The Journal of
Physical Chemistry
0 Copyright 1995 by the American Chemical Society
VOLUME 99, NUMBER 16, APRIL 20,1995
LETTERS Ferromagnetic Behavior of Superhigh Surface Area Carbon C. Ishii, Y. Matsumura,? and K. Kaneko" Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263, Japan Received: November 9, 1994; In Final Form: February 21, 1995@
The magnetic properties of activated mesocarbon microbead (a-MCMB) were examined over the temperature range 1.7-285 K. a-MCMB has a large specific surface area of 3110 m2 g-' whose surface atom ratio is 0.94. The temperature dependence of the magnetic susceptibility from 30 to 285 K is described by the Curie-Weiss equation. The X-T curve shows a maximum at 4.2 f 0.5 K. Distinct magnetic hysteresis and residual magnetization of 0.016 emu Gg-' at 1.7 K is observed. Although the magnetic hysteresis loop becomes small with an increase in temperature, the hysteresis still remains even at 285 K.
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Introduction A carbon whose specific surface area is greater than 2630 m2 g-], Le., the surface area of a single infinite graphite sheet, may be called a superhigh surface area carbon or super surface carbon.' The super surface carbon is mainly composed of minute graphitic crystallites having a stacking structure of two to three layers.2 Almost all carbon atoms of super surface carbon are on the surface, and thereby the super surface carbon should be called a surface solid. Their surface properties have attracted much attenti~n.~ The pore and surface structures have been characterized by small-angle X-ray scattering? X-ray diffraction? XPS,6 and He adsorption at 4.2 K.' The main electronic states of the super surface carbon are the surface states, and characteristic electronic properties are expected. Kuriyama and Dresselhaus reported that activated carbon fibers (ACFs) with large surface area exhibit unusual photoconductivity having a long life.8 Nakayama et al. observed a new ESR relaxation behavior of ACF influenced by He gas.g Both studies indicate the importance of the physical properties of super surface carbon. Activated mesocarbon microbeads (a-MCMBs) prepared from mesocarbon microbeads with an oriented poly+
National Institute of Industrial Health, 6-21-1 Nagao, Tama, Kawasaki
214, Japan.
@Abstractpublished in Advance ACS Absfrucfs,April 15, 1995.
cyclic aromatic structure have greater surface area than ACF, which should be regarded as a representative surface solid. Furthermore, a-MCMB is expected to have a more ordered micrographitic structure than ACF. This Letter describes a remarkable ferromagnetic behavior of a-MCMB at low temperature.
Experimental Section The a-MCMB was produced by Osaka Gas Co. through activation of MCMB with KOH. The N2 adsorption isotherm was measured gravimetrically at 77 K with computer-aided equipment. The micropore structure was analyzed by the subtracting pore effects (SPE) method using high-resolution a, plot for the N2 adsorption isotherm. The detailed SPE method was given in a previous article.2 The micrographitic structure was examined by X-ray diffraction (XRD). The magnetic susceptibility was measured with a SQUID magnetometer (Quantum Design, MPMS2) over the temperature range 1.7285 K in vacuo. A 33.3 mg sample of a-MCMB was sealed in a quartz tube after evacuation at 383 K and 1 mPa for 2 h. The magnetization was also measured in a magnetic field between f 1 0 000 G.
Results and Discussion Figure 1 shows an N2 adsorption isotherm and the a, plot of
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Figure 1. (a) Isotherm of a-MCMB. (b) asplot of a-MCMB. 600
Figure 3. Micrographite structural model of a-MCMB.
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a-MCMB. The N2 adsorption isotherm is basically type I, which is indicative of the presence of uniform micropore.I0 There is a linear increase in adsorption until 0.4 of a relative pressure (PIPo) after the initial uptake, which suggests the presence of larger micropores of more than 1 nm in width. Here, the micropore represents the pore of which pore width is less than 2 nm after the IUPAC classification.'0 The high-resolution or, plot has no filling swing below an or, value of 0.3 due to an enhanced micropore field, but it has a slight capillary swing in the range 0.7- 1.O. Consequently, the specific surface area was correctly determined by the SPE analysis. The specific surface area, the micropore volume, and the pore width were 31 10 m2 g-', 1.97 mL g-l, and 1.3 nm, respectively. The surface atom ratio was 0.94. Here the surface atom ratio was the ratio of carbon atoms at the surface to the total number of carbon atoms. Hence, a-MCMB can be regarded as a typical surface solid. Figure 2 shows an XRD profile of a-MCMB. The maximum position of the broad 002 diffraction peak due to the micrographites coincided with that of graphite, which provides 0.34 nm of the interlayer spacing. The stack height calculated from the half-breadth of the peak was 0.8 nm, which corresponds to the bilayer thickness of the graphitic layer. The combined peak of 100 and 101 peaks was more diffuse than that of ACF, and thereby the micrographitic units of a-MCMB should be smaller than that of ACF. The size of a or b direction was estimated to be 1 nm. Figure 3 shows a simplified bimicrographitic model of a-MCMB estimated from the above results. The micrographitic units are presumed to be combined with each other by sp2 and sp3 bonds. The micropore wall consists of a series of bimicrographitic layers. The ratio of the edge carbon atoms to the whole atoms of the micrographitic unit is 0.7. A part of
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the edge carbon atoms tend to form localized electronic states, and hence there should be many unpaired electrons. The preliminary ESR examination showed the presence of 8 x 1020 spins g-' at room temperature in vacuo. The examination with an electron probe microanalyzer could not detect any heavy metal impurities in the resolution limit. However, we tried to determine the level of Fe with high-sensitivity atomic absorption spectroscopy after dissolution in HNO3 solution,' showing that 80 ppm of Fe was present as impurity in a-MCMB. Figure 4 shows the temperature dependence of the magnetic susceptibility x of a-MCMB with an increase in temperature in vacuo. There is a maximum at 4.2 f0.5 K, and the temperature dependence of the magnetic susceptibility in the temperature range 30-200 K is expressed by the Curie-Weiss law. The Weiss constant was -227 K in the range 30-200 K. However, a-MCMB showed a slow relaxation of magnetization of several hours, and then the observed Weiss constant was not a real equilibrium value. As no maximum in the X-T relation of ferromagnetic carbons at low temperature was reported in the literat~re,I~-'~ super surface carbon has an unusual ferromagnetic behavior. The magnified maximum is also shown in Figure 4. In the magnified figure, a distinct difference between the X-T behavior with the increase and decrease in the measuring temperature is shown. Here the X-T curve with the increase of temperature was measured after the sample cooled to 1.7 K without applying the magnetic field. The value increases with the decrease in temperature, whereas the X-T
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80 ppm originated from bulk Fe304, the estimated residual magnetization should be 0.008 emu Gg-' at maximum, which is smaller than the observed value. Also, there was no ferromagnetic resonance absorption due to Fe304 in the preliminary ESR examination. The fact that the magnetic hysteresis depends sensitively on the measuring temperature is completely different from the ferrimagnetism of Fe304. Hence, we conclude that the observed ferromagnetism of super surface carbon is not extrinsic,but intrinsic. In the super surface carbon, there are clustered and isolated spins arising from the partially ordered micrographitic structures, and therefore both spin glass behavior and mictomagnetism can be assumed to be responsible for the observed ferromagnetism.
Acknowledgment. The authors thank Ministry of Education, Japanese Government for the Grant-in-Aid. We used the SQUID magnetometer at Chemical Analysis Center of Chiba University. References and Notes -0.01
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curve with the increase of temperature has the maximum at 4.2 K. The irreversible temperature dependence of is quite close to that observed in spin glass or mictomagnetic compounds. The spin glass and mictomagnetic behaviors have been usually observed in binary systems like alloy^.'^-^^ The spin glass behavior is observed in the mixed dilute alloy system and in the isolated system of atomic impurities. As micrographitic units having spins are linked with each other to form spin clusters, there is an isolated spin on the micrographite. Therefore, the observed ferromagnetism can be described to the spin glass mechanism or mictomagnetism. The magnetic hysteresis at 1.7 K is shown in Figure 5a. A marked hysteresis is observed in the magnetic field of -2000 to f2000 G. The residual magnetization is 0.016 emu Gg-'. The magnetic moment depends on the magnetic field above the closure of the hysteresis. The magnetic hysteresis becomes less marked with an increase of the measuring temperature. Figure 5b shows the magnetic hysteresis at 285 K. We can observe a clear hysteresis even at 285 K, but the loop is much smaller than that at 1.7 K. We must evaluate the magnetic contribution by the Fe impurities. If we presume that the Fe impurities of
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(1) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075. (2) Kaneko, K.; Ishii, C. Colloid Surf. 1992, 67, 203. (3) Kaneko, K.; Shimizu, K.; Suzuki, T. J . Chem. Phys. 1992,97, 8705. (4) Ruike, M.; Kasu, T.; Setoyama, N.; Suzuki, T.; Kaneko, K. J. Phys. Chem. 1994, 98, 9594. (5) Suzuki, T.; Kasu, T.; Kaneko, K. Chem. Phys. Lett. 1992, 191, 569. (6) Wang, Z. M.; Suzuki,T.; Uekawa, N.; Asakura, K.; Kaneno, K. J . Phys. Chem. 1992, 96, 10917. (7) Setoyama, N.; Ruike, M.; Kasu, T.; Suzuki, T.; Kaneko, K. Langmuir 1993, 9, 2612. (8) Kuriyama, K.; Dresselhaus, M. S. Phys. Rev. B: Solid State 1992, 44, 8256. (9) Nakayama, A.; Suzuki,K.; Enoki, T.; Ishii, C.; Kaneko, K.; Endo, M.; Shindo, N. Solid State Commun., submitted. (10) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A,; RouquCrol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (11) Matsumura, Y.; Ogasawara, M.; Kose, M. Abstract of Conference of Japan Adsorption Society, 5th Hamamatsu, Shizuoka 1991; p 50. (12) Totrance, J. B.; Oostra, S.; Nazzal, A. Synth. Met. 1987, 19, 709. (13) Tanaka, K.: Murashima. M.: Yamabe. T. Svnth. Met. 1988.24.371. (14) Mizogami, S.; Mizutani, M.; Fukuda, M.; Kawabata, K. Synth. Met. 1991, 41-43, 3271. (15) Wohlfarth, E. P. Ferromagnetic Materials; North-Holland: Amsterdam, 1980; p 72. (16) Cannella, V.; Mydosh, J. A. Phys. Rev. B: Solid State 1972, 6, 4220. (17) Shemngton, D.; Kirkpatrick, S. Phys. Rev. iett. 1975, 35, 1792. (18) Kirkpatrick, S.; Shemngton, D. Phys. Rev. B: Solid State 1978, 17, 4384. (19) Sato, H.; Arrott, A. Phys. Rev. 1958, 114, 1427. (20) Kouvel, J. S.; Kasper, J. S. J . Phys. Chem. Solids 1962, 24, 529. JP943024Z
