1 5 0 0

o

1 0 0 0

500 N

Z

F I G . 3 .

, , L , i i i , , , , i , , J i i , I i i I I I i I I o

1 0 0 2 0 0 3 0 0

N o r m a l i z e d P o w e r ( m J )

Plot of area of the v3 band of CC14 vs. laser energy. The area was normalized by dividing by the gas pressure, and the laser energy was corrected for absorption by the CC14. The laser energy was varied between 55 and 280 mJ/pulse. The data represent the signal obtained from a single laser pulse.

cules. This dependence was verified for the measurements of the ~'1 band intensity, to eliminate the possibility of multiphoton effects (Figs. 2 and 3). When these correc- tions were made, the normalized intensity measured for the H2 Q-branch was 8.2 x 10 -5 counts mJ -1 Torr -l, while for the ,1 band of CC14, the average value was 3.4 ± 1.6 counts mj - I Torr- l . Since the instrumental pa- rameters for the data collection were the same, the ratio of these two normalized intensities, 4.1 _+ 1.9 x 104, should be the ratio of the Raman cross sections. The Raman cross sections of both hydrogen and nitrogen have been measured with a KrF laser by Bischel and Black. 7 With these data, the Raman cross section for the Vl band o f CC14 was calculated to be 2.0 _+ 0.9 x 10 -24 cm 2 sr -1.

In order to calculate the resonance enhancement, this value must be normalized with the value for nitrogen measured with a KrF laser since Bischel and Black found a small resonance enhancement for hydrogen relative to nitrogen using 248-nm excitation. This normalized cross section can then be compared with the literature values for the ratio of the normalized Raman cross sections mea- sured with excitation frequencies far from the Cfl 4 UV absorption tabulated by Schrrtter and K16ckner. s On the basis of these data, the resonance enhancement of CC14 at 248 nm is 3.3 ± 1.6 x 104.

CONCLUSION

The resonance enhancement measured herein is ap- proximately three orders of magnitude greater than that determined by Buyken et al. 4 at 253.7 nm, yet it is more in keeping with typical resonance enhancements. This increase in the enhancement is likely due to the proximity of the 248-nm excitation to the absorption maximum of CC14. Of further interest in this study is the lack of a large resonance enhancement of the 760/790-cm-' band. Giv- en the trends evident in the preresonance investigations (Refs. 3 and 4), this result is somewhat surprising. It is possible that the higher backgrounds observed in the cur-

rent work in this region of the spectrum obscure this band, precluding an accurate assessment of the band intensity. The 459-cm -1 band is quite characteristic of carbon tet- rachloride and can itself be used as a quantitative measure of its concentration.

ACKNOWLEDGMENT

This work was performed under the auspices of the U.S. Department of Energy.

1. See, for example, C. L. Schoen, Laser Focus 30, 113 (1994). 2. J. R. Lacher, L. E. Hummel, E. F. Bohmfalk and J. D. Park, J. Amer.

Chem. Soc. 72, 5486 (1950). 3. K. Kaya, N. Mikami, and Y. Udagawa, Chem. Phys. Lett. 13, 221

(1972). 4. H. Buyken, K. Klauss, and H. Moser, Ber. der Bunsenges. Phys. 71,

578 (1967); see also W. Hoffman and H. Moser, Ber. der Bunsenges. Phys. 68, 129 (1964).

5. G. Herzberg, Molecular Spectra and Molecular Structure, II. Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, Prince- ton, 1959).

6. A. A. Gordus and R. B. Bernstein, J. Chem. Phys. 22, 790 (1954). 7. W. K. Bischel and G. Black, "Wavelength Dependence of Raman

Scattering Cross Sections from 200--600 nm", in Excimer Lasers- 1983, AlP Conference Proceedings 100, C. K. Rhodes, H. Egger and H. Pummer, Eds. (American Institute of Physics, New York, 1983), pp. 181-187.

8. H. W. SchrStter and H. W. Klrckner, "Raman Scattering Cross Sections in Gases and Liquids", in Raman Spectroscopy of Gases and Liquids, A. Weber, Ed. (Springer-Verlag, New York, 1979), pp. 123-166.

Characterization of Fullerene Materials and their Oxidative Stability Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy

A M Y M. H U F F M A N and J A N E A. G A N S K E * Natural Science Division, Pepperdine University, Mal- ibu, California 90263

Index Headings: Diffuse reflectance; Fullerene; Soot; Oxidation; Infra- red spectroscopy.

INT R ODUC T ION

Since the generation and IR spectral detection of C60 in laboratory-produced carbon soot a few years ago,t the constituents of the raw fullerene soot have been intensely investigated. The soot, which is prepared by vaporization of graphite with resistive heating under an inert gas$ typ- ically contains < 20% fullerene by mass. With the recent interest in the reactivity of both the raw soot and purified fullerene products, new techniques are needed which can quickly identify changes in composition of these mate- dais. Diffuse reflectance infrared Fourier transform spec-

Received 1 August 1994; accepted 29 December 1994. * Author to whom correspondence should be sent.

834 Volume 49, Number 4, 1995 0003-7028/95/4904-053452.00/0 APPLIED SPECTROSCOPY © 1 9 9 5 S o c i e t y f o r A p p l i e d S p e c t r o s c o p y

O3

t/3

ed

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P. ¢0

ed

troscopy (DRIFTS) is an ideal candidate for the analysis of both raw soot and the pure fullerenes because of its sensitivity, high resolution, sampling versatility, and ap- plicability over a wide range of temperatures and pres- sures. We report here DRIFT spectra of raw fullerene soot, showing clearly the presence of C60 and C70, and a DRIFTS study of the in situ air oxidation of the fullerene soot.

EXPERIMENTAL

Two batches of raw fullerene soot (MER corporation, 11.7% fullerene content by mass; and Strem, 10.1% ful- lerene content) were used in the analysis and gave nearly identical spectra. Buckminsterfullerene (C60, 99.99% pu- rity) and C7o (99+%) were graciously supplied by Pro- fessor D. Huffman. All materials were used as received. Fullerene samples were prepared for DRIFT spectra as mixtures in KBr, and fullerene soot was used neat as supplied. The mixtures were typically prepared by grind- ing the powdered fullerene in a Wig-L-Bug@ grinder for 5 min and then mixing with high-purity KBr (Aldrich, random cuttings) which had been similarly ground. The samples were loaded into either a stainless steel micro- sampling cup (0.01 mL volume) or a 2-mm-deep, 18- mm-wide cup by lightly pressing down the powder with a fiat object. Soot spectra were obtained at 4-cm -~ res- olution on a Nicolet 740 FT-IR equipped with a Spectra Tech "Collector" diffuse reflectance accessory without use of the "Blocker". Mid-IR spectra were collected with a Ge-coated KBr beamsplitter and a wide-band MCT de- tector. Sample spectra were referenced to either an alu- minum-coated mirror background (soot) or powdered al- kali halide background (C6o and C70 comparison spectra). DRIFT spectra were expressed in absorbance units.

For the soot oxidation studies, temperature control was achieved with the Spectra Tech controlled environmental chamber that fits over the sample post and is equipped with either KBr or ZnSe windows. The sample was heated by a heater element in contact with the base of the sample cup, and the temperature was monitored by an internal thermocouple located directly beneath the sample cup. The temperature read by this thermocouple does not ac- curately reflect the sample temperature, so that the sample

~J 0 Z

~C Q

~ soot (c) 7 1

X

C6o • i ,oo a~5o a~oo z~so z~oo lose 1~oo rise ~oo ~5o

NAVENUMBER

Fro. 1. DRIFT spectrum of neat fullerene soot.

C7o

z0oo 1~o 14ao l~zo §60 ~oo ~ o NAVENWMBER

~G. 2. ORIFTspectra(x-scalee×panded) of(a) C7o,(b)C6o, and(c) ~]le~nesoot(y-scale x l0) .

cup temperature was calibrated relative to the tempera- ture read by the internal therrnocouple with an external Type K thermocouple. The chamber was cooled by cir- culating cooled water through the stainless steel base. For the soot oxidation studies, neat soot was exposed to air (ultra-high purity, Alphagaz) in a flowing (10 mL/min) or static (1 atm) mode. The background was taken under the same conditions with the use of an aluminum-coated mirror placed inside the chamber•

RESULTS AND DISCUSSION

Figure 1 shows the DRIFT spectrum of neat fullerene soot from 3500 to 450 cm -~. The soot spectrum contains sharp features characteristic of the molecular solids C6o and C70 superimposed on broad absorptions indicative of the graphitic phase of the soot. The fullerene content of the soot is evidenced by bands in the region from 2000 to 450 cm -L, which are labeled in Fig. 2 and compared to DRIFT spectra of pure C6o and C7o. The four charac- teristic vibrations ~ of icosahedral C6o are clearly resolved in the soot spectrum at 526, 577, 1183, and 1429 cm -~ as well as C70 bands at 641,674, 796, and 1135 cm -~. Several smaller features noted in the soot spectrum, and in the pure fullerene spectra, are due to nonfundamental vibrations of C6o and C7o, and have been observed pre- viously in solid-state and gas-phase studies. 3-6 The pres- ence of small amounts of adsorbed CO2 on the fullerene

APPLIED SPECTROSCOPY 535

(..3 Z <Z

aZ D

m 300°C ~x~kk~L ~: ~ 300°C

e6oo t '~o t~eo t~eo ~so ~oo ,.i.o,o WAVENUMBER

Fro. 3. DRIFT spectra of fullcrene soot (x-scale expanded) exposed to flowing air at the temperatures indicated.

soot, evident at 2335 cm-l , has been reported in other studies.3, 7

The DRIFTS technique is able to detect other IR-active chromophores in the soot as well. Evidence for the am- bient air oxidation of fullerene soot is provided by broad bands centered near 1790 cm -~ (carbonyl moiety) and 1290 cm-t (carbon-oxygen single bond), s The aromatic- ity of the soot matrix is suggested by broad absorptions at 1600 cm -~ (ring stretching) and 760 cm -t (ring bend- ing). 9 The broad nature of these absorptions is indicative of overlapping bands from multiple species and has been noted in infrared spectra of ordinary combustion soot and coals by other workers. 8-~° Over a period of a few weeks, raw soot which had not been stored under vacuum showed little evidence of C70, and the four C60 funda- mental absorptions had decreased considerably in inten- sity. While the pure fullerenes and C6o/C70 mixtures are known to be susceptible to oxidation at elevated tem- peratures/ t,~2 the present report indicates that the two fullerenes are labile to air down to room temperature when left in the soot matrix.

To further examine the oxidative reactivity of raw ful- lerene soot, we used DRIFTS to monitor the temporal behavior of all initial absorptions upon heating in air. These studies were carried out in two different ways: (1) by flowing high-purity air through the cell at a flow rate of 10 mL/min, or (2) by static exposure of the soot to 1 atm of dry, CO2-removed air. Static exposures were pri-

marily used to detect the presence of any gases (CO2, CO, and HzO ) released during the oxidation. Spectra taken under flowing conditions after exposure at 175,300, 425, 450, and 475°C for 1 h are shown in comparison with the room-temperature spectrum in Fig. 3. No significant changes were observed in the spectrum at temperatures below 175°C. Upon heating to 175°C, a very weak feature appears near 1790 cm t, suggestive of an aromatic car- bonyl in an ester or anhydride linkage, 13 which increases in intensity in the 300°C spectrum. The 300°C spectrum also shows enhanced intensities of the 1600- and 760- cm-~ ring stretching and bending absorptions. Concom- itant with the increase in these absorption bands is the reduction in intensity of C7o (peaks at 638, 671,794, and 1134 cm -l) and, to a somewhat lesser degree, the C60 peaks. These results suggest that C60 is more resistant to oxidation than C70, in agreement with gravimetric stud- ies? 4,15 The oxidation of either fullerene may proceed via reaction of the planar s p 2 carbon rings with oxygen, cre- ating pseudo-planar carbonyl-subst i tuted aromatics, which would be supported by the spectra in Fig. 3.

Heating the raw soot also results in mass loss, which becomes apparent at 300°C. Gasification of either fuller- ene or the planar benzenoid building blocks of the soot should produce measurable amounts of carbon dioxide. We note strong absorptions at 2349 and 667 cm -~ in the 300°C static exposure, due to gaseous carbon dioxide. No CO or H20 was detected in the 300°C static exposures; these results are consistent with mass spectraP s and C, H, and O microanalysis studies? 2

At 425°C, the 1790-cm -~ carbonyl absorption has in- tensified, with shoulders developing near 1848 and 1750 cm -l. Vibrational bands near 1850 cm -~ in coals and chars are generally assigned to cyclic anhydrides) ° This band structure was also observed by Vassallo et al. I I in the oxidation of pure C6o under a 25% O2/Ar atmosphere at a lower temperature (350°C). Because of significant mass loss at temperatures >4250C during the exposure, heating times were decreased from 1 to 0.5 h in order to scan the sample without sample repacking or realignment. Further heating to 4500C causes an increase in all oxi- dation product absorptions. No evidence Of CTo, and little C6o, is noted at 475°C, and by 5000C (not shown) all absorption features have decreased significantly because of rapid, indiscriminate gasification of the soot constit- uents. All high-temperature results are consistent with X-ray diffraction, 16 thermal gravimetric analysis, 14,17 and mass spectrometric 15 findings for C6o and fullerene soot.

CONCLUSION

DRIFTS has been shown to be a highly effective single technique for providing information on the composition, purity, and oxidative reactivity offullerene materials. The sensitivity and facile sample preparation inherent to this method make it an excellent choice for the detection of fullerene content in unprocessed fullerene soot, as well as the extent of decomposition of fullerene materials. Most significantly, DRIFTS is uniquely suited to monitoring the fullerene materials' air oxidation over a wide tem- perature range because it enables the detection of both adsorbed and free gases, as well as oxidation products in the bulk of the solid.

536 Volume 49, Number 4, 1995

ACKNOWLEDGMENTS

We are grateful to Professor Don Huffman for providing the C6o, C70, and one of the fullerene soot samples. Partial support for this work from awards made by Research Corporation and Pepperdine University's Research Council is also gratefully acknowledged.

1. W. Kriitschmer, K. Fostiropoulos, and D. R. Huff`man, Chem. Phys. Lett. 170, 167 (1990).

2. W. Kriitschmer, L. D. Lamb, K. Fostiropoulos, and D. R. Huff`man, Nature 347, 354 (1990).

3. B. Chase, N. Herron, and E. Holler, J. Phys. Chem. 96, 4262 (1992). 4. D. S. Bethune, G. Meijer, W. C. Tang, H. J. Rosen, W. G. Golden,

H. Seki, C. A. Brown, and M. S. de Vries, Chem. Phys. Lett. 179, 181 (1991).

5. L. Nemes, R. S. Ram, P. F. Bernath, F. A. Tinker, M. C. Zumwalt, L. D. Lamb, and D. R. Huffman, Chem. Phys. Lett. 218, 295 (1994).

6. M. C. Martin, X. Du, and L. Mihaly, Phys. Rev. B 50, 173 (1994). 7. I. M. K. Ismail and S. L. Rodgers, Carbon 30, 229 (1992). 8. L. B. Ebert, W. H. Davis, D. R. Mills, and J. D. Dennerlein, in

Chemistry of Engine Combustion Deposits, L. B. Ebert, Ed. (Plenum Press, New York, 1985), p. 90.

9. E. L. Fuller, Jr., and N. R. Smyrl, Fuel 64, 1143 (1985). 10. N. R. Smyrl and E. L. Fuller, Jr., "Chemistry and Structure of

Coals: Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy of Air Oxidation in Coal and Coal Products: Ana- lytical Characterization Techniques", in ACS Symposium Series 205, E. L. Fuller, Jr. Ed. (American Chemical Society, Washington, D.C., 1982), p. 132.

11. A. M. Vassallo, L. S. K. Pang, P. A. Cole-Clarke, and M. A. Wilson, J. Am. Chem. Soc. 113, 7820 (1991).

12. J. C. Scanlon, J. M. Brown, and L. B. Ebert, J. Phys. Chem. 98, 3921 (1994).

13. P. Painter, M. Starsinic, and M. Coleman, in Fourier Transform Infrared Spectroscopy, Vol. 4, J. R. Ferraro and L. J. Basile, Eds. (Academic Press, New York, 1985), Chap. 5, p. 178.

14. D. W. McKee, Carbon 29, 1057 (1991). 15. H. Werner, D. Herein, J. BI6cker, B. Henschke, U. Tegtmeyer, Th.

Schedel-Niedrig, M. Keil, A. M. Bradshaw, and R. Schlogl, Chem. Phys. Lett. 194, 62 (1992).

16. J. C. Scanlon and L. B. Ebert, J. Phys. Chem. 97, 7138 (1993). 17. J. D. Saxby, S. P. Chatfield, A. J. Palisano, A. M. Vassallo, M. A.

Wilson, and L. S. K. Pang, J. Phys. Chem. 96, 17 (1992).

Characterization of Crystals in Plant Cells Using FT-IR Microspectroscopy

E D U A R D O L. V A R E T r I * and C A R O L A R. V O L P O N I t QUINOR (Qufmica Inorg~nica) and LANAIS EFO,$ Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, 1900 La Plata, Argentina (E.L. V.); and Museo de La Plata, Universidad Nacional de La Plata, C. Correo 312, 1900 La Plata, Argentina (C.R.V.)

Index Headings: Infrared microspectroscopy; Crystals; Plants.

Received 20 July 1994; accepted 4 November 1994. * Researcher of CONICET, Argentina. Author to whom correspondence

should be sent. t Researcher of CIC-PBA, Argentina.

Laboratorio Nacional de Investigaci6n y Servicios en Espectrofoto- metria Optica, CONICET-UNLP.

I N T R O D U C T I O N

Crystals very often appear in plant cells, being widely distributed. General ly such crystals are compr ised o f hy- drated calcium oxalate, which develops in order to protect the cells f rom the toxicity o f the oxalic acid produced in plant metabol i sm, or act as a calcium reservoir. Calcium carbonate crystals also appear frequently in vegetal tissue, p robably having a regulatory role regarding calcium con- centration. Other substances, such as silica, calcium phos- phate, organic substances, etc., could also appear as crys- talline mater ial in plants.l

Such minute crystals have been characterized in the past by means of chemical, 2 X-ray diffraction, 3 or spec- t roscopic methods . Infrared spectroscopy was used for the first t ime for that purpose by Scurfield et al., 4 who combined infrared measuremen t s on KBr micropellets made with previously isolated crystals with X-ray dif- fraction and scanning electron microscopy to characterize crystals in woody tissues.

T h e d e v e l o p m e n t o f i n f r a r ed m i c r o s p e c t r o s c o p y 5 opened the possibility o f obtaining the spectra o f such minute crystals after a m i n i m u m of sample handling. We present here the results obta ined with this technique on several plant samples and discuss the associated problems of sample preparat ion.

E X P E R I M E N T A L

The spectral data were obtained, generally in the 4000 - 600 cm-~ range and with 4-cm-1 resolution, by means o f a Bruker IFS 66 spec t rophotomete r p rovided with a Bruker infrared microscope. This latter ins t rument was used in the t ransmiss ion mode, with a Cassegranian all- reflecting object ive o f 15 x magnification and binoculars for visual observat ion o f 20 × magnification. The sample region to be studied was optically isolated with the use o f round diaphragms having different apertures. The high- sensitivity, liquid air-cooled M C T detector had a nominal lower spectral l imit o f 600 cm -~.

The plant specimens were studied as thin layers cut with a mic ro tome or cut manual ly with a razor blade. The presence and form of the crystals were de te rmined under a s tandard stereoscopic microscope on samples main ta ined in a humid state. The t rea tment o f samples pr ior to spectral measuremen t s depended to a large extent on the morpho logy of the crystals. In some cases it was sufficient to strongly press the cut tissue between two glass microscope slides, transferring it afterwards to an infrared t ransparent window (AgC1 or ZnS) and leaving it to dry before spectral measurements . This technique could be used when the crystals (generally o f calcium oxalate mon- ohydrate) appeared as long, very thin platelets (raphids) grouped in bundles. However , such sample handling re- sulted in relatively noisy spectra with a practical lower l imit o f ~ 7 0 0 cm 1, as in spectra f and g of Fig. I.

In mos t o f the cases, the crystalline mater ial was too thick (monocrysta ls or druses), giving spectra of poor quality with the above -men t ioned technique. In fact, the o p t i m u m thickness for infrared study o f a pure substance is a round 10 urn. 5 In such cases the best solution was to press the crystal sample, previously isolated with hysto- logical needles, between the faces o f d i amond anvils in a cell devised for high-pressure measurements (Diacell

Volume 49, Number 4, 1 995 0003-7028/95/4904.053752.00/0 APPLIED SPECTROSCOPY 537 © 1995 Society for Applied Spectroscopy