Growth Aspects of Iron-Filled Carbon Nanotubes Obtained by Catalytic

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Growth Aspects of Iron-Filled Carbon Nanotubes Obtained by Catalytic Chemical Vapor Deposition of Ferrocene Christian Mu¨ller,*,† Albrecht Leonhardt, Ma´rcia Cristina Kutz, and Bernd Büchner Leibniz-Institute of Solid State and Material Research Dresden, P.O. Box 270116, D-01171 Dresden, Germany

Helfried Reuther Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany ReceiVed: NoVember 18, 2008

The thermal decomposition of ferrocene combined with an Fe-catalyst nanostructuring on an oxidized Si substrate is investigated in the temperature range of 1015-1200 K. The optimal growth conditions for aligned and homogeneous Fe-filled carbon nanotubes are found at 1100 K. From the nanostructures the corresponding growth rates are determined, and the activation energy of carbon diffusion is calculated to be ∼0.4-0.5 eV/atom. Further, the Fe particle size on the substrate after pretreatment in different gas atmospheres is studied and compared with the nanotube dimensions. With these data the diffusion coefficient of carbon in the catalyst particle amounts to 0.5-1.5 × 10-9 m2/s. Such values prove the formation of liquid catalyst particles during the nanotube growth. Mo¨ssbauer spectroscopy was utilized to analyze and quantify the different Fe phases. In conclusion we propose a simple base-growth model from the experimental results. 1. Introduction Nanostructured magnetic nanowires are promising candidates for many applications ranging from magnetic data storage and sensors for scanning force microscopy to ferromagnetic nanocontainers for biomedical applications. In order to protect the nanowires from oxidation, mechanical damage, and magnetic shortcuts between each other, several groups have encapsulated the nanowires inside carbon nanotubes (CNTs).1-8 Chemical vapor deposition (CVD) has established itself over the last years as the synthesis method for filled CNTs, because it provides material of good quality and can be easily scaled up. Most CVD procedures use metals as the process catalysts (usually Fe, Co, Ni) to produce CNTs. The catalysts can be placed on substrates and/or directly produced via a precursor compound (e.g., ferrocene). The properties of CNTs are strongly influenced by the growth conditions. This makes it difficult to compare CVD results from different groups. During the CVD of CNTs two growth modes exist. When the incorporation of carbon occurs at the substrate the process is called base growth.9,10 When the particles are lifted away from the substrate this is referred to as tip growth.9 The origin of it is often explained in terms of the adhesion forces between the substrate and the catalyst particles. At elevated temperatures most common the vapor-liquid-solid (VLS) mechanism with the catalyst particles entering in the liquid state is favored for the growth of empty or partially filled CNTs.11,12 In fact, the decrease of the melting point of nanometer-sized particles seemed to be a realistic assumption and was demonstrated for other metals (Au,13 Sn14). This means size, carbon content, and the temperature regime should influence the aggregate state of the catalyst particles. In order to obtain defined filled CNTs a * To whom correspondence should be addressed. Tel.: (+49) 351 4659869. E-mail: [email protected]. † Present address: Institute for Integrative Nanosciences, Leibniz-Institute of Solid State and Material Research Dresden, P.O. Box 270116, D-01171 Dresden, Germany.

better understanding of the catalyst nanostructuring, the role of the catalyst particles, and the complex growth of CNTs is necessary. In this work, we want to address these points concerning the growth of Fe-filled CNT and finally summarize the experimental results within a simple growth model. 2. Experimental Section The synthesis of Fe-filled multiwalled CNTs was carried out by catalytic decomposition of ferrocene (Fe(C5H5)2) in a quartz tube reactor inside a dual-zone electrical furnace. The ferrocene acts as the carbon source for the CNTs and the Fe filler. For the CVD process the following parameters were used: the sublimation temperature of the ferrocene in the first furnace (T ) 423 K), the deposition temperature in the second furnace (T ) 1015-1200 K), and the Ar flow rate (120 sccm). For the ferrocene supply a rate of ∼6 mg/min was used. The experiments were performed at atmospheric pressure. Oxidized (100) oriented Si substrates (10 × 10 mm2, 1 µm SiOx) coated with a thin Fe layers (2 nm, sputtered) were used for the deposition. As a reference sample also the bare oxidized Si substrates were utilized. The deposition time was set to 10 min. At the end of each experiment the reactor was slowly cooled down to room temperature (25 K/min). The morphology of the nanotubes was monitored by electron microscopy: scanning electron microscopy (SEM) (Gemini Leo 1530, operated at 10 kV) and transmission electron microscopy (TEM) (Tecnai F30, firm FEI). The as-evaporated and pretreated substrates were characterized by atomic force microscopy (AFM) (Veeco, Dimension 3100) in tapping mode. The fractions of iron phases inside the CNTs were detected by Mo¨ssbauer spectroscopy. The Fe-filled CNTs were studied both in transmission geometry (TM) and by conversion electron Mo¨ssbauer spectroscopy (CEM). Measurements were performed with a conventional spectrometer with a constant acceleration driving system at room temperature. The TM spectra were taken from powder samples (after mechanical removal from the substrate). The CEM spectra were recorded

10.1021/jp8101207 CCC: $40.75  2009 American Chemical Society Published on Web 01/27/2009

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Figure 2. SEM images of Fe-filled CNTs grown with a 2 nm Fe film at (a) 1015, (b) 1100, and (c) 1200 K and CNTs grown on the bare oxidized silicon substrate at (d) 1015, (e) 1100, and (f) 1200 K.

Figure 1. AFM topography images of a 2 nm Fe film on oxidized Si (a) as-evaporated, (b) after 10 min of annealing at 1100 K in pure Ar, and (c) after 10 min of annealing at 1100 K in Ar/H2 (30:1), and the corresponding section analyses. The image sizes are 1 × 1 µm2.

from the aligned CNT film on the substrate which was built in a proportional gas flow detector. 3. Results and Discussion From the literature it is known that thin Fe layers grow on Si/SiO2 in Volmer-Webber mode and give islands.15 The size of these islands is strongly coupled with the conditions of the treatment. To understand the particle formation process on the Fe-coated substrate different pretreatment steps were performed. Figure 1 shows the corresponding AFM images and topography profiles of 2 nm Fe films. AFM analysis of the as-evaporated film (Figure 1a) gives smooth surfaces with a root-mean-square (rms) roughness over the whole area of ∼0.3 nm. The first sample with a 2 nm Fe film was annealed at 1100 K in pure Ar. The Fe film appears discontinuous (rms roughness of 2.5 nm), and large metal islands of about 150 nm can be depicted from Figure 1b. For the second annealing experiment we modified the gas atmosphere to Ar/H2 (30:1), because during CNT growth also small amounts of H2 are involved (from ferrocene decomposition). The used Ar/H2 ratio was based on the typical amounts for the CNT growth experiment. Fe islands with an average lateral extension of 69 nm and a rms roughness of 6.5 nm were calculated (Figure 1c). The reduced island size in the presence of hydrogen might originate from surface reactions. Iron oxide is formed on the top of the catalyst layer, which can easily be reduced in a hydrogen-enriched atmosphere, resulting in Fe particles with an increased surface tension. Due to different annealing parameters it is difficult to make a comparison with data from literature. Nerushev et al.16 investigated the annealing of thin Fe films (1-20 nm) in Ar/H2 (6:

Figure 3. TEM images of Fe-filled CNTs grown at 1100 K (a) on 2 nm Fe and (b) on the bare oxidized silicon substrate. Image (c) shows an individual CNT after scanning along the whole tube length.

1) at 1023 K and reported particle diameters of ∼26 and ∼100 nm, respectively, for 1 and 5 nm Fe films. When the different temperatures are considered then their data are not so far from our results. SEM images reveal for the uncoated as well as for the Fecoated substrates vertically aligned CNTs (Figure 2). Nevertheless, the highest alignment and homogeneity were achieved at T ) 1100 K. At this temperature the average outer/inner CNT diameters were estimated from TEM (Figure 3) to 34 ( 10 nm/ 13 ( 5 nm and 44 ( 18 nm/18 ( 8 nm, respectively, for the bare and the 2 nm Fe-coated substrate. On the one hand the CNT diameter is smaller than the particle size on the Ar/H2annealed substrate. This difference can originate from particle deformations during CNT growth and from an overestimation of the AFM results due to tip curvature. On the other hand, the island density per area is in good agreement with the CNT density per area (∼75-100/µm2 for both). From several experimental runs the average lengths, LCNT, of the Fe-filled CNTs for a deposition time t of 10 min in the temperature range of 1000-1200 K have been measured (Figure 4a). During this period the CNT length increased nearly linearly. In order to evaluate the activation energy EA for the growth of Fe-filled CNTs the growth rate ν was determined. The latter can be expressed by ν ) dLCNT/dt ) A exp(-EA/RT), where A

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Figure 5. Mo¨ssbauer spectra of Fe-filled CNTs grown at 1100 K: (A) transmission mode; (B) CEMS mode. Figure 4. (a) CNT length dependence on growth temperature and (b) Arrhenius plot for the growth rate of Fe-filled CNTs. The growth time was 10 min.

is the pre-exponential factor. Within this calculation it was assumed that the time dependence of the growth rate does not drastically change with the temperature and the CNT growth is diffusion-controlled. From the logarithmic plot of ln ν versus 1/T (Arrhenius plot) the activation energy can be derived (Figure 4b). The linear fits give values of ∼0.5 eV/atom (2 nm Fe) and ∼0.4 eV/atom (bare substrate). The lower activation energy for the bare substrate seems to be reasonable because at the beginning of the CNT growth Fe catalyst has to be accumulated on the substrate surface. The activation energies reported here are much lower as reported for unfilled CNTs, where typical values in the range between 1.2 and 1.8 eV/atom were estimated.17-19 However, in our experiments only ferrocene was used as a carbon source and therefore the C/Fe ratio was much lower. Similar amounts for EA where calculated for the plasmainduced growth of carbon nanofibers (0.35 eV/atom, T ) 523-773 K20) and of tubular CNTs (0.32 eV/atom, T ) 1173-1373 K21). To explain the data from plasma CVD different diffusion models were suggested.20,21 To further clarify our experimental data we estimated the diffusion coefficient of carbon DC in the catalyst particles using the Einstein diffusion equation, DC ) d2/2t ) d2AFGraphitν/2mC; here, d is the diameter of the catalyst particle (assumed to be identical with the CNT diameter), t corresponds with the diffusion time and is accessible from the cross-sectional area A of a CNT, the density of graphite FGraphit, the growth rate ν, and the mass per carbon atom mC. With it the average diffusion coefficients DC at 1100 K were estimated to be 1.5 × 10-9 m2/s (2 nm Fe) and 5 × 10-10 m2/s (bare substrate). Carbon diffusion in liquid metals has not been intensely studied, and data especially for nanoscaled metal particles are rare. In liquid Fe (1823 K, ∼15 wt % C) DC was given with 6 × 10-9 m2/s.22 Taking into account the growth temperature, which is expected to decrease DC by a factor of 2 and experimental uncertainties, then the estimated values for

TABLE 1: Mo¨ssbauer Parameters Obtained from the Fit of the Spectraa Mo¨ssbauer mode

component

IS (mm/s)

H (T)

rel area (%)

transmission

R-Fe γ-Fe Fe3C R-Fe γ-Fe Fe3C

0 0.14 0.14 0 -0.1 -0.1

33

79 6 15 69 28 3

CEMS

21 33 21

a The parameters of the spectra corresponding to the isomer shift (IS) and the magnetic field at the site of the Fe nucleus (H). The Fe-filled CNTs were grown at 1100 K.

DC make the occurrence of liquid Fe particles during the CNT growth probable. It is also no surprise that when iron particles are exposed to an excess of carbon at 1000-1200 K the metastable Fe3C phase can be formed. In the present work relatively large amounts of Fe3C were identified with Mo¨ssbauer spectroscopy (Figure 5 and Table 1). In general, the TM spectra in Figure 5A obtained from Fe-filled CNTs at room temperature show two sextets corresponding to R-Fe and Fe3C and one singlet corresponding to γ-Fe. From the spectral areas the composition amounts to 79% R-Fe, 15% Fe3C, and 6% γ-Fe. The CEM spectrum in Figure 5B provides more information from the surface of the CNT film. The phase ratio R-Fe/Fe3C/γ-Fe is 69%:3%:28%. With other words, near the surface the amount of γ-Fe increases and simultaneously the amount of Fe3C diminishes. This observation is in agreement with other works,23 and a more efficient cooling in this part of the sample after finishing the CVD experiment might be a possible cause for it. Further, these results are supported by the detailed TEM investigations on the encapsulated nanowires.24 Mo¨ssbauer spectroscopy delivers information on the fractions of the different iron phases. The line intensity ratio of the sextet lines of the R-Fe in the CEM spectrum differs from that obtained in the TM spectrum which is characteristic for

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Figure 6. Schematic of the base-growth model via open ends. The labeled steps of the process are particle formation (1), initial base growth (2), and further accumulation of Fe-C particles at the open ends (3).

a randomly magnetic material (3:2:1:1:2:3) and in general from powder samples. It points to a strong crystal alignment of the R-Fe in the tube, reflecting the defined directional growth of the CNTs. Further, the content of Fe3C is insufficient to yield a conclusion. The growth mechanism of Fe-filled CNTs is still under discussion. However, the experimental results enabled us to give a simplified description of it (Figure 6). The growth of Fe-filled CNTs was found to be a catalytically controlled process, where the catalyst and the filling are identical. First, the precursor decomposes into liquid iron clusters, different hydrocarbons, and hydrogen. The presence of hydrogen further supports the activation and island formation of the catalyst on the substrate (Figure 6, step 1). Second, the CNT growth starts from the decomposition of hydrocarbons on the metal surfaces and the diffusion of carbon atoms into the metal particles (Figure 6, step 2). Due to their small size and large amounts of dissolved carbon the melting point of the metal particles is far below the melting point of bulk iron. This means that the catalyst particles are most likely in a liquid state during the growth. After the carbon concentration exceeds supersaturation carbon shells precipitate from the catalyst particles. The layer-by-layer growth on the as-grown innermost tube shells occurs due to periodic shape fluctuations induced by changes of the carbon concentration within the catalyst particle. During this process the initial catalyst particles remain fixed to the substrate, thus favoring the base-growth route. Simultaneously, during the CNT growth, catalyst particles (from precursor decomposition), with diameters in the range of the inner tube diameter or smaller, can continuously accumulate at the exposed open ends (Figure 6, step 3). Assuming again liquid-like particles, then they can easily change their shapes, diffuse along the hollow core, and bind to other nanorods. A similar description for step 3 was assumed by Deck and Vecchio.25 It was also found that the filling decreases from the base to the top of the CNTs. A reason for this could be the higher growth rate of the carbon shell in comparison with the nanowire inside. With it the distance between the encapsulated particles and the open tube end becomes larger, the diffusion paths for carbon atoms increase, finally slowing the growth rate of the CNTs. At this point the CNTs might tend to closure of their freestanding ends. Indeed, such observations could be made in some cases. It is also important to note that the CNT growth must not necessarily stop after the cap formation but can proceed after accumulation of new Fe-C particles (Figure 7). Then it is frequently observable that the growth mode for the CNT changes from base growth into tip growth. 4. Conclusions In this work we have analyzed the growth of Fe-filled multiwalled CNTs on oxidized silicon substrates by using thermal CVD. Thereby the activation energies and the diffusion coefficients of carbon estimated from the growth rates of these filled CNTs have been discussed. The obtained values indicate that the catalyst

Figure 7. TEM image of an Fe-filled CNT grown at 1100 K, which shows tip growth at the freestanding end.

particles are in a liquid state during the growth. Further, the effects of different pretreatments on thin catalyst films were shown and have been compared with the CNT growth. In summary, a VLS-base-growth model via open tube ends was proposed for the initial growth. In addition, it was found that the growth mode can change into tip growth during the process. Acknowledgment. The authors acknowledge G. Kreutzer for her assistance with TEM investigations. We also thank M. H. Ru¨mmeli for manuscript reading. M. C. Kutz thanks CAPES for financial support. References and Notes (1) Saito, Y. Carbon 1995, 33, 979. (2) Rao, C. N. R.; Sen, R.; Satishkumar, B. C.; Govindaraj, A. Chem. Commun. 1998, 15, 1525. (3) Loiseau, A.; Willaime, F. Appl. Surf. Sci. 2000, 164, 227. (4) Grobert, N.; Terrones, M.; Osborne, A. J.; Terrones, H.; Hsu, W. K.; Trasobares, S.; Zhu, Y. Q.; Hare, J. P.; Kroto, H. W.; Walton, D. R. M. Appl. Phys. A: Mater. Sci. Process. 1998, 67, 595. (5) Prados, C.; Cespo, P.; Gonza´lez, J. M.; Hernando, A.; Marco, J. F.; Gancedo, R.; Grobert, N.; Terrones, M.; Walton, R. M.; Kroto, H. W. Phys. ReV. B 2002, 65, 113405. (6) Satishkumar, B. C.; Govindaraj, A.; Vanitha, P. V.; Raychaudhuri, A. K.; Rao, C. N. R. Chem. Phys. Lett. 2002, 362, 301. (7) Mu¨ller, C.; Hampel, S.; Elefant, D.; Biedermann, K.; Leonhardt, A.; Ritschel, M.; Bu¨chner, B. Carbon 2006, 44, 1746. (8) Lo´pez-Urı´as, F.; Mun˜oz-Sandoval, E.; Reyes-Reyes, M.; Romeo, A. H.; Terrones, M.; Mora´n-Lopez, J. L. Phys. ReV. Lett. 2005, 94, 216102. (9) Baker, R. T. K. Carbon 1989, 27, 315. (10) Ru¨mmeli, M. H.; Scha¨ffel, F.; Kramberger, C.; Gemming, T.; Bachmatiuk, A.; Kalenczuk, R. J.; Rellinghaus, B.; Bu¨chner, B.; Pichler, T. J. Am. Chem. Soc. 2007, 129, 15772. (11) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (12) Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1972, 26, 51. (13) Buffat, P.-A. Thin Solid Films 1976, 32, 283. (14) Wronski, C. M. R. Br. J. Appl. Phys. 1967, 18, 1731. (15) Pisana, S.; Cantoro, M.; Parvez, A.; Hofmann, S.; Ferrari, A. C.; Robertson, J. Physica E 2007, 37, 1. (16) Nerushev, O. A.; Sveningsson, M.; Falk, L. K. L.; Rohmund, F. J. Mater. Chem. 2001, 11, 1122. (17) Lee, Y. T.; Kim, N. S.; Park, J.; Han, J. B.; Choi, Y. S.; Ryu, H.; Lee, H. J. Chem. Phys. Lett. 2003, 372, 853. (18) Kim, N. S.; Lee, Y. T.; Park, J.; Han, J. B.; Choi, Y. S.; Choi, S. Y.; Choo, J.; Lee, G. H. J. Phys. Chem. B 2003, 107, 9249.

2740 J. Phys. Chem. C, Vol. 113, No. 7, 2009 (19) Ducati, C.; Alexandrou, I.; Chhowalla, M.; Amaratunga, G. A. J.; Robertson, J. J. Appl. Phys. 2002, 92, 3299. (20) Hofmann, S.; Csa´nyi, G.; Ferrari, A. C.; Payne, M. C.; Robertson, J. Phys. ReV. Lett. 2005, 95, 036101. (21) Bartsch, K.; Biedermann, K.; Gemming, T.; Leonhardt, A. J. Appl. Phys. 2005, 97, 114301. (22) Landolt, H.; Bo¨rnstein, R. Transportpha¨nomene IIsKinetik, Homogene Gasgleichgewichte; Springer: Berlin, Heidelberg, New York, 1968.

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