Role of H Transfer in the Gas-Phase Sulfidation Process of MoO₃: A Quantum Molecular Dynamics Study

Abstract

Layered transition metal dichalcogenide (TMDC) materials have received great attention because of their remarkable electronic, optical, and chemical properties. Among typical TMDC family members, monolayer MoS2 has been considered a next-generation semiconducting material, primarily due to a higher carrier mobility and larger band gap. The key enabler to bring such a promising MoS2 layer into mass production is chemical vapor deposition (CVD). During CVD synthesis, gas-phase sulfidation of MoO3 is a key elementary reaction, forming MoS2 layers on a target substrate. Recent studies have proposed the use of gas-phase H2S precursors instead of condensed-phase sulfur for the synthesis of higher-quality MoS2 crystals. However, reaction mechanisms, including atomic-level reaction pathways, are unknown for MoO3 sulfidation by H2S. Here, we report first-principles quantum molecular dynamics (QMD) simulations to investigate gas-phase sulfidation of MoO3 flake using a H2S precursor. Our QMD results reveal that gas-phase H2S molecules efficiently reduce and sulfidize MoO3 through the following reaction steps: Initially, H transfer occurs from the H2S molecule to low molecular weight Mo x O y clusters, sublimated from the MoO3 flake, leading to the formation of molybdenum oxyhydride clusters as reaction intermediates. Next, two neighboring hydroxyl groups on the oxyhydride cluster preferentially react with each other, forming water molecules. The oxygen vacancy formed on the Mo–O–H cluster as a result of this dehydration reaction becomes the reaction site for subsequent sulfidation by H2S that results in the formation of stable Mo–S bonds. The identification of this reaction pathway and Mo–O and Mo–O–H reaction intermediates from unbiased QMD simulations may be utilized to construct reactive force fields (ReaxFF) for multimillion-atom reactive MD simulations.

Sulfurization of MoO3 in the Chemical Vapor Deposition Synthesis of MoS2 Enhanced by an H2S/H2 Mixture

“…Reaction steps in Scheme are based on the primary reaction pathway observed during our QMD simulations, whereas different reaction pathways may exist depending on the system temperatures, system pressure, and initial precursors used. According to our previous work, MoS 2 can be formed with different pathways when using MoO 3 and H 2 S reactants . However, the reaction pathways, derived by unbiased QMD simulations in this work, provide a new physical insight into the accelerated reduction and sulfurization steps (Figure ) with the level of quantum mechanical accuracy, which may support the observation in the experimental synthesis of MoS 2 layers using the MoO 3 reactants and H 2 S/H 2 mixture.…”

“…According to our previous work, MoS 2 can be formed with different pathways when using MoO 3 and H 2 S reactants. 43 However, the reaction pathways, derived by unbiased QMD simulations in this work, provide a new physical insight into the accelerated reduction and sulfurization steps (Figure 3) with the level of quantum mechanical accuracy, which may support the observation in the experimental synthesis of MoS 2 layers using the MoO 3 reactants and H 2 S/H 2 mixture.…”

Elucidation of Molybdenum Trioxide Sulfurization: Mechanistic Insights into Two-Dimensional Molybdenum Disulfide Growth

“…12 In the case of hydrogen sulfide, adjacent molybdenum oxyhydride clusters resulting from surface interactions with hydrogen sulfides condense into a reduced surface whose interactions with the reduced hydrogen sulfide lead to the formation of Mo−S bonds. 13 Experimental and computational studies of gas phase elemental sulfur have shown the allotrope composition of sulfur in the gas-phase to contain S 2 −S 8 under typical powder vaporization growth conditions. 14−16 Using this information, we previously reported the thermodynamics of the sulfurization by analyzing the gas-phase reaction between molybdenum compounds and the sulfur allotropes present in the growth chamber under typical growth conditions.…”

“…One of the critical parameters is growth temperature. Growth temperatures ranging from approximately 800 to 1275 K have been reported. ,,− In contrast, the mechanism of sulfurization by hydrogen sulfide has been more widely studied both computationally and experimentally. − In addition, the mechanisms of sulfurization of a molybdenum trioxide surface by elemental disulfur and by hydrogen sulfide have recently been investigated from a computational standpoint. In the case of disulfur as the precursor, oxygen molecules leave the surface to form oxysulfide gas-phase molecules that subsequently react with a reduced surface to form Mo–S bonds .…”

“…In the case of disulfur as the precursor, oxygen molecules leave the surface to form oxysulfide gas-phase molecules that subsequently react with a reduced surface to form Mo–S bonds . In the case of hydrogen sulfide, adjacent molybdenum oxyhydride clusters resulting from surface interactions with hydrogen sulfides condense into a reduced surface whose interactions with the reduced hydrogen sulfide lead to the formation of Mo–S bonds …”