Energy minimization of single-walled titanium oxide nanotubes

Bandgap reduction of photocatalytic TiO2 nanotube by Cu doping

We performed the electronic structure calculations of Cu-doped TiO₂ nanotubes by using density functional theory aided by the Hubbard correction (DFT + U). Relative positions of the sub-bands due to the dopants in the band diagram are examined to see if they are properly located within the redox interval. The doping is found to tune the material to be a possible candidate for the photocatalyst by making the bandgap accommodated within the visible and infrared range of the solar spectrum. Among several possibilities of the dopant positions, we found that only the case with the dopant located at the center of nanotube seems preventing from electron-hole recombinations to achieve desired photocatalytic activity with n-type behavior.

Electronic and optical properties of TiO2 nanotubes and arrays: a first-principles study

Recently, the synthesis, properties, modifications, and applications of TiO2 nanomaterials have attracted much research attention. Here, based on extensive density functional theory calculations, we explored the stability, electronic structures and optical absorption properties of single-walled TiO2 nanotubes (SWTONTs) and TiO2 nanotube arrays (TONTAs), which are constructed from anatase TiO2(101) monolayers and bilayers, respectively. We obtained the stable Dnd (n = 3-5) and S2n(-n, n) (n = 3-9) SWTONTs, and found that SWTONTs energetically prefer S2n symmetry. Compared with S2n(-n, n) SWTONTs, the calculated Young's moduli of Dnd(-n, n) SWTONTs are more stiff due to their relatively large strain energies. The band gaps of hexagonal TONTAs are not sensitive to their apertures, which are less than that of TiO2 bilayers. The narrow band gaps of TONTAs originate from the edge states mainly contributed by the Ti and O atoms at the core region. The calculated optical absorptions of both SWTONTs and TONTAs display anisotropic features. These results clearly reveal that the electronic and optical properties of TiO2 nanostructures are strongly associated with their symmetry, dimensions and morphology, which provide useful insights into the understanding of the related experimental observations.

Synergistic modification of electronic and photocatalytic properties of TiO2 nanotubes by implantation of Au and N atoms

The structural and electronic properties of N-doped, Au-adsorbed, and Au/N co-implanted TiO2 nanotubes (NTs) were investigated by performing first-principle density functional theory (DFT) calculations. For all the possible implanted configurations, the radius and bond length do not change significantly relative to the clean NTs. Our results indicate that the introduction of N into NTs is in favor of implantation of Au, and Au pre-adsorption on the NTs can also enhance the N concentration in NTs. The synergistic stability can be mainly attributed to charge transfer between Au and N atoms. In co-implanted configurations, the empty N 2p states in the band gap are occupied by one electron; denoted by Au 5s states. Thus, the associated electron transition among the valence band, the conduction band and the gap states results in redshift of the light absorption. In addition, the disappearance of N 2p empty states can effectively decrease the photogenerated carrier combination. Therefore, the Au/N implanted NTs should be regarded as a promising photocatalytic material under the visible light region.

Monte Carlo simulation and free energies of mixed oxide nanoparticles

A Monte Carlo Exchange technique is used to study the thermodynamic properties of MgO-MnO nanoparticles ranging in size from 1728 to 21,952 ions. The solubility of Mg(2+) is much greater in MnO than the reverse, reflecting the difference in size between the two cations. The solubility, for a given temperature, diminishes with nanoparticle size. As the Mn concentration is progressively increased the Mn(2+) ions occupy the corners, edges and then surface sites of the nanoparticle before entering subsurface layers. We do not observe any pronounced ordering of the cations within the body of the nanoparticles themselves. The enthalpies of forming ternary nanoparticles from particles of MgO and MnO of the same size vary with the size of the nanoparticle and become more positive for a given concentration as the particle size increases. Free energies of mixing of the two end-member nanoparticles have been determined using the semigrand ensemble. The consolute temperature (the temperature above which there is complete miscibility) increases non-linearly with the size of the nanoparticle by approximately 70% over the size range considered.

Atomistic theory and simulation of the morphology and structure of ionic nanoparticles

Computational techniques are widely used to explore the structure and properties of nanomaterials. This review surveys the application of both quantum mechanical and force field based atomistic simulation methods to nanoparticles, with a particular focus on the methodologies available and the ways in which they can be utilised to study structure, phase stability and morphology. The main focus of this article is on partially ionic materials, from binary semiconductors through to mineral nanoparticles, with more detailed considered of three examples, namely titania, zinc sulphide and calcium carbonate.

Ab initio modeling of trititanate nanotubes

Nanotubes with H2Ti3O7 stoichiometry have been studied by ab initio methods in the diameter range of ∼20-60 Å. The distribution of the protons on the unit cell surface and its effect on the structure and electronic properties of the titanium dioxide backbone has been investigated. The preferred protonation pattern was found to be the function of the diameter of the nanotube and the convexity of the surface. Trititanate tubes have been found to be more stable than lepidocrocite nanotubes at all diameters. At diameters<50 Å the dissociation of water from the internal surface and the emergence of a new tube wall structure was observed. Band gaps were found to change only slightly with the curvature, the maximum difference from the flat slab values was ±0.5 eV.