An elevated concentration of MoS2 lowers the efficacy of liquid-phase exfoliation and triggers the production of MoOx nanoparticles

Abstract

It is generally accepted that liquid-phase exfoliation (LPE) enables large-scale production of few-layer MoS₂ flakes. In our work, we studied in detail few-layer MoS₂ oxidation in the course of standard LPE in a water/ethanol solution. We demonstrate that an increase of the initial MoS₂ concentration above a certain threshold triggers a pronounced oxidation and the exfoliation process starts to produce MoO_x nanoparticles. A subsequent decrease of the water pH along with an increased content of SO₄²⁻ suggests an oxidation scenario of few-layer MoS₂ oxidation towards MoO_x nanoparticles. Moreover, the lowered pH leads to agglomeration and sedimentation of the few-layer MoS₂ flakes, which significantly lowers their production yield. We employed a large number of physico-chemical techniques to study the MoS₂-to-MoO_x transformation and found a threshold value of 10 mg ml⁻¹ of the initial MoS₂ concentration to trigger this transformation.