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

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 MoS 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 MoS layer into mass production is chemical vapor deposition (CVD). During CVD synthesis, gas-phase sulfidation of MoO is a key elementary reaction, forming MoS layers on a target substrate. Recent studies have proposed the use of gas-phase HS precursors instead of condensed-phase sulfur for the synthesis of higher-quality MoS crystals. However, reaction mechanisms, including atomic-level reaction pathways, are unknown for MoO sulfidation by HS. Here, we report first-principles quantum molecular dynamics (QMD) simulations to investigate gas-phase sulfidation of MoO flake using a HS precursor. Our QMD results reveal that gas-phase HS molecules efficiently reduce and sulfidize MoO through the following reaction steps: Initially, H transfer occurs from the HS molecule to low molecular weight Mo O clusters, sublimated from the MoO 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 HS 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.