Chalcogen alloying mediated electronic structure modulation in ultrathin Nb(S_xSe_(1-x))₂ nanosheets for the hydrogen evolution reaction

Modulating the electronic structure of transition metal dichalcogenides (TMDs) especially by regulating the d-electrons implicitly escalates their catalytic properties. In this respect, chalcogen alloying has proven to be a prime strategy in engineering the d-band electronic structure of transition metals in TMDs. Herein, we report composition tuning of Nb(SxSe(1-x))(2) alloy nanosheets (NSs) via colloidal synthesis and demonstrate the role of modulating d-electron density and defect engineering in alloy structures for the hydrogen evolution reaction (HER). Introducing sulphur (S) in the NbSe2 lattice optimizes its electronic properties for enhanced HER activity. Both experimental and density functional theory (DFT) calculations uncover the modulation in the d-band electronic structure and emphasize chalcogen vacancies as active sites for facile adsorption-desorption in the HER process. Additionally, substitutional alloying of the smaller size S atom in NbSe2 induced lattice contraction leads to a local polarized electric field in the basal plane. Cumulatively, with modulation of the electronic structure and lowering of the energy barrier, the x = 0.5 (NbSSe) composition exhibits higher HER activity than both NbSe2 and NbS2 NSs. The increase in catalytic activity with chalcogen alloying is also verified by the Gibbs free energy of hydrogen adsorption (Delta G(H*)) being close to thermoneutral for the alloyed catalyst.