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Effective synthesis of surface-modified boron nitride nanotubes and related nanostructures and their hydrogen uptake
ARTICLE IN PRESS
Physica E 40 (2008) 2551–2555 www.elsevier.com/locate/physe
Effective synthesis of surface-modified boron nitride nanotubes and related nanostructures and their hydrogen uptake T. Teraoa,b,, Y. Bandoa,b, M. Mitomeb, K. Kurashimab, C.Y. Zhib, C.C. Tangb, D. Golberga,b a
Graduate School of Pune and Applied Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan b National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Available online 11 October 2007
Abstract The present study is focused on the synthesis of boron nitride nanotubes (BNNTs) with an entirely modified surface structure. We succeeded in the BNNT surface modification by using highly reactive SO2 gas generated in-situ during the tube synthesis. The obtained BNNTs have a high surface area, which is significantly larger than that of standard well-structured BNNT. Therefore, the synthesized nanomaterial is advantageous for the gas adsorption and could be envisaged as a prospective hydrogen-storage material. Herein we report on the preliminary results of the hydrogen accumulation experimental runs. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.De; 68.37.Lp Keywords: Boron nitride (BN) nanotube; Surface modification; Hydrogen storage
1. Introduction Nanoscale inorganic materials have attracted significant attention during the past decade owing to their specific ‘‘nano’’- morphology and its close relationship with the resultant material performance. Hexagonal boron nitride (BN) has a sp2-bonded layered structure, which can easily be interpreted as that of graphite in which C atoms are entirely substituted by alternating B and N atoms. When a layered hexagonal BN is curled or rolled in different ways, various prospective nanostructures can be generated. Just after the discovery of C nanotubes, BN nanotubes (BNNTs) were prepared by Chopra et al. [1] in 1995. Thereafter, other BN nanostructures such as fullerenes [2], nanocones [3], nanobamboos [4] and so forth have been discovered. Similar to C-based nanostructures, those of layered BN were expected to be important gas storage materials, especially for hydrogen storage [5]. There have Corresponding author. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. E-mail address: [email protected] (T. Terao).
1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.014
been several experiments and theoretical studies indicating that due to the strong ionicity of the B–N bonds the BN nanomaterials may rival or even surpass those of C as decent hydrogen accumulators. It is worth mentioning that a BNNT has higher thermal conductivity [6] and oxidation resistance [7] and thus overall structural stability than a C nanotube, which may also be advantageous for this sort of applications presuming high-temperature treatments. Among various BN nanostructures discovered so far, the so-called collapsed BNNTs, first prepared by Tang et al. [8], possess the highest surface area due to numerously broken tubular shell surfaces. Such surface-modified BNNTs were observed to have superb hydrogen storage ability [8]. A collapsed BNNT consists of numerous curved BN fragments, has a large surface area and contains numerous dangling bonds, which may energetically benefit from the saturation with hydrogen molecules and/or atoms. However, there have been some questions/problems related to the Tang’s method [8]. First, the platinum used as a catalyst during the nanotube fabrication has remained as an impurity in the final product. Therefore, the effect of platinum on the observed hydrogen uptake values must not
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be ruled out, but it could hardly be quantitatively elucidated. Secondly, the use of Pt makes such synthesis expensive and restricts the economical yield of a product. Thus, a new synthesis pathway towards these interesting BN surface-modified nanostructures is eagerly needed. To do so, we paid specific attention to the high reactivity of a sulfur-containing gas and developed a direct synthesis method towards high yield and economical production of surface-modified BN nanostructures. These include collapsed BNNTs and novel balloon-like and wool-like nanostructures. The studies on the synthesis mechanism and hydrogen uptake of the prepared BN products are also reported here. 2. Experimental The materials were synthesized by using a highfrequency induction furnace through a carbon-free CVD method [9]. The chemical reactions involved are given by the following Eqs. (1) and (2): 2MgO þ 2BðsÞ ! 2MgðvÞ þ B2 O2 ðvÞ;
(1)
B2 O2 ðvÞ þ 2NH3 ðvÞ ! 2BNðtubeÞ þ 2H2 OðvÞ þ H2 ðvÞ: (2) During these reactions, the growth of BNNTs occurs in line with a vapor–liquid–solid (V–L–S) process with Mg droplets serving as calatysts. The product yield can further be improved by adding other metal oxides [10]. In the present study, we used the advantage of high reactivity of an in-situ formed sulfur-containing vapor. As an additional precursor, ZnS powder was utilized. Therefore, a mixture of B, MgO, SnO and ZnS powders was used as the novel source material. The mixture was heated and the resultant vapors were transported to the reaction zone by an argon flow where they met with ammonia. The synthesis was performed at 1450 1C for 2 h. A sample powder was preliminarily studied by a scanning electron microscope (SEM; JEOL, JSM6700FSEM). Then as-prepared BN compounds were grinded in a mortar and dispersed in ethanol. Several drops of the suspensions were dripped onto carbon-coated copper transmission electron microscope (TEM) grids for TEM (JEOL, JEM-2000EX/T) observations. The hydrogen uptake experiments were carried out at 77 K and at a pressure ranging from ambient to 3000 kPa. 3. Results and discussion The TEM and SEM images of a BN product are shown in Fig. 1(a,c) and (b), respectively. Each tube has a highly developed surface (Fig. 1(c)) and reveals a polycrystal-like diffraction ring pattern (Fig. 1(c) inset). The rows of (0 0 0 2) reflections correspond well to a lattice spacing of BN (0.33 nm). This implies that each tube consists of highly corrugated and entangled BN sheet fragments. Fig. 1(d) shows an energy-dispersive X-ray (EDS) spectrum of the surface-modified BNNTs recorded during
Fig. 1. (a) Low-magnification TEM image of a highly developed BNNT surface, (b) SEM image, and (c) HRTEM image of BNNTs; the inset in (c) is an electron diffraction pattern taken from the tube surface; (d) EDS spectrum taken from purified BNNTs after a heat-treatment.
SEM observations. Though Mg was used as a catalyst, it could be entirely removed by a heat-treatment with nitrogen at 1500–1600 1C [11]. There are no traces of Mg in the purified product. The carbon signal originates from a carbon tape used to fix a sample to a SEM holder. The material is ultimately pure and does not contain any impurities. In addition, we were also able to find other BN nanostructures with correspondingly modified surfaces in the product. Fig. 2(a) demonstrates a balloon-like BN submicron sphere. This sphere is enclosed in 40–50 layers of BN and has a well-developed surface. The surfaces of the observed BN cages were presumably destroyed due to the effect of the regarded sulfur-containing vapor. Some woollike BN structures made of numerous entangled BN
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Fig. 2. TEM images of various surface-modified BN nanomaterials found (a) a balloon-like BN sphere with the modified surface, and a (b) wool-like BN structure made of numerous entangled BN nanosheets.
nanosheets were also discovered in the product (Fig. 2b). Each bunch consists of multilayered BN nanosheets (from several to dozens of layers). The diffraction pattern again shows a lattice spacing of 0.33 nm peculiar to a layered BN. The overall disordered and broken BN layers are composed of highly crystallized BN nanofragments. In addition, we could find BNNTs of an extremely small diameter (Fig. 3). It is worth noting that when sole Mg was used in a precursor mixture (no ZnS additives), the BNNTs with diameters of 20 nm or less have never been observed. In order to clarify the growth mechanism of surfacemodified BNNT, we conducted several control experiments as shown in Table 1. In fact, Mg-containing precursors were found to be essential for the BNNT formation. Importantly enough, surface-modified nanostructures do not form in the sole presence of ZnS without SnO additions. Based on the experimental results, we suggest that SO2 gas was generated when both ZnS and SnO were in use within the precursor mixture along with the following reaction. 2SnO þ ZnS ! 2SnðvÞ þ ZnðvÞ þ SO2 :
(3)
A highly reactive SO2 gas, generated in-situ during the synthesis, leads to enhanced etching of as-produced BN nanostructures and their designated surface modifications. The overall process kinetics may resemble that described by Huang et al. [12] while peeling off BN tubular shells in the presence of S-containing vapors. In the regarded work, BNNTs were peeled off in a polar solvent dimethylsulfoxide (DMSO) under solvothermal conditions. Though a BN bond is very strong, DMSO effectively weakens the BN bond of the BNNT layer through a nucleophilic attack of O on B and electrophilic attack of S on N. The BN bond is then destroyed under a following-up hydrolysis process. In the method presented by us here, SO2 gas works similarly to DMSO in the regarded Huang’s work [12]. It is noted that H2O molecules (essential for hydrolysis) were present in our syntheses as a high-temperature steam formed when BNNTs are generated in line with reaction (2). As a result, the BN bonds were (analogously to Ref. [12]) effectively attacked and dissembled, the tubular surfaces were
Fig. 3. HRTEM image of a small diameter BNNT (5 nm) which consists of only three layers.
Table 1 Raw materials
Product
B+MgO+SnO B+MgO+SnO+ZnS B+MgO+ZnS BNNT+ZnS BNNT+ZnS+SnO
Standard BNNT Surface modified BNNT Standard BNNT Standard BNNT Defective BNNTs/standard BNNTs
destroyed, and surface-modified BNNTs were fabricated. It is also thought here that the rarely observed balloon-like BN spheres and small-diameter BN nanotubes are actually by-products made of completely peeled-off BN layers under SO2 gas attacks on generated BNNTs. Finally, the hydrogen storage capacity of surfacemodified BN nanomaterials was studied. The results are shown in Fig. 4. The uptake value for surface-modified BNNTs at 77 K was measured as 1.2 wt%. Since the report by Dillon et al. [13] on hydrogen storage in CNTs, many experimental and theoretical studies on hydrogen uptake in various CNTs have been reported. In fact, the NTs which possess high surface-to-volume ratios seem to be ideal materials for fast kinetics during hydrogenation–dehydrogenation cycles. However, recent studies [14,15] have shown that the hydrogen storage capacity on pristine CNT at room temperature is less than 0.01 wt%. Lawrence and Xu [16] reported somewhat higher, but still modest value of 0.6 wt% measured on single-walled CNT bundles at 10 MPa and room temperature. On the other hand, safe and efficient hydrogen storage for the needs of the modern technology requires gravimetric hydrogen density exceeding 6 wt% under conditions of fast and reversible kinetics of hydrogenation and dehydrogenation. Unlike CNTs, whose non-polar C–C bonds form a uniform and smooth sidewall structure, BNNTs contain polar B–N bonds with ionic character, which can induce an extra dipole moment within the tube structure. This factor
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structured multi-walled BNNTs (0.5 wt%) [5,8]. In addition, the obtained values are higher than those recently measured in CNTs [14–16]. On the other hand, the presently obtained uptake is lower than that reported by Tang et al. [8] (4.2 wt%) in analogous collapsed BNNTs annealed with Pt. However, as mentioned above, in Tang’s experiments, the existence of Pt nanoparticles in a BNNT product that resulted from the synthetic procedure may be a concern. By contrast, in the present work, Pt was not used and thus the measured values are free of such uncertainty. Finally, we suggest here that there is still a room for the improvement of the presently observed hydrogen adsorption values in surface-modified BN nanomaterials through careful selection of various BN surface-developed nanomaterial fractions (nanotubes vs. nanoballoons) and their usage in separate runs, as well as through the optimization of the system P–T conditions for the H2 adsorption and its activation. Fig. 4. A representative hydrogen absorption curve recorded on surfacemodified BNNTs at 77 K.
may affect bonding of hydrogen with tube surfaces. Theoretically, the binding energy of chemisorbed hydrogen atoms to a zigzag (10,0) single-walled BNNT was calculated at 25%, 50%, 75% and 100% coverage by Han et al. [17] using the density functional theory. The average binding energy (per H atom) was computed to be the highest for a 50% case when the H atoms had been adsorbed on the adjacent B and N atoms along the tube axis. This value was 53.93 kcal/mol, which is equal to half of the H2 binding energy. Interestingly enough, the BN band gap (which was computed to be 4.29 eV in pristine BNNT) decreased to 2.01 eV after 50% tube surface coverage. Ma et al. [5] for the first time experimentally demonstrated that BNNTs may indeed absorb hydrogen at a level equal to or even exceeding that of CNTs. BN nanostructures were found to adsorb up to 2.6 wt% of hydrogen under 10 MPa at room temperature. Additional theoretical calculations by Zhou et al. [18] have demonstrated that a well-structured, perfect BNNT is in fact not a good candidate for hydrogen storage by either physical or chemical absorption mechanisms. Therefore, novel nanostructures in BN have been predicted to be of prime importance. Jhi et al. [19] calculated that an increase in specific surface area for BNNTs may be a solution for an increase in operating temperature and capacity for hydrogen storage. The calculated binding energy of hydrogen on activated BNNTs (those having well-developed pore structures) was computed to reach as much as 22 kJ/mol and thus to lie in the right range for room-temperature hydrogen storage. The present results decently verified this theoretical prediction. In fact, the measured values are 2.5 times higher than that previously measured by us on perfectly
4. Summary We succeeded in direct fabrication of surface-modified BNNTs by using in-situ generated reactive gas SO2 during the BNNT synthesis. This gas is originated through the reactions between SnO and ZnS precursors. SO2 gas attacks weaken the BN bonds of the perfectly structured BNNT layers. A high-temperature H2O steam generated when BNNTs are formed leads to subsequent hydrolysis. As a result, normal BN bonds are weakened and partially cut, and well-structured tubular surfaces are partially destroyed. This results in the effective formation of chemically pure surface-modified BNNTs with a high specific area. In addition, some interesting by-product BN nanomaterials, i.e. nanoballoons, wool-like entangled sheets and small diameter (less than 5 nm) BNNTs, which have presumably been formed from peeled-off BN tubular layers, are discovered. The surface-modified nanostructures were proven to be useful for the improvement of BN material hydrogen storage capacity, yielding a 1.2 wt/% uptake at 77 K in the case of surface-modified BNNTs (compared to 0.5 wt% in perfectly structured BNNTs). Acknowledgments This work was supported by the National Institute for Materials Science. The authors are indebted to Dr. Y. Uemura for continuous technical support. References [1] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269 (1995) 966. [2] O. Stephan, Y. Bando, A. Loiseau, F. Willaime, N. Shramchenko, T. Tamiya, T. Sato, Appl. Phys. A 67 (1998) 107. [3] L. Bourgeois, Y. Bando, W.Q. Han, T. Sato, Phys. Rev. B 61 (2000) 7686.
ARTICLE IN PRESS T. Terao et al. / Physica E 40 (2008) 2551–2555 [4] C.C. Tang, M. Lamy de la Chapelle, P. Li, Y.M. Liu, H.Y. Dang, S.S. Fan, Chem. Phys. Lett. 342 (2001) 492. [5] R. Ma, Y. Bando, H. Zhu, T. Sato, C. Xu, D. Wu, J. Am. Chem. Soc. 124 (2002) 7672. [6] C.W. Chang, Wei-Qiang Han, A. Zett1, Appl. Phys. Lett. 86 (2005) 173102. [7] D. Golberg, Y. Bando, K. Kurashima, T. Sato, Scr. Mater 44 (2001) 1561. [8] C.C. Tang, Y. Bando, X. Ding, S. Qi, D. Golberg, J. Am. Chem. Soc. 124 (2002) 14550. [9] C.C. Tang, Y. Bando, T. Sato, K. Kurashima, Chem. Commun. (2002) 1290. [10] C.Y. Zhi, Y. Bando, C.C. Tang, D. Golberg, Solid State Commun 135 (2005) 67. [11] C.C. Tang, unpublished results.
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[12] Q. Huang, Y. Bando, C.Y. Zhi, D. Golberg, F.F. Xu, L. Gao, Angew. Chem. In. Ed. 45 (2006) 2044. [13] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Klang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [14] S.S. Han, H.M. Lee, Carbon 42 (2004) 2169. [15] A. Anson, M.A. Callejas, A.M. Benito, W.K. Mazer, M.T. Izquierdo, B. Rubio, J. Jagiello, M. Thommes, J.B. Parra, M.T. Martinez, Carbon 42 (2004) 1243. [16] J. Lawrence, G. Xu, Appl. Phys. Lett. 84 (2004) 918. [17] S.S. Han, S.H. Lee, J.K. Kang, H.M. Lee, Phys. Rev. B 72 (2005) 113402. [18] Z. Zhou, J. Zhao, Z. Chen, X. Gao, T. Yan, B. Wen, P. von R. Schleyer, J. Phys. Chem. B 110 (2006) 13363. [19] S.H. Jhi, Phys. Rev. B 74 (2006) 155424.
Physica E 40 (2008) 2551–2555 www.elsevier.com/locate/physe
Effective synthesis of surface-modified boron nitride nanotubes and related nanostructures and their hydrogen uptake T. Teraoa,b,, Y. Bandoa,b, M. Mitomeb, K. Kurashimab, C.Y. Zhib, C.C. Tangb, D. Golberga,b a
Graduate School of Pune and Applied Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan b National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Available online 11 October 2007
Abstract The present study is focused on the synthesis of boron nitride nanotubes (BNNTs) with an entirely modified surface structure. We succeeded in the BNNT surface modification by using highly reactive SO2 gas generated in-situ during the tube synthesis. The obtained BNNTs have a high surface area, which is significantly larger than that of standard well-structured BNNT. Therefore, the synthesized nanomaterial is advantageous for the gas adsorption and could be envisaged as a prospective hydrogen-storage material. Herein we report on the preliminary results of the hydrogen accumulation experimental runs. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.De; 68.37.Lp Keywords: Boron nitride (BN) nanotube; Surface modification; Hydrogen storage
1. Introduction Nanoscale inorganic materials have attracted significant attention during the past decade owing to their specific ‘‘nano’’- morphology and its close relationship with the resultant material performance. Hexagonal boron nitride (BN) has a sp2-bonded layered structure, which can easily be interpreted as that of graphite in which C atoms are entirely substituted by alternating B and N atoms. When a layered hexagonal BN is curled or rolled in different ways, various prospective nanostructures can be generated. Just after the discovery of C nanotubes, BN nanotubes (BNNTs) were prepared by Chopra et al. [1] in 1995. Thereafter, other BN nanostructures such as fullerenes [2], nanocones [3], nanobamboos [4] and so forth have been discovered. Similar to C-based nanostructures, those of layered BN were expected to be important gas storage materials, especially for hydrogen storage [5]. There have Corresponding author. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. E-mail address: [email protected] (T. Terao).
1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.014
been several experiments and theoretical studies indicating that due to the strong ionicity of the B–N bonds the BN nanomaterials may rival or even surpass those of C as decent hydrogen accumulators. It is worth mentioning that a BNNT has higher thermal conductivity [6] and oxidation resistance [7] and thus overall structural stability than a C nanotube, which may also be advantageous for this sort of applications presuming high-temperature treatments. Among various BN nanostructures discovered so far, the so-called collapsed BNNTs, first prepared by Tang et al. [8], possess the highest surface area due to numerously broken tubular shell surfaces. Such surface-modified BNNTs were observed to have superb hydrogen storage ability [8]. A collapsed BNNT consists of numerous curved BN fragments, has a large surface area and contains numerous dangling bonds, which may energetically benefit from the saturation with hydrogen molecules and/or atoms. However, there have been some questions/problems related to the Tang’s method [8]. First, the platinum used as a catalyst during the nanotube fabrication has remained as an impurity in the final product. Therefore, the effect of platinum on the observed hydrogen uptake values must not
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be ruled out, but it could hardly be quantitatively elucidated. Secondly, the use of Pt makes such synthesis expensive and restricts the economical yield of a product. Thus, a new synthesis pathway towards these interesting BN surface-modified nanostructures is eagerly needed. To do so, we paid specific attention to the high reactivity of a sulfur-containing gas and developed a direct synthesis method towards high yield and economical production of surface-modified BN nanostructures. These include collapsed BNNTs and novel balloon-like and wool-like nanostructures. The studies on the synthesis mechanism and hydrogen uptake of the prepared BN products are also reported here. 2. Experimental The materials were synthesized by using a highfrequency induction furnace through a carbon-free CVD method [9]. The chemical reactions involved are given by the following Eqs. (1) and (2): 2MgO þ 2BðsÞ ! 2MgðvÞ þ B2 O2 ðvÞ;
(1)
B2 O2 ðvÞ þ 2NH3 ðvÞ ! 2BNðtubeÞ þ 2H2 OðvÞ þ H2 ðvÞ: (2) During these reactions, the growth of BNNTs occurs in line with a vapor–liquid–solid (V–L–S) process with Mg droplets serving as calatysts. The product yield can further be improved by adding other metal oxides [10]. In the present study, we used the advantage of high reactivity of an in-situ formed sulfur-containing vapor. As an additional precursor, ZnS powder was utilized. Therefore, a mixture of B, MgO, SnO and ZnS powders was used as the novel source material. The mixture was heated and the resultant vapors were transported to the reaction zone by an argon flow where they met with ammonia. The synthesis was performed at 1450 1C for 2 h. A sample powder was preliminarily studied by a scanning electron microscope (SEM; JEOL, JSM6700FSEM). Then as-prepared BN compounds were grinded in a mortar and dispersed in ethanol. Several drops of the suspensions were dripped onto carbon-coated copper transmission electron microscope (TEM) grids for TEM (JEOL, JEM-2000EX/T) observations. The hydrogen uptake experiments were carried out at 77 K and at a pressure ranging from ambient to 3000 kPa. 3. Results and discussion The TEM and SEM images of a BN product are shown in Fig. 1(a,c) and (b), respectively. Each tube has a highly developed surface (Fig. 1(c)) and reveals a polycrystal-like diffraction ring pattern (Fig. 1(c) inset). The rows of (0 0 0 2) reflections correspond well to a lattice spacing of BN (0.33 nm). This implies that each tube consists of highly corrugated and entangled BN sheet fragments. Fig. 1(d) shows an energy-dispersive X-ray (EDS) spectrum of the surface-modified BNNTs recorded during
Fig. 1. (a) Low-magnification TEM image of a highly developed BNNT surface, (b) SEM image, and (c) HRTEM image of BNNTs; the inset in (c) is an electron diffraction pattern taken from the tube surface; (d) EDS spectrum taken from purified BNNTs after a heat-treatment.
SEM observations. Though Mg was used as a catalyst, it could be entirely removed by a heat-treatment with nitrogen at 1500–1600 1C [11]. There are no traces of Mg in the purified product. The carbon signal originates from a carbon tape used to fix a sample to a SEM holder. The material is ultimately pure and does not contain any impurities. In addition, we were also able to find other BN nanostructures with correspondingly modified surfaces in the product. Fig. 2(a) demonstrates a balloon-like BN submicron sphere. This sphere is enclosed in 40–50 layers of BN and has a well-developed surface. The surfaces of the observed BN cages were presumably destroyed due to the effect of the regarded sulfur-containing vapor. Some woollike BN structures made of numerous entangled BN
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Fig. 2. TEM images of various surface-modified BN nanomaterials found (a) a balloon-like BN sphere with the modified surface, and a (b) wool-like BN structure made of numerous entangled BN nanosheets.
nanosheets were also discovered in the product (Fig. 2b). Each bunch consists of multilayered BN nanosheets (from several to dozens of layers). The diffraction pattern again shows a lattice spacing of 0.33 nm peculiar to a layered BN. The overall disordered and broken BN layers are composed of highly crystallized BN nanofragments. In addition, we could find BNNTs of an extremely small diameter (Fig. 3). It is worth noting that when sole Mg was used in a precursor mixture (no ZnS additives), the BNNTs with diameters of 20 nm or less have never been observed. In order to clarify the growth mechanism of surfacemodified BNNT, we conducted several control experiments as shown in Table 1. In fact, Mg-containing precursors were found to be essential for the BNNT formation. Importantly enough, surface-modified nanostructures do not form in the sole presence of ZnS without SnO additions. Based on the experimental results, we suggest that SO2 gas was generated when both ZnS and SnO were in use within the precursor mixture along with the following reaction. 2SnO þ ZnS ! 2SnðvÞ þ ZnðvÞ þ SO2 :
(3)
A highly reactive SO2 gas, generated in-situ during the synthesis, leads to enhanced etching of as-produced BN nanostructures and their designated surface modifications. The overall process kinetics may resemble that described by Huang et al. [12] while peeling off BN tubular shells in the presence of S-containing vapors. In the regarded work, BNNTs were peeled off in a polar solvent dimethylsulfoxide (DMSO) under solvothermal conditions. Though a BN bond is very strong, DMSO effectively weakens the BN bond of the BNNT layer through a nucleophilic attack of O on B and electrophilic attack of S on N. The BN bond is then destroyed under a following-up hydrolysis process. In the method presented by us here, SO2 gas works similarly to DMSO in the regarded Huang’s work [12]. It is noted that H2O molecules (essential for hydrolysis) were present in our syntheses as a high-temperature steam formed when BNNTs are generated in line with reaction (2). As a result, the BN bonds were (analogously to Ref. [12]) effectively attacked and dissembled, the tubular surfaces were
Fig. 3. HRTEM image of a small diameter BNNT (5 nm) which consists of only three layers.
Table 1 Raw materials
Product
B+MgO+SnO B+MgO+SnO+ZnS B+MgO+ZnS BNNT+ZnS BNNT+ZnS+SnO
Standard BNNT Surface modified BNNT Standard BNNT Standard BNNT Defective BNNTs/standard BNNTs
destroyed, and surface-modified BNNTs were fabricated. It is also thought here that the rarely observed balloon-like BN spheres and small-diameter BN nanotubes are actually by-products made of completely peeled-off BN layers under SO2 gas attacks on generated BNNTs. Finally, the hydrogen storage capacity of surfacemodified BN nanomaterials was studied. The results are shown in Fig. 4. The uptake value for surface-modified BNNTs at 77 K was measured as 1.2 wt%. Since the report by Dillon et al. [13] on hydrogen storage in CNTs, many experimental and theoretical studies on hydrogen uptake in various CNTs have been reported. In fact, the NTs which possess high surface-to-volume ratios seem to be ideal materials for fast kinetics during hydrogenation–dehydrogenation cycles. However, recent studies [14,15] have shown that the hydrogen storage capacity on pristine CNT at room temperature is less than 0.01 wt%. Lawrence and Xu [16] reported somewhat higher, but still modest value of 0.6 wt% measured on single-walled CNT bundles at 10 MPa and room temperature. On the other hand, safe and efficient hydrogen storage for the needs of the modern technology requires gravimetric hydrogen density exceeding 6 wt% under conditions of fast and reversible kinetics of hydrogenation and dehydrogenation. Unlike CNTs, whose non-polar C–C bonds form a uniform and smooth sidewall structure, BNNTs contain polar B–N bonds with ionic character, which can induce an extra dipole moment within the tube structure. This factor
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structured multi-walled BNNTs (0.5 wt%) [5,8]. In addition, the obtained values are higher than those recently measured in CNTs [14–16]. On the other hand, the presently obtained uptake is lower than that reported by Tang et al. [8] (4.2 wt%) in analogous collapsed BNNTs annealed with Pt. However, as mentioned above, in Tang’s experiments, the existence of Pt nanoparticles in a BNNT product that resulted from the synthetic procedure may be a concern. By contrast, in the present work, Pt was not used and thus the measured values are free of such uncertainty. Finally, we suggest here that there is still a room for the improvement of the presently observed hydrogen adsorption values in surface-modified BN nanomaterials through careful selection of various BN surface-developed nanomaterial fractions (nanotubes vs. nanoballoons) and their usage in separate runs, as well as through the optimization of the system P–T conditions for the H2 adsorption and its activation. Fig. 4. A representative hydrogen absorption curve recorded on surfacemodified BNNTs at 77 K.
may affect bonding of hydrogen with tube surfaces. Theoretically, the binding energy of chemisorbed hydrogen atoms to a zigzag (10,0) single-walled BNNT was calculated at 25%, 50%, 75% and 100% coverage by Han et al. [17] using the density functional theory. The average binding energy (per H atom) was computed to be the highest for a 50% case when the H atoms had been adsorbed on the adjacent B and N atoms along the tube axis. This value was 53.93 kcal/mol, which is equal to half of the H2 binding energy. Interestingly enough, the BN band gap (which was computed to be 4.29 eV in pristine BNNT) decreased to 2.01 eV after 50% tube surface coverage. Ma et al. [5] for the first time experimentally demonstrated that BNNTs may indeed absorb hydrogen at a level equal to or even exceeding that of CNTs. BN nanostructures were found to adsorb up to 2.6 wt% of hydrogen under 10 MPa at room temperature. Additional theoretical calculations by Zhou et al. [18] have demonstrated that a well-structured, perfect BNNT is in fact not a good candidate for hydrogen storage by either physical or chemical absorption mechanisms. Therefore, novel nanostructures in BN have been predicted to be of prime importance. Jhi et al. [19] calculated that an increase in specific surface area for BNNTs may be a solution for an increase in operating temperature and capacity for hydrogen storage. The calculated binding energy of hydrogen on activated BNNTs (those having well-developed pore structures) was computed to reach as much as 22 kJ/mol and thus to lie in the right range for room-temperature hydrogen storage. The present results decently verified this theoretical prediction. In fact, the measured values are 2.5 times higher than that previously measured by us on perfectly
4. Summary We succeeded in direct fabrication of surface-modified BNNTs by using in-situ generated reactive gas SO2 during the BNNT synthesis. This gas is originated through the reactions between SnO and ZnS precursors. SO2 gas attacks weaken the BN bonds of the perfectly structured BNNT layers. A high-temperature H2O steam generated when BNNTs are formed leads to subsequent hydrolysis. As a result, normal BN bonds are weakened and partially cut, and well-structured tubular surfaces are partially destroyed. This results in the effective formation of chemically pure surface-modified BNNTs with a high specific area. In addition, some interesting by-product BN nanomaterials, i.e. nanoballoons, wool-like entangled sheets and small diameter (less than 5 nm) BNNTs, which have presumably been formed from peeled-off BN tubular layers, are discovered. The surface-modified nanostructures were proven to be useful for the improvement of BN material hydrogen storage capacity, yielding a 1.2 wt/% uptake at 77 K in the case of surface-modified BNNTs (compared to 0.5 wt% in perfectly structured BNNTs). Acknowledgments This work was supported by the National Institute for Materials Science. The authors are indebted to Dr. Y. Uemura for continuous technical support. References [1] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269 (1995) 966. [2] O. Stephan, Y. Bando, A. Loiseau, F. Willaime, N. Shramchenko, T. Tamiya, T. Sato, Appl. Phys. A 67 (1998) 107. [3] L. Bourgeois, Y. Bando, W.Q. Han, T. Sato, Phys. Rev. B 61 (2000) 7686.
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