A Comparison of Defects Produced on Oxidation of Carbon

European Journal of Scientific Research ISSN 1450-216X Vol.33 No.2 (2009), pp.295-304 © EuroJournals Publishing, Inc. 2009 http://www.eurojournals.com/ejsr.htm A Comparison of Defects Produced on Oxidation of Carbon Nanotubes by Acid and UV Ozone Treatment Abu Bakar Sulong Dept. of Mechanical and Materials Engineering, Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi,Selangor, Malaysia E-mail: abubakar@eng.ukm.my Tel: +603-8921-7029; Fax: +603-8925-9569 Che Husna Azhari Dept. of Mechanical and Materials Engineering, Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi,Selangor, Malaysia Tel: +603-8921-7029; Fax: +603-8925-9569 Rozli Zulkifli Dept. of Mechanical and Materials Engineering, Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi,Selangor, Malaysia Tel: +603-8921-7029; Fax: +603-8925-9569 Mohd Roslee Othman Dept. of Chemical Engineering, College of Engineering Universiti Sains Malaysia 14300 Nibong Tebal, Pulau Pinang, Malaysia Joohyuk Park Dept. of Mechanical Engineering, School of Engineering, Sejong University 98 Gunja dong, Gwanjin gu, 143-747 Seoul, South Korea Abstract Oxidation using ozone in the presence of ultraviolet light was used to reduce the number of defects on carbon nanotubes. Fourier transform infra red spectra showed that the treatment promotes milder oxidation in introducing carboxylic functional group than conventional acid treatment. Transmission electron microscope images suggest that ozone treated carbon nanotubes suffered reduced attrition of broken tips, bent tips and bent walls in comparison to that of acid treated carbon nanotubes. Raman spectra indicate that the acid treated carbon nanotubes exhibited a lower IG:ID ratio than these ozone treated carbon nanotubes, confirming that the former samples contain a higher number of defects. While the dispersion stability of the ozone treated carbon nanotubes was found at a level similar to that of acid treated carbon nanotubes, the former showed a slightly larger amount of organic functional groups than the latter as suggested from the thermal gravimetric analysis. Keywords: Materials processing, Nanostructure, Oxidation, Chemical processes, A Comparison of Defects Produced on Oxidation of Carbon Nanotubes by Acid and UV Ozone Treatment 296 1. Introduction Carbon nanotubes (CNTs) have attracted considerable interest due to their properties and potential applications in nanotechnology [1,2]. In the application as reinforcement of fillers in polymer matrix, CNTs must demonstrate good dispersion, high interfacial stress transfer and better alignment of CNTs in the polymer matrix before they can be of notable use. However, in this application, the functionalities of CNTs are frequently limited by their insolubility in most organic solvent [3]. Several methods of chemical functionalization have been developed to enhance the solubility of CNTs. The most widely accepted approach to enhance the solubility of CNTs is through wet chemical oxidation process. This approach is initiated by mixing CNTs with strong acids such nitric acid, sulfuric acid or a mixture of the two. In the process, carboxylic groups are introduced so that CNTs could connect with longer chain functional groups such as amine groups, the charge balance around the CNTs is perturbed and the electronic polarity is induced in order to enhance the solubility of CNTs in the organic solvents [4]. The wet chemical functionalization is also required for purification of as produced (pristine) CNTs from graphite compounds, amorphous carbon, fullerenes, coal and metal nanoparticles. Depending on their applications, CNTs may also be modified through covalent sidewall functionalization, noncovalent exohendral functionalization with surfactants, noncovalent exohendral functionalization with polymers, and endohendral functionalization of CNTs through wet chemical processes [4-9]. While enhanced solubility and dispersion could be realized from the wet chemical functionalization, the unintended outcome from this method would be introduction of carboxylic groups at defects. Typical defects in CNTs following this approach would be; (a) bending tubes caused by five-or sevenmember rings in the framework, (b) sp3-hybridized defects, (c) C framework damaged by oxidative conditions, and (d) emergence of open ended CNTs.In addition, oxidation process with strong acids creates significant physical damages to tube chirality, helicity and micro pathway of CNTs, causing severe degradation of the originally desirable properties of CNTs [10-15]. The process also involves the use of acids which may not be environmentally friendly if certain precautions are not heeded. Dry oxidation such using ozone in the presence of ultraviolet (UV/O3) have been an alternative treatment to resolve the issues associated with the wet oxidation [16-19]. In the photosensitized oxidation incorporating UV/O3, the molecules are excited or dissociated by the absorption of short-wavelength UV radiation. The carbon atoms (preferably at the defect sites on CNTs) react with the atomic oxygen from the continuous dissociations of oxygen molecules to generate ozone molecules for the carboxylation reaction to take place [18]. This oxidation can be enhanced if lattice defects and StoneWales defects exist in the microstructure of CNTs. In this work, we attempted at studying the defects as a result of acid and UV/O3 treatment by observing the physical changes of CNTs and quantifying the defects through Raman spectroscopy. We also conducted experiments in order to learn the effect of the two different oxidation methods on the solubility of CNTs and the formation of the CNT conductive pathways under an applied electrical field. 2. Experimental As produced multiwall carbon nanotubes (MWCNTs) were purchased from ILJIN Nanotech (South Korea). The samples were synthesized by thermo-chemical vapor decomposition of hydrocarbon gases and were advertised as having 5~10 nm diameter, 10~20 μm length, and 95% purity. In this study, MWCNTs are classified into three types: (i) As produced MWCNTs; (ii) UV ozone (UV/O3) treated MWCNTs; and (iii) Acid treated MWCNTs. Prior to UV/O3 and acid treatment, as produced MWCNTs were mechanically cut using ULTRA TURRAX T25 high speed stirrer at 19,000 rpm for 2 hrs. The samples were placed in Ulsso Hi-tech ULH 700S horned type ultrasonicator in methanol for 1 hr. The purpose of performing mechanical cutting was to shorten the length of CNTs in order to achieve better dispersion. The mechanical cutting was also performed to cleanse CNTs from foreign 297 Abu Bakar Sulong, Che Husna Azhari, Rozli Zulkifli, Mohd Roslee Othman and Joohyuk Park objects that might attach on their surfaces. Then, MWCNTs were exposed to UV/O3 for 30, 60, 90 minutes at 38 mW/cm2 of radiation intensity. Ultra-violet radiation was beamed from UV hand lamps. The radiation intensity was measured by a UV Itec UV radiometer with microprocessor. Acid treated MWCNTs were prepared by dispersing MWCNTs in 3M nitric acid for 12 hrs at 70ºC. 3M acid concentration and 12 hrs time period were selected following the previous report that states minimum amount of defects were present at these conditions [20]. The existence of carboxylic groups at MWCNTs was detected using Thermo-electron Nicolet 380 Fourier transformed infrared spectroscopy (FTIR). Image analysis of physical defects at walls and tips of MWCNTs was performed using Phillips Technai F20 transmission electron microscopy (TEM) and Hitachi S-4700 scanning electron microscopic (SEM). The quantification of defects in MWCNTs was measured from the IG:ID ratio utilizing Jobin-Yvon LabRam HR Raman spectrometer. The dispersion stability test for MWCNT in different solvents was performed by dispersing the sample for 30 mins in 50 mL tall glass bottle using a MUJIGAE bath sonicator and then the sample was allowed to settle for 48 hrs. The thermogravimetric property of MWCNTs was analyzed using Scinco STA S-1500 simultaneous thermal analyzer. For analysis of conductive pathway, direct current (DC) electric field was applied to MWCNTs dispersed in isopropyl alcohol (IPA) through 1 mm distance gap of copper electrodes. The movements of MWCNTs were observed by a union optical microscope. 3. Results and Discussion Figure 1 shows the SEM image of as produced MWCNTs taken at random. The TEM image of the sample suggests that the as produced MWCNTs are pristine, allowing the sample be used without further purification. The FTIR spectra of as produced, acid treated and UV/O3 treated MWCNT samples are presented in Figure 2. Carboxylic functional groups were not detected from as produced MWCNTs. The peaks at 1210, 1380, 1460, 1750 and 3500 ~ 3750 cm-1 corresponding to C-O-C, C-O, C=C, C=O, O-H bonds were observed after the samples were acid and UV/O3 treated [18-21]. The spectra of the latter sample indicate that the presence of these groups became more conspicuous with increasing UV/O3 exposure time, providing evidence that UV/O3 was able to successfully introduce carboxylic functional groups into the microstructure of MWCNTs in the same way as the groups were introduced into the acid treated samples. Figure 1: As produced MWCNTs: (a) SEM image; and (b) TEM image (a) (b) 100nm Physical defects at the center’s walls of CNTs were observed by TEM as presented by Figure 3. Bending walls were observed to emanate from the as produced and acid treated MWCNTs samples as shown in Figure 3(b) and 3(c), whereas, virtually no such defect was observed in the UV/O3 treated MWCNTs in Figure 3(d). Figure 4 shows TEM images of MWCNTs at tips. Broken and bending tubes were observed in the acid treated MWCNTs such as shown in Figure 4(b) and 4(c), but such defects were hardly observed in the as produced samples and UV/O3 treated MWCNTs. In order to verify the micro-physical analysis from the TEM and SEM images, we made quantification of the defects using A Comparison of Defects Produced on Oxidation of Carbon Nanotubes by Acid and UV Ozone Treatment 298 Raman Spectrometer. General Raman spectra of CNTs show two phonon peaks; one is called graphite intensity peak (IG: 1590 cm-1) and the other graphite defect intensity peak (ID: 1350 cm-1) [8,22,23]. Figure 2: FTIR spectra of different MWCNTs samples. As produced MWCNTs Acid treated MWCNTs UV/O3 treated MWCNTs (30 min) UV/O3 treated MWCNTs (60 min) UV/O3 treated MWCNTs (120 min) COOH/C=O C-OC-O-C C=C Absorbance (a.u.) O-H 4000 3500 3000 2500 2000 -1 1500 1000 Wave number (cm ) Figure 3: TEM images at center’s walls of: (a) As produced; (b) Acid treated (c) UV/O3 treated MWCNTs: (a) (b) Bending 10 nm 10 nm (c) Bending (d) 10 nm 10 nm 299 Abu Bakar Sulong, Che Husna Azhari, Rozli Zulkifli, Mohd Roslee Othman and Joohyuk Park Figure 4: TEM images at tips of: (a) As produced; (b) Acid treated – i; (c) Acid treated – ii; and (d) UV/O3 treated MWCNTs. (a) (b) 10 nm 10 nm Broken (c) Bending 10 nm (d) 10 nm Figure 5: Raman spectra with IG:ID average ratio for: (a) As produced; (b) Acid treated; and (c) UV/O3 treated MWCNTs As produced MWCNTs (i) As produced MWCNTs (ii) I G As produced MWCNTs (iii) Acid treated MWCNTs (i) Acid treated MWCNTs (ii) Acid treated MWCNTs (iii) (a) Intensity (a.u.) (b) Intensity (a.u.) Average IG:ID = 6.25 ID Average IG:ID = 1.68 ID IG 1000 1100 1200 1300 1400 1500 -1 1600 1700 1800 1000 1100 1200 1300 1400 1500 -1 1600 1700 1800 Raman shift ( cm ) Raman shift ( cm ) UV/O3 treated MWCNTs (i) UV/O3 treated MWCNTs (ii) UV/O3 treated MWCNTs (iii) IG (c) Intensity (a.u.) Average IG:ID = 5.43 ID 1000 1100 1200 1300 1400 1500 -1 1600 1700 1800 Raman shift (cm ) The overlay of the three measurement results of Raman spectra for the different samples of MWCNTs is presented in Figure 5. Acid treated MWCNTs were found to exhibit the least value of IG:ID ratio, indicating that the samples were saturated with the highest amount of defects. In contrast, A Comparison of Defects Produced on Oxidation of Carbon Nanotubes by Acid and UV Ozone Treatment 300 UV/O3 treated MWCNTs demonstrated relatively lower IG:ID ratio than the as produced MWCNTs but significantly higher ratio than the acid treated MWCNTs, indicating that the UV/O3 treated samples contained much less amount of defects than the acid treated samples. The latter results also suggest that UV treatment was a milder oxidation process than acid treatment to introduce carboxylic functional groups into MWCNTs with the least amount of defects. Figure 6: Dispersion stability of: (i) As produced; (ii) Acid treated; and (iii) UV/O3 treated MWCNTs in solvents: (a) Methyl chloride; (b) DMF; and (c) Toluene for 30 mins and 48 hrs. 30 mins a-i a-ii a-iii a-i 48 hrs a-ii a-iii 30 mins b-i b-ii b-iii b-i 48 hrs b-ii b-iii c-i 30 mins c-ii c-iii c-i 48 hrs c-ii c-iii The dispersion stability of MWCNTs in various solvent is shown in Figure 6. The common organic solvent selected in this study was methylene chloride, dimethylformamide (DMF) and toluene with residence time at 30 mins and 48 hrs. UV/O3 treated MWCNTs reveal slightly better dispersion 301 Abu Bakar Sulong, Che Husna Azhari, Rozli Zulkifli, Mohd Roslee Othman and Joohyuk Park stability in methylene chloride than acid treated MWCNTs during the first 30 mins. However, the UV/O3 treated MWCNTs and acid treated MWCNTs in methylene chloride show nearly similar dispersion stability at 48 hrs. While the dispersion of both MWCNTs in DMF remained the same at 48 hrs, the three MWCNTs samples were not dispersed at all in toluene. The thermo-gravimetric analysis of different MWCNTs is presented in Figure 7. The presence of carboxylic functional groups in the samples caused more weigh loss on heating due to the decomposition of the organic functional groups at high temperature. At temperature region between 600 and 655°C, UV/O3 treated MWCNTs experienced the highest weight loss due to the more of the carboxylic functional groups that were burnt off. Further heating at higher temperature resulted in a complete decomposition of the organic compounds from all the three samples. The results suggest that the carboxylic functional groups in UV/O3 treated MWCNTs were more resilient to the extreme temperature condition. Figure 7: Thermal gravimetric analysis of different types of MCWNTs. 110 100 weight degradation (wt%) 90 80 70 60 50 40 30 20 10 0 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 As produced MWCNTs Acid treated MWCNTs UV/O3 treatedMWCNTs Temperature ( C) o When an electric field was applied to CNTs, dispersed in an electrolyte medium, a fraction of the CNTs would move forward to anode under electrophoresis due to the presence of negative surface charges. As soon as these CNTs were close enough to the electrode to allow charge transfer, CNTs discharged and adsorbed onto the anode. Tips of CNTs connecting the electrode then became sources of high field strength and the location for further adsorption of CNTs. As a result, ramified CNTs network structures extended away from the anode, eventually reaching the cathode and providing conductive pathways throughout the samples [15]. These pathways were seen when the as produced MWCNT and UV/O3 treated samples were dispersed in IPA and DC electric field (400V/cm2) was applied as shown in Figure 8 (a) and (c). The CNTs conductive pathway network was not developed for system containing acid treated MWCNTs under similar applied electric field in this work. This might be attributable to the stronger charge effects of the acid treated MWCNTs, causing them to be more affinitive for the cathode than among themselves. As a result, acid treated MWCNTs were more inclined to be adsorbed and deposited onto the cathode surface as shown in Figure 8(b). It was reported that electrical conductivity of UV/O3 treated MWCNTs in polymethylmethacrylate (PMMA) matrix was better than acid treated MWCNTs composites [17]. It also reported significant reduction of the electrical conductivity of as produced MWCNTs epoxy composites, compared to acid treated MWCNTs epoxy composites [13]. A Comparison of Defects Produced on Oxidation of Carbon Nanotubes by Acid and UV Ozone Treatment 302 Figure 8: CNTs conductive pathway network under DC electric field (400 V/cm2): (a) As produced; (b) Acid treated; and (c) UV/O3 treated MWCNTs in IPA (a) (-) (+) (b) (-) (+) (c) (-) (+) 4. Conclusion Carboxylic functional groups were successfully introduced into the microstructure of UV/O3 treated MWCNTs in the same manner as the groups were introduced into the acid treated samples but, the former generated relatively lower amount of defects than the latter samples. Physical defects such as bending structures and broken ended tips that emanated from the acid treated samples might contribute to lower dispersion stability and absence of conductive pathway network under applied electric field. It was also discovered that the stronger charge effects might as well bring about the same effects. Ozone treatment in the presence of UV is considered more favorable than the acid treatment due to the milder oxidation process and the desired characteristics of the functional MWCNTs. 5. Acknowledgement The work was supported by grant No. R01-2003-000-10-72-0 from the Basic Research Program of the Korea Science & Engineering Foundation. Materials provided by ILJIN Nanotech and financial assistances from Sejong University, Universiti Kebangsaan Malaysia and Ministry of Science, Technology and Innovation (Malaysia) are gratefully acknowledged. 303 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] Abu Bakar Sulong, Che Husna Azhari, Rozli Zulkifli, Mohd Roslee Othman and Joohyuk Park Paradise, M., and T., Goswami, 2007. “Carbon nanotubes – production and industrial applications” Material & Design 28, pp. 1477-1489. Popov, V.N., 2004. “Carbon nanotubes: properties and application” Mater Sci Eng R 43, pp. 61-102. Bahr, J.L.,, Mickelson, E.T.,, Bronikowski, M.J., Smalley, R.E., and J.M. Tour, 2001. “Dissolution of small diameter single-wall carbon nanotubes in organic solvents”, Chem. Commun. 2, pp. 193-194. Chen, J., Rao, A.M., Lyuksyutov, S., Itkis, M.E., Hamon, M.A., Hu, H., Cohn, R.W., Eklund, P.C., Colbert, D.T., Smalley, R.E., and R.C. Haddon, 2001. “Dissolution of full-length singlewalled carbon nanotubes” J Phys. Chem. B 105, pp. 525-2528. Banerjee, S., Kahn, M.G.C., and S.S., Wong, 2003. “Rotional chemical strategies for carbon nanotube functionalization” Chem. Eur. Journal, 9, pp. 1898-1908. Sun, Y.P., Fu, K., Lin, Y., and W. Huang, 2002. “Functionalized carbon nanotubes: properties and applications” Acc. Chem. Res., 35, pp. 1096-1104. Hirsh, A. 2002. “Functionalization of single-walled carbon nanotubes”. Ang Chem Int, 41, pp. 1853-1859. Sulong, A.B., Park, J.H., Lee, N.S., and J.H. Goak, 2006 “Wear behavior of functionalized multi-walled carbon nanotubes reinforced epoxy matrix composites” Journal of Composite Material, 40, pp. 1947-1960. Tan, S.H., Goak, J.C., Lee, N.S., Kim, J.Y., and S.C. Hong, 2007 “Functionalization of multiwalled carbon nanotubes with Poly(2-ethyl-2-oxazoline)” Macromol Symp 2007, 249-250, pp. 270-275. Kim, S.H., Kim, J.W., Im, J.S., Kim, Y.H., and Y.S. Lee, 2007 “A comparative study on properties of multiwalled carbon nanotubes (MWCNTs) modified acids and oxyfluorination” J. of Flourine Chem.,128, pp. 60-64. Zhang, Y., Shi, Z., Gu, Z., and S. Iijima, 2000. “Structure modification of single-wall carbon nanotubes” Carbon, 38, pp. 2055-2059. Wang, Y., Wu, J., and F. Wei, 2003. “A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and large aspect ratio” Carbon,41, pp. 2939-2948. Park, J.H., and A.B. Sulong, 2007. “Effect of chemically surface modified MWNTs on the mechanical and electrical properties of epoxy nanocomposites” Studies in Surf. Sci. & Catalysis, 165, pp. 405-408. Martin, C.A., Sandler, J.K.W., Windle, A.H., Schwarz, M.K., Bauhofer, W., Schulte, K., and M.S.P. Shaffer, 2005. “Electric field-induced aligned multi-wall carbon nanotube networks in epoxy composites” Polymer, 46, pp. 877-886 Bubke, K., Gnewuch, H., Hempstead, M., Hammer, J., and M.L.H. Green, 1997. “Optical anisotropy of dispersed carbon nanotubes induced by an electric field” Appl Phys Lett, 71, pp. 1906-1908. Najafi, E., Kim, J.Y., Han, S.H., and K.W. Shin, 2006. “UV-ozone treatment of multi-walled carbon nanotube for enhanced organic solvent dispersion” Colloids and Surfaces A: Physichem Eng Aspect, 284-285, pp. 373-378. Cai, L., Bahr, J.L., Yao, Y., and J.M. Tour, 2002. “Ozonation of single-walled carbon nanotubes & their assemblies of rigid self-assembled monolayers” Chem Mater, 14, pp. 42354241. Sham, M.L., and J.K. Kim, 2006. “Surface functionalities of multi-walled carbon nanotubes after UV/ozone and TETA treatment” Carbon, 44, pp. 768-777. Grujicic, M., Cao, G., Rao, A.M., Tritt, T.M., and S. Nayak, 2003. “UV-light enhanced oxidation of carbon nanotubes UV-light enhaced oxidation of carbon nanotubes” Applied Surface Science, 214, pp. 289-303. References A Comparison of Defects Produced on Oxidation of Carbon Nanotubes by Acid and UV Ozone Treatment [20] [21] [22] 304 [23] Kukovecz, A., Kramberger, Ch., Holzinger, M., Kuzmany, H., Schalko, J., Mannsberger, M., and A. Hirsh, 2002. “On the stacking behavior of functionalized single-walled carbon nanotubes” J. Phys. Chem. B, 106, pp. 6374-6380. Socrates, G., 1980. “Infrared characteristic group frequencies” John Wiley & Sons, New York. Rao, A.M., Richer, E., Bandow, S., Chase, B., Eklund, P.C., Williams, K.A., Fang, S., Subbaswamy, K.R., Menon, M., Rhess, A., Smalley, R.E., Dresselhaus, G., and M.S., Dresselhaus, 1997. “Diameter-selective raman scattering from vibrational modes in carbon nanotubes” Science, 275, pp. 187-191. Dresselhaus, M.S., Dresselhaus, G., Saito, R., and A., Jorio, 2005, “Raman spectroscopy of carbon nanotubes” Physics Reports, 409, pp. 47-99.