Production of carbon nanotubes with marine manganese nodule as a versatile catalyst

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1 Microporous and Mesoporous Materials 81 (2005) Production of carbon nanotubes with marine manganese nodule as a versatile catalyst J.P. Cheng a,b, *, X.B. Zhang a,y.ye b, J.P. Tu a, F. Liu a, X.Y. Tao a, H.J. Geise c, G. Van Tendeloo d a Department of Materials Science and Engineering, Zhejiang University, Hangzhou , PR China b Department of Earth Science, Zhejiang University, Hangzhou , PR China c Department of Chemistry, University of Antwerp(UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium d EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium Received 9 September 2004; received in revised form 12 November 2004; accepted 15 November 2004 Available online 4 March 2005 Abstract Carbon nanotubes can be produced by pyrolysis of acetylene at 750 C, using powdered naturally occurring marine manganese nodules as the catalyst. Helically coiled nanotubes can also be produced in a high yield by changing the experimental conditions. Compared to other catalysts, the mineral catalyst is cheap, convenient and versatile. The porous mineral phase is spontaneously collapsed at elevated temperature, and the catalytic metal cations, originally accommodated in the mineral, moved to the outer surface, where they aggregated to metallic nanoparticles available for the growth of the nanotubes. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Transmission electron microscopy; Coiled nanotube 1. Introduction Manganese oxide materials have recently gained attention for their catalytic and ion-exchange properties. The family of manganese oxide includes layered and one-dimensional tunnel materials, which are composed of corner and edge-shared MnO 6 octahedral building blocks. Todorokite is a naturally hydrated manganese oxide with a rather complicated composition and occurs in large quantities, together with related minerals, in many areas of the world [1]. The so-called marine Mnnodules, occurring in huge numbers at the bottom of the oceans, are mainly composed of todorokite. Its ability to concentrate Ni and Cu to several weight percent * Corresponding author. Tel./fax: address: mseem@zjuem.zju.edu.cn (J.P. Cheng). makes it the focus of crucial economic and environmental significance [2]. Todorokite has a well-defined microporous structure consisting of tunneled, 3 3 arrays of edge-shared MnO 6 octahedra. The presence of tunnels allows the lattice to incorporate foreign metal cations and water molecules, and also gives it the possibility to exchange cations, as zeolites do. The exceptional physicochemical properties of the material and its resemblance to zeolites have stimulated research into its applicability in heterogeneous catalysis and rechargeable battery technology [3,4]. Todorokite has been considered as a sorbent for harmful metal ions [5] and for use in ion and molecular sieves [6]. As far as the catalysis of todorokite is concerned, studies have revealed that it is catalytically active, for e.g., CO oxidation [7], H 2 O 2 decomposition [8], nitrogen oxide reduction [9], cyclohexane functionalization [10], ethanol dehydrogenation [11], and propane /$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi: /j.micromeso

2 74 J.P. Cheng et al. / Microporous and Mesoporous Materials 81 (2005) transformation [12]. However, there is no report of its catalytic potential to grow carbon nanotubes. Carbon nanotubes (CNTs) are globally in the limelight as new fancy materials, broadening their applications to almost all scientific areas. Several methods, such as arc-discharge vaporization [13], laser vaporization [14], chemical vapor deposition (CVD) [15] and others [16,17], have been developed to produce CNTs, including single-wall and multi-wall CNTs. The CVD method, because of its simplicity and low cost, will probably become the method of choice to produce the large quantities of CNTs necessary for fundamental as well as nano-technological purposes. The catalysts for the CNTs production by CVD are usually finely dispersed transition metals (Fe, Co, Ni) supported by inorganic oxides (MgO, Al 2 O 3, SiO 2, etc.); gaseous hydrocarbons are used as the carbon source. Unfortunately, these still are some drawbacks. For example, the synthesis of the catalysts is often complicated, requiring severe conditions. Moreover, high quality, artificial chemical reagents are necessary, which even may induce environmental pollution. We therefore wondered whether it would be possible to use a naturally occurring material as a catalyst for the CNT production. This would greatly simplify the synthesis and could result in an easy and ecologically acceptable production method. Here we report the use of marine Mn-nodules as an integrated catalyst for the decomposition of acetylene in our one-step CVD method to produce multi-wall CNTs. We believe this is to-date the simplest production method, because the catalyst needs neither to be synthesized, nor a prior reduction step. Moreover, the catalyst contains a variety of metallic elements, such as Mn, Fe, Cu and Ni. Some move to the mineral surface during the CNT production, where they catalyze the growth of the nanotubes, as well as the growth of helically coiled tubes. 2. Experimental The marine manganese nodules, used here, originated from Chinese Pioneer Area in the East Pacific. The raw material, black colored rocks, was sonicated for hours in sufficiently pure water to remove the adhering clay. After drying in the open air for several days, the rocks were pulverized and passed through a sieve with 200 mesh (75 lm). The resultant powder was used as the catalyst and characterized by transmission electron microscopy (TEM, Philips CM200) and X-ray diffraction (XRD, Rigaku D/Max 2550) using CuKa radiation. To grow the CNTs, about 100 mg of the mineral powder was uniformly spread in a quartz boat. Then, the boat was placed in the central part of a horizontal fixed bed quartz reactor tube, which was previously heated to 750 C in a stream of nitrogen gas (600 ml/ min). Immediately these after, a flow of acetylene (100 ml/min) was introduced along with the flow of N 2 carrier gas. After 20 min reaction time, the flow of C 2 H 2 was stopped and the tube was allowed to cool to room temperature in a nitrogen atmosphere. In experiments to study the influence of heat on the mineral catalyst, we delayed the introduction of C 2 H 2 for 5 or 20 min after the placement of the catalyst. The morphology of the formed carbon nanotubes was investigated using TEM after sonicating the sample in ethanol and placing a few drops of the resultant dispersion on a holey copper grid, or by gluing the powder directly on a copper TEM-grid. The elemental composition of the samples was determined by energy dispersive X-ray spectroscopy (EDS) inside the TEM. XRD spectra were recorded with D/max-rA, using CuKa radiation. Raman spectra of the rude CNTs were recorded on a Thermo Nicolet, ALMEGA-Dispersive Raman, equipped with a Nd/YAG laser, excitation wavelength k = 532 nm. 3. Results and discussion 3.1. Characterization of the mineral material XRD profile of the powdered manganese nodule powder is shown in Fig. 1. The peaks at 2h =21 and 2h =27 corresponding to lattice distances d = and d = nm (ref JCPDS card No ), respectively, indicate the presence of a quartz impurity in the sample. A vague peak at d = nm ascribed to (0 0 1) of birnessite implies that it is in the minority (JCPDS No ). All other recognizable peaks can be indexed to the minerals buserite and todorokite (JCPDS Nos and , respectively). It is not easy to judge their relative content from the XRD powder profile, because both are manganese oxides Fig. 1. Powder XRD profile of the raw mineral.

3 J.P. Cheng et al. / Microporous and Mesoporous Materials 81 (2005) and have some lattice reflections at identical positions. For example, both have (001) at d = nm and (0 0 2) at d = nm. Nevertheless, by comparing their behaviors in a heat treatment, approximately 20% of the integrated intensity of the (001) peak is due to todorokite [18]. Buserite and todorokite are the major components of the mineral in our case. Fig. 2(a) shows a general overview TEM image at low resolution of the todorokite phase in the powder, and the insert gives a schematic view of its structure, showing that the tunnel consists of triple chains of edge-sharing MnO 6 octahedra. The morphology of crystalline todorokite is that of randomly arranged needles, tens of nanometer in length. Fig. 2(b) shows a high-resolution micrograph of the needle phase, revealing well defined and uniformly distributed nanotunnels, which are formed by well-assembled MnO 6 octahedra. EDS measurements confirm that Mn, Fe, Ni, Cu, are present, several weight percents of Fe, Ni, and Cu are incorporated in the tunnels as foreign metal cations. The content of Fe is much larger than that of Ni, but both play the role of catalytic metal for CNT growth Examination of carbon nanotubes Copious amounts of CNTs are formed in the mineral powder directly after the start of the reaction with acetylene (see Fig. 3(a)). Amorphous carbon is found both on the exterior of the CNTs and as separate conglomerates. Careful analyses of the tubes prove that they all are multi-walled with a diameter distribution ranging from 10 to 80 nm. Their lengths are at the micron level. Just as Liao et al. [19], we observed (in Fig. 3(b)) some catalyst particles fully encapsulated by graphene sheets at the tip of CNT, and the CNT growth stops. Exceptionally, we found a short CNT with an encapsulated catalyst particle at both ends (Fig. 3(c)). The EDS spectrum, recorded under high spatial resolution, reveals that these encapsulated particles are formed by carbon and Fe (Fig. 3(d)). The Cu, also present in the spectrum originates from the sample support. The selected area diffraction pattern (in Fig. 3(e)) of the encapsulated particle shown in Fig. 3(b) indicated that the catalyst particle is a single crystalline iron carbide (Fe 4 C 63 ) rather than an Fe crystal. The diffraction pattern also presents a pair of arcs originating from the carbon (002) reflection. The superimposed pattern of the catalyst can be indexed as the (0 1 3) and ( ) reflection of the iron carbide (marked in the pattern). At present, we do not have a complete picture of the formation process of the catalysts, but the catalyst particles are derived from the mineral powder in the reductive atmosphere at 750 C and that the iron group metal cations are extruded from their hosts, the nanotunnels. The iron carbide, identified in Fig. 3(b), fits in the commonly accepted mechanism of the growth of CNTs. In this mechanism the carbon atoms separate out of the metallic catalyst particles when the latter become supersaturated with carbon. Once diffused to the surface of the catalyst particle the carbon atoms grow into CNTs. The CNTs have been described to grow either from their tip or from their base, depending upon the type of the interaction between the metal and the support [20]. However, Fig. 3(c) provides a unique combination of these modes. We assume that one CNT grows in two opposite directions, at the tip as well as at the base, fed by two catalyst particles close together in space. This dual mode growth may occur at a much higher speed than that of any single mode. A higher Fe content in the catalyst leads to a higher carbon production, rather than to an increased CNT production. The excess carbon atoms accumulate on the catalysts and form graphene layers on their surface. The CNT stops growing once the graphene layers completely cover the catalytic site, which cuts off further carbon supply, resulting in a short CNT. Syntheses of CNTs using zeolite and clays have been reported [21 24], but these materials only act as support for the catalyst metals. In contrast, the manganese Fig. 2. (a) TEM image and (b) a high-resolution micrograph of the todorokite. Insert in (a) is a schematic configuration of todorokite.

4 76 J.P. Cheng et al. / Microporous and Mesoporous Materials 81 (2005) Fig. 3. TEM examination results, (a) aspects of as-prepared CNTs, (b) catalyst particle encapsulated at one end, (c) catalyst particles encapsulatedat both tips, (d) Energy dispersive spectroscopy spectrum of the encapsulated catalyst particle shown in (b), (e) Selected area electron diffraction (SAED) of the encapsulated particle shown in (b). mineral performs the integrated function of catalyst and support simultaneously. This renders the manganese nodules unique. The XRD profile (Fig. 4) of as-prepared CNTs, still containing rests of the catalyst, shows peaks of CNTs and manganese oxide (JCPDS file No , cubic), confirming that the initial mineral phases transform into MnO at 750 C. The weight content of the as-prepared CNTs with respect to the mineral catalyst was measured using a thermal gravimetric analyzer (TGA, Netzsch Sta 409) under an atmosphere of O 2 :N 2 = 12.5:87.5, with a heating rate of 5 C/min from room temperature to 1000 C. The characteristic curve is depicted in Fig. 5. The weight loss is due to the oxidation of carbon, and thus corresponds to the carbon content in the sample. The correlation be- Fig. 4. XRD profile of the raw product, showing the presence of CNTs and MnO. Fig. 5. Typical TGA curve of the as-prepared CNT material.

5 J.P. Cheng et al. / Microporous and Mesoporous Materials 81 (2005) tween TGA oxidation temperature and carbon speciation [25], gave 1 wt% amorphous carbon and 28 wt% CNTs in the sample. Compared to artificial catalysts, the capacity of the mineral catalyst is much lower, which may be due to the presence of many impurities and lack of sufficient, suitable catalyst metal at the right position. Ultimately, only those metal cations are suitable that reach the outer surface, where they must be transformed first in oxide nanoparticles, followed by reduction. Only then, the particles can catalyze the decomposition of C 2 H 2 and nucleate the growth of nanotubes. The metal ions start their journey from inside the mineral tunnels, and possibly may be transferred to the outer surface by mobile water molecules originally accommodated in the tunnels. Alternatively, the cations may migrate through the tunnels by diffusion. However, both transport mechanisms have created problems, at the elevated temperature the mobile water molecules may be scarce, and the tunnels are unstable and tend to collapse. The Raman spectrum of the CNTs with the remnants of the pristine is also characterized. The spectrum shows peaks at 1347 and 1590 cm 1, which are attributed to D and G bands of CNTs, respectively. The D band, at about 1350 cm 1, is related to local defects originating from structural imperfections and the band which corresponds to the tangential C C stretching vibration is at about 1580 cm 1, named as the graphite mode G band. The intensity ratio of I D /I G is known to depend on the structural characteristics of CNTs. The I D /I G in the spectrum is 0.73, which suggests a defective structure or a lower degree of graphitization in the as-prepared CNTs Influence of heat treatment for the mineral Fig. 6. TEM images of the CNTs formed over mineral catalyst calcined for (a) 5 and (b) 20 min prior to CNT growth. In all previous CNT production batches, the formation of catalyst and growth of CNTs occurred simultaneously. Yet, the results obtained above suggest that the formation of the catalytic particles depends strongly on the porous structure of the mineral, which tends to collapse at elevated temperature. Therefore, we decided to investigate the effect of heat treatment on the mineral catalyst prior to the start of CNT growth. The following TEM samples were prepared by gluing powder on copper TEM-grids. Fig. 6(a) shows a typical TEM image of as-prepared CNTs produced over a mineral catalyst, calcined for 5 min before CNT growth. It shows that CNTs already nucleated on the outer surface of the mineral particle. The diameter distribution of the CNTs was found to cover a wide range. It proves the wide range of the size distribution of the catalyst particles, because CNT diameters and catalyst particle sizes are strongly correlated. The formation of the ultimate catalyst particles occurs almost simultaneously with the phase change of the mineral. When the mineral was calcined for 20 min, multi-wall CNTs were still produced. However, a striking feature is that a large fraction, more than 50%, of the CNTs are coiled, forming helixes, as is shown in Fig. 6(b). Some are regularly coiled with a variety of radii and helix pitches. In addition, the mean diameter of these CNTs is much larger than that of those presented in Fig. 6(a). Many more catalyst particles agglomerated into larger sized particles. Also in Fig. 6(b), an arrow points at a catalytic particle placed at the tip of a coiled CNT. The synthesis of helically coiled CNTs by pyrolysis of acetylene over metallic nanoparticles [20,26,27] and a possible growth mechanism [20] have been reported. The synthesis, structure and growth mechanism of helically coiled CNTs have also been discussed elsewhere [28 32]. In our cases, catalyst particles kept at elevated temperature have more chance to transform into a regular shape, than particles kept at that temperature for a short time. Different crystal planes possess different catalytic capabilities for the growth of carbon nanotubes, regularly faceted catalyst particles are suited for the growth of regularly coiled CNTs. Different rates of the catalytic reaction on different places of the surface of the catalyst particle induce different growth speeds around the catalyst particle, and that the asymmetric growth conditions lead to coil growth [20,26 32]. Hence,

6 78 J.P. Cheng et al. / Microporous and Mesoporous Materials 81 (2005) the mineral calcined for 20 min tends to catalyze the production of helically coiled CNTs, whereas the same mineral calcined for only 5 min does not. 4. Conclusions In summary, we have produced multi-wall CNTs with a naturally occurring mineral powder as the catalyst by CVD method. It is believed to be the simplest method for the synthesis of CNTs. The fabrication of helically coiled CNTs can be also accomplished by changing the experimental conditions. Iron-group metal cations initially encapsulated in the mineral nanotunnels migrate to the surface of mineral power and play the role of catalyst. At elevated temperature, the collapse of porous structure of the mineral happened simultaneously with metallic catalyst formation. The as-produced CNTs/MnO composite may be used as a novel electrode material for rechargeable batteries after treatment. All these have proved that marine Mn-nodule is a cheap, convenient and versatile catalyst for producing both curved and helically coiled CNTs. Acknowledgements This work was financially supported by COMRAÕs project (DY ) and the Special Funds for Major States Basic Research Projects of MOST, China (No. G ). References [1] S. Turner, P.R. Buseck, Science 212 (1981) [2] R.G. Burns, V.M. Burns, H.W. Stockman, Am. Mineral. 68 (1983) 972. [3] Y. Yang, D. Shu, H. Yu, X. Xia, Z.G. Lin, J. Power Sources 65 (1997) 227. [4] N. Kumagai, S. Komaba, H. Sakai, N. Kumagai, J. Power Sources (2001) 515. [5] A. Dyer, M. Pillinger, J. Newton, R. Harjula, T. Moller, S. Amin, Chem. Mater. 12 (2000) [6] G. Lei, Mar. Geol. 133 (1996) 103. [7] Y.F. Shen, S.L. Suib, C.L. OÕYoung, J. Catal. 161 (1996) 115. [8] K.M. Parida, P.K. Satapathy, N.N. Das, S.B. Rao, J. Colloid Interface Sci. 179 (1996) 241. [9] A.L. Caberra, M.B. Mapple, G. Arrhenius, Appl. Catal. 64 (1990) 309. [10] J.Y. Yang, G.G. Xia, Y.G. Yin, S.L. Suib, C.L. OÕYoung, J. Catal. 176 (1998) 275. [11] H. Zhou, J.Y. Wang, X. Chen, C.L. OÕYoung, S.L. Suib, Micropor. Mesopor. Mater. 21 (1998) 315. [12] T.K Katranas, A.C. Godelitsas, A.G. Vlessidis, N.P. Evmiridis, Micropor. Mesopor. Mater. 69 (2004) 165. [13] S. Iijima, Nature 354 (1991) 56. [14] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [15] W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou, R.A. Zhao, G. Wang, Science 274 (1996) [16] M. Terrones, N. Grobert, J. Olivares, J.P. Zhang, H. Terrones, K. Kordatos, W.K. Hsu, J.P. Hare, P.D. Townsend, K. Prassides, A.K. Cheetham, H.W. Kroto, D.R.W. Walton, Nature 388 (1997) 52. [17] J.M.C. Moreno, M. Yoshimura, J. Am. Chem. Soc. 123 (2001) 741. [18] Y. Ye, Z.Y. Shen, X.Y. Zhu, J.P. Cheng, X.B. Zhang, Seventh Joint International conference of JICAST, 2002, Hamamatsu, Japan. [19] X.Z. Liao, A. Serquis, Q.X. Jia, D.E. Peterson, Y.T. Zhu, H.F. Xu, Appl. Phys. Lett. 82 (2003) [20] S. Amelinckx, X.B. Zhang, D. Bernacrts, X.F. Zhang, V. Ivanov, J.B. Nagy, Science 265 (1994) 635. [21] J.P. Cheng, J.P. Tu, Y. Ye, et al., Chinese Chem. Lett. 13 (2002) 381. [22] D. Gournis, M.A. Karakassides, T. Bakas, N. Boukos, D. Petridis, Carbon 40 (2002) [23] A.M. Zhang, C. Li, S.L. Bao, Q.H. Xu, Micropor. Mesopor. Mater. 29 (1999) 383. [24] J.P. Cheng, X.B. Zhang, F. Liu, J.P. Tu, Y. Ye, Y.J. Ji, C.P. Chen, Carbon 41 (2003) [25] B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497. [26] X.B. Zhang, X.F. Zhang, D. Bernaerts, G.V. Tendeloo, S. Amelinckx, J.V. Landuyt, V. Ivanov, J.B. Nagy, Ph. Lambin, A.A. Lucas, Europhys. Lett. 27 (1994) 141. [27] V. Ivanov, J.B. Nagy, Ph. Lambin, A. Lucas, X.B. Zhang, X.F. Zhang, D. Bernaerts, G.V. Tendeloo, S. Amelinckx, J.V. Landuyt, Chem. Phys. Lett. 223 (1994) 329. [28] D.Y. Zhong, S. Liu, E.G. Wang, Appl. Phys. Lett. 83 (2003) [29] K. Hernadi, L.T. Nga, L. Forro, J. Phys. Chem. B 105 (2001) [30] S.J. Motojima, Q.Q. Chen, J. Appl. Phys. 85 (1999) [31] Y.K. Wen, Z.M. Shen, Carbon 39 (2001) [32] Y. Qin, Q. Zhang, Z.L. Cui, J. Catal. 223 (2004) 389.