Abstract :
[en] Catalytically driven electrochemical hydrogen evolution reaction
(HER) of monolayered molybdenum disulfide (MoS2) is usually highly suppressed
by the scarcity of edges and low electrical conductivity. Here, we show how the
catalytic performance of MoS2 monolayers can be improved dramatically by catalyst
size reduction and surface sulfur (S) depletion. Monolayered MoS2 nanocrystals
(NCs) (2−25 nm) produced via exfoliating and disintegrating their bulk counterparts
showed improved catalysis rates over monolayer sheets because of their increased
edge ratios and metallicity. Subsequent S depletion of these NCs further improved
the metallicity and made Mo atoms on the basal plane become catalytically active. As
a result, the S-depleted NCs with low mass (∼1.2 μg) showed super high catalytic
performance on HER with a low Tafel slope of ∼29 mV/decade, overpotentials of
60−75 mV, and high current densities jx (where x is in mV) of j150 = 9.64 mA·cm−2
and j200 = 52.13 mA·cm−2. We have found that higher production rates of H2 could
not be achieved by adding more NC layers since HER only happens on the topmost surface and the charge mobility
decreases dramatically. These difficulties can be largely alleviated by creating a hybrid structure of NCs immobilized onto
three-dimensional graphene to provide a very high surface exposure of the catalyst for electrochemical HER, resulting in
very high current densities of j150 = 49.5 mA·cm−2 and j200 = 232 mA·cm−2 with ∼14.3 μg of NCs. Our experimental and
theoretical studies show how careful design and modification of nanoscale materials/structures can result in highly efficient
catalysis. There may be considerable opportunities in the broader family of transition metal dichalcogenides beyond just
MoS2 to develop highly efficient atomically thin catalysts. These could offer cheap and effective replacement of precious
metal catalysts in clean energy production.
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