Spectroscopic Evidence of Carbon Nanotubes' Metallic Character

Besides the efficiency of the functionalization, we show that a loss of metallic character of the carbon nanotubes is initiated by the nitric acid tre...
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J. Phys. Chem. C 2008, 112, 16411–16416

16411

Spectroscopic Evidence of Carbon Nanotubes’ Metallic Character Loss Induced by Covalent Functionalization via Nitric Acid Purification Ce´line Bergeret,† Jack Cousseau,*,† Vincent Fernandez,‡ Jean-Yves Mevellec,‡ and Serge Lefrant*,‡ UniVersity of Angers, CIMA Laboratory, UMR CNRS 6200, BVd LaVoisier, 49045, Angers Cedex 01, France and UniVersity of Nantes, Institut des Mate´riaux Jean Rouxel, UMR CNRS 6502, 2 rue de la Houssinie`re, BP 32229, 44322, Nantes Cedex 03, France ReceiVed: July 25, 2008; ReVised Manuscript ReceiVed: August 23, 2008

A detailed characterization of covalently functionalized HiPco single-walled carbon nanotubes (SWNTs) has been carried out using several physicochemical methods (thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman scattering). The chemical process, finally leading to SWNT-ester derivatives, starts from nitric acid purification of pristine SWNTs. Besides the efficiency of the functionalization, we show that a loss of metallic character of the carbon nanotubes is initiated by the nitric acid treatment of pristine SWNTs and is maintained in the final SWNT ester derivatives. A higher reactivity of the metallic tubes is also demonstrated. Introduction Chemistry of single-walled carbon nanotubes (SWNTs) has been largely developed in the past few years, particularly in the field of the covalent functionalization. This progress has produced a great deal of information about the chemical reactivity of SWNTs and about the solubility of SWNTs in organic or aqueous media, as well as their processing.1-3 A thorough bibliography on this topic has been reported very recently by Imahori et al.,4 to which can be added studies devoted to the iodination of SWNTs5 and to the reaction of SWNTs with peroxy organic acids6 or with a zwitterion derived from dimethyl acetylenedicarboxylate and 4-dimethylaminopyridine.7 Furthermore, from the results issued from works on the chemical properties of SWNTs, the reactivity of metallic or semiconducting tubes that are mixed in any starting SWNTs sample have been differentiated under particular experimental conditions. The use of strong oxidizing conditions via nitronium ions NO2+ as reagents leads to a diameter-selective removal of metallic SWNTs, whereas semiconducting SWNTs remain unaltered.8,9 More recently, halogen oxoanions have also been reported to selectively oxidize metallic SWNTs.10 On the other hand, the selective oxidation of semiconducting SWNTs was observed upon reaction of H2O2 (30 wt %), thus leading to an over 80% metallic SWNT enriched final sample.11 In the case of the covalent functionalization of SWNTs, several situations have been described, as summarized in a recent short review.12 A higher reactivity of metallic tubes was reported when the functionalization is performed through reaction of diazonium salts,13 oxycarbonyl nitrenes,14 dichlorocarbene,15 or organolithium and -magnesium compounds,16 whereas semiconducting SWNTs can selectively cycloadd azomethine ylides, provided the precursors of these reagents derive from trialkylamine-Noxides.17 * Corresponding author. Phone: (33) 241 73 53 75; fax: (33) 241 73 54 05; e-mail: [email protected] (J.C.), [email protected] (S.L.). † University of Angers. ‡ University of Nantes.

In our work, we were interested in the preparation of SWNT derivatives soluble in organic media, in order to obtain thin films with specific properties. For this reason, we have chosen first to oxidize pristine SWNTs because of the high reaction versatility of the carboxylic acid groups that are then generated on SWNTs.18 From this process, one can then easily obtain a large variety of ester derivatives19-22 able to bring about the expected solubility of the final SWNT derivatives in various organic solvents. In this paper, we present the results associated with the characterization of the various SWNT derivatives obtained in the functionalization process. The characterization has been performed using several methods, namely, thermogravimetric analyses (TGA), attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and resonant Raman scattering. We demonstrate that this functionalization is successful, particularly toward the metallic tubes, and we show that a loss of metallic character of such tubes is initiated by the nitric acid treatment of pristine SWNTs and is maintained in the final SWNT ester derivatives. Experimental Section General Information. The thermogravimetric analyses were performed with a TGA/DSC 111 instrument from SETARAM, under an argon flow (1 L/h) at a scan rate of 10 ° min-1. The IR spectra were recorded on a Bru¨ker interferometer model Vertex 70, with an ATR Diamond simple reflection equipment. The XPS measurements were obtained with a Kratos Axis Ultra (Kratos Analytical/Shimadzu Group, Manchester, UK) spectrometer using the monochromatic Al KR X-ray source (1486.6 eV) at 150 W. Samples were deposited on the sample holder by a conductive double-sided tape. A pass energy of 40 eV was used for the valence band measurement, and a pass energy of 20 eV was for the core level one. The instrumental resolution determined using the Fermi edge (between 12 and 88%) was 0.63 eV for a pass energy of 40 and 0.47 eV for a pass energy of 20 eV. Resonance Raman spectra were measured in ambient air at room temperature in a back-scattering geometry. Two excitation

10.1021/jp806602t CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

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Bergeret et al.

SCHEME 1: Synthesis of e-SWNT ester derivatives 5a-c from ap-SWNT HiPco samples.

wavelengths, at 752 and 561 nm, were used to match resonance conditions for semiconducting and metallic HiPco tubes, respectively. The 752 nm line was generated by a Ti sapphire laser, model Spectra-Physics 3900 S, pumped by an Ar+ laser. The 561 nm line was issued from a “CrystaLaser” solid state laser. Both excitation lines were used with power intensity less than 5 mW in order to avoid any damage or modification of Raman spectra due to temperature effects. Raman spectra were recorded with a T64000 Horiba/Jobin-Yvon spectrometer utilized in a triple subtracting configuration. Resolution was set at 1 cm-1, and data were collected with a cooled CCD detector. Chemicals were purchased from Sigma-Aldrich or Acros Organics and were used as received. Dry solvents were distilled over appropriate drying agents before use. The HiPco SWNTs were purchased from Unidym, Inc. (SWNTs, lot R0539 and lot R0559, raw material). Preparation of e-SWNTs Ester Derivatives 5. A 250 mg portion of pristine SWNTs was stirred in 250 mL of aq. nitric acid 2.6 M at 125 °C for 48 h. The reaction mixture was cooled to room temperature; then, the reaction medium was filtered through a polyvinylidene fluoride (PVDF) Millipore membrane (0.22 µm), washed with deionized water up to pH ) 7, then with 5 mL of 0.001 M NaOH solution, and finally with water up to neutrality. Aqueous NaOH treatment is used in order to remove carboxylated carbonaceous fragments.23 The remaining solid was dried at 60 °C under vacuum for 2 d, thus leading to 170 mg of p-SWNT (1). p-SWNTs (1, 170 mg) were added to 170 mL of freshly prepared “piranha solution” (H2SO4 96%/H2O2 30%, 4/1, v/v) previously cooled between 30-35 °C in an ice bath, and the resulting mixture was heated at 45 °C under vigorous stirring for 1.5 h. This reaction medium was then transferred into an Erlenmeyer flask containing 175 g of ice. After cooling for 10 min, the mixture was filtered through a PVDF (0.22 µm) Millipore membrane and washed with deionized water up to pH ) 7. The remaining solid was dried overnight under vacuum to give o-SWNTs (2, 140 mg). o-SWNTs (2, 130 mg) were stirred in 22 mL of SOCl2 with 1 mL DMF at 70 °C for 24 h under nitrogen.31 After cooling to room temperature, the reaction mixture was filtered through a polytetrafluoroethylene (PTFE, 0.1 µm) Millipore membrane. The remaining solid was washed with anhydrous THF and then dried overnight under vacuum to give SWNT-COCl (3, 120 mg). Alcohol 4a-c (7 mL) was added to 3 (170 mg), and this reaction medium was vigorously stirred at 95 °C for 96 h. After cooling to room temperature, the reaction mixture was filtered through a PTFE Millipore membrane (0.1 µm) and washed with ethanol. The remaining solid was dried under vacuum at 60 °C overoneweek,givingesterderivativesSWNT-CO2(CH2-CH2O)nCH3 (5a-c, ca. 150-160 mg). Following this process, the average degree of ester functionalization is ca. 2% carbon atom, as determined by TGA measurements.

as-produced HiPco carbon nanotubes (ap-SWNT), SWNT-ester derivatives 5a-c were obtained after four successive steps (Scheme 1). The first one consists in an oxidative purification of raw SWNTs with boiling 2.6 M aq. nitric acid18,24-34 in order to remove most of the metallic catalyst and amorphous carbon. This process leads to purified carbon nanotubes (1) bearing carboxylic acid groups. In a second step, a further treatment is performed with a “piranha” solution (aq. H2SO4 96%/aq. H2O2 30%, 4/1 mixture) that is expected to increase the content of carboxylic acid sites of the finally oxidized carbon nanotubes (2), due to the cutting property of such a mixture.18,24-27 After conversion of the carboxylic acid sites into acyl chloride groups under the action of thionyl chloride,18,20,34 the obtained SWNTCOCl derivatives (3) are reacted with alcohols HO-(CH2-CH2O)nCH3 (n ) 1-3, 4a-c), thus leading to the corresponding e-SWNT ester derivatives (5a-c, Scheme 1). TGA Analyses. A first evaluation of the SWNT derivatives obtained after the different steps of the above functionalization process was carried out by TGA in argon atmosphere (∆T ) 10 °C min-1). The samples of 1 and 2 display a clear weight loss in the 180-350 °C interval, 12 and 14%, respectively, which is likely to be ascribed to a CO2 release from carboxylic acid sites.35 A more important 25% weight loss was observed from 5c (n ) 3) between 200 and 450 °C (see the profiles in Figure S1 in the Supporting Information), which is then supposed to come from the pyrolysis of the ester groups, as already observed from comparable SWNT derivatives.32 On this basis, the degree of functionalization in 5c can be estimated as one ester group for every ca. 50 nanotube carbons. IR Spectra. ATR-IR spectra of ap-SWNTs, 1, 2, and 5c derivatives were run in the solid state and are presented in Figure 1. As observed in previous studies,36 the IR spectrum of asproduced HiPco samples (Figure 1A, trace a) does not exhibit any band. However, the presence of the carbon nanostructure is revealed after the purification and oxidation steps by bands at 1576 cm-1 in spectra of 1 and 2 (Figure 1A, traces b and c), and at 1583 cm-1 with a stronger intensity in the case of the functionalized sample 5c (Figure 1A, trace d). These bands are likely to be assigned to the activated CdC stretching mode.20,36-39 The process of functionalization is further corroborated by the

Results and Discussion Functionalization Procedure. The synthetic strategy used for functionalizing SWNTs is shown in Scheme 1. Starting from

Figure 1. ATR-IR spectra. A: HiPco samples (a) ap-SWNTs; (b) 1; (c) 2; (d) 5c. B: 5c samples (d) HiPco; (e) Nanoledge; (f) CarboLex.

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Figure 2. XPS C1s experimental curve (black) and fitted components of (a) ap-SWNTs and (b) 5c.

observation of bands at 1733 cm-1 (CdO ester group),6,37,38 1223and1100cm-1 (C-Obondinesterandetherfunctionalities)20,38-40 from 5c (Figure 1A, trace d), as well as at 2870 and 2918 cm-1 (C-H bonds) (see inset in Figure 1A). These bands are consistent with the presence of ester groups covalently bonded to nanotubes. Additional information is provided by the IR spectra shown in Figure 1B, in which a comparison is made between samples bearing identical ester groups (n ) 3) issued from different carbon nanotubes, HiPco tubes (Figure 1B, trace d) and arc discharge tubes Nanoledge and CarboLex (Figure 1B, traces e and f, respectively). It can thus be observed that the frequencies of the IR bands induced by the functionalization (CdC, CdO, and C-O bonds) appear to be clearly dependent on the tube diameters: the lower the frequencies, the larger the diameters. X-ray Photoelectron Spectroscopy. XPS is a very fruitful technique, well adapted to the study of surfaces, and was first used in this work to probe the surface chemical structure of the SWNT derivatives obtained at the different steps of the functionalization process. C1s, O1s, and Fe2p3/2 photoemission peaks have been observed. To quantify the different functional groups present in our samples, we have carried out a curve fitting from the highresolution C1s spectra. In each spectrum, the main peak at 284.5 eV has been assigned to the sp2 nanotube carbons. The other components of the fitting have been assigned to C1s peaks issued from various functional groups, according to the following series: (i) the sp3 C-H (or C-C) bonding at 285.3 eV, (ii) the C-O bonding at 286.2 eV, (iii) the CdO bonding at 287.5 eV, and (iv) the O)C-O(H or R) at 288.7 eV.40,41 Additionally, the “shake-up” of π-π* transitions have been taken into account at 290.2 eV. For the analysis of the ap-SWNT sample, the C1s graphitic carbon component has been fitted with a dissymmetrical profile, this asymmetry being kept all along the C1s analysis. The dissymmetrical part (higher energy) is needed to take into account the excitonic effect in the core-hole screening.42 The C1s curve-fitted spectra of ap-SWNT and 5c and the corresponding component analysis are represented in Figure 2 (the similar C1s spectra provided for 1 and 2 are shown in Figure S2 in the Supporting Information). Table 1 lists the relative proportions (%) of surface functional groups assigned to the fit components associated with the SWNT derivatives studied in this work. For comparison purpose, we also mention the results of the analysis performed on a pure grade HiPco sample as delivered by Unydim Inc. (lot P0234) but not used in our functionalization process. From Table 1, we clearly observe that the nitric acid treatment (1) leads to a significant increase of the carboxylic acid sites

TABLE 1: Relative Proportions (%) of Surface Functional Groups from C1s Curve Fit Analysis pure grade HiPco

ap-SWNTs HiPco

1

2

5c

93.5 0 3.4 2.8 0.4

87.5 0 8.6 3.2 1

84.1 0 5.5 3 7.4

82.7 0 6.7 0.8 9.8

66.4 5.2 20.6 0.1 7.7

Csp2

, SWNTs Csp3, C-H C-O CdO OdC-O(H, R)

TABLE 2: Atomic Concentration of Surface Elemental Constituents sample

C [at %]

O [at %]

Fe [at %]

pure grade HiPco ap-SWNT HiPco 1 2 5c

96.3 94.2 81.7 81.3 83.0

3.1 3.9 18.3 18.7 17.0

0.6 1.9

(7.4%), even higher (9.8%) after the “piranha” treatment (2). These results appear to be in full agreement with the chemical oxidation expected from steps 1 and 2 (Scheme 1). On the other hand, the main observation provided by the data from 5c (Table 1 and Figure 2) consists in two specific features: a larger contribution of the C-O bonding content and the emergence of the carbon sp3 contribution including C-H bonding. Despite the intrinsic uncertainty of the curve fitting, these latter results are fully consistent with the formation of the functional groups expected by the esterification step leading to 5c. The quantitative peak analysis was carried out to determine the surface element concentrations (Table 2). The initial apSWNT HiPco sample exhibits 94.2% of carbon, along with small amounts of oxygen and iron. It can be noticed at this point that the presence of oxygen in both samples, pure grade and ap-SWNT HiPco, is presumably due to air contamination40,41,43 and also to a small proportion of iron oxides, the Fe and O contents being smaller in the case of the pure grade sample. Upon the purification treatment of the ap-SWNT sample with HNO3, a large increase of the oxygen concentration (18.3%) is detected, still slightly higher in 2, obtained after the “piranha” solution treatment. On the other hand, after the last step leading to functionalized samples such as 5c, a noticeable increase of the carbon concentration is observed, which is consistent with the grafting of ester chains on SWNTs in the final product. In addition to the quantitative analysis described above, the high sensitivity of the XPS instrument allowed us to perform valence band energy measurements as a fingerprint of the occupied density of states. The profiles obtained for the SWNT samples successively involved in the process leading to the 5c

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Figure 3. High resolution XPS of the valence band region of HiPco SWNT derivatives (spectra obtained with monochromatized Al KR radiation).

derivative are shown in Figure 3. Several features are to be noticed from these results. A σp band44 is clearly seen at ∼11 eV in the profiles due to 1, 2, and 5c derivatives. This contribution observed after, successively, the purification, the oxidation, and the esterification steps is likely to be related to the CdO bonds issued from carboxylic acid and ester groups, in agreement with the surface elemental analysis. Besides, a slight difference between the density of states curves from 1 and 5c can also be seen at ∼5 eV, as observed in polymers;45 this is a signature of C-H bonds presumably assignable to the ester chains. More importantly, the ap-SWNT sample presents a high density of states near the Fermi level (black line), which is an indication of its high metallic character. On the contrary, the energy profiles associated with 1 (after reaction with HNO3), as well as those of 2 and 5c, exhibit a clear and identical vanishing of the density of states at this Fermi level, compared to that of the ap-SWNT sample. Despite a possible small contribution of iron particles residue to the initial energy associated with the ap-SWNT HiPco sample, such a depletion of the π-band density of states can be compared to that which was previously observed in the case of SWNTs-TCNQ interaction.44 In this latter case, the observed depletion has been ascribed to a p-doping of SWNTs, due to a π-band electrons transfer from SWNTs to the TCNQ dopant molecule. As seen in the Supporting Information, the high resolution XPS of the valence band region has also been recorded from the pure grade HiPco sample (lot P0234) (see Figure S3). This spectrum provides quite similar features to those observed in the spectrum of the ap-SWNT HiPco sample (Figure 3): a high energy is observed near the Fermi level, whereas no σp-band is detected. Although the purification process carried out by CNI is proprietary, these XPS results strongly suggest that in this case the purification does not involve a nitric acid treatment. The interaction of SWNTs with oxidizing and nonoxidizing Brønsted acids has previously been studied.46-51 From spectroscopic results (far IR,47 XPS,48 UV-vis-NIR49), chargetransfer processes were proposed corresponding to a p-doping of SWNTs in which the reversible formation of C:H+ moieties has been postulated as a consequence of a possible intercalation of acids in bundles of SWNTs. In the case of the reactions of aq. HNO3 and H2SO4 solutions at room temperature, the presence of nitrogen and sulfur, respectively, in the resulting p-doped SWNT samples has been established from their XPS spectra.48

Bergeret et al. Such a p-doping process, however, is not likely to occur in our process. No trace of nitrogen has been detected in the XPS analyses of 1, as well as no trace of sulfur has been found in similar analyses of 2 (see Table 2). Moreover, as observed from Figure 3, the energy downshift near the Fermi level is apparently stable in the successive 1, 2, and 5c samples, thus suggesting that any possible p-doping is not reversible in our experimental conditions. This point will be further discussed after the presentation of the results obtained in Raman spectroscopy. Resonant Raman Spectroscopy. Raman spectroscopy has been extensively used to characterize SWNTs, in particular to investigate their electronic and vibrational properties.52 This method is thus well adapted to the study of changes induced by chemical treatments such as those involved in a covalent functionalization process, as shown in several papers.16b,53,54 In this work, we have extensively used Raman scattering to follow each step of the chemical process described above. The excitation wavelengths have been chosen for tuning the resonance with the optical transitions of either metallic or semiconducting tubes, that is, 561 and 752 nm, respectively, in the case of HiPco samples. First-order Raman spectra of SWNTs are characterized by three frequency ranges: (i) below 300 cm-1, where we observe radial breathing modes (RBM) whose frequencies are inversely proportional to the tube diameters; (ii) between 1450 and 1700 cm-1, where the tangential modes (TM) are featured by the G+ and G- modes.55 The G- mode is broad anddissymmetricalonthelowenergysidewithaBreit-Wigner-Fano profile.56 This was interpreted by a phonon-plasmon coupling with a continuum of states as due to the metallic nature of the tubes,56 but recent calculations have also demonstrated that the G- component is strongly dependent on the tube diameters and therefore can lead to this asymmetric profile; 55 and (iii) the 1200-1450 cm-1 range in which is observed the so-called “D” band, assigned to defects in carbon structures in general and resulting from a double resonance process.57 The D band is used most of the time as a probe of the defect concentration. Figure 4A presents a series of Raman spectra of HiPco derivatives using λexc ) 561 nm. Curve (a) is characterized by the usual features exhibited by a HiPco sample, for which this particular wavelength excitation provides a clear and intense component of the G- band at ∼1533 cm-1, as a signature of the metallic character of the tubes. However, it appears that the G- component is dramatically modified all along the chemical procedure used in our work (Scheme 1), being first reduced in intensity after the HNO3 treatment (step 1). Furthermore, an analysis of the tangential (TM) modes, which are now composed of sharp components in curves (b-d), seems to be indicative of a loss of metallic character in the three corresponding samples, 1, 2, and 5c, respectively. An additional observation can be made concerning the D band at ∼1330 cm-1, which has strongly increased after the piranha solution oxidation (step 2). However, its intensity is weaker after esterification (step 4) and thus does not appear to be here a quantitative probe of the degree of functionalization (see curve d), contrarily to what is shown by the other techniques used in this work (TGA, ATR-IR, and XPS). In the RBM bands range, one notes a decrease of the relative intensity as well as a slight upshift from the spectrum provided by the ap-SWNT sample (238 cm-1, curve a) to that of 2 (241 cm-1, curve c) following the piranha solution oxidation step. After the esterification step, this shift is even larger, reaching 244 cm-1, and the intensity of these modes increases again (curve d). These up-shifts could be understood as a consequence of a hole doping, but also as due to changes in the electronic

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Figure 4. Raman spectra of HiPco SWNT derivatives: (a) ap-SWNT; (b) 1; (c) 2; and (d) 5c. The spectra are recorded at room temperature under the excitation wavelengths (A) λ ) 561 nm or (B) λ ) 752 nm.

properties of the metallic tubes. The difference between the final value (244 cm-1) displayed in the spectrum of 5c and that (∼241 cm-1) coming from 1 and 2 suggests a specific influence associated with the formation of ester groups, known to be better electron acceptors than carboxylic acid functions in the solid state. In this latter case, hydrogen bonds are easily formed between -COOH groups, which reduces to some extent the intrinsic electron-withdrawing ability of these groups.58,59 In the case of the semiconducting tubes, the functionalization process induces fewer changes in the Raman spectra (Figure 4B). Nevertheless, one may notice that some RBM bands are no longer observed in the functionalized tubes. On the other hand, a slight shift of the main RBM frequency is also recorded (from 257 to 260 cm-1 for λexc ) 752 nm). This overall smaller shift may invoke a less efficient reaction toward semiconducting tubes. From the two sets of Raman spectra in Figure 4, it also appears that the intensity variation of the D band is very comparable from ap-SWNT to 5c. In particular, this intensity is increasing from ap-SWNT to 2, and becomes very low in 5c. Such a variation is most likely to be related to the functionalization process, from which 2 was found (Table 1) to possess the highest content of carboxylic acid groups formed at the tips and on the sidewalls of SWNTs. The formation of hydrogen-bonded structures from CO2H groups, as indicated above, is then expected to be the most important in 2. As a consequence, these hydrogen bonds may reinforce the interconnections between the SWNT bundles and contribute to the D-band intensity increasing in these samples. In the last step, however, due to the conversion of CO2H groups into esters, such hydrogen bonds will be suppressed, which may account for the weaker D-bands observed in the corresponding spectra of 5c. Conclusions The important change evidenced in the electronic properties of SWNTs following the nitric acid treatment is certainly the most significant feature of this study. This change is clearly shown from the vanishing of the density of states near the Fermi level revealed by the XPS study of 1 (Figure 3). This is also corroborated in Raman spectra from the tangential mode bands profiles that exhibit sharp components following the nitric acid treatment, instead of the broad G- component recorded under the 561 nm excitation wavelength for metallic tubes in pristine

HiPco samples (see Figure 4A). Considering the functionalization process, the formation of CO2H groups in the first step appears to be the key point. On the other hand, the important alteration of the TM modes in Raman spectra of metallic tubes, from the first step of functionalization, strongly suggests that the formation of CO2H groups on metallic tubes deeply changes their electronic properties. Besides, the grafting of these CO2H groups, as well as that of the CO2R ester groups formed in the final step, are nonreversible processes in our experimental conditions. Because of the clear electron-withdrawing ability of these functional groups, an electron charge-transfer from the nanotube to the carboxylic acid and ester groups is most likely to occur in 1, 2, and 5c, creating a permanent p-doping effect and thus lowering the Fermi level of SWNTs, as evidenced in our XPS results. Such a charge-transfer was initially proposed from ab initio results in the case of a theoretical study of the covalent sidewall grafting of CO2H groups on SWNTs60 and was also very recently considered in a study of the covalent functionalization of SWNTs with diazonium salts.54 In addition, the Raman spectra (λexc ) 561 nm) display a definite upshift of the RBM bands, with the upshift starting after the purification step and being slightly amplified after esterification. This latter effect also suggests a particular contribution of the covalently bonded ester groups to the change of the electronic properties of the carbon nanotubes, in relation with the strong electron acceptor ability of these functional groups. On the other hand, the fewer modifications observed here in Raman spectra under 752 nm excitation wavelength are indicative of a weaker chemical reactivity of semiconducting tubes toward nitric acid. The overall functionalization used in this study thus appears to be rather metallic selective, with the nitric acid purification of pristine SWNTs being eventually proven to dramatically affect the electronic properties of the finally derived functionalized carbon nanotubes. Acknowledgment. We thank S. Grolleau (Institut des Mate´riaux Jean Rouxel, Nantes) for the TGA studies. The Conseil Ge´ne´ral de Maine-et-Loire is greatly acknowledged for a grant to C.B. Supporting Information Available: TGA profiles, XPS C1s curves for 1 and 2, and high resolution XPS (valence band region) spectrum for the pure grade HiPco sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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