A New Method to Obtain C60 and Higher Fullerenes

Abstract

In this paper we describe a new experimental method to produce fullerenes with substantial yield, based on the simple vaporisation of graphite rods in a low-frequency plasma jet. The proposed method is sustained by our earlier work on differently configured carbon structures produced by condensing carbon from vapor phase. Our goal is to provide a better-controlled, straightforward and low-cost method for generating large quantities of fullerene-containing carbon soot. A key feature of the vaporisation apparatus is that it is a combination between arc discharge and plasma jet dissociation. The vaporisation of the carbon rod is not driven by ohmic heating but rather by a discharge between the end of graphite rod and the cathode of the plasma-jet system. There are several attractive features of this procedure. First, variations in the parameters of plasma jet may be used to control systematically the size distribution of fullerene clusters. Secondly, this could be used for solid studies of larger fullerenes clusters since large clusters are only produced in low yield by the arc-discharge method. Also, we believe that there is great potential to investigate systematic doping of fullerene cluster using this technique since dopants can be easly introduced into the plasma jet prior to thermal dissociation.     

Synthesis of Fullerenes by Ablation Using Pulsed and cw-Nd:YAG Lasers

Macroscopic quantities of fullerenes were synthesized by ablation of a graphite rod heated with a cw-Nd:YAG laser in Ar gas flow. The ablation was carried out by irradiation of fundamental, second, and third harmonic light emitted from a pulsed-Nd:YAG laser. It was necessary to heat the graphite rod up to a higher temperature than 700 °C for producing macroscopic quantities of fullerenes. The amount of ablation, the fullerene yield and the relative fullerene yield increased with increasing temperature of graphite. The fullerpne yields depended on the pressure of Ar buffer gas as well, in which the j ablation plume was formed, and an optimum pressure range was found to be 53-80 kPa. Relatively high yield of C₇₀ was obtained by ablation method under these conditions.     

Optical Emission Study of RF Thermal Plasma During Fullerene Synthesis

Results on studies of molecular spectra emitted in the initial stages of fullerene synthesis during processing of graphite powder in RF thermal plasma conditions are presented in this work. CN—usually present in carbon plasmas—and C₂ were found as dominant molecular species. The role of CN radicals on the fullerene formation was discussed in detail. Intensities of CN and C₂ lines were studied against the composition of gas phase and the feed rate of graphite powder. The rotational-vibrational temperatures of CN species were calculated by fitting the experimental spectra to the simulated ones. It was concluded that in the plasma region CN radicals could be formed by the reaction of C₂ with atomic nitrogen at smaller loads. This reaction lowered the yield of fullerenes. At larger loads, C₂ formation was decreased due to lower temperature of the particles compared to smaller load. The CN radicals were produced by the surface reaction of the hot carbon particles with nitrogen atoms. Results confirmed that for effective fullerene synthesis, the nitrogen content of the precursors and the plasma gases should be minimized.     

New Approach in Synthesis of Carbon Allotropes in Large Quantities

This work proposes a new method for mass production of carbon allotropes synthesis by arc-plasma. In particular, carbon vapor is produced from graphite electrode by introducing a buffer gas into the electrode gap to generate an intense gas outflow and fast evacuation of the produced vapor from the hot plasma zone. Study of the influence of the gas flow dynamics differently supplied to the hot plasma zone during the fullerenes synthesis was studied and the relationships between different gas flow rates, applied power, diameters of used electrodes and fullerenes productivity and yield were established. The new proposed method allows a significant increase of fullerenes productivity.     

Formation of Various Carbon Nanoclusters from Laser‐Produced Carbon Plasma

Abstract

Laser vaporization of graphite proved to be an unbelievably flexible instrument allowing selective fabrication of various novel forms of carbon (diamond or fullerenes or different types of carbon nanotubes). In this work, the extension of the laser vaporization method to composite fullerene‐containing carbon targets is presented. It is shown that, depending on the pressure in an experimental chamber, different products come out of the ablation plume and get deposited on the substrate. The experimental results of this work allow to suggest a mechanism of formation of different carbon forms from laser‐produced carbon plasma at different experimental conditions.     

乙炔等离子体热解法制备C60的研究

用乙炔高频等离子体热解法连续大批量合成含富勒烯的碳灰。用透射电镜（TEM）、X射线衍射（XRD）、紫外－可见光谱（UV－Vis）、红外光谱（IR）等测试手段对碳灰进行了分析研究，并测定了富勒烯C60的产率为2．5g／h。     

Amorphous shungite carbon: A natural medium for the formation of fullerenes

A comparative analysis of data on the density, porosity, and intermolecular space of high-carbon shungites, graphite, glassy carbon, and C₆₀ fullerite gave an estimate of the fullerene content in the shungite samples which agrees with the values obtained by electrochemical and polar solvent extraction methods. A low yield of fullerenes in the extracts obtained with nonpolar solvents is explained by the high polarity and large adsorption energy of fullerenes and related compounds.     

Extracting of fullerene-like nanoparticles from environmental soots

It is well known that burning graphite electrodes in electric arc is an efficient method for obtaining of fullerenes. However, fullerenes form in any sooting flames. Therefore, detection fullerene in natural burning fuel is of a great interest for understanding of mechanism of soot formation. This research extracted samples of environmental soots in toluene. Soots and products of extraction were characterized by UV-vis spectroscopy, sedimentation analysis, atomic force microscopy. Analysis of extracts found fullerene-like clusters. The concentration of particles decreased in the following sequence: charcoal – carbon black – gas soot.     

Kinetics of Fullerene Formation in a Contact Arc Generator

A kinetic model of the fullerene growth process in a contact arc generator is developed. On the basis of the kinetic model, the yield of the magic fullerenes C₆₀ and C₇₀ is calculated. The fullerene yield is determined by the temperature gradient in the fullerene formation zone, the carbon vapor concentration and the helium jet velocity in the interelectrode space. We found that the upper boundary value of the magic fullerene yield was about 20%.     

ON THE MECHANISM OF CARBON CLUSTERS FORMATION UNDER LASER IRRADIATION. THE CASE OF DIAMOND GRAINS AND SOLID C60 FULLERENE

It is shown by FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometry that carbon clusters considered to be the superior homologues of C₆₀ fullerene are formed by laser irradiation of both synthetic diamond grains or from pure C₆₀ fullerene crystals. The surfaces of the laser irradiated diamond or C₆₀ have been examined by Raman spectroscopy. In the case of diamond the Raman spectrum suggests the superficial formation of mixed carbon nanostructures consisting of disordered graphite, fullerenic nanostructures, onion-like carbon nanostructures and diamond-like carbon. Based on the Raman spectra of the surface and on data taken from the phase diagram of carbon, it is shown that the graphitization is needed in order to produce fullerenes from diamond under laser ablation conditions. In the case of C₆₀ fullerene, it is shown by Raman spectroscopy that the laser irradiation of the crystals causes initially their photopolymerization and after further irradiation their transformation into disordered graphite. Based on these results and on a literature survey on the formation of fullerenes from more than 15 completely different substrates, it is concluded that fullerenes are formed always when laser ablation leads to a graphitization of the laser-irradiated substrate. Some astrochemical implications of the conclusions have been discussed.     

Synthesis of carbon nanotubes by arc discharge in open air

In this work Carbon nanotubes have been synthesized by arc discharge in open air. A TIG welding ac/dc inverter was used as the power source for arc discharge. During each run of the arc discharge based synthesis, the anode was a low purity (approximately 85% C by weight) graphite rod. The effect of varying the atmosphere on the yield of soot of the carbon nanotube containing carbon soot has been studied. Various soots were produced, purified by oxidation and characterized to confirm formation of carbon nanotubes and their relative quality, using transmission electron microscopy, Raman spectroscopy, and XRD. It was found that the yield of soot formed on the cathode is higher when synthesis is carried out in open air than when carried out in a flowing argon atmosphere. When synthesized in open air, using a 7.2-mm-diameter graphite rod as anode, the yield of soot was around 50% by weight of the graphite consumed. Current and voltage for arcing were at identical starting values in all the experiments. This modified method does not require a controlled atmosphere as in the case of a conventional arc discharge method of synthesis and hence the cost of production may be reduced.     

Comparative analysis of two methods for synthesis of fullerenes at different helium pressures

Abstract

The results of the effect of helium pressure in the chamber on the amount and composition of the produced fullerenes (C₆₀, C₇₀, etc.) synthesized in the arc plasma with graphite electrodes are presented. The findings obtained when the arc is powered by a direct (DC) and alternating (AC) currents of low frequency were compared in the same chamber with the electrodes located at the same angle to each other. These two methods are drastically different. The complete conversion of graphite into fullerene soot in AC occurs, but a part of the graphite is converted into a cathode deposit that does not contain fullerenes in DC, the relative amount of which increases when decreasing the helium pressure in the chamber. The highest fullerene content in fullerene soot of 10.2?wt.% is produced at a pressure of 127.5?kPa in AC arc, but in DC arc, the highest content of fullerenes in fullerene soot of 8.3?wt.% is produced at a pressure of 33.3?kPa.     

ON THE MECHANISM OF CARBON CLUSTERS FORMATION UNDER LASER IRRADIATION. THE CASE OF DIAMOND GRAINS AND SOLID C60 FULLERENE

ABSTRACT

It is shown by FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometry that carbon clusters considered to be the superior homologues of C₆₀ fullerene are formed by laser irradiation of both synthetic diamond grains or from pure C₆₀ fullerene crystals. The surfaces of the laser irradiated diamond or C₆₀ have been examined by Raman spectroscopy. In the case of diamond the Raman spectrum suggests the superficial formation of mixed carbon nanostructures consisting of disordered graphite, fullerenic nanostructures, onion-like carbon nanostructures and diamond-like carbon. Based on the Raman spectra of the surface and on data taken from the phase diagram of carbon, it is shown that the graphitization is needed in order to produce fullerenes from diamond under laser ablation conditions. In the case of C₆₀ fullerene, it is shown by Raman spectroscopy that the laser irradiation of the crystals causes initially their photopolymerization and after further irradiation their transformation into disordered graphite. Based on these results and on a literature survey on the formation of fullerenes from more than 15 completely different substrates, it is concluded that fullerenes are formed always when laser ablation leads to a graphitization of the laser-irradiated substrate. Some astrochemical implications of the conclusions have been discussed.     

Kinetics of Fullerene Formation in a Contact Arc Generator

Abstract

A kinetic model of the fullerene growth process in a contact arc generator is developed. On the basis of the kinetic model, the yield of the magic fullerenes C₆₀ and C₇₀ is calculated. The fullerene yield is determined by the temperature gradient in the fullerene formation zone, the carbon vapor concentration and the helium jet velocity in the interelectrode space. We found that the upper boundary value of the magic fullerene yield was about 20%.     

Influence of Carbon Forms and Doped Elements in the Electrodes for the Production of Fullerenes

Production of fullereries using electrodes made of four kinds of pure carbon bearing materials and artificial graphite doped with seven kinds of elements such as B, Y, La, Gd, Ce, Pr and Nd have been examined. The yields of fullerenes were in a range between 12-20% of the soot produced for all pure carbon electrodes, even though the vaporization rate of amorphous carbon electrodes was about ten times higher than that of the natural graphite electrodes. However, for the doped graphite electrodes, the yield of fullerenes was less than 5%. From the above results it is concluded that fullerenes can be effectively produced with the pure carbon electrodes made of any form of carbon bearing materials used in this study and there is little material dependence on the yield of fullerences. Furthermore, the experiments using the artificaial graphite electrodes doped with metal elements suggests that the purity of the carbon electrodes affects the yeld of fullerenes greatly.     

Model of Improved Arc Generator for Fullerene Production

Process of fullerene formation and fragmentation is studied in various zones of fullerene generator. Fullerenes are produced utilizing carbon arc method, in which graphite electrodes are vaporized in a low pressure helium atmosphere by passing an electrical current through the electrodes. The effects of the gas pressure and intensity of the direct current on fullerene yield are investigated. Maximum fullerene yield of 8% is found in a raw soot after the evaporation of graphite by a 60 A direct current, in a helium ambient under the pressure of 8 kPa.     

A STUDY ON THE THERMAL STABILITY TO 1000°C OF VARIOUS CARBON ALLOTROPES AND CARBONACEOUS MATTER BOTH UNDER NITROGEN AND IN AIR

The thermal behavior of graphite, C₆₀ fullerene, fullerene black (carbon soot containing fullerenes), extracted fullerene black and diamond has been analyzed to 1000°C by TGA-DTA (thermogravimetric analysis and differential thermal analysis) under a nitrogen flow at a heating rate of 20°C/min. Very small weight losses have been recorded in the case of graphite and diamond. Furthermore no diamond graphitization has been observed. The sublimation of pure C₆₀ and the fullerene fraction of fullerene black (both pristine and extracted) has been observed and discussed.

The combustion reaction in air flow of graphite, C₆₀ and C₇₀ fullerenes, fullerene black (both unextracted and extracted), carbon nanotubes and diamond has been studied by TGA-DTA at a heating rate of 20°C/min. C₇₀ fullerene and fullerene black have been found to be the most reactive carbon materials with O₂. The role played by C₇₀ in the degradation of fullerites has been discussed. Among the carbon materials examined, the best resistance to O₂ attack has been shown by diamond and carbon nanotubes. The behavior of graphite is intermediate between diamond and fullerene blacks. The behavior of C₆₀ fullerene appears closer to that of graphite although it appears to be more reactive with O₂. Samples of graphite and carbon blacks N375 and N234 have been studied by TGA-DTA in air flow before and after a radiation treatment with neutrons or γ radiation. The effect of the radiation damage in the combustion reaction of these carbon materials has been discussed.     

Synthesis and Laser Desorption Fourier Transform Mass Spectrometry of Massive Fullerenes

Synthesis of buckminsterfullerene (C₆₀) is highly simple and the mechanism of the molecule formation is quite well understood. It makes easier the search for the optimum conditions necessary for formation of massive fullerenes (more than 100 carbon atoms). The most popular graphite arc method has been used for fullerene synthesis in our laboratories. Other available methods, like laser ablation and E-gun evaporation, were not suitable for achieving of the convenient conditions. Results of the synthesis have been analyzed by laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD FT/ICR-MS).

The conditions for massive fullerenes synthesis are discussed in the paper. The special care has been given to the LD FT/ICR-MS analysis. The importance of cooling gas during fullerene nucleation and growth in the very specific vacuum conditions is analyzed. The necessity for a thorough dynamic plasma diagnostic during the giant fullerenes growth is suggested.     

INFLUENCE OF DESIGN AND OPERATIONAL PARAMETERS ON YIELD OF FULLERENES

Fullerene production by arc-discharge method using graphite electrodes has been studied with respect to influence of different design and operational parameters on fullerene yield in a constant arc fullerene reactor. The design parameters like reactor length, diameter, heat transfer area and operational parameters like voltage, current, pressure, coolant flow rate, graphite evaporation rate and electrode diameter etc. have been experimentally studied in detail to establish a relationship between these parameters and fullerene yield. All the parameters affecting the yield have been correlated by dimensional analysis and an equation to calculate the fullerene yield is derived. It was observed experimentally as well as by dimensional analysis that many favorable parameters for getting good yield are linked with other parameters which also get changed if the favorable parameters are changed and thus it is difficult to make a substantial change in the yield of fullerenes.  The relationship established between the yield and parameters is however useful in optimising fullerene yield in a reactor and also helpful in designing a futuristic fullerene reactor of improved yield and productivity. The fullerene yield from different designs of reactors is obtained in the range of 4% to 20%.     

On the Formation and Stability of Fullerenes

Fullerenes (especially the higher ones) degrade to an insoluble material on storage in air. The C_2v(II) isomer of [78]fullerene is completely degraded after storage for 5 months. The extreme instability may account for the variable yields of this isomer obtained by different research groups. Strong heating of KBr discs of these insoluble materials produces CO₂ showing that they are oxygen containing. Given this inherent instability of fullerenes, the question arises as to why they are formed in the first place. It is argued that the formation is not a unique consequence of the need to eliminate dangling bonds produced during carbon vaporisation by the arc-discharge procedure. Rather, fullerene formation is favoured by a higher intramolecular dangling-bond collision frequency (i.e. much higher Arrhenius A-factor) for cage closure compared to the intermolecular collisions that lead to the more stable graphitic sheets.