motion. For very high resolution data, the polarization of the core density cannot be neglected. Several tools to detect systematic errors are described. A validation tool is presented that easily detects when the refinement of additional parameters yields a real improvement in the model or simply overfits the given data. In all investigated structures, it is proved that the multipole parameters of atoms with a comparable chemical environment should be constrained to be identical. The use of restraints could be a promising alternative.

Electronic and optical properties of Zr, N mono-doped and co-doped anatase TiO2: first-principles calculations

The growth conditions, crystal structural, electronic, optical and photocatalytic properties of undoped, N-, Zr mono-doped TiO2 and four kinds of Zr/N co-doped TiO2 (Zr–N1, Zr–N2, Zr–N3 and Zr–N4) with different Zr and N co-doped positions were investigated by first-principles calculations. It is found that N substituted preferentially for O under Ti-rich conditions. While Zr substituted preferentially for Ti under O-rich conditions, and has the lowest formation energies (−4.56 eV) due to Zr having the same valence electron distribution as Ti. And, co-doped samples prepared in the experiment may contain all co-doped modes; owing to the formation energies of them being very close under Ti-rich or O-rich conditions. On the other hand, the optical absorption threshold and the optical absorption intensity of Zr–N1, Zr–N2 and Zr–N4 co-doped TiO2 are more enlarged than the N mono-doped and Zr–N3 co-doped TiO2, due to the smaller carrier effective mass and the curved and wide impurity level (IL) appearing in the band gap, which not only increase the carrier mobility and reduce the electron–hole recombination, but also reduces the electronic transition energies. Thus, Zr–N1, Zr–N2 and Zr–N4 co-doped configurations can explain the mechanism of visible light absorption and photocatalytic activity enhancement for co-doped TiO2 and give a good explanation of the previous experimental observation.

Enhancement of low-temperature activity and sulfur resistance of Fe0.3Mn0.5Zr0.2 catalyst for NO removal by NH3-SCR

A novel Fe0.3Mn0.5Zr0.2 catalyst was prepared by co-precipitation method and used to remove NO at 80–400°C. The physicochemical characteristics of samples were studied by N2 adsorption–desorption, SEM, XRD, XPS and temperature-programmed technology. The results showed that Fe0.3Mn0.5Zr0.2 exhibits better removal ability of NO with NH3 and NO conversion achieves 100% at 200–360°C. Moreover, it gives a good stability and strong SO2 resistance at 200°C. Fe0.3Mn0.5Zr0.2 has 309m2/g of BET surface area and 0.358cm3/g of mesopore volume, which are beneficial for gas adsorption and diffusion. The amorphous MnO2 and Fe2O3 play a dominant role for NO oxidation due to their strong redox property and high lattice oxygen (Oα) concentration. Oα in Fe0.3Mn0.5Zr0.2 can quickly oxidize NO to NO2, which promotes the “Fast SCR” reaction. The strong interactions among Fe, Mn and Zr of Fe0.3Mn0.5Zr0.2 lead to its strong thermal stability and sulfur resistance, but the increase of the absorbed oxygen species changes the valence electron distribution of Mn and Fe after it working at 200°C for 80h. Mn oxides and Fe oxides in Fe0.3Mn0.5Zr0.2 exhibit gradually metallic ions after SO2 with 100/200ppm concentration is induced into simulated gases at 200°C for 53h. In addition, Fe0.3Mn0.5Zr0.2 possesses both Brönsted and Lewis acid sites. They promote the adsorption and activation of NH3, resulting a good low-temperature SCR activity.

The influence of water molecules on NdCl3·3C3H8O in the properties: A DFT study

The catalytic activity of NdCl3-ROH-AlR3 is closely related to the amount of water in NdCl3. Usually NdCl3·6H2O has been dehydrated step by step to NdCl3·2H2O, however, the further dehydration process will be very difficult. In this work, we investigated the effect of added water molecules on structures and properties of NdCl3·3C3H8O, NdCl3·3C3H8O·H2O, and NdCl3·3C3H8O·2H2O, including bond length, angle, mulliken atomic charge, molecular electrostatic potential (MEP), HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy, and natural bond orbital (NBO). All properties were calculated by density functional theory (DFT) using B3PW91 functional and basis set of 6-31G++**/SDDALL. It was found that the bond length of NdCl bond and bond angles of Cl3-NdCl2, O9-NdO5, Cl3-NdCl4 could be directly affected due to the addition of water molecule. The MEP analysis revealed that the molecular reaction site should be located around Cl atom. Furthermore, analysis of mulliken atomic charges showed that charge of Nd atom as the active center of the reaction changed from 0.956 to 1.238 and then to 1.301 after addition of one water molecule and two water molecules respectively. HOMO and LUMO orbitals were carried out to investigate the stability of the system, the addition of two water molecules in the system enhanced the electron flow in the system. Also, NBO analysis was further performed to supply an in depth insight into the electronic structure, the distribution of valence electrons and bond orders, which all changed after the addition of water molecule. Therefore, it was necessary to convert NdCl3·2H2O to NdCl3 in order to achieve a higher catalytic activity.

Spin-polarized antiferromagnetic CaCoSO single crystal: First-principles study

Abstract Comprehensive spin-polarized calculations were performed for the recently synthesized antiferromagnetic CaCoSO single crystal based on the full-potential method plus Hubbard Hamiltonian. The experimental lattice parameters of the CaCoSO single crystal were optimized and the experimental atomic positions were relaxed so as to minimize the forces acting on the atoms. The calculations show that the spin-up (-down) channels exhibit a direct energy band gap Γv- Γc (Kv-Kc) of about 2.187 (1.187) eV and the spin-polarized antiferromagnetic CaCoSO single crystal exhibits an indirect energy band gap (Γv- Kc) of about 0.4 eV. The obtained magnetic moment of 3d electrons within the Co sphere (2.56 μB) is the highest among the others and agrees well with the measured ones (2.75 μB). Further, the charge density distribution of the spin-polarized valence electrons for the spin-up and spin-down channels were investigated in two crystallographic planes. The calculated bond lengths and angles show good agreement with the measured ones.