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Raman validity for crystallite size La determination on reticulated vitreous carbon with different graphitization index
Applied Surface Science 254 (2007) 600–603 www.elsevier.com/locate/apsusc
Raman validity for crystallite size La determination on reticulated vitreous carbon with different graphitization index M.R. Baldan a, E.C. Almeida a, A.F. Azevedo a, E.S. Gonc¸alves b, M.C. Rezende b, N.G. Ferreira a,* a
LAS/INPE, 12.245-970, Instituto Nacional de Pesquisas Espaciais, Sa˜o Jose´ dos Campos, Sa˜o Paulo, Brazil b AMR/CTA, 12.228-904, Sa˜o Jose´ dos Campos, Brazil Received 23 May 2007; received in revised form 20 June 2007; accepted 20 June 2007 Available online 28 June 2007
Abstract The graphitization index provided by X-ray diffraction (XRD) and Raman spectrometry for reticulated vitreous carbon (RVC) substrates, carbonized at different heat treatment temperatures (HTT), is investigated. A systematic study of the dependence between the disorder-induced D and G Raman bands is presented. The crystallite size La was obtained for both X-ray diffraction and Raman spectrometry techniques. Particularly, the validity for La determination, from Raman spectra, is pointed out comparing the commonly used formula based on peaks amplitude ratio (ID/IG) and the recent proposed equation that uses the integrated intensities of D and G bands. The results discrepancy is discussed taken into account the strong contribution of the line broadening presented in carbon materials heat treated below 2000 8C. # 2007 Elsevier B.V. All rights reserved. Keywords: Carbon; Raman spectroscopy; X-ray diffraction
1. Introduction Reticulated vitreous carbon (RVC) is an important material known by its high mechanical resistance, porosity, biocompatibility and relatively high electric conductivity. The morphological and structural properties of such singular material have been extensively explored in the last decades, mainly due to their wide range of applications, for example, in thermal coating of airships, bony prostheses, heart valve and molecular sieves among others. The RVC samples were made from thermosetting resins used to impregnate polyurethane foams. The resin on the foam is hardened through curing reaction and may be carbonized at different heat treatment temperature (HTT). RCV is a kind of graphitic carbon that consists of stacked sheets with the carbons within the layers arranged in a two-dimensional network of regular hexagons. Graphitic carbon and other sp2 bonded amorphous carbons are strong Raman scatterers in spite of their intense optical absorption.
* Corresponding author. E-mail address: [email protected] (N.G. Ferreira). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.038
Raman spectroscopy has become one of the most used tools for the characterization of disordered polycrystalline graphitic carbons. Two Raman active modes are predicted, both vibrating in the plane of the sheets. One is a stretching of the individual sheet and the other is a shear mode of the two adjacent sheets in the unit cell. The Raman spectra show a pair of bands at 1357 and at 1580 cm1 that are the most diagnostic features, which designated by D and G band, respectively. The correlation between the ratio of the D and G band intensities (ID/IG) has been studied by many authors [1–5] but Tuinstra and Koenig (TK) [6] first report that utilization of Raman spectroscopy in carbon crystallinity and later Kinight and White (KW) [7] summarized the Raman spectra of various graphite systems and established the relationship between the crystallite width and the Raman intensity. For carbon materials, this relationship may be related linearly to the inverse of the crystallite size along the basal plane. It has been used to estimate the crystallite size La on the surface layer of different carbon materials. More recently, Canc¸ado et al. [8] have proposed the use of the integrated intensities of D and G Raman bands, introducing small corrections on KW equation [7], as better approximation for La determination. Their study took
M.R. Baldan et al. / Applied Surface Science 254 (2007) 600–603
into account, graphitized material with small disorder structures. On the other hand, the crystallite size La has also been obtained from the width of the X-ray diffraction (XRD) peaks for polycrystalline graphite, Scherrer’s formula [9]. These analyses have shown to be complementary and the La values seem to be in agreement for both techniques, depending on the carbon material studied and/or its graphitization index. However, there are no reports in the literature with a systematic and comparative study for La determination on RVC material as a function of its carbonization temperature. To discuss the results discrepancies and/or similarities using both equations from Raman analysis, a brief review of the carbon spectrum is justified. For crystalline graphite, the Raman spectrum presents only one peak which is called the G band, whereas for samples with some structural disorder that breaks the translational symmetry, an additional peak might be observed, which is called D band. The origin of D band is attributed to breakdown of the selection rule for optical phonos. Such breakdown permits new vibration modes such as A1g which were inactive in the infinite lattice. The A1g mode is attributed to particle size effect [10–12]. A very different explanation for the D band origin is the existence of specific vibrations at the edges. Katagiri et al. [13] have obtained the Raman spectrum of pure edge planes of graphite. Their edge plane spectrum presented the (ID/IG) higher than that of the basal plane that is originated from the discontinuity of the graphite plane. These results showed the influence of the edge plane for D band and suggested that the crystallite size may not be the only responsible for the D band appearance. Although there is not a clear consensus on its origin the reports concur that the D mode is related to structural disorder. For La determination, KW [7] examined the G bandwidth and the ID/IG intensity ratio for various graphitic carbons. Due to a strong dependence of the ratio (ID/IG) with the energy of the laser excitation, the KW final formula is valid only when the experiment is done with l = 514.5 nm. The variation with the energy laser excitation, will change the Raman sampling depth, possibly leading to variation in the spectrum if the carbon structure varies with depth [14,15]. Their general formula is given by: La ðnmÞ ¼ 4:4
ID IG
1 (1)
The first order Raman lines of RVC was used to calculate the ratio ID/IG. The bandwidth was taken as Lorentz distribution with full width at half maximum (FMHW) which depends on the degree of disorder. The first order Raman spectra is especially sensitive to the structural order with the carbon sheets whereas the second order Raman is related to the staking disorder along the crystallographic c-axis [7]. It is important to point out that in KW equation the contribution from band broadening to the measured intensity was not considered. On the other hand, Canc¸ado et al. [8] derived their general formula for any excitation laser energy. Their experimental analysis, using five excitation laser energies, showed that the La
601
crystallite size, obtained from the integrated intensity ratio I 0D =I 0G , is inversely proportional to (El)4, where El is the laser excitation energy. 1 560 I 0D La ðnmÞ ¼ 4 : (2) I 0G El It is important to consider that such formula is very desirable, since direct La measurement from X-ray has somehow a poor definition of the (1 0 0) band. The indirect measurement of La from D and G bands would be easy to provide once there is no overlap of the bands for the most disordered carbons [3]. The information provided by X-ray for crystallite size La, along basal plane, was obtained by evaluating the Scherrer’s formula given by [9] La ðnmÞ ¼
1:84l ; W 1=2 cos u
(3)
where l is the radiation wavelength 0.154 nm, u the position of the peak (1 0 0) and W1/2 is half width half maximum (HWHM) for RCV in 2u (rad) units for this peak. The bandwidth W1/2 is taken as Gaussian distribution. In the present work, La has been investigated for RVC samples by X-ray diffraction and Raman spectroscopy techniques. The samples were treated at different temperatures of 970, 1270, 1570, 1770, 2070 and 2270 K. Special emphasis was given for the comparison of La determination from the equations proposed by KW [7] and Canc¸ado et al. [8]. Particularly, the study of La validity was looked into for carbon materials with low graphitization index, such as RVC samples treated below 2270 K. 2. Experimental details RVC was obtained from polyurethane (PU) foam based on polyether impregnated with furfuryl resin. The resin curing process needed the use of p-toluenosulfonic acid as catalyst, at 3% (w/w). The complete curing occurred after 2 h at 360 and 400 K, assuring mechanical resistance to the material. The samples were submitted to a heat treatment process for obtaining RVC. They were heated at 1 K min1, under inert atmosphere with nitrogen flow of 1 L h1, reaching the maximum temperature of 970, 1270, 1570, 1770, 2070, 2270 K, holding at this maximum temperature for 30 min and then cooling down to room temperature. The furfuryl resin used in this work was a polymer that promotes denser final carbonized foam due to lower losses in volatile cures and carbonization process. This resin produces a more electrically conductive and resistant to flexion material than RVC obtained from phenolic resin. It is important to emphasize that carbonaceous material differ according to their chemical composition, types of heteroatoms, their structure and their micro texture. So, it is interesting to find methods for characterizing them. A Renishaw Microscope system 2270 in backscattering configuration at room temperature, using an argon laser at
602
M.R. Baldan et al. / Applied Surface Science 254 (2007) 600–603
To understand the dependence of RCV at different HTT with the crystallite size La it is important to keep in mind that the main objective of HTT is to promote graphitization process of the RVC structures leading to a better organization of graphene lamellas when analyzed by X-ray or Raman spectroscopy. The first and second order Raman spectra of all RVC samples versus their HTT were analyzed. The Raman spectrum of RVC-970 presented a very broad band. In the range of 970–1570 K the Raman spectra present two broad lines (D and G band) that characterize a strong crystalline disorder. The band broadening decreases as the HTT increases, whereas the band lines are narrow for RCV-2270. The line broadening provides information about the disorder within the carbon sheets. The HTT increase provides an improvement of graphitization index. But, even with the increase of HTT up to 2270 K, it is not enough to reach the first step of graphitization process. The results above give us support for analyzing the HTT influence using the three different equations presented. Fig. 1 compares the results for the three different Eqs. (1)–(3). The results show a good agreement between Eq. (1) (KW) and Xray data (Eq. (3)) for temperature higher than 1770 K. The value disagreements observed in Fig. 1 may be related to different contributions. Firstly, comparing X-ray and Raman measurements, the latter is a local technique that probes only the spot-size area with a penetration of about 5.0 mm from the surface. Due to low laser penetration depth, the information provided by Raman spectroscopy does not take into account the disorder contribution from the bulk, as X-ray measurements
that may probe around 500 mm, for carbon materials. Besides, the ratio ID/IG used in Eq. (1) takes into account only the pick height and does not, therefore, reflect the peak broadening and its influence on the calculation. In fact, for disordered material (lower La), the contribution of the edge graphite planes could give a significant contribution to D band, once a high number of crystallites are exposed to laser. Only a single crystal with all microcrystal perfectly oriented could be absent from the influence of the edge planes [11]. On the other hand, the X-ray beam is scattered by the crystalline part of the RVC. The X-ray spectra for these samples present broad lines that are taken as Gaussian distribution. The lack of a well-defined peak (1 0 0) may induce some discrepancies in the calculation of W1/2. This difficulty, added to the contribution of the edge planes, could explain some of the discrepancies in the results provided by Raman (TK) and X-ray for HTT lower than 1770 K. However, between 1770 and 2270 K the line bands are well defined and the influence of line broadening dispersion decreases with HTT increase. In case of a better definition of the disorder band (low dispersion), the information provided by Raman (Fig. 1) suggests that there is a simple relationship between both Raman and X-ray techniques (La). It is observed that HTT increase has a strong influence in the graphitization index and as a consequence a better definition of both D and G bands. The graphitization index, that relates features in the Raman spectrum with the structural arrangement, may also be identified by the bandwidth of second order phonon at 2970 cm1. Fig. 2 shows the RVC second order Raman spectra as a function of the HTT. The plot presented a clear decrease in the width at 2970 cm1 related as better ordering of lamellar structure. Even for HTT up to 2270 K, where the RVC did not reach the first stage of graphitization, Raman and X-ray started to provide comparable information about the La measurement, because these structures already presented this lamellar ordering. However, the discrepancy of the results provided by Eq. (2) may arise from the fact that Canc¸ado et al. [8]
Fig. 1. La: (&) TK relation (generalized by KW) (Eq. (1)), (~) calculated using general formula (Eq. (2)) and (*) X-ray results using Sherrer’s formula (Eq. (3)).
Fig. 2. Second order Raman spectroscopy as a function HTT.
514.5 nm (El = 2.41 eV) as excitation source, was used to obtain micro-Raman spectra. This powerful and sensible technique permits an analysis of defects, especially the ones associated to graphitic phases. X-ray diffraction measurements were performed from a diffractometer Phillips, PW1210/W/ 380/80. 3. Results and discussion
M.R. Baldan et al. / Applied Surface Science 254 (2007) 600–603
603
Mainly, for RVC structures, it is very important to address that KW [7] formula should be used as a first approximation to calculate La and for a systematic analysis of the crystallite size is highly advisable to use X-ray measurement. 4. Conclusions
Fig. 3. FWHM (vD/vG) and pilling up width L0 0 2 (nm) as a function of HTT.
considered the integrated intensities of the D and G band instead of using the ratio of the peak amplitudes, in other words, the effect of line broadening is included in their calculation. As a consequence, the proportionality constant is higher than that reported by KW [7]. Besides, Canc¸ado et al. [8] have used samples of diamond-like carbon (DLC) films treated between 2070 and 2970 K. Their samples showed no sp3 phases after heat treatment and their X-ray analysis showed symmetric peaks with narrow distribution that is characteristic of better ordering. The same behavior was obtained from the Raman spectroscopy, where the peaks broadening would not represent a strong influence in their results. It is also important to point out that RVC is a disordered kind of carbon and the La must be measured on broad band distribution which may be of questionable accuracy. Essentially, due to these differences in the techniques the La calculated are different. So, it is clear that HTT has a strong influence in the determination of La. For the RVC samples used in this work, with HTT up to 2270 K, the general formula proposed in Eq. (2) showed not to be appropriate while the KW [7] equation remains valid and in good agreement with X-ray results for HTT above 1570 K, taking into account the experimental errors. This temperature is also significant in Raman response because of the hydrogen and oxygen (heteroatoms) release. These heteroatoms, originated from precursor, represent a contribution in surface interaction process that strongly influences the Raman peaks broadening up to 1570 K. Fig. 3 shows a plot of the relative FWHM (vD/vG) and pilling up L0 0 2 as a function of HTT. It is observed the strong influence of HTT in the accommodation of the lamellas in such structure, which is associated to its graphitization index, and as a consequence, an increase of L0 0 2. This behavior was also observed in previous work Gonc¸alvez et al. [16]. They work showed a significant decrease of intralamellar distance at RVC for HTT higher than 1570 K. Apart from the measurable consideration, the results provided by all the equation are quite representatives. The good agreement obtained from Eqs. (1) and (3) may be justified in terms of the graphitization index as a function of HTT.
A systematic analysis to determine the crystallite size La was presented for RVC treated at different HTT. X-ray diffraction results were compared with the values calculated from Raman spectra using Eqs. (1) and (2) to verify their validity. The results showed that Eq. (2) is not a general formula for La determination for RVC treated up to 2270 K compared to Xray results. However, the KW [7] formula remains valid and shows a good agreement with X-ray results for samples treated at HTT higher than 1570 K. It may be concluded that there is an evolution of RVC sample structures, throughout heat treatments, where it is possible to progressively remove different kinds of defects. These defects are heteroatoms, tetrahedral carbons, etc. The elimination of theses defects permits the rearrangement of the polyaromatic structures and the establishment of an organization with a better accommodation of the lamellas in the RVC structure, which is associated to its graphitization index. The good agreement of the results in the range of 1770–2270 K may be associated to the graphitization index. The L0 0 2 increase and vD/vG decrease as a function of HTT show a better orientation level of lamellar structure. It is important to remember that graphite could present different orientation levels, depending on their origin, and that could be the source of their different ID/IG. In summary it is highly advisable to use X-ray diffraction to calculate La. In addition, Eqs. (1) and (2) may be used with special care. References [1] C. Beny-Bassez, J.N. Rouzaud, Scan. Elect. Microsc. I (1985) 119. [2] Y.J. Lee, J. Nucl. Mater. 325 (2004) 174. [3] A. Cuestra, P. Dhamelincourt, J. Laureyns, A. Martinez-Alonso, J.M.D. Tasco´n, J. Mater. Chem. 8 (1998) 2875. [4] T. Gruber, T.M. Zerda, M. Gerspacher, Carbon 32 (1994) 1377. [5] G.A. Zickler, B. Smarsly, N. Gierlinger, H. Peterlik, O. Paris, Carbon 44 (2006). [6] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [7] D.S. Kinight, W.B. White, J. Mater. Res. 4 (1989) 385. [8] L.G. Canc¸ado, K. Takai, T. Enoki, M. Endo, Y.A. Kim, H. Mizusaki, A. Jorio, L.N. Coelho, R. Magalha˜es-Paniago, M.A. Pimenta, Appl. Phys. Lett. 88 (2006) 163106. [9] G.M. Jenkins, K. Kwamura, Polymeric Carbon—Carbon Fibre, Glass and Char, Cambridge University Press, 1976. [10] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [11] M.J. Matthews, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, M. Endo, Phys. Rev. B 59 (1999) R6585. [12] S. Reich, C. Thomsen, Philos. Trans. R. Soc. Lond. A 362 (2004) 2271. [13] G. Katagiri, H. Ishida, A. Ishitani, Carbon 26 (1988) 565. [14] Y. Wang, D.C. Alsmeyer, R.L. McCreery, Chem. Mater. 2 (1990) 557. [15] T.P. Mernagh, R.P. Cooney, R.A. Johnson, Carbon 22 (1982) 39. [16] E.S. Gonc¸alvez, M.C. Rezende, N.G. Ferreira, Bras. J. Phys. 36 (2A) (2006) 264.
Raman validity for crystallite size La determination on reticulated vitreous carbon with different graphitization index M.R. Baldan a, E.C. Almeida a, A.F. Azevedo a, E.S. Gonc¸alves b, M.C. Rezende b, N.G. Ferreira a,* a
LAS/INPE, 12.245-970, Instituto Nacional de Pesquisas Espaciais, Sa˜o Jose´ dos Campos, Sa˜o Paulo, Brazil b AMR/CTA, 12.228-904, Sa˜o Jose´ dos Campos, Brazil Received 23 May 2007; received in revised form 20 June 2007; accepted 20 June 2007 Available online 28 June 2007
Abstract The graphitization index provided by X-ray diffraction (XRD) and Raman spectrometry for reticulated vitreous carbon (RVC) substrates, carbonized at different heat treatment temperatures (HTT), is investigated. A systematic study of the dependence between the disorder-induced D and G Raman bands is presented. The crystallite size La was obtained for both X-ray diffraction and Raman spectrometry techniques. Particularly, the validity for La determination, from Raman spectra, is pointed out comparing the commonly used formula based on peaks amplitude ratio (ID/IG) and the recent proposed equation that uses the integrated intensities of D and G bands. The results discrepancy is discussed taken into account the strong contribution of the line broadening presented in carbon materials heat treated below 2000 8C. # 2007 Elsevier B.V. All rights reserved. Keywords: Carbon; Raman spectroscopy; X-ray diffraction
1. Introduction Reticulated vitreous carbon (RVC) is an important material known by its high mechanical resistance, porosity, biocompatibility and relatively high electric conductivity. The morphological and structural properties of such singular material have been extensively explored in the last decades, mainly due to their wide range of applications, for example, in thermal coating of airships, bony prostheses, heart valve and molecular sieves among others. The RVC samples were made from thermosetting resins used to impregnate polyurethane foams. The resin on the foam is hardened through curing reaction and may be carbonized at different heat treatment temperature (HTT). RCV is a kind of graphitic carbon that consists of stacked sheets with the carbons within the layers arranged in a two-dimensional network of regular hexagons. Graphitic carbon and other sp2 bonded amorphous carbons are strong Raman scatterers in spite of their intense optical absorption.
* Corresponding author. E-mail address: [email protected] (N.G. Ferreira). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.038
Raman spectroscopy has become one of the most used tools for the characterization of disordered polycrystalline graphitic carbons. Two Raman active modes are predicted, both vibrating in the plane of the sheets. One is a stretching of the individual sheet and the other is a shear mode of the two adjacent sheets in the unit cell. The Raman spectra show a pair of bands at 1357 and at 1580 cm1 that are the most diagnostic features, which designated by D and G band, respectively. The correlation between the ratio of the D and G band intensities (ID/IG) has been studied by many authors [1–5] but Tuinstra and Koenig (TK) [6] first report that utilization of Raman spectroscopy in carbon crystallinity and later Kinight and White (KW) [7] summarized the Raman spectra of various graphite systems and established the relationship between the crystallite width and the Raman intensity. For carbon materials, this relationship may be related linearly to the inverse of the crystallite size along the basal plane. It has been used to estimate the crystallite size La on the surface layer of different carbon materials. More recently, Canc¸ado et al. [8] have proposed the use of the integrated intensities of D and G Raman bands, introducing small corrections on KW equation [7], as better approximation for La determination. Their study took
M.R. Baldan et al. / Applied Surface Science 254 (2007) 600–603
into account, graphitized material with small disorder structures. On the other hand, the crystallite size La has also been obtained from the width of the X-ray diffraction (XRD) peaks for polycrystalline graphite, Scherrer’s formula [9]. These analyses have shown to be complementary and the La values seem to be in agreement for both techniques, depending on the carbon material studied and/or its graphitization index. However, there are no reports in the literature with a systematic and comparative study for La determination on RVC material as a function of its carbonization temperature. To discuss the results discrepancies and/or similarities using both equations from Raman analysis, a brief review of the carbon spectrum is justified. For crystalline graphite, the Raman spectrum presents only one peak which is called the G band, whereas for samples with some structural disorder that breaks the translational symmetry, an additional peak might be observed, which is called D band. The origin of D band is attributed to breakdown of the selection rule for optical phonos. Such breakdown permits new vibration modes such as A1g which were inactive in the infinite lattice. The A1g mode is attributed to particle size effect [10–12]. A very different explanation for the D band origin is the existence of specific vibrations at the edges. Katagiri et al. [13] have obtained the Raman spectrum of pure edge planes of graphite. Their edge plane spectrum presented the (ID/IG) higher than that of the basal plane that is originated from the discontinuity of the graphite plane. These results showed the influence of the edge plane for D band and suggested that the crystallite size may not be the only responsible for the D band appearance. Although there is not a clear consensus on its origin the reports concur that the D mode is related to structural disorder. For La determination, KW [7] examined the G bandwidth and the ID/IG intensity ratio for various graphitic carbons. Due to a strong dependence of the ratio (ID/IG) with the energy of the laser excitation, the KW final formula is valid only when the experiment is done with l = 514.5 nm. The variation with the energy laser excitation, will change the Raman sampling depth, possibly leading to variation in the spectrum if the carbon structure varies with depth [14,15]. Their general formula is given by: La ðnmÞ ¼ 4:4
ID IG
1 (1)
The first order Raman lines of RVC was used to calculate the ratio ID/IG. The bandwidth was taken as Lorentz distribution with full width at half maximum (FMHW) which depends on the degree of disorder. The first order Raman spectra is especially sensitive to the structural order with the carbon sheets whereas the second order Raman is related to the staking disorder along the crystallographic c-axis [7]. It is important to point out that in KW equation the contribution from band broadening to the measured intensity was not considered. On the other hand, Canc¸ado et al. [8] derived their general formula for any excitation laser energy. Their experimental analysis, using five excitation laser energies, showed that the La
601
crystallite size, obtained from the integrated intensity ratio I 0D =I 0G , is inversely proportional to (El)4, where El is the laser excitation energy. 1 560 I 0D La ðnmÞ ¼ 4 : (2) I 0G El It is important to consider that such formula is very desirable, since direct La measurement from X-ray has somehow a poor definition of the (1 0 0) band. The indirect measurement of La from D and G bands would be easy to provide once there is no overlap of the bands for the most disordered carbons [3]. The information provided by X-ray for crystallite size La, along basal plane, was obtained by evaluating the Scherrer’s formula given by [9] La ðnmÞ ¼
1:84l ; W 1=2 cos u
(3)
where l is the radiation wavelength 0.154 nm, u the position of the peak (1 0 0) and W1/2 is half width half maximum (HWHM) for RCV in 2u (rad) units for this peak. The bandwidth W1/2 is taken as Gaussian distribution. In the present work, La has been investigated for RVC samples by X-ray diffraction and Raman spectroscopy techniques. The samples were treated at different temperatures of 970, 1270, 1570, 1770, 2070 and 2270 K. Special emphasis was given for the comparison of La determination from the equations proposed by KW [7] and Canc¸ado et al. [8]. Particularly, the study of La validity was looked into for carbon materials with low graphitization index, such as RVC samples treated below 2270 K. 2. Experimental details RVC was obtained from polyurethane (PU) foam based on polyether impregnated with furfuryl resin. The resin curing process needed the use of p-toluenosulfonic acid as catalyst, at 3% (w/w). The complete curing occurred after 2 h at 360 and 400 K, assuring mechanical resistance to the material. The samples were submitted to a heat treatment process for obtaining RVC. They were heated at 1 K min1, under inert atmosphere with nitrogen flow of 1 L h1, reaching the maximum temperature of 970, 1270, 1570, 1770, 2070, 2270 K, holding at this maximum temperature for 30 min and then cooling down to room temperature. The furfuryl resin used in this work was a polymer that promotes denser final carbonized foam due to lower losses in volatile cures and carbonization process. This resin produces a more electrically conductive and resistant to flexion material than RVC obtained from phenolic resin. It is important to emphasize that carbonaceous material differ according to their chemical composition, types of heteroatoms, their structure and their micro texture. So, it is interesting to find methods for characterizing them. A Renishaw Microscope system 2270 in backscattering configuration at room temperature, using an argon laser at
602
M.R. Baldan et al. / Applied Surface Science 254 (2007) 600–603
To understand the dependence of RCV at different HTT with the crystallite size La it is important to keep in mind that the main objective of HTT is to promote graphitization process of the RVC structures leading to a better organization of graphene lamellas when analyzed by X-ray or Raman spectroscopy. The first and second order Raman spectra of all RVC samples versus their HTT were analyzed. The Raman spectrum of RVC-970 presented a very broad band. In the range of 970–1570 K the Raman spectra present two broad lines (D and G band) that characterize a strong crystalline disorder. The band broadening decreases as the HTT increases, whereas the band lines are narrow for RCV-2270. The line broadening provides information about the disorder within the carbon sheets. The HTT increase provides an improvement of graphitization index. But, even with the increase of HTT up to 2270 K, it is not enough to reach the first step of graphitization process. The results above give us support for analyzing the HTT influence using the three different equations presented. Fig. 1 compares the results for the three different Eqs. (1)–(3). The results show a good agreement between Eq. (1) (KW) and Xray data (Eq. (3)) for temperature higher than 1770 K. The value disagreements observed in Fig. 1 may be related to different contributions. Firstly, comparing X-ray and Raman measurements, the latter is a local technique that probes only the spot-size area with a penetration of about 5.0 mm from the surface. Due to low laser penetration depth, the information provided by Raman spectroscopy does not take into account the disorder contribution from the bulk, as X-ray measurements
that may probe around 500 mm, for carbon materials. Besides, the ratio ID/IG used in Eq. (1) takes into account only the pick height and does not, therefore, reflect the peak broadening and its influence on the calculation. In fact, for disordered material (lower La), the contribution of the edge graphite planes could give a significant contribution to D band, once a high number of crystallites are exposed to laser. Only a single crystal with all microcrystal perfectly oriented could be absent from the influence of the edge planes [11]. On the other hand, the X-ray beam is scattered by the crystalline part of the RVC. The X-ray spectra for these samples present broad lines that are taken as Gaussian distribution. The lack of a well-defined peak (1 0 0) may induce some discrepancies in the calculation of W1/2. This difficulty, added to the contribution of the edge planes, could explain some of the discrepancies in the results provided by Raman (TK) and X-ray for HTT lower than 1770 K. However, between 1770 and 2270 K the line bands are well defined and the influence of line broadening dispersion decreases with HTT increase. In case of a better definition of the disorder band (low dispersion), the information provided by Raman (Fig. 1) suggests that there is a simple relationship between both Raman and X-ray techniques (La). It is observed that HTT increase has a strong influence in the graphitization index and as a consequence a better definition of both D and G bands. The graphitization index, that relates features in the Raman spectrum with the structural arrangement, may also be identified by the bandwidth of second order phonon at 2970 cm1. Fig. 2 shows the RVC second order Raman spectra as a function of the HTT. The plot presented a clear decrease in the width at 2970 cm1 related as better ordering of lamellar structure. Even for HTT up to 2270 K, where the RVC did not reach the first stage of graphitization, Raman and X-ray started to provide comparable information about the La measurement, because these structures already presented this lamellar ordering. However, the discrepancy of the results provided by Eq. (2) may arise from the fact that Canc¸ado et al. [8]
Fig. 1. La: (&) TK relation (generalized by KW) (Eq. (1)), (~) calculated using general formula (Eq. (2)) and (*) X-ray results using Sherrer’s formula (Eq. (3)).
Fig. 2. Second order Raman spectroscopy as a function HTT.
514.5 nm (El = 2.41 eV) as excitation source, was used to obtain micro-Raman spectra. This powerful and sensible technique permits an analysis of defects, especially the ones associated to graphitic phases. X-ray diffraction measurements were performed from a diffractometer Phillips, PW1210/W/ 380/80. 3. Results and discussion
M.R. Baldan et al. / Applied Surface Science 254 (2007) 600–603
603
Mainly, for RVC structures, it is very important to address that KW [7] formula should be used as a first approximation to calculate La and for a systematic analysis of the crystallite size is highly advisable to use X-ray measurement. 4. Conclusions
Fig. 3. FWHM (vD/vG) and pilling up width L0 0 2 (nm) as a function of HTT.
considered the integrated intensities of the D and G band instead of using the ratio of the peak amplitudes, in other words, the effect of line broadening is included in their calculation. As a consequence, the proportionality constant is higher than that reported by KW [7]. Besides, Canc¸ado et al. [8] have used samples of diamond-like carbon (DLC) films treated between 2070 and 2970 K. Their samples showed no sp3 phases after heat treatment and their X-ray analysis showed symmetric peaks with narrow distribution that is characteristic of better ordering. The same behavior was obtained from the Raman spectroscopy, where the peaks broadening would not represent a strong influence in their results. It is also important to point out that RVC is a disordered kind of carbon and the La must be measured on broad band distribution which may be of questionable accuracy. Essentially, due to these differences in the techniques the La calculated are different. So, it is clear that HTT has a strong influence in the determination of La. For the RVC samples used in this work, with HTT up to 2270 K, the general formula proposed in Eq. (2) showed not to be appropriate while the KW [7] equation remains valid and in good agreement with X-ray results for HTT above 1570 K, taking into account the experimental errors. This temperature is also significant in Raman response because of the hydrogen and oxygen (heteroatoms) release. These heteroatoms, originated from precursor, represent a contribution in surface interaction process that strongly influences the Raman peaks broadening up to 1570 K. Fig. 3 shows a plot of the relative FWHM (vD/vG) and pilling up L0 0 2 as a function of HTT. It is observed the strong influence of HTT in the accommodation of the lamellas in such structure, which is associated to its graphitization index, and as a consequence, an increase of L0 0 2. This behavior was also observed in previous work Gonc¸alvez et al. [16]. They work showed a significant decrease of intralamellar distance at RVC for HTT higher than 1570 K. Apart from the measurable consideration, the results provided by all the equation are quite representatives. The good agreement obtained from Eqs. (1) and (3) may be justified in terms of the graphitization index as a function of HTT.
A systematic analysis to determine the crystallite size La was presented for RVC treated at different HTT. X-ray diffraction results were compared with the values calculated from Raman spectra using Eqs. (1) and (2) to verify their validity. The results showed that Eq. (2) is not a general formula for La determination for RVC treated up to 2270 K compared to Xray results. However, the KW [7] formula remains valid and shows a good agreement with X-ray results for samples treated at HTT higher than 1570 K. It may be concluded that there is an evolution of RVC sample structures, throughout heat treatments, where it is possible to progressively remove different kinds of defects. These defects are heteroatoms, tetrahedral carbons, etc. The elimination of theses defects permits the rearrangement of the polyaromatic structures and the establishment of an organization with a better accommodation of the lamellas in the RVC structure, which is associated to its graphitization index. The good agreement of the results in the range of 1770–2270 K may be associated to the graphitization index. The L0 0 2 increase and vD/vG decrease as a function of HTT show a better orientation level of lamellar structure. It is important to remember that graphite could present different orientation levels, depending on their origin, and that could be the source of their different ID/IG. In summary it is highly advisable to use X-ray diffraction to calculate La. In addition, Eqs. (1) and (2) may be used with special care. References [1] C. Beny-Bassez, J.N. Rouzaud, Scan. Elect. Microsc. I (1985) 119. [2] Y.J. Lee, J. Nucl. Mater. 325 (2004) 174. [3] A. Cuestra, P. Dhamelincourt, J. Laureyns, A. Martinez-Alonso, J.M.D. Tasco´n, J. Mater. Chem. 8 (1998) 2875. [4] T. Gruber, T.M. Zerda, M. Gerspacher, Carbon 32 (1994) 1377. [5] G.A. Zickler, B. Smarsly, N. Gierlinger, H. Peterlik, O. Paris, Carbon 44 (2006). [6] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [7] D.S. Kinight, W.B. White, J. Mater. Res. 4 (1989) 385. [8] L.G. Canc¸ado, K. Takai, T. Enoki, M. Endo, Y.A. Kim, H. Mizusaki, A. Jorio, L.N. Coelho, R. Magalha˜es-Paniago, M.A. Pimenta, Appl. Phys. Lett. 88 (2006) 163106. [9] G.M. Jenkins, K. Kwamura, Polymeric Carbon—Carbon Fibre, Glass and Char, Cambridge University Press, 1976. [10] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [11] M.J. Matthews, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, M. Endo, Phys. Rev. B 59 (1999) R6585. [12] S. Reich, C. Thomsen, Philos. Trans. R. Soc. Lond. A 362 (2004) 2271. [13] G. Katagiri, H. Ishida, A. Ishitani, Carbon 26 (1988) 565. [14] Y. Wang, D.C. Alsmeyer, R.L. McCreery, Chem. Mater. 2 (1990) 557. [15] T.P. Mernagh, R.P. Cooney, R.A. Johnson, Carbon 22 (1982) 39. [16] E.S. Gonc¸alvez, M.C. Rezende, N.G. Ferreira, Bras. J. Phys. 36 (2A) (2006) 264.