Ethylene physisorption on C60 fullerene

Monte Carlo computer simulation results on the adsorption of ethylene on C₆₀ fullerene are employed to locate the adsorption sites observed for the adsorption of other simple gases. The distributions of molecules according to the gas-solid interaction energy obtained from the simulations are in agreement with experimental results reported in the literature. We focused our attention on the isotherm obtained at 150 K. At this temperature, the molecules with a certain gas-solid energy have been identified and their location employed to find out the adsorption sites. This sort of distribution has been averaged over all the equilibrated configurations generated during the simulation. The results obtained confirm the assignment of adsorption sites previously reported for the adsorption of N₂, Ar, and CO₂. The distribution of molecules over the gas-gas interaction energy is also analyzed and the obtained results suggest that the adsorbed molecules prefer a T-shaped stacking. This conclusion is obtained through the analysis of the distributions with the aid of the gas-gas interaction potential. This observation agrees with recently published results by other authors. The information obtained from the microdensity profiles has also been employed to locate the adsorption sites.     

Reaction of silica-supported fullerene C60 with nonylamine vapor

The reaction of silica-supported [60]fullerene with vaporous nonylamine at 150 °C produces a mixture of addition products. Quantum chemical calculations, at the B3LYP/STO-3G level of theory, support that the addition reaction most likely takes place across the 6,6 bonds of C₆₀ pyracyclene units (and not across the 5,6 bonds). Numerous peaks were found in high-performance liquid chromatograms, apparently due to a large number of possible isomers. According to elemental analysis data (C:N ratio), the number of nonylamine molecules attached to C₆₀ is 3 on average. Thermogravimetric analysis of the nonylamine adduct showed two weight losses, one between 360 and 590 °C due to thermal decomposition of nonylamine moieties, and one between 725 and 840 °C due to decomposition of the remaining fullerene-derived carbonized material. Field-desorption mass spectrometric study revealed a number of molecular and fragment ions corresponding to the adducts with up to six nonylamine moieties attached to [60]fullerene; some of them were observed as multiply-charged ions. The temperature behavior of these peaks is similar to that for TGA, with maxima shifted to lower temperatures due to the cooperative effect of the strong electric field. C₆₀ can be partially regenerated by pyrolysis of the nonylamine adduct, although at very low yields (below 1%, after heating at 350 °C under air for 2 h).     

C60 fullerene inclusions in low-molecular-weight polystyrene-poly(dimethylsiloxane) diblock copolymers

We have synthesized low-molecular-weight diblock copolymers of polystyrene-block-poly(dimethylsiloxane) with total molecular weights <12 kg/mol and PS volume fractions of ∼0.2. We have investigated the phase behavior of the PS-PDMS in its pure state and with up to 10 wt% of C₆₀ added. The C₆₀ was shown to selectively segregate into the PS phase only although its solubility limit is ∼1 wt%. Although the C₆₀ aggregates above 1 wt%, the cylindrical morphology observed in the pure copolymer bulk samples persists in the C₆₀-copolymer composites even up to 10 wt% C₆₀ loading. In thin films, the pure copolymer possesses a highly ordered morphology with grains hundreds of microns across. When C₆₀ is blended with the copolymer the high degree of order rapidly decreases due to increasing numbers of defects observed.     

Surface-enhanced Raman scattering studies on C60 fullerene self-assemblies

Surface-enhanced Raman scattering (SERS) was used to investigate C₆₀ self-assembling in solvents like pyrrolidine (Py) and N-methyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C₆₀ aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C₆₀ and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at ∼342 cm⁻¹. In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I_h C₆₀ symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800 cm⁻¹ is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C₆₀ in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C₆₀ cage. These complexes are noticed in the SERS spectrum by the replacement of the original H_g(1) band at ∼269 cm⁻¹ with two others at ∼255 and ∼246 cm⁻¹. In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118 cm⁻¹, which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C₆₀ clusters of different size.     

Polymeric fullerene oxide (fullerene ozopolymers) produced by prolonged ozonation of C60 and C70 fullerenes

The prolonged ozonation of C₆₀ and C₇₀ fullerene produces light brown to brown amorphous solids which are insoluble in chlorinated and hydrocarbon solvents but which are readily soluble in water, acetone and ethanol where they give dark-brown solutions. The polymeric and polyelectrolytic nature of these solids has been shown by viscosimetric and cryoscopic measurements. Due to the polymeric nature the solids have been called ‘ozopolymers’. The reactivity and the ozone uptake have been measured quantitatively during the ozonation of C₆₀ and C₇₀ fullerene in CCl₄. Three different C₆₀ ozopolymer samples have been produced at different degrees of ozonation: at O₃/C₆₀ molar ratio of 8, 14 and 26. The C₇₀ ozopolymer has been produced at an O₃/C₇₀ molar ratio of 30. All the samples have been studied by FT-IR spectroscopy and the C₆₀ ozopolymers have been easily reduced by the action of Zn dust and acetic acid or by the action of H₂S and studied by FT-IR. The action of oxidizing agents has been studied as well. C₆₀ ozopolymer is a polyelectrolyte rich in carboxyl groups and for this reason it possesses a high metal binding capacity similar to that of humic acids. The thermal stability of C₆₀ ozopolymer and its reduced derivatives as well as the thermal stability of C₇₀ ozopolymer has been checked by thermogravimetric analysis in nitrogen and air flow. Finally, the electrical conductivity of C₆₀ ozopolymer was found to be σ=2.8×10⁻⁸ S/cm which is three orders of magnitude lower than that of pure C₆₀.     

Recoil tritium-C60 interaction: a channel for endohedral encapsulation of tritium in C60

The introduction of tritium (T), with an initial kinetic energy of 2.7 MeV into the cavity of C₆₀ using recoil implantation is studied in a ⁶Li(n,α)T activated homogenized compound matrix of Li₂CO₃ and C_60. Radioactive endohedral T@C₆₀ could be detected through the liquid scintillation spectrometry coupled with high-pressure liquid chromatography (HPLC). Fast atom bombardment mass spectrometry provided evidence for the retainment of C₆₀ cage as against radiation damage. Solid state temperature-programmed desorption based on evolved gas showed the desorption of ³He gas (as a β⁻ decay product from T) from the endohedral C₆₀ cage at a higher temperature than the T₂ and T₂O trapped species at the defect sites and from the ordered crystallographic network of Li₂CO₃, respectively. The non-isothermal kinetics of the helium desorption, as a function of temperature evidenced a diffusion controlled process.     

Preparation and spectroscopic properties of chlorofullerenes C60Cl24, C60Cl28, and C60Cl30

We report easy preparation of recently discovered highly chlorinated fullerenes T_h-C₆₀Cl₂₄, C₁-C₆₀Cl₂₈, and D_3d-C₆₀Cl₃₀ in high-temperature reactions of C₆₀ with PCl₅ and ICl. Formation and interconversion of chlorofullerenes was investigated in details for C₆₀-ICl system. C₆₀Cl₂₈ is the least stable chlorofullerene that undergoes rearrangement (accompanied by partial chlorine elimination) into more stable T_h-C₆₀Cl₂₄ under more drastic reaction conditions (increased temperature and time of chlorination). T_h-C₆₀Cl₂₄ yields D_3d-C₆₀Cl₃₀ at temperatures above 220 °C via a sequence of rearrangements and further addition of chlorine. In contrast to the fullerene reaction with ICl, interaction of C₆₀ with PCl₅ is very selective with respect to formation of C₆₀Cl₂₄ in a wide temperature range. Solid-state electronic (UV-Vis) and vibrational (IR) spectra of chlorinated fullerenes C₆₀Cl₂₄, C₆₀Cl₂₈, C₆₀Cl₃₀ and fluorinated fullerenes C₆₀F₁₈ and C₆₀F₃₆ were recorded in the spectral range between 30 and 45,000 cm⁻¹. Raman spectra were also acquired for all investigated compounds. Moreover, molecular geometry of the C₆₀Cl₂₄ and its theoretical IR-absorption spectrum were calculated using B3LYP/STO-3G chemistry model.     

Electrochemical and chemical redox doping of fullerene (C60) peapods

Different types of redox doping of C₆₀@SWCNT were monitored by Raman spectroscopy. Chemical doping was carried out by gaseous potassium, liquid potassium amalgam and gaseous fluorine diluted with argon. Electrochemical doping was investigated by in situ Raman spectroelectrochemistry in LiClO₄ + acetonitrile solution and in 1-butyl-3-methylimidazolium tetrafluoroborate (ionic liquid). The peapods exhibit characteristic and complex feedback to chemical as well as to electrochemical doping. In contrast to chemical p-doping by F₂, the Raman scattering of intratubular fullerene is selectively enhanced during electrochemical p-doping. Similar selective enhancement is traced at chemical n-doping with gaseous potassium. Doping by gaseous potassium causes deep reduction of intratubular C₆₀ to , which is not fully re-oxidizable upon contact to air. On the other hand, doping with liquid potassium amalgam causes reduction of intratubular C₆₀ to , and complete re-oxidation to neutral fullerene occurs spontaneously upon contact to air. In general, the doping chemistry of peapods is significantly dependent on the applied redox potential, charge-compensating counterions and on the actual doping technique used. A critical review of the current data is provided.     

Positron annihilation in C60 fullerites and fullerene-like nanovoids

C₆₀ fullerites, fatty-acid triglycerides containing nanovoids, and triglycerides with dissolved fullerenes were studied by means of positron annihilation spectroscopy. Two types of nanovoids with mean radii of 0.48 and 0.34 nm were found in the fatty-acid triglycerides. The nanovoids of the latter type change size in the course of structural relaxation and ordering at room temperature. The nanovoid radius is stabilized at the value of 0.355 nm, equal to the radius of C₆₀ fullerenes, when the latter are dissolved in triglycerides. Tetrahedral interstitials in the f.c.c. lattice are shown to be the most probable positron-annihilation sites in C₆₀ fullerites. The shell of the C₆₀ molecule is a potential barrier for positron penetration to the interior of the fullerene. Vice versa, the nanovoid in triglycerides is a potential well for positively charged particles: positrons, protons and cations.     

Electrochemical behavior of C60 films and C60/lipid films in ionic liquids

Cyclic voltammograms (CVs) of C₆₀ films and C₆₀ embedded in cast films of triple-tailed cationic surfactant solutions and salt-free zero-charged cationic/anionic (catanionic) surfactant vesicles on glassy carbon electrode in a typical room-temperature ionic liquid (RT-IL), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), were examined. CVs show typically electrochemical oxidation and reduction. The salt-free zero-charged catanionic surfactant bilayer vesicles were determined by freeze-fracture transmission electron microscopy (FF-TEM) images and small-angle X-ray scattering (SAXS) measurements. The cast films of the salt-free zero-charged catanionic surfactant vesicles incorporated C₆₀ molecules were employed to study the electrochemical properties in RT-ILs, which would open new fields for the bulk electronic properties of fullerenes or their derivatives in ionic liquids.     

Electric current induced light emission from C60

We report the luminescence of C₆₀ crystals and films due to the passage of an electrical current. The current-voltage behavior is highly non-linear with light-emission beyond a threshold voltage. The emission spectrum is featureless and resembles black-body radiation with an effective temperature on the order of 1700 K. We report experiments aimed at distinguishing between electro- and thermoluminescence.     

State of hydrogen molecules confined in C60 fullerene and carbon nanocapsule structures

The combination of PM3 semi-empirical method for geometry optimization, and ab initio DFT (density functional theory) for energy calculation, is used to study the configurations of hydrogen molecules at 0 K within the vacuum of C₆₀ fullerene and carbon nanocapsules. The obtained structural information including the molecular arrangement and structural state of the clusters of H₂ are mainly determined by two kinds of repulsive energies, namely that between the H₂ molecules and the wall of the spheroidal or capsule like carbon structure, and that between interacting H₂ molecules. It is further established that the repulsive energy among the H₂ molecules is not purely a function of the number of encapsulated H₂, and there is a tradeoff between the two kinds of repulsive energies.     

Theoretical calculations of the structure and UV-vis absorption spectra of hydrated C60 fullerene

A combined Monte Carlo simulation with semiempirical quantum mechanics calculations has been performed to investigate the structure of hydrated fullerene (C₆₀HyFn) and the influence of hydration on its UV-vis spectra. The statistical information of the C₆₀ fullerene aqueous solution (C₆₀FAS) is obtained from NPT ensemble including one C₆₀ fullerene immerses in 898 water molecules. To obtain an efficient ensemble average, the auto-correlation function of the energy has been calculated. The analyzed center-of-mass pair-wise radial distribution function indicates that, on average, there are 65 and 151 water molecules around the first and second hydration shells, respectively, of a single C₆₀ molecule. To calculate the average UV-vis transition energies of C₆₀HyFn, only the statistically uncorrelated configurations are used in the quantum mechanical calculations (INDO/CIS). These involve hundreds of supramolecular structures containing one C₆₀ fullerene surrounded by the first hydration shell. The calculated average transitions at 268 and 350 nm are in very good agreement with the experimental prediction.     

Molecular dynamics simulation of triaxial compression of C60 and C80 solids

Present work investigates the triaxial compression behavior of face-centered cubic C₆₀ and C₈₀ solids using molecular dynamics simulation. Second-generation empirical bond-order potential governs the atomic interactions within a C₆₀ or C₈₀ molecule, whereas van der Waals potential dominates the interactions between C₆₀ or C₈₀ molecules. The equilibrium lattice spacings for C₆₀ and C₈₀ solids are obtained as 14.26 Å and 15.56 Å, respectively. Investigation focuses on the effects of: (i) van der Waals potential, (ii) temperature and (iii) loading rate, on the bulk moduli and hydrostatic stress vs. volumetric strain curves of C₆₀ and C₈₀ solids. Our results showed that these properties are dependent on loading rate and the choice of van der Waals potential, but insensitive to temperature change.     

Reaction rate coefficient of fullerene (C60) consumption by soot

The reaction of fullerene molecules with soot was studied by contacting sublimed C₆₀ fullerenes with commercially available carbon black particles at different temperatures in the range 1023-1273 K. Fullerene mass data collected both pre- and post-reaction were fit to a simple first-order kinetic model and yielded a temperature-dependent reaction rate expression. The calculated collision efficiency of the reaction is of the order 10⁻⁸ and the activation energy is ∼9.8 kcal mol⁻¹, which would be consistent with a surface diffusion reaction or a heterogeneous reaction. Simple extrapolation of the observed rate to the conditions of a fullerene forming flame would give a consumption rate six orders of magnitude too small to account for the rate of fullerene consumption observed in the post-flame zone of a fullerene-forming benzene/oxygen/argon flame. Extrapolation of the reaction rate to flame conditions also shows that the rate of consumption calculated here is too small to account for observed oscillations in the fullerene concentration profile which can be related to changes in the relative rates of consumption and formation. Calculation of activation energies required for the extrapolation of rates observed here to match those observed in flames yields significantly larger values than those obtained in the present study and are so large as to suggest that mechanisms other than those studied here control fullerenes consumption in flames. Other mechanistic possibilities for the consumption of fullerenes in flames include reactions of fullerenes with other flame species and fullerene-soot reactions in which soot reactivity depends on soot reactions with other flame species.     

Fluorination of pressure-polymerized C60 phases

The reactivity of O-, T- and R-phases of the high pressure-high temperature (HPHT) polymerized C₆₀ towards gaseous fluorine in the temperature range of 50-250 °C was investigated. The reaction products were characterized by FTIR, powder X-ray diffraction, SEM, EDX, and VTP-EIMS to determine the bulk stoichiometries, bonding patterns, phase compositions, crystalline structures and thermal decomposition behavior of the fluorinated polymers. At 1 h isothermal treatment duration, fluorinated products with various bulk stoichiometries were obtained from different polymer phases with the R-phase showing the highest fluorine uptake. At 250 °C, all C₆₀ polymers showed partial decomposition to unfluorinated C₆₀ monomer under fluorine atmosphere. At 200 °C, the fluorination of R-phase yielded a pure fluoropolymer most likely having a {C₆₀F_x}_n (x = 36-44) composition. The same fluoropolymer was presumably obtained from O- and T-phases in lower yields. The linear chain structure was suggested for this new fluorocarbon polymer in agreement with the molecular mechanics modeling calculations.     

Ecotoxicology of carbon-based engineered nanoparticles: Effects of fullerene (C60) on aquatic organisms

To more fully assess the toxicity of water-soluble fullerene (nC₆₀), acute toxicity assays were performed on several environmentally relevant species. Included were the freshwater crustaceans Daphnia magna and Hyalella azteca, and a marine harpacticoid copepod, and two fish species, fathead minnow Pimephales promelas and Japanese medaka Oryzias latipes. The latter two species were used to assess sublethal effects of fullerene exposure by also assessing mRNA and protein expression in liver. Because prior studies found that both sonication and using tetrahydrofuran to solubilize fullerene increased the toxicity of nC₆₀, the nC₆₀ used in this study was prepared by stirring. For the invertebrate studies, nC₆₀ could not be prepared at high enough concentration levels to cause 50% mortality (LC₅₀) at 48 or 96 h. The maximum concentrations tested were 35 ppm for freshwater and 22.5 ppm for full-strength (35 ppt) seawater, since at higher concentrations the nC₆₀ precipitated out of solution. Daphnia 21-day exposures resulted in a significant delay in molting and significantly reduced offspring production at 2.5 and 5 ppm nC₆₀, which could possibly produce impacts at the population-level. For the fish, we found that neither the mRNA nor protein-expression levels of cytochrome P450 isozymes CYP1A, CYP2K1 and CYP2M1 were changed. The peroxisomal lipid transport protein PMP70 was significantly reduced in fathead minnow, but not medaka, indicating potential changes in acyl-CoA pathways.