STRUCTURAL INFORMATION FROM THE RAMAN SPECTRA OFACTIVATED CARBON MATERIALS

N. Shimodaira, A.Masui, A.Takada, YShinozaki, and N. TomitaResearch Center, Asahi Glass Co. Ltd.

1150 Hazawa-cho, Kanagawa-ku, Yokohama, 221-8755, Japan

Introduction

Raman spectrometry, is one of the most useful characteriz-ing methods for carbon materials, because the spectral shapedrastically changes not only due to the kind of abundant allo-tropic forms of carbon, but to the fine structural changes ofthe individual allotrope. For example, in polycrystallinegraphites, two sharp peaks appear, G-band around 1580cm-1

and D-band around 1355 cm-1, which are generally ascribedto E2g and Aig in-plane vibration modes, respectively. It is wellknown that the intensity ratio of D to G-band (Id/Ig) is corre-lated with the reciprocal of the crystallite size along basalplane (1/La) measured from XRD. [1-4]

In sputtered or evaporated amorphous carbon films (a-C)regardless of hydrogenated or not, a relatively broad bandaround 1550 cm-1 overlapped with a broader band around1400 cm-1 is observed. The Raman spectra are usefullydeconvoluted into two peaks by using gaussian line shapes,which are also called G- and D-band in a-C. The variation inthe position, the width, and the intensity ratio (Id/Ig) has beenoften examined as a function of deposition conditions andsome properties measured from other techniques. Especially,the intensity ratio (Id/Ig) 1s used as the most useful parameterindicating the sp2 cluster size, or the sp3 to sp2 bonding ratioin hydrogenated amorphous carbons. [ 5-11 ]

As described above, the Raman spectra of crystalline graphitesand a-Cs have been studied in details so far, and the relation-ship between the spectral shape and the structure has beenreported theoretically and qualitatively. However, in othernon graphitic carbons such as activated carbons and carbonblacks, despite having the sp2 bonding as a-C or graphite, theRaman spectra haven’t been sufficiently understoodrelationally to the structure.

In this paper, we report a novel characterizing method of theRaman spectra of activated carbon materials by curve fittingtechnique, and propose an interpretation on the microstruc-ture. We also report the structural change before and afteractivation and the difference of the effect of activation on thestructure between by steam and by alkali.

Experimental

Total 29 commercially available activated carbon powders

were prepared. Among them, 16 samples were activatedmainly by steam from non-graphitizable carbons, and 13samples were activated mainly by alkali from graphitizablecarbons. One of the former samples activated by steam froma carbonized phenol resin was annealed at various tempera-tures ranging from 1273 to 1673 K for 3 hours in argon. Asreferences, three types of amorphous carbon films sputteredon silicon at different H2/Ar flow ratio at 6 mTorr, and 16commercially available crystalline graphite powders, wereprepared. All the a-C samples were also annealed at varioustemperatures ranging from 423 to 573 K for 1 hour in argon.

In order to study the structural change before and after acti-vation, a phenol resin and a pitch carbonized at 1173 K,were activated by steam at 1123 K and by KOH at 1023 K,respectively.

Ramah spectra were obtained with Raman or T64000 (Jobin/Yvon) using a 514.5 nm Ar line as an excitation source. Thelaser was focused to about 100 µm in diameter at a power ofless than 5 mW at the sample surface in order to preventthermal degradation of the carbons. The spectra over therange from 800 to 2000 cm-1 at about 0.5 cm-1 intervals weremeasured in a backscattering geometry with a 64 cm singlemonochromater, and detected by 16 bit CCD camera with1024ch. Because of the low signal to noise, 5 to 10 scanswere averaged.

Fitting analysis with four Gaussian curves on a linear back-ground was conducted on all the Raman spectra in the re-gion from 800 to 2000 cm-1 .Raman spectra of a-Cs werealso deconvoluted to G and D-bands by using two Gaussiansand a straight line as the background.

X-ray diffraction was performed with a Rigaku RINT2500with CuKα radiation at 40 kV and 200 mA. Microstructureswere observed with a high resolution transmission electronmicroscope (HITACHI H9000) operating at 300 kV.

Results and Discussion

The examples of Raman spectra with fitting results of theactivated carbon samples are represented in Figure 1, (a) fora non-graphitizable carbon and (b) for a graphitizable car-bon, respectively. Note that this fit using four Gaussians faith-fully reproduces the experimental data, and that two rela-

tively sharp peaks at about 1600 cm-1 and about 1350 cm-1,and two relatively broad peaks around 1560 cm-1 and around1340 cm-1, namely G1, D1, G2 and D2, are observed in boththe samples. All Raman spectra examined in this studyshowed excellent curve-fit (accuracy factor ~0.99) withoutexception.

The behavior of each deconvoluted peak in all the samplesbecomes clear by plotting the Raman shift and the full widthat half maximum (FWHM) in two dimensionally, as shownin Figure 2. Focusing on the annealed samples (closed circle),each peak regularly moves on the 2D-plot space with theincrease of temperature as indicated by arrows. It is quitenotable that the plot points of each peak in all the activatedcarbon samples (closed square) concentrate on almost thesame regions as those in the annealed samples, though thepoints for D2 peak are widely scattered.

In Fig.2, the fit results of a-C films (plus) and polycrystal-line graphites (x) are also shown. All the positions of G1and D1 peaks are almost constant and close to those of Gand D-band of the graphites, respectively. On the other hand,G2 and D2 points disperse in the close regions to G and D-band of a-Cs, respectively, and the upward shift of the posi-tion and the decrease of the FWHM in G2 peak, as annealtemperature increases, is in accord with that in G-band of a-C. This trend is similar to the observation in a-C films re-ported by others [5]. These features give the view that theactivated carbon materials may be composed of both thegraphite crystal and amorphous structure, and that from eachcomponent G-and D-band may appear as G1 and D1 peaks,and G2 and D2 peaks, respectively.

However, XRO diffraction patterns showed no sharp line inany samples (Fig.3). High resolution TEM microstructuresshowed no crystal lattice image (Fig.4), and only winding(002) basal planes (black contrast), which are extremelyunclear in a-C, can be homogeneously observed in thesamples. It is apparent that the crystal grain structure likegraphite does not exist in the activated carbon materials.

We propose that the basic structure of the activated carbonmaterials consists of six-fold aromatic rings having sp2

bondings and includes both bond-angle disorder networksas given in Beeman’s model [12], and order networks, andthat the G and D-band from each the honeycomb networkstructure are exclusively observed in the Raman spectra.Because of the positions, G1 and D1 peaks are respectivelyascribed to E2g and Alg in-plane vibration modes from theorder structure in the sp2 clusters, not from crystal structurelike graphite. On the other hand, the bond-angle disorderstructure behaves like a-C and is the origin of G2 and D2peaks. The upward shift of G2 peak and the decrease ofFWHM of G2 and D2 peaks, as anneal temperature increases,

are due to the removal of bond-angle disorder, as indicatedby R.O.Oillon in a-C film [5].

The peak intensity ratios, ID1/IG1, ID2/IG2, and IG2/IG1, are plot-ted as a function of anneal temperature in Fig. 5. The de-crease of IG2/IG1 with increasing temperature indicates thedecline of disorder structure due to thermal relaxation. Otherpeak ratios, ID1/IG1 and ID2/IG2, don’t clearly show the depen-dence on anneal temperature, probably because the struc-tural transition from disorder to order complicate the sizeeffect of each structure on the Raman spectral shape.

Consequently, it is possible that the Raman spectral shapeof activated carbon materials sensitively changes with thecontent of the disorder honeycomb structure (IG2/IG1) andthe degree of the thermal relaxation. Since the G2 peak po-sition is the most convenient parameter expressing the de-gree of the relaxation, the peak ratios in all the activatedcarbon samples are plotted against it in Fig. 6. It is reason-able that the trend of the decrease of IG2/IG1 with the increaseof the G2 peak position is observed. On the whole, the ID2/IG2 tends to increase with the increase of the G2 peak posi-tion as indicated in a-C films [5, 8, 9], while the ID1/IG1 doesn’tchange so much around 1.0. This suggests that the variety ofthe Raman spectral shape in activated carbon materials ismainly dominated by the disorder structure.

In Fig. 7, the fitted Raman curves before and after activationare represented, (a) for a carbonized phenol resin activatedby steam and (b) for a carbonized pitch activated by KOH.The deconvoluted peak parameters including the peak ra-tios are given in Table. It is worth noting that the change ofthe peak parameters as well as the spectral shape accompa-nied by activation proceeds just in opposite direction be-tween by steam and by KOH. That is, it is concluded on thebasis of the above interpretation that the activation by steamhas a removal effect of the disorder structure, while the acti-vation by KOH induces the disorder structure.

Conclusion

The structure of activated carbon materials is characterizedby using oaussian peaks, G1, G2, D1, and D2, fitted forRaman spectra. In 20-plot of the Raman shift vs. FWHM,the distributed regions of each peak are mutually exclusive,and the G1 position is stable to heat treatment, while G2,D1, and D2 peaks are very sensitive. From the behavior ofthese peaks and the results of XRD and TEM observation, itis suggested that G1 and D1 peaks are due to the order struc-ture of honeycomb networks, while G2 and D2 peaks aredue to the disorder structure with bond-angle distortion inthe networks. The peak ratio, IG2/IG1, can be used as a usefulparameter expressing the content of disorder structure. Thestructural change before and after activation was also exam-

ined by this technique. It is concluded on the basis of theabove interpretation that the activation by steam has a re-moval effect of the disorder structure, while the activationby KOH induces the disorder structure.

References

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Figure 3. XRD patterns of the activated carbons shown in Fig. 1.

Figure 1. Raman spectra with fitting results; (a) a non-graphitizable carbon activated by steam,(b) a graphitizable carbon activated by alkali.

Figure 2. 2D-Plots of decomvoluted peak parameters: peak position vs. FWHM.

Figure 7. Raman spectra (fitted curve) before and after activation; (a) by steam and (b)by KOH.

Figure 4. High resolution TEM images of the activated carbons shown in Fig. 1.

Figure 5. Peak ratios as a function of anneal temperature; (a) ID1/IG1, (b) ID2/IG2, and (c) IG2/IG1.

Figure 6. Peak ratios as a function of G2 peak position; ID1/IG1, (b) ID2/IG2, and (c) IG2/IG1.

Table. Fitting parameters (v: positions, W: widths) of the Raman spectra in Fig.7.