  Electrochemical reactions represent essential processes in fundamental
chemistry that foster a wide range of applications. Although most
electrochemical reactions in bulk substances can be well described by the
classical Marcus-Gerischer charge transfer theory, the realistic reaction
character and mechanism in dimensionally confined systems remain unknown. Here,
we report the multiparametric survey on the kinetics of lateral photooxidation
in structurally identical WS2 and MoS2 monolayers, where electrochemical
oxidation occurs at the atomically thin monolayer edges. The oxidation rate is
correlated quantitatively with various crystallographic and environmental
parameters, including the density of reactive sites, humidity, temperature, and
illumination fluence. In particular, we observe distinctive reaction barriers
of 1.4 and 0.9 eV for the two structurally identical semiconductors and uncover
an unusual non-Marcusian charge transfer mechanism in these dimensionally
confined monolayers due to the limit in reactant supplies. A scenario of band
bending is proposed to explain the discrepancy in reaction barriers. These
results add important knowledge into the fundamental electrochemical reaction
theory in low-dimensional systems.
