J. Phys. Chem. 1996, 100, 755-759
755
Magnetic Spin States of O2 Confined in a Graphitic Slit-Shaped Nanospace at Low Temperature Hirofumi Kanoh† and Katsumi Kaneko* Department of Chemistry, Faculty of Science, Chiba UniVersity, 1-33 Yayoi-cho, Inage, Chiba 263, Japan ReceiVed: July 17, 1995; In Final Form: October 11, 1995X
The magnetic susceptibility of O2 molecules confined in slit-shaped graphitic micropore systems with the slit pore width w of 0.75, 0.96, and 1.45 nm was measured at diffrent coverages F and over the temperature range of 1.7-100 K. For all micropore systems, the adsorbed O2 at a very low coverage showed the paramagnetism without any obvious magnetic transition, which is quite close to the paramagnetism of bulk gaseous oxygen. An antiferromagnetic behavior appeared with an increase in F. This behavior was caused not by the formation of 2D lattices or bulk solid phases but by the random magnetism for the systems of w ) 0.75 and 0.96 nm. The random magnetism indicates the cluster formation of the O2 molecules in the nanospace. The paramagnetism and the random magnetism arise from an enhanced intermolecular interaction of O2 in the unique potential field of the narrow slit space. The O2 assembly in the micropores showed a different magnetic behavior dpending on the pore width. Besides the cluster formation, a phase like the θ phase, which appears in the O2 bilayer on the 2D graphite surface, was observed for 0.4 < F < 0.8 in the micropores of w ) 1.45 nm. This suggest that the micropores with w ) 1.45 nm provide an enhanced formation of the specific 2D phase. These unusual magnetic behaviors were presumed to be caused by the special critical phenomenon in the nanospace.
Introduction The solid oxygen has three polymorphs R (T < 23.9 K), β (23.9 K < T < 43.6 K), and γ (43.8 K < T < 54.4 K).1 The magnetic susceptibility of solid oxygen changes markedly at each phase transition temperature. It is well-known that the bulk R-phase shows antiferromagnetism. The magnetic interactions of O2 adsorbed on the graphite surface have been extensively studied.2-5 The mono- or bilayer of O2 molecules adsorbed on the two-dimensional graphite surface has its own phases such as δ, , and ζ different from the bulk R, β, and γ phases; the phase change depends on both the temperature and the coverage (Figure 1). The R and β phases appear in the multiadsorbed O2 layer on the flat graphite surface. Also, the phase transition of to ζ leads to a magnetic anomaly. Therefore, the molecular assembly structure of O2 can be controlled by the molecule-surface interaction potential and the structural change of the O2 assembly can be sensitively measured by the magnetic method. Activated carbon fibers (ACFs) have uniform slit-shaped micrographitic micropores and great surface area.6,7 Highresolution transmission electron microscopic observation showed that ACF has slit-shaped micropores.8,9 The average micropore width and the micropore volume can be evaluated by the analysis of the N2 adsorption isotherm at 77 K. 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 O2 molecular assembly. As the predominant micropore walls of ACF are the basal planes of the micrographites, the fundamental attractive interaction of O2 with the microporous surface stems from the van der Waals interaction. Since ACF shows diamagnetism,10 we can obtain † Permanent address: Shikoku National Industrial Research Institute, 2217-14 Hayashi, Takamatsu, Kagawa, 761-03 Japan. * Author to whom corespondence should be addressed. Fax: 81-43292-2788. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0755$12.00/0
Figure 1. Phase diagram for the O2/graphite system.4 This is based on the Figure 5 of ref 4, although a few parts are modified.
information on the assembly structure of O2 molecules confined in the micropores through the magnetic measurement. The random magnetism of O2 confined in a slit-shaped nanospace of ACF was reported in a previous paper.11 The magnetism indicates the formation of the magnetic spin clusters and the unusual physical environment for the adsorbed molecules. In this work, the magnetism of O2 adsorbed in the micropores of three types of ACFs was examined and the effect of the pore width on the micropore field was studied. Experimental Section Three kinds of pitch-based ACFs (A5, A10, and A25) were used. The specific surface areas of ACF (as) were determined as 900, 1230, and 1935 m2/g for A5, A10, and A25, respectively, from the high-resolution Rs plot8,12 using the N2-adsorption data. The average slit widths of the micropores were 0.75, 0.96, and 1.45 nm, respectively. In this paper, A5, A10, and A25 are denoted by ACF-0.74, ACF-0.96, and ACF-1.45, respectively. That is, each ACF sample is expressed by the average slit width. O2 was sealed in a quartz tube (90 mm × 5 mm φ) together with ACF at ambient temperature. All O2 molecules were assumed to be adsorbed in micropores of ACF because of great surface area of the ACF sample. The amount of O2 adsorption was controlled by introducing O2 of different pressures. © 1996 American Chemical Society
756 J. Phys. Chem., Vol. 100, No. 2, 1996
Figure 2. Temperature dependence of magnetic susceptibility (χ) of O2 adsorbed in micropores of ACF-0.96 at various coverages (F): O, 0.046; 4, 0.11; 0, 0.24; 3, 0.46; ], 0.75; b, 1.1; 2, 1.3. The contribution of the diamagnetic ACF to χ is subtracted. (a) χ-T curves. (b) χ-1-T curves.
The coverage (F) is defined as follows: F ) nO2NA/6.36/as/ mACF/1018 where nO2 is mole of oxygen dosed, NA is the Avogadro’s constant, 6.36 is the reference density (molecules/ nm2)13 for the registered x3 × x3 commensurate structure, as is the specific surface area (m2/g), and mACF is the weight of ACF (g). The commensurate structure is not observed for adsorbed O2 layers in the ACF micropores but is used to define the standard unit for comparison with results in an O2/graphite system. Fractional filling f, which is the volume fraction filled with adsorbed molecules, is calculated as f ) 0.63F using the pore volumes of ACFs and the liquid density of oxygen (1.14 g/cm3), 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 mPa for 2 h, followed by an introduction of oxygen gas at 298 K, and then sealed. The magnetic susceptibility of ACF itself in vacuo was measured to subtract its contribution to the O2/ACF system (the magnetic susceptibilities of the ACFs at 5 K were (2-8) × 10-6 emu‚g-1). The magnetic susceptibility of O2 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 O2/ACF-0.96 System. The temperature dependence of the magnetic susceptibility χ of O2 molecules adsorbed in micropores of ACF-0.96 is shown as a function of the coverage in Figure 2. No critical change in the χ-T curve for F < 0.2 is observed in the temperature range of 5-100 K. The χ increases steeply as decreasing temperature, which is characteristic of paramagnetism. The χ-T relation is well described by the Curie-Weiss equation. The Curie constant (C) of 2.8 × 10-2 emu‚g-1‚K and the Weiss constant (θ) of -7.4 K are obtained from the χ-1-T curve for F ) 0.046 (the average O2-O2 distance d ) 1.9 nm) (Figure 2b). These Curie and Weiss constants are quite close to those for bulk gaseous O2 (C ) 3.1 × 10-2 emu‚g-1‚K and θ ) 0 K). Although the submonolayer O2 on the graphite surface gives rise to a magnetic anomaly
Kanoh and Kaneko
Figure 3. Curie (CTL-TH) and Weiss (θTL-TH) constants in the several temperature ranges for the O2/ACF-0.96 system at F ) 0.75. The CTL-TH (a) and θTL-TH (b) values were evaluated from the linear approximation for the χ-1-T curves (Figure 2b) in the temperature range of TL to TH K, where TL and TH represent the low and high limit temperatures, respectively, in the temperature range. TL-TH: O, 5-10; 4, 10-20; 0, 30-42; 3, 44-54; ], 60-80.
due to the melting of δ-phase at 26 K for F < 1, the low-coverage O2 layer on ACF-0.96 of F < 0.5 has no evidence for such magnetic transition. Hence, the O2 adsorbed layer of F < 0.5 is completely different from that on the flat graphite surface. Furthermore, the spin system of O2 in the micropore behaves like a free spin regardless of O2 confined in the micropore by the strong surface-molecule interaction potential. The temperature independence of χ with the increase of F is observed, especially for the temperature range of 20-50 K at F > 0.46. However, this change is not due to the effect of antiferromagnetism by the formation of δ-, -, or R-phase, but due to that of random magnetism developed in the micropore as described later. Clear jumps in the χ-T curve of F ) 1.3 (d ) 0.35 nm and f ) 0.82) are observed at 24 and 44 K, which correspond to the transition temperatures of R-β and β-γ of the bulk oxygen, respectively, as shown in the inset of Figure 2a, whereas either phase corresponding to the bulk oxygen does not appear below F ) 3.5 in the O2/graphite system.3 This result shows that even trimolecular layers in the micropore can form the dense structure identical to the bulk solid, since the pore width of ACF-0.96 is almost 3 times as long as the collision diameter of O2. This phenomenon may be ascribed to the strong micropore field which offer a high-pressure effect.14,15 The Curie (CTL-TH) and Weiss (θTL-TH) constants calculated in the several temperature ranges are plotted in Figure 3 to understand how the intermolecular interaction changes with an increase in F, since the χ-1-T curve is not straight at F > 0.2. The CTL-TH and θTL-TH values were evaluated from the linear approximation for the χ-1-T curves in the temperature range of TL to TH K, where TL and TH represent the low- and hightemperature limits, respectively, in the temperature range. The C30-42 and θ30-42 values change markedly at F ) 1.2. The changes in these constants reflect the phase transition of the bulk solid (β to γ). Since the C44-54 and θ44-54 values change steeply at F > 1.2, these constants also correlate to the phase transition. C5-10 and θ5-10, however, show little changes over the whole range of F and indicate the values relatively close to
Spin States of O2 in a Slit Nanospace
Figure 4. Magnetic moment (M) of the O2/ACF-0.96 system at F ) 0.75. (a) M-T curve. The measurement was carried out after cooling down to 1.7 K with the magnetic field (H) of 0 T (zero field cooling (ZFC)), followed by application of the field up to 1 T. 2, heating after ZFC; 3, field cooling (FC). (b) Time course after temperature drop from 30 to 5 K at H ) 1 T. The temperature was dropped at the point indicated by an arrow.
those for gaseous O2. This may be because at the very low temperature region, the contribution from free spins to the susceptibility is predominant, but those from the antiferromagnetic phases or the spin clusters are very small, provided that the mixed states of the free spins and the interacting spins coexist. We can regard C5-10 and θ5-10 as the parameters related to the amount of the free spins. The χ-T curve for F ) 0.75 (d ) 0.46 nm and f ) 0.47) in the low-temperature range is shown in Figure 4a. The curve has a maximum at 2.1 K on heating process after zero-field cooling (ZFC), whereas it has no anomaly and can be described by the Curie-Weiss equation for the field-cooling (FC) process. A time course after temperature drop from 30 to 5 K is also shown in Figure 4b. The magnetic moment slowly increases with time after the temperature drop. This magnetic behavior should be attributed to random magnetism such as spin glass or mictomagnetism.16 This is because O2 molecules form clusters consisting of a few or several molecules and the cluster behaves as the system of an effective spin. As O2 molecules interact with each other through the Lennard-Jones type interaction, the magnetic cluster formation of O2 molecules must come from the dipole-dipole interaction. Since 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 e F e 1.1. The spinspin interaction of the spin clusters gives increases in C30-42 and C44-54 and decreases in θ30-42 and θ44-54 before the bulk phase transition, as are shown at 0.2 < F < 1.0 in Figure 3. The random magnetic behavior was not observed for F < 0.2 because a very small cluster or an isolated molecule behaves like a free spin. On the other hand, the busy packing of the molecules inhibits the free motion of the spin and facilitates the formation of the bulk solid phases, so that the adsorbed O2 of F > 1.3 shows no random magnetism. O2/ACF-0.75 System. Similar results are obtained for O2 adsorbed in the micropore of ACF-0.75 (Figure 5a), although
J. Phys. Chem., Vol. 100, No. 2, 1996 757
Figure 5. O2/ACF-0.75 system. (a) χ-T curves at various coverages (F): O, 0.056; 4, 0.27; 0, 0.76; 3, 1.1; ], 1.3; b, 1.5. (b) CTL-TH-F. (c) θTL-TH-F. Symbols are the same as those in Figure 3.
the dependence of the χ-T curve on F differs from that of ACF0.96. The χ-T curve for F ) 0.056 (d ) 1.7 nm) gives the constants of C ) 2.3 × 10-2 emu‚g-1‚K and θ ) -1.8 K. No 2D magnetic lattice formation is observed at F e 1.1 (d ) 0.38 nm and f ) 0.87), but the bulk phases appear at F > 1.1. CTL-TH and θTL-TH are shown in Figure 5, b and c, respectively. The changes in these two values are not so great relative to those for ACF-0.96. The magnetic relaxation process of the spin cluster in the micropore of ACF-0.75 is also observed at 0.3 < F < 1.2, but the relaxation is much faster than that of ACF0.96 (Figure 6). Therefore, ACF-0.75 offers the restricted molecular environment less suitable for the cluster formation than ACF-0.96. O2/ACF-1.45 System. The χ-T curve of O2 adsorbed in the micropore of ACF-1.45 at F ) 0.021 (d ) 2.7 nm) satisfies the Curie-Weiss law (C ) 2.6 × 10-2 emu‚g-1‚K and θ ) -2.3 K) as is shown in Figure 7a. The curves for F g 0.42, however, give different features from those of other two samples described above. Three curves for F ) 0.42, 0.60, and 0.78 have a broad peak at 28, 36, and 44 K, respectively. Moreover, the bulk phases appear at F ) 1.1 (d ) 0.38 nm and f ) 0.63). This is a smaller coverage than those in both cases of ACF0.96 and ACF-0.75. The changes in CTL-TH and θTL-TH are different from those for ACF-0.96 or ACF-0.75 accordingly (Figure 7, b and c). C30-42 and θ30-42, especially, give discontinuous changes at the region of 0.5 < F < 0.7. The sample of F ) 1.1 gave the slowest spin-relaxation phenomenon in the O2/ACF-1.45 system regardless of the bulk phase formation at the coverage. A maximum at 2.0 K on the heating process after ZFC and the spin relaxation after the temperature drop are observed, as shown in Figure 8. These results indicate the coexistence of the bulk phases and the clusters in micropores of ACF-1.45, which is not observed in the micropores of ACF-0.75 or ACF-0.96. The detailed χ-T curves for F ) 0.42, 0.60, and 0.78 are shown in Figure 9. These are partly similar to a curve for θ phase in the O2/graphite system obtained by Ko¨bler and Marx
758 J. Phys. Chem., Vol. 100, No. 2, 1996
Figure 6. Magnetic moment (M) of O2/ACF-0.75 system at F ) 1.1. The measurement condition is the same as that in Figure 4. (a) M-T curve. 2, heating after ZFC; 3, FC. (b) Time course after temperature drop from 30 to 5 K at H ) 1 T.
Kanoh and Kaneko
Figure 8. Magnetic moment (M) of O2/ACF-1.45 system at F ) 1.1 (a) M-T curve. 2, heating after ZFC; 3, FC. The measurement condition is the same as that in Figure 4a. (b) Time course after temperature drop from 20 to 5 K at H ) 1 T.
Figure 9. χ-T curves of O2/ACF-1.45 system: F ) (a) 0.42, (b) 0.60, and (c) 0.78. Figure 7. O2/ACF-1.45 system. (a) χ-T curves at various coverages (F): O, 0.021; 4, 0.063; 0, 0.42; 3, 0.60; ], 0.78; b, 1.1. (b) CTL-THF. (c) θTL-TH-F. Symbols are the same as those in Figure 3.
(Figure 1). They proposed that liquid and θ phases coexist in the region of 1 e F e 1.8 and 32 < T < 40 (32 K: the melting point of δ phase, 40 K: the melting point of θ phase) for the O2/graphite system. The θ phase is presumed to have a liquid crystalline structure in which molecular axes are ordered like a smectic crystal, but the molecular centers are disordered in the 2D plane. The clusters formed in the micropores of ACF-1.45 should be in a less ordered state than that of θ phase. The values
of C30-42 and θ30-42 at F ) 0.42 (Figure 7) are comparable to those for ACF-0.96 at F ) 0.75 (Figure 3), but the results of the spin relaxation show that the cluster formation proceeds in a lower extent than that of the latter case. This is because the spins in the state of the θ-like phase also contribute to the two constants. Phase Diagrams of O2 in Graphite Micropores. The magnetic behavior of the clusters formed in the micropores of ACFs depends on the pore width and tends to approach that of the O2/graphite system with an increase in the pore width. The C5-10 and θ5-10 values are related to the amount of the free
Spin States of O2 in a Slit Nanospace
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Figure 10. Dependence of C5-10 and θ5-10 on F for three O2/ACF systems: O, ACF-0.75; 4, ACF-0.96; and 0, ACF-1.45. (a) C5-10-F curve, (b) θ5-10-F curve.
spins as described above and are shown in Figure 10. These results indicate that the amount of free spins decreases with increasing F at a constant pore width and with increasing the pore width at a constant F, although the θ5-10 values for ACF0.96 and ACF-1.45 deviate after the bulk phase formation by the influence of the antiferromagnetism of the R phase. Each extrapolation of F to zero gives the Curie and Weiss constants of the free spins confined in the nanospace: C0 ) 2.6 × 10-2 emu‚g-1‚K and θ0 ) -0.2 K for ACF-0.75, C0 ) 3.0 × 10-2 emu‚g-1‚K and θ0 ) -1.2 K for ACF-0.96, and C0 ) 2.9 × 10-2 emu‚g-1‚K and θ0 ) -0.6 K for ACF-1.45. All values obtained are very close to those of gaseous O2. The results for each O2/ACF system also indicate that the spin cluster is formed at C5-10 < 0.015. The phase diagrams of the O2/ACF systems deduced from the whole data in this work are shown in Figure 11. The phase diagrams are much simpler than that of the O2/graphite system because few phase transitions occur before the bulk phases appear. It is very interesting that none of liquid or solid phases are observed for the O2 submonolayer in the low temperature range. We can conclude the anomalous molecular state of O2 confined in the nanospace. Fisher and Nakanishi17,18 and Evans et al.19,20 studied theoretically the fluid behavior in a confined space. They suggested that the strong surface-molecule interaction prevents the bulk phase formation such as liquid or solid in a very narrow pore. It is plausible according to the above mechanism that O2 molecules confined in the slit pore of 1 nm in width cannot form 2D lattice similar to that on the flat graphite surface. The relationship between the disappearance of the 2D lattice formation and the slit width should shed light on the unusual state of molecules confined in a narrow restricted space. Acknowledgment. We are grateful to Prof. Isao Yamada (Department of Physics, Chiba University) for his help concerning the measurements with the SQUID magnetometer at the Analytical Center of Chiba University. We also acknowledge
Figure 11. Phase diagrams for O2/ACF systems. Symbols 2 and 1 in (c) represent the value at the peak and at the trough, respectively, in Figure 9.
support from the Science and Technology Agency, 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, Culture, and Science, Japanese Government. 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) Ko¨bler, 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. (8) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075. (9) Suzuki, T.; Kasuh, T.; Kaneko, K. Chem. Phys. Lett. 1993, 93, 2355. (10) Kaneko, K.; Yamaguchi, K.; Ishii, C.; Ozeki, S.; Hagiwara, S.; Suzuki, T. Chem. Phys. Lett. 1991, 176, 75. (11) Kanoh, H.; Kaneko, K. J. Phys. Chem. 1995, 99, 5746. (12) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982. (13) Pan, R. P.; Etters, R. D.; Kobayashi, K.; Chandrasekharan, V. J. Chem. Phys. 1982, 77, 1035. (14) Imai, J.; Souma, M.; Ozeki, S.; Suzuki, T.; Kaneko, K. J. Phys. Chem. 1991, 95, 9955. (15) Fujie, K.; Minagawa, S.; Suzuki, T.; Kaneko, K. Chem. Phys. Lett. 1995, 236, 427. (16) Mydosh, J. A. Spin glasses: an experimental introduction; Taylor & Francis: London, 1993. (17) Fisher, M. E.; Nakanishi, H. J. Chem. Phys. 1981, 75, 5857. (18) Nakanishi, H.; Fisher, M. E. J. Chem. Phys. 1983, 78, 3279. (19) Evans, R.; Marini Bettolo Marconi, U.; Tarazona, P. J. Chem. Phys. 1986, 84, 2376. (20) Tarazona, P.; Marini Bettolo Marconi, U.; Evans, R. Mol. Phys. 1987, 60, 573.
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