Carbon 41 (2003) 2711–2724

C arbon nanotubes and onions from carbon monoxide usingNi(acac) and Cu(acac) as catalyst precursors2 2

a a b a,b ,*Albert G. Nasibulin , Anna Moisala , David P. Brown , Esko I. KauppinenaCentre for New Materials, Helsinki University of Technology, P.O. Box 1602, FIN-02044 VTT, Espoo, Finland

bVTT Processes, Aerosol Technology Group, P.O. Box 1602, FIN-02044 VTT, Espoo, Finland

Received 17 February 2003; accepted 21 July 2003

Abstract

New catalyst precursors (copper and nickel acetylacetonates) have been used successfully for the synthesis of carbonnanotubes and onion particles from carbon monoxide. Catalyst nanoparticles and carbon products were produced bymetal–organic precursor vapour decomposition and catalytic disproportionation of carbon monoxide in a laminar flowreactor at temperatures between 705 and 12168C. Carbon nanotubes (CNTs) were formed in the presence of nickel particlesat 923–12168C. The CNTs were single-walled, 1–3 nm in diameter and up to 90 nm long. Hollow carbon onion particles(COPs) were produced in the presence of copper particles at 12168C. The COPs were from 5 to 30 nm in diameter andconsisted of several concentric carbon layers surrounding a hollow core. The results of computational fluid dynamicscalculations to determine the temperature and velocity profiles and mixing conditions of the species in the reactor arepresented. The mechanisms for the formation of both CNTs and COPs are discussed on the basis of the experimental andcomputational results. 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon nanotubes, Carbon onions; B. Pyrolysis

1 . Introduction Refs. [2,3]). Importantly, the presence of transition metalshas been found to reduce the temperature required for CNT

Carbon nanomaterials have attracted a great deal of production (e.g. Refs.[4–6]). COPs are quasi-sphericalinterest from the research community since they exhibit carbon nanoparticles consisting of concentric graphiticunique and useful chemical and physical properties. Both shells. Nano-onions have been observed to either encapsu-carbon nanotubes (CNTs) and carbon onion particles late metals[7,8] or to consist only of carbon layers[9].(COPs) have been the subject of intensive research, and Various physical methods have been utilised successfullymany scientific articles and books have been devoted to for their production, such as arc-discharge[7], high-energytheir synthesis, properties and applications. Nanotubes electron irradiation[10] or thermal treatment of carbonace-were first observed by Iijima during the direct-current arc ous materials[11], high-dose carbon ion implantation intodischarge between graphite electrodes in an argon environ- metals[12] and plasma-enhanced chemical vapour deposi-ment [1]. Typical temperatures for carbon nanotube pro- tion[13]. However, all of these methods require highduction by that method are in the range 2000–35008C. energy input and nano-onions have mainly been producedSince their discovery, various authors have described as an unwanted by-product, making the separation ofalternative means of CNT production, allowing increased carbon nano-onions from other carbon-containing productsproduction rates at significantly reduced temperatures (e.g. an unavoidable process step. The research community is

seeking methods for producing carbon nanomaterials withspecific properties and on an industrial scale. Thus, further*Corresponding author. VTT Processes, P.O. Box 1602, FIN-experimental research on their synthesis is needed.02044 VTT, Finland. Tel.:1358-9-456-6165; fax:1358-9-456-

The known techniques for carbon nanomaterial fabrica-7021.tion can be divided into two classes, physical and chemi-E-mail addresses: esko.kauppinen@vtt.fi(E.I. Kauppinen),

albert.nasibulin@vtt.fi(A.G. Nasibulin). cal, according to the method of carbon atomisation in the

0008-6223/03/$ – see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0008-6223(03)00333-6

2712 A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724

initial production stage. The chemical production method value is not reported in the literature); Cu(acac) ,t 52 dec

consists of the catalytic decomposition of carbon-con- 2868C (as provided by the supplier).taining precursor materials. The obvious advantage of this Carbon monoxide (AGA, 99.97 vol.%) was used as themethod is the possibility of producing carbon nanomateri- carbon source and also as a carrier gas, along with nitrogenals at relatively low temperatures. The current work is (AGA, 99.999 vol.%) for metal–organic saturation.devoted to the investigation of the synthesis of carbon Compaction of acetylacetonate powder in the saturatornanomaterials by the chemical method in the presence of was prevented by mixing the powder with an inertcatalyst particles produced in a well-controlled vertical chromatographic carrier, silicon dioxide (Balzers Materi-laminar flow reactor. The use of a laminar flow reactor als, 99.9%), with a grain size of 0.2–0.7 mm. A mixture ofallows the continuous production of carbon nanomaterials 4 g of metal–organic powder and 16 g of silicon dioxideand avoids certain intermediate stages such as catalyst was used in the saturator.preparation (as in CVD methods). In addition, it isrelatively easy to control the experimental conditions 2 .2. Methods(temperature, residence time, and component composition)as compared to physical methods, and the experimental The experimental investigations were carried out using aconditions inside the reactor can be readily determined by vertical laminar flow reactor. A detailed description of thecomputational fluid dynamics (CFD) calculations, giving experimental setup is presented in our previous papersinsight into the mechanisms of carbon product formation. [22,23]. Briefly, the device consisted of a saturator, a

The most widespread precursors among the carbon laminator, and a furnace. A flow of pure filtered nitrogensources are hydrocarbons, which usually tend to produce (or carbon monoxide) carrier gas was supplied to themulti-walled CNTs, amorphous carbon and graphite[14– saturator impregnated with a metal acetylacetonate and17]. A promising carbon precursor is carbon monoxide, silicon dioxide powder mixture. As it passed through thewhich, in combination with various metallic catalysts, is heated precursor, the gas was saturated by the metal–more selective towards the production of the more desir- organic vapour. Carbon monoxide was typically introducedable single-walled CNTs[3,17,18]. In our research, the into the system from the side and separated from thetransition metals nickel and copper have been used as precursor vapour flow by a 1 mm thick stainless tube.catalyst particles. Nickel has been recognised as an effec- Inside the laminator, two separate steady-state laminartive catalyst for CNT growth in many studies (e.g. Refs. flows containing metal–organic precursor vapour and CO[14,16,19]). However, in spite of the similar properties of are established. Inside the furnace the flows are mixed andcopper, there are very few papers[20,21] examining the subsequently heated to a temperature above which pre-use of this metal as an alternative catalyst for CNT and cursor decomposition occurs. Inside the reactor, six knownCOP fabrication. Typically, metallocene or carbonyl com- temperature profiles were maintained with measured maxi-pounds, which are extremely toxic to humans, are used as mum wall temperatures of 705, 813, 923, 1027, 1123 andcatalyst precursors. In this study, for the first time, a new 12168C using the furnace temperature set to 600, 700,class of metal-containing compounds,b-diketonate com- 800, 900, 1000 and 11008C, respectively. Two mainplexes, was utilised for the formation of catalyst particles processes taking place in the heated furnace can befor carbon nanomaterial synthesis. The current work distinguished. First, metal particle formation occurs as ainvestigated carbon product formation from carbon monox- result of precursor decomposition and the subsequentlyide disproportionation in the presence of nickel and copper created supersaturated metal vapour in the system. Thenanoparticles formed from metal–organic compounds: second process is the formation of the carbon productsnickel (II) acetylacetonate and copper (II) acetylacetonate. (such as CNTs, COPs or amorphous carbon) followingThe advantages of these precursors include suitable catalytic disproportionation of carbon monoxide on theequilibrium vapour pressures, convenient decomposition surface of the particles.temperatures and lower toxicity. The concentration of the precursor vapour pressure was

determined by measuring the mass difference of theremovable precursor cartridge over a given period of time.The flow rate of the carrier gas was measured by a flow

2 . Experimental meter (DC-2, BIOS) and was referred to standard con-ditions (t525 8C, P 5 101 325 Pa). Temperatures were

2 .1. Materials measured by nichrome–nickel thermocouples (K-type).The aerosol number size distributions were measured by a

Acetylacetonate compounds of nickel, Ni(acac) , and differential mobility analyser (DMA) system consisting of2

copper, Cu(acac) (Aldrich, 95 and 97%, respectively), a charger, a classifier[24] (modified Hauke, length 11 cm),2

were used as catalyst precursors. The decomposition of a condensation particle counter (CPC, TSI 3027), andthese metal–organic precursors occurs at relatively low supporting software. An electrostatic precipitator (combi-temperatures: Ni(acac) ,t 5 250–3008C (the exact nation electrostatic precipitator, Intox Products) was used2 dec

A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724 2713

to collect the aerosol particles on a carbon-coated copper ment was conducted att 512168C with a low CO flowfurn

grid (SPI Holey Carbon Grid). The morphology and size rate of 0.33 l /min through the saturator with an averageof the product were investigated with a field emission residence time of 3.8 s in the reactor. The averagetransmission electron microscope (TEM, Philips CM200 residence time was calculated as the time in the high-FEG). Electron diffraction (ED) patterns of the products temperature zone, where deviation from the maximumwere used for determination of the crystalline phase. temperature does not exceed 10%.Qualitative elemental analysis of the particles was carriedout with an energy dispersive X-ray spectrometer (EDS, 3 .1. Nickel catalyst particlesNoran Voyager IV) connected to the TEM.

The vapour pressure of the precursor, according to2 .3. Computational fluid dynamics calculations cartridge measurements, was 7 Pa in all our experiments

with nickel acetylacetonate. Precursor decomposition atComputational fluid dynamics (CFD) calculations were temperatures from 705 to 12168C with a total flow rate of

carried out to determine the temperature and velocity 0.66 l /min (0.33 l /min N10.33 l /min CO) yielded2

profiles and mixing conditions of the species in the reactor. nickel particles covered by amorphous carbon at lowTransport and heat transfer of the gas-phase species were temperatures and by graphite carbon layers at the highestcalculated with the StreamWise CFD program[25] for the temperature. No carbon nanotubes were observed when theexperimental Cu(acac) –CO–N system. Grid resolution gaseous atmosphere consisted of the nitrogen and carbon2 2

studies showed that a grid with 100341 points was monoxide mixture.sufficient to resolve the phenomenon under consideration. A dramatic change in the morphology of the productsThe origin was chosen to be the outlet of the Cu(acac) was found when pure carbon monoxide was used as2

filter holder where the precursor was at known saturated suspending gas. It is worth noting that, in these experi-conditions. The calculation was axisymmetric (since gravi- ments, the total flow rate was slightly decreased to 0.419ty is in the axial direction), which allowed the use of l /min (inner 0.33 l /min1outer 0.089 l /min flows). Atsymmetry boundary conditions at the centreline. In addi- t 5 7058C, long-chain agglomerates more than 10mmfurn

tion to conservation of mass, momentum and energy, two in length were observed (Fig. 2a). Careful TEM observa-gas-phase species equations for CO and N were solved. tion revealed that the agglomerates consisted of nickel2

The presence of Cu(acac) was neglected because of the particles covered by amorphous carbon. The observed2

small fraction of the component in the gas phase (the chains could be easily destroyed by a high-intensity TEM25precursor mole fraction was,6310 ). Buoyancy effects electron beam. Removal of the coating revealed a CNT in

were included in the calculation. Inflow boundary con- the core of the chain. The CNTs were also destroyed byditions on mole fractions of the various species, inflow electron bombardment, as shown by the arrows inFig. 2b.velocity, pressure, and temperature were the same as in the Even though CNTs are present under these conditions,described experiments. Wall temperature boundary con- they are in low abundance. A significant increase in theditions were linearly interpolated from experimentally number of CNTs was found when the temperature of themeasured wall values. Inert wall boundary conditions were system was increased to 9238C. Figs. 3–5 show TEMused for the gas-phase species. images of the products synthesised at 923, 1123 and

Fig. 1 shows the heat transfer, velocity field, and the 12168C. At t 59238C the main product was still metalfurn

mixing of CO and N in the reactor for the Cu(acac) –N – particles that remain inactive for CNT initiation (Fig. 3).2 2 2

CO system. There were small recirculation regions at the Increasing the temperature in the system led to a furthercentreline atx50.1 m due to buoyancy effects. The flow increase in the amount of CNTs (Fig. 4). One can see thatbecame fully developed once again beginning atx50.25 the CNTs were single walled and mostly covered withm. The carbon monoxide and nitrogen gases were essen- amorphous carbon. As a rule they had a thickness of 1–3tially completely mixed 0.1 m from the reactor inlet. nm and a typical length of 30–40 nm. Note that the

maximum length of the CNTs did not exceed 90 nm. EDpatterns of the synthesised product and the ED ring pattern

3 . Experimental results simulations performed for nickel are presented inFig. 5.The ED patterns reveal that the crystalline product consists

During the experiments, two carrier gases, carbon of nickel and no appreciable amount of nickel carbide wasmonoxide and nitrogen, were used. The inner flow rate (Q) found.through the saturator was typically maintained constant at It is notable that the conditions of CNT formation0.33 l /min, while the outer flow rate was varied from 0 via require rather small catalyst particles (typically with a0.089 to 0.33 l /min (Table 1). The average residence time geometric mean diameter of 4 nm). Two kinds of nickelwithin the reactor was 3.7–3.0 s at 0.419 l /min total flow particles were produced att 5 12168C depending onfurn

and 2.4–1.9 s at 0.66 l /min total flow at a furnace the gas atmosphere.Fig. 6apresents the particles producedtemperature of 705–12168C, respectively. A single experi- in mixed flow conditions (inner 0.33 l /min N1outer 0.332

2714 A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724

Fig. 1. (a) Velocity vectors and gas temperature contours for the Cu(acac) –CO–N system att 512168C (inner Q 5 0.332 2 furn CO

l /min1outerQ 5 0.33 l /min). (b) Contours of CO mass fraction for the Cu(acac) –CO–N system att 5 12168C (innerQ 5 0.33N 2 2 furn CO2

l /min1outer Q 5 0.33 l /min).N2

T able 1Experimental conditions and corresponding products (AC, amorphous carbon layer; GC, graphitic carbon layer)

Metal–organic Furnace Flow rate (gas) (l /min) Residence Productsprecursor temp. (8C) time (s)

Inner Outer

Ni(acac) 705–923 (N ) 0.33 (CO) 0.33 2.4–2.2 Ni, AC2 2

Ni(acac) 1123–1216 (N ) 0.33 (CO) 0.33 2.0–1.9 Ni, GC2 2

Ni(acac) 705 (CO) 0.33 (CO) 0.09 3.7 Ni, AC2

Ni(acac) 923–1216 (CO) 0.33 (CO) 0.09 3.5–3.0 Ni, CNT2

Cu(acac) 705–1123 (N ) 0.33 (CO) 0.33 2.4–2.1 Cu, AC2 2

Cu(acac) 1216 (N ) 0.33 (CO) 0.33 1.9 Cu, COP, GC2 2

Cu(acac) 1216 (CO) 0.33 0 3.8 COP, GC, Cu2

A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724 2715

Fig. 2. TEM images of the product synthesised at 7058C by Ni(acac) decomposition (innerQ 5 0.33 l /min1outerQ 5 0.089 l /min).2 CO CO

l /min CO), whereas inFig. 6bonly nitrogen (0.33 l /min1 particle size of 12 nm. In a nitrogen atmosphere, almost all0.33 l /min N flows) was used. As can be seen from the particles are faceted with a particle size of 15–30 nm. The2

TEM images, the composition of the carrier gas had a results of the influence of the mole fraction of carbontremendous influence on the morphology of the produced monoxide,x , and nitrogen,x , on the product areCO N2

particles. A mixture of nitrogen and CO resulted in the summarised inTable 2.The reasons for these differencesformation of spherical nickel particles with an average are discussed in the following sections.

Fig. 3. TEM images of products synthesised at 9238C by Ni(acac) decomposition (innerQ 50.33 lpm5outer Q 50.089 lpm).co co

2716 A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724

Fig. 4. TEM images of products synthesised at 11238C by Ni(acac) decomposition (innerQ 50.33 lpm1outer Q 50.089 lpm).2 co co

3 .2. Copper catalyst particles and 12168C. During the experiments, the saturator tem-perature maintained (t51008C) corresponded to a vapour

In a manner similar to nickel particles, copper particles pressure of 0.27 Pa after the saturator. Carbon nanotubeswere produced by the decomposition of the acetylacetonate were not observed under any conditions in the presence ofcompound vapour at furnace temperatures between 705 copper particles.

Fig. 5. TEM images of the product synthesised at 12168C by Ni(acac) decomposition (innerQ 5 0.33 l /min1outerQ 5 0.089 l /min).2 CO CO

ED patterns of the product and ring simulations performed for nickel are included.

A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724 2717

Fig. 6. TEM images of the product synthesised at 12168C by Ni(acac) decomposition: (a) innerQ 5 0.33 l /min1outer Q 5 0.332 N CO2

l /min); (b) inner Q 5 0.33 l /min1outer Q 50.33 l /min. ED patterns of the product and ring simulations performed for nickel areN N2 2

included.

Cu(acac) decomposition at temperatures from 705 to simulations performed for pure copper, thus the existence2

12168C with a total flow rate of 0.66 l /min (0.33 l /min of copper carbide can be ruled out.N 10.33 l /min CO) yielded copper particles covered by2

amorphous carbon at low temperatures and by graphiticcarbon layers at the highest temperature. At the highest 4 . Discussionexperimental temperature, a small fraction of hollow COPswas also produced. The number of COPs increased on 4 .1. Formation of catalyst particles by acetylacetonateincreasing the residence time and the concentration of compound decompositioncarbon monoxide in the system. Experiments with pure COat t 5 12168C produced COPs and copper nanoparticles In our previous paper[26], it was shown that thefurn

(Fig. 7). The COPs consisted of several concentric carbon decomposition of Cu(acac) in an inert nitrogen atmos-2

layers surrounding either a copper particle or a hollow phere leads to the formation of copper (I) oxide at lowcore. Also, copper particles completely covered by a precursor vapour pressures, and that increasing thegraphitic shell were found among the products. The Cu(acac) vapour pressure changes the product from the2

primary size of the copper particles ranged from about 5 to oxide to metallic copper. It was also shown that copper30 nm according to TEM imaging, while carbon particles oxide formation occurs via the reaction between copperwere from 5 to 30 nm in diameter and thoroughly dimers and the products of metal–organic precursor de-agglomerated. The ED pattern of the product synthesised composition (such as carbon dioxide and/or water vapour)under these conditions fitted with the ED ring pattern on the surface of growing particles:

T able 2Influence of gas composition on the product att 5 12168Cfurn

x 5 0, x 5 0.5, x 5 1,CO CO CO

x 51 x 5 0.5 x 50N N N2 2 2

Residence time (s) 1.9 1.9 3.0Particle size (nm) 15–30 12 4Product Faceted Spherical nickel CNTs initiated

nickel particles covered by nickelparticles by carbon particles

2718 A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724

Fig. 7. TEM images of the product synthesised at 12168C by Cu(acac) decomposition (Q 50.42 l /min). ED patterns of the product and2 CO

ring simulations performed for copper are included.

Cu 1CO ⇔Cu O 1CO , DH 5 does not play an important role in metal oxidation due to2(g) 2(g) 2 (s) (g)

the very low concentration of water vapour in our experi-2 360 kJ/mol,DG 5 2220 kJ/mol (1)

ments. However, if copper oxide was formed in thesystem, oxide reduction by the main gaseous component

Cu 1H O ⇔Cu O 1H , DH 52(g) 2 (g) 2 (s) 2(g) CO must be taken into account:

2 395 kJ/mol,DG 5 2224 kJ/mol (2) Cu O 1CO ⇔2Cu 1CO , DH 52 (s) (g) (s) 2(g)

2 115 kJ/mol,DG 5 2101 kJ/mol (3)where the values of the enthalpy change,DH, and the freeenergy change,DG, are given at 7058C. From a thermo-dynamic point of view it is obvious that prevention of During the current experiments, conditions for copperCu O formation can be realised by introducing carbon oxide formation existed in the furnace. However, the2

monoxide into the system where, according to Le overwhelming presence of CO is presumed to have pre-Chatelier’s principle, reaction (1) is inhibited. Reaction (2) vented the formation of metal oxide particles. According to

A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724 2719

the ED patterns (Fig. 7) obtained from the produced produced at low temperatures (t,4008C) [29], whileparticles, it can be concluded that no crystalline metal copper does not react directly with carbon[31]. Twooxides were formed in the reactor. Thus, the ED patterns known copper carbides (Cu C and CuC ) can be formed,2 2 2

support the assumption that decomposition of the pre- but only by indirect chemical methods and they are verycursors leads to the formation of pure copper particles as unstable and extremely explosive. Another very importantthe primary product. difference in the properties of the metals is carbon

Analogously, the same situation can be deduced for solubility. The bulk carbon solubility in the liquid iscrystalline nickel particle formation during the decomposi- 0.0002 at% in copper as compared to 10.7 at% in nickel.tion of Ni(acac) . If nickel oxide was formed in the system We propose that the latter feature may be one of the2

it would be reduced by carbon monoxide in a fashion reasons for the formation of the different products ob-similar to reaction (3). In accordance with this assumption, served, as will be discussed further in the followingdiffraction rings corresponding to crystalline nickel and not sections.to nickel oxide were detected under various conditions(Figs. 5 and 6). 4 .3. CNT formation catalysed by nickel particles

4 .2. Comparison of the properties of nickel and copper FromTable 1,it can be seen that CNTs are formed evenat high temperatures. Nevertheless, the CO disproportiona-

The effects of the experimental conditions on the tion reactionproduct are summarised inTable 1. It can be seen thatdecomposition of the nickel precursor led to the synthesis CO 1CO ⇔C 1CO , DH 5 2 171 kJ/mol (4)(g) (g) (s) 2(g)

of CNTs, while the presence of copper initiated COPgrowth. An understanding of the different behaviour of the which provides the carbon source, is exothermal and iscatalysts can be inferred from a comparison of the physico- reversed at high temperatures.Fig. 8 shows thermody-chemical characteristics of these metals. In spite of the fact namic data for the disproportionation reaction, includingthat the elements are situated very close in the Periodic the temperature dependence of the free energy change,DG,Table, they have very different properties (Table 3) except and the equilibrium mole fraction of CO in the gaseousfor crystallographic data and density. The surface tension phase[32]. As can be seen, the reaction is inhibited atof nickel is more than 50% greater than that of copper. The temperatures higher than about 9008C (CO and CO2

equilibrium vapour pressure of nickel is more than two equilibrium composition is 97 and 3%, respectively).orders of magnitudes lower than that of copper at the Moreover, kinetic investigations[33] of carbon monoxidehighest experimental temperature. The melting temperature disproportionation on the surface of high-porosity nickelof copper is almost 5008C lower than that of nickel. (Fig. 8) showed that appreciable reaction rates were

Another very important difference between these metals achieved in the temperature interval from 520 to 8008Cis related to the binary metal–carbon phase diagrams and with the maximum rate at a temperature of 6708C. Thisthe solubility of carbon in the metals. The nickel–carbon means that, under the experimental condition oft 5furn

phase diagram has a eutectic point, while the diagram for 12168C, reaction (4) is likely to occur in the reactor eitherthe copper–carbon system is quite simple without any before or after the high-temperature zone, where the localsingular features. Additionally, these metals have very temperature is lower than about 9008C.different affinities for carbon: although nickel carbide The most widely accepted model for the formation ofexists only in a metastable condition, it can still be carbon filaments, the Vapour–Liquid–Solid model[34],

T able 3Comparison of the physico-chemical properties of nickel and copper

Macroscopic properties Nickel Copper

¯ ¯Crystallographic space group F4/m32/m (f.c.c.) F4/m32/m (f.c.c.)˚Lattice parameters (A) 3.5167[27] 3.6151[27]

Melting temperature (T ) (8C) 1453 1083melt3Density at 208C (liquid state) (kg/m )[28] 8902 (7780) 8960 (7940)

Surface tension at 14558C (N/m) 1.80 1.17Equilibrium vapour pressure

23 21(at t512168C) (Pa) 4.0310 7.131024Carbon solubility (at%)[29] at T 10.7 2310melt

Gas solubility (at%) atTmelt23 23Nitrogen [29] 5.4310 5.0310

22Hydrogen[30] 0.12 5.2310

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Fig. 8. Thermodynamic data for CO disproportionation: dependence of the free energy change,DG, and CO mole fraction in the gaseousphase on temperature. Kinetic data: CO concentration after disproportionation on the surface of a high-porosity nickel catalyst.

assumes that the filaments arise via the dissolution of bient. We assume for the time being that reaction (4)carbon into metal particles, followed by segregation of occurs mainly in the first zone. After the evaporation ofexcess carbon at the surface of the particles in the form of Ni(acac) and heating of the vapour to a temperature above2

faceted cylinders. Such an explanation can also be adopted 3008C, nickel vapour formation occurs inside the furnaceto our system if the particles are liquid. However, the as a result of the Ni(acac) decomposition reaction. The2

maximum experimental temperature was much lower than next stage is the formation of metallic nickel particles bythe bulk metal melting points. Also, according to the vapour nucleation and condensation, and particle coagula-equilibrium phase diagram of binary Ni–C alloys, the tion processes.eutectic temperature of this system is 13148C [29], which In the high-temperature zone, the process of the dissolu-is about 1008C higher than the maximum gas temperature tion of the released carbon in the nickel particles occurs.achieved in the reactor. Nevertheless, it is well known that Note that, at some point, the particles become saturated bythe melting temperature of small particles can be sig- carbon due to either the carbon solubility limit or thenificantly reduced by decreasing the particle size (e.g., Ref. temperature decrease. Also, the possibility of the occur-[35]). A similar sensitivity to particle size is observed, for rence of an inverse disproportionation reaction, which isexample, for solubility. Thus, the macroscopic properties favoured at temperatures higher than 9008C, should beare not strictly applicable to nanometer-sized particles and mentioned. Nevertheless, this reaction requires two com-it is likely that, even at temperatures below 13148C, there ponents, CO and C, and, hence, is not significant under2

is significant dissolution of carbon in molten metal par- the experimental conditions due to the very small con-ticles. centration of carbon dioxide.

Let us consider the processes that might lead to the The cooling zone of the reactor determines the forma-growth of CNTs in detail.Fig. 9a summarises the main tion of the final product and is related to carbon segrega-stages that result in CNT formation. In the scheme, only tion in the supersaturated particles due to the reduction inthe most important species taking part in subsequent stages solubility as the system temperature decreases. The dis-and the relevant processes are indicated. As mentioned solved carbon contributes to the flux of carbon atoms topreviously, the disproportionation reaction (4) occurs only the particle surface upon cooling. This is called theat temperatures below 9008C. Let us divide the reactor segregation flux. Additionally, there is another flux, thespace into three parts: the first section is an initial zone surface flux, which is limited by the diffusion of carbonwith the temperature less than 9008C; the second section is atoms on the surface of the particles seeking their lowesta high-temperature reactor zone; and the third section is a energy states. If the cooling rate was infinitely slow thencooling zone, where the temperature is dropping to am- the achieved particle structure after carbon segregation

A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724 2721

Fig. 9. Schematic representation of the mechanisms of carbon nanomaterial production: (a) CNTs and (b) COPs.

would be graphitic carbon around a nickel particle, i.e. the [36] have shown the existence of such conditions. In ourequilibrium structure. However, due to competition be- case, substitution of a mixture of N /CO by a pure CO2

tween the segregation and surface fluxes, two situations flow and a sufficiently long residence time (3.0 s) atcan occur in the system upon cooling. t 5 12168C results in the conditions for a sufficientlyfurn

If the segregation flux is greater than the surface flux, high carbon concentration in particles, favouring theinitiation of CNT growth can occur, since there is in- segregation flux after the particles enter the cooling zone.sufficient time for the creation of the equilibrium structure. When the segregation flux is smaller than the surfaceIn this case, nucleation of CNTs is likely to occur from flux, carbon forms the thermodynamically most stableislands where the segregation flux is larger. Gavillet et al. system—particles surrounded by graphitic layers. This

2722 A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724

situation can be realised experimentally at residence times the released gas most likely only fills voids during emptyof less than 3.0 s and in the presence of nitrogen (x 5 space formation, but cannot be considered as a drivingCO

0.5), where particles appear with graphitic shells. force in void formation, since it is very doubtful thatBy examining the available experimental data, it can be carbon layers would possess the high flexibility and tensile

concluded that the disproportionation reaction essentially strength needed for bubble formation.occurs in the initial zone of the reactor. Even insufficient The voids were most probably initially formed afterCO disproportionation in the initial zone would lead to evaporation of copper through defects in the graphiticsurface passivation by carbon, preventing the occurrence layers surrounding the copper particles. Indeed, theof reaction (4) in the cooling section. An analysis of the equilibrium vapour pressure of copper att512168C (0.71change in catalyst particle size supports this hypothesis. Pa) is higher than the vapour pressure of copper afterThe gas composition has a very strong effect on the complete precursor decomposition in the reactor (0.27 Pa).product (Table 2). Increasing the amount of carbon A similar situation could not occur in the presence ofmonoxide in the system leads to a change from faceted nickel, because the equilibrium vapour pressure of nickel

23nanoparticles (with a particle size of 15–30 nm) via (4.0310 Pa) att512168C is much lower than that ofspherical nickel particles covered by carbon (about 12 nm) copper and the experimental vapour pressure after pre-to CNTs (with a metal particle geometric mean diameter of cursor decomposition and mixing of the carrier gases4 nm). Note that the residence time in the latter case is the (about 5 Pa). A similar mechanism of COP formation in anlongest, which would extend the coagulation and sintering electric arc discharge has already been reported by Saitotime for the formed particles. Nevertheless, the size of the [31]. He reported that hollow COP formation initiallyparticles is the smallest in the latter case, which implies consisted of Ni (or carbide) particles covered by graphiticthat a process preventing particle growth must take place. layers. In those experiments, nickel atoms subsequentlyThis process is surface passivation by carbon, which evaporated through defects in the outer graphitic carbon onoccurs in the initial zone of the reactor, following reaction the hot surface of the cathode. Gadd et al.[38] demon-(4). strated void formation undergoing a similar mechanism

during the electron beam irradiation of a mixture of nickel4 .4. Carbon onion formation catalysed by copper particles and C powder. Furthermore, Banhart et al.[39]60

were able to show void formation by migration of metalIn the case of copper, three different kinds of carbon (gold) though carbon shells.

onion particles were formed. The carbon onion core was Fig. 9bsummarises the proposed mechanism resulting ineither completely or partially filled with metal, or com- the formation of COPs. All the processes occurring in thepletely empty. The obtained products are shown under initial zone can be presented in the same manner as forhigh magnification inFig. 7b–d. nickel: Cu(acac) decomposition reaction; copper particle2

As shown above, the formation of CNTs in the presence formation via vapour nucleation and condensation, andof nickel becomes possible due to the relatively high particle coagulation processes; CO adsorption and dis-solubility of carbon in nickel. In the case of copper, the proportionation on the surface of copper particles.carbon solubility is so small that it can be neglected. On The differences arise in the high-temperature zonethe other hand, the solubility of various gases in the metals where, instead of dissolving, carbon forms graphitic layersat the studied temperatures is rather high. Due to the around the particles. Significant evaporation of copperabsence of CO solubility data in the literature for nickel through the carbon shell is expected to occur near theand copper, we assume a likely value from the data for maximum temperature in the reactor. However, not allhydrogen and nitrogen (Table 3) based on the solubility enclosed copper particles evaporate because of differentbehaviour of CO in iron. Note that, in iron, CO solubility levels of defects in the shells. Some of the copper is leftis even greater than that of H and N[37]. If CO inside the graphitic layers. It is worth noting that the2 2

dissolves in copper particles, void formation can be temperature in this section is higher than the bulkT ,melt

assumed to take place due to the pressure increase after the which implies the existence of copper particles in liquidliberation of gas from the particles upon cooling and its form.subsequent penetration between copper particles and In the cooling region, the only important process is thegraphitic layers, reminiscent of liquid soap-bubble forma- supersaturation of copper vapour and its release by meanstion. A quantitative estimation of the available CO dis- of homogenous nucleation and condensation on existentsolved in the copper particles showed that the amount of particles. TEM investigations revealed some small copperreleased gas would be adequate for void formation. On the particles (typically about 4 nm) covered by amorphousbasis of the TEM images of individual agglomerates, e.g. carbon.presented inFig. 7a,the maximum gas solubility in copper To our knowledge, the method described in this paper isneeded for void formation would be approximately 0.047 a completely new approach to the production of hollowat%. This value is potentially attainable, especially taking carbon nano-onions. In this method, the disproportionationinto account the effect of size on solubility. Nevertheless, of carbon monoxide in the presence of copper catalyst

A.G. Nasibulin et al. / Carbon 41 (2003) 2711–2724 2723

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