CARBON PERGAMON Carbon39(2001)1575-1587 Surface characterization of submicron vapor grown carbon fibers by scanning tunneling microscopy J I. Paredes. A. Martinez-Alonso. J.M.D. Tascon Instituto Nacional del Carbon, CSIC, Apartado 73, 33080 Oviedo, Spain Received 6 September 2000; accepted 20 October 2000 Abstract The surface structure of submicron vapor grown carbon fibers has been investigated by means of scanning tunneling microscopy (STM)at the nanometer and atomic scale. At the nanometer scale the fibers were found to have a relatively smooth topography consisting of more or less rounded platelets just a few nm in diameter. Atomic scale imaging revealed the absence of long range graphitic order. Instead, the fiber surface was comprised for the most part of small structured regions, with lateral sizes typically between I and 4 nm. Within these structured regions, the observed atomic arrangement differed in most of the cases from that of perfect graphite. Likewise, linear structures and ring-like superstructures were also observed on the surface of the fibers at the atomic scale. Some possible interpretations are proposed to account for these observations. The scanning tunneling microscopy characterization of the fibers is complemented with scanning electron microscopy, X-ray diffraction and nitrogen physical adsorption measurements. The results are discussed in terms of the ecific structure and growth process of this type of fiber. o 2001 Elsevier Science Ltd. All rights reserved Keywords: A Carbon fibers; Vapor grown carbon, C Scanning tunneling microscopy (STM); Scanning electron microscopy (SEM); X-ray diffraction 1. Introduction attention in recent years in the search for their type or grown carbon fibers(VGCFS)are a relatively new materials. For example, the use of VGCEs as rein- pyrolysis of a hydrocarbon gas in the presence of hydrogen for their potential application as plasma-facing components at temperatures around 1000-1200.C. The fiber growth is in nuclear fusion reactors and also as thermal management initiated by ultrafine transition metal catalyst particles, material in spacecraft [9, 10]. Likewise, metal matrix(e.g usually iron or iron-containing, deposited on a substrate aluminum) composites reinforced with VGCFs have been (seeded catalyst method) or directly injected into the gas found to exhibit an excellent thermal conductivity, not ration conditions, VGCFs can be made with diameters forcement. This makes these composites firm candidates between several tens of nanometers and several tens of for their use in electronic devices, where the increasing microns and lengths from several microns up to many power level and density of microelectronic chips has centimeters [4-7] prompted the need for efficient thermal managemen Due to their potentially low cost of production and [11, 12]. Composites with excellent mechanical properties the fact that some of their physical properties, such made up of VGCFs and polymeric matrices have also been thermal and electrical conductivity, often surpass those of prepared and studied recently [13] other, more conventional, carbon fibers(PAN-based or In another field of research, nanometer-sized vapor pitch-based )[8], VGCFs have been receiving increasing grown carbon fibers with tailor-made structures(attained by controlling a number of processing parameters)have *Corresponding author. Tel. + 34-985-280-800; fax: +34- ently proved to be capable of sorbing and retaining 985-297-66 amount of hydrogen far exceeding that of conventional E-mail address: tascon @incar csic es (J.M. D. Tascon) hydrogen storage materials, such as metals or metal alloy 0008-6223/01/s-see front matter 2001 Elsevier Science Ltd. All rights reserved PII:S0008-6223(00)00286-4
PERGAMON Carbon 39 (2001) 1575–1587 Surface characterization of submicron vapor grown carbon fibers by scanning tunneling microscopy J.I. Paredes, A. Martınez-Alonso, J.M.D. Tascon ´ ´ * Instituto Nacional del Carbon´ , CSIC, Apartado 73, 33080 Oviedo, Spain Received 6 September 2000; accepted 20 October 2000 Abstract The surface structure of submicron vapor grown carbon fibers has been investigated by means of scanning tunneling microscopy (STM) at the nanometer and atomic scale. At the nanometer scale the fibers were found to have a relatively smooth topography consisting of more or less rounded platelets just a few nm in diameter. Atomic scale imaging revealed the absence of long range graphitic order. Instead, the fiber surface was comprised for the most part of small structured regions, with lateral sizes typically between 1 and 4 nm. Within these structured regions, the observed atomic arrangement differed in most of the cases from that of perfect graphite. Likewise, linear structures and ring-like superstructures were also observed on the surface of the fibers at the atomic scale. Some possible interpretations are proposed to account for these observations. The scanning tunneling microscopy characterization of the fibers is complemented with scanning electron microscopy, X-ray diffraction and nitrogen physical adsorption measurements. The results are discussed in terms of the specific structure and growth process of this type of fiber. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; Vapor grown carbon; C. Scanning tunneling microscopy (STM); Scanning electron microscopy (SEM); X-ray diffraction 1. Introduction attention in recent years in the search for their possible applications, principally as reinforcement in composite Vapor grown carbon fibers (VGCFs) are a relatively new materials. For example, the use of VGCFs as reintype of carbon fibers which are produced from the forcement in carbon/carbon composites has been explored pyrolysis of a hydrocarbon gas in the presence of hydrogen for their potential application as plasma-facing components at temperatures around 1000–12008C. The fiber growth is in nuclear fusion reactors and also as thermal management initiated by ultrafine transition metal catalyst particles, material in spacecraft [9,10]. Likewise, metal matrix (e.g. usually iron or iron-containing, deposited on a substrate aluminum) composites reinforced with VGCFs have been (seeded catalyst method) or directly injected into the gas found to exhibit an excellent thermal conductivity, not (floating catalyst method) [1–3]. Depending on the prepa- achieved with any other type of carbon fiber as reinration conditions, VGCFs can be made with diameters forcement. This makes these composites firm candidates between several tens of nanometers and several tens of for their use in electronic devices, where the increasing microns and lengths from several microns up to many power level and density of microelectronic chips has centimeters [4–7]. prompted the need for efficient thermal management Due to their potentially low cost of production and to [11,12]. Composites with excellent mechanical properties the fact that some of their physical properties, such as made up of VGCFs and polymeric matrices have also been thermal and electrical conductivity, often surpass those of prepared and studied recently [13]. other, more conventional, carbon fibers (PAN-based or In another field of research, nanometer-sized vapor pitch-based) [8], VGCFs have been receiving increasing grown carbon fibers with tailor-made structures (attained by controlling a number of processing parameters) have *Corresponding author. Tel.: 134-985-280-800; fax: 134- recently proved to be capable of sorbing and retaining an 985-297-662. amount of hydrogen far exceeding that of conventional E-mail address: tascon@incar.csic.es (J.M.D. Tascon). ´ hydrogen storage materials, such as metals or metal alloys 0008-6223/01/$ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(00)00286-4
1576 J 1, Paredes et al./ Carbon 39(2001)1575-1587 [14-16]. This property of the fibers may have very as pellets by agglomerating small quantities of the fibers in important implications for portable energy devices, where a hydraulic press. The specimens thus produced were the take-off of hydrogen as a clean and renewable energy mounted on a metallic STM sample holder using carbon source has been restricted due to the lack of a suitable adhesive tape and transferred to an STM/AFM apparatus hydrogen storage system and to volume and weight (Nanoscope Multimode Illa, Digital Instruments) where limitations. These materials would allow the transport of imaging was performed in air at room temperature. All the hydrogen in an efficient way. Also, TaCls and Mocis images were obtained in constant current mode(variable intercalated VGCFs have been shown to possess a high height) with mechanically prepared Pt/Ir (80/20)tips environmental stability (better than that of other types of Concerning the tunneling parameters, for non atomic scale carbon fibers), constituting very interesting materials for engineering applications such as electromagnetic interfer- positive) and tunneling currents between 0.4 and 1.0 nA ence shielding or lightning-strike protection [17, 18] were used. On the other hand atomic scale imaging Another potential application for VGCFs would be as an required more demanding settings. Specifically, bias volt electrode material for lithium-ion batteries, which req ges as low as 10-20 mv were normally employed and an electrically conducting and easily intercalable (with tunneling currents of 3-6 na were needed in order to ions)anode VGCFs are good candidates for this purpose resolve atomic features on the fiber surface. Low bias [ 19). Likewise, boron doping of this type of fiber is voltages and high tunneling currents mean that the Stm currently being investigated [20, 21] tip scans closer to the surface, so the sensitivity to minute In this context, and considering some of the potential urface corrugations(such as those of atomic features)is uses of VGCFs, it becomes clear that a thorough know higher, implying that they can be more readily detected edge of the fiber surface is a requirement in order to be In order to verify the reproducibility of the measure- able to subsequently tailor the properties of the material for ments and rule out possible artifacts affecting the images, a specific purpose. To this end, scanning probe microscopy several different STM tips were employed. Furthermore, to (SPM), in particular scanning tunneling microscopy ensure that the STM investigation provided a representa (STM) but also atomic force microscopy(AFM), provides tive depiction of the surface of the material, several the capability of investigating surface structures with a hundred images were obtained on different areas of the resolution(down to the atomic scale in the most favorable fibers. This approach was especially necessary for atomic cases)not attainable by other microscopic techniques [22]. scale imaging, due to the tiny fraction of surface that can The utility of STM/AFM for the study of PAN-based and be examined within a single image at that level of pitch-based carbon fibers has been demonstrated in the past magnification. in a number of examples [23-28 SEM measurements were carried out using a Zeiss dsm This work is aimed to study the surface structure of 942 microscope equipped with an ultrathin Si/Li detector submicrometric vapor grown carbon fibers, i.e. fibers with for energy dispersive X-ray(EDX)microanalysis. In this diameters around 0. 1-0. 2 um, by scanning tunneling case, small amounts of the fibers were deposited on a microscopy both at the nanometer and the atomic scale. To highly oriented pyrolytic graphite substrate and SEM he authors knowledge this is the first stm study on this maging was performed on this sample without further pe of carbon fiber. Two articles on VGCFs using AFM preparation. X-ray powder diffractograms were recorded have appeared very recently in the literature [29, 30], but in with a Siemens d 5000 diffractometer using Cu K these cases the diameters of the fibers were about two radiation(A=0.1504 nm)at a step size of 0.015(20)and orders of magnitude greater than those of the fibers studied a time per step of 3 s. Peak broadening(0.1475)was ere, and they were not studied at the atomic scale. To measured using a reference Si crystal. N, adsorption complement the present STM investigation of submic- isotherms(77 K) for the fibers were obtained with a Nova rometric VGCFs, their characterization by scanning elec- 1200(Quantachrome)automatic adsorption apparatus tron microscopy (SEM), X-ray diffraction (XRD)and nitrogen physical adsorption was also carried out. 2. Experime SEM micrographs of the submicron VGCF his work are presented in Fig. 1. As mentioned ers are typically between 100 and 200 nm, can be seen in Fig. la. Along with the fibers, some more or a density of 1.95 g cm. Their elemental analysis was less rounded particles a few hundred nm in diameter can performed and revealed the following amounts: 96.4%C, also be distinguished. These particles are probably soot. 2.1%O,0.8%H, 0.3%N and 0.2%S As the size of the Fig. 1b shows a single fiber with what seems to be a fibers was so small that their imaging as loose particles by particle located at its end. EDX microanalysis on that area STM was rather impracticable, the samples were prepared indicated the presence of iron(presumably in an oxidized
1576 J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 [14–16]. This property of the fibers may have very as pellets by agglomerating small quantities of the fibers in important implications for portable energy devices, where a hydraulic press. The specimens thus produced were the take-off of hydrogen as a clean and renewable energy mounted on a metallic STM sample holder using carbon source has been restricted due to the lack of a suitable adhesive tape and transferred to an STM/AFM apparatus hydrogen storage system and to volume and weight (Nanoscope Multimode IIIa, Digital Instruments) where limitations. These materials would allow the transport of imaging was performed in air at room temperature. All the hydrogen in an efficient way. Also, TaCl and MoCl - images were obtained in constant current mode (variable 5 5 intercalated VGCFs have been shown to possess a high height) with mechanically prepared Pt/Ir (80/20) tips. environmental stability (better than that of other types of Concerning the tunneling parameters, for non atomic scale carbon fibers), constituting very interesting materials for imaging bias voltages from 100 mV to 1.0 V (sample engineering applications such as electromagnetic interfer- positive) and tunneling currents between 0.4 and 1.0 nA ence shielding or lightning-strike protection [17,18]. were used. On the other hand, atomic scale imaging Another potential application for VGCFs would be as an required more demanding settings. Specifically, bias voltelectrode material for lithium-ion batteries, which require ages as low as 10–20 mV were normally employed and an electrically conducting and easily intercalable (with Li tunneling currents of 3–6 nA were needed in order to ions) anode. VGCFs are good candidates for this purpose resolve atomic features on the fiber surface. Low bias [19]. Likewise, boron doping of this type of fiber is voltages and high tunneling currents mean that the STM currently being investigated [20,21]. tip scans closer to the surface, so the sensitivity to minute In this context, and considering some of the potential surface corrugations (such as those of atomic features) is uses of VGCFs, it becomes clear that a thorough knowl- higher, implying that they can be more readily detected. edge of the fiber surface is a requirement in order to be In order to verify the reproducibility of the measureable to subsequently tailor the properties of the material for ments and rule out possible artifacts affecting the images, a specific purpose. To this end, scanning probe microscopy several different STM tips were employed. Furthermore, to (SPM), in particular scanning tunneling microscopy ensure that the STM investigation provided a representa- (STM) but also atomic force microscopy (AFM), provides tive depiction of the surface of the material, several the capability of investigating surface structures with a hundred images were obtained on different areas of the resolution (down to the atomic scale in the most favorable fibers. This approach was especially necessary for atomic cases) not attainable by other microscopic techniques [22]. scale imaging, due to the tiny fraction of surface that can The utility of STM/AFM for the study of PAN-based and be examined within a single image at that level of pitch-based carbon fibers has been demonstrated in the past magnification. in a number of examples [23–28]. SEM measurements were carried out using a Zeiss DSM This work is aimed to study the surface structure of 942 microscope equipped with an ultrathin Si/Li detector submicrometric vapor grown carbon fibers, i.e. fibers with for energy dispersive X-ray (EDX) microanalysis. In this diameters around 0.1–0.2 mm, by scanning tunneling case, small amounts of the fibers were deposited on a microscopy both at the nanometer and the atomic scale. To highly oriented pyrolytic graphite substrate and SEM the authors’ knowledge, this is the first STM study on this imaging was performed on this sample without further type of carbon fiber. Two articles on VGCFs using AFM preparation. X-ray powder diffractograms were recorded have appeared very recently in the literature [29,30], but in with a Siemens D 5000 diffractometer using Cu Ka these cases the diameters of the fibers were about two radiation (l50.1504 nm) at a step size of 0.0158 (2u ) and orders of magnitude greater than those of the fibers studied a time per step of 3 s. Peak broadening (0.14758) was here, and they were not studied at the atomic scale. To measured using a reference Si crystal. N adsorption 2 complement the present STM investigation of submic- isotherms (77 K) for the fibers were obtained with a Nova rometric VGCFs, their characterization by scanning elec- 1200 (Quantachrome) automatic adsorption apparatus. tron microscopy (SEM), X-ray diffraction (XRD) and nitrogen physical adsorption was also carried out. 3. Results 2. Experimental SEM micrographs of the submicron VGCFs studied in this work are presented in Fig. 1. As mentioned previously, The submicron vapor grown carbon fibers used in this their diameters are typically between 100 and 200 nm, as study were obtained from Applied Sciences, Inc., and have can be seen in Fig. 1a. Along with the fibers, some more or 23 a density of 1.95 g cm . Their elemental analysis was less rounded particles a few hundred nm in diameter can performed and revealed the following amounts: 96.4% C, also be distinguished. These particles are probably soot. 2.1% O, 0.8% H, 0.3% N and 0.2% S. As the size of the Fig. 1b shows a single fiber with what seems to be a fibers was so small that their imaging as loose particles by particle located at its end. EDX microanalysis on that area STM was rather impracticable, the samples were prepared indicated the presence of iron (presumably in an oxidized
J.I. Paredes et al. Carbon 39(2001)1575-1587 the images to some extent when those features have a considerable height, as is the case of Fig. 2a. Thus, the real diameters of the fibers obably somewhat maller than those measured by STM. Closer inspection of individual fibers(Fig. 2b) reveals that their topography is smooth only to a certain degree, and that they certainly display identifiable features: individual platelets with a lateral size from about 1 to 7 nm and heights between 0.3 and 2.5 nm were observed (i.e. one to eight grapher no specific orientation of the surface features along the fiber axis. This is in contrast to the case of, e.g. PAN-based carbon fibers, which tend to display elongated grains oriented preferentially along the fiber axis [26] Fig. 3a presents another general view of several fibers Interestingly, in the upper left part of the image, there is a a fiber whose end can be seen protruding outwards. This was a very rarely found feature in the normally disposed with their axis more or less parallel to the plane of the image, as can be noticed for the rest of the fibers of Fig. 3a and in Fig. 2a, so their end sections cannot be observed. Fig. 3b shows the aforementioned fiber nore detail evidencing a hollow core, the fiber section itself being only a ring of carbon a few tens of nanometers thick. Further magnification on top of the fiber section did not reveal higher detail. This could be due to the fact that the edge of the fiber is quite rough, precluding very high resolution imaging. Next, the aforementioned platelets comprising the fibers surface structure were examined at the atomic scale. Some of the typically encountered images at this level of magnification are shown in Figs. 4-7. The first evident characteristic that can be noticed, which perhaps could also be expected from the nanometer resolution images(Fig 2b), is the absence of long range atomic order virtually b anywhere on the surface. Only small structured regions with varying lateral sizes(for example, between I and 4 Fig. 1. SEM micrographs of the submicron VGCFs.(a)General nm in the images of Figs. 4 and 5) are found. Also, the ppearance of a bundle of fibers. (b) Detail of the end of a fiber degree of atomic roughness changes from one place to showing a particle located at its apex another. For instance, while Fig. 5c exhibits a relatively smooth region, the area shown in Fig. 5b is atomically very rough(notice the :-scale), and much more disordered state), suggesting that the particle contained the iron the region of Fig. Sa being of an intermediate roughness catalyst that promoted the fiber growth. It has to be noted In addition to this, in most of the cases there is not a that not all the fibers observed presented a particle at their perfectly ordered atomic arrangement within the structured regions such as that normally observed in graphite, i.e.a Fig. 2 shows two STM images of the fiber surface at the perfect triangular array of spots. Instead, there is a nanometer scale. In a general view of a few fibers(Fig. departure from that ideal structure, as can be seen in Fig. 6 a), they appear as having an extremely smooth morpholo- Several types of distortions are found: square or rectan- gy free of any specific features. The fiber running from the ular lattices(in some parts of Fig. 6a), elongated rather middle left to the top right of the image has a diameter of than rounded spots(top of Fig. 6b)and even no clearly about 90 nm, whereas that running parallel to it in botton identifiable periodicity of the spots( Fig. 6c). The distances right is 130 nm thick. However, these values must be between spots are in the range of 0. 20 to 0. 45 nm, whereas considered only as rough estimates of the actual diameters, those in graphite are 0.25 nm. Among these three images, since convolution between STM tip and sample surface the perfect triangular array of spots typical of graphite can topography is expected to enlarge the features appearing in only be observed in the central left part of Fig. 6b
J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 1577 the images to some extent when those features have a considerable height, as is the case of Fig. 2a. Thus, the real diameters of the fibers are most probably somewhat smaller than those measured by STM. Closer inspection of individual fibers (Fig. 2b) reveals that their topography is smooth only to a certain degree, and that they certainly display identifiable features: individual platelets with a lateral size from about 1 to 7 nm and heights between |0.3 and 2.5 nm were observed (i.e. one to eight graphene layers). The platelets are more or less rounded, so there is no specific orientation of the surface features along the fiber axis. This is in contrast to the case of, e.g. PAN-based carbon fibers, which tend to display elongated grains oriented preferentially along the fiber axis [26]. Fig. 3a presents another general view of several fibers. Interestingly, in the upper left part of the image, there is a fiber whose end can be seen protruding outwards. This was a very rarely found feature in the images: the fibers are normally disposed with their axis more or less parallel to the plane of the image, as can be noticed for the rest of the fibers of Fig. 3a and in Fig. 2a, so their end sections cannot be observed. Fig. 3b shows the aforementioned fiber in more detail evidencing a hollow core, the fiber section itself being only a ring of carbon a few tens of nanometers thick. Further magnification on top of the fiber section did not reveal higher detail. This could be due to the fact that the edge of the fiber is quite rough, precluding very high resolution imaging. Next, the aforementioned platelets comprising the fibers’ surface structure were examined at the atomic scale. Some of the typically encountered images at this level of magnification are shown in Figs. 4–7. The first evident characteristic that can be noticed, which perhaps could also be expected from the nanometer resolution images (Fig. 2b), is the absence of long range atomic order virtually anywhere on the surface. Only small structured regions with varying lateral sizes (for example, between 1 and 4 nm in the images of Figs. 4 and 5) are found. Also, the Fig. 1. SEM micrographs of the submicron VGCFs. (a) General appearance of a bundle of fibers. (b) Detail of the end of a fiber degree of atomic roughness changes from one place to showing a particle located at its apex. another. For instance, while Fig. 5c exhibits a relatively smooth region, the area shown in Fig. 5b is atomically very rough (notice the z-scale), and much more disordered, state), suggesting that the particle contained the iron the region of Fig. 5a being of an intermediate roughness. catalyst that promoted the fiber growth. It has to be noted In addition to this, in most of the cases there is not a that not all the fibers observed presented a particle at their perfectly ordered atomic arrangement within the structured ends. regions such as that normally observed in graphite, i.e. a Fig. 2 shows two STM images of the fiber surface at the perfect triangular array of spots. Instead, there is a nanometer scale. In a general view of a few fibers (Fig. departure from that ideal structure, as can be seen in Fig. 6. 2a), they appear as having an extremely smooth morpholo- Several types of distortions are found: square or rectangy free of any specific features. The fiber running from the gular lattices (in some parts of Fig. 6a), elongated rather middle left to the top right of the image has a diameter of than rounded spots (top of Fig. 6b) and even no clearly about 90 nm, whereas that running parallel to it in bottom identifiable periodicity of the spots (Fig. 6c). The distances right is |130 nm thick. However, these values must be between spots are in the range of 0.20 to 0.45 nm, whereas considered only as rough estimates of the actual diameters, those in graphite are 0.25 nm. Among these three images, since convolution between STM tip and sample surface the perfect triangular array of spots typical of graphite can topography is expected to enlarge the features appearing in only be observed in the central left part of Fig. 6b
1578 J.I. Paredes et al. Carbon 39(2001)1575-1587 100 d00 40.0 10.D 0.0 10.0 20.0 Fig. 2. Typical nanometer scale STM images of the submicron VGCFs surface.(a) General view of several fibers. (b) Detail of the surface of an individual fiber revealing a structure made up of platelets with a lateral size of a few na (delimited by a rectangle)and this arrangement takes place rhomboid shows a linear rather than a triangular or only on a very local scale(-1.7X10 nm") spots. This type of feature The images of Fig. 7 present some features that deserve occasionally observed on the fiber surface. The distance special attention. In Fig. 7a, the area marked by a between spots is -0. 20-0. 25 nm, slightly lower than that
1578 J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 Fig. 2. Typical nanometer scale STM images of the submicron VGCFs surface. (a) General view of several fibers. (b) Detail of the surface of an individual fiber revealing a structure made up of platelets with a lateral size of a few nanometers. (delimited by a rectangle) and this arrangement takes place rhomboid shows a linear rather than a triangular or 2 only on a very local scale (|1.731.0 nm ). rectangular array of spots. This type of feature was The images of Fig. 7 present some features that deserve occasionally observed on the fiber surface. The distance special attention. In Fig. 7a, the area marked by a between spots is |0.20–0.25 nm, slightly lower than that
J.I. Paredes et al. Carbon 39(2001)1575-1587 300.0nN 0.0nM 400 00 150.0nN 200 75,0n 0,0 b 100 Fig. 3.(a) Nanometer scale STM image of an area of a VGCF sample with a fiber protruding outwards (upper left part of the image)and evidencing its hollow core. (b) STM image of the protruding fiber of (a) in more detail of graphite. In the case of Fig. 7b, the region within the that this region is a(v3xv3)R30 superstructure. Ur ellipse is made up of ring-like spots with a periodicity of fortunately, the regular triangular lattice of graphite is not about 0.44 nm, which is approximately equal to 3 times discernible in the image, so the angle between the super- the periodicity of perfect graphite. This strongly suggest structure and the regular lattice cannot be measured
J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 1579 Fig. 3. (a) Nanometer scale STM image of an area of a VGCF sample with a fiber protruding outwards (upper left part of the image) and evidencing its hollow core. (b) STM image of the protruding fiber of (a) in more detail. ] ] of graphite. In the case of Fig. 7b, the region within the that this region is a sŒ Œ 3 3 3dR308 superstructure. Unellipse is made up of ring-like spots with a periodicity of fortunately, the regular triangular lattice of graphite is not ] about 0.44 nm, which is approximately equal to 3 times discernible in the image, so the angle between the super- Œ the periodicity of perfect graphite. This strongly suggests structure and the regular lattice cannot be measured
J 1, Paredes et al. / Carbon 39(2001)1575-1587 4.0n 10.0 2.0 7.5 10.0 3.5 10,0 1,8 7.5 0.0 5.0 2.5 1D.0 Fig. 4. Typical atomic scale STM images of the VGCFs surface. Only small structured regions with varying lateral sizes are found. In(b) these regions tend to be larger than in(a) X-ray diffractograms of the fibers are presented in Fi (100)and(110) peaks, which are detected as a single 8. Only two regions are shown: one, from 16 to 36, band. It is seen from Fig 8 that the material consists of corresponding to the (002) graphite diffraction peak and two phases. Indeed, deconvolution of both regions of the the other, from 38 to 50, corresponding to the graphite diffractograms indicates that the fibers comprise a highly
1580 J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 Fig. 4. Typical atomic scale STM images of the VGCFs surface. Only small structured regions with varying lateral sizes are found. In (b) these regions tend to be larger than in (a). X-ray diffractograms of the fibers are presented in Fig. (100) and (110) peaks, which are detected as a single 8. Only two regions are shown: one, from 168 to 368, band. It is seen from Fig. 8 that the material consists of corresponding to the (002) graphite diffraction peak and two phases. Indeed, deconvolution of both regions of the the other, from 388 to 508, corresponding to the graphite diffractograms indicates that the fibers comprise a highly
J.I. Paredes et al. Carbon 39(2001)1575-1587 2,0 0.0 2.50 5.00 7 8.0n 6.00 4.0nM 2.D0 b 6.00 8.00 8.00 2.0n 6,00 2,0 Fig. 5. STM images of the VGCFs showing different degrees of atomic scale roughness
J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 1581 Fig. 5. STM images of the VGCFs showing different degrees of atomic scale roughness
582 J.I. Paredes et al. Carbon 39(2001)1575-1587 2.5 3 2,50 0.0n 2.50 5 2,0 2 2.50 5n 0. 0.0nM 2.50 Fig. 6. STM images of the fiber surface revealing a variety of atomic scale arrangements: rectangular pattens in parts of (a), very elongated spots in the top part of (b), triangular array of spots typical of perfect graphite within the rectangle in(b), and no clearly identifiable patterns in(c)
1582 J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 Fig. 6. STM images of the fiber surface revealing a variety of atomic scale arrangements: rectangular patterns in parts of (a), very elongated spots in the top part of (b), triangular array of spots typical of perfect graphite within the rectangle in (b), and no clearly identifiable patterns in (c)
J.I. Paredes et al. Carbon 39(2001)1575-158 4.0 2 0.7 0.0nM 2,D 1.00 Fig. 7. Atomic scale STM images of the fibers showing singular features. (a)A linear array of spots within the rhomboid. (b)A ringlike superstructure graphitic phase, with its(002) peak centered at about between the areas of the(002) peaks of the graphitic and 26.3, and a much less ordered phase((002) peak at 24.70). disordered phases and will be commented on subsequently apparent crystallite Finally, the BEt specific surface area measured by N the c axis, Le, and apparent crystallite size along the basal gas adsorption at 77K yielded a value of 16.5 m" g planes, La, are listed in Table I together with the ratio This value can be compared with that calculated assun
J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 1583 Fig. 7. Atomic scale STM images of the fibers showing singular features. (a) A linear array of spots within the rhomboid. (b) A ringlike superstructure within the ellipse. graphitic phase, with its (002) peak centered at about between the areas of the (002) peaks of the graphitic and 26.38, and a much less ordered phase ((002) peak at 24.78). disordered phases and will be commented on subsequently. The interlayer spacing, d002, apparent crystallite size along Finally, the BET specific surface area measured by N2 2 21 the c axis, L , and apparent crystallite size along the basal gas adsorption at 77 K yielded a value of 16.5 m g . c planes, La, are listed in Table 1 together with the ratio This value can be compared with that calculated assuming
1584 J.I. Paredes et al. Carbon 39(2001)1575-1587 1600 200 150 00 100 200 8404244464850 Diffraction angle 20(degrees Fig. 8. X-ray diffractogram for the VGCFs. Only the regions between 16 and 36, corresponding to the(002)peak, and between 38 and 50, corresponding to the band arising from the(100)and(110) peaks, are depicted. The result of the deconvolution of both regions is also a cylindrical shape for the fibers and that their length is aspect ratio graphitic filament which continuously carries much greater than their diameter(which is supported by the catalytic particle at its end( Fig. Ib), thus giving rise to the SEM micrographs). With these assumptions, the spe- a hollow core. As this happens, the catalyst particle is cific surface area, A, is given by the formula A=2/pr, covered by an increasingly thicker layer of carbon until it where p is the density of the fibers and r their cylinder is isolated from the surrounding atmosphere, halting the dius of Taking a density of 1.95 assuming for r a value of 75 nm as a rough estimate, then moment, the fiber starts growing in diameter(rather than in A is about 13.7 m-g. This figure is very close to that length) by carbon deposition from the gas phase, the measured by N, gas adsorption, indicating that the fibers pyrolytic carbon layer being considerably less graphitic re devoid of a significant poro than the filament. Thus, VGCFs possess a duplex structure consisting of a highly graphitic inner hollow filament, and a layer of less ordered surrounding pyrolytic carbon [4, 311 4. Discussion The hollow core of the fibers was visible in the stm images of Fig. 3. The duplex structure was rey In the As mentioned previously, VGCFs are produced from the XRD results(Fig 8), the graphitic phase corresponding to pyrolysis of a hydrocarbon gas in a hydrogen atmosphere the inner filament and the disordered phase to the outer and their growth is catalysed by transition metal particles pyrolytic carbon layer (normally iron). According to the literature [2, 31-33], the Taking into account these considerations it becomes fiber growth starts when carbon from the gas phase clear that, since STM probes exclusively the surface of the dissolves and diffuses at the surface of the catalyst fibers, only the disordered pyrolytic carbon layer can be particles to form carbides. When the catalyst particles are explored by this technique. This explains why the surface saturated with carbon, the surplus carbon is exhausted, of the fibers was found to be very poorly organized on the creating a cylindrical and highly graphitic deposit around atomic scale(Figs 4-7). These images suggest that the the particle. The lengthening of the deposit produces a high carbon atoms deposited pyrolytically on the filament tend to aggregate into aromatic structures and form carbon Table I planes(graphenes) parallel to the surface. However, since Structural parameters of the VGCFs deduced from XRD data the production temperature of the fibers(-1000-1200.C) Phase dooz(nm) L(nm) L,(nm) Area ratio [1, 3, 5, 7] is relatively low(graphitization of carbon materi- als starts only above -2000C [34]), a great number of disordered lattice defects. both nar and interplanar, are ex pected to be present. These include atomic vacancies Graphitic 109 interstitial carbon atoms. heteroatoms and others. Further- Disordered 0.359 more, as STM reflects the local electronic density of the sample near the Fermi energy(Ep)[35], and defects on the
1584 J.I. Paredes et al. / Carbon 39 (2001) 1575 –1587 Fig. 8. X-ray diffractogram for the VGCFs. Only the regions between 168 and 368, corresponding to the (002) peak, and between 388 and 508, corresponding to the band arising from the (100) and (110) peaks, are depicted. The result of the deconvolution of both regions is also shown. a cylindrical shape for the fibers and that their length is aspect ratio graphitic filament which continuously carries much greater than their diameter (which is supported by the catalytic particle at its end (Fig. 1b), thus giving rise to the SEM micrographs). With these assumptions, the spe- a hollow core. As this happens, the catalyst particle is cific surface area, A, is given by the formula A 5 2/rr, covered by an increasingly thicker layer of carbon until it where r is the density of the fibers and r their cylinder is isolated from the surrounding atmosphere, halting the 23 radius of curvature. Taking a density of 1.95 g cm and catalytic growth (elongation) of the fiber. From this assuming for r a value of 75 nm as a rough estimate, then moment, the fiber starts growing in diameter (rather than in 2 21 A is about 13.7 m g . This figure is very close to that length) by carbon deposition from the gas phase, the measured by N gas adsorption, indicating that the fibers pyrolytic carbon layer being considerably less graphitic 2 are devoid of a significant porosity. than the filament. Thus, VGCFs possess a duplex structure consisting of a highly graphitic inner hollow filament, and a layer of less ordered surrounding pyrolytic carbon [4,31]. 4. Discussion The hollow core of the fibers was visible in the STM images of Fig. 3. The duplex structure was revealed in the As mentioned previously, VGCFs are produced from the XRD results (Fig. 8), the graphitic phase corresponding to pyrolysis of a hydrocarbon gas in a hydrogen atmosphere the inner filament and the disordered phase to the outer and their growth is catalysed by transition metal particles pyrolytic carbon layer. (normally iron). According to the literature [2,31–33], the Taking into account these considerations it becomes fiber growth starts when carbon from the gas phase clear that, since STM probes exclusively the surface of the dissolves and diffuses at the surface of the catalyst fibers, only the disordered pyrolytic carbon layer can be particles to form carbides. When the catalyst particles are explored by this technique. This explains why the surface saturated with carbon, the surplus carbon is exhausted, of the fibers was found to be very poorly organized on the creating a cylindrical and highly graphitic deposit around atomic scale (Figs. 4–7). These images suggest that the the particle. The lengthening of the deposit produces a high carbon atoms deposited pyrolytically on the filament tend to aggregate into aromatic structures and form carbon planes (graphenes) parallel to the surface. However, since Table 1 Structural parameters of the VGCFs deduced from XRD data the production temperature of the fibers (|1000–12008C) [1,3,5,7] is relatively low (graphitization of carbon materi- Phase d002 c a (nm) L (nm) L (nm) Area ratio als starts only above |20008C [34]), a great number of graphitic/ lattice defects, both intraplanar and interplanar, are ex- disordered pected to be present. These include atomic vacancies, Graphitic 0.338 9.4 10.9 interstitial carbon atoms, heteroatoms, and others. Further- 0.60 more, as STM reflects the local electronic density of the Disordered 0.359 1.7 10.2 sample near the Fermi energy (E ) [35], and defects on the F