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 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.