Metallic Electronic States Research Articles

Evolution of Oxygen–Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X‐Ray Spectroscopy

AbstractManganese oxide (MnOx) electrocatalysts are examined herein by in situ soft X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L‐edge XAS data with previous mechanistic studies indicates MnIV is the highest oxidation state involved in the catalytic mechanism. MnOx is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn−O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K‐edge provide complementary evidence for such a transition. This step is crucial for the formation of O2 from water.

Evolution of Oxygen-Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X-Ray Spectroscopy.

Manganese oxide (MnOx ) electrocatalysts are examined herein by in situ soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L-edge XAS data with previous mechanistic studies indicates MnIV is the highest oxidation state involved in the catalytic mechanism. MnOx is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn-O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K-edge provide complementary evidence for such a transition. This step is crucial for the formation of O2 from water.

Energetics and optimized ferroelectric polarization with metallic electronic states in ATiO3 (A = Ba and Pb): Layer doping strategy

Density functional theory is used to investigate the formation energetics (Ef), the spontaneous polarization (SP), and electronic properties of layer doped ATiO3 (A = Ba and Pb) systems having 10 dopants at A-site. We found that Ef and SP show a strong interdependence of dopant sizes (atomic radii) and both increases gradually with the decrease of dopant sizes. Along with this, the energy band gap substantially reduced for lengthening bond lengths. Interestingly, our results show that P@Pb, As@Pb, and Sb@Pb PTO doped systems are promising candidates for device applications because of optimized SP due to large octahedral distortion and n-type conductivity as compared to the pristine one. Furthermore, Bi@Pb PTO doped system also exhibits metallicity but have little lower magnitude of SP than that of undoped. The metallic electronic states induced due to charge transfer from (P+3O−2)+1, (As+3O−2)+1, (Sb+3O−2)+1, and (Bi+3O−2)+1 polar doped to (Ti+4O−4)0 non-polar undoped layers owing to the polar discontinuity. Hence, our findings open a new perspective to enhance SP with conducting electronic states in perovskite oxides by layer doping strategy.

Exciton-plasmon coupling in two-dimensional plexitonic nano grating

The proximity of metal and semiconductor nanostructures leads to the emergence of new optical features for many tunable applications, which affects the electromagnetic modes in metallic nanostructure and electronic states in semiconductor nanostructure in nanometer scales. Thus, it will create some changes in the transition matrix elements and the absorption and emission properties. Therefore, absorption and emission properties can be designed and controlled by exciton-plasmon interaction. In the present study, Rhodamine-B and 6G were used as organic dyes in Polyvinylpyrrolidone as host medium and two-dimensional crystal as plasmonic ones. To this aim, Nano imprint lithography was used to produce two dimensional crystals and its deposit gold was utilized to harvest plasmonic mold in the proximity of excitonic media. Then, the dispersion relation was measured and the polar diagram was plotted for different coupling regime. Based on the results, this system has a poor capability for overcoming the difficulties of obtaining strong coupling although different figures of merit were observed for increasing coupling strength, which is very useful for designing and constructing new generation of plexitonic structures.

Magnetic ground state of orthorhombically distorted CaCoO3 perovskite oxide: GGA and GGA+U calculations

Cobalt-based perovskite oxides have been attracted considerable attention due to their fascinating properties, especially entangled magnetic spin structures. Here, we comprehensively study the various magnetic ordering in recently reported orthorhombically distorted CaCoO3 perovskite oxide by employing the density functional theory calculations. A strong ferromagnetic coupling due to double exchange interactions between neighbouring Co atoms within the CoO2 bilayers is found than the anti-ferromagnetic coupling, which results in ferromagnetic spin ordering as a magnetic ground state with metallic electronic ground state (even Co is in its +4 state) for both exchange-correlation potentials (generalized gradient approximation (GGA) and GGA+U). The estimated magnetic transition temperature TC = 116/139 K for GGA/GGA+U, which is close to the experimentally observed value of 95 K for helimagnetic ground state.

Highly efficient hydrogen release from formic acid using a graphitic carbon nitride-supported AgPd nanoparticle catalyst

Bimetallic AgPd nanoparticles with various molar ratios immobilized on graphitic carbon nitride (g-C3N4) were successfully synthesized via a facile co-reduction approach. The powder XRD, XPS, TEM, EDX, ICP-AES and BET were employed to characterize the structure, size, composition and loading metal electronic states of the AgPd/g-C3N4 catalysts. The catalytic property of as-prepared catalysts for the dehydrogenation of formic acid (FA) with sodium formate (SF) as the additive was investigated. The performance of these catalysts, as indicated by the turnover frequency (TOF), depended on the composition of the prepared catalysts. Among all the AgPd/g-C3N4 catalysts tested, Ag9Pd91/g-C3N4 was found to be an exceedingly high activity for decomposing FA into H2 with TOF up to 480h−1 at 323K. The prepared catalyst is thus a potential candidate for triggering the widespread use of FA for H2 storage.

Atomic-scale epitaxial aluminum film on GaAs substrate

Atomic-scale metal films exhibit intriguing size-dependent film stability, electrical conductivity, superconductivity, and chemical reactivity. With advancing methods for preparing ultra-thin and atomically smooth metal films, clear evidences of the quantum size effect have been experimentally collected in the past two decades. However, with the problems of small-area fabrication, film oxidation in air, and highly-sensitive interfaces between the metal, substrate, and capping layer, the uses of the quantized metallic films for further ex-situ investigations and applications have been seriously limited. To this end, we develop a large-area fabrication method for continuous atomic-scale aluminum film. The self-limited oxidation of aluminum protects and quantizes the metallic film and enables ex-situ characterizations and device processing in air. Structure analysis and electrical measurements on the prepared films imply the quantum size effect in the atomic-scale aluminum film. Our work opens the way for further physics studies and device applications using the quantized electronic states in metals.

Synthesis of flower-like molybdenum sulfide/graphene hybrid as an efficient oxygen reduction electrocatalyst for anion exchange membrane fuel cells

Abstract Nanostructured transition metal chalcogenides (TMCs) have significant interest towards electrochemical devices such as fuel cells, metal-ion batteries, due to their unique physical and electrochemical properties. Herein, we report a facile hydrothermal synthesis of flower-like nanostructured molybdenum sulphide and its incorporation on to graphene as a potential oxygen reduction reaction catalyst in alkaline medium. The phase purity and morphological evolution of MoS 2 is systematically studied through X-ray diffraction and scanning electron microscopic techniques. The electronic states of metal and non-metallic species are deeply studied by X-ray photoelectron spectroscopy. The effect of annealing temperatures and TMC concentrations are also investigated by electrochemical techniques such as cyclic and linear sweep voltammograms. The optimised electrocatalyst (MoS 2 /G-500) delivers significant ORR activity with onset and half-wave potentials of 0.91 and 0.80 V (vs. RHE), respectively. Superior durability compared to state-of-art Pt/C catalyst is ascertained by repeating potential cycles for about 5000 times and also by chronoamperometric technique. Finally, the hybrid catalyst is evaluated in AEMFC as cathode catalyst which delivers peak power density of about 29 mW cm −2 under ambient temperature and pressure. The present findings emphasis that MoS 2 /G catalyst is promising as cost-effective and alternative to noble metal-based catalysts for fuel cell applications.

RuCo NPs supported on MIL-96(Al) as highly active catalysts for the hydrolysis of ammonia borane

Bimetallic RuCo nanoparticles (NPs) were successfully deposited on the highly porous and hydrothermally stable nanofibrous metal-organic framework MIL-96(Al) by using a simple liquid impregnation strategy, and the powder XRD, FT-IR, BET, TEM, EDX, ICP-AES and XPS were employed to characterize the structure, size, composition and loading metal electronic states of the RuCo@MIL-96 catalysts. The catalytic property of RuCo@MIL-96 for hydrogen generation from the hydrolysis of ammonia borane (AB) at room temperature was investigated. The results show that Ru1Co1@MIL-96 exhibits much enhanced catalytic activity compared with monometallic Ru, Co counterparts loadings and RuCo NPs due to the uniform distribution of metal NPs and synergetic effect between Ru and Co particles as well as bi-functional effects between RuCo NPs and the host of MIL-96. The turn over frequency (TOF) value of the Ru1Co1@MIL-96 catalyst is determined to be 320.7 mol H2 min−1 (mol Ru)−1, which is higher than most of the reported TOF values for the same reaction, and the activation energy (Ea) is 36.0 kJ mol−1. Moreover, this catalyst exhibits satisfied durability after five cycles for the hydrolytic dehydrogenation of ammonia borane.

Ligancy-Driven Controlling of Covalency and Metallicity in a Ruthenium Two-Dimensional System

The homopolar network and conjugation in d-block single elements can materialize a highly anisotropic and robust structure of a noble-metal system. Here, we have prepared ruthenium (Ru) atomic monolayers of a nonmetallic hexagonal lattice, and determined their layering scheme and metallization. The two-dimensional (2D) network is retained at the first stacking of the monolayer, while maintaining the nonmetallic features. We find out that the atop (AA) related stacking structure of bilayered Ru nanosheets occurs due to the ligancy-driven covalency, and the inception of the metallic electronic states is from trilayered stacking. These results indicate that the metallic states can be separated from covalent-bonding linkage and unpaired electrons in spd hybrid orbital systems. Our approach enables the molecular structure of noble-metal atoms to be induced via controlling the ligancy of d-block atomic bonds.

Insulator-metal transition in deep Sr-vacant spin-orbit Mott insulator [formula omitted

Sr2IrO4 exhibits a novel insulating state assisted by spin-orbit interactions. A series of polycrystalline samples of Sr2−xIrO4 have been synthesized. It is found that deep Sr-vacancies of Sr2−xIrO4 greatly reduce the rotation of IrO6 octahedral, and more importantly, a significant structural change occurs around x = 0.48 in both the lattice constants and the Ir−O2 bond length. An insulator-metal transition (IMT) appears and a non-Fermi-liquid metallic electronic state has been proved at x > 0.48 in Sr2−xIrO4. Furthermore, a sudden drop emerges of the localization temperature T0 and the antiferromagnetic (AFM) transition temperature TN in Sr1.5IrO4, together with the Curie-Weiss temperature reversing its sign. These abrupt changes are closely related with the reduction of the rotation crystal structure.

Structural and Vibrational Properties of CuPc/Ag(111) Ultrathin Films

The initial stages of copper(II)–phthalocyanine (CuPc) thin film growth on Ag(111) have been investigated using Fourier transform infrared absorption spectroscopy (FT-IRAS), spot profile analysis low-energy electron diffraction (SPA-LEED), and thermal desorption spectroscopy (TDS). Starting at (sub)monolayer coverages up to 5 monolayers (ML), a number of ordered overlayers are found. Vibrational spectroscopy shows characteristic spectroscopic signatures for the individual submonolayer phases as well as for the bilayer, trilayer, and multilayers. Highly asymmetric line shapes of the in-plane vibrational modes of submonolayer CuPc provide unequivocal evidence for interfacial dynamical charge transfer between the metal electronic states and CuPc molecular orbitals, indicative for a partially filled LUMO at the Fermi energy as well as the prevalence of severe nonadiabaticity in the electron–vibron coupling. Growth of the second and third CuPc layers proceeds in a layer-by-layer fashion (Frank van der Merwe gr...

On the structural origin of the single-ion magnetic anisotropy in LuFeO3

The electronic structure for the conduction bands of both hexagonal and orthorhombic LuFeO3 thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectral features and the linear dichroism are observed. Thesedifferences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states have not changed, the spectra are sensitive to the changes in the local environments of the Fe3+ and Lu3+ sites in the hexagonal and orthorhombic structures. Using the crystal-field splitting and the hybridizations that are extracted from the measured electronic structures and the structural distortion information, we derived the occupancies of the spin minority states in Fe3+, which are non-zero and uneven. The single ion anisotropy on Fe3+ sites is found to originate from these uneven occupancies of the spin minority states via spin–orbit coupling in LuFeO3.

Variable-Rung Design for a Mixed-Valence Two-Legged Ladder System Situated in a Dimensional Crossover Region

Ladder systems situated in a crossover from one dimensionality to two dimensionalities have been an attractive research target, because the physical properties, which are associated with dimensionality, are strongly dependent on the number of constituent legs. However, control of the intraladder configuration and electronic properties based on the substitution of structural components remain challenging tasks in materials science. On the other hand, structural design using coordination chemistry offers crucial advantages for architectural and electronic variations through substitutions of metal-organic building blocks. Here, we show the rational design and electronic properties of novel metal complex-based two-legged ladder compounds with several organic rung units: 4,4'-bipyridine, trans-1,4-diaminocyclohexane, and 4,4'-azopyridine. Single-crystal X-ray studies show that these two-legged ladder compounds are composed of halogen-bridged mixed-valence one-dimensional chains (MX chains) as their constituent legs. Depending on the molecular shape of the organic rung units, unique configurations of two-legged ladder lattices with periodic distortion of the legs are achieved. In addition, the electronic absorption spectra show that intervalence charge-transfer (IVCT) band gap of the two-legged ladder system increases with increasing degree of distortion of the leg. We have demonstrated for the first time that a two-legged ladder system shows a unique relationship between IVCT energy and the distortion parameter of the leg, as distinct from a single MX chain system. These systematic investigations, not only of configurations based on the rung variation but also of electronic states in metal-organic ladder system, provide the possibility for wide and rational tunings of physical and electronic properties of metal complex-based functional materials.

Limits of the Transition Time Between Electron States in Metals and Quantum Wells Obtained from the Energy-Time Uncertainty Principle

Limits of the Transition Time Between Electron States in Metals and Quantum Wells Obtained from the Energy-Time Uncertainty Principle

Synthesis of nanosized thin-film bimetallic catalysts based on mesoporous TiO2 for microstructured reactors

A synthetic procedure for the production of effective catalysts for the selective hydrogenation of organic substrates was developed, and the samples of thin-film Pt-Sn/TiO2 materials, which are characterized by high dispersity and a narrow size distribution of bimetallic particles in a mesoporous matrix of titanium dioxide, were prepared. With the use of a set of physicochemical techniques (low-temperature nitrogen adsorption, X-ray diffraction analysis, thermal analysis, and transmission electron microscopy), the formation and surface properties of the mesoporous Pt-Sn/TiO2 materials as powder and coatings on the surfaces of silica capillaries and plates were studied. Based on the effects of the sol composition and calcination temperature on the texture properties of TiO2, an optimum sol composition (1Ti(O-iPr)4: 0.009 F127: 0.13 HNO3: 1.3 H2O: 25 C2H5OH) and heat treatment conditions (calcination at 673 K; stepwise increase in the temperature) were determined to form a mesoporous coating with a specific surface area of 130 m2/g and an average pore diameter of 5.4 nm. It was found that the concentrations of metals (0.5–4 wt %), average particle sizes (1.0–3.5 nm), and particle size distributions in the Pt-Sn/TiO2 materials can be mainly regulated by varying the nature of the solvent, the concentration of Pt-Sn carbonyl complexes, and the time of adsorption, whereas the electronic state of metals can be fine-tuned by thermal treatment conditions. In an oxidizing atmosphere, the Pt-Sn carbonyl complexes decomposed with the formation of a two-phase system (Pt○ and SnO2). Thermal treatment in a vacuum and an inert or reducing atmosphere led to the formation of bimetallic phases (PtSn and Pt3Sn), whose stucture was regulated by the composition of a gas atmosphere and by the calcination temperature.

Binding sites and electronic states of group 3 metal-aniline complexes probed by high-resolution electron spectroscopy

Group 3 metal-aniline complexes, M(aniline) (M = Sc, Y, and La), are produced in a pulsed laser-vaporization molecular beam source, identified by photoionization time-of-flight mass spectrometry, and investigated by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and quantum chemical calculations. Adiabatic ionization energies and several low-frequency vibrational modes are measured for the first time from the ZEKE spectra. Metal binding sites and electronic states are determined by combining the ZEKE measurements with the theoretical calculations. The ionization energies of the complexes decrease down the metal group. An out-of-plane ring deformation mode coupled with an asymmetric metal-carbon stretch is considerably anharmonic. Although aniline has various possible sites for metal coordination, the preferred site is the phenyl ring. The metal binding with the phenyl ring yields syn and anti conformers with the metal atom and amino hydrogens on the same and opposite sides of the ring, respectively. The anti conformer is determined to be the spectral carrier. The ground electronic state of the anti conformer of each neutral complex is a doublet with a metal-based electron configuration of nd(2)(n + 1)s(1), and the ground electronic state of each ion is a singlet with a metal-based electron configuration of nd(2). The formation of the neutral complexes requires the nd(2)(n + 1)s(1) ← nd(1)(n + 1)s(2) electron excitation in the metal atoms.

Imaging the evolution of metallic states in a correlated iridate

The Ruddlesden-Popper series of iridates (Srn+1IrnO3n+1) have been the subject of much recent attention due to the anticipation of emergent phenomena arising from the cooperative action of spin-orbit-driven band splitting and Coulomb interactions. However, an ongoing debate over the role of correlations in the formation of the charge gap and a lack of understanding of the effects of doping on the low-energy electronic structure have hindered experimental progress in realizing many of the predicted states. Using scanning tunnelling spectroscopy we map out the spatially resolved density of states in Sr3Ir2O7 (Ir327). We show that its parent compound, argued to exist only as a weakly correlated band insulator, in fact possesses a substantial ~ 130 meV charge excitation gap driven by an interplay between structure, spin-orbit coupling and correlations. We find that single-atom defects are associated with a strong electronic inhomogeneity, creating an important distinction between the intrinsic and spatially averaged electronic structure. Combined with first-principles calculations, our measurements reveal how defects at specific atomic sites transfer spectral weight from higher energies to the gap energies, providing a possible route to obtaining metallic electronic states from the parent insulating states in the iridates.

Photoelectron spin-flipping and texture manipulation in a topological insulator

Recently discovered materials called three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the material's surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spin-polarized electrons with tunable polarization in three dimensions.

Electrochemical behavior and electrodeposition of dysprosium in ionic liquids based on phosphonium cations

The electrochemical behavior and the electrodeposition of dysprosium (Dy) in phosphonium-cation-based ionic liquid were investigated in this study. A new group of the room-temperature ionic liquids (RTILs) based on phosphonium cations with bis(trifluoromethylsulfonyl)amide anions was applied as novel electrolytic solutions. The cyclic voltammetric measurements resulted in one step reduction of the trivalent dysprosium ion in phosphonium-cation-based ionic liquid. On the other hand, no anodic peak ascribed to the oxidation of dysprosium metal was observed in this electroanalytical study. The diffusion coefficient and the activation energy for diffusion of the trivalent Dy complex in IL were estimated using semi-integral analysis, because it is important to analyze the diffusion properties to recover Dy through electrowinning methods. The diffusion coefficient of Dy(III) which was calculated to be 2.0 × 10−12 m2 s−1 at 25 °C, closed to that of the trivalent lanthanoid ion such as Eu(III) and Sm(III) in phosphonium-cation-based ionic liquid. In addition, the activation energy for diffusion was estimated to be 65 kJ mol−1 (0.5 M) and 49 kJ mol−1 (0.075 M). The estimated activation energy for diffusion was affected by the concentration of the electrolytic solution, since the RTILs had relatively strong electrostatic interactions between the metal cations and the solvent anions. Furthermore, the electrodeposition of Dy in phosphonium-cation-based IL was carried out using a two-electrode system constructed with a copper plate cathode and dysprosium metal anode. Energy dispersive X-ray analysis of electrodeposits showed a sharply peaked spectrum corresponding to the characteristic X-ray lines of Dy. In addition, the obtained Dy, with the exception of the surface layer, was confirmed to be in the metallic electronic state by X-ray photoelectron spectroscopy.