J. Phys. Chem. C 2007, 111, 10213-10216
10213
Magnetism of Organic Radical Molecules Confined in Nanospace of Single-Wall Carbon Nanohorn Taku Matsumura,† Hideki Tanaka,† Katsumi Kaneko,† Masako Yudasaka,‡ Sumio Iijima,‡,§ and Hirofumi Kanoh*,† Department of Chemistry, Graduate School of Science, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan, JST/SORST, c/o NEC Corporation, Miyukigaoka, Tsukuba 305-8501, Japan, and Department of Physics, Meijo UniVersity, Nagoya 468-8522, Japan ReceiVed: February 28, 2007; In Final Form: May 10, 2007
The temperature and magnetic field dependences of magnetic characteristics of an organic radical (4-hydroxy2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL)) adsorbed on oxidized single-wall carbon nanohorn (ox-NH) were studied with a SQUID magnetometer and an electron spin resonance (ESR) spectrometer. TEMPOL molecules adsorbed on ox-NH do not form any ordered structures, but the formation of disordered clusters was suggested because of intensive restriction by nanoconfinement. Exchange interaction of TEMPOL adsorbed on ox-NH was weaker than that of bulk TEMPOL. Slow relaxation phenomena of magnetization and paramagnetic behaviors were observed by confinement of TEMPOL in nanospace.
1. Introduction Quite recently, specific assembly structures of molecules confinedinlow-dimensionalnanoporessuchascarbonnanotubes,1-6 zeolites,7-12 molecular crystals,13 and metal organic frameworks14 have been studied extensively. Particularly the structure of water molecules incorporated in single-wall carbon nanotubes has received much attention because of their unique structure consisting of hydrogen bonds.15,16 Magnetic behaviors are also interesting from the viewpoint of quantum phase transitions in low dimensionality.17 Molecules can interact strongly with nanopores, which induce a quasi-high-pressure effect on the molecules, leading to their self-assembly structure formation and the acceleration of the chemical reactions. We found the cluster formations of several kinds of molecules such as water, argon, oxygen, and alcohols confined in slit-shaped graphitic nanopores.18 Oxygen is a good probe molecule for studying the molecular assembly state in nanopores because oxygen has a unique magnetism corresponding to the assembly structures. We studied intermolecular states of O2 confined in the slit-shaped graphitic nanospaces of activated carbon fiber by the magnetic susceptibility measurement as a function of the slit-pore width and the pore-wall coverage of O2.19-21 It was found that low-temperature magnetism of the confined O2 in the graphitic slit nanospaces is substantially different from that of bulk solid O2 as well as that of two-dimensional O2; the magnetism of adsorbed O2 shows the presence of free spin, spin cluster, and solidlike structure depending on the fractional filling and pore width. Organic radical molecules can be also probe molecules to monitor their assembly structures.22 4-Hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) is a typical radical molecule used as a spin probe.23 Since TEMPOL shows a magnetic transition to antiferromagnetism from paramagnetism around 6 * Corresponding author. Telephone: +81-43-290-2784. Fax: +81-43290-2788. E-mail:
[email protected]. † Chiba University. ‡ JST/SORST. § Meijo University.
K,24 if this is confined in nanospaces, magnetic anomalies are expected to be observed as for oxygen molecules confined in nanospaces. Single-wall carbon nanohorn (SWNH) is one of the singlewall nanotubulites and is suitable for interfacial chemical study. A primary SWNH particle is a tubule with a cone cap, resembling a horn. SWNHs aggregate to form a colloidal assembly with an 80 nm diameter. SWNH colloids with a dahliaflower-like structure are produced with a high yield (>90%) and purity.25 An SWNH can offer a huge specific surface area and inherent nanoscale internal spaces when we can open the single carbon wall.26 In the present study, we examined the magnetism of TEMPOL confined in the nanopores of an oxidized single-wall carbon nanohorn (ox-NH) to clarify assembly properties of organic molecules in nanospaces. 2. Experimental Section A dahlia-flower-type SWNH was synthesized by CO2 laser ablation of graphite under Ar gas. Detailed discussion on the synthesis of SWNH is found in previous articles.25 Oxidation treatment was performed on SWNH samples for 10 min at 823 K in an atmosphere of O2 gas to form nanoscale pores (nanowindows) at the carbon walls of SWNH.26 The obtained sample, oxidized SWNH (ox-NH), has more space for adsorption because its inner space is available for molecular adsorption through the nanowindows. After preevacuation of ox-NH at 423 K and 10-2 Pa for 2 h, 50 mg of the sample was immersed in 20 mL of a TEMPOL ethanol solution using a vacuum impregnation method, and then settled for 24 h. TEMPOL molecules were adsorbed on the nanopores of ox-NH during this process. Then, the sample was sonicated for 10 min. After the heat treatment at 338 K caused evaporation of ethanol solvent, TEMPOL-adsorbed ox-NH (T-ox-NH-φ) was obtained, where “φ” means the fractional filling. Fractional filling of TEMPOL adsorbed on the pores of the assembly composed of particles of ox-NH was evaluated by using a specific pore volume (0.49 cm3/g) of the ox-NH assembly and the density (1.14 g cm-3) of TEMPOL. The
10.1021/jp071636g CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
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Figure 1. N2 adsorption isotherm of ox-NH and TEMPOL-adsorbed ox-NH with different φ values. The number after the hyphen denotes each φ.
Figure 4. Temperature dependence of magnetic susceptibility (χ) of TEMPOL adsorbed on ox-NH at φ ) 0.55. (3) FC, measured on field cooling; (4) ZFC, measured on heating after zero-field cooling; (O) equilibrium, each measurement done after waiting for enough time for equilibration.
Figure 2. Nanopore volume of TEMPOL-adsorbed ox-NH at different fractional filling φ (circles) and ox-NH (dotted line).
Figure 5. Relaxation time of bulk TEMPOL (O) and TEMPOL adsorbed on ox-NH at different φ values: 0.03 (4), 0.30 (3), 0.55 (0), and 1.00 (]).
Figure 3. ESR spectra of TEMPOL crystal and TEMPOL adsorbed on ox-SWNH at different φ values: (a) bulk, (b) 0.03, (c) 0.30, (d) 0.55, and (e) 1.00.
specific pore volume was determined by N2 adsorption at 77 K with a volumetric adsorption apparatus (Autosorb MP-1, Quantachrome). 10-2
T-ox-NH-φ was put into a quartz cell, evacuated at Pa at room temperature, and sealed for magnetic measurement. Electron spin resonance spectra were measured with an ESR apparatus (JEOL, JES-TE200) at room temperature. Magnetic susceptibility was measured with a magnetometer (MPMSR2, Quantum Design) at 2-100 K. The magnetization curves were also measured at 2 and 10 K over the magnetic field range from -5 to 5 T.
Figure 6. χ-T plot of bulk TEMPOL (O) and TEMPOL adsorbed on ox-NH at different φ values: 0.03 (4), 0.30 (3), 0.55 (0), and 1.00 (]).
3. Results and Discussion N2 adsorption isotherms of ox-NH and T-ox-NH-φ in terms of 1 g of ox-NH are shown in Figure 1. In the adsorption isotherm of ox-NH, a steep rise at very low relative pressure, gradual increment at the medium range of P/P0, and rise at high P/P0 show the presence of micropores, mesopores, and macropores, respectively, in the assembly of ox-NH. The nanopore volume of all samples was calculated from the Dubinin-Radushkevich analysis.18 Figure 2 shows the nanopore volume of T-ox-NH-φ as a function of fractional filling of TEMPOL. The nanopore volume of T-ox-NH-φ decreased with
Magnetism of TEMPOL Adsorbed on ox-NH
Figure 7. Magnetization curves of TEMPOL adsorbed on ox-NH at φ ) 0.55. Nonequilibrium state (square) and equilibrium state (circle) at T ) 2 K. Closed and open symbols denote magnetization and demagnetization processes.
increasing fractional filling of TEMPOL. The nanopore volume of T-ox-NH-100 is smaller than that of as-grown SWNH (0.15 cm3 g-1). These results indicated that TEMPOL molecules are adsorbed inside the nanopores and in the interstitial pores of ox-NH. An ESR spectrum of TEMPOL in solution ordinarily shows three lines caused by the hyperfine coupling with the nuclear magnetic moment of a nitrogen atom of TEMPOL. However, ESR spectra of bulk TEMPOL crystal and TEMPOL adsorbed on ox-NH indicated a broad single line, as shown in Figure 3. These results indicate that TEMPOL adsorbed on ox-NH should be in a state similar to its solid state and quite different from the liquid state. The g-value of adsorbed TEMPOL is found to be 2.005 or 2.006, which is very close to that of the TEMPOL crystal, whereas the g-value of ox-NH is 2.002. Therefore, ESR signals obtained from TEMPOL adsorbed on ox-NH should originate from the spins of TEMPOL. The full width at halfmaximum of each peak in the spectra for adsorbed TEMPOLs is broader than that of bulk TEMPOL. These results indicate that TEMPOL molecules adsorbed on ox-NH have a weaker exchange interaction than that of bulk TEMPOL because adsorbed TEMPOLs gather more compactly and were more restricted than bulk TEMPOL. The spins of TEMPOLs under such a confined condition could be difficult to interact with each other. The weak exchange interaction between the spins of TEMPOLs adsorbed on ox-NH could hardly lead to small exchange narrowing. A similar ESR spectrum was found in that of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) accommo-
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10215 dated in one-dimensional channels of a tris(o-phenylenedioxy)cyclotriphosphazene crystal.22 Temperature dependence curves of magnetic susceptibility (χ-T) of T-ox-NH-φ were measured at 2-100 K, as shown in Figure 4, where χ of ox-NH was subtracted from that of T-oxNH-φ. These curves are normalized by the weight of TEMPOL. Magnetic moment is ordinarily measured just after a sample temperature becomes constant. The χ-T curves for zero-fieldcooling (ZFC) and field-cooling (FC) processes were obtained in such an ordinary way. However, the two curves do not agree well with each other, especially at T < 20 K. Thus, the difference of χ-T curves between heating and cooling processes can be attributed to a very slow relaxation process on temperature change because if the interval between each measurement is long enough for equilibration, the same curves are obtained for the two different processes. It takes about 2 h to reach the equilibrium of magnetic susceptibility of adsorbed TEMPOL. However, such a slow relaxation was not observed for bulk TEMPOL. The relaxation time (τ) was calculated by fitting with (χ(t) - χmax)/(χ(0) - χmax) ) exp(-τ/t). Figure 5 shows temperature dependence curves of relaxation time of T-ox-NH-φ and bulk TEMPOL. The longest relaxation time is observed at lower temperature and larger φ. The intermolecular interaction is likely to affect the spin-relaxation phenomena because the slowest relaxation is observed at the condition of the strongest confinement of TEMPOL in ox-NH. The χ-T curve of T-oxNH-φ at equilibrium state and bulk TEMPOL are shown in Figure 6. TEMPOL adsorbed on ox-NH shows a higher value of χ at lower temperature, especially at T < 20 K, compared with that of bulk TEMPOL. The χ-T curve of bulk TEMPOL has a maximum at 6 K due to antiferromagnetism, but TEMPOL adsorbed on ox-NH has no maximum. The Weiss constant θ and Curie constant C were evaluated by fitting the χ-T curve to the Curie-Weiss equation: χ ) C/(T - θ). The Curie constant of adsorbed TEMPOL was almost the same as that of bulk TEMPOL. However, the Weiss constants of TEMPOL adsorbed on ox-NH and bulk TEMPOL were quite different. The Weiss constant of bulk TEMPOL was θ ) -8 K, whereas that of adsorbed TEMPOL was nearly 0 K. Therefore, TEMPOL adsorbed on ox-NH does not show antiferromagnetism like bulk TEMPOL, but shows paramagnetism. These results indicate that TEMPOL molecules do not form any ordered structures in nanoconfinment and hardly interact magnetically with each other. These results are consistent with the results from the ESR measurement. The magnetization curves at 2 K that were normalized by the weight of TEMPOL are shown in Figure 7. Measurements
Figure 8. Magnetization curves of bulk TEMPOL (O) and TEMPOL adsorbed on ox-NH at different φ values: 0.03 (4), 0.30 (3), 0.55 (0), and 1.00 (]) at T ) 2 K (a) and T ) 10 K (b).
10216 J. Phys. Chem. C, Vol. 111, No. 28, 2007 were performed after the external magnetic field become constant. The magnetization of ox-NH was subtracted from that of T-ox-NH-φ. The magnetization curve and demagnetization curve of T-ox-NH-φ were not in agreement (square symbols). The magnetic moment of the demagnetization curve is greater than that of the magnetization curve. These results were attributed to slow relaxation like a χ-T curve because, in the demagnetization curve measurement, the magnetization at the previous larger magnetic field remained when the magnetization at the next magnetic field was measured. The two curves show good agreement when the equilibration is enough. Thus, the nanoconfinement effect between adsorbed TEMPOLs and oxNH can be responsible for these relaxation phenomena. Magnetization curves at 2 and 10 K after waiting for equilibration are shown in Figure 8. Magnetization curves are consistent with demagnetization curves at equilibrium state. Magnetization curves of bulk TEMPOL and adsorbed TEMPOLs at 10 K have almost the same value. However, magnetization of adsorbed TEMPOL is larger than that of bulk TEMPOL at 2 K. This behavior can be explained by the following fact: bulk TEMPOL shows antiferromagnetism at 2 K, whereas adsorbed TEMPOL shows not antiferromagnetism but paramagnetism because of the weakness of the exchange interaction. The agreement between the magnetization curves at 10 K results from the disappearance of antiferromagnetism of bulk TEMPOL over 6 K. These results are consistent with the χ-T curve at equilibrium state. 4. Conclusion TEMPOL molecules in the nanopore cannot form a structure similar to that of the bulk solid but form a disordered structure consisting of clusters. The specific spin-spin interaction between TEMPOL molecules, which can be found in the bulk phase at low temperature, is not observed in the disordered structure in such a highly confined situation. Therefore, under nanoconfinement, the low-temperature magnetic susceptibility is large, indicating paramagnetism, but it takes a long time to reach an equilibrated state because of the restricted freedom of magnetization. These magnetic behaviors are likely to be caused by a weaker exchange interaction. Acknowledgment. This work was supported by Grants-inAid for Fundamental Scientific Research (B) (Grant 19350100)
Matsumura et al. by the Japan Society for the Promotion of Science, and Scientific Research (Chemistry of Coordination Space) (Grant 18033008) by the Ministry of Education, Culture, Sports, Science and Technology, Japan. References and Notes (1) Koga, K.; Gao, G. T.; Tanaka, H.; Zeng, X. C. Nature 2001, 412, 802. (2) Maniwa, Y.; Kataura, H.; Abe, M.; Udaka, A.; Suzuki, S.; Achiba, Y.; Kira, H.; Matsuda, K.; Kadowaki, H.; Okabe, Y. Chem. Phys. Lett. 2005, 401, 534. (3) Steele, W. A.; Bojan, M. J. AdV. Colloid Interface Sci. 1998, 76, 153. (4) Tanaka, H.; El-Merraoui, M.; Steele, W. A.; Kaneko, K. Chem. Phys. Lett. 2002, 352, 334. (5) Wang, Q.; Johnson, J. K. J. Phys. Chem. B 1999, 103, 4809. (6) Ying, M. J.; Xia, Y. Y.; Liu, X. D.; Li, F.; Huang, B. D.; Tan, Z. Y. Appl. Phys. A 2004, 78, 771. (7) Demontis, P.; Stara, G.; Suffritti, G. B. Microporous Mesoporous Mater. 2005, 86, 166. (8) Floquet, N.; Coulomb, J. P.; Dufau, N.; Andre, G. J. Phys. Chem. B 2004, 108, 13107. (9) Martin, C.; Tosi-Pellenq, N.; Patanin, J.; Coulomb, J. P. Langumuir 1998, 14, 1774. (10) Morishige, K.; Ogisu, Y. J. Chem. Phys. 2001, 114, 7166. (11) Tanaka, H.; El-Merraoui, M.; Kodaira, T.; Kaneko, K. Chem. Phys. Lett. 2002, 351, 417. (12) Tanaka, H.; El-Merraoui, M.; Steele, W. A.; Kaneko, K. Chem. Phys. Lett. 2002, 352, 334. (13) Hertzsch, T.; Budde, F.; Weber, E.; Hullinger, J. Angew. Chem., Int. Ed. 2002, 41, 2281. (14) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762. (15) Koga, K.; Gao, G. T.; Tanaka, H.; Zeng, X. C. Nature 2001, 412, 802. (16) Maniwa, Y.; Kataura, H.; Abe, M.; Udaka, A.; Suzuki, S.; Achiba, Y.; Kira, H.; Matsuda, K.; Kadowaki, H.; Okabe, Y. Chem. Phys. Lett. 2005, 401, 534. (17) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762. (18) Kaneko, K. Carbon 2000, 38, 287. (19) Kanoh, H.; Kaneko, K. J. Phys. Chem. 1995, 99, 5746. (20) Kanoh, H.; Kaneko, K. Chem. Phys. Lett. 1995, 237, 329. (21) Kanoh, H.; Kaneko, K. J. Phys. Chem. 1996, 100, 755. (22) Kobayashi, H.; Ueda, T.; Miyakubo, K.; Toyoda, J.; Eguchi, T.; Tani, A. J. Mater. Chem. 2005, 15, 872. (23) Berliner, J. L.; Fujii, H. Science 1985, 227, 517. (24) Yamauchi, J.; Fujito, T.; Ando, E.; Nishiguchi, H.; Deguchi, Y. J. Phys. Soc. Jpn. 1968, 25, 1558. (25) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takahashi, K. Chem. Phys. Lett. 1999, 309, 165. (26) Utsumi, S.; Urita, K.; Kanoh, H.; Yudasaka, M.; Suenaga, K.; Iijima, S.; Kaneko, K. J. Phys. Chem. B 2005, 109, 14319.
