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MAX phases have exhibited diverse physical properties, inspiring their promising applications in several important research fields. The introduction of a chalcogen atom into a phase of MAX has further facilitated the modulation of their physical properties and the extension of MAX family diversity. The physical characteristics of the novel chalcogen-containing MAX 211 phase Zr2SeB and Zr2SeN have been systematically investigated. The present investigation is conducted from a multi-faceted perspective that encompasses the stability, electronic structure, and mechanical properties of the system, via the employment of the first-principles density functional theory methodology. By replacing C with B/N in the chalcogen-containing MAX phase, it has been shown that their corresponding mechanical properties are appropriately tuned, which may offer a way to design novel MAX phase materials with enriched properties. In order to assess the dynamical and mechanical stability of the systems under investigation, a thorough evaluation has been carried out based on the analysis of phonon dispersions and elastic constants conditions. The predicted results reveal a strong interaction between zirconium and boron or nitrogen within the structures of Zr2SeB and Zr2SeN. The calculated band structures and electronic density of states for Zr2SeB and Zr2SeN demonstrate their metallic nature and anisotropic conductivity. The theoretically estimated Pugh and Poisson ratios imply that these phases are characterized by brittleness. [ABSTRACT FROM AUTHOR]

Developing efficient hydrogen evolution reaction (HER) electrocatalysts is extremely important for large-scale hydrogen production from water splitting. In this work, ultra-small MoS 2 nanosheets with 5% Ni doping grown on both sides of hollow carbon microtubes (5%-Ni-MoS 2 /aCMT) are prepared by a hydrothermal method. The vertically oriented Ni-doped MoS 2 nanosheets provide more active sites for HER, while open porous structure promotes ions transport, and hollow carbon microtube substrate accelerates electrons transport as well as hydrogen release. Density functional theory (DFT) analysis proves that the optimal amount of Ni doping can effectively adjust the electronic structure of MoS 2. As a result, 5%-Ni-MoS 2 /aCMT electrode exhibits enhanced kinetics for HER in both acidic and alkaline conditions. To deliver the current density of 10 mA cm−2, it requires overpotentials of 140 and 88 mV in 1 M KOH and 0.5 M H 2 SO 4. 5%-Ni-MoS 2 /aCMT exhibits better stability than commercial Pt/C at a high current density of 200 mA cm−2. Moreover, 5%-Ni-MoS 2 /aCMT/NF (nickel foam) || RuO 2 /NF electrolyzer exhibits ultra-long stability of 30 h at the current density of 100 mA cm−2 and excellent overall water splitting performance with requiring low cell voltages of 1.52 and 1.71 V to generate current densities of 10 and 50 mA cm−2. [Display omitted] • The Ni doping with an optimized content adjusted the electronic structure of MoS 2 , and improved the intrinsic activity. • The ultra-small vertically oriented and defect-rich Ni-doped MoS 2 nanosheets provide more active sites for HER. • The natural N-doped aCMT substrate promotes electron transport during the HER process. • 5%-Ni-MoS 2 /aCMT behaves an enhanced kinetics process based on EIS and DFT analysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • CoSe 2 -MoSe 2 (1–1)/rGO has good HER activity in both acidic and alkaline media. • The CoSe 2 -MoSe 2 (1–1) nanosheets uniformly decorate the rGO matrix. • DFT results verify CoSe 2 -MoSe 2 (1–1) heterostructure owns small band gap and ΔG H*. • The rGO matrix and heterojunction structure improve the electronic conductivity. It is important to develop novel energy to solve energy shortage and environmental problems. Hydrogen evolution reaction (HER) is envisaged as a viable technology that can be used to develop sustainable clean energy. Herein, we report a catalyst with CoSe 2 -MoSe 2 heterostructure grown on reduced graphene oxide with an optimum Co/Mo proportion of 1:1 (CoSe 2 -MoSe 2 (1–1)/rGO). It exhibits good HER activities in both acidic and alkaline conditions. The CoSe 2 -MoSe 2 (1–1)/rGO shows an overpotential of 107 mV at 10 mA cm−2 with a Tafel slope of 56 mV dec−1 under acidic condition. Meanwhile, CoSe 2 -MoSe 2 (1–1)/rGO also presents an overpotential of 182 mV at 10 mA cm−2 and with a Tafel slope of 89 mV dec−1 under alkaline condition. These impressive performances of the catalyst are mainly due to the excellent electronic transmission capability of rGO and the abundant active sites of CoSe 2 -MoSe 2 heterostructure as well as the optimized hydrogen adsorption energy of CoSe 2 -MoSe 2 interface. The design of CoSe 2 -MoSe 2 (1–1)/rGO provides a meaningful guide for manufacturing electrode in energy storage and conversion. [ABSTRACT FROM AUTHOR]

• The electronic structures of multilayer MXenes under strains are investigated. • The conversion from a semiconductor to a metal appears in semiconducting MXenes. • Multilayer Hf 2 CO 2 and Zr 2 CO 2 can be used in pressure sensors. Since it has been shown that the electronic properties of the multilayer Ti 2 CO 2 can be efficiently manipulated by strain and that the multilayer configurations could be applied in strain sensors, more MXene materials deserve further investigations based on their excellent performance and a wide range of potential applications. In this work, the theoretical study on the electronic characteristics of different MXene multilayer structures under strains is carried out using first principles calculations. The calculation results show that Ti 2 CF 2 and Ti 2 C(OH) 2 remains metallic under different strains, and thus the oxygen terminated Ti 2 C MXene might be more suitable for application in pressure sensing devices. Moreover, strain stress curves and electronic structures under strains are calculated. From the results, the conversion from semiconductors to metals for Hf 2 CO 2 and Zr 2 CO 2 is determined to be allowed under compressive strains. Therefore, the current in a device can be more readily controlled using Hf 2 CO 2 and Zr 2 CO 2 than Nb 2 CO 2 and V 2 CO 2 under pressures in an appropriate range. It is expected that the current study can provide new clues for expanding the potential applications of MXenes such as in pressure sensors and flexible electronic devices. [ABSTRACT FROM AUTHOR]