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Oxidation and hydrogenation of monolayer MoS2 with compositing agent under environmental exposure: The ReaxFF Mo/Ti/Au/O/S/H force field development and applications

Scope

New ReaxFF parameters for Mo–Ti–Au–O–S–H with reactive MD of monolayer MoS\(_2\) under O\(_2\) and H\(_2\)O, highlighting how Ti clusters alter oxidation and hydrogenation pathways.

Summary

Transition metal dichalcogenides such as MoS\(_2\) degrade under ambient oxygen and water when integrated with Ti/Au-rich processing stacks. The paper develops a ReaxFF parameterization for Mo, Ti, Au, O, S, and H, then applies it in reactive MD with temperature ramps to compare pristine MoS\(_2\) surfaces with Ti-cluster-containing models under O\(_2\) and H\(_2\)O exposure. The scientific target is to separate intrinsic TMD oxidation from metal-seed-mediated pathways that appear when fabrication leaves titanium-rich debris near the flake.

Methods

1 — MD application (atomistic dynamics)

  • Engine / code: LAMMPS with a ReaxFF reax pair style for the Mo/Ti/Au/O/S/H set (article Methods).
  • System size & composition: Periodic 2H monolayer MoS\(_2\) supercells; the authors list an example with 225 Mo and 450 S in a hexagonal cell (\(\alpha=\beta=90°\), \(\gamma=120°\)), plus gas-phase O\(_2\) or H\(_2\)O; systems with Ti use a preparation stage at 1,800 K for 200 ps in NVT to let Ti cluster, followed by energy minimization after cooling to 300 K; ~200 O\(_2\) or H\(_2\)O molecules are added above/below the sheet with reported gas densities in the O\(_2\)- vs H\(_2\)O-cases (see Table 1 in the PDF).
  • Boundaries / periodicity: 3D PBC for the monolayer + gas; no fixed “wall” geometry beyond the supercell in the text excerpted.
  • Ensemble: NVT (canonical) for equilibration and for heating/annealing steps described in the PDF.
  • Timestep: 0.1 fs in production trajectories to maintain stable covalent dynamics at the temperatures used.
  • Duration / stages: After initial minimization, 500 ps at 300 K (NVT); cases are then heated 300 K → 1,000 K at 10 K/ps, with additional NVT anneals at high temperature (article lists multi-step schedules per case, including 1,000 K hold segments); full tables are in the Frontiers Methods.
  • Thermostat / barostat / pressure: Nose–Hoover (or scheme stated alongside NVT in the PDF); N/A — NPT (constant lateral stress) and independent hydrostatic pressure control for these gas–surface NVT production runs; barostat not used for the summarized protocol.
  • Temperature: Ramped to 1,000 K (and higher regimes discussed for oxidation limits) in staged heating programs; Ti-cluster preparation uses 1,800 K transient heating as quoted above.
  • Electric field / replica or enhanced sampling: N/A — no applied electric field or umbrella / metadynamics; chemistry is thermal and concentration-driven.

2 — Force-field training (ReaxFF)

  • Parent FF / elements: New ReaxFF parameters for Mo, Ti, Au, O, S, and H tailored to monolayer MoS\(_2\)/Ti–Au stack scenarios; the article combines/extends training against non-periodic and periodic DFT targets (energies, structures, key bond/angle data in figures) with the authors’ stated agreement tolerances to QM (e.g. ~5 kcal/mol scale mentioned for selected bonds in the main text; see PDF figures and SI for full sets).
  • QM reference / basis: DFT-level reference data (functional and level per article/SI) for bond-dissociation, angles, and cluster/oxide motifs used in the fit; bond-order cutoff values for product identification are set via a bond-scan procedure (0.1 Å steps) in the PDF.
  • Training set & optimization: Fitting uses compiled QM dissociation and conformational data for Mo–S, Ti–S, Au–O, and related fragments as enumerated in the article; optimization follows the group’s ReaxFF least-squares / combined-parameter workflow (see SI for tables).
  • Validation: Bond-order–based reactivity and comparators to DFT in main-text figures; additional metrics in SI.

3 — Static QM / DFT and experiments

  • DFT (training only): Primary new science is the ReaxFF; DFT entries support parameterization (see §2), not a separate DFT free-energy study in this paper.
  • Experiments: N/A — the reported evidence is computational; the introduction cites experimental literature for MoS\(_2\) oxidation in air/humidity as motivation only.

Findings

Oxidation / hydrogenation without Ti: During heating/ramping in O\(_2\), the MoS\(_2\) basal surface is oxidized and S is lost along pathways involving O\(_2\) adsorption and dissociation; in H\(_2\)O, hydrogenation of the surface is the favored initial channel, then oxidation and hydroxylation in the order stated in the abstract. With Ti clusters: at low–intermediate temperature, the clusters scavenge O- and H-bearing species, sequestering O\(_2\) and H\(_2\)O-derived fragments and slowing MoS\(_2\) oxidation/hydrogenation; as temperature rises in H\(_2\)O, the clusters release OH\(^-\)-like and H\(_3\)O\(^+\)-type species, accelerating hydrogenation of MoS\(_2\) at very high temperature. The article frames this as a context-dependent role of Ti near TMDs in device-like Ti/Au process stacks, where “protective” vs “activating” behavior depends on temperature and gas identity.

Comparisons and design levers: The study contrasts O\(_2\) vs H\(_2\)O environments and with/without Ti clusters; gas density is elevated relative to ambient air in several builds to isolate each gas–surface reaction channel. Limitations (as in paper): Parameter training and reactive kinetics are classical—long ambient aging and photo-chemistry are outside the MD model.

Limitations

Specific numerical convergence of the ReaxFF fit to every DFT reference should be verified in the article tables; finite MD trajectories sample only part of long-term ambient degradation. Frontiers open-access formatting makes SI bond-scan and ramp protocol tables the authoritative references when reconciling qualitative oxidation/hydrogenation sequences with quantitative barrier claims.

Relevance to group

Group-led ReaxFF extension for 2D TMD integration with metallization and environmental exposure.

Citations and evidence anchors

Reader notes (navigation)