Two-dimensional (2D) transition-metal monochalcogenides have been recently predicted to be potential photo(electro)catalysts for water splitting and photoelectrochemical (PEC) reactions. Differently from the most established InSe, GaSe, GeSe, and many other monochalcogenides, bulk GaS has a large band gap of ∼2.5 eV, which increases up to more than 3.0 eV with decreasing its thickness due to quantum confinement effects. Therefore, 2D GaS fills the void between 2D small-band-gap semiconductors and insulators, resulting of interest for the realization of van der Waals type-I heterojunctions in photocatalysis, as well as the development of UV light-emitting diodes, quantum wells, and other optoelectronic devices. Based on theoretical calculations of the electronic structure of GaS as a function of layer number reported in the literature, we experimentally demonstrate, for the first time, the PEC properties of liquid-phase exfoliated GaS nanoflakes. Our results indicate that solution-processed 2D GaS-based PEC-type photodetectors outperform the corresponding solid-state photodetectors. In fact, the 2D morphology of the GaS flakes intrinsically minimizes the distance between the photogenerated charges and the surface area at which the redox reactions occur, limiting electron–hole recombination losses. The latter are instead deleterious for standard solid-state configurations. Consequently, PEC-type 2D GaS photodetectors display a relevant UV-selective photoresponse. In particular, they attain responsivities of 1.8 mA W^–1 in 1 M H₂SO₄ [at 0.8 V vs reversible hydrogen electrode (RHE)], 4.6 mA W^–1 in 1 M Na₂SO₄ (at 0.9 V vs RHE), and 6.8 mA W^–1 in 1 M KOH (at 1.1. V vs RHE) under 275 nm illumination wavelength with an intensity of 1.3 mW cm^–2. Beyond the photodetector application, 2D GaS-based PEC-type devices may find application in tandem solar PEC cells in combination with other visible-sensitive low-band-gap materials, including transition-metal monochalcogenides recently established for PEC solar energy conversion applications.

Metallic two-dimensional transition metal dichalcogenides (TMDs) of the group-5 metals are emerging as catalysts for efficient the hydrogen evolution reaction (HER). The HER-activity of the group-5 TMDs originates from the unsaturated chalcogen-edges and the highly active surface basal planes, whereas the HER-activity of the widely studied group-6 TMDs originates solely from the chalcogen- or metal-unsaturated edges. However, the batch production of such nanomaterials and their scalable processing into high-performance electrocatalysts is still challenging. Herein, we report the liquid phase exfoliation of the 2H-TaS2 crystals by using 2-propanol to produce single/few-layer (1H/2H) flakes, which are afterward deposited as catalytic films. A thermal treatment-aided texturization of the catalytic films is used to increase their porosity, promoting the ion access to the basal planes of the flakes, as well as the number of catalytic edges of the flakes. The hybridization of the H-TaS2 flakes and H-TaSe2 flakes tunes the Gibbs free energy of the adsorbed atomic hydrogen onto the H-TaS2 basal planes to the optimal thermo-neutral value. In 0.5 M H2SO4, the heterogeneous catalysts exhibit a low overpotential (versus -RHE- reversible hydrogen electrode) at the cathodic current of 10 mA cm-2 (ƞ10) of 120 mV and high mass activity of 314 A g-1 at overpotential of 200 mV. In 1 M KOH, they show a ƞ10 of 230 mV and a mass activity of 220 A g-1 at overpotential of 300 mV. Our results provide new insight into the usage of the metallic group-5 TMDs for the HER through scalable material preparation and electrode processing.

The development of niobium disulphide (NbS₂)-based heterogeneous electrocatalysts for an efficient hydrogen evolution reaction.