Characterizing and Understanding Divalent Adsorbates on Carbon Nanotubes with Ab Initio and Classical Approaches: Size, Chirality, and Coverage Effects

Abstract

The study of oxygen chemisorption on single-walled carbon nanotubes generally relies on simple atomistic models and hence hampers the possibility to understand whether nanotube size or adduct concentration have a role in determining the surface-adsorbate interaction. Our large-scale DFT-based simulations show that structural and electronic properties as well as diffusion barriers strongly depend on both nanotube diameter and adsorbate concentration. Our atomistic models cover nanotube of different chirality with diameters from 0.6 to 1.5 nm and oxygen concentration from 0.1 to 1%. In particular, the tendency to cluster increases with concentration and stabilizes ether (ET) groups but affects hopping barriers only to a minor extent. Significant differences with graphene are found, also for 1.5 nm diameter nanotubes. Extension to species isoelectronic to oxygen reveals dissimilarities, and especially for sulfur that tends to form epoxides (EP), to diffuse more easily and to rapidly close the energy gap for increasing concentration. The relative ET-EP stability can be described in terms of the bare-bond curvature, a concentration-dependent chemical descriptor here introduced. Comparison of these DFT calculations-using different exchange-correlation functionals-and our additional investigation with a reactive force-field (ReaxFF) clarifies several similarities but also discrepancies between the predictions of the two schemes.

Atom Vacancies on a Carbon Nanotube: To What Extent Can We Simulate their Effects?

“…On one hand, we find a close agreement between the results obtained with the DFT-GGA functionals PBE and BLYP as already pointed out in the study of hydrogen and oxygen chemisorption . Also the partial addition of exact exchange, as in the PBE0 functional, has a negligible effect on the structural characteristics and the ranking of the different vacancy configurations considered here.…”

“…Moreover, in order to represent highly diluted defects, the cell size must be chosen sufficiently large so as to avoid spurious effects of their interaction. This same issue was first emphasized in previous work on hydrogen and oxygen chemisorption on CNTs . As pointed out in ref , as a consequence of the periodic boundary conditions, possible torsional deformations of the tube induced by the defects will be limited to those compatible with the symmetry of the supercell.…”

“…Only spin states of the lowest multiplicity (singlets) are considered. Details are the same as in our previous papers . More can be found in the (SI).…”

The response of single-walled carbon nanotubes to NO2 and the search for a long-living adsorbed species

“…The reason is however clear: those calculations referred to "chemisorbed states," for which the bond strength depends on the degree of sp 2 -sp 3 rehybridization of the carbon atoms involved, and thus decreases with the decreasing curvature of the nanotube. 31 In conclusion, the analysis of our experimental data on contact-passivated individual SWNTs with diameters in the range of 1.5-3.5 nm exposed to 1 ppm NO 2 leads us to establish that the long-living species on the surface of the nanotube have binding energies on the order of 1 eV. Our calculations show that, unless defects are present, it is not necessary to invoke chemisorption to account for it.…”

“…This is the motivation of the work we present here. A series of DFT-based calculations were performed, 25,31 in the frame of several exchange-correlation (xc) functionals, namely, gradient-corrected GGA functionals (PBE 23 and BLYP 32,33 ) including semi-empirical corrections for dispersion forces, 34 and the hybrid parameter-free PBE0 functional. 35 More sophisticated descriptions of the dispersion forces within DFT have been developed, [36][37][38][39][40][41] but their use is still prohibitive for the model sizes we need to consider here.…”