Salt-Assisted Growth of Transition Metal Dichalcogenide Nanotubes—Mechanisms from Molecular Dynamics

The use of inorganic salts to increase the yield and quality of 2D transition metal dichalcogenide (TMD) monolayers during CVD has become well established in recent years^1-2. While 2D van der Waals (vdW) heterostructures have been extensively studied since the late 2000’s, their 1D counterpart, the 1D nanotube vdW heterostructure, was first reported in 2020.³ Salt assisted growth has become a strategy for synthesising 1D heterostructures as well, highlighting the importance of a better understanding of these salts in the synthesis of TMD nanomaterials. Proposed mechanisms for this type of salt assisted growth feature lower melting temperature salts (compared to for example MoO₃/WO₃) such as Na₂WO₄ and NaMoO₃, which provide a steady flux of TMD metal to the substrate, thereby increasing yield and quality.^2,4 Similarly, Na₂MoO₄ has been proposed to form a eutectic with the chalcogen, before monolayer TMDs are precipitated.³The formation of transition metal oxychlorides as intermediates that more readily reduces has also been investigated as an explanation of these effects.⁴ However, the precise atomistic mechanism by which the salt acts to make these improvements remains unclear.<br/><br/>Here we present quantum chemical molecular dynamics (MD) simulations using the GFN1-xTB⁵ method to explore the role of NaCl in the formation of MoS₂. MoO₃, S₂ and varying amounts of NaCl were modelled in the gas phase to better understand the intermediates that form and the individual role of Na and Cl on the formation of these intermediates. We show that adding NaCl doubles the rate of oxidation/reduction of Mo and O, with gas-phase Na⁺ cations being the main facilitator of this change. Our results show new insights highlighting the role of Na, which has so-far been neglected in attempting to understand salt assisted growth, as a key reduction agent and catalyst driving bond rearrangement between Mo, O and S.<br/><br/><b>References</b><br/>[1] S. Li, et al., <i>Applied Materials Today</i>, 2015, <b>1</b>, 1.<br/>[2] S. Li, et al., <i>Chem. </i><i>Mater.</i>, 2021, <b>33</b>, 18.<br/>[3] R. Xiang, et al., <i>Science</i>, 2020, <b>367</b>, 6477.<br/>[4] J. Lei, et al., <i>J. of American Chem. Soc.</i>, 2022, <b>144</b>, 16.<br/>[5] S. Grimme, et al., <i>Journal of Chemical Theory and Computation</i>, 2017, <b>13</b>, 5