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Complexity of Intercalation in MXenes: Destabilization of Urea by Two-Dimensional Titanium Carbide

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Authority of statements

Prose below summarizes the JACS article identified by doi, title, and pdf_path. The corpus PDF is a Just Accepted manuscript; formatting may differ from the final issue.

Summary

Ti\(_3\)C\(_2\)T\(_z\) MXene treated with aqueous urea is compared to reference solids using inelastic neutron scattering (INS) and infrared spectroscopy, alongside Ti(urea)\(_6\) complexes and NH\(_4\)-intercalated MXene prepared separately. The data support urea decomposition under intercalation-relevant conditions, with ammonium signatures in INS and CO\(_2\) evolution detected by IR, rather than persistence of intact urea between layers as often assumed from c-axis expansion alone. Reactive MD (ReaxFF) simulations (detailed in Supporting Information) provide atomistic reaction pathways and energetics consistent with the experimental observations. The central cautionary message is that gallery expansion after urea treatment is not sufficient evidence for intact urea intercalation—vibrational fingerprints and gas-phase products matter.

Methods

  • Materials: HF-etched Ti\(_3\)C\(_2\) MXene from Ti\(_3\)AlC\(_2\), urea-treated MXene (e.g. 50 wt% urea, 15 h, 60 °C), vacuum-dried variants; NH\(_4\)HF\(_2\)-etched ammonium-intercalated reference; Ti(urea)\(_6\)I\(_3\) complex synthesized for vibrational references.
  • Spectroscopy: INS (ORNL; instrument details in SI) on dried samples; IR for gas-phase CO\(_2\) signatures during controlled treatments as described in the paper/SI.
  • Modeling: ReaxFF-based reactive MD to explore urea–MXene reaction pathways and products; parameters and protocols in Supporting Information (per manuscript).

Reactive MD (SI-first in this corpus). The main text points to Supporting Information for ReaxFF parameters and simulation setup for reactive molecular dynamics of urea with Ti\(_3\)C\(_2\)-family MXene models. Engine: ReaxFF MD as implemented in the SI workflow (N/A — whether LAMMPS vs another engine is not stated in the p1–2 Just Accepted extract used here). System / composition: MXene slab/supercell models with intercalated urea-derived species as described in SI (N/A — exact atom counts should be copied from SI tables). PBC: three-dimensional periodic supercells assumed for the interlayer gallery models unless the SI specifies otherwise. Ensemble / timestep / duration / thermostat / barostat: N/A — not recoverable from the indexed p1–2 extract for this Just Accepted PDF; use the Supporting Information and final JACS issue Methods for NVT/NPT labels, timestep, temperature ramps, and production lengths. External electric field in MD: N/A — not used as a bias in the excerpted description. Enhanced sampling: N/A — umbrella / metadynamics / replica exchange not indicated in the indexed excerpt.

Findings

  • INS of urea-exposed MXene matches ammonium-containing references more closely than intact urea or the Ti–urea complex, supporting decomposition of urea and intercalation of NH\(_4^+\)-derived species rather than pristine urea as the primary intercalant.
  • IR detects CO\(_2\), consistent with oxidative decomposition of urea during treatment.
  • Reactive simulations corroborate feasible bond-breaking pathways and energetics for urea destabilization at the MXene interface, motivating caution when inferring intercalant identity from interlayer spacing alone.
  • Together, experiment and modeling argue that urea-based MXene processing recipes should be interpreted as reactive infiltration chemistry, not merely physical intercalation of a neutral molecule.

Limitations

Just-Accepted PDF may differ slightly from the final edited article; INS/IR are bulk probes and may miss minority species. Urea chemistry is sensitive to temperature, time, and residual HF-etch byproducts; the experiments therefore should be read together with the sample preparation paragraphs when inferring intercalant speciation.

Reader notes (navigation)

  • Just Accepted manuscript PDF in corpus; confirm final pagination against the journal issue when available. ReaxFF details: Supporting Information (as cited in the article).