microwave-assisted chemical functionalization of single-walled carbon nanotubes with organic...

Chinese Journal of Chemistry, 2009, 27, 359—364 Full Paper

* E-mail: [email protected] (X. Wang), [email protected] (J. Cheng); Tel.: 0086-027-88662132; Fax: 0086-027-88665610 Received February 2, 2008; revised June 10, 2008; accepted September 10, 2008. Project supported by the National Natural Science Foundation of China (No. 50772031), the Chinese Program for New Century Excellent Talents in

University (No. NCET-05-0678), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and from Hubei Provincial Department of Education (No. Q200610005) and Science & Technology (No. 2006ABA020), China.

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Microwave-Assisted Chemical Functionalization of Single-Walled Carbon Nanotubes with Organic Peroxides

WAN, Lia(万丽) WANG, Xianbao*,a,b(王贤保) LI, Shaoqinga(李少卿) LI, Qina(李琴) TIAN, Ronga(田蓉) LI, Mingjiana(李名剑) CHENG, Jing*,c(程静)

a Faculty of Materials Science and Engineering, Hubei University, Wuhan, Hubei 430062, China b Ministry-of -Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei

University, Wuhan, Hubei 430062, China c Key Laboratory of Pesticide and Chemical Biology, Ministry-of-Education, College of Chemistry, Central China

Normal University, Wuhan, Hubei 430079, China

Microwave-assisted chemical functionalization of single-walled carbon nanotubes, with undecyl groups de-composed from lauroyl peroxides was reported. This rapid efficient procedure reduced the reaction time to 10 min, and obtained the products with higher functionalized degree than that by the conventional refluxing method. The in-fluence of different reaction time and microwave power on the functionalized degree has been explored by using FT-IR, TGA and Raman analyses. The results show that longer treatment time will lead to partial defunctionaliza-tion, and higher microwave power (higher than 900 W) can reduce the functionalized degree by removing some ini-tially-attached functional groups. Dispersion stability images and HRTEM images show that the resulting SWNT has enhanced dispersivity in organic solvents compared to the pristine nanotubes.

Keywords single-walled carbon nanotube, functionalization, microwave radiation

Introduction

Carbon nanotubes (CNT) have emerged as an inno-vative and new class of nanoscale carbon-based materi-als that are currently under intensive investigation. The unique structures and properties of single-walled carbon nanotubes (SWNT) have attracted a lot of attention over the last decade due to their potential in applications, such as molecular electronics, gas sensors and storage, field emission devices, and reinforcing materials in high-performance composites.1-3 However, insolubility of SWNT in water and organic solvent has extremely limited their applications. The chemical functionaliza-tion can improve the solubility or dispersability of SWNT,4 which is critical for their applications to pro-ducing mechanically reinforced materials.5,6 Many of the efforts so far have been devoted to conventional chemical techniques, such as refluxing7,8 and sonica-tion,9,10 which are time-consuming. For example, radical functionalization of SWNT with organic peroxide has been carried out by refluxing in o-dichlorobenzene for up to 24—120 h.11,12 So there is an urgent need to de-velop techniques for rapid chemical functionalization of SWNT. Recently microwave radiation, as a noninvasive and clean processing tool,13 has been used to activate or accelerate functionalization of single-walled carbon

nanotubs in a mixture of nitric and sulfuric acid.14,15 Such activation can provide an expanded reaction range with lower temperatures and reduced reaction time, and also can obviate unwanted side reactions and products due to thermal effects and lead to accrued cost savings. Microwave radiation has also been explored to synthe-size CNT on a variety of supports,16 to join sin-gle-walled CNT through defect generation and recon-struction,17 to assist derivatization of CNT with gold nanoparticles,18 and to enable side-wall cycloaddition to CNT19 and radical functionalization with 4-methoxy- phenylhydrazine .20

Herein, we present the microwave-assisted function-alization of SWNT with lauroyl peroxide, which mark-edly reduced the reaction time from 24—120 h to 10 min and obtained the products with higher functional-ized degree than that by a conventional refluxing method. We have also studied the influence of the reac-tion time and microwave power on the functionalization reaction.

Experimental

General

All chemicals were obtained commercially and used without further purification unless otherwise noted.

360 Chin. J. Chem., 2009, Vol. 27, No. 2 WAN et al.


SWNT samples (OD: 1—2 nm) were produced at Chengdu Organic Chemical Co., Ltd., Chinese Acad-emy of Science by a chemical catalytic vapor deposition process. FT-IR spectra were obtained on a Perkin-Elmer Spectrum One spectrophotometer. Thermo gravimetric analyses were performed by a Perkin-Elmer TGA-7 thermal analysis system. Raman spectra were deter-mined on a LabRAM HR 800 UV spectrometer (excited by 632.8 nm He-Ne radiation). TEM images were taken using a Philip Technai-20 transmission electron micro-scope.

Synthesis

The reaction was carried out by both the micro-wave-assisted chemical functionalization and the con-ventional refluxing method (Scheme 1). All the result-ing products were characterized and analyzed by FT-IR and TGA measurements, from which we could confirm this microwave-assisted rapid efficient procedure.

Scheme 1 Synthetic route of undecyl group functionalized SWNT by both conventional refluxing and microwave-assisted methods

Microwave-assisted chemical functionalization The experiments were carried out in a microwave oven (MAS-I) with a 100 mL reaction chamber. Firstly, SWNT (20 mg, 1.6 mmol) was dispersed in o-dichlorobenzene (50 mL) under sonication for 3 min, followed by adding lauroyl peroxide (1.2756 g, 3.2 mmol). Then the mixture was heated to 100 ℃ in a setting time un-der argon atmosphere, with the microwave power being set respectively to 500, 600, 700, 800, 900, or 1000 W. The functionalized SWNT was isolated by washing off the unreacted peroxide with a large amount of chloro-form on a 0.2 µm Teflon membrane. The produced bucky-film was dried under vacuum for 12 h.

Conventional refluxing chemical functionaliza-tion A mixture of SWNT (20 mg, 1.6 mmol) and o-dichlorobenzene (50 mL) was added to a 100 mL re-action flask, under sonication for 3 min, followed by adding lauroyl peroxide (1.2756 g, 3.2 mmol). Then the mixture was refluxed at 100 ℃ in a setting time under argon atmosphere, stirred using a magnetic stirring ap-paratus. The functionalized SWNT was isolated by washing off the unreacted peroxide with a large amount of chloroform on a 0.2 µm Teflon membrane. The pro-duced bucky-film was dried under vacuum for 12 h.

Results and discussion

IR spectra

The FT-IR spectra with pressing potassium bromide (KBr) troches of the functionalized SWNT were ob-tained to determine the chemical group on the nanotubes. As SWNT has good adsorptivity, and can adsorb water in the circumstance, all the curves in FT-IR spectra have the broad peaks centered at about 3420 cm－1, which were assigned to the —OH stretching mode of the H2O, and were not the characteristic peaks of the undecyl groups, not among the subjects to be discussed.

In the spectrum of the pristine nanotubes (Figure 1A-a), the infrared absorption is extremely low. After functionalization, the typical FT-IR spectra (Figure 1B—1G) show some characteristic peaks, assigned as fol-lows: The peaks in 2980—2840 cm－1 characterize the antisymmetric and symmetric stretching modes of the methyl and methylene groups. The antisymmetric modes of —CH3 and —CH2— bending vibrations were observed in 1460—1440 cm－1. The peak at about 1380 cm－1 belongs to the methyl symmetric bend, which is isolated from other peaks. The weak vibrations in 850—720 cm－1 were observed for more than four —CH2— groups. All these peaks are owed to the at-tachment of the undecyl functional group.

Compared to the FT-IR spectra of the conventional refluxing method (Figure 1G), the characteristic peaks in the spectrum of the microwave-assisted chemical functionalization (Figure 1B—1F) are more prominent, which qualitatively indicates the efficiently micro-wave-assisted procedure.

A series of comparison experiments show that the intensities of the characteristic peaks, which reflect the functionalized degree, vary with the reaction time and the microwave power. With increasing the reaction time (from 30 min to 8 h), the intensities of the peaks, which are located in 2980—2840 cm－1 and at about 1380 cm－1, become stronger and stronger. However, the peak intensities of the samples at the reaction time of 10 min are extremely prominent. We also carried out the reac-tion for 5 min, but its FT-IR spectrum shows not-so- prominent peaks (Figure 1A-b), which indicates lower functionalized degree for this period of time. Thus the results reveal that 10 min of reaction is enough to func-tionalize the SWNT (Figure 1F). Our results also indi-cate that the microwave power has an influence on the functionalization reaction. The spectra of the samples at different power show differences in the intensities of the peaks in 2980—2840 cm－1. This change was obvious in the spectra of the products at the reaction time of 8 and 4 h (Figure 1B and 1C), while the change for the re-duced reaction time (Figure 1D and 1E) became vaguer due to the lower functionalized degree. And this influ-ence of different reaction time and microwave power will be further indicated and analyzed in the following TGA measurement.

Single-walled carbon nanotube Chin. J. Chem., 2009 Vol. 27 No. 2 361


Figure 1 FT-IR spectra of (A) pristine SWNT, (B)—(F) SWNT functionalized by undecyl groups at different reaction time and mi-crowave powers, and (G) SWNT functionalized by a conventional refluxing method.



TGA measurement

The functionalization of SWNT has also been inves-tigated by TGA measurement and compared with the pristine SWNT (Figure 2A). The heating was carried out under nitrogen at a heating rate of 10 ℃/min from room temperature to 800 ℃. The TGA trace showed that the weight loss of the pristine SWNT is negligible below 500 ℃ and then increases linearly to 800 ℃,

Figure 2 (A) TGA of SWNT functionalized by undecyl groups for 4 h; (B) weight loss curve (150 to 500 ℃ in nitrogen) as a function of the microwave power for the functionalized SWNT with different reaction time, and (C) TGA of SWNT functionalized by a conventional refluxing method. Experiments were carried out in nitrogen (hearting rate 10 ℃/min).

indicating the tube decomposition, while the functional-ized SWNT has a strong weight loss between 150 and 500 ℃ (Figure 2), which corresponds to the destruc-tion of the undecyl groups grafted on the SWNT sur-face.

As the indication in FT-IR spectral analysis, the TGA measurement quantitatively confirmed that the microwave-assisted chemical functionalization was a rapid efficient procedure. The TGA weight loss data (150—500 ℃) of the conventional refluxing function-alizing reactions for 8 h, 2 h and for 10 min are 10%, 10% and 9%, respectively (Figure 2C and Table 1), while the results from the microwave-assisted chemical functionalization for the same reaction time are larger. And comparing the TGA weight loss data of the con-ventional refluxing functionalized reaction for 120 h (13%) with those of the microwave-assisted chemical functionalization for 10 min (15% on average), indicates that using the microwave-assisted method, we can ob-tain the products with higher functionalized degree at shorter reaction time. All of those prove that the micro-wave-assistd chemical functionalization is a rapid effi-cient procedure.

Table 1 TGA weight loss data (150 to 500 ℃) of undecyl group functionalized SWNT by a conventional refluxing method

Product TGA weight loss (150—500 ℃)/%

120 h 13

8 h 10

2 h 10

10 min 9

TGA analysis verifies the influence of different re-

action conditions on the functionalized degree as indi-cated by FT-IR spectral measurement (Figure 2). The weight losses of the products obtained from 8 and 4 h reactions are much larger than those from the 2 h and 30 min reactions, while the products obtained from the 10 min reaction have similar TGA weight loss as those from the 8 h reaction. This indicates that, at the begin-ning of the reaction, microwaves mainly assist to func-tionalize the SWNT; however, as the reaction continues, the reactant is consumed and at some point the kinetics of the functionalization process is overcome by that of the defunctionalization process. Therefore, the micro-wave-assisted protocol involves a competitive functionalization with defunctionalization of undecyl groups. 10 min of reaction time were optimal, and longer treat-ment led to partial defunctionalization. The TGA weight losses as a function of microwave power (Figure 2B) show that under the same reaction time, the weight losses first decrease from the microwave power of 500 to 700 W, and then increase at 800 and 900 W, while at the microwave power of 1000 W, the weight losses de-crease again, which is in good agreement with the result of the FT-IR spectra (Figure 1B—1F). The data thus suggest a competitive effect of microwave power: it can

Single-walled carbon nanotube Chin. J. Chem., 2009 Vol. 27 No. 2 363


promote functionalization, but too higher microwave power will remove some initially-attached functional groups, causing the undecyl radical coupling and mak-ing the degree of the functionlization very low.

Raman spectra

Raman analysis (Figure 3), on the other hand, gives confirmative evidence to the functionalization of SWNT. Several representative products were chosen for this measurement. Pristine SWNT (Figure 3a) exhibits a stronger C—C tangential mode band at 1578 cm－1 (G) and a weak band at 1317 cm－1 (D) attributed to sp3 hy-bridized carbon atoms in the hexagonal framework of the nanotube walls. After functionalization the disorder mode band near 1350 cm－1 (D) shows an obvious en-hancement in intensity (Figure 3b－3f). A decrease in the relative intensities, using the G band (at about 1593 cm－1) as an internal reference, of these bands was ob-served upon undecyl functionalizing reaction. In addi-tion, the G band and D band are noticeably up-field shifted in the spectra of functionalized SWNT relative to those of the pristine SWNT, as expected as that the groups are attached to the sidewalls of the nanotubes. The slight differences between the relative intensities of D and G bands (Figure 3b－3f) give indications of the extent of disorder introduced in the functionalization steps, which was influenced by different reaction condi-tions.

Figure 3 Raman spectra of SWNT functionalized by undecyl groups for different reaction time at different microwave powers.

Dispersion stability images

The dispersibility of undecyl functionalized SWNT obtained after microwave treatment was enhanced in some organic solvents, extremely in chloroform (CHCl3). A dispersed concentration of 0.75 mg/mL was obtained in CHCl3 under sonication for 15 min (Figure 4A). The experiment of dispersion stability showed very few SWNT precipitaed after prolonged standing (24 h) under ambient conditions, whereas the pristine nano-tubes precipitated, just after sonication, to the bottom of the container (Figure 4B).

Figure 4 Dispersion stability images of (A) the raw nanotube materials and the functionalized nanotubes, after being dispersed in CHCl3 under sonication for 15 min; (B) the raw nanotube ma-terials and the functionalized nanotubes, after being kept still for 24 h.

HRTEM images

Additional evidence for surface modification of the nanotubes was provided by inspection of the HRTEM images presented in Figure 5. All the TEM samples were prepared by deposition on TEM grids from a dilute CHCl3 solution of the raw materials and functionalized nanotubes. A typical HRTEM image from microwave functionalized SWNT (Figure 5B) showed individual and thin SWNT bundles, while the raw materials (Fig-ure 5A) exhibit thick bundles. Compared to the smooth and clean surfaces of tube walls of the raw materials, the surfaces of out-walls of most functionalized nano-tubes were coarse (Figure 5C), which resulted from the attachment of the functionalization groups to the wall surfaces of the nanotubes.

Figure 5 HRTEM images of the raw nanotube materials (A) and the functionalized nanotubes (B and C).



Conclusion

In conclusion, microwave-assisted chemical func-tionalization of SWNT with organic peroxides has been shown to give a rapid and highly efficient processing to produce undecyl group functionalized SWNT with an optimal reaction time of 10 min. FT-IR spectra, TGA and Raman analyses show the high functionalized de-gree. The weight loss as a function of microwave power together with FT-IR spectra confirms the influence of different reaction time and microwave power on the functionalized degree. The results show that longer treatment time will lead to partial defunctionalization, and higher microwave power (higher than 900 W) can reduce the functionalized degree by removing some ini-tially-attached functional groups.

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microwave-assisted chemical functionalization of single-walled carbon nanotubes with organic...

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