Showing papers on "Valence (chemistry) published in 2016"

Nanoscale cation motion in TaO x , HfO x and TiO x memristive systems

Abstract: A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

Band Edge Energies and Excitonic Transition Probabilities of Colloidal CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals

Abstract: Colloidal CsPbX3 (X = Cl, Br, and I) nanocrystals have recently emerged as preferred materials for light-emitting diodes, along with opportunities for photovoltaic applications. Such applications rely on the nature of valence and conduction band edges and optical transitions across these edges. Here we elucidate how halide compositions control both of these correlated parameters of CsPbX3 nanocrystals. Cyclic voltammetry shows that the valence band maximum (VBM) shifts significantly to higher energies by 0.80 eV, from X = Cl to Br to I, whereas the shift in the conduction band minimum (CBM) is small (0.19 eV) but systematic. Halides contribute more to the VBM, but their contribution to the CBM is also not negligible. Excitonic transition probabilities for both absorption and emission of visible light decrease probably because of the increasing dielectric constant from X = Cl to Br to I. These band edge properties will help design suitable interfaces in both devices and heterostructured nanocrystals.

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental–Theoretical Study

Abstract: We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic sta...

High Power Factor and Enhanced Thermoelectric Performance of SnTe-AgInTe2: Synergistic Effect of Resonance Level and Valence Band Convergence

Abstract: Understanding the basis of electronic transport and developing ideas to improve thermoelectric power factor are essential for production of efficient thermoelectric materials. Here, we report a significantly large thermoelectric power factor of ∼31.4 μW/cm·K2 at 856 K in Ag and In co-doped SnTe (i.e., SnAgxInxTe1+2x). This is the highest power factor so far reported for SnTe-based material, which arises from the synergistic effects of Ag and In on the electronic structure and the improved electrical transport properties of SnTe. In and Ag play different but complementary roles in modifying the valence band structure of SnTe. In-doping introduces resonance levels inside the valence bands, leading to a significant improvement in the Seebeck coefficient at room temperature. On the other hand, Ag-doping reduces the energy separation between light- and heavy-hole valence bands by widening the principal band gap, which also results in an improved Seebeck coefficient. Additionally, Ag-doping in SnTe enhances the...

The In-Gap Electronic State Spectrum of Methylammonium Lead Iodide Single-Crystal Perovskites

Abstract: The density of trap states within the bandgap of methylammonium lead iodide single crystals is investigated. Defect states close to both the conduction and valence bands are probed. Additionally, a comprehensive electronic characterization of crystals is carried out, including measurements of the electron and hole mobility, and the energy landscape (band diagram) at the surface.

N-Electron Valence State Perturbation Theory Based on a Density Matrix Renormalization Group Reference Function, with Applications to the Chromium Dimer and a Trimer Model of Poly(p-Phenylenevinylene).

Abstract: The strongly contracted variant of second-order N-electron valence state perturbation theory (NEVPT2) is an efficient perturbative method to treat dynamic correlation without the problems of intruder states or level shifts, while the density matrix renormalization group (DMRG) provides the capability to address static correlation in large active spaces. We present a combination of the DMRG and strongly contracted NEVPT2 (DMRG-SC-NEVPT2) that uses an efficient algorithm to compute high-order reduced-density matrices from DMRG wave functions. The capabilities of DMRG-SC-NEVPT2 are demonstrated on calculations of the chromium dimer potential energy curve at the basis set limit, and the excitation energies of a trimer model of poly(p-phenylenevinylene) (PPV(n = 3)).

Tuning of Thermal Stability in Layered Li(NixMnyCoz)O2

Abstract: Understanding and further designing new layered Li(NixMnyCoz)O2 (NMC) (x + y + z = 1) materials with optimized thermal stability is important to rechargeable Li batteries (LIBs) for electrical vehicles (EV). Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM(Ni, Mn, Co)3-O-Li3–x′): each O atom bonds with three transition metals (TM) from the TM-layer and three to zero Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed. The synergistic effect between Li vacancies and raised valence state of Ni during delithiation process can aggravate instability of oxygen, a...

Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory

Abstract: Tin sulfide solar cells show relatively poor efficiencies despite attractive photovoltaic properties, and there is difficulty in identifying separate phases, which are also known to form during Cu2ZnSnS4 depositions. We present X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy measurements of single crystal SnS, SnS2, and Sn2S3, with electronic-structure calculations from density functional theory (DFT). Differences in the XPS spectra of the three phases, including a large 0.9 eV shift between the 3d5/2 peak for SnS and SnS2, make this technique useful when identifying phase-pure or mixed-phase systems. Comparison of the valence band spectra from XPS and DFT reveals extra states at the top of the valence bands of SnS and Sn2S3, arising from the hybridization of lone pair electrons in Sn(II), which are not present for Sn(IV), as found in SnS2. This results in relatively low ionization potentials for SnS (4.71 eV) and Sn2S3 (4.66 eV), giving a more comprehensive explanation as to...

Electronic structure, photovoltage, and photocatalytic hydrogen evolution with p-CuBi2O4 nanocrystals

Abstract: As a visible light active p-type semiconductor, CuBi2O4 is of interest as a photocatalyst for the generation of hydrogen fuel from water. Here we present the first photovoltage and photocatalytic measurements on this material and DFT results on its band structure. Single crystalline CuBi2O4 nanoparticles (25.7 ± 4.7 nm) were synthesized from bismuth and cupric nitrate in water under hydrothermal conditions. Powder X-ray diffraction (XRD) confirms the CuBi2O4 structure type and UV-Vis spectroscopy shows a 1.75 eV optical band gap. Surface photovoltage (SPV) measurements on CuBi2O4 nanoparticle films on fluorine doped tin oxide yield 0.225 V positive photovoltage at >1.75 eV photon energy confirming holes as majority carriers. The photovoltage is reversible and limited by light absorption. When dispersed in 0.075 M aqueous potassium iodide solution, the CuBi2O4 particles support photochemical hydrogen evolution of up to 16 μmol h−1 under ultraviolet but not under visible light. Based on electrochemical scans, CuBi2O4 is unstable toward reduction at −0.2 V, but a pH-dependent photocurrent of 6.45 μA cm−2 with an onset potential of +0.75 V vs. NHE can be obtained with 0.01 M Na2S2O8 as a sacrificial electron acceptor. The photoelectrochemical properties of CuBi2O4 can be explained on the basis of the band structure of the material. DFT calculations show that the valence and conduction band edges arise primarily from the combination of O 2p and Cu 3d orbitals, respectively, with additional contributions from Cu 3d and Bi 6s orbitals just below the Fermi level. Trapping of photoelectrons in the Cu 3d band is the cause for reductive photocorrosion of the material, while the p-type conductivity arises from copper vacancy states near the VB edge. These findings provide an improved understanding of the photophysical properties of p-CuBi2O4 and its limitations as a proton reduction photocatalyst.

A review of the structural architecture of tellurium oxycompounds

Abstract: Relative to its extremely low abundance in the Earth's crust, tellurium is the most mineralogically diverse chemical element, with over 160 mineral species known that contain essential Te, many of them with unique crystal structures. We review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals. The dataset is restricted to compounds where oxygen is the only ligand that is strongly bound to Te, but most of the Periodic Table is represented in the compounds that are reviewed. The dataset contains 375 structures that contain only Te cations and 302 with only Te, with 26 of the compounds containing Te in both valence states. Te was almost exclusively in rather regular octahedral coordination by oxygen ligands, with only two instances each of 4- and 5-coordination. Conversely, the lone-pair cation Te displayed irregular coordination, with a broad range of coordination numbers and bond distances. A threshold was applied for Te-O links of ~2.45 A or 0.3 valence units with some flexibility, as a criterion to define strongly bound Te-O polymers and larger structural units. Using this criterion, Te cations display one-sided 3-, 4- or 5-coordination by oxygen (with rare examples of coordination numbers 2 and 6). For both valence states of Te, examples are known of TeO complexes which are monomeric (m = 1; neso), noncyclic finite oligomers (soro), rings (cyclo), infinite chains (ino), layers (phyllo) and frameworks (tecto tellurates). There is a clear analogy to the polymerization classes that are known for silicate anions, but the behaviour of Te is much richer than that of Si for several reasons: (1) the existence of two cationic valence states for Te; (2) the occurrence of multiple coordination numbers; (3) the possibility of edge-sharing by TeO polyhedra; (4) the possibility for oxygen ligands to be 3-coordinated by Te; and (5) the occurrence of TeO polymers that are cationic, as well as neutral or anionic. While most compounds contain only one or two symmetrically distinct types of Te atom, Pauling's Fifth Rule is frequently violated, and stoichiometrically simple compounds such as CaTeO can have polymorphs with up to 18 distinct Te sites. There is a tendency for local symmetry features such as the threefold axis of a TeO octahedron or the acentric symmetry of a TeO polyhedron to be inherited by the host structure; the latter in particular can lead to useful physical properties such as nonlinear optical behaviour. We develop for the first time a hierarchical taxonomy of Te-oxysalt structures, based upon (1) valence state of Te; (2) polymerization state of TeO complexes; (3) polymerization state of larger strongly-bound structural units that include non-Te cations. Structures are readily located and compared within this classification.

Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules III: A Benchmark of GW Methods.

Abstract: The performance of different GW methods is assessed for a set of 24 organic acceptors. Errors are evaluated with respect to coupled cluster singles, doubles, and perturbative triples [CCSD(T)] reference data for the vertical ionization potentials (IPs) and electron affinities (EAs), extrapolated to the complete basis set limit. Additional comparisons are made to experimental data, where available. We consider fully self-consistent GW (scGW), partial self-consistency in the Green's function (scGW0), non-self-consistent G0W0 based on several mean-field starting points, and a "beyond GW" second-order screened exchange (SOSEX) correction to G0W0. We also describe the implementation of the self-consistent Coulomb hole with screened exchange method (COHSEX), which serves as one of the mean-field starting points. The best performers overall are G0W0+SOSEX and G0W0 based on an IP-tuned long-range corrected hybrid functional with the former being more accurate for EAs and the latter for IPs. Both provide a balanced treatment of localized vs delocalized states and valence spectra in good agreement with photoemission spectroscopy (PES) experiments.

Valence and conduction band tuning in halide perovskites for solar cell applications

Abstract: We performed density functional calculations aimed at identifying the atomistic and electronic structure origin of the valence and conduction band, and band gap tunability of halide perovskites ABX3 upon variations of the monovalent and bivalent cations A and B and the halide anion X. We found that the two key ingredients are the overlap between atomic orbitals of the bivalent cation and the halide anion, and the electronic charge on the metal center. In particular, lower gaps are associated with higher negative antibonding overlap of the states at the valence band maximum (VBM), and higher charge on the bivalent cation in the states at the conduction band minimum (CBM). Both VBM orbital overlap and CBM charge on the metal ion can be tuned over a wide range by changes in the chemical nature of A, B and X, as well as by variations of the crystal structure. On the basis of our results, we provide some practical rules to tune the valence band maximum, respectively the conduction band minimum, and thus the band gap in this class of materials.

Octahedral Rotation Preferences in Perovskite Iodides and Bromides

Abstract: Phase transitions in ABX3 perovskites are often accompanied by rigid rotations of the corner-connected BX6 octahedral network. Although the mechanisms for the preferred rotation patterns of perovskite oxides are fairly well recognized, the same cannot be said of halide variants (i.e., X = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (a0a0c+ in Glazer notation). To discern the chemical factors stabilizing this unique phase, we investigated a series of 12 perovskite bromides and iodides using density functional theory calculations and compared them with similar oxides. We find that in-phase tilting provides a better arrangement of the larger bromide and iodide anions, which minimizes the electrostatic interactions, improves the bond valence of the A-site cations, and enhances the covalency between the A-site metal and Br– or I– ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizin...

On the Nature of Support Effects of Metal Dioxides MO2 (M = Ti, Zr, Hf, Ce, Th) in Single-Atom Gold Catalysts: Importance of Quantum Primogenic Effect

Abstract: Fundamental understanding of support effects and metal–support interaction is critical in heterogeneous catalysis. In this work, theoretical investigations are carried out to study the nature of support effects of different tetravalent-metal dioxides of MO2 (M = Ti, Zr, Ce, Hf, Th) in single-atom gold catalysts using density functional theory with on-site Coulomb interactions (DFT+U). The properties of gold adatom on the stoichiometric (MO2) and reduced (MO2–x) surfaces as well as CO adsorption on Au1/MO2 and Au1/MO2–x have been investigated systematically. Our calculations indicate that the fundamental quantum primogenic effect that causes the radial contraction and low orbital energies of 3d and 4f orbitals in these MO2 oxides plays a vital role in determining the valence states and charge distribution of single-atom gold as well as the adsorption modes of CO on various MO2 supports. We find that gold atoms supported on different surfaces exhibit oxidation states from Au(−I) to Au(0) to Au(I), depending...

Aggregation behavior of aqueous cellulose nanocrystals: the effect of inorganic salts

Abstract: Natural anisotropic building-blocks such as cellulose nanocrystals (CNCs) have attracted considerable attention due to their biodegradability and nanometer-size. In this work the colloidal behavior of CNCs, obtained from sulfuric acid hydrolysis of microcrystalline cellulose, has been studied in presence of salts of different valences. The influence on the colloidal stability and nature of aggregates has been investigated for monovalent salts (LiCl, NaCl, KCl, CsCl), divalent salts (CaCl2 and MgCl2), and a trivalent salt (AlCl3), both experimentally by means of turbidity and small angle X-ray scattering (SAXS) measurements, as well as by Monte Carlo simulations using a simple coarse-grained model. For the entire salt series, a critical aggregation concentration (CAC) could be determined by turbidity measurements, as a result of the reduction of effective Coulomb repulsions due to the presence of sulfate groups on the CNC surface. The CACs also followed the Schulze–Hardy law, i.e. the critical aggregation concentration decreased with increasing counterion valence. For the monovalent ions, the CACs followed the trend Li+ > Na+ > K+ > Cs+, which could be rationalized in terms of matching affinities between the cation and the sulfate groups present at the surface of CNCs. From the SAXS measurements it was shown that the density of the aggregates increased with increasing salt concentration and ion valence. In addition, these findings were rationalized by means of simulation, which showed a good correlation with experimental data. The combination of the experimental techniques and the simulations offered insight into interaction-aggregation relationship of CNC suspensions, which is of importance for their structural design applications.

X-ray Photoemission Spectroscopy Study of Cationic and Anionic Redox Processes in High-Capacity Li-Ion Battery Layered-Oxide Electrodes

Abstract: Electrode materials based on Li-rich layered oxides are of growing interest for high-energy Li-ion battery applications because of their staggering capacities associated with the emergence of a novel, reversible anionic process. However, the fundamental science at work behind this new process needs to be well understood for further optimization. Here we report on the redox mechanisms in high-capacity Li-rich materials Li2Ru1–xMxO3 and Li2Ir1–xMxO3, by combining X-ray photoemission spectroscopy (XPS) core peaks and valence intensity analyses. We fully confirm that these materials electrochemically react with Li via cumulative reversible cationic/anionic redox processes, but more importantly we reveal that, depending on the nature of the metal (Ru or Ir), there is a delicate balance between metal and oxygen contributions. For instance, we show a greater implication of oxide ions for Ir-based electrodes, consistent with the higher covalent character of Ir–O bonds compared to Ru–O bonds. We equally provide ev...

Effect of Pt Particle Size and Valence State on the Performance of Pt/TiO2 Catalysts for CO Oxidation at Room Temperature

Abstract: A series of Pt/TiO2 catalysts with various Pt particle sizes and valence states were prepared and tested for CO oxidation at room temperature. Field-emission transmission electron microscopy and X-ray photoelectron spectroscopy analyses confirmed that the activity of the Pt/TiO2 catalyst was influenced by the particle size and valence state of the catalyst particles. Excellent CO oxidation activity was observed at room temperature using highly dispersed, small metallic Pt particles. Increasing the Ptmetallic/Pttotal ratio resulted in an increase of turnover frequencies. According to the Fourier-transform infrared spectroscopy results, the linear CO species that was adsorbed on metallic Pt sites at 25 °C reacted with atmospheric O2 and was easily desorbed. However, linear CO species adsorbed on PtOx (x ≥ 2) sites was only desorbed at temperatures ≥100 °C, confirming the lack of CO oxidation activity at room temperature with ionic Pt catalysts.

Gold as a 6p-Element in Dense Lithium Aurides

Abstract: The negative oxidation state of gold (Au) has drawn a great attention due to its unusual valence state that induces exotic properties in its compounds, including ferroelectricity and electronic polarization. Although monatomic anionic gold (Au–) has been reported, a higher negative oxidation state of Au has not been observed yet. Here we propose that high pressure becomes a controllable method for preparing high negative oxidation state of Au through its reaction with lithium. First-principles calculations in combination with swarm structural searches disclosed chemical reactions between Au and Li at high pressure, where stable Li-rich aurides with unexpected stoichiometries (e.g., Li4Au and Li5Au) emerge. These compounds exhibit intriguing structural features like Au-centered polyhedrons and a graphene-like Li sublattice, where each Au gains more than one electron donated by Li and acts as a 6p-element. The high negative oxidation state of Au has also been achieved through its reactions with other alkali...

Valence Change Ability and Geometrical Occupation of Substitution Cations Determine the Pseudocapacitance of Spinel Ferrite XFe2O4(X = Mn, Co, Ni, Fe)

Abstract: Cations Determine the Pseudocapacitance of Spinel Ferrite XFe2O4 (X = Mn, Co, Ni, Fe) Chao Wei,†,⊥ Zhenxing Feng,‡,⊥ Murat Baisariyev,† Linghui Yu,† Li Zeng, Tianpin Wu, Haiyan Zhao, Yaqin Huang, Michael J. Bedzyk, Thirumany Sritharan,† and Zhichuan J. Xu*,† †School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States Graduate Program of Applied Physics, Northwestern University, Evanston, Illinois 60208, United States Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States X-ray Science Divisions, Argonne National Laboratory, Lemont, Illinois 60439, United States Chemical and Materials Engineering Department, University of Idaho, Idaho Falls, Idaho 83401, United States College of Materials Science and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

Valence state heterojunction Mn3O4/MnCO3: Photo and thermal synergistic catalyst

Abstract: The valence state Mn3O4/MnCO3 heterojunctions were synthesized using a facile, wet chemical oxidation method during hydrothermal process. Their crystal structures, morphologies and optical properties were systematically investigated. The obtained Mn3O4/MnCO3 exhibited photo and thermal synergistic catalysis which have been estimated via degradation of methylene blue (MB) and HCHO under visible light (λ > 420 nm) irradiation at different temperature (20 °C, 60 °C and 80 °C). Mn3O4 and MnCO3 possessed matching band potentials that could prevent the recombination rate of photogenerated electrons and holes in the reaction, which proved by the PL intensity of the samples, resulting in a great enhancement on their catalytic efficiency. Catalytic activity of Mn3O4/MnCO3 under photo and thermal effect was not the simple summation of photocatalysis and thermocatalysis, but a synergistic effect was existed in Mn3O4/MnCO3 composites which possessed much lattice oxygen to capture the holes leading to efficiently oxidizing reaction at high temperature. A possible mechanism was proposed based on valence state heterojunction to illustrate the synergistic effect between photo and thermal catalysis on degradation of organic pollutions.

Elucidation of the highest valence band and lowest conduction band shifts using XPS for ZnO and Zn0.99Cu0.01O band gap changes

Abstract: ZnO and Zn0.99Cu0.01O nanostructures were prepared by a simple sol–gel method. The band gaps of the materials were systematically studied based on the dependence of the dimensions of the nanostructures as well as the presence of a dopant material, Cu. ZnO and Zn0.99Cu0.01O nanostructures were found to exhibit band gap widening whilst substitution of Cu in the lattice of ZnO caused its band gap to narrow with respect to the pure ZnO materials. In order to understand the phenomenon of band gap change, structural, spectroscopic, particle size and morphological studies were done. The band gap change occurring when the materials were in the nanostructured phase was proven to be mainly due to the downward shift of the valence band. Interestingly, when the band gaps of the pure ZnO and Cu doped ZnO were compared, the band gap changes were due to different shifts of the valence bands.

Formation, electronic structure, and defects of Ni substituted spinel cobalt oxide: A DFT+U study

Abstract: Nickel substituted spinel cobalt oxide is a promising technological material with complex electronic and magnetic structures. Understanding these structures is important for improving the material’s performance in various applications. We have carried out first-principles calculations on the formation, electronic properties, and defects of bulk NiCo2O4 using density functional theory (DFT) with on-site Hubbard U terms on the transition metal d states. Analysis of the electronic structure of NixCo3-xO4 as a function of x = 0–1 shows that Ni acts as a p-type dopant in Co3O4, gradually transforming the minority spin channel from insulating to conducting. As a result, the inverse spinel NiCo2O4 (NCO) is found to have a ferrimagnetic half-metallic ground state with fractional valence on Ni and Co cations at tetrahedral sites (Td), in agreement with experimental observations. Projected densities of states confirm that the states around the Fermi energy originate from Ni and Co(Td) 3d states hybridized with oxyg...

Periodic Trends in Lanthanide Compounds through the Eyes of Multireference ab Initio Theory

Abstract: Regularities among electronic configurations for common oxidation states in lanthanide complexes and the low involvement of f orbitals in bonding result in the appearance of several periodic trends along the lanthanide series. These trends can be observed on relatively different properties, such as bonding distances or ionization potentials. Well-known concepts like the lanthanide contraction, the double–double (tetrad) effect, and the similar chemistry along the lanthanide series stem from these regularities. Periodic trends on structural and spectroscopic properties are examined through complete active space self-consistent field (CASSCF) followed by second-order N-electron valence perturbation theory (NEVPT2) including both scalar relativistic and spin–orbit coupling effects. Energies and wave functions from electronic structure calculations are further analyzed in terms of ab initio ligand field theory (AILFT), which allows one to rigorously extract angular overlap model ligand field, Racah, and spin–...

Valence Electronic Structure of Aqueous Solutions: Insights from Photoelectron Spectroscopy

Abstract: The valence orbital electron binding energies of water and of embedded solutes are crucial quantities for understanding chemical reactions taking place in aqueous solution, including oxidation/reduction, transition-metal coordination, and radiation chemistry. Their experimental determination based on liquid-photoelectron spectroscopy using soft X-rays is described, and we provide an overview of valence photoelectron spectroscopy studies reported to date. We discuss principal experimental aspects and several theoretical approaches to compute the measured binding energies of the least tightly bound molecular orbitals. Solutes studied are presented chronologically, from simple electrolytes, via transition-metal ion solutions and several organic and inorganic molecules, to biologically relevant molecules, including aqueous nucleotides and their components. In addition to the lowest vertical ionization energies, the measured valence photoelectron spectra also provide information on adiabatic ionization energies and reorganization energies for the oxidation (ionization) half-reaction. For solutes with low solubility, resonantly enhanced ionization provides a promising alternative pathway.

Viewing the Valence Electronic Structure of Ferric and Ferrous Hexacyanide in Solution from the Fe and Cyanide Perspectives

Abstract: The valence-excited states of ferric and ferrous hexacyanide ions in aqueous solution were mapped by resonant inelastic X-ray scattering (RIXS) at the Fe L2,3 and N K edges. Probing of both the central Fe and the ligand N atoms enabled identification of the metal- and ligand-centered excited states, as well as ligand-to-metal and metal-to-ligand charge-transfer excited states. Ab initio calculations utilizing the RASPT2 method were used to simulate the Fe L2,3-edge RIXS spectra and enabled quantification of the covalencies of both occupied and empty orbitals of π and σ symmetry. We found that π back-donation in the ferric complex is smaller than that in the ferrous complex. This is evidenced by the relative amounts of Fe 3d character in the nominally 2π CN– molecular orbital of 7% and 9% in ferric and ferrous hexacyanide, respectively. Utilizing the direct sensitivity of Fe L3-edge RIXS to the Fe 3d character in the occupied molecular orbitals, we also found that the donation interactions are dominated by...

Mixed Valence Tin Oxides as Novel van der Waals Materials: Theoretical Predictions and Potential Applications

Abstract: Van der Waals (vdW) heterostructures, which can be assembled by combining 2D atomic crystals in a precisely chosen sequence, enable a wide range of potential applications in optoelectronics, photovoltaics, and photocatalysis. However, the difficulty of peeling isolated atomic planes and the lattice mismatch between different materials is the main obstacle to hinder vdW materials from more practical applications. In this work, the mixed valence tin oxides, SnxOy (0.5 < x/y < 1), are proposed as a new member of vdW materials and these mixed valence tin oxides show promise to overcome the above-mentioned obstacle. Density-functional theory calculations are combined with an evolutionary algorithm to predict the crystal structures of a series of previously reported tin oxides (Sn2O3, Sn3O4, Sn4O5, and Sn5O6), unreported compositions (Sn7O8, Sn9O10, and Sn11O12), and a new β-SnO phase. These structures consist of β-SnO, Sn2O3, and Sn3O4 monolayers. Their band gaps can be engineered in the 1.56–3.25 eV range by stacking the monolayers appropriately. The band gap depends linearly on the interlayer distance, as understood from interlayer Sn2+–Sn2+ and intralayer Sn2+–O interactions. SnxOy structures exhibit high photoabsorption coefficients and suitable band-edge positions for photoexcited H2 evolution; this indicates potential for environmentally benign solar energy conversion in photovoltaic and photocatalytic applications.

Chemical state determination of molecular gallium compounds using XPS

Abstract: A series of molecular gallium compounds were analyzed using X-ray photoelectron spectroscopy (XPS). Specifically, the Ga 2p3/2 and Ga 3d5/2 photoelectron binding energies and the Ga L3M45M45 Auger electron kinetic energies of compounds with gallium in a range of assigned oxidation numbers and with different stabilizing ligands were measured. Auger parameters were calculated and used to generate multiple chemical speciation (or Wagner) plots that were subsequently used to characterize the novel gallium-cryptand[2.2.2] complexes that possess ambiguous oxidation numbers for gallium. The results presented demonstrate the ability of widely accessible XPS instruments to experimentally determine the chemical state of gallium centers and, as a consequence, provide deeper insights into reactivity compared to assigned oxidation and valence numbers.