INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 39 (2006) 320–326 doi:10.1088/0022-3727/39/2/012

RF thermal plasma processing offullerenesB Todorovic-Markovic1, Z Markovic1,7, I Mohai2, Z Nikolic3,Z Farkas4, J Szepvolgyi2, E Kovats5, P Scheier6 and S Feil6

1 ‘Vinca’ Institute of Nuclear Sciences, POB 522, 11001 Belgrade, Serbia and Montenegro2 Institute of Materials and Environmental Chemistry, Chemical Research Center,Hungarian Academy of Sciences, POB 17, H-1525 Budapest, Hungary3 Faculty of Physics, University of Belgrade, POB 316, 11001 Belgrade, Serbia andMontenegro4 Department of Silicate Chemistry and Materials Engineering, Veszprem University,Egyetem u. 2, H-8200 Veszprem, Hungary5 Research Institute of Solid State Physics and Optics, Hungarian Academy of Sciences,POB 49, H-1525 Budapest, Hungary6 Leopold Franzens Universitat Innsbruck, Institut fur Ionenphysik, Technikerstr. 25,A-6020 Innsbruck, Austria

E-mail: zormark@vin.bg.ac.yu

Received 18 July 2005, in final form 8 November 2005Published 6 January 2006Online at stacks.iop.org/JPhysD/39/320

AbstractThis paper presents results on fullerene formation during the processing ofdifferent graphite powders in an RF thermal plasma reactor. Graphitepowders of different particle size and purity were fed into the reactor(Aldrich, KS4). Optical emission spectroscopy of atomic and molecularspecies is used as a diagnostic tool of the RF plasma flame. It was found thatrotational and vibrational temperatures of C2 radicals depend on the feedrate of the precursor used. The volumes of RF plasma flame duringprocessing of graphite powders have been calculated. By analysingscanning electron microscopy micrographs of soot, the evaporation rate ofthe precursors used has been evaluated as well. Based on the obtainedvalues of volumes of plasma flame and evaporation rate of starting powders,the concentration of C2 radicals has been calculated.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The many potential uses of fullerenes and nanotubes fornano-engineered materials include a possible electronicsuper/semiconductor, self-aligned electron field emission tips,catalyst cradles for highly selective chemical reactions and atransport medium for medical drug delivery [1, 2]. However,the high price of purified fullerenes has inhibited theirwidespread use up to now. Fullerenes are usually produced inhigh-temperature gases such as electric arcs, gases which areproduced by resistive heating or flames [3]. All these methodshave in common a very low fullerene production capacity.

Only a few reports on fullerene synthesis by plasmatorch using various carbon powders as precursors have been

7 Author to whom any correspondence should be addressed.

published up to now. Yoshie et al has synthesized fullerenesin a hybrid plasma which is characterized by the superpositionof an RF plasma and a dc arc jet operated at atmosphericpressure [4]. In Wang’s report, an RF inductively coupledthermal plasma was used to fabricate fullerenes by directevaporation of C or C–Si mixture powder by high enthalpyof the plasma [5]. Fulcheri et al concluded that plasmatechnologies are particularly adapted to the production ofnanoparticles. Being highly flexible they allow the use ofa wide range of carbon feed stock (solid, liquid, gaseous,alone or associated with a catalyst element) access to veryhigh temperatures and enthalpy densities unreachable withconventional combustion processes [6]. Dubrovsky et alsynthesized fullerenes by injecting carbon black precursor intoa dc plasma torch [7].

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RF thermal plasma processing of fullerenes

Figure 1. Experimental set-up.

Our first results on fullerene synthesis in an RF reactorhave promising results [8]. The synthesis conditions wereestimated by a kinetic model of fullerene formation constructedpreviously [9]. This model assumes that fullerenes form in aninert atmosphere at temperatures above 2000 K [10].

Besides carbon concentration and the velocity of theplasma flame, the temperature of species present in hot plasmais a significant parameter that affects the fullerene yield. In thisway, the measurement of temperature of the generated speciesis very important for understanding the processes that occurredin the RF reactor.

In this work, the process of fullerene formation in anRF thermal reactor by using different precursors as startingmaterial has been described. Optical emission spectroscopy(OES) of atomic and molecular species is used as a diagnostictool of particular effects occurring in a hot plasma flame.Rotational and vibrational temperatures of C2 radicals arecalculated as a function of the feed rate of precursors used.The concentration of C2 radicals present in plasma flame hasbeen calculated for both of the graphite precursors used.

2. Experimental procedure

Different graphite precursors (Aldrich, 99.3%, TimcalKS4-99.93%) were subjected to thermal evaporation followedby recombination in an RF thermal plasma reactor, atatmospheric pressure.

The RF power was produced by a generator operatingat 3–5 MHz. The plate power of 27 kW was inductivelycoupled to a TEKNA PL-35 torch connected to a water-cooledplasma reactor, cyclone and dust filter [8]. The scheme ofthe experimental set-up has been presented in figure 1. Theexperimental conditions are given in table 1. Both the plasmachamber and the precursor feeder were evacuated prior tothe experiments to avoid the presence of N2 and O2 in thesystem. Graphite powder was injected axially to the top of theplasma flame. Feed rates were from 60 to 190 g h−1 duringthe processing of the Aldrich powder. As for the KS4 powder,feed rates were in the range of 18–150 g h−1. Under the sameexperimental conditions, feed rates of the Aldrich powderwere larger compared with the KS4 powder due to the smallerstrength of the inter-particle forces.

Table 1. Experimental conditions of fullerene synthesis in an RFreactor.

Sheath Sheath Central Central Carriergas Ar gas He gas Ar gas He gas He He P

Powder (slpm) (slpm) (slpm) (slpm) (slpm) (v/v%) (kW)

Aldrich 16 52 7 12 6 75 27.9KS4 9 53 6 12 3 82 27.2

Emissions of plasmas were detected perpendicularly to theaxis of the plasma flame, through a quartz glass window, at adistance of 10 cm below the tip of the injecting nozzle. At thisposition, phenomena occurring at an early stage of fullereneformation can be observed. Upwards from this position,heating, activation and decomposition of graphite particles arethe dominant processes, while downwards, the combinationof active species to more complex structures takes place. Thestructure of the plasma flame was recorded by a Nikon digitalcamera through a quartz window on the chamber.

The wavelength was selected by a 55 cm focallength monochromator (medium resolution monochromator,TRIAX 550 Jobin-Yvon). Holographic gratings with 1200 and1800 grooves/mm have been used. The reciprocal dispersionswere 1.55 nm mm−1 and 1 nm mm−1, respectively. Light wascollected and transferred to the entrance slit (6 µm) by a multi-legged fibre bundle. The exposure time varied in the range of2–20 ms. Plasma emission was detected by an optical multi-channel analyser (CCD-3000, 1024 × 256 pixels). Spectrawere recorded and processed by PC.

The main impurities of the precursors were determined byICP-AES (Thermo Jarrel Ash Atomscan 25). The inorganiccontent of the graphite powders was dissolved by cc. HNO3/HFmixture using a microwave digestion technique.

The mean particle size of the precursors was measured bya laser diffraction technique using a Malvern Master ParticleSizer M6.10.

The morphology of soot was characterized by scanningelectron microscopy (SEM, Philips XL30 ESEM). Twosoftware packages, the Diameter Distribution Analyzer andthe Volume Analyzer, were developed in order to determinethe evaporation rate of precursor and volume of plasma flameduring processing of the precursor [11, 12].

XRD measurements were carried out for phase and crystalstructure identification with a Philips PW1710 diffractometerusing Cu Kα radiation and a graphite monochromator (45 kV,35 mA and divergence 1◦). Average crystallite size (L002) wasdetermined from a single-line profile Fourier (Voight method)analysis [13–15].

Fullerene content of the extracts was determined bymass spectroscopy (MS) and high performance liquidchromatography (HPLC). The MS measurements wereperformed by a high resolution, double focusing massspectrometer (Varian MAT CH5-DF) of reversed Nier-Johnsontype BE1 geometry combined with a second electrostaticanalyser E2. The mixture of fullerenes which consisted mainlyof C60, C70, C76, C78 and C84 was evaporated in a vacuumof about 10−6 Pa in an oven operating in the temperaturerange of 550–800 ◦C. In these conditions both negative andpositive ions could be formed. To produce positive ions, theeffusive beam was intersected by an electron beam having

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Table 2. Results of the XRD analysis.

Precursor FWHMa Lb002 (Å) D50/L002 Other phases

Aldrich 0.387 140 840 FexNy

Timcal KS4 0.429 118 310 None

a FWHM: width at the half peak height of reflection (002).b L002: mean crystallite size.

energy of 100 eV and current of 370 µA. The resulting ions areextracted perpendicularly to the fullerene and electron beamsand accelerated to the analysing part of the mass spectrometerby a potential drop of 3 kV.

The HPLC analysis was made by a Jasco LC-1500 HPLCsystem equipped with a UV/visible detector and an analyticalCosmosil Buckyprep (4.6 mm × 250 mm) column. Thechromatograms were detected at 330 and 525 nm in tolueneas eluent.

A Soxlet extraction method was used to separate fullerenesfrom the soot. Toluene solutions were dried in the rotaryevaporator. The mass of dried fullerenes was compared withthe mass of soot inserted in the Soxlet thimble in order todetermine the fullerene yield.

3. Results and discussion

Laser diffraction analysis has revealed that particle sizedistribution of Aldrich graphite was considerably widerthan that of KS4 graphite. The particle size of both theprecursors varied in the ranges of (1–27.5 µm) and (1–6.4 µm),respectively. The Aldrich graphite had a mean particle of11.8 µm, while that of the KS4 graphite was 3.7 µm [11, 16].

The main impurities of the graphite are Fe (0.41% and135 ppm), Si (0.18% and 50 ppm) and Ca (640 and 50 ppm)in Aldrich and KS4 powders, respectively. The content ofimpurities in the Aldrich powder is significantly larger than inthe KS4 powder.

The graphite structure that can be characterized by themean crystallite size is not the same for Aldrich and KS4(table 2). The crystallite size of KS4 powder is smallercompared to the crystallite size of Aldrich. The ironcontent of the Aldrich graphite can be assigned as ironnitrides. The content of impurities in KS4 powder is so smallthat the phase composition of the impurities could not beregistered.

During feeding of the graphite powders into the plasma,the registered optical emission spectra are characterized by thedominance of the C2 Swan system (transition d 3�g → a 3�u,sequence �v = +2, +1, 0, −1, −2) [17]. Molecular bandsof CN radicals were not present in the spectra due to carefulevacuation of the system before the experiments.

The rotational temperature of C2 radicals (Trot) wasdetermined by fitting the simulated and experimental spectra(d 3�g → a 3�u, �v = 0) [18]. Figures 2(a) and (b) showtypical emission spectra of C2 radicals during the feedingof Aldrich and KS4 graphite. The dashed lines representexperimental data, while the solid lines stand for the simulatedones. The bandhead widths in the experimental and simulatedspectra are equal to each other. According to figures 2(a) and(b), self-absorption phenomena are not detected [19]. Figure 3shows the effect of feed rate of precursors on the rotational

(a)

(b)

Figure 2. Fitting of simulated and experimental spectra of C2

radicals of (0–0) band at 516.5 nm. (a) Feed rate of KS4 was150 g h−1. Trot(KS4) = 5500 K. (b) Feed rate of Aldrich was105 g h−1. Trot(Aldrich) = 5700 K.

temperature of C2 radicals. The rotational temperature of C2

radicals for Aldrich and KS4 powders was in the range of4500–5800 K and 3500–5500 K, respectively.

The vibrational temperature of C2 radicals was determinedby computer code NMT [20]. Figure 4 shows the effect ofthe feed rate of precursors on the vibrational temperatureof C2 radicals. The vibrational temperature is in the rangeof 3400–4300 K for KS4 graphite and 3300–4200 K duringplasma processing of the Aldrich graphite powder. Vibrationaltemperatures of C2 radicals have similar values for bothprecursors.

The temperature of C2 radicals evaporating from thegraphite surface is equal to the sublimation temperature ofgraphite. Our results indicate that the measured vibrationaltemperature of C2 radicals is almost constant versus the feedrate for both precursors. Since at least 1000 collisions arerequired to change vibrational temperature, we concludedthat the vibrational temperature of C2 corresponds to thesublimation temperature of graphite. Therefore, the processof evaporation of graphite particles does not get completed inthe vicinity of the midpoint of the reactor window.

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Figure 3. Effect of graphite feed rate on rotation temperatures of C2

radicals during processing of KS4 ( ) and Aldrich (◦) powders.

Figure 4. Effect of graphite feed rate on vibration temperatures ofC2 radicals during processing of KS4 ( ) and Aldrich (�) powders.

The results presented in figure 3 indicate that dependencesof the rotational temperature of C2 radicals versus the feed rateare almost identical during processing of both precursors. Therotational energy of C2 radicals depends on heating (transferof energy from inert atoms) and cooling (non-radiative transfertowards cold graphite particles). Rotational–translationalequilibrium typically requires only ≈10 collisions [21].Despite the fact that the Aldrich plasma contains cold bigparticles of highly non-equilibrium plasma (figure 5(a)) andKS4 plasma contains mostly C2 radicals in the gas phaseof near-equilibrium plasma, the dependence of the rotationaltemperature of C2 radicals versus the feed rate is commonfor both precursors. At smaller loads, C2 radicals have timeto thermalize with the inert environment. At larger loads,the thermal gradient between hot inert gas and cold graphiteparticles is extremely large. In order to evaporate a large load,plasma tends to deliver a larger portion of energy to graphitei.e. a higher flux of inert atoms is directed towards graphite.In this way, collisions with C2 radicals occur, which result ina higher rotational energy of C2 radicals.

In our previous work, it was established that fullerenesynthesis is most effective when the mean rotationaltemperature of C2 radicals is lower than 3000 K [18]. In thoseconditions the evaporation of graphite is the most intensive.It is also known that caged structures could react mutuallyand form fullerenes in the most favourable temperature rangeof 2000–4000 K [22]. Since the temperature of C2 radicalsduring processing of the KS4 powder decreased to 3500 K,one could expect a very large fullerene yield when the feedrate of the precursor is low. This presumption has beenconfirmed experimentally. When the feed rate of powderhas low values (5.4–12.6 g h−1), rotational and vibrationaltemperatures of C2 radicals have almost the same values. Inthat case, thermodynamic equilibrium has been established.The KS4 powder is heated and evaporated much faster than theAldrich because the mean particle size of KS4 is almost threetimes lower than that of Aldrich. The thermal conductivityof KS4 is higher because the mean number of crystalliteboundaries is lower than that of the other powder (table 2).Favourable particle properties (small particle size and goodthermal conductivity) and lower starting particle velocity ofKS4 which affect the C2 radicals are created very fast anddetected already in the region 5 cm below the plasma nozzle(figure 5(b)). In the case of Aldrich powder, C2 radicals aredetected 10 cm below the nozzle (figure 5(a)). As can be seen,the green colour of the plasma flame which originates fromthe resonant band of C2 radicals is much more intense duringprocessing of KS4 powder. In figure 5(b), one can see theregion (cold channel) in which the particles are heated. Out ofthat region, only evaporation of C2 radicals from the surfaceof the graphite particles can be registered.

The volumes of plasma flame during feeding of theAldrich and KS4 powders have been calculated by analysingthe recorded photographs of hot gas. Several hundreds ofphotographs have been recorded during feeding of both thepowders. The mean values of volumes were 180 ± 50 cm3 forAldrich and 110 ± 20 cm3 for KS4, respectively. The volumeof plasma flame during processing of KS4 is smaller than theAldrich volume because of the following [23]:

– The particle diameter of KS4 powder is smaller than theAldrich particle diameter. In this way, the heat transfer tothe particle is proportional to the particle surface whereasthe required heat to evaporate the particle is proportionalto the particle volume.Therefore, smaller injected particles evaporate faster inKS4 plasma.

– During feeding of KS4 powder, carrier gas flow is low(3 slpm). In that case, the residence time of the particlesin the plasma is raised by decreasing the particle velocity.

In figures 6(a) and (b), SEM micrographs of the Aldrichgraphite powder and fullerene soot are presented. Infigures 7(a) and (b), SEM micrographs of KS4 graphite powderand fullerene soot are presented.

In fullerene mixtures separated from the soot by tolueneextraction, the individual fullerenes were identified by MS andHPLC. A positive mass spectrum of fullerenes produced byprocessing KS4 graphite is presented in figure 8. The mostabundant cations are C+

60, C+70 and C+

84 with ion signals of620.1 kHz, 136 kHz and 4.6 kHz, respectively. Peaks detectedbetween mass per charge of 720 and 840 can be attributed to the

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(a) (b)

Figure 5. RF plasma flame during processing of Aldrich (a) and KS4 (b) graphite precursors. Exposure time is 2.5 ms.

(b)(a)

Figure 6. SEM micrographs of (a) graphite powder (Aldrich) and (b) fullerene soot after processing in an RF reactor. Feedrate = 63.6 g h−1. The content of helium was 75%.

(a) (b)

Figure 7. SEM micrographs of (a) graphite powder (KS4) and (b) fullerene soot after processing in an RF reactor. Feed rate = 18 g h−1.The content of helium was 82%.

well-known neutral C2 loss which follows the electron impactionization and excitation [24].

The HPLC chromatogram of toluene extracts from KS4soot is presented in figure 9. Two major peaks (C60 and C70)

were detected in a weight ratio of 18 : 7 at retention times of7.7 min and 12.9 min, respectively. Oxygen derivates of C60

and C70 such as C60O, C60Ox and C70O are also present inthe mixture in small amounts (peaks 2, 3 and 5, respectively).There are several peaks at higher retention times (6, 7, 9, 10)that can be assigned to higher fullerenes (C76, C78, C84 andC86). Peak 8 presumably corresponds to C80 or C82. In the lackof calibration, quantitative analysis of higher fullerenes cannotbe performed. The chromatogram of toluene extracts fromAldrich soot was similar. The detailed results are presented intable 3.

Since the volume of plasma flame is smaller for KS4, onecould expect that the concentration of C2 radicals is higher. Inorder to check this presumption the degree of evaporation rateof used precursors has been calculated.

By analysing SEM micrographs of the fullerene soot, theevaporation rate of used graphite powders was calculated.The evaporation rate presents volume fraction of particlessmaller than 1 µm. We have assumed that all sphericallyshaped particles with diameters smaller than 1 µm have beencreated by the coalescence of carbon clusters and fullerenes.A significant amount of small particles was deposited on largenon-evaporated particles. Based on this presumption, it wasfigured that the evaporation rate for Aldrich powder is inthe range of 2–6 vol% depending on experimental conditions.As for the KS4 powder, the evaporation rate is from 4 to

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Figure 8. The positive mass spectrum of fullerene extract fromKS4. The spectrum was registered at electron energy of 100 eV andvaporization temperature of 863 K.

Figure 9. HPLC chromatogram (Cosmosil Buckyprep column,toluene eluent, 1 mL min−1 flow rate, 525 nm) of extracted soot fromprocessing of KS4 graphite. Assignation: 1: C60, 2: C60O, 3: C60Ox ,4: C70, 5: C70O, 6: C76, 7: C78, 8: C80 or C82, 9: C84, 10: C86.

11 vol%. The total concentration of C2 radicals in the plasmaflame was calculated using the values of evaporation rate,feed rate and volumes of plasma flame per unit time. Theobtained values of the total concentration of C2 radicals inplasma when the Aldrich powder was processed are in therange of (2–7.5)×1020 m−3. The concentration of C2 radicalsduring processing of the KS4 powder was in the range of(1.5–3.88) × 1020 m−3. The main parameter which affectsthe concentration of C2 radicals is the feed rate of the usedprecursor. In our experiments, the feed rate during processingof the Aldrich powder is larger compared with the feed rateof the KS4 powder. Therefore, concentrations of C2 radicalsduring processing of these different powders have almost thesame value.

The evaporation of C2 radicals and their depositionon large non-evaporated graphite particles are dynamicprocesses. The deposition of carbon clusters on coldgraphite particles (they do not reach the graphite sublimationtemperature) occurred simultaneously with the formation of

Table 3. Peak ratios from the total peak area.

C60 C70 C70Ox C76 C78 C80/82 C84 C86

Retention 7.8 12.8 15.38 18.2 19.7 20.6 24.8 30.3time

Aldrich 80.18 15.61 0.39 1.13 0.71 0.47 1.38 0.13KS4 83.93 13.13 0.40 0.84 0.51 0.35 0.79 0.06

larger carbon clusters and finally fullerenes. For this reasonthe effective concentration has been introduced. The effectiveconcentration of C2 radicals is defined as the concentration ofC2 radicals which participate in fullerene formation directly.The effective concentration of C2 radicals is smaller thanthe calculated values. Since Aldrich powder contains largerparticles, it can be assumed that the effective concentration ofC2 radicals is larger in the case of the KS4 flame comparedwith the Aldrich flame. As a result, the largest fullerene yieldduring processing of KS4 was 7.4%. By applying the Aldrichgraphite powder, fullerene yield was 1.8% under the sameconditions. Also, the efficiency of fullerene synthesis wasreduced by numerous impurities present in the latter powder.

4. Conclusion

In this work, two different graphite precursors have been usedas starting material during fullerene processing in the RFthermal plasma reactor. The particle diameters of the usedpowders were not the same. As a result, the powder withsmaller particles (KS4) had better evaporation rate. Therefore,the heat transfer from inert gas to graphite particles is moreeffective during processing of KS4 powder. Furthermore, itgave rise to the lowest rotational temperature of C2 radicals ofabout 3500 K and the process of fullerene formation alreadystarted 10 cm from the nozzle of the plasma torch. Betterevaporation of KS4 graphite powder results in higher fullereneyield.

Acknowledgments

The authors thank the Hungarian Scientific Research Fund(Project No T047360 and T046700) and the Ministry ofScience and Environmental Protection of the Republic ofSerbia (Project No 2018) for their support. Special thanks arealso given to Timcal Co. for the use of their graphite powders.

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