Origin of Catalytic Activity in MoS2 Nanostructures upon Chemical Transformation
Authors: Gupta Gautam , Cummins Dustin R. , Martinez Ulises , Mohite Aditya , Chhowalla Manish , Sunkara Mahendra Kumar 
Published: February 2020

The need for alternative energy sources to replace the combustion of fossil fuels continues to grow as a global priority. Hydrogen production from electrochemical or electrocatalytic/photoelectrocatalytic water splitting offers long-term solution for obtaining clean hydrogen. However, Hydrogen is currently obtained using thermal steam reforming of fossil fuels1, such as coal and natural gas2-4 , mainly due to numerous limitations in renewable technologies for generation of hydrogen. Currently, platinum metal and platinum based materials are most commonly used in water splitting applications and have an ideal overpotential (0 V vs. RHE). There is an important need to obtain hydrogen by developing inexpensive, earth abundant, and efficient catalysts, which possess low overpotential as well as durable in harsh acidic conditions.
Layered transition metal dichalcogenides present opportunities for efficient electrocatalytic systems, 5 however this materials class suffers from anisotropic crystal properties, i.e. increased chemical activity from edge planes, compared to the chemically inert basal planes. Here, we report the modification of MoS2 nanostructures and the corresponding changes in morphology, structure, and mechanism of H2 evolution. The MoS2 structures exhibit significant improvement in H2 evolution characteristics after chemical transformation, however the origin of high activity depends drastically on the type of nanostructure such as nanosheets vs nanowires. The catalytic activity also depends on the choice of chemical intercalant. In the case of nanowires, the high electrochemical activity results from a disruption of the single crystalline MoS2 layers (shell), leading to an increase number of active edge sites. However, in the case of 2D sheets, a crystal phase transition from hexagonal (2H) to trigonal (1T) structure is responsible for the high activity. To test the origin of catalytic activity, gate-dependent HER catalysis experiments on single flakes of MoS2 will be discussed.
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