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A simple method to synthesize polyhedral hexagonal boron nitride nanofibers
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Solid State Sciences 9 (2007) 1099e1104 www.elsevier.com/locate/ssscie
A simple method to synthesize polyhedral hexagonal boron nitride nanofibers Liang-xu Lin a, Ying Zheng a,*, Zhao-hui Li b, Xiao-nv shen a, Ke-mei Wei b b
a College of Chemistry & Materials Science, Fujian Normal University, Fuzhou, 350007, PR china National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, 350002, PR China
Received 19 April 2007; received in revised form 13 July 2007; accepted 17 July 2007 Available online 27 July 2007
Abstract Hexagonal boron nitride (h-BN) fibers with polyhedral morphology were synthesized with a simple-operational, large-scale and low-cost method. The sample obtained was studied by X-ray photoelectron spectrometer (XPS), electron energy lose spectroscopy (EELS), X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), etc., which matched with h-BN. Environment scanning electron microscopy (ESEM) and transmission electron microscope (TEM) indicated that the BN fibers possess polyhedral morphology. The diameter of the BN fibers is mainly in the range of 100e500 nm. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Hexagonal boron nitride; Nanofibers; Polyhedral
1. Introduction h-BN is a material which has received considerable attention due to its properties such as hardness, chemical inertness, high thermal conductivity and the capability of hydrogen uptake [1e3]. Since the BN nanotube was formally reported by Chopra et al. [4], many researches on the BN fibers, especially the BN nanotubes (BNNTs) have been published due to their potential applications in prospective electronic and mechanical devices [5e12]. As an inorganic fiber, the BN fibers also have potential applications to improve the performance of ceramics [13]. The study of BN fibers was not suspended on this stage. Many methods were developed to synthesize functional BN fibers to make better purification of BN fibers, develop new functional BN fibers which have a special application and so on [14e16]. From now on, there are only little investigations on the practical application of BN nanofibers, mainly because there are no reliable methods to produce
* Corresponding author. E-mail address: [email protected] (Y. Zheng). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.07.026
bulk BN nanofibers with high purity [16]. This bewildering complexion has promoted the essentiality to develop a new facile method to produce bulk amounts and high purity hBN nanofibers. The synthesis of h-BN fibers with high performance was considered as a technological challenge and needs appropriate method for a good processibility [17,18]. BN nanotubes have been synthesized using different methods, such as chemical vapor deposition [5], arc discharge [4], ball-milling [8], laser ablation [9], carbon nanotube substitution reaction [10] and pyrolysis[11,18]. Among them, the ball-milling/annealing method was considered as an effective way to synthesize BN nanofibers bulkily. But most of these growth methods have introduced metal catalysts and could not synthesize BN fibers with high purity. The other optional method is pyrolysis. Cylindrical BN fibers without tubular morphology could be synthesized using this method. However, the h-BN nanofibers with polyhedral morphology reported here is, to our knowledge, hitherto unknown for h-BN. This is a novel morphology of BN nanofibers and is different from the cylindrical and tubular BN nanofibers. Hamilton et al. have reported the first synthesis of tubular BN fiber in 1993 [19]. Their BN fibers
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L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
Fig. 1. XRD patterns of BN sample. Fig. 2. FT-IR spectra of BN sample.
Fig. 3. (aec) ESEM images of the sample. (dei) Amplificatory ESEM images of local fibers.
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
1101
Fig. 4. XPS spectra of the products. (a) The survey spectrum; (b) B 1 s; (c) N 1 s.
also revealed the presence of a little facets. But this morphology is not usual in these tubular BN fibers. Herein, we designed a two steps route to synthesize h-BN fibers with unique morphology. This is a large-scale, lowcost and simple-operational synthesis method. The present paper reports the synthesis and the characterization of new h-BN fibers. Although the mechanism of polyhedral h-BN nanofibers formed in the high-temperature reaction is difficult to be understood, it appears that the crucial factor is the presence of KCl and the potential factor is the presence of our precursor with facet (pseudo-polyhedron morphology). 2. Experimental An intermediate material was prepared for synthesizing BN nanofibers. The synthesis of intermediate was conducted in a 500 mL three-neck flask. The synthesis process comprises four steps. (1) An appropriate amount of KBH4 powder (21.58 g) was dissolved in 100 mL of deionized water in the three-neck flask and then stirred for 30 min. (2) NH4Cl (40 g) was dissolved in 200 mL of deionized water, and the
NH4Cl solution was added to system slowly when the system was cooled down to 0 C temperature condition. Then the system was heated to 90 C slowly and allowed to react for 4 h. (3) The system was then heated to 120 C temperature condition and regurgitate for 48 h when the air produced was not so violent. (4) Forty one gram of intermediate was obtained when water was distilled. All the reactions were produced in N2 condition for making an appropriate pressure. We define the precursor is the material, which purified from intermediate and does not contains any NH4Cl and KCl. Some experimental details proven that the intermediate material consists of pure precursor, NH4Cl and KCl. Some pure precursors were obtained when the intermediate was washed with icecold deionized water for some times. Until the washed water was tested by AgNO3 solution, no white substance produced obviously. For synthesizing h-BN fibers, a furnace containing an Al2O3 tube with one end connected to the N2 source and the other end to a bubbler was utilized. Place Al2O3 boat, which was loaded 6 g intermediate material into an Al2O3 tube. Then heat it to 1250 C and react for 10 h. The crude production was treated
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
1102
Fig. 5. EELS spectrum of a h-BN fiber.
with a solution some times, which contains 10 mL 40% aqueous HF and 5 mL EtOH, at 25 C to remove any boron oxides, SiO2, KCl and NH4Cl. A 10% NaOH solution also was introduced in the once treated production to remove potential Al2O3 at mild temperature. Then the remaining product was washed with deionized water and dried with pump at 60 C. BN sample (0.8 g) was obtained after simple purification. 3. Result and discussion The purity of the sample was examined by XRD, FT-IR and XPS. The XRD pattern (Fig. 1) of the sample was highly ordered
h-BN with lines indexed from left to right as (002), (100), (101), (102), (004), (110) and (112) [20]. FT-IR (Fig. 2) was used to distinguish the sp2 hexagonal and sp3 cubic phases [21]. The peak at 1409 cm 1 is attributed to the TO mode of the sp2bonded h-BN [21], and the peak at 804 cm 1 could be attributed to the BeNeB out-of-plane bending vibration [22]. ESEM images (Fig. 3) of h-BN reveal that the h-BN fibers possess a unique distinct morphology. From Fig. 3a, the diameter of the BN fibers is mainly in the range of 100e500 nm and the length of most fibers is more than 5 mm. The diameter value reported here is larger than that reported in previous papers [5e7]. Fig. 3bed also show that the BN nanofibers possess polyhedral morphology clearly. This morphology is, to the best of our knowledge, hitherto unknown for BN. Recent reports have reported polyhedral graphite particles and graphite fibers with facets [23,24]. Considering the h-BN was similar with graphite, it is likely that the h-BN nanofibers also possess some polyhedral morphology. But there are no reports about this kind of h-BN nanofibers. The proximate report only proven that the presence of little facets in BN fibers [19]. However, in the case of BN fibers presented here have more regular uniform facets and possess a regular fibrous morphology. The h-BN nanofiber’s compositional information was further detected by XPS. The O 1s XPS peak at 531 eV is a signature of surface oxidization. The binding energies near the 190.3 eV for B 1s (Fig. 4b) and 398 eV for N 1s (Fig. 4c) are in good agreement with the values of bulk BN in the published literature [22]. B 1s and N 1s binding energy spectrums
Fig. 6. (aec,e,f) TEM images and (d) electro diffraction patterns of the BN fiber.
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
1103
Fig. 7. High resolution TEM micrograph of sample.
also provided qualitative evidence for the presence of sp2 (hBN) bonding through the identification of a p plasmon loss peak at approximately 9 eV away from the B 1s and N 1s peaks [25]. The quantification of peaks confirmed that the atomic ratio of B:N was 0.975:1, which agrees well with the chemical stoichiometric relation between B and N. Fig. 5 shows a typical EELS spectra taken on a h-BN nanofiber. Two distinct absorption features are observed at 189 and 404 eV, which correspond to boron K-edge and nitrogen K-edge, respectively. The EELS result also indicates the BN nanofiber with a B/N ratio of around 0.95. The sharp p*-peaks on the left side of the B and N K-edges and the sharp s*-bands on the right side confirmed that the nanofibers were composed of sp2-bonded B and N atoms [26]. To further study the morphologies of these h-BN fibers, TEM characterization was introduced. The polyhedral morphology of the fiber was indicated by Fig. 6c. Fig. 6f shows
a BN fiber with a length about 5 mm. Figs. 6a and 7a and c exhibit sharp lattice fringes indicating that the interplanar distance is well ordered with the value of w0.33 nm, which is ˚ in bulk hconsistent with the interplanar distance of 3.33 A BN [4]. Selected-area electron diffraction (Fig. 6d) pattern from one fiber is indexed by the h-BN which was corresponding with XRD. Fig. 7 shows a tip’s morphology of a h-BN nanofiber. In Fig. 7a, the indicated q is the angle between the fiber axis and the h-BN basal planes. This is an all-pervading phenomenon in these BN fibers from our TEM results. Particularly, not all the tips are well ordered as Fig. 7a shows. Fig. 7c shows some defects in the tips of BN fiber. The numeric signs on these local lattice fringes correspond with what in the inserted electron diffractions. These local lattice fringes were parallel with each other no more. As a result of this morphologic defect, the mild flexural fiber could grow in the reaction
1104
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
as is indicated in Figs. 3a and 7b. It is possible that some morphologic defects might present in local reactor area where the reactants were not well mixed. Further more, SEM and TEM images also reveal some flat tips present in the fiber. The bigger white ring sign in Fig. 3b indicated an even end. The formation of flat end could be a result of potential rupture of fiber. Without considering the defect discussed above, one interesting result could be concluded. The manner of fiber’s growth was distinguished from that reported before [4,5,7]. We were interested how these h-BN fibers with facets formed in heating reaction. There are two factors to be considered. As the first factor, the presence of KCl appears here to be a vital. In fact, the KCl presents here could be a fluxing agent. In the absence of the KCl, only some ruleless conglomeration and elementary character of facet appeared. The BN with different morphologies could be formed at different temperature conditions. However, there are no obvious evidences of uniform facet formed until the presence of fluxing agent. In other word, the presence of KCl at 1000e1250 C created a melting environment. The fluid capability of reactant was enhanced. Many solid materials exhibit a one-dimensional growth habit as a result of the inherently anisotropic feature in their crystal lattice [12]. The BN fiber could grow from this melting environment easily. Contrarily, ruleless BN nanoparticle formed in the reaction when the absence of melting KCl at 1000 C temperature. Besides the KCl factor, there is another factor, the morphology of precursor also should be considered. The morphology of h-BN also could formed from the polyhedral structure of precursor. 4. Conclusions In summary, bulk amount of hexagonal boron nitride fibers with polyhedral morphology and a diameter of 100e500 nm was synthesized from an intermediate successfully. Contrasting with conventional synthesis methods such as chemical vapor deposition, arc discharge, laser ablation and so on, this is a simple-operational, large-scale and low-cost method. The practical applications of BN nanofibers were hindered by the reality that no reliable method is present to produce BN nanofibers with bulk amounts and high purity. The study in this paper is a significative try to synthesize high purity h-BN nanofibers bulkily and economically. The BN nanofibers with polyhedral morphology reported here possess an uniform morphology. Although the mechanism of formation of this new morphology is unclear and most of the high-temperature reactions are difficult to be understood, it appears that the presence of KCl is one crucial factor and the presence of precursor is a potential factor.
Acknowledgement The authors are grateful to the financial support from National Natural Science Foundation of China (20576021) and Science Foundation of Fujian Province Education Commission of China (2005K015). Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.solidstatesciences. 2007.07.026. References [1] R.T. Paine, C.K. Narula, Chem. Rev. 90 (1990) 73e91. [2] C. Tang, Y. Bando, C. Liu, S. Fan, J. Zhang, X. Ding, D. Golberg, J. Phys. Chem. B 110 (2006) 10354e10357. [3] R. Ma, Y. Bando, H. Zhu, T. Sato, C. Xu, D. Wu, J. Am. Chem. Soc. 124 (2002) 7672e7673. [4] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269 (1995) 966e967. [5] C. Zhi, Y. Bando, C. Tang, S. Honda, K. Sato, H. Kuwahara, D. Golberg, Angew. Chem., Int. Ed. 44 (2005) 7929e7932. [6] S. Bernard, D. Cornu, P. Miele, H. Vincent, J. Bouix, J. Organomet. Chem. 657 (2002) 91e97. [7] J. Yu, Y. Chen, R. Wuhrer, Z. Liu, S.P. Ringer, Chem. Mater. 17 (2005) 5172e5176. [8] Y. Chen, J.F. Gerald, J.S. Williams, S. Bulcock, Chem. Phys. Lett. 299 (1999) 260e264. [9] L. Thomas, M. Yoshio, M. Alain, J. Bernard, Appl. Phys. Lett. 76 (2000) 3239e3241. [10] W. Han, Y. Bando, K. Kurashima, T. Sato, Appl. Phys. Lett. 73 (1998) 3085e3087. [11] O.R. Lourie, C.R. Jones, B.M. Bartlett, P.C. Gibbons, R.S. Ruoff, W.E. Buhro, Chem. Mater. 12 (2000) 1808e1810. [12] Y. Xiong, B.T. Mayers, Y. Xia, Chem. Commun. (2005) 5013e5022. [13] V. Valca´rcel, A. Souto, F. Guitia´n, Adv. Mater. 10 (1998) 138e140. [14] W. Han, A. Zettl, J. Am. Chem. Soc. 125 (2003) 2062e2063. [15] S. Xie, W. Wang, F.K.A. Shiral, X. Wang, Y. Lin, Y. Sun, Chem. Commun. (2005) 3670e3672. [16] C. Zhi, Y. Bando, C. Tang, R. Xie, T. Sekiguchi, D. Golberg, J. Am. Chem. Soc. 127 (2005) 15996e15997. [17] T.F. Cooke, J. Am. Ceram. Soc. 74 (1991) 2959e2978. [18] B. Toury, D. Cornu, F. Chassagneux, P. Miele, J. Eur. Ceram. Soc. 25 (2005) 137e141. [19] E.J.M. Hamilton, S.E. Dolan, C.M. Mann, H.O. Colijn, C.A. Mcdonald, S.G. Shore, Science 260 (1993) 659e661. [20] N.L. Coleburn, J.W.J. Forbes, Chem. Phys. 48 (1968) 555e559. [21] J. Hu, Q. Lu, K. Tang, S. Yu, Y. Qian, G. Zhou, X. Liu, J. Wu, J. Solid State Chem. 148 (1999) 325e328. [22] C. Tang, Y. Bando, T. Sato, K. Kurashima, Adv. Mater. 14 (2002) 1046e1049. [23] F. Kokai, A. Koshio, D. Kasuya, K. Hirahara, K. Takahashi, A. Nakayama, M. Ishihare, Y. Koga, S. Iijima, Carbon 42 (2004) 2515e2520. [24] H. Okuno, A. Palnichenko, J.-F. Despres, J.P. Issi, J.-C. Charlier, Carbon 43 (2005) 692e697. [25] D.H. Berns, M.A. Cappelli, J. Mater. Res. 12 (1997) 2014e2026. [26] Y. Bando, K. Ogawa, O. Stephan, K. Kurashima, Chem. Phys. Lett. 347 (2001) 349e354.
Solid State Sciences 9 (2007) 1099e1104 www.elsevier.com/locate/ssscie
A simple method to synthesize polyhedral hexagonal boron nitride nanofibers Liang-xu Lin a, Ying Zheng a,*, Zhao-hui Li b, Xiao-nv shen a, Ke-mei Wei b b
a College of Chemistry & Materials Science, Fujian Normal University, Fuzhou, 350007, PR china National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, 350002, PR China
Received 19 April 2007; received in revised form 13 July 2007; accepted 17 July 2007 Available online 27 July 2007
Abstract Hexagonal boron nitride (h-BN) fibers with polyhedral morphology were synthesized with a simple-operational, large-scale and low-cost method. The sample obtained was studied by X-ray photoelectron spectrometer (XPS), electron energy lose spectroscopy (EELS), X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), etc., which matched with h-BN. Environment scanning electron microscopy (ESEM) and transmission electron microscope (TEM) indicated that the BN fibers possess polyhedral morphology. The diameter of the BN fibers is mainly in the range of 100e500 nm. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Hexagonal boron nitride; Nanofibers; Polyhedral
1. Introduction h-BN is a material which has received considerable attention due to its properties such as hardness, chemical inertness, high thermal conductivity and the capability of hydrogen uptake [1e3]. Since the BN nanotube was formally reported by Chopra et al. [4], many researches on the BN fibers, especially the BN nanotubes (BNNTs) have been published due to their potential applications in prospective electronic and mechanical devices [5e12]. As an inorganic fiber, the BN fibers also have potential applications to improve the performance of ceramics [13]. The study of BN fibers was not suspended on this stage. Many methods were developed to synthesize functional BN fibers to make better purification of BN fibers, develop new functional BN fibers which have a special application and so on [14e16]. From now on, there are only little investigations on the practical application of BN nanofibers, mainly because there are no reliable methods to produce
* Corresponding author. E-mail address: [email protected] (Y. Zheng). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.07.026
bulk BN nanofibers with high purity [16]. This bewildering complexion has promoted the essentiality to develop a new facile method to produce bulk amounts and high purity hBN nanofibers. The synthesis of h-BN fibers with high performance was considered as a technological challenge and needs appropriate method for a good processibility [17,18]. BN nanotubes have been synthesized using different methods, such as chemical vapor deposition [5], arc discharge [4], ball-milling [8], laser ablation [9], carbon nanotube substitution reaction [10] and pyrolysis[11,18]. Among them, the ball-milling/annealing method was considered as an effective way to synthesize BN nanofibers bulkily. But most of these growth methods have introduced metal catalysts and could not synthesize BN fibers with high purity. The other optional method is pyrolysis. Cylindrical BN fibers without tubular morphology could be synthesized using this method. However, the h-BN nanofibers with polyhedral morphology reported here is, to our knowledge, hitherto unknown for h-BN. This is a novel morphology of BN nanofibers and is different from the cylindrical and tubular BN nanofibers. Hamilton et al. have reported the first synthesis of tubular BN fiber in 1993 [19]. Their BN fibers
1100
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
Fig. 1. XRD patterns of BN sample. Fig. 2. FT-IR spectra of BN sample.
Fig. 3. (aec) ESEM images of the sample. (dei) Amplificatory ESEM images of local fibers.
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
1101
Fig. 4. XPS spectra of the products. (a) The survey spectrum; (b) B 1 s; (c) N 1 s.
also revealed the presence of a little facets. But this morphology is not usual in these tubular BN fibers. Herein, we designed a two steps route to synthesize h-BN fibers with unique morphology. This is a large-scale, lowcost and simple-operational synthesis method. The present paper reports the synthesis and the characterization of new h-BN fibers. Although the mechanism of polyhedral h-BN nanofibers formed in the high-temperature reaction is difficult to be understood, it appears that the crucial factor is the presence of KCl and the potential factor is the presence of our precursor with facet (pseudo-polyhedron morphology). 2. Experimental An intermediate material was prepared for synthesizing BN nanofibers. The synthesis of intermediate was conducted in a 500 mL three-neck flask. The synthesis process comprises four steps. (1) An appropriate amount of KBH4 powder (21.58 g) was dissolved in 100 mL of deionized water in the three-neck flask and then stirred for 30 min. (2) NH4Cl (40 g) was dissolved in 200 mL of deionized water, and the
NH4Cl solution was added to system slowly when the system was cooled down to 0 C temperature condition. Then the system was heated to 90 C slowly and allowed to react for 4 h. (3) The system was then heated to 120 C temperature condition and regurgitate for 48 h when the air produced was not so violent. (4) Forty one gram of intermediate was obtained when water was distilled. All the reactions were produced in N2 condition for making an appropriate pressure. We define the precursor is the material, which purified from intermediate and does not contains any NH4Cl and KCl. Some experimental details proven that the intermediate material consists of pure precursor, NH4Cl and KCl. Some pure precursors were obtained when the intermediate was washed with icecold deionized water for some times. Until the washed water was tested by AgNO3 solution, no white substance produced obviously. For synthesizing h-BN fibers, a furnace containing an Al2O3 tube with one end connected to the N2 source and the other end to a bubbler was utilized. Place Al2O3 boat, which was loaded 6 g intermediate material into an Al2O3 tube. Then heat it to 1250 C and react for 10 h. The crude production was treated
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
1102
Fig. 5. EELS spectrum of a h-BN fiber.
with a solution some times, which contains 10 mL 40% aqueous HF and 5 mL EtOH, at 25 C to remove any boron oxides, SiO2, KCl and NH4Cl. A 10% NaOH solution also was introduced in the once treated production to remove potential Al2O3 at mild temperature. Then the remaining product was washed with deionized water and dried with pump at 60 C. BN sample (0.8 g) was obtained after simple purification. 3. Result and discussion The purity of the sample was examined by XRD, FT-IR and XPS. The XRD pattern (Fig. 1) of the sample was highly ordered
h-BN with lines indexed from left to right as (002), (100), (101), (102), (004), (110) and (112) [20]. FT-IR (Fig. 2) was used to distinguish the sp2 hexagonal and sp3 cubic phases [21]. The peak at 1409 cm 1 is attributed to the TO mode of the sp2bonded h-BN [21], and the peak at 804 cm 1 could be attributed to the BeNeB out-of-plane bending vibration [22]. ESEM images (Fig. 3) of h-BN reveal that the h-BN fibers possess a unique distinct morphology. From Fig. 3a, the diameter of the BN fibers is mainly in the range of 100e500 nm and the length of most fibers is more than 5 mm. The diameter value reported here is larger than that reported in previous papers [5e7]. Fig. 3bed also show that the BN nanofibers possess polyhedral morphology clearly. This morphology is, to the best of our knowledge, hitherto unknown for BN. Recent reports have reported polyhedral graphite particles and graphite fibers with facets [23,24]. Considering the h-BN was similar with graphite, it is likely that the h-BN nanofibers also possess some polyhedral morphology. But there are no reports about this kind of h-BN nanofibers. The proximate report only proven that the presence of little facets in BN fibers [19]. However, in the case of BN fibers presented here have more regular uniform facets and possess a regular fibrous morphology. The h-BN nanofiber’s compositional information was further detected by XPS. The O 1s XPS peak at 531 eV is a signature of surface oxidization. The binding energies near the 190.3 eV for B 1s (Fig. 4b) and 398 eV for N 1s (Fig. 4c) are in good agreement with the values of bulk BN in the published literature [22]. B 1s and N 1s binding energy spectrums
Fig. 6. (aec,e,f) TEM images and (d) electro diffraction patterns of the BN fiber.
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
1103
Fig. 7. High resolution TEM micrograph of sample.
also provided qualitative evidence for the presence of sp2 (hBN) bonding through the identification of a p plasmon loss peak at approximately 9 eV away from the B 1s and N 1s peaks [25]. The quantification of peaks confirmed that the atomic ratio of B:N was 0.975:1, which agrees well with the chemical stoichiometric relation between B and N. Fig. 5 shows a typical EELS spectra taken on a h-BN nanofiber. Two distinct absorption features are observed at 189 and 404 eV, which correspond to boron K-edge and nitrogen K-edge, respectively. The EELS result also indicates the BN nanofiber with a B/N ratio of around 0.95. The sharp p*-peaks on the left side of the B and N K-edges and the sharp s*-bands on the right side confirmed that the nanofibers were composed of sp2-bonded B and N atoms [26]. To further study the morphologies of these h-BN fibers, TEM characterization was introduced. The polyhedral morphology of the fiber was indicated by Fig. 6c. Fig. 6f shows
a BN fiber with a length about 5 mm. Figs. 6a and 7a and c exhibit sharp lattice fringes indicating that the interplanar distance is well ordered with the value of w0.33 nm, which is ˚ in bulk hconsistent with the interplanar distance of 3.33 A BN [4]. Selected-area electron diffraction (Fig. 6d) pattern from one fiber is indexed by the h-BN which was corresponding with XRD. Fig. 7 shows a tip’s morphology of a h-BN nanofiber. In Fig. 7a, the indicated q is the angle between the fiber axis and the h-BN basal planes. This is an all-pervading phenomenon in these BN fibers from our TEM results. Particularly, not all the tips are well ordered as Fig. 7a shows. Fig. 7c shows some defects in the tips of BN fiber. The numeric signs on these local lattice fringes correspond with what in the inserted electron diffractions. These local lattice fringes were parallel with each other no more. As a result of this morphologic defect, the mild flexural fiber could grow in the reaction
1104
L.-x. Lin et al. / Solid State Sciences 9 (2007) 1099e1104
as is indicated in Figs. 3a and 7b. It is possible that some morphologic defects might present in local reactor area where the reactants were not well mixed. Further more, SEM and TEM images also reveal some flat tips present in the fiber. The bigger white ring sign in Fig. 3b indicated an even end. The formation of flat end could be a result of potential rupture of fiber. Without considering the defect discussed above, one interesting result could be concluded. The manner of fiber’s growth was distinguished from that reported before [4,5,7]. We were interested how these h-BN fibers with facets formed in heating reaction. There are two factors to be considered. As the first factor, the presence of KCl appears here to be a vital. In fact, the KCl presents here could be a fluxing agent. In the absence of the KCl, only some ruleless conglomeration and elementary character of facet appeared. The BN with different morphologies could be formed at different temperature conditions. However, there are no obvious evidences of uniform facet formed until the presence of fluxing agent. In other word, the presence of KCl at 1000e1250 C created a melting environment. The fluid capability of reactant was enhanced. Many solid materials exhibit a one-dimensional growth habit as a result of the inherently anisotropic feature in their crystal lattice [12]. The BN fiber could grow from this melting environment easily. Contrarily, ruleless BN nanoparticle formed in the reaction when the absence of melting KCl at 1000 C temperature. Besides the KCl factor, there is another factor, the morphology of precursor also should be considered. The morphology of h-BN also could formed from the polyhedral structure of precursor. 4. Conclusions In summary, bulk amount of hexagonal boron nitride fibers with polyhedral morphology and a diameter of 100e500 nm was synthesized from an intermediate successfully. Contrasting with conventional synthesis methods such as chemical vapor deposition, arc discharge, laser ablation and so on, this is a simple-operational, large-scale and low-cost method. The practical applications of BN nanofibers were hindered by the reality that no reliable method is present to produce BN nanofibers with bulk amounts and high purity. The study in this paper is a significative try to synthesize high purity h-BN nanofibers bulkily and economically. The BN nanofibers with polyhedral morphology reported here possess an uniform morphology. Although the mechanism of formation of this new morphology is unclear and most of the high-temperature reactions are difficult to be understood, it appears that the presence of KCl is one crucial factor and the presence of precursor is a potential factor.
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