First-Principles Evaluation of the Morphology of WS2 Nanotubes for Application as Visible-Light-Driven Water-Splitting Photocatalysts.

One-dimensional tungsten disulfide (WS2) single-walled nanotubes (NTs) with either achiral, i.e., armchair (n, n) and zigzag-type (n, 0), or chiral (2n, n) configuration with diameters d NT > 1.9 nm have been found to be suitable for photocatalytic applications, since their band gaps correspond to the frequency range of visible light between red and violet (1.5 eV < Δεgap < 2.6 eV). We have simulated the electronic structure of nanotubes with diameters up to 12.0 nm. The calculated top of the valence band and the bottom of the conduction band (εVB and εCB, respectively) have been properly aligned relatively to the oxidation (εO2/H2O) and reduction (εH2/H2O) potentials of water. Very narrow nanotubes (0.5 < d NT < 1.9 nm) are unsuitable for water splitting because the condition εVB < εO2/H2O < εH2/H2O < εCB does not hold. For nanotubes with d NT > 1.9 nm, the condition εVB < εO2/H2O < εH2/H2O < εCB is fulfilled. The values of εVB and εCB have been found to depend only on the diameter and not on the chirality index of the nanotube. The reported structural and electronic properties have been obtained from either hybrid density functional theory and Hartree-Fock linear combination of atomic orbitals calculations (using the HSE06 functional) or the linear augmented cylindrical waves density functional theory method. In addition to single-walled NTs, we have investigated a number of achiral double-walled (m, m)@(n, n) and (m, 0)@(n, 0) as well as triple-walled (l, l)@(m, m)@(n, n) and (l, 0)@(m, 0)@(n, 0) nanotubes. All multiwalled nanotubes show a common dependence of their band gap on the diameter of the inner nanotube, independent of chirality index and number of walls. This behavior of WS2 NTs allows the exploitation of the entire range of the visible spectrum by suitably tuning the band gap.

Water Adsorption on Clean and Defective Anatase TiO2 (001) Nanotube Surfaces: A Surface Science Approach.

We use ab initio molecular dynamics simulations to study the adsorption of thin water films with 1 and 2 ML coverage on anatase TiO2 (001) nanotubes. The nanotubes are modeled as 2D slabs, which consist of partially constrained and partially relaxed structural motifs from nanotubes. The effect of anion doping on the adsorption is investigated by substituting O atoms with N and S impurities on the nanotube slab surface. Due to strain-induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While the introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water, with a proton transfer from the water film and the formation of an NH group at the N site. At 2 ML coverage, a further surface-assisted proton transfer takes place in the water film, resulting in the formation of an OH- group and an NH2+ cationic site on the surface.

First-principles calculations on Fe-Pt nanoclusters of various morphologies.

Bimetallic FePt nanoparticles with L1 0 structure are attracting a lot of attention due to their high magnetocrystalline anisotropy and high coercivity what makes them potential material for storage of ultra-high density magnetic data. FePt nanoclusters are considered also as nanocatalysts for growth of carbon nanotubes of different chiralities. Using the DFT-LCAO CRYSTAL14 code, we have performed large-scale spin-polarized calculations on 19 different polyhedral structures of FePt nanoparticles in order to estimate which icosahedral or hcp-structured morphology is the energetically more preferable. Surface energy calculations of all aforementioned nanoparticles indicate that the global minimum corresponds to the nanocluster possessing the icosahedron "onion-like" structure and Fe43Pt104 morphology where the outer layer consists of Pt atoms. The presence of the Pt-enriched layer around FePt core explains high oxidation resistance and environmental stability, both observed experimentally.

First-principles calculations of oxygen interstitials in corundum: a site symmetry approach.

Using site symmetry analysis, four possible positions of interstitial oxygen atoms in the α-Al2O3 hexagonal structure have been identified and studied. First principles hybrid functional calculations of the relevant atomic and electronic structures for interstitial Oi atom insertion in these positions reveal differences in energies of ∼1.5 eV. This approach allows us to get the lowest energy configuration, avoiding time-consuming calculations. It is shown that the triplet oxygen atom is barrierless displaced towards the nearest regular oxygen ion, forming a singlet dumbbell (split interstitial) configuration with an energy gain of ∼2.5 eV. The charge and spatial structure of the dumbbell is discussed. Our results are important, in particular, for understanding the radiation properties and stability of α-Al2O3 and other oxide crystals.

Evidence for interfacial-storage anomaly in nanocomposites for lithium batteries from first-principles simulations.

We present theoretical support for a mass storage anomaly proposed for nanocomposites in the context of lithium batteries which forms the transition between an electrostatic capacitive mechanism and an electrode mechanism. Ab initio atomic and electronic structure calculations, performed on the Ti(0001)/Li2O(111) model interface, indicate the validity of the phenomenological model of interfacial Li storage and provide a deeper insight into the local situation. Beyond the specific applicability to storage devices, the possibility of a two-phase effect on mass storage generally highlights the availability of novel degrees of freedom in materials research when dealing with nanocomposites.