- Email: [email protected]
Evidence for growth mechanism and helix-spiral cone structure of stacked-cup carbon nanofibers
Available online at www.sciencedirect.com
Carbon 45 (2007) 2751–2758 www.elsevier.com/locate/carbon
Evidence for growth mechanism and helix-spiral cone structure of stacked-cup carbon nanofibers Jose Vera-Agullo a, Helena Varela-Rizo a, Juan A. Conesa a, Cristina Almansa b, Ce´sar Merino c, Ignacio Martin-Gullon a,* a
Department of Chemical Engineering, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain b Analytical Technical Services, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain c Grupo Antolı´n Ingenierı´a, Crta. Iru´n 244, E-09007 Burgos, Spain Received 14 May 2007; accepted 17 September 2007
Abstract New evidence for the structure of Ni-catalyzed stacked-cup carbon nanofibers (CNFs) has been found. This type of carbon nanofiber exhibits a wide hollow core as well as a large diameter (between 40 and 140 nm). The fibers have been produced by the floating catalyst method using natural gas as carbon feedstock, a sulfur compound, and a nickel catalyst. It was found that the catalytic particles are heterogeneous with two different parts: one composed of metallic Ni, which is the catalytically active portion of the particle, and another composed of NiS, which allows for the hollow nanofiber structure. The hollow core of the fibers has similar dimensions to the NiS volume of the particle and the graphitic layers grow from the rear nickel region of the particle. Nevertheless, the NiS component seems to be indispensable in producing the helix-spiral formation of the graphitic structure, as clearly shown by the TEM studies. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Among the different types of carbon nanofilaments that can be produced by catalytic chemical vapor deposition (cCVD) are those that have a wide hollow core. In 1972, Baker et al. [1], while studying the production of carbon filaments from acetylene over nickel as a catalyst, pointed out that a small amount of the produced filaments were exceptionally thick (100 nm) and had a wide hollow core. The dimensions of these exceptional filaments were an outer diameter of 94–87 nm and a central core diameter of 58– 56 nm. In 1993, Tibbetts et al. [2,3] reported that the carbon nanofibers produced by floating catalyst using ferrocene, methane, and hydrogen sulfide also had a wide hollow core. Also, the graphitic planes were not parallel to the fiber axis as was observed in the substrate grown herringbone nanofibers [4,5]. Nevertheless, the structure of
*
Corresponding author. Fax: +34 96 590 3826. E-mail address: [email protected] (I. Martin-Gullon).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.09.040
stacked-cup like carbon nanofiber was first fully described by Endo et al. [6], working with carbon nanofibers produced from ferrocene/iron pentacarbonyl, natural gas, and hydrogen sulfide in a floating catalyst reactor. They defined stacked-cup CNFs as a unique structure of large diameter nanofibers with a large inner hollow core formed by an arrangement of truncated graphitic cones. Moreover, Kim et al. [7] claimed that this stacked-cup CNF was a different type of fiber than herringbone CNF. Although both structures would look the same in transmission electron microscopy (TEM), the main difference was in the almost circular cross section of stacked-cup CNFs, presumably due to the effect of a molten catalyst particle. The mechanism of growth of catalytic carbon nanofilaments has been investigated since the 70s [1,8]. Baker et al. [1] grew carbon nanofibers from acetylene and a nickel catalyst in a gas reaction cell built within a TEM. These authors pointed out that the ‘‘pear-shaped’’ catalyst particles have the properties of a liquid in some steps of the growth model that they proposed, and this liquid behavior is responsible for the interior hollow core of the filament. In
2752
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
addition, the rate of filament formation is very sensitive to both the reaction temperature and the composition of the catalyst [9]. Nevertheless, Yang and Chen [10], while studying the growth of nanofibers from methane over Fe, Co, and Ni, reached the conclusion that the catalyst particles must be faceted, where the Ni(1 1 1) and Ni(3 1 1) faces form metal/graphite interfaces with the graphite planes growing parallel to them, and the Ni(1 0 0) and Ni(1 1 0) are gas/ metal interface, active for the decomposition of hydrocarbons. Rodriguez et al. [4] analyzed three different types of carbon nanofibers and proposed one three-dimensional model for each type with the catalyst particles faceted; several faces were attributed to the gas/metal interaction and others to the graphite filament/metal interaction. Another key factor in the production of stacked-cup carbon nanofibers is the need for sulfur or phosphorous in the reactor [11–14]. Martin-Gullon et al. [15] reported the formation of stacked-cup CNFs in a floating catalyst reactor with nickel as a catalyst, natural gas, and a sulfur-containing feedstock. These nickel based CNFs were only produced when sulfur was added. Sulfur seemed to the form a binary catalytic particle, with one side composed mainly of nickel and the other of an alloy of nickel and sulfur. Others authors also found that sulfur is not homogeneously dispersed in the metal catalyst particles [16]. A combination of a floating catalyst system, methane, and a presence of sulfur in the feedstock is essential to the formation of the stacked-cup nanofiber structure, regardless of the metal catalyst (Fe [2] or Ni [15]). It seems that the same structure is also formed at equivalent conditions with cobaltocene in a floating catalyst system [14]. Nevertheless, the effect of sulfur in the formation of carbon nano-
filaments is not yet clear since there are different explanations in the literature [16–20]. Its concentration in the feedstock stream cannot be random and must be similar to that of the metal in order to really promote the CNF growth and to avoid the formation of soot [2,12,20]. This latter aspect might indicate that sulfur is required to change the metal composition to a certain degree, and that a higher concentration completely poisons the catalytic activity of the metal [20]. A very similar behavior was found when replacing sulfur by phosphorus [19,21,22], when a very specific amount promotes CNF formation from methane. Meanwhile Endo’s group [6,23] described the structure of this stacked-cup CNF as discontinuous and individual truncated cones, Eksioglu and Nadarajah [24] pointed out that an arrangement of discontinuous truncated cones might have only very selective angles, which does not correspond to the variety of angles found, most of them around 25 with respect to the CNF axis [25]. These values can only be explained if the structure is a cone-helix arrangement of a continuous graphene sheet. In addition, this cone-helix structure is the only one that might explain high electrical conductivity found for this CNF. An arrangement of truncated cones cannot explain high conductivity along the fibril axis, due to the fact that the conductivity in the direction perpendicular with the graphene planes is five orders of magnitude lower. Tibbetts et al. [26] and Endo et al. [27] pointed out that stacked-cup carbon nanofibers exposed to 2800 C high temperature graphitization did not modify much of the electrical conductivity, although it introduced deep structural changes in the crystalline orientation. This aspect might indicate that the structure is formed is a continuous graphene helix,
Fig. 1. Overall TEM view of GANF1. Very few Ni particles are found at the fibril tips.
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
2753
Fig. 2. SEM micrograph of two nanofiber ends.
where the large changes in crystal size and 002 distance will not change the conductivity. Tibbetts et al. [25] left opened the discussion for a discontinuous cone arrangement or a continuous cone-helix structure, but pointed out that 1500 C was the best high temperature treatment due to the continuity formed with the thin pyrolitic CVD layer. TEM exploration failed to shed more light about the stacked-cup CNF structure. The present communication tries to deepen the understanding of the effect that Ni plays on the formation mechanism and structure of stacked cup CNFs. As stated before, a sulfur source, methane as the main carbon feedstock, and a floating catalyst system were always present when stacked-cup nanofibers were formed. Sulfur alters the catalytic particles producing a binary particle. How do the graphitic layers grow from these particles? Depending on the growth mechanism, one type of possible structure would make sense. 2. Experimental The sample analyzed in this study is GANF1 grade D&S, manufactured by Grupo Antolin Ingenieria (Burgos, Spain). GANF1 is a nickel derived stacked-cup carbon nanofiber produced using natural gas and a sulfur feedstock at a temperature above 1100 C. This sample has been deeply characterized elsewhere [15].
Fig. 3. Stacked-cup CNF with graphene layers oblique to the fiber axis at around 25.
The chemical structure of the CNF, as well as the metal catalyst particle morphology and composition was studied through TEM, using a model JEOL JEM 2010. The equipment is provided with an energy dispersive spectroscopy system (EDS), Oxford instruments model INCA Energy TEM100. The metal composition in a given area of the exploration was found through this technique. Important information about the crystalline structure from selected area electron diffraction (SAED) was also obtained. The electrical resistivity of carbon nanofibers in the bulk state was determined for pressures up to 60 bar. In this case, the experimental electrical resistivity of Pyrograf IIITM grade PR-24, Fe grown stacked-cup CNF (Applied Sciences Inc., Cedarville, OH), and substrate grown multi wall nanotubes (MWNT) from ferrocene/xylene/Ar [28] were performed for comparison purposes.
3. Results Fig. 1 shows a general overview of the GANF1 material. Fibrils are long and straight, with a typical tubular structure with a huge hollow core (lower electronic density in the middle of the fiber), which corresponds with a stacked-cup CNF structure. Fig. 2 shows a SEM micrograph with evidence for the tubular structure.
Fig. 4. TEM image of a metal particle at the tip of a carbon nanofiber.
2754
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
Very few Ni particles are located at the nanofiber tips. Since all the fibrils were produced from a particle at one of the extremes, it seems that the catalytic particle could have dropped off the fibril, indicative of a molten behavior. Fig. 3 shows the detail of one fibril, with an average angle of 21–25 from the axis, very similar to the angle found and reported for Fe-based Pyrograf IIITM [25]. At the bottom right corner, a higher magnification clearly shows the graphene planes. In general, most of the nanofibers do not have particles located at the tip. Very few fibers show a catalytic particle inside. Our analysis will be focused on those fibrils where the catalytic particle remained within the nanofiber. These particles are inside the fiber with tips coated by a graphite layer. According to Rodriguez et al. [4], this preserves the shape of the metal catalyst in the reactive state during subsequent cooling to room temperature, and therefore, postreaction TEM examinations give an accurate picture of the morphological characteristics of the particle. Fig. 4 shows a TEM micrograph of an unusual carbon nanofiber with a catalytic particle at its tip. This catalytic particle formed a nanofiber with stacked-cup structure at reaction conditions, and the particle did not drop off after the nanofiber growth. EDS analysis has been done at two different positions in the particle (1 and 2). Results are shown in Table 1. The right portion of the particle is composed mainly of nickel with a negligible amount of sulfur, the left part contains an appreciable quantity of sulfur with respect to the quantity of nickel (47 %). The phase diagram of nickel and sulfur shows that the left side of the particle was molten when the nanofiber grew at the production temperature (above 1100 C) due to either the formation of NiS (melting point of 797 C) or the formation of an eutectic between nickel and sulfur at 35 at.% S (635 C). The right part of the particle (Ni, bulk melting point
1455 C) might have been solid at reaction conditions, although it might behave as a liquid due to its nano-dimensions [29]. In any case, the Ni side would be less fluid than the other part. Furthermore, it can be observed in Fig. 4 that the graphene layers originate from the outer part of the particle (composed of Ni) corresponding to the outer diameter; and the diameter of the inner channel corresponds to the other section of the particle (see Table 2). Most of the particles located at the tip of the carbon nanofibers exhibit this behavior. Fig. 5a shows the same
Table 1 Atomic composition of the metal particle encapsulated at the tip of the carbon nanofiber (Fig. 4) by energy dispersive X-ray spectrometry Left side of the particle (1)
Right side of the particle (2)
Element
at.%
Element
at.%
C S Ni
80.26 9.18 10.56
C S Ni
61.43 0.58 37.99
S/(S + Ni) · 100
46.5
S/(S + Ni) · 100
1.5
Table 2 Atomic composition of the metal particle encapsulated at the tip of the carbon nanofiber (Fig. 5a) by energy dispersive X-ray spectrometry Left side of the particle
Right side of the particle
Element
at.%
Element
at.%
C S Ni
41.07 0.00 58.93
C S Ni
90.94 4.42 4.64
S/(S + Ni) · 100
48.8
S/(S + Ni) · 100
0.0
Fig. 5. (a) TEM image of a metal particle at the tip of a carbon nanofiber; (b) detail of the right part of the particle, with a characteristic crystallinity; (c) diffraction pattern of the particle; (d) dark field image of the Ni network; and (e) dark field image of the NiS network.
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
2755
Fig. 6a. A ribbon of carbon comes out from the nanofiber end.
results in another catalytic particle. The left side is composed of nickel only, and the right side is a mixture of sulfur and nickel at 49 at.%. The right part of the particle exhibits a characteristic crystallinity (Fig. 5b). A diffraction pattern of a selected area of Fig. 5a is shown in Fig. 5c. There are two independent networks in the diffraction pattern. One corresponds exactly to nickel sulfide and the other to metallic nickel, according to the calculated interlayer spacing. This fact clearly indicates that NiS is formed in the production mechanism. The dark field image of the nickel network (Fig. 5d) shows the left side illuminated, and the dark field image of the nickel sulfide network (Fig. 5e) shows the right part illuminated. Sulfur in large quantities is well known for being inactive as a catalyst in the formation of carbon nanofilaments for any light hydrocarbon reaction [20]. As a consequence, the stacked-cup carbon nanofiber must grow from the nickel part. The huge hollow core corresponds to the size of the NiS side. Next, TEM exploration shows that the structure is a continuous cone-helix. Fig. 6a shows the end of a CNF where an apparent layer attached to the nanofiber wall is unraveling from the fiber. The width of the layer is equal to the length of the graphene layers at the wall of the CNF (21–25 nm). Fig. 6b shows a closer and tilted image of the filament, where it is clearly observed that the layer comes out from the CNF wall in a spiral fashion, as if the carbon layer were unrolling from the cone-helix. Fig. 6c shows the complete image of this carbon layer that unraveled from the nanofiber, which has a length over 1 lm. This unrolled carbon ribbon shows bends and curves, which are repeated periodically. The lower and upper diameters of the hypothetical truncated cones of this
Fig. 6b. Closer look at the ribbon spiral.
specific filament, which correspond with the diameter of the hollow core and the external diameter of the nanofiber, respectively, are around 56 and 80 nm, respectively. The circular perimeters that correspond to those values are 177 and 280 nm. Each periodical length of the carbon sheet is an average of 214 nm, which exactly matches the average of the circular perimeter. Moreover, the width of the ribbon at each cycle matches the length of the graphene layers at the wall of the CNF. As a consequence, it is evident that this continuous ribbon corresponds to the cone-helix structure of a carbon nanofiber, which unraveled. To determine the thickness of this layer, an attempt was made to observe the edge of this layer, although it presented many
2756
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
Fig. 6c. Complete view of the ribbon of graphene layers that is unrolled.
difficulties (Fig. 6d). It seems that the layer is formed by at least five graphene planes. Fig. 7 shows the bulk electrical volume resistivity of stacked-cup carbon nanofibers (GANF1 and Pyrograf III) and high quality, crystalline MWNTs. The lowest resistivity is, as expected, MWNTs, although it is only one order of magnitude lower than the resistivity of stackedcup CNFs. If the structure were isolated cones, the difference between MWNTs and CNFs should be much higher.
There are no significant differences between Fe-grown Pyrograf III and Ni-grown GANF1, affirming that the cone-helix structure is similar and that the slight amorphous carbon coating, present in Pyrograf III, does not affect the resistivity.
4. Discussion This work shows that the structure of stacked-cup carbon nanofibers is a continuous helix-spiral, rolled along the axis to yield pseudo stacked-cup CNFs. It is not sur-
Fig. 6d. Side view of the carbon layer, composed of at least five graphene planes.
Fig. 7. Bulk volume resistivity of MWNT and stacked-cup Pyrograf III and GANF versus the applied pressure.
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
prising that Baker et al. [1] in 1971 commented, in relation to the oxidation behavior of hollow core carbon filaments: ‘‘During oxidation, the filament wall appeared to react in spiral fashion suggesting that the tube might have the form of a scroll extending continuously along the length of the filament.’’ And it is clear that this seminal paper described stacked-cup carbon nanofibers. The next topic to discuss is the formation mechanism of stacked-cup carbon nanofibers, since they are only formed when sulfur is present. As shown in Fig. 4, the carbon layers come from the Ni component of the particle. We have argued that this Ni component produced a continuous ribbon of several graphene sheets, as in the case of CNTs, which is much more likely than producing discontinuous cones. As a consequence, the growth mechanism must be different to the one described by Owens et al. [20], which applies to the growth of discontinuous graphene sheets parallel to a faceted particle. Now, the role of the NiS component could be, due to its fluid state, favoring (or making possible) the spiral alignment of the graphitic ribbons, with a somewhat circular cross section. This aspect could be a possibility. Another possibility is that a component of sulfur at high temperature avoids the sintering of nickel. Sulfur is known to weaken the adhesion between metal crystallites [20]. This fact allows a certain Ni crystal size that, at those temperatures, yields continuous graphite. The question of why the continuous layer of graphite formed has the exact hexagonal arrangement which yields a cone-helix formation is still unknown. 5. Conclusions • The structure of stacked-cup carbon nanofibers is formed by a continuous helix-spiral graphite layer and not by individual truncated cones, which explains the electrical resistivity of this material. • The role of sulfur in the Ni-based mechanism is to partially poison the metallic particle, yielding a binary particle formed of Ni and NiS, where the Ni catalyzes the graphite formation and the NiS allows the spiral arrangement.
Acknowledgements The authors fully acknowledge Rodney Andrews and Daniel Bortz from the University of Kentucky-Center for Applied Energy Research for kindly supplying MWNT, and for their valued help in the preparation of the manuscript.
References [1] Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ. Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. J Catal 1972;26(1):51–62.
2757
[2] Tibbetts GG, Gorkiewicz DW, Alig RL. A new reactor for growing carbon fibers from liquid- and vapor-phase hydrocarbons. Carbon 1993;31(5):809–14. [3] Tibbetts GG, Doll GL, Gorkiewicz DW, Moleski JJ, Perry TA, Dasch CJ, et al. Physical properties of vapor-grown carbon fibers. Carbon 1993;31(7):1039–47. [4] Rodriguez NM, Chambers A, Baker RTK. Catalytic engineering of carbon nanostructures. Langmuir 1995;11:3862–6. [5] Audier M, Oberlin A, Oberlin M, Coulon M, Bonnetain L. Morphology and crystalline order in catalytic carbons. Carbon 1981;19(3):217–24. [6] Endo M, Oshida K, Terrones M, Yanagisawa T, Higaki S, Dresselhaus MS. Structural characterization of cup-stacked-type nanofibers with an entirely hollow core. Appl Phys Lett 2002;80(7):1267–9. [7] Kim Y-A, Hayashi T, Naokawa S, Yanagisawa T, Endo M. Comparative study of herringbone and stacked-cup carbon nanofibers. Carbon 2005;43(14):3005–8. [8] Oberlin A, Endo M, Koyama T. Filamentous growth of carbon through benzene decomposition. J Cryst Growth 1976;32(3):335– 49. [9] Kim MS, Rodriguez NM, Baker RTK. The role of interfacial phenomena in the structure of carbon deposits. J Catal 1992;134(1): 253–68. [10] Yang RT, Chen JP. Mechanism of carbon filament growth on metal catalysts. J Catal 1989;115(1):52–64. [11] Collins S, Brydson R, Rand B. Structural analysis of carbon nanofibres grown by the floating catalyst method. Carbon 2002; 40(7):1089–100. [12] Ci L, Li Y, Wei B, Liang J, Xu C, Wu D. Preparation of carbon nanofibers by the floating catalyst method. Carbon 2000;38(14): 1933–7. [13] Ci L, Zhu H, Wei B, Liang J, Xu C, Wu D. Phosphorus – a new element for promoting growth of carbon filaments by the floating catalyst method. Carbon 1999;37(10):1652–4. [14] Singh C, Quested T, Boothroyd CB, Thomas P, Kinloch IA, AbouKandil AI, et al. Synthesis and characterization of carbon nanofibers produced by the floating catalyst method. J Phys Chem B 2002;106:10915–22. [15] Martin-Gullon I, Vera J, Conesa JA, Gonzalez JL, Merino C. Differences between carbon nanofibers produced using Fe and Ni catalysts in a floating catalyst reactor. Carbon 2006;44(8):1572–80. [16] Chen X, Saito T, Kusunoki M, Motojima S. Three-dimensional vapor growth mechanism of carbon microcoils. J Mater Res 1999;14(11):4329–36. [17] Tibbetts GG, Bernardo CA, Gorkiewicz DW, Alig RL. Role of sulfur in the production of carbon fibers in the vapor phase. Carbon 1994;32(4):569–76. [18] Tibbetts GG, Balogh MP. Increase in yield of carbon fibres grown above the iron/carbon eutectic. Carbon 1999;37(2):241–7. [19] Motojima S, Hasegawa I, Kagiya S, Momiyama M, Kawaguchi M, Iwanaga H. Preparation of coiled carbon fibers by pyrolysis of acetylene using a Ni catalyst and sulfur or phosphorus compound impurity. Appl Phys Lett 1993;62(19):2322–3. [20] Owens WT, Rodriguez NM, Baker RTK. Effect of sulfur on the interaction of nickel with ethylene. Catal Today 1994;21(1):3–22. [21] Benissad-Aissani F, Ait-Amar H, Schouler M-C, Gadelle P. The role of phosphorus in the growth of vapour-grown carbon fibres obtained by catalytic decomposition of hydrocarbons. Carbon 2004;42(11): 2163–8. [22] Ci L, Zhu H, Wei B, Liang J, Xu C, Wu D. Correspondence: phosphorus – a new element for promoting growth of carbon filaments by the floating catalyst method. Carbon 1999;37(10): 1652–4. [23] Choi Y-K, Gotoh Y, Sugimoto K-i, Song S-M, Yanagisawa T, Endo M. Processing and characterization of epoxy nanocomposites reinforced by cup-stacked carbon nanotubes. Polymer 2005;46(25):11489. [24] Eksioglu B, Nadarajah A. Structural analysis of conical carbon nanofibers. Carbon 2006;44(2):360–73.
2758
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
[25] Tibbetts GG, Lake ML, Strong KL, Rice BP. A review of the fabrication and properties of vapor-grown carbon nanofiber/polymer composites. Compos Sci Technol 2007;67(7–8):1709–18. [26] Tibbetts GG, Kwag C, Howe JY, Lake ML. In: Proceedings of the 48th international SAMPE symposium (Long Beach, 2003). [27] Endo M, Kim YA, Hayashi T, Yanagisawa T, Muramatsu H, Ezaka M, et al. Microstructural changes induced in stacked cup carbon nanofibers by heat treatment. Carbon 2003;41(10):1941–7.
[28] Andrews R, Jacques D, Rao AM, Derbyshire F, Qian D, Fan X, et al. Continuous production of aligned carbon nanotubes: a step closer to commercial realization. Chem Phys Lett 1999;303(5–6): 467–474. [29] Benissad F, Gadelle P, Coulon M, Bonnetain L. Formation de fibres de carbone a partir du methane II: germination du carbone et fusion des particules catalytiques. Carbon 1988;26(4):425–32.
Carbon 45 (2007) 2751–2758 www.elsevier.com/locate/carbon
Evidence for growth mechanism and helix-spiral cone structure of stacked-cup carbon nanofibers Jose Vera-Agullo a, Helena Varela-Rizo a, Juan A. Conesa a, Cristina Almansa b, Ce´sar Merino c, Ignacio Martin-Gullon a,* a
Department of Chemical Engineering, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain b Analytical Technical Services, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain c Grupo Antolı´n Ingenierı´a, Crta. Iru´n 244, E-09007 Burgos, Spain Received 14 May 2007; accepted 17 September 2007
Abstract New evidence for the structure of Ni-catalyzed stacked-cup carbon nanofibers (CNFs) has been found. This type of carbon nanofiber exhibits a wide hollow core as well as a large diameter (between 40 and 140 nm). The fibers have been produced by the floating catalyst method using natural gas as carbon feedstock, a sulfur compound, and a nickel catalyst. It was found that the catalytic particles are heterogeneous with two different parts: one composed of metallic Ni, which is the catalytically active portion of the particle, and another composed of NiS, which allows for the hollow nanofiber structure. The hollow core of the fibers has similar dimensions to the NiS volume of the particle and the graphitic layers grow from the rear nickel region of the particle. Nevertheless, the NiS component seems to be indispensable in producing the helix-spiral formation of the graphitic structure, as clearly shown by the TEM studies. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Among the different types of carbon nanofilaments that can be produced by catalytic chemical vapor deposition (cCVD) are those that have a wide hollow core. In 1972, Baker et al. [1], while studying the production of carbon filaments from acetylene over nickel as a catalyst, pointed out that a small amount of the produced filaments were exceptionally thick (100 nm) and had a wide hollow core. The dimensions of these exceptional filaments were an outer diameter of 94–87 nm and a central core diameter of 58– 56 nm. In 1993, Tibbetts et al. [2,3] reported that the carbon nanofibers produced by floating catalyst using ferrocene, methane, and hydrogen sulfide also had a wide hollow core. Also, the graphitic planes were not parallel to the fiber axis as was observed in the substrate grown herringbone nanofibers [4,5]. Nevertheless, the structure of
*
Corresponding author. Fax: +34 96 590 3826. E-mail address: [email protected] (I. Martin-Gullon).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.09.040
stacked-cup like carbon nanofiber was first fully described by Endo et al. [6], working with carbon nanofibers produced from ferrocene/iron pentacarbonyl, natural gas, and hydrogen sulfide in a floating catalyst reactor. They defined stacked-cup CNFs as a unique structure of large diameter nanofibers with a large inner hollow core formed by an arrangement of truncated graphitic cones. Moreover, Kim et al. [7] claimed that this stacked-cup CNF was a different type of fiber than herringbone CNF. Although both structures would look the same in transmission electron microscopy (TEM), the main difference was in the almost circular cross section of stacked-cup CNFs, presumably due to the effect of a molten catalyst particle. The mechanism of growth of catalytic carbon nanofilaments has been investigated since the 70s [1,8]. Baker et al. [1] grew carbon nanofibers from acetylene and a nickel catalyst in a gas reaction cell built within a TEM. These authors pointed out that the ‘‘pear-shaped’’ catalyst particles have the properties of a liquid in some steps of the growth model that they proposed, and this liquid behavior is responsible for the interior hollow core of the filament. In
2752
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
addition, the rate of filament formation is very sensitive to both the reaction temperature and the composition of the catalyst [9]. Nevertheless, Yang and Chen [10], while studying the growth of nanofibers from methane over Fe, Co, and Ni, reached the conclusion that the catalyst particles must be faceted, where the Ni(1 1 1) and Ni(3 1 1) faces form metal/graphite interfaces with the graphite planes growing parallel to them, and the Ni(1 0 0) and Ni(1 1 0) are gas/ metal interface, active for the decomposition of hydrocarbons. Rodriguez et al. [4] analyzed three different types of carbon nanofibers and proposed one three-dimensional model for each type with the catalyst particles faceted; several faces were attributed to the gas/metal interaction and others to the graphite filament/metal interaction. Another key factor in the production of stacked-cup carbon nanofibers is the need for sulfur or phosphorous in the reactor [11–14]. Martin-Gullon et al. [15] reported the formation of stacked-cup CNFs in a floating catalyst reactor with nickel as a catalyst, natural gas, and a sulfur-containing feedstock. These nickel based CNFs were only produced when sulfur was added. Sulfur seemed to the form a binary catalytic particle, with one side composed mainly of nickel and the other of an alloy of nickel and sulfur. Others authors also found that sulfur is not homogeneously dispersed in the metal catalyst particles [16]. A combination of a floating catalyst system, methane, and a presence of sulfur in the feedstock is essential to the formation of the stacked-cup nanofiber structure, regardless of the metal catalyst (Fe [2] or Ni [15]). It seems that the same structure is also formed at equivalent conditions with cobaltocene in a floating catalyst system [14]. Nevertheless, the effect of sulfur in the formation of carbon nano-
filaments is not yet clear since there are different explanations in the literature [16–20]. Its concentration in the feedstock stream cannot be random and must be similar to that of the metal in order to really promote the CNF growth and to avoid the formation of soot [2,12,20]. This latter aspect might indicate that sulfur is required to change the metal composition to a certain degree, and that a higher concentration completely poisons the catalytic activity of the metal [20]. A very similar behavior was found when replacing sulfur by phosphorus [19,21,22], when a very specific amount promotes CNF formation from methane. Meanwhile Endo’s group [6,23] described the structure of this stacked-cup CNF as discontinuous and individual truncated cones, Eksioglu and Nadarajah [24] pointed out that an arrangement of discontinuous truncated cones might have only very selective angles, which does not correspond to the variety of angles found, most of them around 25 with respect to the CNF axis [25]. These values can only be explained if the structure is a cone-helix arrangement of a continuous graphene sheet. In addition, this cone-helix structure is the only one that might explain high electrical conductivity found for this CNF. An arrangement of truncated cones cannot explain high conductivity along the fibril axis, due to the fact that the conductivity in the direction perpendicular with the graphene planes is five orders of magnitude lower. Tibbetts et al. [26] and Endo et al. [27] pointed out that stacked-cup carbon nanofibers exposed to 2800 C high temperature graphitization did not modify much of the electrical conductivity, although it introduced deep structural changes in the crystalline orientation. This aspect might indicate that the structure is formed is a continuous graphene helix,
Fig. 1. Overall TEM view of GANF1. Very few Ni particles are found at the fibril tips.
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
2753
Fig. 2. SEM micrograph of two nanofiber ends.
where the large changes in crystal size and 002 distance will not change the conductivity. Tibbetts et al. [25] left opened the discussion for a discontinuous cone arrangement or a continuous cone-helix structure, but pointed out that 1500 C was the best high temperature treatment due to the continuity formed with the thin pyrolitic CVD layer. TEM exploration failed to shed more light about the stacked-cup CNF structure. The present communication tries to deepen the understanding of the effect that Ni plays on the formation mechanism and structure of stacked cup CNFs. As stated before, a sulfur source, methane as the main carbon feedstock, and a floating catalyst system were always present when stacked-cup nanofibers were formed. Sulfur alters the catalytic particles producing a binary particle. How do the graphitic layers grow from these particles? Depending on the growth mechanism, one type of possible structure would make sense. 2. Experimental The sample analyzed in this study is GANF1 grade D&S, manufactured by Grupo Antolin Ingenieria (Burgos, Spain). GANF1 is a nickel derived stacked-cup carbon nanofiber produced using natural gas and a sulfur feedstock at a temperature above 1100 C. This sample has been deeply characterized elsewhere [15].
Fig. 3. Stacked-cup CNF with graphene layers oblique to the fiber axis at around 25.
The chemical structure of the CNF, as well as the metal catalyst particle morphology and composition was studied through TEM, using a model JEOL JEM 2010. The equipment is provided with an energy dispersive spectroscopy system (EDS), Oxford instruments model INCA Energy TEM100. The metal composition in a given area of the exploration was found through this technique. Important information about the crystalline structure from selected area electron diffraction (SAED) was also obtained. The electrical resistivity of carbon nanofibers in the bulk state was determined for pressures up to 60 bar. In this case, the experimental electrical resistivity of Pyrograf IIITM grade PR-24, Fe grown stacked-cup CNF (Applied Sciences Inc., Cedarville, OH), and substrate grown multi wall nanotubes (MWNT) from ferrocene/xylene/Ar [28] were performed for comparison purposes.
3. Results Fig. 1 shows a general overview of the GANF1 material. Fibrils are long and straight, with a typical tubular structure with a huge hollow core (lower electronic density in the middle of the fiber), which corresponds with a stacked-cup CNF structure. Fig. 2 shows a SEM micrograph with evidence for the tubular structure.
Fig. 4. TEM image of a metal particle at the tip of a carbon nanofiber.
2754
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
Very few Ni particles are located at the nanofiber tips. Since all the fibrils were produced from a particle at one of the extremes, it seems that the catalytic particle could have dropped off the fibril, indicative of a molten behavior. Fig. 3 shows the detail of one fibril, with an average angle of 21–25 from the axis, very similar to the angle found and reported for Fe-based Pyrograf IIITM [25]. At the bottom right corner, a higher magnification clearly shows the graphene planes. In general, most of the nanofibers do not have particles located at the tip. Very few fibers show a catalytic particle inside. Our analysis will be focused on those fibrils where the catalytic particle remained within the nanofiber. These particles are inside the fiber with tips coated by a graphite layer. According to Rodriguez et al. [4], this preserves the shape of the metal catalyst in the reactive state during subsequent cooling to room temperature, and therefore, postreaction TEM examinations give an accurate picture of the morphological characteristics of the particle. Fig. 4 shows a TEM micrograph of an unusual carbon nanofiber with a catalytic particle at its tip. This catalytic particle formed a nanofiber with stacked-cup structure at reaction conditions, and the particle did not drop off after the nanofiber growth. EDS analysis has been done at two different positions in the particle (1 and 2). Results are shown in Table 1. The right portion of the particle is composed mainly of nickel with a negligible amount of sulfur, the left part contains an appreciable quantity of sulfur with respect to the quantity of nickel (47 %). The phase diagram of nickel and sulfur shows that the left side of the particle was molten when the nanofiber grew at the production temperature (above 1100 C) due to either the formation of NiS (melting point of 797 C) or the formation of an eutectic between nickel and sulfur at 35 at.% S (635 C). The right part of the particle (Ni, bulk melting point
1455 C) might have been solid at reaction conditions, although it might behave as a liquid due to its nano-dimensions [29]. In any case, the Ni side would be less fluid than the other part. Furthermore, it can be observed in Fig. 4 that the graphene layers originate from the outer part of the particle (composed of Ni) corresponding to the outer diameter; and the diameter of the inner channel corresponds to the other section of the particle (see Table 2). Most of the particles located at the tip of the carbon nanofibers exhibit this behavior. Fig. 5a shows the same
Table 1 Atomic composition of the metal particle encapsulated at the tip of the carbon nanofiber (Fig. 4) by energy dispersive X-ray spectrometry Left side of the particle (1)
Right side of the particle (2)
Element
at.%
Element
at.%
C S Ni
80.26 9.18 10.56
C S Ni
61.43 0.58 37.99
S/(S + Ni) · 100
46.5
S/(S + Ni) · 100
1.5
Table 2 Atomic composition of the metal particle encapsulated at the tip of the carbon nanofiber (Fig. 5a) by energy dispersive X-ray spectrometry Left side of the particle
Right side of the particle
Element
at.%
Element
at.%
C S Ni
41.07 0.00 58.93
C S Ni
90.94 4.42 4.64
S/(S + Ni) · 100
48.8
S/(S + Ni) · 100
0.0
Fig. 5. (a) TEM image of a metal particle at the tip of a carbon nanofiber; (b) detail of the right part of the particle, with a characteristic crystallinity; (c) diffraction pattern of the particle; (d) dark field image of the Ni network; and (e) dark field image of the NiS network.
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
2755
Fig. 6a. A ribbon of carbon comes out from the nanofiber end.
results in another catalytic particle. The left side is composed of nickel only, and the right side is a mixture of sulfur and nickel at 49 at.%. The right part of the particle exhibits a characteristic crystallinity (Fig. 5b). A diffraction pattern of a selected area of Fig. 5a is shown in Fig. 5c. There are two independent networks in the diffraction pattern. One corresponds exactly to nickel sulfide and the other to metallic nickel, according to the calculated interlayer spacing. This fact clearly indicates that NiS is formed in the production mechanism. The dark field image of the nickel network (Fig. 5d) shows the left side illuminated, and the dark field image of the nickel sulfide network (Fig. 5e) shows the right part illuminated. Sulfur in large quantities is well known for being inactive as a catalyst in the formation of carbon nanofilaments for any light hydrocarbon reaction [20]. As a consequence, the stacked-cup carbon nanofiber must grow from the nickel part. The huge hollow core corresponds to the size of the NiS side. Next, TEM exploration shows that the structure is a continuous cone-helix. Fig. 6a shows the end of a CNF where an apparent layer attached to the nanofiber wall is unraveling from the fiber. The width of the layer is equal to the length of the graphene layers at the wall of the CNF (21–25 nm). Fig. 6b shows a closer and tilted image of the filament, where it is clearly observed that the layer comes out from the CNF wall in a spiral fashion, as if the carbon layer were unrolling from the cone-helix. Fig. 6c shows the complete image of this carbon layer that unraveled from the nanofiber, which has a length over 1 lm. This unrolled carbon ribbon shows bends and curves, which are repeated periodically. The lower and upper diameters of the hypothetical truncated cones of this
Fig. 6b. Closer look at the ribbon spiral.
specific filament, which correspond with the diameter of the hollow core and the external diameter of the nanofiber, respectively, are around 56 and 80 nm, respectively. The circular perimeters that correspond to those values are 177 and 280 nm. Each periodical length of the carbon sheet is an average of 214 nm, which exactly matches the average of the circular perimeter. Moreover, the width of the ribbon at each cycle matches the length of the graphene layers at the wall of the CNF. As a consequence, it is evident that this continuous ribbon corresponds to the cone-helix structure of a carbon nanofiber, which unraveled. To determine the thickness of this layer, an attempt was made to observe the edge of this layer, although it presented many
2756
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
Fig. 6c. Complete view of the ribbon of graphene layers that is unrolled.
difficulties (Fig. 6d). It seems that the layer is formed by at least five graphene planes. Fig. 7 shows the bulk electrical volume resistivity of stacked-cup carbon nanofibers (GANF1 and Pyrograf III) and high quality, crystalline MWNTs. The lowest resistivity is, as expected, MWNTs, although it is only one order of magnitude lower than the resistivity of stackedcup CNFs. If the structure were isolated cones, the difference between MWNTs and CNFs should be much higher.
There are no significant differences between Fe-grown Pyrograf III and Ni-grown GANF1, affirming that the cone-helix structure is similar and that the slight amorphous carbon coating, present in Pyrograf III, does not affect the resistivity.
4. Discussion This work shows that the structure of stacked-cup carbon nanofibers is a continuous helix-spiral, rolled along the axis to yield pseudo stacked-cup CNFs. It is not sur-
Fig. 6d. Side view of the carbon layer, composed of at least five graphene planes.
Fig. 7. Bulk volume resistivity of MWNT and stacked-cup Pyrograf III and GANF versus the applied pressure.
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
prising that Baker et al. [1] in 1971 commented, in relation to the oxidation behavior of hollow core carbon filaments: ‘‘During oxidation, the filament wall appeared to react in spiral fashion suggesting that the tube might have the form of a scroll extending continuously along the length of the filament.’’ And it is clear that this seminal paper described stacked-cup carbon nanofibers. The next topic to discuss is the formation mechanism of stacked-cup carbon nanofibers, since they are only formed when sulfur is present. As shown in Fig. 4, the carbon layers come from the Ni component of the particle. We have argued that this Ni component produced a continuous ribbon of several graphene sheets, as in the case of CNTs, which is much more likely than producing discontinuous cones. As a consequence, the growth mechanism must be different to the one described by Owens et al. [20], which applies to the growth of discontinuous graphene sheets parallel to a faceted particle. Now, the role of the NiS component could be, due to its fluid state, favoring (or making possible) the spiral alignment of the graphitic ribbons, with a somewhat circular cross section. This aspect could be a possibility. Another possibility is that a component of sulfur at high temperature avoids the sintering of nickel. Sulfur is known to weaken the adhesion between metal crystallites [20]. This fact allows a certain Ni crystal size that, at those temperatures, yields continuous graphite. The question of why the continuous layer of graphite formed has the exact hexagonal arrangement which yields a cone-helix formation is still unknown. 5. Conclusions • The structure of stacked-cup carbon nanofibers is formed by a continuous helix-spiral graphite layer and not by individual truncated cones, which explains the electrical resistivity of this material. • The role of sulfur in the Ni-based mechanism is to partially poison the metallic particle, yielding a binary particle formed of Ni and NiS, where the Ni catalyzes the graphite formation and the NiS allows the spiral arrangement.
Acknowledgements The authors fully acknowledge Rodney Andrews and Daniel Bortz from the University of Kentucky-Center for Applied Energy Research for kindly supplying MWNT, and for their valued help in the preparation of the manuscript.
References [1] Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ. Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. J Catal 1972;26(1):51–62.
2757
[2] Tibbetts GG, Gorkiewicz DW, Alig RL. A new reactor for growing carbon fibers from liquid- and vapor-phase hydrocarbons. Carbon 1993;31(5):809–14. [3] Tibbetts GG, Doll GL, Gorkiewicz DW, Moleski JJ, Perry TA, Dasch CJ, et al. Physical properties of vapor-grown carbon fibers. Carbon 1993;31(7):1039–47. [4] Rodriguez NM, Chambers A, Baker RTK. Catalytic engineering of carbon nanostructures. Langmuir 1995;11:3862–6. [5] Audier M, Oberlin A, Oberlin M, Coulon M, Bonnetain L. Morphology and crystalline order in catalytic carbons. Carbon 1981;19(3):217–24. [6] Endo M, Oshida K, Terrones M, Yanagisawa T, Higaki S, Dresselhaus MS. Structural characterization of cup-stacked-type nanofibers with an entirely hollow core. Appl Phys Lett 2002;80(7):1267–9. [7] Kim Y-A, Hayashi T, Naokawa S, Yanagisawa T, Endo M. Comparative study of herringbone and stacked-cup carbon nanofibers. Carbon 2005;43(14):3005–8. [8] Oberlin A, Endo M, Koyama T. Filamentous growth of carbon through benzene decomposition. J Cryst Growth 1976;32(3):335– 49. [9] Kim MS, Rodriguez NM, Baker RTK. The role of interfacial phenomena in the structure of carbon deposits. J Catal 1992;134(1): 253–68. [10] Yang RT, Chen JP. Mechanism of carbon filament growth on metal catalysts. J Catal 1989;115(1):52–64. [11] Collins S, Brydson R, Rand B. Structural analysis of carbon nanofibres grown by the floating catalyst method. Carbon 2002; 40(7):1089–100. [12] Ci L, Li Y, Wei B, Liang J, Xu C, Wu D. Preparation of carbon nanofibers by the floating catalyst method. Carbon 2000;38(14): 1933–7. [13] Ci L, Zhu H, Wei B, Liang J, Xu C, Wu D. Phosphorus – a new element for promoting growth of carbon filaments by the floating catalyst method. Carbon 1999;37(10):1652–4. [14] Singh C, Quested T, Boothroyd CB, Thomas P, Kinloch IA, AbouKandil AI, et al. Synthesis and characterization of carbon nanofibers produced by the floating catalyst method. J Phys Chem B 2002;106:10915–22. [15] Martin-Gullon I, Vera J, Conesa JA, Gonzalez JL, Merino C. Differences between carbon nanofibers produced using Fe and Ni catalysts in a floating catalyst reactor. Carbon 2006;44(8):1572–80. [16] Chen X, Saito T, Kusunoki M, Motojima S. Three-dimensional vapor growth mechanism of carbon microcoils. J Mater Res 1999;14(11):4329–36. [17] Tibbetts GG, Bernardo CA, Gorkiewicz DW, Alig RL. Role of sulfur in the production of carbon fibers in the vapor phase. Carbon 1994;32(4):569–76. [18] Tibbetts GG, Balogh MP. Increase in yield of carbon fibres grown above the iron/carbon eutectic. Carbon 1999;37(2):241–7. [19] Motojima S, Hasegawa I, Kagiya S, Momiyama M, Kawaguchi M, Iwanaga H. Preparation of coiled carbon fibers by pyrolysis of acetylene using a Ni catalyst and sulfur or phosphorus compound impurity. Appl Phys Lett 1993;62(19):2322–3. [20] Owens WT, Rodriguez NM, Baker RTK. Effect of sulfur on the interaction of nickel with ethylene. Catal Today 1994;21(1):3–22. [21] Benissad-Aissani F, Ait-Amar H, Schouler M-C, Gadelle P. The role of phosphorus in the growth of vapour-grown carbon fibres obtained by catalytic decomposition of hydrocarbons. Carbon 2004;42(11): 2163–8. [22] Ci L, Zhu H, Wei B, Liang J, Xu C, Wu D. Correspondence: phosphorus – a new element for promoting growth of carbon filaments by the floating catalyst method. Carbon 1999;37(10): 1652–4. [23] Choi Y-K, Gotoh Y, Sugimoto K-i, Song S-M, Yanagisawa T, Endo M. Processing and characterization of epoxy nanocomposites reinforced by cup-stacked carbon nanotubes. Polymer 2005;46(25):11489. [24] Eksioglu B, Nadarajah A. Structural analysis of conical carbon nanofibers. Carbon 2006;44(2):360–73.
2758
J. Vera-Agullo et al. / Carbon 45 (2007) 2751–2758
[25] Tibbetts GG, Lake ML, Strong KL, Rice BP. A review of the fabrication and properties of vapor-grown carbon nanofiber/polymer composites. Compos Sci Technol 2007;67(7–8):1709–18. [26] Tibbetts GG, Kwag C, Howe JY, Lake ML. In: Proceedings of the 48th international SAMPE symposium (Long Beach, 2003). [27] Endo M, Kim YA, Hayashi T, Yanagisawa T, Muramatsu H, Ezaka M, et al. Microstructural changes induced in stacked cup carbon nanofibers by heat treatment. Carbon 2003;41(10):1941–7.
[28] Andrews R, Jacques D, Rao AM, Derbyshire F, Qian D, Fan X, et al. Continuous production of aligned carbon nanotubes: a step closer to commercial realization. Chem Phys Lett 1999;303(5–6): 467–474. [29] Benissad F, Gadelle P, Coulon M, Bonnetain L. Formation de fibres de carbone a partir du methane II: germination du carbone et fusion des particules catalytiques. Carbon 1988;26(4):425–32.