Oxygen Interaction With Carbon Nanotubes

Despite the importance of oxygen adsorption on SWNTs the theoretical work existing in the literature about this interaction is controversial and can be briefly summarized in the next few paragraphs.

Jhi et al. combining pseudopotentials together with local density approximation (LDA) and local spin density approximation studied the oxygenation of an (8,0) SWNT [43]. They found that O2 binds to a bridge position between to carbon atoms of the tube with adsorption energy of 0.25 eV and a distance of 2.7 A. Nevertheless the O-O distance was kept frozen.

Zhu et al. performed tight binding (TB) DFT calculations to study the adsorption and desorption of an oxygen molecule to the edges of a (5,5) and a (9,0) SWNT [44]. They found that an O2 molecule arriving at the tube wall edges will not adsorb on the wall but instead will glide into the edges with a binding energy of 4.61 eV [44, 45].

Sorescu et al. using pseudopotential DFT studied the adsorption of oxygen atom and molecules on an (8,0) SWNT [46]. The atomic O performed epoxide-like structures with adsorption energy up to 44 kcal/mol, while the molecular O2

is predicted to weakly physisorb (0.9 kcal/mol) to the SWNT surface. It is also found to perform unstable chemisorbed species 10.2 kcal/mol above the reactants [46].

Park et al. using the pseudopotential LDA method investigated atomic and molecular oxygen adsorption in the fullerene type caps of a (5,5) SWNT [47]. They observed epoxide-type bridge positions for the atomic oxygen with binding energies of 6.6 to 6.7 eV and square-type four-center geometries for the binding of the O2 molecule with binding energies of 1.66 to 3.02 eV [47].

Even though all the references presented [43-47] use methods based on similar approaches like pseudopotential or TB, a big variety of binding energies is reported, as is also pointed out by Park et al. [47]. On top of this the question whether the molecular oxygen reacts exothermically with carbon nanotubes remains unanswered.

6.1. Application of the Cluster Model

In order to answer this question we performed a fully ab initio study of the atomic and the molecular oxygen adsorption to a carbon nanotube [48]. We use the DFT method in the cluster approximation. A large enough part of an armchair (4,4) SWNT containing 56 carbon atoms is separated and treated as an individual system. The dangling bonds at the end of the tube are saturated by hydrogen atoms (Fig. 1c).

The RI-DFT as implemented in the TURBOMOLE program package in combination with an auxiliary basis set was used for studding the atomic and molecular hydrogen interaction with the 56-atom SWNT [49]. The efficiency and the computational speed of the methodology we use made the geometry optimization of a C56H16O2 system possible in a Linux personal computer.

6.2. Atomic Oxygen Interaction with Carbon Nanotubes

In the beginning we investigated the atomic oxygen adsorption on SWNTs [48]. As can be seen in Figure 9 there are only two stable positions. These positions are epoxide-like with the oxygen bridging two neighboring carbon atoms of the tube. All other possibilities of the O sitting on top of a single carbon atom or in the center of a C hexagon are unstable and relax to the bridge positions after geometry optimization. The binding energy for the 9a isomer is

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