4522
J. Phys. Chem. 1992, 96, 4522-4526
Reaction of Oxygen with Graphite: X-ray Absorption Spectroscopy of Carbonaceous Materials F. Atamny, J. Blocker, B. Henschke, R. Schlogl,* Institut fur Anorganische Chemie der Universitat, Niederurseler Hang, 0-6000 Frankfurt, Germany
Th. Schedel-Niedrig, M. Keil, and A. M. Bradshaw Fritz Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 33, Germany (Received: June 24, 1991)
The reaction of molecular oxygen at 1000 K with the (001) face of highly oriented pyrolytic graphite (HOPG)has been studied by X-ray absorption spectroscopy (XAS). At 10% weight loss the reaction produces additional defects but leaves the overall electronic structure of graphite unchanged. Reference experiments on open surfaces of various carbon black materials showed that, despite morphological similarities,an oxidized graphite surface has little in common with these disordered materials.
Introduction The reaction between graphite and oxygen has been extensively studied both for theoretical reasons (anisotropic substrate, ease of kinetic simulation due to the lack of a solid reaction product) and for practical purposes (nuclear reactor material, reentry shields for space crafts). In the many studies performed, the course of the reaction has been followed with various microscopic techniques. Atomic oxygen was found to react with graphite nonspecifically giving rise to three-dimensionally isotropic etch pit@ and a surface covered with oxygen functional groups. Molecular oxygen is much less reactive due to its low sticking coefficient3 and acts as a selective etching agent for defects. These are enlarged during the reaction and show a pronounced anisotropy parallel to the (001) ~ u r f a c e . ~More recent studies using transmission electron microscopy combined with a gold decoration techniques,6indicated that both atomic and molecular oxygen react initially at defect sites and that a Langmuir-Hinshelwood mechanism modified by a surface diffusion term provides an adequate description of the oxidation process. Scanning tunneling microscopy has since confirmed the anisotropic mode of attack giving rise to shallow pits at low partial pressures of All these local observations do not, however, provide a representative description of the process, Le., it is not known how "typical" they are for large portions of the graphite surface. This kind of information, together with a description of the electronic structure of the reacting surface, can be supplied by spectroscopic techniques which sample up to 50 mm2 surface compared to 50 nm2 studied with high-resolution microscopy. One technique of choice, X-ray photoelectron spectroscopy (XPS), is not suitable for this purposeg due to its lack of chemical sensitivity for differently coordinated carbon atoms bound to other carbon atoms. More suitable techniques are high-energy electron loss spectroscopy,loinverse photoelectron spectroscopy," and carbon K-edge X-ray absorption spectroscopy (XAS),I2.l3all of which appear (1) Marsh, H.; OHair, E.; Reed,R.;Wynne-Jones, W. F. R. Nature 1963, 198, 1195. (2) Marsh, H.; O'Hair, T. E.; Wynne-Jones, W. F. R.Trans. Faraday Soc.
1965, 61,274. (3) Barber, M.; Evans, E. L.; Thomas, J. M. Chem. Phys. Lett. 1973, 18, 423. (4) Thomas, J. M. In Chemisrry and Physics of Carbon;Walker, P., Ed.; Marcel Dekker: New York, 1965; Vol. I, p 121. ( 5 ) Yang, R.T.; Wong, Chor J . Chem. Phys. 1981, 75, 4471. (6) Wong, Chor; Yang, R. T.; Halpern, B. L. J . Chem. Phys. 1983, 78, 3325. (7) Chang, H.; Bard, A. J . J. Am. Chem. SOC.1990, 112, 4589. (8) Kim, D. P.; Labes, M. M.; Siperko, L. M. Mater. Res. Bull., in press. (9) Schlogl, R. Surf. Sci. 1987, 189, 861. (IO) Kincaid, B. M.; Meixner, A. E.; Platzman, P. M. Phys. Reo. Lett. 1978, 40, 1296. (11) Reihl, B.; Gimzewski, J. K.; Nicholls, J. M.; Tosatti, E. Phys. Reo. 1986, B33, 5770. (12) Denley, D.; Perfetti, P.; Williams, R.S.; Shirley, D. A,; Stohr, J. Phys. Rev. 1980, B21, 2267.
to be sensitive to the chemical environment of carbon atoms. The present paper describes a study in which the (001) surface of HOPG has been severely oxidized in situ and analyzed before and after treatment by XAS. Microscopic characterization after the measurements established the degree of oxidation. Experimental Section A sample of HOPGi4of dimensions 12 X 12 X 1 mm3 was analyzed a t the Berlin synchrotron radiation facility BESSY in an ultrahigh-vacuum surface science chamber attached to the HE-PGM 2 monochromator. The latter was operated a t a resolution of 0.4 eV a t the carbon edge. The photon energy was calibrated using the graphite sample itself by taking the value of the lowest energy C 1s-a transition from EELS data (285.4 eV, R ) . Spectra in the carbon (270-370 eV) and oxygen regions (500-570 eV) were recorded in the total yield mode and normalized to the primary photon current measured by a freshly copper-coated metal grid in front of the sample. A sample manipulator allowed the c axis to be oriented at angles between 0 and 70' with respect to the incident beam. The sample was cleaved in air immediately before insertion into an attached preparation chamber where it was sputtered with Ar and reacted with molecular oxygen at mBar and loo0 K for 20 min. The progress of oxidation was monitored by a quadrupole mass spectrometer with the m / e 44 peak for COz. No gas-phase contaminants such as water which might have influenced the reaction mechanism were detected. The reference powder samples were suspended as aqueous slurry on a copper foil, dried in air at ca. 400 K, and then fixed onto the sample holder block. A control experiment with powdered graphite showed no influence of the preparation technique on the XAS data. Optical reflection microscopy was performed with polarized light near grazing incidence illumination. After the XAS experiments the samples were inspected in a JEOL JSM 40 instrument at 5-keV acceleration voltage without any metal evaporation. Powder X-ray diffraction was performed on a SIEMENS D 5000 instrument in reflection geometry using Cu Ka radiation and a background-free Si( 100) sample holder. Accurate d-spacing measurements were made by the admixture of silicon as an internal standard. Results and Discussion To obtain a substantial difference between the spectra for clean and oxidized HOPG, its surface was extensively treated with oxygen giving rise to an integral weight loss of ca. 10 wt %. At this reaction level the formerly smooth surface is rough, showing a richly terraced surface. The micrographs of Figure 1 give a ( I 3) Rosenberg, R. A.; Love, P. J.; Rehn, V. Phys. Rev. 1986,833, 4034. (14) Union Carbide, grade ZYH, lot no. 4197.
0022-3654/92/2096-4522%03.00/0 0 1992 American Chemical Society
Reaction of Oxygen with Graphite
The Journal of Physical Chemistry, Vol. 96, No. I I, 1992 4523
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Figure 2. Series of angle-resolved XAS data of cleaved HOPG.
Figure 1. Scanning electron micrograph of the oxidized HOPG surface revealing etch pits and on terraces highly anistropic etch marks.
visual impression of the morphology which can be described as a superposition of large circular etch pits and small residual terraces. The anisotropy of the oxidation process can be seen clearly in that the reaction is faster parallel to the surface (with, e.g., dangling bonds of broken aromatic sextets) than perpendicular to it. The light contrast in the SEM image is due to isolated very thin (10 nm) islands of graphite which are transparent to the electron beam. A similar topography for an oxidized surface of HOPG has recently been reported using the STM.15 The polarization dependence of the XAS spectra for the clean and oxidized surfaces are presented in Figure 2 and 3, respectively. The resolution in the present experiment is considerably higher than in the early study of Denley et a1.12 In a simple picture there are two kinds of resonances arising from transitions into flat portions of the various unoccupied u and ?r bands. These transitions are polarized perpendicular and parallel to the c axis, (1 5) Schliigl, R.; Loose, G.; Wesemann, M. Solid State Zonics 1990,43, 183.
respectively. The first, very sharp feature at 285.4 eV with maximum intensity at 8 = 70°, arises from an excitation into the antibonding ?r-band in the region of the M point. In inverse photoemission experiments it has been located 1.7 eV above EP1 A further sharp feature occurs at 291.8 eV and has the opposite Narization dependence (maximum intensity at 8 = Oo, or E I c). It is clearly of u symmetry and can also be observed in the secondary electron energy distribution 2.8 eV above the vacuum level.' This feature is known to be very sensitive to structural9 and electronic16 modifications of the graphite. The valley at ca. 315 eV in the spectra of Figures 2 and 3 separates the near-edge C 1s transitions from what might be regarded as the first EXAFS oscillation at ca. 330 eV. Denley et a1.I2 have shown that the intensity of this feature is dependent on the degree of crystallinity of the graphite sample. Because of the possibility of spurious structure due to carbon contamination of the monochromator optics, several spectra were also taken on SX-700-4, a relatively new monochromator. A spectrum for 6 = 60' is shown in Figure 4. A plausible assignment, at least for the first six features, is possible using the calculated band structures of Willis et al.," Tatar and Rabii,18 and Holzwarth et al.19 This is given in Table I together with the symmetry character apparent from the polarisation dependence. Of particular interest is the location of the lowest d band (16) Schlijgl, R.; Geiser, V.; Oelhafen, P.; Giintherodt, H.-J. Phys. Rev. 1987, B35,6414.
(17) Willis, R. F.; Fitton, B.; Painter, G. S. Phys. Rev. 1974, B9, 1926. (18) Tatar, R. C.; Rabii, S. Phys. Rev. 1982, B25,4126. (19) Holzwarth, N.A. W.; Louie, S. G.; Rabii, S. Phys. Rev. 1982, B26, 5382.
Atamny et al.
4524 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
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HOPG surface. TABLE I: Assignment of the Features in the Carbon K Near-Edge X-ray Absorption Spectrum of H O E
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energy, eV 285.4 289.1 291.8 293.0 296.0 297.6 301.6 304.2 308.2 316.5
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'Here is also a possible intensity contribution from the split antibonding 'II bands at r (I'2+,r2-). at I? (feature 2), which has been the cause of some controversy in the past.20*21This so-called interlayer band is observed at 3.5 eV above EFin the inverse photoemission experiment." The data in Table I agree fairly well with those reported by Rosenberg et al.13 Onset (285.4 eV) and width (31.4 eV) of the conduction band are identical; some peak positions are, however, more clearly defined in the high-resolution spectrum of Figure 4. Feature 2 is experimentally observed for the first time with the XAS technique. Qualitative comparison of the spectral shapes in Figures 2 and 3 reveals no significant differences. Oxidation thus does not affect (20) Pasternak, M.; Baldereschi, A.; Freeman, A. J.; Wimmer, E.; Weinert, M. Phys. Rev. Lett. 1983, 50, 761. (21) Holzwarth, N. A. W.; Louie, S. G.; Rabii, S. Phys. Rev. 1984,830, 2219.
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HOPG after various treatments. The spectra are all normalized in intensity to a point near 310 eV. For definition of the angle see insets in Figures 2 and 3. the electronic structure of the remaining surface. Even the structuresensitivefeature at 291.8 eV and the high-energy features are completely unchanged. The delocalized r-electron system of graphite is not affected by oxygen attack, and there is no significant formation of carbon-oxygen covalent bonds. This is due to the ease with which initially present C-O groups can be further oxidized to carbonate species leaving the surface as gaseous C02. The concentration of intermediate C-0 groups is thus low on a reacting surface. This result means that the surface of oxidizing graphite retains essentially its electronic structure of a semimetal. A possible reaction mechanism involving the destruction of the "aromatic" stabilization of the surface graphene layer into the electronic structure of turbostratic carbon described in ref 22 is now excluded on the basis of a surface-averaged experiment. Experimental evidence for this statement was up to now either rather indirect or obtained with bulk-sensitive techniques.22 Spectra taken in partial yield to enhance surface sensitivity at the oxygen K-edge confirmed that the number of oxygen functions had increased by only a factor less than 2 without any changes in the line shape of the resonance. A quantitative analysis of the polarization dependence of the lowest energy C 1s-r transition for clean and oxidized graphite is presented in Figure 5 . The scatter in the data is due to background subtraction problems. The finite intensity for E I c (e = Oo) is due to misorientation of crystallites in the HOPG sample, mainly introduced by the cleavage process. The increase (22) Spain, I. L. In Chemisrry and Physics of Carbon, Walker, P. L., Thrower, P. A., Eds.; Marcel Dekker: New York, 1981; Vol. 16, pp 119,256.
Reaction of Oxygen with Graphite
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4525
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Figure 6. Comparison of XAS data for cleaved well-ordered HOPG and
the same surface after Ar+ ion bombardment.
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Figure 7. XPS data (Mg Ka,binding energy reference Au 4fTI2at 84.0 eV, analyser at pass energy 50 eV, samples at 300 K in loose powder form
for natural graphite and the FLA 101 carbon black) comparing the C 1s emission for well ordered graphite and carbon black powders.
of this intensity for the oxidized surface indicates an overall loss of stacking order in the mosaic structure of the crystal. The reduced slope of the least squares fit pints to a reduced number of r states in the oxidized surface which may be taken as indication for an increased defect density producing extra u states which were also identified by photoemis~ion.~ To evaluate the significance of these rather small spectral differences between clean and oxidized H O E , it is useful to test the sensitivity of the XAS technique to structural defects in graphitic carbon. This was done by measuring spectra of Ar+ etched HOPG which are shown in Figure 6. Gentle sputtering with 5-keV Ar+ ions for 20 s destroys the crystalline structure of the graphite within the sampling depth of XAS. Consequently, the polarization dependence of the first 7 resonance is completely lost, as can be seen from Figures 5 and 6. That the intensity of this feature is still quite high indicates
Figure 8. Sections of the powder diffraction patterns (Cu Ka, Guinier transmission geometry) for well-ordered graphite (top) and the FLA 101 carbon black (center) samples. The bottom curves are sections from the high-energy parts of Figures 2 and 9 at an angle of 50°. The intensities were normalized to a point near 310 eV just outside the section displayed.
that a large fraction of the carbon atoms are still sp2 hybridized and not transformed into, e.g., diamond-like configurations. The band structure features of graphite are, however, widely lost leading to a structureless u-type resonance feature.22 More naturally open surfaces that serve as possible models for oxidized graphite are provided by carbon black, the turbostratic modification of graphite containing a large number of layer defects in the form of sp3 carbon atoms. C 1s photoemission data were taken for graphite and the FLA 10123 carbon black sample (Figure 7) and demonstrate that XPS is rather insensitive to the details of carbon-carbon bonding. The large structural difference between the two samples is illustrated in Figure 8 by relevant sections of the powder X-ray diffraction profiles. The short-range order indicated by the (100) and the (101) reflections (weak, at ca. 2 2 O 0) is quite similar and allows us to understand the similarity in the C 1s binding energies by simple initial state arguments. The stacking order is, however, very poor in the carbon black sample (wide (002) profile at ca 14 deg 0) and the increased interlayer (23) Lamp black FLA 101, furnace black FW 1, industrial designations of Degussa.
4526 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Atamny et al. more disordered carbon black material. Moreover, the peak at 316.5 eV due to a state of A symmetry (see Table I) is missing in the carbon black spectrum, indicating a sp3 bonding arrangement.
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