Simple rules for the understanding of Heusler compounds

Development of a non-noble-metal hydrogen-producing catalyst is essential to the development of solar water-splitting devices. Improving both the activity and the stability of the catalyst remains a key challenge. In this Communication, we describe a two-step reaction for preparing three-dimensional electrodes composed of CoSe2 nanoparticles grown on carbon fiber paper. The electrode exhibits excellent catalytic activity for a hydrogen evolution reaction in an acidic electrolyte (100 mA/cm2 at an overpotential of ∼180 mV). Stability tests though long-term potential cycles and extended electrolysis confirm the exceptional durability of the catalyst. This development offers an attractive catalyst material for large-scale water-splitting technology.

CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction

Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX2) might offer properties superior to those of graphene. So far, only very few MX2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA...

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Stable, Single-Layer MX2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure

The development of efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing challenge. We report metallic cobalt pyrite (cobalt disulfide, CoS2) as one such high-activity candidate material and demonstrate that its specific morphology—film, microwire, or nanowire, made available through controlled synthesis—plays a crucial role in determining its overall catalytic efficacy. The increase in effective electrode surface area that accompanies CoS2 micro- and nanostructuring substantially boosts its HER catalytic performance, with CoS2 nanowire electrodes achieving geometric current densities of −10 mA cm–2 at overpotentials as low as −145 mV vs the reversible hydrogen electrode. Moreover, micro- and nanostructuring of the CoS2 material has the synergistic effect of increasing its operational stability, cyclability, and maximum achievable rate of hydrogen generation by promoting the release of evolved gas bubbles from the electrode surface. The benefits of ca...

High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS2) Micro- and Nanostructures

Recent development in 2D materials beyond graphene

X-ray photoelectron spectra of 3d transition metal pyrites

Electronic band-structure calculations are presented for the ferromagnetic compounds CrTe, Cr3Te4 and Cr2Te4 and Cr2Te3. In these compounds the Cr3d-Te5p covalency and the Cr3d(z2)-Cr3d(z2)-overlap along the c axis are the most important interactions. The magnetic polarisation of Te is parallel to the Cr local moment in CrTe, antiparallel to it in Cr2Te3 and about zero in Cr2Te4. Measurements of electronic transport properties (resistivity, Hall effect and thermo-electric power) and magnetic properties of Cr1- delta Te( delta =0.1) and Cr3+xTe4(x=0.2) indicate that these chromium tellurides are p-type metals, with strong interaction between the holes in the Te5p band and the Cr magnetic moments. In the literature the variation of the magnetic properties of Cr3.2Te4 near Ts approximately=100 K has been attributed to a change from a canted antiferromagnetic to a collinear ferromagnetic structure. However, our Hall-effect measurements indicate that the spin structure is not collinear ferromagnetic above Ts.

https://iopscience.iop.org/article/10.1088/0953-8984/1/46/008/pdf

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

For pt.I see ibid., vol.1, p.2341 (1989). The electrical resistivity and the Hall effect of inter-metallic compounds XMnSb (X=Pt, Ni, Au) and PtMnSn were investigated in the temperature region 4-1000 K. The results for the anomalous Hall effect were analysed in terms of skew scattering and side-jump contributions. This analysis is possible in a half-metallic ferromagnet because the conduction electron spin polarisation at T=0 K is known.

https://iopscience.iop.org/article/10.1088/0953-8984/1/13/008/pdf

Half-metallic ferromagnets. II. Transport properties of NiMnSb and related inter-metallic compounds

The authors present investigations of the crystal structure the microstructure and the magnetic properties of the inter-metallic compounds XMnSb (X=Pt, Ni, Co, Au, Cu) and PtMnSn. It was found that several of these materials contain precipitates of other phases and/or large atomic disorder, which can be influenced by heat treatment. The magnetic properties show an effective paramagnetic moment which differs from the value corresponding to the saturation moment at 0 K. This effect is attributed to a decrease of the conduction electron spin polarisation at high temperature.

https://iopscience.iop.org/article/10.1088/0953-8984/1/13/007/pdf

Half-metallic ferromagnets. I. Structure and magnetic properties of NiMnSb and related inter-metallic compounds

We present a study of angle-resolved photoemission of the layered compound Ti${\mathrm{Te}}_{2}$. The results are compared to augmented-plane-wave band-structure calculations which predict Ti${\mathrm{Te}}_{2}$ to be a semimetal with an overlap of valence and conduction bands of 0.6 eV. Reasonable agreement with the calculations is found except for an almost dispersionless band found just below the Fermi level which is not present in the theory. This band is argued to result from either a narrow impurity band or localization of electrons due to the formation of polarons. From an analysis of resistivity, thermoelectric power, and Hall effect, we deduced the carrier concentrations and mobilities for ${\mathrm{Ti}}_{1.01}$${\mathrm{Te}}_{2}$. The large value of the Pauli susceptibility indicates large exchange enhancement. At 150 K, a phase transition was detected in magnetic susceptibility measurements. The corresponding lattice distortion could not be observed directly.

C.F. van Bruggen

Papers

X-ray photoelectron spectra of 3d transition metal pyrites

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Half-metallic ferromagnets. II. Transport properties of NiMnSb and related inter-metallic compounds

Half-metallic ferromagnets. I. Structure and magnetic properties of NiMnSb and related inter-metallic compounds

Titanium ditelluride : Band structure, photoemission, and electrical and magnetic properties