Study of the Thermal Stability of Supported Catalytic Nanoparticles for

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Study of the Thermal Stability of Supported Catalytic Nanoparticles for the Growth of Single-Walled Carbon Nanotubes with Narrow Diameter Distribution by Chemical Vapor Deposition of Methane Pierre Petit,*,† Diana Salem,† Maoshuai He,†,∥ Matthieu Paillet,‡,§ Romain Parret,‡,§,⊥ Jean-Louis Sauvajol,‡,§ and Ahmed Zahab‡,§ †

Institut Charles Sadron, UPR22 CNRS-University of Strasbourg, 23 rue du Loess BP8407, 67034 Strasbourg, cedex 02, France Université Montpellier 2, Laboratoire Charles Coulomb UMR 5221, F-34095 Montpellier, France § CNRS, Laboratoire Charles Coulomb UMR 5221, F-34095 Montpellier, France ‡

S Supporting Information *

ABSTRACT: The thermal stability of catalytic precursors is of major importance for the synthesis of single-walled carbon nanotubes (SWNTs) with narrow diameter distributions by chemical vapor deposition at high temperature. We report a study of the thermal stability of iron (Fe) and iron oxide (Fe2O3) nanoparticles with narrow size distribution grown onto Si/SiO2 hydroxylated flat substrates. Whereas the Fe2O3 nanoparticles are thermally stable up to 900 °C, above their reducing temperature of ∼700 °C, the size distribution of Fe nanoparticles increases and broadens. An accurate analysis of atomic force microscopy data shows that this instability is due to a diffusion−coalescence mechanism, ruling out a possible Ostwald’s ripening mechanism. The origin of this behavior is attributed to the dewetting of the nanoparticles. Using the thermally stable oxide nanoparticles as catalytic precursors, SWNTs can be successfully grown using a methane−hydrogen mixed gas. Statistical analysis of the SWNTs Raman radial breathing modes measured using four excitation wavelengths indicates that the diameter distribution of the nanotubes is centered at 1.3 ± 0.03 nm with a narrow full width at half-maximum of 0.3 ± 0.05 nm. The analysis of our results compared with other results recently published suggests that the metal−surface interaction could play a key role in the catalytic decomposition of methane and in the growth of SWNTs.

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substrates with a size distribution on the order of 1 ± 0.3 nm.15 The main advantages of this method are: (i) the metal oxide nanoparticles grow directly on the surface, avoiding handlings such as precipitation or deposition of a colloidal suspension and (ii) the size of the nanoparticles is governed by the thermodynamical equilibrium of the system, preventing operator-dependent parameters such as time of annealing or concentration of dissolved metal salts.16−19 Surface densities as high as 103 particles per μm2 can be reached with this approach, allowing one to get good statistics for reliable size distribution determinations. In the present article, we focused on the thermal stability of iron oxide nanoparticles obtained by this route and their use for the growth of SWNTs by CVD of methane, aiming at obtaining a tube diameter distribution as narrow as the size distribution of the nanoparticles and at elucidating the mechanism originating the sintering of the catalytic nanoparticles, which is still an open question.20

INTRODUCTION To date, it is generally admitted that the diameter of single or multiwalled carbon nanotubes (SWNTs or MWNTs) synthesized by chemical vapor deposition (CVD) is correlated to the size of the metal nanoparticles that catalyze their growth.1−4 Aiming at controlling the tube diameter, many works have been carried out to control the catalytic particle size. Narrow nanoparticle size distributions have been obtained using solution-based nanoparticles such as ferritin,2 dendrimers,5 and self-assembly techniques.6−9 However, sintering of the catalytic nanoparticle occurs during the CVD process at high temperature and results in the increase and broadening of the tube diameter distributions. The main approach to avoid the particle sintering is to use porous substrates such as porous silica,10 alumina,11 or zeolites,12 for example. Another approach consisting of lowering drastically the density of supported particles and controlling their size by thermal annealing13,14 has been recently reported to be an efficient tool for growing SWNTs with narrow diameter distribution. However, very little attention has been devoted to date to elucidate whether the sintering of the particles is induced by thermal diffusion− coalescence or by Ostwald’s ripening, and both mechanisms are still mentioned in the literature. We have recently published a new route for the synthesis of transition metal oxide nanoparticles supported on Si/SiO2 © 2012 American Chemical Society

Received: August 1, 2012 Revised: October 9, 2012 Published: October 25, 2012 24123

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Figure 1. (a) Typical AFM image of supported Fe2O3 nanoparticles. Scale bar: 400 nm. (b) Corresponding statistics of the particle heights; solid line is a Gaussian fit. (c) XPS spectrum for Fe 2p region of Fe2O3 nanoparticles.

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working temperature was reached, the argon flow was switched off and replaced by a pure hydrogen flow (200 sccm) for a given time, after which the hydrogen flow was switched off and the sample was quenched at room temperature under argon. AFM experiments were performed immediately after these treatments. According to this experimental procedure, the physical characteristics of the nanoparticles, particularly their shapes, were kept unchanged by the quench of the samples from the temperature of treatment to room temperature, except for a possible overestimation of their height due to partial surface reoxidation upon air exposure of reduced samples. The synthesis of SWNTs was performed by CVD of methane using a quartz tube reactor of 2.2 cm internal diameter placed in a cylindrical furnace (Vecstar Furnace, ref VTF4). Gas flows were controlled by mass flow controllers (MKS ref 1179BX52CR1BV) connected to a multigas controller (MKS 647C). Raman spectra were recorded with four laser lines (457 (2.71 eV), 532 (2.33 eV), 561 (2.21 eV), and 633 nm (1.96 eV)) as excitations and using an iHR550 Jobin-Yvon spectrometer in a micro Raman backscattering configuration. The sample was scanned under a homemade microscope equipped with a 50× or 100× objective (numerical apertures 0.5 and 0.9, respectively) with a piezoelectric stage (PIMars P-563, PI). The power density impinging on the sample was kept below 0.5 mW/μm2 to avoid laser heating.

EXPERIMENTAL METHODS Polished oxidized silicon wafers with an oxide layer of 500 nm were first cleaned by sonication in acetone for 10 min, then rinsed with pure water. In a second step, wafers were dipped in a mixture of either H2SO4 or H2O2 (ratio 3 to 1) at 40 °C for 20 min to hydroxilate them (formation of surface Si−OH groups). This hydroxilated wafer was dipped 5 min in a solution of 1 mL of a 10−2 M solution of sodium dodecylsulfate (SDS) in pure water added to 10 mL of a 10−3 M solution of anhydrous FeCl3 at pH 2, then copiously rinsed with pure water, dried in pure argon, and finally calcinated at 300 °C for 20 min under an ambient atmosphere. All surfaces were investigated in tapping mode using a Nanoscope III atomic force microscope (AFM) from Digital Instruments. The root-mean-square roughness of the hydroxilated wafers determined by AFM on a one-by-one micrometer image was found to be ∼0.2 nm. The size distributions of the nanoparticles were determined using the AFM software by measuring the difference in heights indicated by two cursors, respectively, located on the top and at the root of the nanoparticles on the same scanning line, and that for each nanoparticle counted in the distributions. For each temperature, height distributions were measured on different samples and on ∼100 of features on different locations for each sample. The results reported here are the averages of the mean values and widths of these distributions analyzed using Gaussian fits. X-ray photoelectron spectra (XPS) were acquired on a Multilab 2000 spectrometer (Thermo VH Scientific) using Al Kα radiation (1486.6 eV). The aluminum anode was operated at an accelerating voltage of 15 kV, 15 mA, pass energy 20 eV. Base pressure in the analysis chamber was maintained in the range of 5 × 10−9 mbar. Spectra were obtained for the C 1s, O 1s, Fe 2p, Na 1s, S 2p regions. Because of the charging effect, the XPS peaks were found to shift toward higher binding energies. For this reason, C 1s binding energy (284.6 eV) was taken as an internal reference to correct peak positions. Before all experiments, the reactor was purged in pure argon (200 sccm) for 5 min. For the study of the stability of the nanoparticles in their oxide phase, the samples were heated at a rate of 75 °C/min to the desired working temperature under an argon flow (200 sccm). Once this temperature was reached, the sample was kept under the argon flow for a given time; it was then quenched at room temperature by rapidly removing it from the heating zone of the furnace. For the study of the stability of the nanoparticles in their metallic phase, once the

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RESULTS AND DISCUSSION Characterization of the Nanoparticles. The route and the mechanism of formation of iron oxide nanoparticle are detailed elsewhere.15 Basically, the method lies on the complexation of Fe3+ cations with both anionic surfactant and hydroxilated surfaces, which results in the formation of small aggregates onto the surface. At thermodynamical equilibrium, the resulting balance between the loss of translational entropy due to the aggregation and the gain in enthalpy due to hydrophobic interactions between the alkyl chains of the surfactant governs the size of these aggregates. After calcination in air, metal oxide nanoparticles with very narrow size distribution are obtained. AFM images of the obtained samples show a dense collection of nanoparticles (Figure 1a). Because of their size being smaller than the AFM tip (radius ≈ 5−10 nm), large convolution effects impair the lateral resolution and only the heights of the particles are reliably measurable. Figure 1b displays a statistical analysis of iron oxide nanoparticle heights measured from AFM 24124

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Figure 2. Typical AFM images of iron nanoparticles reduced under hydrogen during 5 min at 400 (left), 700 (middle), and 900 °C (right).

Figure 3. (a) Height distributions and Gaussian fits of iron nanoparticles reduced under hydrogen during 5 min at 400, 500, 600, and 700 °C. (b) Height distributions and Gaussian fits of iron nanoparticles reduced under hydrogen during 5 min at 800 (black) and 900 °C (red). Solid lines are fits of the data achieved with a set of two (black) and three (red) Gaussians functions (dashed lines). (c) Height mean values of iron nanoparticles reduced under hydrogen during 5 min as a function of the temperature of reduction; error bars correspond to the widths of the distributions obtained by Gaussian fits.

result is consistent with the work of Ermakova et al.,23 who showed by thermogravimetric experiments that Fe2O3 nanoparticles impregnated by SiO2 are reduced in Fe0 at 676 °C. The stability of the particles at 700 °C also indicates that they fulfill partial wetting conditions up to this temperature, indicating that their contact angle with the surface is lower than 90°. This means that their shape is not spherical, contrary to what is generally assumed in the literature (Scheme 1).

images. The mean value and the width of the particle height distributions using a Gaussian fit are found to be 1.1 ± 0.3 nm. In the XPS spectrum of the nanoparticles for the Fe 2p region, the presence and the position of satellite peaks are the signature of the Fe2O3 chemical composition21 (Figure 1c). Thermal Stability of the Iron Nanoparticles for the Growth of SWNTs. As the growth of carbon nanotube is thought to be catalyzed by nanoparticles in their metallic phase, we have first study the thermal stability of the reduced nanoparticles. As can be observed in Figure 2, upon increasing the temperature of reduction and after 5 min of treatment, the nanoparticle size first decreases. Above 700 °C, a strong increase in the nanoparticle size is observed, accompanied by a decrease of their density. After thermal treatment at 1000 °C, no particles are found anymore on the substrates, probably because they are embedded in the surface of SiO2, as shown by Jeong et al.22 The analysis of AFM images recorded on samples treated at temperatures ranging from room temperature to 700 °C shows a single particle size distribution (Figures 1b and 3a) for each temperature. A slight decrease in the mean particle height with temperature is noticeable and can be attributed to the progressive reduction of the particles. At 700 °C, there is no change in the particle height distribution if the reduction time is increased up to 10 min, indicating that the reduction of the nanoparticles is already complete after 5 min of treatment. This

Scheme 1. Schematic Representation of the Nanoparticle Shapes at Room Temperature (left, nanoparticles fulfilling partial wetting conditions in their oxide phase), 700 °C (middle, nanoparticles fulfilling partial wetting conditions in their metallic phase), and 900 °C (right, dewetting of nanoparticles in their metallic phase)

Upon heating above 700 °C, the mean values of the nanoparticle heights increase, and their size distribution broadens. However, a careful analysis of AFM images shows that above 700 °C there is a multimodal size distribution of the iron nanoparticles (Figure 3b). At 800 °C, two modes are observed centered at 1 and 2 nm, whereas at 900 °C, three modes are distinguishable centered at 1, 2, and 3 nm. The 24125

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Figure 4. (a) Mean height values of Fe2O3 nanoparticles as a function of temperature. (b) Mean height values of Fe2O3 nanoparticles as a function of time at 900 °C. Error bars correspond to the widths of the distributions obtained by Gaussian fits.

diminution of the number of particles of the first mode, whose position is almost temperature-independent within the experimental errors, to the benefit of the other ones is indicative of the coalescence of the particles. The analyses of the distributions by Gaussian fits show that the number of modes increases with the temperature and that their positions and distribution widths increase by integer multiples of the value of the position and distribution width of first mode (Figure 3b,c). Such splitting in the height distributions of the nanoparticles at high temperature in several equidistant modes, the first one being temperature-independent, demonstrates that their increase in size is due to a thermal diffusion instead of Ostwald’s ripening. Indeed, in the latter case, features of larger curvature have the larger adatom vapor pressure at equilibrium, and a gradient of concentration is established between features of different sizes. Adatoms are then diffusing from particles of large curvature to those of small curvature. In other words, large particles grow to the detriment of small ones, which progressively disappear. Thus, contrary to what is observed, a shift of the first mode toward lower height values would have been expected, with no particular structuring of the distribution at higher values. Assuming that the classical theory of wetting24 holds on such small scales, the slightly higher value of the mean particle height of the first mode compared with the one measured at 700 °C could be originated by an increase in their angle of contact due to their dewetting prior to diffusion (Scheme 1). From these results, the highest temperature for the synthesis of SWNTs aiming at getting the smallest diameter distribution is 700 °C. However, as expected, we did not succeed in growing SWNTs because the temperature of 700 °C is too low for an efficient catalytic decomposition of methane.23,25 Thermal Stability of the Iron Oxide Nanoparticles for the Growth of SWNTs. An alternate for growing carbon nanotubes using iron oxide nanoparticles as catalytic precursors is to perform CVD in a reductive medium (mixture of hydrocarbon and hydrogen).26 The thermal stability of iron oxide nanoparticles is shown in Figure 4a. Whatever the temperature and for a thermal treatment of few seconds, a single height distribution is determined from AFM images. As observed for nanoparticles in the metallic phase, after thermal treatment at 1000 °C, no particles are found anymore onto the substrates. Embedment of the particles in the surface may be responsible for that disappearance because a possible change from the oxide phase into the metallic one may occur at high

temperature; for example, the reduction of CoO nanoparticles has been observed.27 Nevertheless, a possible evaporation of the nanoparticles cannot be ruled out. Both the mean values and the distribution widths are temperature-independent. Moreover, at 900 °C, this height distribution is time-independent up to 15 min, indicating a strong anchorage of the nanoparticles onto the surface. The observed highest thermal stability of the iron oxide nanoparticles compared with that of the nanoparticles in their metallic phase is due to their stronger metal−support interaction (MSI) with hard-to-reduce oxide substrates, such as SiO2, TiO2, Al2O3, or ZrO2,25,28 with the MSI becoming even stronger for hydroxylated surfaces.29 SWNTs were catalytically grown through atmospheric CVD using CH4−H2 mixed gas. As already observed, the hydrocarbon feed has a strong influence on the SWNT diameters.30−32 In our experiments, the smallest SWNTs diameter distribution has been obtained at 900 °C using CH4−H2 mixed gas with flow values of 75 and 150 sccm, respectively. SEM image (Figure 5) of the as-grown SWNTs during 10 min allows one to estimate the yield of the synthesis to be of ∼5% (number of SWNTs divided by the number of catalytic precursors). Statistics on unreacted nanoparticles from the AFM image (not shown) gives a size distribution of 2 ± 0.5

Figure 5. SEM image of SWNTs grown from Fe2O3 nanoparticles in a mixture of CH4 (75 sccm) and H2 (150 sccm) at 900 °C. 24126

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enough to ensure that only a few nanotubes are probed on each location. (Typically, 0, 1, or 2 RBM peaks are measured on each spectrum.) The diameter of the SWNTs has been estimated from the RBM frequencies using the relation of Meyer et al.34 that has been recently used for the identification of SWNTs on Si/SiO2 substrates.35 The diameter distributions presented in Figure 7a were obtained by assigning one count to each occurrence of RBM peak (>100 data points were used for green and red lines and ∼50 data points were used for blue and yellow lines). The method used, that is, counting the number of times a given diameter is found on the sample without any consideration related to RBM intensities, enables us to get rid of possible sources of errors such as hypothesis on the Raman cross sections and complicated intensity estimation.36 Furthermore, the sampling of the distributions (0.2 nm) was chosen as well above the possible error due to the uncertainty of the RBM frequency/nanotube diameter relation. Overall, all results show that 95% of the produced SWNTs have diameters below 2 nm and that more than 70% have diameters between 1.1 and 1.5 nm. As expected, however, the diameter distributions obtained with different laser excitations are different because they are biased by Raman resonance conditions.37 Indeed, the results obtained by Raman are a convolution of the diameter distribution of the sample with the one of the photoselected SWNTs at each laser energy. To circumvent this effect, we estimated from an adapted Kataura plot38 the relative number of observable SWNTs at each wavelength and for each diameter intervals of the distribution in the range 0.9 to 1.9 nm. (See the Supporting Information.) Doing so, the first observation one can make is that the number of photoselected nanotubes overall increases with their diameter. This means that raw Raman data are overestimating the number of SWNTs with diameters superior to 1.5 nm. We thus revaluate that actually 85% of the nanotubes have diameters between 1.1 and 1.5 nm. To go further, we normalized the diameter distribution by the determined numbers of observable SWNTs for each diameter interval and for each laser energy. The diameter distributions thus obtained are displayed in Figure 7b. All four normalized histograms are equivalent, which validates the procedure used to obtain a reliable diameter distribution on this sample from Raman data. We conclude that the sample diameter is centered

nm. With iron oxide being a catalyst for the decomposition of methane in H2 and Cα species, this result suggests that unreacted particles are poisoned prior to their reduction in the metallic phase. Raman spectroscopy rather than AFM has been used for the characterization of the nanotubes. Indeed, AFM does not allow one to discriminate between SWNTs of large diameter and bundles of SWNTs. Moreover, because of their deformation induced by surface van der Waals forces,33 nanotubes of large diameters cannot be characterized accurately using AFM. Typical Raman spectra acquired with two excitation wavelengths (633 and 532 nm) are presented in Figure 6.

Figure 6. Typical Raman spectra recorded with 532 (green lines) and 633 nm (red lines) laser excitation. Left: RBM region; right: D and G bands region.

The G band of tangential modes is observed in the frequency range 1500−1600 cm−1 and the defect D band is observed around 1340 and 1320 cm−1 at 532 and 633 nm, respectively. The ratio between the integrated intensity of the G band and of the D band is found to be typically ∼10 (respectively, 6) for excitation at 532 nm (respectively, 633 nm), indicating that the tubes are of relatively high crystalline quality. For each of the four excitation wavelengths, the lowfrequency region (radial breathing modes, RBMs) has been recorded on different locations on the sample to get a reliable determination of the SWNTs diameter distribution. To ensure that different tubes were probed on each spectrum, we used a 0.2 μm (respectively 10 μm) step for Raman mapping along the X (respectively, Y) axis. Indeed, the density of the sample is low

Figure 7. (a) Diameter distributions obtained from Raman data. (b) Diameter distributions obtained by the normalization of the data presented in panel a by the corresponding number of observable nanotubes (i.e., fulfilling Raman resonance conditions) probabilities for each diameter intervals. Blue diamonds, green squares, yellow crosses, and red circles correspond to data obtained with laser excitation at 457, 532, 561, and 633 nm, respectively. The lines correspond to Gaussian fits of the data with center at 1.3 ± 0.03 nm and fwhm of 0.3 ± 0.05 nm. In panel a, the fit was done by considering all data together; in panel b, each line is a Gaussian fit of the data with the corresponding color. 24127

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at 1.3 ± 0.03 nm with a rather narrow distribution with a full width at half-maximum (fwhm) of 0.3 ± 0.05 nm. Moreover, fitting the raw data as whole or their average gives analogous results. (See the Supporting Information.) This extremely narrow distribution of nanotube diameters grown by CVD of methane has to be compared with the one recently reported by Ago et al. from Raman spectroscopy raw data.13 We should, however, notice that the accuracy obtained here is helped by the adequate density of the sample and its narrow diameter distribution and that the method presented here cannot be used in the most general case. From our experiments, it is difficult to correlate tentatively the tube diameters with the nanoparticle heights because the latter ones, even if determined immediately before the tube synthesis, are estimated from the oxidized phase of the nanoparticles. However, the present results show a remarkable difference between the full width at half-maximum of the initial nanoparticle height distribution (0.6 nm) and that of the tube diameter distribution (0.3 nm). Given the low yield of the SWNTs synthesis, our results show that only a fraction of the nanoparticles has been active to catalyze the growth of SWNTs. These results are consistent with those reported recently29,30 that show that the diameters of the nanotubes grown by CVD depend on the couple (nanoparticle size−carbon feeding). Moreover, Ermakova et al.23 have shown that the yield of carbon filaments and nanotubes obtained by CVD of methane on porous iron oxide nanoparticles is optimal for a given concentration of impregnated SiO2. Our results, together with those reported by Ermakova et al.,23 Picher et al.,30 and Lu et al.31 suggest that the size of both Fe nanoparticles and the Fe/ SiO2 interface, where the oxidation state of iron atoms is not strictly 0,25 could play a major role in the catalytic decomposition of methane and consequently in the growth of carbon nanotubes.

Present Addresses ∥

Department of Biotechnology and Chemical Technology, School of Science and Technology, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland. ⊥ Laboratoire Pierre Aigrain, Ecole normale supérieure de Paris, 24 rue Lhomond 75017 Paris, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. C. Petit is thanked for helpful discussions and comments during this work. This work was supported by a CNRS postdoctoral fellowship (M.H.) and by a CNRS-Région Alsace Ph.D. grant (D.S.). The Agence Nationale de la Recherche is thanked for financial support (grant no. ANR-06-NANO-060025). This work has been done in the framework of the GDRI GNT no. 3217 “Graphene and Nanotubes: Sciences and Application”.

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(1) Ivanov, V.; Nagy, J. B.; Lambin, Ph.; Lucas, A.; Zhang, X. B.; Zhang, X. F.; Bemaerts, D.; Van Tendeloo, G.; Amelinckx, S.; Van Landuyt, J. Chem. Phys. Lett. 1994, 223, 329−335. (2) Li, V; Kim, W.; Zhang, Y.; Rolandi, M.; Wang, D.; Dai, H. J. Phys. Chem. B 2001, 1050, 11424−11431. (3) Cheung, C. L.; Kurtz, A.; Park, H.; Lieber, C. M. J. Phys. Chem. B 2002, 106, 2429−2433. (4) Nasibulin, A. G.; Pkhista, P.; Jiang, H.; Kauppinen, E. I. Carbon 2005, 43, 2251−2257. (5) Choi, H. C.; Kim, W.; Wang, D.; Dai, H. J. Chem. Phys. B 2002, 106, 12361−12365. (6) Aiken, J. D., III; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1−44. (7) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550− 575. (8) Hamley, W. Nanotechnology 2003, 14, R39−R54 and references therein.. (9) Charrault, E.; He, M.; Muller, P.; Maaloum, M.; Petit, C.; Petit, P. Langmuir 2009, 25, 11285−11288. (10) Bachilo, S. M.; Balzano, L.; Herrera, J. E.; Pompeo, F.; Resasco, D. E.; Weisman, R. B. J. Am. Chem. Soc. 2003, 125, 11186−11187. (11) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. J. Phys. Chem. B 1999, 103, 6484−6492. (12) Miyauchi, Y.; Chiashi, S.; Murakami, Y.; Hayashida, Y.; Maruyama, S. Chem. Phys. Lett. 2004, 387, 198−203. (13) Ago, H.; Ayagaki, T.; Ogawa, Y.; Tsuji, M. J. Phys. Chem. C 2011, 115, 13247−13253. (14) Kim, J.-J.; Lee, B.-J.; Lee, S.-H.; Jeong, G.-H. Nanotechnology 2012, 23, 105607-1−105607-8. (15) Salem, D.; Smolyakov, G.; Schosseler, F.; Petit, P. Chem. Phys. Lett. 2012, 537, 113−117. (16) Yang, S.-M.; Jang, S. G.; Choi, D.-G.; Kim, S.; Yu, H. K. Small 2006, 2, 458−475 and references therein.. (17) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599−5611. (18) Glass, R.; Möller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153−1160. (19) Bennett, R. D.; Miller, A. C.; Kohen, N. T.; Hammond, P. T.; Irvine, D. J.; Cohen, R. E. Macromolecules 2005, 38, 10728−10735. (20) Journet, C.; Picher, M.; Jourdain, V. Nanotechnology 2012, 23, 142001-1−142001-19. (21) Descostes, M.; Mercier, F.; Thromat, N.; Beaucaire, C.; GautierSoyer, M. Appl. Surf. Sci. 2000, 165, 288−302 and references therein.. (22) Jeong, G.-H.; Yamazaki, A.; Suzuki, S.; Kobayashi, Y.; Homma, Y. Chem. Phys. Lett. 2006, 422, 83−88. (23) Ermakova, M. A.; Ermakov, D. Y.; Chuvilin, A. L.; Kuvshinov, G. G. J. Catal. 2001, 201, 183−197.

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CONCLUSIONS We have shown that the thermal instability of supported metal nanoparticles is a diffusion−coalescence process, and we proposed that it is a consequence of their dewetting. However, SWNTs with a diameter distribution as narrow as 1.3 ± 0.15 nm can be grown by CVD of methane at high temperature onto flat substrates by the use of strongly anchored iron oxide nanoparticles as catalytic precursors. Our results compared with other results recently published confirm that the diameters of the nanotubes grown by CVD depend on the couple (nanoparticle size−carbon feeding)30,31 and suggest that the metal−surface interaction may play a key role in the catalytic decomposition of methane and in the growth of SWNTs.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the method used for normalizing the diameter distribution measured by Raman. Additional discussion on the diameter distribution deduced from Raman measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: ++ 33 3 88 41 41 53. Fax: ++ 33 3 41 40 99. 24128

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