Showing papers on "Valence (chemistry) published in 2015"
TL;DR: A survey of the literature for ca. one thousand B-site substituted perovskite oxides can be found in this article, together with their electronic and magnetic properties and properties.
815 citations
Journal Article•
TL;DR: In this article, the authors analyzed the electronic structure and optical properties of perovskite solar cells based on CH3NH3PbI3 with the quasiparticle self-consistent GW approximation.
Abstract: The performance of organometallic perovskite solar cells has rapidly surpassed those of both traditional dye-sensitized and organic photovoltaics, e.g. solar cells based on CH3NH3PbI3 have recently reached 18% conversion efficiency. We analyze its electronic structure and optical properties within the quasiparticle self-consistent GW approximation (QSGW ). Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density approximation (LDA) or GW based on the LDA. Several characteristics combine to make the electronic structure of this material unusual. First, there is a strong driving force for ferroelectricity, as a consequence the polar organic moiety CH3NH3. The moiety is only weakly coupled to the PbI3 cage; thus it can rotate give rise to ferroelectric domains. This in turn will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and may contribute to the current-voltage hysteresis found in perovskite solar cells. Second, spin orbit modifies both valence band and conduction band dispersions in a very unusual manner: both get split at the R point into two extrema nearby. This can be interpreted in terms of a large Dresselhaus term, which vanishes at R but for small excursions about R varies linearly in k. Conduction bands (Pb 6p character) and valence bands (I 5p) are affected differently; moreover the splittings vary with the orientation of the moiety. We will show how the splittings, and their dependence on the orientation of the moiety through the ferroelectric effect, have important consequences for both electronic transport and the optical properties of this material.
418 citations
TL;DR: In this article, the authors show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient.
Abstract: SnTe, a lead-free rock-salt analogue of PbTe, having valence band structure similar to PbTe, recently has attracted attention for thermoelectric heat to electricity generation. However, pristine SnTe is a poor thermoelectric material because of very high hole concentration resulting from intrinsic Sn vacancies, which give rise to low Seebeck coefficient and high electrical thermal conductivity. In this report, we show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient. Mg (2–10 mol %) alloying in SnTe modulates its electronic band structure by increasing the band gap of SnTe and results in decrease in the energy separation between its light and heavy hole valence bands. Thus, solid solution alloying with Mg enhances the contribution of the heavy hole valence band, leading to significant improvement in the Seebeck coefficient in Mg alloyed SnTe, which in turn results in re...
364 citations
TL;DR: The transition metal-semiquinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in MOFs.
Abstract: A three-dimensional network solid composed of FeIII centers and paramagnetic semiquinoid linkers, (NBu4)2FeIII2(dhbq)3 (dhbq2–/3– = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone), is shown to exhibit a conductivity of 0.16 ± 0.01 S/cm at 298 K, one of the highest values yet observed for a metal–organic framework (MOF). The origin of this electronic conductivity is determined to be ligand mixed-valency, which is characterized using a suite of spectroscopic techniques, slow-scan cyclic voltammetry, and variable-temperature conductivity and magnetic susceptibility measurements. Importantly, UV–vis–NIR diffuse reflectance measurements reveal the first observation of Robin–Day Class II/III mixed valency in a MOF. Pursuit of stoichiometric control over the ligand redox states resulted in synthesis of the reduced framework material Na0.9(NBu4)1.8FeIII2(dhbq)3. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are investigated and correlated to the relative r...
298 citations
TL;DR: In this article, the valence and conduction bands of am- and γ-Al2O3 films grown by the atomic layer deposition technique were studied simultaneously in identical experimental conditions using high-resolution near-edge X-ray absorption fine structure and soft x-ray photoelectron spectroscopy.
Abstract: The valence and conduction bands of am- and γ-Al2O3 films grown by the atomic layer deposition technique were studied simultaneously in identical experimental conditions using high-resolution near -edge X-ray absorption fine structure and soft X-ray photoelectron spectroscopy. The valence band maximum was found to be centered at 3.64 ± 0.04 eV for am-Al2O3 and 3.47 ± 0.04 eV for γ-Al2O3. The band gap of Al2O3 was determined to be 7.0 ± 0.1 and 7.6 ± 0.1 eV for measured am- and γ-Al2O3, respectively. The main role in changing the band gap belongs to a shift of the bottom of conduction band depending on Al2O3 crystalline form. The position of the bottom of the conduction band is governed by the charge transfer from Al atom to the oxygen that depends strongly on the Al atom coordination symmetries. A strong p–d hybridization allowed for Td symmetry but forbidden for Oh symmetry plays the decisive role in the formation of the bottom of the conduction band.
274 citations
TL;DR: Using ab initio calculations combined with experiments, it is clarified how the kinetics of Li-ion diffusion can be tuned in LiNixMnyCozO2 (NMC, x + y + z = 1) materials.
Abstract: Using ab initio calculations combined with experiments, we clarified how the kinetics of Li-ion diffusion can be tuned in LiNixMnyCozO2 (NMC, x + y + z = 1) materials. It is found that Li-ions tend to choose oxygen dumbbell hopping (ODH) at the early stage of charging (delithiation), and tetrahedral site hopping (TSH) begins to dominate when more than 1/3 Li-ions are extracted. In both ODH and TSH, the Li-ions surrounded by nickel (especially with low valence state) are more likely to diffuse with low activation energy and form an advantageous path. The Li slab space, which also contributes to the effective diffusion barriers, is found to be closely associated with the delithiation process (Ni oxidation) and the contents of Ni, Co, and Mn.
262 citations
TL;DR: In this paper, the polarized transmittance and reflectance spectra of β-Ga2O3 crystals are investigated, and the data are interpreted in terms of the monoclinic crystal band structure.
Abstract: The polarized transmittance and reflectance spectra of β-Ga2O3 crystals are investigated, and the data are interpreted in terms of the monoclinic crystal band structure. The energies of the absorption edge can be divided into six ranges, and these ranges can be assigned to the transitions from the valence bands to the conduction band minimum according to the selection rules. The indirect bandgap-energy of 4.43 eV is smaller than the direct bandgap-energy of 4.48 eV at RT; and the energy difference of 0.05 eV nearly matches the theoretically calculated values of 0.03–0.04 eV.
246 citations
TL;DR: In this paper, it was shown that the alignment of the O 2p valence bands and the unoccupied Co 3d conduction bands improves the conductivity of the La1-xSrxCoO3 perovskite series.
Abstract: The bulk electronic structure, surface composition, conductivity, and electrochemical activity toward the oxygen evolution reaction for the La1–xSrxCoO3 perovskite series (with x = 0, 0.2, 0.4, 0.6, 0.8, 1) are investigated experimentally and theoretically. It is found that Sr substitutions have the effect of straightening the octahedral cage, aligning atoms along the Co–O–Co axis, and increasing the average oxidation state of the Co cations. As a consequence, both the ex situ electronic conductivity as well as the activity toward the oxygen evolution reaction are considerably improved. According to density-functional theory calculations, the alignment of the Co–O–Co bonds and the oxidation of the Co cations enhance the overlap between the occupied O 2p valence bands and the unoccupied Co 3d conduction bands, rationalizing the improvement of the conductivity as a function of the Sr fraction. Additionally, a study of the surface properties as a function of the Sr fraction, carried out by X-ray photoelectro...
238 citations
TL;DR: In this paper, the band offsets of methylammonium lead halides are reported, including relativistic corrections and using the Pb 1s core level as a reference state.
Abstract: Organic–inorganic halide perovskites efficiently convert sunlight to electricity in solar cells. The choice of halide (Cl, Br or I) can be used to chemically tune the spectral response of the materials and the positions of the valence and conduction bands (i.e. the ionisation potential and electron affinity). Here the band offsets of the methylammonium lead halides are reported, including relativistic corrections and using the Pb 1s core level as a reference state. The binding energy of the valence band decreases monotonically down the series, primarily due to the change from 3p to 4p to 5p valence orbitals of the halide. Type I band alignments are predicted, which implies that Br and Cl secondary phases in CH3NH3PbI3 thin-films would act as barriers to charge transport in photovoltaic devices.
230 citations
TL;DR: The parametrization approach transfers the accuracy of the valence force field model to the Stillinger-Weber potential, which supports stable molecular dynamics simulations, as each potential term is at an energy-minimum state separately at the equilibrium configuration.
Abstract: We propose parametrizing the Stillinger-Weber potential for covalent materials starting from the valence force-field model. All geometrical parameters in the Stillinger-Weber potential are determined analytically according to the equilibrium condition for each individual potential term, while the energy parameters are derived from the valence force-field model. This parametrization approach transfers the accuracy of the valence force field model to the Stillinger-Weber potential. Furthermore, the resulting Stilliinger-Weber potential supports stable molecular dynamics simulations, as each potential term is at an energy-minimum state separately at the equilibrium configuration. We employ this procedure to parametrize Stillinger-Weber potentials for single-layer MoS2 and black phosphorous. The obtained Stillinger-Weber potentials predict an accurate phonon spectrum and mechanical behaviors. We also provide input scripts of these Stillinger-Weber potentials used by publicly available simulation packages including GULP and LAMMPS.
213 citations
TL;DR: In this paper, an innovative and effective method is reported to significantly boost the durability and capacitance of VOx through tuning the valence state of vanadium, which is optimized through a very facile electrochemical oxidation method.
Abstract: Vanadium oxides (VOx) have been intensely investigated as cathode materials for SCs due to the multiple stable oxidation states (III–V) of vanadium in its oxides and typical layered structure. Nevertheless, fast capacity fading is always observed for VOx upon cycling in aqueous electrolyte. Developing an efficient strategy to essentially promote the durability of VOx in mild aqueous electrolyte remains a crucial challenge. Here, an innovative and effective method is reported to significantly boost the durability and capacitance of VOx through tuning the valence state of vanadium. The valence state of vanadium is optimized through a very facile electrochemical oxidation method. A superior electrochemical performance and an ultralong cyclic stability of 100 000 cycles are obtained for these electrodes. An in-depth study on the variation for the valence state of vanadium during the oxidation process and the cyclic stability test indicates that the long cyclic stability has an important relationship with the distribution of the valence state of vanadium.
TL;DR: In this paper, the average valence of the oxygen anions in the perovskite oxide BaTiO3 was found using O1s photoelectron spectra to be −1.55.
Abstract: The average valence, ValO, of the oxygen anions in the perovskite oxide BaTiO3, was found using O1s photoelectron spectra to be −1.55. This experimental result is close to the theoretical value for BaTiO3 (−1.63) calculated by Cohen [Nature 358, 136 (1992)] using density functional theory. Using the same approach, we obtained values of ValO for several monoxides, and investigated the dependence of ValO and the ionicity on the second ionization energy, V(M2+), of the metal cation. We found that the dependence of the ionicity on V(M2+) in this work is close to that reported by Phillips [Rev. Mod. Phys. 42, 317 (1970)]. We therefore suggest that O1s photoelectron spectrum measurements should be accepted as a general experimental method for estimating the ionicity and average valence of oxygen anions.
TL;DR: In this paper, the authors describe In+ species using photoluminescence (PL) and X-ray absorption fine structure (XAFS) analysis, and demonstrate that In+ exists in a metastable amorphous network, which is the origin of the observed luminescent properties.
Abstract: Valence control of polyvalent cations is important for functionalization of various kinds of materials. Indium oxides have been used in various applications, such as indium tin oxide in transparent electrical conduction films. However, although metastable In+ (5 s2 configuration) species exhibit photoluminescence (PL), they have attracted little attention. Valence control of In+ cations in these materials will be important for further functionalization. Here, we describe In+ species using PL and X-ray absorption fine structure (XAFS) analysis. Three absorption bands in the UV region are attributed to the In+ centre: two weak forbidden bands (1S0 → 3P1, 1S0 → 3P2) and a strong allowed band (1S0 → 1P1). The strongest PL excitation band cannot be attributed to the conventional allowed transition to the singlet excited state. Emission decay of the order of microseconds suggests that radiative relaxation occurs from the triplet excitation state. The XAFS analysis suggests that these In+ species have shorter In–O distances with lower coordination numbers than in In2O3. These results clearly demonstrate that In+ exists in a metastable amorphous network, which is the origin of the observed luminescent properties.
TL;DR: The analysis elucidats that the band structures are mainly governed by the orbits of phosphorus, oxygen and europium, and the sharp peaks of theEuropium f-orbit occur at the top of the valence bands.
Abstract: In this study, the Ba3Eu(PO4)3 and Sr3Eu(PO4)3 compounds were synthesized and the crystal structures were determined for the first time by Rietveld refinement using powder X-ray diffraction (XRD) patterns. Ba3Eu(PO4)3 crystallizes in cubic space group I3d, with cell parameters of a = 10.47996(9) A, V = 1151.01(3) A3 and Z = 4; Ba2+ and Eu3+ occupy the same site with partial occupancies of 3/4 and 1/4, respectively. Besides, in this structure, there exists two distorted kinds of the PO4 polyhedra orientation. Sr3Eu(PO4)3 is isostructural to Ba3Eu(PO4)3 and has much smaller cell parameters of a = 10.1203(2) A, V = 1036.52(5) A3. The bandgaps of Ba3Eu(PO4)3 and Sr3Eu(PO4)3 are determined to be 4.091 eV and 3.987 eV, respectively, based on the UV–Vis diffuse reflectance spectra. The photoluminescence measurements reveal that, upon 396 nm n-UV light excitation, Ba3Eu(PO4)3 and Sr3Eu(PO4)3 exhibit orange-red emission with two main peaks at 596 nm and prevailing 613 nm, corresponding to the 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3+, respectively. The dynamic disordering in the crystal structures contributes to the broadening of the luminescence spectra. The electronic structure of the phosphates was calculated by the first-principles method. The analysis elucidats that the band structures are mainly governed by the orbits of phosphorus, oxygen and europium, and the sharp peaks of the europium f-orbit occur at the top of the valence bands.
TL;DR: An analysis of hot-carrier lifetimes from electron-phonon interaction in lead iodide perovskites using first-principles calculations shows that the holes in CsPbI3 have very long lifetimes in the valence band region situated 0.6 eV below the top of theValence band, while no long lifetime is predicted in PbI 3(-).
Abstract: We report on an analysis of hot-carrier lifetimes from electron–phonon interaction in lead iodide perovskites using first-principles calculations. Our calculations show that the holes in CsPbI3 have very long lifetimes in the valence band region situated 0.6 eV below the top of the valence band. On the other hand, no long lifetime is predicted in PbI3–. These different results reflect the different electronic density of states (DOSs) in the valence bands, that is, a small DOS for the former structure while a sharp DOS peak for the latter structure. We propose a reduction of the relaxation paths in the small valence DOS as being the origin of the slow hot-hole cooling. Analyzing the generalized Eliashberg functions, we predict that different perovskite A-site cations do not have an impact on the carrier decay mechanism. The similarity between the DOS structures of CsPbI3 and CH3NH3PbI3 enables us to extend the description of the decay mechanism of fully inorganic CsPbI3 to its organic–inorganic counterpart...
TL;DR: In this paper, the authors show that zone-melted SnTe systems with additional Mn (1−7 mol%) can control the hole concentration by reducing the Sn vacancies, and modulate the electronic band structure by increasing the band gap and decreasing the energy separation between the light and heavy hole valence bands.
Abstract: Tin telluride (SnTe) has recently attracted lots of interest due to its potential thermoelectric application as a lead-free rock-salt analogue of PbTe. However, pristine SnTe samples have high hole concentration due to the presence of intrinsic Sn vacancies, and shows a low Seebeck coefficient and high electrical thermal conductivity, resulting in poor thermoelectric performance. In this report, we show that zone-melted SnTe systems with additional Mn (1–7 mol%) can control the hole concentration by reducing the Sn vacancies, and modulate the electronic band structure by increasing the band gap and decreasing the energy separation between the light and heavy hole valence bands. Therefore, alloying with additional Mn enhances the contribution of the heavy hole valence band and significantly improves the Seebeck coefficient in SnMnxTe with the highest value of ∼270 μV K−1. A record power factor of 31.9 μW cm−1 K−2 has been obtained at 820 K. The maximum thermoelectric figure of merit ZT of ∼1.25 is found at 920 K for the high quality crystalline ingot of p-type SnMn0.07Te.
TL;DR: In this paper, a series of Lewis acid metal salts were used for glucose dehydration to 5-hydroymethylfurfural (HMF) in water, and the experimental results showed that the valence state played an important role in determining catalytic activity and selectivity.
Abstract: A series of Lewis acidic metal salts were used for glucose dehydration to 5-hydroymethylfurfural (HMF) in water. Effect of valence state, ionic radii of Lewis acidic cation, and the type of anions on the catalytic performance have been studied systematically. The experimental results showed that the valence state played an important role in determining catalytic activity and selectivity. It was found that a higher glucose conversion rate and HMF selectivity could be obtained over high valent Lewis acid salts, where the ionic radii of these Lewis acidic metal salts are usually relatively small. Analysis on the effect of the anions of Lewis acid salts on the catalytic activity and the selectivity suggested that a higher glucose conversion and HMF selectivity could be readily obtained with Cl−. Furthermore, the recyclability of high valence state Lewis acid salt was also studied, however, inferior catalytic performance was observed. The deactivation mechanism was speculated to be the fact that high valence state Lewis acid salt was comparatively easier to undergo hydrolysis to yield complicated metal aqua ions with less catalytic activity. The Lewis acidic activity could be recovered by introducing a stoichiometric amount of hydrochloric acid (HCl) to the catalytic before the reaction.
TL;DR: In this paper, a combination of measurements using photoelectron spectroscopy and calculations using density functional theory (DFT) was applied to compare the detailed electronic structure of the organolead halide perovskites CH3NH3PbI3 and CH3 NH 3PbBr3, which are used to absorb light in mesoscopic and planar heterojunction solar cells.
Abstract: A combination of measurements using photoelectron spectroscopy and calculations using density functional theory (DFT) was applied to compare the detailed electronic structure of the organolead halide perovskites CH3NH3PbI3 and CH3NH3PbBr3. These perovskite materials are used to absorb light in mesoscopic and planar heterojunction solar cells. The Pb 4f core level is investigated to get insight into the chemistry of the two materials. Valence level measurments are also included showing a shift of the valence band edges where there is a higher binding energy of the edge for the CH3NH3PbBr3 perovskite. These changes are supported by the theoretical calculations which indicate that the differences in electronic structure are mainly caused by the nature of the halide ion rather than structural differences. The combination of photoelectron spectroscopy measurements and electronic structure calculations is essential to disentangle how the valence band edge in organolead halide perovskites is governed by the intr...
TL;DR: The proposed mechanism governing Fermi-level pinning is applicable to the systems of adsorbates on recently developed two-dimensional materials, such as graphene and transition metal dichalcogenides.
Abstract: Density functional theory calculations were performed to assess changes in the geometric and electronic structures of monolayer WS2 upon adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO). The most stable configuration of the adsorbed molecules, the adsorption energy, and the degree of charge transfer between adsorbate and substrate were determined. All evaluated molecules were physisorbed on monolayer WS2 with a low degree of charge transfer and accept charge from the monolayer, except for NH3, which is a charge donor. Band structure calculations showed that the valence and conduction bands of monolayer WS2 are not significantly altered upon adsorption of H2, H2O, NH3, and CO, whereas the lowest unoccupied molecular orbitals of O2, NO, and NO2 are pinned around the Fermi-level when these molecules are adsorbed on monolayer WS2. The phenomenon of Fermi-level pinning was discussed in light of the traditional and orbital mixing charge transfer theories. The impacts of the charge transfer mechanism on Fermi-level pinning were confirmed for the gas molecules adsorbed on monolayer WS2. The proposed mechanism governing Fermi-level pinning is applicable to the systems of adsorbates on recently developed two-dimensional materials, such as graphene and transition metal dichalcogenides.
TL;DR: Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions, and the N3(-) ligand is shown to behave as a strong σ and π donor.
Abstract: The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)-azido complex (1), which has been characterized experimentally (magnetically and by electronic d-d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = -10.7 cm(-1), E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)-azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3(-) ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co-N bond distances) of the axial ligand on the N3(-) site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.
TL;DR: Three new mixed valence trinuclear Co(II/III) compounds synthesized by reacting a di-Schiff base ligand with cobalt perchlorate hexahydrate and sodium azide show catecholase-like activities in the aerial oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone.
Abstract: Three new mixed valence trinuclear Co(II/III) compounds cis-[Co3L2(MeOH)2(N3)2(μ1,1-N3)2] (1), trans-[Co3L2(H2O)2(N3)2(μ1,1-N3)2]·(H2O)2 (2) and [Co3LR2(N3)3(μ1,3-N3)] (3) have been synthesized by reacting a di-Schiff base ligand (H2L) or its reduced form [H2LR] (where H2L = N,N′-bis(salicylidene)-1,3-propanediamine and H2LR = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine) with cobalt perchlorate hexahydrate and sodium azide. All three products have been characterized by IR, UV-Vis and EPR spectroscopies, ESI-MS, elemental, powder and single crystal X-ray diffraction analyses. Complex 1 is an angular trinuclear species in which two terminal octahedral Co(III)N2O4 centers coordinate to the central octahedral cobalt(II) ion through μ2-phenoxido oxygen and μ1,1-azido nitrogen atoms along with two mutually cis-oxygen atoms of methanol molecules. On the other hand, in linear trinuclear complex 2, in addition to the μ2-phenoxido and μ1,1-azido bridges with terminal octahedral Co(III) centres, the central Co(II) is bonded with two mutually trans-oxygen atoms of water molecules. Thus the cis–trans configuration of the central Co(II) is solvent dependent. In complex 3, the two terminal octahedral Co(III)N2O4 centers coordinate to the central penta-coordinated Co(II) ion through double phenoxido bridges along with the nitrogen atom of a terminal azido ligand. In addition, the two terminal Co(III) are connected through a μ1,3-azido bridge that participates in pnicogen bonding interactions (intermolecular N–N interaction) as an acceptor. Both the cis and trans isomeric forms of 1 and 2 have been optimized using density functional theory (DFT) calculations and it is found that the cis configuration is energetically more favorable than the trans one. However, the trans configuration of 2 is stabilized by the hydrogen bonding network involving a water dimer. The pnicogen bonding interactions have been demonstrated using MEP surfaces and CSD search which support the counter intuitive electron acceptor ability of the μ1,3-azido ligand. Complexes 1–3 exhibit catecholase-like activities in the aerial oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone. Kinetic data analyses of this oxidation reaction in acetonitrile reveal that the catecholase-like activity follows the order: 1 (kcat = 142 h−1) > 3 (kcat = 99 h−1) > 2 (kcat = 85 h−1). Mechanistic investigations of the catalytic behaviors by X-band EPR spectroscopy and estimation of hydrogen peroxide formation indicate that the oxidation reaction proceeds through the reduction of Co(III) to Co(II).
TL;DR: Valence-to-core (VtC) X-ray emission spectroscopy (XES) holds promise as an emerging probe of chemical structure for the ligands bound to a metal center, enabling access to chemical information that can be difficult to obtain with other methods.
Abstract: A long-standing goal of inorganic chemists is the ability to decipher the geometric and electronic structures of chemical species. This is particularly true for the study of small molecule and biological catalysts, where this knowledge is critical for understanding how these molecules effect chemical transformations. Numerous techniques are available for this task, and collectively they have enabled detailed understanding of many complex chemical systems. Despite this battery of probes, however, challenges still remain, particularly when the structural question involves subtle perturbations of the ligands bound to a metal center, as is often the case during chemical reactions. It is here that, as an emerging probe of chemical structure, valence-to-core (VtC) X-ray emission spectroscopy (XES) holds promise. VtC XES begins with ionization of a 1s electron from a metal ion by high energy X-ray photons. Electrons residing in ligand-localized valence orbitals decay to fill the 1s hole, emitting fluorescent photons in the process; in this manner, VtC XES primarily probes the filled, ligand-based orbitals of a metal complex. This is in contrast to other X-ray based techniques, such as K-edge X-ray absorption and EXAFS, which probe the unoccupied d-manifold orbitals and atomic scatterers surrounding the metal, respectively. As a hard X-ray technique, VtC XES experiments can be performed on a variety of sample states and environments, enabling application to demanding systems, such as high pressure cells and dilute biological samples. VtC XES thus can offer unique insights into the geometric and electronic structures of inorganic complexes. In recent years, we have sought to use VtC XES in the study of inorganic and bioinorganic complexes; doing so, however, first required a thorough and detailed understanding of the information content of these spectra. Extensive experimental surveys of model compounds coupled to the insights provided by DFT calculated spectra of real and hypothetical compounds allowed the development of a framework whereby VtC XES spectra may be understood in terms of a molecular orbital picture. Specifically, VtC spectra may be interpreted as a probe of electronic structure for the ligands bound to a metal center, enabling access to chemical information that can be difficult to obtain with other methods. Examples of this include the ability to (1) assess the identity and number of atomic/small molecule ligands bound to a metal center, (2) quantify the degree of bond activation of a small molecule substrate, and (3) establish the protonation state of donor atoms. With this foundation established, VtC has been meaningfully applied to long-standing questions in bioinorganic chemistry, with the potential for numerous future applications in all areas of metal-mediated catalysis.
TL;DR: In this paper, the effect of oxygen doping on the electronic and geometric structures of monolayer graphitic carbon nitride was calculated by first principle and revealed the favorable O doping configurations over all the Fermi levels utilizing the Ab initio thermodynamics approach.
TL;DR: The present results imply that as doping increases, thus subsequently increasing the Ce3+/Ce4+ ratio, antioxidant potential decreases, suggesting that differences in reactivity of CeO2 are due to the ability of Ce to transition between the two valence states and the presence of increased oxygen vacancies, rather than dependent on a specific valence state.
Abstract: Cerium oxide (CeO2) nanoparticles, which are used in a variety of products including solar cells, gas sensors, and catalysts, are expected to increase in industrial use. This will subsequently lead to additional occupational exposures, making toxicology screenings crucial. Previous toxicology studies have presented conflicting results as to the extent of CeO2 toxicity, which is hypothesized to be due to the ability of Ce to exist in both a +3 and +4 valence state. Thus, to study whether valence state and oxygen vacancy concentration are important in CeO2 toxicity, CeO2 nanoparticles were doped with gadolinium to adjust the cation (Ce, Gd) and anion (O) defect states. The hypothesis that doping would increase toxicity and decrease antioxidant abilities as a result of increased oxygen vacancies and inhibition of +3 to +4 transition was tested. Differences in toxicity and reactivity based on valence state were determined in RLE-6TN rat alveolar epithelial and NR8383 rat alveolar macrophage cells using enhanced dark field microscopy, electron paramagnetic resonance (EPR), and annexin V/propidium iodide cell viability stain. Results from EPR indicated that as doping increased, antioxidant potential decreased. Alternatively, doping had no effect on toxicity at 24 h. The present results imply that as doping increases, thus subsequently increasing the Ce3+/Ce4+ ratio, antioxidant potential decreases, suggesting that differences in reactivity of CeO2 are due to the ability of Ce to transition between the two valence states and the presence of increased oxygen vacancies, rather than dependent on a specific valence state.
Electronic supplementary material
The online version of this article (doi:10.1007/s12011-015-0297-4) contains supplementary material, which is available to authorized users.
TL;DR: This work designed and synthesized a novel n-n heterojunction photocatalyst, namely CdS-ZnWO4 heterojunctions, in which ZnW O4 has more negative conduction band and more positive valence band than those of CdCdS.
Abstract: In most of the reported n-n heterojunction photocatalysts, both the conduction and valence bands of one semiconductor are more negative than those of the other semiconductor. In this work, we designed and synthesized a novel n-n heterojunction photocatalyst, namely CdS-ZnWO4 heterojunctions, in which ZnWO4 has more negative conduction band and more positive valence band than those of CdS. The hydrogen evolution rate of CdS-30 mol %-ZnWO4 reaches 31.46 mmol h(-1) g(-1) under visible light, which is approximately 8 and 755 times higher than that of pure CdS and ZnWO4 under similar conditions, respectively. The location of the surface active sites is researched and a plausible mechanism of performance enhancement by the tuning of the structure is proposed based on the photoelectrochemical characterization. The results illustrate that this kind of nonconventional n-n heterojunctions is also suitable and highly efficient for solar hydrogen evolution.
TL;DR: In this paper, the influence of dopant size and valence state on the reduction properties, oxygen defects, and lattice strain in doped ceria has been studied fastidiously.
TL;DR: In this paper, a new V4+ ions formation mechanism by means of the electron trapped at single-electron-trapped oxygen vacancy (SETOV, Vo ) not Ti3+ was proposed, and the origin of the observed remarkable improved photocatalytic activity of the V-TiO2[NTA] samples has also been investigated.
Abstract: The novel V-doped TiO2 (V-TiO2[NTA]) samples with different (0–10%) concentration have been successfully prepared by a facile solid state sintering method using the nanotubular titanic acid (NTA) as titanium precursor for the first time, which shows remarkable photocatalytic activity for degradation of propylene under visible-light irradiation. Based on the density function theory (DFT) calculations in conjunction with a series of experimental characterization techniques including Raman spectra, TEM, XPS, ESR, and UV–vis DRS, a new V4+ ions formation mechanism by means of the electron trapped at single-electron-trapped oxygen vacancy (SETOV, Vo ) not Ti3+ has been proposed, and the origin of the observed remarkable improved photocatalytic activity of the V-TiO2[NTA] samples has also been investigated. At low concentration, the enhancement of photocatalytic activity for the V-TiO2[NTA] samples firstly come from the synergistic effect of V and Vo co-doping: a part of V5+ is reduced to V4+ by Vo into TiO2 lattice, others exist on the surface in the form of V2O5. The incorporation of V4+ in TiO2 lattice induces some new states (around the top of the valence band, due to O 2p and V 3d orbitals; around the bottom of the conduction band, due to Ti 3d, O 2p, and V 3d orbitals) near the edge of the valence and conduction bands, respectively, causing an effective narrowing of the band gap. The narrowing of gap is responsible for the red-shifted and increased light-absorption. The presence of V5+/V4+ redox couple facilitates the efficient separation and migration of photo-induced e−/h+ pairs to generate active species. Secondly, the enhancement of photocatalytic activity with low doping concentration may also be ascribed to the increased surface area. At high doping concentration, the reasons for the decreased photocatalytic activity is that the same trapping sites may act as the recombination centers and excess V dopant may occupy the active sites of surface.
TL;DR: In this paper, an integrated approach that combines synthesis, X-ray photoelectron spectroscopy (XPS) studies, and theoretical calculations for the investigation of active unsaturated metal sites (UMS) in copper-based metal-organic frameworks (MOFs) was developed.
Abstract: We have developed an integrated approach that combines synthesis, X-ray photoelectron spectroscopy (XPS) studies, and theoretical calculations for the investigation of active unsaturated metal sites (UMS) in copper-based metal–organic frameworks (MOFs). Specifically, extensive reduction of Cu+2 to Cu+1 at the MOF metal nodes was achieved. Introduction of mixed valence copper sites resulted in significant changes in the valence band structure and an increased density of states near the Fermi edge, thereby altering the electronic properties of the copper-based framework. The development of mixed-valence MOFs also allowed tuning of selective adsorbate binding as a function of the UMS oxidation state. The presented studies could significantly impact the use of MOFs for heterogeneous catalysis and gas purification as well as foreshadow a new avenue for controlling the conductivity of typically insulating MOF materials.
TL;DR: Further new alluaudite type transition metal sulphates can only be expected to yield a high rate performance, if their synthesis ensures the presence of a comparable transition metal sub-stoichiometry and/or a suitably tailored concentration of sodium/transition metal antisite defects.
Abstract: Bond-valence site energy modelling, classical molecular dynamics and DFT simulations were employed to clarify Na+ ion migration in monoclinic Na2+δFe2−δ/2(SO4)3, the recently reported first representative of a new promising class of alluaudite-type high voltage cathode materials for sodium-ion batteries. Empirical potential parameters derived from our softBV bond valence parameter set reproduce experimental unit-cell parameters. Migration energy barrier calculations based on both these empirical and on ab initio approaches consistently show a strongly anisotropic and fairly fast Na+ ion mobility along partially occupied Na(3) channels in the c-direction. Nominally fully occupied Na(1) sites are attached to these paths with a moderate activation energy as sources of mobile ions. At elevated temperatures separate parallel Na(2) channels contribute to the ionic conductivity. As such one-dimensional pathways are highly vulnerable to blocking by structural defects, the experimentally observed favourable rate performance can only be understood as a consequence of cross-linking of the channels to a more robust higher-dimensional migration pathway network. Our static and dynamic bond valence pathway models for representative local structure models reveal that this cross-linking is achieved by the iron deficiency of the compound: iron vacancies act as low-lying interstitial sites that can be reached from both types of channels with moderate activation energies. Structural relaxations around the vacancies however reduce the sodium mobility along the channels. An analogous dual effect of blocking migration along the channels and promoting perpendicular migration would result from Na+/Fe2+ antisite defects. Hence, further new alluaudite type transition metal sulphates can only be expected to yield a high rate performance, if their synthesis ensures the presence of a comparable transition metal sub-stoichiometry and/or a suitably tailored concentration of sodium/transition metal antisite defects.
TL;DR: A general scheme for disentangling donation and back-donation in the CD function of both symmetric and non-symmetric systems is presented and illustrated through applications to M-ethyne (M = Au, Ni and W) coordination bonds, including an explicative study on substrate activation in a model reaction mechanism.
Abstract: We recently devised a simple scheme for analyzing on quantitative grounds the Dewar-Chatt-Duncanson donation and back-donation in symmetric coordination complexes. Our approach is based on a symmetry decomposition of the so called Charge-Displacement (CD) function quantifying the charge flow, upon formation of a metal (M)-substrate (S) bond, along the M–S interaction axis and provides clear-cut measures of donation and back-donation charges in correlation with experimental observables [G. Bistoni et al., Angew. Chem., Int. Ed. 52, 11599 (2013)]. The symmetry constraints exclude of course from the analysis most systems of interest in coordination chemistry. In this paper, we show how to entirely overcome this limitation by taking advantage of the properties of the natural orbitals for chemical valence [M. Mitoraj and A. Michalak, J. Mol. Model. 13, 347 (2007)]. A general scheme for disentangling donation and back-donation in the CD function of both symmetric and non-symmetric systems is presented and illustrated through applications to M–ethyne (M = Au, Ni and W) coordination bonds, including an explicative study on substrate activation in a model reaction mechanism.