Layered Transition Metal Dichalcogenides

2D matierlas of layered transition metal dichalcogenides (LTMDs) are shown to have the unque feature of polymorphism, which means two different phases of the material. The phases of a metal and a semiconductor coexist in a monolayer or several layers of MoS2 (Molybdenum Disulfide) and WS2 (Tungsten Disulfide), which are common LTMDs.




The key challenge to use MoS2 or WS2 as a catalyst for Hydrogen Evolution Reaction is to increase the catalytic active sites. To increase the electroactive sites of the layered transition metal dichalcogenides, scientists have tried changing its substrate to conductive graphene or carbon nanotubes.

In order to obtain a monolayer or several layers of MoS2 and WS2, you can buy the bulks from Aldrich, and open up the stacks of the layers by mixing the solutions with n-Butyl Lithium solution, which is highly flammable. So be careful, if you're using it. Basically, the positive lithium ions get attracted to the Sulfur atoms which are slightly negative when they are stacked on top of each other. This process changes the structure of MoS2 from 2H = semiconducting (bulk) to 1T = metallic. The thickness of the resulting monolayer of MoS2 was approximately 1 nm. The figure (a) shows the Atomic Force Microscopy image of a monolayer MoS2. The figure (b) shows  WS2's metallic structure and a slightly changed metallic structure due to the charge interaction with Lithium. The figure (c) shows the semiconducting 2H phase of WS2.




As the figure (b) shows, after a monolayer of LTMD was obtained, they have a metallic phase, but the metallic structure in molecules is slightly distorted, which turned out to be effective in evolving hydrogen from acidic solution.