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Catalyzed growth of encapsulated carbyne

Evidence and attribution

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Prose below summarizes the Carbon article identified by doi, title, and pdf_path.

Summary

Linear carbon chains (carbyne) inside carbon nanotubes are of fundamental interest, but growth mechanisms from small hydrocarbon feeds on catalyst clusters require atomistic detail. This work combines ReaxFF molecular dynamics (Zou et al. parameters) with DFT (VASP, GGA-RPBE, PAW) to study Ni-catalyzed insertion of C\(_2\), C\(_2\)H, and C\(_2\)H\(_2\) into a (5,5)@(10,10) double-walled nanotube at 500 K versus 1700 K effective carbon fluxes. Sticking and adsorption energies anticorrelate with H/C ratio in the feed; step-like Ni facets favor C–H cleavage compared with flatter (111)-like surfaces. DFT finds stronger binding magnitudes than ReaxFF but consistent trends.

The double-walled CNT framework confines Ni and hydrocarbon feeds to a one-dimensional reaction environment, amplifying facet-dependent selectivity that would be harder to resolve on open surfaces with many competing sites.

Methods

1 — MD application (RMD, ReaxFF)

Engine / code: ReaxFF (parameters of Zou et al., as in the Carbon Methods), periodic PBC along the nanotube axis only. System / composition: (5,5)@(10,10) double-walled nanotube (inner/outer diameters ~0.70 / ~1.39 nm); Ni cluster in the inner channel; feed C\(_2\), C\(_2\)H, C\(_2\)H\(_2\). Ensemble and thermostats: NpT equilibration with a Berendsen thermostat and barostat, then NVT production with a Bussi thermostat; 500 K vs 1700 K target temperatures for RMD as reported. Feed protocol: insertion every 25 ps at 1700 K and every 250 ps at 500 K to match low vs high carbon flux. Timestep (fs): N/A — not stated in the Carbon Methodology text in the PDF (confirm in SI if reproducing). Barostat in production: N/A when NVT—NPT only in the preequilibration stage as written. Mean hydrostatic pressure control in NVT: N/A (constant volume NVT). Electric field: N/A. Replica / enhanced sampling: N/A—sequential RMD only. Long-range electrostatics / ReaxFF QEq update schedule: per Zou et al. / ReaxFF defaults in the cited implementation (not re-tabulated here).

2 — Force-field training

N/A — this work uses a published ReaxFF for C/H and Ni-containing carbon environments (Zou et al.); the new science is the MD+DFT application, not a reoptimization of the field in this article.

3 — Static QM (VASP, supporting DFT and NEB)

Functional / theory level: GGA RPBE in VASP with PAW; 400 eV cutoff; in-plane ~20×20 Å-scale supercells with Γ-centered (1×1×1) k mesh as described; spin polarization; no symmetry constraints; Gaussian smearing as in the paper. Dispersion: PBE+TS tests (Tkatchenko–Scheffler) are reported as negligible for the quantities considered; main DFT data in the manuscript are without vdW corrections for those comparisons. Structures / pathways: slab/cluster geometries for adsorption and NEB reaction paths (initial dimer nucleation with barriers ~0.1–0.3 eV in the Results). Properties computed: adsorption energies and barrier heights used to compare trends with ReaxFF.

Findings

Dimerization competes with dehydrogenation during early growth; Ni-terminated chains allow hydrogen tuning of electronic response; hydrocarbon identity controls whether C–C or C–H scission dominates on curved Ni facets. ReaxFF and DFT adsorption energies can differ by roughly 2× in places while preserving qualitative ordering.

Cross-validation message. The authors use DFT not only for barriers but also as a check on ReaxFF trends, which is important when absolute adsorption strengths feed into rate estimates.

Confinement effects. The inner (5,5) tube sets a strong curvature and electronic perturbation on Ni clusters compared with flat surface models, so facet arguments should be read as curvature-dependent selectivity rather than universal Ni catalysis rules.

Chain-growth outlook. The paper focuses on early insertion steps; long chain stability under beam or oxidizing environments is outside the present ReaxFF scope.

Repository use. Treat papers/ReaxFF_others/Khalilov_Neyts_Carbon_2019_carbyne_growth.pdf as the evidence anchor for barrier ranges and cluster geometries when aligning follow-on kinetic Monte Carlo models.

Limitations

Long carbyne stability is not the primary focus; results emphasize growth mechanism trends. Parameter transfer to other metals or tube chiralities needs separate validation.

Relevance to group

Reactive carbon nanotube chemistry adjacent to broader ReaxFF carbon materials work.

Citations and evidence anchors