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

Abstract

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.

Surface-Bound and Volatile Mo Oxides Produced During Oxidation of Single MoS2 Crystals in Air and High Relative Humidity

“…It has been confirmed both experimentally and theoretically that bulk MoS 2 oxidation is not readily observed at ambient conditions due to high energy barriers for such reactions [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. Nevertheless, all kinds of thin and thick microscopic MoS 2 crystals are etched by oxygen within the time scales of minutes when heated to temperatures of at least 320 °C.…”

“…Nevertheless, all kinds of thin and thick microscopic MoS 2 crystals are etched by oxygen within the time scales of minutes when heated to temperatures of at least 320 °C. Such etching progresses according to a following stoichiometry [ 8 , 9 , 10 , 11 , 12 ]: 2MoS 2 + 7O 2 → 2MoO 3 + 4SO 2 (g) …”

“…Therein, MoO 3 was reported to be a non-magnetic material. In one of our earlier AFM and micro-Raman studies we pointed out that during initial 10–15 min of heating at temperatures ranging from 320 to 390 °C, in air, a predominantly observed outcome on thick microscopic MoS 2 flakes was triangular etch pits within the basal MoS 2 surface [ 11 ]. Similar triangular etch pits were observed several years earlier on very thin MoS 2 samples heated either in oxygen or in oxygen/argon flow or in air [ 8 , 9 , 10 ].…”

Surface Characterization of MoS2 Atomic Layers Mechanically Exfoliated on a Si Substrate

“…In the current work, we characterized thin Mo disulfide crystals obtained using mechanical exfoliation [26,27] and deposited on Si substrates. The surface chemical composition, morphology and electron transport phenomena of prepared Mo disulfide flakes were characterized ex-situ by the combination of AES, SEM and EPES.…”

“…#429MM-AB), and then transferred on fine polished and basically undoped <111> Si crystals with resistivity of more than 10,000 Ω•cm (ITME, Warsaw, Poland). This preparation procedure is essentially identical to that used earlier to obtain thick MoS 2 flakes [26,27].…”