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Carbon Xerogel-supported Iron as a Catalyst in Combustion Synthesis of Carbon Fibrous Nanostructures
J. Mater. Sci. Technol., 2012, 28(4), 294–302.
Carbon Xerogel-supported Iron as a Catalyst in Combustion Synthesis of Carbon Fibrous Nanostructures Wojciech Kici´ nski† and Joanna Lasota Institute of Chemistry, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland [Manuscript received October 3, 2011, in revised form February 10, 2012]
The catalytically assisted self-propagating high-temperature synthesis of carbon fibrous nanostructures, where the iron-doped colloidal carbon xerogel is proposed as a catalyst system, was examined. The carbon xerogel was prepared through carbonization of an iron doped organic xerogel at temperatures ranging from 600 to 1050 ◦ C. The reaction between calcium carbide and hexachloroethane in the presence of sodium azide is exothermic enough to proceed at a high temperature, self-sustaining regime. The combustion reactions of those mixtures enriched with iron-doped carbon xerogels were conducted in a stainless steel reactor—calorimetric bomb under an initial pressure of 1 MPa of argon. Scanning electron microscopy analysis of the combustion products revealed low yield of various type of carbon fibers (presumably nanotubes), which grew via the tip-growth mechanism. The fibrous nanostructures were found in the vicinity of the spot of ignition, while in the outer and cooler area of the reactor, dusty products with soot-like morphology dominated. No significant correlation between the pyrolysis temperature of the carbon xerogel and the morphology of the obtained carbon fibrous nanostructures was observed. KEY WORDS: Combustion synthesis; Carbon xerogel; Carbon fibrous nanostructures; Carbon nanotubes; Scanning electron microscopy
1. Introduction Discovered in 1991[1] , albeit probably observed for the first time in 1952[2] , carbon nanotubes (CNTs) are recognized as one of the milestones in the development of materials science and nanotechnology[3] . Even though research concerning CNTs’ synthesis seems to be colossal, there is still interest in finding new, simple, selective and efficient production techniques. Among the three basic production routes of CNTs, i.e. arc discharge, laser ablation and chemical vapor deposition (CVD), the CVD method is the most popular one and it is used to produce CNTs in large scale[4–7] . Originally used to obtain carbon fibers[8] , the catalytic CVD method was introduced to produce CNTs in 1993[9] . Transition metals from the iron subgroup, i.e. Fe, Co and Ni are most often used as the catalysts for growing CNTs through CVD † Corresponding author. Ph.D.; Tel.: +48 22 6839382; E-mail address: [email protected] (W. Kici´ nski).
method. Metal catalysts are routinely supported on [10] substrates like SiO2 . The size of the catalyst grain and the properties of the support (substrate) allow control of the properties of the nanotubes obtained. In spite of many advantages, CVD remains an energy-consuming method and it would be profitable to develop new, more energy-efficient ones. From this point of view interesting research was recently presented by Koch[11,12] and Szala[13] . They successfully applied the so-called self-propagating hightemperature combustion synthesis (SHS, or more generally, combustion synthesis)[14] for CNTs synthesis. Szala proved that multi-walled CNTs can be formed through the self-sustaining combustion of a mixture of calcium carbide CaC2 , hexachloroethane C2 Cl6 , sodium azide NaN3 and ferrocene[13] . Ferrocene acts as a source of iron, a catalyst for CNTs growth. Combustion of mixtures without ferrocene did not produce any nanotubes. As it was proved, the catalyst0 s properties (size, structure, morphology) determine the properties
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of the obtained carbon fibrous nanostructures— nanotubes and nanofibers[15] . In the procedure proposed by Szala the catalyst is produced in situ as a result of the thermal decomposition of its organic precursor, and thus its properties cannot be precisely controlled. In contrast, using a catalyst supported on specific substrates that possess predetermined properties may allow control of the structure of the carbon fibrous nanostructures (i.e. nanotubes) produced via combustion synthesis. From a range of possible catalyst supports, gels (especially aerogels) have been proved to be particularly interesting[16–18] . They possess continuous, 3D structures made up of interconnected nanoparticles and are characterized by well developed porosity and large specific surface areas. This specific structure enables a high dispersion of uniform catalyst grains within the gel0 s matrix. Baumann and coworkers[19,20] showed that iron- or nickel-doped carbon aerogels are effective substrates to produce CNTs through the CVD method. They were able to obtain a new composite material: CNTs/carbon aerogel (entangled networks of CNTs throughout the aerogel architecture). The critical aspect of this work is the attempt to utilize the advantages of both: combustion synthesis and carbon gels as catalyst supports for the production of carbon fibrous nanostructures, e.g. nanotubes. The mixtures proposed in literature[13] were used in this research, with ferrocene replaced by an iron-doped carbon xerogel (CX, xerogel is a type of gel obtained via ambient pressure drying). The properties of the CX/Fe composite were controlled through the carbonization temperature of the corresponding organic xerogel (OX). 2. Experimental 2.1 Characterization A scanning electron microscope (SEM) operating at an accelerating voltage of 2 kV (LEO 1530 apparatus) was used to study the microstructure of the products of combustion of the mixtures of CaC2 , NaN3 (reductors), C2 Cl6 (oxidizer) and iron-doped carbon xerogel (catalyst system). To study the structural appearance of the OX and corresponding CXs, X-ray diffraction (XRD) was performed (using a Siemens D500 powder diffractometer with CuKα radiation) with a scan range of 2θ from 10 to 60 deg. The average values of the crystallite size of Fe supported on CXs were obtained using Scherrer0 s equation. The obtained carbon xerogels are referred to in the paper as CX-600, -700, -800, and -1050, respectively, where the numbers refer to carbonization temperatures. 2.2 Synthesis of the catalyst system (iron-doped carbon xerogel) Iron catalysts supported on CXs were obtained via the carbonization of an organic iron-doped resorcinol-
[21]
furfural xerogel. As described in literature , condensation of resorcinol and furfural carried out in a solution of FeCl3 yields an organic gel with permanently entrapped iron within the gel matrix. Organic xerogels were obtained from the condensation of resorcinol with furfural (R-F gel) carried out in a watermethanol solution of anhydrous Fe(III) chloride. The synthesis was carried out at room temperature. Resorcinol (7.5 g) and FeCl3 (12 g) were dissolved in a water (70 g)/methanol (31.5 g) mixture. The obtained solution exhibits a dark purple color since phenols create colorful complexes with Fe3+ ions. Furfural (15 g) was then added. The obtained sol mixture was sealed in a beaker and sol-gel polymerization was carried out by holding the mixture at 60 ◦ C. Fe(III) chloride, which is prone to hydrolysis, lowers pH of the solution and as a result acts as a catalyst for the sol-gel polymerization of resorcinol and furfural. Aqua-alcogel was obtained within 30 min. The gel was aged at 60 ◦ C for 3 d. The wet gel was then dried under a perforated foil at 60 ◦ C for 3 d, and subsequently without any confinement at 100 ◦ C for more 3 d. Eventually, a bulky organic xerogel was obtained. The monolithic xerogel saturated with FeCl3 was then exposed to a distilled water purification process in order to remove the unbounded iron. The xerogel samples were boiled three times in an excess of water. The removal of FeCl3 resulted in a 35% reduction in mass. After being purified, the wet gel was dried again. As we presented elsewhere, FeCl3 can be totally removed from the gel matrix by simple water purification, however some portion of iron remains permanently bonded (entrapped) within the organic gel network[21] . The iron-doped carbon xerogel was obtained by carbonizing the organic xerogel at 600, 700, 800 or 1050 ◦ C for 1 h with a heating rate of 5 ◦ C/min under a flow of pure N2 . This process yields a monolithic sample containing a reduced form of iron, as revealed by the attraction of the xerogels obtained to a magnet. 2.3 Preparation and combustion of the reactive mixtures enriched with carbon xerogels The starting mixtures were prepared by mixing calcium carbide (CaC2 technical 80%) and sodium azide NaN3 (reductors) powders, hexachloroethane C2 Cl6 (oxidizer) and the carbon xerogel/iron system (catalyst). The powder mixture was pounded in a ceramic mortar until a fine powder was obtained. In each case, 3.5 g of the pulverized samples of the mixtures was placed in a graphite crucible and put into a calorimetric bomb (275 cm3 in volume), which was used as a reactor. The calorimetric bomb was filled with argon at an initial pressure of 1.0 MPa. The combustion process was initiated with an electrically heated resistance wire. The compositions of the all investigated mixtures are presented in Table 1. Dur-
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Table 1 Compositions of starting mixtures No. Mixture Composition/wt% 1 CaC2 /C2 Cl6 /NaN3 /CX-FeCl3 -700C 37.2/45.1/10/7.7 2 CaC2 /C2 Cl6 /NaN3 /CX-700 37.2/45.1/10/7.7 3 CaC2 /C2 Cl6 /NaN3 /CX-800 37.2/45.1/10/7.7 4 CaC2 /C2 Cl6 /NaN3 /CX-1050 37.2/45.1/10/7.7 5 CaC2 /C2 Cl6 /NaN3 /0.5CX-800 38.7/46.9/10.4/4.0A 6 CaC2 /C2 Cl6 /NaN3 /2CX-800 34.5/41.9/9.3/14.3B 7 CaC2 /C2 Cl6 /NaN3 /CX-600 37.2/45.1/10/7.7 8 CaC2 /C2 Cl6 /NaN3 /OXD 37.2/45.1/10/7.7 9 CaC2 /C2 Cl6 /NaN3 /OX-FeClE 37.2/45.1/10/7.7 3 A Notes: half the mass of the catalyst, B double the mass of the catalyst—in comparison to the rest of the mixtures, C carbon xerogel from which FeCl3 was not removed, D water-purified organic xerogel, E organic xerogel from which FeCl3 was not removed
2CaC2 +C2 Cl6 +2NaN3 → 2NaCl+2CaCl2 +6C+3N2 Solid products of combustion were found on the reactor walls as well as inside the graphite crucible. There was a clear difference between their appearances. The deposit from the reactor walls was a dusty, hydrophobic light powder, while the deposit inside the crucible was a monolith made up of combustion products bonded together by the by-products of the combustion (presumably inorganic salts: NaCl, CaCl2 ). For that reason, as in previous research[13] , the two types of products were analyzed separately. Solid products deposited on the reactor walls were removed with water and then washed with plenty of ethanol and water, filtered off and dried. The graphite crucible filled with the combustion product was immersed in water, the monolith disintegrated and the precipitate was washed in water to remove the inorganic salts, ethanol and finally in an excess of 15% HCl followed by 15% HNO3 to remove any iron remains. 3. Results 3.1 Characterization of the catalyst system (irondoped carbon xerogel) Resorcinol is a complexing agent towards Fe3+ ions[22] . Complexed iron ions cannot be effectively removed from the gel framework by washing with water. Additionally, as suggested in literature[23] , iron ions in the xerogel can be chelated by two phenolic groups of the polymeric matrix. Moreover, FeCl3 undergoes hydrolysis in aqueous solutions, which may yield amorphous, water insoluble iron hydroxides (FeOOH). As revealed by the XRD analysis, the water-purified RF xerogel is a totally amorphous material (Fig. 1). No crystalline phases of iron or iron compounds were detected. Complexed iron (or entrapped iron compound) is reduced to metallic state during carbonization and in
Intensity / a.u.
ing the combustion, solids as well as gas products are released. It was assumed that the combustion reaction proceeds according to the theoretical schema:
20
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30
35
40 2
45
50
55
60
/ deg.
Fig. 1 XRD profile of the organic R-F xerogel, from which FeCl3 was removed by water purification. Any crystalline form of Fe compounds is not found
this form it causes partial carbon xerogel graphitization. After carbonization the mass of the CX was reduced by about 50%. Ferromagnetic properties were revealed after carbonization at temperatures as low as 600 ◦ C. FeCl3 -doped xerogel (not washed in water) was also subject to carbonization. The obtained carbon xerogels presented a morphology inherent to the parent organic xerogels, i.e. a colloidal structure built up of interconnected micron-sized spherical particles with diameters of 3–6 µm (Fig. 2(a) and (b)). In order to determine the iron content of the carbon xerogel, gel pyrolyzed at 800 ◦ C was burned in air at 800 ◦ C. This process yields a rusty, red powder—Fe2 O3 with a morphology of spherical particles about 100 nm in size, shown in Fig. 2(c) and (d) (at different magnification). The uniform size of these particles suggests that iron particles are evenly distributed in the carbon matrix and possess nanoscopic size. Taking into account the mass of the xerogel burn out residues it was determined that the iron trapped within the gel matrix constitutes about 3.45 wt% of the carbon xerogel. XRD analysis revealed that the carbonization of amorphous organic xerogel leads to the formation of
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Fig. 2 SEM images: (a) water-purified organic xerogel; (b) carbon xerogel (CX-800) obtained after carbonization (800 ◦ C) of water-purified organic xerogel; (c) and (d) residues of its burn out (at 800 ◦ C) in air (most probably Fe2 O3 )
C(002)
Intensity / a.u.
Fe(110) Fe O 2
(c)
3
Fe= 30 nm
Fe O 2
3
(b)
Fe = 40 nm (a)
Fe = 36 nm
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35
2
40
45
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/ deg.
Fig. 3 XRD profile of the carbon xerogels obtained through carbonization at 700 ◦ C (CX-700) (a), 800 ◦ C (CX-800) (b) and 1050 ◦ C (CX-1050) (c)
lites produced in situ was not observed. The XRD analysis presented in Fig. 3 suggests that the average size of iron particles supported on xerogel obtained via carbonization at 700 ◦ C (diameter of Fe≈36 nm) does not differ significantly from those obtained at a temperature 350 ◦ C higher (diameter of Fe≈30 nm). Nonetheless it is obvious that an increase in carbonization temperature modifies the structure of the iron nanocrystallites produced. The iron oxide γFe2 O3 (maghemit) detected in all carbon xerogels may be formed during the sol-gel polymerization (as a result of hydrolysis). Alternatively it could be the result of the oxidation of nanometric iron when the carbon xerogel is exposed to air. Additionally, as noticed in literature[19,24] , during the carbonization of the organic gels, iron compounds trapped in the gel matrix are first transformed to Fe2 O3 and FeOOH at around 300 ◦ C and then are reduced to iron. 3.2 Characterization of the combustion products
crystalline forms of iron (α-Fe, Fe2 C, Fe3 C, Fe2 O3 ) and carbon (graphite) (Fig. 3). Carbonization at 700 ◦ C enables the formation of crystalline iron (αFe), but no graphitic carbon is created. After carbonization at 800 ◦ C broad reflection (2θ ≈26 deg.) appeared, which is characteristic for graphite (002). Carbonization at 1050 ◦ C leads to the effective graphitization of the xerogel matrix by iron particles formed in situ (intensive (002) band at 2θ≈26 deg.). Unfortunately, a direct correlation between the carbonization temperature and the size of iron nanocrystal-
The combustion products collected from the reactor walls exhibit identical morphology for all combusted mixtures. As revealed by SEM analysis (Fig. 4) they are spheroidal particles with sizes up to 200 nm. This morphology resembles typical carbon soot. Figs. 5–8 exhibit SEM images of products presented in the graphite crucible (the container where the mixtures were ignited). The combustion products of mixtures enriched with xerogels carbonized at 700,
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Fig. 4 SEM images (at two different magnifications (a) and (b)) of combustion products of the CaC2 /C2 Cl6 /NaN3 /CX-700 mixture collected from the walls of the reactor. Carbon deposited on the reactor walls possesses soot-like morphology
Fig. 5 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-700 mixture
800 and 1050 ◦ C contain numerous fibrous structures. Nonetheless, the filamentous structures vary significantly in length and diameter even within one sample. Fig. 5(a) and (b) show combustion products derived from the mixture of CaC2 /C2 Cl6 /NaN3 /CX700. Long, straight fibers with lengths of a few micrometers and diameters of tens of nanometers were found. However, shorter, thicker (300 nm) and strongly kinked fibers with numerous constrictions were also observed next to such structures (Fig. 5(c) and (d)). SEM images of products derived from mixtures with CX-800 (CaC2 /C2 Cl6 /NaN3 /CX-800) are presented in Figs. 6 and 7. They also contain various types of nanometric fibers. Many of these fibers are a few microns long with diameters below 100 nm (Fig. 6(a)). Some of the fibers are much more than 10 µm long with diameters below 100 nm (Fig. 7(a)).
On the other hand, structures shown on Fig. 6(b) are short (up to 2 µm) and thick with diameters of up to 300 nm. SEM images of products obtained upon combustion of CaC2 /C2 Cl6 /NaN3 /CX-1050 are presented in Fig. 8. They possess a similar morphology to products obtained from mixtures with CX-800. A particular morphology feature of the observed fibrous nanostructures, presented in all investigated mixtures, is their bulb-like tip (indicated on the SEM images by arrows in Figs. 5(c), 6(b), 8(a) and particularly in Fig. 6(c) and (d)). These fibers0 endings suggest that the observed structures arose according to the so-called tip-growth mechanism. The characteristic lumps probably consist of the catalyst—iron particles enclosed at the tip of fiber from which the fiber growth started. SEM analysis does not allow us to determine the type of observed fibers (carbon nanotubes or nanofibers). However, taking into ac-
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Fig. 6 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-800 mixture
Fig. 7 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-800 (a) and the CaC2 /C2 Cl6 /NaN3 /0.5CX-800 mixture (b)
count results presented in literature[13,19] and the characteristic tips of the fibers, one can assume that they are nanotubes. On the other hand, it should be noted that the only way to determine if there are nanotubes or nanofibers is to perform transmission electron microscopy. Fig. 7(b) presents the morphology of combustion products derived from a mixture that contained half the amount of carbon xerogel (by mass) in comparison to the other mixtures (CaC2 /C2 Cl6 /NaN3 /0.5CX800). No significant differences in the morphology of the products were observed for either mixtures with half or double the amount of carbon xerogel catalyst. Fig. 9 shows the products recovered from the graphitic crucible after combustion of the CaC2 /C2 Cl6 /NaN3 /2CX-800 mixture, where double the amount of the xerogel catalyst system was used. The presence of colloidal xerogel particles untouched
by the grinding and the combustion processes suggests some heterogeneity of the iron distribution within the starting mixtures. Additionally, Fig. 9 implies that part of the catalyst system (Fe nanoparticles trapped within colloidal carbon xerogel) survives during the combustion process, consequently some of the iron particles could remain inactive in the catalyticassisted growth of the carbon fibrous structures. No catalytic activity of organic xerogels filled with FeCl3 (mixture No. 9, Table 1), water purified organic xerogels (mixture No. 8, Table 1), CX-600 or a carbonized sample from which FeCl3 was not removed before carbonization (mixture No. 1, Table 1) was observed. In all these cases, fibrous nanostructures were not found in the combustion products. 4. Discussion The catalytically assisted, self-propagating, high-
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Fig. 8 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-1050 mixture
Fig. 9 SEM image of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /2CX-800 mixture. Intact xerogel spheres are present in the combustion products
temperature synthesis of carbon fibrous nanostructures described herein differs from the previous synthetic methods in terms of the catalyst applied. In the method presented by Szala[13] the catalyst is produced in situ during the combustion process. Iron released via ferrocene decomposition constitutes 0.6 wt% of the starting mixture[13] . In our procedure iron is introduced into the starting mixtures as pre-prepared nanoparticles supported on substrates—carbon xerogels. It constitutes about 0.26 wt% of the starting mixtures. The main advantage of the proposed method is the ability to design iron (catalyst) particle size and to some extent the chemical composition (i.e. carbide, oxide, pure metal) as well, by selecting the appropriate carbonization temperature of the xerogel. Additionally, the carbon gel provides a support (substrate) with 3D porous microstructure, which ensures a high level of catalyst dispersion. This assumption is indirectly supported by the morphology of the residues of CX-800 oxidation in air (Fig. 2(c) and (d)). As revealed by SEM analysis, part of the carbon xerogel undergoes the combustion process intact. This suggests that some portion of iron remains isolated from the gaseous products of combustion. Additionally, carbonization of iron-doped resorcinolfurfural gel causes its partial graphitization, which
starts at 800 ◦ C and becomes very effective at 1050 ◦ C (Fig. 3). For this reason some part of iron nanoparticles may become isolated by the graphene layers. Fibrous carbon structures arise from the surface on xerogels where the Fe catalyst is anchored. For their high reactivity, carbon moieties (i.e. carbenes) released during the vigorous combustion do not penetrate the interior of the xerogels0 colloids. Carbon fibrous nanostructures must have been created where the catalyst was directly exposed to the products of the combustion wave (i.e. on the support surface). The same phenomenon was observed by Steiner et al.[19] —they observed that carbon nanotubes were created strictly on the surface of iron-doped carbon aerogels and not within their volume. They observed that, even though the CVD process was performed for 10 min, (a mixture of gases was passed above the aerogel), much longer than the combustion of the examined mixtures, CNTs did not penetrate the aerogel network. As the authors stated, the limiting factor for nanotube growth within the porosity of the carbon gel is the diffusion of CVD gases rather than the activity of the catalyst particles within the gel matrix[19] . They noticed that the shallow depth of CVD growth is most likely due to the short mean free path of diffusion inherent to porous materials such as carbon gels. Furthermore, it was observed that for carbon aerogels carbonized at 600 ◦ C nanotubes were not created[19] . This observation is in agreement with the results presented herein. In the combustion products of mixtures with CX-600 no fibrous structures were found. One can conclude that a carbonization temperature of 600 ◦ C does not permit the transfer of iron precursors into their catalytically active form. The self-sustaining combustion of reductors (CaC2, NaN3 , where calcium carbide acts not only as a reductor, but also as a source of carbon) and an oxidizer (C2 Cl6 ) yields two different types of carbonaceous products, depending on the reactor zone. Sootlike carbon powder was collected from the reactor walls. Carbon nanofibers formed only in the graphite crucible where the mixture was ignited. High temperature conditions created in the crucible neighborhood (the combustion reaction zone) enhance the formation
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is crucial for the formation of carbon structures. For bigger catalyst crystallites only carbon nanocapsules can be created and the growth of nanotubes would be restricted; for smaller particles single walled nanotubes are usually formed[27,28] . 5. Conclusions
Fig. 10 Schematic of tip growth mechanisms: (a) deposit of Fe catalyst particle on support—carbon xerogel/Fe, (b) diffusion of carbon into catalyst, (c) lift-off of catalyst particle due to weak interaction with substrate, (d) diffusion of carbon on exposed end of catalyst—growth, (e) catalyst covered with carbon layers and growth arrest[25]
of active chemical vapors (containing carbon radicals and carbenes among others) and may cause the transformation of iron nanoparticles into a liquid state[13] . The vapors may be the key factor determining the creation of carbon fibrous nanostructures. Vapors which do not interact with iron active particles within the high-temperature reaction zone condense into amorphous soot-like carbon after reaching cooler areas of the reactor. SEM analysis of the products recovered from the crucible and results of the investigation concerning carbon nanotubes presented by Szala[13] and Steiner et al.[19] lead us conclude that the growth mechanism is the same in all cases, i.e. the catalytically assisted tip growth mechanism. Catalyst particles were observed on the tip—end of the nanotubes (Fig. 6(c) and (d)). The weak interaction of the catalyst particles with the support (xerogel) causes the catalyst particles to lift off with the concentric cylindrical structures formed below them[25] . As the catalyst particle moves away from the xerogel, it gets trapped within the inner core of the tube/fiber. The tip growth stops when the catalyst particle is covered with carbon layers or when the supply of feed gas is cut off, i.e. ending of the combustion. The scheme of this process is presented in Fig. 10. The type of catalyst support determines the catalyst-support interactions, and as a result the mechanism for the growth of nanotubes and nanofibers. For catalyst particles weakly bonded to the support, the tip-growth route occurs. On the contrary, if the catalyst is well bonded to the support, the root-growth mechanism occurs. Within a single process a combination of root-tip-growth mechanisms may also occur[25,26] . The fibrous structures manifest a wide range of diameters: from tens of nanometer up to 300 nm (Fig. 6(a)). The wide range of fibers diameters is likely a consequence of the size distribution of iron catalyst particles formed during the carbonization step. It should be noted that the size of the iron particles
The pursuit of new, more economic methods for the production of carbon nanofibrous structures remains a very active area of materials research. The catalytically assisted, self-propagating, hightemperature synthesis became recently one of the state-of-the-art energy-saving methods for the production of carbon nanotubes[11–13] . In recent research porous supports, especially carbon gels are considered attractive catalyst substrates for catalytic methods of synthesis of carbon nanotubes[19,20,28] . As presented, iron-doped carbon xerogels can be used as catalyst systems for the synthesis of carbon fibers via the self-sustaining combustion of mixtures of inorganic reductors and organic oxidizers. A CX as a porous, microstructured support enables a high dispersion of the catalyst[28] . Previous research presented elsewhere showed in detail that porous supports like activated carbon are very valuable substrates for synthesis of carbon fibers with varied morphology[28] . The morphology of the obtained carbon fibrous nanostructures indicates that they arise according to the catalytically assisted tipgrowth mechanism. It was observed that iron-doped CXs can be applied as active, specific catalyst systems for the synthesis of carbon fibrous structures via combustion. The fibrous structures were obtained with low yield and they manifest varied morphology. Some of them were short, thick (300 nm) and curved; yet within the same sample long (above 10 µm), thin ones were also observed. The low yield is caused by the limited availability of the iron catalyst—during the vigorous combustion some iron trapped inside CX particles may remain inaccessible for the formation of carbon elongated nanostructures. The substrate (CX) does not provide enhanced diffusion efficiency of the gases (generated via combustion synthesis) throughout the xerogel structure and hinders the formation of carbon fibrous nanostructures. As already suggested, increasing combustion temperature may improve the efficiency of this synthetic route[13] . In spite of the low yield, the presented results show that carbon nanofibrous structures can be produced through the self-sustaining combustion synthesis as first stated in the pioneering research presented by Koch[11,12] and Szala[13] and catalysts supported on carbon gels are active catalyst systems in this method. This method is characterized by simplicity and what is more important, energy efficiency. Acknowledgements This work was supported by the internal grant of the
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Carbon Xerogel-supported Iron as a Catalyst in Combustion Synthesis of Carbon Fibrous Nanostructures Wojciech Kici´ nski† and Joanna Lasota Institute of Chemistry, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland [Manuscript received October 3, 2011, in revised form February 10, 2012]
The catalytically assisted self-propagating high-temperature synthesis of carbon fibrous nanostructures, where the iron-doped colloidal carbon xerogel is proposed as a catalyst system, was examined. The carbon xerogel was prepared through carbonization of an iron doped organic xerogel at temperatures ranging from 600 to 1050 ◦ C. The reaction between calcium carbide and hexachloroethane in the presence of sodium azide is exothermic enough to proceed at a high temperature, self-sustaining regime. The combustion reactions of those mixtures enriched with iron-doped carbon xerogels were conducted in a stainless steel reactor—calorimetric bomb under an initial pressure of 1 MPa of argon. Scanning electron microscopy analysis of the combustion products revealed low yield of various type of carbon fibers (presumably nanotubes), which grew via the tip-growth mechanism. The fibrous nanostructures were found in the vicinity of the spot of ignition, while in the outer and cooler area of the reactor, dusty products with soot-like morphology dominated. No significant correlation between the pyrolysis temperature of the carbon xerogel and the morphology of the obtained carbon fibrous nanostructures was observed. KEY WORDS: Combustion synthesis; Carbon xerogel; Carbon fibrous nanostructures; Carbon nanotubes; Scanning electron microscopy
1. Introduction Discovered in 1991[1] , albeit probably observed for the first time in 1952[2] , carbon nanotubes (CNTs) are recognized as one of the milestones in the development of materials science and nanotechnology[3] . Even though research concerning CNTs’ synthesis seems to be colossal, there is still interest in finding new, simple, selective and efficient production techniques. Among the three basic production routes of CNTs, i.e. arc discharge, laser ablation and chemical vapor deposition (CVD), the CVD method is the most popular one and it is used to produce CNTs in large scale[4–7] . Originally used to obtain carbon fibers[8] , the catalytic CVD method was introduced to produce CNTs in 1993[9] . Transition metals from the iron subgroup, i.e. Fe, Co and Ni are most often used as the catalysts for growing CNTs through CVD † Corresponding author. Ph.D.; Tel.: +48 22 6839382; E-mail address: [email protected] (W. Kici´ nski).
method. Metal catalysts are routinely supported on [10] substrates like SiO2 . The size of the catalyst grain and the properties of the support (substrate) allow control of the properties of the nanotubes obtained. In spite of many advantages, CVD remains an energy-consuming method and it would be profitable to develop new, more energy-efficient ones. From this point of view interesting research was recently presented by Koch[11,12] and Szala[13] . They successfully applied the so-called self-propagating hightemperature combustion synthesis (SHS, or more generally, combustion synthesis)[14] for CNTs synthesis. Szala proved that multi-walled CNTs can be formed through the self-sustaining combustion of a mixture of calcium carbide CaC2 , hexachloroethane C2 Cl6 , sodium azide NaN3 and ferrocene[13] . Ferrocene acts as a source of iron, a catalyst for CNTs growth. Combustion of mixtures without ferrocene did not produce any nanotubes. As it was proved, the catalyst0 s properties (size, structure, morphology) determine the properties
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of the obtained carbon fibrous nanostructures— nanotubes and nanofibers[15] . In the procedure proposed by Szala the catalyst is produced in situ as a result of the thermal decomposition of its organic precursor, and thus its properties cannot be precisely controlled. In contrast, using a catalyst supported on specific substrates that possess predetermined properties may allow control of the structure of the carbon fibrous nanostructures (i.e. nanotubes) produced via combustion synthesis. From a range of possible catalyst supports, gels (especially aerogels) have been proved to be particularly interesting[16–18] . They possess continuous, 3D structures made up of interconnected nanoparticles and are characterized by well developed porosity and large specific surface areas. This specific structure enables a high dispersion of uniform catalyst grains within the gel0 s matrix. Baumann and coworkers[19,20] showed that iron- or nickel-doped carbon aerogels are effective substrates to produce CNTs through the CVD method. They were able to obtain a new composite material: CNTs/carbon aerogel (entangled networks of CNTs throughout the aerogel architecture). The critical aspect of this work is the attempt to utilize the advantages of both: combustion synthesis and carbon gels as catalyst supports for the production of carbon fibrous nanostructures, e.g. nanotubes. The mixtures proposed in literature[13] were used in this research, with ferrocene replaced by an iron-doped carbon xerogel (CX, xerogel is a type of gel obtained via ambient pressure drying). The properties of the CX/Fe composite were controlled through the carbonization temperature of the corresponding organic xerogel (OX). 2. Experimental 2.1 Characterization A scanning electron microscope (SEM) operating at an accelerating voltage of 2 kV (LEO 1530 apparatus) was used to study the microstructure of the products of combustion of the mixtures of CaC2 , NaN3 (reductors), C2 Cl6 (oxidizer) and iron-doped carbon xerogel (catalyst system). To study the structural appearance of the OX and corresponding CXs, X-ray diffraction (XRD) was performed (using a Siemens D500 powder diffractometer with CuKα radiation) with a scan range of 2θ from 10 to 60 deg. The average values of the crystallite size of Fe supported on CXs were obtained using Scherrer0 s equation. The obtained carbon xerogels are referred to in the paper as CX-600, -700, -800, and -1050, respectively, where the numbers refer to carbonization temperatures. 2.2 Synthesis of the catalyst system (iron-doped carbon xerogel) Iron catalysts supported on CXs were obtained via the carbonization of an organic iron-doped resorcinol-
[21]
furfural xerogel. As described in literature , condensation of resorcinol and furfural carried out in a solution of FeCl3 yields an organic gel with permanently entrapped iron within the gel matrix. Organic xerogels were obtained from the condensation of resorcinol with furfural (R-F gel) carried out in a watermethanol solution of anhydrous Fe(III) chloride. The synthesis was carried out at room temperature. Resorcinol (7.5 g) and FeCl3 (12 g) were dissolved in a water (70 g)/methanol (31.5 g) mixture. The obtained solution exhibits a dark purple color since phenols create colorful complexes with Fe3+ ions. Furfural (15 g) was then added. The obtained sol mixture was sealed in a beaker and sol-gel polymerization was carried out by holding the mixture at 60 ◦ C. Fe(III) chloride, which is prone to hydrolysis, lowers pH of the solution and as a result acts as a catalyst for the sol-gel polymerization of resorcinol and furfural. Aqua-alcogel was obtained within 30 min. The gel was aged at 60 ◦ C for 3 d. The wet gel was then dried under a perforated foil at 60 ◦ C for 3 d, and subsequently without any confinement at 100 ◦ C for more 3 d. Eventually, a bulky organic xerogel was obtained. The monolithic xerogel saturated with FeCl3 was then exposed to a distilled water purification process in order to remove the unbounded iron. The xerogel samples were boiled three times in an excess of water. The removal of FeCl3 resulted in a 35% reduction in mass. After being purified, the wet gel was dried again. As we presented elsewhere, FeCl3 can be totally removed from the gel matrix by simple water purification, however some portion of iron remains permanently bonded (entrapped) within the organic gel network[21] . The iron-doped carbon xerogel was obtained by carbonizing the organic xerogel at 600, 700, 800 or 1050 ◦ C for 1 h with a heating rate of 5 ◦ C/min under a flow of pure N2 . This process yields a monolithic sample containing a reduced form of iron, as revealed by the attraction of the xerogels obtained to a magnet. 2.3 Preparation and combustion of the reactive mixtures enriched with carbon xerogels The starting mixtures were prepared by mixing calcium carbide (CaC2 technical 80%) and sodium azide NaN3 (reductors) powders, hexachloroethane C2 Cl6 (oxidizer) and the carbon xerogel/iron system (catalyst). The powder mixture was pounded in a ceramic mortar until a fine powder was obtained. In each case, 3.5 g of the pulverized samples of the mixtures was placed in a graphite crucible and put into a calorimetric bomb (275 cm3 in volume), which was used as a reactor. The calorimetric bomb was filled with argon at an initial pressure of 1.0 MPa. The combustion process was initiated with an electrically heated resistance wire. The compositions of the all investigated mixtures are presented in Table 1. Dur-
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Table 1 Compositions of starting mixtures No. Mixture Composition/wt% 1 CaC2 /C2 Cl6 /NaN3 /CX-FeCl3 -700C 37.2/45.1/10/7.7 2 CaC2 /C2 Cl6 /NaN3 /CX-700 37.2/45.1/10/7.7 3 CaC2 /C2 Cl6 /NaN3 /CX-800 37.2/45.1/10/7.7 4 CaC2 /C2 Cl6 /NaN3 /CX-1050 37.2/45.1/10/7.7 5 CaC2 /C2 Cl6 /NaN3 /0.5CX-800 38.7/46.9/10.4/4.0A 6 CaC2 /C2 Cl6 /NaN3 /2CX-800 34.5/41.9/9.3/14.3B 7 CaC2 /C2 Cl6 /NaN3 /CX-600 37.2/45.1/10/7.7 8 CaC2 /C2 Cl6 /NaN3 /OXD 37.2/45.1/10/7.7 9 CaC2 /C2 Cl6 /NaN3 /OX-FeClE 37.2/45.1/10/7.7 3 A Notes: half the mass of the catalyst, B double the mass of the catalyst—in comparison to the rest of the mixtures, C carbon xerogel from which FeCl3 was not removed, D water-purified organic xerogel, E organic xerogel from which FeCl3 was not removed
2CaC2 +C2 Cl6 +2NaN3 → 2NaCl+2CaCl2 +6C+3N2 Solid products of combustion were found on the reactor walls as well as inside the graphite crucible. There was a clear difference between their appearances. The deposit from the reactor walls was a dusty, hydrophobic light powder, while the deposit inside the crucible was a monolith made up of combustion products bonded together by the by-products of the combustion (presumably inorganic salts: NaCl, CaCl2 ). For that reason, as in previous research[13] , the two types of products were analyzed separately. Solid products deposited on the reactor walls were removed with water and then washed with plenty of ethanol and water, filtered off and dried. The graphite crucible filled with the combustion product was immersed in water, the monolith disintegrated and the precipitate was washed in water to remove the inorganic salts, ethanol and finally in an excess of 15% HCl followed by 15% HNO3 to remove any iron remains. 3. Results 3.1 Characterization of the catalyst system (irondoped carbon xerogel) Resorcinol is a complexing agent towards Fe3+ ions[22] . Complexed iron ions cannot be effectively removed from the gel framework by washing with water. Additionally, as suggested in literature[23] , iron ions in the xerogel can be chelated by two phenolic groups of the polymeric matrix. Moreover, FeCl3 undergoes hydrolysis in aqueous solutions, which may yield amorphous, water insoluble iron hydroxides (FeOOH). As revealed by the XRD analysis, the water-purified RF xerogel is a totally amorphous material (Fig. 1). No crystalline phases of iron or iron compounds were detected. Complexed iron (or entrapped iron compound) is reduced to metallic state during carbonization and in
Intensity / a.u.
ing the combustion, solids as well as gas products are released. It was assumed that the combustion reaction proceeds according to the theoretical schema:
20
25
30
35
40 2
45
50
55
60
/ deg.
Fig. 1 XRD profile of the organic R-F xerogel, from which FeCl3 was removed by water purification. Any crystalline form of Fe compounds is not found
this form it causes partial carbon xerogel graphitization. After carbonization the mass of the CX was reduced by about 50%. Ferromagnetic properties were revealed after carbonization at temperatures as low as 600 ◦ C. FeCl3 -doped xerogel (not washed in water) was also subject to carbonization. The obtained carbon xerogels presented a morphology inherent to the parent organic xerogels, i.e. a colloidal structure built up of interconnected micron-sized spherical particles with diameters of 3–6 µm (Fig. 2(a) and (b)). In order to determine the iron content of the carbon xerogel, gel pyrolyzed at 800 ◦ C was burned in air at 800 ◦ C. This process yields a rusty, red powder—Fe2 O3 with a morphology of spherical particles about 100 nm in size, shown in Fig. 2(c) and (d) (at different magnification). The uniform size of these particles suggests that iron particles are evenly distributed in the carbon matrix and possess nanoscopic size. Taking into account the mass of the xerogel burn out residues it was determined that the iron trapped within the gel matrix constitutes about 3.45 wt% of the carbon xerogel. XRD analysis revealed that the carbonization of amorphous organic xerogel leads to the formation of
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Fig. 2 SEM images: (a) water-purified organic xerogel; (b) carbon xerogel (CX-800) obtained after carbonization (800 ◦ C) of water-purified organic xerogel; (c) and (d) residues of its burn out (at 800 ◦ C) in air (most probably Fe2 O3 )
C(002)
Intensity / a.u.
Fe(110) Fe O 2
(c)
3
Fe= 30 nm
Fe O 2
3
(b)
Fe = 40 nm (a)
Fe = 36 nm
20
25
30
35
2
40
45
50
55
/ deg.
Fig. 3 XRD profile of the carbon xerogels obtained through carbonization at 700 ◦ C (CX-700) (a), 800 ◦ C (CX-800) (b) and 1050 ◦ C (CX-1050) (c)
lites produced in situ was not observed. The XRD analysis presented in Fig. 3 suggests that the average size of iron particles supported on xerogel obtained via carbonization at 700 ◦ C (diameter of Fe≈36 nm) does not differ significantly from those obtained at a temperature 350 ◦ C higher (diameter of Fe≈30 nm). Nonetheless it is obvious that an increase in carbonization temperature modifies the structure of the iron nanocrystallites produced. The iron oxide γFe2 O3 (maghemit) detected in all carbon xerogels may be formed during the sol-gel polymerization (as a result of hydrolysis). Alternatively it could be the result of the oxidation of nanometric iron when the carbon xerogel is exposed to air. Additionally, as noticed in literature[19,24] , during the carbonization of the organic gels, iron compounds trapped in the gel matrix are first transformed to Fe2 O3 and FeOOH at around 300 ◦ C and then are reduced to iron. 3.2 Characterization of the combustion products
crystalline forms of iron (α-Fe, Fe2 C, Fe3 C, Fe2 O3 ) and carbon (graphite) (Fig. 3). Carbonization at 700 ◦ C enables the formation of crystalline iron (αFe), but no graphitic carbon is created. After carbonization at 800 ◦ C broad reflection (2θ ≈26 deg.) appeared, which is characteristic for graphite (002). Carbonization at 1050 ◦ C leads to the effective graphitization of the xerogel matrix by iron particles formed in situ (intensive (002) band at 2θ≈26 deg.). Unfortunately, a direct correlation between the carbonization temperature and the size of iron nanocrystal-
The combustion products collected from the reactor walls exhibit identical morphology for all combusted mixtures. As revealed by SEM analysis (Fig. 4) they are spheroidal particles with sizes up to 200 nm. This morphology resembles typical carbon soot. Figs. 5–8 exhibit SEM images of products presented in the graphite crucible (the container where the mixtures were ignited). The combustion products of mixtures enriched with xerogels carbonized at 700,
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Fig. 4 SEM images (at two different magnifications (a) and (b)) of combustion products of the CaC2 /C2 Cl6 /NaN3 /CX-700 mixture collected from the walls of the reactor. Carbon deposited on the reactor walls possesses soot-like morphology
Fig. 5 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-700 mixture
800 and 1050 ◦ C contain numerous fibrous structures. Nonetheless, the filamentous structures vary significantly in length and diameter even within one sample. Fig. 5(a) and (b) show combustion products derived from the mixture of CaC2 /C2 Cl6 /NaN3 /CX700. Long, straight fibers with lengths of a few micrometers and diameters of tens of nanometers were found. However, shorter, thicker (300 nm) and strongly kinked fibers with numerous constrictions were also observed next to such structures (Fig. 5(c) and (d)). SEM images of products derived from mixtures with CX-800 (CaC2 /C2 Cl6 /NaN3 /CX-800) are presented in Figs. 6 and 7. They also contain various types of nanometric fibers. Many of these fibers are a few microns long with diameters below 100 nm (Fig. 6(a)). Some of the fibers are much more than 10 µm long with diameters below 100 nm (Fig. 7(a)).
On the other hand, structures shown on Fig. 6(b) are short (up to 2 µm) and thick with diameters of up to 300 nm. SEM images of products obtained upon combustion of CaC2 /C2 Cl6 /NaN3 /CX-1050 are presented in Fig. 8. They possess a similar morphology to products obtained from mixtures with CX-800. A particular morphology feature of the observed fibrous nanostructures, presented in all investigated mixtures, is their bulb-like tip (indicated on the SEM images by arrows in Figs. 5(c), 6(b), 8(a) and particularly in Fig. 6(c) and (d)). These fibers0 endings suggest that the observed structures arose according to the so-called tip-growth mechanism. The characteristic lumps probably consist of the catalyst—iron particles enclosed at the tip of fiber from which the fiber growth started. SEM analysis does not allow us to determine the type of observed fibers (carbon nanotubes or nanofibers). However, taking into ac-
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Fig. 6 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-800 mixture
Fig. 7 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-800 (a) and the CaC2 /C2 Cl6 /NaN3 /0.5CX-800 mixture (b)
count results presented in literature[13,19] and the characteristic tips of the fibers, one can assume that they are nanotubes. On the other hand, it should be noted that the only way to determine if there are nanotubes or nanofibers is to perform transmission electron microscopy. Fig. 7(b) presents the morphology of combustion products derived from a mixture that contained half the amount of carbon xerogel (by mass) in comparison to the other mixtures (CaC2 /C2 Cl6 /NaN3 /0.5CX800). No significant differences in the morphology of the products were observed for either mixtures with half or double the amount of carbon xerogel catalyst. Fig. 9 shows the products recovered from the graphitic crucible after combustion of the CaC2 /C2 Cl6 /NaN3 /2CX-800 mixture, where double the amount of the xerogel catalyst system was used. The presence of colloidal xerogel particles untouched
by the grinding and the combustion processes suggests some heterogeneity of the iron distribution within the starting mixtures. Additionally, Fig. 9 implies that part of the catalyst system (Fe nanoparticles trapped within colloidal carbon xerogel) survives during the combustion process, consequently some of the iron particles could remain inactive in the catalyticassisted growth of the carbon fibrous structures. No catalytic activity of organic xerogels filled with FeCl3 (mixture No. 9, Table 1), water purified organic xerogels (mixture No. 8, Table 1), CX-600 or a carbonized sample from which FeCl3 was not removed before carbonization (mixture No. 1, Table 1) was observed. In all these cases, fibrous nanostructures were not found in the combustion products. 4. Discussion The catalytically assisted, self-propagating, high-
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Fig. 8 SEM images of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /CX-1050 mixture
Fig. 9 SEM image of products of the combustion of the CaC2 /C2 Cl6 /NaN3 /2CX-800 mixture. Intact xerogel spheres are present in the combustion products
temperature synthesis of carbon fibrous nanostructures described herein differs from the previous synthetic methods in terms of the catalyst applied. In the method presented by Szala[13] the catalyst is produced in situ during the combustion process. Iron released via ferrocene decomposition constitutes 0.6 wt% of the starting mixture[13] . In our procedure iron is introduced into the starting mixtures as pre-prepared nanoparticles supported on substrates—carbon xerogels. It constitutes about 0.26 wt% of the starting mixtures. The main advantage of the proposed method is the ability to design iron (catalyst) particle size and to some extent the chemical composition (i.e. carbide, oxide, pure metal) as well, by selecting the appropriate carbonization temperature of the xerogel. Additionally, the carbon gel provides a support (substrate) with 3D porous microstructure, which ensures a high level of catalyst dispersion. This assumption is indirectly supported by the morphology of the residues of CX-800 oxidation in air (Fig. 2(c) and (d)). As revealed by SEM analysis, part of the carbon xerogel undergoes the combustion process intact. This suggests that some portion of iron remains isolated from the gaseous products of combustion. Additionally, carbonization of iron-doped resorcinolfurfural gel causes its partial graphitization, which
starts at 800 ◦ C and becomes very effective at 1050 ◦ C (Fig. 3). For this reason some part of iron nanoparticles may become isolated by the graphene layers. Fibrous carbon structures arise from the surface on xerogels where the Fe catalyst is anchored. For their high reactivity, carbon moieties (i.e. carbenes) released during the vigorous combustion do not penetrate the interior of the xerogels0 colloids. Carbon fibrous nanostructures must have been created where the catalyst was directly exposed to the products of the combustion wave (i.e. on the support surface). The same phenomenon was observed by Steiner et al.[19] —they observed that carbon nanotubes were created strictly on the surface of iron-doped carbon aerogels and not within their volume. They observed that, even though the CVD process was performed for 10 min, (a mixture of gases was passed above the aerogel), much longer than the combustion of the examined mixtures, CNTs did not penetrate the aerogel network. As the authors stated, the limiting factor for nanotube growth within the porosity of the carbon gel is the diffusion of CVD gases rather than the activity of the catalyst particles within the gel matrix[19] . They noticed that the shallow depth of CVD growth is most likely due to the short mean free path of diffusion inherent to porous materials such as carbon gels. Furthermore, it was observed that for carbon aerogels carbonized at 600 ◦ C nanotubes were not created[19] . This observation is in agreement with the results presented herein. In the combustion products of mixtures with CX-600 no fibrous structures were found. One can conclude that a carbonization temperature of 600 ◦ C does not permit the transfer of iron precursors into their catalytically active form. The self-sustaining combustion of reductors (CaC2, NaN3 , where calcium carbide acts not only as a reductor, but also as a source of carbon) and an oxidizer (C2 Cl6 ) yields two different types of carbonaceous products, depending on the reactor zone. Sootlike carbon powder was collected from the reactor walls. Carbon nanofibers formed only in the graphite crucible where the mixture was ignited. High temperature conditions created in the crucible neighborhood (the combustion reaction zone) enhance the formation
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is crucial for the formation of carbon structures. For bigger catalyst crystallites only carbon nanocapsules can be created and the growth of nanotubes would be restricted; for smaller particles single walled nanotubes are usually formed[27,28] . 5. Conclusions
Fig. 10 Schematic of tip growth mechanisms: (a) deposit of Fe catalyst particle on support—carbon xerogel/Fe, (b) diffusion of carbon into catalyst, (c) lift-off of catalyst particle due to weak interaction with substrate, (d) diffusion of carbon on exposed end of catalyst—growth, (e) catalyst covered with carbon layers and growth arrest[25]
of active chemical vapors (containing carbon radicals and carbenes among others) and may cause the transformation of iron nanoparticles into a liquid state[13] . The vapors may be the key factor determining the creation of carbon fibrous nanostructures. Vapors which do not interact with iron active particles within the high-temperature reaction zone condense into amorphous soot-like carbon after reaching cooler areas of the reactor. SEM analysis of the products recovered from the crucible and results of the investigation concerning carbon nanotubes presented by Szala[13] and Steiner et al.[19] lead us conclude that the growth mechanism is the same in all cases, i.e. the catalytically assisted tip growth mechanism. Catalyst particles were observed on the tip—end of the nanotubes (Fig. 6(c) and (d)). The weak interaction of the catalyst particles with the support (xerogel) causes the catalyst particles to lift off with the concentric cylindrical structures formed below them[25] . As the catalyst particle moves away from the xerogel, it gets trapped within the inner core of the tube/fiber. The tip growth stops when the catalyst particle is covered with carbon layers or when the supply of feed gas is cut off, i.e. ending of the combustion. The scheme of this process is presented in Fig. 10. The type of catalyst support determines the catalyst-support interactions, and as a result the mechanism for the growth of nanotubes and nanofibers. For catalyst particles weakly bonded to the support, the tip-growth route occurs. On the contrary, if the catalyst is well bonded to the support, the root-growth mechanism occurs. Within a single process a combination of root-tip-growth mechanisms may also occur[25,26] . The fibrous structures manifest a wide range of diameters: from tens of nanometer up to 300 nm (Fig. 6(a)). The wide range of fibers diameters is likely a consequence of the size distribution of iron catalyst particles formed during the carbonization step. It should be noted that the size of the iron particles
The pursuit of new, more economic methods for the production of carbon nanofibrous structures remains a very active area of materials research. The catalytically assisted, self-propagating, hightemperature synthesis became recently one of the state-of-the-art energy-saving methods for the production of carbon nanotubes[11–13] . In recent research porous supports, especially carbon gels are considered attractive catalyst substrates for catalytic methods of synthesis of carbon nanotubes[19,20,28] . As presented, iron-doped carbon xerogels can be used as catalyst systems for the synthesis of carbon fibers via the self-sustaining combustion of mixtures of inorganic reductors and organic oxidizers. A CX as a porous, microstructured support enables a high dispersion of the catalyst[28] . Previous research presented elsewhere showed in detail that porous supports like activated carbon are very valuable substrates for synthesis of carbon fibers with varied morphology[28] . The morphology of the obtained carbon fibrous nanostructures indicates that they arise according to the catalytically assisted tipgrowth mechanism. It was observed that iron-doped CXs can be applied as active, specific catalyst systems for the synthesis of carbon fibrous structures via combustion. The fibrous structures were obtained with low yield and they manifest varied morphology. Some of them were short, thick (300 nm) and curved; yet within the same sample long (above 10 µm), thin ones were also observed. The low yield is caused by the limited availability of the iron catalyst—during the vigorous combustion some iron trapped inside CX particles may remain inaccessible for the formation of carbon elongated nanostructures. The substrate (CX) does not provide enhanced diffusion efficiency of the gases (generated via combustion synthesis) throughout the xerogel structure and hinders the formation of carbon fibrous nanostructures. As already suggested, increasing combustion temperature may improve the efficiency of this synthetic route[13] . In spite of the low yield, the presented results show that carbon nanofibrous structures can be produced through the self-sustaining combustion synthesis as first stated in the pioneering research presented by Koch[11,12] and Szala[13] and catalysts supported on carbon gels are active catalyst systems in this method. This method is characterized by simplicity and what is more important, energy efficiency. Acknowledgements This work was supported by the internal grant of the
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