NANO LETTERS
Site-Specific Fabrication of Fe Particles for Carbon Nanotube Growth
2009 Vol. 9, No. 2 689-694
Renu Sharma,*,†,‡ Edward Moore,† Peter Rez,‡,§ and Michael M. J. Treacy§ LeRoy Eyring Center for Solid State Science, Arizona State UniVersity, Tempe, Arizona 85287-9605, School of Materials, Arizona State UniVersity, Tempe, Arizona 85287-8706, and Department of Physics, Arizona State UniVersity, Tempe, Arizona 85287-1504 Received October 20, 2008; Revised Manuscript Received December 22, 2008
ABSTRACT We report a method for site-specific fabrication of Fe catalyst particles on silica (SiO2) substrate by electron beam induced decompositionat 650 (EBID) of iron nonacarbonyl. The unobstructed, atomic level in situ observations of the catalyst particles, recorded °C in 8-15 mTorr of acetylene, reveal the structural transformations during reduction, sintering, carburization of Fe nanoparticles and subsequent CNT growth.
Carbon nanotubes are a versatile material with numerous potential applications such as scanning probes, elements of nanocircuits and biosensors, provided that their structure, morphology and placement can be controlled. Therefore during the last two decades, much research, with limited success, has been dedicated to understanding the nucleation and growth processes, and exploring conditions for selective synthesis of carbon nanotubes. For example, catalytic chemical vapor deposition (CVD) and plasma enhanced CVD, using transition metal catalysts such as nickel, cobalt, molybdenum, or iron, are found to be the most adaptable techniques for synthesizing large quantities of single-walled carbon nanotubes (SWCNTs) and for incorporating CNTs in nanocircuits.1-4 These techniques are also very suitable for the large-scale synthesis of vertically aligned tubes for nanotechnology applications.5,6 However, the resultant samples are a often mixture of tubes with varying diameter and length. Several mechanisms for carbon nanotube’s nucleation and growth have also been proposed on the basis of ex situ measurement and theoretical consideration.7-9 Recently, in situ transmission electron microscopy has been employed to observe the catalytic CVD process, and has revealed that the mechanisms for nucleation and growth of carbon nanotubes (CNTs) are different for Ni and Fe.10-14 For example, Helveg et al.10 showed that carbon nanofibers nucleate at surface steps and that surface diffusion of C is energetically favored over bulk diffusion for Ni metal particles.1,8 Yoshida et al.14 have reported that both single-walled and multiwalled CNTs grow from crystalline cementite (Fe3C) catalyst * Corresponding author. E-mail:
[email protected]. † LeRoy Eyring Center for Solid State Science, Arizona State University. ‡ School of Materials, Arizona State University. § Department of Physics, Arizona State University. 10.1021/nl803180e CCC: $40.75 Published on Web 01/22/2009
2009 American Chemical Society
particles via bulk diffusion, even though earlier reports suggested that cementite nanoparticles are inactive for CNT synthesis.15,16 We have shown that the structure and the morphology of CNTs is controlled by the substrate temperature and precursor pressure.11,17 In our earlier experiments, straight SWCNTs (over 90% yield) formed at temperatures above 600 °C in 1 mTorr of acetylene, whereas tangled CNTs grew in a zigzag manner at temperatures below 500 °C at a higher precursor pressure from the same catalyst samples.17 Despite these successes, the complexity of the interactions between the catalyst, the support and the growing nanotubes are still not completely resolved. There are open questions about the chemical transformations occurring in the catalyst particle before and during nanotube growth. Moreover, controlling the dispersion and size of active catalyst particles is undoubtedly essential for the site-specific fabrication of CNTs in nanotechnology applications such as field emission displays.18,19 To overcome these difficulties we have employed site-specific fabrication of active catalyst by electron beam induced decomposition (EBID) of organometallic precursor, giving us control over size and placement of individual particles. In situ, atomic resolution images are used to reveal the phase transformations occurring during catalytic conversion of acetylene to CNTs. We use the column of a Tecnai F20 FEG-E(S)TEM as a flow reactor for both Fe particle and CNT synthesis.20 First, we fabricate arrays of equidistant Fe particles on a perforated SiO2 thin film supported on a 3-mm diameter silicon wafer (SPI Supplies) by electron-beam-induced decomposition (EBID) of nonacarbonyldiiron vapors (Fe2(CO)9; Alfa Aesar) at room temperature in the column of the E(S)TEM. Electron energy-loss spectroscopy (EELS) was used for chemical analysis of deposited particles. The particle size was con-
Figure 1. (a) Annular dark field image of 20 nm particles, separated by 100 nm, deposited by electron beam induced decomposition of Fe2(CO)9. The bar is 100 nm. (b) Electron energy-loss spectra collected from as deposited particle (top spectra) show that the particles contain carbon, oxygen, and Fe. The bottom spectrum was collected at 650 °C after heating in 90 mTorr of hydrogen.
Figure 2. (a) High-resolution image of a deposited particle extracted from a video sequence recorded at 650 °C in 10 mTorr of flowing C2H2 using Gatan Orius SC600 camera; (b) same as (a) with several nanocrystals oriented along different zone axes marked by dashed lines. (A, B) Diffractograms from the regions marked A and B, respectively, in (b). The bar is 5 nm.
trolled by varying microscope imaging parameters such as electron beam current, size of the probe, and electron beam exposure time (dwell time).21 For 5, 4, and 3 s dwell times, we obtained particles with average diameters of 20, 18.7, and 16.9 nm, respectively, indicating a nonlinear relationship between particle diameter and the dose. Figure 1a shows a dark-field STEM image of a typical array of particles, separated by 100 nm, deposited by a 5 s electron beam exposure time near the edge of a hole in the silica support. The TEM column was evacuated to remove traces of iron precursor after depositing 30-50 particles. Electron energy-loss spectroscopy (EELS) reveals the presence of iron, carbon (from the deposition process), and oxygen from the support and/or particle as shown in Figure 1b (top spectrum). The particles became active after heating the sample up to 650 °C in ∼90 mTorr of flowing H2. Position-resolved energy-loss spectra (Figure 1b, bottom spectrum), collected after heating, confirm that any residual carbon in as-deposited particles is below the detection limit. This observation indicates that the carbon is codeposited with Fe during EBID without forming a chemical compound. This carbon removal is an essential step to activate the particles for CNT growth. CNTs grew from most of the particles, with the exception of a few larger particles, when hydrogen was replaced by 13 mTorr of flowing acetylene at 650 °C. The deposited particles were often observed to be agglomerates of smaller 690
crystals (Figure 2a) orientated in different crystallographic directions. For example, approximately six different crystals can be identified from the high-resolution image shown in Figure 2b. Diffractograms from two regions (marked A and B in Figure 2b), recorded at different time intervals, are shown in panels A and B in Figure 2, respectively. The orientations of individual crystals changed with time under reaction conditions, and not all of them were in the correct orientation to simultaneously provide the two lattice vectors needed for unambiguous structure identification. In certain orientations, it is not possible to assign a definitive structure to different crystals. For example, measurements from the region marked A in Figure 2b matched, within 2% error (Table 1), face-centered cubic (fcc) magnetite (Fe3O4). But the measured lattice vectors and angles from region B could be matched to FeO as well as to Fe3O4, with best fit to FeO. However, most of the nanocrystals in the vicinity were identified as fcc Fe3O4. We conclude that the heating at low hydrogen pressure (90 mTorr) did not prevent partial oxidation of Fe particles by residual water vapor in the column. Most of the Fe-rich agglomerates required an incubation period of 1-2 min after the introduction of acetylene (C2H2) before CNT growth begins (see the Supporting Information). Digital videos, recorded using Gatan Orius SC600 camera, revealed several structural transformations during this period. First, after approximately 50 s of heating in C2H2, the central Nano Lett., Vol. 9, No. 2, 2009
Table 1. Measured and Reported (JCPDS) Lattice Vectors and Angles From Figures 2 and 3 lattice vector (Å) and index angles (deg) structure (hkl) (JCPDS) j 1) Fe3O4 (fcc) [110] (11 4.8463 (22j 0) 2.9678 (11j 1)/(22j 0) 35.3 j1 j) FeO (fcc) [110] (11 2.4843 (02j 0 2.1515 j )/(02j 0) (11j 1 54.7 BCC iron oxide [012]a (200) 2.5 (12j 1) 2.041 (121) 2.041 j 1) (200/12 65.91 (12j 1/(121) 48.18 j 1) (200/(2j 1 65.91 R-Fe (BCC) [111] (1j 01) 2.0268 (1j 10) 2.0268 (011j ) 2.0268 (1j 01)/(1j 10) 60 j) (1j 10)/(011 60 (011j )/(101j ) 60 Fe3C [100] (002) 2.263 (03j 1) 2.0217 (03j 1j ) 2.0217 j 1) (002)/(03 63.6 (03j 1)/(03j 1j ) 52.8 (03j 1j )/(002j ) 63.6 Fe3C [100] (002) 2.263 (03j 1) 2.0217 (03j 1j ) 2.0217 j 1) (002)/(03 63.6 (03j 1)/(03j 1j ) 52.8 (03j 1j )/(002j ) 63.6 a Calculated values assuming bcc structure with a ) 0.5 nm.
crystal A in the agglomerate (images a and b in Figure 2) transformed to a body-centered cubic (bcc) structure (Figure 3a), as confirmed by the high-resolution image and diffractogram (image b and diffractogram c in Figure 3). The lattice vectors from the boxed region in Figure 3a could not be matched to any known oxide or carbide structure of iron but did fit a bcc structure, with lattice parameter 0.50 nm, oriented along the [012] zone axis. The calculated lattice spacing and angles using a(bcc) ) 0.50 nm for the observed {200} and {211} planes are given in Table 1 for comparison. Because the structures of small particles are frequently dominated by the surface energy contribution to the total energy of the particle, it is possible that a metastable bcc structure formed during reduction. Moreover, Bhattacharya et al.22 have shown that a fcc f bcc structural transformation can occur for certain values of the uniaxial stretch, generated because of formation of point defects, grain boundary, or dislocation movements. Other individual nano crystals in the agglomerate were observed to follow similar transformations. During the next ∼27 s of heating, all of the nanocrystals in the agglomerate were reduced and sintered to form an R-Fe (ferrite) single crystal with bcc structure, oriented along a 〈111〉 direction and bound by {110} planes (Figure 3d) as confirmed by the high-magnification image (Figure 3e), lattice vectors, and angles measured from the diffractogram (Figure 3f; Table 1) of the boxed region shown in Figure 3d. Sintering occurs as the surface mobility of Fe atoms encourages nanocrystals to reduce their surface energies (Gibbs-Thomson effect) by forming large crystals. The particle remained in crystalline form during the reduction Nano Lett., Vol. 9, No. 2, 2009
lattice vector (Å) and angles (deg) (measured)
% error
4.76 3 35.5 2.51 2.19 57.4 2.48 2.01 2.04 64.6 48.4 66.9 2.04 2.03 2.06 60.1 59.1 60.9 2.28 2.05 2.04 63.7 52.4 63.9 2.27 2.05 2.04 63.6 53.2 63.2
-1.8 1.1 0.6 1.0 1.8 4.9 -0.8 -1.5 0.0 -2.0 0.5 1.5 0.7 0.2 1.6 0.2 -1.5 1.5 0.8 1.4 0.9 0.2 -0.8 0.5 0.3 1.4 0.9 0.0 0.8 -0.6
reference figure Figure 2a Figure 2b Figure 3a
Figure 3b
Figure 3c
Figure 3d
and sintering but movement of lattice planes and strain contrast were observed (see the Supporting Information), confirming that the orientation of the particles changed episodically to accommodate both sintering and reduction. After another ∼28 s, the size, shape, and the structure of the particle transformed suddenly, losing its facets to become a more rounded shape (see the Supporting Information; Figure 3g), within one frame of the video recording (0.11 s), before nucleation and growth of carbon nanotubes (Figure 3j). A magnified image of the boxed area is shown in Figure 3h. Formation of Fe3C (cementite, space group Pnma)23 is consistent with the lattice vector measurements from the diffractogram (Figure 3i), as given in Table 1. The time interval between formation of cementite and the spurt of CNT growth (Figure 3j) is short, less than a second (0.11 s). However, the structure and composition of the particle remained Fe3C during CNT growth, as identified from the high-resolution image (Figure 3k) and diffractogram (Figure 3l). A close inspection of the diffractograms from R-Fe and Fe3C reveals that (10j1)Fe is parallel to (03j1)Fe3C as marked by red arrows in Figure 4a. The transformation of ferrite to cementite can be represented as a shearing of the hexagons in 〈111〉 projection of the bcc-Fe structure (Figure 4a) as (002) lattice vector of cementite (Figure 4c) is ∼11% longer than that of {011} of ferrite, whereas (031j) is rotated by 5° with respect to {110}Fe, as seen in the overlapping diffractograms (Figure 4a). It is easier to accommodate carbon atoms in trigonal prism sites than in octahedral interstices due to their larger size. The atomic level mechanism of the 691
Figure 3. Continuation of frames extracted from the same digital video sequence (see the Supporting Information). (a) The same particle as shown in Figure 2, (b) high-magnification image of the central (boxed) part of the particle, and (c) FFT of the boxed rigion. (d) Particle after 34.7 s. (e) High-magnification image from the boxed area and (f) the FFT of the boxed region. (g) The shape change of the particle with time, (h) high-magnification image, and (i) corresponding FFT confirmed the formation of Fe3C. (j) Growth of multiwalled CNTs from the particle. Note that the particle after CNT nucleation retained Fe3C structure confirmed by (k) high-magnification image and (l) FFT. The relative time lapsed in seconds is given in the upper left corner. The bar is 5 nm.
Figure 4. (a) Overlaid diffractograms of R-Fe along the [111] axis (red) and Fe3C along the [100] axis (blue). (b) Structure model of R-Fe along the [111] axis and (c) Fe3C along [100] showing the change in symmetry as result of C incorporation in R-Fe. Fe is red, carbon is black, and open and filled circles represent atoms at different heights. Colored arrows in (a) correspond to lattice planes marked in (b) and (c).
transformation can be deduced by comparing the two structures oriented along [111]Fe and [100]Fe3C, as shown in b and c in Figure 4, respectively. The bcc-Fe structure (Figure 692
4b) has Fe atoms located at three different levels with a central atom (checkered) surrounded by a set of three above (solid circles), three below (hollow circles), with a height difference of 0.85 nm along {111}Fe (Figure 4b). The inclusion of carbon (small solid circle in Figure 4c) pushes the Fe atoms (hollow and solid balls) above and below the original positions in bcc-Fe structure, increasing lattice spacing for {110}Fe from 0.2023 to 0.226 nm for {031}Fe3C. As a result, the 3-fold symmetry for bcc structure is destroyed. The reconstructive transformation is consistent with the absence of any symmetry relationship between the two structures (Fm3m and Pnma).31 The rapid reaction to form cementite and nucleation of CNTs is consistent with our estimates of the diffusion rate and availability of carbon. Diffusion constants (D) for C in both Fe and Fe3C, although reported to vary with the temperature and the carbon activity, are on the order of 1 × 107 and 1 × 102 nm2/s, respectively.24,25 On the basis of the Nano Lett., Vol. 9, No. 2, 2009
Scheme 1. H2/H2O/650 °C
C2H2O/650 °C
C2H2/650 °C
Fe2(CO)9fParticles containing Fe and C 98 Fe3O4 (fcc) 98 Iron oxide(bcc) 98 C2H2/650 °C
C2H2/650 °C
R-Fe + (CO2 + H2) 98 Fe3C 98 Fe3C + CNT
shape of the Fe particle, bound by {110} surfaces (Figure 3d), we calculate the volume and surface area of the single crystal Fe, to be approximately 2.3 × 103 nm3 and 8.6 × 102 nm2, respectively. Assuming that the number of Fe atoms (images d and g in Figure 3) is conserved, we estimate that 6.9 × 104 carbon atoms were incorporated within 0.11 s to form Fe3C. From the expression for the relationship between Dc and temperature proposed by da Silva and McLellan,26 we obtain a diffusion constant (Dc) of 4.5 × 107 nm2/s for C in bcc R-Fe at our reaction temperature (650 °C). From the pressure and temperature of the acetylene precursor, assuming that all acetylene molecules decompose to give carbon atoms, we find that 3.5 × 106 C atoms hit the particle surface in the same time period. Clearly, both the number of carbon atoms available and the number diffusing through the particle are much larger than the number needed to form cementite. Therefore for nanoparticles, as fabricated here, neither the carbon availability nor its diffusion is the rate limiting step for cementite formation. Cementite formation was followed by CNT formation and the catalyst particles frequently became mobile, as has been reported for earlier in situ observations,10,11,14 and split during this period (see the Supporting Information). Bulk cementite melts at 300 °C below pure bulk Fe, therefore cementite particles formed at high temperature from pure Fe particles may partially melt because of surface premelting, and may allow greater C absorption. Moreover, melting point of nanoparticles also decreases with size with smaller clusters melting at lower temperature.32 Such premelting results in a high atomic mobility, which in turn results in the particle splitting into smaller fragments during CNT growth as observed here (see the Supporting Information). All catalytically active particles for CNT growth were observed to have the Fe3C structure. Our in situ observations provide an atomic level mechanism for the formation of various stable and metastable phases from selectively fabricated iron-containing particles that catalyze the decomposition of acetylene to form CNTs. The sequence of formation of the various phases observed can be written as shown in Scheme 1. The body-centered cubic (bcc) structure for any of the iron oxide phases has not been reported before. However, lattice-vector measurements from atomic resolution images confirm fcc-bcc transformation. Because we could not match the twodimensional pattern (Figure 3a) to any of the known iron, iron oxide, or iron carbide structures, we have assumed it to be a metastable oxide phase. Compositional analysis of this structure has not been possible as it is not a stable phase but most probably is formed because of vacancy formation during reduction of fcc-Fe3O4 to bcc-Fe structure.22 It is clear from these observations that Fe nanoparticles do not reduce during heating in low pressures of flowing Nano Lett., Vol. 9, No. 2, 2009
H2. Small amounts of water vapor present in the sample column or carried in the column by hydrogen gas flow are sufficient for oxygen activity to be high enough to keep the particles from reducing. However, particles are reduced when hydrogen is replaced by C2H2, implying that C2H2 is a better reducing agent. Reduction steps observed here are slightly different than the structural transformations previously reported from the X-ray diffraction patterns collected during heating of a Fe catalyst in a mixture of H2-CH4-Ar (35, 55, and 10% by volume, respectively).15 The difference can easily be attributed to the difference in gas compositions. We show that site-selective CNT formation can be achieved by site-specific depositin of catalyst particles using EBID. Moreover, these particles provide an undisrupted view for observing the structural transformations that occur during catalytic CVD of CNTs. Our optimized catalyst, which was designed and prepared in situ, provides us with a valuable tool for direct determination of catalytic process responsible for nanotube growth. High-resolution lattice images unambiguously reveal both stable and unstable intermediate phases formed during the catalytic growth of CNTs. CNTs nucleate and grow from F3C particles as was recently reported by Yoshida al.14 However, we show that cementite is formed during the heating of iron oxide nanoparticles in acetylene via reduction of iron oxide. Schapper et al.33 have suggested that cementite phase may be an intermediate step for CNT growth but we show that cementite is quite stable and its decomposition is not necessary for CNT nucleation and growth as the enclosed particles after the growth retain cementite structure (see the Supporting Information). There are two possible reasons for the stability of cementite structure; (a) we have achieved an equilibrium state due to high arrival rate of carbon from catalytic decomposition of C2H2 and/or (b) we are below the cementite decomposition temperature as suggested by Zhang et al.15 Because the number of carbon atoms arrival rate on the particle surface is much higher than consumed for either Fe3C formation or CNT growth, it is safe to conclude that CNTs are formed from the supersaturated solutions. The ability to fabricate site-specific catalytically active particles opens a route for direct fabrication of a 2D or 3D network of carbon nanotubes for technological applications. It is also clear that carbon diffusion and the CNT growth mechanism is controlled by the nature of both the catalyst and the precursor. Acknowledgment. Funding from the National Science Foundation (NSF-CBET 0625340), the use of facilities in the LeRoy Eyring Center for Solid State Science at Arizona State University, and in depth discussions about phase 693
transformation with Prof. Emily A. Carter and Prof. Eberhard Schweda are gratefully acknowledged. Supporting Information Available: Edited version of a typical digital video recorded with a time resolution of 9 frames per second (0.11 s), showing one of the several Fe particles fabricated in the ESTEM column by electron beam induced decomposition of iron precursor, at 650 °C in 13 mTorr of acetylene flow, using a Gatan Orius 600 camera. Incubation period with no or minimal structure changes is removed to reduce the file size. Individual frames extracted from the video, as shown in Figure 2 and 3, are used to follow the phase transformations occurring before the CNT growth (AVI). Second part of the (unedited) digital video as shown in previous video used to follow the structural transformations during CNT gowth (AVI). High-resolution image extracted from the growth sequence in the videos showing the splitting of cementite (Fe3C) particle and CNTs grown of individual particles. Diffractogram inset in the upper right corner is from the boxed region and is indexed as cementite structure (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Kitiyanan, B.; Alvarez, W. E.; Harwell, J. H.; Resasco, D. E. Controlled production of single-wall carbon nanotubes by catalytic decomposition of CO on bimetallic Co-Mo catalysts. Chem. Phys. Lett. 2000, 317, 497–503. (2) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Large-scale synthesis of aligned carbon nanotubes. Science 1996, 274 (5293), 1701–1703. (3) Abdi, Y.; Koohshorkhi, J.; Mohajerzadeh, S.; Darbari, S.; Sanaee, Z. Embedded vertically grown carbon nanotubes for field emission applications. J.Vac. Sci. Technol., B 2007, 25 (3), 822–828. (4) Melechko, A. V.; Merkulov, V. I.; McKnight, T. E.; Guillorn, M. A.; Klein, K. L.; Lowndes, D. H.; Simpsona, M. L. Vertically aligned carbon nanofibers and related structures: Controlled synthesis and directed assembly. J. Appl. Phys. ReV. 2005, 97, 39. (5) Devaux, X.; Vergnat, M. On the low-temperature synthesis of SWCNTs by thermal CVD. Phys. E 2008, 40 (7), 2268–2271. (6) Qu, L.; Du, Feng; Dai, L. Preferential Syntheses of Semiconducting Vertically Aligned Single-Walled Carbon Nanotubes for Direct Use in FETs. Nano Lett. 2008, 8 (9), 2682–2687. (7) Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. Nucleation and growth of carbon deposits from the Ni catalyzed decomposition of acetylene. J. Catal. 1972, 26, 51–62. (8) Abild-Pedersen, F.; Nørskov, Jens K.; Rostrup-Nielsen, R.; Sehested, J.; Helveg, S. Mechanisms for catalytic carbon nanofiber growth studied by ab initio density functional theory calculations. Phys. ReV. B 2006, 73, 115419. (9) Perez-Cabero, M.; Romeo, E.; Royo, C.; Monzon, A.; Guerrero-Ruiz, A.; Rodriguez-Ramos, I. Growing mechanism of CNTs: a kinetic approach. J. Catal. 2004, 224 (1), 197–205. (10) Helveg, S.; Lopez-Cartes, C.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abild-Pedersen, F.; Norskov, J. Atomicscale imaging of carbon nanofibre growth. Nature 2004, 427, 426. (11) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Mirco, C.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. In situ Observations of Catalyst Dynamics during Surface-Bound Carbon Nanotube Nucleation. Nano Lett. 2007, 7 (3), 602–608.
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NL803180E
Nano Lett., Vol. 9, No. 2, 2009
