Hydrogen evolution activity of individual mono-, bi-, and few-layer MoS₂ towards photocatalysis

We investigate the hydrogen evolution activity in the dark and under illumination above the band gap of individual mono-, bi- and few-layer (bulk) MoS₂ flakes. We demonstrate that the electrocatalytic activity of 2H-MoS₂ immersed in 1 M H₂SO₄ increases with decreasing number of layers. For monolayers, we observe the highest exchange current density, which is one magnitude larger than in the bulk case. The onset potential scales with the number of layers, which is consistent with a previous report, suggesting that hopping transport across inter-layer barriers within the MoS₂ flakes is responsible for this scaling. A specially designed micro-sized catalytic cell enables us to investigate individual MoS₂ flakes with well-known geometry and edge-to-surface ratio. Taking these geometric parameters into account, we tentatively attribute the catalytic activity mainly to sulfur vacancies in the basal planes acting as active sites. The associated turn over frequencies (TOF) for mono- and bi-layer MoS₂ yield values higher than 10³ s⁻¹ at an overpotential of −0.2 V vs. RHE. In view of light driven hydrogen evolution as a means of solar energy conversion, we investigate the photocatalytic activity of few-layer MoS₂ under white light illumination.