Mass Spectrometric, Thermal, and Separation Studies of Fullerenes

Chapter 8

Mass Spectrometric, Thermal, and Separation Studies of Fullerenes 1

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Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: January 6, 1992 | doi: 10.1021/bk-1992-0481.ch008

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Donald M. Cox , Rexford D. Sherwood, Paul Tindall , Kathleen M. Creegan , William Anderson , and David J. Martella 2

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Corporate Research Laboratories, Exxon Research and Engineering Co., Annandale, NJ 08801 Department of Chemistry, Lehigh University, Bethlehem, PA 18018 Exxon Chemicals Co., Linden, NJ 07036 2

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In this chapter, we describe resultsfromplasma desorption mass spectrometric studies of fullerenes, studies of the thermal and oxidative properties of the fullerenes, and the use of sublimation both for extraction of fullerenes from raw soot and for separation into purified fractions.

Following the initial disclosure of a technique to produce macroscopic quantities of CgQ (2), the production, extraction, and separation of fullerenes is now being routinely performed by many research groups around the world, as indicated by the chapters in this book. There is a growing need for nondestructive mass spectrometric techniques with which parent fullerene molecular species can be identified, information about the thermal and oxidative properties of the fullerenes, and alternatives to liquid chromatographic separation (2—4). Considering these needs, we will discuss our preliminary results from plasma desorption mass spectrometric studies of evaporated deposits of purified and mixed fullerene samples, thermogravimetric studies of fullerenes and carbon soot with both N and Ô sweep gases, and application of sublimation-condensation not only to extract fullerenes from raw soot, but also as a means to separate fullerenes into highly purified components. 2

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Plasma Desorption Mass Spectrometry Figure 1 shows a schematic of plasma desorption mass spectrometry (PDMS) apparatus. Briefly, PDMS works as follows: A sample is placed on a thin film (—2 μτη thick) of aluminized Mylar. The fissioning of a C f atom produces two high-energy, nearly equal mass products. One fission fragment produces a start pulse for data acquisition. The second fragment strikes the film, deposits 2 5 2

0097-6156/92/0481-0117S06.00/0 © 1992 American Chemical Society

In Fullerenes; Hammond, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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FULLERENES: SYNTHESIS, PROPERTIES, AND CHEMISTRY 00

HV


Start Detector

End Detector

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650 700 750 800 850(M/Z)

sample

Figure 1. Schematic of plasma desorption mass spectrometer and the positive ion time-of-flight mass spectrum of purified prepared by evaporative depo sition of Ο from a dilute solution of toluene onto a thin nitrocellulose coating on the aluminized Mylar fUm. The accelerating voltage is +15 keV. ω

its kinetic energy, and produces ions. The exact details of the ion production are uncertain and the subject of much discussion (5). The sample ions are accelerated by an electric field set up between the substrate held at high voltage and a grounded screen about 2 mm away. The ions then travel in a field-free region for about 14 cm before being detected by a channel-plate multiplier. Simply changing the sign of the accelerating voltage allows either positive or negative ions to be detected. This technique has advantages and disadvantages. One advantage is that fullerenes can be detected with this technique. Also shown in Figure 1 is a typ ical mass spectrum obtained from a "purified" sample of C ^ . In this instance a low concentration of the fullerene molecule was evaporatively deposited onto the Mylar film from a weak solution of in toluene. is by far the dom inant peak in the mass spectrum, a result indicating that the parent molecules are being detected. Figures 2a and 2b show spectra obtained when thicker films of "puri fied" and raw fullerene extract (a fullerene mixture containing mostly C^Q and C Q) are used, respectively. In both Figures 2a and 2b there is evidence for parent molecules (the strongest peaks), but also for "all" larger size fullerenes; that is, although not shown here, even atom carbon clusters containing 60 to more than 250 carbons are mass resolved. Our interpretation of these effects is that plasma desorption can be used to selectively detect parent fullerene molecules only when the fullerenes are highly dispersed on the film, that is, when the probability of the fission fragment interacting with more than one molecule at a time is very low. For concentrated coverages, on the other hand, a significant fraction of the energy deposited by the fission fragment presumably is deposited into this "carbon film", and in analogy with laser vaporization of graphite (6), simply "synthesizes" the large family of fullerenes. 7



8. COX ET AL. Mass Spectrometric, Thermal, and Separation Studies 119

Figure 2. Positive ion time-of-flight mass spectra for more concentrated samples of (a) purified C^Q and (b) fullerene extract These samples were prepared by evaporative deposition onto aluminized Mylar fUms from concentrated solutions offullerene in toluene. The accelerating voltage is +14 keV. The PDMS of the fullerene extract shows higher intensities for C ^ , C , C , C g, and Cg,, exactly those fullerenes that can be extracted by chromatography (2—4, 7). Tnis finding suggests that such intensity oscillations are indicative of the presence of parent molecular species in the sample, because such oscillations are not present in the PDMS of a thick sample of purified (or C ) . The most intense peak in this instance is (or C ), while the intensities of the other peaks are nearly equal except for a slow decrease with increasing cluster size. 7 0

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FULLERENES: SYNTHESIS, PROPERTIES, AND CHEMISTRY

Separation of Fullerenes The disclosure (1) that fullerenes can be produced in macroscopic amounts by operating a carbon arc in an inert gas atmosphere has led to a demand for purified materials. The initial problems involved extraction of fullerenes from the raw soot and separation of the mixture of fullerenes into pure cuts of C , C ^ , etc. A typical recipe for these steps involves combining the raw soot and a solvent such as benzene or toluene in a flask. The fullerenes are somewhat soluble in aromatic solvents, and thus after filtering out the insoluble material, a deep burgundy solution (the extract) containing only soluble material is obtained. This solution is found to contain a mixture consisting primarily of fullerenes, but may also contain other fullerenelike materials such as the fullerene mono- and dioxides and possibly some polyaromatic hydrocarbon species. Typically, under our "synthesis" conditions the fullerenes make up 8-15% of the carbon soot. In order to separate the fullerenes from each other, liquid chromatography has been employed (2-4, 7). In this manner, purified samples of C ^ , C , Cg^, etc., have been obtained. In what follows we will describe appropriate sublimation conditions that not only allow fullerenes to be extracted from raw soot, and thus obviate the solvent-extraction step, but also yield highly purified deposits of (and C ) , and thus obviate the chromatographic step. The method exploits the properties that and C are thermally stable (above 800 °C in our tests) in vacuum or under inert gas, sublime without decomposing, and have different heats of desorption (8). The general behavior is that the larger fullerenes condense onto warmer surfaces than do the smaller ones. Thus, fullerene extraction and separation from the raw soot by controlling the heating and condensation conditions can be a viable alternative to solvent extraction and liquid chromatographic separation.


7 0

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Thermal Properties of Fullerenes. The thermal and oxidative properties of fullerene-containing soots as well as separated fullerenes were examined by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA) using nitrogen, helium, or air. Figure 3 displays a typical T G A plot for a sample of toluene-Soxhlet-extracted fullerenes. With N~, no appreciable weight loss is observed for temperatures less than -700 °C. Most of the weight loss occurs between 750 and 850 °C. Little weight loss occurs above 850 °C, with approximately 13% of the initial fullerene sample still remaining even at 1100 °C. This residual material is insoluble in toluene and presumably represents conversion of fullerenes or fullerene oxides into some carbonaceous material, possibly larger fullerenes with more graphitelike properties, or amorphous carbon. When synthetic air (20% 0 , 80% N ) is used as the carrier gas, weight loss occurs at substantially lower temperatures; that is, almost all weight loss now occurs between 550 and 650 °C, and nearly all the material is oxidized, as evidenced by 98% weight loss. Similar studies of raw soot, that is, the carbon soot prior to fullerene extraction, show only about 10% weight loss with N as the carrier gas, but with air the entire sample can be oxidized. The 10% loss 2

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8. COX ET AL. Mass Spectrometric, Thermal, and Separation Studies 111 (b)


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Figure 3. The TGA plots (% weight loss versus temperature) for the fullerene extract for N (solid line) and synthetic air (dashed line, 20% 0 , 80% N ) are compared in panel a. The heating rate was 20 °C/min. The upper panel (b) plots the derivatives of the curves in panel a. 2

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with N not only reflects the fraction of sublimable material (fullerenes), but is consistent with the weight fraction of fullerenes obtained by toluene extraction. These results are interpreted as follows: With N as the carrier gas, weight loss occurs by sublimation. With air as the carrier gas, weight loss occurs via oxidation (burning) before any significant weight is lost by sublima tion. These results are confirmed by D T A and DSC on the extracted ful lerenes. A typical DTA scan is shown in Figure 4. With N , no evidence of an endo- or exothermic reaction is observed. With air, however, the reaction is exothermic. The shape of the curve suggests that the sample contains two major components, presumably Ο and C™. In addition, we suspect that the shoulder toward the lower temperature reflects the oxidation of C , and the 2

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FULLERENES: SYNTHESIS, PROPERTIES, AND CHEMISTRY

DTA Of C / C 6 0

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Figure 4. DTA of the fullerene extract in air. The heat flow (mttlicalories per second) is plotted such that exothermic processes give negative values. large peak corresponds to oxidation of C ^ , because the ratio of the signal in the shoulder to that of the peak is consistent with the to ratio of about 0.2 in this sample. Furthermore, a similar effect is observed in the derivative of the T G A with oxygen (see Figure 3b). If this assignment is correct, then C oxidizes more easily than C ^ . Early mass spectrometric characterizations of fullerene extracts in many instances were carried out by thermal desorption from heated probes where probe temperatures were held between 300 and 500 °C (1, 2—4, 9), temperatures significantly below that for which significant weight loss is observed. In these instances the mass spectrometric sensitivity is sufficiently high that even at the lower vapor pressures sufficient material is sublimed for detection. 7 0

Separation by Sublimation. In order to test the feasibility of separation using sublimation, an initial experiment is carried out by placing a known amount of fullerene extract (toluene-extracted as usual) into the bottom of a %-inch-diameter by 12-inch-long quartz tube. The tube is evacuated and sealed off. It is then placed in an oven such that a 4-inch extension out of the end of the oven remains near room temperature. The tube containing the soot is then heated to 700 °C for 60 h. A thick deposit is formed on the inner surface of the tube over about a 2-inch-long region located where the tube exits the oven, that is, the region where the temperature drops from near 700 °C to somewhere near room temperature. Characterization of this deposit reveals that the material nearest the cool end is highly enriched in C ^ , containing greater than 95% C ^ . The percent C ^ in the extract prior to sublimation is 83%. Further



8. COX ET AL. Mass Spectrometric, Thermal, and Separation Studies

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evidence of selective sublimation is obtained by examination of the extract that did not sublime. This material is found to be significantly depleted in C ^ , containing only 70% C U . To better define the thermal conditions and to demonstrate that fullerenes can be extracted and separated directly from soot, a three-zone oven is used. Figure 5 schematically shows the details of this setup. Each stage of the oven is nominally 6 inches long and has a thermocouple located near the center of each stage. Raw carbon soot (200 mg) is placed in the bottom of a 24-inch-long by 0.5-inch-diameter quartz tube, which is then positioned in the oven such that the lower 5 inches containing the soot is in the 800-°C zone. The three zones of the oven are then heated to 800, 600, and 400 °C. The top 8 inches of the tube outside the oven remains near room temperature. Our observations are as follows: No appreciable material is deposited on the room-temperature surfaces outside the oven or in the 600-°C region. A coating builds up along the tube surface in the 400-°C region. The morphology of the coating varies along the tube length and is found to reflect both compositional (Cgo/Cyo ratio changes) and concentration (film thickness) changes. The differing compositions and concentrations of sublimed material appear to be controlled mostly by the temperature profile along the length of the quartz tube. The deposited material is then removed from different areas along the length of the tube, weighed, and characterized by U V absorption. From these measurements the percent and the yield of sublimable material is determined (see Table I). The two spectra shown at the bottom of Figure 5 dramatically illustrate the differing concentrations of and C in the cooler and hotter regions along the tube. ? 0

Table I. Quantity (%) of and C in Sublimed Films as a Function of Position along Length of Quartz Tube ? 0

Position

C ^70

C

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Mass Spectrometric, Thermal, and Separation Studies of Fullerenes

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