A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio

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1 Carbon 41 (2003) A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio * Yao Wang, Jun Wu, Fei Wei Department of Chemical Engineering, Tsinghua University, Beijing, PR China Received 20 June 2003; accepted 13 August 2003 Abstract CNT agglomerates, prepared by catalytic chemical vapor deposition in a nano-agglomerate fluidized-bed reactor are separated and dispersed. The effects of shearing, ball milling, and ultrasonic and chemical treatments on the dispersing of the carbon nanotubes were studied using SEM, TEM/ HRTEM and a Malvern particle size analyser. The resulting microstructures of the agglomerates and the efficiency of the different dispersion methods are discussed. Representative results of annealed CNTs are highlighted. The as-prepared CNT product exists as loose multi-agglomerates, which can be separated by physical methods. Although a concentrated H2SO 4/ HNO 3 (v/ v53:1) treatment is efficient in severing entangled nanotubes to enable their dispersion as individuals, damage to the tube-wall layers is serious and unavoidable. A high temperature annealing (2000 8C, 5 h) before the acid treatment (140 8C, 0.5 h) is recommended and can give well separated nanotubes with a high aspect ratio and 99.9% purity. These highly dispersed CNTs contain few impurities and minimal defects in their tube-bodies and will be of use in further research and applications Published by Elsevier Ltd. Keywords: A. Carbon nanotubes; B. Chemical treatment; Heat treatment; C. Electron microscopy; D. Particle size 1. Introduction than other processes which hence opens a door to produce nanotubes in bulk [4]. Carbon nanotubes (CNTs) are examples of nanomateri- However, the as-manufactured CNTs produced by als with a very high aspect ratio and many unusual CCVD ordinarily exist as agglomerates of several hundred properties. They have aroused a great deal of interest for micrometers, which is an obstacle to most applications. For their intrinsic properties and their possible applications [1]. example, CNT agglomerates do not provide the three- Among a wide range of uses of CNTs, the use of their dimensional networks needed to efficiently carry mechanimechanical properties and intriguing transport properties cal loads or provide useful transport properties for a mix or are of special interest because of their potential engineer- composite, that is, they do not yet have the desired ing and product applications in composites [2]. properties. Highly entangled products that are difficult to The morphology of the nanotubes and the way they disperse uniformly in fluids and melts would give fluid come together vary with the synthesis method. Typical suspensions and composites that have only modest imaspect ratios of CNTs vary from 500 to CNTs provements in their mechanical or transport properties. produced by catalytic chemical vapor deposition (CCVD) Thus one of the important challenges in developing can have lengths up to several centimeters [3]. In addition, the CCVD process is simple and has a higher productivity *Corresponding author. Tel.: ; fax: address: wangyao@flotu.org (Y. Wang) / 03/ $ see front matter 2003 Published by Elsevier Ltd. doi: / S (03) applications for CNTs lies in their uniform and reproduc- ible dispersion, that is, to separate the agglomerates into individual fibrils in order to realize their potential and attain unique mechanical or transport properties for materials modified by them. At the same time, a high dispersion of CNTs is also a useful system for evaluating the use of nanotube additives to liquids and polymeric solids to effect

2 2940 Y. Wang et al. / Carbon 41 (2003) significant improvements in their bulk properties at low volume loadings. (1) A raw MWNT material (r-mwnts) was prepared Two different approaches to separate CNTs are usually using the CCVD method in a nano-agglomerate used: physical methods and chemical methods. Physical fluidized-bed reactor (NAFBR). The MWNTs were dispersion methods include ultrasonication, ball milling, grown by passing mixtures of C3H 6/N 2/H2 over a grinding and high speed shearing. It was reported by fluidized catalyst bed of Fe/ Mo/Al2O 3. The con- Hilding et al. that physical dispersion methods not only ditions of the preparation of the catalyst and the CNTs separate CNT agglomerates, but also fragment the have been given elsewhere [4,15]. nanotubes [5]. They proposed a model for the analysis of (2) A heat-treated sample (a-mwnts) was obtained by a the lengths and agglomerate size distribution based on high temperature annealing of the r-mwnts. A total SEM and optical microscopy. Lu et al. found that ulgraphite of 100 g of the r-mwnts was placed inside a trasonication makes CNTs shorter, and also thinner as a pot within a vacuum furnace and heated to function of time due to the expansion and peeling of C. After holding at C and Pa for graphene layers [6]. Chemical methods use surfactants [7] 5 h, the sample was cooled to room temperature under or functionalization [8] to change the surface energy of the the vacuum. nanotubes to improve their wetting or adhesion characteristreating (3) A ball milled sample (m-mwnts) was obtained by tics and to reduce their tendency to agglomerate in the 250 g of the dry r-mwnts in a ball mill for continuous solvent phase. In research on the purification 3.5 h. The porcelain balls in the mill are 20 mm in and chemical modification of CNTs, it has been found that diameter and 11 g in mass. an acid-treatment can result in short lengths in multitreating (4) A highly sheared sample (s-mwnts) was obtained by walled carbon nanotubes (MWNTs) [5,9] and singleshearing 5 g of the dry r-mwnts in a r/min walled carbon nanotubes (SWNTs) [10 13]. These shortened machine for 5 min. CNTs can modulate the properties of the CNTs (5) An ultrasonic-treated sample (r-u-mwnts) was ob- including their dispersion property. In various solvents, tained by using a 80 W and 59 khz ultrasonication functionalized CNTs are solvated as individual tubes, bath to disperse 100 mg of the r-mwnts in 100 ml of which makes it possible to carry out further solution deionised water containing a little ionic dispersant of chemistry [14]. Therefore, both physical and chemical sodium dodecyl sulfate (SDS) for 10 h. methods can alter the aspect ratio distribution of the (6) An ultrasonic-treated sample (m-u-mwnts) was ob- nanotubes, and result in changes to the dispersion prop- tained by using ultrasonication dispersion of the m- erties. MWNTs. The treatment conditions are the same as for However, only highly conductive, high aspect ratio the r-u-mwnts. solids can produce three-dimensional networks with fast (7) An acid-soaked sample (r-s-mwnts) was obtained by transport properties. When nanotube fillers are badly an acid treatment of the r-mwnts. The typical digested during dispersions, the electrical and thermal procedure is as follows: 500 mg of the r-mwnts was conductivities of the composites containing these CNTs are manually ground, suspended in 50 ml of a 3:1 (v/v) not improved as significantly as hoped for, and neither is mixture of concentrated H2SO 4 (98%) and HNO3 their mechanical strength. Thus, an efficient dispersion of (68%) and ultrasonicated in a water bath for several CNTs with limited fragmentation and damage is urgently hours at C. After centrifugation, the upper required for applications. colorless liquid was decanted and the resulting black This article reports on the separating and dispersing of solid was washed thoroughly with deionized water CNTs with a high aspect ratio. MWNTs have been used until the ph of the water was about 7. for the experimental work because they are available in (8) An acid-boiled sample (r-b-mwnts) was obtained by engineering-scale quantities. New data are provided on the an acid treatment of the r-mwnts. A total of 500 mg effects of high speed shearing, ball milling, and ultrasonic of the r-mwnts was manually ground, suspended in and acid treatments on the morphology of the carbon 50 ml of a 3:1 (v:v) mixture of concentrated H2SO4 nanotubes and their interactions in the solid and liquid (98%) and HNO 3 (68%) and boiled at 140 8C for 0.5 phases. The objective is to elucidate the effects of the h in a high pressure tank. This was followed by different methods and to obtain a better dispersion of several cycles of centrifugation/ deionized water individual long CNTs. washing until the ph was around 7. (9) A heat-treated, acid-boiled sample (a-b-mwnts) was obtained with an acid treatment of the a-mwnts. The 2. Experimental treatment conditions are basically similar to that described for the r-b-mwnts Materials and treatments Genealogical relationships between the samples are The materials investigated are as follows: summarized an in Fig. 1.

3 Y. Wang et al. / Carbon 41 (2003) Fig. 1. Genealogical relationship between the samples SEM and TEM operating conditions be exactly that of the original. However, when the same measurement conditions were used and measurements The structures of the dry material, both untreated and were completed in a short time, this is still an important mechanical separated, have been examined by scanning way to directly evaluate the dispersion effect of the electron microscopy. Samples were attached to a copper different approaches. stub and directly examined at an accelerating voltage of 3 10 kv. TEM analysis of the dispersed CNTs is especially useful 3. Results in providing both an overview at low magnification and a detailed examination at high-resolution. The CNT suspen Physical separation sions were dropped onto a copper grid and imaged at 200 kv in order to investigate the organization and microscopic The as-manufactured sample (r-mwnts) is a black structure of the nanotubes after the chemical treatment. 3 powder with a density of 50 kg/ m. As obtained from the Ultrasonic treatment was required to disperse the untreated reactor, the raw powders from the synthesis process exist nanotubes (r-mwnts) in solvents such as alcohols for mainly in the form of large irregular agglomerates (Fig. 2a) comparison. A special carbon film without holes was used which look like loose felt. Further observation of the for the TEM sample of the r-b-mwnts. agglomerate surface shows that numerous nanotubes are physically entangled and loosely associated into agglomer Agglomerate size detection experimental condition ates (Fig. 2b). The nanotubes are not coiled but generally curved with an outer diameter of nm. The lengths of The size distributions of the suspended CNT agglomer- the nanotubes are difficult to determine definitively due to ates were measured by a laser particle size instrument the entangled arrangement, but are at least of the order of (Malvern Mastersizer Micro-Plus). The reliability of the microns. agglomerate size was confirmed by SEM observations. The SEM photos of the ball milled sample (m-mwnts) and results from such analyses offer the most direct data of the the highly sheared sample (s-mwnts) are shown in Fig. CNT agglomerates in the liquid phase. 2c f. These indicate that agglomerates still exist after ball Measurements were carried with 500 ml of the water milling (Fig. 2c) and shearing (Fig. 2e). However, on suspension, which was stirred at 2300 r/min under 70 W comparing Fig. 2c,e with Fig. 2a, we find that the typical ultrasonic treatment in order to avoid sedimentation and agglomerate sizes have decreased from several hundred to conglomeration. Since the as-manufactured CNTs are tens of microns after the mechanical treatments, and the rather hydrophobic, a very small amount of SDS was used highly sheared agglomerates are rather smooth and regular. to give nanotubes with hydrophilic properties in some The surface morphology of the m-mwnt agglomerates cases. For samples that had undergone the acid treatment, (Fig. 2d) and the s-mwnt agglomerates (Fig. 2f) seem SDS was not required. not as loose as that of the r-mwnts (Fig. 2b). It is quite It should be mentioned that the samples were measured interesting that the bulk density of the m-mwnts is about when they are in a liquid phase under ultrasonic stirring. two times higher than that of the r-mwnts, while the Thus the agglomerate morphology in the suspension is that density of the s-mwnts is the same as that of the developed during the measurement processes, and may not r-mwnts. Therefore, although both ball milling and high

4 2942 Y. Wang et al. / Carbon 41 (2003) Fig. 2. SEM photographs of CNT agglomerates and their surface morphology. (a) r-mwnts; (b) enlargement of the selected area in (a); (c) m-mwnts; (d) enlargement of the selected area in (c); and (e) s-mwnts. (f) Enlargement of the selected area in (e). speed shearing of the dry powder can break up and r-mwnt agglomerates dispersed in water are mm, separate the large agglomerates, the resulting granules are as shown in Fig. 3a. The average diameter is mm. not quite the same. For the s-mwnts, the average diameter is mm, The decrease in agglomerate size caused by the me- while it is mm for the m-mwnts. These results are chanical methods was further confirmed by the particle consistent with the SEM observations in Fig. 2. size measurements. The log-normal size distributions of Ultrasonication is an extremely common method used to the CNT agglomerates are presented in Fig. 3. The sizes of break up agglomerates in solution processing techniques.

5 Y. Wang et al. / Carbon 41 (2003) Fig. 3. Size distributions of the CNT agglomerates. To make the CNT samples wettable in water, a little SDS 3.2. Chemical dispersion was added. As shown in Fig. 3a, some small agglomerates of several tens nanometers to several micrometers were The r-b-mwnts were obtained by boiling raw MWNTs present in the suspensions for the ultrasonic-treated sam- in a H2SO 4/ HNO3 mixture at 140 8C for 30 min. After ples (both r-u-mwnts and m-u-mwnts). By comparing centrifugation and washing, it remained in quiescent the results of the r-u-mwnts with that of the m-u- suspension with significant concentrations for several MWNTs in Fig. 3a, we find that ball milling followed by months. The analysis of the r-b-mwnts in Fig. 3a shows ultrasonic dispersion can give many more agglomerates that almost all the suspended objects are smaller than 100 smaller than 10 mm. However, for both these two samples, nm. It should be mentioned that the Malvern laser particle stable suspensions could not be obtained even after ul- size instrument is not useful for analyzing the size distrasonic treatment for several hours. tributions of separated nanotubes. One reason is that these

6 2944 Y. Wang et al. / Carbon 41 (2003) nanotubes have a high aspect ratio. Another reason is that aspect ratios of the r-b-mwnts and the r-s-mwnts is the lower limit of the instrument is 50 nm, which is larger due to the different temperatures. than the CNT diameter. However, it is still an effective For the CNTs heavily damaged by the acid treatment, instrument for the examination of the CNT agglomerates such as the sample r-b-mwnts, the yield is rather low and their transformation in suspensions. The results of the because many CNTs have been severed into pieces. It is r-s-mwnt, acid-soaked at C for several hours, are difficult to give accurate yield data in this case due to the shown in Fig. 3b. It can be observed that with the aid of lack of a reliable detection method. For the r-s-mwnts, ultrasonic treatment and with increasing time, more and the loss of CNTs mainly comes from the liquid solid more large agglomerates are separated into smaller units separation and the solid washing procedures. With careful and further dispersed. When the sample was soaked in the operation, the yield of the r-s-mwnts can be higher than acid mixture for more than 8 h, few agglomerates larger 90%. than 200 nm remained, and no further evident change could be detected. In these cases, TEM observation is 3.3. Dispersion of annealed MWNTs needed. The TEM images directly reflect the dispersion status of Although an acid treatment causes the nanotubes to the nanotubes. Although the as-observed images actually disperse much better, damage to the tube-walls cannot be represent the structure of the nanotubes dried on the TEM avoided in this process if the as-manufactured sample is grid, any observed difference does indicate the structure in directly treated in a concentrated H2SO 4/ HNO3 mixture, the suspensions. Fig. 4a shows a typical TEM image of the as has been shown by both our experiments and other r-mwnts. This indicates that the nanotubes aggregate into workers in the literature. agglomerates on the scale of several hundred microns by It has been reported that a high temperature annealing wrapping around cores of catalysts, which lead to poor may heal defect structures of the graphene layers, and also dispersion. If the nanotubes had not entangled or aggre- eliminate catalyst impurity [16]. The a-b-mwnts were gated in solution, they might be expected to form a obtained after annealing at C and Pa for 5 uniform suspended phase. This may be achieved by h, and then similarly treated with a boiling H2SO 4/ HNO3 decreasing the length of the nanotubes to reduce physical mixture as were the r-b-mwnts. The purity of the entanglements. annealed sample is about 99.9%, from a TGA analysis The TEM image of the r-b-mwnts shows that [17]. TEM observation shows that in the absence of nanotubes had been broken into smaller pieces and are mechanical entanglements, the a-b-mwnts tend to exist evenly spread on the carbon film (Fig. 4b). The average as well-dispersed nanotubes in suspensions. The length of length of these MWNTs, deduced from the TEM observa- the a-b-mwnts in Fig. 4d is considerably longer than that tion, was shorter than 500 nm. This means that an of the r-b-mwnts in Fig. 4b. Under the same aggressive aggressive chemical treatment can seriously digest the conditions, the annealed sample cannot be severed into nanotubes. The resulting shortened tubes are less likely to smaller pieces as was the case with the unannealed entangle and form agglomerates. Our experiment indicates material. This may be due to the annealing of defects in the that shorter MWNTs, severed by the acid mixture, can graphitization after the high temperature treatment. The form a stable dispersion in water without the help of high aspect ratio of these CNTs may give three-dimensionsurfactants. However, although this is an efficient way to al networks that can increase transport properties, such as improve the dispersion of the nanotubes, that is, to shorten electrical and thermal conductivity. Moreover, only very the tubes, there are some serious disadvantages with few catalyst particles were found in Fig. 4d, which breaking up the tubes into such smaller pieces. When the confirms that catalyst particles in the raw product have tube-walls are severed, the wall may be damaged in other been almost completely removed by the vacuum high ways as well, which will significantly decrease the quality temperature treatment. With the removing of the catalyst of the mechanical and transport properties. Thus a moder- impurity, 99.9% purity CNTs have been highly dispersed ate chemical treatment needs to be developed. with a yield of about 86%. Fig. 4c is a typical TEM image of the r-s-mwnts, For some applications, it is not only a high aspect ratio which were soaked in an acid mixture with ultrasonic but also complete graphene layers that are desirable. Our treatment at C for 10 h. It can be observed that HRTEM-based study indicates that a pre-annealing step is individual tubes, free from significant entanglement, are useful for reducing the damage done by the acid on the dispersed on the whole grid and no large aggregates are tube-walls (Fig. 5). Fig. 5a of the r-b-mwnts shows that found, which is quite different from the raw sample (Fig. the raw unannealed CNTs have been badly etched by the 4a) and the acid-boiled sample (Fig. 4b). We believe that acid-treatment. In contrast, the considerably complete certain breaks resulting from the oxidization of the acid appearance of the a-b-mwnts in Fig. 5b suggests that the mixture also contribute to the good dispersion, although acid attack is initiated on and only limited to the external the breakage of the CNTs cannot be observed directly from layers and local defects. As reported in the literature, an the TEM images. The obvious difference between the acid oxidation may occur by layer-by-layer thinning and

7 Y. Wang et al. / Carbon 41 (2003) Fig. 4. TEM photographs of the CNT samples. (a) r-mwnts; (b) r-b-mwnts; (c) r-s-mwnts; and (d) a-b-mwnts. tion has been achieved [20]. Therefore, while the aggre- gates were converted into well-dispersed tubes by the aggressive acid treatment, the inner graphene layers of the a-b-mwnts are still complete and their high aspect ratio remains. The different degree of damage observed in the attack at the sites of pre-existing side defects [18,19]. The former contributes to a reduction of the CNT diameter, while the later to the rupture of the CNTs. Due to the annealing step before the acid treatment, defects along the tubes have been decreased and a high degree of graphitiza-

8 2946 Y. Wang et al. / Carbon 41 (2003) Fig. 5. HRTEM photographs of the acid-boiled samples. (a) r-b-mwnts; and (b) a-b-mwnts. r-b-mwnts and a-b-mwnts is due to the different crystallinities of the nanotubes. Ten TEM microphotographs, like Fig. 4a,d, were taken for each sample, and from these a minimum of 200 MWNT diameters were measured and the diameter distributions in Fig. 6 were deduced. The outer diameter distribution of the MWNTs after the chemical treatment (a-b-mwnts) is approximately the same as before (r- MWNTs). However, the acid-treatment has narrowed the diameter distribution by digesting MWNTs with small diameters and thinning others to some extent. This result suggests that thinner tubes are more reactive than tubes Fig. 6. Outer diameter distributions of the CNTs before and after the acid treatment. (a) r-mwnts; and (b) a-b-mwnts.

9 Y. Wang et al. / Carbon 41 (2003) with a larger diameter, which has also been reported by Our results indicate that the physical methods, such as other workers [21,22]. shearing, ball milling and ultrasonic treatment, can only break up the multi-agglomerates into smaller parts or single-agglomerates. A stable suspension containing sepa- 4. Discussion rated individual tubes is obtained only by chemical methods, such as acid treatment. The excellent dispersion is due The separation and dispersion of CNTs mainly depends to the reduction in length of the nanotubes and the effect of on the CNT materials and dispersion methods adopted. the hydrophilic group [23,24], i.e., the carboxyl group, The microstructure of aggregates/ agglomerates and the introduced by the H2SO 4/ HNO3 mixture. The breakage degree of entanglement are very important. Several factors rate of agglomerates and nanotubes depends on their sizes may result in the formation of CNT agglomerates. and degree of crystallinity of the nanotubes. Larger agglomerates and longer tubes with poorer graphene layers (1) Thin-long and curved CNTs easily entangle together experience the highest breakage rate. The different methto form congeries. ods and their effects are collected in Table 1 for a detailed (2) Solid bridges exist between nanotubes and catalyst comparison. particles. (3) Van der Waals forces between nanotubes may attract them together. 5. Conclusion The r-mwnts were grown on agglomerated Fe/ Mo/ CNTs manufactured by CCVD in a NAFBR, are me- Al2O3 catalyst particles in a NAFBR. The nanotubes are chanically entangled into aggregates (named single-agconnected to the catalyst at one end and form a loose ball glomerate ) which then self-associate into large agglomerwith the catalyst kernel, as shown in Fig. 4a. We name this ates (named multi-agglomerate ) of up to a few tens of an aggregate or a single-agglomerate. Due to the entangle- mm. Entangled CNTs can be difficult to disperse without ment of thin-long and curved tubes, many single-agglom- damage. Physical separation methods, such as ball milling, erates come together to form a large particle, as shown in shearing and ultrasonic treatment, can only break up large Fig. 2a. We name this a multi-agglomerate. multi-agglomerates into small parts or single-agglomerates. Van der Waals forces between the nanotubes may not be On the other hand, a chemical treatment with a 3:1 a primary obstacle to the separation of CNTs because concentrated H2SO 4/ HNO3 mixture gives a good dispereither a single-agglomerate or a multi-agglomerate is in a sion by effectively severing the entangled CNTs. Once quite loose form (Fig. 2b). On the other hand, the joining they exist in the form of separated individual tubes, the together of the nanotubes and catalysts is not a major nanotubes spontaneously disperse in water and remain factor affecting the dispersion. The a-mwnts have been stable for many months with little material precipitation highly purified by eliminating the catalyst components, but over time. In the process of rending CNT agglomerates after ultrasonication in an alcohol solution, large amounts into individual tubes by chemical oxidation, damage to the of single-agglomerates without catalyst kernels can still be tube-walls is unavoidable. After annealing at C and found during TEM observations. Therefore, we suggest Pa for 5 h, CNTs of 99.9% purity and with good that entanglement of the nanotubes is the dominant cause graphene layers are obtained. They can then be well for the aggregation and agglomeration. This is why dispersed into long individual tubes by boiling in a severing is an effective route to get well-dispersed CNTs. H2SO 4/ HNO3 mixture at 140 8C for 0.5 h. Such highly Table 1 Comparison of the physical and chemical dispersion methods Method Scale Mechanism Main treatment conditions Sample code Shearing mm Breaking up the multi r/ min, 5 min s-mwnts agglomerates Ball milling mm Breaking up the multi- Porcelain ball (20 mm, 11 g), m-mwnts agglomerates 3.5 h Ultrasonic mm Dispersing of the single- 59 khz, 80 W, 10 h r-u-mwnts agglomerates m-u-mwnts Concentrated Nano-scale Dispersing the MWNTs 3:1 concentrated H SO / HNO r-s-mwnts H SO / HNO (individual by shortening their length ultrasonicated for 0 10 h at r-b-mwnts nanotubes) and adding carboxylic C or boiled for 0.5 h a-b-mwnts groups at 140 8C

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