J. Phys. Chem. 1995,99, 5746-5748
5746
Random Magnetism of Temperature
0 2
Confined in a Slit-Shaped Graphitic Nanospace at Low
Hirofumi Kanoht and Katsumi Kaneko" Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263, Japan Received: January 4, 1995; In Final Form: March 14, I995@
The temperature dependence of the magnetic susceptibility 01) of 0 2 molecules adsorbed in slit-shaped carbon micropores of 1 nm in width was measured as a function of the coverage r at 1.7-100 K. The adsorbed 0 2 molecules below = 0.2 showed the paramagnetism without any obvious magnetic transition, which was quite close to the magnetism of bulk gaseous oxygen. An antiferromagnetic behavior appeared with an increase in This behavior was not caused by the formation of 2D lattices or bulk solid structures, but by the random magnetism. These results indicated that 0 2 molecules adsorbed in the micropore do not form any 2D magnetic lattice, but clusters before the formation of the bulk solid structure.
e
e.
Introduction The solid oxygen has three polymorphs: a (T < 23.9 K), /3 (23.9 K c T < 43.8 K), and y (43.8 K c T < 54.4 K).l The magnetic susceptibility of solid oxygen changes markedly at each phase transition temperature. It is well-known that the bulk a-phase shows antiferromagnetism. The magnetic interactions of 0 2 adsorbed on the graphite surface have been extensively The mono- or bilayer of 0 2 molecules adsorbed on the graphite surface has its own phases such as 6 , E , and 5 different from the bulk a , /3, and y phases; the phase change depends on both temperature and the coverage. The a and /3 phases appear in the multiadsorbed 0 2 layer. Also, the phase transition of 6 to 5 leads to a magnetic anomaly. Therefore, the intermolecular structure of 0 2 can be controlled by the solid surfaces, and the structural change can be sensitively measured by the magnetic method. Activated carbon fibers (ACFs) have uniform slit-shaped micropores and great surface area.6,7 The adsorption of vapor molecules is enhanced by the micropore field in the extremely low relative pressure region. Thus, ACF can offer an unusual field for an 0, molecular assembly. Since ACF shows diamagnetism,8 we can obtain information conceming the intermolecular structure of 0 2 molecules confined in the micropores through the magnetic measurement. Experimental Section ACF (pitch A10) prepared by Osaka Gas Co., Ltd., was used without further chemical treatment. The specific surface area and the pore volume of ACF were determined as 1230 m2/g (a,) and 0.58 cm3/g, respectively, from the high-resolution a, plot9J0using the NZadsorption data. The average slit width of the micropores was 0.96 nm. The pore can accommodate trimolecular layers because the pore width of the adsorbent is almost 3 times as long as the collision diameter of 0 2 . 0 2 was sealed in a quartz tube (90 mm x 5 mm diameter) together with ACF at ambient temperature. All 0 2 molecules were assumed to be adsorbed in micropores of ACF because of great surface area of the ACF sample (surface area of 10 mg sample, 12.3 m2, >> surface area of the cell wall, 0.1 m2). The amount
* To whom correspondence
should be addressed.
t Permanent address: Shikoku National Industrial Research Institute,
2217-14 Hayashi-cho, Takamatsu 761-03, Japan. @Abstractpublished in Advance ACS Abstracts, April 15, 1995.
of 0 2 adsorption was controlled by introducing 0 2 of different pressures. The coverage (e)is defined as follows: e = no2N~/6.36/a$ mAm/1018,where no2is mole of oxygen dosed, NA is Avogadro's constant, 6.36 is the reference density (molecules/nm2)" for registered 4 3 x 4 3 commensurate structure, a, is the specific surface area (m2/g) , and mACF is the weight of ACF (grams). The commensurate structure is not observed for adsorbed 0 2 layers in the ACF micropores but is used to define the standard unit for comparison with results in a graphite-02 system. Fractional filling f is calculated as f = 0 . 6 3 ~using 0.58 cm3/g and 1.14 g/cm3 for the pore volume of ACF and the liquid density of oxygen, respectively. Samples for the magnetic measurement were prepared as follows. The fiber specimen was weighed, inserted into the quartz tube, evacuated at 383 K and 10 "a for 2 h, followed by an introduction of oxygen gas at 298 K, and then sealed. The magnetic susceptibility of ACF in vacuo was measured to subtract its contribution to the ACF-02 system. (The magnetic emu g-l.) susceptibility of ACF at 5 K was 7 x The magnetic susceptibility of 0 2 adsorbed on ACF after sealing was measured with a SQUID magnetometer system MPMS (Quantum Design, CA) over a temperature range of 1.7-100 K and at a magnetic field of 1 T. Results and Discussion The temperature dependence of the magnetic susceptibility of 0 2 molecules adsorbed in micropores of ACF is shown as a function of the coverage in Figure 1. No critical change in the X-T curve for e < 0.2 is observed in the temperature range 5-100 K. The increases steeply as decreasing temperature, which is characteristic of paramagnetism. The Curie constant of 2.8 x emu g-' K and the Weiss constant of -7.4 K are obtained from the experimental data for Q = 0.046 (the average 0 2 - 0 2 distance d = 1.85 nm). These Curie and Weiss constants are very close to those for bulk gaseous 0 2 (3.1 x lov2 emu g-' K and 0 K, respectively). Although the submonolayer 0 2 on the graphite surface gives rise to a magnetic anomaly due to the melting of 8-phase at 26 K for e < 1, the low coverage 0 2 layer on ACF of e < 0.5 has no evidence for such magnetic transition. Hence, the 0 2 adsorbed layer of e < 0.5 is completely different from that on the flat graphite surface. Furthermore, the spin system of 0 2 in the micropore behaves like a free spin regardless of 0 2 confined in the micropore by a strong overlapped molecular potential.
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0022-3654/95/2099-5746$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99,No. 16, 1995 5747
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Figure 1. Temperature dependence of magnetic susceptibility 01) of 0 2 adsorbed on micropores of ACF at various coverages (e):0,0.046; A, 0.24; 13,0.46; V,0.75; 0,1.3; 0 , pure oxygen. The contribution of the diamagnetic ACF to x is subtracted.
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Figure 2. Temperature dependence of the magnetic moment (M) of 0 2 adsorbed on micropores of ACF at e = 0.75. The magnetic moment was measured after cooling to 1.7 K with the magnetic field of 0 T, followed by application of the field up to 1 T. A, heating; V, cooling.
x
We observe the temperature independence of with the increase in e, especially for the temperature range 20-50 K. However, this change is not due to the effect of antiferromagnetism by the formation of 6-, E - , or a-phase, but due to that of random magnetism developed in the micropore as described below. Clear jumps in the x-T curve of e = 1.3 cf = 0.82) are observed at 24 and 44 K, which correspond to the transition temperatures of a-P and B-y of the bulk oxygen, respectively, as is shown in the inset of Figure 1, whereas either phase corresponding to the bulk oxygen does not appear below e = 3.5 in the graphite-02 ~ y s t e m .This ~ result shows that even bimolecular layers in the micropore can form the dense structure identical to the bulk solid. This phenomenon may be ascribed to the strong micropore field which offer a high pressure effect. The x-Tcurve for e = 0.75 (d = 0.458 nm andf= 0.47) in the low-temperature range is shown in Figure 2. The curve has a cusp at 2.1 K on heating process, whereas it has no anomaly and can be described by the Curie-Weiss equation on the cooling process. A time course after temperature drop from 30 to 5 K is also shown in Figure 3. The magnetic moment slowly changes with time after the temperature drop. This magnetic behavior should be attributed to random magnetism such as spin glass or mict0magneti~m.l~This is because 0 2 molecules form clusters consisting of a few or several molecules, and the cluster behaves as the system of an effective spin. Since
0
5
10
t l l o2 s
15
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Figure 3. Time course of the magnetic moment (M) of 0 2 adsorbed on ACF after temperature drop from 30 to 5 K at the magnetic field of 1 T and e = 0.75. The temperature was dropped at the point indicated by an arrow.
such spins are frozen in the low-temperature range, we can observe the relaxation phenomenon for spin orientation in the magnetic field. Similar results were obtained for 0.24 I e 5 1.1. The random magnetic behavior was not observed for e -= 0.2 because a very small cluster or an isolated molecule behaves like a free spin. On the other hand, the strong packing of the molecules inhibits the free motion of the spin and facilitates the formation of the bulk solid phases, so that the adsorbed 0 2 of e > 1.2 shows no random magnetism. Evans et al. and Nakanishi and Fisher studied theoretically the fluid behavior in a confined s p a ~ e . ' ~ JTheir ~ studies suggested that the strong surface-molecule interaction prevents the phase formation such as liquid or solid in a very narrow pore. It is plausible according to the above mechanism that 0 2 molecules confined in the slit pore of 1 nm in width cannot form the 2D adsorbed phase which is observed in the case of 0 2 on the flat graphite surface. The relationship between the disappearance of the 2D lattice and the slit width should shed light on the unusual state of molecules confined in a narrow restricted space.
Conclusion Oxygen molecules do not form any 2D magnetic lattice in the micropore of ACF but show random magnetism, which is completely different from that of 0 2 molecules adsorbed on a flat graphite surface. This result suggests that cluster formation of 0 2 molecules in the micropore is due to a quite strong micropore field. Acknowledgment. We are grateful to Prof. Isao Yamada (Department of Physics, Chiba University) for his help conceming the measurement with the SQUID magnetometer. We also acknowledge support from the Science and Technology Agency and the Japanese Government for H.K. to study in Chiba University for a year. This work was supported in part by the Grant-in-Aid for scientific research from the Ministry of Education, Japanese Government. We used the SQUID magnetometer at Chemical Analysis Center of Chiba University. References and Notes (1) DeFotis, G . C. Phys. Rev. B 1981,23, 4714. (2) Gregory, S.Phys. Rev. Lett. 1978,40, 723. (3)Awschalom, D. D.; Lewis, G. N.; Gregory, S . Phys. Rev. Lett. 1983,
51, 586. (4)Kabler, U.;Marx, R. Phys. Rev. B 1987,35, 9809. (5) Suematsu, H.;Murakami, Y . J. Magn. Magn. Mater. 1990,90/91,
749. (6)'Kaneko, K.;Shimizu, K.;Suzuki, T. J. Chem. Phys. 1992,97,8705. (7)Ruike, M.; Kasu, T.; Setoyama, N.; Suzuki, T.; Kaneko, K.J. Phys. Chem. 1994,98,9594.
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(10) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area andPorosity, 2nd ed.; Academic Press: New York, 1982. (1 1) Pan, R. P.; Etters, R. D.; Kobayashi, K.; Chandrasekharan, V. J. Chem. Phys. 1982, 77, 1035. (12) Imai, J.; Souma, M.; Ozeki, S.; Suzuki, T.; Kaneko, K. J. Phys.
Chem. 1991, 95, 9955. (13) Fujie, K.; Minagawa, S.; Suzuki, T.; Kaneko, K. Chem. Phys. Lett., in press. (14) Mydosh, J . A. Spin Glasses: An Experimental Introduction; Taylor Francis: 1993. (15) Nakanishi, H.; Fisher, M. E. J. Chem. Phys. 1983, 78, 3279. (16) Evans, R.; Marini Bettolo Marconi, U.; Tarazona, P. J. Chem. Phys. 1986, 84, 2376,
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