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The atomic scale details of single-walled carbon nanotube (SWNT) nucleation on metal catalyst particles are elusive to experimental observations. Computer simulation of metal-catalyzed SWNT nucleation is a challenging topic but potentially of great importance to understand the factors affecting SWNT diameters, chirality, and growth efficiency. In this work, we use nonequilibrium density functional tight-binding molecular dynamics simulations and report nucleation of sp(2)-carbon cap structures on an iron particle consisting of 38 atoms. One C(2) molecule was placed every 1.0 ps around an Fe(38) cluster for 30 ps, after which a further 410 ps of annealing simulation without carbon supply was performed. We find that sp(2)-carbon network nucleation and annealing processes occur in three sequential and repetitive stages: (A) polyyne chains on the metal surface react with each other to evolve into a Y-shaped polyyne junction, which preferentially form a five-membered ring as a nucleus; (B) polyyne chains on the first five-membered ring form an additional fused five- or six-membered ring; and (C) pentagon-to-hexagon self-healing rearrangement takes place with the help of short-lived polyyne chains, stabilized by the mobile metal atoms. The observed nucleation process resembles the formation of a fullerene cage. However, the metal particle plays a key role in differentiating the nucleation process from fullerene cage formation, most importantly by keeping the growing cap structure from closing into a fullerene cage and by keeping the carbon edge "alive" for the addition of new carbon material.
ACS Nano 2009 Nov 24
PMID:Quantum chemical molecular dynamics simulation of single-walled carbon nanotube cap nucleation on an iron particle. 1982 61

Carbon nitride materials have extraordinary potential in various applications, including catalysts, filled-particles, and superhard materials. Carbon nitride nanoclusters have been prepared under mild solvothermal conditions by a reaction between 1,3,5-trichlotriazine and sodium azide in toluene. The bulk material formed has a C(3)N(4) composition and consists of spheres with diameters ranging from approximately 1 nm to 4 mum. Nanometer-sized clusters of C(3)N(4) stoichiometry have been isolated on surfaces by sublimation or simple physicochemical methods. The clusters have then been characterized by atomic force microscopy and X-ray photoelectron spectroscopy. The laser desorption ionization mass spectra show peaks assignable to the C(12)N(16), C(21)N(28), and C(33)N(44) molecules which could correspond to cage structures with 4, 7, and 11 units of the C(3)N(4) subunit, respectively. The structure and stability of these new nitrogen-rich carbon nitride nanocages has been investigated using density functional theory calculations.
ACS Nano 2009 Nov 24
PMID:Azafullerene-like nanosized clusters. 1986 Mar 86

Several strategies aimed at sorting single-walled carbon nanotubes (SWNT) by diameter and/or electronic structure have been developed in recent years. A nondestructive sorting method was recently proposed in which nanotube bundles are dispersed in water-surfactant solutions and submitted to ultracentrifugation in a density gradient. By this method, SWNTs of different diameters are distributed according to their densities along the centrifuge tube. A mixture of two anionic amphiphiles, namely sodium dodecylsulfate (SDS) and sodium cholate (SC), presented the best performance in discriminating nanotubes by diameter. We present molecular dynamics studies of the water-surfactant-SWNT system. The simulations revealed one aspect of the discriminating power of surfactants: they can actually be attracted toward the interior of the nanotube cage. The binding energies of SDS and SC on the outer nanotube surface are very similar and depend weakly on diameter. The binding inside the tubes, on the contrary, is strongly diameter dependent: SDS fits best inside tubes with diameters ranging from 8 to 9 A, while SC is best accommodated in larger tubes, with diameters in the range 10.5-12 A. The dynamics at room temperature showed that, as the amphiphile moves to the hollow cage, water molecules are dragged together, thereby promoting the nanotube filling. The resulting densities of filled SWNT are in agreement with measured densities.
ACS Nano 2010 Feb 23
PMID:Role of surfactants in carbon nanotubes density gradient separation. 2005 84

Y(3)C(80) obtained in the synthesis of nitride clusterfullerenes Y(3)N@C(2n) (2n = 80-88) by the reactive atmosphere method is found to be a genuine trimetallofullerene, Y(3)@C(80), with low ionization potential and divalent state of yttrium atoms. DFT studies of the electronic structure of Y(3)@C(80) show that this molecule mimics Y(3)N@C(80) with the pseudoatom (PA) instead of the nitrogen atom. Topology analysis of the electron density and electron localization function show that yttrium atoms form Y-PA bonds rather than direct Y-Y bonds. Molecular dynamics simulations show that the Y(3)PA cluster is as rigid as Y(3)N and rotates inside the fullerene cage as a single entity.
ACS Nano 2010 Feb 23
PMID:A pseudoatom in a cage: trimetallofullerene Y(3)@C(80) mimics y(3)n@c(80) with nitrogen substituted by a pseudoatom. 2007 1

The assembly, structure, and stability of DNA nanocages with the shape of truncated octahedra have been studied. The cages are composed of 12 double-stranded B-DNA helices interrupted by single-stranded linkers of thymidines of varying length that constitute the truncated corners of the structure. The structures assemble with a high efficiency in a one-step procedure, compared to previously published structures of similar complexity. The structures of the cages were determined by small-angle X-ray scattering. With increasing linker length, there is a systematic increase of the cage size and decrease of the twist angle of the double helices with respect to the symmetry planes of the cage structure. In the present study, we demonstrate the length of the single-stranded linker regions, which impose a certain degree of flexibility to the structure, to be the important determinant for efficient assembly. The linker length can be decreased to three thymidines without affecting assembly yield or the overall structural characteristics of the DNA cages. A linker length of two thymidines represents a sharp cutoff abolishing cage assembly. This is supported by energy minimization calculations suggesting substantial hydrogen bond deformation in a cage with linkers of two thymidines.
ACS Nano 2010 Mar 23
PMID:Structure of nanoscale truncated octahedral DNA cages: variation of single-stranded linker regions and influence on assembly yields. 2014 42

The amount of radioactive iodine generated from nuclear power plants is expected to increase with the proliferation of nuclear energy production, and long-term immobilization methods for such radioactive elements need to be developed to make nuclear energy sustainable. The standard immobilization method of radioactive elements, vitrification, is not very effective for radioactive iodine-129 because of the low solubility of iodine in silicate melts, its very high volatility at standard vitrification process temperatures, and its instability in the alkaline environment of deep geological layers below 300 m. We have developed a novel three-phase ceramic composite produced by a sintering process. Iodine adsorbed onto Ca-type zeolite A was covered with a hydroxyapatite nanolayer through the exchange reaction of ammonium with calcium. Clusters of iodine of 30 nm within the zeolite structure were found to be thermally stable up to 1253 K because of the partial blockage of the alpha-cage apertures by ammonium ions and the partial change from a crystalline phase to an amorphous phase at 473 K. No gasification of iodine molecules was found to occur during the sintering process. The outer phase was highly crystalline hydroxyfluorapatite in which the hydroxyapatite nanolayer plays an important role for successful sintering. The elution of iodine in low-dioxygen water, similar to that found within the Earth's crust, was investigated and was found to occur only in the surface layer of the sintered body.
ACS Appl Mater Interfaces 2009 Jul
PMID:Novel long-term immobilization method for radioactive iodine-129 using a zeolite/apatite composite sintered body. 2035 64

New cobalt(III) bis(dicarbollide) complexes covalently linked to two 2-oligothienyl units have been synthesized and electropolymerized in acetonitrile electrolyte in order to produce the corresponding polythiophene films containing in-chain metallic centers. The polymer films electrogenerated from the bithienyl (4b) and terthienyl (4c) derivatives display redox processes attributed to the Co(III)/Co(II) couple at ca. -1.1 V vs SCE and to the p-doping/undoping of the expected quaterthienyl and sexithienyl segments at ca. 0.8 V vs SCE. In contrast, the anodic oxidation of the thienyl (4a) derivative leads to passivation of the electrode surface. As the length of the oligothiophene substituents increases, the metallic and dicarbollide cage carbon atoms contributions in the HOMO decrease dramatically so that the highest occupied frontier orbitals of 4b and 4c can be considered as almost purely oligothiophene-based. From further UV-vis spectroscopy analysis, it is demonstrated that the polymer incorporating the sexithienyl segments is more conjugated than that with the quaterthienyl segments as the absorption maximum for the interband pi-pi* transition was observed at 410 and 448 nm for poly(4b) and poly(4c) respectively. Furthermore, these polymers display a more extended degree of conjugation than the parent oligothiophenes. Such features indicate a significant electronic delocalization through the cobaltabisdicarbollide moiety. Their conducting probe atomic force microscopy characterization indicates that poly(4b) and poly(4c) behave like heavily doped semiconductors rather than pure semiconductors. Mean conductivity values extracted from the current-voltage profiles are 1.4 x 10(-4) and 7.5 x 10(-4) S cm(-1) for poly(4b) and poly(4c), respectively. Such materials are found to be efficient for the electrocatalytic reduction of protons to dihydrogen, as exemplified for poly(4b). The overpotential for hydrogen evolution is significantly decreased by ca. 230 mV with respect to that obtained with the bare electrode (measured for a current density of 1.4 mA cm(-2) in the presence of 20 mM HBF(4)).
ACS Appl Mater Interfaces 2010 Mar
PMID:Polythiophenes containing in-chain cobaltabisdicarbollide centers. 2035 70

It is shown, for the first time, that mixtures of sulfonated polyhedral silsesquioxane cage structures (sPOSS) and poly(dimethyl siloxane) (PDMS) with silicone oil exhibit significant electrorheological (ER) activity. At low sPOSS concentrations, less than 10 wt %, the viscosity is enhanced by approximately 100, which is comparable to the viscosity enhancements exhibited by conventional ER fluids, under the influence of comparable applied electric fields, E = 2 kV/mm. Measurements of the shear stress, sigma, dependencies on E, the conductivities, and relative permittivities reveal that the properties of these POSS/PDMS systems cannot be reconciled with theory developed to explain the behavior of conventional ER fluids.
ACS Appl Mater Interfaces 2010 Apr
PMID:Electrorheological phenomena in polyhedral silsesquioxane cage structure/PDMS systems. 2038 59

The magnetic titanate nanotube (Fe(3)O(4)-TN composite) was prepared and functionalized with C(18) groups and then coated by a hydrophilic alginate polymer cage. This material (ALG@C(18)-Fe(3)O(4)-TN) exhibited the properties of large surface area, superparamagnetism, high adsorption ability, and good dispersibility in water. ALG@C(18)-Fe(3)O(4)-TNs possessed high extraction efficiency to phthalate esters di-n-propyl-phthalate (DPP), di-n-butyl-phthalate (DBP), dicyclohexyl-phthalate (DCP), and di-n-octyl-phthalate (DOP). The dispersed adsorbents in solution could be collected with an external magnetic field within 10 min. The extraction could be conducted under some extreme conditions such as high salinity, acid or alkali solution or humic acid-rich samples without the decrease in extraction efficiency. In the presence of 100 mg L(-1) humic acid (HAs), the recoveries of analytes were not affected at pH >6, and decreased recoveries of DPP and DCP were observed only in acid solution. Potential analysis and comparison study with the biphthalate acid or n-decanoic acid-rich matrix indicated that the anti-interference ability of adsorbents to HAs mainly resulted from the repulsion interaction and size exclusion provided by alginate cage to HAs. Under the optimized conditions, ALG@C(18)-Fe(3)O(4)-TNs were used to analyze several environmental water samples, a concentration factor of 1000 and detection limits of phthalate esters ranging in 11-46 ng L(-1) were achieved, and the recoveries of analytes were in the range of 84-109% for all samples.
ACS Appl Mater Interfaces 2010 Apr
PMID:Alginate-polymer-caged, C(18)-functionalized magnetic titanate nanotubes for fast and efficient extraction of phthalate esters from water samples with complex matrix. 2042 35

A detailed study on the relaxation mechanisms of higher cage fullerene sizes is done as a prerequisite for studies of the influence of the endohedral structures on fullerene cage carbon relaxation. Recent studies of the dynamic behavior of C(60) and C(70) in aromatic solvents and CS(2) solution show the influence of the shape and the symmetry of the cage to be highly important as well as the influence of the solvent to be negligible. As higher fullerene cages have more than one stable isomer, the isolation of isomeric pure structures is of high importance for a detailed study of the dynamic behavior of such fullerenes. Here we investigated the three higher fullerene cage isomers D(2)-C(76), C(2v)(3)-C(78), and D(2)-C(80) with respect to the relaxation rate of the carbons measured in their temperature dependence. Thus, we study the influence on the relaxation of the carbons and the dynamic behavior of these fullerenes in solution. Besides the diffusion dependence on the shape of the carbon cage, the relaxation behavior at lower temperatures is found to be dependent on the difference in chemical shift anisotropy within the carbon cage. This difference is originated from the changes of symmetry and results in polarization of electron density. Furthermore, the mobility of the carbons is influenced by their pyramidalization.
ACS Nano 2010 Jun 22
PMID:Influence of the cage size on the dynamic behavior of fullerenes: a study of (13)c NMR spin-lattice relaxation. 2045 13


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