A simple electrochemical route to metallic phase trilayer MoS2: evaluation as electrocatalysts and supercapacitors

The development of a simple, scalable and reproducible technique for the synthesis of two-dimensional MoS2 nanosheets is of paramount importance in the field of catalysis and energy storage devices. Current routes to produce MoS2 nanosheets in reasonable quantities involve either solution exfoliation of bulk MoS2 or intercalation of organo-lithium into bulk MoS2, which is then subsequently exfoliated by immersing it in water. The former process produces semiconducting 2H-MoS2 nanoplatelets with smaller lateral flake sizes whereas the latter process produces highly conducting metallic (1T) phase monolayer MoS2. 1T-MoS2 nanosheets have high catalytic activity for the hydrogen evolution reaction (HER) and are efficient electrode materials for supercapacitors when compared to the 2H phase. However, the feasibility of producing 1T-MoS2 by organolithium intercalation is undermined by the long reaction time (2-3 days) and by its pyrophoric nature. We report a simple, bench-top electrochemical process to produce exfoliated metallic phase MoS2 within two hours. By using an inert Li salt (LiClO4) as a source of lithium and a Pt counter electrode, an electrochemically lithium intercalated MoS2 electrode was obtained, which was subsequently exfoliated by imit in water. Characterization of the exfoliated product using a variety of methods confirmed the formatiomersing n of the 1T phase. Remarkably, flake thickness measurement using atomic force microscopy and high-resolution transmission electron microscopy revealed that the majority of the nanosheets are trilayers. The 1T-MoS2 nanosheets showed enhanced electrocatalytic activity towards hydrogen evolution compared to 2H-MoS2 and are efficient materials for supercapacitor applications. Coin cells constructed from a 1T-MoS2-graphene composite achieved a volumetric capacitance of over 560 F cm(-3) in an aqueous system and 250 F cm(-3) in a nonaqueous electrolyte with capacity retention of over 90% after 5000 cycles. This process is readily scalable and should ultimately support the production of metallic MoS2 for various applications. It can also be extended to produce 2H-MoS2 nanosheets by heating the exfoliated 1T phase.