Influence of Air Oxidation on the Surfactant-Assisted Purification of

ARTICLE pubs.acs.org/Langmuir

Influence of Air Oxidation on the Surfactant-Assisted Purification of Single-Walled Carbon Nanotubes Alejandro Ans
on-Casaos,* M
onica Gonz
alez, Jos
e M. Gonz
alez-Domínguez, and M. Teresa Martínez Instituto de Carboquímica ICB-CSIC, Miguel Luesma Cast
an 4, 50018 Zaragoza, Spain

bS Supporting Information ABSTRACT: Arc discharge single-walled carbon nanotube (SWCNT) soot was treated under different experimental conditions including gas- and liquid-phase oxidation, heat treatment in an inert gas, and hydrogen gasification. Afterward, the samples were dispersed in a surfactant and centrifuged at a moderately high speed. Near-infrared spectra of all the dispersions were compared with that of raw SWCNT soot. The relative intensity of SWCNT characteristic spectral bands strongly increased for air-oxidized samples after centrifugation, while it did not substantially change for samples oxidized with nitric acid or reduced with hydrogen. The relative SWCNT spectral intensity was associated to the sample purity through the so-called purity index, which was calculated from the S22 band transition of semiconducting SWCNTs. Air-oxidized samples experienced a 7-fold increase in the purity index during centrifugation, while it increased by only 2
3 times for nonoxidized samples. Air oxidation specifically improves the preferential stability of SWCNTs over carbonaceous impurities in the dispersions, leading to the highest purity index values reported so far.

1. INTRODUCTION Optical spectroscopy has been often applied to the study of single-walled carbon nanotubes (SWCNTs), which show characteristic absorption bands in the near-infrared (NIR)
visible region. The wavelength of the absorption peaks depends on the SWCNT diameter, chirality, and conducting properties.1,2 Currently available SWCNT samples contain nanotubes with heterogeneous diameter distributions, in which the most intense absorption bands correspond to the S11, S22, and M11 transitions of semiconducting (S) and metallic (M) nanotubes. Additionally, pristine SWCNT samples contain impurities, including metallic catalyst particles, amorphous carbon, and graphitic materials. Samples with the highest SWCNT purity demonstrate the most prominent characteristic absorption features. It was proved that the relative intensity of the S22 band transition can be applied to the reliable evaluation of SWCNT purity,3
5 giving an estimate of the amount of carbonaceous impurities present in the sample. SWCNT optical spectra are usually measured on samples previously dispersed in liquid media, either an organic solvent or water containing a surfactant.6
8 The ability of the surfactant to disperse SWCNTs can be evaluated in terms of the total mass suspended in the liquid, the proportion of individual SWCNTs, and the intensity of absorption/fluorescence spectral features.7,8 Sodium dodecylbenzene sulfonate (SDBS) is an anionic surfactant that has been widely utilized to disperse and debundle SWCNTs.8,9 Ultracentrifugation in a sodium cholate surfactant can be utilized to obtain SWCNT samples free from graphitic impurities.10 Alternatively, the separation of SWCNTs from graphitic materials can be effected by moderate-speed centrifugation in SDBS or in Pluronic F68 solutions.11 Pluronic F68 is a r 2011 American Chemical Society

block copolymer based on polyethylene oxide and polypropylene oxide that works as a nonionic surfactant. In the present article, we demonstrate that classical oxidation treatments in air strongly increase the relative intensity of SWCNT NIR
visible features for samples subsequently centrifuged in SDBS or Pluronic F68 solutions; however, other oxidation or gasification processes do not substantially change the SWCNT relative absorbance. The surface properties of carbon materials have been studied for years, particularly those regarding the effects of covalent oxygen functionalities such as carbonyl, quinone, phenol, ether, anhydride, lactone, and carboxylic groups. 12,13 It is also known that acidic or basic groups on SWCNT surfaces modify the energy of nanotube
surfactant interactions.14 We hypothesize that surface chemistry could strongly influence the final relative intensity of SWCNT features in the centrifuged dispersions. The significance of the present work is to easily achieve ultrahigh-purity SWCNT suspensions in water with relatively high yields and mild processing conditions. The method will be particularly useful when the presence of the surfactant is beneficial for the subsequent application of the SWCNTs. For example, suspensions of carbon nanotubes in SDBS can be utilized for the preparation of TiO2/carbon nanotube composites,15 while Pluronic F68 works as a compatibilizing agent in the synthesis of high-toughness SWCNT/epoxy composite materials.16

Received: February 24, 2011 Revised: April 12, 2011 Published: April 29, 2011 7192

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2. EXPERIMENTAL SECTION 2.1. Preparation of the SWCNT Samples. SWCNT samples were synthesized by the arc discharge method with graphite electrodes and a Ni/Y catalyst. Characterization studies on this SWCNT material, hereafter AG-SWCNTs, have been previously reported.17
19 AGSWCNT samples contain metallic particles, amorphous carbon, and graphitic materials. An evaluation of the AG-SWCNT purity will be given below. AG-SWCNT samples were air oxidized in an oven at 350 °C for various times, ranging from 30 min to 4 h. Samples oxidized with air are labeled as Ax-SWCNTs, where x indicates the treatment time in hours. Besides air oxidation, other oxidation or gasification processes were performed to modify SWCNT materials. Nitric and hydrochloric acid treatments were performed under a reflux. Gasification with hydrogen (ultra-high-purity H2) at 700
800 °C was performed in a horizontal quartz reactor heated by a Carbolite furnace. 2.2. SWCNT Sample Dispersion. SDBS and Pluronic F68 were purchased from Aldrich and dissolved in water at 1 and 0.5 wt/v%, respectively. A 100
200 mg amount of the SWCNT samples was dispersed in 50 mL of the solutions by sonication in a UP 400S Hielscher tip for 30
60 min. On the basis of the literature and our previous experience, it was assumed that such sonication conditions lead to a sufficient dispersion of the SWCNT samples without causing severe nanotube damage.20 However, thermal and nitric acid treatments could produce changes in the sample agglomeration that lead to differences in the final dispersion quality and stability. Some of the dispersions were centrifuged at 13 000 rpm (23 000g) in 50 mL vials using a Hermle Z383 equipment. The possible influence of centrifugation volumes in the resultant supernatant characteristics is not a topic of discussion in the present work. The supernatants were carefully decanted and analyzed. The centrifuged dispersions were observed to be stable for at least several months. Some supernatant dispersions were filtered to determine the centrifugation mass yield, comparing the mass of dry supernatants with the SWCNT sample mass in the initial dispersions. Centrifugation yields typically range between 15% and 30%.11 2.3. Analytical Techniques. Near-infrared (NIR) absorption spectroscopy was performed in the liquid phase using a Bruker VERTEX 70 spectrometer and 2 mL quartz cubettes. Absorbance measurements in the visible region were performed in a Shimadzu UV-2401PC spectrometer. All dispersions were diluted with 1% SDBS or 0.5% Pluronic F68 solutions to adjust the absorbance into an appropriate range (absorbance ≈ 0.4 at 12 000 cm
1). SDBS and Pluronic solutions were utilized for the blank measurements. Transmission electron microscopy (TEM) images were obtained in a JEOL-200FXII microscope. Thermogravimetric analysis (TGA) was carried out in a Setaram balance, model Setsys Evolution. Thermalprogrammed desorption-mass spectrometry (TPD-MS) experiments were performed using a Thermostar GSD 301T Balzers spectrometer connected to a quartz reactor. The spectrometer was calibrated with 5% CO2/Ar and 5% CO/Ar standards purchased from Air Liquide. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a ESCAPlus Omicron equipment provided with a Mg anode (1253.6 eV) working at 150 W.

3. RESULTS AND DISCUSSION 3.1. Purity Assessment by NIR Spectroscopy. The relative intensity of SWCNT optical features was evaluated by means of the NIR purity index (PI). The PI was calculated as follows3

PI ¼

Ab At

Figure 1. Calculation of the purity index (PI) from the NIR spectrum of an A2-SWCNT sample centrifuged in a 1% SDBS solution.

Figure 2. Baseline-subtracted S22 band in the NIR spectra of SWCNT samples dispersed in 1% SDBS: (a) AG-SWCNTs (PI = 0.034), (b) A3SWCNTs (PI = 0.050), (c) centrifuged AG-SWCNTs (PI = 0.068), and (d) centrifuged A3-SWCNTs (PI = 0.209). The spectra were measured at almost identical optical thickness (absorbance ≈ 0.4 at 12 000 cm
1).

where At is the total area under the S22 band transition, between 7750 and 11 750 cm
1, and Ab is the baseline-subtracted S22 area (Figure 1). The numerator (Ab) represents the strength of resonant transitions in a fixed diameter distribution of SWCNTs and is proportional to the mass of SWCNTs. The denominator (At) is background signal caused by a number of factors such as carbonaceous impurity absorption or light scattering in turbid samples. The relative intensity of SWCNT spectral features and the PI increase as the carbon impurities content decreases. The 7193

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Table 1. Purity Index (PI) of Various SWCNT Samples Dispersed and Centrifuged in 1% SDBS sample

treatment conditions

AG-SWCNT

a

surfactant

PI

PI/PI0a

SDBS

0.061

1.8

H-SWCNT N-SWCNT

H2 750 °C, 2 h HNO3 1.5M, 2 h

SDBS SDBS

0.057 0.076

1.7 2.2

A2-SWCNT

air, 2 h

SDBS

0.203

6.0

A2-SWCNT

air, 2 h

Pluronic F68

0.243

7.1

PI0 is the PI of the starting AG-SWCNT sample (PI0 = 0.034).

Figure 3. NIR purity index for Ax-SWCNT samples prepared at various oxidation times: (a) Ax-SWCNTs directly sonicated in 1% SDBS and (b) Ax-SWCNTs dispersed in 1% SDBS and centrifuged. Black circles indicate the air oxidation burnoff (right axis).

experimental determination of PI is performed at low dispersion concentrations to avoid light-scattering contributions. In the present work, dispersions were diluted with the surfactant solution until the spectra fell into the appropriate absorbance range (∼0.4 at 12 000 cm
1). Therefore, the measurements were performed in dispersions of an almost identical optical thickness. The PI of AG-SWCNTs dispersed in SDBS or Pluronic F68 solutions increases during centrifugation due to the sedimentation of graphitic materials.11 Also, certain amounts of amorphous carbon and metal impurities are selectively removed from the liquid phase during centrifugation.11,21 The preferential stabilization of SWCNTs over carbon impurities in a surfactant can be explained by steric considerations based on the SWCNT monodimensional character (high aspect ratio).22 However, air oxidation treatments on AG-SWCNTs produce an additional increase in the NIR absorbance of the centrifuged SWCNT dispersions. Figure 2 shows the baseline-subtracted S22 bands of AG- and Ax-SWCNT dispersions in SDBS (the original spectra were measured at the same optical thickness). The PI of an AG-SWCNT sample directly sonicated in the surfactant (without centrifugation) was 0.034. The PI of a centrifuged AG-SWCNT dispersion was 0.068, which is twice the value of the noncentrifuged dispersion. In contrast, centrifugation of an Ax-SWCNT dispersion led to a 4-fold increase in the PI, from 0.050 to 0.209. Thus, the PI of the centrifuged AxSWCNT dispersion was approximately 6 times the PI of the noncentrifuged AG-SWCNT dispersion. The shape and position of the S22 band did not substantially change with centrifugation, indicating that the SWCNT diameter distribution was not modified. The absorbance increase that takes place during centrifugation is not specific toward any SWCNT diameter. All of the above observations are in good agreement with a previously published article about the centrifugation of arc discharge SWCNT samples in hexadecyltrimethylammonium bromide solutions.23

Figure 4. Visible
NIR absorption spectrum for an A2-SWCNT sample centrifuged in 0.5% Pluronic F68 (AG-SWCNT PI0 = 0.050).

The PI of centrifuged Ax-SWCNT dispersions depends on the oxidation time (Figure 3). The PI increased from nearly 0.07 for nonoxidized SWCNTs to higher than 0.2 for SWCNTs oxidized for 2
3 h and centrifuged in 1% SDBS. Intermediate PIs of 0.1
0.2 were obtained for the centrifuged dispersions after air oxidation intervals between 30 and 120 min. PIs of higher than 0.2 corresponded to 50
60% burnoff during air oxidation (Figure 3). The PI decreased for treatments of longer than 3 h (burnoff >60%) due to SWCNT destruction. The PI of noncentrifuged Ax-SWCNT dispersions in SDBS ranged between 0.03 and 0.05 (Figure 3). It is commonly assumed that amorphous carbon is more chemically active than SWCNTs and reacts first during air oxidation treatments. However, air oxidation by itself only produced a slight increase in the NIR absorbance with respect to AG-SWCNTs, while centrifugation was necessary to obtain dispersions with high PI (∼0.2). Therefore, the increase in the SWCNT purity took place during centrifugation, and the effect was maximized by previous air oxidation. Table 1 shows the PI of various SWCNT samples after centrifugation in SDBS or Pluronic F68 solutions. The same starting material (PI = 0.034 in 1% SDBS) was utilized in all cases. PIs of SWCNT samples treated with nitric acid (NSWCNTs) or hydrogen (H-SWCNTs) were near that obtained 7194

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Langmuir for the centrifuged AG-SWCNT dispersion (PI ≈ 0.07). Hydrogen treatments at 700
750 °C did not produce any increase in the PI of noncentrifuged or centrifuged SWCNT dispersions, although the burnoff levels registered during H2 gasification (data not shown) reached those of air oxidation. According to NIR absorption measurements, H2 gasification and air oxidation do not cause any substantial reduction of the amorphous carbon content. However, thermal treatments in air create oxygen groups on the surfaces of carbon materials and lead to PIs of higher than 0.2 after dispersion/centrifugation. Nitric acid oxidation generates surface groups as well, but the subsequent dispersion/centrifugation protocol does not lead to high PIs. The PI of SWCNTs centrifuged in Pluronic F68 is usually higher than that of samples centrifuged in SDBS (Table 1). To date, the highest PI (=0.322) we have found for a SWCNT dispersion corresponds to an A2-SWCNT sample centrifuged in

Figure 5. TEM image of an A2-SWCNT sample centrifuged in 0.5% Pluronic F68 (AG-SWCNT PI0 = 0.050) and filtered.

ARTICLE

Pluronic F68. Figure 4 shows the visible
NIR spectrum of the A2-SWCNT dispersion with PI = 0.322. Both metallic (M11) and semiconducting (S22) bands demonstrate high relative intensities, so the interaction of the dispersion medium with air-oxidized SWCNTs is not selective toward metallic or semiconducting tubes. The highest PI value that can be found in the literature for a pristine arc discharge SWCNT sample is 0.141.3 For purified arc discharge SWCNTs, PIs of 0.186 and 0.253 have been reported,4,24 while a maximum of 0.319 was found for purified laser-grown SWCNTs.24 These literature data were measured on SWCNT dispersions in N,N0 -dimethylformamide. Additionally, it was estimated that analytically pure arc discharge SWCNT samples should reach a PI ≈ 0.325.5 This expected value (PI ≈ 0.325) was closely approximated by the experiment presented in Figure 4 (PI ≈ 0.322). In order to confirm the high purity of the sample, the Pluronic dispersion was filtered and the sample was redispersed in ethanol and dropped onto a grid for TEM observation. The images (Figure 5) showed a highly pure SWCNT sample; however, some metal catalysts and polymeric particles can still be seen on SWCNT bundles. Bundles are thick, probably because SWCNTs aggregated during the filtration stage for TEM sample preparation. Finally, we performed preliminary experiments utilizing two commercial SWCNT samples, synthesized by the electric arc method and the CoMoCAT chemical vapor deposition method. The results (see the Supporting Information) indicated that air oxidation always stabilizes SWCNTs in a Pluronic F68 solution. However, optimization of the protocol conditions would be required for the efficient purification of each type of sample. 3.2. Characterization of Surface Chemistry. NIR spectra showed that air oxidation treatments specifically improve the separation of SWCNTs from other carbon materials during centrifugation. Other gasification and oxidation treatments did not lead to substantial increases in the PI. We hypothesize that surface chemistry has an important role in PI improvement. We will show that the amount of oxygen functional groups depends on the oxidation treatment time. Additionally, air oxidation

Figure 6. TGA plots (N2, 5 °C/min) of SWCNT samples air oxidized for increasing time intervals: (a) AG-SWCNTs, (b) A0.5-SWCNTs, (c) A1SWCNTs, (d) A1.5-SWCNTs, (e) A3-SWCNTs, and (f) A4-SWCNTs. 7195

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Figure 7. TGA (N2, 5 °C/min) plots for the samples: (a) N-SWCNT, (b) A2-SWCNT, and (c) A2-SWCNT refluxed 4 h in 3 M HCl.

produces different functionalities from those obtained by liquidphase oxidation. According to NIR results, air oxidation is quite unspecific toward SWCNTs or amorphous carbon in the samples. However, we propose that oxygen groups inserted by air oxidation could specifically improve the relative stability of SWCNTs in certain surfactant dispersions. The study of the oxidation process and identification of oxygen surface groups was carried out by TGA, TPD, and XPS. Figure 6 shows TGA plots of AG-SWCNTs and a series of AxSWCNT samples prepared at various oxidation times. The most important feature in Ax-SWCNT sample plots is the steep weight loss around 500
600 °C. Both the temperature and the height of the step increase with the oxidation time. The relative amount of oxygenated groups evolving at 500
600 °C progressively increased with the oxidation time. As air oxidation treatments were carried out at 350 °C, weight losses below 350 °C can be assigned to ambient exposure. The A0.5-SWCNT sample (oxidized for 30 min) showed important weight losses above 500 °C. These losses correspond to chemical groups that are very stable on carbon surfaces and have low oxygen contents (low O/C atomic ratio). When the oxidation time becomes longer, stable groups are further oxidized and converted into groups with a high O/C ratio that can eventually evolve as CO2 during heating. Air oxidation treatments of longer than 1 h can be utilized to insert oxygen functional groups with a well-defined thermal stability. While the TGA plot for A2-SWCNTs shows a steep weight loss at ∼530 °C, N-SWCNTs show continuous weight losses up to 800 °C (Figure 7). Surface functional groups generated by nitric acid treatment are totally different from those created by air oxidation. Nitric acid oxidation generates a variety of chemical groups with different thermal stabilities. Oxygen groups inserted through air oxidation and evolving at 500
600 °C in the TGA could be the cause of the NIR absorbance enhancement during centrifugation. In fact, dispersions of AxSWCNTs treated at high temperatures or refluxed in HCl

Figure 8. TPD-MS profiles (Ar, 10 °C/min) of (a) A2-SWCNT and (b) N-SWCNT samples.

demonstrated much lower PIs than Ax-SWCNT dispersions after centrifugation. When Ax-SWCNTs are treated at high temperatures (∼ 700 °C, N2, data not shown) or refluxed in hydrochloric acid, oxygenated groups are removed from carbon surfaces and the steep weight loss in the TGA at 500
600 °C disappears (Figure 7). Identification of oxygen surface groups on the oxidized SWCNT samples was carried out by TPD-MS (Figure 8). CO2 and CO desorption profiles of A2-SWCNTs (Figure 8a) mostly consist of sharp peaks centered at ∼590 and 620 °C, respectively. Both the CO2 and CO peaks are in part overlapped, indicating the evolution of anhydride surface groups (C2O3 f CO2 þ CO) between ∼600 and 650 °C.13 The rest of the CO2 is released between ∼550 and 600 °C, and must be assigned to the evolution of lactone groups. The CO peak shows a shoulder at around 700
750 °C that could be due to the presence of a small amount of carbonyl/quinone groups. TPD-MS identification of oxygen groups on the A2-SWCNT sample was in agreement with XPS results (Figure 9). The O1s spectrum of A2-SWCNTs shows a maximum at ∼532 eV and a shoulder at ∼530 eV. The profile can be decomposed into 4 subbands according to the literature.13,25 The sub-band at 529.5 eV can be assigned to metal oxides that are present in the sample after oxidation of Ni/Y catalyst nanoparticles.25,26 Such metal impurities remain in AG-SWCNT materials after their synthesis. 7196

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samples increases 6
7 times after air oxidation and centrifugation in SDBS or Pluronic F68 aqueous solutions. The purity improvement mostly occurs during the centrifugation stage, but the result is strongly dependent on the previous air oxidation conditions. The purification mechanism could consist of a preferential stabilization of air-oxidized SWCNTs over the carbonaceous impurities in the dispersion. SWCNT preferential stability in the surfactant is caused by the SWCNT monodimensionality and would be improved by the anhydride and lactone groups generated during air oxidation.

’ ASSOCIATED CONTENT

bS Supporting Information. Comparison of visible
NIR spectra of SWCNTs synthesized by different methods; Raman spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Figure 9. High-resolution O1s XPS spectrum of an A2-SWCNT sample.

Both sub-bands at 532.3 and 533.3 eV correspond to different oxygen environments in anhydride/lactone functional groups.13 Finally, the sub-band at 531.1 eV can be assigned to the carbonyl/ quinone groups previously identified by TPD-MS. CO2 and CO desorption profiles of N-SWCNTs (Figure 8b) show broad signals with maxima at ∼290 and ∼685 °C, respectively. The CO2 release up to 400 °C is due to the presence of surface carboxylic acid groups (
COOH).13 CO2 evolution at temperatures higher than 400 °C must be assigned to lactone and anhydride groups. The CO maximum at ∼700 °C could correspond to phenol/ether groups.13 Nitric acid oxidation produces a wide variety of surface oxygen groups with different thermal stabilities. In contrast, air oxidation is a specific process, as it mostly generates two types of functional groups (anhydride and lactone) with narrow ranges of thermal stability. These anhydride/lactone groups increase the SWCNT relative stability in SDBS or Pluronic F68 aqueous solutions, leading to the relative increase in the spectral features of centrifuged SWCNT dispersions. The impact that the different treatments exert on the SWCNT carbon lattice was studied by Raman spectroscopy (see the Supporting Information). The radial breathing modes were substantially affected; this could be related to the changes observed in the SWCNT sidewall interactions with the surfactants. The shape of longitudinal tangential modes remained almost unchanged.

4. SUMMARY The relative intensity of SWCNT spectral features in the visible
NIR region increases after subsequent processes of air oxidation and centrifugation in a surfactant. The relative increase in SWCNT visible
NIR band intensity is associated with a SWCNT purity enhancement. The purity index of SWCNT

’ ACKNOWLEDGMENT This work was supported by the Spanish Ministry of Science and Innovation through projects ref. EUI2008-00153 and TEC2010-15736. Special thanks are directed towards M. Vico and colleagues at the ICB analysis services. ’ REFERENCES (1) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555–2558. (2) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. B 2000, 61 (4), 2981–2990. (3) Itkis, M. E.; Perea, D. E.; Niyogi, S.; Richard, S. M.; Hamon, M. A.; Hu, H.; Zhao, B.; Haddon, R. C. Nano Lett. 2003, 3, 309–314. (4) Zhao, B.; Itkis, M. E.; Niyogi, S.; Hu, H.; Perea, D. E.; Haddon, R. C. J. Nanosci. Nanotechnol. 2004, 4 (8), 995–1004. (5) Itkis, M. E.; Perea, D. E.; Jung, R.; Niyogi, S.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 3439–3448. (6) Landi, B. J.; Ruf, H. J.; Worman, J. J.; Raffaelle, R. P. J. Phys. Chem. B 2004, 108, 17089–17095. (7) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003, 3 (10), 1379–1382. (8) Tan, Y.; Resasco, D. E. J. Phys. Chem. B 2005, 109, 14454–14460. (9) Cathcart, H.; Coleman, J. N. Chem. Phys. Lett. 2009, 474, 122–126. (10) Miyata, Y.; Yanagi, K.; Maniwa, Y.; Tanaka., T.; Kataura, H. J. Phys. Chem. C 2008, 112, 15997–16001. (11) Ans
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