We have studied the kinetics of microscopic heat-induced oxidative etching in the case of thick, mechanically exfoliated, geological MoS 2 crystals in air. We have measured spatial dimensions of microscopically obtained triangular etch pits during a series of sample heating increments at a given temperature. The data have been collected for the samples heated at 320, 350, 370, and 390 °C. Using our data, we have extracted an Arrhenius apparent activation energy, E a = 1.15 ± 0.25 eV, as well as an Arrhenius kinetic constant, A = 10 x s −1 with x = 9.09 ± 2.03. The obtained value of E a compares extremely well with another study of oxidative etching but carried out via in situ Raman spectroscopy on a collection of thin MoS 2 flakes. We notice that apparent activation energy relates to a weighted average of microscopic Arrhenius-like processes. It might need a correction because of as yet unknown fractions of removed MoO x species at the investigated temperatures. Based on the existing literature, the most expected reaction is a series of etching events proceeding along zig-zag (ZZ) Mo edges and with each event being comprised of three stages. First, an oxygen molecule reacts with unsaturated Mo atomsaccessed via abundant single sulfur vacancies (SSVs)to produce MoO 3 as well as Mo vacancies with exposed S atoms. The unsaturated S-terminated layer reacts subsequently with two O 2 molecules to produce two SO 2 molecules and to expose new unsaturated Mo atoms along the Mo ZZ edge. Finally, the value of E a obtained here suggests that oxidative etching competes with two other surface reactions with very similar apparent activation energies. These are dissociative O 2 adsorption on defected MoS 2 surfaces and oxygen-induced SSV creation.

The chemical presence of the MoOx species on single microscopic MoS2 flakes is shown at two conditions, which are of interest for future MoS2‐based devices and where their presence is not previously confirmed. First, the case of thick MoS2 flakes oxidatively etched at 350–370 °C in air is treated. Atomic force microscopy (AFM), high resolution X‐ray photoelectron spectroscopy, and X‐ray absorption spectroscopy are combined to unambiguously confirm the chemical presence of the α‐MoO3 species on such samples, mostly in the form of loose particles. Second, it is shown that MoS2 flakes heated at temperatures of only 220 °C display a quite uniform ≈2 nm thick MoOx layer at already 10% relative humidity. The presence of such MoOx oxide layers is confirmed by scratching the sample with AFM tips and performing comparative Kelvin probe force microscopy and Auger photoelectron spectroscopy on scratched‐out and untouched parts of the flakes.

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