HIGHLY DISPERSED MULTI-WALLED CARBON NANOTUBES IN WATER/ETHANOL AND ITS APPLICATION FOR PVA COMPOSIT

ACTA METALLURGICA SINICA (ENGLISH LETTERS)Vol. 18 No. 3 pp 389-396 June 2005HIGHLY DISPERSED MULTWALLED CARBON NANOTUBESIN WATERETHANOL AND ITS APPLICATIONFOR PVA COMPOSITEY Li2,XB. Zhang2,XY. Tao), J.M. Xu, F. Liu and G.L. Xu21) State Key laboratory of Silicon Materials, Department of Materials Science and Engineering, ZhejiangUniversity, Hangzhou 310027, China2) Institute of Materials Physics and Microstructures, Department of Materials Science and Engineering,Zh3)College of Electronic Information, Hangzhou Dianzi University, Hangzhou 310018, ChinaManuscript received 13 December 2004Two carboxylation processes were carried out to modify multi-walled carbon nanotube bundles (m-MWNTs). The results showed that both of the m-MWNT's could be highly dispersedin water with the concentration up to 25 and 15mg/mL in ethanol without obvious depositionor conglomeration over 6 months. FTIR, Raman, TGA and TEM characterizations of the mMWNTs showed that hydroxyl and carboxyl groups have been attached with MWNTS, andthat the nanotube structure was only damaged a little after modification treatments in mixedacids over 24h. Above prepared nanotube suspensions in water were successfully used as afiller of thermoplastic Pva composite to enhance its electrical conductivity. The resultshowed that the addition of m-MWNTs greatly improved the conductivity and the thresholdof mass fraction was about 5wr%. The results also proved that purification process was notneeded because of high purity of the raw MWNTs prepared in our groupKEY WORDS carbon nanotube, polyvinylalcohol composite, conductivity1. IntroductionCarbon nanotubes(CNTs) are of great interest from both the fundamental and practical points ofviewsIl-31. Electronic structures4), polymer composites 67 and biological systems s) are some of the impor-tant application fields that being intensively studied. Previous electrical conductivity measurements ofindividual CNT showed that CNTs containing defects showed higher resistance than graphite CNTS,which was attributed to an increase in electron scattering by the internal defects 9, 10. Theoretical studiesby Rochefort et al. proposed that the introduction of defects into CNTs lattices would reduce CNTs conductivity by 30%-50%, which is consistent with the experimental results However, recently it wasfound that the defective CNTs made by chemical vapor depositVi) nrocess exhibited a higherconductivity than that of the Cnts by arc discharge and las中国煤化工cially, a big breakthrough has been obtained in mass production of multi-waCnMH Gy CVD method(4)Therefore, it is very attractive and necessary to fully take above advantages of the CVD-made mwntsin application for making conductive composites with polymers390However, the research and applications of carbon nanotubes are often hindered by processing andmanipulation difficulties due to their inherent insolubility or poor dispersion in water, common organicsolvents and polymeric matrixes. From the literature it is reported that sulfuric and nitric mixed acid canmodify the surface of the MWNTs to improve the solubility of nanotubes because of the formation of hydrophilic groups such as hydroxyl and carboxyl groups on the nanotube sidewalls and ends 5, 6. These at-tached hydroxyl and carboxyl groups on tubes have many potential advantages for further functionization of nanotubes to form amides 9), acid base chemistry and ester linkages 2), and so on. Moreoverthe carboxyl groups on the tubes can be used for biosensors in water and for the reinforcing compo-nent of composites. Hence, it is valuable to modify CNTs surface with above groups in a large scale,which can be homogeneously dispersed in different solvents in order to further synthesize CNT-compos-ites with designed functionsWe herein introduced that MWNTs modified by the mixture of sulfuric and nitric acids could behighly dispersed in water with a concentration over 25mg/mL and in ethanol over 15mg/mL for morethan 6 months without obvious deposition or conglomeration in glass tubes. And the modified nanotubescould be uniformly dispersed in Pva to greatly improve its electrical conductivity2. Experimental2. 1 Nanotube carboxylationMulti-walled carbon nanotube bundles produced by CVd in our group with diameter about10-20nm were used in the experiment 4. Firstly, the common nitric acid treatment was carried out to pirify the raw MWNTs bundles(r-MWNTs, purity over 97wt%). One gram of r-MWNTs was bath-soni-cated in 100mL concentration nitric acid (65%) for 8h at room temperature, followed by filtering in aglass filter and washing with abundant distilled water. The nitric acid-oxidation removed the metal catalyst and its support used for the synthesis of MWNTs. The purity of the MWNTs (p-MWNTs) couldreach over 99wt% after this process. However, upon TEM observations, the p-MWNTs were still bundled together and could not suspend in water. In order to make a uniform suspension, the p-MWNTsbundles were put in a mixed solution of concentrated H SO, and HNO, acids (3: 1, 98% and 65% respectively) and then were bath-sonicated for the given time at room temperature. After the reaction, the mix-ture was diluted with distilled water and filtered by a 0. 22um millipore polycarbonate filter membraneuntil the pH value of the filtrate was neutral. The black materials were dried at 120C in vacuum forovernight. The above method was designated as a two-step carboxylation(m-2-MWNTs) schemed inig. 1. Afterwards, it was found that directly sonicate of the nanotubes bundles in the mixed acids couldalso make a uniform suspension, which was shown in Fig. I and called as one-step carboxylation(m-1-MWNTs)below.r-MWNTs COncentrated HNO, acidp-MWNTsConcentrated HSOA and HNO acids中国煤化工oncentrated H2SO and HNO3 acidsm-1CNMHGN-2-MNTsFig 1 Schematic modification process of the r-MWNTs bundles3912.2 PVA composites with carbon nanotubesThe modified nanotubes were dispersed in water/ethanol by bath-sonication to form suspensionand transferred to glass tubes to deposit for a week. The deposited materials at the bottom of the glasstubes were collected to weigh and evaluate the concentration of the nanotubes. Then the topper of theglass tubes was transferred to the others glass tubes with the same volume to deposit. The poly (vinyl al-cohol)(PvA)(from Shanghai, with an average molar mass of 27000g/mol, 99+% hydrolyzed) was dis-solved in distilled water at 80C (to give 40mg/mL solution) and subsequently cooled down to roomtemperature. Then the nanotubes suspension was mixed with different volume of PVa aqueous solutionby bath-sonication for 4h. The m-MWNTS-PVA composites were dropped on a 2cmx2cm glass flakeand dried in vacuum overnight at 50C for further conductivity test2. 3 Measurements and apparatusThe acid modified nanotubes were characterized by Ftir (Nieolet, AVATAR 360), scanning electron microscopy(JEOL, JSM-5610, 5kV; Sirion, FEL, 5kV), transmission electron microscopy (TEmJEM-200CX, 160kV; HRTEM, Philips CM200, 200k V), Raman spectroscopy (Thermo Nieolet,ALMEGA- Dispersive Raman, Nd: Y AG, excitation wavelength: 532nm) and thermal gravimetric analy-(GA)(NETZSCH STA 409 PG/PC, 10C/min, N2 atmosphere). The room-temperature electricalconductivity of the MWNTs-PVA composite films was measured by impedance spectroscopy in afour-point configuration3. Results and discussion3. 1 Characterization of the nanotube carboxylationBoth the r-MWNTs and m- MWNTs with different modification time were analyzed by ftir. Anillustration of the changes occurred to the nanotubes due to carboxylation is shown in Fig 2a. It was confirmed by literature that the frequency at 1630cm'(indicated by the arrow) corresponds to the carbonylof the carboxyl groups and the frequency at 3420cm to hydroxyl groupss, I. From Fig 2a, it is apparentthat with the reaction time expanded the absorption of carbonyl and hydroxyl groups becomes stronger,indicating that more carboxyl groups and hydroxyl groups have been attached on the tubesRaman spectra of both the raw MWNts bundles and the carboxylated nanotubes are demonstratedin Fig 2b. The higher D-band (1347cm") intensity of the modified nanotubes than that of the raw nanotubes can be attributed to an increased number of sp-hybridized carbons in the nanotube frameworkand can be considered as a crude measure of the degree of carboxylation. But the G-band (1578cm )in-tensity is still higher than that of the D-band, indicating that the structure is not seriously damaged forthe modified time below 16h. When the modification time prolonged to 24h, the D-band intensity seemsa little higher than that of G-band showing that the nanotube structure is partially damaged (not shown)The existing two peaks up-shifted a little comparing to the case of the raw nanotubes because of the en-ergy dispersion after modifiedAdditionally, TGA was done on the carboxylated nanda\中国煤化工 vely the degree ofcarboxylation. Fig 2c shows the thermogram of the carboxyCNMHGtrates that with theincrease of modified time, more carboxyl groups have been attached on the tubes. from the weight losscurves the concentration of the carboxyl groups for m-2-MWNTs was estimated to be 3. 8wt%, 7. 6wt%,12. 1wt% and 157wt% for the modification time of 2, 8, 16 and 24h respectively. whereas for the one-stepa r-MWNTS助800160024003200400016002000Wave numbers. cmRaman shift cm1008h. m-1-MWNTs·8h,m-2 MWNTs.16h, m-1-MWNT16h, m-2-MWNTs≥8020nm75200Fig 2 Characterization of the carboxylated nanotubes:(a)FTIR spectra of r-MWNTs and m-2-MWNTsat 4h and 8h, respectively; (b) Raman spectra of r- MWNTs, m-1-MWNTs at &h and m-2-MWNTsat 8h;(c) TGA analyses of m-1-MWNTs and m-2-MWNTs;(d) high magnification transmissionelectron microscope(HRTEM) image of the m-2-MWNTS at 8hcarboxylation, the concentrations of the carboxyl groups were estimated to be 3.6wt%, 7. 2wt%, 123wt%and 155wt%for the same modification time as mentioned above respectivelyThe TEM image of the m-2-MWNTs for 8h is displayed in Fig 2d, showing that the outer sidewallsand the ends of the tubes are partially etched, while the graphite layers are not damaged badly and thelength of the nanotubes is kept over lpm, i.e. the nanotubes should still keep the structure well and holdtheir good conductivity, which is consistent with the Raman spectra. After the modification time in-creased to 24h, most of the carbon nanotubes were cut into shorter ones with the length below 500nmThe same result is also observed for the specimen of m-l-MWNts3.2 Dispersion of the nanotubes in water/ethanolTable 1 shows the resulted concentrations of m-MWNTs aqueous and ethanol suspensions withdifferent modification time by the above two methods. It is surprising that the nanotubes modified for 8hcan be dispersed in water to a concentration over lOmg/TV中国煤化 IFh can even be up to25mg/mL. For the ethanol suspensions, the highest concermL. It was noted thatCNMHno any deposition or conglomeration of the topper m- iS suspeisIuus nappened after they kept stillover 6 months, indicating that amounts of carboxyl and hydroxyl groups have been linked on the tubesToour knowledge, it is the first time for such a high concentration of nanotube suspension in water/ethanol393Table 1 Suspension of m- MWNTs at different modification timeModification Water suspension, mg/mL Ethanol suspension, mg/mLtime. hm-1-MWNTs m-2-MWNTs m-1-MWNTs m-2-MWNTs1.05.06.3.87.016.020.020.214.62425.816.717.2obtained. Water can ionize H and Oh interacting with the attached groups, which should be the reasonfor the higher concentration suspensions formed in water than those in ethanol. The concentrations ofm-1-MWNTs suspensions are similar to those of the two-step carboxylation, indicating that the purifica-tion process is not important to modify the MWNTs bundles and to make high concentration of MWNtssuspension in water/ethanol. The high purity ratio of the MWNTs bundles should be the main cause ofthe result3.3 Conductivity of MWNTs- PVA compositeSuch a high concentration of above m-MWNTs suspensions prepared in this work enabled us tofabricate m-MWNT/polymer composites. The r-MWNTs or p-MWNTs were firstly mixed with PVA asreferences. The phase separation between nanotubes and Pva matrix can be obviously seen even by thenaked eyes. As mentioned previously, the m-MWNTs suspensions obviously showed better homogeneityand interfacial compatibility than that of the r-MWNTs or p-MWNTs, which may lead to better compos-ite performance. So, a set of m-MWNTs-PVA composites with different mass fractions of m-MWNTswas made by drop-cast of the formed aqueous solution of the mixture on clean glass flakes. After thesolution dried, a black homogeneous film formed, which was rather uniform to the eyes. Upon SEM in-vestigations, the nanotubes spread very well with the full length for low addition of the nanotubes owing5kUX18,98000 JSM-56 10中国煤化工CNMHGFig 3 SEM images of m-MWNTs and PVA composites: (a)0.5wt% m-2-MWNTS inPVA; (b)10wt% m-2-MWNTs in PVAm-2-MWNTs16hnanotubes loading, w%Nanotubes loading, wt%Fig 4 The electronic properties of m-MWNTs-PVA composites:(a)m-1-MWNTs-PVA andm-2-MWNTs-PVA, the modification time of m- MWNTs is 8h and(b) m-l-MWNTS-Pva at different modification timeto their good hydrophilic surface as shown in Fig. a; however, they twisted and overlapped together forhigher concentration of nanotubes in the composite as shown in Fig. 3b for the case of 10wt%The effect of modification method (one-or two-step both modified for 8h) on the properties ofsynthesized composites with PVA was compared, their electrical conductivity property with the additionof the m-MWNTs to Pva at room temperature was also measured(Fig 4a), which shows that the onemade of the m-l-MWNTs is slightly better than that of the m-2-MWNTS. Then the samples ofm-l-MWNTs modified with different time were chosen to fabricate composites with PVa in the following experiments. Fig 4b demonstrates the measured results of electrical conductivity for the compositesThe changing trend of the curve slope indicates that the content of nanotubes in the composites affectedthe property dramatically when the nanotube addition was less than 5wt%; the conductivity increased 8orders of the magnitude after 5wt% of m-1-MWNTs was added to the polymer and then gradually getting saturated, which means the conductivity is a characteristic of the tube percolation in composite sys-tems with a threshold of 5wt% for the mass fraction. Above experimental result also indicates that smalladdition of m-MWNTs is enough to improve the electronic conductivity of PVA remarkably, which ismost probably due to the better electrical conductivity of MWNTs and the better homogeneous dispersion of m-MWNTs in our experiments On contrast with the literature, one noted that the conductivity ofthe m-MWNTs-PVA composites is lower than that of epoxy resins/ 2) but higher than that of PS e4l for thesame amount loading of nanotubes. This can be understood because the nanotubes functioned differentlywith matrixes in the different polymers. The result still shows that the modified time for 2 and 8h almostdisplays a same conductivity, which is unexpected. Generally, the modified time for 2h should show bet-ter conductivity than that of modified for 8h because of its better graphite structure kept. Then our resultapproves that the conductivity of the m-MWNTS-PVA composites is determined both by the nanotubesstructure, the polymer matrixes and their miscibility. So the better dispersion of the modified time for 8hin pva should be one reason for the result. more detailed中国煤化工 nake it clear.4. ConclusionsCNMHGTwo carboxylation methods were carried out to modify multi-walled carbon nanotubes bundlesFTIR, Raman, TGA and TEM were used for characterization of the m-MWNTs attached with hydroxyl395and carboxyl groups. The TEM observations show that after sonicated in mixed acids for over 24h, thenanotubes structures were damaged only a little The m-MWNTs were well dispersed by sonication inwater/ethanol. It is a big breakthrough that the m-MWNTs can be highly dispersed in water with the concentration over 25mg/mL and in ethanol over 15mg/mLfor excess 6 months without obvious depositionor conglomeration at the bottom of the glass tubes. This high concentration of nanotube suspensions inwater was used as a filler to enhance the conductivity of the thermoplastic PVa composite. The resultsshow that the mass fraction threshold of m-MWNTs-PVa composite is approximate 5wt%. And our results also reveal that the purification process is not necessary for making high dispersion carbon nanotubes bundles in water/ethanol because of the high purity of the raw MWNTs(97wt%)Acknowledgements-This work was supported by the National Natural Science Foundation of China(No. 50172043), National High Technical Research and Development Programmeof China(No. 2002AA334020) and National Key Basic Research and DevelopmentProgramme of China (No. G20000264-06)REFERENCESI P. M. Ajayan, Chem. Rev. 99(1999)17872 Y.P. Sun, K. Fu, Y Lin and w. Huang, Acc. Chem. Res. 35 (2002)123R. H. Baughman, A. Zakhidov and W.A. de Heer Science 297(2002)787.4 P.G. Collins, A. Zettl, H. Bando, A. Thess and R E. Smalley, Science 278(1997)1005 M.S. 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