The adsorption of small gaseous molecules to the metal center in Pt-doped (5,5) single-walled carbon nanotubes has been explored within density functional theory. A model system consisting of a single Pt atom residing in the middle of a carbon nanotube with capping H atoms is used for our investigation. For all gases studied, the overall process of adsorption was found to be exothermic, where the affinity strongly depended on the orientation of the molecule. By examining the density of states and molecular orbitals of these nanotube−adsorbate complexes in comparison to the bare Pt-doped nanotube, we show that the electronic structure of these materials is strongly influenced by the presence of gases. Hence, we propose an application of Pt-doped single-walled carbon nanotubes as gas sensors and hope to motivate experimental work in this field.

In this work, we applied a two-layered ONIOM (B3LYP/6-31G(d):UFF) method to study the reaction of nitric oxides with a 5-1DB defect on the sidewall of the single-walled carbon nanotube (SWCNT). We have chosen a suitable ONIOM model for the calculation of the SWCNT based on the analyses of the frontier molecular orbitals, local density of states, and natural bond orbitals. Our calculations clearly indicate that the 5-1DB defect is the chemically active center of the SWCNT. In the reaction of nitric oxides with the defected SWCNT, the 5-1DB defect site can capture a nitrogen atom from nitric oxides, yielding the N-substitutionally doped SWCNT. We have explored the reaction pathway in detail. Our work verifies the chemical reactivity of the 5-1DB defects of the SWCNTs, indicates that the 5-1DB defect is a possible site for the functionalization of the SWCNTs, and demonstrates a possible way to fabricate position controllable substitutionally doped SWCNTs with a low doping concentration under mild conditions via some simple chemical reactions.

The ozonization at the vacancy defect site of the single-walled carbon nanotube has been studied by static quantum mechanics and atom-centered density matrix propagation based ab initio molecular dynamics within a two-layered ONIOM approach. Among five different reaction pathways at the vacancy defect, the reaction involving the unsaturated active carbon atom is the most probable pathway, where ozone undergoes fast dissociation at the active carbon atom at 300 K. Complementary to the experiments, our work provides a microscopic understanding of the ozonization at the vacancy defect site of the single-walled carbon nanotube.

The structures of the (5,5) single-walled carbon nanotube (SWCNT) segments with hemispheric carbon cages capped at the ends (SWCNT rod) and the Pt-doped SWCNT rods have been studied within density functional theory. Our theoretical studies find that the hemispheric cages introduce localized states on the caps. The cap-Pt-doped SWCNT rods can be utilized as sensors because of the sensitivity of the doped Pt atom. The Pt-doped SWCNT rods can also be used as catalysts, where the doped Pt atom serves as the enhanced and localized active center on the SWCNT. The adsorptions of C(2)H(4) and H(2) on the Pt atom in the Pt-doped SWCNT rods reveal different adsorption characteristics. The adsorption of C(2)H(4) on the Pt atom in all of the three Pt-doped SWCNT rods studied (cap-end-doped, cap-doped, and wall-doped) is physisorption with the strongest interaction occurring in the middle of the sidewall of the SWCNT. On the other hand, the adsorption of H(2) on the Pt atom at the sidewall of the SWCNT is chemisorption resulting in the decomposition of H(2), and the adsorption of H(2) at the hemispheric caps is physisorption.

Interactions between atomic Pt and pristine or Stone-Wales-defective (5,5) single-walled boron nitride nanotubes (BNNTs) were studied using density functional theory (DFT) with truncated nanotube models. The binding energy of Pt on a pristine BNNT is about 20 kcal/mol with little dependency on the binding site. On the other hand, when the Stone-Wales (SW) defect is presented, the atomic Pt is preferentially inserted between the B-B bond in the SW defect region with a large binding energy of 58 kcal/mol. On an SWdefective BNNT, the atomic Pt, even placed away from the defect site, may eventually (thermodynamically) move toward the defect area until being trapped between the B-B bond, and the final adduct has decreased reactivity toward both electrophiles and nucleophiles compared with Pt adsorption to pristine BNNTs. Pt adsorption on pure or SW-defective BNNTs makes the hosting nanotube wide-gap semiconductive by introducing the valence states of the absorbed Pt into the band gap of the nanotube. In comparison, the Pt atom filling into a B or N single vacancy on a BNNT changes the electronic structure of the vacancy-defective BNNT so dramatically that the Pt-doped BNNT becomes semiconducting with improved reactivity.

The structures, stabilities, and electronic properties of the singlevacancy-defected fullerenes, C 60 and C 70 , and the single-and double-vacancy-defected single-walled carbon nanotubes (SWCNTs) were studied within density functional theory. The isomerization barriers for the single-vacancy-defected C 60 on the triplet potential energy surface (PES) are lower than those on the singlet PES. The symmetric double-vacancy-defected (10,0) SWCNT is the most stable one among the models investigated. According to the analyses of frontier molecular orbitals (FMOs), nature bond orbitals, and local density of states, introduction of vacancy on the SWCNT decreases the band gap of semiconducting SWCNT, increases the band gap of conducting SWCNT, destructs the conjugation of the FMOs, and gives rise to enhanced chemical activity.Because of icosahedral (I h ) symmetry, all the carbon atoms are equivalent in C 60 . While in C 70 , there are five different types of symmetrically distinct carbon atoms, thus resulting in many possibilities for vacancy. For simplicity, we only considered the situation that the missing carbon atom is located in the pentagon at the pole of C 70 . Geometry optimizations were performed with the hybrid DFT method, B3LYP [36,37], with the standard 6-31G(d) basis set, and were followed by single-point calculations with B3LYP/6-311G(d). Natural bond or-LIU, TIAN, AND WANG 3442 INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY

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We have studied single-walled carbon nanotubes (SWCNTs) doped with transition metal (TM) atoms with both exo and endo doping configurations. The electronic and geometric properties of these TM-doped SWCNTs were calculated within density functional theory. It was found that the endo-doped SWCNTs are less stable than the exo-doped counterparts due to the large geometric strain of the deformation in the endo-doped nanotubes. On the basis of partial charge analysis, the TM-doped SWCNTs have localized net charge distributions, whereas the spin densities in Sc-, Co-, and Cu-doped SWCNTs are delocalized over the entire nanotubes. The TM-doped SWCNTs are mostly metallic or narrow-gap semiconductive. With highly localized frontier molecular orbitals, the exo-doped SWCNTs are better electron donors than the corresponding endo-doped systems. As the dopant TM changes from Sc to Zn in the same row of the periodic table or from the top to the bottom in the same Pt group, the energy of the highest occupied crystal orbital of the TM-doped SWCNTs decreases, indicating a reduced electron donating ability.