Effects of Oxidation by Hydrogen Peroxide on the Structures of

Aug 18, 2006 - Several functional groups such as carboxylic (−COOH), carbonyl (−C O), and hydroxyl (−OH) groups were formed on the surface of MW...
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Ind. Eng. Chem. Res. 2006, 45, 6483-6488

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Effects of Oxidation by Hydrogen Peroxide on the Structures of Multiwalled Carbon Nanotubes Yun Peng† and Hewen Liu*,†,‡ Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China, and State Key Laboratory of Polymer Physics and Chemistry, Changchun, 130022, People’s Republic of China

We studied the effects of H2O2 oxidation without metal catalysts and under neutral conditions on morphologies and structures of the multiwalled carbon nanotubes (MWNTs). The formation of surface functional groups and changes in nanotube structures, morphology, and thermal stability during oxidation were analysized by X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectra, high-resolution transmission electron microscopy, and thermogravimetric analysis. Several functional groups such as carboxylic (-COOH), carbonyl (-CdO), and hydroxyl (-OH) groups were formed on the surface of MWNTs; however, hydroxyl groups were preferentially formed and reached a maximum atomic concentration of about 46% in 4 days of oxidation. The graphitization degree decreased in the first day of oxidation; however, it readily increased in the continued oxidation days. Introduction Oxidation of carbon nanotubes (CNTs) has been a powerful tool for tailoring CNT’s surface characteristics and electronic states1,2 and has been used for very diverse purposes such as the heterogeneous oxidation catalysis,3 purification, and functionalization of CNTs4 and chemical (for example H2O2) sensors for space industry or biosensing based on oxidase,5,6 etc. Many liquid-phase oxidizing agents have been utilized in the oxidative treatments of CNTs, including permanganate (KMnO4),7 nitric acid (HNO3),8,9 a nitric acid-sulfuric acid (H2SO4) mixture,10 and hydrogen peroxide (H2O2),11,12,13 etc. Oxidation of CNTs particularly occurred at defect sites, for example, 5,7-ring defects, edges, dangling bonds, and kink sites, etc. It has been observed that in the oxidation process several functional groups such as carboxylic (-COOH), carbonyl (-CdO), and hydroxyl (-OH) groups are formed on the surface of nanotubes. Due to the different natures, some oxidants produce preferably acidic groups (-COOH), while others produce preferably “basic” groups (-OH). Surface functions can be tailored to meet demands for applications in different environments. It is important to investigate systematically the functional groups introduced by the oxidation reactions. Effects of oxidation by HNO3/H2SO4 or permanganate on the structures of CNTs have been systematically studied.7-10 However, systematic work on the oxidation of CNTs by H2O2 seems to have been ignored. Compared with other liquid-phase oxidizing agents, H2O2 is a relatively mild oxidant, and it is often used with a catalyst of ferrous ion. However, the advantage of hydrogen peroxide over other oxidants in the oxidation of CNTs lies in that H2O2 itself does not incorporate foreign metal elements into the carbon surface and can be used under neutral conditions. Thus, H2O2 oxidation could show special performances for purification and modification of CNTs. Very recently, H2O2 was used to in-situ oxidize impurities in the aligned CNT (ACNT) arrays. It is found that the in-situ purification by H2O2 is an efficient way to purify ACNT arrays * To whom correspondence should be addressed.. Tel.: +86-5513607780. E-mail: [email protected]. † University of Science and Technology of China. ‡ State Key Laboratory of Polymer Physics and Chemistry.

without damaging their alignment.11 Kataura et al. found H2O2 can selectively oxidize semiconducting single-wall carbon nanotubes (SWCNTs), and a higher than 80% concentration of metallic SWCNTs was obtained in the final product of oxidation.12 Besides purification and functionalization of CNTs, the oxidation of CNTs by H2O2 was also a central topic when using CNTs as H2O2 sensors or sensitive glucose sensors based on oxidase,14,15 which seems of high interest in biological and medical fields. Excess H2O2 is produced in the pathogenesis of brain injuries and neurodegenerative diseases, for example, Parkinson’s disease. H2O2 may damage cells through direct oxidation of lipids, proteins, and DNA, or it can act as a signaling molecule to trigger intracellular pathways leading to cell death.16 In this work, we studied the effects of oxidation by H2O2 without metal catalysts and acidic conditions on morphologies and structures of the multiwalled carbon nanotubes (MWNTs). The formation of surface functional groups and changes of nanotube structures and morphology were monitored by X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), Raman spectra, and high-resolution transmission electron microscopy (HR-TEM). The effects of oxidation on thermal stability were analysized by thermogravimetric analysis (TGA). Experimental Section Multiwalled carbon nanotubes of high purity were purchased from the Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, with outside diameters of 8-15 nm, inside diameters of 3-5 nm, and lengths of 10-50 µm. MWNTs were synthesized by chemical vapor deposition (CVD) methods with Fe/Co/Ni group catalysts. The MWNTs are of high purity with C of 99.76%, Al of 0.03%, Cl of 0.09%, and S of 0.12%, measured with energy-dispersive X-ray spectroscopy. MWNTs were used as received. Oxidation of MWNTs by H2O2 was carried out by stirring a MWNT of 0.5 g in 30% H2O2 of 10 mL for 1-6 days at 65 °C. To keep sufficient concentration of H2O2, 5 mL of H2O2 was added to the reaction mixture every day. The oxidized MWNTs were washed with deionized water, filtered through

10.1021/ie0604627 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006

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Figure 1. XPS spectra of CNTs oxidized by H2O2 in (A) 0, (B) 1, (C) 2, (D) 3, (E) 4, and (F) 6 days.

0.45 µm Milli-pore PVDF membrane. The filtrate was dried overnight at 110 °C, and then was vacuum-dried for 6 h at 150 °C to remove possible adsorbed H2O2 and attached peroxide groups on the surface of the MWNTs. XRD analysis was performed on an X’PERT PRO (PHILIPS) D/Max-Ra X-ray diffractionmeter equipped with graphitemonochromatized Cu KR radiation (λ ) 0.154 18 nm). XPS measurement was performed on an ESCALAB 250 (ThermoVG Scientific) Sspectrometer with monochromatized Al KR X-ray radiation as the X-ray source for excitation. The data were obtained at room temperature, and the operating pressure in the

analysis chamber was below 10-10 mbar with an analyzer pass energy of 50 eV. The resolution of the spectrometer was 0.6 eV. TGA was performed on a Diamond TG/DTA (PerkinElmer) with heating rate of 10 K/min under N2 atmosphere. HR-TEM pictures were taken on a JEOL JEM-2010 electron microscope with an accelerating voltage of 200 kV. Raman spectroscopy with the laser excitation line of 488 nm was performed on a LabRam-010 Raman spectrometer (Dilor, France). Raman spectroscopy with the different laser excitation lines (325 and 514.5 nm) was performed on RAMALOG 6 (SPEX, USA) at room temperature.

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Figure 2. Surface functionalities of the oxidized MWNTs in different times.

Figure 4. Change of the intensity ratio of XRD 100 to 002 in different oxidation times.

Figure 3. XRD pattern of the raw MWNT. Table 1. XPS Analysis of Oxidized Nanotubes by H2O2 C-C

C-OH

CdO

COOH

oxidation/ fwhm/ fwhm/ fwhm/ fwhm/ days at. % eV at. % eV at. % eV at. % eV 0 1 2 3 4 6

96.09 45.33 34.40 26.68 24.74 24.61

0.77 0.72 0.59 0.51 0.62 0.74

0 29.82 34.49 44.32 46.05 28.64

1.89 1.31 1.32 1.63 0.96

0 9.43 10.62 8.96 9.61 17.04

3.50 2.21 1.22 1.63 1.70

0 8.61 7.23 9.64 7.70 8.05

3.50 2.64 2.03 1.53 1.19

Results and Discussion 1. Surface Functionalization. XPS is a powerful technique for surface analysis providing chemical bonding information on the surface. XPS was performed on the raw CNT and each oxidized nanotube sample to get the information of functional groups on the nanotube surface (Figure 1). In the curve fitting, the overall peak in the range of 283-292 eV could be fitted by a superposition of four peaks. The main binding-energy peak (284.6 eV) was attributed to the C-C 1s, while the other three peaks were assigned to -C-OH (285.2 eV), -CdO (286.8 eV), and -COOH (289.2 eV), respectively. The XPS spectra confirmed the attachment of functional groups to the nanotube surface after oxidation. It is clear from Figure 1 that the peak heights and fwhm (full width at half-maximum) have obvious changes depending on the oxidation time; i.e., these peaks arising from functional groups were getting sharper with increasing oxidation time. The intensity I of photoelectrons of kinetic energies E is given by17

I ) Nσλ(E) T(E) where N is the atomic density, σ is the ionization cross-section of the observed photoelectron line according to a defined core

Figure 5. (a) Raman spectra of MWNT oxidized for 6 days under excitation of 325 (3.82 eV), 488 (2.54 eV), and 514.5 (2.41 eV) nm, respectively. (b) Integration ratios of G peaks to the D band in Raman spectra of MWNTs under excitation of 488 nm.

level and the photon energy hν, λ(E) is the inelastic mean free path, and T(E) is the transmission function of the spectrometer used. Thus, I ∝ N, and we can quantitatively analyze the atomic concentration of carbon atoms functionalized in the oxidizing process. Atomic composition of functionalities listed in Table 1 was obtained from curve fitting results based on ratios of peak areas. These values could only give us rough information about the functional groups attached to nanotubes. However, it is quite clear that much more carbon atoms were bonded with -OH groups than those bonded with -COOH groups, and a change of surface functionalities became much slower after about 1-2 days. Much more accurate quantitative analysis of functionalized nanotubes is going to be performed in the future. The change of the composition of functionalities versus oxidation time is illustrated in Figure 2. According to Figure 2, considerable oxidation effects have been reached in the first day of oxidation

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Figure 7. TGA and DTA of the raw MWNT (a, a′), and MWNTs oxidized in 1 day (b, b′), 2 days (c, c′) measured under N2 atmosphere with a heating rate of 10 °C/min.

Figure 6. (a) G peaks can be fitted by two simple Lorentz peaks of 1580 (G band) and 1610 cm-1 (D′ band). (b) Integration ratios of the G band to the D′ band under excitation of 488 nm were almost constant during oxidation days.

and slow changes happened in the continued oxidation days. The atomic concentration of C-OH reached the maximum of 46 at. % in 4 days, however, then decreased to 28.6 at. % on the sixth day. Meanwhile the surface concentration of CdO increased from the value of 9.6 at. % on the fourth day to 17 at. % on the sixth day. This is perhaps because lots of C-OH groups have been oxidized to CdO groups; however, the atom concentration of COOH changed little after the first day. A sharp peak at 3584 cm-1 in the FTIR of the MWNT oxidized for 6 days corroborated with -OH groups on the surfaces of the MWNTs. 2. Carbon Ordering. Multiwalled nanotubes can be considered to consist of several concentrically arranged single-wall carbon tubes nested into each other like a Russian doll. Figure 3 shows the XRD pattern of the MWNT used in this work where the two characteristic peaks of the MWNT arising from diffractions from in-plane regularity (100, with a peak maximum at 2θ ) 43.33°) and diffractions from interlayer spacing (002, with a peak maximum at 2θ ) 26.07°) were well-resolved. Calculated from Figure 3, the separation of the interlayer in the MWNTs was 3.42 Å, larger than 3.35 Å of the interlayer spacing in ordered graphite.18 We investigated the changes of the integration ratios of peak 100 to peak 002 (I100/I002) in the oxidation processes. The results were illustrated in Figure 4 which clearly showed that I100/I002 decreased in the first 2 days of oxidation; however, they increased readily after 2 days in the experimental time range. The value of I100/I002 is dependent on the planar order of the MWNT powder. If all parts of the MWNT sample are absolutely parallel to the 002 plane, the intensity of the 100 peak vanished.

Figure 8. Thermal stability of MWNTs oxidized in different times.

However, some parts of the long MWNT could curl to the 100 direction, which increased the contribution of diffractions from in-plane regularity to signals of the 100 peak. We guess that the change of I100/I002 illustrated in Figure 4 was due to the breaking of MWNT samples. For a long tube, breaking into shorter length can increase the degree of alignment to the 002 plane. Thus, I100/I002 decreased at first. However, further breaking would increase the content of tube ends (100 plane) and the possibility of warping. Hence I100/I002 increased in the elongated oxidation days. The carbon ordering can be determined using Raman spectroscopy. Figure 5A shows the Raman spectra of carbon nanotubes oxidized for 6 days under excitation at wavelengths λ of 325, 488, and 514.5 nm. The same sample showed very different Raman spectra under different excitation wavelengths. Two strong peaks at 1580 and 1362 cm-1 could be attributed to the G band and disorder-induced D band, respectively, analogous to those of graphite. The Raman spectrum under excitation of 325 nm showed a very low disordered content, whereas higher disordered contents were shown in Raman spectra under excitation of 488 and 514.5 nm. Lights with longer wavelength reached a deeper depth of the sample, while shorter wavelengths were shallower. The difference might indicate that the disordered parts were mainly distributed in deeper parts of the sample after long-time oxidation, for example deep inside cracks or inside the CNT. Peak G in Figure 5A excited by 488 and 514.5 nm could be fitted with two simple Lorenz peaks at 1580 and 1610 cm-1 (Figure 6A). The latter peak (often named the D′ band in the literature) was believed to arise from defective graphite and

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Figure 9. HR-TEM pictures of the original MWNT (A) and the MWNT oxidized by H2O2 for 6 days (B). Panels C and D illustrate the graphite structures of MWNT oxidized by H2O2 for 6 days. The scaling bar represents 60 nm in both A and B and 5 nm in C and D.

disordered carbons, as well as the D band. The peak of the G band (1580 cm-1) corresponded to graphite in-plane vibrations with E2g symmetry. The D peak (1362 cm-1) was generally associated with breathing vibrations of sp2 rings. The D′ peak (1610 cm-1) was believed to correspond to a strong maximum in the vibrational density of states (VDOS) of graphite. It is well-known that, in amorphous materials with no close order, the breakdown of the selection rules leads to Raman spectra that reflect the VDOS in their crystalline counterparts.19 The position of the D and D′ bands may vary considerably, depending on the structure of the disordered carbon and the excitation wavelength. When changing the excitation wavelength from 325 to 514.5 nm, the intensity D band increased, while the peak position moved to low frequency. Except for waving intensities, however, the peak positions of the D′ peak did not change. Under excitation of 325 nm, both D and D′ peaks of the Raman spectra of MWNT oxidized for 6 days were suppressed, and the D′ peak actually disappeared (Figure 5A). However, upon peak fitting results, the ratios of G band to D′

band of Raman spectra of the MWNTs oxidized in different days were almost constant within the error range (Figure 6B). Waving of data in Figure 6B can be partially attributed to a curve-fitting error. In contrast, the relative intensities of D bands vs those of G peaks (including G and D′ bands) were significantly changed during oxidation processes. The changes of the ratios of G to D are illustrated in Figure 5B. IG/ID decreased in the first day; however, they increased in the following 3 days, and reached a constant value on the sixth day (Figure 5B). The different behaviors of D and D′ bands are attributed perhaps to the different Raman-allowed modes from which D and D′ bands were generated. According to Figure 5B, the contents of ordered graphite structures seemed to decrease in the first day of oxidation by hydrogen peroxide; however, they readily increased in the continued oxidation days. After careful analysis of absorbances and scanning electron microscopy (SEM), the physical natures of the CNT samples contributed little to the variance of the ratios of IG/ID. Thus, Figure 5B indicates that oxidation simultaneously occurred at

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ordered and disordered parts; however, disordered parts or low organized carbon materials were preferentially removed under longer time oxidation. Although the fact that oxidation could increase the graphitization degree of the MWNT was also reported in the literature,12 however, the initial decrease in the contents of ordered graphite structures seemed to be ignored, perhaps because the experimental work reported in the literature was carried out under faster oxidation conditions, for example under acidic conditions. 3. Thermal Stability. TGA provides a straightforward means of characterizing the thermal stability of an oxidized nanotube sample. Lee et al. reported the thermal stability of MWNTs was related to the temperature at which the MWNT was synthesized according to their TGA results. Higher synthesis temperatures resulted in more highly crystalline nanotubes.20 It was believed that the stability of MWNT is due to the presence of defect sites along the walls and at the ends of the nanotubes. Selegue et al. found high-temperature annealing can significantly increase the oxidative stability of MWNTs, and the effect of annealing on stability enhancement in MWNTs is much more remarkable than in graphite and diamond, which suggests that raw MWNTs contain more defects than unannealed samples of diamond and graphite.21 The raw MWNT used in this work showed a decomposition temperature of 628 °C under N2 atmosphere, and the highest decomposition rate occurred at 740 °C. The TGA and DTA of the raw MWNT and MWNTs oxidized in 1 and 2 days by H2O2 are illustrated in Figure 7. A remarkable decrease in thermal stability (about 128 °C) occurred in the first day of oxidation; however, the thermal stability of the oxidized MWNT kept increasing in the continued oxidation days, from 500 °C for the MWNT oxidized in 1 day to 580 °C on the sixth day. Introducing functionalities to MWNT made the MWNT more “organic”, and the thermal stability could significantly decrease (Figure 8). However, disordered parts or low organized carbon materials were removed by oxidation in the elongated oxidation days; thus, thermal stability was increased, which is quite reasonable considering the above-mentioned Raman and XRD results. 4. Morphology and Graphitization. Electron microscopy provides a useful visual method to analyze MWNTs. HR-TEM pictures of the raw MWNT and the MWNT oxidized in 6 days are illustrated in Figure 9A,B, respectively. According to Figure 9B, the number of tube ends was increased, and the walls of the nanotubes were eroded during oxidation, which corresponded with the above discussions of the XRD and Raman results based on preferential oxidation at defect sites and breaking of nanotubes. Though the wall structures of the MWNT were eroded in oxidation in 6 days, however, the remaining parts retained good graphite structures (Figure 9C,D). Conclusions The effects of oxidation by H2O2 under neutral conditions on morphologies and structures of the multiwalled carbon nanotubes (MWNTs) were systematically studied in this work. H2O2 used in this work was 30% solution. XPS analysis showed that several functional groups such as carboxylic (-COOH), carbonyl (-CdO), and hydroxyl (-OH) groups were formed on the surface of MWNTs, however, the formation of hydroxyl groups prevailed. Hydroxyl groups reached a maximum atomic concentration of about 46% in 4 days of oxidation; however, it could be further oxidized into carbonyl groups in elongated oxidation time. Analysis of XRD patterns demonstrated that the ratios of the 100 diffraction peak to the 002 diffraction peak

decreased in the first 2 days but readily increased in the continued oxidation days within the experimental range. Raman spectra of excitation of 325, 488, and 514.5 nm were performed. Investigation of the G band and the disorder-induced D and D′ bands suggested that the graphitization degree decreased in the first day of oxidation; however, it readily increased in the continued oxidation days. TGA results showed that remarkable decrease in thermal stability (about 128 °C) occurred in the first day of oxidation; however, the thermal stability of the oxidized MWNTs kept increasing in the continued oxidation days, from 500 °C for MWNT oxidized in 1 day to 580 °C on the sixth day. High-resolution TEM was performed for the analysis of the change of morphology. All the experimental work indicated that the functionalization of MWNTs and the removal of disordered parts or low organized carbon materials occurred simultaneously during oxidation of H2O2. Overall, our work suggested that good effects could be reached in just 1 or 2 days for the purpose of surface functionalization of CNTs under the conditions described in this work. However, for long-run applications in oxidation circumstances, breaking of MWNT should be taken into account. Acknowledgment Project 50573072 supported by National Natural Science Foundation of China. Literature Cited (1) Collins, P. G.; Ishigami, K. B. M. Science 2000, 287, 1801. (2) Lim, S. C.; Jo, C. S.; JEONG, H. J.; Shin, Y. M.; Lee, Y. H.; Samayoa, I. A.; Choi, J. Jpn. J. Appl. Phys. 2002, 41, 5635. (3) Ovejero, G.; Sotelo, J. L.; Romero, M. D.; Rodriguez, A.; Ocana, M. A.; Rodriguez, G.; Garcia, J. Ind. Eng. Chem. Res. 2006, 45, 2206. (4) Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi, G. Anal. Chem. 2003, 75, 5413. (5) Zhao, Y. D.; Bi, Y. H.; Zhang, W. D.; Luo, Q. M. Talanta 2005, 65, 489. (6) Song, C. H.; Pehrsson, P. E.; Zhao, W. J. Phys. Chem. B 2005, 109, 21634. (7) Zhang, N.; Xie, J.; KVaradan, V. Smart Mater. Struct. 2002, 11, 962. (8) Zhang, X.; Sreekumar, T. V.; Liu, T.; Kumar, S. J. Phys. Chem. B 2004, 108, 16435. (9) Rosca, I. D.; Watari, F.; Uo, M.; Akasaka, T. Carbon 2005, 43. 3124. (10) Many references can be found in the functionalization of CNT, for example: Kong, H.; Gao, C.; Yan, D. J. Am. Chem. Soc. 2004, 126, 412. (11) Wang, Y.; Bai, X. D.; Liang, J. New Carbon Mater. 2005, 20, 103. (12) Miyata, Y.; Maniwa, Y.; Kataura, H. J. Phys. Chem. B 2006, 110, 25. (13) Hernadi, K.; Siska, A.; Thien-Nga, L.; Forro, L.; Kiricsi, I. Solid State Ionics 2001, 141-142, 203. (14) Zhu, Y. N.; Peng, T. Z.; Li, J. P. Chem. J. Chin. UniV. 2004, 25, 1637. (15) Lim, S. H.; Wei, J.; Lin, J. Y.; Li, Q. T.; KuaYou, J. Biosens. Bioelectron. 2005, 20, 2341. (16) Zhu, D. H.; Tan, K. S.; Zhang, X. L.; Sun, A. Y.; Sun, G. Y.; Lee, J. C. M. J. Cell Sci. 2005, 118, 3695. (17) Hesse, R.; Streubel , P.; Szargan Surf , R. Interface Anal. 2005, 37, 589. (18) Reznik, D.; Olk, C. H.; Neumann, D. A.; Copley, J. R. D. Phys. ReV. B 1995, 52, 116. (19) Hoffman, E. N.; Yushin, G.; Barsoum, M. W.; Gogotsi, Y. Chem. Mater. 2005, 17, 2317. (20) Lee, C. J.; Park, J.; Huh, Y.; Lee, J. Y. Chem. Phys. Lett. 2001, 343, 33. (21) Bom, D.; Andrews, R.; Jacques, D.; Anthony, J.; Chen, B.; Meier, M. S.; Selegue, J. P. Nano Lett. 2002, 2, 615.

ReceiVed for reView April 12, 2006 ReVised manuscript receiVed July 10, 2006 Accepted July 22, 2006 IE0604627