Filling Carbon Nanotubes with Co9S8 Nanowires through in Situ

Jan 18, 2008 - Engineering Research Institute, Florida International UniVersity, Miami, Florida 33174. ReceiVed: NoVember 1, 2007; In Final Form: NoVe...
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J. Phys. Chem. C 2008, 112, 1890-1895

Filling Carbon Nanotubes with Co9S8 Nanowires through in Situ Catalyst Transition and Extrusion Gaohui Du,† Wenzhi Li,*,† and Yanqing Liu‡ Department of Physics, Florida International UniVersity, Miami, Florida 33199, and AdVanced Materials and Engineering Research Institute, Florida International UniVersity, Miami, Florida 33174 ReceiVed: NoVember 1, 2007; In Final Form: NoVember 15, 2007

We describe the synthesis of novel Co9S8-nanowire-filled carbon nanotubes (CNTs) by a simple method involving the pyrolysis of thiophene on cobalt catalyst in a conventional chemical vapor decomposition system. The encapsulated Co9S8 nanowires are single-crystalline, and their lengths are about 10 µm with their [110] direction parallel to the axis of the CNTs. Detailed investigation suggests that the filling of the Co9S8 nanowires results from the volume increase induced by a phase transition from cobalt to cobalt sulfide together with the in situ extruding action of CNTs as nanomolds. A new filling mechanism is thus found and proposed.

Introduction Considerable efforts have been made to functionalize carbon nanotubes (CNTs) via inserting molecules or nanostructures into the CNTs to achieve new functionality by combining the properties of both the CNTs and the inserted materials. It has been reported that fullerenes as well as other organic and inorganic compounds can be introduced in CNTs.1-3 Different methods to realize the encapsulation of exotic materials within CNTs have been developed. The general approach employs a capillary technique4 or wet chemical techniques,5,6 which involve opening the CNTs and depositing the filling material by impregnation of a molten precursor or precursor solution followed by a subsequent heat treatment. Alternatively, the encapsulation of elements in CNTs can be achieved by preparing the CNTs in a carbon arc-discharge process with the presence of metals.7-9 Among the reported research, magnetic material-filled CNTs are especially attractive because they have potential applications in high-density magnetic storage,10 magnetic inks, microwave adsorption,11 biomedicine,12 probes of magnetic force microscopy,13 spintronics,14 etc. A previous study on Fe-filled CNTs has demonstrated that the outer graphite layers not only prohibit the oxidation of the inner metals, but also enhance the magnetic coercivity.15 These magnetic nanoparticles or nanorods enclosed inside CNTs have been frequently observed in samples made by the chemical vapor deposition (CVD) method by continuous feeding of the carbon source and catalyst precursor (e.g., ferrocene). Since the typical catalysts for CNT growth are Co, Fe, and Ni, the encapsulated material is usually either a metal or an alloy made of two of them. For example, FeCo nanowires inside CNTs have been prepared by aerosol thermolysis of C7H8-FeCP2-CoCP2 solutions, and they show large coercive fields at room temperature.16 So far, there are few reports about magnetic sulfide nanowires-filled CNTs. Among a variety of metal sulfides, Co9S8 shows interesting metallic17,18 and magnetic17,19 properties, and it has a better stability than the metals, especially in some specific environments and conditions, e.g., * To whom correspondence should be addressed. Fax: (305) 348-6700. E-mail: [email protected]. † Department of Physics. ‡ Advanced Materials and Engineering Research Institute.

in oxygen or H2S at elevated temperature. In this paper we demonstrate a method to prepare Co9S8-nanowire-filled CNTs by catalytic CVD and present a new growth mechanism based on the volume increase induced by phase transition of the catalyst nanoparticles. Experimental Section For the production of CNTs filled with Co9S8 nanowires, the Co/MgO catalyst powder was placed on a Si wafer, which was inserted into a horizontal quartz tubular reaction chamber which was heated to 1000 °C with Ar at a flow rate of 1700 sccm and H2 at a flow rate of 100 sccm. H2 was switched to bubble through thiophene (C4H4S, liquid) to initiate the nanotubes growth. After reaction for 15 min, H2 was closed and the chamber was cooled to room temperature. The preparation procedure of the catalyst is described as follows. Co(NO3)2‚ 6H2O and Mg(NO3)2‚6H2O were mechanically mixed, ground, and then calcined at 600 °C for 1 h in air to decompose the precursor and yield the cluster made of cobalt and magnesium oxides. The resulting powder was then reduced in H2 (100 sccm) and Ar (200 sccm) for 30 min at 600 °C to form Co nanoparticles supported on MgO substrate, which were collected and used as a catalyst. The weight ratio of Co in the catalyst powder is 25%. The morphology and structure of the CNT samples were examined using X-ray diffraction (XRD; Mo KR radiation, λ ) 0.71 Å), field emission scanning electron microscopy (FESEM; JEOL JSM-6330F), and transmission electron microscopy (TEM; JEOL 2010 and Hitachi HF-2000 FEG). Chemical composition analysis was performed by an energy dispersive X-ray spectrometer equipped with a JEOL 2010 TEM instrument. Results and Discussion The synthesis of the filled CNTs was achieved by the pyrolysis of thiophene in our approach. Thiophene is a heterocyclic compound consisting of four carbon atoms and one sulfur atom in a five-membered ring. It was used in our experiments as a carbon and sulfur source for the in situ growth of Co9S8-nanowire-filled CNTs. Figure 1a is a scanning electron

10.1021/jp710543u CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

Filling Carbon Nanotubes with Co9S8

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Figure 1. (a) Low-magnification and (b) high-magnification SEM images of Co9S8-filled CNTs. (c, d) Low-magnification TEM images of the Co9S8-nanowire-filled CNTs. (e, f) High-magnification TEM images of the CNTs, showing that the CNTs can be filled with continuous or discrete Co9S8 nanowires.

microscopy (SEM) image of the as-synthesized product, showing a large number of straight CNTs grown on the catalyst particles. A high-magnification SEM image shown in Figure 1b reveals that these CNTs generally have thick roots and thin tips. The diameter of the tip is about 40-100 nm, and the diameter of the root ranges from 80 to 250 nm. Figure 1c is a typical TEM image showing some CNTs filled with Co9S8 nanowires. The majority of Co9S8 nanowires are 8-12 µm in length and about 30 nm in diameter, exhibiting a very high aspect ratio (>300). Figure 1d shows a TEM image revealing the roots of the CNTs. A big lump is generally present at the root of each Co9S8-filled CNT. A few CNTs which are not filled with Co9S8 tend to grow curly (as indicated with an arrowhead in Figure 1d). Most CNTs were found to be fully filled with Co9S8 (Figure 1e), leading to a long nanowire inside each CNT, while some CNTs were discretely filled, as shown in Figure 1f. In Figure 1f, the CNT is filled with two segments of Co9S8 nanowires with a separation of 350 nm. The wall thickness of this CNT is 18 nm. A representative energy dispersive X-ray (EDX) spectrum of the filled CNT is shown in Figure S1 in

the Supporting Information, and it confirms the sheath-corelike nanostructures are made of cobalt, sulfur, and carbon. The XRD patterns of the samples are shown in Figure 2. Figure 2a is the XRD pattern of the catalyst powder before reaction, confirming that it is Co particles supported on MgO. Co9S8 and MgS were formed after CNT growth at 1000 °C (Figure 2b). The results demonstrated that there was a sulfidation reaction during the growth of the CNTs. To investigate the role of MgO powder during CNT growth, we tried to use a Co film deposited on a Si wafer by electron-beam evaporation as a catalyst to grow CNTs. The filled CNTs were also successfully grown on the Si wafer. It is concluded that the growth of the filled CNTs is attributed to the presence of a Co catalyst; the support (MgO or Si wafer) has little influence on it. A high-resolution TEM image of a filled CNT is shown in Figure 3a. Its core is well crystallized with a clear lattice fringe. The fringe spacing is 0.35 and 0.57 nm, corresponding to the (220) and (1,-1,1) planes of cubic Co9S8 with a unit cell of a ) 0.992 nm (JCPDF file 86-2273). The interlayer spacing of the sheath is 0.34 nm, which matches the (002) plane of graphite

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Figure 2. (a) XRD pattern of the catalyst powder before the CNT synthesis and (b) XRD pattern of the sample after the CNT growth at 1000 °C.

Figure 4. TEM images of the CNTs prepared with different Ar flow rates: (a) 1100 and (b) 2400 sccm. The insets in (a) and (b) are TEM images of the characteristic catalyst particles in each sample. The scale bars in the insets are 100 nm.

Figure 3. (a) High-resolution TEM image of a Co9S8-filled CNT. (b, c) SAED patterns taken from representative CNTs filled with Co9S8. The characteristic spots are marked.

carbon. The interface between the CNT and Co9S8 nanowire is clearly revealed in this HRTEM image: the (1,-1,1) plane fringes of Co9S8 are parallel to the (002) fringes of the CNT, and these two phases adjoin closely and match well. The growth direction of the Co9S8 nanowire is along the [110] direction; in other words, the [110] direction of the Co9S8 nanowire is parallel to the axis of the CNT. Several filled CNTs were checked by selected area electron diffraction (SAED), and the results demonstrated that the Co9S8 nanowires are single crystals. A typical single-crystalline diffraction pattern is shown in Figure 3b. The sharp spots could be indexed as the reflections of cubic Co9S8 with the [-1,4,1] axis parallel to the electron beam. Another SAED pattern is shown in Figure 3c, which was taken along the [1,0,-1] zone axis of a Co9S8 nanowire. TEM results confirmed that the nanowires within the CNTs are indeed Co9S8, consistent with the XRD analysis. We carried out a set of experiments to investigate the influence of the Ar flow rate on the CNT growth. The H2 flow

rate was kept constant at 100 sccm in all runs while Ar at a varied flow rate was introduced as a dilute gas in our experiments. When the Ar flow rate is low, the thiophene concentration is high. The calculation of the thiophene concentration was described elsewhere.20 If the Ar flow rate is lower than 1700 sccm, the corresponding vapor concentration of thiophene will be higher than 0.51%, which will result in the growth of CNTs without filling materials inside their channels. Figure 4a shows a TEM image of the CNTs grown with an Ar flow rate of 1100 sccm, which corresponds to a thiophene vapor concentration of 0.76%. The diameters of the CNTs vary from 60 to 200 nm, and their lengths can reach 0.5-1 mm after growing for 15 min (see Figure S2 in the Supporting Information). A few Y-shaped (Y-junction) CNTs were found in this sample. The advantage of the Y-junction CNTs in our samples is that each branch can be as long as several micrometers, which makes them good candidates for application as a main component in a nanoscale electronic devise.21,22 The catalyst particle after CNT growth is in a shape like a prolate spheroid with tapered ends (inset of Figure 4a). EDX and SAED analysis revealed that the catalyst particles after CNT growth are Co9S8. The growth process can be described by a tip-growth mode.23 Figure 4b is a TEM image of the CNTs prepared with an Ar flow rate of 2400 sccm, which corresponds to a thiophene vapor concentration of 0.37%. These CNTs are short, and their lengths are about 2-3 µm. There is a catalyst particle on the tip of each CNT, revealing also a tip-growth mechanism. In contrast to the CNT product obtained with a high thiophene vapor concentration in Figure 4a, the catalyst particles have different morphologies (inset of Figure 4b). They are in a nanorod-like or cylindrical shape. A catalyst nanorod was ejected out from a CNT end (marked with an arrowhead), indicating a strong

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Figure 5. (a) TEM image of a Y-junction CNT filled with Co9S8 nanowires. The inset is a magnified TEM image of the junction. (b, c) TEM images showing the CNTs containing two different parts: the bottom part is empty, and the top part is filled. The inset of (b) is a magnified TEM image of the catalyst particle. The scale bars in the insets are 200 nm.

stress between the carbon shell and the catalyst particles during the CNT growth. Detailed TEM analysis demonstrates that the catalyst particle is cubic cobalt (Co) with a unit cell dimension of a ) 3.54 Å (JCPDS file 15-0806), which is identical to the original structure before reaction. The results indicate that the growth of Co9S8-filled CNTs is very sensitive to the vapor concentration of thiophene. The optimal Ar flow rate for the growth of Co9S8-filled CNTs ranges from 1700 to 2200 sccm; the corresponding thiophene vapor concentration in the reaction chamber is 0.51-0.40%. The above results reveal that the thiophene vapor concentration influences greatly the microstructure of the catalyst and thus determines the morphology and structure of the CNT product. The kinetics of the thermal decomposition of thiophene has been studied previously.24 It was concluded that pyrolysis of thiophene is initiated by C-S bond fission at high temperature and then produces a mixture which is mainly composed of CxHy molecules and sulfur-containing species, e.g., H2S. Therefore, there are at least two types of reactions on the cobalt catalyst. One is the decomposition of CxHy and the sequent diffusion of carbon atoms on the catalyst to grow a CNT; the other is the reaction of sulfidation between the Co catalyst and sulfide species, leading to the formation of Co9S8. According to our results, these two types of reactions are indispensable to the formation of Co9S8-filled CNTs. When the thiophene vapor concentration is lower than 0.40%, the sulfidation effect becomes trivial because of the extremely lower sulfur atom concentration. This is the reason why the catalyst is still cobalt

after CNT growth. The CNTs can grow, but they are short, thin, and in poor quality. On the contrary, the sulfidation reaction becomes influential when the thiophene vapor concentration is higher than 0.51%. It was reported that sulfur can serve as a growth promoter for producing long single-walled CNT bundles,25 double-walled CNTs,26 or branched CNTs.20,27 Hong and coworkers successfully synthesized CNTs using Co9S8 particles as the catalyst and C2H2 as the carbon source. They found that Co9S8 particles possess high catalytic activity, and branched CNTs were prone to form on the sulfide catalyst.28 In our experiment, the formation of long CNTs can be ascribed to the presence of sulfur-containing species in the reaction chamber, and the growth of Y-junction CNTs is probably attributed to the formation of Co9S8 particles due to the strong sulfidation. What we are concerned with here is how sulfur leads to the filling of CNTs. The thiophene vapor concentration for the growth of filled CNTs is 0.51-0.40%, which is just between the two conditions discussed above. It can be inferred that the reaction between Co and S is moderate. In other words, the reaction is mild and slow so that it takes from several seconds to tens of seconds to complete the phase transition from Co to cobalt sulfide. This Co sulfidation duration is commensurate with the growth time of the CNTs so that the Co9S8 can form and fill the CNTs in situ while the CNTs grow. Figure 5 shows some filled CNTs with different morphologies in contrast with that in Figures 1 and 3. Figure 5a is a Y-junction CNT filled with Co9S8 nanowires. There exists a big particle at the root of each Co9S8-filled CNT branch. The inset is a

1894 J. Phys. Chem. C, Vol. 112, No. 6, 2008 magnified TEM image to demonstrate the junction clearly. Figure 5b is a CNT containing two different parts. The bottom part is a CNT with an empty channel, while the top part is fully filled. There is a bump between these two parts (inset of Figure 5b) which looks like the catalyst particle responsible for the growth of these two parts. These images are inspiring for us to understand the filling mechanism. A plausible explanation for the formation of filled CNTs is linked to the capillary action: the liquid or melted precursor is sucked into the CNTs from one end. However, this mechanism cannot explain the filled CNTs in our experiments. For example, if this mechanism is correct, the root part of the CNT shown in Figure 5b should be filled first because the capillary action occurred there. The channels of the two top branches in Figure 5a are discrete from that of the root CNT; it is difficult to explain how the capillary action occurred in the top branches. In addition, the reaction temperature is much lower than the melting point (1495 °C) of cobalt; it is impossible to form the liquid cobalt that is necessary for a capillary action. Here we propose a new growth mechanism based on a volume increase induced by a phase transition. The initial catalyst is cubic cobalt with a unit cell containing four Co atoms. Its volume increases by 144% after it transforms to cubic Co9S8, whose unit cell contains 36 Co atoms and 32 S atoms. Recently, Banhart et al. reported that the multiwalled CNTs can cause large pressure buildup inside their cores under electron irradiation, and the internal pressure can reach values higher than 40 GPa. The large pressure can plastically deform and extrude solid materials that are encapsulated inside the cores.29 During the CNT growth in our experiment, a high pressure will build up between the enclosed catalyst particle and the carbon shell due to the volume expansion of the catalyst particle when it undergoes the phase transition from cobalt to cobalt sulfide. The CNT or the carbon shell will act as a robust nanoscale mold to extrude and deform the cobalt sulfide into a nanowire. For example, the size of the oval catalyst particle in Figure 5b is about 480 × 140 nm, corresponding to a volume of about 4 926 017 nm3. Suppose it is initially Co, and the increase in its volume will be about 7 093 464 nm3 after it transforms to Co9S8. The volume increase is just right to form a Co9S8 nanowire with a diameter of 30 nm and a length of about 10 µm. An illustration is shown in Figure 6 to depict the growth process of the Co9S8-filled CNTs. Thiophene thermally decomposes at 1000 °C and releases CxHy and H2S, resulting in a coexistence of the sulfidation of the Co catalyst and the growth of CNT. The microstructures and morphologies of the final products are controlled by the extent of Co sulfidation. Sulfidation reaction is neglectable if the thiophene vapor concentration is lower than 0.40%, leading to the formation of short CNTs with Co catalyst particles on their tips. The long CNTs and branched CNTs are formed due to the strong sulfidation when the thiophene vapor concentration is higher than 0.51%. The sulfidation affects the CNT product by changing the activity and structure of the catalyst. The thiophene vapor concentration suitable for the growth of the Co9S8-filled CNTs is 0.51-0.40%, which provides the appropriate amount of sulfur for the sulfidation. In the beginning of CNT growth, CxHy molecules dissociate on the Co catalyst and release carbon atoms to grow carbon shells (Figure 6a). In the meantime, H2S reacts slightly with the Co particle, and a tiny sulfide nucleates on it (Figure 6b). An alternative explanation for this process can be that carbon atoms diffusing through the Co catalyst are partially displaced by sulfur, resulting in both the formation of a tiny

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Figure 6. Schematic illustration showing the growth mechanism of CNTs filled with Co9S8 nanowires: (a-d) CNT growth route 1 and (e-h) CNT growth route 2.

cobalt sulfide on the catalyst particle and the precipitation of a carbon shell at the metal surface. Cobalt sulfide is also a catalyst for CNT growth, so a carbon shell can form and CNT can grow from the sulfide nucleus. The cobalt sulfide nucleus grows slowly, leading to a gradual increase in its volume; subsequently, it is extruded into the CNT which is growing on the catalyst particle (Figure 6c). The sulfidation of Co and the extrusion of the sulfide take place during the growth of CNTs; consequently, a long nanowire will be formed within the CNT (Figure 6d). The sulfide nanowire also directs the CNT to grow straight. To obtain the long and continuous Co9S8 nanowires, the growth rate of CNTs should be consistent with that of the Co9S8 nanowires. If the CNT growth rate is higher, the overgrown CNT part will not be filled by Co9S8; instead, the as-grown CNT will be partly filled or filled with several short nanowires. In some cases, empty CNTs form in the beginning when the instant vapor concentration around some catalyst particles deviates from 0.51% to 0.40% due to the local concentration fluctuation (Figure 6e-g); in addition, the deviation is inevitable at the moment when the thiophene vapor is just opened because the thiophene vapor concentration in the reaction chamber suffers a variation from none to the designed value. When the local vapor concentration falls back in the range of 0.51-0.40%, the sulfidation begins to affect CNT growth. As a result, a Co9S8-filled CNT will grow on the top of an empty CNT part (Figure 6h), as observed in Figure 5b. Conclusions In summary, Co9S8-nanowire-filled CNTs have been prepared and structurally analyzed for the first time. The Co9S8 nanowires are single-crystalline, and their lengths are up to about 10 µm. The key factor in the formation of the Co9S8-nanowire-filled CNTs in our approach is the introduction of the appropriate concentration of thiophene as the carbon source. Thiophene constantly supplies sulfur and carbon during the cobalt-catalyzed

Filling Carbon Nanotubes with Co9S8 CNT growth, leading to a reliable production of Co9S8-filled CNTs with a high filling ratio. The CNT growth is sensitive to the thiophene vapor concentration. The optimal thiophene vapor concentration for the formation of the Co9S8-filled CNTs is 0.51-0.40%; a higher thiophene concentration leads to the formation of long hollow CNTs and branched CNTs, while a low thiophene concentration results in short hollow CNTs with Co particles on their tips. The filling of Co9S8 nanowires is ascribed to the volume increase induced by the phase transition from cobalt to cobalt sulfide together with the extruding action of CNTs as nanomolds. This filling mechanism can be employed to synthesize the CNTs filled with other materials. Acknowledgment. This work was supported by NSF Grant No. DMR-0548061. We thank Prof. Z. L. Wang and Dr. Y. Ding (Georgia Institute of Technology) for their help with the TEM experiment. Supporting Information Available: EDX spectrum and SEM images of CNTs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323. (2) Yanagi, K.; Miyata, Y.; Kataura, H. AdV. Mater. 2006, 18, 437. (3) Hsin, Y. L.; Hwang, K. C.; Chen, F. R.; Kai, J.-J. AdV. Mater. 2001, 13, 830. (4) Ajayan, P. M.; Lijima, S. Nature 1993, 361, 333. (5) Sloan, J.; Cook, J.; Heesom, J. R.; Green, M. L. H.; Hutchison, J. L. J. Cryst. Growth 1997, 173, 8187. (6) Tsang, S.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (7) Ajayan, P. M.; Colliex, C.; Lambert, J. M.; Bernier, P.; Barbedette, L.; Tence, M.; Stephan, O. Phys. ReV. Lett. 1994, 72, 1722. (8) Subramoney, S.; Ruoff, R. S.; Lorents, D. C.; Chan, B.; Malhotra, R.; Dyer, M. J.; Parvin, K. Carbon 1994, 32, 507. (9) Guerret-Piecoun, C.; Le Bouar, Y.; Loiseau, A.; Pascard, H. Nature 1994, 372, 761.

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