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Effect of sulfur on the growth of carbon nanotubes by detonation-assisted chemical vapor deposition
Applied Surface Science 257 (2010) 932–936
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of sulfur on the growth of carbon nanotubes by detonation-assisted chemical vapor deposition Can Wang, Liang Zhan ∗ , Yan-li Wang, Wen-Ming Qiao, Xiaoyi Liang, Li-Cheng Ling State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 5 February 2010 Received in revised form 17 June 2010 Accepted 27 July 2010 Available online 4 August 2010 Keywords: Carbon nanotubes Sulfur Dynamic reshaping Bamboo knots
a b s t r a c t Thiophene was introduced as an additive in detonation-assisted chemical vapor deposition to investigate the effect of sulfur on the growth of carbon nanotubes. The results reveal that sulfur promoted the growth of hollow tubes, instead of bamboo-like carbon nanotubes without sulfur addition. Structural characterization of products indicates that the dynamic reshaping of the catalyst assisted bamboo-like carbon nanotube growth and the bamboo knots preferentially nucleated on the Ni–graphite step edges. It is suggested that sulfur suppressed the bamboo knot growth through blocking the step sites. The findings are important for understanding of nanotube growth mechanism and the role of sulfur often involved in catalytic reactions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The extraordinary thermal, electronic and mechanical properties of carbon nanotubes (CNTs) make them promising candidates for nanoelectronic devices, energy storage media and composites [1]. Recently, the quality of catalytically grown CNTs has improved steadily and large quantities of fairly clean CNTs can be produced [2], but there is a lack of definitive model for the growth of CNTs and structural control for CNTs. Existing growth mechanisms for CNTs are common to the vapour–liquid–solid (VLS) model [3], which assumes a fixed number of stationary active sites for solid carbon deposition. Recently, in situ observation of the CNT growth process reveals that the nucleation and growth of graphene layer are assisted by a dynamic formation and restructuring of step edges at the catalyst surface, i.e., spatiotemporal dynamic growth [4–7]. The finding that metallic step edges act as spatiotemporal dynamic growth sites may give great impact on the conventional scenario. In chemical vapor deposition (CVD) of CNTs, sulfur-containing compounds (such as thiophene and sulfurated hydrogen) have frequently been used as an additive to improve the yield of CNTs [8,9], control the diameter and shell number of CNTs [10–12], or synthesize Y-junction CNTs [13,14]. Sulfur is proposed to exert its
∗ Corresponding author at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 519 #, 130 Meilong Road, Shanghai 200237, PR China. Tel.: +86 21 64252924; fax: +86 21 64252914. E-mail address: [email protected] (L. Zhan). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.07.094
influence by (i) blocking active sites on the catalyst [15], (ii) lowering the melting point of the catalyst [10–12,16], or (iii) interacting with the growing tube [17]. However, there is no consensus yet on the mechanism of the effects of sulfur on CNT formation. Zhu et al. synthesized CNTs by detonation-assisted CVD [18–20], in which the energy required for the assembly of carbon species to nanotubes was derived from the detonation of explosives on a micro-second time scale. Some intermediate objects would survive within the fast thermal annealing, which enabled us to trace the CNT formation. In this paper, we present the synthesis of CNTs by detonation-assisted CVD and investigate the effect of sulfur on the morphology and microstructure of CNTs. It is found that CNT growth follows the dynamic growth model and the role of sulfur in promotion of hollow tube growth was discussed. 2. Experimental details All of the chemical reagents were analytical grade including picric acid (PA), liquid paraffin, nickel acetate (Ni(AC)2 ) and thiophene. Thiophene was dissolved in paraffin and sonicated to form a homogeneous solution. Before the detonation experiments, the starting materials (2 g PA, 0.1 g Ni(AC)2 , 0.5 g paraffin) were premixed physically. The detonation-assisted CVD was performed in a sealed stainless steel pressure vessel (10 ml) connected with a pressure gauge, and induced by external heating (20 ◦ C/min) to 310 ◦ C. When the detonation occurs, about 40 MPa pressure (shock wave) and 900 ◦ C temperature are produced inside the vessel. After the detonation, the vessel was cooled in air and emptied of gaseous products, and then the solid products were collected.
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Table 1 The effect of sulfur addition amounts on the structures of CNTs. Exp. no.
Sulfur/nickel (molar ratio)
Structure feature of the producta
1 2 3 4
0 1/8 1/4 1/2
BCNTs BCNTs + hollow tubes 50% hollow tubes + nanoparticles 30% hollow tubes + nanoparticles
a
Estimated from TEM observations.
Scanning electron microscopy (SEM) of the as-synthesized samples was examined using a FEI Quanta 200FEG operated at 5 kV. Transmission electron microscopy (TEM) was carried out on a JEOL 2010F operated at 200 kV, equipped with an energy dispersive Xray spectrometer (EDS). Powder X-ray diffraction (XRD) patterns were performed on a Rigaku D/max 2500x X-ray diffractometer using Cu K␣ radiation.
Fig. 2. XRD patterns of the products obtained with the molar ratio of sulfur/nickel = 0 (Exp. 1) and 1/4 (Exp. 3).
3. Results and discussion 3.1. Morphology and structure of CNTs Table 1 presents the corresponding structures of the products obtained with different sulfur addition amounts. If there is no addition of sulfur (Exp. 1), most of the product was bamboo-like carbon nanotubes (BCNTs), and few amorphous carbon and catalyst particles encapsulated by graphite layers, as shown in Fig. 1a and c. SEM image shows that bamboo-like tubes were up to tens of microns long with catalyst particles at the ends. TEM observations reveal that the hollow compartments of BCNTs were normally spaced by curved knots made from few graphene sheets.
In a certain sulfur addition range (Exps. 2–4), CNTs with hollow channels or few bamboo knots were obtained, whose yield was reduced with the sulfur addition amounts increasing. In Exp. 3, CNTs were mostly conventional tubes with hollow cores (Fig. 1d). Unlike the entangled bamboo tubes, hollow tubes were relatively isolated and straight. These tubes were just a few micrometers, and many of them contained open ends, as shown in Fig. 1b. According to above experimental results, it is concluded that sulfur plays an important role in the structure transformation of CNTs from bamboo to hollow channel.
Fig. 1. SEM and TEM images of CNTs obtained with different sulfur addition amounts: (a) and (c) molar ratio of sulfur/nickel = 0 (Exp. 1); (b) and (d) molar ratio of sulfur/nickel = 1/4 (Exp. 3).
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Fig. 3. (a and b) Typical TEM images of BCNTs obtained without sulfur addition. (c) HRTEM image and (d) schematic model of the bamboo knot marked by the rectangle in (b). White arrows indicate where knot is formed. (e) TEM image of an incompletely contracted catalyst particle. (f and g) TEM and HRTEM images of a bamboo knot within nucleation period.
3.2. Structures of the catalyst particles Fig. 2 shows the XRD patterns of two typical samples obtained with different sulfur addition amounts. Two peaks at 44.5◦ and 51.8◦ were indexed to the diffraction lines from the (1 1 1) and (2 0 0) planes of face-centered cubic (fcc) nickel, respectively. Signals for nickel sulfides, carbides or oxides were not detected in the samples. Moreover, the crystalline structure of catalyst for the growth of bamboo-shaped tubes was the same as that for the growth of hollow tubes, which indicates that the structure transformation of CNTs induced by sulfur is independent on the catalyst crystalline structure. Therefore, the effect of sulfur may be estab-
lished by the interaction between the adsorbed sulfur and metal surface. 3.3. The role of sulfur TEM and high resolution TEM (HRTEM) were used to analyze the growth mechanism of BCNTs and clarify the role of sulfur. Fig. 3a and b shows the typical TEM images of BCNTs. It is found that the core of BCNT was divided into compartments with different dimensions and shapes by bamboo knots. And the morphologies of the adjective bamboo knots were distinctive: flat, conical, complete and incomplete, especially subdomes contained in the main
C. Wang et al. / Applied Surface Science 257 (2010) 932–936
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Fig. 4. (a and b) TEM images of hollow tubes obtained from Exp 3. (c) HRTEM image of a section of tube wall marked by the rectangle in (a). (d) TEM image of a spherical catalyst located at the end. (e) EDS spectrum of the region marked in (d) indicates the presence of sulfur on the catalyst surface. (f) Scheme of the mechanism of hollow tube growth with sulfur addition. The dashed indicates the blocking sulfur.
compartments. From the growth perspective, the compartments in bamboo tubes could be visualized as the trace of the catalyst particle moving along the tube length. Therefore, the distinctive compartments and bamboo knots suggest that the catalyst particle was dynamic reshaping, i.e., elongation and contraction, during the CNT growth. HRTEM image of the bamboo knot (Fig. 3c) shows that the outmost graphene sheets were structured continuously along the tube length, and there was a step between the outer wall and the knot (as indicated by white arrows). The structure model of the bamboo knot was illustrated in Fig. 3d. Combined with the finding of incomplete bamboo knot (shown in Fig. 3b), the bamboo knot was suggested to nucleate and grow on the multistep Ni–graphite edges, rather than precipitate on the top surface of the catalyst. Previous density functional theory (DFT) calculations also show that, the Ni–graphite step edge acted as nucleation site is energetically preferred because it is stabilized by the interaction with both the external graphene wall and the Ni catalyst particle surface [4,21]. Detailed observation of the as-product could find some intermediate objects of BCNT growth. Fig. 3e shows an elongated catalyst particle which started to contract but was encapsulated by the graphene layer, suggesting a tip-growth model. The compartment between its asymmetric growth front and the bamboo knot indicates the fluctuation of the catalyst during growth. Fig. 3f and g shows a bamboo knot within nucleation period. It is obvious that graphene sheets (bamboo knot) terminate at a stepped Ni surface, demonstrated the above interpretation. Therefore, we can conclude that the dynamic reshaping of the catalyst particle assists the tube growth, and graphene sheets preferentially nucleate on the mul-
tistep Ni–graphite edges. This result is consistent with the in situ HRTEM observations [4–7]. With the addition of sulfur, carbon nanotubes were produced with pure tube-like carbon (showing in Fig. 4a) or few bamboo knots (showing in Fig. 4b). We find some closed ends with spherical catalysts in the products, indicating a tip-growth mode for hollow tubes. Fig. 4c shows the HRTEM image of a section of tube wall marked in Fig. 4a. It is found that the outmost graphene sheets were aligned parallel to the tube axis, similar to that of BCNTs. Combined with occasionally existed bamboo knot within the tube, the structure similarity suggests that the formation mechanism of hollow tubes is closely correlative with that of BCNTs: the catalyst reshaping assists the tube growth. The role of sulfur in the structure transformation seems to suppress the bamboo knot growth. Previous researches indicated that CNT growth was influenced by the carbon supply rate for the catalyst [22], or the carbon diffusion rate on/through the catalyst [23]. In detonation-assisted CVD, Cn species for building nanotubes are available without metal catalysis [24,25]. In other words, the effect of sulfur on the carbon supply is eliminated. According to XRD results, metal sulfides were not detected in bulk crystallinity. Fig. 4e shows the EDS spectroscopy of the interface region between catalyst and out wall, and small amount of sulfur was indeed detected. Thus, it can be concluded that sulfur adsorbed on and interacted with the metal surface. Previous studies in the steam reforming process on Ni catalyst show that, the nucleation of graphite is initiated at the step sites, and sulfur promoter preferentially binds to Ni step edges to suppress graphite formation [21,26]. In our system, we suggest that sulfur inhibits the formation of bamboo knot through a step-blocking mechanism. Sulfur atoms were adsorbed on the catalyst surface
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and diffuse through the Ni–graphene interface to bind at the multistep edges. Since sulfur showed a strong affinity with the metal step, the nucleation site for bamboo knot was blocked, and hollow tubes were synthesized. Fig. 4f shows the scheme of the mechanism of hollow tube growth with sulfur addition. Another evidence for such an explanation is that: there was no apparent graphene lattice found on either the growth front or the free facet of the catalyst particle (as shown in Fig. 4e). In addition, it should be noted that the yield of hollow tubes was decreased with the addition amount of sulfur increasing, and its length was shorter in comparison to that of BCNTs. And much more open ends were observed in the SEM and TEM images of the product obtained with sulfur addition. Adsorbed sulfur in general can act as “surfactant”, minimizing surface energies and dewetting catalyst [27]. Such an effect makes the Ni particle be easily ejected from the tube channel when the Ni particle contracts, resulting in open ends and growth termination. The quicker the growth termination, the shorter the tube length and the lower the tube yield. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Carbon nanotubes with hollow channels were synthesized instead of BCNTs due to the sulfur addition. According to the structural characterization and dynamic analysis, it is found that the catalyst was continuously reshaping during CNT growth, and bamboo knot preferentially nucleated at the Ni–graphite step edges. The role of sulfur is suggested to prevent bamboo knot nucleation and growth by blocking the step sites. The findings are important for understanding of nanotube growth mechanism and the role of sulfur often involved in catalytic reactions. Acknowledgements This work is supported by the National Natural Science Foundation of China (NO. 50730003, 50672025 and 20806024) and the Research Fund of China 863 (No. 2008AA062302) and Shanghai International Cooperation Project (No. 08160704000).
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of sulfur on the growth of carbon nanotubes by detonation-assisted chemical vapor deposition Can Wang, Liang Zhan ∗ , Yan-li Wang, Wen-Ming Qiao, Xiaoyi Liang, Li-Cheng Ling State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 5 February 2010 Received in revised form 17 June 2010 Accepted 27 July 2010 Available online 4 August 2010 Keywords: Carbon nanotubes Sulfur Dynamic reshaping Bamboo knots
a b s t r a c t Thiophene was introduced as an additive in detonation-assisted chemical vapor deposition to investigate the effect of sulfur on the growth of carbon nanotubes. The results reveal that sulfur promoted the growth of hollow tubes, instead of bamboo-like carbon nanotubes without sulfur addition. Structural characterization of products indicates that the dynamic reshaping of the catalyst assisted bamboo-like carbon nanotube growth and the bamboo knots preferentially nucleated on the Ni–graphite step edges. It is suggested that sulfur suppressed the bamboo knot growth through blocking the step sites. The findings are important for understanding of nanotube growth mechanism and the role of sulfur often involved in catalytic reactions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The extraordinary thermal, electronic and mechanical properties of carbon nanotubes (CNTs) make them promising candidates for nanoelectronic devices, energy storage media and composites [1]. Recently, the quality of catalytically grown CNTs has improved steadily and large quantities of fairly clean CNTs can be produced [2], but there is a lack of definitive model for the growth of CNTs and structural control for CNTs. Existing growth mechanisms for CNTs are common to the vapour–liquid–solid (VLS) model [3], which assumes a fixed number of stationary active sites for solid carbon deposition. Recently, in situ observation of the CNT growth process reveals that the nucleation and growth of graphene layer are assisted by a dynamic formation and restructuring of step edges at the catalyst surface, i.e., spatiotemporal dynamic growth [4–7]. The finding that metallic step edges act as spatiotemporal dynamic growth sites may give great impact on the conventional scenario. In chemical vapor deposition (CVD) of CNTs, sulfur-containing compounds (such as thiophene and sulfurated hydrogen) have frequently been used as an additive to improve the yield of CNTs [8,9], control the diameter and shell number of CNTs [10–12], or synthesize Y-junction CNTs [13,14]. Sulfur is proposed to exert its
∗ Corresponding author at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 519 #, 130 Meilong Road, Shanghai 200237, PR China. Tel.: +86 21 64252924; fax: +86 21 64252914. E-mail address: [email protected] (L. Zhan). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.07.094
influence by (i) blocking active sites on the catalyst [15], (ii) lowering the melting point of the catalyst [10–12,16], or (iii) interacting with the growing tube [17]. However, there is no consensus yet on the mechanism of the effects of sulfur on CNT formation. Zhu et al. synthesized CNTs by detonation-assisted CVD [18–20], in which the energy required for the assembly of carbon species to nanotubes was derived from the detonation of explosives on a micro-second time scale. Some intermediate objects would survive within the fast thermal annealing, which enabled us to trace the CNT formation. In this paper, we present the synthesis of CNTs by detonation-assisted CVD and investigate the effect of sulfur on the morphology and microstructure of CNTs. It is found that CNT growth follows the dynamic growth model and the role of sulfur in promotion of hollow tube growth was discussed. 2. Experimental details All of the chemical reagents were analytical grade including picric acid (PA), liquid paraffin, nickel acetate (Ni(AC)2 ) and thiophene. Thiophene was dissolved in paraffin and sonicated to form a homogeneous solution. Before the detonation experiments, the starting materials (2 g PA, 0.1 g Ni(AC)2 , 0.5 g paraffin) were premixed physically. The detonation-assisted CVD was performed in a sealed stainless steel pressure vessel (10 ml) connected with a pressure gauge, and induced by external heating (20 ◦ C/min) to 310 ◦ C. When the detonation occurs, about 40 MPa pressure (shock wave) and 900 ◦ C temperature are produced inside the vessel. After the detonation, the vessel was cooled in air and emptied of gaseous products, and then the solid products were collected.
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Table 1 The effect of sulfur addition amounts on the structures of CNTs. Exp. no.
Sulfur/nickel (molar ratio)
Structure feature of the producta
1 2 3 4
0 1/8 1/4 1/2
BCNTs BCNTs + hollow tubes 50% hollow tubes + nanoparticles 30% hollow tubes + nanoparticles
a
Estimated from TEM observations.
Scanning electron microscopy (SEM) of the as-synthesized samples was examined using a FEI Quanta 200FEG operated at 5 kV. Transmission electron microscopy (TEM) was carried out on a JEOL 2010F operated at 200 kV, equipped with an energy dispersive Xray spectrometer (EDS). Powder X-ray diffraction (XRD) patterns were performed on a Rigaku D/max 2500x X-ray diffractometer using Cu K␣ radiation.
Fig. 2. XRD patterns of the products obtained with the molar ratio of sulfur/nickel = 0 (Exp. 1) and 1/4 (Exp. 3).
3. Results and discussion 3.1. Morphology and structure of CNTs Table 1 presents the corresponding structures of the products obtained with different sulfur addition amounts. If there is no addition of sulfur (Exp. 1), most of the product was bamboo-like carbon nanotubes (BCNTs), and few amorphous carbon and catalyst particles encapsulated by graphite layers, as shown in Fig. 1a and c. SEM image shows that bamboo-like tubes were up to tens of microns long with catalyst particles at the ends. TEM observations reveal that the hollow compartments of BCNTs were normally spaced by curved knots made from few graphene sheets.
In a certain sulfur addition range (Exps. 2–4), CNTs with hollow channels or few bamboo knots were obtained, whose yield was reduced with the sulfur addition amounts increasing. In Exp. 3, CNTs were mostly conventional tubes with hollow cores (Fig. 1d). Unlike the entangled bamboo tubes, hollow tubes were relatively isolated and straight. These tubes were just a few micrometers, and many of them contained open ends, as shown in Fig. 1b. According to above experimental results, it is concluded that sulfur plays an important role in the structure transformation of CNTs from bamboo to hollow channel.
Fig. 1. SEM and TEM images of CNTs obtained with different sulfur addition amounts: (a) and (c) molar ratio of sulfur/nickel = 0 (Exp. 1); (b) and (d) molar ratio of sulfur/nickel = 1/4 (Exp. 3).
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Fig. 3. (a and b) Typical TEM images of BCNTs obtained without sulfur addition. (c) HRTEM image and (d) schematic model of the bamboo knot marked by the rectangle in (b). White arrows indicate where knot is formed. (e) TEM image of an incompletely contracted catalyst particle. (f and g) TEM and HRTEM images of a bamboo knot within nucleation period.
3.2. Structures of the catalyst particles Fig. 2 shows the XRD patterns of two typical samples obtained with different sulfur addition amounts. Two peaks at 44.5◦ and 51.8◦ were indexed to the diffraction lines from the (1 1 1) and (2 0 0) planes of face-centered cubic (fcc) nickel, respectively. Signals for nickel sulfides, carbides or oxides were not detected in the samples. Moreover, the crystalline structure of catalyst for the growth of bamboo-shaped tubes was the same as that for the growth of hollow tubes, which indicates that the structure transformation of CNTs induced by sulfur is independent on the catalyst crystalline structure. Therefore, the effect of sulfur may be estab-
lished by the interaction between the adsorbed sulfur and metal surface. 3.3. The role of sulfur TEM and high resolution TEM (HRTEM) were used to analyze the growth mechanism of BCNTs and clarify the role of sulfur. Fig. 3a and b shows the typical TEM images of BCNTs. It is found that the core of BCNT was divided into compartments with different dimensions and shapes by bamboo knots. And the morphologies of the adjective bamboo knots were distinctive: flat, conical, complete and incomplete, especially subdomes contained in the main
C. Wang et al. / Applied Surface Science 257 (2010) 932–936
935
Fig. 4. (a and b) TEM images of hollow tubes obtained from Exp 3. (c) HRTEM image of a section of tube wall marked by the rectangle in (a). (d) TEM image of a spherical catalyst located at the end. (e) EDS spectrum of the region marked in (d) indicates the presence of sulfur on the catalyst surface. (f) Scheme of the mechanism of hollow tube growth with sulfur addition. The dashed indicates the blocking sulfur.
compartments. From the growth perspective, the compartments in bamboo tubes could be visualized as the trace of the catalyst particle moving along the tube length. Therefore, the distinctive compartments and bamboo knots suggest that the catalyst particle was dynamic reshaping, i.e., elongation and contraction, during the CNT growth. HRTEM image of the bamboo knot (Fig. 3c) shows that the outmost graphene sheets were structured continuously along the tube length, and there was a step between the outer wall and the knot (as indicated by white arrows). The structure model of the bamboo knot was illustrated in Fig. 3d. Combined with the finding of incomplete bamboo knot (shown in Fig. 3b), the bamboo knot was suggested to nucleate and grow on the multistep Ni–graphite edges, rather than precipitate on the top surface of the catalyst. Previous density functional theory (DFT) calculations also show that, the Ni–graphite step edge acted as nucleation site is energetically preferred because it is stabilized by the interaction with both the external graphene wall and the Ni catalyst particle surface [4,21]. Detailed observation of the as-product could find some intermediate objects of BCNT growth. Fig. 3e shows an elongated catalyst particle which started to contract but was encapsulated by the graphene layer, suggesting a tip-growth model. The compartment between its asymmetric growth front and the bamboo knot indicates the fluctuation of the catalyst during growth. Fig. 3f and g shows a bamboo knot within nucleation period. It is obvious that graphene sheets (bamboo knot) terminate at a stepped Ni surface, demonstrated the above interpretation. Therefore, we can conclude that the dynamic reshaping of the catalyst particle assists the tube growth, and graphene sheets preferentially nucleate on the mul-
tistep Ni–graphite edges. This result is consistent with the in situ HRTEM observations [4–7]. With the addition of sulfur, carbon nanotubes were produced with pure tube-like carbon (showing in Fig. 4a) or few bamboo knots (showing in Fig. 4b). We find some closed ends with spherical catalysts in the products, indicating a tip-growth mode for hollow tubes. Fig. 4c shows the HRTEM image of a section of tube wall marked in Fig. 4a. It is found that the outmost graphene sheets were aligned parallel to the tube axis, similar to that of BCNTs. Combined with occasionally existed bamboo knot within the tube, the structure similarity suggests that the formation mechanism of hollow tubes is closely correlative with that of BCNTs: the catalyst reshaping assists the tube growth. The role of sulfur in the structure transformation seems to suppress the bamboo knot growth. Previous researches indicated that CNT growth was influenced by the carbon supply rate for the catalyst [22], or the carbon diffusion rate on/through the catalyst [23]. In detonation-assisted CVD, Cn species for building nanotubes are available without metal catalysis [24,25]. In other words, the effect of sulfur on the carbon supply is eliminated. According to XRD results, metal sulfides were not detected in bulk crystallinity. Fig. 4e shows the EDS spectroscopy of the interface region between catalyst and out wall, and small amount of sulfur was indeed detected. Thus, it can be concluded that sulfur adsorbed on and interacted with the metal surface. Previous studies in the steam reforming process on Ni catalyst show that, the nucleation of graphite is initiated at the step sites, and sulfur promoter preferentially binds to Ni step edges to suppress graphite formation [21,26]. In our system, we suggest that sulfur inhibits the formation of bamboo knot through a step-blocking mechanism. Sulfur atoms were adsorbed on the catalyst surface
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and diffuse through the Ni–graphene interface to bind at the multistep edges. Since sulfur showed a strong affinity with the metal step, the nucleation site for bamboo knot was blocked, and hollow tubes were synthesized. Fig. 4f shows the scheme of the mechanism of hollow tube growth with sulfur addition. Another evidence for such an explanation is that: there was no apparent graphene lattice found on either the growth front or the free facet of the catalyst particle (as shown in Fig. 4e). In addition, it should be noted that the yield of hollow tubes was decreased with the addition amount of sulfur increasing, and its length was shorter in comparison to that of BCNTs. And much more open ends were observed in the SEM and TEM images of the product obtained with sulfur addition. Adsorbed sulfur in general can act as “surfactant”, minimizing surface energies and dewetting catalyst [27]. Such an effect makes the Ni particle be easily ejected from the tube channel when the Ni particle contracts, resulting in open ends and growth termination. The quicker the growth termination, the shorter the tube length and the lower the tube yield. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Carbon nanotubes with hollow channels were synthesized instead of BCNTs due to the sulfur addition. According to the structural characterization and dynamic analysis, it is found that the catalyst was continuously reshaping during CNT growth, and bamboo knot preferentially nucleated at the Ni–graphite step edges. The role of sulfur is suggested to prevent bamboo knot nucleation and growth by blocking the step sites. The findings are important for understanding of nanotube growth mechanism and the role of sulfur often involved in catalytic reactions. Acknowledgements This work is supported by the National Natural Science Foundation of China (NO. 50730003, 50672025 and 20806024) and the Research Fund of China 863 (No. 2008AA062302) and Shanghai International Cooperation Project (No. 08160704000).
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