Synthesis of fullerenes from carbon powder by using high power induction thermal plasma

Radio frequency (rf) inductively coupled thermal plasma (ICTP) was used to fabricate fullerenes (C₆₀,C₇₀, etc.) by direct evaporation of carbon powder injected into the plasma. Spectroscopic observation of the plasma was made for molecular band spectra of C₂ and atomic lines of C. The formation of fullerenes C₆₀ and C₇₀ as well as higher fullerenes were checked and recognized by high performance liquid chromatography (HPLC) and time-of-flight mass spectrometer (TFMS). The suitable conditions for the synthesis of fullerenes within the experimental conditions adopted were 10-kPa plasma pressure, with a considerably higher flow rate of approximately 150 l/min for mixed-gas condition of Ar, He and CO₂, with carbon powder of average diameter 20 μm. The results showed that the productivity of fullerenes has a relation to the intensity of C₂ molecular and C atomic spectra from the induction plasma. Mixing of Si with C particles has a kind of role in enhancing the synthesis rate of fullerenes C₆₀, as well as the higher order fullerenes.     

Modifying the buckyball

Structural and electronic properties of carbon clusters, in particular the C₆₀ “buckyball” molecule as well as structurally and chemically modified fullerenes, are calculated using a combination of predictive ab initio techniques and parametrized total energy schemes. These calculations indicate that single- and multi-shell fullerenes are the most stable C_n isomers at T = 0 for n < 20. More open structures are favored by entropy at higher temperatures. Upon interaction with donor elements, C₆₀ molecules form stable M@C₆₀ endohedral complexes; analogous acceptor-based complexes are unstable. Solid C₆₀ reacts with alkali metals and forms a stable intercalation compound which shows superconducting behavior. The relatively high value of the critical temperature for superconductivity can be explained quantitatively within the Bardeen-Cooper-Schrieffer formalism.     

Which Fullerene Is Appropriate to Host C20? A Molecular Modeling Study

Molecular mechanics (MM) and molecular dynamics (MD) study of fullerene complexes of the short-lived “smallest fullerene” C₂₀ with icosahedral C₆₀, C₈₀, C₁₄₀, C₁₈₀, and C₂₄₀ of I_h, I_h, I, I_h, I_h symmetry, respectively, using CVFF and ESFF forcefields revealed that the most stable complexes can be formed only if the intershell distance between fullerenes is similar to that in graphite or in experimentally observed buckyonions. The fullerenes C₁₄₀ and C₁₈₀ are the smallest suitable candidates to form such complexes with C₂₀. Similarly to the stabilization of short-lived cyclobutadiene or benzyne in hemicarcerands, C₂₀ is expected to be stabilized inside the C₁₄₀ and C₁₈₀ cages which would enable a deeper understanding of the highly nonplanar system of conjugated bonds of this exotic molecule.     

The Dark Reaction of C60 and of C70 with Molecular Oxygen at Atmospheric Pressure and Temperatures between 300 K and 800 K

The solid fullerenes C₆₀ and C₇₀ react in deliberate and adventitious situations with molecular oxygen in a wide range of temperatures. Using thin films and polycrystalline bulk samples with well-defined structures and chemical histories we investigated the complex process from molecular intercalation over atomic adduct formation to deep oxidation and polymerisation which together describe the oxidation reaction. A combination of photoemission (UPS, XPS) photoabsorption (XAS), FT-IR, temperature-programmed and isothermal gravimetric measurements and DSC allowed to identify the existence and gradual interconversion of the three types of solid products besides the gas phase products CO and CO₂. Both fullerenes react along the same reaction path. The difference of the molecular structures results in different activation barriers in the initial step of intercalation and in the final step of cage-opening. The overall reactivity of both fullerenes is quite similar at temperatures above the gasification onset of 570 K. The formation of the various adduct compounds was found to exert a detectable influence upon the overall molecular shape of the fullerenes and a small effect on the electronic structure was observed. The only moderate differences in electronic and geometric structures of pristine and initially oxidised fullerenes precludes a pronounced molecule-by-molecule reaction control and allows the topochemistry of the intercalation to control the shape of the reaction interface. This control is less effective for C₇₀ than for the C₆₀ fullerene.     

Addition of “Living” Polymers onto C60.

Various “living” polymers were grafted onto C₆₀ The number of arms of the so obtained “star” molecules can be controlled by stoechiometry and/or by varying the reactivity of the carbanion on the “living” chain against a double bond on the C₆₀. Even the oxanion of “living” polyethylenoxide is able to add onto the reactive double bonds on C₆₀. In some conditions, the carbanions present on these alkaline salts of grafted fullerenes becomes able to initiate anionic polymerization of vinyl monomers. Using “living” poly(phenylvinylsulfoxide) as a precursor polymer for PA, polyacetylene chains could be attached to the fullerene.     

Pyrolytic Trifluoromethylation of [76]-, [78]-, [84]-, and Aza[60]Fullerenes with Silver Trifluoroacetate; Evidence for Coordination of Fullerenes to Silver

Pyrolytic trifluoromethylation of [76], [78], [84], and aza[60]fullerenes with silver trifluoroacetate at 300°C results in extensive polyaddition of up to 18, 18, 20 and 20 CF₃ groups, respectively. In contrast to trifluoromethylation of [60]- and [70]fullerenes that give a full range of derivatives ranging upwards from C_n(CF₃)₂, [76]-, [78]-, and [84]-fullerenes only give C_n(CF₃)_6-18 derivatives, largely in the 10-12 CF₃ range; reaction with [76]fullerene is accompanied by formation of C₆₀(CF₃)₆ attributed to cage fragmentation. For aza[60]fullerene the hexa-addition level dominates, in contrast to its other reactions which give predominantly penta-addition products. All the compounds showed peaks at 1256±2 and 1180-1190 cm^-1, due to the CF₃ group, and peaks in this region are shown also by the soluble extract obtained on trifluoromethylation of nanotubes. As in trifluoromethylation of [60]- and [70]-fullerenes, the products obtained initially are involatile, attributed to formation os silver complexes; these are decomposed on subsequent solution in toluene. Mixed isomeric trifluoromethylated C₆₀F₈ derivatives viz. C₆₀F₇CF₃, C₆₀F₆(FG₃)₂, C₆₀F₅(CF₃)₃ and C₆₀F₄(CF₃)₄, and C₆₀F₄CF₃CF₂CF₃ (a C₆₀F₆ derivative) have been isolated from fluorination of [60]fullerene with MnF₃/K₂NilF₆ at 510°C.     

The Dark Reaction of C60 and of C70 with Molecular Oxygen at Atmospheric Pressure and Temperatures between 300 K and 800 K

The solid fullerenes C₆₀ and C₇₀ react in deliberate and adventitious situations with molecular oxygen in a wide range of temperatures. Using thin films and polycrystalline bulk samples with well-defined structures and chemical histories we investigated the complex process from molecular intercalation over atomic adduct formation to deep oxidation and polymerisation which together describe the oxidation reaction. A combination of photoemission (UPS, XPS) photoabsorption (XAS), FT-IR, temperature-programmed and isothermal gravimetric measurements and DSC allowed to identify the existence and gradual interconversion of the three types of solid products besides the gas phase products CO and CO₂. Both fullerenes react along the same reaction path. The difference of the molecular structures results in different activation barriers in the initial step of intercalation and in the final step of cage-opening. The overall reactivity of both fiiHerenes is quite similar at temperatures above the gasification onset of 570 K. The formation of the various adduct compounds was found to exert a detectable influence upon the overall molecular shape of the fiiHerenes and a small effect on the electronic structure was observed. The only moderate differences in electronic and geometric structures of pristine and initially oxidised fullerenes precludes a pronounced molecule-by-molecule reaction control and allows the topochemistry of the intercalation to control the shape of the reaction interface. This control is less effective for C₇₀ than for the C₆₀ fullerene.     

Electronic structure and electrical transport characteristics of C60, 2C60 and 4C60 fullerene molecules

The extended H?ckel method and the Green s function method were used to calculate the electronic structure and electrical transport of Au electrode-C₆₀, 2C₆₀ or 4C₆₀ fullerene-Au electrode systems. Furthermore, their electronic structure and electrical transport characteristics were compared and analyzed. The results show that (i) owing to the contact with the Au electrodes, the C₆₀, 2C₆₀ and 4C₆₀ molecules change in their electronic structures significantly, and their energy gaps between LUMO and HOMO are narrow; (ii) the bonding between C₆₀, 2C₆₀ or 4C₆₀ fullerene and Au electrodes is partially covalent and partially electrovalent; and (iii) the conductance of the three fullerenes conforms to the order of C₆₀>2C₆₀>4C₆₀.     

Characterization of Fullerenes Obtained from Boron Nitrite Containing Graphite Electrodes: Electronic Structure of C60-X-YBXNY and Deformed C60

The electronic structure of C₆₀ molecules with carbon substituted by other elements such as boron or nitrogen has been calculated for molecules in the singlet state with even number of electrons. Therefore, in the case of boron and nitrogen substituents, calculations have been performed for ionized molecules. The results obtained from our density functional calculations prove than in the C_60-x-yB_xN_y molecules both HOMO and LUMO levels are split due to the lower symmetry of the molecule Influence of deformation of C₆₀ molecules on electronic states, which is interesting because fullerenes may undergo deformation in solvate crystals, is also shown These C₅₉B and C₅₉N, as well as C₅₈BN molecules are of special interest because samples containing such species can be prepared The Electron Spin Resonance (ESR) in those samples differs from the results obtained in pure C₆₀ samples, it consists of several lines which are very sensitive to the temperature.     

Preparation of Fullerene Derivatives by Resistive Heating With Graphite Crucible

We report here the preparation of both endohedral fullerenes (La°C₈₂, La₂°C₈₀) and exohedral fullerenes (C₆₀ (C_mO_n)) by the resistive heating with graphite crucible filled with lanthanum oxide.     

Raman Spectra of Chlorinated C60 and C70 Fullerenes

Raman spectra of chlorinated C₆₀ and C₇₀ fullerenes prepared by photochlorination have been reported and discussed. The Raman lines suggest structural analogies with already fully characterized brominated fullerenes. Intense laser light irradiation cause a decomposition of fullerene chloro-derivatives leading to the formation of C₆₀ polymer.     

Fullerene Research in Poland

Main results of the investigations of fullerene and its derivatives are briefly reviewed. Such topics as plasma spectroscopy, fullerenes and nanotubes formation, C₆₀ carbyne knots, fullerene reduction and doping, charge transfer states and electroabsorption of C₆₀, electrical conductivity, superconductivity, ESR properties, fullerene clathrates, C₆₀/C₇₀ complexes with organic donors, fullerene adducts, hydrogenated fullerenes, metallofullerenes and carbon nanotubes are discussed.     

Imaging Fullerene C60 with Atomic Resolution Using a Scanning Tunnelling Microscope

C₆₀ was purified and imaged utilizing scanning tunnelling microscopy (STM) in a constant current mode. By fixing the Fullerenes on the substrate (“frozen state” - no movement or rotation), direct imaging of C₆₀ with atomic resolution was possible, showing one pentagon and one hexagon carbon ring of C₆₀.     

UNSTABLE PRODUCTS FROM THE OZONATION OF C60 IN SOLVENTS

The ozonation of a solution of C₆₀ in toluene results in the formation of several unstable compounds C₆₀X and C₆₀Y[1-5] which decay completely at room temperature within about 1 h. The products are oxides C₆₀O[6,6], C₆₀O₂[I], C₆₀O₂[II], and possibly an isomer of C₆₀O₃. The transformations are not due to oxidation by atmospheric oxygen but are spontaneous. The ozonation of a solution of C₆₀O[6,6] in toluene results in the formation of two unstable compounds C₆₀Z[1-2] which also decay completely to C₆₀O₂[I] and C₆₀O₂[II]. It is suggested that all unstable “parents” are ozonides.     

Instrumental Neutron Activation Analysis of Trace Element Impurities in Graphite Soot, Gold Grade C60 and C70 and in Super Gold Grade C60

Actually the purity of fullerenes is in general characterized as the percentage of the main component (e.g. C₆₀) versus the higher fullerenes (e.g. C₇₀, etc.) and/or in a few cases of some other polycyclic organic components.

We are reporting here for the first time detailed data on trace element impurities in fullerene precursors and in the main fullerenes. For studying the element impurities instrumental neutron activation analysis (INAA) was used. Our measurements have shown that all these materials contain trace elements at ppb (ng/g) ami ppm (μg/g) levels. From the results it comes (Hit that in the future investigations of the elemental purity of the C₆₀ and C₇₀, and even of the higher fullerenes would have to be also considered when their electrical, magnetic, etc. properties are studied.     

C60 Dimers Revisited

Several dimers: C₁₂₀, C₁₄₀, C₁₃₀ and their oxygen-containing derivatives as well as some oligomers or polymers of the well known spherical fullerenes have been synthesized so far. Some of the formulas attributed to such compounds represent the first steps in a more complex fate leading to trivalent capped tubulenes. Such a way could even be followed by functionalized fullerenes. A detailed network transformation is given for C₆₀ and its mono- and di-epoxy derivatives. Semiempirical calculations support the presented findings.     

Computations of metal-covered C60 and C70

Semiempirical quantum-chemical PM3 calculations are reported for a new class of exohedral metallo-fullerenes – metal-coated or metal-covered fullerenes: C₆₀M_n and C₇₀M_n. The exohedral species have been observed in gas phase, however, their geometrical and electronic structures are not known yet. Relatively-even metal-atom distributions over the fullerene rings are considered – such regular forms are computed for M = Be, Mg, Al. Three selected types of stoichiometries are treated in particular: C₆₀M₁₂/C₇₀M₁₂ (metal atoms above all pentagons), C₆₀M₂₀/C₇₀M₂₅ (metal atoms above all hexagons), and C₆₀M₃₂/C₇₀M₃₇ (metal atoms above all rings). If an odd number of electrons should result (Al), the related cation is computed, or one metal atom added or removed. This interesting arrangement above the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational technique used – the PM3 semiempirical method is parametrized only for some selected metals. Metal-layer atomization heats are suggested as a stability measure. Structural characteristics are also presented and interesting reductions of the cage symmetry are found. Their relationship to Jahn–Teller effect is discussed. The metal covered fullerenes can represent models for metal catalysis in the nanotube synthesis and could eventually lead to new interesting materials.     

An Invariant of Carbon Arc Synthesis of Fullerenes

The yields of fullerenes C₆₀ and C₇₀ were determined in a wide range of controlled parameters. The total yield of fullerenes varied from 3 to 24%. The molar relative contents C₆₀/C₇₀ appeared to be constant for all samples of toluene extracts of the soot and equal to 5.06 ± 0.1. The accuracy of this constancy (± 2%) was determined by application of a special mathematical processing to the spectra of toluene extracts.     

Photophysical and Photochemical Properties of the Fullerenes

This article primarily reviews the recent work of this laboratory on the photophysical and photochemical properties of the fullerenes, C₆₀ and C₇₀, in solution. Properties of the excited singlet and triplet states of them have been characterised by static and transient fluorescence spectroscopy. Charge and/or proton transfer dynamics in ground and excited singlet state of the fullerenes with the amines have been investigated in detail. Strong evidence has been obtained for weak interaction between the fullerenes and the aromatic solvents, like benzene, toluene etc. A novel method has been described to solubilise C₆₀ in aqueous solution via formation of inclusion complex with γ-cylodextrin.     

Fullerane, the Hydrogenated C60 Fullerene: Properties and Astrochemical Considerations

Hydrogenated C₆₀ fullerene, C₆₀H₃₆ was prepared in different solvents using Zn/HCl as reducing agents. The structure of C₆₀H₃₆ was confirmed both by electronic and FT-IR spectroscopy and the purity of the reaction product was checked by HPLC analysis. It has been confirmed that C₆₀H₃₆ is not stable in air, especially in presence of light which enhances the oxidation. The oxidation of C₆₀H₃₆ was studied by FT-IR spectroscopy and by differential scanning calorimetry (DSC) in air; the formation of hydroxyl groups on the fullerene cage and ketonic groups (involving cage breakdown) have been detected. Furthermore, the action of O₃ on C₆₀H₃₆ was investigated and it has been found that O₃ exerts practically the same effect of air but causing an enhanced cage breakdown. The thermal stability of C₆₀H₃₆ was checked by a thermogravimetric analysis (TGA) coupled with a differential thermal analysis (DTA) under N₂ flow. The vaporization of C₆₀H₃₆ occurs at very high temperature: the DTA trace has shown an endothermic peak at 540°C (at a heating rate of 20°C/min). C₆₀H₃₆ shows an electronic absorption spectrum with a maximum at about 217 nm and it is able to match both in position and in half width the peak at 217.5 nm observed in the spectrum of the interstellar extinction of light which was attributed to hydrogenated, radiation processed and thermally annealed carbon dust. Similarly, the absorption spectrum of C₆₀H₃₆ is able to match several infrared emission bands (called UIBs) detected from certain astrophysical objects like the protoplanetary nebulae (PPNe). It is proposed that hydrogenated fullerenes can be used as model compounds in the laboratory simulation studies of interstellar carbon dust.