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Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn

This work reexamines the effect of the exchange screening parameter ω on the performance of the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. We show that variation of the screening parameter influences solid band gaps the most. Other properties such as molecular thermochemistry or lattice constants of solids change little with ω. We recommend a new version of HSE with the screening parameter ω=0.11bohr−1 for further use. Compared to the original implementation, the new parametrization yields better thermochemical results and preserves the good accuracy for band gaps and lattice constants in solids.

Influence of the exchange screening parameter on the performance of screened hybrid functionals.

Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM). The performance of these devices is currently limited by the modest (< approximately 70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to approximately 220% at room temperature and approximately 300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of approximately 85%, which rivals that previously observed only using half-metallic ferromagnets. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 degrees C) will accelerate the development of new families of spintronic devices.

Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers

Although non-aqueous Li-ion batteries possess significantly higher energy density than their aqueous counterparts, the latter can be more feasible for grid-scale applications when cost, safety and cycle life are taken into consideration. Moreover, aqueous Zn-ion batteries have an energy storage advantage over alkali-based batteries as they can employ Zn metal as the negative electrode, dramatically increasing energy density. However, their development is plagued by a limited choice of positive electrodes, which often show poor rate capability and inadequate cycle life. Here we report a vanadium oxide bronze pillared by interlayer Zn2+ ions and water (Zn0.25V2O5⋅nH2O), as the positive electrode for a Zn cell. A reversible Zn2+ ion (de)intercalation storage process at fast rates, with more than one Zn2+ per formula unit (a capacity up to 300 mAh g−1), is characterized. The Zn cell offers an energy density of ∼450 Wh l−1 and exhibits a capacity retention of more than 80% over 1,000 cycles, with no dendrite formation at the Zn electrode. High-performing positive electrode materials are crucial for the development of aqueous Zn-ion batteries. Here the authors report a battery based on reversible intercalation of Zn ions in a layered Zn0.25V2O5⋅nH2O-based positive electrode, which exhibits high-capacity and long-term cycling stability.

A high-capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode

Hybrid Fock exchange/density functional theory functionals have shown to be very successful in describing a wide range of molecular properties. For periodic systems, however, the long-range nature of the Fock exchange interaction and the resultant large computational requirements present a major drawback. This is especially true for metallic systems, which require a dense Brillouin zone sampling. Recently, a new hybrid functional [HSE03, J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)] that addresses this problem within the context of methods that evaluate the Fock exchange in real space was introduced. We discuss the advantages the HSE03 functional brings to methods that rely on a reciprocal space description of the Fock exchange interaction, e.g., all methods that use plane wave basis sets. Furthermore, we present a detailed comparison of the performance of the HSE03 and PBE0 functionals for a set of archetypical solid state systems by calculating lattice parameters, bulk moduli, heats of formation, and band gaps. The results indicate that the hybrid functionals indeed often improve the description of these properties, but in several cases the results are not yet on par with standard gradient corrected functionals. This concerns in particular metallic systems for which the bandwidth and exchange splitting are seriously overestimated.

Screened hybrid density functionals applied to solids

Quantitative chemical state X-ray photoelectron spectroscopic analysis of mixed nickel metal, oxide, hydroxide and oxyhydroxide systems is challenging due to the complexity of the Ni 2p peak shapes resulting from multiplet splitting, shake-up and plasmon loss structures. Quantification of mixed nickel chemical states and the qualitative determination of low concentrations of Ni(III) species are demonstrated via an approach based on standard spectra from quality reference samples (Ni, NiO, Ni(OH)2, NiOOH), subtraction of these spectra, and data analysis that integrates information from the Ni 2p spectrum and the O 1s spectra. Quantification of a commercial nickel powder and a thin nickel oxide film grown at 1-Torr O2 and 300 °C for 20 min is demonstrated. The effect of uncertain relative sensitivity factors (e.g. Ni 2.67 ± 0.54) is discussed, as is the depth of measurement for thin film analysis based on calculated inelastic mean free paths. Copyright © 2009 John Wiley & Sons, Ltd.

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X-ray photoelectron spectroscopic chemical state quantification of mixed nickel metal, oxide and hydroxide systems

The mechanisms of pyrite oxidation and leaching: A fundamental perspective

This paper reviews evidence for the assignments of components of the S 2p XPS spectra from sulphide mineral surfaces under different conditions of preparation, oxidation and reaction. Evidence from other techniques confirming assignment of high-binding-energy S 2p components to metal-deficient sulphide surfaces, polysulphides, elemental sulphur and electronic defect structures is considered for specific cases. Reliable assignment of S 2p 3/2 components at 163.6-164.0 eV to elemental sulphur S n 0 can be confirmed by evaporative loss at 295 K and/or observation of S-S bonding by x-ray absorption fine structure (XAFS), x-ray diffraction or vibrational spectroscopy. Assignment to polysulphides S n 2- at 162.0-163.6 eV requires confirmation of S-S bonding by XAFS or vibrational spectroscopy. Metal-deficient lattices can be represented as electronic defects (e.g. vacancies) or restructured surface phases confirmed by diffraction or XAFS evidence. High-binding-energy S 2p 3/2 components can also result from Cu(I) substitution into ZnS with associated oxidation of sulphur as electronic defect sites without S-S bonding, metal deficiency or restructuring. This assignment is confirmed by XAFS evidence.

XPS of sulphide mineral surfaces: metal‐deficient, polysulphides, defects and elemental sulphur

Bulk properties of the isostructural oxides MgO, NiO, and CoO have been calculated quantum chemically with periodic models and compared with experimental data from the literature. Ab initio Hartree-Fock, gradient-corrected density-functional methods, and hybrid approaches have been used for the calculation of the lattice constants, heats of atomization, and electronic structures. General trends of the effects of electron correlation and the treatment of exchange on the calculated properties are observed. None of the standard methods considered provided results in agreement with experimental data for all properties. A combination of Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) leads to a consistent description of the electronic and structural properties in comparative studies of the three compounds. Other methods are more accurate than B3LYP if certain properties or compounds are considered. The combination of the Hartree-Fock exchange functional with the Lee-Yang-Parr density-functional correlation is the best method for the open-shell transition-metal oxides NiO and CoO in terms of relative stability and geometry and the electronic structure of the valence band. The absolute values of calculated heats of atomization, however, are generally too small. The density-functional method based on the Perdew-Wang generalized gradient approximation (PWGGA) is preferable for the calculation of thermodynamic properties for all compounds but is less reliable in the prediction of structural and electronic properties. A hybrid approach based on the PWGGA method is proposed that improves the results for bulk geometries and electronic properties while maintaining the high quality of calculated energetic results.

Andrea R. Gerson

Papers

Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn

X-ray photoelectron spectroscopic chemical state quantification of mixed nickel metal, oxide and hydroxide systems

The mechanisms of pyrite oxidation and leaching: A fundamental perspective

XPS of sulphide mineral surfaces: metal‐deficient, polysulphides, defects and elemental sulphur

Effect of exchange and correlation on bulk properties of MgO, NiO, and CoO