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Compositional Studies on the Electronic and Magnetic Properties of Potassium Fulleride Mesoporous Niobium Oxide Composites Bing Ye,† Michel Trudeau,‡ and David Antonelli*,† Department of Chemistry and Biochemistry, The University of Windsor, 401 Sunset Avenue, Windsor, Ontario N9B-3P4, Canada, and M. L. Trudeau, Emerging Technologies, Hydro-Que´ bec Research Institute, 1800 Boul. Lionel-Boulet, Varennes, Quebec J3X 1S1, Canada Received January 24, 2002. Revised Manuscript Received April 10, 2002
Mesoporous niobium oxide potassium fulleride composites were synthesized by the treatment of mesoporous niobium oxide with 0.3, 0.6, or 1.0 equiv of potassium naphthalene followed by stirring with either excess K3C60 or C60. The reaction of potassium-reduced mesoporous niobium oxide with neutral C60 is the first reported example of a mesoporous oxide functioning as an electron donor to a guest molecule. These new routes allow for greater flexibility in the K:Nb and C:Nb ratios in the composite than our previous method and for this reason enables a more comprehensive study of the effect of absolute carbon and potassium content on electron transport properties. Materials were characterized by elemental analysis, nitrogen adsorption, XRD, XPS, SQUID magnetometry, and roomtemperature conductivity measurements in an effort to relate density of states at the Fermi level, temperature-independent paramagnetism, and conductivity patterns to the composition of the composite. Variable temperature resistivity measurements showed that the composites are metallic, semiconducting, or insulating, depending on the composition. The main factors governing conductivity were the absolute carbon content and the oxidation state of the intercalated fulleride, while the absolute potassium content had little effect on the electronic properties.
Introduction The exploration of fullerene materials1,2 is one of the most exciting areas of materials science. While carbon nanotubes have already shown promise in hydrogen storage3,4 and the fabrication of ultrathin wires,5,6 alkali fullerides continue to be an area of great interest and controversy because of their superconducting and metallic properties.7-11 The role of crystal structure and alkali metal dopant level in the electronic properties of * To whom correspondence should be addressed. † The University of Windsor. ‡ Hydro-Que ´ bec Research Institute. (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Kraetschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffmann, D. R. Nature 1992, 347, 354. (3) Iijima, S. Nature 1991, 354, 56. (4) Dillon, A. C.; Jones, K. M.; Bekkedah, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (5) Bockrath, M.; Cobden, D. H.; McEuen, P. L.; Chopra, N. G.; Zettl, A.; Thess, A.; Smalley, R. E. Science 1997, 275, 1922. (6) Hong, B. H.; Bae, S. C.; Lee, C. W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348. (7) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (8) Holczer, K.; Klein, O.; Huang, S. M.; Kaner, R. B.; Fu, K. J.; Whetten, R. L.; Dederich, F. Science 1991, 252, 1154. (9) Tanigaki, K.; Hirosawa, I.; Ebbesen, T. W.; Mizuki, J.; Shimakawa, Y.; Kubo, Y.; Tsai, J. S.; Kuroshima, S. Nature 1992, 356, 419. (10) Tanigaki, K.; Ebbesen, T. W.; Saito, S.; Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222. (11) Rosseinsky, M. J. Chem. Mater. 1998, 10, 2665.
these materials is not fully understood, as K3C60 is predicted to be a Mott-Hubbard insulator because the t1u conduction band is half full.12 Accidental K vacancies have been invoked to explain this observation; however, there is still no general consensus on the role of structure and stoichiometry in the physical properties of this class of materials.11 Because the structure of known superconducting alkali fullerides is largely limited to face-centered cubic (fcc) systems, there are not yet enough empirical examples to fully develop a model relating crystal space group to electronic properties. To this end, we recently synthesized mesoporous niobium oxide composites of K3C60 in which the pore size of the mesostructure (≈23 Å) is just large enough to accommodate one unit cell (14 Å) of the fulleride.13,14 This reaction depends on the capacity of mesoporous transition-metal oxides15-19 to function as electron donor20-26 (12) Lof, R. W.; van Veenendaal, M. A.; Koopmans, B.; Jonkman, H. T.; Sawatzky, G. A. Phys. Rev. Lett. 1992, 68, 3924. (13) Ye, B.; Trudeau, M.; Antonelli, D. M. Adv. Mater. 2001, 13, 29. (14) Ye, B.; Trudeau, M.; Antonelli, D. M. Adv. Mater. 2001, 13, 561. (15) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014. (16) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 35, 426. (17) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (18) Tian, Z.-R.; Tong, W.; Wang, J.-Y.; Duan, N.-G.; Krishnan, V. V.; Suib, S. L. Science 1997, 276, 926.
10.1021/cm020057f CCC: $22.00 © 2002 American Chemical Society Published on Web 05/16/2002
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and the well-documented capacity of mesoporous oxides27-35 to act as host for a wide variety of guest species.36,37 The oxidation state and potassium loading level of the composite can be tuned by further addition of potassium and the oxidation state of the fulleride units monitored by Raman spectroscopy. The confinement of the fulleride in one-dimensional channels in this system leads to different physical properties because the breaking down of the cubic symmetry of the fulleride units should lead to a reduction in available electronic pathways. The unusual results obtained so far are that the room-temperature conductivity shows two maxima at n ) 2.5 and n ) 4.0 for C60n- occluded in the pores. The maxima at n ) 2.5 suggests that this may actually be the phase responsible for metallic behavior in pure K3C60, while the maximum at above n ) 4 has no parallel in bulk fullerides, as K4C60 is an insulator. In a previous report we investigated the effect of changing the pore size of the mesostructure to accommodate two units across the channel rather than one, as well as the composition of the wall to titanium oxide and tantalum oxide, on the electronic properties of these composites.38 The results of this study indicated that the conductivity maxima occur at roughly n ) 2.5 and n ) 4.0 in all systems and that composition of the walls and pore size do not strongly influence the electronic behavior of the fulleride phase. The maximum at above n ) 4.0 was always accompanied by an increase in the density of states near the Fermi level as measured by XPS, consistent with an increase in electrical conductivity. In this report we present new synthetic routes into potassium fulleride composites of mesoporous niobium oxide with a wide range of Nb:K:C60 ratios and compare the temperature-dependent conductivity and magnetic properties in an effort to further probe into the nature (19) Antonelli, D. M.; Trudeau, M. Angew. Chem., Int. Ed. 1999, 38, 1471. (20) Vettraino, M.; Trudeau, M.; Antonelli, D. M. Adv. Mater. 2000, 12, 337 (21) Vettraino, M.; Trudeau, M.; Antonelli, D. M. Inorg. Chem. 2001, 40, 2088. (22) Murray, S.; Trudeau, M.; Antonelli, D. M. Adv. Mater. 2000, 12, 1339. (23) Murray, S.; Trudeau, M.; Antonelli, D. M. Inorg. Chem. 2000, 39, 5901. (24) Vettraino, M.; Trudeau, M.; Antonelli, D. M. J. Mater. Chem. 2001, 11, 1755. (25) He, X.; Trudeau, M.; Antonelli, D. M. Adv. Mater. 2000, 12, 1036. (26) He, X.; Trudeau, M.; Antonelli, D. M. Chem. Mater. 2001, 13, 4808. (27) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (28) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Shepard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (29) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J.; Schramm, S. E. Chem. Mater. 1994, 6, 1816. (30) Davis, M. E. Nature 1993, 364, 391. (31) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (32) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. (33) Ciesla, U.; Demuth, D.; Leon, R.; Petroff, P.; Stucky, G. D.; Unger, K.; Schuth, F. J. Chem. Soc., Chem. Commun. 1994, 1387. (34) Tanev, P.; Pinnavaia, T. J. Science 1995, 267, 865. (35) Pinnavaia, T. J.; Prouzet, E.; Bagshaw, S. A. Science 1995, 262, 1241. (36) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (37) Wu, C.-G.; Bein, T. Chem. Mater. 1994, 6, 1109. (38) Ye, B.; Trudeau, M.; Antonelli, D. M. Chem. Mater. 2001, 13, 2730.
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of charge transport in these materials and the role of absolute potassium content on this phenomenon. Experimental Section Materials and Equipment. All chemicals unless otherwise stated were obtained from Aldrich. Samples of mesoporous niobium oxide (Nb-TMS1) were obtained from Alfa-Aesar and used without further purification. Conducting silver adhesive was obtained from Alfa-Aesar. Trimethylsilyl chloride was obtained from Aldrich and distilled over calcium hydride. Mesoporous niobium oxide samples were dried at 100 °C overnight under vacuum and then stirred with excess trimethylsilyl chloride in dry ether for 12 h under nitrogen. Nitrogen adsorption and desorption data were collected on a Micromeritics ASAP 2010. X-ray diffraction (XRD) patterns (Cu KR) were recorded in a sealed glass capillary on a Siemens D-500 θ-2θ diffractometer. All X-ray photoelectron spectroscopy (XPS) peaks were referenced to the Carbon C-(C,H) peak at 284.8 eV and the data were obtained using a Physical Electronics PHI-5500 using charge neutralization. The roomtemperature conductivity measurements were recorded on a Jandel four-point universal probe head combined with a Jandel resistivity unit. The equations used for calculating the resistivity were as follows: For pellets of 0.5-mm thickness, the following equation is used:
V I
F ) 2π(S)
where F ) resistivity, π/(log n2) ) sheet resistivity, V ) volts, I ) current, t ) thickness of the pellet, and S ) the spacing of the probes (0.1 cm). Measurements were done in triplicate in an inert atmosphere, allowing the system to stabilize for 30 min for each reading to ensure maximum reliability. Magnetic measurements were conducted on a Quantum Design superconducting quantum interference device (SQUID) magnetometer MPMS system with a 5-T magnet. Diamagnetic correction terms for the constituent elements were added to the data where appropriate.39 Variable temperature conductivity measurements were conducted on a Quantum Design SQUID MPMS system in the absence of an external field on epoxycoated pellets of the materials with four copper wires affixed to the surface with silver paste. The methyl ethyl ketone solvent in the silver paste was evaporated and replaced by THF because the ketone oxidizes the surface of the pellets and hinders conductivity measurements. The Raman spectra were recorded on a Renishaw Ramascope using a Renishaw 780nm diode laser system. All elemental analysis data were conducted under an inert atmosphere by Galbraith Laboratories (2323 Sycamore Drive, Knoxville, TN 37921-1700). Metal analysis was conducted by inductively coupled plasma (ICP) techniques. Synthesis. (a) From K3C60 and K-NbTMS. To a THF suspension of Nb-TMS1 previously reduced by either 0.3, 0.6, or 1.0 molar equiv (based on percentage of Nb in the structure as determined by ICP) of potassium naphthalene according to the literature procedure21 was added excess K3C60 (synthesized by heating C60 and K together in a sealed tube at 200400 °C and characterized by XRD). After several days and additional stirring to ensure complete absorption of the fulleride, the reduced material was collected by suction filtra(39) Cheetham, A. K.; Day, P. Solid State Chemistry: Techniques; Oxford University Press: New York, 1987.
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Figure 1. X-ray powder diffraction pattern of samples of mesoporous niobium oxide reduced with 0.3, 0.6, and 1.0 equiv of K-naphthalene and then treated with K3C60 (NbI, NbII, NbIII). tion and washed several times with benzene. The material was then dried in vacuo at 10-3 Torr on a Schlenk line until all condensable volatiles had been removed. This procedure is normally sufficient to remove all solvent from the composite. All materials were characterized by Raman, XPS, and elemental analysis to ensure sample quality before proceeding with SQUID and conductivity studies. (b) From C60 and K-NbTMS. To a THF suspension of NbTMS1 previously reduced by either 0.3, 0.6, or 1.0 molar equiv of potassium naphthalene (see above) was added excess C60. After several days and additional stirring to ensure complete absorption of the fullerene, the material was collected by suction filtration and washed several times with benzene. The material was dried in vacuo at 10-3 Torr on a Schlenk line until all condensable volatiles had been removed. All materials were characterized by Raman, XPS, and elemental analysis to ensure sample quality before proceeding with SQUID and conductivity studies.
Results and Discussion To expand the compositional diversity of mesoporous niobium oxide potassium fulleride composites, two new methods of synthesis were developed. The first of these involves the treatment of potassium-reduced mesoporous niobium oxide with K3C60. Samples of mesoporous niobium oxide were reduced with either 0.3, 0.6, or 1.0 equiv of potassium naphthalene and then treated with excess K3C60 in THF. After 3 days of stirring, new grayto-black materials were isolated and washed with excess benzene and dried in vacuo to ensure that residual solvent was removed. Figure 1 shows the XRD patterns of these new composites (NbI, NbII, and NbIII). The d spacing for the main peak in each sample falls at 36 Å, virtually identical to those for the unreduced starting materials, demonstrating that the mesoporous structure was fully retained upon absorption of the fulleride. The nitrogen adsorption/desorption isotherms of the starting material and products are shown in Figure 2. The surface areas of the composites dropped from 925 m2/g in the starting material to 569, 371, and 323 m2/g, respectively for NbI, NbII, and NbIII, while the HK pore sizes decreased from 23 to 18, 17, and 17 Å. This is consistent with adsorption of the fulleride and partial occlusion of the pore channels by the fulleride units.
Figure 2. Nitrogen adsorption and desorption isotherms for samples from Figure 1 compared to unreduced mesoporous niobium oxide. Table 1. Table of Elemental Analysis Values and BET Surface Area for Samples of Fulleride Composites Synthesized from Mesoporous Niobium Oxide Reduced by 0.3, 0.6, and 1.0 equiv of K-Naphthalene and Then Treated with Either K3C60 (NbI, NbII, NbIII) or C60 (NbIV, NbV, NbVI)a BET surface sample niobium potassium carbon molar ratio area ID % % % (Nb:K:C60 (m2/g) 1 2 3 4 5 6 7 8 9 10
NbI NbII NbIII NbIV NbV NbVI NbVII NbVIII NbIX NbX
37.16 40.34 36.17 37.46 40.81 30.32 30.71 34.36 36.07 34.12
6.79 11.33 14.91 7.42 9.05 11.47 4.20 4.40 5.12 7.35
19.67 12.20 10.82 10.48 11.92 26.94 29.88 15.9 16.56 20.16
14.8:6.4:1 25.6:17.1:1 25.9:25.4:1 27.6:12.8:1 25.6:13.3:1 8.8:7.8:1 9.9:3.0:1 16.7:5.1:1 16.5:5.6:1 13.1:6.7:1
568 371 323 521 412 274 264 325 288 227
a Values for materials synthesized previously14 by treatment of unreduced mesoporous Nb oxide with K3C60 (NbVII) followed by reduction with K-naphthalene to n ) 2.5, 3.0, and 4.1 (NbVIII, NbIX, NbX) are shown for comparison.
Elemental analysis of these materials provided Nb:K: C60 ratios of 14.8:6.4:1, 25.6:17.1:1, and 25.9:25.4:1, respectively, for NbI, NbII, and NbIII. The nitrogen adsorption and elemental analysis data are summarized in Table 1. For comparison, the elemental analysis and nitrogen adsorption data for samples of unreduced mesoporous niobium oxide treated with excess K3C60 (NbVII) and then further reduced by potassium naphthalene to levels of n ) 2.5 (NbVIII), n ) 3.0 (NbIX), and n ) 4.0 (NbX) are shown. Because the absorption of fulleride by mesoporous niobium oxide is driven by the reduction of the porous structure by the electronrich fulleride,13 the carbon content drops as the level of prereduction by potassium naphthalene increases from 0.3 to 1.0 equiv. There is also a difference of almost 10% C between the materials reduced with 0.6 (NbII) and
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Table 2. Table of Raman Ag Mode, Oxidation State of Fulleride, Room-Temperature Conductivity, Molar Nb:K:C Ratios, and XPS Nb 3/2, 5/2 Emissions for Materials from Table 1a
1 2 3 4 5 6 7 8 9 10 a
sample ID
Raman shift of Ag mode of C60 anion (cm-1)
oxidation state of C60 anion
conductivity at room temperature (Ω-1 cm-1)
molar ratio (Nb:K:C60)
XPS 3d Nb 3/2, 5/2 emissions (eV)
NbI NbII NbIII NbIV NbV NbVI NbVII NbVIII NbIX NbX
1459 1453 1449 1464 1463 1461 1462 1449 1447 1438
0.8 2.0 2.8 0.2 0.4 0.6 0.5 2.6 3.0 4.1
3.72 × 10-5
