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Grafting Boron Nitride Nanotubes: From Polymers to Amorphous and Graphitic Carbon Chunyi Zhi,*,† Yoshio Bando,† Chengchun Tang,† Hiroaki Kuwahara,‡ and Dmitri Golberg† Nanoscale Materials Center, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and InnoVation Research Institute, Teijin Ltd., 2-1, Hinode-cho, Iwakuni, Yamaguchi 740-8511, Japan ReceiVed: September 16, 2006; In Final Form: NoVember 4, 2006
In this article, using an atom transfer radical polymerization approach, for the first time polystyrene (PS) and polymethyl methacrylate (PMMA) were grafted covalently on boron nitride nanotubes (BNNTs). Then, a technique was developed to fabricate novel amorphous or graphitic carbon-BNNTs composites with a perfect physical contact between the two phases using the polymer-functionalized BNNTs. BNNTs-carbon-balls composite structures were first successfully fabricated. Other experimental route revealed that a C-BN compound nanotubular nanostructure can be fabricated by using polyvinyl pyrrolidone wrapped BNNTs as an effective precursor, which possess well-defined composition variations and a clear interface between the carbon and BN tubular fragments. The obtained polymer-functionalized BNNTs and carbon-BNNTs composites are useful for the mechanical enhancement of polymeric matrixes and in various electrochemical devices.
Introduction Boron nitride nanotubes (BNNTs) have analogous layered structures and competitive mechanical properties and thermal conductivity with carbon nanotubes (CNTs). Importantly, compared to CNTs, BNNTs are much more thermally stable and possess a constant wide band gap independent of tube morphology and geometry.1-3 These characteristics are suggested to promote BNNT potential applications in energy storage devices, polymeric composites, and so forth.4-9 However, the related studies on BNNTs have been hindered because of some problems, such as bad conductance and poor dispersibility in solvents.8-10 Chemical functionalization is an effective way to modify nanotubes, manipulate them, and extend their applications.11 Proper techniques like polymer grafting or graphitic carbon coating may change BNNTs’ properties.11-14 For example, our recent experiments10 indicate that it is still a challenge to improve interfacial interaction between BNNTs and a polymer matrix. To achieve better affinity between BNNTs and polymers, the polymer-grafted BNNTs should be fabricated.15-17 In addition, a BN-C nanotube compound structure may open a wide horizon for tuning nanostructure electrical and electrochemical properties. Although the BN-C sandwich structures induced by phase separation have been observed,18-19 the production cannot be controllable because the phase separation is spontaneous. It is suggested that covalently grafting carbon containing species on BNNTs may be a reliable way for the BN-C nanotubular composites. In this paper, for the first time BNNTs were covalently grafted with polystyrene (PS) and polymethyl methacrylate (PMMA) via surface-initiated atom transfer radical polymerization (ATRP). The BNNTs functionalized by polymers can be perfectly dispersed in some organic solvents. On the basis of the PS * Author to whom correspondence should be addressed. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Innovation Research Institute.
functionalized BNNTs, a novel carbon-BNNTs composite was fabricated, in which numerous amorphous carbon balls were grafted on BNNTs. In addition, noncovalent fuctionalization of BNNTs through wrapping with polyvinyl pyrrolidone (PVP) was found to also yield a qualitative graphitized coating on tubes after designed proper treatments. Experimental Section Recently, ATRP method has gained more and more attention because it is compatible with a variety of monomers, thereby permitting the controllable synthesis of a wide range of polymers.20-24 In the present work, BNNTs were synthesized via a chemical vapor deposition method using a boron powder and a metal oxide as reactants (BOCVD).25-26 The general strategy for grafting PS on the BNNTs via ATRP is described in Figure 1. Our previous studies indicate that there are some amino groups on BOCVD-fabricated BNNTs because of a reductive NH3 atm used during their growth.12-14 Required initiating sites BNNT-Cl for ATRP were formed by the amidation between chloroacetyl chloride and amino groups on BNNTs. Synthesis of BNNT-Cl. Chloroacetyl chloride (Aldrich, 98%) was used as received. Typically, 100 mg of BNNTs was immersed in 100 mL chloroacetyl chloride, and the mixture was stirred, heated to 150 °C, and annealed over 120 h. After centrifugation, the remaining solid was washed with N,Ndimethylformamide (Wako, 99.8%) repeatedly to afford BNNTCl. Synthesis of BNNT-PS. Typically, 10 mg of a BNNT-Cl material was mixed with a 10 mL styrene (Aldrich, 99%) solution in an Ar-gas-filled glass bottle. Thirty milligrams of CuCl (Aldrich, 99.99%), 420 mg 4,4′-dinonyl-2,2′-dipyridyl (DIDIPY) (Aldrich, 97%), and 5 mL xylene (Aldrich, 99.0%) were mixed and added to the glass bottle. The bottle was immersed in an oil-filled beaker, was heated to 130 °C, and was kept stirring for 48 h. At the end of the reaction, the
10.1021/jp066052d CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006
Grafting Boron Nitride Nanotubes
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Figure 1. The strategy for grafting PS on BNNTs via ATRP.
Figure 3. (a) Low-magnification and (b) high-resolution TEM images of BNNTs-PS.
Figure 2. FTIR spectra of pure BNNTs, BNNTs-Cl, and BNNTsPS.
viscosity increased drastically. After cooling down, the mixture was diluted and washed with chloroform and was filtered with a filter paper (mesh size 11 µm) repeatedly to remove nongrafted polymers and free reagents. Fabrication of Carbon-Balls-BNNTs Composite. In a typical run, 3 mg PS-BNNT was put in an alumina crucible and then the crucible was placed at the center of a horizontal alumina tube. The furnace was heated to 960 °C and was kept over 5 h under protection of Ar gas. After cooling down, a composite was obtained. Fabrication of Carbon-BNNTs Core-Shell Structure. Ten milligrams of BNNT and 90 mg of PVP (Wako, no impurities) were mixed in ethanol and were sonicated for 90 min, and then the mixture was poured into an alumina crucible. The ethanol was evaporated away by heating the crucible to 90 °C and by keeping for 2 h. After that, the obtained PVP-BNNT composite was heated to 960 °C and was annealed over 5 h under protection of Ar gas. After cooling down, a carbonBNNTs core-shell composite was obtained. Measurement. An SEM (JEOL SM67F) was used to characterize the morphology of the products. Transmission electron microscopy analysis and elemental mapping were performed using a JEOL-3000F and JEOL-3100FEF highresolution field-emission transmission electron microscopy (TEM) operated at 300 kV. Fourier transform infrared (FTIR) spectra were collected using a Perkin-Elmer FTIR spectrometer with a laser wavelength of 1024 nm. In addition, using a benefit of ATRP compatibility with various monomers, PMMA grafted BNNTs were also fabricated utilizing the similar sequence of operations. Results and Discussion The dried BNNT-PS products were investigated by Fourier transform infrared spectroscopy (FTIR), as shown in Figure 2. A typical FTIR spectrum of crude BNNTs is dominated by two peaks at around 820 and 1366 cm-1, which are assigned to A2u (BN vibration perpendicular to the tube axis) and E1u (BN
vibration parallel to the tube axis) modes.27 For BNNT-Cl, the peaks related to the C-H (866, 1019, 1096, 1263 cm-1) and C-Cl groups (470 cm-1) appeared. After PS grafting, the characteristic peaks of PS at 2700-3500, 1000-1400, and 700 cm-1 were observed, which indicated that PS was successfully attached to the nanotubes. The resultant products exhibit much better dispersibility in the organic solvents, such as chloroform, N,N-dimethylformamide, and so forth. So, it is thought that PS grafted BNNTs may possess better affinity to polymers, especially PS. The experiments on the property evaluations of PS/BNNTs bulk composites are underway. Figure 3a shows a low-magnification TEM image of BNNTPS. BNNTs produced by the BOCVD method possess a doublehelix structure,28 which results in regular dark spot appearance on the nanotube sidewalls in bright-field TEM images. It is visible on the images that BNNTs are surrounded by a light amorphous-like polymer shield, as particularly highlighted in a high-resolution image, Figure 3b. The BNNT walls keep good crystallinity after the functionalization process, as confirmed in the inset to Figure 3b. Because of the polymer chain entanglements during solvent evaporation, some BNNTs are bonded together. In addition, it is noticed that the thickness of a PS shell is not uniform along an individual nanotube; it ranges from 0.8 to 5 nm. We speculate that this is caused by the defect concentration variations along a BNNT since the adopted functionalization strategy is based on defect chemistry. Interestingly enough, the detailed studies on the polymer coating thickness variations may shed light on BNNT surface defect distribution. Thermal polymerization of styrene occurred at the experimental temperature adopted. To confirm that polymers are connected onto BNNTs covalently, crude BNNTs were used in the same polymerization experiments instead of BNNT-Cl. No polymer shells were observed during TEM when the samples were washed by chloroform repeatedly. Also, no characteristic PS-related peaks were discernible in the FTIR spectra. A carbon-coated BNNT composite should exhibit better transport properties and electrochemical activities than pure BNNTs. The key characteristic of such a composite is perfect physical contact between a BNNT and a carbon shield. A PS grafted BNNT may serve as a proper precursor for a carbonBNNT composite since PS and BNNTs are covalently connected. Figure 4a displays the resultant morphology of carbonBNNT composites obtained from a BNNT-PS precursor, as revealed by TEM. It is clear that carbon balls are successfully immobilized on BNNTs. The carbon balls are amorphous-like and possess a good physical contact to BNNTs, as depicted in Figure 4b. In Figure 4c, a bundle of BNNTs is characteristically
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Figure 4. (a) Low-magnification and (b) high-resolution TEM images of a carbon-balls-BNNTs composite; (c) TEM image of a BNNT bundle glued by amorphous-like carbon balls.
glued by means of such sticky carbons. We estimated that around 10 vol % BNNTs have been joined in bundles because of such phenomena in the present carbon-balls-BNNT composite. A TEM sample was made through sonicating of a carbon-BNNT composite in ethanol for 3 min and dripping a few drops of the mixture onto a TEM holey grid. Importantly, a TEM sample of pure BNNTs prepared under exactly the same conditions solely displays random distribution of nanotubes; no BNNT bundles were observed. Keeping in mind the facts that PS was connected to BNNTs covalently and that a high temperature (960 °C) could not break the C-N bonds existing in PS-BNNTs, it is believed that the carbon balls are also attached to BNNT covalently. Our experiments reveal that the key issue for fabrication of carbon-BNNT composites is to find a polymer which has strong interaction with BNNTs. To do so, other polymers were investigated. During this screening, PVP was found to be a perfect candidate because of a good affinity to BNNTs.29 The fabrication, which only includes dispersing and heating up, is much simpler than in the case of BNNT-PS composites. Scanning electron microscopy (SEM) indicates that BNNTs in the resultant carbon-BNNTs composites are in fact covered and become thicker then the pristine tubes. Under TEM, Figure 5 and its inset, a well-defined graphitic carbon-BNNT coreshell structure was revealed. The covering layers resemble a CVD-grown-CNT-like nanostructure because of high-temperature annealing utilized. It is interesting that a BNNT edge can be clearly identified because of the characteristic dark contrast domain caused by the existing BNNT double-helix structure. This enables us even to accurately count the number of BN layers within the composite. For example, in Figure 5b, there are 40 internal BN layers, covered with turbostratic-like carbon layers. It is well-known that in a given experiment it is rather difficult to identify the chemical composition of the atomic layers constituting the nanotube walls using standard electron energy loss spectroscopy because of insurmountable spatialresolution limitations.29 Interestingly enough, the chemical composition within the present composite tube can be understood while simply looking at the TEM image contrast variations induced by structural peculiarities of the phases forming the
Figure 5. (a) Low-magnification and (b) high-resolution TEM images of a carbon-BNNT core-shell nanostructure. Inset is the schematic model of the structure; (c) B, C, and N elemental maps of a BNNT covered with graphitic-like carbon.
tube. Figure 5c presents the constructed spatially resolved B, C, and N elemental maps of a composite nanotube, which further
Grafting Boron Nitride Nanotubes unambiguously reveal the well-defined composition variations and a clear interface between the carbon and BN tubular fragments. As mentioned above, the interactions between a polymer and BNNTs are very important for the fabrication of carbonBNNTs composites. For example, PVP has good affinity to BNNTs, which was proved by checking the solubility of PVPBNNTs. It is suggested that during heating up, the melted PVP can wrap around BNNTs. This ensures that a degenerated carbon can be physically close enough to BNNTs to set up strong BN/C intralayer interactions and characteristic sp2 chemical bonding. A similar mechanism is in effect for covalently PS grafted BNNTs. However, in this case, there is no strong interaction between the polymer and a tube (such as mixing PS and BNNTs simply, no covalent connection between PS and BNNTs), and carbon and BNNTs are well separated in the resultant composite. In addition, the resultant carbon-BNNTs structure may be also influenced by a carbon-BNNT weight ratio and thermal properties of a polymer. At a high temperature, melted PS shrinks to form balls on BNNTs, while PVP may be flat, correspondingly yielding either ball-shaped or tube-shaped carbon coatings. Therefore, it is proposed here that, on one hand, carbon-BNNT composites can be fabricated by using polymerwrapped BNNTs, and on the other hand, the resultant composites’ morphology and structure shed an additional light on the relative thermal properties of a polymer and a tube and the interactions between them. Conclusion In conclusion, for the first time PS and PMMA were grafted on BNNTs by an ATRP approach. On the basis of polymerfunctionalized BNNTs, a technique was developed to fabricate novel amorphous and graphitic carbon-BNNTs composites with a perfect physical contact between the two phases. BNNTscarbon-balls composite structures were first successfully fabricated using BNNT-PS as a precursor. Another experimental route revealed that PVP wrapped BNNTs can be an effective precursor for C-BN compound nanotubular nanostructures, which possess well-defined composition variations and a clear interface between the carbon and BN tubular fragments. The obtained polymer-functionalized BNNT and carbon-BNNTs composites may be useful for the mechanical enhancement of polymeric matrixes and in various electrochemical devices. Acknowledgment. The authors thank Drs. Y. Uemura and M. Mitome for cooperation and fruitful discussions. Supporting Information Available: Synthesis of BNNTs; synthesis of BNNT-PMMA; images of purified BNNTs; elemental maps of BNNT-PS; TEM image of two BNNTs glued by PS; TEM and FTIR results of BNNT-PMMA; FTIR of pure PMMA and PS; solubility of the samples; EELS of
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