Study of Carbon Nanotube Purification and Modification
Kuo-Shien Huang*,Bo-Shain Tsai,Bo-Chi Huang,
Department of Polymer Materials, Kun Shan University, Yung Kang,
Tainan, 71003 Taiwan
*e-mail:
Tel: 886-6-2050317, Fax:886-6-2728944
Abstract
This experiment adopted mixed acid (H2SO4:HNO3 = 3:1) to purify multi-walled carbon nanotube, and used cationic surfactant hexadecyltrimethylammonium bromide (HTAB), and silane coupling agent n-octyltriethoxysilane (OTES) to modify carbon nanotube, respectively. The experimental result showed that, through treatment of mixed acid, carbon nanotube surface could have more hydroxyl (-OH) and carboxyl (-COOH) groups, and specific surface area of carbon tube could be enlarged, carbon nanotube specific surface area was maximal when purifying for 40min, but excessive oxidation time lowered specific surface area. Using OTES or HTAB to modify carbon nanotube could improve carbon nanotube dispersibility in xylene or water, with excellent dispersibility in MOC5 and MHC1.Excessive amount of modifier had no positive improvement on carbon tube dispersibility.
Keywords: carbon nanotube, silane coupling agent, modification, specific surface area, dispersibility
1. Introduction
Carbon nanotube, since being discovered in 1991, has been always favored by chemical, physical and material science researchers. However, carbon nanotube coagulates very easily, and is difficult to be dispersed, thus having bottleneck in practical application. Applying surface chemical modification on carbon nanotube to improve its surface property is an effective solution to the problem[1-4].Chemical modification enables chemical reaction between carbon nanotube and modifier, and changesthe carbon nanotube surfacestructure and state. Since the initially prepared carbon nanotube contains excessiveforeign substances, such as metal catalyst particles, amorphous carbon, and graphite fragment, these would influence the carbon nanotube quality, so it has to be purified before modification. According to past studies, there are three common purification methods as follows: (1) acid treatment, put carbon nanotube in mixture of sulfuric acid and nitric acid,heat under reflux while stirring, filter and rinse with pure water until reaching a neutral pH to obtainmulti-walled carbon nanotube, so that multi-walledcarbon nanotube has rich hydroxyl and carboxylfunctional groups, and higher surface activity and hydrophilicity [5-6],treatment in mixed acid of sulfuric acid and nitric acid not only derives clean multi-walled carbon nanotube, but also allows multi-walledcarbon nanotube to have rich hydroxyl and carboxylfunctional groups, and higher surface activity and hydrophilicity, thus facilitating surface modification of multi-walledcarbon nanotube [7-9]. (2) Oxidation of sulfuric acid and hydrogen peroxide is utilized to introduce carboxyl group;without surfactant aid, direct ultrasonication in water can get stable suspension. (3) Chromatography is used to disperse carbon nanotube and impure material in interface surfactant by ultrasonicating, then guide to chromatography column to obtain carbon tubes of different lengths with different time lengths. Theadvantage of this method is the ability to isolate independent carbon tube, but the disadvantage is time-consuming and small output [10-12].
Unique structure of carbon nanotubedetermines its special property and usage, due to its excellent mechanical, electrical, optical, thermal properties [13-17].It shows good application in electronic devices, composite material, chemical sensors and biosensors. However, since carbon nanotube is hardly soluble in organic solvent and water, research of its chemical property is hard to proceed, therefore, scientists have exerted great effects on carbon nanotube solubility.New methods of chemical functionalization have beendeveloped so as to enhance the dispersion of SWCNTs in theaqueous medium [18-22]. These functionalized CNTs showedimproved solubility/dispersion in water and various commonorganic solvents/polymer matrices [23-27].Common modifiers can be divided into surfactant and coupling agent, the surfactant is a kind of substance acting on surface and interface with capability and efficiency to decrease surface/interface tension greatly.In solution with certain concentration, it can form ordered molecular assembly, thus having a series of practical functions.Coupling agent is a kind of organic compoundwith two chemical property groups in one molecule, which are organophilic group and inorganophilic group at each end. Thus, organic material can combine inorganic material throughcoupling agent.Common coupling agents include titanate coupling agents and silane coupling agents.In this experiment, cationic surfactant and silane coupling agent were used to modify carbon nanotube.
2. Experimental method
2-1 Reagent
Multi-walledcarbon nanotubes (95%, MW carbon nanotube) with anaverage particle diameterof 10~40nm weresupplied by Desunnano Co., Ltd., Taiwan.Sulfuric acid, nitric acid, ammonium, and isopropanol of reagent grade werepurchased from Nihon Shiyaku Reagent Co., Japan.Both Octyltriethoxysilane (96%, OTES), and Hexadecyltrimethylammanium bromide (99.0%, HTAB) werereagent grade, and purchased from Sigma-Aldrich Chemie Co..
2-2 Experiment procedure
2-2-1 Purification of carbon nanotube
The SWCNTs were purified using the following method.The SWCNT (2 mg) was Soxhlet extracted with 50 ml oftoluene for 6 h. After Soxhlet extraction, the nanotubes wererinsed with acetone and finally dried in the air oven at100℃.Mixed acid(20%HNO3 and 20%H2SO4 in 1:3 V/V) was first prepared. Thecarbon nanotubewas added to the mixed aciduntil full submergence, and then heated to 100℃and stirred for 10, 20, 40, 60, 80 min, respectively. The specimens were filtered and rinsed with distilled wateruntil reaching a neural pH, and baked at 100℃.Thecarbon nanotubewere labeled as Cn (n=10, 20, 40, 60 or 80).
2-2-2 Preparation of OTES-modified carbon nanotube
The purified carbon nanotube (C40) 1g and 2.5mlisopropanol, 2.5ml2%sulfuric acid and 2.5ml distilled water, were mixed with various volumes (2.5, 5, 10 and 20ml) of OTES in 4-mouth flask, which was fastened and mounted with a stirring motor and condenser. The specimen was heated to70℃and stirred under reflux for 120min.The treated SWCNTs were vacuum filtered using anultrafiltration membrane and washed repeatedly with deionizedwater until the washings showed no acidity. The filtered solidwas dried under vacuum for 24 h at 40℃. The modified carbon nanotubewas labeled as MOCn (where n=2.5, 5, 10 or 20)
2-2-3 Preparation of HTAB-modified carbon nanotube
The purified carbon nanotube (C40) 1g was added with40mlof concentrated ammonium, ultrasonicatedfor 15min, and added with various weights of (0.25, 0.5, 1, 5 and 10g)HTAB, respectively.Ultrasonication was resumed for 15min. The specimen was heated to100℃and stirred under reflux for 120 min.The treated SWCNTs were vacuum filtered using anultrafiltration membrane and washed repeatedly with deionizedwater until the washings showed no acidity. The filtered solidwas dried under vacuum for 24 h at 40℃. The modified carbon nanotubewas labeled as MHCn (where n=0.25, 0.5, 1, 5, 10).
2-3 Analysis and inspection
FT-IR/ATR spectra of the samples were recorded with a Bio-Rad Digilab FTS-200 spectrometer using an MCT detector. A diamond crystal was used as internal reflectance element. Single beam spectra were the result of 64 scans. The spectral resolution was 4 cm-1. Specimen dispersibility test was conducted as follows: take 7.5mgcarbon nanotube sample, add to 50mL distilled water or xylene solvent, ultrasonicatefor 15 min, rest in on tabletop, and observe the changes of solution dispersion in time.The surface area and pore volume of the dried alumina powder samples were determined through nitrogen physisorption analysis, using a Quantachrome Autosorb-1 system. The five-point BET calaulation was used to determine the surface area. The surface morphologies of the films were observed with a JEOL Model JSM 6400 scanning electron microscopic. A gold coating was deposited on the samples to avoid charging the surface.A standard method (ASTM D-
974) was followed to determine the number of carboxylgroups produced on the carbon nanotubes surface. Preciselyweighed quantity of functionalized carbon nanotubes wasdispersed in water. The dispersion was titrated against standardsodium hydroxide solution and the extent of functionalizationwas determined in terms of number of acid groups from thefollowing relation.
No of acid groups=
[(titration reading−blank titration)×molarity of aq NaOH]/weight in g of SWCNT
3. Results and Discussion
3-1 Purification of carbon nanotube
Different CNT synthesis methods are known to yieldCNTs with different types and amounts of impurities, (e.g.amorphous carbon and catalyst particles). Several strategieshave already been developed over the past decade to purifyCNTs without significantly damaging the structure of nanotubes.The optimum conditions reported in the literature [28] wereused. The SWCNTswere then treated with acid solution forremoval of amorphous carbon. This treatment removes thecarbonaceous particles that are shelled off from the SWCNTs.The reflux treatment with sulfuric acid and functional reagents has been reported foroxidative functionalization of SWCNTs. However, suchsevere treatment conditions are also reported to damage thestructure of nanotube walls and reduce their aspect ratio.
(1)SEM of purified carbon nanotube
Figure 1 shows the SEM images of carbon nanotube before and after purification. The original carbon nanotube image in Figure 1-(a) shows that, the primary original product is carbon nanotube, but it contains excessive impurities, and carbon nanotube is encapsulated by foreign substances. The SEM images of purified carbon nanotube (C40) in Figure 1-(b) show that, acid treatment removes metalcatalyst particles, amorphous carbons, and graphite fragment from carbon nanotube. Moreover, as seen inFigure 1-(b),carbon nanotube has typical hollow structure, with tube diameter of dozens of nm andlength of dozens of μm, entangled with each other.
(2)FTIR and number of carboxylgroupsof purified carbon nanotube
The formation of carbon nanotube surfacefunctional groups is due to the oxidization of foreign substances in of carbon nanotube by strong acid.The erosion of carbon nanotube wall also destroys carbon nanotube wall structure so that carbon nanotube is broken or fractured, thus resulting in hydroxyl(-OH) and carbonyl (-COOH) groups.
Figure 2shows the FTIR spectra of carbon nanotube after various treatment times. As seen, purified carbon tube (C40) hydroxyl(-OH) peak at 3429cm-1 was improved significantly, while distinct carboxyl(-COOH) absorption peak appeared at 1730cm-1, and C-Cstructure peak at 1588 cm-1showed no change. The peak at 1730cm-1 is clearly assigned to the C=O stretching vibration in acid treated SWCNT. The broad peak at ~3429 cm-1 is assigned to the O-H stretching vibration while the peak in the frequencyrange of 1380-1430 cm-1 may be due to the bending vibrations of O-H groups[25]. These observations are a direct evidence forintroduction of a carboxylic acid group on the nanotubes. The degree of functionalization (a measure of the numberof carboxyl groups present on the surface of the carbon nanotubes) depends on the oxidative treatment time and temperature. Table 1 indicates purification time vs. quantity of the carboxyl group on carbon nanotube. When the purification time is longer, the carboxyl group quantity increases. But when the purification time is up to 80 minutes, the carboxyl group quantity obviously reduces. This is caused by break of nanotube during purification. Therefore, surface area is reduced.
This indicated that treatment of H2SO4+HNO3mixed acid could make carbon nanotube surface have rich surface functional groups, increase carbon nanotube surface activity, which is very favorable to carbon nanotube surface modification.
(3) Specific surface area of purified carbon nanotube
As shown in Table 1, carbon tube specific surface area increased with purified time, reaching the maximal specific surface area at 40min, but decreasing beyond 40min. From the start, pores were formed in carbon tube surface due to erosion of mixed acid.As treatment time increased, carbon tube pores not only increased but also enlarged, thus specific surface area increased.After 40min, many carbon tubes gradually were broken due to large pores, leading to smaller specific surface area.
3-2 Modification of carbon nanotube
(1) FTIR of modified carbon nanotube
Figure 3shows the FTIR spectra of purified carbon nanotube(C40) after modified by OTES in various ratios. As shown in Figure 3, for OTES modified carbon nanotube, as OTES ratio increased, 2925cm-1 (C-H, >CH2, -CH3)absorptionbecame stronger, and 3435cm-1 and 1553cm-1 hydroxyl(-OH)absorption intensities weakened.This is because hydroxyl(-OH) group in purified carbon nanotube (C40) reacted with OTES, which could be proved by the increasingly strong carbon tubeabsorption at 1107cm-1 (Si-O-C). The analysis results showed that carbon nanotube(C40) was successfully modified by OTES.
Figure 4shows the FTIR spectra of HTAB and HTAB-modified carbon nanotube. As seen, after HTAB modification of carbon nanotube (C40), distinct C-H(>CH2, -CH3) absorption appeared at 2924cm-1, intensity of hydroxyl(-OH)absorption at 3435cm-1was weakened significantly. The results suggested that, carbon nanotube was indeed modified by HTAB with long chain alkane molecule.
(2) Dispersibility of modified carbon nanotube
As for the dispersion of carbon nanotube modified by various volume ratios of OTES in xylene solvent, as shown in Figure 5-(a), after setting MOC2.5for 24hrs, distinct separation appeared in solution,while MOC5 solution had no distinct separation. Further experiment also proved that, MOC5, MOC10and MOC20had no distinct separation observed in solution after half a month. This indicated that an appropriate amount of silanecoupling agentOTES would improve dispersibility of modified carbon nanotube in xylene solventsignificantly.
As for the dispersion of carbon nanotube modified by cationicsurfactantHTAB in various weight ratios in water, as shown in Figure 5-(b), MHC0.5solution had distinct separation after setting for 24hrs, while MHC1 had no distinctseparation. Further experiment found that, no distinctseparation was observed in MHC1, MHC5 and MHC10 solution after setting half a month. This indicated that an appropriate amount of cationicsurfactantHTAB would improve dispersibility of modified carbon nanotube inwater significantly.
Based on above findings, when using OTES or HTAB to modify carbon nanotube, if the amount was appropriate, the modified carbon nanotubewould have good dispersibility in solventbecause more lipophilic or hydrophilic groups in above modifiers would lead to more functional groups in carbon nanotube, thus exhibiting good dispersibility. However, when modifierwas excessive, more modifier content was unnecessaryas the number of active group on carbon tubewasfixed.
4. Conclusions
The purpose of this work was to use mixed acid (H2SO4:HNO3 = 3:1) to purify carbon nanotube, and use cationic surfactant hexdecyltrimethylammonium bromide (HTAB) and silanecoupling agent n-octyltriethoxysilane (OTES) to modify carbon nanotube respectively. Based on the experimental result, the conclusions are as follows:
1. Purification not only could obtain pure carbon nanotube, but also allow carbon nanotube surface to have rich hydroxyl(-OH) and carboxyl (-COOH) functional groups.
2. Purifying carbon nanotube could increase carbon nanotubespecific surface area, carbon nanotube has the maximal specific surface area when purified for 40min, while longer reaction will decrease specific surface area.
3. After modified by OTES or HTAB, carbon nanotube dispersibility in xylene or water could be significantly improved, and dispersibilityis good at MOC5 and MHC1, higher modifier level has no positive improvement of carbon tube dispersibility.
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Captions:
Figure 1: SEM image of carbon nanotube (C40) (a: before purified, b: after purified)
Figure 2; FTIR spectra of carbon nanotube. (a: before purified, b:after purified)
Figure 3: FTIR spectra of carbon nanotube modified by OTES in various
ratios. (a: C40, b: MOC2.5, c: MOC5, d: MOC10, e: MOC20)
Figure 4: FTIR spectra of carbon nanotube before and after modification by HTAB (a: MHC1, b:C40)
Figure 5: Carbon nanotube dispersed in xylene after 24hrs. (a: C40, b: MOC2.5, c: MOC5)
Figure 6: Carbon nanotube dispersed in water after 24hrs. (a: C40, b: MHC0.5, c: MHC1)
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