~. Cellulose 11: 475-480, 2004. " © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 475

Microfibrillar carbon from native cellulose

O. Ishida l , D.-Y. Kim l , S. Kuga l ,*, Y. Nishiyama l and R. Malcolm Brown, 1r. 2

I Department of Biomaterials Science, Graduate School of Agriculture, University of Tokyo, Yajoi 1-1-1, Bunkyo-ku, 113-8657 Tokyo, Japan; 2 Molecular Genetics and Microbiology, University of Texas at Austin, 1 University Station A5000, Austin, TX 78712-0162, USA; *Author for correspondence (e-mail: skugaCi:~sbpfp·a.u-tokyo.acjp)

Received 4 November 2003; accepted in revised form 12 April 2004

Key words: Carbon, Freeze-drying, Graphite, Microfibril, Pyrolysis, Surface area

Abstract

Use of pyrolytic carbon from cellulose has been limited in practice to activated adsorbent carbon, but cellulose-derived carbon retaining the nanoscale microfibrillar morphology offers rich possibilities as an advanced material. Here we developed novel methods to prepare such materials by an improved drying of wet cellulose prior to pyrolysis. This procedure is an adaptation from electron microscopy techniques, i.e. rapid freeze drying of suspension and solvent exchange drying, both being effective in preventing coagu­lation of cellulose microfibrils/microcrystals. Pyrolytic carbon from such material has a large external surface area, with the graphitic carbon crystallites roughly aligned along the fiber axis. These features are potentially useful in developing novel carbon nanomaterials for electrodes, catalyst supports, or composite material elements.

Introduction On the other hand, however, it is known that the structure of carbon material is strongly dependent

Organic substances can be carbonized by heating on that of the original organic solid. Regarding the to 250-600 °c in an inert atmosphere, and this raw material, there is a distinction between process is utilized in the production of various 'graphitizable' and 'nongraphitizable' carbons. In carbon materials. While cellulosic materials are general, nonmelting solids such as cellulose belong used as carbon precursors on a large scale, their to the latter. While the common sources of cellu­use is mostly limited to activated carbon as an losic carbon are those of higher plants, there are adsorbent. High-strength graphite fibers were special types of native cellulose with unusually produced from rayon in the early days, but it was high crystallinity. replaced by poly(acrylonitrile) (Bahl et al. 1998). Figure I shows schematically the shape of native The reason is twofold: (i) even though cellulose has microfibrils of higher plants, algaljtunicate, and a carbon content of 44.4%, its pyrolytic carbon bacterial celluloses (see e.g. Brown 1982). Car­yield is as low as 10-20% at 500°C and even less bonization of these highly crystalline celluloses can than 10% at 1000 °C; (ii) cellulose does not soften give novel forms of pyrolytic carbon. In fact, we or melt by heating except under special conditions previously found strong correlations between (Nordin et al. 1973; Back et al. 1974), and there­ crystallinities of the starting cellulose and the de­fore cannot be stretched during pyrolysis for rived high-temperature-treated carbon (Kim et al. improving crystallinity and orientation of graphite 200Ia). Here we report attempts to develop novel, crystallites. nanofibrillar forms of carbon from native cellulose

476

Higher plants

6 x 6 = 36 chains

Algali tunicate

40 x 40 =1600 chains

10 nm

Acetobacter

16 x 100 = 1600 chains

Figure I. Schematic drawing of microfibrils of higher plants, algae, lunicate, and bacteria.

through improvements in preparation of starting materials, and discuss properties of these materials and the possibilities of their high value-added uses.

Experimental

I. Cellulose materials (a) Cotton-derived microcrystal: Whatman CF I ICellulose Powder was hydrolyzed by heating in 60% sulfuric acid at 60C for 2 h. The solid residue was recovered by centrifugation and dial­ysis, giving an aqueous suspension of highly dis­persed microcrystals. (b) Tunicate cellulose: the mantle of Halocinlhia rorelzi was obtained from a local food market and was washed with water and treated alternately with 4% NaOH and 0.3% sodium chlorite several times, each overnight. (c) Bacterial cellulose: the cellulose pellicle of Acelobacler xylinum, BPR 2001 was purified as in (b). 2. Drying (a) Rapid freeze drying of the microcrystal sus­pension: the microcrystal suspension of 0.2 1.0% solid was sprayed onto a liquid nitrogen-cooled

coppe,~\>?I SPcaY9'"

Figure 2. Rapid freeze drying by suspension spraying.

copper plate as shown in Figure 2. The resulting frost was scraped off repeatedly, collected in a cooled glass bottle, and subjected to a regular freeze dryer. (b) Carbon dioxide critical point drying: the never-dried, water-swollen cellulose mass (tunicate or bacterial) was subjected to solvent exchange of ethanol -liquid CO2, and the latter was evaporated at its critical point using an apparatus for electron microscopy specimens. (c) Solvent exchange drying: the water swollen material was subjected to methanol-i-butyl alcohol exchange, and then dried in vacuo.

The ferred 3. Pyre The cel furnace nitroge lmprov using a gravim in nitfi sample! nace (F for I h graphitl 4. Nitn Surface their ca sorp 101 (Quanta 5. Eject Scannin, formed mlcrosC( EM 420 tern. The grinding mountin

Results ~

Figure 3 Jose mic

Figure 3. Mi (b,c) Scannin mass. (c) Raj

raying.

resulting eted in a

regular

ing: the (tunicate hange of aporated electron

swollen I alcohol

The dried materials obtained as above are re­ferred to as cellulose aerogels in the following. 3. Pyrolysis and thermogravimetry The cellulose aerogel was pyrolyzed in an infrared furnace (RHL-E45PN, Ulvac Co., Tokyo) under nitrogen flow up to 600°C. For some samples, improvement of carbon yield was achieved by using a hydrogen chloride atmosphere. Thermo­gravimetry was conducted by an Ulvac TGD 9600 in nitrogen or HCI atmosphere. Some carbon samples were treated by a high-temperature fur­nace (Fujidempa Kogyo, Tokyo) up to 2200'C for I h under argon atmosphere for developing the graphite-like structures. 4. Nitrogen adsorption measurement Surface characteristics of cellulose aerogels and their carbon samples were evaluated by an Omni­sorp 100 (Beckman-Coulter, USA) or Nova 4000 (Quantachrome, USA). 5. Electron microscopy Scanning electron microscopy (SEM) was per­formed by a Hitachi S4000. Transmission electron microscopy (TEM) was performed by a Philips EM 420 equipped with a GATAN 620 video sys­tem. The specimen for the latter was prepared by grinding the carbon sample in acetone and mounting on a carbon membrane.

Results and discussion

Figure 3 shows TEM/SEM micrographs of cellu­lose microcrystals from Whatman CF 11. The

477

original microcrystals are rod-like particles, 20­50 nm wide and 100 -300 nm long (Figure 3a). This width is significantly greater than that of native microfibril, which is approx. 3.5 nm. This differ­ence is considered to result from secondary coag­ulation during hydrolysis. Regular freeze drying of this suspension resulted in the formation of flake­like mass (Figure 3b). This loss of fibrillar mor­phology is apparently due to coagulation of mi­crocrystals caused by ice crystal formation. In contrast, the spray-dried aerogel retained the fibrillar morphology, approx. 50 nm wide (Figure 3c).

The spray dry technique cannot be applied to the gel-like materials of tunicate and bacterial cellulose; instead these materials can be subjected to solvent exchange by simple immersion-transfer procedures. The cellulose aerogels obtained by CO2 critical point drying or I-BuOH drying gave also much better preservation of fibrillar mor­phology than by regular freeze drying (data not shown). The effectiveness of these drying methods is seen in the results of nitrogen adsorption (Ta­ble 1). Native tunicate and bacterial celluloses consist of 10- 50 nm wide microfibrils (also re­ferred to as 'ribbons' for the latter), correspond­ing to a specific surface area of 100· 200 m2/g. The values for the aerogels in Table I fall in this range, indicating that the microfibrillar entity is well preserved.

The carbon yield of cellulose in nitrogen or vacuum is usually as low as 15% at the 600 JC stage, against the theoretical value of 44.4%. This

(a) (b) (c)

Surface area 2.9 m2/g Surface area 83.7 m2/g

Figure 3. Microscopic morphology of starting cellulose. (a) Transmission electron micrograph of microcrystals from Whatman CFll. (b,c) Scanning electron micrographs of cellulose aerogels. (b) Regular freeze drying caused aggregation of microcrystals into film-like mass. (c) Rapid freeze drying could preserve the microfibrillar morphology.

478

Table 1. Specific surface area of cellulose aerogels from "12

adsorption.

Starting material Drying Surface area method (m 2/g)

Callan micro-crystal suspension FD" 32 Spray FDb 81

Tunicate block FD 25 I-BuOHc 130

Bacterial cellulose block I-BuOH 178

"Regular freeze drying. bSprayed onto cooled copper plate. C Solvent exchange-dried as water· ethanol-r-butyl alcohol.

loss is considered to result from evaporation of levoglucosan (Essig et al. 1989) and its secondary degradation into volatile species such as CO, CO2,

methanol, acetic acid, and furfural. We observed significant improvement in yield by the addition of a small amount of sulfuric acid, which is consid­ered to catalyze dehydration instead of levo­glucosan formation (Kim et al. 2001 b). This method, however, was not applicable to the pres­ent material, since the addition of sulfuric acid caused collapse of cellulose aerogels due to the nonvolatility of sulfuric acid. Therefore we applied pyrolysis in gaseous HCI, which can act on cellu­lose without wetting it, as reported in an old study (Shindo et al. 1969). The effect was remarkable, as shown in Figure 4. All cellulose samples gave a carbon yield of 20 -25% in HCl gas, against 10­15% in nitrogen. This method, as expected, was also effective in preserving the straight microfi­brillar morphology as in Figure 5.

100

80

'#. 60 <J)­

::J "D'w 40 <J)

0::

20 ... --...... ... .,. ...... 0

0 200 Temperalure,'C

Figure 4. Thermogravimetric curve of Whatman CFtl cellu­lose powder in nitrogen (- - - -) and in HCI (-). Dehydrating action of HCI improves carbon yield significantly.

400 600 800

As is generally the case for pyrolytic carbon materials, the ex-cellulose carbon obtained by 600°C treatment is noncrystalline; but further treatment at above 2000 °C results in development of graphene planes and graphitic carbon crystal­lites (Kuga et al. 2002). When the 600C carbon was treated at 2200'C for 60 min, the apparent fibrillar morphology was not changed, but nitro­gen adsorption analysis indicated significant changes in nanoscopic structure (Table 2). The 600°C carbons have BET surface areas of 500­650 m2 jg, several times greater than those of ori­ginal cellulose aerogels. This extra surface area is obviously due to micropore formation by 10w­temperature carbonization (see changes in micro­pore volume in Table 2). The 2200 'c treatment caused significant losses in surface area because of carbon crystallite formation. The behavior, however, is remarkably different for the regular freeze-dried material and cellulose aerogel-based materials. While the former's surface area was re­duced to 84.3 m2,.'g, those of the latter were 137 m2jg for N2 treatment, and 239 m2jg for HCI treatment. This maintenance of surface area obviously results from the fibrillar morphology as seen in Figure 6.

High-magnification ohservation of the 2200 °c_ treated tunicate cellulose (/-BuOH drying) showed the lattice image of carbon crystallites. The lateral size of these crystallites is typically several na­nometers, i.e. one order smaller than the original celulose microfibrils, but they were roughly aligned along the fibrillar axis. This indicates a structure memory effect of the original cellulose crystals. The observed spacing between carbon crystallites is supposedly filled with disordered parts or crys­tallites having the graphene lattices not aligned parallel to the electron beam. Stronger heat treat­ments are expected to promote further develop­ment and better alignment of the carbon crystallites.

To our knowledge, such a microfibrillar form of graphite-like carbon has not been observed in conventional materials. Table 3 shows the ob­served and expected properties of the present material in contrast with other forms of carbon. Large proportions of edge face in graphite-like structures would be advantageous in intercalating complexation (utilized in lithium ion batteries), and adhesion with composite matrices. The special drying method developed here may involve some extra costs for large scale productions, but the

F(~ure .

Jl11prOVI

Table 2.

Drying

FD,HC I-BuOH. I-BlIOH.

"Atmosp hI J1 argo

"Microp<

Tahle 3. (

Natural g;

Conventio Fullerenc Carbon m Fibrous/rc

*Expectecl

479

carbon ned by further

lopment crystal­

, carbon lpparent It nitro­~nificant

2). The of SOO ­~ of ori­ Figure 5. SE:vl image of 600 0(" carbon from spray-dried cotton microcrystal pyrolyzed in (a) nitrogen and (b) HC\. In addition to ~ area is improvement of carbon yield, microfibrillar morphology is presen0d better in (b). Scale applies to both.

by low­1 micro­

Table 2. Surface area and pore volume of carbon from lunicate cellulose.'eatment :cause of Dryil1g method, atmosphere" 600 C I rea ted 2000 DC lreated b

lehavior, Surface Microporec Mesoporec Surface 'vi icropore Mesopore

regular area (m2/g) (cm}/g) (cm}/g) area (m 2/g) (cm3/g) (cm 3/g)

~el-based FD. HCI 533 0.255 0.332 84 0.031 0.247 1-8LlOI-I. N 2 667 0.309 0.468 137 0.051 0364

was re­137 m 2,'g

'-8uOI-l, HCI 549 0.258 0.626 239 0090 0.644 ·eatment.

a:\ tmosphere for pyrolysis. ly results bIn argon atmosphere.

igure 6. cMicropore: 0.5-2 111n: mesojJore: 2 50 11111.

2200'C­) showed Ie lateral feral na­original

~ aligned structure crystals. stallites or crys­aligned

:at treat­develop­

carbon (3)

Figure 6. 2200 °C-treated carbon frol11 I-BuOI-l-dried tunicate cellulose. (a) SEM, (b) TEM, and (c) TEM lattice image.. form of erved in the ob­ Table J. Comparison between carbon materials. present

Morphology Surface area Edge face Adhesibility, carbon. lhite-like Natural graphite Flake Small Small Weak

rcalating Conventional pyrolytic carbon Aggregate Small Small Weak Fullerene Sphere Large None Weakatteries), Carbon nanotubes Tube Large None Weak .e special Fibrous/rod-like carbon Fibril Large (Large)* (Strong)"tve some • Expected plOperties. but the

(b) (c)

480

material would still be several orders less expen­sive than carbon nanotubes or fullerenes, which are based on chemica! vapor deposition. These features would give possibilities of utilization of native cellulose for novel carbon materials for high-strength composites, electrode, or catalyst supports.

Acknowledgements

The authors thank the following for support of this research: S.K. (Grant-in-Aid for Scientific Research, No. 13556022); R.:Y1.B. (Welch Foun­dation Grant F-1217).

References

Back E., "fordin S. and \lyren J. 1974. fndication of mollen cellulose produced in a laser-beam. Text. Res. J. 44: 915-917.

Bahl O.P.. Shen Z, Lavin J.G and Ross RA. 1998. Manu­facture of carbon fibers. In: Don net J.-B., Wang T.K., Peng J.C.M. and Rebouillat S. (eds), Carbon Fibers, 3rd ed. Marcel Dekker. New York, pp. [-83.

Brown R.M. Jr. (ed.) 1982. Cellulose and Other !\atuml Poly­mer Systems. Plenum Press, "J"ew York, Chapters 4, 14 and 19.

Essig 'vr., Richards G.N. and Schenck E. 1989. Mechanism of formation of the major volatile products from the pyrolysis of cellulose. In: Schuerch C. (ed.), Cellulose and Wood ­Chemistry and Technology. Wiley Interscience, New York, pp. 841-862

Kim D.-Y, "J"ishiyama Y., Wada :\1. and Kllga S. 2001a. Graphitization of highly crystalline cellulose. Carbon 39: 1051-1056.

Kim D.-Y., J\ishiyama Y., Wada M. and Kuga S. 2001b. High­yield carbonization of cellulose by sulfuric acid impregnation. CelJulos<: 8: 29-33.

Kuga S., Kim D.-Y., :"Jishiyama Y. and Brown R.M. Jr. 2002. "J"anofibrillar carbon from native cellulose. Mol. Cryst. Liq Cryst. 387: IJ-19.

I\ordin S.B., Nyren J.O. and Back LL. 1973. Molten cellulose produced in a laser-beam. Sv. Papperstidn. 76: 609-610.

Shindo A., Nakanishi Y. and Soma I. 1969. Carbon fibers from cellulose fibers. J. Appl. Polym. Sci.-Appl. Polym. Symp. 9: 271-284

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