An elevated concentration of MoS lowers the efficacy of liquid-phase exfoliation and triggers the production of MoO nanoparticles.

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