Enhanced visible-light activity of titania via confinement inside carbon nanotubes

Titania confined inside carbon nanotubes (CNTs) was synthesized using a restrained hydrolysis method. Raman spectra and magnetic measurements using a SQUID magnetometer suggested the formation of remarkable oxygen vacancies over the encapsulated TiO(2) in comparison with nanoparticles dispersed on the outer surface of CNTs, extending the photoresponse of TiO(2) from the UV to the visible-light region. The CNT-confined TiO(2) exhibited improved visible-light activity in the degradation of methylene blue (MB) relative to the outside titania and commercial P25, which is attributed to the modification of the electronic structure of TiO(2) induced by the unique confinement inside CNTs. These results provide further insight into the effect of confinement within CNTs, and the composites are expected to be promising for applications in visible-light photocatalysis.     

Fullerenes inside carbon nanotubes and multi-walled carbon nanotubes: optimum and maximum sizes

The potential energies of interaction between carbon nanotubes and internal fullerenes of spherical and ellipsoidal shape, as well as between nanotubes in multi-walled nanotubes were calculated using the Lennard–Jones (LJ) potential for carbon–carbon interactions. The optimum and maximum size of internal fullerenes and multi-walled nanotubes are determined as a function of the external nanotube radius. It was found that at the potential energy minimum, the van der Waals distance is close to that in graphite for all studied cases. The calculated results agree with available experimental observations and could be used as a guide for future experiments.     

Temperature and size effects on diffusion in carbon nanotubes

We study the self-diffusion of simple gases inside single-walled carbon nanotubes at the zero-loading limit by molecular dynamics simulations. The host-framework flexibility influence is taken into account. In particular, we study the influences of nanotube size and temperature. For the carbon-nanotube radius-dependent self-diffusivities, a maximum is observed, which resembles the so-called levitation effect. This occurs for pores having a radius comparable to the position of the interaction-energy minimum. Surprisingly, the temperature influence is not uniform throughout different pore sizes. Diffusivities are expected to increase with temperature. This effect is observed for carbon nanotubes distinctly larger than the guest molecules. Remarkably, for smaller pores, the self-diffusivities decrease with increasing temperature or exhibit a maximum in the temperature dependence. This effect is caused by competing influences of collision frequency and temperature.     

Effects of molecular confinement inside single walled carbon nanotubes on chemical reactivity--atomic H + 1-heptene

It has been demonstrated that the confinement of physisorbed 1-heptene molecules inside of carbon single walled nanotubes (SWNTs) results in lowering of their reactivity to atomic hydrogen compared to 1-heptene molecules adsorbed on external SWNT sites.     

Fabrication of polyimide nanotubes and carbon nanotubes containing magnetic iron oxide in confinement

Polyimide nanotubes with tunable wall thickness were fabricated by a precursor impregnation method using an AAO template, and carbon nanotubes containing magnetic iron oxide were obtained using ferric chloride-embedded polyimide precursor by a carbonization process.     

Interaction of double-stranded DNA inside single-walled carbon nanotubes

Deoxyribonucleic acid (DNA) is the genetic material for all living organisms, and as a nanostructure offers the means to create novel nanoscale devices. In this paper, we investigate the interaction of deoxyribonucleic acid inside single-walled carbon nanotubes. Using classical applied mathematical modeling, we derive explicit analytical expressions for the encapsulation of DNA inside single-walled carbon nanotubes. We adopt the 6–12 Lennard–Jones potential function together with the continuous approach to determine the preferred minimum energy position of the dsDNA molecule inside a single-walled carbon nanotube, so as to predict its location with reference to the cross-section of the carbon nanotube. An analytical expression is obtained in terms of hypergeometric functions which provides a computationally rapid procedure to determine critical numerical values. We observe that the double-strand DNA can be encapsulated inside a single-walled carbon nanotube with a radius larger than 12.30 ?, and we show that the optimal single-walled carbon nanotube to enclose a double-stranded DNA has radius 12.8 ?.     

Staggered alignment of quadrupolar molecules inside carbon nanotubes

Grand canonical Monte Carlo molecular simulations of the adsorption of three quadrupolar moleculesnitrogen (N(2)), carbon dioxide (CO(2)), and perfluoroethane (C(2)F(6))within single walled carbon nanotubes are reported. A unique slanted ordering is seen in the nanotubular systems (1-D geometry) which has not been reported before nor is present in slit nanopores (2-D geometry), and is due to the particular combination of steric effects and the unique anisotropic attraction experienced by these fluids.     

Fluid structure and transport properties of water inside carbon nanotubes

The fluid structure and transport properties of water confined in single-walled carbon nanotubes (CNTs) with different diameters have been investigated by molecular-dynamics simulation. The effects of CNT diameter, density of water, and temperature on the molecular distributions and transport behaviors of water were analyzed. It is interesting that the water molecules ordered in helix inside the (10, 10) CNT, and the layered distribution was clearly observed. It was found that the axial and radial diffusivities in CNTs were much lower than that of the bulk, and it ever decreased as the diameter of CNT decreases. The axial thermal conductivity and shear viscosity in CNTs are obviously larger than that of the bulk and those in the radial direction, they increase sharply as the diameter of CNT decreases, which is clearly in contrast to the diffusivity. The inner space of CNT and the interactions between water molecules and the confining walls play a key role in the structure and transport properties of water confined in the CNTs.     

Electronic properties of single-walled carbon nanotubes inside cyclic supermolecules

Possible ways for manipulating carbon nanotubes (CNTs) with cyclic supermolecules are studied using density functional theory. Electronic structure calculations with structure optimizations have been performed for the (4,4) and (8,0) single-walled carbon nanotubes (SWNTs) complexed with crown ethers as well as for the (4,0) SWNT with beta-cyclodextrin. A slight polarization of charge in both the nanotube and the supermolecule is observed upon rotaxane complexation, but the interaction is mainly repulsive, and the systems stay 2.8-3.5 A apart. The supermolecule does not affect the electronic band structure of the nanotube significantly within such a configuration. The situation differs noticeably for chemically cross-linked SWNTs and crown ethers, where a peak arises at the Fermi energy in the density of states. As a result, the band gap of semiconducting CNT(8,0) (0.5 eV) vanishes, and a new conduction channel opens for the metallic CNT(4,4).     

Anomalous vibrational energy diffusion in carbon nanotubes

We study the vibrational energy diffusion in single-walled carbon nanotubes by using the molecular-dynamics method. It is found that energy transports ballistically at low temperature and superdiffusively at room temperature. The velocity of energy transport along the axis in carbon nanotube at room temperature is about 0.10 A/fs. It is also found that energy transport in carbon nanotube is different from that one in one-dimensional carbon lattice with the same interaction potential.     

Photoluminescence of carbon quantum dots: coarsely adjusted by quantum confinement effects and finely by surface trap states

Photoluminescence (PL) mechanism of carbon quantum dots (CQDs) remains controversial up to now even though a lot of approaches have been made. In order to do that, herein a PL color ladder from blue to near infrared of CQDs with the absolute quantum yields higher than 70% were prepared via a one-pot hydrothermal synthesis route and separated by silica gel column. Time-correlated single photon counting measurements suggest that the electron transition takes in effect in the PL progress of the crystalline core-shell structured CQDs, and the PL properties could be coarsely adjusted by tuning the size of the crystalline carbon core owing to quantum confinement effects, and finely adjusted by changing the surface functional groups consisted shell owing to surface trap states, respectively. Both coarse and fine adjustments of PL, as optical and photoelectrical characterizations and density-functional theory (DFT) calculations have demonstrated, make it possible for top-level design and precise synthesis of new CQDs with specific optical properties.     

Phase separation of Lal3 inside single-walled carbon nanotubes.

Simple binary solids can be found to adopt unprecedented structures when confined into nanometre-sized cavities, such as the inner cylindrical bore of single-walled carbon nanotubes (SWNT). In the case of the discussed Lal(x)@SWNT encapsulation composite, the Lal2 "crystal" fragment adopts the structure of bulk Lal3, with one third of the iodine positions unoccupied. A complete characterisation of the encapsulation composite was achieved using an enhanced digital restoration approach of high-resolution transmission electron microscopy (HRTEM) images. The resulting complex exit surface wave provides information about the precise structural data of both filling material and host SWNT, establishing the SWNT's chirality and thus enabling a prediction of the composite's overall electron-transport properties.     

Hydrogen bond dynamics and microscopic structure of confined water inside carbon nanotubes

We have investigated the density and temperature dependences of microscopic structure and hydrogen bond dynamics of water inside carbon nanotubes (CNTs) using molecular dynamics simulation. The CNTs are treated as rigid, and smoothly truncated extended simple point charge water model is adopted. The results show that as the overall density increases, the atomic density profiles of water inside CNTs become sharper, the peaks shift closer to the wall, and a new peak of hydrogen atomic density appears between the first (outermost) and second layer. The intermittent hydrogen bond correlation function C(HB)(t) of water inside CNTs decays slower than that of bulk water, and the rate of decay decreases as the tube diameter decreases. C(HB)(t) clearly decays more slowly for the first layer of water than for other regions inside CNTs. The C(HB)(t) of the interlayer hydrogen bonds decays faster than those of the other regions and even faster than that of the bulk water. On the other hand, the hydrogen bond lifetimes of the first layer are shorter than those of the inner layer(s). Interlayer hydrogen bond lifetimes are clearly shorter than those of the constituent layers. As a whole, the hydrogen bond lifetimes of water inside CNTs are shorter than those of bulk water, while the relaxation of C(HB)(t) is slower for the confined water than for bulk water. In other words, hydrogen bonds of water inside CNTs break more easily than those of bulk water, but the water molecules remain in each other's vicinity and can easily reform the bonds.     

Confined water inside single-walled carbon nanotubes: global phase diagram and effect of finite length

Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.     

Studies on the adsorption of organic materials inside thick carbon nanotubes

We clarified the adsorption sites inside the single-wall carbon nanohorns (SWNHs), a type of single-wall carbon nanotubes having thick diameters of 2-5 nm, through the thermogravimetric analysis of the desorption of xylene and benzene from SWNHs. The influence of the sizes of holes piercing through the SWNH walls was also examined. Three types of adsorption sites were found inside the SWNH tubes, which were assigned to the tube tips, the sidewalls, and central regions of the hollow spaces. The experimental results also suggested that the adsorbed xylene and benzene were stabilized mainly by weak self-interactions at the central regions and their quick desorption was caused by the weak self-interactions.     

A theoretical study of the dihydrogen molecule confined inside carbon nanotubes

The aim of this work is to better understand the interaction between the confined dihydrogen molecule and armchair (2,2), (3,3) (4,4), (5,5), and (6,6) single‐walled carbon nanotubes (SWNT) using Restricted Hartree–Fock (RHF) and Density Functional Theory (DFT) methods using B3LYP and CAM‐B3LYP functionals. Depending on the calculation method and its orientation inside the nanotube, H₂ binds differently. We found that H? H bond length increases when H₂ is trapped in CNT (2,2) and decreases for CNT (3,3) and (4,4). The characteristics of confined H₂ in (5,5) and (6,6) nanotubes are similar to H₂ in a free state. © 2013 Wiley Periodicals, Inc.     

Bulk synthesis of linear carbon chains confined inside single-wall carbon nanotubes by vacuum discharge

Carbyne, an infinite carbon chain, has attracted much interest and induced significant controversy for many decades. Recently, the presence of linear carbon chains (LCCs), which were confined stably inside double-wall carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs), has been reported. In this study, we present a novel method to produce LCCs in a film of carbon nanotubes (CNTs). Our transmission electron microscopy and Raman spectroscopy revealed the formation of a bulk amount of LCCs after electric discharge of CNT films, which were used as field emission cathodes. The LCCs were confined inside single-wall CNTs as well as DWCNTs. Furthermore, two or three LCCs in parallel with each other are encapsulated when the inner diameter of CNT is larger than approximately 1.1 nm.