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.
