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Nature of the Catalyst Particles in CCVD Synthesis of Multiwalled Carbon Nanotubes Revealed by the Cooling Step Study Vasile Heresanu,*,† Celia Castro,‡ Julien Cambedouzou,† Mathieu Pinault,‡ Odile Stephan,† Cecile Reynaud,‡ Martine Mayne-L’Hermite,‡ and Pascale Launois*,† Laboratoire de Physique des Solides, UMR CNRS 8502, UniVersite´ Paris Sud, 91405 Orsay, France and Laboratoire Francis Perrin, URA CEA-CNRS 2453, DSM-DRECAM-SPAM, CEA Saclay, 91191 Gif-sur-YVette, France ReceiVed: October 8, 2007; In Final Form: January 30, 2008
The true chemical nature and physical state of the catalyst particles in Catalytic Chemical Vapor Deposition (CCVD) synthesis of carbon nanotubes are the subject of intense discussions, as it is one of the keys to understand their growth mechanisms. The CCVD method considered in this article involves pyrolysis of mixed liquid aerosols and leads to the synthesis of large carpets of multiwalled nanotubes (MWNTs) partially filled with iron-based materials. The experimental approach consists in studying the influence of the cooling procedure applied at the end of the synthesis. Both slow standard cooling or quenching were performed, and the structure and chemical state of the iron-based particles were compared through complementary local and global investigations involving X-ray diffraction, electron microscopy, electron diffraction, as well as electron energy loss spectroscopy. We clearly demonstrate that iron-based catalyst particles are carbon-rich and oxygenfree in quenched samples, and that they oxidize during the slow cooling step. It is inferred that they are very probably molten supersaturated carbon-metal particles during the NT growth.
I. Introduction Carbon nanotubes (NT) have been intensively studied for almost two decades1 because of their original properties resulting from their one-dimensional rolled graphene structure. As a consequence, various potential applications2,3 are evaluated and a huge economical impact is expected. Among the synthesis processes, Catalytic Chemical Vapor Deposition (CCVD) methods are versatile and offer the possibility of large scale production.4-7 According to the type of CCVD process used, carbon nanotubes can be randomly distributed or aligned as in a carpet. Vertically aligned multiwalled carbon nanotubes (VAMWNT) carpets, in which nanotubes can be partly filled with magnetic nanowires, are of great interest, for applications in the field of composites,4,8 chemical separation and sensing,9 magnetic storage,10 or field emission displays.11-13 The properties of the carpets are strongly dependent on their structural characteristics which are governed by their synthesis conditions. The knowledge of the mechanisms involved in nanotube growth is fundamental in order to adjust the synthesis conditions toward the production of nanotubes with well-controlled characteristics. This thorough understanding is crucial to design procedures which might be directly used in different nanotechnology fields. There is an abundant literature focusing on nanotube growth mechanisms during CVD processes. In situ experiments (during nanotube growth) have been developing for about three years,14-22 but they are still technically difficult to achieve. Generally, nanotubes are analyzed after their growth (ex situ experiments).7,14,15,23-30 Ex situ experiments are mainly based on the location of the catalyst particles on the nanotubes and * To whom correspondence should be addressed. Tel: 33 1 69 15 60 56. Fax: 33 1 69 15 60 86. E-mail:
[email protected]. Tel: 33 1 69 15 60 56. Fax: 33 1 69 15 60 86. E-mail:
[email protected]. † Universite ´ Paris Sud. ‡ Laboratoire Francis Perrin.
on their chemical and structural analysis. Particles can be located at the nanotube tip, which corresponds to a tip growth mechanism,17 or at the base which is identified as a base growth mechanism.29 Even mixed mechanisms have been proposed.18 Particles can be considered to be either liquid15,17,23,24 or solid14,16,20,23b,25,26 and different catalyst phases have been reported: pure metallic particles,16,20,27 metal carbides25,26 or metal oxides.27,28 In particular, for aligned nanotubes grown by aerosol-assisted CCVD, we have found by ex situ study, that catalyst particles located at the base of nanotubes were iron oxides.28,30 We suggested that the occurrence of such oxides is mainly related to the non reductive atmosphere (Ar), but we did not determine in which step of the synthesis process such oxide phase is actually formed. In particular, the CVD reactor cooling step, which lasts several hours, could induce phase transformation of the catalyst particles. In this context, the objective of our work is to identify the origin of the formation of iron oxide and to get information on the physical state of catalyst particles. The experimental approach consists in changing the cooling procedure applied at the end of the aerosol-assisted CCVD synthesis in order to obtain information about the real state of the catalytic particle during the growth. The analysis approach combines global and local studies in order to determine the chemical composition and the crystalline structure of both catalyst particles at the nanotube base and nanowires filling nanotube core. Global techniques (such as X-ray diffraction (XRD)) and local ones (such as transmission electron microscopy (TEM and STEM) coupled with electron diffraction (ED), or electron energy loss spectroscopy (EELS)) have been used. The morphology and the arrangement of nanotube carpets were examined by scanning electron microscopy (SEM). A comparative analysis of samples cooled to room temperature following the standard procedure (samples were cooled at the natural cooling rate of the furnace
10.1021/jp709825y CCC: $40.75 © 2008 American Chemical Society Published on Web 04/18/2008
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Figure 1. Temperature inside the reactor versus time for the two cooling procedures (triangles: standard procedure, squares: quenching procedure).
following power switch off), or quenched through a fast shift of the furnace far from the reaction zone was performed. The crucial role of the cooling step on the chemical nature of the catalyst particles in their final state is pointed out. Indeed, iron oxide is never observed for samples cooled with the quenching procedure but is only occurring for the standard one. Therefore, the catalytic particles oxidize certainly during the slow cooling procedure. In addition, the morphology of catalyst particles is found to be generally not faceted for quenched samples, while facets are most often present in samples cooled with the standard procedure. It is furthermore inferred that they are probably in the form of molten supersaturated carbon-metal particles. Finally, the structure and chemical composition of the encapsulated anisotropic nanoparticles (nanowires) formed inside MWNT is investigated and their formation mechanism is discussed with respect to the state of the catalyst particles. II. Experimental Section MWNT carpets were prepared using an aerosol-assisted CCVD device, equipped with an ultrasonic aerosol generator already described elsewhere.28 The device is first purged by an argon flow (purity: 99.995) in order to eliminate the air trapped inside the reactor. Then, the aerosol is generated from a 5 wt % ferrocene solution in toluene and carried by an argon flow through a quartz reactor placed in a tubular furnace at 850 °C. The MWNTs are growing on the quartz reactor walls and on thin Si substrates (i.e., 10 µm) placed in the isothermal area of the reactor. The product is subsequently cooled to room temperature. For the present study, the design of the experimental device has been adjusted in order to allow two different cooling procedures named as standard cooling procedure and quenching procedure. For standard cooling procedure the quartz reactor is kept inside the furnace and the carbonaceous product is cooled at the natural cooling rate of the furnace following power switch off, and reaches room temperature in more than 2 h (Figure 1). The quenching procedure consists in exposing the outside of the quartz reactor to the air at room temperature, immediately after the synthesis, by rapidly pushing the furnace out of the reaction area. The latter operation was made possible by mounting the furnace on rails. For the quenching procedure, carbonaceous product reaches room temperature very rapidly (in less than 10 min) (Figure 1). For both cooling procedures, the carbonaceous product (inside of the quartz reactor) is always only exposed to flowing Ar until temperature reaches room temperature.
Heresanu et al. The morphology and the arrangement of the products obtained were examined by scanning electron microscopy (SEM) (Le´oGe´mini 1525, field emission gun). A SETARAM thermogravimetric analyzer was used to measure the iron content after heating the samples in air at a rate of 10 °C/min up to 800 °C.28 The structure of MWNT samples was analyzed by X-ray diffraction (XRD). Experiments were performed in transmission, under vacuum to minimize air scattering, and using a rotating anode with Mo KR radiation (λ ) 0.711 Å), which is sufficiently far away from the iron absorption edge to avoid iron fluorescence. The X-ray beam is circular with a diameter of about 1 mm. Samples were kept on their thin silicon substrates (thickness ≈ 10 µm in order to minimize parasitic diffraction and absorption effects), thus preserving the catalyst particles. The scattering patterns were recorded in transmission on a planar imaging plate. Due to the small amount of iron content (a few % in weight), large exposure times, up to 65 h were necessary in order to obtain pertinent signals coming from the catalytic particles. The structure and chemical composition of individual catalyst particles were analyzed by transmission electron microscopy coupled with Electron Diffraction or Electron Energy Loss Spectroscopy (EELS). Samples were dispersed in alcohol by sonication for 30 min and subsequently deposited on holey carbon grids. High-resolution electron microscopy images and diffraction patterns were acquired with a TOPCON 002B TEM fitted with UHR pole-pieces on a homemade CCD camera. Diffraction patterns were acquired in the select area mode allowing one to isolate a circular area ∼300 nm in diameter. Spatially resolved chemical analyses were carried out in a scanning transmission electron microscope (STEM) VG HB 501 with a field emission gun operated at 100 kV and fitted with a Gatan 666 spectrometer, optically coupled to a CCD camera. Such an STEM instrument delivers a 0.5 nm electron probe of high brightness and offers a 0.3-0.5 eV energy resolution.31 Under typical experimental conditions, the smallest probe (0.5 nm diameter) was used with a convergence angle of 7.5 mrad and a collecting angle of 24 mrad. Line scans of 64 to 128 spectra were acquired with an acquisition time per spectrum of typically 0.1 to 0.5 s. Spatially resolved EELS analysis in the spectrum-imaging mode was employed to investigate the content and the spatial distribution of the relevant chemical elements in the nanotubes. III. Results III.1. Synthesis and Morphology of the Samples. Samples were synthesized either during 4 min or 15 min. They were cooled through the standard procedure or the quenching one described above. Main characteristics of the products formed under the different synthesis durations and cooling procedures are exemplified in Table 1. For quenched samples, the global chemical yield is lower than the one of standard samples. The residual iron content is found to be higher for quenched sample (6.9 wt %) as compared to the one for standard samples (5.8 wt %). Therefore, the catalytic yield for quenched samples is lower than the standard ones. SEM observations of the samples show that product is mainly composed of clean nanotubes being aligned perpendicularly to the carpet basis (Figure 2a). For 15 min synthesis duration, the mean thickness (i.e., carpet height) of quenched sample (450 µm; average growth rate ≈ 30 µm.min-1) is lower than the one of standard sample (560 µm) (Table 1). A typical TEM image of these MWNT samples is shown in Figure 2b. The sample is
Catalyst Particles in CCVD Synthesis of MWNTs
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Figure 2. (a) SEM image showing the aligned MWNT synthesized from a 5 wt % ferrocene solution in toluene at 850 °C for 15 min and cooled with the standard procedure (b) STEM image of MWNTs cooled with the standard procedure and deposited on a holey carbon membrane, solid arrow point toward a catalytic particle and dotted arrow points toward an encapsulated particle.
Figure 3. (a) Characteristic XRD pattern obtained from a carpet of iron filled MWNTs (15 min synthesis duration, followed by subsequent standard cooling). (b) Linear scan as indicated in the Figure 1a. The peaks are indexed as follows: 1 for MWNTs, 2 for Fe3O4, 3 for Fe3C, 4 for γ-Fe, and 5 for R-Fe.
TABLE 1: Synthesis Conditions and Characteristics of Synthesis (global and catalytic yields) and of MWNT Carpets (thickness)a,b,c synthesis conditions at 850 °C duration (min) 4 4 15 15
cooling
main characteristics global yield (%)
catalytic yield
thickness (µm)
1 0,7 3,6 3
16,3 13,6
560 ( 20 450 ( 20
standard quenching standard quenching
a Global yield is the ratio between the total amount of product and the amount of liquid precursor consumed. Catalytic yield is the ratio between the amount of carbon and the amount of iron in the product. For samples synthesized during 4 min, catalytic yield and carpet thickness are not reported because of the small amount of sample which does not allow a precise measurement of these characteristics. b The occurrence of metastable γ-Fe, and not of stable R-Fe only, may appear surprising. It is attributed to confinement effect inside nanotubes.25,33 Indeed, the transition from γ to R phase requires bi-dimensional expansion, which would be impeded inside cylindrical MWNTs. c Thermodynamical arguments can be advanced in favor of magnetite rather than maghemite. First, maghemite is not stable at high temperature, hexagonal R-Fe2O3 being the stable phase (see, e.g., ref 34, p 363), and second, the oxygen vapor pressure is expected to be low in our experimental setup and so more favorable to magnetite formation. The occurrence of the Fe3O4 phase would be in concordance with X-ray photoelectron spectroscopy (XPS) observations, where the signal of Fe3+ and Fe2+ was detected.28 However, determination of the chemical composition of the oxide is not within the scope of this article, and it will not be discussed further.
composed of straight MWNT with particles located at one end of the NTs, which are probably catalyst particles, together with elongated particles encapsulated inside the NTs. III.2. XRD Experiments. X-ray diffraction study of nanotube carpets gives information about the different crystalline phases in the carpets and about their relative amounts. Such information, obtained on large samples (beam area is 1 mm2), is global and statistically relevant.
Let us first establish which phases are observed using X-ray scattering. A typical diffraction pattern, obtained for a standard cooled carpet synthesized for 15 min long, the carpet basis being perpendicular to the incident beam, is shown in Figure 3. Radial analysis of the diffraction pattern (see Figure 1b) enables one to identify five phases: MWNTs,32 R and γ iron (see footnote b of Table 1), cementite Fe3C and iron oxide. Indexation of the iron oxide diffraction peaks can be performed for the two nearly
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Figure 4. XRD patterns of MWNT carpets for standard cooling (filled circles) and quenching (crosses) procedures. Left: short synthesis duration (4 min), right: long synthesis duration (15 min). Solid arrows point toward Fe3O4 diffraction peaks and dotted arrows toward Fe3C ones.
isostructural phases: maghemite R-Fe2O3 or magnetite Fe3O4.28 It is not possible to distinguish between these two phases on the basis of the diffraction results (see footnote c of Table 1). On the basis of X-ray scattering analysis, which is a global method, one cannot state in principle if these phases correspond to nanoparticles inside or outside the nanotubes. In the case of γ iron nanoparticles only, preferred orientation effects reported in ref33 show that large amount of these particles is inside the nanotubes. Analysis of peak positions allowed identifying the different phases coexisting in nanotube carpets. Comparison between peak intensities corresponding to the different phases will now allow one to determine their relative amounts. Figure 4 shows XRD diagrams of four different samples, obtained for either short or long synthesis durations and then cooled down by air quenching or by standard cooling. Special attention is paid to the wavevector region between 2 and 2.9 Å-1, where peaks characteristic of iron oxide and of cementite particles are clearly visible. The iron oxide peaks are the 220 and 331 peaks, located at 2.12 Å-1 and 2.48 Å-1, respectively. They are the most intense peaks of iron oxide. The cementite peaks are located at 2.63 and 2.82 Å-1. They correspond respectively to 121 plus 210, and to 002 plus 201 diffraction peaks of cementite (resolution is not sufficient to separate peak pairs). For both short (Figure 4a) and long (Figure 4b) synthesis durations, one finds that the peaks corresponding to iron oxide are dramatically reduced in the quenched samples by comparison with the standard cooled samples. Intensities of the diffraction peaks of iron-based particles have also been compared to that of MWNTs. For this purpose, we choose the 11 peak of MWNTs around 5.1 Å-1, because it is only contaminated to a small extent by a cementite contribution (at the opposite, for instance, diffraction peaks for all iron-based phases contribute around the position of the 10 peak of MWNTs, which cannot be used to calibrate the amount of the different phases). Within experimental precision, we find that the amount of Fe3C increases for quenched samples compared to standard cooled ones. No significant effect of the cooling down procedure is observed on the amount of γ-Fe as compared to R-Fe. In summary, comparative analysis samples obtained with different cooling down procedures unambiguously shows that iron oxide particles form during the standard cooling procedure. Excess of cementite is also found after quenching by comparison with standard cooling. One should finally note that, in the case of standard cooled samples, the amount of oxide is found to be proportional to the carpet basis surface, and not to the volume of the carpet. Indeed, similar intensities of oxide are found for short and long growth durations (corresponding to small and large carpet volumes) if equal basis surfaces (silicon substrate) are exposed to the X-ray
Figure 5. (a) TEM image of an iron oxide particle observed in a sample cooled with the standard procedure. (b) High-resolution Transmission Electron Microscopy (HRTEM) image of the region 1 indicated in frame a showing that the particle is well connected to the NT (apparent rotation is due to the helical trajectory of electrons in the microscope). (c) HRTEM image of the region 2 in frame a), where atomic arrangement can be clearly seen. (d) Fast Fourier Transform (FFT) of image c indexed as iron oxide (Fe3O4 or γ-Fe2O3) with the zone axis [0-11]. The calibration of the FFT was done using the image of the MWNT where the distance between planes was assumed to be 3.41 Å. (e) Electron diffraction pattern of the whole particle indexed as iron oxide. A combination of two images was used to show diffraction spots masked by the beam stop.
beam. This shows that oxidized particles are situated at the basis of the carpets, and not in volume. This result is in agreement with previous microdiffraction results.30 Carpets grow through a base growth mechanism and particles at their basis are mainly catalyst particles.28,29 It follows that oxidized particles observed in standard cooled samples correspond to particles which were catalytic during growth. III.3. Electron Microscopy and Electron Diffraction, Electron Energy Loss Spectroscopy. X-ray diffraction gives global information about the crystalline phases in NT carpets, as underlined in section III.2. However, after XRD investigations, open questions remained. What is for instance the location of R-iron and cementite particles detected by XRD: are they inside, outside, or at the tip of nanotubes? In order to obtain local information about the particles inside and at the tips of the nanotubes, TEM, HRTEM, ED, and EELS have been undertaken. TEM and electron diffraction results obtained on catalyst particles observed in samples cooled with the standard procedure and with the quenching one are reported in Figures 5-7, TEM and EELS investigations of catalyst particles in samples cooled with the standard procedure and with the quenching one are reported in Figure 8. Finally, Figure 9 shows
Catalyst Particles in CCVD Synthesis of MWNTs
Figure 6. (a) TEM image of a Fe3C particle observed in a sample cooled with the standard procedure. (b) HRTEM image of the region 1 indicated in frame a showing that the particle is connected to the NT. (c) Electron diffraction pattern of the whole particle indexed as Fe3C.
Figure 7. (a) TEM image of a Fe3C particle observed in a sample cooled with the quenching procedure. (b) HRTEM image of the region indicated in frame a showing that the particle is connected to the NT. (c) Electron diffraction pattern of the whole particle indexed as Fe3C.
TEM and electron diffraction results obtained for different anisotropic nanoparticles inside the nanotubes. Let us first consider the catalyst particles. The NT carpets being synthesized through a base-growth mechanism, catalyst particles are expected to be located at the end of the nanotube corresponding to the carpet basis, while the other one should be closed and empty. However, it could happen that a particle inside the tube is located close to its end, or some catalytic particles may have been lost when scratching the sample from its substrate to prepare the grid for microscopy. As a consequence, it is not possible to directly state that a particle has been catalytic because it is observed at a nanotube end. Careful analysis of the size and of the connection of the particle with the NT helps determining whether a particle was catalytic or not. Particles with sizes too different from that of the nanotubes or which are badly connected to them have been rejected. TEM images of tip particles typical of samples cooled with the standard procedure are shown in Figures 5a, 6a, and 8a. HRTEM images in Figures 5b and 6b show that particles are rightly connected with the nanotubes. The particle crystalline organization is illustrated by the HRTEM image in Figure 5c. FFT and electron diffraction allow us to unambiguously identify the particle in Figure 5 as an iron oxide (Fe3O4 or γ-Fe2O3) particle and the one in Figure 6 as a Fe3C particle. For the particle shown in Figure 5, one may also notice that 111 planes of the iron oxide particle are parallel to the MWNTs walls. The distance between 111 planes of Fe3O4 is 4.84 Å so the double of this distance is very close to the triple of the NTs inter walls distance which is 3 × 3.4 ) 10.2 Å. In this case, no strong deformation of MWNTs is observed at the interface between the nanotube and particle. In Figure 8c, carbon, oxygen, and iron elements are shown on the EELS spectrum. Moreover, zoom of the EELS spectrum around oxygen K edge, detailed in the inset in Figure 8c, is characteristic of an iron oxide spectrum.35 It is compatible with both γ-Fe203 and Fe3O4 spectra in Figure 3 in ref 35, but further experiments with higher resolution in energy are needed to discriminate between these two oxides. Line profiles of iron and oxygen elements across the particle (Figure 8b) also show that the catalyst particle contains both oxygen and iron. Carbon line profile, minimum at the center of the particle, indicates that carbon is located around and not inside the particle. The particle in Figure 8a is
J. Phys. Chem. C, Vol. 112, No. 19, 2008 7375 probably iron oxide. In brief, in samples cooled with the standard procedure, catalyst particles are found to be either iron oxide particles or cementite particles. TEM images of tip particles typical of samples cooled with the quenching procedure are shown in Figures 7a and 8d. Indexation of the electron diffraction diagram in Figure 7c shows that the particle is a cementite particle. EELS investigation of the particle in Figure 8a show only the presence of iron and carbon. No oxygen is detected. The fact that the signal of carbon, at right in Figure 8e, decreases in the same way as the signal of iron suggests that the particle contains both iron and carbon. To summarize, tip particles in samples cooled with the quenching procedure were found to have the cementite structure by electron diffraction and no oxygen was detected by EELS. It follows that combined TEM, HRTEM, electron diffraction, and electron spectroscopy investigations, performed on both samples cooled with both procedures, corroborate the XRD experiments, where very little oxide phase is found in quenched samples and were iron oxide nanoparticles were detected in samples cooled with the standard procedure. Moreover, they show that particles at the tip of the nanotubes are iron carbide in quenched samples and that they can be either iron carbide or iron oxide particles in samples cooled with the standard procedure. Another important effect of the cooling procedure concerns the shape of the catalyst particles and the formation or not of a large carbon shell around them. For samples cooled with the standard procedure, the particles present regular facets and no large carbon shell is observed, as is illustrated in Figures 5a, 6a, and 8a. The existence of a thin carbon shell cannot be excluded from our observation, because no systematic HRTEM observation was done. For quenched samples, the particles have generally no regular facet and they are embedded in a large carbon shell (Figures 7a and 8d). Let us now consider the encapsulated particles inside the nanotubes. They have been studied using electron diffraction and EELS. Whatever the sample cooling procedure, electron microscopy shows the presence inside the NTs of single-crystal encapsulated particles that are either as R-Fe, γ-Fe, or Fe3C as evidenced from their diffraction patterns (Figure 9). Our results are in agreement with those reported in refs 9, 18, and 36-38, where one at least among R-Fe, γ-Fe, and Fe3C phases were observed inside the nanotubes. Moreover, we show here for the first time that, for the aerosol-assisted CCVD method used, these phases are all present in the form of encapsulated nanowires within the nanotubes. We finally underline that, based on both electron diffraction and EELS measurements, no oxide particles have been found inside the nanowires, even for samples cooled with the standard procedure. IV. Discussion Regarding the synthesis, the increase of global and catalytic yields as well as of carpet thickness for sample cooled with the standard procedure indicates that NT growth is prolonged during the slow cooling step, while the quenching procedure would stop almost immediately nanotube growth. This is certainly related to the furnace temperature remaining close to 850 °C during the first minutes of the standard cooling step (Figure 1) and to the presence of residual reactant gas at the beginning of the cooling step. The height of the standard carpet obtained after a standard cooling procedure is roughly 100 µm higher than the one of the quenched carpet. Considering that the mean nanotube growth rate at 850 °C is 30 µm.min-1, it can be estimated that nanotube growth is prolonged for 3 min during
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Figure 8. (a,d) Scanning Transmission Electron Microscopy (dark field) images of tip particles observed in a sample cooled with the standard procedure a and in a sample cooled with the quenching procedure d. (b,e) EELS profiles along the lines indicated in frames a and d, showing the presence of oxygen in the particle for the sample cooled with the standard procedure and its absence for the quenched sample. Spectra have been normalized to their maxima in order to facilitate the comparison. (c,f) EELS spectra taken in the rectangles in frames a and d. The inset in frame c shows the detailed oxygen K spectrum, which is characteristic for iron oxides.
Figure 9. TEM images of iron nanowires confined in the NTs (standard cooling) in a,b,c) and corresponding electron diffraction patterns in a′, b′, c′. The particles are indexed as follows: (a) γ-Fe, (b) R-Fe and (c) Fe3C. The modulated ring with maximum intensity perpendicular to the tube axis is situated at about 1.8 Å-1 and corresponds to 002 diffraction peak of nanotubes; the two following rings at about 3 and 5.1 Å-1 correspond to their 10 and 11 reflections.
the cooling step (it is a minimal time since growth rate decreases with decreasing temperature (e.g., 22 µm.min-1 at 800 °C) and amount of reactant39). Regarding the study of the structure and of the chemical composition of the iron-based nanoparticles, comprehensive investigation involving local (transmission electron microscopy, electron diffraction) and global probes (X-ray diffraction), as
well as complementary chemical composition analysis using electron energy loss spectroscopy, allowed us to obtain new results about particles at the nanotube tips or inside them. At the nanotube tips, nanoparticles that have catalyzed nanotube growth (“catalyst particles”), are in the form of iron oxide particles or cementite particles after standard cooling, while only cementite particles are observed after quenching. Moreover,
Catalyst Particles in CCVD Synthesis of MWNTs catalyst particles present a rather round shape and are surrounded by large carbon shells after quenching, while they have most often well-defined facets and much less carbon around them after standard cooling. Inside the nanotubes, three sorts of elongated particles are observed: R-Fe, γ-Fe, or Fe3C particles. In the following, we show how these ex situ results allow one to infer reasonable hypothesis about the state of catalyst particles during growth. Almost no oxide being observed in the quenched samples, we conclude that for standard samples the oxidation of catalyst particles occurs during the cooling step. This suggests that particles are not oxidized during the main steps of nanotube growth. This conclusion is supported by the fact that the reactor atmosphere has most probably a reducing character, during the growth, due to hydrogen coming from both hydrocarbon and ferrocene.14 Such argument no more applies during the standard cooling step under Ar flow since the decomposition of toluene/ ferrocene mixture is progressively stopped while the temperature is maintained high enough for the oxidation of metal-based particles. In addition, the traces of oxygen in Ar flow and on the oxide substrates do not exclude oxidation of catalyst particles during the cooling step. In quenched samples, the very low amount of iron oxide together with the occurrence of cementite (Fe3C) at the nanotube tips suggests that catalyst particles are metal (Fe)-carbon particles. Different authors proposed, on the basis of both experimental15,23,40,41 or theoretical42,43 investigations, that metalcarbon catalyst nanoparticles could be in a liquid state at relatively low temperatures, below their equilibrium eutectic temperature especially because of their nanometric size.40,42 In molten metal nanoparticles, carbon solubility is strongly enhanced, up to 50%44 or 68% C.24 The observation, for quenched samples, of Fe3C particles encapsulated in carbon shells, suggests that the catalyst particles exhibit, during the growth, a C/Fe ratio greater than 33% (which correspond to the Fe3C composition). The maximum solubility of carbon in solid iron at 850 °C being of about 5% atom, the catalytic particle would not be solid iron and the hypothesis of molten iron particles supersaturated in carbon thus appears most appealing. In addition, the diffusion of carbon is enhanced when the particle is liquid, thus involving a rapid nanotube growth.45 The high growth rate (∼30 µm.min-1) in the aerosol-assisted CCVD process is certainly due to both the continuous feeding of catalyst precursor29 and to a rapid diffusion of carbon in molten metalbased particles. The shape, the chemical composition and the structure of such particles, liquid-like during NT synthesis, is modified during the cooling step. For quenching, carbon in excess in the catalyst particle precipitates around them during the fast cooling step, and a stable Fe3C phase wrapped in carbon shells is formed (see, e.g., ref 3, p 101). As the cooling rate is very high and considering that the particles are supersaturated in reductive element (e.g., carbon) and that they are embedded in carbon layers, they cannot be oxidized during the cooling step. In addition, their shape is frozen by the fast cooling, giving rise to the non-faceted shape observed by TEM which is in favor of molten particles during NT growth. For standard cooling, nanotube growth is prolonged during the first few minutes of the cooling step. Then, the reactant partial pressure decreases at the same time as the temperature decreases which involves finally a definitive extrusion of carbon from the catalytic particles while they solidify, thus forming few graphene layers around faceted particles. The particles which solidify during the slow cooling step are thus surrounded by much less carbon
J. Phys. Chem. C, Vol. 112, No. 19, 2008 7377 shells which are not enough protective against oxidation occurring because of the relatively high temperature and the oxygen impurities of the Ar gas flow or of the oxide substrates. The orientational relationship between the MWNTs and the oxide particles, as observed in Figure 5, is the result of crystallographic matching allowed by the slow cooling rate. It is important to note that such an orientational relationship is not the signature of epitaxial growth mechanisms of the nanotubes on the catalyst particles. Inside nanotube cores, nanoparticles are either pure iron (RFe or γ-Fe) or iron carbide (Fe3C) elongated particles. This can be understood within our hypothesis that catalyst particles are molten supersaturated carbon-metal particles. They would wet the growing nanotubes and detach themselves from the catalyst particle, during nanotube growth. Their further solidification would lead to the formation of either iron carbide or pure iron nanowires, the carbon in excess condensing close to the nanowires as was observed in ref 15 for in situ heating experiments. The proposed mechanism is in agreement with the absence of iron oxide nanowires inside nanotubes since nanowires form from the metal-carbon catalytic nanoparticle during the nanotube growth, before the cooling step when the particle can get oxidized. Iron nanowires inside the nanotubes are perfectly protected against oxidation. In summary, our experimental results all converge toward the same hypothesis about the state of catalyst particles at 850 °C, making it most credible: catalyst particles would be molten supersaturated carbon-metal particles. V. Conclusions The relations between the NT growth and the catalyst particle structure deduced from ex situ measurements are in principle not straightforward. However, we show that conclusive results can be inferred by investigating the effect of the cooling procedure on the state of catalyst particles in aerosol-assisted CCVD synthesis. Complementary global and local analyses, using XRD, electron microscopy, and EELS, show that (i) NT growth is prolonged during the beginning of the cooling step, (ii) that the oxidation of the catalyst particles takes place during the cooling step, (iii) that catalyst particles are in the form of cementite particles surrounded by large carbon shells after quenching, and (iv) that nanowires inside nanotubessa priori formed from the catalyst particles during NT growthsare either in the form of iron (R or γ-Fe) or of cementite. These results converge toward the conclusion that in the CCVD process used, during the growth at 850 °C, catalytic particles are very likely molten supersaturated iron-carbon particles. Acknowledgment. M. Kociak is gratefully acknowledged for many helpful discussions and for his help with some of the electron microscopy experiments. The authors thank D. Porterat for technical assistance in synthesis device, and S. Poissonnet, P. Bonnaillie (CEA Saclay, DEN/SRMP) for SEM observations. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Berlin: Springer, 2001. (3) Loiseau, A.; Launois, P.; Petit, P.; Roche, S.; Salvetat, J. P. Understanding Carbon Nanotubes; Springer: Berlin Heidelberg, 2006. (4) Singh, C.; Shaffer, M. S. P.; Koziol, K. K. K.; Kinloch, I. A.; Windle, A. H. Chem. Phys. Lett. 2003, 372, 860. (5) Andrews, R.; Jacques, D.; Rao, A. M.; Derbyshire, F.; Qian, D.; Fan, X.; Dickey, E. C.; Chen, J. Chem. Phys. Lett. 1999, 303, 467.
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