Polymeric fullerene oxide films produced by decomposition of hexanitro[60]fullerene

A new poly(fullerene oxide) thin film material has been fabricated by thermal activation and electron bombardment on hexanitro[60]fullerene (HNF) film deposited on a Au substrate, all under vacuum conditions. The reaction products in the polymerization process are analyzed by XPS, UPS, IR, TGA-MS and LDI-MS techniques. It is found that the main effect of thermal and radiation treatments is to induce cleavage of -NO bonds from HNF molecules resulted in the release of nitric oxide gas and the formation of fullerene-bound oxyradicals, C₆₀-O₆. Spectroscopic evidence strongly suggests that rearrangement of fullerenic nitro moieties into nitrito groups is involved in the HNF decomposition process prior to the generation of reactive oxyradical intermediates. Consequently, the intermolecular coupling reaction of these oxyradicals leads to carbon polymer networks containing oxygen-bridged fullerenes. The thermally generated polymeric thin film is stable up to 900 K. Electron bombardment is also effective in both the decomposition of -NO₂ groups and the removal of -OH groups present in HNF films. UV irradiation at 365 nm alone is shown to be not as efficient for the polymer formation.     

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.     

New solid solvates of C60 and C70 fullerenes: The relationship between structures and lattice energies

The goal of this study was to establish the relationship between the X-ray and the thermodynamic data on fullerene’s solvated crystals. X-ray diffraction study of 12 solid solvates of C₆₀ and C₇₀ with different aromatic solvents have been performed. It has been demonstrated that the solid solvates under consideration were typical van-der-Waals complexes with the negative excess volumes, packing coefficients from 0.72 to 0.78, stable due to the formation of the fullerene to solvent bonds, reasonably described by six to 12 Lennard-Jones potentials. The atom-atom potential method has been used to describe both the crystal structures and the thermodynamics of the solid solvates. The minimization of the lattice energy with respect to the cell and rigid body parameters (T=0 K) has led to the crystal structures very close to the experimental ones. The minimum energies found have reasonably reproduced the calorimetric lattice energies of the solvates. The theory has also demonstrated its ability to account for the trends in thermodynamic stability of solid solvates, e.g., has predicted correctly the low stability of the hypothetical monoclinic solvate C₆₀·2C₆H₁₄.     

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.     

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.     

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).     

Soot-free synthesis of C60

A new synthetic medium for the production of C₆₀ has been found that does not produce soot. C₆₀ was produced in the liquid phase of an aerosol of precursor soot at 700 °C. The precursor soot aerosol, a high temperature stable form of hydrocarbon, was produced by pyrolysis of acetylene at atmospheric pressure in a flow tube reactor. At 700 °C, the effluent particles were found to contain PAHs, small hydrocarbons and fullerenes but no observable black material. However, when the reactor temperature was changed to 800 °C, soot was also produced in the effluent particles along with PAHs and other small hydrocarbons, and the fullerene product disappeared. These results show a clear competition between the production of fullerenes and other forms of carbon. The filter-collected effluent was shown to be completely soluble in conventional solvents suggesting the possibility of an efficient cyclic synthetic process. Fullerenes were only found in the particle phase implying the first observed liquid phase synthesis of C₆₀.     

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.     

Stability of hexagonal modification of fullerite C60

Stability of the hcp modification of fullerite C₆₀ under heating, compression, and mechanical grinding was studied. It was established that uniaxial load and mechanical grinding caused transition of the hcp modification into more stable fcc modification, while no transition was observed upon heating or hydrostatic pressure. The results were discussed in terms of the theory of the deformation of solids.     

Synthesis of a hexagonal modification of C60 using cryoextraction

Samples of the hexagonal close-packed phase (h.c.p.) of fullerite C₆₀ with a low (less then 7%) content of more stable face-centered cubic (f.c.c.) phase were synthesized using cryoextraction with n-hexane. X-Ray diffraction analysis and thermogravimetry were applied for characterization of the samples. The role of n-hexane in the formation of the h.c.p. structure is discussed.     

Dimerisation and polymerisation of C70 after thermobaric treatment

Four crystalline phases have been identified by TEM-analysis of C₇₀-samples after high-pressure-high-temperature (HPHT) treatment. All the phases are triclinic and exhibit distorted fcc lattices with a doubled c₀-parameter. Decrease of lattice parameters and increased density are explained by a two-stage process: dimerisation at the first stage and the formation of chains consisted of C₇₀-dimers at the second stage of thermobaric treatment.     

Soot-free synthesis of C60

A new synthetic medium for the production of C₆₀ has been found that does not produce soot. C₆₀ was produced in the liquid phase of an aerosol of precursor soot at 700 °C. The precursor soot aerosol, a high temperature stable form of hydrocarbon, was produced by pyrolysis of acetylene at atmospheric pressure in a flow tube reactor. At 700 °C, the effluent particles were found to contain PAHs, small hydrocarbons and fullerenes but no observable black material. However, when the reactor temperature was changed to 800 °C, soot was also produced in the effluent particles along with PAHs and other small hydrocarbons, and the fullerene product disappeared. These results show a clear competition between the production of fullerenes and other forms of carbon. The filter-collected effluent was shown to be completely soluble in conventional solvents suggesting the possibility of an efficient cyclic synthetic process. Fullerenes were only found in the particle phase implying the first observed liquid phase synthesis of C₆₀.     

Solid state studies of the C60. 2(CH3)CCl3 solvate

At room temperature, the hexagonal C₆₀. 2(CH₃)CCl₃ solvate (a = 10.13(1) Å, c = 10.84(1) Å), made of alternating layers of C₆₀ and solvent molecules, forms with a negative excess volume, and its desolvation enthalpy is virtually the same as the sublimation enthalpy of the pure solvent. Crystallographic and calorimetric studies vs temperature indicate that hexagonal C₆₀. 2(CH₃)CCl₃ changes at 211.7 K (1.3 kJ mol⁻¹ of solvate) into an intermediate triclinic phase which transforms at 189.7 K (4.1 kJ mol⁻¹ of solvate) into another triclinic phase.A crystallographic analysis in the series of hexagonal C₆₀. 2 YCCl₃ solvates (Y = H, Cl, Br, CH₃) reveals that: (i) the change in the unit-cell volume values is due to a change in axis c whose value depends on the size of Y, (ii) the molar volume of the solvates depends linearly on the molar volume of the solvents.Ageing studies at room temperature show that C₆₀. 2(CH₃)CCl₃ loses its solvent molecules within a few days or a few months, depending on storage conditions.     

Effective photopolymerization of C60 films under simultaneous deposition and UV light irradiation: Spectroscopy and morphology study

An effective method for uniform photopolymerization of C₆₀ films using simultaneous deposition and irradiation with ultraviolet (UV) light is described. The photopolymerization process was monitored as a function of irradiation time using Raman and infrared (IR) spectroscopy. New features appeared in the Raman (near the pentagonal pinch A_g(2) mode) and IR spectra (400-1500 cm⁻¹) after more than 20 h of UV irradiation testifying to the transformation of pristine C₆₀ to polymerized C₆₀ phases. Band shape analysis of the vibrational data revealed: (i) the degree of photopolymerization to be ∼90% after 20 h of irradiation, and (ii) the presence of orthorhombic, tetragonal, and rhombohedral phases in the photopolymerized films. Electron microscopy and diffraction studies revealed the amorphous nature of the photopolymerized films which comprised of crystals with a linear dimension of ∼40-60 nm. No evidence for cracks in the surface of the polymerized film was found. The proposed route for photopolymerization provides an opportunity to prepare extended polymeric C₆₀ films suitable for technological applications.     

Rheological examination of C60 in low density solutions

Fullerene (C₆₀) in solutions of decahydronaphthalene (decalin) and a petroleum solvent viscous standard (PSVS) were studied to understand the rheological properties of fullerene-laden solutions. Although fullerene solubility limits have been published for a variety of solvents, little has been reported on the effect that fullerenes have on flow properties of fluids. In this study, the solvents were studied up to the point of saturation, whereby measurements of solubility, density, viscosity and elasticity were conducted varying the concentration level of C₆₀. Rheological measurements based on molecular interactions and on distortion of the flow were studied. No significant elastic contribution from the fullerenes resulted for the solutions below saturation. A pseudoplastic behavior with a lubrication effect imparted by the C₆₀ molecules was observed in the decalin solutions at concentrations below the saturation level. The PSVS solutions remain Newtonian for all C₆₀ concentrations while leading to an increase in viscosity.     

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.