Cellulose (2011) 18:1481–1485 DOI 10.1007/s10570-011-9596-x
Fabrication of carbon nanofiber by pyrolysis of freeze-dried cellulose nanofiber Ehsan Jazaeri • Liyuan Zhang • Xungai Wang Takuya Tsuzuki
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Received: 4 May 2011 / Accepted: 22 September 2011 / Published online: 15 October 2011 Ó Springer Science+Business Media B.V. 2011
Abstract The possibility of fabricating carbon nanofibers from cellulose nanofibers was investigated. Cellulose nanofiber of *50 nm in diameter was produced using ball milling in an eco-friendly manner. The effect of the drying techniques of cellulose nanofibers on the morphology of carbon residue was studied. After pyrolysis of freeze-dried cellulose nanofibers below 600 °C, amorphous carbon fibers of *20 nm in diameter were obtained. The pyrolysis of oven-dried precursors resulted in the loss of original fibrous structures. The different results arising from the two drying techniques are attributed to the difference in the spatial distance between cellulose nanofiber precursors. Keywords Carbon nanofiber Cellulose nanofiber Cellulose pulp Pyrolysis Freeze drying
Introduction Due to their excellent mechanical, thermal and electrical properties, carbon nanofibers (CNFs) have attracted much attention in many applications E. Jazaeri L. Zhang X. Wang T. Tsuzuki (&) Center for Material and Fiber Innovation, Institute for Technology Research and Innovation, Deakin University, Geelong, VIC 3217, Australia e-mail:
[email protected]
including nanocomposites in automotive and aerospace and sensors in electronics (Sevilla and Fuertes 2010; Tran et al. 2009). Various methods have been used to obtain CNFs including chemical vapor deposition (CVD; Uchida et al. 2006) and pyrolysis (Yoshino et al. 1990). The latter is a convenient and simple approach which can reduce the cost of CNF production and environmental concerns associated with CVD (Oh et al. 2005). Micron-diameter carbon fibers are normally produced by the pyrolysis of precursors such as polyacrylonitrile (PAN), oil/coalbased pitch and re-generated cellulose fibers. Unlike PAN and oil/coal based pitch, cellulose is a renewable raw material and widely available on earth with the estimated yearly-production of 1010 tons (Nogi et al. 2010). Cellulose has a high carbon content of 44.4 wt% in addition to 49.4 wt% of oxygen and 6.2 wt% of hydrogen (Khezami et al. 2005). Hence cellulose nanofiber (CellNF) is expected to be an eco-friendly precursor to fabricate carbon nanofibers by pyrolysis. Several groups investigated carbonization of bacterial and tunicate CellNFs and found that the carbon residue retained its fibrous morphology due to the high crystallinity (Ishida et al. 2004; Yoshino et al. 1990; Kim et al. 2001). However, to date, there has been little research conducted on the carbonization of CellNF originated from plant fibers that constitute the majority of cellulose available on Earth. This is due to the scarcity of appropriate fabrication methods. Recently, our group has successfully developed the technology to produce CellNF from natural plant
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products, using an eco-friendly and scalable technique via a top down approach (Zhang et al. 2010). Here we report fabrication of CNF from such CellNF by simple pyrolysis in an inert atmosphere.
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50°, step size of 0.01, time per step of 3 s, and Cu ka ˚ ) radiation generated at 40 kV and (k = 1.54 A 30 mA. Chemical bonds were analysed using a BRUKER VERTEX 70 Fourier transform infrared (FT-IR) spectrometer with the OPUS 5.5 analysis software.
Methods CellNF preparation
Results and discussion
In order to prepare CellNF, dried soft-wood pulp (NIST standard material RM 8495 Northern Softwood Bleached Kraft Pulp) was suspended in water for 48 h and then ball-milled for 90 min using zirconia balls of 0.5 mm in diameter (Zhang et al. 2010). The resulting CellNF aqueous suspension was subsequently dried using two methods; (i) oven-drying in a glass container at 50 °C over night under ambient atmosphere and (ii) freeze-drying by first dipping the CellNF aqueous suspension in liquid nitrogen and then vacuum-drying for 24 h.
Figure 1 shows thermo-gravimetric graphs of the oven-dried and freeze-dried CellNFs. They follow the usual trend in cellulosic decomposition temperature (Cao and Tan 2002), starting from around 250 °C. The mass loss of carbon residues obtained by ovendried and freeze-dried CellNFs are 70 and 93%, respectively, as shown in Fig. 1. Figure 2 shows X-ray diffraction profiles of the dried CellNF and their carbon residue after pyrolysis. The peak at 22.5° for the carbon residue from oven-dried and freeze-dried CellNF is broad, indicating that the crystalline structure of cellulose was destroyed during heat treatment and amorphous carbon was formed. The diffraction peak associated with (002) lattice plane of graphitic carbon was not observed at 26°, indicating that the heat treatment up to 600 °C did not induce graphitization. Figure 3 shows Fourier transform infrared (FT-IR) absorption spectra of the freeze-dried and oven-dried samples before and after pyrolysis. Freeze-dried and oven-dried CellNF show identical spectra, indicating that drying technique does not change characteristic chemical bonds of the cellulose. Both carbon residues show a peak at 1,540 cm-1 which is assigned to C=C bands (Shin et al. 1997), suggesting that the
Pyrolysis Dried CellNF samples were then carbonized using a Lindberg/Blue tube furnace under N2 gas with the flow rate of 60 ml/min. The samples were heat-treated in three stages: (1) increase the temperature from 20 to 240 °C with the rate of 5 °C/min and held for 1 h, (2) increase the temperature to 350 °C with the heating rate of 1 °C/min and (3) increase the temperature to 600 °C with the heating rate of 10 °C/min. The specimens were then cooled down to room temperature. Characterization Morphological study of CNF was undertaken using a Supra 55 VP scanning electron microscopy (SEM) instrument. Carbon samples were mounted on a carbon tape and vacuumed for 2 h before being placed in the SEM chamber for observation. Thermal analysis was undertaken by simultaneous thermogravimetric (TG) and differential scanning Calorimetry (DSC) analysis using a Netzsch 407 PC Luxx instrument with the NETZSCH Proteus analysis software. CellNF samples were mounted on the crucible and heated with the same stages as the pyrolysis under N2 atmosphere. The crystallinity of the CNF was studied using a Panalytical XRD instrument with 2h between 5° and
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Fig. 1 TG graphs of CellNFs and their carbon residues
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Fig. 2 XRD diffraction spectra of CellNFs and their carbon residues a freeze-dried and b oven dried
carbonization of cellulose is progressed. The peaks at 880 and 1,336 cm-1 attributed to CH2 and O–H groups are still visible in both carbon materials, and suggest that the carbonization has not been completed at 600 °C. Figure 4a shows the scanning electron microscopy (SEM) image of oven-dried CellNF. It is evident that the sample consisted of fibers with diameters smaller than 100 nm. The Oven-dried CellNF network appears greatly compacted and little spatial distance exists between the fibers. Figure 4b shows the SEM image of freeze-dried CellNF. The fibers have greater spatial distance between each other than in oven-dried samples. Figure 4c is the SEM image of the carbon residue from oven-dried CellNF, showing that the original fiber structure is destroyed during pyrolysis. On the contrary, when CellNF was freeze-dried, original fibers tend to keep their morphology after
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Fig. 3 FTIR spectra of CellNFs and their carbon residues a freeze-dried and b oven-dried
pyrolysis (Fig. 4d). The fiber-diameter distribution of the CNFs obtained from freeze-dried CellNF was estimated from SEM images (Fig. 5) with a mean diameter of 20 nm. The difference in fibrous morphology of the carbon residues between the two drying methods can be explained from the viewpoint of the spatial distance between the fibers. It is known that, cellulose goes through thermal decomposition between 150 and 400 °C and further aromatization above 400 °C starting with a dehydration (150–240 °C) followed by depolymerisation (240–400 °C; Sekiguchi et al. 1983). Tang and Bacon (1964) suggested that between 240 and 400 °C cellulosic rings start to break down which leads to the formation of highly active free radicals. They cause formation of gaseous products such as CO2 and CO as well as H2O and tar. Due to the hydrophilic property of cellulose and upon oven-
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Fig. 4 SEM images of a carbon residue from oven-dried CellNF, b oven-dried CellNF, c freeze-dried CellNF, and d carbon residue from freeze-dried CellNF
hand, freeze drying creates sufficient space between the individual fibers whereby removal of gaseous and tarry by-products from the sample becomes faster by the flow of an inert gas, which allows individual fibers to remain relatively isolated from adjacent fibers.
Conclusion
Fig. 5 Fiber diameter distribution of the CNFs obtained from freeze-dried CellNF
drying the CellNF, the hydroxyl groups tend to bridge between the cellulosic chains of neighbouring nanofibers via hydrogen bonds. Thus the fibers tend to be in close contact with each other in the oven-dried sample when the free radical mechanism is initiated. Viscous tar may connect fibers with each other when their distance is very short, leading to the disappearance of fibrous structure in oven-dried samples. On the other
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In this study, the fabrication of amorphous carbon nanofibers of *20 nm in diameter from a renewable material was demonstrated via pyrolysis of freezedried cellulose nanofibers made from softwood pulp. It is shown that the method to dry cellulose nanofibers prior to pyrolysis largely affects the final fibrous structure of the carbon residue, mainly due to the fact that different drying conditions lead to different spatial distance between the cellulose nanofibers; short distance between cellulose nanofibers results in the disappearance of fibrous morphology during carbonization. Optimization of the carbonization conditions and graphitization at higher temperatures are subject to further study. The present work opens up the possibility of economical and eco-friendly production
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of CNF with diameters less than 50 nm for many applications. Acknowledgments The authors thank Dr Batchelor at the Australian Pulp and Paper Institute in Monash University for providing the dried soft-wood pulp.
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