Article pubs.acs.org/IECR
Simultaneous Effect of the Catalyst Precursor Concentration and the Longitudinal Position on the Growth Patterns of Multiwalled Carbon Nanotubes Morteza Maghrebi,*,† Kourosh Esfandiari,† Ali Sane,‡ Abbasali Khodadadi,‡ and Yadollah Mortazavi‡ †
Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, POB 91775-1111, Mashhad, Iran Catalysis & Nanostructured Lab, School of Chemical Engineering, University of Tehran, Tehran, Iran
‡
ABSTRACT: The effect of concentration of the catalyst precursor on the longitudinal profile of multiwalled carbon nanotube (MWCNT) arrays, synthesized by using xylene and ferrocene in a floating catalyst reactor, is reported. Optical microscopy, Raman spectroscopy, high resolution transmission electron microscopy, and field emission scanning electron microscopy were employed for point-to-point analyses of the grown MWCNTs in the so-called “growth window” (GW). It was found that at lower catalyst concentrations longer MWCNT arrays are obtained. Furthermore, higher catalyst concentrations resulted in MWCNTs with a lower diameter at the beginning and the end of the GW; the reverse order was seen in the middle. These trends are attributed to the relative saturation of catalyst nanoparticles, increased diameter of the catalyst nanoparticles, and competition between catalytic and pyrolytic reactions. It was also found that the diameter of MWCNTs is influenced by both catalyst concentration and longitudinal position inside the GW. Therefore, for better understanding of these simultaneous effects, a more careful comparison is suggested in the study of MWCNT synthesis by floating catalyst reactors.
1. INTRODUCTION Multiwalled carbon nanotubes (MWCNTs) with unique specifications are considered to be utilized in various applications such as highly efficient catalysts,1 super capacitors,2 and fuel cells.3 MWCNTs are usually synthesized by different versions of chemical vapor deposition including a floating catalyst, which has attracted much attention during recent years, because of the one-pot process and acceptable purity of the products.4 In this method, an organometallic, a hydrocarbon, and a carrier gas are simultaneously introduced into a reactor to form metal nanoparticles, which then catalyze the synthesis process on a substrate or just the inner wall of the reactor. Despite the simple process of the floating catalyst method, it is highly sensitive to different operating parameters such as carbon source composition,5 catalyst precursor concentration,6 carrier gas percentage,7 reaction time,8 and temperature variation.9 These dependencies provide several unpredictable situations in which just changing one parameter could lead to quite new results. Although a vast number of studies, concerning the influence of the above-mentioned parameters, have been reported, no comprehensive investigation on the effect of catalyst concentration on specification of the synthesized MWCNTs has been performed. Furthermore, the reported results of these studies are inconsistent in terms of quality and quantity of the grown MWCNTs. For instance, Chai et al.10 found that increasing the catalyst weight leads to lower CNT production. In contrast, Castro et al.11 observed a significant increase in the weight of CNTs by increasing the catalyst content in the feed solution. On the other hand, regarding the CNT diameters, Bai et al.12 and also Castro et al.11 revealed a reverse relationship between the catalyst concentration and the CNT diameters, contrary to results reported by Glerup et al.13 One reason for these © 2014 American Chemical Society
inconsistencies may be due to the fact that representative CNT samples were normally collected from a specific point of the reactor. Jeong et al.14 were the first who reported qualitative and quantitative variation of grown CNT arrays along the reactor using 10% H2/Ar as a carrier gas. We also reported a similar longitudinal profile or the so-called “growth window” (GW), which may even shift along the reactor.15 Zeng et al.16 showed that the deposition rate of CNTs has an increasing− decreasing trend with a local maximum which is influenced by the operating conditions along the carrier gas flow direction. He et al.17 also investigated the effect of axial direction with a temperature profile on diameter, density, and the growth rate of CNT/Al2O3 hybrids. They demonstrated that the optimum Al2O3 position inside the reactor and the local temperature could alter the decomposition rate of the xylene precursor. It is worth mentioning that different research groups propose different mechanisms for their observations. Thus, there is a need for a systematic investigation to resolve the inconstancies and contradictions between the results reported in the literature. In this investigation, we studied the effect of catalyst precursor concentration on the morphology and amorphous content of the MWCNT arrays using a point-to-point analysis along the GW. The obtained results are discussed in order to shed some light on the progress of MWCNT growth. Received: Revised: Accepted: Published: 1293
October 27, 2013 December 21, 2013 December 22, 2013 January 3, 2014 dx.doi.org/10.1021/ie403624b | Ind. Eng. Chem. Res. 2014, 53, 1293−1300
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Figure 1. The schematic of the experimental setup.
2. EXPERIMENTAL SECTION
3. RESULT AND DISCUSSION 3.1. Height Profile of the MWCNT Arrays. Figure 2 illustrates the height variation of MWCNT arrays for different concentrations of the catalyst precursor (ferrocene) as well as the temperature profile along the reactor. The GW contains a 29-cm-long region inside the reactor considering MWCNT arrays longer than about 0.5 mm. For all samples, the beginning and the end of this region are in the vicinity of 620 and 840 °C, respectively.
The reactor used in this study was a 21.7 mm i.d. and 100-cmlong quartz tube which was located in two thermal zones, i.e., a preheating/evaporating zone and a main furnace. The liquid feed was a solution of 1.0, 4.0, and 6.0 wt % of ferrocene in xylene. A syringe pump was used to supply 10 mL of this mixture (0.2 mL/min) into the first zone of the reactor, which was heated up to 300 °C in order to vaporize the solution. Subsequently, the ferrocene−xylene vapor was swept into the main furnace, maintained at 850 °C. A mass flow controller was used to introduce argon (900 sccm) for carrying the vaporized solution along the reactor. Figure 1 depicts a schematic of the used MWCNT growth apparatus. Following the injection of the feed mixture, the MWCNTs began to grow on the substrate (4.0 mm o.d. and 50.0-cm-long quartz rod) as well as the inner wall of the tube. After 50 min of CNT growth, the reactor was cooled gradually to ambient temperature under an argon atmosphere. The height of MWCNT arrays was highest at the topmost position of the rod and vanished almost monotonically toward the adjacent reactor inner wall. In order to be precise with regard to the characterization of the grown MWCNTs in a specific longitudinal position of the reactor, we decided to collect samples from the above-mentioned quartz rod. The packed forest-like growth of MWCNTs dictates MWCNTs to increase volume in one direction, i.e., their height. However, to have a more comprehensive conclusion, we also included measuring the MWCNT diameters. The grown MWCNT samples were collected from different parts of the quartz rod and characterized with optical microscopy, micro-Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscopy (FESEM). An optical microscope (Olympus BX 51) was used to measure the height of MWCNT arrays along the GW. A microRaman spectroscope (WITec TS-150) was operated with a 488 nm laser of 1.2 μm spot size with an acquisition time of 30.0 s and a laser power of 3.0 mW. In Raman spectroscopy and FESEM (Jeol JSM 6340F and HITACHI S4160, respectively) analyses, the middle height of the MWCNTs was considered to avoid imperfect structures in the top/bottom of the MWCNT arrays. For HRTEM (JEM-2010F), a sample of MWCNT arrays was sonicated for 90 min. The suspension was then applied on a carbon-coated copper grid.
Figure 2. The height variation of MWCNT arrays and the temperature profile along the GW.
As can be seen, a general longitudinal ascending−descending height profile for the MWCNT arrays could be observed. Such a trend resembles findings by Li et al.,18 despite their having used quite a different carbon source and reaction temperatures. The height of all MWCNT arrays increases up to about 790 °C, followed by a second peak around 845 °C. The first ascending trend may imply the increasing rate of the catalytic reactions, as a result of increased temperature and residence time along the GW.15,19 In the case of the second peak, as we discussed before,20 it may be due to the catalytic reactions of xylenedecomposition byproducts such as acetylene and ethylene. The rapid height reduction of the MWCNT arrays at the end of GW may be related to the catalyst deactivation resulting from the deposition of a large amount of amorphous carbon (a-C).21 As is shown, by increasing the catalyst concentration, the height of the arrays is shortened almost to half; i.e., by 1294
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Figure 3. (a) Typical Raman spectra of MWCNT arrays at different longitudinal positions of 4 wt % sample. (b) The variation of ID/IG ratios of all samples along the GW.
Figure 4. High-resolution TEM micrographs of MWCNTs grown in the middle of the GW: (a) 1 wt % and (a, inset) interlayer distance of MWCNTs grown with 1 wt %, (b) 4 wt % and (b inset) a contraction inside the MWCNT structures grown with 4 wt %, and (c,d) 6 wt % and (d inset) polycrystalline structure of catalyst nanoparticles grown with 6 wt % samples.
GW compared to the samples synthesized with 1 and 4 wt % catalyst concentrations. 3.2. Variation of Amorphous/Defected Species. Raman spectra for 4 wt % samples collected from the beginning, middle, and end of the GW are presented in Figure 3a. They all show D and G bands at 1346 and 1573 cm−1, representative of amorphous/defected and graphitic carbons, respectively.10,22 The intensity ratios of the D band relative to the G band (ID/ IG) are shown in Figure 3b. As is seen, the ID/IG ratio increases
increasing the catalyst concentration from 1 to 6 wt %, the maximum height of the arrays decreased from 1.1 to 0.57 mm, respectively. Similarly, Chai et al.10 reported a decrease in the height of MWCNTs with increasing the catalyst weight. They also observed that a higher catalyst concentration was responsible for the higher catalyst lifetime. Interestingly, Figure 2 shows a slow drop in the height of arrays at the end of the GW for the 6 wt % sample. Such a slight drop could be translated as relatively higher catalyst activity at the end of the 1295
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Figure 5. Typical FESEM images of the MWCNT arrays at different longitudinal positions for 1 wt % (a, d, g), 4 wt % (b, e, h), and 6 wt % (c, f and f inset, i) samples.
such variations to a difference in CNT curvature and atomic vibrations, which led to different intertube interactions among the graphitic walls. Therefore, to reach a conclusive explanation for this behavior, further investigations are needed. Another observation is the growth of a bamboo-like structure on one side of the catalyst nanoparticle encapsulated inside the MWCNT structure (Figure 4c). Regarding the absence of these structures in the MWCNT samples with catalyst concentrations lower than 6 wt %, one may find a direct relation between the catalyst concentration and the formation of bamboo-like structures. Similarly, Yadav et al.25 showed that the formation of this structure is influenced by ferrocene concentration. Son et al.26 found an inverse relation between MWCNT growth rate and the formation of bamboo-like MWCNTs. This finding is in agreement with our observation in which the catalyst-reach sample, i.e., the sample with 6 wt % catalyst concentration, in contrast to others, resulted in the formation of bamboo-like structures (Figure 4c) albeit with quite a low height of the MWCNT array (Figure 2). In the samples synthesized with 4 wt % (Figure 4b inset) and 6 wt % (Figure 4c) of the catalyst concentration, one can see a contraction inside the MWCNT structures. This variation in internal diameter may be due to either elongation of the encapsulated catalyst27 or compressive forces of outer graphitic layers on the inner ones.28 Figure 4c,d depict the catalyst crystalline structure inside the MWCNT for 6 wt % sample. As can be seen, the catalyst nanoparticles were polycrystalline (Figure 4d inset), and the lattice fringes make an angle of about 45° with the graphitic sheets (Figure 4c), similar to the observations made by Golberg et al.29 3.4. Morphology Variation along the GW. The SEM images of the grown MWCNTs for different catalyst concentrations and the average diameter of the MWCNT arrays along the GW are illustrated in Figures 5a−i and 6, respectively. It is clearly seen that the CNT diameters are in the range of 70−150 nm (Figure 6). Such thick diameters were
toward the end of the GW for all samples, indicating an increase of a-C deposition along the reactor. Despite the observed trend (Figure 3b), the slight change of the ID/IG ratios at the end of the GW may be attributed to a higher local temperature in this region. In other words, while increasing the local temperature at the end of the GW is responsible for the deposition of a-C, it also results in higher crystallinity in the deposited a-C. This could be translated as a slower increase in ID/IG toward the end of the GW. Similarly, Segupta et al.22 observed a decrease in defected and an increase in crystallinity of the as-grown MWCNTs by increasing the synthesis temperature. From the viewpoint of catalyst concentration, one could generally see that the higher the catalyst concentration, the greater the ID/IG ratio (Figure 2b). This trend is more evident at the beginning of the GW. Such an increased ID/IG ratio at this position of the GW may be mainly attributed to the reduction of graphitic content (i.e., graphitic layers) of MWCNT arrays, and not to a relative increase of a-C. The aC deposition is known to be developed by multistep pyrolytic reactions which are normally provided at higher local temperatures and residence times.21 The reduction of graphitic content of CNT arrays at the beginning of the GW will be thoroughly discussed in section 3.4. 3.3. Internal Structure of Grown MWCNTs. Figure 4a−d illustrate high resolution TEM micrographs for all three samples in the middle of the GW. Obviously, all cylindrical nanostructures observed in TEM micrographs show a characteristic “hollow cylinder” which is well-known for nanotube moieties. Interestingly, it can be seen that the number of tubular graphitic walls increases with increasing the ferrocene content, similar to the results reported by Xu et al.23 Furthermore, the interlayer distance of the produced MWCNTs increases monotonically from ∼3.44 Å in 1 wt % to ∼3.63 Å in the 6 wt % sample (Figure 4a−c). It should be mentioned that there are relatively rare studies around variation of the interlayer distance of MWCNTs. Singh et al.24 attributed 1296
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MWCNT diameter in very low concentrations, and an inverse order in high catalyst concentrations. In striking contrast, Singh et al.34 reported an opposite order. They observed an ascending trend of the mean diameter by increasing the concentration of the catalyst precursor in concentrations as high as about 9 wt %. Our findings suggest that such discrepancies in the effect of catalyst concentration may not be merely attributed to the range of employed concentrations but also to the longitudinal position of the MWCNT arrays along the GW. For a better understanding of the simultaneous effects of the catalyst concentration and the position on the MWCNT diameters, one should consider that the overall diameter of MWCNTs is comprised of inner graphitic and outer amorphous layers. These layers are known to be produced by the catalytic and pyrolytic reactions, respectively. In the presence of a catalyst, xylene (C8H10) could be consumed as a hydrocarbon precursor in the catalytic process of the MWCNT growth. On the other hand, as the reactor local temperature increases, xylene molecules are “decomposed” to form C2 byproducts such as acetylene and ethylene. These species could be later consumed in two separate pathways, i.e., catalytic and pyrolytic reactions. The catalytic reactions lead to more crystalline MWCNT layers, whereas the pyrolytic reactions in competition with the catalytic ones could be responsible for a-C deposition on the surface of the grown MWCNT arrays. The rates of these competitive reactions could determine the overall MWCNT diameter and ID/IG ratio in different locations along the GW. Since the MWCNT growth stops immediately after discontinuing the catalyst precursor entering the reactor and resumes just after re-entering the catalyst precursor, one can conclude that the catalyst precursor is continuously consumed by the catalytic process related to the growth of newly evolved MWCNTs. In other words, if the catalyst precursor does not participate in the catalytic process for MWCNT growth, the contribution of the pyrolytic reactions becomes dominant. In this part, we separately investigate the effect of the catalyst concentration on the overall MWCNT diameters (observed by SEM) and ID/IG ratio (calculated from the Raman spectra) along the GW (Table 1).
Figure 6. The average diameter of MWCNT arrays along the GW. The error bars represent standard deviation of measured diameters of MWCNT arrays.
already reported by Liu et al.,6 who investigated the effect of ferrocene/xylene feed ratio and the flow rate of xylene solution on the morphology of grown CNTs. They reported CNTs with very high purity and a diameter range of 40−70 nm. Although, the data reported by Liu et al. are similar to what we observed at the beginning of the GW, as could be seen from Figure 5, the CNT diameters in the middle and the end of the GW are thicker than what was reported by Liu. We believe that this finding might be due to the position inside the reactor from which the samples were collected. As can be seen, the mean diameter of MWCNTs increases along the GW. These observations confirm Raman results (Figure 3b), regarding the increase of the a-C deposition along the GW. Nevertheless, different trends are observed for variation of the MWCNT diameters with an increase in the catalyst concentration. At the beginning of the GW, the MWCNT diameters decrease monotonically by increasing the catalyst concentration (Figure 5a−c). This observation confirms the Raman results about the decrease of the graphitic content of MWCNTs at the beginning of the GW (Figure 3b). In an inverse order, the MWCNT diameter has a direct relationship with the catalyst concentration in the middle of the GW (Figure 5d−f), in agreement with the TEM results (Figure 4d− f). At the end of the GW, the MWCNT diameters decrease with the increase of catalyst concentration similar to the beginning of the GW. One should consider that apparent diameters of the MWCNTs originate from amorphous as well as crystalline layers, so the above trends should be viewed as a competition between catalytic and pyrolytic reactions, as discussed below. 3.5. Discussion around the Effect of the Catalyst Concentration along the GW. Actually, there has been a debate about the influence of the catalyst concentration on the MWCNT diameters. Bai et al.12 observed a drop in the MWCNT diameters by an increase in the catalyst concentration. In contrast, Lupo et al.30 and also Sinnott et al.31 suggested a strong direct relationship between these two parameters. Furthermore, there are even some reports about weak dependency of the mean diameter on the catalyst concentration.32 McKee et al.33 tried to reconcile such discrepancies based on different ranges of the catalyst concentration each group used in their experiments. Accordingly, they suggested a direct relationship between the catalyst concentration and the
Table 1. Effect of Increasing the Catalyst Concentration along the GW
rate of catalytic reactions (mainly responsible for growth of graphitic layers) rate of pyrolytic reactions (mainly responsible for growth of amorphous layers) overall diameter of CNTs (sum of graphitic and amorphous layers)
beginning of GW
middle of GW
decrease
increase
later deactivation
constant
slight decrease
alleviating
decrease
increase
end of GW
slight decrease
At the beginning of the GW, the overall diameter of MWCNTs shows a decrease upon an increase in the catalyst concentration (Figure 5a−c). Similarly, Bai et al.12 and McKee et al.33 reported the same trend for the MWCNT diameters. While Bai et al. attributed this finding to carbon deficiency, McKee et al. rejected this idea by observing the same trend in the excess carbon condition. They believed that in the high catalyst concentration, the catalyst nanoparticles enlarged to some extent so that multiple small MWCNTs grew from each 1297
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Accordingly, the effect of catalyst concentration should be discussed in light of later deactivation of the catalytic reactions. Chai et al.10 showed that increasing the catalyst concentration delays deactivation of the catalyst. The later catalyst deactivation for higher catalyst concentrations may also be supported by relatively slower GW termination for the 6 wt % sample (Figure 2). This observation indicates the relatively slower pyrolytic reactions, due to the consumption of C2 byproducts mainly by catalytic reactions. Such low pyrolytic formation of an a-C deposit may explain the smaller overall diameter of MWCNTs (Figure 5g−i) at the end of the GW. Finally, as stated before, the sensitive process of the floating catalyst necessitates more detailed studies to have a thorough understanding of the simultaneous effect of catalyst concentration and longitudinal position along the reactor on the mechanism of MWCNT growth.
nanoparticle. However, McKee et al.’s assumption may not be applicable here, because the diameter of the catalyst nanoparticles is generally small at the beginning of the GW.35 We believe that this trend may be ascribed to the hindered saturation of the catalyst nanoparticles with the carbon species, i.e., the higher the catalyst concentration, the later the saturation of catalyst with carbon species and so the later the nucleation of MWCNT growth.36 A decrease of the MWCNT diameter (Figure 4a−c) and length (Figure 2) at the beginning of the GW by increasing the catalyst concentration may point to such weakened catalytic activity. Weakening of the catalytic growth of MWCNTs upon the addition of the catalyst concentration could also justify the corresponding increase of ID/IG ratio (Figure 3b). As discussed before, the local temperature in this region is not so high for significant development of pyrolytic reactions responsible for deposition of a-C on the MWCNT arrays. So, the observed decrease of the MWCNT diameters (Figure 5a− c) may be mainly attributed to the decrease of the graphitic content of the MWCNT arrays. In other words, the higher catalyst concentration leads to less development and so slower catalytic reactions at the beginning of the GW, without a noticeable change in the rate of pyrolytic reactions. In the middle of the GW, one can see a direct relationship between the catalyst concentration and the MWCNT diameters (Figures 4a−c and 5d−f). Castro et al.11 suggested that as the catalyst concentration increases, the catalyst nanoparticles become larger. Such large catalyst nanoparticles could lead to larger MWCNT diameters.37 Furthermore, a higher local temperature may enhance the catalytic growth of the MWCNTs in the middle of the GW.35 Hence, the increased catalyst concentration in the middle of the GW could result in the higher rate of catalytic reactions, distinguished in the form of larger diameter (Figures 4a−c and 5d−f) and height of MWCNTs (Figure 2). Moreover, a higher rate of catalytic reaction may also lead to higher relative consumption of byproducts of the xylene decomposition such as acetylene and ethylene. As we suggested before,20 these C2 byproducts are also key intermediates in the polymerization of the pyrolytic products and a-C deposits. Therefore, their competitive consumption in the catalytic reactions could lead to slower development of pyrolytic reactions and so lower deposition of a-C deposits. Consequently, higher catalyst concentration could lead to relatively lower deposition of a-C on MWCNTs. As could be seen in the Raman spectra (Figure 3b), the increased catalyst concentration could decelerate the steep increase of ID/IG ratio at the middle of the GW. Unexpectedly, at the end of the GW, the MWCNT diameters decreased slightly with the increase of catalyst concentration (Figures 5g−i). Before discussing the catalyst effect, one should realize the dependence of contribution of catalytic and pyrolytic reactions on the position within the GW. Unlike the middle of the GW, the high local temperature at this position does not lead mainly to the catalytic growth but rather to the higher rate of pyrolytic side reactions.21 The latter are most frequently reported as a reason for the deactivation of catalytic growth of MWCNTs and eventually termination of the GW.38,39 The catalyst deactivation might be responsible for larger CNT diameter and lower array height (limited aspect ratio) at the end of the GW as suggested by Hart and Slocum.40 This is also in agreement with Figure 2 where the CNT arrays get shorter at the end of the GW.
4. CONCLUSION Carbon nanotube (MWCNT) arrays were grown in a floating catalyst reactor in a temperature range of 620−840 °C, using different feed concentrations of ferrocene in a xylene mixture. Point-to-point variations of quantity and quality of the MWCNT arrays were monitored along the GW, using optical microscopy, Raman spectroscopy, HRTEM, and FESEM. It was revealed that both catalyst concentration and longitudinal position affect the growth of MWCNT arrays. The higher the catalyst concentration, the lower the height of MWCNT arrays, but the wider the GW. In addition, by increasing the catalyst concentration, the overall diameter of MWCNT arrays which comprises both crystalline and amorphous layers, increased in the middle but decreased at the beginning and the end of GW. These observations may be attributed to a slower saturation of catalyst nanoparticles, an increased diameter of the catalyst nanoparticles, and a larger contribution of catalytic reactions compared to the pyrolytic ones. These findings may help to better understand the mechanism of MWCNT growth as well as optimize the operating parameters of the reactors for the production of MWCNTs. It is also suggested to be cautious in interpreting the effect of the catalyst concentration, especially by considering the longitudinal position of the samples inside the reactor.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Iran Nanotechnology Initiative Council and also Research Deputy of Ferdowsi University of Mashhad for supporting this project with grant no. 22788-20/04/1391. Also, M. Maghrebi would like to thank Stevin Snellius Pramana for TEM assistance.
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