TiS2 and ZrS2 single‐ and double‐wall nanotubes: First‐principles study

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. © 2013 Wiley Periodicals, Inc.     

AlN nanotube: round or faceted?

Theoretical studies on one-dimensional AlN nanostructures have been performed. A faceted instead of cylindric model is proposed to reasonably understand the synthesized hexagonal AlN nanotubes. The close correlation is established that the nanotube structure should possess similar symmetry to that of the corresponding bulk crystal. This is also suitable for the few other faceted nanotubes and could be used to predict the morphology for the increasing nanotubes from nonlayered materials.     

First-Principles Calculations of Phonons and Thermodynamic Properties of Zr(Hf)S2-Based Nanotubes

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.     

A hydrogen storage mechanism in single-walled carbon nanotubes.

We have carried out systematic calculations for hydrogen-adsorption and -storage mechanism in carbon nanotubes at zero temperature. Hydrogen atoms first adsorb on the tube wall in an arch-type and zigzag-type up to a coverage of theta = 1.0 and are stored in the capillary as a form of H(2) molecule at higher coverages. Hydrogen atoms can be stored dominantly through the tube wall by breaking the C--C midbond, while preserving the wall stability of a nanotube after complete hydrogen insertion, rather than by the capillarity effect through the ends of nanotubes. In the hydrogen-extraction processes, H(2) molecule in the capillary of nanotubes first dissociates and adsorbs onto the inner wall and is further extracted to the outer wall by the flip-out mechanism. Our calculations describe suitably an electrochemical storage process of hydrogen, which is applicable for the secondary hydrogen battery.     

Structural stability and vibrational analysis of beryllium sulfide BeS from the bulk to the (n,0) nanotubes. An ab initio description

Beryllium sulfide (BeS) in different forms from molecule, bulk, monolayer (h-BeS), to the single-walled nanotube obtained by wrapping the h-BeS monolayer along the (n,0) hexagonal lattice vector for n varying from n = 6 to 64, has been examined. Density functional theory (DFT) with an all electron basis set and the hybrid DFT/B3LYP level have been applied to compute energetic and geometrical, electronic and vibrational, elastic and piezoelectric properties, where the trend towards the hexagonal monolayer (h-BeS) in the limit of large tube radius is obtained. The vibrational properties including Raman spectra and polarizabilities are evaluated via the Coupled Perturbed Hartree–Fock and Kohn–Sham (CPHF/KS) computational schemes. For the first time, the IR vibrational modes at higher tube diameter and their convergence to the h-BeS monolayer limit are reported where the vibrational and electronic contributions to both perpendicular and parallel components of polarizability tensor are additionally discussed. For the (n,0) BeS nanotube family, three ranges of IR active phonon modes are determined. The first one located in the 0–300 cm⁻¹frequency domain, goes regularly to zero when the tube diameter increases. Both the second (300–400 cm⁻¹) and the third (700–800 cm⁻¹) ones tend smoothly with different slope, towards the two optical vibrational modes of the h-BeS layer. These theoretical models can be extended to investigate further issues, such as the effect caused by the addition of a dopant, the influence of the substitutional fraction of nanotube atoms and the interaction of molecules outside and/or inside of the nanotube.     

Demonstrating structural deformation in an inorganic nanotube

There is much current interest in nanostructured materials (nanotubes, nanobelts, nanospheres, etc.). Their crystal structures can differ from those of the equivalent bulk materials. Determining these differences is important in understanding how the properties of nanomaterials differ from those of the bulk. Established methods of X-ray structure determination become increasingly difficult or impossible to apply on reducing the dimensions to a few nanometers. Here we show that, by combining the Debye equation for X-ray scattering (which relates an ensemble of atoms to their diffraction pattern without recourse to symmetry) with a model of the crystal structure, generated by folding the ideal crystal structure into a nanotube, the severely broadened/distored powder diffraction pattern may be described. This procedure reveals the significant structural deformations necessary to accommodate the nanotube shape. The importance of knowing the (deformed) crystal structure is discussed.     

二氧化钛纳米管储氢能力的分子动力学研究

二氧化钛纳米管是一种有前景的储氢材料,因此,在本文中通过卷曲锐钛矿单分子层,获得锯齿型(Zig-zag)和手性型(Chiral)二氧化钛纳米管结构。并采用分子动力学方法(Molecular dynamics)研究了氢分子分别在锯齿型和手性型二氧化钛纳米管和碳纳米管中的分布情况,并计算其储氢能力。结果表明,与碳纳米管一样,锯齿型和手性型二氧化钛纳米管存在管间储氢和管内储氢情况,并且氢分子在管间和管内的分布与二氧化钛纳米管内、外两侧的氧原子相关。Lennard-Jones势能模型表明:氢分子向纳米管内部和管间隙处的低能处聚集,形成氢分子环结构。储氢量计算结果表明,虽然锯齿型和手性型二氧化钛纳米管储存的氢分子数目较多,但由于系统重量较大,储氢量较低,低于美国能源部6%的商业标准,不能满足实际需要,而碳纳米管储氢量接近这一标准。     

Studies of iridium nanoparticles using density functional theory calculations

The energetics and the electronic and magnetic properties of iridium nanoparticles in the range of 2-64 atoms were investigated using density functional theory calculations. A variety of different geometric configurations were studied, including planar, three-dimensional, nanowire, and single-walled nanotube. The binding energy per atom increases with size and dimensionality from 2.53 eV/atom for the iridium dimer to 6.09 eV/atom for the 64-atom cluster. The most stable geometry is planar until four atoms are reached and three-dimensional thereafter. The simple cubic structure is the most stable geometric building block until a strikingly large 48-atom cluster, when the most stable geometry transitions to face-centered cubic, as found in the bulk metal. The strong preference for cubic structure among small clusters demonstrates their rigidity. This result indicates that iridium nanoparticles intrinsically do not favor the coalescence process. Nanowires formed from linear atomic chains of up to 4-atom rings were studied, and the wires formed from 4-atom rings were extremely stable. Single-walled nanotubes were also studied. These nanotubes were formed by stacking 5- and 6-atom rings to form a tube. The ring stacking with each atom directly above the previous atom is more stable than if the alternate rings are rotated.     

The calculations of phonon dispersion relations for single-wall carbon armchair and zigzag nanotubes

A 3D single-wall carbon nanotube can be viewed as a 2D graphite sheet rolled into a 3D cylinder. In the study of dispersion relations of carbon nanotubes, the consistent force parameters for 2D graphite sheets have to be modified to include the curvature effect. The present paper reports a series of calculations of phonon dispersion relations for single-wall carbon armchair, zigzag nanotube in which the curvature effect has been properly treated. The symmetry of crystal vibration mode at the centre of Brillouin zone is analyzed based on our numeric results and the structure symmetry of the nanotubes.     

Mechanistic Insights into the Formation of Lithium Fluoride Nanotubes

Born–Oppenheimer molecular dynamics (BOMD) and periodic density functional theory (DFT) calculations have been applied for describing the mechanism of formation of lithium fluoride (LiF) nanotubes with cubic, hexagonal, octagonal, decagonal, dodecagonal, and tetradecagonal cross-sections. It has been shown that high energy structures, such as nanowires, nanorings, nanosheets, and nanopolyhedra are transient species for the formation of stable nanotubes. Unprecedented (LiF)_n clusters (n≤12) were also identified, some of them lying less than 10 kcal mol^−[1] above their respective global minima. Such findings indicate that stochastic synthetic techniques, such as laser ablation and chemical vapor deposition, should be combined with a template-driven procedure in order to generate the nanotubes with adequate efficiency. Apart from the stepwise growth of LiF units, the formation of nanotubes was also studied by rolling up a planar square sheet monolayer, which could be hypothetically produced from the exfoliation of the FCC crystal structure. It was shown that both pathways could lead to the formation of alkali halide nanotubes, a still unprecedented set of one-dimensional materials.     

Generation and growth mechanism of metal (Fe, Co, Ni) nanotube arrays.

Nanotubes composed of layered or nonlayered materials have been synthesized through various methods, among which template-based electrodeposition technology provides a versatile technique for synthesizing one-dimensional nanostructured materials. However, the growth mechanism of nanotubes using the template method is seldom clarified. Herein, we present the systematic preparation of metal nanotube arrays and put forward the growth mechanism, termed current-directed tubular growth (CDTG), for template-based electrodeposition. There are competitive growth rates for metal atoms entering the crystal lattice, that is, v( parallel) (growth rate parallel to current direction) and v( perpendicular) (growth rate perpendicular to current direction). Metal nanotubes can be obtained at v( parallel)>v( perpendicular), while nanowires can be obtained at v( parallel) approximately v( perpendicular). The as-synthesized metal (Fe, Co, Ni) nanotubes are constructed from nonlayered materials, which are of body-centered cubic iron structure, hexagonal close packed cobalt structure, and face-centered cubic nickel structure, respectively. The CDTG mechanism is expected to have applications in designing and synthesizing other metal nanotubes and even compound nanotubes via template-based electrodeposition technology.     

Distribution patterns and controllable transport of water inside and outside charged single-walled carbon nanotubes

The density distribution patterns of water inside and outside neutral and charged single-walled carbon nanotubes (SWNTs) soaked in water have been studied using molecular dynamics simulations based on TIP3P potential and Lennard-Jones parameters of CHARMM force field, in conjunction with ab initio calculations to provide the electron density distributions of the systems. Water molecules show different electropism near positively and negatively charged SWNTs. Different density distribution patterns of water, depending on the diameter and chirality of the SWNTs, are observed inside and outside the tube wall. These special distribution patterns formed can be explained in terms of the van der Waals and electrostatic interactions between the water molecules and the carbon atoms on the hexagonal network of carbon nanotubes. The electric field produced by the highly charged SWNTs leads to high filling speed of water molecules, while it prevents them from flowing out of the nanotube. Water molecules enter the neutral SWNTs slowly and can flow out of the nanotube in a fluctuating manner. It indicates that by adjusting the electric charge on the SWNTs, one can control the adsorption and transport behavior of polar molecules in SWNTs to be used as stable storage medium with template effect or transport channels. The transport rate can be tailored by changing the charge on the SWNTs.     

Computational Evidence for the Smallest Boron Nanotube

One-dimensional boron nanostructures have being actively studied experimentally1 and theoretically2 in the recent years, due to their promising applications in field emission devices and high-temperature electronics. Amorphous3 and crystalline1 nanowires,…     

Oxygen gas-induced lip-lip interactions on a double-walled carbon nanotube edge

We have investigated adsorption of an O(2) molecule on a double-walled carbon nanotube (DWCNT) edge using density functional theory calculations. An O(2) molecule adsorbs exothermally without an adsorption barrier at open nanotube edges that are energetically favorable with a large adsorption energy of about -9 eV in most cases. Dissociative adsorption of an O(2) molecule induces various spontaneous lip-lip interactions via the bridged carbon atoms, generating the closed tube ends. This explains why the DWCNTs are chemically more stable than the single-walled nanotubes during observed field emission experiments. The field emission takes place via the localized states of the bridged carbon atoms, not via those of the adsorbed oxygen atoms particularly in the armchair nanotubes. We also find that some O(2) precursor states exist as a bridge between tube edges.     

A computer simulation study of stick-slip transitions in water films confined between model hydrophilic surfaces. 1. Monolayer films

The shear behavior of monolayer water films confined in a slit-like pore between hydrophilic walls is simulated in the quasistatic regime using the grand canonical Monte Carlo technique. Each wall is represented as a hexagonal lattice of force sites that interact with water through an orientation-dependent hydrogen-bonding potential. When the walls are in registry, the water oxygen atoms form either a crystal- or fluid-like structure, depending on the period of the wall's lattice. In both cases, however, the monolayer structure is orientationally disordered. Both the crystal- and fluid-like monolayers prove to be capable of experiencing well-defined stick-slip transitions, with the largest yield stress occurring in the crystal-like case. Beyond the yield point, the crystal-like monolayers "melt", but their structure and molecular motion differ in many respects from those characteristic of normal fluids.     

Phase transition of alkylsilane monolayers studied by temperature-dependent grazing incidence X-ray diffraction

The phase transition of organosilane monolayers on Si-wafer substrate surfaces prepared from octadecyltrichlorosilane (OTS) or docosyltrichlorosilane (DOTS) was investigated on the basis of grazing incidence X-ray diffraction (GIXD) at various temperatures. The OTS monolayer was prepared by a chemisorption method. The DOTS monolayer was prepared by a water-cast method (DOTS). The GIXD measurement clarified that the OTS monolayer also changed from hexagonal phase to amorphous state above a melting point of otadecyl groups. The GIXD measurements also clarified that the molecular aggregation state of the DOTS monolayer changes from an anisotropic phase to an isotropic phase with an increase in temperature. An estimated linear thermal expansion coefficient of the lattice lengths of a and b of the DOTS monolayer in the rectangular crystalline state assigned a similar value to those of bulk polyethylene with an orthorhombic crystalline lattice. The setting angle of the ab plane of the rectangular DOTS monolayer also showed similar behavior to that of the ab plane of bulk polyethylene.     

Studies on the encapsulation of F- in single walled nanotubes of different chiralities using density functional theory calculations and Car-Parrinello molecular dynamics simulations

In this study, the encapsulation of F(-) in different nanotubes (NTs) has been investigated using electronic structure calculations and Car-Parrinello molecular dynamics simulations. The carbon atoms in the single walled carbon nanotube (CNT) are systematically doped with B and N atoms. The effect of the encapsulation of F(-) in the boron nitride nanotube (BNNT) has also been investigated. Electronic structure calculations show that the (7,0) chirality nanotube forms a more stable endohedral complex (with F(-)) than the other nanotubes. Evidence obtained from the band structure of CNT calculations reveals that the band gap of the CNT is marginally affected by the encapsulation. However, the same encapsulation significantly changes the band gap of the BNNT. The density of states (DOS) derived from the calculations shows significant changes near the Fermi level. The snapshots obtained from the CPMD simulation highlight the fluctuation of the anion inside the tube and there is more fluctuation in BNNT than in CNT.     

Computer Simulation of the Initial Stage of Water Vapor Nucleation on a Silver Iodide Crystal Surface: 1. Microstructure

The nucleation of water vapors on the surface of a fragment of silver iodide crystal is simulated by the Monte Carlo method under the conditions similar to natural conditions in a humid atmosphere. A stable monolayer island of water molecules with clearly pronounced features of hexagonal symmetry and low orientational order is formed at the initial stage, when the vapor pressure is still lower than the saturating pressure. The island readily grows over the surface and, in the unsaturated vapor, does not grow in the direction perpendicular to the surface. The formed monolayer represents a substrate for further growth of a condensed phase and, eventually, is responsible for the mechanism of nucleation on the crystal surface. Water molecules are held by the substrate mainly owing to the directional electrostatic interaction between the negatively charged oxygen atoms and positively charged silver ions. The interaction with iodine ions lowers the binding of the island (nucleus) and the substrate. A point defect in the form of an extra ion on the surface does not change the planar shape of the nucleus and virtually does not distort its hexagonal structure. Indirect experimental data supporting the formation of a water monolayer at the stage preceding nucleation, as well as the data of observations indicating the important role of defects on a crystal surface, are reported.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 548–560.Original Russian Text Copyright © 2005 by Shevkunov.     

Topology, connectivity, and electronic structure of C and B cages and the corresponding nanotubes

After a brief discussion of the structural trends which appear with an increasing number of atoms in B cages, a one-to one correspondence between the connectivity of B cages and C cage structures will be proposed. The electronic level spectra of both systems from Hartree-Fock calculations is given and discussed. The relation of curvature introduced into an originally planar graphitic fragment to pentagonal "defects" such as are present in buckminsterfullerene is also briefly treated. A study of the structure and electronic properties of B nanotubes will then be introduced. We start by presenting a solution of the free-electron network approach for a "model boron" planar lattice with local coordination number 6. In particular the dispersion relation E(k) for the pi-electron bands, together with the corresponding electronic Density Of States (DOS), will be exhibited. This is then used within the zone-folding scheme to obtain information about the electronic DOS of different nanotubes obtained by folding this model boron sheet. To obtain the self-consistent potential in which the valence electrons move in a nanotube, "the March model" in its original form was invoked, and the results are reported for a carbon nanotube. Finally, heterostructures, such as BN cages and fluorinated buckminsterfullerene, will be briefly treated, the new feature here being electronegativity difference.     

Boron‐Double‐Ring Sheet,Fullerene, and Nanotubes: Potential Hydrogen Storage Materials

Similar to carbon‐based graphene, fullerenes and carbon nanotubes, boron atoms can form sheets, fullerenes, and nanotubes. Here we investigate several of these novel boron structures all based on the boron double ring within the framework of density functional theory. The boron sheet is found to be metallic and flat in its ground state. The spherical boron cage containing 180 atoms is also stable and has I symmetry. Stable nanotubes are obtained by rolling up the boron sheet, and all are metallic. The hydrogen storage capacity of boron nanostructures is also explored, and it is found that Li‐decorated boron sheets and nanotubes are potential candidates for hydrogen storage. For Li‐decorated boron sheets, each Li atom can adsorb a maximum of 4 H₂ molecules with g_d=7.892 wt %. The hydrogen gravimetric density increases to g_d=12.309 wt % for the Li‐decorated (0,6) boron nanotube.