Microscopic Kinetics of Heat-Induced Oxidative Etching of Thick MoS2 Crystals

We have studied the kinetics of microscopic heat-induced oxidative etching in the case of thick, mechanically exfoliated, geological MoS2 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, Ea = 1.15 ± 0.25 eV, as well as an Arrhenius kinetic constant, A = 10x s–1 with x = 9.09 ± 2.03. The obtained value of Ea compares extremely well with another study of oxidative etching but carried out via in situ Raman spectroscopy on a collection of thin MoS2 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 MoOx 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 MoO3 as well as Mo vacancies with exposed S atoms. The unsaturated S-terminated layer reacts subsequently with two O2 molecules to produce two SO2 molecules and to expose new unsaturated Mo atoms along the Mo ZZ edge. Finally, the value of Ea obtained here suggests that oxidative etching competes with two other surface reactions with very similar apparent activation energies. These are dissociative O2 adsorption on defected MoS2 surfaces and oxygen-induced SSV creation.