Multiwalled Carbon Nanotube Microspheres from Layer-by-Layer

J. Phys. Chem. C 2008, 112, 11617–11622

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ARTICLES Multiwalled Carbon Nanotube Microspheres from Layer-by-Layer Assembly and Calcination Jiahua Shi,†,‡ Zhiyong Chen,§ Yujun Qin,*,† and Zhi-Xin Guo† Department of Chemistry, Renmin UniVersity of China, Beijing 100872, China, Institute of Fine Chemical and Engineering, Henan UniVersity, Kaifeng 475001, China, and The Key Laboratory of Bioorganic Chemistry and Molecular Engineering, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed: December 16, 2007; ReVised Manuscript ReceiVed: March 21, 2008

Multiwalled carbon nanotube (MWNT) microspheres were fabricated by layer-by-layer (LBL) assembling of modified MWNTs and poly(diallyldimethylammonium chloride) (PDDA) via electrostatic interactions on poly(methacrylic acid) (PMAA) microspheres and subsequent calcination. The MWNT/PDDA film growth on planar substrates monitored by scanning electron microscopy (SEM) and UV-vis spectrophotometry demonstrated the success of uniform LBL assembly. SEM observations of the stepwise increase in MWNT thickness on PMAA microspheres with the increase of LBL cycle number indicated the efficiency of the spherical assembly. Upon calcination of the (MWNT/PDDA)/PMAA microspheres in air at 400 °C for 2 h to remove the PMAA templates, the resulting MWNTs exhibited microsphere morphology with diameters that are half those of their precursors. Transmission electron microscopic images revealed that the MWNT microspheres were not solid, but loose and porous. It is proposed that the MWNT layers shrink during the calcination and form such microsphere structure due to the van der Waals attractions between the nanotubes. 1. Introduction Carbon nanotubes (CNTs) have attracted tremendous interests because of their unique optical, mechanical, and electrical properties.1 In recent years, various techniques have been used to construct different CNT-based superstructures to explore their applications.2–10 The research of CNTs, however, has been hampered by the poor solubility/dispersion of pristine CNTs in most solvents and matrices. To overcome such difficulties, considerable efforts have been focused on the chemical functionalizations of the CNTs with organic groups or polymers.11,12 For example, chemical oxidation of CNTs results in the attachment of carboxylic acids on the nanotubes, which allows further amidization and esterification reactions with various moieties. Such chemical modification helps achieve a uniform distribution and tight matrix connectivity of the nanotubes in the construction of CNT-based materials. Layer-by-layer (LBL) assembly, a well-established approach to functional multilayer assemblies from two- to three-dimensional architectures,13–16 is another efficient technique to resolve the problems of phase separation and inhomogeneity in CNT-based composites. In particular, fabrication of CNT/polymer films through LBL assembly has been widely used recently. In this way, alternating adsorption of CNTs and polymer attracted to each other through donor-acceptor interactions,17 van der Waals attractions,18,19 hydrogen bond attractions,20,21 or electrostatic attractions22–31 results in uniform growth of composite films. * To whom correspondence should be addressed. E-mail: [email protected]. † Renmin University of China. ‡ Henan University. § Peking University.

LBL technique has also been utilized in the fabrication of core-shell microspheres by using colloidal cores as templates onto which multilayers are deposited.15 The thickness of the shell on the template core could be controlled by the number of the LBL cycles. In the past few years, reports on the LBL fabrication of CNT-related microspheres have been published. Sano et al. first demonstrated the fabrication of single-walled carbon nanotube (SWNT) “cages” by LBL adsorption of SWNTs on silica gel template and removal of the template via HF etching.32 Thereafter, LBL deposition of CNTs on the polymer templates has been reported by other groups.33–37 Beside LBL technique, CNT-based microspheres could also be prepared by simple deposition of CNTs on the polymer templates with the aid of cross-linkers or through water-in-oil emulsion systems without solid templates.38–40 In general, the polymer or silica templates of the abovementioned CNT-based core-shell structures could be removed by etching or dissolution, yielding CNT microspheres or microcapsules. However, the morphologies of the resulting CNTs are difficult to control in such chemical processes. Herein, we report the electrostatic LBL assembly of MWNTs and polyelectrolytes onto poly(methacrylic acid) (PMAA) microsphere template to fabricate MWNT-based composite microspheres. Upon removal of the PMAA template by thermal calcination, MWNT microspheres of uniform size are obtained. 2. Experimental Section The polyelectrolytes, poly(diallyldimethylammonium chloride) (PDDA; Mw, 400000-500000) and poly(sodium 4-styrenesulfonate) (PSS; Mw, ∼70000) were obtained from Sigma-

10.1021/jp711793c CCC: $40.75  2008 American Chemical Society Published on Web 07/10/2008

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Figure 1. SEM images of (MWNT/PDDA)1 (a), (MWNT/PDDA)3 (b), and (MWNT/PDDA)5 (c) assembled on a silicon wafer and UV-vis absorption spectra (d) of MWNT/PDDA multilayer films fabricated on a quartz wafer. The number of assembly cycles is 0, 1, 2, 3, 4, 5, and 6 from bottom to top. The inset shows the absorbance at 500 nm vs the number of cycles. The scale bar is 1 µm in all images.

Aldrich. The cross-linked poly(methacrylic acid) (PMAA) microspheres of 6 µm diameter were prepared using one-step swelling and polymerization method according to a previously described method.41 MWNTs were purchased from Shenzhen Nanotech Port Co. Ltd., China, and used as received. To carry out the electrostatic LBL assembly with the polyelectrolytes, the pristine MWNTs were sonicated in nitric-sulfuric acid to attach carboxylic acid groups.42,43 Then the modified MWNTs were sonicated in a dilute NaOH aqueous solution to convert the carboxylic groups into the sodium salt form. The resulting negatively charged MWNTs could be dispersed in water homogeneously. Prior to planar LBL assembly on quartz slides, the substrates were immersed in a freshly prepared piranha solution and heated until no bubbles were released. Afterward, the slides were thoroughly washed with deionized water and dried in air. The MWNT/PDDA multilayer films were fabricated by first perpendicularly immersing a treated slide into the aqueous solution of PDDA (2 mg/mL) for 5 min. Then the slide was rinsed with deionized water and blow-dried with air. In this way, a layer of PDDA was adsorbed onto each side of the substrate. The quartz slide coated with PDDA was then immersed in an aqueous dispersion of MWNTs (2 mg/mL) for 5 min, rinsed with deionized water, and blow-dried with air to complete an assembly cycle. By repeating the above procedure in an alternating fashion, a multilayer MWNT/PDDA thin film was fabricated. LBL assembly on silicon wafers for SEM measure-

ments was carried out in the same way as described above. Here we use (MWNT/PDDA)n to denote the film after n assembly cycle(s). To provide a uniformly charged surface and to facilitate subsequent CNT adsorption, a precursor polyelectrolyte PDDA/ PSS two-layer film (PE2) was first deposited on the PMAA microspheres. The PDDA layer was deposited by adding 5 mg of PMAA microspheres to 20 mL of aqueous PDDA solution (2 mg/mL). The dispersion was stirred for 20 min before removing excess polyelectrolyte by three repeated centrifugation (1000g, 3 min)/water wash/re-dispersion cycles. The PSS layer was then deposited by using the same procedure, thereby producing PE2-coated PMAA microspheres (PE2-PMAA). Then another PDDA layer was deposited on the PE2-PMAA microspheres before the microspheres were added into 5 mL of aqueous MWNT suspension (0.5 mg/mL). After 15 min of MWNT adsorption, the excess (unadsorbed) MWNTs were removed by three centrifugation (1000g, 3 min)/water wash/ re-dispersion cycles. By repeating the assembly steps for PDDA and MWNTs, the MWNT/PDDA multilayers with a desired number were adsorbed on the PE2-coated PMAA microspheres. Here we use (MWNT/PDDA)n/PMAA to denote the PE2-PMAA microsphere covered with MWNT/PDDA multilayers after n assembly cycle(s). Scanning electron microscopic (SEM) characterization was conducted using a Hitachi S-4300 scanning electron microscope at an accelerating voltage of 5 kV. Transmission electron

MWNT Microspheres from LBL Assembly

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Figure 2. Schematic illustration of the assembly of MWNT/PDDA multilayers on PMAA microspheres and the preparation of MWNT microspheres.

Figure 3. SEM images of uncoated PMAA microspheres (a) and PE2-coated PMAA microspheres coated with one (b), two (c), three (d), four (e), and five (f) MWNT/PDDA multilayer(s). The scale bar is 2 µm in all images.

microscopic (TEM) images were taken on a JEOL 2010 transmission electron microscope operating at an accelerating voltage of 200 kV through a Gatan model 780 CCD camera. UV-vis spectra were obtained on UV-2501PC spectrometers. Thermogravimetric analysis (TGA) measurement was performed on a Universal V2.6D TA Instruments. Samples were analyzed in platinum pans from room temperature to 800 °C under steady air at a heating rate of 5 °C/min. 3. Results and Discussion In the previous reports, the CNTs used for the assembly are modified with carboxylic acid groups by oxidation. The

electrostatic attractions between the carboxylic acids and positively charged polyelectrolytes enable the LBL assembly. Herein, we transform the carboxylic acids on MWNTs to sodium carboxylates, which makes the surfaces of the nanotubes more negatively charged. The strong electrostatic interactions between MWNTs bearing sodium carboxylates and PDDA would facilitate the LBL process in this study. Before conducting the spherical assembly of MWNT/PDDA on PMAA microspheres, we investigated the planar assembly of MWNT/PDDA on quartz slides and silicon wafers, which could be easily monitored by SEM and UV-vis spectrophotometry. The morphologies of the MWNT/PDDA films on

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Figure 4. TEM images of uncoated PMAA microsphere (a) and (MWNT/PDDA)5/PMAA microsphere (b).

Figure 5. TGA curves of PMAA microspheres (s), MWNTs (- -), and (MWNT/PDDA)5/PMAA microspheres (--).

silicon wafers with different assembly cycles observed by SEM are shown in Figure 1. After one cycle of MWNT/PDDA adsorption, the randomly distributed MWNTs cover most of the substrate surface (Figure 1a), which indicates a satisfactory efficiency of the assembly. With an increase of the cycle numbers, the MWNT coverage increases dramatically. The nanotubes of (MWNT/PDDA)3 film almost fully cover the substrate surface (Figure 1b). After five assembly cycles, the tightly overlapping nanotube film demonstrates the success of uniform LBL assembly (Figure 1c). The LBL assembly procedure of the MWNT/PDDA could be further monitored and confirmed by UV-vis spectrum. Figure 1d shows the UV-vis absorption spectra of MWNT/ PDDA films of 1, 2, 3, 4, 5, and 6 assembly cycle(s) on a quartz wafer. The quartz substrate and PDDA exhibit no absorbance in the spectral region monitored, whereas the MWNTs show a typical featureless absorption at the range of 200-800 nm. Thus the measured absorbance of multilayer film is only due to the MWNTs. The absorbance curve is found to stepwisely increase with the growth of the film. From the inset in Figure 1d, we can clearly see that the absorption at 500 nm increases linearly with increasing cycle number, implying the growth of the MWNT/PDDA film in each assembly cycle is effective and uniform. The electrostatic interactions between MWNTs and PDDA realize the LBL assembly not only on the planar substrate but

also on the spherical templates, which enables the fabrication of MWNT shell structure via a microspheric core. Removal of the template (by calcination in this study) could result in the formation of MWNT microsphere. The construction and calcination process of (MWNT/PDDA)n/PMAA microsphere is summarized in Figure 2. In the first step, a two-layered polyelectrolyte film (PSS/PDDA) is deposited on the PMAA template to make its surface negatively charged. Afterward, another layer of positively charged PDDA and subsequent layer of negatively charged MWNTs were deposited, which complete a cycle of MWNT/PDDA assembly on the PE2-coated microsphere (step 2). The subsequent PDDA and MWNTs adsorption cycles result in a further increase of the multilayer shell thickness. At the last step, the PMAA template and polyelectrolytes are removed after calcination of the (MWNT/PDDA)/ PMAA microsphere, yielding a unique MWNT microsphere (step 3). Direct evidence for the stepwise growth of a MWNT/PDDA multilayer shell on the PMAA microspheres can be provided by SEM. Figure 3 exhibits representative SEM images of the uncoated PMAA microspheres and the PE2-coated PMAA microspheres after one to five MWNT/PDDA assembly cycles. The uncoated PMAA microspheres prepared using one-step swelling and polymerization method feature a clean surface with some tiny holes (Figure 3a). After one MWNT/PDDA bilayer was absorbed, some flocky threadlike MWNTs appear on the surfaces of the PMAA microspheres. With the increase of the assembly number, more and more MWNTs assemble and tightly wrap on the microspheres, which makes the microspheres produce an increased surface roughness in comparison with the uncoated PMAA microspheres (Figure 3b-f). From the SEM images, we can estimate that the diameter of the microspheres increases by ∼200 nm after one more MWNT/PDDA bilayer deposition. That is, the thickness of each MWNT layer is ∼100 nm. To further investigate the surface morphology transformation of microspheres in the procedure of electrostatic assembly, the uncoated PMAA microsphere and the (MWNT/PDDA)5/PMAA microsphere were analyzed using TEM. As shown in Figure 4, compared with the uncoated PMAA microsphere with smooth edge, the (MWNT/PDDA)5/PMAA microsphere is covered with compact MWNTs. The hollow structures of the protruding nanotubes (indicated by arrows) are very clear in Figure 4b,

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Figure 6. SEM images of porous MWNT microspheres produced by calcination of (MWNT/PDDA)5/PMAA microspheres.

Figure 7. TEM images of (MWNT/PDDA)5/PMAA microsphere (a) and MWNT microspheres (b). Inset of b: TEM image of MWNT microspheres at low magnification (scale bar is 2 µm).

which further confirms that those flocky threads observed under SEM are MWNTs assembled on PMAA microspheres. Various core-shell materials can be constructed through multilayer assembly of organic or inorganic materials on the surface of colloid particles functioning as a template. Subsequent

removal of the templates through chemical or thermal methods results in hollow shell materials.15 Although CNT microspheres or microcapsules could be fabricated via etching or dissolution of the template cores, the morphologies of the resulting CNT materials are not satisfactory. In this study, a PMAA micro-

11622 J. Phys. Chem. C, Vol. 112, No. 31, 2008 sphere template of (MWNT/PDDA)/PMAA is removed by calcination in a TGA instrument, which gives interesting and novel MWNT microspheres. To determine the calcination conditions, the thermal properties of PMAA microspheres, modified MWNTs, and (MWNT/PDDA)5/PMAA were investigated. Figure 5 presents the mass loss curves of the samples upon heating in air. It indicates that the MWNTs begin to decompose at ∼560 °C. The weight loss before this temperature should be attributed to the loss of organic groups. PMAA microspheres display a mass decrease of 92% from 250 to 350 °C due to their oxidation decomposition. The decomposition temperature of (MWNT/PDDA)5/PMAA microspheres is ∼10 °C higher than that of PMAA, which should be ascribed to the MWNT covering. On the basis of the TGA results, (MWNT/PDDA)5/PMAA microspheres were heated to 400 °C and kept at this temperature for 2 h to remove the PMAA templates and obtain MWNT microspheres. To investigate the morphologies of the final product, a drop of its acetone suspension after 10 min of ultrasonication was deposited on a glass slide for SEM. As shown in Figure 6, the microsphere structure of MWNTs is well kept after the removal of the PMAA templates and PDDA. At low magnification, SEM images show that the CNT microspheres are independent microparticles with uniform size (Figure 6a,b). This proves the stability of MWNT microstructure after calcination and sonication. The high-magnification SEM images reveal the MWNT microstructure is porous MWNT microspheres with an average size of ∼3 µm. In comparison with the originally assembled microspheres of 6 µm in diameter, the resulting MWNT microspheres show remarkable size shrinking after calcination. Further structure analysis of MWNT microspheres was carried out with TEM. As shown in Figure 7a, before calcination, (MWNT/PDDA)5/PMAA microsphere displays a solid structure with MWNTs assembled compactly on the surface. After the polymers are removed, the resulting MWNT microspheres shrink almost half in diameter. The MWNTs are entangled together to form porous and loose morphologies. Although we cannot say the MWNT microsphere is hollow, the inner nanotubes are not compact. The inset of Figure 7b clearly indicates that the MWNT microspheres are not solid. It is proposed that, during the calcination, the electrostatic attractions between MWNTs and polyelectrolyte gradually disappear as PMAA and PDDA decompose. Meanwhile, MWNTs collapse toward the center of the microspheres and wrap together induced by van der Waals attractions. As a result, porous MWNT microspheres are formed. 4. Conclusion In summary, we have demonstrated that layer-by-layer assembly method could be used to fabricate the MWNT-based thin films on planar substrate and microspheres on PMAA colloids. The strong electrostatic attractions between negatively charged MWNTs and polyelectrolytes enable the effective LBL assembly process. The thickness of the MWNT on the template core could be controlled by the number of the LBL cycles. Removal of PMAA microspheric templates via calcination yields novel MWNT microspheres with uniform size. The MWNT microspheres are expected to open the application possibilities in many fields due to their unique structure. Acknowledgment. This work was supported by the Starting Foundation of Renmin University of China, the National Natural Science Foundation of China (Grant No. 50433020), and the

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