Raman spectra of Graphon carbon black

Crirhon Vol. 22, No. 1. PP. 3942, Printed in Great Britain.

1984

ooO&6223/84 $3.00 + .W IT 1984 Pergamon Press Ltd.

RAMAN SPECTRA OF GRAPHON

CARBON BLACK

P. MERNAGH,RALPH P.COONEY and ROBERTA. JOHNSON Chemistry Department, University of Newcastle, N.S.W. Australia 2308

TERRENCE

(Received 1 September

1982)

Abstract-The Raman spectrum of Graphon carbon black has been recorded using rotating cell techniques. Angular dependence of scattering at 1360, 1580 and 2700 cm ’ are reported and these data suggest that the 1360 cm -. i line is associated with non-planar microstructure distortions. The excitation frequency dependence of the intensity ratio of the bands at 1360 (D) and 1580 cm-’ (G) is interpreted in terms of resonance (vibronic) interaction. This dependence is primarily the result of an increase in the intensity of the 1360 cm- I line. The disorder-associated line(D) exhibits a significant excitation-dependent shift from 1378 cm _ ’ (457.9 nm Ar +) to 1330 cm ’ (647.1 nm Kr +). The “graphite” (G) line position is less sensitive to changes in excitation frequency. The spectral features are discussed in terms of factor group, C&, and layer site symmetry, C,,,. Also the possible role of localized alkene-like structure in zones of structural distortion is considered. 1. I~TRODU~ION

For disordered carbons, which exhibit more complex Raman spectra (Table I), the assumption of space group, D$, would appear to be less justified. The layer-stacking disorder in microstructures of carbon blacks[4] would tend to eliminate interlayer centres of symmetry invoked for crystalline graphite. AIso Vidano et a/.[21 consider that all the bands observed for graphite and carbons are in-plane vibrations. This would indicate that any interlayer centre of inversion is of secondary importance to the spectrum, assuming weak interlayer vibrational coupling. In this report the angular dependence of the scattering in the dominant first-order features in the

The vibrational spectrum of crystalline graphite has been analyzed in terms of the centrosymmetric space group, D&, by Brillson et al. [l]. This analysis has been the basis of many of the subsequent studies of disordered carbons which have been reviewed by Vidano et ai.[2]. The factor group analysis based on D:h, which requires the presence of an interlayer cell-edge centre of inversion [ l] appears to explain the single line Raman spectrum of crystalline graphite. However Fateley et af.[3] point out that the Wycoff space group, C&, can explain the observed spectrum if weak interiayer interaction and no coupling of vibrational modes is assumed.

‘fable 1. Suggested internal vibrational assignments” and frequencies for disordered carbons Carbons

Graphite: Layer Site Assignment

Factor Group Assignments’

1

A,,JIW Bpg(-)

A;(R)

A;(R,IR)

81(-)

AI,z(R,IR;-I

Structural

FrWpXIKy

Dlstortlon r.eatures

(cm- i )

868C

*+rC kg(R)

1580(0~~’

E(R, IR)

Ez(R,IR)

‘._\ ,* Elu(IR)

E(R,IH)

El(R)

135O(D,ld

,-*(TSI

1588C

137U(D~)

_/--US1 ‘,

::t*

.( h’TS) _.

16’0(D’) ._

a.

b.

liotation: IR, infrared actlve; R, plane; TS, totally symmetric; NTS, band designation for carbons [Z]. These modes,

representat;ons e.g. for Cc”,

c.

Reference

d.

TM features is unresolved

encIude acoustic TA: A1 + E, and

Ramanactive; non-totally

T:

-_( NTS)

l, out-of-

-, inactive; symmetric;

(T,A) and optical Bi * C2.

G,

P,,

Liz,

translatory

(II, observed in this

for slightly study.

damaged

39

graphrtcs

[2].

The

prafllc

I’!)

[I’,

d

et al.

T. P. MERNAGH

40

spectrum of Graphon carbon (a graphitized channel black) together with other observations, are related to symmetry and structural features. 2.EXPERIMENTAL

2.1 Spectroscopic equipment Raman spectroscopic data were obtained using an Anaspec-Cary 81 spectrometer and exciting lines selected from the emissions from Coherent Radiation Laboratories 52A argon ion and 500 K krypton lasers. The spectra were detected and processed using an RCA C31034A/02 phototube and a Brookdeal-Ortec 95 11 quantum photometer. The quantum photometer was connected via a Scitec (Australia) interface to a Nicolet 1074 hard-wired computer and a NIC 283A magnetic tape system. A rotating sample cell (designed for resonance Raman experiments) was employed to avoid laserinduced degradation processes. The carbon sample was ground under a N2 atmsophere and then pressed into a metal annulus which was rotated at cu. 2000 rpm. Experiments carried out on static samples in air revealed that degradation processes are occurring under the laser beam (see Results and Discussion). Angular dependence measurements were recorded using the rotating cell (above) seated on a x-y-z-0 stage which slowly rotated (360”/60min) about the point of laser focus. Excitation curves for the lines at cu. 1360, 1580 and 2700cm-’ were recorded with the Spheron 6 dispersed in a pressed solid sodium sulphate matrix. Individual line intensity variations were measured relative to the intensity of v, (SO,*-) at 990 cm- r. Materials

Graphon (Cabot Corporation) is produced by heat treatment of Spheron 6 carbon at temperatures in the vicinity of 3000”[5]. This heat treatment results in the removal of oxygen containing surface groups and in increased ordering of layers[5]. 3.RESULTSANDDlSCUSSlON

The results of the factor group analysis based on D& and C$, are given in Table 1. The representation of modes for the former group was deduced by Brillson et al. [ l] whi!e the analysis based on the latter group was concluded as part of the present study using the factor group tables of Newton and Adams[6]. The suggested assignment of observed frequencies to symmetry species of these groups is also given in Table 1. While a change in space group symmetry from D& for graphite to C& for disordered carbons leads to predictions of changes in the appearance of the Raman spectrum, it appears unlikely that this factor by itself would account for the appearance of disorder-associated features at 1360 cm-’ (the D band) and ca. 1620cm-’ (the D’ band). The A, (C&) mode for carbons would be expected to coincide approximately with the out-of-plane, IR-active mode

//

1400

1200

1600 RAMAN

//

I

(

2600

2800

SHIFTkmY

Fig. 1. Low resolution

Raman spectrum of graphon. line, 514.5 nm Ar+, 240mW; rotating cell, 2000 rpm).

(exciting

(&, D&J assigned to an absorption at 868 cm-’ by Nemanich et al. for graphite[7]. The E, (Ci,) mode may not be resolvable from the 1580 cm-’ line given the proximity of the IR absorption of graphite (1588 cm-‘) assigned[7] to the associated D& mode. The optical translations not represented in Table 1 appear likely to occur at very low frequencies. The shear mode, for example, was observed at 48 cm-’ [8]. Reducing the problem to one of layer-site symmetry (C,,) also does not immediately generate an explanation of the disorder-associated features at 1360 and 162Ocm-’ observed for Graphon and other carbons. To clarify the origin of the 1360cm-’ line which (unlike the 1620cm-’ line) is clearly resolved in the spectrum of Graphon, the angular dependence of scattering in these bands was examined. The results which are meaningful over the angle range, lo” 50 I 80”, are given in Fig. 2. The preparation under pressure, of the carbon-black annulus for the rotating-cell may lead to a preferred orientation of the capsular or polyhedral particles[4] perpendicular to the applied pressure. The increase in intensity over the range, 60” I 0 I 80”, may be understandable in terms of an approach to “edge-on” excitation of the carbon layers within polyhedral faces [4]. It appears that scattering at 1360 cm - ’ is not angular dependent whereas scattering at 1580 and

I

I

.

80

70

60

SO

40

30

20

IO

B(DEGREESI

Fig. 2. Angular dependence of Raman scattering for graphon (exciting line 514.5 nm Ar+ , 500 mW; band pass, 10.7cn-’ rotating cell, 2OOOrpm; 0 rotation, 90”/15min;

polarization is parallel to the surface).

Raman spectra of graphon carbon black is angular dependent. It is tentatively concluded that the 1360 cm- ’ feature is associated with the non-planar zones of microstructure distortion (e.g. zones of curvature) evident in highresolution electron micrographs of graphitized carbons (see Fig. 5 in Ref. [4]). The localized reduction in symmetry associated with structural distortion would split the degenerate modes, 2E, associated with the layer site, C,,[3] (see Table 1). Depending on the local symmetry in the zone of deviation from planarity each degenerate layer vibration would split into a totally-symmetric and a non-totally symmetric pair (e.g. Cs: A’ + A”). The disorder-associated lines at 1360 and 1620 cm-’ could both be interpreted as arising from nondegenerate (possibly totally-symmetric) components of the degenerate layer modes (see Table 1). The question unanswered in the assignment above (and earlier assignments) relates to the magnitude of the displacement of the (assigned) split component 1360 cm-‘, from the original degenerate layer mode at 1580cm-‘. Such a displacement suggests major changes in carbon-ring force constants. This displacement could be explained in terms of disruption of the delocalized electronic structure at the point of deviation from planarity (e.g. curvature, etc.) or at the layer edge. Localized electronic structure would have alkene-type characteristics. Hexene-1, for example, exhibits Raman lines at 1300 and 1638 cm-’ with the former cu. three-fold more intense than the latter[9]. This pattern is not dissimilar to the spectral features at 1360 and 1620 cm - ’ arising from disordered zones in Graphon and other carbons[lO]. The excitation curves for the major features in the Graphon spectrum (1360, 1580 and 2700 cm - ‘) were recorded with intensities measured relative to v, (SO:-) of the sodium sulphate solid matrix. With increasing excitation wavelength there is a slight decrease in line intensity at 1580cm-’ (G) and a better-defined increase in intensity at 1360 cm-’ (D) (see Table 2). The result of these two effects is that the [Z(D)/Z(G)] ratio changes in a relatively smooth fashion with excitation frequency (Fig. 3). Similar changes are not apparent in the spectra of some other carbons reported by Vidano et al.[2] and as they are unlikely to arise from differences in sampling method (e.g. rotating sample) they are presumably a potentially informative characteristic of the carbon samples. These data for Graphon support the suggestion of Vidano et al.[2] that carbon spectra arise from

41

27OOcm-’

I 16

17

18 19 y, WV10

20

21

22

Fig. 3. Intensity ratio for graphon [Z(D, 1360)/Z(G, ISSO)], as a function of excitation frequency (Q.

resonance (vibronic) enhancement. Further, they indicate that the vibronic interaction (and the resonance enhancement) is different for the D and G modes with the former favoured at longer wavelengths and the latter at shorter wavelengths. Further evidence for the different origins of the D line (cu. 1360cm-‘) and the G line (cu. 1580cm-‘) emerges from the excitation-frequency dependence of Graphon (see Fig. 4). Unlike the other features, the G line is relatively insensitive to changes in excitation frequency. Similar observations for other carbons have been reported by Vidano et a1.[2]. In order to examine the efficacy of rotating the carbon sample when spectra are recorded in air, data have also been obtained for static Graphon samples. In addition to the usual dominant D, G features, broad lines were detected at cu. 1470, 2810 and 2960cm-’ as well as additional high-frequency broadening on the 1580 and 2700 cm- ’ features after longer exposure to the laser. The new lines are attributed tentatively to the following characteristic groups: 1470cm-’ to CYO vibrations of surface carboxylates or related oxidized species; 2800-2900 cm-’ to C-H bonds or non-fundamentals rendered Raman-active by symmetry reduction dur-

G

1600 &op3*G UIMO

I

Table 2. Excitation data for graphon

45T.9 4.x.n 514.', 5bd.l L4'.1

u.w 0.:'5 1.11 l.SO I.50

2.10 2.05 2.45 2.00 1.14

2.10 1.75 2.11 1.75 1.07

L

16

17

. 18

. 19

. 20

21

, 22

% krn“)/103

Fig. 4. Frequency displacement with excitation frequency (f,,) for the D, 13 and G’ bands of graphon.

42

T. P. MERNAGHet al.

ing laser degradation. It appears that reduction localized laser flux is essential in obtaining producible spectra from carbons in air.

of re-

They are also indebted to Dr. C. G. Barraclough for helpful discussion and to J. Wright and Co. for the donation of carbon materials. REFERENCES

4.

CONCLUSION

The observation of excitation sensitive changes in line intensity and position indicate that the spectrum of Graphon carbon black behaves like the combination of two distinct resonance Raman spectra, which arise from the disordered and ordered regions of the material. Resonance enhancement would account for the intensities observed, given that the depth of laser penetration is expected to be of the order of tens of nanometers[2]. The difference in excitation behaviour for the 1580 and 1360 cm - ’ lines appears to rule out an explanation based on optical skin depth. This parameter increases 30% in changing from 488 to 647 nm excitation [Z].

Acknowledgements-The authors are grateful to the Australian Research Grants Committee for providing the Raman spectroscopic equipment and also to the Australian De-

partment of Education for a Postgraduate award to T.P.M.

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2. R. P. Vidano, D. B. Fischbach,-L. J. Willis and T; M. Loehr. Solid State Commun. 39. 341 (19811. 3. W. G..Fateley, F. R. Dollish, N.‘T. McDevitt and F. F. Bentley, Infrared and Raman Selection Rules for Molecular and Lattice Yibrations : The Correlation Method, pp. 162-164. Wiley-Interscience, New York (1972). 4. A. I. Medaha and D: Rivin, Characterization of Powder Surfaces (Edited by G. D. Par&t and K. S. -W. Sing) Chap. 7. Academic Press, New York (1976). 5. R. L. Gale and R. A. Beebe. J. Phvs. Chem. 68. 555 (1964). 6. D. M. Adams and D. C. Newton, Tables for Factor Group and Point Group, Analysis. Beckmann-RIIC (1970). 7. R. J. Nemanich, G. Lucovsky and S. A. Soiin, Solid State Commun. 23, I 17 (1977). 8. R. J. Nenamich and S. A. Solin, PhVs. Rev. 820. 392 (1979). 9. J. Behringer, Raman Spectroscopy (Edited by H. Szy-

manski), p. 186. Plenum Press, New Jersey (1976). 10. R. Vidano and D. B. Fishbach, J. Am. Cermic Sot. 61,

13 (1978).