Two-dimensional MoSe2 has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), although its catalytic activity needs to be further improved. Herein, we report Se-rich MoSe2 nanosheets synthesized using a hydrothermal reaction, displaying much enhanced HER performance at the Se/Mo ratio of 2.3. The transition from the 2H to the 1T′ phase occurred as Se/Mo exceeded 2. Structural analysis revealed the presence of Se adatoms as well as the formation of Se–Se bonding. Based on first-principles calculations, we propose two equally stable Se-rich structures. In the first one, excess Se atoms bridge two MoSe2 layers via the interlayer Se–Se bonds. In the second one, the Se atoms substitute for the Mo atoms, and extra Se atoms are added closest to the Mo-substituted Se. Calculation of Gibbs free energy along the reaction path indicates that the Se adatoms of the second model are the most active sites for HER.

Few layered GeAs nanosheets exhibit a band gap of 2.1 eV, corresponding to that of monolayer predicted from first-principles calculations.

Two-dimensional ReSe2 has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), but its catalytic activity needs to be further improved. Herein, we synthesized Re1–x Mo x Se2 alloy nanosheets with the whole range of x (0–100%) using a hydrothermal reaction. The phase evolved in the order of 1T″ (triclinic) → 1T′ (monoclinic) → 2H (hexagonal) upon increasing x. In the nanosheets with x = 10%, the substitutional Mo atoms tended to aggregate in the 1T″ ReSe2 phase with Se vacancies. The incorporation of the 1T′ phase makes the alloy nanosheets more metallic than the end compositions. The 10% Mo substitution significantly enhanced the electrocatalytic performance toward HER (in 0.5 M H2SO4), with a current of 10 mA cm–2 at an overpotential of 77 mV (vs RHE) and a Tafel slope of 42 mV dec–1. First-principles calculations of the three phases (1T″, 2H, and 1T′) predicted a phase transition of 1T″-2H at x ≈ 65% as well as the production of a 1T′ phase along the composition tuning, which are consistent with the experiments. At x = 12.5%, two Mo atoms prefer to form a pair along the Re4 chains. Gibbs free energy along the reaction path indicates that the best HER performance of nanosheets with 10% Mo originates from the Mo atoms that form Mo–H when there are adjacent Se vacancies.

The novel properties of two-dimensional materials have motivated extensive studies focused on transition metal dichalcogenides (TMDs), which led to many interesting findings in recent years. Further advances in this area would require the development of effective methods for producing nanostructured TMDs with a controlled structure. Herein, we report unique MoS2 layered nanostructures intercalated with dimethyl-p-phenylenediamine (DMPD) with various concentrations, synthesized by a one-step hydrothermal reaction. The MoS2 layers possess a significantly expanded interlayer spacing. Remarkably, as the concentration of DMPD increases, the MoS2 preferentially adopts a unique metallic 1T' (distorted 1T) phase. The intercalated MoS2 exhibits excellent catalytic performance in the hydrogen evolution reaction. First-principles calculations show that the phase transition from 2H to 1T' phase occurs with increasing concentrations of DMPD, which can be accelerated by the S vacancies. A significant charge transfer from the DMPD molecules to MoS2 stabilizes the 1T' over the 2H phase, driving the 2H-1T' phase conversion. The DMPD and the S vacancies increased the carrier concentration, which leads to the enhanced catalytic performance. The present work illustrates how the phase control of TMDs can be effectively achieved by the intercalation of electron-donating molecules.