  The ability to efficiently evolve hydrogen via electrocatalysis at low
overpotentials holds tremendous promise for clean energy. Hydrogen evolution
reaction (HER) can be easily achieved from water if a voltage above the
thermodynamic potential of the HER is applied. Large overpotentials are
energetically inefficient but can be lowered with expensive platinum based
catalysts. Replacement of Pt with inexpensive, earth abundant electrocatalysts
would be significantly beneficial for clean and efficient hydrogen evolution.
Towards this end, promising HER characteristics have been reported using 2H
(trigonal prismatic) XS2 (where X = Mo or W) nanoparticles with a high
concentration of metallic edges as electrocatalysts. The key challenges for HER
with XS2 are increasing the number and catalytic activity of active sites. Here
we report atomically thin nanosheets of chemically exfoliated WS2 as efficient
catalysts for hydrogen evolution with very low overpotentials.
Atomic-resolution transmission electron microscopy and spectroscopy analyses
indicate that enhanced electrocatalytic activity of WS2 is associated with high
concentration of strained metallic 1T (octahedral) phase in the as-exfoliated
nanosheets. Density functional theory calculations reveal that the presence of
strain in the 1T phase leads to an enhancement of the density of states at the
Fermi level and increases the catalytic activity of the WS2 nanosheet. Our
results suggest that chemically exfoliated WS2 nanosheets could be interesting
catalysts for hydrogen evolution.
