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Symmetry and models of single-walled TiO2 nanotubes with rectangular morphology

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Evarestov R., Zhukovskii Y., Bandura A., Piskunov S.

Open Physics

2011

vol. 9

issue 2

492-501

The formalism of line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry has been applied for symmetry analysis of single-walled titania nanotubes (SW TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of anatase structure. Either six- or twelve-layer (101) slabs have been cut from TiO2 crystal in a stable anatase phase. After structural optimization, the latter keeps the centered rectangular symmetry of initial slab slightly compressed along a direction coincided with large sides of elemental rectangles. We have considered two sets of SW TiO2 NTs with optimized six- and twelve-layer structures, which possess chiralities (−n, n) and (n, n) of anatase nanotubes. To analyze the structural and electronic properties of titania slabs and nanotubes, we have performed their ab initio LCAO calculations, using the hybrid Hartree-Fock/Kohn-Sham exchange-correlation functional PBE0. The band gaps (Δɛ gap) and strain energies (E strain) of six-layer nanotubes have been computed and analyzed as functions of NT diameter (D NT). As to models of 12-layer SW TiO2 NTs of both chiralities, their optimization results in structural exfoliation, i.e., the multi-walled structure should be rather formed in nanotubes with such a number of atomic layers.

Simulations on the mechanism of CNT bundle growth upon smooth and nanostructured Ni as well as θ-Al2O3 catalysts

100%

Zhukovskii Y., Piskunov S., Kotomin E., Bellucci S.

Open Physics

2011

vol. 9

issue 2

530-541

In the current study, we have performed ab initio DFT calculations on the gradually growing 2D periodic models of capped single-wall carbon nanotubes (SW CNTs) upon their perpendicular junctions with the Ni(111) substrate, in order to understand the peculiarities of the initial stage of their growth on either smooth or nanostructured catalytic particles. Appearance of the adsorbed carbon atoms upon the substrate follows from the dissociation of CVD hydrocarbon molecules, e.g., CH4: (CH4)ads → (CH)ads+3Hads and (CH)ads → Cads+Hads. (Since the effective growth of CNTs upon Ni nanoparticles occur inside the nanopores of amorphous alumina, we have also simulated analogous surface reactions upon the θ-Al2O3(010) slabs). Association of the adsorbed carbon atoms upon the catalyst surface precedes further swelling of the (Cn)ads islands after appearance of pentagonal defects within a honeycomb sheet which are more probable upon the catalyst surface containing either defects or nanoclusters (as in the case of the nanostructured substrate). The gradual growth of the capped CNTs is considerably more effective upon the nanostructured Ni(111) substrate compared to a smooth nickel substrate (cf. values of CNT adhesion energy per boundary C atom for chiralities of either armchair-type, 4.04 vs. 2.51 eV, or zigzag-type, 4.61 vs. 2.14 eV, respectively). The electronic charge transfer from the Ni catalyst towards the CNTs has been calculated for both chiralities (> 1 e per C atom), i.e., quite strong chemical bonds are formed within the CNT/Ni(111) interconnects.

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2 Open Physics

2 2011

2 Piskunov S.

2 Zhukovskii Y.

1 Bandura A.

1 Bellucci S.

1 Evarestov R.

1 Kotomin E.

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Symmetry and models of single-walled TiO2 nanotubes with rectangular morphology

Simulations on the mechanism of CNT bundle growth upon smooth and nanostructured Ni as well as θ-Al2O3 catalysts