KMnCuTe2: a layered antiferromagnetic semiconductor with long metal-metal distance

The magnetic semiconductor in a two-dimensional system is a major subject for both theoretical and experimental investigations. Here we report the synthesis of a new quaternary manganese chalcogenide KMnCuTe₂, which shows layered structure and antiferromagnetic (AFM) semiconducting features. Single crystals of KMnCuTe₂ were obtained using a self-flux method and based on single-crystal X-ray diffraction, KMnCuTe₂ adopts the ThCr₂Si₂-type structure composed of edge-sharing tetrahedral layers separated by K⁺ cations. The Mn and Cu atoms randomly distribute in the centre of tetrahedral units. Attributed to the large radius of Te, KMnCuTe₂ has large lattice parameters (a = 4.3115(3) Å and c = 14.9360(20) Å), leading to a long metal–metal distance (3.049 Å) in the tetrahedral layers. Based on the experiments and theoretical calculations, KMnCuTe₂ exhibits a G-type AFM interaction with the transition temperature at around 225 K and an indirect semiconducting nature with the band gap of 0.95 eV. The magnetic semiconducting property of KMnCuTe₂ is unique in AMnMCh₂ systems (A = Li, Na, K, M = Cu, Ag and Ch = S, Se, Te), which could be associated with the large metal–metal distance. Our work not only highlights the role of metal–metal interactions on regulating the properties of ThCr₂Si₂-type compounds, but also provides a feasible strategy to obtain the layered magnetic semiconductor.

We synthesized a new quaternary telluride KMnCuTe₂ with long metal–metal distance in tetragonal layers, which shows an indirect band gap of 0.95 eV and long-range antiferromagnetic ordering.

Reversed Active Sites Boost the Intrinsic Activity of Graphene-like Cobalt Selenide for Hydrogen Evolution

Optimizing the hydrogen adsorption Gibbs free energy (ΔG_H ) of active sites is essential to improve the overpotential of the electrocatalytic hydrogen evolution reaction (HER). We doped graphene-like Co_0.85 Se with sulfur and found that the active sites are reversed (from cationic Co sites to anionic S sites), which contributed to an enhancement in electrocatalytic HER performance. The optimal S-doped Co_0.85 Se composite has an overpotential of 108 mV (at 10 mA cm^-2 ) and a Tafel slope of 59 mV dec^-1 , which exceeds other reported Co_0.85 Se-based electrocatalysts. The doped S sites have much higher activity than the Co sites, with a hydrogen adsorption Gibbs free energy (ΔG_H ) close to zero (0.067 eV), which reduces the reaction barrier for hydrogen production. This work provides inspiration for optimizing the intrinsic HER activity of other related transition metal chalcogenides.