Langmuir 1992,8, 1695-1697
1695
Electrical Conductivity of a Single Micrographitic Carbon Fiber with a High Surface Area under Various Atmospheres Jun Imai and Katsumi Kaneko' Department of Chemistry, Faculty of Science, Chiba University, Chiba 263, Japan Received January 21, 1992. I n Final Form: May 23,1992 The temperature and frequency dependencies of the electrical conductivity of a single microporous carbon fiber with a high surface area were determined from 150 to 303 K in the frequency range 20 Hz-1 MHz. The effecta of gas adsorption of 02, NO, N20, SO2, and HzO on the electrical conductivity were also examined. 02 exposure enhanced the conductivity, indicating that the charge carrier is a hole. SO2 adsorption also increased the conductivity,while NO adsorption decreased it. The electrical conductivity change upon gas adsorption was closely associated with the electron affinity of the adsorbate molecule.
Introduction Graphite is one of the representative layer compounds. The electronic structure of graphite can be approximated by a two-dimensional structure consisting of 2s,2p,, v d 2p, atomic orbitals, which are associated with a extensive conjugated system of *-electrons.l Activated carbon fiber (ACF) is mainly composed of microcrystallites of graphite whose size is less than 5 nm, as determined by X-ray diffraction,2small angle X-ray scattering: high-resolution transmission electron microscopic observation,4 and diamagnetic susceptibility meas~rements.~ ACF has uniform slit-shaped micropores,6 whose walls are mostly made of basal planes of micrographite. The micropore field of ACF stabilizes the NO dimer,'*e inducing an unusual high pressure disproportionation reaction of (NO)z in the presence of SOz.g Photoconduction was observed in ACF,l0J1 which is probably caused by the existence of many localized states due to dangling bonds and defects. Recent in situ X-ray diffraction measurements2J2showed that the interlayer spacing of the micrographites of ACF were changed with the adsorption of HzO, benzene, and Nz. Therefore, the relationship between electrical conductivity and molecular adsorption can lead to the elucidation of the molecular processes in slit-shaped micropores. Electronic properties of various types of carbon materials have been investigated.'3-16 In particular the electrical conduction of graphite intercalation compounds has been extensively studied.1618 The effects of gas adsorption on carbons on their electrical conductivity have (1) Minot, C. J. Phys. Chem. 1987,91, 6380. (2) Suzuki, T.; Kaneko, K. Carbon 1988,26, 743. (3) Kaneko, K.; Suzuki, T.; Fujiwara, Y.; Nishikawa, K. Characterization of Porous Solid II; Rodriguez-Reinoso,F., et al., Eds.;Elsevier Science Publishers B.V.: Amsterdam, 1991; p 389. (4) Kakej, K.; Ozeki, S.; Suzuki, T.; Kaneko, K. Characterization of
Porous Solid II; Rodnguez-Reinoso, F., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1991; p 429. (5) Kaneko, K.; Yamaguchi, K.; Iahii, C.; Ozeki, S.; Hagiwara, S.; Suzuki, T. Chem. Phys. Lett. 1991, 176, 75. (6) Kakei, K.; Ozeki, S.; Suzuki, T.; Kaneko, K. J.Chem. SOC.,Faraday Trans. 1990,86,371. (7) Kaneko, K.; Fukuzaki, N.; Ozeki, S. J . Chem. Phys. 1987,87,776. (8)Kaneko, K.; Fukuzaki, N.; Kakei, K.; Suzuki, T.;Ozeki, S.Langmuir 1989,5,960. (9) Imai, J.; Souma, M.; Ozeki, S.; Suzuki, T.; Kaneko, K. J. Phys. Chem. 1991,95,9955. (10) Kuriyama, K.; Dresselhaus, M. S.J. Mater. Res. 1991, 6, 1040. (11) Kuriyama, K.; Dresselhaus, M. S.Phys. Rev. B 1991, 44, 8256. (12) Suzuki, T.; Kaneko, K. J. Colloid Interface Sci. 1990, 138, 590. (13) Mrozowski, S.Carbon 1965,3,305. (14) Mrozowski, S.Carbon 1971,9, 97. (15) Mrozowski, S.Carbon 1973,11, 433. (16) Dresselhaus, M. S.; Dresselhaus, G. Adu. Phys. 1981, 30,139. (17) Shioya, J.; Mataubara, H.; Murakami, S. Synth. Met. 1986, 14, 115. (18) Sugihara, K. Phys. Reu. B. 1988, 37, 4752.
Table I. Adsorption Parameters of ACF surface area pore volume pore width sample a , (m2/g) W (mL/g) w (nm) ACF 996 0.39 0.86
been but these effects are not sufficiently understood yet, especially in the case of activated carbons. Systematic investigations of adsorption and structural properties of ACFs have been carried 0 ~ t . ~ ~ ~ 4 -As 2 7ACFs are homogeneous in quality according to the previous investigations, electrical conductivity measurements of ACFs can probably give useful information on the electronic properties of microporous carbons. In this work, we determined the dependencies of the electrical conductivity of single well-characterized ACF on temperature, frequency, and gas adsorption; we derived a simple relationship between the electrical conductivity change of ACF upon gas adsorption and the electronic affinity of an adsorbate molecule.
Experimental Section Pitch-based ACF (Osaka Gas Co., LM., Osaka) was used in this study. Specific surface area determined by the as method, micropore volume by DR plot, and micropore width by MSMF method?*which are shown in Table I, were determined by NZ adsorptionat 77 K with a computer-aidedgravimetric adsorption apparatus. A single fiber of the ACF sample of 5 mm length and 10pm diameterwas used for electricalconductivity measurement (4terminal method) after pre-evacuation at 1 mPa and 383 K for 2 h. The temperaturedependenceof dc and ac conductivities was measured in the temperature range from 150 to 303 K, with the aid of a micro-ohmmeter (Keithley 580) and an LCR meter (Hewlett-Packard4284A). The frequency dependence of ac conductivity was measured in the frequency range of 20 Hz-1 MHz at 303 K. The effecta of SOz, 02, NzO, NO, and H20 adsorption on the dc and ac electrical conductivities were examined at 303 K. (19) Lukaszewicz, J.; Siedlewski,J.; Rychlicki, G.Pol. J. Chem. 1982, 56, 761. (20) Lukaszewicz, J.; Siedlewski, J. Pol. J. Chem. 1985,56, 855. (21) Siebel, K., 2.Phys. 1921,4, 288. (22) Dacey, J. R.; Frohnadorff, G.J. C.; Gallagher, J. T. Carbon 1964, 2, 41. (23) Barton, S. S.; Koresh, J. E. Carbon 1984,22,481. (24) Kaneko, K.; Nakahigaehi, Y.; Nagata, K. Carbon 1988,26, 327. (25) Kakei, K.; Ozeki, S.;Suzuki, T.;Kaneko, K. J. Chem. SOC., Faraday Trans. 1990,86,371. (26) Kuwabara, H.; Suzuki, T.; Kaneko, K. J. Chem. SOC.,Faraday Trans. 1991,87, 1915. (27) Sato, M.; Sukegawa, T.; Suzuki, T.; Hagiwara, S.; Kaneko, K. Chem. Phys. Lett. 1991,181,526. (28) Kakei, K.; Ozeki, S.; Suzuki, T.;Kaneko, K. In Characterization of Porous Solids II; Rodriguez-Reiaono,F., et al., Eds.; Elsevier Science Publishers B.V.: Amstardam, 1991; p 429.
0743-746319212408-1695$03.00/0 0 1992 American Chemical Society
Letters
1696 Langmuir, Vol. 8,No. 7, 1992
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Results and Discussion Figure 1 shows the temperature dependencies of dc and ac electrical conductivities of a single ACF; electrical conductivity increases with temperature. The activation energy of ACF for electrical conduction is 13 meV. This semiconductive behavior coincides with the results of lessgraphitized granular activated c a r b o n ~ . ~ ~The $ ~ 8ac electrical conductivity scarcely shows any temperature dependence at lower temperature, indicating the coexistence of hopping conduction. Figure 2 shows the frequency dependence of ac conductivity a t 303 K. Here uac= a# Udc, and u# and Udc are the conductivities at frequency f and dc conductivity. This region is situated between f" (nC 1)andf2 dependence regions.16tm The ac conductivity obeying the power law of frequency arises from hopping conduction between localized electronic states.30 No frequency dependence is observed at lower frequency region owing to the predominant contribution by dc conduction, which is mainly caused by band conduction. However, the hopping conduction increases with the frequency according to a power law, predominating over the band conduction. The conductivity above 100 kHz is governed by hopping conduction which probably results from the disordered stacking structure of the micrographitic layers andlor dangling sp3 bonds at the edge of the graphitic crystallites. The effect of gas adsorption on the dc electrical conductivity at 303 K is shown in Figure 3. Here u, is the initial conductivity. Exposure of ACF to 02 gas leads to (29) Carmona, F.; Delhaes, P.; Keryer, G.;Manceau, J. P. Solid State Commun. 1974,14,1183. (30) Mott., N. F.; Davis, E. A. Electronic Processes in Non-crystalline Materials; Clarendon Press: Oxford, 1971; Chapter 4.
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a clear increase of conductivity, indicating that the dominant charge carrier is a hole. This is because the 0 2 molecule accepts an electron from the surface carbon atom to produce 02- and creates a hole. This p-type semiconductive nature of ACF agrees with the results of the thermoelectric power measurement of ACFlO and the results of other disordered ~ a r b o n s . ~ lThe J ~ electrical conductivity increases rapidly upon exposing to SO2 at low pressure and then slightly decreases above 2 kPa. An adsorbed SO2 molecule should also trap an electron to create a hole, because an SO2 molecule has greater electron affinity than an 02 m ~ l e c u l e . ~On ~ * ~the ~ contrary, adsorption of H20, NO, and N2O lowers the electrical conductivity. Graphite changes its electrical conductivity on forming intercalation compounds according to the electron affinity of intercalant. In this study, probably almost all molecules are not intercalated, but physically adsorbed; a small number of molecules can be chemisorbed on the dangling bonds or functional groups at the edge surface of the graphitic crystallites or the micropore wall, inducing the initial conductivity changes. The relationship between the initial electrical conductivity change of ACF due to chemisorption and the electron affinity of the adsorbed molecule is as expected. Figure 4 shows a relationship between the logarithm of the maximum electrical conductivity change and the electron affinity of the adsorbate molecule. Here the ordinate is the logarithm of the ratio of the initial maximum electrical conductivity umarupon adsorption to the original conductivity a,; the (31) Jonscher, A. K. Electronic and Structural Properties of Amorphous Semiconductor; Le Comber, P. G.,Mort, J., Eds.; Academic Press: New York, 1973; Chapter 8. (32) Mrozowski, S.; Chaberski, A. Phys. Reu. 1956, 104, 74. (33) Klein, C. A. J . Appl. Phys. 1964, 35,2947. (34) Tothe, E. W.; Tang, S. Y.; Reck, G . P. J. Chem. Phys. 1975,62, 3829.
Letters
electron affinity values in the l i t e r a t ~ r e ~are " ~used. ~ This relationship is almost linear, indicating that the initial electrical conductivity change is attributed to charge transfer adsorption of adsorbate molecules on the micrographite surface. The direction of the charge transfer should depend on the difference between the surface electronic level of the micrographite and the electron affinity of the adsorbate molecule. We can determine the electron affinity value corresponding to a m d u o= 1from the linear relationship, where the direction of charge transfer is converted; this electron affinity (0.25 f 0.05 eV) indicates the surface electronic level of microcrystallites of ACF. As both H2O and SO2 fill the micropores at higher pressure, the electrical conductivity of ACF tends to decrease gradually. Figure 5 shows the relationships between the relative electrical conductivity ala, and the fractional filling (ratio of the micropore volume filled by adsorbate molecules at 303 K to the micropore volume by N2 adsorbed at 77 K)for H2O and S02. The rapid initial increase of the electrical conductivity and the successive gradual decrease are observed in the adsorption of S02, while H2O adsorption gradually lowers the conductivity. These conductivity decreases are not caused by chemisorption, but by the structural changes of ACF. As the gas adsorption proceeds, the interlayer distance and/or (35) Celotta, R. J.; Bennet, R. A.; Hall,J. L.; Siegel, M. W.; Levine, J. Phys. Rev. A 1972,6,631. (36) Hughes, B. M.; Lifshitz, C.; Tierrnan, T. 0.J. Chem. Phvs. 1973, 59, 3829. (37) ChiDman. D. M. J. Phvs. Chem. 1978.82 (9). 1080. (38) Hopper, D.G.; Wahl, A: C.;Wu, R. L. C.; Tierman,T. 0.J. Chem. Phys. 1976, 65 (19), 5474.
Langmuir, Vol. 8, No. 7, 1992 1697 I
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orientation of the micrographites is changed, according to previous in situ X-ray diffraction examinations.le The electrical conduction in ACF depends on the in-plane ordered state of the micrographites and the electronic contact between the micrographitic crystallites at the edge of the crystallites. The progress in the filling of micropores leads to the significant distortion in the graphitic layer and disorientation of the micrographitic crystallites, giving rise to the observed decrease in the electrical conductivity due to the breaking of electronic contact.
Acknowledgment. The financial support of a Science Research Grant from the Ministry of Education, Japanese Government, and Junkosha Co., Ltd., is greatly appreciated.
