Showing papers on "Valence (chemistry) published in 2019"
TL;DR: In this article, the ionization energy and electron affinity values of all primary tin and lead-based perovskites were determined using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis.
Abstract: Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.
270 citations
TL;DR: The poster presented by as discussed by the authors at the 2019 Frontiers of Science Congress Series (MFS) celebrated in the Parque de las Ciencias de Granada (Espana) del 11 al 13 de septiembre de 2019.
Abstract: Resumen del poster presentado al Microscopy at the Frontiers of Science Congress Series (MFS), celebrado en el Parque de las Ciencias de Granada (Espana) del 11 al 13 de septiembre de 2019.
256 citations
TL;DR: A theoretical calculation shows that Fe will be more energetically favorable in the tetrahedral site than Ni and Co, which can further regulate their electronic structure of binary NiCo spinel oxides for optimal adsorption energies of OER intermediates and improved electronic conductivity and hence boost their OER performance.
Abstract: A major challenge that prohibits the practical application of single/double-transition metal (3d-M) oxides as oxygen evolution reaction (OER) catalysts is the high overpotentials during the electro...
185 citations
01 Jan 2019
TL;DR: Energy decomposition analysis (EDA) is a powerful method that connects the results of accurate quantum chemical calculations with the Lewis electron-pair bonding model as discussed by the authors, which can provide a physically sound picture of chemical bonding between any atoms.
Abstract: Breaking down the calculated interaction energy between two or more fragments into well-defined terms enables a physically meaningful understanding of chemical bonding. Energy decomposition analysis (EDA) is a powerful method that connects the results of accurate quantum chemical calculations with the Lewis electron-pair bonding model. The combination of EDA with natural orbitals for chemical valence (NOCV) links the heuristic Lewis picture with quantitative molecular orbital theory complemented by Pauli repulsion and Coulombic interactions. The EDA-NOCV method affords results that provide a physically sound picture of chemical bonding between any atoms. We present and discuss results for the prototypical main-group diatomics H2, N2, CO and BF, before comparing bonding in N2 and C2H2 with that in heavier homologues. The discussion on multiply bonded species is continued with a description of B2 and its N-heterocyclic carbene adducts. This Perspective introduces energy decomposition analysis as a means of providing a quantum chemically derived bonding model that we can use to rationalize molecular geometries and bonding. The model serves as a bridge between the simple Lewis electron-pair bond and the complicated quantum theoretical nature of the chemical bond.
164 citations
TL;DR: In this article, ruthenium (Ru) catalysts supported on CeO2 nanorods (NR), nanocubes (NC), and nanoctahedra (NO) were comparatively investigated to correlate the shape and exposed surface planes.
Abstract: In this work, ruthenium (Ru) catalysts supported on CeO2 nanorods (NR), nanocubes (NC), and nanoctahedra (NO) were comparatively investigated to correlate the shape and exposed surface planes ({100...
125 citations
TL;DR: A new electronegativity scale for atoms, based consistently on ground-state energies of valence electrons, is introduced, closely related to (yet different from) L. C. Allen's, which is based on configuration energies.
Abstract: We introduce a new electronegativity scale for atoms, based consistently on ground-state energies of valence electrons. The scale is closely related to (yet different from) L. C. Allen's, which is based on configuration energies. Using a combination of literature experimental values for ground-state energies and ab initio-calculated energies where experimental data are missing, we are able to provide electronegativities for elements 1-96. The values are slightly smaller than Allen's original scale, but correlate well with Allen's and others. Outliers in agreement with other scales are oxygen and fluorine, now somewhat less electronegative, but in better agreement with their chemistry with the noble gas elements. Group 11 and 12 electronegativities emerge as high, although Au less so than in other scales. Our scale also gives relatively high electronegativities for Mn, Co, Ni, Zn, Tc, Cd, Hg (affected by choice of valence state), and Gd. The new electronegativities provide hints for new alloy/compound design, and a framework is in place to analyze those energy changes in reactions in which electronegativity changes may not be controlling.
124 citations
TL;DR: In this article, the compositional element ratio of all prepared samples is analyzed by energy-dispersion X-ray spectroscopy technique, and the optical parameters of thin films are studied.
122 citations
TL;DR: This work reports a record-high average ZT value of ∼1.6 at 300-793 K with maximum ZT values ranging from 0.8 at 300 K to 2.1 at 793 K in p-type SnSe crystals, which arises from the enhanced power factor and lowered lattice thermal conductivity through crystal structure modification via Te alloying.
Abstract: The simple binary compound SnSe has been reported as a robust thermoelectric material for energy conversion by showing strong anharmonicity and multiple electronic valence bands. Herein, we report a record-high average ZT value of ∼1.6 at 300-793 K with maximum ZT values ranging from 0.8 at 300 K to 2.1 at 793 K in p-type SnSe crystals. This remarkable thermoelectric performance arises from the enhanced power factor and lowered lattice thermal conductivity through crystal structure modification via Te alloying. Our results elucidate that Te alloying increases the carrier mobility by making the bond lengths more nearly equal and sharpening the valence bands; meanwhile, the Seebeck coefficient remains large due to multiple valence bands. As a result, a record-high power factor of ∼55 μW cm-1 K-2 at 300 K is achieved. Additionally, Te alloying promotes Sn atom displacements, thus leading to a lower lattice thermal conductivity. Our conclusions are well supported by electron localization function calculations, the Callaway model, and structural characterization via aberration-corrected scanning transmission electron microscopy. Our approach of modifying crystal structures could also be applied in other low-symmetry thermoelectric materials and represents a new strategy to enhance thermoelectric performance.
122 citations
TL;DR: In this article, a quantum model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment was presented, which can assist in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts.
Abstract: We present a quantum mechanical model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment. Studies of 93 atoms predict drastic changes to ground-state electronic configurations and electronegativity in the pressure range of 0-300 GPa. This extension of atomic reference data assists in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts. For example, we can speculate on the existence of pressure-induced polarity (red-ox) inversions in various alloys. Our study confirms that the filling of energy levels in compressed atoms more closely follows the hydrogenic aufbau principle, where the ordering is determined by the principal quantum number. In contrast, the Madelung energy ordering rule is not predictive for atoms under compression. Magnetism may increase or decrease with pressure, depending on which atom is considered. However, Hund's rule is never violated for single atoms in the considered pressure range. Important (and understandable) electron shifts, s→p, s→d, s→f, and d→f are essential chemical and physical consequences of compression. Among the specific intriguing changes predicted are an increase in the range between the most and least electronegative elements with compression; a rearrangement of electronegativities of the alkali metals with pressure, with Na becoming the most electropositive s1 element (while Li becomes a p group element and K and heavier become transition metals); phase transitions in Ca, Sr, and Ba correlating well with s→d transitions; spin-reduction in all d-block atoms for which the valence d-shell occupation is d n (4 ≤ n ≤ 8); d→f transitions in Ce, Dy, and Cm causing Ce to become the most electropositive element of the f-block; f→d transitions in Ho, Dy, and Tb and a s→f transition in Pu. At high pressure Sc and Ti become the most electropositive elements, while Ne, He, and F remain the most electronegative ones.
114 citations
TL;DR: Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene-induced charge transfer in this 2D mixed-valence SAC.
Abstract: Single-atom catalysts (SACs) aim at bridging the gap between homogeneous and heterogeneous catalysis. The challenge is the development of materials with ligands enabling coordination of metal atoms in different valence states, and preventing leaching or nanoparticle formation. Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene-induced charge transfer. Inspired by nature's selection of Cu(I) in enzymes for oxygen activation, this 2D mixed-valence SAC performs flawlessly in two O2 -mediated reactions: the oxidative coupling of amines and the oxidation of benzylic CH bonds toward high-value pharmaceutical synthons. High conversions (up to 98%), selectivities (up to 99%), and recyclability are attained with very low metal loadings in the reaction. The synergistic effect of Cu(II) and Cu(I) is the essential part in the reaction mechanism. The developed strategy opens the door to a broad portfolio of other SACs via their coordination to various functional groups of graphene, as demonstrated by successful entrapment of FeIII /FeII single atoms to carboxy-graphene.
114 citations
TL;DR: In this paper, the stability of single-atom catalysts in the hydrogen reducing environment was extensively investigated by using in situ diffuse reflectance infrared Fourier transform spectroscopy CO chemisorption measurements.
Abstract: Single-atom catalysts (SACs) have recently attracted broad attention in the catalysis field due to their maximized atom efficiency and unique catalytic properties. An atomic-level understanding of the interaction between the metal atoms and support is vital for developing stable and high-performance SACs. In this work, Pt1 single atoms with loadings up to 4 wt.% were fabricated on ceria nanorods using the atomic layer deposition technique. To understand the Pt–O–Ce bond interfacial interactions, the stability of Pt1 single atoms in the hydrogen reducing environment was extensively investigated by using in situ diffuse reflectance infrared Fourier transform spectroscopy CO chemisorption measurements. It was found that ceria defect sites, metal loadings and high-temperature calcination are effective ways to tune the stability of Pt1 single atoms in the hydrogen environment. X-ray photoemission spectroscopy further showed that Pt1 single atoms on ceria are dominantly at a +2 valence state at the defect and step edge sites, while those on terrace sites are at a +4 state. The above tailored stability and electronic properties of Pt1 single atoms are found to be strongly correlated with the catalytic activity in the dry and water-mediated CO oxidation reactions.
TL;DR: This work demonstrated that the photoactivity of MIL-53(Fe) can be improved by adjusting the ratio of FeII/FeIII, and the feasibility of using as all-day-active catalyst is demonstrated.
Abstract: One of vital issues that inhibit photoactivity of metal–organic frameworks is the poor electrical conductivity. In this work, one-dimensional mixed-valence iron chains are used to improve this poor...
TL;DR: Iridium oxide (IrOx)-based materials are the most suitable oxygen evolution reaction (OER) catalysts for water electrolysis in acidic media as discussed by the authors, and there is a strong demand from industry for improved perf...
Abstract: Iridium oxide (IrOx)-based materials are the most suitable oxygen evolution reaction (OER) catalysts for water electrolysis in acidic media. There is a strong demand from industry for improved perf...
TL;DR: High-quality monolayer and bilayer samples enable us to resolve, in the photoluminescence spectra, the band-edge exciton from the exciton involving holes in a pair of deeper valence bands, degenerate at Γ, with a splitting that agrees with both μARPES data and the results of DFT modeling.
Abstract: Atomically thin films of III-VI post-transition metal chalcogenides (InSe and GaSe) form an interesting class of two-dimensional semiconductors that feature a strong variation of their band gap as a function of the number of layers in the crystal and, specifically for InSe, an expected crossover from a direct gap in the bulk to a weakly indirect band gap in monolayers and bilayers. Here, we apply angle-resolved photoemission spectroscopy with submicrometer spatial resolution (μARPES) to visualize the layer-dependent valence band structure of mechanically exfoliated crystals of InSe. We show that for one-layer and two-layer InSe the valence band maxima are away from the Γ-point, forming an indirect gap, with the conduction band edge known to be at the Γ-point. In contrast, for six or more layers the band gap becomes direct, in good agreement with theoretical predictions. The high-quality monolayer and bilayer samples enable us to resolve, in the photoluminescence spectra, the band-edge exciton (A) from the exciton (B) involving holes in a pair of deeper valence bands, degenerate at Γ, with a splitting that agrees with both μARPES data and the results of DFT modeling. Due to the difference in symmetry between these two valence bands, light emitted by the A-exciton should be predominantly polarized perpendicular to the plane of the two-dimensional crystal, which we have verified for few-layer InSe crystals.
TL;DR: The progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds is reviewed and the consequences of the unpaired electron residing in the Ln–Ln bonding orbital for the magnetic interactions between Ln ions are highlighted.
Abstract: A characteristic phenomenon of lanthanide-fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln-Ln bonding orbital in the second half of the lanthanide row limits the Ln2 → fullerene transfer to only five electrons. One electron remains in the Ln-Ln bonding orbital, whereas the fullerene cage with a formal charge of -5 is left electron-deficient. Such Ln2@C80 molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln2@C79N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp2 carbon into an sp3 carbon and yielding the monoadduct Ln2@C80(CH2Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln2@C79N and Ln2@C80(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides. In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds and highlight the consequences of the unpaired electron residing in the Ln-Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln-Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln2 dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln2@C79N and Ln2@C80(CH2Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln3+-e-Ln3+] fragment behaves as a single giant spin. Furthermore, the Ln-Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln-Ln orbital affects the magnetic structure of the molecule.
TL;DR: In this article, the substitutional doping of tungsten oxide (WO3) with metal ions demonstrates a promising approach to enhance its photoelectrochemical (PEC) water splitting efficiency.
Abstract: The substitutional doping of tungsten oxide (WO3) with metal ions demonstrates a promising approach to enhance its photoelectrochemical (PEC) water splitting efficiency. In this article, the substitutional doping of Sn ions into WO3 lattice and its effect on optical, electrical, band edge, and PEC water splitting properties are explored. Sn-doped WO3 thin films were synthesized using a facile hydrothermal method. The characterization data reveal that the doping of Sn alters the morphology, induces multiple crystal phases, effects the crystal orientation, reduces the band gap, and increases the carrier density of WO3. With the uniform distribution of Sn ions in WO3 and the decreased charge transfer resistance at the electrode/electrolyte interface, the doped WO3 show notable enhancement in its PEC activity compared to the undoped WO3. The band edge study revealed that the introduction of Sn in WO3 lattice causes an increase in the energy distance between the valence band edge and Fermi level and, at the same time, induces a downward shift in both the valence and conduction band edges towards higher potentials with respect to reversible hydrogen electrode (RHE). Conclusively, this work shows significant and new insights about Sn-doped WO3 photoanodes and their influence on PEC water splitting efficiency.
TL;DR: Using density functional theory prediction, it is found that the reaction Gibbs free energy of permonosulfate activation can be rapidly reduced by the polyvalent molybdenum with low valence.
Abstract: Utilization of photocatalytic reactions to trigger persistent large-scale reactions could be an alternative path for practical solar energy conversion to relieve environmental pressure nowadays. We took the view that the photoinduction of transition states was critical for improving the activity of catalytic reactions. On the basis of theoretical predictions, the reaction Gibbs free energy of permonosulfate (PMS) activation can be rapidly reduced by molybdenum with low valence. We therefore constructed a multiphasic molybdenum dichalcogenide (MoS2) heterostructure-based photosystem that enabled generation of Mo transition states by visible light excitation. According to combination results of electron paramagnetic resonance, photoelectrochemical analysis, and X-ray photoelectron spectroscopy, we confirmed that the optimized 2H/1T heterojunction permitted the transport of excited interfacial electrons from the semiconductive 2H phase to the metallic 1T phase, and synchronously partially reduced Mo(IV) to Mo(III) at the interface. This intensified the charge transfer between the MoS2 and PMS-containing solution, thereby efficiently splitting the PMS molecules into •OH and SO4•- radicals. In this system, a type of refractory herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), can be degraded within 60 min at a rate constant of 6.20 × 10-2 min-1 using multiphasic MoS2 with a 1T/2H ratio of 1:1.
TL;DR: In this paper, the XPS spectrum showed that the ZnZr05 film can produce copper vacancies VCu and mixed valence structures for Cu+/Cu2+ and Ti3+/Ti4+ inside the crystal.
TL;DR: In this paper, a kind of CoFe-oxide nanocubes with tunable valence composition was rationally designed for boosting water splitting electrocatalysis by partially chemical tailoring Prussian blue analogue.
Abstract: Engineering valence state of active center in non-noble metal-based electrocatalysts is of prime importance to enhance the performance for different catalytic reactions. However, studies on optimized valence configuration with extremely high activity remains a great challenge because of scanty chemical approaches. Herein, a kind of CoFe-oxide nanocubes with tunable valence composition was rationally designed for boosting water splitting electrocatalysis by partially chemical tailoring Prussian blue analogue. The resulting Co2+-rich CoFe-oxide nanocube (CoIIFe-ONC) exhibited higher OER and HER catalytic performance than the well-balanced CoFe-oxide. It demanded overpotentials of only 289 mV and 284 mV to drive a current density of 50 mA cm−2 for OER and HER in 1.0 M KOH, respectively. The DFT calculations revealed that CoIIFe-ONC is more favorable for OER and HER since the higher capacity of water adsorption, optimized route for electrons transferring, and lower energy barrier for water dissociation by the active valance configuration.
TL;DR: In this paper, three 2D π-conjugated polymer network (i.e., covalent organic frameworks, COFs) materials based on pyrene, porphyrin, and zinc-porphyrin cores connected via diacetylenic linkers were investigated at the density functional theory global-hybrid level.
Abstract: We have considered three two-dimensional (2D) π-conjugated polymer network (i.e., covalent organic frameworks, COFs) materials based on pyrene, porphyrin, and zinc-porphyrin cores connected via diacetylenic linkers. Their electronic structures, investigated at the density functional theory global-hybrid level, are indicative of valence and conduction bands that have large widths, ranging between 1 and 2 eV. Using a molecular approach to derive the electronic couplings between adjacent core units and the electron-vibration couplings, the three π-conjugated 2D COFs are predicted to have ambipolar charge-transport characteristics with electron and hole mobilities in the range of 65–95 cm2 V−1 s−1. Such predicted values rank these 2D COFs among the highest-mobility organic semiconductors. In addition, we have synthesized the zinc-porphyrin based 2D COF and carried out structural characterization via powder X-ray diffraction, high-resolution transmission electron microscopy, and surface area analysis, which demonstrates the feasibility of these electroactive networks. Steady-state and flash-photolysis time-resolved microwave conductivity measurements on the zinc-porphyrin COF point to appreciable, broadband photoconductivity while transmission spectral measurements are indicative of extended π-conjugation.
TL;DR: The theoretical calculations suggest both cation and boron doping can effectively optimize adsorption energy of OER intermediates, promote the charge transfer among the heteroatoms, and improve their OER property.
Abstract: Valence engineering has been proved an effective approach to modify the electronic property of a catalyst and boost its oxygen evolution reaction (OER) activity, while the limited number of elements restricts the structural diversity and the active sites. Also, the catalyst performance and stability are greatly limited by cationic dissolution, ripening, or crystal migration in a catalytic system. Here we employed a widely used technique to fabricate heteroepitaxial pyrite selenide through dual-cation substitution and a boron dopant to achieve better activity and stability. The overpotential of Ni-pyrite selenide catalyst is decreased from 543 mV to 279.8 mV at 10 mA cm-2 with a Tafel slope from 161 to 59.5 mV dec-1. Our theoretical calculations suggest both cation and boron doping can effectively optimize adsorption energy of OER intermediates, promote the charge transfer among the heteroatoms, and improve their OER property. This work underscores the importance of modulating surface electronic structure with the use of multiple elements and provides a general guidance on the minimization of activity loss with valence engineering.
TL;DR: In this paper, the incorporation of a valence isoelectronic Al atom into binary boron clusters was explored, and it was shown that the Al atom can be incorporated into a binary binary BORON-based compound to obtain unique molecular architecture and chemical bonding.
Abstract: Binary boron-based compounds are expected to possess unique molecular architecture and chemical bonding. Here, we explore how incorporation of a valence isoelectronic Al atom into boron clusters co...
01 May 2019
TL;DR: In this article, the authors surveyed models that have been developed to understand the optical properties of bismuth-based materials and compiled a few case studies on the luminescence of trivalent Bi 3+ and simple oxide hosts.
Abstract: Bismuth is a non-toxic post transition metal with a large number of possible valence states which, together with to its tendency to form clusters, make it a versatile but complex dopant for possible luminescence applications. The outer orbitals responsible for luminescence are unshielded and strongly influenced by the surrounding environment, so that their energies are host dependent. In this review article we surveyed models that have been developed to understand the optical properties of bismuth based materials and compiled a few case studies on the luminescence of bismuth doped materials, with some emphasis on trivalent Bi 3+ and simple oxide hosts (e.g. CaO:Bi; Y 2 O 3 :Bi; ZnO:Bi; La 2 O 3 :Bi; SrO:Bi; YPO 4 :Bi and LaPO 4 :Bi). Special emphasis was given to the compilation of simplified energy diagrams under different conditions. For the most stable trivalent Bi 3+ state it has been experimentally found by many researchers that the absorption spectra consist of medium, weak and strong bands in order of increasing energy (designated the A, B and C bands, respectively), which were interpreted in terms of transitions to the states associated with the 3 P 1 , 3 P 2 and 1 P 1 levels of the excited 6s 1 p 1 configuration. Traditionally above the C band energy, but before the host absorption, a further D band is generally encountered, which is attributed to a metal-to-metal charge transfer (MMCT) transition corresponding to the transfer of an electron from the Bi 3+ ion to the bottom of the conduction band. Recent modelling of this MMCT band in different materials has clarified that it may occur at energies below the C band and be an important influence on excitation spectra at lower energies. The challenge of unambiguously identifying the nature of emissions is still an ongoing challenge, as illustrated by some of the case studies.
TL;DR: The calculations reveal that transition metal compounds with ligands, that are typically classified as donor–acceptor complexes possessing dative bonds or as TM compounds with electron‐sharing bonds, exhibit significant differences between the orbital interactions when closed‐shell or open shell fragments are used.
Abstract: Quantum chemical calculations using density functional theory at the BP86-D3(BJ)/def2-TZVPP level of theory are reported for transition metal compounds [TM]-L in high and low oxidation states involving carbene, carbyne, alkene, and alkyne ligands L. The nature of the [TM]-L bond is analyzed with the energy decomposition analysis - natural orbitals for chemical valence (EDA-NOCV) method. The calculations reveal that transition metal compounds with ligands, that are typically classified as donor-acceptor complexes possessing dative bonds (Fischer-type carbenes and carbynes, alkene, and alkyne complexes) or as TM compounds with electron-sharing bonds (Schrock-type carbenes and carbynes, metallacyclopropanes, and metallacyclopropenes), exhibit significant differences between the orbital interactions when closed-shell or open shell fragments are used. Fischer-type carbene complexes have much lower orbital interaction (ΔEorb ) values when singlet fragments are employed compared to triplet fragments. In contrast, singlet and triplet fragments of Schrock-type carbene complexes give similar ΔEorb values. The best description for Fischer-type carbyne complexes is found for neutral fragments in their electronic doublet state, which engage in a mixture of dative bonding (σ donation and π backdonation) and one electron-sharing π bond. The EDA-NOCV calculations of Schrock-type carbynes using open-shell species in their quartet electronic state give similar ΔEorb values as neutral fragments in their electronic doublet state. Alkene and alkyne complexes, but also metallacyclic species, are best described with singlet fragments, but the difference between the ΔEorb values for dative bonding and electron-sharing bonding using triplet fragments becomes much smaller for molecules that are considered as metallacycles. © 2018 Wiley Periodicals, Inc.
TL;DR: In this article, the effects of Sr2+ substitution for Ba2+ on microwave dielectric properties and crystal structure of Ba3-xSrx(VO4)2 (0 ≤ x ≤ 3, BSVO) solid solution were investigated.
Abstract: The effects of Sr2+ substitution for Ba2+ on microwave dielectric properties and crystal structure of Ba3-xSrx(VO4)2 (0 ≤ x ≤ 3, BSVO) solid solution were investigated. Such Sr2+ substitution contributes to significant reduction in sintering temperature from 1400 °C to 1150 °C. Both permittivity (∑r) and quality factor (Q × f) values decreased with increasing x value, which was determined to be related with the descending values of average polarizability and packing fraction, whereas the increase in τf value was explained by the decreased average V O bond length, A-site bond valence. BSVO ceramics possessed encouraging dielectric performances with ∑r = 12.2–15.6 ± 0.1, Q × f = 44,340 - 62,000 ± 800 GHz, and τf = 24.5–64.5 ± 0.2 ppm/°C. Low-temperature sintering was manipulated by adding B2O3 as sintering additive for the representative Sr3V2O8 (SVO) ceramic and only 1 wt.% B2O3 addition successfully contributed to a 21.7% decrease in sintering temperature to 900 °C, showing good chemical compatibility with silver electrodes, which render BSVO series and SVO ceramics potential candidates in multilayer electronic devices fabrication.
TL;DR: A new rare-earth compound is prepared, Eu9MgS2B20O41 (EMSBO), and its structural and physical properties exhibit very similar features characteristic of quantum spin liquid behaviors observed in other materials.
Abstract: We prepared a new rare-earth compound, Eu9MgS2B20O41 (EMSBO), and characterized its structural and physical properties. EMSBO consists of triple-Kagome-layer slabs separated by nonmagnetic ions and groups. Within each slab, intervalence charge transfer has been found to occur between the Eu2+ and Eu3+ Kagome layers, a new channel for quantum fluctuation of magnetic moments. The measured magnetic susceptibilities and the specific heat capacity exhibit very similar features characteristic of quantum spin liquid behaviors observed in other materials.
TL;DR: In this article, the first monomeric Cp-based Al(I) species could be realized in a pure form via a three-step reaction sequence (salt elimination/adduct formation and adduct cleavage) starting from readily available AlBr3.
Abstract: With the isolation of Cp3tAl (1), the first monomeric Cp-based Al(I) species could be realized in a pure form via a three-step reaction sequence (salt elimination/adduct formation/adduct cleavage) starting from readily available AlBr3. Due to its monomeric structure, reactions involving 1 were found to proceed more selectively, faster, and under milder conditions than for tetrameric (Cp*Al)4. Thus, 1 readily formed simple Lewis acid–base adducts with tBuAlCl2 (6) and AlBr3 (7), reactions that before have always been interfered with by the presence of aluminum halide bonds. In addition, the 2 : 1 reaction of 1 with AlBr3 enabled the realization of the very rare trialuminum adduct species 8. 1 also reacted rapidly with N2O and PhN3 at room temperature to afford Al3O3 and Al2N2 heterocycles 9 and 10, respectively. With the structural characterization of products 4 and 5, the reaction of monovalent 1 with Cp3tAlBr2 (2) provided the first experimental evidence for the concept of valence isomerism between dialanes and their Al(I)/Al(III) Lewis adducts.
TL;DR: The role of valence and semicore correlation in differentially stabilizing the intermediate spin state of Fe(II)-porphyrins is analyzed and accounting for valence, semicore, and correlation beyond the active space enlarges the (3Eg–5A1g) spin gap to −5.7 kcal/mol.
Abstract: The role of valence and semicore correlation in differentially stabilizing the intermediate spin state of Fe(II)-porphyrins is analyzed. For CASSCF treatments of valence correlation, a (32,34) active space containing metal 3 d, d' orbitals and the entire π system of the porphyrin is necessary to stabilize the intermediate spin state. Semicore correlation provides a minor (-1.6 kcal/mol) but quantitatively significant correction. Accounting for valence, semicore, and correlation beyond the active space enlarges the (3 E g-5 A1 g) spin gap to -5.7 kcal/mol.
TL;DR: In this paper, the authors used 3D metal Co as a dopant of Mo2C to construct Co-Mo bimetallic carbide and found an enhanced HER activity after Co incorporation.
Abstract: Efficient catalysts for the alkaline hydrogen evolution reaction are continuously pursued to accelerate the kinetics of water splitting and enhance the conversion of renewable energy to chemical feedstock. Among numerous candidates, noble-metal-free Mo2C catalysts are favored because of their low cost, abundance and similar d-orbital electronic state to the state of the art platinum. However, due to the high empty d-orbital density of high-valence Mo species in Mo2C, the HER performance was impaired. We reason that introducing modulators into the Mo2C framework that could decrease the empty d-orbital density and valence states of Mo may be an efficient way to optimize the HER energetics. Herein, inspired by the versatile electronic structures of 3d metals, we carried out first principles calculations using 3d metal Co as a dopant of Mo2C to construct Co–Mo bimetallic carbide and found an enhanced HER activity after Co incorporation. We further synthesized a Co–Mo bimetallic carbide catalyst. The obtained catalysts achieved excellent alkaline HER performance with the lowest overpotential of −46 mV at −10 mA cm−2, a low Tafel slope of 46 mV dec−1 and great stability without any decay after a 500 hour reaction. The X-ray adsorption spectroscopy study showed that the valence state of Mo was decreased by the Co dopant and we propose that the decrease of Mo valence states should be the reason for the better HER performance of the bimetallic carbide than bulk Mo2C.
TL;DR: A method for efficiently catalyzing the oxygen evolution reaction (OER) represents a top priority for water electrolysis due to its multistep electron transfer pathway and sluggish kinetics.
Abstract: A method for efficiently catalyzing the oxygen evolution reaction (OER) represents a top priority for water electrolysis due to its multistep electron transfer pathway and sluggish kinetics. The OE...