1842 Letters to the Editor

Correlation between adsorption and x-ray diffraction studies on viscose rayon based activated carbon cloth

K. KUMAR. R.K. SAXENA, R. KOTHARI, D.K. SURI, N. K. KAUSHIK~ and J.N. BOHRA* Materials Characterisation Division, National Physical Laboratory, New Delhi - 110012, India

‘Department of Chemistry, University of Delhi, Delhi 110007

(Received 22 August 1997: accepted in revised form 7 November 1997)

Key words - A. activated carbon, C. adsorption, x-ray diffraction

Activated carbon fibres, possessing super high surface area, are fascinating materials with novel applications. These materials are of recent origin and therefore require detailed investigation. Dresselhaus et. al. [1] used several techniques for the characterisation of large specific surface area activated carbon fibres (ACF’s). It is now well established that the ACF’s are made up of randomly oriented graphitic microcrystalline units [2,3]. X-ray diffraction is one of the techniques commonly used for the structural characterisation of ACF’s [l-4]. Activated carbons in general show a 002 diffraction maxima at 28 - 25’ and overlapped 100 and 101 maxima (usually identified as the 10 peak) at 28 - 42’. The stack height L,, of the graphitic microcrystallites is determined by the width of the 002 peak. On the basis of x-ray diffraction studies on cellulose-based ACF’s heated under argon, Kaneko et. al. [Z] found that both diffraction peaks near 25 and 42’ grow with the increase of heating temperature. They also proposed a geometric relationship between the microcrystalline structures and specific surface area of ACF’s. The size, mutual orientation and stacking of the microcrystallites are intimately associated with the microporosity and the high surface area. A small crystallite thickness and short range ordering between crystallites is necessary for the realization of super high surface area activated carbons (BET surface area more than 2000 m2g-I).

Nitrogen adsorption studies at 77 K have been performed for high surface area activated carbon cloth separately prepared under a carbon dioxide and nitrogen atmosphere [5]. It was found that above a maximum heat treatment temperature (MHTT) of 1123 K pore widening took place for samples prepared under carbon dioxide.

In the present study an attempt has been made to correlate the results of adsorption measurements on activated carbon cloth [5] with x-ray diffraction investigation of the structure in the same samples.

Samples of viscose rayon cloth were treated with a 4% (v/v) solution of AR-grade ortho-phosphoric acid at 90°C. The treated samples were dried in a hot air oven maintained at 80°C. Pretreated and dried samples were pyrolysed under CO, and N2 atmospheres at different maximum heat treatment temperatures with 30 minutes soaking time at the MHTT. Details of sample preparation and adsorption analysis are given elsewhere [51.

X-ray diffraction patterns of activated carbon cloth (ACC) samples were taken on a Siemens D-500 powder x-ray diffractrometer using CuKa radiation at

Table 1. Conditions of sample preparation, char yield, and BET surface area of the samples as determined with

nitrogen adsorption at 77 K.

40 kV, 14 mA in conjunction with a scintillation counter as detector and graphite monochromator in the diffracted beam. The ACC samples were cut into fine pieces for x-ray diffraction measurements. The thickness of the microcrystallites, denoted by t (or L,), was calculated from the broadening of the 002 peak using the Scherrer equation and a shape factor of 0.94 [6].

Table 1 summarizes preparation conditions, yield

Fig. I: X-ray diffraction patterns of samples prepared under Cop; A (1123 K), B (1173 K), C (1223 K).

Letters to the Editor 1843

Table 2. Inter-layer spacing (d,,,,) and graphitic micro- crystallite thickness (t) as determined by different methods.

I ] data ] model model A I 3.52 I 9.8 I 8.60 I 8.32

B 5.71 4.60 C 4.28 3.55 . D 3.65 10.5 12.0 E 3.65 11.6 - 9.6

I I I 1 , 1 K) 20 30 40 YJ so

2e(Dep)

Fig. 2: X-ray diffraction patterns of samples prepared under N2; D (1123 K), E (1223 K).

and BET surface area of samples investigated by x-ray diffraction. The x-ray diffraction patterns for samples prepared under carbon dioxide atmosphere (samples A, B and C) and under nitrogen (samples D and E) are shown in Figs. 1 and 2, respectively.

It is usually expected that as the heat treatment temperature is increased, a sharpening of the x-ray diffraction peaks is observed due to growth of the graphitic microcrystallites [2]. However in the present study one sees from Fig. 1 that the 10 diffraction peak becomes increasingly broader and the 002 peak disappears completely as the Mm is increased (from 1123 to 1223 K) for samples prepared under carbon dioxide. The broadening of the peaks is an indication that there is a progressive increase in the structural disorder in the samples that were activated under carbon dioxide. The BET surface areas of the activated carbon cloth samples also increased with increasing structural disorder (1145, 2071 and 2682 m*g-1 for samples A, B and C respectively).

The values of the BET surface areas determined with nitrogen adsorption for samples B and C were used to calculate the thickness of the graphitic microcrystallites according to Kaneko’s model proposed for super high surface area ACF’s [2]. The thicknesses of the micro-crystallites were calculated as 5.71A and 4.28A in samples B and C respectively. The stack height (thickness) of the graphitic microcrystallites in ACF’s is usually in the range 9 - lo.&. Thus the disappearance of the 002 peaks in the x-ray diffractrograms of samples B and C is due to the extremely small crystallite thickness. In addition the values of crystallite thickness for samples B and C according to the equation SSA = 2/pt, used by Dresselhaus et. al. [l] are extremely small (Table 2). In this equation SSA is the specific surface area, p is the density of ACF (2.1 g.cm-3) and t is the microcrystallite thickness.

The microcrystallite thickness, t, determined for sample A from the 002 peak (Fig. 1) using the Scherrer equation, is 9.8A. The value oft for sample A was also calculated by using the equation SSA = 2/pt [l] and by the model proposed by Kaneko et.al. [2]. The calculated values of t are in reasonable agreement with the value obtained from the XRD pattern (Table 2). It is interesting to note from Fig. 1 that the overlapped 10 peak does not disappear even at highest bum-off

(sample C), indicating that some short range ordering still remains in the direction parallel to the basal plane (La).

It is now well known that the pore walls in ACF’s are made up of graphitic microcrystallites. Hence a decrease in crystallite thickness may lead to widening of the pores. In the present study it is seen that for samples A, B and C, activated under carbon dioxide at increasing MH?Ts, the microcrystallite thickness (t) follows the order: tA > tB > k. Therefore the widening of the pores as the MHTI’ is increased from 1123 to 1223 K for samples prepared under C02, as reported earlier using adsorption techniques [5], is probably due to the decrease in the thickness of the graphitic microcrystallites. The gradual widening of the pores with increasing activation (decreasing value of t) is shown schematically in Fig. 3. Thus the proposed explanation of the mechanism of pore widening, in samples prepared under carbon dioxide atmosphere, using x-ray and adsorption studies [S], are in good agreement.

Kaneko et. al. [2] observed significant sharp- ening of the diffraction peaks for ACFs heated at high temperatures (1473 - 3173 K) in an inert atmosphere. The MHTT’ of samples D and E, prepared under inert atmosphere (Nz), is not very different. Thus no significant difference in the structures of samples D and E is expected. Sharpening of the 002 peak in the x-ray diffractrograms of these samples is not obvious from Fig. 2. However an indication of the slight sharpening of the 002 peak, when the MHTT is increased from 1123 (sample D) to 1223 K (sample E) is obtained from the FWHM (p) values for these samples, (fi = 8.1” for sample D and 7.4’ for sample E).

From Table 1 it can be noted that when the MHTT is increased from 1123 K (sample D) to 1223 K (sample E) under nitrogen atmosphere, the BET surface area increases from 791 mzg-1 to 989 m*g-1. Crystallite

TA TB Tc =

WAt= co, = =t+,Activatlon

T* C T, cTc

tA > To > Tc

WI< WI < WC

Fig. 3: Schematic mechanism of pore widening in ACF’s prepared under CO2 atmosphere at progressively higher temperatures. T, t, and w are the MHTT, microcrystallite thickness and pore width respectively.

I844 Letters to the Editor

thicknesses (t) as calculated from XRD data for samples D and E also increase from 10.5A to 11.6A. According to the relation SSA = 2/pt, the specific surface area should show an inverse relationship with t. However, the inverse relationship between SSA and t may not be valid when crystallite growth is taking place at higher temperatures under inert atmosphere. The increase in surface area while growth in crystallite thickness is taking place (increase in t value) may be due to the rearrangement of microcrystallites leading to the opening of some closed/partially closed pores thereby increasing surface area. Thus it is not necessesarily true that an increase in the surface area is always accompanied with a decrease in microcrystallite thickness.

The results of the x-ray diffraction studies on activated carbon cloth prepared under a carbon dioxide atmosphere indicate that the ACC becomes progressively disordered as the MH’IT is increased. This disorder leads to pore widening and increasing BET surface areas. These results are in good agreement with our earlier investigation of ACC based on adsorption measurements [5]. In the case of samples prepared under an inert atmosphere, the x-ray and adsoprtion studies seem to indicate that above a MI-ITT of 1123 K

rearrangement of the graphitic microcrystallites takes place leading to the opening up of some closed/partially closed pores.

Acknowledgements The authors are thankful to Dr. Krishan LaI, Head, Materials Characterization Division and Prof. E.S.R. Gopal, Director, National Physical Laboratory, New Delhi, India, for their kind permission to publish this work.

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REFERENCES

Dresselhaus M.S., Fung A.W.P., Rao A.M., di Vittorio S.L., Kuriyama K., Dresselhaus G. and Endo M., Curbon, 1992, 30, 1065. Kaneko K., Ishii C., Ruike M. and Kuwabara H., Carbon, 1992.30, 1075. Ermolenko I.N., Lyubliner I.P. and Gulko N.V., in Chemically Modified Carbon Fibres and Their Applications, VCH Verlagsgesellschaft mbH, D-6940 Weinheim, Germany, 1990. Suzuki T. and Kaneko K., Carbon, 1988,26,743. Kumar K., Kothari R. and Bohra J.N., Carbon, 1997, 35, 703. Bokros J.C., in Chemistry and Physics of Carbon, Vol. 5, Ed. P.L. Walker, Jr., Dekker, New York , 1971, p. 1.

Concentric shell carbon: curling process of graphitic layers

K. YAMADA’, G. BURKHARD*, Y. TANABE~ and A.B. SAWAOKA* 1 Department of Chemistry, The National Defense Academy

Hashirimizu, Yokosuka, 239 Japan ’ Materials and Structure Laboratory, Tokyo Institute of Technology

Midori, Yokohama, 227 Japan

(Received 22 September 1997: accepted in revised form 29 October 1997)

Key Words - A. Fullerene C. transmission electron microscopy (TEM)

Since onion-like carbon spheres were synthesized [I], several investigators have proposed the formation mechanisms of the concentric shell carbon (hereafter spherical carbon) particles on the basis of the microstructural information proved by high-resolution electron microscopy (HREM). The first formation mechanism is formation of a corannulene carbon framework followed by a spiral-shell growth [2]. The second formation mechanism is that the regular concentric arrangements of carbon layers in the spherical particles occur through solidification process of carbon droplet under ultrafast condensation [3]. The third formation mechanism is due to a solid + quasi-liquid + solid process: that is, reorganization of a soot containing tubular and polyhedral graphitic particles by irradiation of an intense high energy electron beam [4]. On the other hand, it has been believed that when the sphere formed by the spiral shell growth grows larger, graphitic flakes are nucleated on the surface due to the formation of paired pentagonal-heptagonal carbon rings [5].

In the present paper we describe a formation process of the spherical carbon produced from an oriented pyrolytic graphite by a planar shock wave technique. For this study the HREM technique was used.

The starting material was oriented pyrolytic

graphite cube of side 5mm which was annealed in a argon flow at 2995’C for 1 hour in a carbon resistance furnace. This sample was then placed in a sample chamber 5. lmm in diameter in a copper capsule. In this case the carbon layers in the sample were perpendicular to the capsule axis. Subsequently, the copper capsule was placed inside a 12mm diameter, 60mm long cylinder of 2041 aluminum. The cylinder was embedded in the cavity of a momentum trap recovery assembly [6]. Then a 3.2mm thick pure iron flyer-plate was impinged at a velocity of 2.0km/s explosively driven against the assembly. The shock-compressed sample was removed from the retrieved container mechanically. The electron microscopy was carried out by a JEOL-200CX high- resolution electron microscope.

There were several allotropic forms of carbon formed due to phase transformation of the pyrolytic graphite in the recovered sample, but we shall here concentrate on the study of spherical carbon particle. The results on other carbon allotropes will be reported elsewhere. The size of the obtained spherical carbon particles was in the range of 8 - 70nm in diameter. Figure 1 depicts a HREM image of small spherical carbon particles. As seen in the photograph, the particle exhibits clearly a spiral-shell structure. The interlayer distance was about 0.34nm corresponding approximately