  The quantitative correlation of the catalytic activity with microscopic
structure of heterogeneous catalysts is a major challenge for the field of
catalysis science. It requests synergistic capabilities to tailor the structure
with atomic scale precision and to control the catalytic reaction to proceed
through well-defined pathways. Here we leverage on the controlled growth of
MoS2 atomically thin films to demonstrate that the catalytic activity of MoS2
for the hydrogen evolution reaction decreases by a factor of ~4.47 for the
addition of every one more layer. Similar layer dependence is also found in
edge-riched MoS2 pyramid platelets. This layer-dependent electrocatalysis can
be correlated to the hopping of electrons in the vertical direction of MoS2
layers over an interlayer potential barrier, which is found to be 0.119V and
consistent with theoretical calculations. Our results point out that increasing
the hopping efficiency of electrons in the vertical direction is a key for the
development of high-efficiency two-dimensional material catalysts.
