- Email: [email protected]
Properties and formation mechanisms of branched carbon nanotubes from polyvinylidene fluoride fibers
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Properties and formation mechanisms of branched carbon nanotubes from polyvinylidene fluoride fibers Han-Ik Joh 1, Heung Yong Ha
*
Energy Storage Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Branched carbon nanotubes (b-CNTs) were synthesized via a very simple process where
Received 6 March 2013
polyvinylidenefluoride (PVDF) fibers containing H2PtCl6 were partly dehydrofluorinated
Accepted 20 June 2013
and then subjected to carbonization at high temperature under a nitrogen atmosphere.
Available online 28 June 2013
During the process, the solid polymeric fibers were converted to carbon nanotubes with branches growing on the surface of the tubes. The carbon branches started to grow at around 500 °C, and the growth terminated at around 700 °C. The gaseous species generated during the carbonization process were identified using in situ mass spectroscopy. Based on the analytical data, a mechanism for the formation of the b-CNTs is proposed. Ó 2013 Elsevier Ltd. All rights reserved.
One-dimensional nanostructures, such as carbon nanofibers (CNFs) and carbon nanotubes (CNTs), have received increased scientific and technological interest because of their unique properties and potential applications as electrocatalysts and energy storage devices [1,2]. Recent reports have unfolded the fabrication of carbon/carbon (branch/stem) composite materials with complicated structures that are much different from conventional CNTs and CNFs. The composites have branches growing on carbon stems, for example, CNT/CNF, CNT/CNT, and CNF/CNF [3–5]. Usually, the carbon materials corresponding to a stem are first prepared using various methods like chemical vapor deposition, arc discharge, and dry or wet spinning. Then, the catalyst particles such as Ni, Co, Fe, Cu, or their alloys are dispersed on the surface of the carbon stems as growing points for branches. As a carbon source, a mixture of hydrocarbon gases is subsequently supplied to the stems that are placed in a carbonization reactor to grow branches. As reported in the literature, the morphology and properties of the carbon composites varies with fabrication conditions and the growth feature of the carbon materials is also affected by the degree of interaction between the substrate and the catalyst particle, as summarized in Table S1. We previously reported the synthesis of branched carbon nanotubes (b-CNTs) by carbonizing electrospun polymeric fi-
bers containing Pt precursor [6]. The solid polymeric fibers turned into carbon nanotubes, and carbon branches simultaneously grew on the surfaces of the nanotubes in a carbonization reactor in the absence of additional steps to disperse catalysts on the carbon substrate and to supply external carbon sources. In the present study, we have carried out systematic studies to investigate the effects of various fabrication parameters on the morphology and properties of the b-CNTs in more detail, and the sophistication of the analytical work permits the proposal of a mechanism for the formation of b-CNTs. In the first place, a thermo gravimetric analysis (TGA) was conducted to observe the weight changes in the Pt precursor (H2PtCl6Æ6H2O), the pristine PVDF fibers, and the DHF–PVDF fibers without (DHF–PVDF) or with (DHF–PVDF-Pt) Pt precursor by increasing the temperature to 1000 °C under a nitrogen atmosphere. As shown in Fig. 1, the weight of the H2PtCl6Æ6H2O decreases through four steps with the removal of the water and the chlorine step-by-step as the temperature is increased [7]. Decomposition of the Pt compound is completed at around 540 °C. In the case of the pristine PVDF, there is no change in the weight until the temperature of 450 °C is reached, and then the weight drops rapidly until the temperature of 500 °C is reached, which is followed by a gradual decrease afterwards until 1000 °C is reached. The DHF–PVDF
* Corresponding author: Fax: +82 2 958 5229. E-mail address: [email protected] (H. Yong Ha). 1 Present address: Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, San 101 Eunha-ri, Bongdong-eup, Wanju-gun, Jeollabukdo 565-905, Republic of Korea. 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.06.072
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Fig. 1 – Thermal gravimetric analysis of the Pt precursor, pristine PVDF fibers, DHF–PVDF and DHF–PVDF-Pt.
shows similar, but slightly deformed, profiles at temperatures below 450 °C compared with that of the pristine PVDF. The total weight loss of the DHF–PVDF-Pt is about 7% larger than those of the other PVDFs. Fig. 2a shows an image of the as-spun PVDF fibers. The fibers (DHF–PVDF) that were subjected to the DHF treatment have almost the same morphology as the as-spun fibers [6]. When the DHF–PVDF is carbonized, carbon branches begins to appear on the surface of the carbon nanofibers at 500 °C (Fig. 2b), and these branches grow longer as the temperature increases to 700 °C. The branches at 500 °C looks like sprouts on the trunk of a tree. The branches formed at 600 °C (Fig. 2c)
are still sparse, while at 700 °C (Fig. 2d) the branches become denser and longer to the point of covering the carbon stems. Above 700 °C, only a minor change is observed (Fig. 2e). The cross-sectional view of the b-CNTs (Fig. 2f) reveals the hollow nature of the CNTs. This means that the solid polymer fibers containing a Pt compound turn into hollow carbon fibers (tubes) with branches on their outer surface through a simple process that includes DHF and carbonization steps. This is a very interesting result considering the fact that in the absence of Pt the DHF–PVDF fibers simply turn into solid carbon fibers with no branches growing on the surface. Carbonization temperature among the synthetic parameters exhibited the biggest effect on the pore structure of the b-CNTs (Figs. 3, S1 and S2). As shown in Fig. 3, the surface area of the b-CNTs increases with an increase in temperature until 800 °C, while the average pore size remains almost the same regardless of the temperature tested. This indicates that either the number of pores or the total pore volume increases with the temperature without an appreciable change in the pore size. The surface area increment is remarkable at temperatures ranging between 500 and 700 °C. This increment might be due to the formation of pores and branches during the carbonization process. The shapes of the pores could be estimated using the nitrogen adsorption isotherms (Fig. 3b). All the isotherms have slanting adsorption and desorption curves with slopes that increases with an increase in the carbonization temperature. These types of isotherms have ink-bottle-shaped pores with bodies and necks that comprise a wide range of radii [8]. At
Fig. 2 – SEM images of (a) the electrospun PVDF fibers, and the carbon fibers carbonized at (b) 500 °C, (c) 600 °C, (d) 700 °C, (e) 900 °C, and (f) a cross-sectional view of the b-CNTs.
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Fig. 3 – (a) Effect of carbonization temperature on the surface area and average pore diameter of the b-CNTs; (b) adsorption and desorption isotherms of N2 on the b-CNTs.
800 °C and above, however, the isotherms are distorted, which suggests that the necks of the ink-bottle pores become narrower. In order to identify the carbon sources that contribute to the formation of branches on the surface of b-CNTs, the gaseous effluents from the carbonization reactor were analyzed using an in situ mass spectroscope. As the temperature was increased from room temperature to 900 °C, many species evolved from the samples, as shown in Figs. 4 and S3. The DHF–PVDF releases H2O, H2, F , HF, CO, ethylene, C10H18O2, and CO2, and all the above-mentioned species plus 1,2 chloro isobutene are detected with the DHF–PVDF-Pt [9]. In Fig. 4a, 1,2-chloro isobutene appears below 350 °C in the DHF–PVDFPt sample. The compound, however, was unexpected because chlorine originates only from H2PtCl6, and not from PVDF. Therefore, it is speculated that the chlorinated hydrocarbons must have been formed during the carbonization process that produces chlorine and a carbonaceous species because of the catalytic activity of the chlorinated platinum toward the isomerization of alkanes [10]. Above 350 °C, the chlorinated compounds are no longer detected, and instead, CO and ethylene are produced. Therefore, Pt particles in this temperature range are believed to participate in the decomposition of the pristine PVDF present in the core part of the fibers and thus to provide carbon sources for the growth of the carbon branches. Carbonaceous species such as CO, ethylene, C10H18O2, and CO2 are produced above 350 °C by the decomposition of DHF–
PVDF. In the presence of Pt, the amount of the carbonaceous species produced is larger than in the absence of Pt, and the peak maximum for C10H18O2 is shifted to a higher temperature of 750 °C. Considering that the pristine PVDF present in the core part of the DHF–PVDF fibers is removed by catalytic decomposition, it is against our expectation that the relatively small amount of carbonaceous species from the sample with Pt is evolved at temperatures ranging between 450 and 700 °C. This might be because almost all the carbonaceous species generated is consumed as carbon sources to grow carbon branches until a temperature of 700 °C is reached, and thus only a small fraction of the carbonaceous species that are produced is exhausted out of the reactor. Above 700 °C, these carbon sources might no longer be used for building carbon branches, but would be eluted out of the reactor, showing peak maxima for C10H18O and CO2 at around 750 °C. Based on the analytical data shown above, a probable mechanism for the formation of b-CNTs is proposed as illustrated in Fig. 5. First, solid PVDF fibers containing Pt precursor are prepared by electrospinning (Fig. 5a). Then, the fibers are treated with a NaOH–TBAB mixture solution to remove hydrogen and fluorine from the shell side of the PVDF fibers (the DHF process), while the PVDF in the core remains intact (Fig. 5b). When the resultant DHF–PVDF-Pt fibers are subjected to carbonization, the Pt precursor is partially decomposed until a temperature of 450 °C is reached, and an additional amount of weight is lost (Fig. 1) because of the removal of the water, OH and H present in the DHF–PVDF. At
Fig. 4 – In situ mass spectra of PVDF fibrous polymer with and without Pt precursor during carbonization with an increasing reactor temperature: (a) m/z = 28, (b) m/z = 44, and (c) m/z = 85.
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Fig. 5 – A growth mechanism of the b-CNTs: (a) Electrospun fibrous polymer with a Pt precursor; (b) Partial dehydrofluorination from the shell side of the PVDF fiber, (c) Reduction of Pt precursor and partial decomposition of PVDF polymer at temperatures from 200 to 500 °C; (d) Decomposition of the pristine PVDF present in the core part (500–700 °C); and (e) simultaneously, the growth of carbon branches at the Pt particles dispersed on the CNT stems.
temperatures between 450 and 500 °C, there is a rapid weight loss caused mostly by the release of H2O, F , HF and carbonaceous species (Fig. 5c). H2PtCl6 is also decomposed step-bystep until a temperature of 530 °C is reached. At around 450 °C, the pristine PVDF present in the core part of the fibers starts to decompose through mediation of the Pt catalyst to release CO, C2H4 and C10H18O2, leaving a hollow space in the fibers (Fig. 5d). Simultaneously, the gaseous carbonaceous species thus formed are used as sources for growing carbon branches at the Pt seeds that spread over the fibers (Fig. 5e). The growth of the branches continues until a temperature of 700 °C is reached. Considering the fact that the PVDF fibers without Pt are converted to simple solid carbon fibers and no branches are formed, it is certain that the Pt particles function as a catalyst to decompose the pristine PVDF in the core of the fibers to form hollow spaces and, as seeds, to grow carbon branches by utilizing the gaseous carbonaceous species originating from the decomposition of the PVDF. In conclusion, carbon nanotubes with branches were synthesized by carbonization of the solid PVDF fibers containing H2PtCl6 that were fabricated using an electrospinning method without introducing external carbon sources. The DHFtreated PVDF fibers were converted into carbon nanotubes with branches that started to grow at around 500 °C, and the growth continued until around 700 °C. During the carbonization process, many species were evolved: H2, H2O, F-, HF, CO2, 1,2-chloro isobutene, CO, ethylene, HF, and 4,5-dihydro-5-hexyl-2(3H)-furanone (C10H18O2). Based on the analytical data, the following mechanism is proposed for the formation of b-CNTs. The outer shell of the DHF–PVDF fibers is carbonized to leave a carbon body while the pristine PVDF that remains intact in the core of the fibers decomposes into a gaseous carbonaceous species via Pt catalyst, leaving a hollow space in the fibers. The carbonaceous species, in turn, is used as a carbon source to grow carbon branches
at the Pt seeds that are dispersed on the surface of the CNTs.
Acknowledgements The authors acknowledge financial support for this work from a Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. 2012-0008931) and also from the Korea Evaluation Institute of Industrial Technology (KEIT) through a Redox Flow Battery Program under contract number 10043787.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.06.072.
R E F E R E N C E S
[1] Park I-S, Park K-W, Choi J-H, Park CR, Sung Y-E. Electrocatalytic enhancement of methanol oxidation by graphite nanofibers with a high loading of PtRu alloy nanoparticles. Carbon 2007;45(1):28–33. [2] Joh H-I, Song HK, Yi K-B, Lee S. The production of porosity in carbon nanofibers by the catalytic action of Ni nanoparticles in low temperature activation. Carbon 2013;53:409–13. [3] Hou H, Reneker D. Carbon nanotubes on carbon nanofibers: a novel structure based on electrospun polymer nanofibers. Adv Mater 2004;16(1):69–73. [4] AuBuchon JF, Chen L-H, Daraio C, Jin S. Multibranching carbon nanotubes via self-seeded catalysts. Nano Lett 2006;6(2):324–8. [5] Xia W, Su D, Birkner A, Ruppel L, Wang Y, Wo¨ll C, et al. Chemical vapor deposition and synthesis on carbon
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nanofibers: sintering of ferrocene-derived supported iron nanoparticles and the catalytic growth of secondary carbon nanofibers. Chem Mater 2005;17(23):5737–42. [6] Joh H-I, Ha HY, Prabhuram J, Jo SM, Moon SH. Synthesis of branched carbon nanotubes by carbonization of solid polyvinylidene fluoride fibers. Carbon 2011;49(13):4601–3. [7] Hernandez JO, Choren EA. Thermal stability of some platinum complexes. Thermochim Acta 1983;71(3): 265–272.
[8] Thomas JM, Thomas WJ. Introduction to the principles of heterogeneous catalysis. London: Academic Press; 1967. [9] Mass Spectrometry Data Centre. Eight peak index of mass spectra. 3rd ed. Nottingham: Royal Society of Chemistry; 1991. [10] Melchor A, Garbowski E, Mathieu M-V, Primet M. Physicochemical properties and isomerization activity of chlorinated Pt/Al2O3 catalysts. J Chem Soc Faraday Trans 1986;82(12):3667–79.
New superhard carbon allotropes based on C20 fullerene Jianfu Li, Rui-Qin Zhang
*
Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region
A R T I C L E I N F O
A B S T R A C T
Article history:
Three new carbon allotropes have been uncovered by compressing hexagonal C20 solid
Received 17 April 2013
based on density functional tight binding level simulations. They are identified to be ener-
Accepted 24 June 2013
getically more favorable and stable than C20 molecule, with the phase III structure being the
Available online 3 July 2013
energetically most favorable. The volume compression calculations suggest that the phase III has a very high anti-compressibility with a bulk modulus of 427 GPa. All of the predicted structures are semiconductors with varied bandgaps and the phase III has a direct band gap of 5.1 eV. Our results indicate that solid C20 can be converted to a transparent superhard material under cold compression. Ó 2013 Elsevier Ltd. All rights reserved.
Graphite, amorphous carbon and fullerene are considered as most promising starting materials for the synthesis of new carbon phases. Without catalysts, graphite can be converted to lonsdaleite and diamond at pressure above 15 GPa and high temperature (1600–2500 K) [1]. On the contrary, cold compression of graphite at room temperature produces transparent and hard phases [2], different from lonsdaleite and diamond. However, the cold compressing of graphite needs large lattice distortions to buckle adjacent graphite carbon layers. And it is difficult to identify new carbon phases in experiments of cold graphitic compression because small new phases are often not detectable in large amounts of many products (e.g., raw material, generative graphite, and amorphous carbon). Fullerenes have attracted many attentions as excellent building blocks since C60 can be easily synthesized in experiment. Under compression, pure C60 forms different polymerized structures [3]. The smallest member of the fullerene family is C20 molecule which offers the extreme curvature of the cage surface that makes the dihedral angles between bonds (108°) more appropriate to sp3 hybridization (109.5°) than sp2. Therefore it can be expected that C20 changes hybridization type especially easily.
Compared with the other fullerene family members, the C20 molecule has the largest carbon mass density (2.34 g/cm3). High degree of sp3 hybridization and large mass density of carbon are considered to be important prerequisites for a superhard carbon phase. There are still some difficulties in synthesis of high purity C20 in large quantity. Nevertheless, the difficulty could be reduced in the future with the further technology development. Based on the above, we choose C20 as the starting material to build new functional carbon materials. In this letter, we predict three stable new carbon phases formed from the C20 molecules as starting materials based on density functional tight binding theory calculations. All the uncovered structures have been confirmed to be semiconductors with different bandgaps and one of the phases (phase III) is identified to be a transparent superhard material with a bulk modulus of 427 GPa. All the calculations are performed based on a self-consistent-charge density functional tight binding (SCC-DFTB) method with the DFTB+ code [4]. The SCC-DFTB approach is an approximate DFT scheme, which is derived from a second-order expansion of the Kohn–Sham total energy in DFT
* Corresponding author: Fax: +852 3442 0538. E-mail address: [email protected] (R.-Q. Zhang). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.06.086
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Properties and formation mechanisms of branched carbon nanotubes from polyvinylidene fluoride fibers Han-Ik Joh 1, Heung Yong Ha
*
Energy Storage Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Branched carbon nanotubes (b-CNTs) were synthesized via a very simple process where
Received 6 March 2013
polyvinylidenefluoride (PVDF) fibers containing H2PtCl6 were partly dehydrofluorinated
Accepted 20 June 2013
and then subjected to carbonization at high temperature under a nitrogen atmosphere.
Available online 28 June 2013
During the process, the solid polymeric fibers were converted to carbon nanotubes with branches growing on the surface of the tubes. The carbon branches started to grow at around 500 °C, and the growth terminated at around 700 °C. The gaseous species generated during the carbonization process were identified using in situ mass spectroscopy. Based on the analytical data, a mechanism for the formation of the b-CNTs is proposed. Ó 2013 Elsevier Ltd. All rights reserved.
One-dimensional nanostructures, such as carbon nanofibers (CNFs) and carbon nanotubes (CNTs), have received increased scientific and technological interest because of their unique properties and potential applications as electrocatalysts and energy storage devices [1,2]. Recent reports have unfolded the fabrication of carbon/carbon (branch/stem) composite materials with complicated structures that are much different from conventional CNTs and CNFs. The composites have branches growing on carbon stems, for example, CNT/CNF, CNT/CNT, and CNF/CNF [3–5]. Usually, the carbon materials corresponding to a stem are first prepared using various methods like chemical vapor deposition, arc discharge, and dry or wet spinning. Then, the catalyst particles such as Ni, Co, Fe, Cu, or their alloys are dispersed on the surface of the carbon stems as growing points for branches. As a carbon source, a mixture of hydrocarbon gases is subsequently supplied to the stems that are placed in a carbonization reactor to grow branches. As reported in the literature, the morphology and properties of the carbon composites varies with fabrication conditions and the growth feature of the carbon materials is also affected by the degree of interaction between the substrate and the catalyst particle, as summarized in Table S1. We previously reported the synthesis of branched carbon nanotubes (b-CNTs) by carbonizing electrospun polymeric fi-
bers containing Pt precursor [6]. The solid polymeric fibers turned into carbon nanotubes, and carbon branches simultaneously grew on the surfaces of the nanotubes in a carbonization reactor in the absence of additional steps to disperse catalysts on the carbon substrate and to supply external carbon sources. In the present study, we have carried out systematic studies to investigate the effects of various fabrication parameters on the morphology and properties of the b-CNTs in more detail, and the sophistication of the analytical work permits the proposal of a mechanism for the formation of b-CNTs. In the first place, a thermo gravimetric analysis (TGA) was conducted to observe the weight changes in the Pt precursor (H2PtCl6Æ6H2O), the pristine PVDF fibers, and the DHF–PVDF fibers without (DHF–PVDF) or with (DHF–PVDF-Pt) Pt precursor by increasing the temperature to 1000 °C under a nitrogen atmosphere. As shown in Fig. 1, the weight of the H2PtCl6Æ6H2O decreases through four steps with the removal of the water and the chlorine step-by-step as the temperature is increased [7]. Decomposition of the Pt compound is completed at around 540 °C. In the case of the pristine PVDF, there is no change in the weight until the temperature of 450 °C is reached, and then the weight drops rapidly until the temperature of 500 °C is reached, which is followed by a gradual decrease afterwards until 1000 °C is reached. The DHF–PVDF
* Corresponding author: Fax: +82 2 958 5229. E-mail address: [email protected] (H. Yong Ha). 1 Present address: Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, San 101 Eunha-ri, Bongdong-eup, Wanju-gun, Jeollabukdo 565-905, Republic of Korea. 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.06.072
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Fig. 1 – Thermal gravimetric analysis of the Pt precursor, pristine PVDF fibers, DHF–PVDF and DHF–PVDF-Pt.
shows similar, but slightly deformed, profiles at temperatures below 450 °C compared with that of the pristine PVDF. The total weight loss of the DHF–PVDF-Pt is about 7% larger than those of the other PVDFs. Fig. 2a shows an image of the as-spun PVDF fibers. The fibers (DHF–PVDF) that were subjected to the DHF treatment have almost the same morphology as the as-spun fibers [6]. When the DHF–PVDF is carbonized, carbon branches begins to appear on the surface of the carbon nanofibers at 500 °C (Fig. 2b), and these branches grow longer as the temperature increases to 700 °C. The branches at 500 °C looks like sprouts on the trunk of a tree. The branches formed at 600 °C (Fig. 2c)
are still sparse, while at 700 °C (Fig. 2d) the branches become denser and longer to the point of covering the carbon stems. Above 700 °C, only a minor change is observed (Fig. 2e). The cross-sectional view of the b-CNTs (Fig. 2f) reveals the hollow nature of the CNTs. This means that the solid polymer fibers containing a Pt compound turn into hollow carbon fibers (tubes) with branches on their outer surface through a simple process that includes DHF and carbonization steps. This is a very interesting result considering the fact that in the absence of Pt the DHF–PVDF fibers simply turn into solid carbon fibers with no branches growing on the surface. Carbonization temperature among the synthetic parameters exhibited the biggest effect on the pore structure of the b-CNTs (Figs. 3, S1 and S2). As shown in Fig. 3, the surface area of the b-CNTs increases with an increase in temperature until 800 °C, while the average pore size remains almost the same regardless of the temperature tested. This indicates that either the number of pores or the total pore volume increases with the temperature without an appreciable change in the pore size. The surface area increment is remarkable at temperatures ranging between 500 and 700 °C. This increment might be due to the formation of pores and branches during the carbonization process. The shapes of the pores could be estimated using the nitrogen adsorption isotherms (Fig. 3b). All the isotherms have slanting adsorption and desorption curves with slopes that increases with an increase in the carbonization temperature. These types of isotherms have ink-bottle-shaped pores with bodies and necks that comprise a wide range of radii [8]. At
Fig. 2 – SEM images of (a) the electrospun PVDF fibers, and the carbon fibers carbonized at (b) 500 °C, (c) 600 °C, (d) 700 °C, (e) 900 °C, and (f) a cross-sectional view of the b-CNTs.
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Fig. 3 – (a) Effect of carbonization temperature on the surface area and average pore diameter of the b-CNTs; (b) adsorption and desorption isotherms of N2 on the b-CNTs.
800 °C and above, however, the isotherms are distorted, which suggests that the necks of the ink-bottle pores become narrower. In order to identify the carbon sources that contribute to the formation of branches on the surface of b-CNTs, the gaseous effluents from the carbonization reactor were analyzed using an in situ mass spectroscope. As the temperature was increased from room temperature to 900 °C, many species evolved from the samples, as shown in Figs. 4 and S3. The DHF–PVDF releases H2O, H2, F , HF, CO, ethylene, C10H18O2, and CO2, and all the above-mentioned species plus 1,2 chloro isobutene are detected with the DHF–PVDF-Pt [9]. In Fig. 4a, 1,2-chloro isobutene appears below 350 °C in the DHF–PVDFPt sample. The compound, however, was unexpected because chlorine originates only from H2PtCl6, and not from PVDF. Therefore, it is speculated that the chlorinated hydrocarbons must have been formed during the carbonization process that produces chlorine and a carbonaceous species because of the catalytic activity of the chlorinated platinum toward the isomerization of alkanes [10]. Above 350 °C, the chlorinated compounds are no longer detected, and instead, CO and ethylene are produced. Therefore, Pt particles in this temperature range are believed to participate in the decomposition of the pristine PVDF present in the core part of the fibers and thus to provide carbon sources for the growth of the carbon branches. Carbonaceous species such as CO, ethylene, C10H18O2, and CO2 are produced above 350 °C by the decomposition of DHF–
PVDF. In the presence of Pt, the amount of the carbonaceous species produced is larger than in the absence of Pt, and the peak maximum for C10H18O2 is shifted to a higher temperature of 750 °C. Considering that the pristine PVDF present in the core part of the DHF–PVDF fibers is removed by catalytic decomposition, it is against our expectation that the relatively small amount of carbonaceous species from the sample with Pt is evolved at temperatures ranging between 450 and 700 °C. This might be because almost all the carbonaceous species generated is consumed as carbon sources to grow carbon branches until a temperature of 700 °C is reached, and thus only a small fraction of the carbonaceous species that are produced is exhausted out of the reactor. Above 700 °C, these carbon sources might no longer be used for building carbon branches, but would be eluted out of the reactor, showing peak maxima for C10H18O and CO2 at around 750 °C. Based on the analytical data shown above, a probable mechanism for the formation of b-CNTs is proposed as illustrated in Fig. 5. First, solid PVDF fibers containing Pt precursor are prepared by electrospinning (Fig. 5a). Then, the fibers are treated with a NaOH–TBAB mixture solution to remove hydrogen and fluorine from the shell side of the PVDF fibers (the DHF process), while the PVDF in the core remains intact (Fig. 5b). When the resultant DHF–PVDF-Pt fibers are subjected to carbonization, the Pt precursor is partially decomposed until a temperature of 450 °C is reached, and an additional amount of weight is lost (Fig. 1) because of the removal of the water, OH and H present in the DHF–PVDF. At
Fig. 4 – In situ mass spectra of PVDF fibrous polymer with and without Pt precursor during carbonization with an increasing reactor temperature: (a) m/z = 28, (b) m/z = 44, and (c) m/z = 85.
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Fig. 5 – A growth mechanism of the b-CNTs: (a) Electrospun fibrous polymer with a Pt precursor; (b) Partial dehydrofluorination from the shell side of the PVDF fiber, (c) Reduction of Pt precursor and partial decomposition of PVDF polymer at temperatures from 200 to 500 °C; (d) Decomposition of the pristine PVDF present in the core part (500–700 °C); and (e) simultaneously, the growth of carbon branches at the Pt particles dispersed on the CNT stems.
temperatures between 450 and 500 °C, there is a rapid weight loss caused mostly by the release of H2O, F , HF and carbonaceous species (Fig. 5c). H2PtCl6 is also decomposed step-bystep until a temperature of 530 °C is reached. At around 450 °C, the pristine PVDF present in the core part of the fibers starts to decompose through mediation of the Pt catalyst to release CO, C2H4 and C10H18O2, leaving a hollow space in the fibers (Fig. 5d). Simultaneously, the gaseous carbonaceous species thus formed are used as sources for growing carbon branches at the Pt seeds that spread over the fibers (Fig. 5e). The growth of the branches continues until a temperature of 700 °C is reached. Considering the fact that the PVDF fibers without Pt are converted to simple solid carbon fibers and no branches are formed, it is certain that the Pt particles function as a catalyst to decompose the pristine PVDF in the core of the fibers to form hollow spaces and, as seeds, to grow carbon branches by utilizing the gaseous carbonaceous species originating from the decomposition of the PVDF. In conclusion, carbon nanotubes with branches were synthesized by carbonization of the solid PVDF fibers containing H2PtCl6 that were fabricated using an electrospinning method without introducing external carbon sources. The DHFtreated PVDF fibers were converted into carbon nanotubes with branches that started to grow at around 500 °C, and the growth continued until around 700 °C. During the carbonization process, many species were evolved: H2, H2O, F-, HF, CO2, 1,2-chloro isobutene, CO, ethylene, HF, and 4,5-dihydro-5-hexyl-2(3H)-furanone (C10H18O2). Based on the analytical data, the following mechanism is proposed for the formation of b-CNTs. The outer shell of the DHF–PVDF fibers is carbonized to leave a carbon body while the pristine PVDF that remains intact in the core of the fibers decomposes into a gaseous carbonaceous species via Pt catalyst, leaving a hollow space in the fibers. The carbonaceous species, in turn, is used as a carbon source to grow carbon branches
at the Pt seeds that are dispersed on the surface of the CNTs.
Acknowledgements The authors acknowledge financial support for this work from a Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. 2012-0008931) and also from the Korea Evaluation Institute of Industrial Technology (KEIT) through a Redox Flow Battery Program under contract number 10043787.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.06.072.
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New superhard carbon allotropes based on C20 fullerene Jianfu Li, Rui-Qin Zhang
*
Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region
A R T I C L E I N F O
A B S T R A C T
Article history:
Three new carbon allotropes have been uncovered by compressing hexagonal C20 solid
Received 17 April 2013
based on density functional tight binding level simulations. They are identified to be ener-
Accepted 24 June 2013
getically more favorable and stable than C20 molecule, with the phase III structure being the
Available online 3 July 2013
energetically most favorable. The volume compression calculations suggest that the phase III has a very high anti-compressibility with a bulk modulus of 427 GPa. All of the predicted structures are semiconductors with varied bandgaps and the phase III has a direct band gap of 5.1 eV. Our results indicate that solid C20 can be converted to a transparent superhard material under cold compression. Ó 2013 Elsevier Ltd. All rights reserved.
Graphite, amorphous carbon and fullerene are considered as most promising starting materials for the synthesis of new carbon phases. Without catalysts, graphite can be converted to lonsdaleite and diamond at pressure above 15 GPa and high temperature (1600–2500 K) [1]. On the contrary, cold compression of graphite at room temperature produces transparent and hard phases [2], different from lonsdaleite and diamond. However, the cold compressing of graphite needs large lattice distortions to buckle adjacent graphite carbon layers. And it is difficult to identify new carbon phases in experiments of cold graphitic compression because small new phases are often not detectable in large amounts of many products (e.g., raw material, generative graphite, and amorphous carbon). Fullerenes have attracted many attentions as excellent building blocks since C60 can be easily synthesized in experiment. Under compression, pure C60 forms different polymerized structures [3]. The smallest member of the fullerene family is C20 molecule which offers the extreme curvature of the cage surface that makes the dihedral angles between bonds (108°) more appropriate to sp3 hybridization (109.5°) than sp2. Therefore it can be expected that C20 changes hybridization type especially easily.
Compared with the other fullerene family members, the C20 molecule has the largest carbon mass density (2.34 g/cm3). High degree of sp3 hybridization and large mass density of carbon are considered to be important prerequisites for a superhard carbon phase. There are still some difficulties in synthesis of high purity C20 in large quantity. Nevertheless, the difficulty could be reduced in the future with the further technology development. Based on the above, we choose C20 as the starting material to build new functional carbon materials. In this letter, we predict three stable new carbon phases formed from the C20 molecules as starting materials based on density functional tight binding theory calculations. All the uncovered structures have been confirmed to be semiconductors with different bandgaps and one of the phases (phase III) is identified to be a transparent superhard material with a bulk modulus of 427 GPa. All the calculations are performed based on a self-consistent-charge density functional tight binding (SCC-DFTB) method with the DFTB+ code [4]. The SCC-DFTB approach is an approximate DFT scheme, which is derived from a second-order expansion of the Kohn–Sham total energy in DFT
* Corresponding author: Fax: +852 3442 0538. E-mail address: [email protected] (R.-Q. Zhang). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.06.086