Carbon Nanofibers Synthesized from Electrospun Cellulose for

Materials Science Forum Vols. 730-732 (2013) pp 903-908 Online available since 2012/Nov/12 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.730-732.903

Carbon Nanofibers Synthesized from Electrospun Cellulose for Advanced Material Applications Olga Naboka1,a, Katia Rodriguez2,b, A. Farshad Toomadj1,c, Anke Sanz-Velasco1,d, Guillermo Toriz2,e, Per Lundgren1,f, Peter Enoksson1,g and Paul Gatenholm2,h 1

Wallenberg Wood Science Center and Department of Microtechnology and Nanoscience – MC2, BioNano Systems Laboratory, Micro and Nanosystems group, Chalmers University of Technology, SE-412 96, Göteborg, Sweden 1

Wallenberg Wood Science Center and Department of of Chemical and Biological Engineering, Biopolymer Technology, Chalmers University of Technology, SE-412 96, Göteborg, Sweden a

[email protected], [email protected], [email protected], [email protected], [email protected] [email protected], g [email protected], [email protected]

d

Keywords: Carbon nanofibers, electrospun cellulose, alkaline regeneration.

Abstract. Carbon nanofibrous sheets (conductivity 1.9 to 35.5 S×cm-1, water contact angle up to 137°) consisting of amorphous fibers with diameter of 20 – 150 nm (C:O atomic ratio 25.4 – 86.0) were synthesized by carbonization of cellulose regenerated from electrospun cellulose acetate mats with three methods of alkaline deacetylation. It was established that C:O atomic ratio, conductivity and hydrophobicity depended on the regeneration method and on the temperature of carbonization. The highest flexibility, lowest conductivity and lowest water contact angle was observed for carbon synthesized from cellulose regenerated with NaOH in ethanol (0.05 mol/l) for 24 hours at room temperature. The highest conductivity, highest water contact angle and lowest flexibility was observed for carbon synthesized from cellulose regenerated with water solution of NaOH/NaCl (3.75 M NaOH, 2.1 M NaCl) during 15 minutes at 65°C. Introduction Carbon nanofibrous materials are among the most important representatives of nanostructured materials. Synthesis of carbon nanofibers with predefined structure and properties allows their extensive application in various fields ranging from high tech smart materials for aerospace and electronics to automobile, sport and housing applications [1]. At present, the most used precursors for carbon fiber synthesis are coal tar pitch and polyacrylonitrile [1,2] but in order to approach a sustainable society renewable resources such as wood derived cellulose should be considered. Besides the chemical structure of the carbonization precursor and exposure to external shape forming factors (such as the use of matrices and control of carbonization conditions) macromolecular organization of polymers (aggregate formation in solutions, crystalline structure) could also significantly affect the properties and morphology of synthesized carbon [3,4]. It is well established that the crystalline structure and mechanical performance of regenerated cellulose is affected by the chemical composition, time, and temperature used in the hydrolysis of cellulose derivatives [5-7]. In this paper carbonization of electrospun cellulose (EC) mats regenerated from electrospun cellulose acetate (CA) in alkaline medium by three different methods was carried out in order to study the effect of cellulose regeneration conditions on the properties of the synthesized carbon nanofibers.

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Experimental Materials. Cellulose acetate (CA) with Mn~30,000 and 39.9 wt% acetyl content (Aldrich), NaOH (analytical grade, Riedel-de-Haën), NaCl (analytical grade, Riedel-de-Haën), acetone (analytical grade, Solveco), ethanol (analytical grade, Solveco), dimethylacetamide (99.9%, Sigma Aldrich) were used as received without additional purification. Electrospinning of cellulose acetate. Cellulose mats were produced by electrospinning of CA. CA solution of 17% Wt/Wt in acetone/dimethylacetamide 2:1 ratio was used. CA solution was fed at 1 ml/h through a stainless steel needle with a diameter of 0.643 mm (Howard Electronic Instruments Inc.) at 25 kV applied voltage. The distance between needle and collector was 25 cm. The electrospun CA fibers were deposited on steel mesh covered with aluminum foil at 22°C and 60% of relative humidity. Regeneration to Cellulose. Three methods of alkaline treatment were chosen for deacetylation of electrospun cellulose acetate: 1. NaOH in ethanol (0.05 mol/l) for 24 hours at room temperature (EC1); 2. NaOH in ethanol (0.05 mol/l) for 24 hours at room temperature with additional treatment with a water solution of NaOH at pH=8 and 80°C during 2 hours (EC2); 3. Water solution of NaOH/NaCl (3.75 M NaOH, 2.1 M NaCl) during 15 minutes at 65°C (NaCl was introduced in order to avoid swelling of cellulose [8]). 5 ml of ethanol was added to each 100 ml of solution for better wetting of hydrophobic CA (EC3). Regenerated cellulose sheets were thoroughly rinsed and kept overnight in deionized water. Drying was carried out in air on polystyrene Petri dishes. Carbonization. The carbonization of EC samples placed between silicon wafers was carried out in a Tempress quartz tube furnace in N2 flow (1l/minute) by heating up to 750, 800, 900, 1000 and 1100°C (heating rate was 5°C/min). Samples were kept at the highest temperature for 2 hours before cooling down. Before any heating the furnace was purged with N2 for 5 minutes. Scanning Electron Microscopy (SEM). To perform SEM, native (uncoated) samples were placed on sample holders with conductive double-sided carbon glue tape. The acceleration voltage was chosen to be in the range of 1.0 to 1.5 kV. Energy dispersive X-Ray microanalysis (EDX) was performed with Oxford Inca EDX system at 5.0 kV. Fourier-Transform Infrared Spectroscopy (FTIR) was performed with Perkin Elmer System 2000 FT-IR spectrometer in 4000-370 cm-1 (resolution 4 cm-1, averaging over 20 scans) in transmission mode. Spectra were recorded using tablets prepared from 0.5 mg of EC and 100.0 mg of KBr. X-Ray Diffraction Analysis (XRD) was carried out with Philips X'Pert Materials Research Diffractometer (MRD). Radiation was generated with an X-ray tube with Cu anode (Kα radiation, λ=1.54184 Å) at 45 kV and 40 mA. An X-Ray lense with Ni filter was used as incident optics, and a parallel beam collimator was used as diffracted optics. The 2θ range was 10-60°, and the resolution was 0.05° with 10 seconds averaging time per step. The Electrical Conductivity measurements were performed by using a 4-point probe system (CMT-SR2000N, AIT, Korea). Water Contact Angle measurements were carried out by the sessile droplet technique (10 µm droplet volume). Results and Discussion Flexible thin mats of 65-95 µm thickness were produced by electrospinning of CA. Mats consist of randomly oriented fibers with 0.3-1.5 µm in diameter. Regenerated EC1 and EC2 fibers had the same morphology as electrospun CA fibers (Fig. 1 a) whereas EC3 fibers were twisted and melted together (Fig. 1 b). High starting temperature of deacetylating solution and presence of ethanol could be reason for the partial melting and sintering of cellulose acetate fibers before they turned to thermosetting cellulose fibers.

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Fig. 1. EC obtained by different regeneration methods: a – EC1, b – EC3

FTIR spectroscopy showed that CA fibrous mats (Fig.2 a) were characterized by bands assigned to stretching vibration of C=O (1756 cm-1) and alkoxyl stretch of the C-O-C (1241 cm-1) from ester groups, C-CH3 bending for groups of the acetate substituent (1383 cm-1), acetal linkages of cellulose backbone (1161 cm-1) and hydroxyl group adsorption (3492 cm-1) [9]. FTIR spectra of all regenerated EC samples were characterized by the absence of carbonyl peaks and significant broadening of the hydroxyl peak which points to complete deacetylation of CA [5, 9]. XRD patterns of regenerated samples have peaks around 12°, 20° and 22° (fig 2b) that could be assigned to the crystal structure of cellulose II which usually forms upon cellulose regeneration [5, 9, 10]. EC3 sample diffraction peaks from 20 to 22° were narrower and better defined (sharper) than the corresponding peaks of the EC2 and EC1 samples. This fact suggested that the EC3 sample had the highest degree of crystallinity of the three samples.

Fig. 2. Characterization of regenerated EC with FTIR spectroscopy (a) and XRD (b) Carbonization of EC leads to formation of carbon sheets with thicknesses in the range 25 – 42 µm. Carbon sheets obtained from EC1 and EC2 were rather flexible (Fig. 3a) whereas sheets obtained from EC3 were quite brittle. The total weight loss for 750 – 1100°C carbonization temperature was 82.7 – 86.6% which is 30.5 – 39.3% from the nominal content of carbon in cellulose. The yield of carbon received from EC3 was higher than the yield of carbon received from EC1 and EC2 (for example 39.3% compared to 31.7 and 32.3% respectively for 1000°C carbonization temperature) which could be a consequence of the higher degree of crystallinity of the EC3 starting material. The SEM investigation showed that carbon samples consisted of fibrils of 20 – 150 nm in diameter (Fig. 3 b-d). In general fibers maintained the morphology of the cellulose precursor – nanofibers of carbonized EC1 and EC2 were straight (Fig. 3 b,c), nanofibers received from EC3 were twisted and melted together (Fig. 3. d).

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Fig. 3. Carbon, synthesized by carbonization of EC1-EC3: a – flexible carbon sheet carbonized at 750°C, b – carbon fibers from EC1 (synthesized at 800°C), c – carbon fibers from EC2 (synthesized at 800°C), d – carbon fibers from EC3 (synthesized at 1000°C).

All observed carbon nanofibers (regardless of precursor and carbonization temperature) display a granular structures with grain size in the range 8 – 16 nm (Fig. 4a) similar to that observed in [11]. The size and morphology of the fibers do not depend on the carbonization temperature. According to XRD analysis carbon synthesized from EC1-EC3 at 750 - 1100°C is amorphous (Fig. 4b).

Fig. 4. a – structure of carbon nanofiber synthesized from EC2 (carbonization at 1000°C), b – XRD patterns of carbon samples synthesized at 1100°C.

Oxygen content in carbon decreased with carbonization temperature, as determined by EDX (Fig. 5a). The lowest oxygen content (C:O atomic ratio 36.3 – 86.0) was observed for carbon synthesized from EC3 (highest crystallinity), the highest (C:O atomic ratio 25.4 – 56.0) for EC1 (lowest crystallinity) (Fig. 5a).

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Fig. 5. Dependence of atomic C:O ratio determined by EDX (a) and conductivity (b) on the carbonization temperature. The same dependence of conductivity (Fig. 5b) and water contact angle (Fig.6) on the carbonization temperature and method of regeneration was found for carbon sheets. The temperature dependence of the electrical conductivity is suggested to be related to the increased quality, size and density of conductive carbon nanoclusters formed in the fibers with higher carbonization temperatures; resulting in clusters of higher intrinsic conductivity as constituents of a more dense percolation network [12]. The lowest conductivity was observed for carbon synthesized from EC1 at 750°C (1.9 S×cm-1) and the highest for carbon from EC3 carbonized at 1100°C (35.5 S×cm-1).

Fig. 6. Water contact angle of carbon samples received at 1000°C from EC1 (a) and EC 2 (b). Carbon sheets with a broad range of contact angles were obtained depending upon the type of regeneration and on the carbonization temperature: from highly hydrophilic (EC1 carbonized at 800°C immediately and completely adsorbed water during contact angle measurements) to highly hydrophobic (EC2, EC3 carbonized at 1100°C had contact angle 132-137°, Fig. 6). The oxygen content determined by EDX corroborated the contact angle behavior. Summary In the present work the influence of electrospun cellulose regeneration method and carbonization temperature on the properties of the resulting carbon nanofibers was investigated. It was found that synthesized carbon consists of nanofibers of 20 – 150 nm in diameter which have a granular structure (grain size of 8-16 nm). By varying the methods of regeneration and the carbonization temperature, the properties of the synthesized carbon could be controlled in a wide range: conductivity – from 1.9 to 35.5 S×cm-1; water contact angle – from complete water adsorption to 137°; from flexible to brittle sheets.

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Carbon nanostructured materials with controlled properties synthesized in this project are prospective for use as membranes, adsorbents, thermal insulators and electrodes for fuel cells and supercapacitors. Acknowledgement WWSC Center funded by Knut and Alice Wallenberg Foundation is greatly acknowledged for the financial support. References [1] P.Morgan, Carbon Fibers and their Composites, Taylor&Fransis, Boca Raton, 2005. [2] G. Gellerstedt, E. Sjöholm, I. Brodin,The wood-based biorefinery: a source of carbon fiber?, The Open Agriculture J. 3 (2010) 119-124. [3] L. Dubrovina et al., Carbon-loaded porous composites produced by matrix carbonization of Poly(vinylidene fluoride), Inorganic Materials 44 (2008) 697-704. [4] O. Ishida, D.-Y. Kim, S.Kuga et al., Microfibrillar carbon from native cellulose, Cellulose 11 (2004) 475-480. [5] Y. Chen, X.-p. Xiong, G. Yang, Characterization of regenerated cellulose membranes hydrolyzed from cellulose acetate, Chinese J. of Polymer Sci. 20 (2002) 369-375. [6] I.S. Kim, J.P. Kim, S.Y. Kwak et al., Novel regenerated cellulosic material prepared by an environmentally-friendly process, Polymer 47 (2006) 1333-1339. [7] M. Zimmerley et al. Molecular orientation in dry and hydrated cellulose fibers: a coherent antistokes raman scattering microscopy study, J. Phys. Chem. B 114 (2010) 10200-10208. [8] V.A. Bakunov, G.A. Budnitskii, L.F. Maiboroda Hollow cellulose acetate fibers for hemodialysis, Khimicheskie Volokna, No. 6 (1979) 483-485 [9] K Rodriguez, S Renneckar, P. Gatenholm, Biomimetic calcium phosphate crystal mineralization on electrospun cellulose-based scaffolds, ACS Appl. Mater. Interfaces 3 (2011) 681689. [10] D. Klemm, B. Heublein, H.-P. Fink, A. Bohn, Cellulose: fascinating biopolymer and sustainable raw material, Angew. Chem. Int. Ed. 44 (2005) 3358-3393. [11] Y. Hishiyama, A. Yoshida, Y. Kaburagi, Resistivity, Hall coefficient, magnetoresistance, and microtexture of cellulose carbon films, Carbon, 31 (1993), 1265-1272. [12] Y.-R. Rhim et al. Changes in electrical and microstructural properties of microcristalline cellulose as function of carbonization temperature, Carbon, 48 (2010), 1012-1024.

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Carbon Nanofibers Synthesized from Electrospun Cellulose for Advanced Material Applications 10.4028/www.scientific.net/MSF.730-732.903 DOI References [3] L. Dubrovina et al., Carbon-loaded porous composites produced by matrix carbonization of Poly(vinylidene fluoride), Inorganic Materials 44 (2008) 697-704. http://dx.doi.org/10.1134/S0020168508070054 [4] O. Ishida, D. -Y. Kim, S. Kuga et al., Microfibrillar carbon from native cellulose, Cellulose 11 (2004) 475-480. http://dx.doi.org/10.1023/B:CELL.0000046410.31007.0b [6] I.S. Kim, J.P. Kim, S.Y. Kwak et al., Novel regenerated cellulosic material prepared by an environmentally-friendly process, Polymer 47 (2006) 1333-1339. http://dx.doi.org/10.1016/j.polymer.2005.12.070 [9] K Rodriguez, S Renneckar, P. Gatenholm, Biomimetic calcium phosphate crystal mineralization on electrospun cellulose-based scaffolds, ACS Appl. Mater. Interfaces 3 (2011) 681- 689. http://dx.doi.org/10.1021/am100972r [10] D. Klemm, B. Heublein, H. -P. Fink, A. Bohn, Cellulose: fascinating biopolymer and sustainable raw material, Angew. Chem. Int. Ed. 44 (2005) 3358-3393. http://dx.doi.org/10.1002/anie.200460587 [11] Y. Hishiyama, A. Yoshida, Y. Kaburagi, Resistivity, Hall coefficient, magnetoresistance, and microtexture of cellulose carbon films, Carbon, 31 (1993), 1265-1272. http://dx.doi.org/10.1016/0008-6223(93)90085-O [12] Y. -R. Rhim et al. Changes in electrical and microstructural properties of microcristalline cellulose as function of carbonization temperature, Carbon, 48 (2010), 1012-1024. http://dx.doi.org/10.1016/j.carbon.2009.11.020