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2008, 112, 7552–7556 Published on Web 04/26/2008
Fine Nanostructure Analysis of Single-Wall Carbon Nanohorns by Surface-Enhanced Raman Scattering Toshihiko Fujimori,† Koki Urita,† Yuusuke Aoki,† Hirofumi Kanoh,†,‡ Tomonori Ohba,†,‡ Masako Yudasaka,§ Sumio Iijima,§,| and Katsumi Kaneko*,†,‡ Graduate School of Science and Technology, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, Department of Chemistry, Faculty of Science, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, Japan Science and Technology Agency, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, and Department of Physics, Meijo UniVersity, 1-501 Shiogamaguchi, Tenpaku, Nagoya 468-8502, Japan ReceiVed: February 18, 2008; ReVised Manuscript ReceiVed: March 27, 2008
Surface-enhanced Raman scattering (SERS) was applied to investigate the fine nanostructure of single-wall carbon nanohorns (SWCNHs) and partially oxidized SWCNHs of which Raman D bands are predominant. SERS of SWCNH samples was measured in vacuo by deposition of SWCNH particles on silver foil with evaporation of SWCNHs-dispersed solvent. New peaks were observed over the wavenumber range of 200 to 1700 cm-1 in addition to peaks observed in normal Raman scattering. The new SERS peaks are tentatively assigned to the vibrational mode due to topological defects such as the pentagon and heptagon. Introduction Open single-wall carbon nanotube (SWCNT) have super high surface area, which is evaluated geometrically to be 2630 m2 g-1.1,2 Also, SWCNTs form the bundle structure having characteristic nanoporosity. Therefore, one of the promising applications of SWCNTs should be for surface chemistry. Recently, many theoretical studies have pointed out the importance of defect structures in the surface chemical processes of SWCNTs.3 However, SWCNTs contain abundant metal catalysts, perturbing the fundamental understanding of the surface chemical properties of SWCNTs. On the other hand, singlewall carbon nanohorns (SWCNHs)4,5 should be more promising in surface chemical studies. SWCNHs have no metallic impurities and a unique spherical assembly structure, offering nanoporosity different from SWCNTs. Furthermore, SWCNHs are highly defective,6 exhibiting n-type semiconductivity7 different from SWCNTs. Accordingly, an exact structural characterization of SWCNHs can extend the application potentials of singlewall nanocarbons. Surface-enhanced Raman scattering (SERS) is a very powerful technique to provide a spectrum intensity enhanced by orders of magnitude.8 It is essential for the SERS effect that the analyte is adsorbed on an effective metal surface. Gold, silver, and copper are typically used as SERS-active metals. Many SERS studies on SWCNTs have been conducted, showing the intensity enhancement.9–16 However, SWCNTs have an ordered structure, and basically, the fundamental structure can be well-understood. On the contrary, SWCNHs have many defect-associated structures which cannot be well-characterized with the established * To whom correspondence should be addressed. Tel: +81-43-290-2779. Fax: +81-43-290-2788. E-mail:
[email protected]. † Graduate School of Science and Technology, Chiba University. ‡ Department of Chemistry, Faculty of Science, Chiba University. § Japan Science and Technology Agency, NEC Corporation. | Meijo University.
10.1021/jp801416b CCC: $40.75
technique. The SERS can offer an appropriate tool to elucidate the nanostructures of SWCNHs. In this study, we observed the SERS activity on SWCNHs dispersed on silver foil to characterize the defective nanostructure. Experimental Methods Dahlia-flower-type SWCNHs, which were synthesized by the CO2 laser ablation of pure graphite under an Ar gas atmosphere, were used (as-grown SWCNHs). SWCNHs were oxidized in oxygen at 663 and 823 K, which were denoted ox-SWCNHs/ 663 and ox-SWCNHs/823, respectively. The detailed procedures of the oxidation treatment are shown in the preceding literature.17 Silver foil (purity 99.95%, 0.015 mm thickness) was purchased from Wako Pure Chemical Industries, Ltd., and used after washing by ethanol with sonication. A quantity of 0.5 mg of the samples was added to 12.5 mL of toluene, and then, the mixtures were sonicated for 5 min. The SWCNHs-dispersed toluene solution was dropped onto the silver foil under an ambient atmosphere, and then, toluene was evaporated. SWCNHs dispersed on the silver foil were pretreated using the in situ Raman cell (Japan High Tech Co. Ltd., model MVH-5) at 423 K under 2 × 10-3 Pa for 2 h. Raman measurements were carried out at ambient temperature in vacuo with the 532 nm line from a Nd:YAG laser (JASCO NRS-3100). As no SERS effect was obtained from the aggregated SWCNH, we conducted Raman measurement on the isolated SWCNH with scanning of the SWCNH-dispersed silver foil. In SERS experiments, the laser power was approximately 3 mW at the sample, and 20× long-distance objectives were used because of the requirement for the in situ Raman cell. Raman measurements were also conducted under the ambient condition by using 0.1 mW of the laser power and 100× objectives for comparison with SERS spectra and normal Raman spectra. 2008 American Chemical Society
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Figure 1. Normal Raman spectrum of as-grown SWCNHs (a), oxSWCNHs/663 (b), and ox-SWCNHs/823 (c) obtained with 532 nm excitation. Heavy and dashed lines indicate the fitting curve and its Lorentzian components, respectively.
Results/Discussion The Raman spectra of as-grown SWCNHs, ox-SWCNHs/ 663, and ox-SWCNHs/823 under the ambient conditions are shown in Figure 1. The spectrum (a) of as-grown SWCNHs exhibits two bands at 1592 and 1337 cm-1, which are assigned to the G band and D band, respectively. On the other hand, ox-SWCNHs/663 and ox-SWCNHs/823 have an additional weak band at 1617 cm-1, indicating the presence of partially opened SWCNHs and the oxygen-terminated holes.18,19 These defective structure can activate Raman scattering of the forbidden phonon mode. The intrinsic modes of graphite and the SWCNT were theoretically studied with double resonance theory by Saito et al.;20 they showed that the peaks centered at 1617 cm-1 could be assigned to the phonon mode, usually denoted as the D′ band. SERS spectra at around 1000-1900 cm-1 are shown for asgrown SWCNHs, ox-SWCNHs/663, and ox-SWCNHs/823 in Figure 2a,c, and d, respectively. The spectra were measured in 60 s of collection time. The spectra shown were measured on the same focal point, and each was sequentially obtained from bottom to top. For as-grown SWCNHs, SERS spectra could be obtained with 10 s of collection time (Figure 2b) but not for ox-SWCNHs/663 nor ox-SWCNHs/823. It can be clearly seen in SERS spectra of as-grown SWCNHs that 60 s of collection time was too long to detect the fine peaks observed by 10 s of collection time; therefore, the SERS spectra shown in Figure 2a are averaged ones, and their features are close to those of the normal Raman spectrum, except for the appearance of the enhanced signals. SERS spectra at around 200-1200 cm-1 are shown for asgrown SWCNHs and ox-SWCNHs/663 in Figure 3a and b, respectively. Though the signals can be observed for oxSWCNHs/663 (Figure 3b), the signal-to-noise ratio (S/N ratio) seems to be less than that of as-grown SWCNHs, despite its long collection time. According to this phenomenon, the S/N ratio for ox-SWCNHs/823 may be less than that of oxSWCNHs/663, and indeed, no signals were observed for ox-
J. Phys. Chem. C, Vol. 112, No. 20, 2008 7553 SWCNHs/823 at this Raman shift range. Comparing SERS spectra with normal spectra, the fine peaks were detected for every SERS spectrum. The peaks in normal Raman spectra are observed in SERS spectra for all samples. It is obvious that both signal intensities and Raman shifts were fluctuated in every SERS spectrum, as shown in Figures 2 and 3. This phenomenon has been known as the blinking effect, which is characteristic of SERS spectrum.21–25 It is meaningful to compare the SERS spectra of SWCNHs and the Raman feature of the defectcive SWCNTs. IR spectroscopy can reveal the functional groups such as C-O and OH on the surface of SWCNTs. However, it is difficult to detect these functional groups by non-SERS Raman spectroscopy. Kim et al.26 have reported the assignment of the IR bands due to the functional groups on the SWCNTs. According to their assignment, the C-O stretching mode must be observed at around 1100 or 1230 cm-1, depending on its origin. The OH stretching mode can be observed at around 1230 cm-1. Interestingly, SERS spectra of as-grown SWCNHs exhibit signals at 1102-1119 and 1213-1217 cm-1, and those of ox-SWCNHs/663 exhibit weak signals at 1143-1145 and 1206-1225 cm-1. Further investigation is required for available assignment of the fine peaks appearing on SERS spectra of SWCNHs to the functional groups. However, such assignment is indispensable to design highly functionalized single-wall nanocarbons. Recently, the weak Raman signals from graphitic materials, which had been not well-assigned, have been explained by double resonance Raman scattering.20 Consequently, we can refer to the theoretical study. First, the novel SERS signals from SWCNHs should be compared with the intrinsic phonon modes of graphite. In Figures 2b-d and 3, theoretically obtained frequency values20 are shown by the bars (bottom). The reported values20 are calculated using 2.41 eV excitation, whereas the excitation energy used in our experiment is 2.33 eV (532 nm). Here, we corrected the peak positions due to the exitation energy difference. The calculated peak position of the G band is 1580 cm-1, which is close to the observed value (1592 cm-1) for as-grown SWCNHs. The very weak SERS peaks were also observed around predicted values by Saito et al., although the observed SERS spectra give more unpredictable peaks. Then, we need another aspect to interpret our experimental results. Wu et al.27 showed a theoretical study on Raman spectra for SWCNTs with the topological defects. According to the literature, a pentagon or heptagon induced between different chirality SWCNTs ((10, 10) and (17, 0) SWCNTs are connected through a pentagon and heptagon, denoted as SWCNT intramolecular junctions (IMJs)) are used as their structural model. Asgrown SWCNHs possess complex nanostructures due to the presence of pentagons or heptagons, as previously mentioned. We assume that the unassignable SERS peaks could be explainable by the vibrational modes from the topological defects. In this sense, the Raman feature of SWCNHs can be associated with the theoretically obtained Raman spectra of defective SWCNTs. Many peaks are predicted in the frequency range of 1400-2100 cm-1 in the theoretically derived Raman spectra, whereas the ideal SWCNT shows only three peaks at this frequency region (e.g., 1650, 1662, 1691 cm-1 for (10, 10) SWCNT). In Figures 2b-d and 3, the calculated peak positions for a SWCNT with pentagons and heptagons are shown by the bars (top). In order to compare the experimental results with the calculated Raman spectrum of the topological defect induced on the SWCNT, theoretically obtained values can be red shifted for ∼40-100 cm-1 because the calculated G band is at 1691 and 1626 cm-1 for the (10, 10) and (17, 0) SWCNT, respec-
7554 J. Phys. Chem. C, Vol. 112, No. 20, 2008
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Figure 2. SERS spectra of as-grown SWCNHs by 60 s of collection time (a) and that of 10 s (b); ox-SWCNHs/663 (c) and ox-SWCNHs/823 (d) in the 1000-1900 cm-1 region, obtained with 532 nm excitation. A collection time was 60 s for (c) and (d). Each spectrum shown in the same figure was obtained on the same focal point obtained from the bottom to the top of the spectrum. The bars indicate the peak position of (top) the calculated modes of SWCNT IMJs,27 (middle) that of the localized modes of graphene with a Stone-Wales defect,27 and (bottom) the calculated phonon modes of graphite.20
tively, whereas the G band usually appears at around 1590 cm-1 for SWCNHs. The exact assignment of the fine SERS spectrum is difficult due to the fluctuation of the intensity and peak position on the SERS spectra of SWCNHs. However, it is possible to note the similarity of the Raman feature between the calculated spectra for pentagon- or heptagon-induced SWCNTs and the experimental results for as-grown SWCNHs possessing many pentagons or heptagons if attention is especially given to the vibrational modes localized on the topological defects. According to the calculated Raman spectrum of SWCNT IMJs, localized modes on the pentagon are predicted at 1813, 1849, 1948, 2022, and 2024 cm-1. On the other hand, those of the heptagon are predicted at 1452 and 1494 cm-1. Considering the red shift for calculated values, the additional fine peaks around the Dband (1311-1348 and 1370-1382 cm-1) could be assigned to the vibrational modes localized on heptagons of SWCNT IMJs. Furthermore, Wu et al.27 also showed the Raman feature of the graphene with a Stone-Wales defect. The localized vi-
brational modes on pentagons or heptagons are also shown in Figures 2b-d and 3 by the bars (middle). Though the SWCNH shows a tube-like structure, its diameters are much wider than those of typical SWCNTs. Assuming that the localized modes on topological defects of wider tube diameters are correlated with that of graphene with the topological defects, the other SERS signals of SWCNHs could be also assignable. According to the calculated Raman spectrum of the graphene with Stone-Wales defects, localized modes on the pentagon are predicted at 1505, 1706, 1708, 1710, 1726, and 1820 cm-1. On the other hand, those of the heptagon are predicted at 597, 1129, 1180, 1298, 1338, and 1356 cm-1. Since the calculated G band for a perfect graphene is obtained at 1591 cm-1, the calculated values could be compared without compensation. The peak centered at 1490 cm-1 is close to the predicted 1505 cm-1 mode. However, graphite theoretically shows the peaks centered at 1490 cm-1.20 Therefore, it is difficult to distinguish it at the moment. Though it is a very rare case for our results, ox-SWCNHs/663 shows a broad peak at 1697 cm-1, and it
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J. Phys. Chem. C, Vol. 112, No. 20, 2008 7555 measurements should be caused by topological defects in the single graphene-like structure of SWCNHs. The oxidation process causes “nano-windows”17 on SWCNHs, mainly opening around the geometrical region near the pentagons or heptagons. There are several fine peaks near the D-band region for as-grown SWCNHs and ox-SWCNHs/663. For ox-SWCNHs/823, however, it seems that only a broad D band centered at 1339-1346 cm-1 is observed. Adopting one hypothesis that localized vibrational modes of the pentagon or heptagon are quenched when the “nano-windows” are generated, the disappearance of the SERS signals around the D band, which are assigned as localized modes on the heptagons, could be explainable. Furthermore, this hypothesis also explains the experimental results for ox-SWCNHs/823 in that almost no SERS signals are observed at around 200-1200 cm-1. The higher oxidation temperature, the more the topological defects; the signal intensities of the localized modes quench. The other characteristic Raman signal of SWCNTs is the socalled the radial breathing mode (RBM), whose peak position can be correlated with its diameter. Assuming that the empirical equation to estimate the diameter of SWCNT, which is expressed as ωRBM ) 248/dt (ωRBM and dt denote the peak position, in cm-1, and diameter, in nm, respectively),28–33 can be applied to SWCNHs (2-5 nm for dt), RBM positions are expected to be from 50 to 124 cm-1; however, the tube diameter thicker than 2-3 nm does not exhibit characteristic features as those known for the SWCNT. Indeed, no RBM feature is reported for the SWCNH yet. The theoretical calculations of the defective SWCNTs27 show more peaks than nondefective SWCNTs around the RBM region. There are no references for defective SWCNTs with large diameters, such as the SWCNH. Therefore, the qualitative assignment is still the subject for SERS spectra of SWCNHs in the RBM region, as shown in Figure 3a and b. Interestingly, it is worth mentioning that very weak peaks are observed for both as-grown SWCNHs and ox-SWCNHs/ 663 in the frequency range of 200-500 cm-1. Supporting Information Available: Summary of signal assignments for as-grown SWCNHs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 3. SERS spectra of as-grown SWCNHs (a), and ox-SWCNHs/ 663 (b) at the 200-1200 cm-1 region, obtained with 532 nm excitation. The collection times were 10 and 60 s for as-grown SWCNHs and ox-SWCNHs/663, respectively. Each spectrum shown in the same figure was obtained on the same focal spot obtained from the bottom to the top of the spectrum. The bars indicate the peak position of (top) the calculated modes of SWCNT IMJs,27 (middle) that of the localized modes of graphene with a Stone-Wales defect,27 and (bottom) the calculated phonon modes of graphite.20 The bars of SWCNT IMJs at 200-1200 cm-1 are not shown because no signals were shown in the reference at this region.
might be related to the mode from the pentagon. It is clearly seen in Figures 2b and 3a that as-grown SWCNHs show sharp SERS peaks centered at 1102-1119 and 1149-1174 cm-1; these peaks could be assigned to the modes from heptagons. The predicted 1298-1356 cm-1 peaks are overlapped with the predicted peaks for SWCNT IMJs. The SERS peaks around the D band should be related to the modes from heptagons. Assignment for the SERS signals from the narrow diameter of the SWCNH (tip) might be adequate by the SWCNT IMJs model, whereas graphene with the Stone-Wales defects model might be adequate for signals from a much wider diameter region. Accordingly, the fine signals obtained by the SERS
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