J Mater Sci DOI 10.1007/s10853-013-7793-8
One-pot synthesis of carbon nanotubes from renewable resource: cellulose acetate Lyubov Dubrovina • Olga Naboka • Volodymyr Ogenko • Paul Gatenholm Peter Enoksson
•
Received: 17 July 2013 / Accepted: 1 October 2013 Ó Springer Science+Business Media New York 2013
Abstract In the present work, we report for the first time one-pot synthesis of carbon nanotubes (CNTs) by pyrolysis of cellulose acetate (CA) cross-linked with polyisocyanate in the fumed silica template. NiCl2 was chosen as precatalyst for CNT growth. The diameter of CNTs is 24–38 nm and their wall thickness is 9–11 nm. The main role in the formation of CNTs by the pyrolysis of CA may be attributed to combination of closed macropores in the template formed by evolved CO2 during cross-linking reaction and mesopores formed by silica particles. The macropores acted as microreactors while the mesopores templated catalytic nanoparticles. The importance of this method for CNT synthesis reported here consists of the utilization of readily available renewable resource—CA. Moreover the method does not require preliminary synthesis of catalyst, it is technologically simple (can be performed in the conventional tube furnace), and hence it is energetically efficient.
L. Dubrovina V. Ogenko V.I. Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Prospekt Palladina 32/34, Kyiv 03142, Ukraine O. Naboka (&) P. Enoksson Department of Microtechnolgy and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden e-mail:
[email protected] O. Naboka P. Gatenholm P. Enoksson Wallenberg Wood Science Center, Chalmers University of Technology, 41296 Gothenburg, Sweden P. Gatenholm Department of Chemical and Biological Engineering, Chalmers University of Technology, 41296 Gothenburg, Sweden
Introduction Even though carbon nanotubes (CNTs) have been widely known for more than two decades [1] they still attract interest as prospective material for wide range of applications such as micro- and nano-electronics, energy conversion, health care, and constructional materials [2]. While there are a lot of works reporting methods for synthesis of CNTs most of them require precise control of experimental conditions such as atmosphere, pressure, temperature of reaction, and morphology of catalyst [3, 4]. All the above mentioned obviously makes the process not easily scalable, which results in very expensive product. Other important issue for the synthesis of carbon nanostructures that should be taken into the consideration is the availability of carbon source. At present, the main precursors for the synthesis of carbon materials are of nonrenewable origin, namely hydrocarbons such as methane, acetylene, or benzene [3]. Biomass offers a large variety of organic products which could be used for the synthesis of carbon nanostructures. However, only few works report formation of tubular carbon structures during pyrolysis of plant derivatives, mainly from lignocellulosic materials [5–7] and oils [8]. It is difficult to expect repeatable results since composition and structure of starting materials strongly depend on species belongings and planting conditions (e.g., presence of particular heavy metals in soil [6]). Refined cellulose derived from plant sources could offer more reliable results in the synthesis of carbon materials. However to the best of our knowledge there are no reports on synthesis of CNTs from pure cellulose so far. Cellulose is not readily soluble in conventional solvents, which significantly restricts its potential for fabrication of various carbon nanostructured materials. In contrary, cellulose
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derivatives are soluble in a wide range of solvents [9], which allows their controllable arrangement into materials with various morphologies. However, thermoplastic properties of majority of cellulose derivatives [10] do not allow direct synthesis of carbon nanostructures from them due to melting during heat treatment. Regeneration to cellulose is required prior to the carbonization in order to retain morphology of precursor in the carbon product [11]. This increases overall production time and cost of end product. Addition of, the cross-linking agent which can thermally stabilize cellulose derivative could be a prospective alternative to the cellulose regeneration. Isocyanates are commonly used as cross-linking agents particularly for cellulose derivatives [12, 13]. In the present work, we utilized thermoplastic polymer, cellulose acetate (CA) cross-linked with polyisocyanate for one-pot catalytic synthesis of CNTs.
Materials and methods Materials CA (Mw = 30000, bound acetic acid content 53.1 %), polymeric diphenylmethane diisocyanate (PIC) (IsoPMDI 92140, Elastogran GmbH, Germany) based on 4,40 -diphenylmethane diisocyanate containing oligomers of high functionality and isomers (the average functionality is 2.7, NCO content is 31.5 %), Nickel (II) chloride hexahydrate (99.3 % metals basis, Alfa Aesar), fumed silica with specific surface area 300 m2/g (State enterprise ‘‘Kalush Test Experimental Plant of Surface Chemistry Institute National Academy of Sciences of Ukraine’’), acetone and ethanol of analytical grade (Solveco) were used as received without additional purification. Synthesis of carbon nanotubes CA was used as the precursor for CNT synthesis. PIC was used in order to cross-link CA through the reaction between OH-groups of CA and NCO-groups of PIC. Fumed silica was used as a structure-forming agent. NiCl26H2O was used as a precatalyst for CNT growth. NiCl26H2O was dissolved in ethanol (C=0.25 g/ml). CA was dissolved in acetone (C=5 g/100 ml). To obtain cross-linked CA in the silica template, 12.5 ml of CA solution was mixed with 1.25 g of PIC, 1 ml of NiCl2 9 6H2O solution, and 1.000 g of fumed silica until a homogeneous stable gel was formed. To study the influence of template, freshly prepared mixture of CA, PIC and NiCl2 was film casted without using the fumed silica. To investigate the role of catalyst, CA cross-linked with PIC in silica template was prepared without addition of NiCl2. The
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samples were dried at 15 °C until stable weight was reached, then they were carbonized in a tube oven in a nitrogen atmosphere by heating it up to 750 °C with the heating rate 10 °C/min and keeping at the highest temperature for 20 min. Scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis Microscopy examination of synthesized carbon was performed with Leo Ultra 55 FEG SEM. Carbonized samples (without applying any conductive layer) were placed on sample holders with conductive double-sided carbon glue tape. The acceleration voltage was chosen to be in the range of 1.2–1.5 kV. Secondary electron detectors were used for the topography investigation; the in-column energy selective back-scattered detector with a filtering grid potential of 1.1–1.3 kV was used for low loss backscattered electron (Low Loss BSE) imaging for the investigation of material composition. CNT morphology was studied with scanning transmission electron microscopy (STEM) detector in the bright field mode at 25 kV incident voltage. To prepare samples for STEM, the silica template was etched away by heating the composite in a mixture of 40 wt% water solution of hydrofluoric acid (HF) and 98 wt% water solution of sulfuric acid (H2SO4) with HF:H2SO4 volume ratio 1:0.15. Heating was applied until the complete evaporation of the liquid. Then the sample was washed 10 times with distillated water and dried. Dried samples were dispersed in acetone using sonication in ultrasonic bath (Transsonic T700, Elma) at 35 kHz and temperature 20 °C during 15 min. Drop of the freshly prepared dispersion was put on a carbon-coated copper grid and allowed to dry. Elemental composition of samples was investigated using energy dispersive X-ray microanalysis (EDX). EDX was performed with the Oxford Inca EDX system at 10 kV. X-ray diffraction X-ray diffraction (XRD) patterns were recorded with Philips X’Pert Materials Research Diffractometer. Radiation was generated using an X-ray tube with Cu anode (Ka radiation, ˚ ) at 45 kV and 40 mA. An X-ray lens (glass k = 1.54184 A poly-capillary optics) with Ni filter was used as incident optics, a thin film collimator was used as diffracted optics. The 2h range was 24–78°, and the resolution was 0.05° with 20 s averaging time per step. Phase analysis was performed with X’Pert HighScore 3.0 (PANalytical BV) using ICDD databases (release 2008/2009).
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Low-temperature nitrogen sorption Specific surface area and pore-size distribution of synthesized composites were determined from low-temperature nitrogen sorption/desorption with the TriStar 3000 surface area and pore size analyzer, Micromeritics. Pore-size distribution was determined according to desorption isotherm. Outgassing of samples before measurements was performed in vacuum at 200 °C during 3 h.
Results and discussion Several reactions are possible after mixing the starting components [see Reactions (1)–(4)] [12]. CA through remaining hydroxyl groups reacts with isocyanate groups of PIC. Cross-linking of two different molecules of CA and the different cellulose structural units within the same molecule occur. As a result of the reaction, proceeding at the room temperature, urethane groups are formed (1). ½CAOH þ OCN½PIC ! ½CAOCðOÞNH½PIC
ð1Þ
Since solvents were not dried prior to use gels contained water which reacted with NCO– groups of PIC (2) leading to the CO2 evolution and formation of NH2-groups. H2 O þ OCN½PIC ! NH2 ½PIC þ CO2
ð2Þ
Sufficient quantities of H2O and PIC in the reaction system could lead to considerable CO2 emission which results in the development of macroporosity in gels which remains in cross-linked xerogels [12, 14, 15]. Reaction between freshly formed –NH2 groups and NCO-groups (3) is among other possible reactions: ½PIC NH2 þ OCN ½PIC ! ½PICNHCðOÞNH½PIC
ð3Þ
As a result of the above mentioned reactions, formation of three-dimensional CA cross-linked with-PIC occurs through urea and urethane groups. Fumed silica also contains hydroxyl groups on its surface, which react with remaining NCO-groups thus immobilizing cross-linked polymer. Si OH þ OCN½PIC ! SiOCðOÞNH½PIC
ð4Þ
Heating of the xerogel in an inert atmosphere leads to pyrolysis of CA cross-linked with PIC resulting in the formation of carbon and gaseous low-molecular organic compounds. One can expect CA to be responsible mainly for the formation of the majority of low-molecular organic compounds [16] while immobilized PIC is expected to carbonize directly during heat treatment [17] resulting in
the porous three-dimensional carbon network within silica template. According to SEM (Fig. 1a, b) and low-temperature nitrogen adsorption (Fig. 2), thermally treated template samples are porous carbonaceous composites containing macropores (10–50 lm, Fig. 1a, b) and mesopores (18–26 nm, Fig. 2). Using NiCl2 does not affect surface area of carbonaceous composite specific surface area of Ni-containing composite is 231 m2/g which is close to specific surface area of control sample synthesized without addition of precatalyst—225 m2/g. NiCl2 transforms to metallic Ni under heat treatment in the presence of products of precursor pyrolysis. Formation of metallic Ni was proven with XRD (Fig. 3). Macropores of carbonaceous composite synthesized with addition of NiCl2 contain bundles of bamboo-like CNTs (Fig. 1a, c). According to SEM (Fig. 1a, c) and STEM (Fig. 1b), CNTs are 24–38 nm in diameter and their wall thickness is 9–11 nm. Low loss BSE SEM images reveal compositional contrast between body of CNTs and their tips (Fig. 1c). Taking into account XRD data showing presence of metallic nickel (Fig. 3) and shape of contrasting inclusions one can assume that tips of CNTs contain Ni nanoparticles thus demonstrating a typical catalytic tip growth of the CNTs [18, 19]. Most probably Ni nanoparticles were templated by mesopores of composite since size of Ni nanoparticles and CNTs is close to the size of mesopores. Absence of graphitic peaks in the XRD patterns of CNT-containing composites is most probably due to relatively low weight ratio of CNT to the amorphous silica template and porous carbon. Carbonization of CA cross-linked with PIC in the fumed silica template in the absence of NiCl2 did not result in the growth of CNTs (Fig. 1b). Formation of CNTs was not observed upon carbonization of non-templated films of cross-linked CA even though NiCl2 was used as precatalyst. Heat treatment of casted films resulted in formation of carbon films containing Ni particles with the size of 100–200 nm which is significantly larger than the size of Ni nanoparticles formed in fumed silica templates (Fig. 4). XRD confirmed formation of metallic Ni in films (Fig. 3b). It is worth noting that films do not have considerable macroporosity that might be explained by lower viscosity of casted polymer solutions comparing to gels containing fumed silica: non-viscous solutions allowed escaping of formed CO2 from reaction media. To our point of view, combination of closed macropores formed by evolved CO2 (reaction [3]) and mesopores is essential for the catalytic formation of CNTs from CA. Macropores collect gases emitted during pyrolytic decomposition of CA while mesopores present on the walls of macropores provide formation of Ni nanoparticles. Thus,
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Fig. 1 Composites synthesized by carbonization of CA cross-linked with PIC in fumed silica template. a SEM image of composite synthesized with addition of NiCl2. b SEM image of composite
synthesized in the absence of NiCl2. c SEM image of bundle of CNTs in the macropore of the composite synthesized with addition of NiCl2. d STEM image of CNT
Fig. 3 XRD patterns of carbonized CA cross-linked with PIC in the presence of NiCl2. a composite containing fumed silica. b casted nontemplated film
Fig. 2 Isotherms of low-temperature Nitrogen adsorption and the pore-size distribution of CA cross-linked with PIC carbonized in fumed silica templates
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macropores in the composite act as chemical vapor deposition (CVD) microreactors allowing to maintain atmosphere necessary for reaction without using special equipment required for conventional CVD process. Previously we reported formation of CNTs by pyrolysis of non-carbonizing polymer polystyrene in silica template [20], however, diameter of tubes was considerably larger because of larger pore size and the yield of nanotubes was
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while carbon formed on the surface of fumed silica particles has up to 2.1 atomic % of nitrogen (Fig. 5b, c) which apparently originates from –NCO groups of PIC. The method of CNT synthesis reported here is technologically simple and energetically efficient since it does not require performing the multiple stages and does not require special equipment for CVD synthesis. Produced CNT can be used either in conventional way after template removal or within received CNT containing porous composites. Such CNT containing porous composite is expected to be prospective for use in supercapacitors or lithium-ion batteries, or for hydrogen storage. Fig. 4 Carbon synthesized from casted non-templated CA–PIC film containing NiCl2
Conclusions In the present work, we have reported for the first time onepot synthesis of CNTs by pyrolysis of CA cross-linked with polyisocyanate in the fumed silica template in the presence of pre-catalyst NiCl2. The diameter of CNTs is 24–38 nm and the wall thickness is 9–11 nm. The main value of the method is utilization of readily available plant-derived material—CA. In addition, the method is technologically simple, does not require special equipment, and is energetically efficient. Acknowledgements Wallenberg Wood Science Center (WWSC) funded by Knut and Alice Wallenberg Foundation is greatly acknowledged for the financial support. This work was performed in the frame of Wallenberg Wood Science Center (WWSC) activities funded by Knut and Alice Wallenberg Foundation and Chalmers area of advance for Production.
References
Fig. 5 EDX spectra (a, b) and elemental composition (c) of bundle of CNTs (a, c) and Ni containing porous composite synthesized by carbonization of CA cross-linked with PIC in fumed silica template (b, c). Error bars represent standard deviation calculated from three measurements
much lower since no catalyst was utilized and no closed space allowing collecting of pyrolitic gases (such as macropores in the present work) was provided. The role of CA as a main precursor for CNTs could be proven by elemental composition of CNTs measured with the EDX—no presence of nitrogen was observed (Fig. 5a);
1. Iijima S (1991) Nature 354:56 2. Schnorr JM, Swager TM (2011) Chem Mater 23:646 3. Prasek J, Drbohlavova J, Chomoucka J, Hubalek J, Jasek O, Vojtech A et al (2011) J Mater Chem 21:15872 4. Coleman KS (2012) Annu Rep Prog Chem Sect A 108:478 5. Goodell B, Xie X, Qian Y, Daniel G, Peterson M, Jellison J (2008) J Nanosci Nanotechnol 8:2472 6. Qu J, Luo C, Cong Q, Yuan X (2012) Environ Chem Lett 10:153 7. Kang Z, Wang E, Mao B, Su Z, Chen L, Xu L (2005) Nanotechnology 16:1192 8. Paul S, Samdarshi SK (2011) New Carbon Mater 26:85–88 9. Klemm D, Heublein B, Fink HP, Bohn A (2005) Angew Chem Int Ed 44:3358 10. Edgar KJ, Buchanan CM, Debenham JS, Rundquist PA, Seiler BD, Shelton MC, Tindall D (2001) Prog Polym Sci 26:1605 11. Naboka O, Rodriguez K, Toomadj AF, Sanz-Velasco A, Toriz G, Lundgren P, Enoksson P, Gatenholm P (2013) Mater Sci Forum 730–732:903 12. Saunders JH, Frisch KC (1962) Polyurethanes. Chemistry and Technology. Part1, Chemistry. Wiley, New-York 13. Chen C, Wang L, Huang Y (2009) Mater Lett 63:569 14. Lee J, Kim G-H, Ha C-S (2012) J Appl Polym Sci 123:2384
123
J Mater Sci 15. Ashida K, Iwasaki K (1995) In: Landrock AH (ed) Handbook of plastic foams: types, properties, manufacture and applications. Noyes Publications, Park Ridge, New Jersey 16. Brown WP, Tipper CFH (1978) J Appl Polym Sci 22:1459 17. Biesmans G, Mertens A, Duffours L, Woignier T, Phalippou (1998) J Non Cryst Solids 225:64
123
18. Gavanini FA, Lopez-Damian M, Rafieian D, Svensson K, Lundgren P, Enoksson P (2011) Sensors Actuators A 172:347 19. Zhu Z, Jia J, Kwong F, Ng DHL (2012) Carbon 50:2504 20. Ogenko VM, Dubrovina LV, Goldun OV, Volkov SV, Senkevich AI, Danilenko NI (2006) Inorg Mater 42:515
