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X-ray absorption fine structure study on residue bromine in carbons with different degrees of graphitization
Carbon 41 (2003) 2931–2938
X-ray absorption fine structure study on residue bromine in carbons with different degrees of graphitization Hidetaka Yoshikawa a , Katsuya Fukuyama a , Yoichiro Nakahara a , Takehisa Konishi b , Nobuyuki Ichikuni c , Yasuko Yoshikawa d , Noboru Akuzawa e , Yoichi Takahashi d , Keiko Nishikawa a , * a
Graduate School of Science and Technology, Chiba University, Yayoi, Inage-ku, Chiba 263 -8522, Japan b Faculty of Science, Chiba University, Yayoi, Inage-ku, Chiba 263 -8522, Japan c Faculty of Engineering, Chiba University, Yayoi, Inage-ku, Chiba 263 -8522, Japan d Department of Applied Chemistry, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112 -8551, Japan e Department of Chemical Science and Engineering, Tokyo National College of Technology, Kunugidamachi, Hachioji-shi, Tokyo 193 -0997, Japan Received 20 March 2003; accepted 5 August 2003
Abstract The structure of bromine residue compounds was investigated by X-ray absorption fine structure (XAFS) in order to interpret where and how bromine is present in carbons with different degrees of graphitization. The residue compounds can be classified into three groups, as obtained from X-ray absorption near edge structure (XANES) spectra and the values of the intramolecular distance r Br – Br determined by extended X-ray absorption fine structure (EXAFS). In Group I, prepared from the host carbons heat treated at temperatures higher than 1900 8C, bromine exists in the interlayer space of graphite in the form of Br 2 molecules with interaction of the p electrons of graphite. In Group III, from carbon heat treated at 1000 8C, most of the bromine probably reacts with carbon atoms having a dangling bond or functional groups. For Group II, where the host carbons are heat treated at intermediate temperatures, it is likely that bromine exists in undeveloped defects with a unique electronic state. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Intercalation compounds; B. Graphitization, Intercalation; D. Chemical structure
1. Introduction Graphite intercalation compounds (GICs) with bromine have been studied extensively because of their ease of synthesis [1–3] and special physical and chemical properties [4]. In particular, the high electrical conductivity and the unusual stability of the residue compounds has attracted much attention from various fields of technology as well as basic science [5]. For the bromine GICs including the residue compounds, even when they are prepared from graphite or highly graphitized carbons, their structures are very complex, because there are several phases present in relation to the arrangement of bromine in the graphite *Corresponding author. Fax: 181-43-290-3939. E-mail address: [email protected] (K. Nishikawa).
layers, depending on the preparation conditions and the concentration of doped bromine [6,7]. Namely, there is complex disorder in the layer structure, or two-dimensional phase transitions in the arrangement of the bromine molecules [8,9], etc. Their structures have been discussed from various viewpoints for various structure determination experiments, such as X-ray diffraction for a single crystal [10], Raman spectroscopy [11,12] and X-ray absorption fine structure (XAFS) [13–16]. On the other hand, there have only been a few studies on the bromine–carbon system with emphasis on the effect of the degree of graphitization of the host carbon [17–19]. From this standpoint, some of the present authors have recently made a systematic investigation of the behavior of bromine doping and de-bromination by wide-angle X-ray diffraction, Raman spectroscopy focusing on the G and D bands, and electrical conductivity experiments [20]. From
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00400-7
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the results, the carbon materials can be classified into at least two groups with respect to their behavior with bromine doping and de-bromination depending on the heat-treatment temperature (HTT). The boundary of the two groups lies at a HTT of 2000 8C. The carbons of the higher-HTT group form intercalation compounds with compositions up to C 10 Br, and a considerable fraction (13–25%) of bromine remains after the de-bromination process. On the other hand, the lower-HTT carbons, except for HTT1000, absorb only a small amount of bromine, which is almost all expelled by the de-bromination process. In this paper, bromine–carbon compounds after debromination are designated as bromine residue compounds, irrespective of how much and how the bromine remains in the carbons. We have carried out XAFS experiments on bromine residue compounds, not focusing on the host carbon, but on the guest bromine. The present aim is to investigate where and how bromine is present in host carbons with different degrees of graphitization after debromination. The bromine residue compounds are highly stable, while most GICs decompose on contact with air or moisture. The goal was to determine the reason for this unusual stability of bromine residue compounds.
2. Experiment The carbon samples were taken from artificial graphite provided by SEC Corp., which were derived from petroleum coke particles with pitch binder and heat treated at about 1000 8C (hereafter designated HTT1000). Small blocks of HTT1000 were then heat treated at temperatures of 1500, 1750, 1900, 2000, 2200, 2400 and 2800 8C, which are designated HTT1500–HTT2800. Specimens (233320 mm 3 ) of the carbon samples were doped with bromine to saturation. The bromine residue compounds were prepared by placing each saturated carbon in a vacuum of 10 23 Torr at 300 8C for several hours. The present samples are the same as those whose detailed preparation is reported in Ref. [20]. Measurements of XAFS spectra were carried out using the apparatus at the BL-10B [21], Photon Factory (PF) at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. White X-rays were monochromatized by use of primary diffraction from Si(311) with lattice ˚ and the energies of incident X-rays spacing d 311 51.637 A were changed by rotating the monochromator. The energy resolution at 13.5 keV, which refers to the energy of the Br K-edge, was estimated as 3 eV under the present experimental conditions. A transmission mode measurement was applied. The incident X-ray intensity and the transmitted intensity were detected by ion chambers placed in front of and at the back of the sample, respectively. The flow gas in the former ion chamber was N 2 and that in the
latter was N 2 (85%)1Ar (15%). Bromine K-edge absorption spectra for the residue compounds were measured. Each of the samples was powdered and encapsulated in sample containers with windows of Mylar film of 5 mm thickness. Three kinds of containers were prepared for each sample, with thicknesses of 0.5, 1.0 and 1.5 mm. The thickness of the sample (t) in each measurement was chosen by stacking the containers with the sample so that the value of Dm t becomes an appropriate experimental condition, where Dm is the difference in the absorption coefficients below and above the Br K-edge. The accumulated time of each run was 30 min.
3. Results and discussion
3.1. Host carbons and bromine residue compounds In order to discuss the present XAFS results, the characterization of the host carbons and bromine residue compounds is briefly reviewed [20]. Fig. 1 shows the concentration of remaining bromine after the de-bromination process as the atomic ratio of bromine to carbon (n Br /n C ). As for the bromine-saturated compounds, they are classified into two groups. For host carbons prepared at HTT$2000 8C, the amounts of doped bromine gradually increase with the HTT, and the values of n Br /n C are 0.08–0.10. The saturation composition for HTT2800 carbon is C 10 Br. This can be compared with the literature value for artificial graphite [17]. On the other hand, for carbons prepared at HTT#1900 8C, the amounts of bromine decrease with the HTT and the values of n Br /n C are less than 0.03. For residue compounds from host carbons of higher HTT, the concentration of remaining bromine is 0.01–0.02. Those for host carbons of low HTT are fairly small, virtually zero; the exception is the residue compound from HTT1000 with a ratio of about 0.01.
Fig. 1. Bromine concentration of residue compounds.
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The graphite (004) diffraction peak appears at 2u 554.78 ˚ where 2u is the scattering angle for Cu Ka ( l 51.54 A), and l is the wavelength. Fig. 2 shows the X-ray diffraction patterns around the scattering angle for host carbons (thin curves) and bromine residue compounds (thick curves). The usual graphitization behavior with increasing HTT was observed for the host carbons. For the residue compounds, the HTT#1900 8C samples showed very similar patterns to the respective host carbons, while the peak positions of the HTT$2000 8C samples shifted appreciably towards lower scattering angles, indicating that the interlayer distance was expanded slightly by the remaining bromine.
3.2. X-ray absorption near edge structure spectra X-ray absorption near edge structure (XANES) spectra of the Br K-edge for residue compounds are shown in Fig. 3. As for the residue compounds prepared from HTT1500, 1750 and 1900, remaining bromine is confirmed by the X-ray absorption spectra, although the concentrations are very low. The XANES spectra of the residue compounds can be classified into three groups according to the characteristics of the first absorption peak (the so-called white line),
Fig. 3. XANES spectra of the Br K-edge for residue compounds.
which appears at about 13 475 eV. Group I refers to the residue compounds prepared from HTT2000–2800, Group II from HTT1500 and 1750, and Group III from HTT1000. A sharp white line appears in the spectra of Group I, while it is not observed or appears as a very weak shoulder in the spectra of Group II. The second peak is also different; namely, the peak of Group II is more prominent than that of Group I. At this stage, it is estimated that the residue compound from HTT1900 is likely a mixture of Groups I and II states. Further consideration is given below in the discussion on the results from extended X-ray absorption fine structure (EXAFS). Although the white line appears in the spectrum of Group III, it is shifted towards higher energy as compared with those of Group I, and the patterns of the valleys are quite different. As a result, we classify Group III as a different group from either Group I or Group II. The white line was assigned by Heald et al. [13] and Feldman et al. [15] as being due to electronic transition to the unfilled 4p states of bromine. Judging from the significant difference in the peaks of the XANES spectra, the electronic state of bromine for Group II is different from that for Group I. For further discussion it is necessary to simulate the observed XANES spectra by constructing structural models. This simulation is difficult, however, because the structures of the host carbons are too complex and disordered to be interpreted. A quantitative analysis of near-edge absorption spectra is left for future investigations.
3.3. Study by extended X-ray absorption fine structure Fig. 2. X-ray diffraction patterns of host carbons (thin curves) and bromine residue compounds (thick curves).
Fig. 4 shows EXAFS spectra in the form of k 3 x (k) functions, where k is the photoelectron wave number. The
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Fig. 4. EXAFS oscillation of residue compounds.
EXAFS function, x (k), is derived by subtracting the absorption spectrum of an isolated Br atom from the experimental spectrum in the range 50–1000 eV from the absorption edge and then normalizing it by the former spectrum. For samples from HTT1500, 1750 and 1900, the S / N’s are bad, because of the very low concentrations of bromine. Fig. 5 shows the radial structure functions, which are obtained by Fourier transform of k 3 x (k ) in the range k54– ˚ 21 . Some of the peaks correspond to distances 16 A between the absorbing Br atom and various neighboring
Fig. 6. k 3 x (k) functions for the residue compounds from HTT 2800, 1900, 1750 and 1000 obtained by reverse Fourier transforms of the r Br – Br values (thick curves with circles) and the respective experimental functions (thin curves).
shells. It is likely that the dominant peaks, enclosed by a box, are due to the intramolecular Br–Br bond lengths, ˚ are obtained after from which bond lengths of 2.28–2.34 A the phase shift correction. It can be seen from Fig. 5 that this peak increases with the HTT of the host carbons. By carrying out a reverse Fourier transformation, the EXAFS function xBr – Br (k), which is due to the Br–Br contribution only, was extracted. As typical examples, the reverse Fourier transforms for residue compounds from HTT2800, 1900, 1750 and 1000 are shown by thick curves with dots in Fig. 6, compared with the respective experimental k 3 x (k) functions (thin curves). Using theoretical amplitudes and phases calculated by the FEFF code [22], a leastsquares analysis was performed. Table 1 shows values for the bond length r Br – Br , the coordination number N, the Debye–Waller factor s, and the reliability factor R. The factor R in the present study is defined as
Table 1 Intramolecular bromine–bromine distances (r Br – Br ), coordination number (N), Debye–Waller factor (s ) and reliability factor (R) for residue compounds. The corresponding r Br – Br value for a free Br 2 ˚ [23] molecule is 2.281 A
Fig. 5. Radial structure functions of residue compounds.
Host carbon
˚ r Br – Br (A)
N
˚ s (A)
R
Group III
HTT1000
2.31 (1)
0.3
0.07
0.366
Group II
HTT1500 HTT1750
2.28 (1) 2.29 (1)
1.2 1.1
0.10 0.09
0.125 0.157
Group I
HTT1900 HTT2000 HTT2200 HTT2400 HTT2800
2.34 (,1) 2.33 (,1) 2.33 (,1) 2.34 (,1) 2.34 (1)
1.0 1.3 1.3 1.3 1.0
0.07 0.07 0.06 0.07 0.07
0.156 0.110 0.120 0.089 0.152
H. Yoshikawa et al. / Carbon 41 (2003) 2931–2938
FO
( x iexp 2 x icalc )2 R 5 ]]]]] ( x exp )2 i
O
G
1/2
,
where x exp and x calc are the experimental and calculated EXAFS functions, respectively. The values for the bond length r Br – Br are plotted as a function of the HTT of the host carbons in Fig. 7. For the determination of N, the value for the residue compound from HTT2800 was first assumed to be 1.0 and then those for the others were obtained in comparison with the value from HTT2800. Although the standard deviations for the coordination number are fairly large, about 60.3, the value from HTT1000 only is significantly smaller than those of the others, which can be considered to be about 1.0. The poor R values are due to the fact that the contribution of r Br – Br only is included in the x calc functions. As in the case of the XANES spectra, the residue compounds can be classified into three groups from the viewpoint of the bond length r Br – Br . The classification of Groups I–III is the same as that for the XANES spectra. In Group I, which includes the residue compounds from ˚ these HTT2000–2800, the distances are 2.33–2.34 A; values are considerably longer than that of gaseous Br 2 ˚ [23]. For Group II, which consists of (r Br – Br 5 2.281 A) the residue compounds from HTT1500 and 1750, errors in the value of r Br – Br are included, because the EXAFS spectra are very noisy. However, the values of 2.28–2.29 ˚ are shorter than those of Group I and are almost the A same as that of the gas. For Group III of residue com˚ pounds from HTT1000, the distance is about 2.31 A, longer than that of the gas, and the feature of the r Br – Br peak in Fig. 5 is weak and broad in spite of the considerable amount of bromine remaining in the residue compound. The residue compound from HTT1900 appears to be on the boundary between Groups I and II. From the
Fig. 7. Intramolecular bromine–bromine distances (r Br – Br ) for residue compounds.
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standpoint of the bromination and de-bromination behavior and the diffraction peak due to graphite (004), it is classified into Group II. However, from the viewpoint of the bond length r Br – Br , it is included in Group I. Generally for carbons, graphitization starts at around 1900 8C. It is believed that a small amount of the present host carbon prepared at 1900 8C grows into graphite crystallites, in which most of the bromine will be present after the bromination and debromination processes, while most of the carbon is at a low degree of graphitization, where litte bromine is present. As a result, for the residue compound from HTT1900, the macroscopic averaged properties for carbon show the behavior of Group II. On the other hand, the structure of the residue bromine exhibits properties of Group I, because most of the bromine in the compound is in graphite. From the viewpoint of the structure of the residue bromine, the residue compound is included in Group I, as shown in Table 1.
3.4. Where and how does bromine remain? Although we tried to extract the peaks attributed to neighboring carbon atoms around the Br atom, we could not identify them in the small peaks of the radial structure function. This is because the EXAFS oscillation due to r Br – C decreases much more suddenly than that due to r Br – Br , the bromine molecules may not be located at a fixed site in the quasi-graphite or non-graphitized carbon, and / or the Debye–Waller factor may be fairly large. Moreover, the present samples were in powder form so that we could not derive information on r Br – C by means of the anisotropic factor obtained from polarized EXAFS data [15]. As a result, we must speculate where and how bromine is present in the carbon from the information on r Br – Br and the XANES spectral patterns by comparison with previous results for bromine GICs and other bromine-doping materials. In Group I, bromine most certainly exists in the interlayer space of the graphite or carbon with a high degree of graphitization in the form of Br 2 molecules, judging from the following two facts. First, the layer distances of the residue compounds are expanded slightly in comparison with those of the respective host carbons, as shown in Fig. 2. Secondly, the XANES patterns are very similar to those of bromine GICs prepared from HOPG and Grafoil, where bromine is present in the interlayer space. In the present results, the intramolecular distances of Br 2 ˚ which is about 2.4% longer for Group I are 2.33–2.34 A, ˚ [23]. Table 2 shows the than that of gaseous Br 2 (2.281 A) values of r Br – Br that have been reported as a result of EXAFS experiments. Although there is little difference with respect to the host carbon and the amount of doped ˚ bromine, the values of r Br – Br lie in the range 2.31–2.34 A. Heald et al. reported exceptionally longer r Br – Br values of ˚ [13] and 2.53 A ˚ [24] for the bromine GIC from 2.44 A Grafoil with n Br /n C 50.0054. The latter value approaches
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Table 2 Comparison of r Br – Br determined by EXAFS for various bromine GICs and residue compounds Ref.
Host carbon
State of bromine and its concentration
r Br – Br ˚ (A)
[13]
Grafoil
Intercalated, n Br /n C 5 0.0054 Adsorbed on surface with coverage of 0.6 or 0.9
2.44 2.31
[15]
HOPG Highly graphitized carbon fiber
Residue, n Br /n C 5 0.05
2.31
Residue, n Br /n C 5 0.025
2.31
[24]
Grafoil
Higher concentration Residue, n Br /n C 5 0.0054
2.34 2.53
Present work Group I
Artificial graphite HTT>1900 8C
n Br /n C 5 0.015–0.02
2.33–2.34
the distance of r Br – Br for a polybromine ion in brominedoped polyacetylene [25]. Although it is very interesting, the report [24] is very short and details are not given. The exceptionally longer values of r Br – Br reported by Heald et al. [13,24] were for the residue compound with 0.27 mol% Br 2 (n Br /n C 50.0054), the preparation and chemical characterization of which were not described. The sample may be too different to be compared with the present samples. Except for this sample, it seems that the bromine is present as Br 2 molecules with an intramolecular distance slightly longer than that of a free Br 2 molecule. The expansion of r Br – Br is due to a partial electron transfer from a p orbital in the graphite sheets to an anti-bonding orbital of an intercalated Br 2 molecule. The high electronic conductivity [20] originates from this charge transfer. The same expansion is observed in IBr and ICl GICs [26,27], where the values of r I – Br and r I – Cl are increased 2.4% (r I – Br ), 5.6% (r I – Cl in Stage I) and 10.3% (r I – Cl in Stage II) than for the respective free molecules. For the host carbons of Group II, it is necessary to determine whether or not the peaks enclosed by the box in Fig. 5 can be assigned to the intramolecular Br 2 distance, because the peaks are too low and broad to distinguish them from ghost peaks originating from the Fourier transformation. As shown by the curves for HTT1750 in Fig. 6, the phase of the oscillation of the reverse Fourier transform due to r Br – Br is the same as that of the experimental curve, although the latter is very noisy. This suggests that the peaks in Fig. 5 for Group II are not due to the termination error of the Fourier transformation. The coordination numbers are nearly 1.0. From the above two reasons, we conclude that the peaks are due to the Br–Br distances. The second item to be discussed, as for Group II, is where the Br 2 remains. The graphite crystallites are not sufficiently grown, as shown by the (004) diffraction peak in Fig. 2. This is the reason why only a small amount of bromine is doped and most is expelled by the de-bromination process. However, a very small amount of bromine
certainly remains. Judging from the fact that the interlayer distances of the residue compounds are the same as the respective host carbons (Fig. 2), the remaining Br 2 molecules are probably present in defects or interstitial spaces between small graphite crystallites. This assumption is also supported by the following facts: first, the values of r Br – Br in Group II are almost the same as that of a free Br 2 molecule, and, secondly, the electric conductivity of the residue compounds does not change in comparison with those of the host carbons [20]. These two facts correspond to the situation where there is no interaction between the p electrons of undeveloped graphite and Br 2 molecules. The third factor is how the remaining bromine of Group II is present in the carbons. It is reasonable to consider the bromine as being confined in defects or voids in the grains in the form of free molecules. However, the XANES spectra of bromine for Group II are entirely different from that of a free Br 2 molecule [13,16]. The latter has a strong and sharp white line, while there is no or a very weak shoulder peak in the former spectra (Fig. 3). The XANES spectra of Group II seem to imply a peculiar state of Br 2 , the nature of which is left for future studies. The HTT1000 carbon sample (Group III) shows a unique character in the behavior of bromine doping. It absorbed an appreciable amount of bromine under saturated bromine pressure, and a considerable fraction of the bromine remained after the de-bromination process. However, little effect on the electric conductivity [20] and no X-ray diffraction peak attributed to (004) are observed, and the value of r Br – Br is different from those of Groups I and II. It is assumed that bromine possibly exists on the edges of the crystallites by forming weak bonding [20]. However, the present XAFS results suggest that most of the bromine is probably decomposed and forms a strong chemical bond with a dangling bond of carbon or a functional group of the petroleum cokes. This is indicated by the radial structure function of Group III in Fig. 5, in which the peak attributed to r Br – Br is very weak and broad in spite of a considerable amount of bromine remaining.
H. Yoshikawa et al. / Carbon 41 (2003) 2931–2938
The decrease in peak intensity may indicate that all the remaining bromine molecules are not present as C? ? ?Br– Br. This is also supported by the exceptionally small value of the coordination number N for the residue compound from HTT1000 compared with those from the others, which are considered to be about 1.0, implying the existence of Br 2 molecules.
4. Conclusion The structure of bromine residue compounds was investigated by EXAFS and XANES spectra, focusing on the effect of the degree of graphitization of the host carbons, although the results from XANES are still qualitative. The residue compounds can be classified into three groups. In Group I, prepared from host carbons of HTT$1900 8C, bromine is present in the interlayer space of graphite in the form of Br 2 molecules. The bond length r Br – Br is longer than that of gaseous Br 2 , which appears to originate from the interaction of the p electrons of graphite. For Group II, prepared from host carbons of HTT51500 and 1750 8C, it is likely that bromine remains in defects in or between the small crystallites in the undeveloped graphite with a unique electronic state. In Group III, from carbon heat treated at 1000 8C, most of the bromine probably reacts with carbon atoms with a dangling bond or functional groups. EXAFS and XANES experiments provide a very sensitive analysis of bromine residue compounds, and the spectra reflect how and where bromine remains in the carbons depending on the heat-treatment temperature of the host carbon.
Acknowledgements The authors wish to express their thanks to PF at KEK for providing them with the opportunity to perform the XAFS experiments. They also thank Mr. H. Ohmura (Graduate School of Science and Technology, Chiba University) for help with the analysis of the least-square refinements. Part of this work was supported by a grant-inaid for the ‘Research for Future’ Program (NANO Carbon) from the Japan Society for the Promotion of Science (JSPS).
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X-ray absorption fine structure study on residue bromine in carbons with different degrees of graphitization Hidetaka Yoshikawa a , Katsuya Fukuyama a , Yoichiro Nakahara a , Takehisa Konishi b , Nobuyuki Ichikuni c , Yasuko Yoshikawa d , Noboru Akuzawa e , Yoichi Takahashi d , Keiko Nishikawa a , * a
Graduate School of Science and Technology, Chiba University, Yayoi, Inage-ku, Chiba 263 -8522, Japan b Faculty of Science, Chiba University, Yayoi, Inage-ku, Chiba 263 -8522, Japan c Faculty of Engineering, Chiba University, Yayoi, Inage-ku, Chiba 263 -8522, Japan d Department of Applied Chemistry, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112 -8551, Japan e Department of Chemical Science and Engineering, Tokyo National College of Technology, Kunugidamachi, Hachioji-shi, Tokyo 193 -0997, Japan Received 20 March 2003; accepted 5 August 2003
Abstract The structure of bromine residue compounds was investigated by X-ray absorption fine structure (XAFS) in order to interpret where and how bromine is present in carbons with different degrees of graphitization. The residue compounds can be classified into three groups, as obtained from X-ray absorption near edge structure (XANES) spectra and the values of the intramolecular distance r Br – Br determined by extended X-ray absorption fine structure (EXAFS). In Group I, prepared from the host carbons heat treated at temperatures higher than 1900 8C, bromine exists in the interlayer space of graphite in the form of Br 2 molecules with interaction of the p electrons of graphite. In Group III, from carbon heat treated at 1000 8C, most of the bromine probably reacts with carbon atoms having a dangling bond or functional groups. For Group II, where the host carbons are heat treated at intermediate temperatures, it is likely that bromine exists in undeveloped defects with a unique electronic state. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Intercalation compounds; B. Graphitization, Intercalation; D. Chemical structure
1. Introduction Graphite intercalation compounds (GICs) with bromine have been studied extensively because of their ease of synthesis [1–3] and special physical and chemical properties [4]. In particular, the high electrical conductivity and the unusual stability of the residue compounds has attracted much attention from various fields of technology as well as basic science [5]. For the bromine GICs including the residue compounds, even when they are prepared from graphite or highly graphitized carbons, their structures are very complex, because there are several phases present in relation to the arrangement of bromine in the graphite *Corresponding author. Fax: 181-43-290-3939. E-mail address: [email protected] (K. Nishikawa).
layers, depending on the preparation conditions and the concentration of doped bromine [6,7]. Namely, there is complex disorder in the layer structure, or two-dimensional phase transitions in the arrangement of the bromine molecules [8,9], etc. Their structures have been discussed from various viewpoints for various structure determination experiments, such as X-ray diffraction for a single crystal [10], Raman spectroscopy [11,12] and X-ray absorption fine structure (XAFS) [13–16]. On the other hand, there have only been a few studies on the bromine–carbon system with emphasis on the effect of the degree of graphitization of the host carbon [17–19]. From this standpoint, some of the present authors have recently made a systematic investigation of the behavior of bromine doping and de-bromination by wide-angle X-ray diffraction, Raman spectroscopy focusing on the G and D bands, and electrical conductivity experiments [20]. From
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00400-7
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the results, the carbon materials can be classified into at least two groups with respect to their behavior with bromine doping and de-bromination depending on the heat-treatment temperature (HTT). The boundary of the two groups lies at a HTT of 2000 8C. The carbons of the higher-HTT group form intercalation compounds with compositions up to C 10 Br, and a considerable fraction (13–25%) of bromine remains after the de-bromination process. On the other hand, the lower-HTT carbons, except for HTT1000, absorb only a small amount of bromine, which is almost all expelled by the de-bromination process. In this paper, bromine–carbon compounds after debromination are designated as bromine residue compounds, irrespective of how much and how the bromine remains in the carbons. We have carried out XAFS experiments on bromine residue compounds, not focusing on the host carbon, but on the guest bromine. The present aim is to investigate where and how bromine is present in host carbons with different degrees of graphitization after debromination. The bromine residue compounds are highly stable, while most GICs decompose on contact with air or moisture. The goal was to determine the reason for this unusual stability of bromine residue compounds.
2. Experiment The carbon samples were taken from artificial graphite provided by SEC Corp., which were derived from petroleum coke particles with pitch binder and heat treated at about 1000 8C (hereafter designated HTT1000). Small blocks of HTT1000 were then heat treated at temperatures of 1500, 1750, 1900, 2000, 2200, 2400 and 2800 8C, which are designated HTT1500–HTT2800. Specimens (233320 mm 3 ) of the carbon samples were doped with bromine to saturation. The bromine residue compounds were prepared by placing each saturated carbon in a vacuum of 10 23 Torr at 300 8C for several hours. The present samples are the same as those whose detailed preparation is reported in Ref. [20]. Measurements of XAFS spectra were carried out using the apparatus at the BL-10B [21], Photon Factory (PF) at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. White X-rays were monochromatized by use of primary diffraction from Si(311) with lattice ˚ and the energies of incident X-rays spacing d 311 51.637 A were changed by rotating the monochromator. The energy resolution at 13.5 keV, which refers to the energy of the Br K-edge, was estimated as 3 eV under the present experimental conditions. A transmission mode measurement was applied. The incident X-ray intensity and the transmitted intensity were detected by ion chambers placed in front of and at the back of the sample, respectively. The flow gas in the former ion chamber was N 2 and that in the
latter was N 2 (85%)1Ar (15%). Bromine K-edge absorption spectra for the residue compounds were measured. Each of the samples was powdered and encapsulated in sample containers with windows of Mylar film of 5 mm thickness. Three kinds of containers were prepared for each sample, with thicknesses of 0.5, 1.0 and 1.5 mm. The thickness of the sample (t) in each measurement was chosen by stacking the containers with the sample so that the value of Dm t becomes an appropriate experimental condition, where Dm is the difference in the absorption coefficients below and above the Br K-edge. The accumulated time of each run was 30 min.
3. Results and discussion
3.1. Host carbons and bromine residue compounds In order to discuss the present XAFS results, the characterization of the host carbons and bromine residue compounds is briefly reviewed [20]. Fig. 1 shows the concentration of remaining bromine after the de-bromination process as the atomic ratio of bromine to carbon (n Br /n C ). As for the bromine-saturated compounds, they are classified into two groups. For host carbons prepared at HTT$2000 8C, the amounts of doped bromine gradually increase with the HTT, and the values of n Br /n C are 0.08–0.10. The saturation composition for HTT2800 carbon is C 10 Br. This can be compared with the literature value for artificial graphite [17]. On the other hand, for carbons prepared at HTT#1900 8C, the amounts of bromine decrease with the HTT and the values of n Br /n C are less than 0.03. For residue compounds from host carbons of higher HTT, the concentration of remaining bromine is 0.01–0.02. Those for host carbons of low HTT are fairly small, virtually zero; the exception is the residue compound from HTT1000 with a ratio of about 0.01.
Fig. 1. Bromine concentration of residue compounds.
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The graphite (004) diffraction peak appears at 2u 554.78 ˚ where 2u is the scattering angle for Cu Ka ( l 51.54 A), and l is the wavelength. Fig. 2 shows the X-ray diffraction patterns around the scattering angle for host carbons (thin curves) and bromine residue compounds (thick curves). The usual graphitization behavior with increasing HTT was observed for the host carbons. For the residue compounds, the HTT#1900 8C samples showed very similar patterns to the respective host carbons, while the peak positions of the HTT$2000 8C samples shifted appreciably towards lower scattering angles, indicating that the interlayer distance was expanded slightly by the remaining bromine.
3.2. X-ray absorption near edge structure spectra X-ray absorption near edge structure (XANES) spectra of the Br K-edge for residue compounds are shown in Fig. 3. As for the residue compounds prepared from HTT1500, 1750 and 1900, remaining bromine is confirmed by the X-ray absorption spectra, although the concentrations are very low. The XANES spectra of the residue compounds can be classified into three groups according to the characteristics of the first absorption peak (the so-called white line),
Fig. 3. XANES spectra of the Br K-edge for residue compounds.
which appears at about 13 475 eV. Group I refers to the residue compounds prepared from HTT2000–2800, Group II from HTT1500 and 1750, and Group III from HTT1000. A sharp white line appears in the spectra of Group I, while it is not observed or appears as a very weak shoulder in the spectra of Group II. The second peak is also different; namely, the peak of Group II is more prominent than that of Group I. At this stage, it is estimated that the residue compound from HTT1900 is likely a mixture of Groups I and II states. Further consideration is given below in the discussion on the results from extended X-ray absorption fine structure (EXAFS). Although the white line appears in the spectrum of Group III, it is shifted towards higher energy as compared with those of Group I, and the patterns of the valleys are quite different. As a result, we classify Group III as a different group from either Group I or Group II. The white line was assigned by Heald et al. [13] and Feldman et al. [15] as being due to electronic transition to the unfilled 4p states of bromine. Judging from the significant difference in the peaks of the XANES spectra, the electronic state of bromine for Group II is different from that for Group I. For further discussion it is necessary to simulate the observed XANES spectra by constructing structural models. This simulation is difficult, however, because the structures of the host carbons are too complex and disordered to be interpreted. A quantitative analysis of near-edge absorption spectra is left for future investigations.
3.3. Study by extended X-ray absorption fine structure Fig. 2. X-ray diffraction patterns of host carbons (thin curves) and bromine residue compounds (thick curves).
Fig. 4 shows EXAFS spectra in the form of k 3 x (k) functions, where k is the photoelectron wave number. The
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Fig. 4. EXAFS oscillation of residue compounds.
EXAFS function, x (k), is derived by subtracting the absorption spectrum of an isolated Br atom from the experimental spectrum in the range 50–1000 eV from the absorption edge and then normalizing it by the former spectrum. For samples from HTT1500, 1750 and 1900, the S / N’s are bad, because of the very low concentrations of bromine. Fig. 5 shows the radial structure functions, which are obtained by Fourier transform of k 3 x (k ) in the range k54– ˚ 21 . Some of the peaks correspond to distances 16 A between the absorbing Br atom and various neighboring
Fig. 6. k 3 x (k) functions for the residue compounds from HTT 2800, 1900, 1750 and 1000 obtained by reverse Fourier transforms of the r Br – Br values (thick curves with circles) and the respective experimental functions (thin curves).
shells. It is likely that the dominant peaks, enclosed by a box, are due to the intramolecular Br–Br bond lengths, ˚ are obtained after from which bond lengths of 2.28–2.34 A the phase shift correction. It can be seen from Fig. 5 that this peak increases with the HTT of the host carbons. By carrying out a reverse Fourier transformation, the EXAFS function xBr – Br (k), which is due to the Br–Br contribution only, was extracted. As typical examples, the reverse Fourier transforms for residue compounds from HTT2800, 1900, 1750 and 1000 are shown by thick curves with dots in Fig. 6, compared with the respective experimental k 3 x (k) functions (thin curves). Using theoretical amplitudes and phases calculated by the FEFF code [22], a leastsquares analysis was performed. Table 1 shows values for the bond length r Br – Br , the coordination number N, the Debye–Waller factor s, and the reliability factor R. The factor R in the present study is defined as
Table 1 Intramolecular bromine–bromine distances (r Br – Br ), coordination number (N), Debye–Waller factor (s ) and reliability factor (R) for residue compounds. The corresponding r Br – Br value for a free Br 2 ˚ [23] molecule is 2.281 A
Fig. 5. Radial structure functions of residue compounds.
Host carbon
˚ r Br – Br (A)
N
˚ s (A)
R
Group III
HTT1000
2.31 (1)
0.3
0.07
0.366
Group II
HTT1500 HTT1750
2.28 (1) 2.29 (1)
1.2 1.1
0.10 0.09
0.125 0.157
Group I
HTT1900 HTT2000 HTT2200 HTT2400 HTT2800
2.34 (,1) 2.33 (,1) 2.33 (,1) 2.34 (,1) 2.34 (1)
1.0 1.3 1.3 1.3 1.0
0.07 0.07 0.06 0.07 0.07
0.156 0.110 0.120 0.089 0.152
H. Yoshikawa et al. / Carbon 41 (2003) 2931–2938
FO
( x iexp 2 x icalc )2 R 5 ]]]]] ( x exp )2 i
O
G
1/2
,
where x exp and x calc are the experimental and calculated EXAFS functions, respectively. The values for the bond length r Br – Br are plotted as a function of the HTT of the host carbons in Fig. 7. For the determination of N, the value for the residue compound from HTT2800 was first assumed to be 1.0 and then those for the others were obtained in comparison with the value from HTT2800. Although the standard deviations for the coordination number are fairly large, about 60.3, the value from HTT1000 only is significantly smaller than those of the others, which can be considered to be about 1.0. The poor R values are due to the fact that the contribution of r Br – Br only is included in the x calc functions. As in the case of the XANES spectra, the residue compounds can be classified into three groups from the viewpoint of the bond length r Br – Br . The classification of Groups I–III is the same as that for the XANES spectra. In Group I, which includes the residue compounds from ˚ these HTT2000–2800, the distances are 2.33–2.34 A; values are considerably longer than that of gaseous Br 2 ˚ [23]. For Group II, which consists of (r Br – Br 5 2.281 A) the residue compounds from HTT1500 and 1750, errors in the value of r Br – Br are included, because the EXAFS spectra are very noisy. However, the values of 2.28–2.29 ˚ are shorter than those of Group I and are almost the A same as that of the gas. For Group III of residue com˚ pounds from HTT1000, the distance is about 2.31 A, longer than that of the gas, and the feature of the r Br – Br peak in Fig. 5 is weak and broad in spite of the considerable amount of bromine remaining in the residue compound. The residue compound from HTT1900 appears to be on the boundary between Groups I and II. From the
Fig. 7. Intramolecular bromine–bromine distances (r Br – Br ) for residue compounds.
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standpoint of the bromination and de-bromination behavior and the diffraction peak due to graphite (004), it is classified into Group II. However, from the viewpoint of the bond length r Br – Br , it is included in Group I. Generally for carbons, graphitization starts at around 1900 8C. It is believed that a small amount of the present host carbon prepared at 1900 8C grows into graphite crystallites, in which most of the bromine will be present after the bromination and debromination processes, while most of the carbon is at a low degree of graphitization, where litte bromine is present. As a result, for the residue compound from HTT1900, the macroscopic averaged properties for carbon show the behavior of Group II. On the other hand, the structure of the residue bromine exhibits properties of Group I, because most of the bromine in the compound is in graphite. From the viewpoint of the structure of the residue bromine, the residue compound is included in Group I, as shown in Table 1.
3.4. Where and how does bromine remain? Although we tried to extract the peaks attributed to neighboring carbon atoms around the Br atom, we could not identify them in the small peaks of the radial structure function. This is because the EXAFS oscillation due to r Br – C decreases much more suddenly than that due to r Br – Br , the bromine molecules may not be located at a fixed site in the quasi-graphite or non-graphitized carbon, and / or the Debye–Waller factor may be fairly large. Moreover, the present samples were in powder form so that we could not derive information on r Br – C by means of the anisotropic factor obtained from polarized EXAFS data [15]. As a result, we must speculate where and how bromine is present in the carbon from the information on r Br – Br and the XANES spectral patterns by comparison with previous results for bromine GICs and other bromine-doping materials. In Group I, bromine most certainly exists in the interlayer space of the graphite or carbon with a high degree of graphitization in the form of Br 2 molecules, judging from the following two facts. First, the layer distances of the residue compounds are expanded slightly in comparison with those of the respective host carbons, as shown in Fig. 2. Secondly, the XANES patterns are very similar to those of bromine GICs prepared from HOPG and Grafoil, where bromine is present in the interlayer space. In the present results, the intramolecular distances of Br 2 ˚ which is about 2.4% longer for Group I are 2.33–2.34 A, ˚ [23]. Table 2 shows the than that of gaseous Br 2 (2.281 A) values of r Br – Br that have been reported as a result of EXAFS experiments. Although there is little difference with respect to the host carbon and the amount of doped ˚ bromine, the values of r Br – Br lie in the range 2.31–2.34 A. Heald et al. reported exceptionally longer r Br – Br values of ˚ [13] and 2.53 A ˚ [24] for the bromine GIC from 2.44 A Grafoil with n Br /n C 50.0054. The latter value approaches
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Table 2 Comparison of r Br – Br determined by EXAFS for various bromine GICs and residue compounds Ref.
Host carbon
State of bromine and its concentration
r Br – Br ˚ (A)
[13]
Grafoil
Intercalated, n Br /n C 5 0.0054 Adsorbed on surface with coverage of 0.6 or 0.9
2.44 2.31
[15]
HOPG Highly graphitized carbon fiber
Residue, n Br /n C 5 0.05
2.31
Residue, n Br /n C 5 0.025
2.31
[24]
Grafoil
Higher concentration Residue, n Br /n C 5 0.0054
2.34 2.53
Present work Group I
Artificial graphite HTT>1900 8C
n Br /n C 5 0.015–0.02
2.33–2.34
the distance of r Br – Br for a polybromine ion in brominedoped polyacetylene [25]. Although it is very interesting, the report [24] is very short and details are not given. The exceptionally longer values of r Br – Br reported by Heald et al. [13,24] were for the residue compound with 0.27 mol% Br 2 (n Br /n C 50.0054), the preparation and chemical characterization of which were not described. The sample may be too different to be compared with the present samples. Except for this sample, it seems that the bromine is present as Br 2 molecules with an intramolecular distance slightly longer than that of a free Br 2 molecule. The expansion of r Br – Br is due to a partial electron transfer from a p orbital in the graphite sheets to an anti-bonding orbital of an intercalated Br 2 molecule. The high electronic conductivity [20] originates from this charge transfer. The same expansion is observed in IBr and ICl GICs [26,27], where the values of r I – Br and r I – Cl are increased 2.4% (r I – Br ), 5.6% (r I – Cl in Stage I) and 10.3% (r I – Cl in Stage II) than for the respective free molecules. For the host carbons of Group II, it is necessary to determine whether or not the peaks enclosed by the box in Fig. 5 can be assigned to the intramolecular Br 2 distance, because the peaks are too low and broad to distinguish them from ghost peaks originating from the Fourier transformation. As shown by the curves for HTT1750 in Fig. 6, the phase of the oscillation of the reverse Fourier transform due to r Br – Br is the same as that of the experimental curve, although the latter is very noisy. This suggests that the peaks in Fig. 5 for Group II are not due to the termination error of the Fourier transformation. The coordination numbers are nearly 1.0. From the above two reasons, we conclude that the peaks are due to the Br–Br distances. The second item to be discussed, as for Group II, is where the Br 2 remains. The graphite crystallites are not sufficiently grown, as shown by the (004) diffraction peak in Fig. 2. This is the reason why only a small amount of bromine is doped and most is expelled by the de-bromination process. However, a very small amount of bromine
certainly remains. Judging from the fact that the interlayer distances of the residue compounds are the same as the respective host carbons (Fig. 2), the remaining Br 2 molecules are probably present in defects or interstitial spaces between small graphite crystallites. This assumption is also supported by the following facts: first, the values of r Br – Br in Group II are almost the same as that of a free Br 2 molecule, and, secondly, the electric conductivity of the residue compounds does not change in comparison with those of the host carbons [20]. These two facts correspond to the situation where there is no interaction between the p electrons of undeveloped graphite and Br 2 molecules. The third factor is how the remaining bromine of Group II is present in the carbons. It is reasonable to consider the bromine as being confined in defects or voids in the grains in the form of free molecules. However, the XANES spectra of bromine for Group II are entirely different from that of a free Br 2 molecule [13,16]. The latter has a strong and sharp white line, while there is no or a very weak shoulder peak in the former spectra (Fig. 3). The XANES spectra of Group II seem to imply a peculiar state of Br 2 , the nature of which is left for future studies. The HTT1000 carbon sample (Group III) shows a unique character in the behavior of bromine doping. It absorbed an appreciable amount of bromine under saturated bromine pressure, and a considerable fraction of the bromine remained after the de-bromination process. However, little effect on the electric conductivity [20] and no X-ray diffraction peak attributed to (004) are observed, and the value of r Br – Br is different from those of Groups I and II. It is assumed that bromine possibly exists on the edges of the crystallites by forming weak bonding [20]. However, the present XAFS results suggest that most of the bromine is probably decomposed and forms a strong chemical bond with a dangling bond of carbon or a functional group of the petroleum cokes. This is indicated by the radial structure function of Group III in Fig. 5, in which the peak attributed to r Br – Br is very weak and broad in spite of a considerable amount of bromine remaining.
H. Yoshikawa et al. / Carbon 41 (2003) 2931–2938
The decrease in peak intensity may indicate that all the remaining bromine molecules are not present as C? ? ?Br– Br. This is also supported by the exceptionally small value of the coordination number N for the residue compound from HTT1000 compared with those from the others, which are considered to be about 1.0, implying the existence of Br 2 molecules.
4. Conclusion The structure of bromine residue compounds was investigated by EXAFS and XANES spectra, focusing on the effect of the degree of graphitization of the host carbons, although the results from XANES are still qualitative. The residue compounds can be classified into three groups. In Group I, prepared from host carbons of HTT$1900 8C, bromine is present in the interlayer space of graphite in the form of Br 2 molecules. The bond length r Br – Br is longer than that of gaseous Br 2 , which appears to originate from the interaction of the p electrons of graphite. For Group II, prepared from host carbons of HTT51500 and 1750 8C, it is likely that bromine remains in defects in or between the small crystallites in the undeveloped graphite with a unique electronic state. In Group III, from carbon heat treated at 1000 8C, most of the bromine probably reacts with carbon atoms with a dangling bond or functional groups. EXAFS and XANES experiments provide a very sensitive analysis of bromine residue compounds, and the spectra reflect how and where bromine remains in the carbons depending on the heat-treatment temperature of the host carbon.
Acknowledgements The authors wish to express their thanks to PF at KEK for providing them with the opportunity to perform the XAFS experiments. They also thank Mr. H. Ohmura (Graduate School of Science and Technology, Chiba University) for help with the analysis of the least-square refinements. Part of this work was supported by a grant-inaid for the ‘Research for Future’ Program (NANO Carbon) from the Japan Society for the Promotion of Science (JSPS).
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