Large-Scale Production of Aligned Long Boron Nitride Nanofibers by

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Large-Scale Production of Aligned Long Boron Nitride Nanofibers by Multijet/ Multicollector Electrospinning Yejun Qiu,† Jie Yu,*,† Javed Rafique,† Jing Yin,† Xuedong Bai,‡ and Enge Wang‡ Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Xili, Shenzhen 518055, China and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: February 11, 2009; ReVised Manuscript ReceiVed: May 4, 2009

Boron nitride nanofibers (BNNFs) with alignment and large length have been prepared on a large scale by electrospinning the precursor fibers and subsequent nitridation. A multijet/multicollector electrospinning system is constructed by simply arranging three jets and three tip collectors abreast. By this system the aligned precursor fibers with lengths over 13 cm are collected throughout a very large length of 68 cm. The alignment of the electrospun precursor fibers is mainly induced by the converging electric field generated by the applied tip collectors. Composite B2O3/polyvinylbutyral (PVB) fibers (BOFs) were used as the precursor fibers, where PVB was added for increasing the spinnability of the electrospinning solutions. However, the presence of PVB in the BOFs results in the formation of carbon residue in the nitrided products, which is a stubborn problem for preparing the BNNFs. We successfully solved this problem by introducing a small amount of O2 at appropriate temperature during nitridation with NH3. The morphology of the nitrided products is strongly dependent on the diameter and B2O3/PVB ratio of the BOFs and the BNNFs with diameters as small as below 100 nm were obtained by controlling them. The obtained BNNFs were crystallized with (002) base planes preferentially oriented in parallel to the fiber axis. A high onset oxidation temperature of 890 °C was demonstrated for the BNNFs. 1. Introduction One-dimensional nanomaterials including nanotubes, nanowires, nanobelts, and nanofibers have attracted significant attention due to their potential applications in a broad range of areas such as electronics, optoelectronics, sensors, and composites.1-4 The preparation of such one-dimensional nanomaterials with controlled size and orientation has been a major challenge in nanoscale science and technology. Recently, electrospinning aroused much interest due to its capability of producing continuous nanofibers with diameters down to a few nanometers.5,6 Numerous polymers and polymer-based hybrids have been made into nanofibers by electrospinning. Many promising applications have been demonstrated for the electrospun nanofibers, the examples of which include chemical sensors,7 supercapacitors,8 nanomechanical devices,9 and tissue engineering.10 Inorganic nanofibers such as carbon,11 metals,12 carbides,13,14 and many oxides15-18 have been synthesized by calcining the electrospun precursor fibers. To date, the inorganic compound nanofibers prepared by electrospining are mostly oxides15-18 and carbides,13,14 while the synthesis of boron nitride nanofibers (BNNFs) has not been reported yet. The reason is mainly that it is not easy to eliminate the carbon residue originating from the polymers used for increasing the spinnability of the electrospinning solutions during the nitriding process. Hexagonal boron nitride (hBN) is an important nitride material and possesses many extraordinary properties mainly including high chemical stability, high onset temperature of * To whom correspondence should be addressed. Phone: 86-75526033478. Fax: 86-755-26033504. E-mail: [email protected], msejyu@ yahoo.com. † Harbin Institute of Technology. ‡ Institute of Physics, Chinese Academy of Sciences.

oxidation (>850 °C), high electrical resistance, high thermal conductivity, and wide band gap (>5.5 eV).19-21 The BNNFs are of great importance not only for the conventional application areas of boron nitride such as high-temperature ceramic composites but also for the developing nanotechnology mainly due to their unique semiconducting properties.22,23 As the main member of BN nanomaterials, BN nanotubes (BNNTs) have been synthesized and investigated intensively and found to have many promising applications such as polymeric composites, gas adsorption, electrical nanoinsulators, electron field emission, and ultraviolet nanoelectronics, etc.4,20,24-30 It is expected that most of these applications are available for the BNNFs due to their nanoscale diameters. Especially, the continuous BNNFs have obvious advantages over the BNNTs for some applications such as catalyst support, filtration, and composite reinforcement because they can be easily made into webs and other desired shapes. On the other hand, the conventional electrospinning setup can only produce randomly distributed nanofibers. However, well-aligned nanofibers are generally required for many applications, especially for device fabrication, scaffolds, and composite reinforcement. Much success has been achieved in controlling the spatial orientation of the electrospun nanofibers by both mechanical and electrostatic methods. The mechanical methods include moving the collector of spaced wooden splints,31 rotating circular collector,32-35 and moving plate collector,36 and the electrostatic methods include using frame electrode collector,37 conductive collector with gap,15 and dual rings collector.38 However, these methods can only produce aligned nanofibers of a few centimeters in length over a small area. Recently, we developed a simple electrospinning technique of controlling the spatial orientation of the nanofibers based on the inducing effect of the converging electrical field generated

10.1021/jp901267k CCC: $40.75  2009 American Chemical Society Published on Web 06/04/2009

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by a tip collector. By using this technique the aligned polymer nanofibers with length over 20 cm and aligned helical polymer nanofibers have been prepared over a large area.39,40 However, effective techniques of producing aligned long nanofibers on a large scale are still urgently required. In this article, we report the large-scale production of aligned long BNNFs by multijet/multicollector electrospinning. Here both preparations of the BNNFs and the multijet/multicollector technique for preparing the aligned long nanofibers are reported for the first time. Composite B2O3/polyvinylbutyral (PVB) fibers (BOFs) were used as the precursors for preparing the BNNFs. The BNNFs were prepared by nitriding the BOFs with NH3 and N2 at high temperatures. By introducing a small amount of O2 during the nitriding process the carbon residue from PVB was effectively removed and high purity was achieved for the BNNFs. In order to increase the productivity of the aligned nanofibers a multijet/multicollector electrospinning technique was developed, which could produce the aligned long nanofibers over a very large area. This method of producing the aligned long nanofibers is of high promise in commercialization due to its simplicity, efficiency, and easiness in scaling up. The present work also provides an effective way for preparing other nitride nanofibers from electrospun precursors. 2. Experimental Section The electrospinning solutions were prepared by dissolving B2O3 and PVB (Mw ) 60 000) in ethanol solvent at different concentrations, where the addition of PVB is for increasing the viscosity of the solutions. The electrospinning was conducted at an applied voltage of 20 kV. No syringe pump was used all throughout the experiments. The nitriding process of the precursor BOFs was conducted in a high-temperature tube furnace. During nitridation the precursor BOFs were first heated to 800 °C in pure NH3 and whereafter kept for 2 h in the mixture atmosphere of NH3/O2 at a volume ratio of 9:1. Subsequently, the O2 flow was stopped and the temperature increased to 1100 °C in pure NH3 and kept for 6 h. In the following step the temperature was increased to 1500 °C in pure N2 and kept for 2 h. Finally, the samples were cooled down to room temperature. The heating rate from room temperature to 800 °C and from 800 to 1100 °C is 2 °C/min, and that from 1100 to 1500 °C is 1 °C/min. Scanning electron microscopy (SEM, HITACHI S-4700), transmission electron microscopy (TEM, JEM-2010), Fourier transform infrared spectroscopy (FTIR, NICOLET-380), X-ray diffractometer (XRD, Rigaku D/Max 2500 PC), and Raman spectroscopy (Renishaw RM-1000) were used to characterize the structures of the samples. An electron energy loss spectroscope (EELS) equipped in the TEM and energy-dispersive X-ray spectrometer (EDS) equipped in the SEM system were used to determine the sample compositions. The thermal stability of the samples in air was determined by a WCT-2C thermogravimetric (TG) analyzer. During TG measurements, the samples were heated in air from room temperature to 1000 °C at a heating rate of 10 °C/min. 3. Results and Discussion Preparation of the BNNFs includes two main steps, i.e., electrospinning of the precursor fibers and subsequent nitridation. In this work the precursor fibers were prepared by the multijet/multicollector electrospinning technique. Figure 1a shows the schematic of the multijet/multicollector electrospinning system, where three needles were placed in parallel to each other with equal interval and three ground electrodes acting as

Figure 1. (a) Schematic of the multijet/multicollector electrospinning system. (b) Macrophoto of the BOFs collected by the multijet/ multicollector electrospinning system. (The steel rule in front of the BOFs is 30 cm in length.) (c) Typical SEM image of the BOFs collected by the multijet/multicollector electrospinning system.

the collectors were placed in the holes of an electrode holder with a similar interval and facing the needle nozzles. The ground electrodes are thin metal wires of 2 mm in diameter and named ‘tip collectors’ due to their small size. This multijet/multicollector system is obviously a combination of three single-jet/ single-collector setups, the details of which have been presented elsewhere.39,40 Figure 1b shows a typical macrophoto of the precursor BOFs prepared by the three-jet/three-collector electrospinning system, where a wooden board of 68 cm in length was used as the electrode holder. The white arrows indicate the positions where the tip collectors are located in the wooden board, and the interval between adjacent tip collectors is 17 cm. This photo was taken under illumination of an electric torch in order to make the fibers more clear. It is clearly observed that the BOFs were collected along the full length of the wooden board. The suspended part of the BOFs between the wooden board and the support plate were measured to be over 13 cm in length. Figure 1c is the typical SEM image of the BOFs shown in Figure 1b. It is indicated that high alignment was obtained for the BOFs. Although we made an explanation elsewhere,39,40 the detailed aligning mechanism is not very clear now. In brief, three factors may account for the alignment of the BOFs, i.e., application of the tip collectors, sideward ejection, and electrospinning of single nanofibers one by one. In the conventional

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electrospinning process the fibers are ejected continuously, where one end of the fiber is fixed at the needle tip. In the present case, it is inferred that the fibers are electrospun individually and one by one due to the low solution flow rate from the syringe reservoir to needle nozzle, which has been confirmed by the real-time images presented in our previous paper.39 This is mainly because the syringe pumps were not used during electrospinning, and the solution flow from the syringe reservoir to the needle tip was only driven by gravitation. In the case of a low solution flow rate, once a droplet was exhausted a single fiber with two free ends formed. We calculated the electrical field distribution generated by a tip collector and a larger area collector.40 It was found that the electrical field lines are almost parallel to each other near the large collector while the field lines are converging toward the tip collector. As a consequence, the front ends of the free fibers are dragged toward the collector tip by the converging electrical field, which tends to direct the fiber flight with its axis in parallel to the line connecting the needle tip and the ground electrode tip. In conjunction with the repelling force from the formerly deposited fibers with charge, the electrospun single fibers were deposited throughout the electrode holder with alignment. It was observed that the fiber deposition started from the three electrode positions and then extended outward due to accumulation of charge. A relatively uniform distribution of the BOFs along the wooden board was achieved with increasing deposition time. However, slightly higher densities were observed around the electrode positions because of the strong converging electric field between the needles and the collecting electrodes. The highest density of the nanofibers was found around the central electrode, and the reason is that more fibers produced by the central needle were collected around the central electrode while for the two side electrodes some of the fibers land beyond the length of the electrode holder due to the repulsion from the middle jet. Obviously, the uniformity of the fiber collection could be improved further with increasing numbers of needles and collector tips. An important factor influencing the fiber alignment, productivity, and distribution uniformity of the multijet/multicollector electrospinning is the interval between adjacent ground electrodes. The fiber productivity and distribution uniformity increases with decreasing interval, but the alignment degree decreases if the interval is too small. This is because with decreasing the interval between adjacent ground electrodes the converging degree of the electric field lines decreases and thus the aligning effects on the electrospun fibers decreases. We found that the fiber alignment deteriorates obviously when the interval is below 13 cm. We demonstrated here that the three-jet/three-collector electrospinning system is capable of producing the aligned long nanofibers on a large scale. It is obvious that numerous jets and tip collectors could be assembled together very easily in the above manner, showing the tremendous commercializing potential of this multijet/multicollector technique for producing the aligned long nanofibers. The electrospinning behavior and diameter of the precursor BOFs are strongly dependent on the concentrations of B2O3 and PVB in ethanol. The pure B2O3 solution is not spinnable at any concentrations. Both B2O3 and PVB concentrations influence the diameter of the BOFs, which increases rapidly with increasing PVB concentration but slowly with the B2O3 concentration (Table 1). This is obviously caused by the viscosity changes with solution concentration and commonly observed during electrospinning.41,42

Qiu et al. TABLE 1: Diameters of the Precursor BOFs (dBOF) and BNNFs (dBNNF) Prepared at Different B2O3/PVB Concentrations (w/v %) B2O3/PVB concentrations

dBOF(nm)

dBNNF(nm)

0.5%/5.0% 2.0%/5.0% 7.0%/5.0% 7.0%/3.5% 7.0%/2.5% 2.0%/2.5% 0.5%/2.5%

700–1700 900–1900 1300–2400 350–580 190–340 150–280 130–230

no products necklace-like bark-like 250–350 110–200 80–130 no products

Figure 2 shows the SEM images of the nitrided products. The morphology of the nitrided products is strongly dependent on the solution concentrations of B2O3 and PVB where the precursor BOFs were electrospun, which determines the diameter and composition of the BOFs. For the PVB concentration of 5%, the BN products were bark-like and necklace-like at the B2O3 concentration of 7% and 2% (Table 1 and Figure 2a and 2b), respectively, rather than the common fibrous shape. Formation of the bark-like morphology could be ascribed to the large diameter and high B2O3 concentration of the precursor BOFs. According to previous reports,43,44 addition compound (B2O3)n · NH3 forms during nitridation when the temperature exceeds 200 °C and (BN)x(B2O3)y · (NH3)z forms when the temperature exceeds 350 °C. These addition compounds are thermally stable and thus prevent the B2O3 fibers from fusing during nitridation. Obviously, the formation of the addition compounds should start from the fiber surface to form a surface protecting layer. In the case of high B2O3 concentration this surface layer is continuous and dense, leading to limited channels for gas release. Simultaneously, because of the large diameter too much volatile products originating from PVB decomposition was evolved in a short time inside the fibers during heat treatment. Consequently, the surface layer bursted during heating, forming the bark-like morphology. Formation of the necklace-like morphology may be caused by the low B2O3 concentration, where B2O3 is not continuous in the BOFs.

Figure 2. SEM images of the nitrided products prepared at the B2O3/ PVB concentrations (w/v%) of (a) 7.0%/5.0%, (b) 2.0%/5.0%, and (c) 2.0%/2.5%.

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Figure 4. (a) Typical TEM image of the BNNFs. (b) High-resolution TEM image taken from the same sample.

Figure 3. (a) FTIR spectra of the products nitrided under different conditions: (I) nitrided at 1500 °C, (II) nitrided at 800 °C. (b) Typical XRD pattern of the BNNFs.

Because of the discontinuous distribution B2O3 cannot shrink uniformly when PVB decomposes during heat treatment but tends to congregate to form a ball shape for decreasing surface energy. As a result the diameter of the obtained BN fibers is not uniform along the fiber axis and the necklace-like morphology formed. When decreasing B2O3 concentration to 0.5%, no products were obtained, maybe because the products are very little and blown away by the gas flow at this low B2O3 concentration. It can be seen that the BNNFs could not be obtained at the PVB concentration of 5%, where the precursor BOFs are very thick, generally over 1 µm (Table 1). We thus decrease the PVB concentration to 3.5% and 2.5% while keeping the B2O3 concentration at 7%, where the BNNFs were successfully obtained. By further decreasing the B2O3 concentration to 2% while keeping the PVB concentration at 2.5% the BNNFs with diameters as small as below 100 nm were obtained (Table 1 and Figure 1c). However, if further decreasing the B2O3 ratio to 0.5% at the PVB concentration of 2.5% no products were obtained. From the SEM image we observed that the BNNFs are very uniform in diameter and smooth in surface (Figure 1c). FTIR spectra were measured for identifying the BN phase formation during the nitriding reaction. Figure 3a-I is the FTIR spectrum of the final BNNFs, and Figure 3a-II is that of the sample only nitrided at 800 °C in NH3/O2 mixture atmosphere. For both spectra two main absorption peaks characteristic of hBN were observed around 815 and 1370 cm-1, ascribed to the out-of-plane bending of sp2-bonded B-N-B and the in-

plane stretching of sp2-bonded B-N, respectively.19,45,46 This indicates that the conversion of B2O3 to BN occurs even at 800 °C and the BN phase forms for the nitrided products. The FTIR measurements were also performed for the products synthesized without introducing O2 during the nitriding process. Apart from the two absorption peaks of hBN a weak peak at 1106 cm-1 was also observed (Figure S1, Supporting Information). This peak is likely related to B-C bonds originating from the carbon residue,47,48 demonstrating that the carbon residue could be removed efficiently by introducing O2 during the nitriding process. Figure 3b shows a typical X-ray diffraction (XRD) pattern of the BNNFs, which exhibits the characteristic diffraction peaks of hBN. The well-defined intense peak at 25.9° corresponds to (002) reflection, and the other weak peaks at 42.5°, 54.0°, and 77.4° correspond to (100), (004), and (110) reflections of hBN,19,44,49,50 indicating a relatively well-crystallized structure. The present XRD pattern is similar to that of the conventional BN fibers prepared from the boric oxide and polymeric precursors as reported previously.43,50 The interplanar spacing d002 was calculated to be 3.44 Å, larger than the theoretical value of hBN. It is believed that the crystallinity of the BNNFs could be improved by further increasing the nitriding temperature. Raman spectra were also measured for characterizing the structure of the BNNFs. However, no peaks were observed due to very strong fluorescence interference. The typical TEM images of the BNNFs are shown in Figure 4. From the high-resolution TEM image (Figure 4b) the lattice fringes can be clearly observed. The average spacing between the lattice fringes was measured to be 3.49 nm, indicating that the lattice fringes are (002) crystal planes of hBN. It is found that in some areas the (002) planes are very straight and strictly in parallel to each other, but discontinuities were also observed in some other areas. Interestingly, it is clearly observed that the base planes of (002) are preferentially oriented in parallel to the fiber axis (Figure 4b). However, it was reported that for the conventional B2O3 melt-spinning process of preparing the BN fibers the crystallite orientation was only obtained by applying tension at high temperature over 2000 °C.42,51 The preferential orientation of the basal planes may be induced by the alignment of the polymer chains, which is caused by the large elongation of the fibers during electrospinning. Obviously, the preferential orientation of the basal planes is of great

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Figure 5. Typical EELS spectrum of the BNNFs.

Figure 6. Macrophoto of the nitrided products prepared at different conditions: (a) 800 °C without introducing O2, (b) 800 °C with introducing O2, and (c) 1500 °C with introducing O2 at the 800 °C step.

importance for obtaining high mechanical strength for the BNNFs. Testing on many nanfibers shows similar results that the BNNFs are crystallized with the (002) planes in parallel to the fiber axis. EELS spectra were recorded during TEM measurements to determine the stoichiometry and chemical states of the products. The typical EELS spectrum from single nanofibers exhibits two

Figure 7. TG traces of the BNNFs.

Qiu et al. distinct absorption features starting at 188 and 401 eV (Figure 5), corresponding to the known K-shell ionization edges of boron (B-K) and nitrogen (N-K), respectively. The sharp π* peaks on the left side of the B and N K edges and the sharp σ* bands on the right side are typical of the sp2-hybridized layered BN.25-27 Only B and N K edges were observed with the B/N ratio calculated to be about 1.0. EELS measurements on numerous single BNNFs gave similar results, indicating that the obtained BNNFs are boron nitride with high purity. On the contrary, the obvious C K absorption edge at 284 eV was observed for the products synthesized without introducing O2 during the nitriding process (Figure S2, Supporting Information). Because of the high sensitivity of EELS to light elements it can be concluded that the carbon residue was effectively removed and the obtained products are pure BNNFs. We also used the EDS to confirm the chemical composition of the products and obtained similar results, i.e., only B and N peaks were observed (Figure S3, Supporting Information). Considering the detecting limits of EELS and EDS we could not absolutely exclude the presence of a trace of carbon. However, the consistent results of FTIR, XRD, TEM, EELS, and EDS strongly support that the synthesized products are pure BNNFs. As stated above, synthesis of the BNNFs by electrospinning has not been reported so far yet. This is mainly due to the lack of appropriate precursors and the complexity in the thermal conversion process. Generally, the oxide nanofibers were prepared by calcining the electrospun precursor fibers containing the desired metal salts and polymers in air. The addition of the polymers is for increasing the viscosity and thus the spinnability of the electrospinning solutions. During calcination in air, the polymers were burned out fully and the salts were converted into oxides. However, the thermal conversion process is carried out in NH3 and N2 when preparing the BNNFs. Clearly, the key problem in synthesizing the BNNFs is how to remove the carbon residue originating from the polymers in the precursor fibers in the nitriding atmosphere. It can be seen that we successfully solved this problem and well established the process of preparing the BNNFs. By introducing a small amount of O2 during the low-temperature nitriding stage the carbon element was effectively removed and the pure BNNFs were obtained.

Production of Aligned Long Boron Nitride Nanofibers Oxidation treatment has also been used for synthesizing the pure BN nanotubes, which were obtained by heating BxCyNz nanotubes in air at 700 °C.52 In this work, color changes of the BN products were observed for different nitriding processes (Figure 6). The color of the sample nitrided at 800 °C without introducing O2 is nearly black (Figure 6a), suggesting the presence of a carbon residue originating from PVB, while the sample nitrided at 800 °C with introducing O2 is yellow (Figure 6b), indicating that the carbon residue was decreased greatly or removed completely. The final products of BNNFs after 1500 °C nitridation with introducing O2 at 800 °C step were white (Figure 5c), the color of the pure BN, confirming the successful synthesis of the BNNFs. TG analysis was carried out in air for testing the oxidation resistance of the BNNFs. The onset temperature of oxidation is about 890 °C, and the weight increase is due to the formation of B2O3 (Figure 6). The oxidation resisting properties of the present BNNFs is comparable to that reported for the conventional BN fibers, BNNTs, and bulk BN materials,19-21,44 demonstrating the outstanding high-temperature stability of the BNNFs. The high onset temperature of oxidation indicates that the BNNFs are greatly promising for high-temperature applications. The present work established a complete set of processes for preparing the BNNFs by the simple electrospinning technique and using common precursor materials with the advantage of alignment, large fiber length, and high productivity. The multijet/multicollector electrospinning technique could be easily scaled up by only arranging more jets and collectors and shows great promise in commercial applications. 4. Conclusion In summary, the aligned long BNNFs were successfully prepared on a large scale from the electrospun precursor fibers by high-temperature nitridation. The multijet/multicollector electrospinning system was constructed by arranging three jets and three tip collectors abreast for producing the aligned long precursor fibers on a large scale. The aligned precursor BOFs with lengths over 13 cm were collected over a very large area with a length of 68 cm, demonstrating the productivity of the multijet/multicollector electrospinning technique in producing the aligned long precursor fibers. This method has the advantage of easily scaling up and high promise in commercialization. The BNNFs with high purity were prepared by nitriding the precursor BOFs electrospun from B2O3/PVB/ethanol solutions with NH3 and N2. The morphology of the nitrided products is strongly dependent on the diameter and B2O3/PVB ratio of the precursor fibers, where highly pure BNNFs with diameters ranging from 80 to 350 nm were obtained depending on the composition and diameter of the BOFs. The BNNFs were crystallized with the (002) planes preferentially oriented in parallel to the fiber axis. The introduction of O2 during the nitriding process is crucial for removing the carbon residue originating from PVB and obtaining high-purity BNNFs. An onset oxidation temperature as high as 890 °C was demonstrated for the BNNFs. The present work opens up a way for producing the aligned long BNNFs on a large scale. Acknowledgment. This work was supported by the NSFC (grant no. 50572019), New Century Excellent Talents in University (NCET060343), SRF for ROCS, SEM, S&T Program of Shenzhen government, and the China Postdoctoral Science Foundation funded project (grant no. 20080440847). Supporting Information Available: FTIR and EELS spectra of the nitrided products synthesized without introducing O2

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