J. Phys. Chem. 1996, 100, 4503-4506
4503
Studies of C60 Oxidation and Products J. Adelene Nisha, V. Sridharan, J. Janaki, Y. Hariharan, V. S. Sastry, C. S. Sundar, and T. S. Radhakrishnan* Materials Science DiVision, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India ReceiVed: NoVember 6, 1995X
The oxidation of C60 solid was investigated using TGA, DTA, X-ray diffraction, FTIR, and optical microscopy. Oxidation reaction of solid C60 commences at a temperature below 220 °C, resulting in the formation of a polycondensate (PCS) with C:O ) 5:1. The associated heat of formation ∆Hf is determined to be 9.2 kJ/g. From the isothermal investigations carried out between 220 and 240 °C, the reaction mechanism is identified to be a two-dimensional nucleation and growth process. The PCS is found to undergo a structural modification without weight change beyond 220 °C with an enthalpy change of 4.6 kJ/g. The PCS itself is found to consist of (1) a glassy carbon with an inter-microfibril distance of 3.78 Å obtained with the destruction of the C60 cage, (2) an amorphous C60-O-C60 complex characterized by the intermolecular distance of 15.04 Å with an oxygen atom forming a bridge between two C60 molecules, and (3) an unidentified crystalline phase with carbonyl bonds.
I. Introduction
II. Experimental Details
C60 undergoes a host of reactions which can be broadly classified as addition,1-4 photopolymerization,5,6 intercalation,7-9 endohedral complexation,10-12 thermal amorphization,13 etc., leading to an array of products. Depending upon the conditions, the reaction of solid C60 with oxygen can lead to intercalation of oxygen molecule into C60 solid without chemical bonding,14 photoassisted transformations leading to C60O epoxide at room temperature,15,16 and a carbonyl-bonded amorphous product even at low temperatures17 and the formation of an amorphous polycondensate (PCS) at temperatures greater than 200 °C.18 While intercalation and photoassisted transformation leave the C60 cage intact,14-16 it is reported that the PCS formation proceeds via cage opening with the ultimate destruction of the cage.19 A study of these reactions and the resulting products could provide important information on the stability of C60 in oxygen atmosphere. Vassallo et al.,19 from the FTIR studies, report the destruction of the C60 cage in O2 atmosphere from 200 °C onward, leading to the formation of an unspecified “complex”. Chen et al.20,21 have also reported the breakage of C60 molecules in O2 atmosphere above 200 °C, resulting in the formation of a PCS with a C:O ratio of 60:12. They have observed two exothermic peaks in the differential scanning calorimetry (DSC) thermogram at 306 and 427 °C. They have attributed the first peak to the composite effect of the formation of the PCS and the subsequent breakage of the bonds of the carbon-oxygen species. The second peak was attributed by them to the burning of the PCS into CO or CO2 gases. The formation reaction was determined to proceed through a two-dimensional nucleation and growth process. Although Chen et al.20 had conjectured more than one reaction under the first peak of DSC, no attempts were made to delineate them. In this paper, we report the thermogravimetric analysis (TGA) and differential thermal analysis (DTA) studies of the different stages of the C60-oxygen reaction. We believe that it is comprised of the PCS formation accompanied by a structural modification and followed by a two-step burning process. The corresponding values of the heat of reaction and the energy changes associated with the burning are also reported.
C60 was synthesized by striking a plasma arc between two graphite electrodes in a helium atmosphere.22 From the resultant soot, C60 was extracted into toluene using a Soxhlet extracter and chromatographically separated using a neutral grade alumina column. The samples were vacuum annealed at 250 °C for 24 h to remove any trapped solvents. The purity of the C60 was confirmed using X-ray diffraction and UV-vis spectroscopic studies. Thermal analysis investigations were carried out in a simultaneous thermal analyzer (Model STA 1500) of the Polymer Laboratory, U.K. The temperature and energy calibrations were carried out using high-purity aluminum. C60 samples were in the form of powder. In all the runs platinum crucibles were used with calcined fine Al2O3 powder as the reference material. High-purity oxygen at a gas flow rate of 20 cm3/min was maintained throughout the investigations. Powder X-ray diffraction studies were carried out at room temperature with Cu KR radiation using a Siemens diffractometer (Model D500) in the θ-2θ Bragg-Brentano arrangement coupled to a Philips X-ray generator. To reduce the air scattering at low angles, 0.3 mm slits in the crossed geometry were used at both the source and the detector sides. The FTIR spectra were recorded on a Nicolet Model IMPACT 400 having a resolution of 4 cm-1. The FTIR spectra of solid samples were recorded using KBr powder. Optical microscopic observations were carried out in a Reichert microscope (Model Polyvar 2 MET) under daylight conditions.
X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4503$12.00/0
III. Results and Discussions The TGA, DTA, and the computed differential TGA (DTGA) thermograms of pristine C60 in oxygen atmosphere at the heating rate of 1 °C/min are shown in Figure 1. In the TGA trace, a weight pickup has been observed from 250 °C onward, reaching a maximum value of 15% at 310 °C. With further increase of temperature, the weight initially decreases almost linearly up to 400 °C and then more precipitously, marked by total weight loss at 455 °C. In the corresponding DTGA thermogram, three extrema, P1, P2, and P3, at 292, 356, and 428 °C, respectively, are observed. On the other hand, the DTA thermogram exhibits © 1996 American Chemical Society
4504 J. Phys. Chem., Vol. 100, No. 11, 1996
Adelene Nisha et al.
Figure 1. Dynamic TGA and DTA and computed DTGA thermogram of C60 solid in oxygen atmosphere at the heating rate of 1 °C/min.
Figure 2. 220 °C TGA and DTA isotherm of C60 solid in oxygen atmosphere for the annealing time of 700 min.
TABLE 1: Peak Temperature of DTA and DTGA Peaks of Pristine C60 for Heating Rates of 1 and 5 °C/min and Their Corresponding Peak Shifts DTGA peak no. P1 P2 P3
heating rate (°C/min) 1 5 292 356 428
337 419 458
DTA ∆Tp (°C) 45 63 30
heating rate (°C/min) 1 5 310
399 not resolvable 426 453
∆Tp (°C) 89 27
only two prominent exothermic peaks, at 310 and 426 °C, corresponding to P1 and P3. The analogue of the second DTGA extremum P2, if any, is either nonexistent or unresolvable in the DTA trace. The positions of DTGA extrema and DTA peaks for two different heating rates, 1 and 5 °C/min, are presented in Table 1. It is seen that while the first peak of DTA shifts to a higher temperature by 89 °C, the corresponding DTGA extremum shifts only by 45 °C for the same increase in the heating rate. On the other hand, P3 of DTGA and DTA shift by similar magnitudes. From the area under the DTA curve, the enthalpy change for the total reaction is determined to be 23.6 kJ/g, while Chen et al.21 reports a value of 19 kJ/g. The implications of these observations are discussed below. We interpret the weight pickup observed from 250 °C onward as due to a C60-oxygen reaction leading to the formation of a PCS in accordance with Chen et al.20 We also ascribe the subsequent weight loss to the burning of the PCS in the oxygen atmosphere, with the final burning temperature being 455 °C. The onset temperatures of the weight pickup and that of the first peak of DTA are the same. Hence, we ascribe the first DTA peak to the C60-O reaction leading to the formation of the PCS. For the heating rate of 1 °C/min the latter leads the corresponding DTGA peak by 18 °C. This was also observed by Chen et al.,20 but no interpretation was offered. It is pertinent to note here that the first peak of DTA (Figure 1) spans a temperature range wherein weight loss is also observed. Taking this together with the observation of a differing heating rate dependent shift of P1 between DTA and DTGA, it is brought out for the first time that more than one reaction is involved under the first peak of DTA. Nonetheless, we were not able to resolve the peaks even up to a heating rate of 12 °C/min. This may probably be due to the sequential nature of the reactions. To delineate the different reactions under the first peak of DTA, we have carried out isothermal investigations in oxygen atmosphere in the temperature range 220-240 °C. Figure 2 shows the TGA and DTA isotherms of pristine C60 at 220 °C in oxygen atmosphere up to an annealing time of 700 min. The percentage of product conversions as estimated from the TGA trace and the cumulative area under the DTA closely match
Figure 3. Dynamic TGA and DTA thermogram of the PCS formed at 220 °C in oxygen atmosphere at 1 °C/min.
with each other. This implies that the 220 °C annealing leads to the formation of PCS alone. From the area under the DTA isotherm, the heat of formation of PCS is determined to be 9.2 kJ/g. This value is slightly smaller than the reported value of 11.8 kJ/g by Chen et al.21 From the TGA isothermal studies carried out between 220 and 240 °C and using the Avrami equation,23 the order of the reaction n is estimated to be approximately 2. The profile of the TGA isotherm is sigmodal with a saturation weight pickup of approximately 26%. This corresponds to a carbon to oxygen ratio of 5:1, which agrees well with the report of Chen et al.20,21 The sigmodal profile indicates a nucleation and growth process for the PCS formation. To investigate the thermal stability of the PCS, a dynamic run of the PCS formed at 220 °C was carried out in oxygen atmosphere at a heating rate of 1 °C/min. The corresponding TGA and DTA thermograms are shown in Figure 3. As already observed, the total weight loss temperature of 455 °C and the second DTA peak at 430 °C correspond to the final burning temperature of the PCS. Although the onset temperature for the first peak of DTA is 210 °C, there is no associated weight change until 260 °C. Hence, the first DTA peak is attributed to a structural modification of the PCS without weight change prior to its burning. This modification of the PCS is being reported for the first time. The area under the DTA curve of the PCS corresponds to an enthalpic change of 14.3 kJ/g. As expected, the total area under the DTA has decreased in relation to the corresponding quantity of the pristine C60 by 9.3 kJ/g, which is the enthalpy of formation of the PCS as determined from Figure 2. To estimate the enthalpy change due to the structural modification of the PCS, pristine C60 was annealed for 700 min at 240 °C, during which period the structural modification is expected to be completed. The dynamic DTA and TGA
Studies of C60 Oxidation and Products
J. Phys. Chem., Vol. 100, No. 11, 1996 4505
Figure 4. Dynamic TGA and DTA thermogram of the PCS formed at 240 °C in oxygen atmosphere at 1 °C/min.
thermograms of this 240 °C annealed product at 5 °C/min is shown in Figure 4. It exhibits a major peak at 475 °C and a satellite peak at 350 °C. It is pertinent to note that the DTA thermogram is devoid of the first exothermic peak (P1), which was assigned to the combined effect of the formation of the PCS and its structural modification (see Figures 1 and 3). This confirms the completion of the structural modification of the PCS at 240 °C for an annealing time of 700 min. From the area under the DTA isotherm at 240 °C (figure not shown), the total heat of reaction is determined to be 13.8 kJ/g. This is smaller by 4.6 kJ/g than the corresponding value determined from the 220 °C isotherm, which is attributed to the structural modification of the PCS. The DTA curve in Figure 4 thus represents the two-step burning process of PCS alone, and the corresponding change in enthalpy is determined to be 9.3 kJ/g. This value is slightly higher than 7.5 kJ/g reported by Wiedemann and Bayer.18 It is of interest to examine the nature of the PCS formed around 220 °C. We have carried out optical microscopy, X-ray diffraction, and FTIR investigations on the PCS. In Figure 5a the optical micrograph of the C60 powder is shown, and in Figure 5b that of the PCS. It is interesting to note that even after grinding the C60 grains exhibit faceting and are golden in color under daylight conditions. Under the same conditions, the PCS shows the presence of two phases: a fine powdered phase of brown color and golden-colored particulates embedded in the fine powdered phase. The size of the brownish particles is considerably less than that of pristine C60. The X-ray diffraction pattern of the 220 °C annealed sample is shown in Figure 6. It exhibits two broad prominent peaks representative of amorphous nature centered around 2θ values of ≈6° and 23.5° and a few relatively weak crystalline peaks. From the area under the crystalline peaks, the volume of the crystalline phase is estimated to be less than 16%. The PCS was added to toluene, subjected to ultrasonic agitation for 30 min, and subsequently centrifuged at 400 rpm for 5 min. This resulted in a residue and a clear solution of brownish hue. The X-ray diffraction pattern of the residue is quite similar to the pattern in Figure 6, but the crystalline peaks were absent. Absence of crystalline peaks suggests that the crystalline phase has dissolved in toluene. It was also seen from the optical micrograph (not shown) that both the fine powdered brown phase and the golden particulates were retained in the residue. This indicates that both these phases seen in Figure 5b are amorphous in nature. Failure to detect the crystalline phase in Figure 5b is probably due to the low volume fraction. In Figure 7, the FTIR spectra of the residue and the crystalline phase (which was obtained in the form of a thin film on a KBr pellet by evaporating toluene) are compared with that of C60. The FTIR spectrum of the crystalline phase does not correspond to that of either C60 or C60O.15,24 The absorption bands above 1600
Figure 5. Optical micrographs of (a, top) C60 and (b, bottom) the PCS formed at 220 °C under daylight conditions.
Figure 6. X-ray diffraction pattern of the PCS formed at 220 °C exhibiting two broad peaks centered around 2θ ) 6° and 2θ ) 23.5° with crystalline peaks superposed.
cm-1 seen in the case of both the residue and the crystalline phase correspond to carbonyl bonds.25 Although the interplanar distances of the crystalline peaks (Figure 6) closely match with that of the pristine C60 phase, the brownish color of the solution and the FTIR studies indicate the presence of an unidentified crystalline phase with carbonyl bonds. The FTIR spectrum of the residue exhibits four prominent absorption bands at 1237, 1400, 1620, and 1720 cm-1 with a shoulder at 1780 cm-1 and is similar to the result of Vassallo et al.19 The absorption bands are relatively broad compared to that of the crystalline phase of the PCS and that of C60, indicating a broad distribution in the bond lengths. The 1620 cm-1 absorption band corresponds to an aromatic C-C stretch enhanced by a functional group.25 The broad peak around 1720 cm-1 with a shoulder at 1780 cm-1 indicates the presence of a cyclic anhydride with a five-membered ring.25,26 The afore-
4506 J. Phys. Chem., Vol. 100, No. 11, 1996
Adelene Nisha et al. 3.78 Å obtained by the destruction of the C60 cage and a C60O-C60 complex characterized by an average intermolecular distance of 15.04 Å with the C60 cage intact. It is also seen that the burning of the PCS itself is a two-step process, and the heat of burning is determined to be 9.3 kJ/g. Acknowledgment. We thank G. V. N. Rao for the X-ray diffraction pattern and A. Bharathi for fruitful discussions. We also thank Padma Gopalan and M. Premila for their collaboration in the preparation of the sample. Our sincere thanks are to K. S. V. Srinivasan and Hyder Ali, Central Leather Research Institute, Madras, for providing access to FTIR measurements. One of us (J.A.N.) acknowledges the grant of the Junior Research Fellowship of the Department of Science and Technology, India, under the National Superconductivity Programme. References and Notes
Figure 7. FTIR spectra of C60, residue of the PCS on desolution, and toluene desolvable crystalline portion of the PCS.
mentioned absorption bands suggest the presence of a phase with a sheetlike structure obtained via complete destruction of the C60 cage. The broad absorption bands at 1400 and 1237 cm-1 overlap partly with the characteristic emission peaks of C60. The 1237 cm-1 band is assigned to sp2-hybridized carbon singly bonded to oxygen.25,26 The broad X-ray diffraction pattern of Figure 6 indicates the presence of an amorphous product in the PCS. From the positions of the broad X-ray diffraction peaks, the average nearest neighbor (nn) distances were estimated to be approximately 15.04 and 3.78 Å. From the available data in the literature, the nn distances of 15.04 and 3.78 Å could not be associated with a single carbonaceous product. The distance 3.78 Å is larger than the interplanar distance of graphite and is assigned to a glassy carbon with an inter-microfibril distance of 3.78 Å.27 This lends support to our assignment of 1720 and 1620 cm-1 FTIR absorption bands to a sheetlike local structure with a five- or six-membered ring. As the X-ray diffraction studies were carried out in the interval 0.3 e Q e 3 Å-1, the C-C distance could not be estimated. To the best of our knowledge, the nn distance of 15.04 Å so far has not been reported in the literature for the C60-O reaction product. It should be noted that this distance is even larger than the intermolecular distance of pristine C60 solid (14.14 Å). This is unlikely to be associated with any atomic species but can only correspond to a molecular species. Thus, at this juncture, we believe that the distance of 15.04 Å might correspond to the intermolecular distance of a C60-O-C60 type of complex, with an oxygen atom forming a bridge between two intact C60 molecules. A similar bridge involving an oxygen atom in higher metallofullerenes has also been reported.28 Further work is under way in elucidating the composition and the structure of these phases. IV. Conclusions The present investigation confirms the C60-oxygen reaction leading to the formation of a polycondensate (PCS) with C:O ) 5:1. The mechanism of the reaction has been identified to be a surface-controlled two-dimensional nucleation and growth process, and the heat of formation is estimated to be 9.2 kJ/g. We have identified a hitherto unreported structural modification of the PCS at temperatures greater than 220 °C without change in weight. The associated enthalpic change has been determined to be 4.6 kJ/g. We report for the first time that the PCS consists of a carbonyl-bonded crystalline phase and two amorphous phases: a glassy carbon with an inter-microfibril distance of
(1) Hoke, S. H., II; Dillettato, D.; Molstad, M. J.; Jay, M. J.; Carlson, D.; Khar, B.; Cooks, R. G. J. Org. Chem. 1992, 57, 5069. (2) Wang, H. H.; Schlueter, J. A.; Cooper, A. C.; Smart, J. L.; Whitten, M. E.; Geiser, U.; Carlson, K. D.; Williams, J. M.; Welp, U.; Dudek, J. D.; Caleca, M. A. J. Phys. Chem. Solids 1993, 54, 1655. (3) Tuinman, A. A.; Mukerjee, P.; Adcock, J. L.; Hettich, R. L.; Compton, R. N. J. Phys. Chem. Solids 1992, 96, 7584. (4) Wuld, F. Acc. Chem. Res. 1992, 25, 106. (5) Rao, A. M.; Zhou, P.; Wang, K.-A.; Hager, G. T.; Holden, J. M.; Wang, Y.; Lee, W. T.; Bi, X.-X.; Eklund, P. C.; Cornett, D. S.; Dunkan, M. A.; Amster, I. J. Science 1993, 259, 955. (6) Wang, Y.; Holden, J. M.; Bi, X.-X.; Eklund, P. C. Chem. Phys. Lett. 1994, 217, 413. (7) Hebbard, 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) Winter, J.; Kuzmany, H. Solid State Commun. 1992, 84, 935. (9) Tanigaki, K.; Hirosawa, I.; Ebbesen, T. W.; Mizuki, J.-I.; Tsai, J.-S. J. Phys. Chem. Solids 1994, 54, 1645. (10) Weiss, F. D.; Elkind, J. L.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. J. Am. Chem. Soc. 1988, 110, 4464. (11) Guo, T.; Diener, M. D.; Chai, Y.; Alford, M. J.; Haufler, R. E.; McClure, S. M.; Ohno, T. R.; Weaver, J. H.; Scuseria, G. E.; Smalley, R. E. Science 1992, 257, 1661. (12) Smith, A. B.; Strongin, R. M.; Brard, L.; Romanow, W. J. J. Am. Chem. Soc. 1994, 116, 1083. (13) Sundar, C. S.; Bharathi, A.; Hariharan, Y.; Janaki, J.; Sastry, V. S.; Radhakrishnan, T. S. Solid State Commun. 1992, 84, 823. (14) Assink, R. A.; Schirber, J. E.; Loy, D. A.; Morosin, B.; Carlson, G. A. J. Mater. Res. 1992, 7, 2136. (15) Creegan, K. M.; Robins, J. L.; Robins, W. K.; Miller, J. M.; Sherwood, R. D.; Tindall, P. J.; Cox, D. M. J. Am. Chem. Soc. 1992, 114, 1103. (16) Wood, J. M.; Kahr, B.; Hoke, S. H., II; Dejarme, L.; Cooks, R. G.; Amotz, D. B. J. Am. Chem. Soc. 1991, 113, 5907. (17) Kroll, G. K.; Benning, P. J.; Chen, Y.; Ohno, T. R.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Chem. Phys. Lett. 1991, 181, 112. (18) Wiedemann, H. G.; Bayer, G. Thermochim. Acta 1993, 214, 85. (19) Vassallo, A. M.; Pans, L. S. K.; Cole-Clarke, P. A.; Wilson, M. A. J. Am. Chem. Soc. 1991, 113, 7820. (20) Chen, H. S.; Kortan, A. R.; Haddon, R. C.; Fleming, D. A. J. Phys. Chem. 1992, 96, 1016. (21) Chen, H. S.; Kortan, A. R.; Haddon, R. C.; Kaplan, M. L.; Chen, C. H.; Mujsce, A. M.; Chou, H.; Fleming, D. A. Appl. Phys. Lett. 1991, 59, 2956. (22) Sundar, C. S.; Hariharan, Y.; Bharathi, A.; Sastry, V. S.; Rao, G. V. N.; Janaki, J.; Geethakumari, T.; Radharishnan, T. S.; Arora, A. K.; Shakuntala, T.; Yousuf, M.; Sahu, P. Ch.; Subramanian, N.; Raghunathan, V. S.; Valsakumar, M. C. Ind. J. Chem. 1992, 31, F92. (23) Avrami, M. J. Chem. Phys. 1939, 7, 1103. (24) Elemes, Y.; Silverman, S. K.; Sheu, C.; Kao, M.; Foote, C. S.; Alvarez, M. M.; Whetten, R. L. Angew. Chem., Int. Ed. Engl. 1992, 31, 351. (25) Atlas of Spectral Data and Physical Constants for Organic Compounds; Grasselli, J. G., Ritchey, W. M., Eds.; CRC Press Inc.: Ohio, 1975; Vol. I. (26) Scanlon, J. C.; Brown, J. M.; Ebert, L. B. J. Phys. Chem. 1994, 98, 3921. (27) Spain, I. L. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Decker Inc.: New York, 1981; Vol. 16. (28) Soderholm, L.; Wurz, P.; Lykke, K. R.; Parker, D. H. J. Phys. Chem. 1992, 96, 7153.
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