Core-Level X-Ray Photoemission Satellites in Ruthenates: A New Mechanism Revealing the Mott Transition

Ru 3d core-level x-ray photoemission spectra of various ruthenates are examined. They show in general two-peak structures, which can be assigned as the screened and unscreened peaks. The screened peak is absent in a Mott insulator, but develops into a main peak in the metallic regime. This spectral behavior is well explained by the dynamical mean-field theory calculation for the single-band Hubbard model with on-site core-hole potential using the exact diagonalization method. The new mechanism of the core-level photoemission satellite can be utilized to reveal the Mott transition phenomenon in various strongly correlated electron systems, especially in nano-scale devices and phase-separated materials.Comment: 4 pages, 5 figures, submitted to PR

Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency

Recent rapid progress in efficiencies for solar water splitting by photoelectrochemical devices has enhanced its prospects to enable storable renewable energy. Efficient solar fuel generators all use tandem photoelectrode structures, and advanced integrated devices incorporate corrosion protection layers as well as heterogeneous catalysts. Realization of near thermodynamic limiting performance requires tailoring the energy band structure of the photoelectrode and also the optical and electronic properties of the surface layers exposed to the electrolyte. Here, we report a monolithic device architecture that exhibits reduced surface reflectivity in conjunction with metallic Rh nanoparticle catalyst layers that minimize parasitic light absorption. Additionally, the anatase TiO2 protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution. An initial solar-to-hydrogen efficiency of 19.3 % is obtained in acidic electrolyte and an efficiency of 18.5 % is achieved at neutral pH condition (under simulated sunlight)

Cathodic NH₄⁺ leaching of nitrogen impurities in CoMo thin-film electrodes in aqueous acidic solutions

Electrocatalytic reduction of dinitrogen (N₂) to ammonium (NH₄⁺) in acidic aqueous solutions was investigated at ambient temperature and pressure using a cobalt–molybdenum (CoMo) thin-film electrode prepared by magnetron reactive sputtering. Increased concentrations of ammonium ions (NH₄⁺) were consistently detected in the electrolyte using ion chromatography (IC) after constant-potential electrolysis at various potentials (≤−0.29 V vs. RHE). Using a newly developed analytical method based on ammonia derivatization, performing the experiments with ¹⁵N₂-labelled gas led however to the detection of increased ¹⁴NH₄⁺ concentrations instead of ¹⁵NH₄⁺. X-ray photoelectron spectroscopic (XPS) analysis of the electrode surface revealed the presence of Mo N and Mo–NH_x species. Several contamination sources were identified that led to substantial increases in the concentration of ammonium ions, including ¹⁵NH₃ impurities in ¹⁵N₂ gas. The observed ammonium concentrations can be consistently ascribed to leaching of nitrogen (¹⁴N) impurities incorporated in the CoMo film during the sputtering process. Researchers in the field are therefore urged to adopt extended protocols to identify and eliminate sources of ammonia contamination and to very carefully monitor the ammonium concentrations in each experimental step

Cathodic NH₄⁺ leaching of nitrogen impurities in CoMo thin-film electrodes in aqueous acidic solutions

Electrocatalytic reduction of dinitrogen (N₂) to ammonium (NH₄⁺) in acidic aqueous solutions was investigated at ambient temperature and pressure using a cobalt–molybdenum (CoMo) thin-film electrode prepared by magnetron reactive sputtering. Increased concentrations of ammonium ions (NH₄⁺) were consistently detected in the electrolyte using ion chromatography (IC) after constant-potential electrolysis at various potentials (≤−0.29 V vs. RHE). Using a newly developed analytical method based on ammonia derivatization, performing the experiments with ¹⁵N₂-labelled gas led however to the detection of increased ¹⁴NH₄⁺ concentrations instead of ¹⁵NH₄⁺. X-ray photoelectron spectroscopic (XPS) analysis of the electrode surface revealed the presence of Mo N and Mo–NH_x species. Several contamination sources were identified that led to substantial increases in the concentration of ammonium ions, including ¹⁵NH₃ impurities in ¹⁵N₂ gas. The observed ammonium concentrations can be consistently ascribed to leaching of nitrogen (¹⁴N) impurities incorporated in the CoMo film during the sputtering process. Researchers in the field are therefore urged to adopt extended protocols to identify and eliminate sources of ammonia contamination and to very carefully monitor the ammonium concentrations in each experimental step

Density of Superfluid Helium Droplets

The classical integral cross sections of large superfluid 4He_N droplets and the number of atoms in the droplets (N=10^3-10^4) have been measured in molecular beam scattering experiments. These measurements are found to be in good agreement with the cross sections predicted from density functional calculations of the radial density distributions with a 10-90 % surface thickness of 5.7\AA. By using a simple model for the density profile of the droplets a thickness of about 6-8\AA is extracted directly from the data.Comment: 27 pages, REVTeX, 5 postscript figure

Efficient Water-Splitting Device Based on a Bismuth Vanadate Photoanode and Thin-Film Silicon Solar Cells

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2 % under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten–bismuth vanadate (W:BiVO_4) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO_4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO_4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO_4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO_4 and the nc-Si:H cell

Microscopic study of the He2-SF6 trimers

The He2-SF6 trimers, in their different He isotopic combinations, are studied both in the framework of the correlated Jastrow approach and of the Correlated Hyperspherical Harmonics expansion method. The energetics and structure of the He-SF6 dimers are analyzed, and the existence of a characteristic rotational band in the excitation spectrum is discussed, as well as the isotopic differences. The binding energies and the spatial properties of the trimers, in their ground and lowest lying excited states, obtained by the Jastrow ansatz are in excellent agreement with the results of the converged CHH expansion. The introduction of the He-He correlation makes all trimers bound by largely suppressing the short range He-He repulsion. The structural properties of the trimers are qualitatively explained in terms of the shape of the interactions, Pauli principle and masses of the constituents.Comment: 17 pages, 5 figures. Submitted to PR

High-quality variational wave functions for small 4He clusters

We report a variational calculation of ground state energies and radii for 4He_N droplets (3 \leq N \leq 40), using the atom-atom interaction HFD-B(HE). The trial wave function has a simple structure, combining two- and three-body correlation functions coming from a translationally invariant configuration-interaction description, and Jastrow-type short-range correlations. The calculated ground state energies differ by around 2% from the diffusion Monte Carlo results.Comment: 5 pages, 1 ps figure, REVTeX, submitted to Phys. Rev.

An Electrochemical, Microtopographical and Ambient Pressure X-Ray Photoelectron Spectroscopic Investigation of Si/TiO_2/Ni/Electrolyte Interfaces

The electrical and spectroscopic properties of the TiO_2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO_2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO_2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO_2/Ni electrodes and to contextualize the AP-XPS results

Absence of system xc⁻ on immune cells invading the central nervous system alleviates experimental autoimmune encephalitis

Background: Multiple sclerosis (MS) is an autoimmune demyelinating disease that affects the central nervous system (CNS), leading to neurodegeneration and chronic disability. Accumulating evidence points to a key role for neuroinflammation, oxidative stress, and excitotoxicity in this degenerative process. System x(c)- or the cystine/glutamate antiporter could tie these pathological mechanisms together: its activity is enhanced by reactive oxygen species and inflammatory stimuli, and its enhancement might lead to the release of toxic amounts of glutamate, thereby triggering excitotoxicity and neurodegeneration. Methods: Semi-quantitative Western blotting served to study protein expression of xCT, the specific subunit of system x(c)-, as well as of regulators of xCT transcription, in the normal appearing white matter (NAWM) of MS patients and in the CNS and spleen of mice exposed to experimental autoimmune encephalomyelitis (EAE), an accepted mouse model of MS. We next compared the clinical course of the EAE disease, the extent of demyelination, the infiltration of immune cells and microglial activation in xCT-knockout (xCT(-/-)) mice and irradiated mice reconstituted in xCT(-/-) bone marrow (BM), to their proper wild type (xCT(+/+)) controls. Results: xCT protein expression levels were upregulated in the NAWM of MS patients and in the brain, spinal cord, and spleen of EAE mice. The pathways involved in this upregulation in NAWM of MS patients remain unresolved. Compared to xCT(+/+) mice, xCT(-/-) mice were equally susceptible to EAE, whereas mice transplanted with xCT(-/-) BM, and as such only exhibiting loss of xCT in their immune cells, were less susceptible to EAE. In none of the above-described conditions, demyelination, microglial activation, or infiltration of immune cells were affected. Conclusions: Our findings demonstrate enhancement of xCT protein expression in MS pathology and suggest that system x(c)- on immune cells invading the CNS participates to EAE. Since a total loss of system x(c)- had no net beneficial effects, these results have important implications for targeting system x(c)- for treatment of MS