Role of H Transfer in the Gas-Phase Sulfidation Process of MoO₃: 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 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 _xO _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.