Search

For the first time, the dependences of the strain energy and the band gap of achiral nanotubes obtained by folding monolayers of gallium(II) sulfides and selenides on their diameter have been calculated nonempirically. The calculations were performed using the CRYSTAL17 software package using an atomic basis set within the hybrid density functional theory with a thirteen-percent Hartree–Fock exchange. To take into account the dispersion interactions between the layers in the crystal, the Grimme empirical correction is included in the calculations. As a result of simulation of nanotubes with different chirality and different diameters, the minimum diameters of single-walled nanotubes, at which the integrity of chemical bonds on their outer surface is preserved, are established. It is shown that the dependence of the strain energy on the diameter satisfies the classical law of inverse squares and is the same for nanotubes of the zigzag and armchair types.

Comparative hybrid density functional calculations on the structure, stability, and phonon frequencies of monolayers and single-walled nanotubes are performed for Zr(Hf)S2 disulfides. The first-principles calculations of HfS2 -based nanotubes are made for the first time. The symmetry analysis of infrared and Raman active vibrational modes in ZrS2 and HfS2 nanotubes is made using the induced representations of the isogonal point groups of line groups. It is shown that the number of infrared and Raman active modes is constant for NTs with the same chirality type. The correlation of the phonon modes of the nanotubes of relatively large diameters with those of monolayer is analyzed. The thermodynamic functions of monolayers and nanotubes with various chirality and diameters are calculated on the basis of the obtained phonon frequencies. It is established that the phonon contribution to the nanotube strain energy is small, but may be important for an accurate estimate of the stability of the nanotubes of small diameters. The calculated results show that the thermal contributions to Helmholtz free energy are positive; thereby they slightly reduce the stability of ZrS2 and HfS2 nanotubes at elevated temperatures. © 2019 Wiley Periodicals, Inc.

We used hybrid density functional theory to investigate 50% cubic solid solutions based on Fe-doped LaGaO3 and LaAlO3. The supercell composed of 8 primitive cells was used. All the possible configu...

In this paper we report the classification of all the 81 magnetic line group families into seven spin splitting prototypes, in analogy to the similar classification previously reported for the 1651 magnetic space groups, 528 magnetic layer groups, and 394 magnetic rod groups. According to this classification, electrically induced (Pekar–Rashba) spin splitting is possible in the antiferromagnetic structures described by magnetic line groups of type I (no anti-unitary operations) and III, both in the presence and in the absence of the space inversion operation. As a specific example, a group theoretical analysis of spin splitting in CoO (8, 8) nanotube is carried out and its predictions are confirmed by ab initio density functional theory calculations.

A force field is proposed that reproduces with a high accuracy a large number of properties of the bulk crystal MoS2 phases, monolayers, and nanotubes. The reproduced values are both the experimental results and the results of quantum chemical calculations. The elaborated interaction potential can be useful primarily for investigation of multiwall MoS2 nanotubes and their thermodynamic properties, especially, since the potential is able to reproduce the frequencies of the crystal phonon spectrum. In this study the proposed potential is applied to simulate the temperature dependence of a number of properties of the armchair and zigzag nanotubes. The calculations have been performed using molecular mechanics method within the framework of quasi harmonic approximation which is carried out through the estimation of the temperature dependence of the Helmholtz free energy.

A new analytical expression was derived for the chemical potential of a hard sphere dipole in hard sphere dipole fluid at infinite dilution of the solute using the mean spherical approximation (MSA). A set of Monte Carlo (MC) simulations has been carried out to investigate the scope of applicability of the derived equation. The mean reaction field (MRF) approach was used in our MC computations. Two different MC methods (Widom particle insertion and thermodynamic integration) were applied for obtaining the chemical potential change associated with the dipole creation at the solute particle to provide adequate accuracy of the MC simulations. Also, corresponding changes in the mean potential energy were calculated by direct method and by thermodynamic integration. The solvation energies have been obtained for the systems of dipolar hard spheres with reduced dipole moment 1.0 at the reduced densities 0.2, 0.5, and 0.8. Computations have been made for solute particles with the reduced dipole moment varied from 0.0 to 1.5 and the hard sphere diameter varied from 0.5 to 2.0. The variation of those quantities with the molecular parameters was analyzed and compared with the MSA equation and Kirkwood classical expressions. It was found that the MSA calculations agree relatively well with MC simulations at densities less than 0.5 and solute dipole moment less than 1.0.

An increasing number of industrial applications rely on controlling solutes in water above and below its critical point. Processes such as hydrothermal synthesis, steam power generation and ultra-high enthalpy geothermal power are all influenced by factors such as mineral precipitation, pH and solute speciation. The supercritical point of water is remarkable in that slight changes in temperature and pressure can cause dramatic changes in some solute properties. Here, it was found that our approach reliant on molecular statistical thermodynamic expressions for hard sphere (HS), ion-dipole and dipole-dipole interactions via mean spherical approximation (MSA) provided excellent agreement to available experimental data. In addition to model parameters having some physical meaning, this approach used less adjustable parameters than the well-known Helgeson-Kirkham-Flowers (HKF) model. Furthermore, the model was used to obtain standard thermodynamic values for HCl0(aq), KCl0(aq) and NaOH0(aq) ion pairs. In total, modeling parameters for 10 different aqueous species were obtained to demonstrate the capabilities of the approach.

The possibility to use the axial point group dynamical representations for the infrared and Raman active modes classification in nanotubes is analyzed. The method proposed allows one to obtain the results of phonon symmetry analysis for nanotubes in Mulliken designations, which are traditional for molecules and crystallographic point groups. The approach suggested is applied to the phonon symmetry analysis in the single-wall carbon and MoS2 -based nanotubes. First-principles calculations of phonons in a bulk MoS2 crystal and a monolayer S-Mo-S are made. The results obtained are in reasonable agreement with the existing experimental data and other published results. The first-principles calculations of the phonon frequencies for armchair and zigzag MoS2 nanotubes are performed for the first time. It is shown that the number of infrared and Raman active modes becomes fixed starting from the relatively small nanotube diameters. The correlation of the phonon modes of MoS2 nanotubes with diameters up to 3.64 nm with phonon modes of the S-Mo-S monolayer is analyzed. It is demonstrated that the interpretation of the nature of nanotube A-type modes in the crystallographic factorization of the line group L = TF is the same as for m = 0 modes in the "polymer type" factorization L = ZP where P is the subgroup of the isogonal point group F, T is the translation subgroup of line group and the cyclic group Z includes the one-dimensional translations and the rotations around the screw axes or the reflections in the glide planes. © 2018 Wiley Periodicals, Inc.

MoS2 nanostructures, especially mono-, multilayer nanothin films as well as single- and multiwall nanotubes are rather interesting popular objects in nanomaterials chemistry. The thermodynamic properties of inorganic nanotubes, and the temperature dependence of their properties can be efficiently investigated by first-principles and molecular mechanics methods in the framework of harmonic approximation. At the same time, only thin single-wall nanotubes are available for the first-principles calculations. The classical mechanics is suitable to simulate very large atomic systems and their phonon frequencies, but developing sufficiently accurate force field is rather tedious work. Herein, we report the force field fitted to the experimental and first-principles data on the structure of 2H- and 3RMoS2 polytypes of bulk crystal, structure of monolayer and several bilayers, vibrational frequencies of 2HMoS2 bulk and monolayer, relative energetic stability of polytypes experimental and first-principles data, elastic constants, strain energy of a (12, 12) MoS2 nanotube. The thermodynamic functions and their temperature dependence for the armchair and zigzag nanotubes are calculated within the formalism of molecular mechanics using elaborated interatomic potential. The results of molecular mechanics and first-principles method application to the thinnest nanotubes are compared.

For the first time, the structural and stability properties of multi-walled GaS, GaTe, and Janus Ga2STe nanotubes have been simulated using molecular-mechanics calculations. The armchair and zigzag nanotubes with different wall numbers and diameters have been considered. The force field used for GaS, GaTe and Ga2STe multi-walled nanotubes is based on the interaction potentials recently developed to describe monolayers of the transition and post-transition metal chalcogenides. The Buckingham potentials have been introduced to model the interlayer and interwall interactions. The results of quantum mechanical calculations for a number of bulk crystals, mono and bilayers, as well as small diameter single-wall nanotubes have been employed to check the validity of the proposed force field. With the help of an adapted force field, large eight-walled nanotubes with an outer diameter of 32 nm were simulated, which at present cannot be studied using the first principles of quantum mechanical methods. Molecular mechanics simulations predict the faceted structure of armchair GaS, GaTe, and Ga2STe multi-walled nanotubes with a sufficiently large diameter and a cylindrical structure for all zigzag nanotubes. The Janus nanotubes' stability is superior to that of binary nanotubes and may probably achieve a maximum at certain wall numbers and diameters.

Adsorption of water on (001)-terminated α-V2O5 surface has been investigated in the framework of density functional theory. According to our calculations, molecular adsorption is preferred, and two types of adsorption sites (Lewis acid center on V atom and Lewis base center on O atoms) have been found. Water adsorption on Lewis acid site does not change the electronic structure of V2O5 surface. Adsorption on Lewis base site is less favorable but reduces the band gap value by half (from 2.5 to 1.3 eV). The reason of this decreasing is the localization of the non-bonding 1b2 electron state of water molecule within the forbidden gap of V2O5 surface. The idealized structure of V2O5∙nH2O xerogel has been computationally studied at n = 0 and 1. It is proved that only Lewis base sites are suitable for water positions. At n = 1, the water absorption energy in xerogel is close to adsorption energy of water on the oxygen atoms of α-V2O5 surface. The presence of the non-bonding states of physically absorbed water in the forbidden gap is also found in the xerogel case. This results in decreasing of the band gap from 2.7 eV to 0.7 eV.

Hybrid density functional theory calculations are performed for the first time on the phonon dispersion and thermodynamic properties of WS2-based single-wall nanotubes. Symmetry analysis is presented for phonon modes in nanotubes using the standard (crystallographic) factorization for line groups. Symmetry and the number of infra-red and Raman active modes in achiral WS2 nanotubes are given for armchair and zigzag chiralities. It is demonstrated that a number of infrared and Raman active modes is independent on the nanotube diameter. The zone-folding approach is applied to find out an impact of curvature on electron and phonon band structure of nanotubes rolled up from the monolayer. Phonon frequencies obtained both for layers and nanotubes are used to compute the thermal contributions to their thermodynamic functions. The temperature dependences of energy, entropy, and heat capacity of nanotubes are estimated with respect to those of the monolayer. The role of phonons in the stability estimation of nanotubes is discussed based on Helmholtz free energy calculations. © 2017 Wiley Periodicals, Inc.

Atomistic model has been proposed to describe the structure, phonon frequencies, and thermodynamic properties of multi-walled nanotubes based on molybdenum disulfide.

Comparative hybrid density functional calculations on the structure, stability, and phonon frequencies of monolayers and single-walled nanotubes are performed for Zr(Hf)S

A zone-folding approach is applied for the estimation of phonon contributions to the thermodynamic properties of carbon nanotubes and nanotubes based on transition metal oxides (TiO2, V2O5) and sulfides (TiS2, ZrS2) with different morphology and various chiralities. The results obtained are compared with those from the direct calculation of the thermodynamic properties of nanotubes in the harmonic approximation. All calculations have been made using the PBE0 hybrid exchange–correlation functional. It is found that the zone-folding approach allows a sufficiently accurate estimation of phonon contributions to internal energy and heat capacity and shows worse but acceptable results for Helmholtz free energy and entropy.

Using the laser ablation method, films comprised of alternating layers of AgI and (GeSe 2 ) 30 (Sb 2 Se 3 ) 30 (AgI) 40 glass were obtained. Individual layer thickness amounts to 10 ÷ 15 nm, and the total number of layers is about 100. X-ray diffraction (XRD) and film conductivity measurements were carried out during several cycles of heating up to 200 °C and cooling to room temperature. It was established that after three cycles of thermal processing specific lateral conductivity of the film is equal to 0.3 S cm − 1 and conductivity activation energy is equal to 0.07 eV at room temperature. Attempts to explain such a high conductivity value based on XRD results did not yield satisfactory results. However, our first-principle calculations within the density functional theory (DFT) showed that in the free layer composed of four AgI planes a rearrangement occurs, resulting in formation of the stable structure of two silver planes on the inside and two iodine planes on the outside (I–Ag–Ag–I). Rearrangement of similar stack of eight or twelve atomic planes results in formation of two or three I–Ag–Ag–I layers loosely bound to each other, accordingly. This suggests that increase in specific conductivity growth of multilayer film as a consequence of cyclic heating and cooling may be connected with AgI stratification on its boundary with chalcogenide glass and following stabilization of layered phases mentioned above. The existence of an empty space between the layers that is constrained by iodine ion planes should facilitate silver ion diffusion along the layers.

We employed first-principles simulations using a hybrid exchange-correlation density functional PBE0 within an LCAO approximation to investigate the properties of InS single layers and nanotubes constructed from its stable orthorhombic and hypothetical hexagonal phases. We have found two types of 4-plane layers with relatively low formation energy, Rec4 and Hex4, which have been extracted from the orthorhombic and hexagonal phases, respectively. By rolling up Rec4 and Hex4 layers, the initial structures of single- and double-walled nanotubes have been generated. The nanotube formation and strain energies calculated after atomic relaxation show that the most stable structures can be obtained from the rectangular Rec4 nanosheets. At the same time, the double-walled nanotubes folded from the Rec4 nanosheets may be potentially useful for photocatalytic water splitting if they can really be synthesized.

Results of carbon nanoribbons and nanotubes simulation by means of hybrid density functional method and using empirical potentials have been compared. Energy of the nanoribbons formation and their citting from graphene sheet as well as energy of the nanotubes folding from graphene and nanoribbons have been determined. The REBO force field satisfactorily reflects the result of quantum chemical simulations; however, it cannot reproduce the formation of triple bonds between the edge atoms of the nanoribbons in the armchair conformation and thus leads to underestimated stability of the latter. Energy of the nanotubes folding from the nanoribbons is linear with the nanotube diameter.

The properties of multi-walled MoS2 nanotubes have been investigated by the first principles calculations and by molecular mechanics (MM) simulations using a revised three-body force field. The density functional theory (DFT) calculations have been performed on single-, double- and triple-walled MoS2 nanotubes. The new version of the force field is able to reproduce the structure integrity of the MoS2 nanotubes at temperatures up to 700 K through the molecular dynamics simulations. Comparison of the results of first principles and MM simulations of the multi-walled nanotubes demonstrates satisfactory agreement. The results of DFT and MM simulations indicate that the difference between chirality indices of adjacent shells of a multi-walled nanotube is the main factor that determines a possibility of the nanotube to be synthesized. The structure of zigzag 12-walled nanotubes with chirality indices difference 12 and 13, simulated by MM method and using the proposed force field, is the most close to the structure of experimentally detected nanotubes.

Calculations of the cubic models of two solid solutions LaGa0.5Fe0.5O3 and LaAl0.5Fe0.5O3 have been performed within the hybrid density functional theory. Multiple configurations have been considered for the solid solutions resulting from a different distribution of Fe atoms over the p-metal (Ga or Al) positions accounting for different spin orientation for Fe atoms themselves. It was demonstrated that 27 structures for both LaGa0.5Fe0.5O3 and LaAl0.5Fe0.5O3 should be treated to account for all the possible configurations in case of cubic 2 × 2 × 2 supercell. Optimized geometry, energy, and electron properties were calculated for all the obtained configurations. Statistical weights and probabilities were estimated for each symmetry non-equivalent configuration of the solid solutions within the canonical ensemble. A new parameterization for the dilute Ising model has been proposed. In this model, we account for non-magnetic contributions, which are absent in the simple Ising model, using the lattice approach based on the concept of interchange energy. Two model parameters (the magnetic coupling constant and interchange energy) were fitted to the calculated total energies of all considered configurations of both solid solutions. The dilute Ising model confirmed the benefit of Fe-clustering in doped lanthanum gallate against aluminate. Different signs of the estimated interchange energy enable us to explain the reasons for such differences.

MoS

Temperature dependence of the Young’s modulus of cylindrical single-wall nanotubes with zigzag and armchair chiralities and consolidated-wall nanotubes has been studied by the molecular mechanics method with the use of the atom–atom potential. The nanotubes have been obtained by rolling up of crystal layers (111) of TiO2 with fluorite structure. Calculations have been performed for isothermal conditions on the basis of calculating the Helmholtz free energy of the system. The dependence of the Helmholtz free energy of nanotubes on the period has been calculated in the quasi-harmonic approximation as a result of calculation of phonon frequencies. It has been shown that the temperature dependence of the stiffness of nanotubes is determined by their chirality, and some nanotubes exibit anomalous behavior of both the Young’s modulus and the period of unit cell with variation in temperature.

Hybrid density functional method PBE0 which mixes the 75% Perdew–Burke–Ernzerhof and 25% Hartree–Fock exchange functional has been applied for investigation of the electronic and atomic structures of nanotubes obtained by rolling up of hexagonal layers of tin disulfide. Calculations have been performed on the basis of the localized atomic functions by means of the CRYSTAL09 computer code. The calculated strain energy of SnS2 single-wall nanotubes approximately obeys the R− 2 law (R is nanotube radius) of the classical elasticity theory. The SnS2 nanotube electronic band structures yield a semiconducting behavior. Band gap of single-wall nanotubes decreases linearly with R− 1. The dispersion force correction is found to be important for prediction of the multi-wall nanotube stability. The distance and interaction energy between the single-wall components of the double-wall nanotubes are proved to be close to the distance and interaction energy between layers in the bulk crystal. Analysis of the relaxed nanotube shape using the offered method demonstrates a small but noticeable deviation from completely cylindrical cross-section of the external walls in the armchair-like double- and triple-walled nanotubes.

A zone-folding (ZF) approach is applied for the estimation of the phonon contributions to thermodynamic properties of carbon-and ZrS2 -based nanotubes (NTs) of hexagonal morphology with different chiralities. The results obtained are compared with those from the direct calculation of the thermodynamic properties of NTs using PBE0 hybrid exchange-correlation functional. The phonon contribution to the stability of NTs proved to be negligible for the internal energy and small for the Helmholtz free energy. It is found that the ZF approach allows us an accurate estimation of phonon contributions to internal energy, but slightly overestimates the phonon contributions to entropy. © 2015 Wiley Periodicals, Inc.

Adsorption of Rb+ to the quartz(101)–aqueous interface at room temperature was studied with specular X-ray reflectivity, resonant anomalous X-ray reflectivity, and density functional theory. The interfacial water structures observed in deionized water and 10 mM RbCl solution at pH 9.8 were similar, having a first water layer at height of 1.7 ± 0.1 A above the quartz surface and a second layer at 4.8 ± 0.1 A and 3.9 ± 0.8 A for the water and RbCl solutions, respectively. The adsorbed Rb+ distribution is broad and consists of presumed inner-sphere (IS) and outer-sphere (OS) complexes at heights of 1.8 ± 0.1 and 6.4 ± 1.0 A, respectively. Projector-augmented planewave density functional theory (DFT) calculations of potential configurations for neutral and negatively charged quartz(101) surfaces at pH 7 and 12, respectively, reveal a water structure in agreement with experimental results. These DFT calculations also show differences in adsorbed speciation of Rb+ between these two conditions. At pH 7, the lowe...

The analytical relation between k-points in the primitive-cell Brillouin zone and reduced supercell Brillouin zone is reported for supercell-zone-folding transformation. Examples are given for symmetry points of square and cubic simple, face-centered and body-centered lattices. The cyclic cluster symmetry is considered as a particular case of supercell-zone-folding transformation. The results of the first principles calculations of LiCl crystal in the supercell model as well as the symmetry of one-electron states are discussed using the supercell-zone-folding concept. The first principles calculations of copper impurity in LiCl crystal are made using different supercells. The site symmetry method is applied to find the space group representations induced by Cl p-states and copper d-states. The zone-folding transformation for the two-dimensional layer unit cell is considered in relation to the single wall nanotubes modeling. The results of zone-folding method application to electron states of WS2-based nanotubes and to phonon calculations of carbon and ZrS2-based nanotubes are presented and discussed.

First principles periodic calculations based on the density functional theory within the localized atomic orbital approach (DFT-LCAO) using the hybrid exchange–correlation potential PBE0 have been performed in order to simulate the structural and electronic properties of both stoichiometric and nonstoichiometric [001]-oriented four-faceted SrTiO3 (STO) nanowires (NW) of cubic structure. Their diameters have been varied from 0.3 up to 2.4 nm with a corresponding consequent change of NW cross-section from 2 � 2t o 5� 5 extension of the lattice constant in bulk. Energetic stability of STO NW (both stoichiometric and non-stoichiometric) has been found to increase with the decrease of their formation energies together with the increase of NW diameter. The electronic structure calculations have shown that the width of the band gap changes in STO NWs of different structural types as compared to that in bulk being consequently reduced with the growth of NW diameter although the character of such a decrease depends on the morphology of the nanowire. Analysis of these changes shows that stoichiometric and non-stoichiometric TiO2-terminated strontium titanate nanowires can be quite promising candidates for further applications in photocatalytic processes under solar irradiation whereas SrO-terminated NWs are rather not suitable for this purpose.

A large-scale first-principles simulation of the structure and stability of SrZrO 3 and BaZrO 3 single- and double-wall nanotubes with different chiralities and diameters was performed using the periodic PBE0 method and the basis set of localized Gaussian-type atomic orbitals. The initial structures of nanotubes were obtained by the rolling up of slabs cut from perovskite bulk phases and consisting of two or four alternating (0 0 1) ZrO 2 , SrO or BaO layers. Significant structural reconstruction was found in 4-layer singe-walled and in double-walled nanotubes. If the distance between the single-wall components of double-walled nanotubes is less than approximately 5.0 A, they inclined to merge to stable polyhedron-shaped tubular objects consisting of blocks with distorted cubic perovskite structure. A comparison of the data obtained with the results of our previous works shows that the stability of perovskite nanotubes with merged walls increases in the following sequence of the parent phases: SrZrO 3 3 ≈ SrTiO 3 3 . Calculated stability correlates with a ratio R II / R IV of ionic radii of group II and IV metals.

We have performed first-principles calculations to study the atomic and electronic structures of single wall nanotubes (NTs) of two possible chirality types rolled up from monolayers of α- and γ-V2O5 phases. We have used a hybrid exchange–correlation PBE0 functional within density functional theory and a basis set of localized atomic orbitals. A dispersion correction has been taken into account. All the lattice parameters and atomic positions have been totally optimized. The strain energies calculated for the nanotubes folded from the layers of both phases along the [100] direction are close to zero. This reflects the unique flexibility of the layers for folding in the [100] direction. The electronic structure of the nanotubes of both phases appeared to be similar to that of the parent layer. It was found that for both considered phases, the nanotubes of the same chirality are energetically equivalent but the shape of γ-NTs is closer to the cylindrical form than that of α-NTs. Young's moduli calculated for (6,0) α- and γ-NTs were found to be 172 GPa and 148 GPa, respectively. The phonon mode frequencies of (6,0) α- and γ-NTs have been calculated and compared with those of α- and γ-V2O5 free layers.

Hybrid density functional theory calculations are performed for the first time on the phonon dispersion and thermodynamic properties of WS

Hybrid density functional theory calculations were performed for the first time on structure, stability, phonon frequencies, and thermodynamic functions of hafnia-based single-wall nanotubes. The nanotubes were rolled up from the thin free layers of cubic and tetragonal phases of HfO2 . It was shown that the most stable HfO2 single-wall nanotubes can be obtained from hexagonal (111) layer of the cubic phase. Phonon frequencies have been calculated for different HfO2 nanolayers and nanotubes to prove the local stability and to find the thermal contributions to their thermodynamic functions. The role of phonons in stability of nanotubes seems to be negligible for the internal energy and noticeable for the Helmholtz free energy. Zone folding approach has been applied to estimate the connection between phonon modes of the layer and nanotubes and to approximate the nanotube thermodynamic properties. It is found that the zone-folding approximation is sufficiently accurate for heat capacity, but less accurate for entropy. The comparison has been done between the properties of TiO2 , ZrO2 , and HfO2 . © 2017 Wiley Periodicals, Inc.

Hybrid density functional theory calculations are performed for the first time to compare the stability, structural and electronic properties of monolayers and single-wall nanotubes based on pure Mo(W)S(Se)2 and mixed (Janus) Mo(W)SSe dichalcogenides. The stability, structural and electronic properties of Mo and W dichalcogenide nanotubes have been compared at different wall compositions, chiralities and diameters using the same calculation scheme. Different types of mixed nanotubes are considered – with S or Se atoms on the outer (inner) shell of the nanotube. It was found that nanotubes Se(out)WS(in) with average diameter (Davr) greater than ≈40 A have the negative strain energy. Our calculations show that the band gap is direct for zigzag MS2 and S(out)MSe(in) nanotubes (M = Mo, W) but it becomes indirect in armchair nanotubes. For the MSe2 and Se(out)MS(in) nanotubes of both chiralities, the band gap is mostly direct, except the armchair tubes with Davr

We have performed first-principles calculations of the atomic and electronic structures and stability of three (α, β and γ) layered V 2 O 5 phases and their free layers within the same computational approach. In computations, we have used a hybrid exchange–correlation functional within the density functional theory and a basis set of localized atomic orbitals. All the lattice parameters and the atomic positions have been totally optimized and the phonon frequencies at the Γ-point of the Brillouin zone have been obtained. The calculated relative stability of considered bulk phases decreases as α > γ > β. The calculated relative stability of layers differs from that of bulk phases and decreases as β > α ⩾ γ. The possibility of the folding of V 2 O 5 layers into nanotubes is discussed.

Hybrid density functional theory has been applied for investigations of the electronic and atomic structure of bulk phases, nanolayers, and nanotubes based on titanium and zirconium disulfides. Calculations have been performed on the basis of the localized atomic functions by means of the CRYSTAL-2009 computer code. The full optimization of all atomic positions in the regarded systems has been made to study the atomic relaxation and to determine the most favorable structures. The different layered and isotropic bulk phases have been considered as the possible precursors of the nanotubes. Calculations on single-walled TiS2 and ZrS2 nanotubes confirmed that the nanotubes obtained by rolling up the hexagonal crystalline layers with octahedral 1T morphology are the most stable. The strain energy of TiS2 and ZrS2 nanotubes is small, does not depend on the tube chirality, and approximately obeys to D(-2) law (D is nanotube diameter) of the classical elasticity theory. It is greater than the strain energy of the similar TiO2 and ZrO2 nanotubes; however, the formation energy of the disulfide nanotubes is considerably less than the formation energy of the dioxide nanotubes. The distance and interaction energy between the single-wall components of the double-wall nanotubes is proved to be close to the distance and interaction energy between layers in the layered crystals. Analysis of the relaxed nanotube shape using radial coordinate of the metal atoms demonstrates a small but noticeable deviation from completely cylindrical cross-section of the external walls in the armchair-like double-wall nanotubes.

This paper reports the results of a classical molecular dynamics (CMD) study of molecular water adsorption on MO2- and SrO-terminated SrMO3 (001) surfaces (M = Ti, Zr) at 300 K with ½ ML and 1 ML coverage. Models of the force fields for the water–crystalline oxide interfaces have been proposed. These force fields describe the oxide–oxide, water–oxide and water–water interactions, as well as interactions within the water molecule itself. The water–water and flexible water intramolecular potentials have been adopted from Toukan and Rahman (Phys. Rev. B 31 (1985) 2643–2648). The results of CMD simulations of the structure of the water layers at the oxide surfaces are discussed in terms of the most probable configurations. The power spectra of the water hydrogen velocity autocorrelation functions have been calculated using the CMD trajectories. The calculated power spectra permit us to consider the vibrational modes of the different structural species formed by the water molecules at the oxide surfaces and to analyse the level of binding of these species, both between themselves and to the surface.

The set of three pair atom–atom potentials for zirconia crystals and nano objects was developed. Besides Buckingham potential and Coulomb interaction, these force fields employ the additional functional forms: Fermi-Dirac, Inverse Gaussian or Morse potential. The developed force fields are capable to reproduce the structural, mechanical and thermodynamic properties of five zirconia phases: monoclinic, tetragonal, cubic, brookite and cotunnite with acceptable accuracy. The proposed force fields give monoclinic phase as a ground state structure and predict the correct energy ordering of the phases. The Raman and IR phonon frequencies, temperature dependencies of the thermodynamic properties of monoclinic and tetragonal zirconia are predicted in good and moderate agreement with the experimentally observed data and results of first-principles calculations. To test the transferability of proposed potentials, the zirconia-based nanosheets and nanotubes were simulated. Results of structure optimisation and calculation of the nanotubes’ strain energies are well compared with the corresponding data of the first-principles calculations.

The first-principles calculations of the atomic and electronic structure of double-walled nanotubes (DWNTs) of γ-V2O5 have been performed and the obtained properties have been compared with those of α-V2O5 ones. The DWNT structure relaxation leads to the formation of two types of local regions: (1) adhesion regions and (2) puckering regions. Although the structure of adhesion regions of α-V2O5 DWNTs is close to the structure of bulk α-V2O5, this is not the case for γ-V2O5 DWNTs. The resulting structure of adhesion regions in γ-V2O5 SWNTs allows us to assume the existence of hypothetical stable phases, with one of them resembling the experimentally observed R-Nb2O5 and (V0.7Mo0.3)2O5 crystals.

We employed first-principles simulations using hybrid exchange–correlation density functional PBE0 within LCAO approximation to investigate the properties of ZrO2 nanotubes constructed from the different zirconia polymorphs, in an attempt to understand the relation between the bulk zirconia atomic structure and the stability of zirconia nanotubes. For the rolling of nanotubes we used the relaxed or reconstructed thin slabs obtained by cleaving cubic, tetragonal and monoclinic zirconia parallel to all possible low-index faces. The structure of nanotubes has been optimized with respect to unit cell constant and intracell degrees of freedom consistent with the initial rotohelical symmetry. The calculated nanotube formation and strain energies show that the most stable tubes with thin (one ZrO2 layer) walls originated from the hexagonal (1 1 1) nanosheets of the cubic fluorite phase. The tubes with walls composed of two ZrO2 layers, most likely have lepidocrocite morphology. The tubes with thicker walls can possess a different structure originating from cubic, tetragonal or orthorhombic phases. The comparison of similar zirconia and titania nanotubes provides evidence that zirconia nanotubes are more stable relative to the precursor nanosheets, while titania nanotubes are more stable relative to the parent bulk phases.

A novel mechanism for protonating bridging O atoms (Obr) and dissolving silica is proposed that is consistent with experimental data and quantum mechanical simulations of the α-quartz (101)/water i...

The calculations based on the linear combination of atomic orbitals have been performed for the low-temperature phase of BaTiO(3) crystal. Structural and electronic properties, as well as phonon frequencies were obtained using hybrid PBE0 exchange-correlation functional. The calculated frequencies and total energies at different volumes have been used to determine the equation of state and thermal contribution to the Helmholtz free energy within the quasiharmonic approximation. For the first time, the bulk modulus, volume thermal expansion coefficient, heat capacity, and Grüneisen parameters in BaTiO(3) rhombohedral phase have been estimated at zero pressure and temperatures form 0 to 200 K, based on the results of first-principles calculations. Empirical equation has been proposed to reproduce the temperature dependence of the calculated quantities. The agreement between the theoretical and experimental thermodynamic properties was found to be satisfactory.

The calculations based on linear combination of atomic orbitals basis functions as implemented in CRYSTAL09 computer code have been performed for cubic, tetragonal, orthorhombic, and rhombohedral modifications of BaTiO(3) crystal. Structural and electronic properties as well as phonon frequencies were obtained using local density approximation, generalized gradient approximation, and hybrid exchange-correlation density functional theory (DFT) functionals for four stable phases of BaTiO(3). A comparison was made between the results of different DFT techniques. It is concluded that the hybrid PBE0 [J. P. Perdew, K. Burke, M. Ernzerhof, J. Chem. Phys. 1996, 105, 9982.] functional is able to predict correctly the structural stability and phonon properties both for cubic and ferroelectric phases of BaTiO(3). The comparative phonon symmetry analysis in BaTiO(3) four phases has been made basing on the site symmetry and irreducible representation indexes for the first time.

The line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry have been applied for symmetry analysis of double-wall boron nitride and titania nano- tubes (DW BN and TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of hexagonal structure with the same or opposite orientation of translation and chiral vectors. We have considered the two sets of commensurate DW BN and TiO2 NTs with either armchair- or zigzag-type chiralities, i.e., (n1,n1)@(n2,n2 )o r (n1,0)@(n2,0), respectively. To establish theequilibriuminterwalldistancescorrespondingtotheminimaof energy, we have varied chiral indices n1 and n2 of the constituent single-wall (SW) nanotubes. To analyze the structural and elec- tronic properties of hexagonal DW NTs, we have performed ab initio LCAO calculations using the hybrid HartreeFock/KohnSham exchange-correlation functional PBE0 as implemented in CRYSTAL-09 code. The inversely stacked structure of zigzag-type DW BN NT, characterized by arrangement of positively and negatively charged rings in each atomic cross section (consisting of either B or N atoms, respectively), has been found to be energetically more preferable as compared to the straightly stacked structure containing nanotube rings consisting of the same type ofatomsincrosssections,i.e.,B(N)andB(N).Inarmchair-typeDWBNNTs,eachatomicringcontainsthewholenumberofBN bonds,whichreducestheelectrostaticinteractionbetweenbothwalls.Ontheotherhand,maincontributiontointerwallbondingin DW TiO2 NTs is provided by interaction between the nearest oxygen and titanium ions of neighboring shells. The interaction between the walls results in a decrease of band gaps for double-wall NTs as compared to those for SW NTs, which is substantially larger for TiO2.

Adsorption of Zn2+ at the rutile TiO2 (110)-aqueous interface was studied with Born–Oppenheimer molecular dynamics at 300 K. Simulations were carried out using the periodically repeated slab model with vacuum gap of 15 A filled with 72 H2O molecules. Two possible adsorption sites, monodentate above bridging oxygen (Ti–O–Ti or Obr) and bidentate above terminal oxygens (Ti–O), were investigated. Sites with different coordination environment for adsorbed Zn2+ differ from each other by the position of Zn2+ above surface level and by characteristic Zn–O distances. Obtained results gave evidence that 4-fold coordination of adsorbed Zn2+ is more probable than the 6-fold coordination found for aqueous species. The hydrolysis of H2O molecules was observed in the first coordination shell of adsorbed ion, resulting in formation of OH– groups attached to Zn2+. Calculated energies favor the tetrahedral bidentate structure of hydrated Zn2+ on the rutile surface. The model structures are compared to observed positions o...

The different ways of water molecule adsorption were simulated at (001) ZrO2-terminated surfaces of Y-doped BaZrO3 using hybrid HF-DFT method. The water adsorption energies were calculated after the full optimization of atomic positions within the water-slab systems. The proton releasing and hydroxyl group formation was established as result of the exothermic water molecule dissociation. The most favorable proton positions in the surface layers of Y-doped BaZrO3 were specified. To study the proton diffusion from surface to the deeper layers, the energy barriers were estimated for the proton jumping from one oxygen atom to another.

Plane wave density functional theory (DFT) calculations have been performed to study the atomic structure, preferred H2O adsorption sites, adsorption energies, and vibrational frequencies for water adsorption on the α-quartz (101) surface. Surface energies and atomic displacements on the vacuum-reconstructed, hydrolyzed, and solvated surfaces have been calculated and compared with available experimental and theoretical data. By considering different initial positions of H2O molecules, the most stable structures of water adsorption at different coverages have been determined. Calculated H2O adsorption energies are in the range −55 to −65 kJ/mol, consistent with experimental data. The lowest and the highest O−H stretching vibrational bands may be attributed to different states of silanol groups on the water-covered surface. The dissociation energy of the silanol group on the surface covered by the adsorption monolayer is estimated to be +80 kJ/mol. The metastable states for the protonated surface bridging O...