Carbon Vol. 20. ‘Uo. I, pp. s-39. 1982 Printed in Great Britam.

ooO8-6223182/010035~S$03.wi0 Pergamon Press Ltd.

ANGULAR DISTRIBUTION XPS STUDIES ON CARBON FOIL

VANEICA YOUNG Department of Chemistry, Texas A & M University, College Station, TX77843, U.S.A.

(Received 29 May 1981)

Abstract-A search of the literature reveals that oxygen corrosion of carbons appears to take many diverse pathways. In this study, the nature of the oxygen corrosion product on a commercially obtained carbon foil has been investigated using angular distribution XPS and ion bonbardment. The untreated foil shows an oxygen doublet with binding energies of 531.9 and 533.6 eV and with an intensity ratio of 1.5: 1 for all values of 8. Hence, the corrosion product is present to at least a depth of 20 A. The product is destroyed when the foil is heated to 531°C in ultrahigh purity nitrogen and forms ether type functional groups when bombarded with 5OOeV argon ions to damage saturation. When heat treated carbon foil is oxidized at 0°C according to the Staudenmaier method, an intense Or, peak at 531.8 eV coupled with a much weaker peak at 533.7 eV is observed. All data is consistent with the assignment of hydrated graphite oxide as the corrosion product.

1. INTRODUCTION

Oxygen corrosion of various forms of carbon has been intensively studied by a number of techniques. Within the last 10 yr, X-ray photoelectron spectroscopy, a valu- able tool for elucidating the structure of surface func- tional groups, has been used to study the electronic structure of the corrosion products. The Or, level for graphite exposed to atomic oxygen at room temperature is fairly symmetric with a binding energy -532.3 eV and a FWHM of 2.1 eV[l]. By contrast the Or, level of graphite exposed to atomic oxygen at 676 K is asym- metric with a shoulder on the low binding energy side (- 530eV) of the main peak (- 532 eV)[l]. The O,, level of argon ion bombarded graphite exposed to microwave excited oxygen and nitric oxide is a broad asymmetric peak with a binding energy of 532.0 eV and a FWHM of 3 eV [2]. The Or, level of the oxygen corroded prismatic surfaces of pyrolytic graphite is broad with a shoulder on the high binding energy side (- 532eV) of the main peak (- 530 eV)[3]. The interaction of a 500 eV beam of NO’ ions with polycrystalline and pyrolytic graphites gives an oxygen doublet at 531.4 and 533.1 eV with FWHM of 2.2 and 2.3 eV, respectively[4]. Recently, we have observed that the oxygen corrosion product on a commercially obtained carbon foil gives an oxygen doublet at 531.9 and 533.6eV (FWHM = 2.2 eV). However, as for other graphitic carbons, the C,, level shows a narrow peak at 284.3 eV. Since evidence for a peak at 533.6 eV has not been previously reported, we have used angular dis- tribution XPS to study the depth distribution of oxygen as well as any changes in the O,, level energy dis- tribution of oxygen as well as any changes in the O,, level energy distribution. This technique is superior to depth profiling techniques employing ion beams because the latter can lead to surface alterations. In addition, both heat treated and argon ion bombarded foils have been studied in order to better elucidate the nature of the oxygen species. The former have also been oxidized at 0°C according to the Staudenmaier method[5], a method

shown to produce graphite oxide from powdered graphite.

2. EXPERIMENTAL

Light tight carbon foil, 99.8 t % (Goodfellow Metals), serves as the substrate in these experiments. The foil has a total metal impurity level of 1.2 ppt with iron being the major impurity (- 58%). It has a density of 1.2 g/cm3 (by comparison crystalline graphite has a density of 2.20g/cmg at 20°C and bulk amorphous carbon has a density in the range 1.8-2.1 g/cm’[6]). Substrates are prepared by cutting 10 x 12 mm rectangles from the foil. Survey scans and high resolution scans of the Or, and C,, levels of untreated substrates, substrates heated for 30min at 531°C on a hot plate in ultra-high purity nitrogen, and substrates bombarded for 30 minutes with a 500eV beam of argon ions (generated from ultra-high purity argon) are recorded on a HP5950A spectrometer, which has a base pressure of 1 x lo-’ Torr. The analyzer chamber has a base pressure of 1 X 10M9 Torr. The spec- trometer is equipped with an angular distribution probe (Surface Science Laboratories Model 259) and spectra have been obtained at 0 values of 18, 38, 58 and 78”, where 0 is the angle between the substrate surface and the exiting photoelectrons. A HP982SA computer, inter- faced to the system, has been used for background subtraction and peak deconvolution. It utilizes a Surface Science Laboratories software package (9825 ESCA Data System Rev. E).

Substrates heated for 30 min at 531°C on a hot plate in ultra-high purity nitrogen are also placed in individual glass vials and covered with 2.0 ml of a 67 : 33 V/V solu- tion concentrated sulfuric acid and 60% nitric acid. The vials are placed in an agitated icebath and S-10mg. of KClO, is added to each. The reaction is quenched by gently transferring the foils into distilled water. The foils are then washed in four successive 2OOml. aliquots of distilled water. They are dried overnight in a dessicator containing anhydrous calcium chloride. Survey scans on

36 v. YOUNG

the HP5950A are conducted to evaluate the completeness of the washing procedure (i.e. we look for Szp, Cl,, and N,, peaks) and high resoJutjon scans of the 01, and Cc, levels are obtained as indicated above.

3. RESULTS AND DISCUSSION

In Fig. I, the survey scans of untreated carbon foil are shown for B = 18”, 38,58 and 78”. Note that for 6 = 18”, a weak NI, level is evident. This peak is absent at larger values of 8. Such behavior is expected for a gas adsorbed only at the surface of a substrate. By contrast, an O,, level is evident at all values of 8. A plot of the Cl, O,,

8: 38’

--.-_--I e-58*

i

J

-- BOO 800 400 200 0

RIND@NG ENERGY C&f

Fig. 1. Survey scans of untreated carbon foil. 8 = (a) 18” (b) 38” (c) 58” and (d) 78”.

intensity ratio as a function of B is shown in Fi. 2. The value remains essentially constant from @ = 18” to 6’ = 38”. This ratio would be expected to increase mono- tonically with increasing 6 if oxygen is present only at the surface. The ratio of atoms of oxygen to atoms of carbon at each 0 may be calculated from the equation[7,t]

No _ lo Wo UC NC Zc Wcz

(0

tPrivate communication.

1 UNTREATED / ‘C

Is 30 i /

18” 38O 58” 7Ef”

e

Fig. 2. I&o,, vs 8. 0 untreated carbon foil XAr” etched foil.

where lo and Zc are the time normalized intensities of the O,, and C,, levels, W. and WC are the scan window widths, and Q and u. are the Scofield photoionization cross sections[81. We obtain 0.024,0.026,0,018 and 0.016 for B = 18, 38, 58 and 78”, respectively. The stoi- chiometry corresponding to the ratio 0.026 is Cloo 02.6, hence the extent of oxygen corrosion is very small. In order to estimate the depth to which oxygen corrosion occurs, we proceed as follows. For a homogeneous sub- strate, the depth probed at any angle is approximately given by the equation[9]

d = A sin e

where A is the electron mean escape depth for inelastic scattering. Since the extent of corrosion is diminuitive, we may take A equal to the electron mean escape depth for pure carbon, ho In the limit B +90°, d = AC; this approximation holds essentially for 0 = 78” (sin 8 = 0.97815). Several values for AC can be found in the literature. For electron kinetic energies of 262-l 169 eV, Klasson et aL[lO] reported h,‘s of 8-18 A. These values were supported by the results of Voreades, who found A < 20 1$, for secondary electrons produced when carbon films are bombarded with 15-25 keV primary electrons [ Ill. Likewise, the theoretical model of Penn[ 121 predicts that hc = 15.8 A for an electron with a kinetic energy of 1202 eV (Cls using Al Kcu). On the other hand, Cadman et at.[13] reported a Ihc value of 44A for %7eV electrons in graphite (GJ using MgKe). If we average the values for electron kinetic energies - 1000 eV, we obtain AC = 23 A with a standard deviation of 14A. Thus we conservatively estimate that oxygen is present to a depth of -23 A in the periphery of the foil.

In Fig. 3, high resolution scans of the 01, level are shown. All spectra exhibit an asymmetric level which can be deconvoluted into two peaks. The intensity ratio

I

i O=?*’ -A

Angular distribution XPS studies on carbon foil 31

(HOOCCH(NHJCH& to a doublet with an energy separation of 1.2 eV, e.g. the 532.0 and 533.2 eV peaks of CI~C~SCH~COOH have been abserved[l7]. However, the intensity ratio should be 1: 1 whereas the observed experimental value for oxygen on carbon foil is 1.5: 1. Organic carbonates also exhibit high Jinding energy

Fig. 3. Oxygen 1s level of untreated carbon foil B = (a) 18” (b) 38” (c) 58” and (d) 78”.

of the two peaks is constant with angle, 1.5 : I, indicating that the stoichiometry of the oxygen species is constant. X-Ray diffraction studies using a Bragg diffraction scin- tillation detector show that the c axis of the foil proces- ses with an angle of 9ir 1” off the surface normali’. Furthermore, a weak 7~ electron plasmon loss satellite -6.5 eV from the main C,, peak (characteristic of well ordered graphite samples [2]) is observed. Hence the foil has large individual graphitic layers, probably tilted out of the sheet plane[l4]. Its density is -45% below that of ideal graphite; this indicates a large number of edge defects and pores[lSl. The peak at 533.6 eV is charac- teristic of bound water, and Lang and Magnier [ 161 have shown that capillary adsorption of water on carbons can not be reversed even after 200 hr exposure to vacuum. They have shown that minute traces of moisture in a combustion gas can change the entire behavior of cor- rosion even for very pure graphites. Furthermore, they have shown that corrosion of carbon by dry oxygen is insignificant at room temperature (a result confirmed by other investigators [ 1,2]. while corrosion proceeds readily at room temperature in the presence of moisture. They have suggested that water together with O2 gives rise to carboxyl groups. The electronic structure of the carboxyl group leads us to expect two peaks in the O,, spectrum since the carbonyl and hydroxyl groups should be chem- ically inequivalent. Experimentally, single symmetrical peaks with a binding energy of 531.5 eV e.g.

II oxygens, e.g. th oxygen peaks of 0-O-C-O-0 occur at 533.5 and 534.8eV[18]. However, we do not observe an O,, peak at -535eV, and furthermore an oxygen in- tensity ratio of 2: 1 would be expected. Thus we feel that the obvious assignment of the peak at 533.6eV is to bound water.

The probable identity of the peak at 531.9 eV is now addressed. Barton, et aL[l9] have shown by diazomethane methylation that active hydrogens on car- bon and graphite surfaces containing acidic oxides occur as OH groups. A stoichiometric compound involving hydroxyl groups and water has been described; it is C: (OH-),(H,O),, designated graphite oxide in the com- pendium by Ub~lohde and Lewis[ 151. This compound is expected to be formed when graphite is oxidized in the presence of water. Evans el al.[5] have published XPS spectra for graphite oxide. They obtain a symmetric O,, peak with a binding energy -531.7 eV and a FWHM of 2.2eV. They have taken great pains to make sure that their products are moisture free, because as they note, graphite oxide is very hygroscopic. Thus we hypothesize that the corrosion product on carbon foil is hydrated graphite oxide.

According to Ubbelohde and Lewis[lS], graphite

c

Fig. 4. Survey scans of heated carbon foil. 6 =(a) 18” (b) 38” (c) _.. 58” and (d) 78”. tPrivate communication.

38 v. YOUNG

oxide is stable at room temperature but decomposes to CO, and CO when heated above 200°C. As a test of our hypothesis, we have heated carbon foil to 531°C for 30 min in an atmosphere of ultra-high purity nitrogen. As can be seen from the survey scans in Fig. 4, the oxygen is reduced below the detection level of the XPS tech- nique. Observe that the plasmon loss satellite is much more intense, indicating an increase in ordering. The C,, peak becomes narrower by -0.1 eV, although the binding energy is unchanged. When the untreated foil is bom- barded for 30 min with 500 eV argon ions (an Ar+ dose commensurate with damage saturation[2]), significant changes are observed for all spectra. Survey scans show that nitrogen is present for all values of B (Fig. 5) and oxygen is found to be uniformly distributed for all values of 0 studied. High resolution scans of the O,, level reveal a single symmetric peak at 532.8 eV with FWHM = 2.2 eV (Fig. 6). We note that the broad peak obtained by Evans and Thomas[2] for argon ion etched graphite exposed to microwave excited O2 and NO can be resolved into two peaks of equal intensity with FWHM = 2.2. They lie at 531.5 and 532.8 eV. Our peak is thus analogous to their high binding energy component. They attribute their peaks to C=O and C-O-C functional groups. We share the belief that the high binding energy component is due to C-O-C functional groups, however the peak at 531.5 eV is probably graphite oxide. In our case, we believe that Ar’ bombardment leads to a scis- sion of the O-H bond of graphite oxide and water followed by an attack of the oxygen on neighboring

e.w

2 h I ,

Fig. 5. Survey scans of Ar+ bombarded carbon foil. 8 = ((I) 18” (b) 38” (c) 58” and (d) 78”.

.9=38’

a=5!3”

e=?a’

54c 1 530 520

Fig. 6. Oxygen Is level of Ar+ bombarded carbon foil. 0 = (a) 18” (b) 38” (c) 58” and (d) 78”.

carbon atoms. Since untreated carbon foil shows a N,, peak only at 0 = 18”, it is hard to imagine that the source of nitrogen and additional oxygen is due to ion induced diffusion from the bulk substrate. Rather, it may be attributed to trace amounts of N; and 0; produced from residual air in the accessory chamber. As reactive ions, their sticking coefficients on carbon foil are large relative to ground state oxygen. They are expected to enter the foil as atomic N and 0 where they react after ther- malization.

Survey scans of the heat treated carbon foils oxidized by the Staudenmaier method show no detectable chlorine or nitrogen and a barely detectable SZp peak. The latter has a binding energy of 167.7 eV, while sulfate has a binding energy of -168.5 eV. We conclude that a barely detect- able amount of adsorbed SO3 is present. These foils show considerable physical damage and the surfaces have a grayish cast, in contrast to the shiny black sur- faces of the heat treated foils. High resolution scans of the C,, and O,, levels are shown in Fig. 7. The Cr, level shows three peaks at 283.6, 285.1 and 286.4eV, respec- tively. The peak at 283.6eV is associated with massive physical damage and has been observed for carbon foil anodized in 1F H,SO, and carbon foil recoil implanted with Au atoms. The details of these studies will be published elsewhere[20]. The peak at 285.1 eV is also associated with damage; for example it is formed when carbon foil is bombarded with argon ions. The peak at 286.4eV is characteristic of graphite oxidel51. The O,,

BINDING ENERGY (eV)

Fig. 7. Carbon 1s and oxygen 1s levels for heated carbon foil oxidized by the Staudenmaier method.

level shows two peaks, one at 531.7 eV (characteristic of anhydrous graphite oxide[S]) and one at 533.7 eV (characteristic of bound water). The fact that the latter peak is only 8.3% of the total oxygen reflects the rather anhydrous conditions under which the oxidation occur- red. The binding energies of these peaks are within experimental error of those observed for untreated car- bon foils.

4. CONCLUSIONS

It has been shown that the Or, level for oxygen corroded carbon foil can be deconvoluted into two peaks of identical FWHM at 531.9 and 533.6eV, respectively. The parameters of the former peak are within experi- mental error of the analogous parameters of the Or, peak of anhydrous graphite oxide and the main Or, peak of carbon foil heated to remove the original oxygen con- tamination and then oxidized by the Standenmaier method. The higher binding energy peak is characteristic of bound water. Its presence would be expected in moisture laden environments because the capillary ad- sorption of water on carbons is strong. Thus, the most probable identity of the oxygen corrosion product on carbon foil is hydrated graphite oxide, C: (OH-),(H20)2. Bombardment of the foil with a beam of inert gas ions offers further corroboration, since a single Or, peak at

Angular distribution XPS studies on carbon foil 39

532.8 eV, assignable to C-O-C function groups, is present. We postulate that the corrosion behavior of the foil is a consequence of the large number of defects present, i.e. the foil can be regarded as a kind of semi- ordered aggregate of graphite powder. Thus, while oxi- dation of metal foils leads eventually to uniform coverage of the foils with monolayer of metal oxides; the oxidation of carbon foil leads to aggregates of hydrated graphite oxide confined to defect sites. We suggest that defect sites in other carbons should be vulnerable to oxidation leading to the formation of graphite oxide, particularly in the presence of moisture.

Acknowledgement- The author wishes to thank the Robert A. Welch Foundation, grant number A-771, for support of this research.

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