Effect of Surface Modification on the Hansen Solubility Parameters of

Article pubs.acs.org/IECR

Effect of Surface Modification on the Hansen Solubility Parameters of Single-Walled Carbon Nanotubes Jing Ma* and Raino Mikael Larsen Department of Mechanical and Manufacturing Engineering, Aalborg University, Fibigerstræde 16, DK-9220 Aalborg Øst, Denmark ABSTRACT: In this work, seven types of surface-modified single-walled carbon nanotubes (SWNTs) were studied by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to investigate the functional groups and extent of functionalization. Hansen solubility parameters were determined based on observations of the sedimentation and swollen states of the SWNTs in solvents after ultrasonication, and the results were compared with the hydrodynamic sizes of the SWNTs evaluated by the dynamic light scattering method. We found that the solubility of SWNTs is related to their functional groups and degree of functionalization, as well as their size.

1. INTRODUCTION

2. MATERIALS AND METHODS 2.1. Surface Functionalization of SWNTs. Seven different types of nanotubes were studied: (1) as-received SWNTs, (2) plasma COOH-functionalized SWNTs, (3) plasma NH2functionalized SWNTs, (4) 8 M HNO3-functionalized SWNTs, (5) concentrated HNO3-functionalized SWNTs, (6) DDAfunctionalized SWNTs, and (7) dichlorocarbene-functionalized SWNTs. (1) SWNTs produced by the combustion chemical vapor deposition (CCVD) method were purchased from Cheaptubes, Inc. According to the information from Cheaptubes Inc., the SWNTs were purified in 3 M HNO3, giving a purity of >90 wt %, and had diameters of 1−2 nm and lengths of 5−30 μm. The SWNTs were used as-received without any further purification. (2) Plasma COOH-functionalized SWNTs were purchased from Cheaptubes, Inc. (3) Plasma NH2-functionalized SWNTs were purchased from Cheaptubes, Inc. (4) 8 M HNO3-functionalized SWNTs were prepared according to the method reported in our previous article13 by refluxing in 8 M HNO3 for 1 h. (5) Concentrated HNO3-functionalized SWNTs were prepared according to the method reported in our previous article13 by refluxing in concentrated HNO3 for 2 h. (6) To prepare DDA-functionalized SWNTs, 2 g of dodecyl amine (DDA) was added to 200 mg of 8 M HNO3functionalized SWNTs, and the mixture was held at 90 °C for 4 days. Then, the mixture was washed with ethanol to remove the remaining DDA and filtered through a 0.45-μm filter. Finally, the powder was dried at 100 °C for 24 h. (7) To prepare dichlorocarbene-functionalized SWNTs, asreceived SWNTs (100 mg) were dried at 110 °C for 8 h to remove absorbed water and then ultrasonicated in 6 mL of chloroform for 1 h. In addition, 40 g of potassium tert-butoxide was dissolved in 60 mL of tetrahydrofuran (THF) and cooled

Carbon nanotubes (CNTs) have exceptional mechanical, thermal, and electrical properties.1,2 However, CNTs are very challenging to disperse homogeneously in most organic solvents and water, because of the van der Waals forces between the tiny tube-shaped molecules. Many researchers3−6 have found that the pristine CNTs, independent of the production method, disperse better in highly polar solvents such as dimethyl formaldehyde (DMF) and N-methyl-2pyrrolidone (NMP) than in other solvents. To improve the dispersibility of CNTs in solvents, physical and chemical methods have been developed such as ultrasonication and surface functionalization (covalent or noncovalent interactions)4,7,8 Hansen solubility parameter (HSP) theory is an effective way of investigating the solubility and dispersibility of pigments and polymers. In three-dimensional space, the Hansen solubility parameters δd, δp, and δh represent dispersion forces, dipolar interactions, and hydrogen bonding, respectively. The HSPs of a solute represent the center of the HSP sphere, and the radius of this sphere, R0, indicates the maximum tolerance of the solution. In the ideal case, all good solvents are included within the sphere and bad solvents are excluded. For a specific solvent in HSP space, Ra is the distance between the solvent and the center of the solute. The dispersibility/solubility of CNTs in various solvents can be evaluated using HSPs.4,9−12 However, until now, the effect of surface modification on the HSPs of single-walled carbon nanotubes (SWNTs) has rarely been reported. In this study, we investigated seven different types of surfacemodified SWNTs, using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to evaluate the degree of functionalization and determine the functional groups. In addition, we compared the HSPs and hydrodynamic sizes of the SWNTs in different solvents. The HSPs of SWNTs obtained from different suppliers and produced by different methods were also compared and are discussed in this article. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 3514

October 27, 2012 February 12, 2013 February 13, 2013 February 13, 2013 dx.doi.org/10.1021/ie302950u | Ind. Eng. Chem. Res. 2013, 52, 3514−3521

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Figure 1. High-resolution C 1s XPS spectra of SWNTs.

with dry ice/ethanol solution to a temperature of around −75 °C. Then, the purified SWNT/chloroform suspension was added to the potassium tert-butoxide/THF solution drop by drop. Next, the mixture was poured into ice, washed with water to remove potassium chloride, and washed three times with ethanol and once with THF to remove butanol and other chemicals. Finally, the powder was dried at 100 °C for 24 h. 2.2. Methods. 2.2.1. X-ray Photoelectron Spectroscopy (XPS). An ESCALAB 250 XPS instrument produced by Thermo Fisher Scientific with a high-resolution spectrometer was used for analysis. The instrument employs monochromatic Al Kα 200 W as the X-ray source and formed a spot size of 500 μm on the sample. For the overall survey and the high-resolution scans, pass energies of 200 and 30 eV, respectively, were used.

The lens mode was set to Large Area XL, and the analyzer mode was Constant Analyzer Energy (CAE) during analysis. Spectral data were recorded and analyzed by computer to give the surface composition. 2.2.2. Raman Spectroscopy. A Raman spectrometer (Renishaw InVia Raman microscope) was used to investigate the structural changes in the CNTs upon functionalization, using a He−Ne laser (632.8 nm) focusing through a ×50 objective lens onto the sample. 2.2.3. HSPs and Dynamic Light Scattering (DLS). To evaluate the HSPs of the SWNTs, SWNT powders were added to 20 different solvents with known HSPs.14 After tip sonication, each SWNT/solvent suspension (0.1 mg/mL) was analyzed with a Zetasizer Nano ZS system from Malvern 3515

dx.doi.org/10.1021/ie302950u | Ind. Eng. Chem. Res. 2013, 52, 3514−3521


Article

Figure 2. High-resolution O 1s XPS spectra of SWNTs.

optimization algorithm based on nonlinear programming was used to determine the HSP values of the SWNTs.15 The algorithm simply finds the position and size of the HSP sphere that encloses as many good solvents and excludes as many bad solvents as possible.

Instruments using DLS to characterize the hydrodynamic size of the SWNTs in the solvent. The Zetasizer Nano ZS instrument illuminates the particles with a laser and measures the rate of intensity fluctuation from the scattered light to calculate the size of the particles. The software calculates the particle size according to ISO standard 13321:1996 (E). The average hydrodynamic particle diameter (Zavg) is calculated by the Zetasizer software from the rate of diffusion. DLS results are highly dependent on the holding time after sonication: For a good solvent, the standard variation is low, but for a bad solvent, the DLS results vary significantly as a result of reaggregation and sedimentation of SWNTs. In the HSP experiments, the solvents were divided into two groups, namely, “good” and “bad” solvents, based on the sedimentation and swollen state of the SWNT/solvent suspension after sonication. A recently developed Matlab

3. RESULTS AND DISCUSSION 3.1. XPS. 3.1.1. Qualitative Measurements. XPS data for each SWNT sample were analyzed to determine the peak locations in relation to specific binding energies. From the highresolution C 1s spectra shown in Figure 1, the main peak at 284.8 eV corresponds to the graphitic carbon (sp2 carbon) peak in SWNTs,16 and additional photoemission peaks present at higher binding energies indicate the presence of carbon atoms bonded to other functional groups.17,18 The peak centered at 3516



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around 532.5 eV from the high-resolution O 1s spectra in Figure 2 is for the oxygen component. The C 1s and O 1s spectra of the plasma COOHfunctionalized SWNTs are distinctly different from those of the other SWNTs, as can be seen in Figures 1 and 2. The C 1s binding energy at 282.5 eV might represent carbon−metal bond groups.19,20 The O 1s peaks below 531.0 eV can be attributed to some metallic oxides such as FeO, Fe2O3,21 and NiO,22 and the O 1s peak at 531.7 eV represents CO groups.23 This analysis implies that some residual metal catalyst particles are still present after the purification by the manufacturer Cheaptubes Inc. The as-received SWNTs were purified by the manufacturer using 3 M HNO3. The treatment was evidenced by the presence of the CO, COH, COOR, and OCO groups.23,24 Likewise, for the 8 M HNO3- and concentrated HNO3-treated SWNTs, some oxidized groups, such as CO and CO groups,17,23 were attached to the SWNTs. The plasma NH2- and DDA-functionalized SWNTs showed traces of a nitrogen peak at around 400 eV on the nanotubes (see Figure 3). Nitrogen originates from the NH2 groups.17 In

Figure 5. XPS survey spectra of SWNTs.

Table 1. Atomic Contents of Different SWNTs by XPS material as-received SWNTs plasma COOH-functionalized SWNTs plasma NH2-functionalized SWNTs 8 M HNO3-functionalized SWNTs concentrated HNO3functionalized SWNTs DDA-functionalized SWNTs dichlorocarbene-functionalized SWNTs

Figure 3. High-resolution N 1s XPS spectra of SWNTs.

addition to the nitrogen content, some CO25,26 and CO groups17,23 remained on the nanotubes according to the O 1s peak (see Figure 2). These XPS results are consistent with those of other studies presented in the literature.23,27 In Figures 1 and 4, the presence of CCl17 indicated the success of the reaction of dichlorocarbene-functionalized SWNTs in the C 1s and Cl 2p peaks. The binding energies of 290.9 eV (Figure 1) and 688.8 eV representing covalent CF groups that were observed for the as-received, NH2-modified, and dichlorocarbene-modified SWNT surface might have been caused by impurities in the products. The survey scans (as shown in Figure 5 and Table 1) also identified fluorine in these three samples. It might be noted

C 1s (%)

O 1s (%)

96.1 87.2

2.8 12.8

90.6

5.2

95.2

4.8

91.7

8.3

95.2 94.2

3.6 4.6

N 1s (%)

2.3

Cl 2p (%)

F 1s (%)

0.2

0.9

0.4

1.6

0.6

0.7

1.2

that the binding energy at 290.9 eV in these samples probably overlaps with the binding energy peaks of some oxidized groups such as OCOO groups.17 3.1.2. Elemental Analysis of SWNTs. Large variations were observed in the O 1s intensity (around 532.5 eV) from the survey spectra in Figure 5. The atomic concentration of each element listed in Table 1 provides a quantitative measure of the extent of functionalization.27 The higher oxygen content of 12.8% for the plasma carboxylic acid-functionalized SWNTs compared to the as-received SWNTs (2.8%) is evidence of COOH groups. The oxygen content of the plasma aminefunctionalized SWNTs increased to 5.2%, and the nitrogen content increased to 2.3%, indicating that the amine groups attached on the SWNTs surface. Oxygen might come from

Figure 4. High-resolution Cl 2p XPS spectrum of SWNTs. 3517



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chemically treated SWNTs. The plasma treatment is thus more effective and reactive for functionalization than the common chemical treatment.31 The most frequently used techniques for the characterization of the surface functional groups on CNTs are Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and XPS. However, FTIR spectroscopy is not a quantitative technique for determining the concentrations of functional groups. When the concentration of a functional group is low, it is difficult to acquire a sufficient FTIR signal. Moreover, it is difficult to confirm the presence of covalently attached groups using FTIR spectroscopy. Raman spectroscopy can be used to quantify functionalization, but it is sensitive to the structural defects and sp2 character in CNTs detected by D and G band and not all functional groups can be detected by using Raman spectroscopy alone. Furthermore, the FTIR and Raman characterizations lack information of direct binding energy information. XPS is a better quantitative surface analysis technique used to characterize the functional groups, as it is more sensitive than FTIR spectroscopy. XPS can reveal the binding energy of the carbon atoms and discern the hybrid sp3 and sp2 bonds. It is thus a very powerful nondestructive method for evaluating the structure of CNTs. 3.3. HSP Prediction. Table 2 illustrates the Hansen solubility parameters and dynamic light scattering (DLS) data for SWNTs in the solvents. On comparison of the DLS data and the solubility/dispersibility results it was found that the smaller Zavg sizes (normally under 500) and PDI (normally below 0.5) were generally associated with the solvents with stable dispersion as indicated by the solubility/dispersibility (S) in Table 2. In Table 3, the HSPs of the studied SWNTs calculated based on the data in Table 2 are reported and compared with those of the SWNTs used in our previous studies; details can be found in refs 32 and 33. The hydrodynamic sizes of the COOH plasma-treated SWNTs and NH2 plasma-treated SWNTs dispersed in DMF were smaller than that of the as-received SWNTs, whereas in the majority of the other solvents, the sizes were larger (see Table 2). Despite the high degree of functionalization for the plasma-treated SWNTs as evidenced by the XPS and Raman analyses, the HSP results for the plasma-treated SWNTs were the same as those for the as-received SWNTs. On one hand, because DMF is the only good solvent among the 20 solvents used in our experiments, the analysis program will naturally acquire the same HSP position. The HSP calculation is based only on the quality of dispersion in limited solvents, which might lead to shortcomings. On the other hand, the interaction radii of these three powders are unknown, because only one solvent was good among the limited solvents. Even for the same HSP position as the as-received SWNTs, the interaction radius of the plasma-treated SWNTs might differ. According to HSP theory, the sizes of the solute and solvent have an influence on the interaction; specifically, when the size of a solute increases, the interaction radius decreases.14 The plasmatreated SWNTs had larger molecular sizes than the as-received SWNTs, as indicated by the larger hydrodynamic sizes in bad solvents. The sizes of nanotubes in the bad solvents do not change significantly from the original state because of poor interactions. Because of this size effect, the interaction radius will decrease for the plasma-treated SWNTs. The use of a chemical treatment, such as HNO3, DDA, and dichlorocarbene modification, changes the HSP parameters to

adsorbed water on the SWNTs taking part in the reaction during the plasma treatment. For the 8 M and concentrated HNO3-treated SWNTs, the oxygen content increased to 4.8% and 8.3%, respectively. We used the 8 M HNO3-treated SWNTs to produce the DDA-functionalized SWNTs, and the oxygen content of the DDA-functionalized SWNTs decreased slightly from 4.8% to 3.6%. At the same time, the nitrogen content increased to 1.2%, indicating that some of the COOH groups reacted with DDA to form the amino acid groups. The chlorine content for dichlorocarbene-functionalized SWNTs was only 0.6%, indicating a low degree of dichlorocarbene functionalization. 3.2. Raman Spectroscopy. The Raman spectra of SWNTs shown in Figure 6 provide information on the structures of the

Figure 6. Raman spectra of SWNTs (Cheaptubes Inc.).

nanotubes. The feature at ∼1300 cm−1 is called the “D band” because of sp3-hybridized carbon components and indicates the presence of impurities and defects. The feature at ∼1580 cm−1 is called the “G band” and is attributed to well-ordered graphite. The ratio of the intensities of the D and G bands (ID/IG) can be used to evaluate the density of defects due to covalent functionalization. The ID/IG ratio of plasma COOH- and plasma NH2-modified SWNTs increased very significantly compared to that of the asreceived SWNTs. However, for the chemically treated SWNTs, ID/IG did not show any increase, and even a slight decrease was observed. The decreased ID/IG value might reflect the removal of some impurities, such as amorphous carbon, from the asreceived SWNTs after HNO3 treatment. However, the chemical environment of both 8 M HNO3 and concentrated HNO3 was not harsh enough to oxidize or change the SWNTs significantly. We also tried acidic oxidation of the SWNTs with a mixture of concentrated nitric acid and sulfuric acid (3:1 HNO3/H2SO4, by volume). However, after a vigorous reaction in the strong acids, the nanotubes became soluble in the mixture, and even in water, it became very difficult to collect the powders by filtration and centrifugation. This mixture has been used to cut SWNTs or functionalize MWNTs in many studies.28−30 HNO3 treatment might preferentially oxidize sites on the SWNTs surface with a high degree of disordering such as cap ends. Both XPS elemental analysis and Raman measurements indicated that the plasma-treated SWNTs had a higher degree of covalent functionalization than the 3518


a

3519

methanol ethanol 2-propanol acetone tetrahydrofuran cyclohexanone ethyl acetate toluene DMF triethylamine dicloromethan chloroform tetrachloromethane hexane decahydronaphthalene benzene xylol acrylonitrile tetrachloroethylene trithorethylene

0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0

Sa 717 667 389 569 1543 2560 790 2389 233 471 6148 6380 2084 8574 1534 1188 1743 1263 1444 1971

Zavg (nm) 0.5 0.7 0.7 0.3 0.6 1.0 0.4 0.8 0.4 0.4 1.0 1.0 0.6 1.0 1.0 0.6 0.6 0.6 0.6 0.8

PDI 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0

Sa 1426 4748 2003 2023 4041 5610 2883 3048 142 2469 7198 2639 4550 4927 2116 3583 2160 2272 1194 985

Zavg (nm) 0.8 0.5 0.6 0.7 0.8 1.0 1.0 0.9 0.3 0.7 1.0 0.8 1.0 1.0 0.4 1.0 0.5 0.8 0.6 1.0

PDI

plasma COOHfunctionalized SWNTs 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0

Sa 1729 1380 2552 2396 1089 6617 3415 11460 206 8514 2457 2014 3327 11670 8338 2974 6487 2479 563 1138

Zavg (nm) 0.8 1.0 1.0 0.8 0.5 1.0 1.0 1.0 0.4 1.0 0.7 0.9 1.0 1.0 1.0 1.0 0.0 0.7 0.5 1.0

PDI

plasma NH2functionalized SWNTs 0 0 0 0 1 1 1 0 1 0 0 0 0 0 0 0 0 1 0 1

Sa 3155 1816 964 1325 339 631 762 1801 181 1993 3834 5902 18090 4482 2110 1377 3858 567 2242 184

Zavg (nm) 1 1 0.6 0.5 0.4 0.5 0.4 0.7 0.4 0.7 1.0 1.0 1.0 1.0 0.4 0.7 1.0 0.3 0.6 0.8

PDI

8 M HNO3functionalized SWNTs 0 1 1 0 1 1 1 0 1 1 0 0 0 0 0 0 0 1 0 1

Sa 1169 682 209 778 148 1022 279 2159 129 562 2067 730 1590 3670 628 1977 890 615 1856 179

Zavg (nm) 1.0 1.0 0.6 0.9 0.4 0.9 0.4 1.0 0.5 0.4 1.0 0.7 1.0 1.0 0.8 1.0 0.9 0.5 0.5 0.8

PDI

concentrated HNO3functionalized SWNTs 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 1 1 0 1

Sa 1801 2335 1667 1399 470 2617 1381 2302 234 1405 5916 1468 5891 1995 1294 779 1737 546 4256 247

Zavg (nm) 0.7 1.0 1.0 0.5 0.3 1.0 0.6 1.0 0.5 0.5 1.0 0.7 1.0 0.4 0.7 0.6 0.6 0.3 1.0 0.9

PDI

DDA-functionalized SWNTs 1 1 1 1 1 1 1 0 1 0 0 0 0 0 0 0 0 1 0 1

Sa

566 408 593 817 1049 3107 439 1778 430 1039 2017 1321 1930 4259 3259 2352 2116 258 2233 147

Zavg (nm)

0.5 0.6 0.8 0.4 0.5 0.8 0.2 0.6 0.4 0.5 0.8 0.5 0.6 0.7 0.4 1.0 0.9 0.4 0.7 0.6

PDI

dichlorocarbenefunctionalized SWNTs

S represents the solubility/dispersibility based on the observation of sedimentation and swollen state, where values of 1 and 0 indicate good and bad solvents for dispersion, respectively.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

solvent

as-received SWNTs

Table 2. Hansen Solubility Parameter and Dynamic Light Scattering Experimental Results

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SWNTs might be a consequence of different degrees of functionalization caused by varying amounts of defects in the SWNTs, as shown by the Raman D/G intensity ratio, and also probably a varying surface area (size effect) of the SWNTs due to different SWNT production techniques. On the whole, functionalization of the SWNTs from Cheaptubes Inc. did not significantly change the HSP values, but the other SWNTs from other manufactures were more responsive to chemical treatment. The functionalization of SWNTs from Cheaptubes Inc. was accomplished, as evidenced by XPS; however, the amount of defects in the nanotubes used in this study (Cheaptubes Inc.) was probably less than the amounts in SWNTs from other manufacturers, as confirmed by Raman spectroscopy and XPS.

Table 3. Calculated HSPs of SWNTs SWNT type

δd

Cheaptubes Inc. as-received SWNTs 17.4 plasma COOH-functionalized SWNTs 17.4 plasma NH2-functionalized SWNTs 17.4 8 M HNO3-functionalized SWNTs 18.9 Concentrated HNO3 SWNTs 13.5 DDA-functionalized SWNTs 20.6 dichlorocarbene-functionalized SWNTs 16.0 Bucky USA purified SWNTs32 16.0 HNO3-functionalized SWNTs32 14.8 DDA-functionalized SWNTs32 16 dichlorocarbene-functionalized SWNTs32 14.2 Carbolex A purified SWNTs33 19.4 HNO3-functionalized SWNTs33 15.2 octadecylamine- (ODA-) functionalized 17.0 SWNTs33

δp

δh

R0

13.7 13.7 13.7 10.7 8.4 11.6 10.2

11.3 11.3 11.3 11.4 11.9 9.8 13.1

0.0 0.0 0.0 9.2 11.3 10.6 9.6

4.3 10 2.9 5.2

2.1 13.7 6.0 11.2

5.2 9.8 5 9.5

10.4 14.0 4.7

15.2 14.1 7.0

11.3 9.0 2.9

4. CONCLUSIONS The functional groups of modified SWNTs were detected by the XPS method. The extent of functionalization for plasmatreated SWNTs increased compared to those of the as-received and chemically treated SWNTs. The solubility parameters of SWNTs depend on the functional groups, degree of functionalization of the SWNT, and size of the nanotube bundles. SWNTs obtained from different suppliers and produced by different methods have different amounts of defects, leading to significant variations in the degree of functionalization in their response to the same chemical treatment and again large variations in the Hansen solubility parameters.

some extent, but DMF still remains the best solvent. The degree of functionalization for HNO3-treated SWNTs (both 8 M and concentrated HNO3) increased according to the XPS analysis, but the HSP parameters changed only slightly. The interaction radius of the HSP sphere was enlarged, and solubility in the other solvents was improved by the chemical modification treatment. In addition, we note that the solubility of the nanotubes presented in this work (Cheaptubes Inc.) differs from that of the SWNTs used in our previous work. In those cases, SWNTs produced by the CCVD method (Bucky USA)32 and SWNTs produced by the arc discharge method (Carbolex A)33 were used (see Table 3). The purified SWNTs used in those studies were compatible with several solvents, and after HNO3 treatment, the HSP centers changed significantly. The polar and hydrogen-bonding parameters increased because of functional groups such as carboxylic acid, alcohol, and ketone groups. DDA- and octadecylamine- (ODA-) modified SWNTs became less polar. Higher hydrogen-bonding parameters were found for dichlorocarbene-modified SWNTs. The Raman D/G intensity ratio for purified Carbolex A SWNTs was 0.23, and that for purified Bucky USA SWNTs was 0.22, indicating that these SWNTs had a slightly higher amount of defect sites than the nanotubes used in this study (Cheaptubes). Defects on the nanotubes make chemical functionalization more efficient, so that the change in solubility parameters becomes more significant. In our previous study on Carbolex A SWNTs34 functionlized by HNO3, the Raman D/G intensity ratio increased from 0.23 to 0.36, and for ODA-functionalized SWNTs, the Raman D/G intensity ratio increased to 0.39. From the XPS analysis of the Bucky USA SWNTs,32 it was found that the surface oxygen content of the HNO3functionalized SWNTs was 11%, which was higher than that of the purified SWNTs (3.1%). The oxygen content of DDAfunctionalized SWNTs decreased slightly from 11% to 8.1%, whereas the nitrogen content increased to 2.4%. The oxygen content (11.2%) and chlorine content (0.5%) increased when the purified SWNTs were functionalized withdichlorocarbene. In comparation with the Cheaptubes SWNTs, the degrees of functionalization of Carbolex A and Bucky USA SWNTs are higher. The different solubility behaviors of the different

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +45 9940 9296. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X-ray photoelectron spectroscopy measurements were performed with the help of Prof. Wenbin Cao of the University of Science and Technology Beijing. The authors acknowledge financial support from the China Scholarship Council.

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REFERENCES

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Effect of Surface Modification on the Hansen Solubility Parameters of

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