commercially available carbon black materials were characterized by Raman spectro- scopy. The Raman .... Raman spectrum of Vulcan (V) with the correspond-.
Carbon Vol. 33, No. 11, pp. 1561-1565,1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved ooO8-6223/95 $9.50 + 0.00
Pergamon OOOS-6223(95)00117-4
RAMAN SPECTROSCOPIC CHARACTERIZATION OF SOME COMMERCIALLY AVAILABLE CARBON BLACK MATERIALS T. JAWHARI,*~ A. ROID~ and J. CASADO~ “Serveis Cientifico-Tkcnics, Universitat de Barcelona, 08028 Barcelona, Spain *Institut de Citncia de Materials de Barcelona (C.S.I.C.), 08193 Bellaterra, Spain ‘Departamento de Investigacibn, S.E. de Carburos Metalicos S.A., Pa Zona Franca 14-20, 08038 Barcelona, Spain (Received 14 February 1995; accepted in revised form 5 July 1995)
Abstract-Some
commercially available carbon black materials were characterized by Raman spectro-
scopy. The Raman spectra were recorded between 1000 and 1800 cm-‘, which corresponds to the spectral region that provides the most valuable data on the microstructure of carbons. A comparative study of the intensity, bandwidth and frequency shifts of the D and G bands, as well as the broad amorphous feature, is presented here. A curve fitting method is also proposed in order to improve the accuracy in determining the spectroscopic parameters of the main Raman bands. Correlations between the Raman spectra and the structure were established. The samples were found to correspond to low order sp’ bonded carbons, but cannot be considered as truly amorphous since they have some degree of order in the basal plane.
Key Words-Amorphous
carbons,
graphite,
Raman
spectroscopy,
1. INTRODUCTION Graphitic materials are used in a wide range of industrial applications. Good thermal and electrical conductivity, low atomic number, high superficial area and high mechanical strength are some of the characteristic properties of these materials. One of the most suitable methods to study the properties of carbon blacks is Raman spectroscopy. Raman modes can be predicted from group theory. Carbon has several allotropes and can exist in a wide range of disordered forms. Raman spectroscopy was found to be quite sensitive to these structures and to changes that perturb the translational symmetry of the analyzed sample, such as those that take place in small-dimensional crystals or small grain size (polycrystalline). It is thus a powerful method for characterizing carbons [ l-41. Since the Raman signal is sensitive to short range disorder, it can also reveal different forms of amorphous carbons. However, the interpretation of the Raman spectra of carbonaceous materials has generated considerable debate in the literature, mainly resulting from the complex microstructure of these systems and the rich information that can be extracted from these spectra. Therefore, we shall briefly review the different structural determinations of carbon-based materials that can be obtained by Raman spectroscopy. Carbon has two crystalline forms: diamond and graphite. Diamond belongs to the face-centered cubic lattice, and its Raman active phonon is a single, triply degenerated zone-centered vibrational mode, which is characteristic of C-C single bonds between sp3hybridized carbon atoms in the cubic structure. This *To whom all correspondence
should be addressed.
microstructure.
mode is only Raman active, and the first order band appears at 1332 cm-’ as a sharp line. The diamond structure has no first order infrared absorption. Graphites are formed of stacked sheets with the carbons atoms in the basal plane arranged in sp’ configuration. This two-dimensional tetragonal network consists of regular hexagons. The fourth electron is in a 7~orbital, perpendicular to the weakly bonded hexagonal sheets. The distance between layers is much longer than the typical short carbon-carbon distance of hexagonal rings. Because of the weak intersheet bonding, graphite crystals are mainly affected by disorder along the c-axis while along the basal plane strong carbon-carbon bonding preserves a relatively high degree of order. The Raman spectrum of single hexagonal crystal graphite (space group O&) presents a main first order E 2g2 band at 1582cm-’ with weaker bands at 42 (E,,,) and 2724 cm-‘[2,5]. The mode at 1582 cm-‘, often referred as the G mode, is assigned to ‘in plane’ displacement of the carbons strongly coupled in the hexagonal sheets. When disorder is introduced into the graphite structure, the bands broaden and additional lines are found at about 1357, 1620 (the so-called D’ mode, which appears as a shoulder of the graphite E,,, mode at 1582cm-‘), 2450 and 3250 cm-‘, which have been assigned to nonzero-center phonons. A broad band around 1500-1550 cm-’ was also noticed in several carbonbased materials and was associated with amorphous $-bonded forms of carbon [ 1,2,4,6]. More precisely, it has been suggested that these disordered carbons can be attributed to interstitial defects[7-91. Further, it was observed that the spectrum of well-crystallized graphites formed of small grain size (polycrystalline) also present the band near 1357 cm-‘. This mode is
1561
T.
1562
JAWHARI
usually called the ‘disorder-induced’ or D mode, and corresponds to a second maximum in the graphite vibrational density of states near the M point of the Brillouin zone boundary, which becomes active in small graphite crystallites when the lack of a long range translation symmetry leads to a breakdown of the k-momentum conservation rule [ 2,10,11]. Other lines have been reported[ 121 at 870, 1230 cm-’ and at lower frequencies, but most of the interesting information on the microstructure of graphitic materials has been obtained analyzing the spectral features (i.e. intensity, peak position, lineshape and bandwidth) between 1300 and 1650 cm-‘. For example, the integrated intensity ratio ID/IG is inversely proportional to the microcrystalline planar size L,, which corresponds to the in-plane dimension of the single microcrystalline domain in graphites [ 4,131. Both the peak position and bandwidth of G and D modes are sensitive to L,. Further, the down-shifting of the G line position has been correlated to the presence of bond-angle disorder [ 121. Studies on amorphous hydrogenated carbons show that the D and G modes are also found to be sensitive to the type of carbon bonding, i.e. sp’ or sp3. Information on sp3Jsp’ bonded fraction of carbon was obtained from the band position of D and G[14,15], and from the integral intensity of the D band[ 121. The purpose of this work is to analyze precisely the spectral region between 1000 and 1800cm-‘, which produces the most interesting structural information on carbonaceous materials. A curve fitting method is here proposed for the study of these carbon blacks that allows us to follow small spectroscopic changes with accuracy.
2. EXPERIMENTAL The Raman spectrometer used here was a Jobin Yvon T64000 instrument using an argon ion laser as a illumination source, and consisted of a subtractive dispersion double monochromator combined with a spectrograph that disperses the light onto a bi-dimensional CCD detector cooled at 140 K. The Raman instrument was coupled to a standard Olympus microscope and the collection optics system was used in the backscattering configuration. The analyzed region was directly visualized through a x 50 or a x 100 microscope objective. The samples were analyzed without any sample treatment or preparation. The laser frequency used was the 457.9 nm line, since the other argon ion lines were found to affect the structure of the carbon as a result of a higher absorption effect. Even with the 457.9 nm laser line, it was necessary to use low laser power density with a x 50 microscope objective, in order to avoid laser heating effects. The laser spot diameter reaching the sample was about 2 pm. The laser power at the sample used in this study was 1 mW. The spectra were recorded at 4 cm-’ resolution. A fitting program
et al.
was used in order to follow with more accuracy the evolution of the different contributions. The four samples analyzed here were powdered carbon blacks that will be referred to below as V for Vulcan XC-72 (Cabot), M for Merck (Merck), L for Printex L and XE for Printex XE2 (both Degussa). A reference powder sample R of polycrystalline graphite was also analyzed. The specimens studied here consist of aggregates of nearly spherical particles of average diameter of about 60 nm, as can be observed in the SEM micrograph of Fig. 1. Therefore, the Raman microprobe averages a large number of randomly distributed crystallites.
3. RESULTS AND DISCUSSION The Raman spectra in the range 1200-1800 cm-’ of the four samples analyzed in this study are presented in Figs 2-5. The bands of these spectra are relatively broad, indicating that the crystallite size of these materials is small. In fact, a rapid and qualitative comparison of these spectra separates them into two groups: the spectra of Figs 2 and 3, corresponding to L and V, which are very similar to the published spectra[ 1,2,6] of coal, and the spectra of XE and M, which can be related to glassy carbon. These four samples. which show a high degree of disorder, are non-hydrogenated carbons and therefore can be assumed to consist of only sp2 bonds[ 11. In order to improve the accuracy in the determination of spectroscopic parameters such as peak position, bandwidth, lineshape (i.e. Gaussian, Lorentzian or a mixture of both) and band intensity, a curve fitting was carried out for each spectrum. The result of this line decomposition is also indicated in Figs 2-5. Several fits were tried leaving all the spectroscopic parameters free to progress and the best fitting was invariably obtained for the four samples with two Lorentzian lines around 1360 and 1600 cm- 1 and a broad Gaussian band at about 1540-1550 cm-‘. The result of this fitting is in good agreement with a recent work[l5] carried out on amorphous carbon that has proposed a decomposition of the bands between 1200 and 1800 cm-‘: the two bands D and G of Lorentzian shape and a third component is used in the fitting, which corresponds to a broad Gaussian band between 1500 and 1550 cm ’ This broad feature was assigned to amorphous graphitic phase[ 15,161. Carbons bonded at sp3 have vibrational features at frequencies below 1500 cm- ‘. and the fact that the G mode is close to the main E,,, band of crystal graphite is further evidence in favour of sp2 bonding[ 1,161. The reference sample (R) spectrum is also shown as an example in Fig. 6. Clearly, this spectrum indicates a graphitic structure typical of polycrystalline graphites, the degree of order being much higher than that of the carbon blacks. Further, no apparent broad band about 1540 cm-r is observed in the spectrum of Fig. 6, revealing that no manifest interstitial disorder sites are present in sample R. The deconvolution
Raman spectroscopic characterization of carbon black
1563
Fig. 1. SEM micrograph of Printex XE2. The laser spot at the sample is nearly twice as large as the surface shown here.
I
1200
I
1400
1600
1800
Wavenumber (cm-‘)
1
1200
I
1400
,
1600
1800
Wavenumber (cm-‘)
Fig. 2. Raman spectrum of Printex L (L) with the corresponding curve fitted bands.
Fig. 3. Raman spectrum of Vulcan (V) with the corresponding curve fitted bands.
was obtained in this case using only two Lorentzian lines. A slight deviation in the fitting can be noted and is probably due to the weak shoulder present at about 1620 cm-‘, which corresponds to the D'mode. Table 1 shows the peak position, integrated intensity and bandwidth of the three bands obtained after curve fitting for the samples analyzed here. Owing to experimental error and the strongly overlapping bands, the precise determination of the spectroscopic parameters presented in Table 1 is complex and will restrict the structural analysis. However, keeping this limitation in mind, these parameters are found to be useful when comparing the different samples. It can first be noticed that for all samples the
linewidth of the G mode is narrower than that of the other two bands, confirming that the G mode is related to the crystalline component in carbons. Further, the bandwidth of the D mode is clearly greater in the case of the V and L samples when compared to that of the XE and M specimens, which is clear evidence that the degree of ordering of the former samples is lower than that of samples XE and M. It is also known[ l] that an increase of order in carbonaceous materials is reflected by an increase in the frequency of the G mode as well as a decrease of its bandwidth. However, the comparative study of the bandwidth of the G band does not allow conclusive remarks. This is probably due to the fact that the D'
1564
T. JAWHARI et al
I
I
I
1200
1400
1600
Wavenumber Fig. 4. Raman
spectrum corresponding
1800
(cm-‘)
of Printex XE2 (XE) curve fitted bands.
with
the
I
1400 Wavenumber Fig. 5. Raman
1800
spectrum of Merck (M) with the corresponding curve fitted bands.
1400
Wavenumber Fig. 6. Raman
1600 (cm-‘)
spectrum sponding
of graphite
1600
1800
(cm-‘) (R) with
comparing the bandwidth of the amorphous component of these four samples; however, the analysis of its intensity is much more fruitful since it can be observed that both the XE and M samples have relatively low amorphous contribution when compared to the G or D band. Whereas the L sample and to a lesser extent the V sample have a higher degree of amorphous phase. This amorphous contribution may be correlated to interstitial disorder, i.e. along the c-axis between the in-plane crystallites determined by L,. When comparing the intensity of the D band to that of G, XE shows the lowest value, L the highest, the two other samples having similar intensity ratios. In the case of the peak position, it is also possible to classify the samples into two groups, i.e. XE and M in one group and V and L in the other. For example, the D mode appears at lower frequency in the case of XE and M. It can also be seen that sample M shows the highest peak position for the G mode, which may indicate a lower degree of bond angle disorder when compared to the other samples. This result confirms the fact that the spectrum of sample M shows the lowest amorphous band intensity. No clear conclusions were obtained from the position of the amorphous band. As mentioned above, it is now generally accepted that the dependence between the integrated intensity ratio ID/I, and the microcrystalline planar size L, shows inversely proportional behaviour. However, the relationship may not be straightforwardly applicable when different types of graphites[ l] are compared. Since we analyze similar varieties of graphites here, i.e. low crystalline degree of graphitic carbons, the relationship may be used to obtain relative information on the microcrystalline planar crystal size: L,=
44 [1(D)/1(G)]-’
(1)
where I(D) and I(G) are the integrated intensities of the D and G bands, respectively. The crystal size L, calculated from eqn (1) is also added in Table 1. As expected, these samples show low microcrystalline planar size, i.e. L, varies between 1.4 and 2.5 nm. Note here the sensitivity of the technique to detect short range order. The reference spectrum of Fig. 6 shows a smaller bandwidth for the G mode than the other samples analyzed here. The bandwidth of the D mode is, however, slightly greater than that of sample XE. This result may indicate that the distribution of crystallite size is greater in the case of sample R than that of sample XE. The band frequency of sample R is quite similar to previously published graphite spectra, and the corresponding value of L, is 5.6 nm.
the corre-
curve fitted bands. 4. CONCLUSION
mode, which usually appears at about 1620 cm-’ as a shoulder, is not observed in these four spectra and probably interferes with the G mode, thus complicating the interpretation. No clear differences can be extracted when
Both M and XE, which have spectra similar to glassy carbons, show higher crystallite size and a lesser degree of amorphous phase than samples V and L, which correspond to the group of coal specimens. Furthermore, these materials correspond to
Raman Table 1. Raman
spectroscopic
parameters
spectroscopic obtained
characterization after curve fitting
of carbon
black
the experimental
1565 spectra
by using two lorentzian
bands (D and G) and a gaussian band (A).
Sample Microcrystalline graphite (R; reference) D G Printex XE2 (XE; Degussa) D A G Merck (M; Merck) D A G Vulcan XC-72 (V; Cabot) D A G Printex L (L; Degussa) D A G
Peak position v (cm-‘)
Bandwidth wliZ (cm-‘)
Intensity (peak area)
1369 1582
115 39
184 235
5.6
1364 1539 1603
105 162 67
480 78 275
2.5
1359 1548 1608
134 135 50
440 51 205
2.1
1371 1551 1604
174 165 77
314 64 145
2.0
1371 1559 1602
176 181 65
289 91 94
1.4
low order sp2 bonded carbons but cannot be considered as truly amorphous materials (such as evaporated amorphous carbons or amorphous hydrogenated carbons) since they still have some degree of order in the basal plane, L, being between 1.4 and 2.5 nm. The material R has a graphitic structure typical of polycrystalline graphite of relatively high crystalline degree (nearly no amorphous content was detected) with a lateral crystal size of the order of 5.6 nm. Acknowledgments-Dr E. Brillas is thanked for providing some of the samples. A. R. would like to thank the Direcci6 General de Recerca (Comissionat per a Universitats i Recerca) de la Generalitat de Catalunya and Carburos Metalicos S.A. for financial support, scholarship numbers CIRIT RE92/2 and CIRIT RE93/7.
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