  Quantifying anisotropy in the chemical reactions of mesoscopic crystals has
mostly resorted on the combination of electron microscopy and diffraction. In
this work, we established crystal-facet-resolved kinetic measurements of
oxidation reactions in 2D transition metal dichalcogenides (TMDs) using optical
second-harmonic generation spectroscopy and scanning probe microscopy. We show
the in-plane anisotropy of their bond-breaking reactions is governed by their
structure and strongly material-dependent among four TMDs. The facet-resolved
analysis directly revealed that the reactions proceed fastest (slowest) for
chalcogen (metal)-terminated zigzag edges with armchair edges in the middle.
The degree of the anisotropy inducing trigonal oxidation patterns was much
higher in MoS2 and MoSe2 than WS2 and WSe2. Kinetic Wulff construction based on
edge-specific reaction rates verified the material-dependent mesoscopic
reaction patterns. We also show that the reactions are initiated at
substrate-mediated defects located on the bottom and top surfaces of 2D TMDs.
