Effect of Reaction Temperature and Catalyst Type on the Formation of

Article pubs.acs.org/IECR

Effect of Reaction Temperature and Catalyst Type on the Formation of Boron Nitride Nanotubes by Chemical Vapor Deposition and Measurement of Their Hydrogen Storage Capacity Burcu Saner Okan,† Züleyha Ö zlem Kocabaş,‡ Asli Nalbant Ergün,‡ Mustafa Baysal,‡ Ilse Letofsky-Papst,§ and Yuda Yürüm*,‡ †

Sabancı University Nanotechnology Research and Application Center, Tuzla, Istanbul 34956, Turkey Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, Istanbul 34956, Turkey § Institute for Electron Microscopy, Graz University of Technology, Steyrergasse 17, A-8010, Graz, Austria ‡

ABSTRACT: Boron nitride nanotubes (BNNT) were synthesized over both Fe3+ impregnated MCM-41 (mobil composition of matter no. 41) and Fe2O3/MCM-41 complex catalyst systems at relatively low temperatures for 1 h by the chemical vapor deposition technique in large quantities. The formation of BNNT was tailored at different reaction temperatures by changing catalyst type. The use of Fe3+-MCM-41 and Fe2O3 as a complex catalyst system led to thin and thick tube formations. The diameters of BNNTs were in the range of 2.5−4.0 nm for thin tubes and 20−60 nm for thick tubes. The thin tube formation originated from the growth of BNNT over Fe3+-MCM-41 due to its average pore size of 4 nm. Higher reaction temperatures caused both BNNT and iron-based side product formations. The hydrogen uptake capacity measurements by the Intelligent Gravimetric Analyzer at room temperature showed that BNNTs could adsorb 0.85 wt % hydrogen which was two times larger than that for commercial carbon nanotubes.

1. INTRODUCTION The recent studies on the development of hydrogen storage materials have been with the nanostructured materials which adsorb large amounts of hydrogen by physisorption.1 According to the commercial standards presented by the US Department of Energy, hydrogen storage capacity of CNTs is comparably lower than boron nitride nanotubes (BNNTs) due to highly complex electronic properties of CNTs.2 Therefore, BNNTs are promising hydrogen storage materials since their electronic properties are independent of helicity, diameter, and number of walls compared to CNTs.3 There are numerous attempts about the synthesis and characterization of BNNTs due to their high mechanical strength, good resistance to corrosion, low density, and excellent thermal and electrical properties.4−6 Multiwalled-BNNTs7 and single walled-BNNTs8 were first synthesized by an adapted arc discharge technique. In the previous experimental studies, Tang et al.9 demonstrated the synthesis of multiwalled-BNNTs in tubular form, iron oxide-assisted chemical vapor transport at 1350 °C under ammonia atmosphere. Bando et al.10 produced nanotubular BN materials via a chemical vapor deposition (CVD) method using B−N−O precursors at a high temperature of 1700 °C. Cai et al.11 reported a convenient synthesis route to BNNT by the reaction of boron powder, iron oxide, and ammonium chloride at 600 °C for 12 h. In addition, the morphologies of BNNTs crystallize in single- and multiwalled structures by changing the reactants and tailoring the reaction conditions.12,13 Wang et al.14 prepared BNNTs, BN-bamboos, and BN-fibers from borazine oligomer under the confinement of alumina anodic membrane as a template. Furthermore, Li et al.4 produced BNNTs with a uniform diameter of about 7 nm using BCl3 and NH3 at relatively low temperatures (650−850 °C) within the channels of © 2012 American Chemical Society

mesoporous silica SBA-15. So far, the growth of BNNTs over mesoporous templates by CVD carries a significant importance to produce high-quality and high-yield BNNTs. Therefore, mesoporous MCM-41 (mobil composition of matter no. 41) as a template is a good candidate in BNNT synthesis since it has a regular hexagonal array of uniform pore openings with diameters between 2 and 10 nm.15 In the present work, a simple and shorter synthesis technique for the production of BNNT was conducted over iron impregnated mesoporous silica MCM-41 at a relatively low reaction temperature of 600 °C by the CVD method. In addition, the structural changes of BNNTs were tailored at different reaction temperatures and using different catalyst systems. To the best of our knowledge, this is the first comprehensive work in the literature about the controllable synthesis of BNNTs over MCM-41 templates. Hydrogen storage properties of BNNTs were investigated by an Intelligent Gravimetric Analyzer at room temperature in the pressure range of 1000−9000 mbar.

2. EXPERIMENTAL SECTION 2.1. Materials. Boron powder (Sigma-Aldrich, 99%), carbon nanotubes (CNT, Baytubes, purity >95%), hexadecyltrimethylammonium bromide (HDTMABr, Merck, 99%), sodium silicate (Na2SiO3, Aldrich, 27 wt % SiO2), iron(III) nitrate hexahydrate [Fe(NO3)3·6H2O, Merck, 99%], nitric acid (HNO3, Merck, 65%), hydrochloric acid (HCl, Merck, 37%), argon gas Received: Revised: Accepted: Published: 11341

June 19, 2012 August 7, 2012 August 17, 2012 August 17, 2012 dx.doi.org/10.1021/ie301605z | Ind. Eng. Chem. Res. 2012, 51, 11341−11347

Industrial & Engineering Chemistry Research

Article

The fibers were separated from the solution and kept in an oven overnight at 80 °C to dry. 2.5. Measurement of Hydrogen Storage Capacity. Intelligent Gravimetric Analyzer (IGA) is specifically designed as a versatile gravimetric analysis system to accurately measure gas sorption isoterms from vacuum to high pressure. The IGA system includes a conventional microbalance head (sensitivity, ± 1.0 μg) mounted in a stainless steel vacuum-pressure reactor. Hydrogen adsorption studies were carried out for BNNTs and Baytubes CNTs by Hiden Isochema 001-IGA at room temperature in the pressure range of 1000−9000 mbar. All samples were outgassed for 24 h at room temperature. Bulk densities of BNNTs and CNTs were estimated as 0.11 and 0.16 g/cm3, respectively. 2.6. Characterization. The surface morphologies of samples were analyzed by a Leo Supra 35VP field emission scanning electron microscope (SEM). Imaging was generally performed at 2−15 keV accelerating voltage, using the secondary electron and inlens imaging technique. Elemental analysis was conducted using an energy-dispersive X-ray (EDX) analyzing system. High resolution transmission electron microscopy (TEM) analysis was performed by JEOL 2100 Lab6 HRTEM. Fourier transform infrared spectroscopy (FTIR) measurements of the samples were conducted using a Nicolet iS10 spectrometer ranging from 525 to 4000 cm−1. X-ray diffraction (XRD) measurements of all samples were done with a Bruker axs advance powder diffractometer fitted with a Siemens X-ray gun and equipped with Bruker axs Diffrac PLUS software. The sample was swept from 2θ = 5° to 2θ = 90°. The X-ray generator was set to 40 kV at 40 mA. Thermogravimetric analyses (TGA) were performed with a NETZSCH 449C thermogravimetric analyzer from room temperature to 1000 °C at a heating rate of 10 °C/min under air flow. Surface area was measured by a Quantachrome NOVA 2200e series surface analyzer. The adsorption isotherms of nitrogen at 77 K were investigated using the Brunauer− Emmett−Teller (BET) method in the P/P0 range of 0.05−0.3.17 All samples were outgassed for 24 h at 150 °C. The pore size distribution (PSD) was obtained from the desorption isotherms using the Barrett−Joyner−Halenda (BJH) method.18

(Ar, 99.99%), ammonia gas (NH3, 99.99%), and hydrogen gas (H2, 99.99%) were used. 2.2. Synthesis of Fe3+ Impregnated MCM-41. The mesoporous silica MCM-41 was synthesized by mixing 6.6 g of hexadecyltrimethylammonium bromide in 43 mL of H2O, and then, 5.65 g of sodium silicate was added subsequently. Iron(III) nitrate hexahydrate was added to the mixture to form the final mixture including 25:75 mol ratio of Si/Fe. The pH of solution was adjusted to 11 using 1 M HCl, and the whole mixture was stirred for 2 h at 40 °C. Then, the resultant 120 mL volume, homogeneous reaction mixture was placed in a Teflon autoclave. The autoclave was then placed in a domestic microwave oven. The microwave synthesis was performed with the irradiation under reflex conditions at 120 W for 30 min. The product (Fe3+-MCM-41) was filtered, washed thoroughly with distilled water, dried at 100 °C for 12 h, and finally calcined in a tube furnace at 550 °C for 6 h under an air atmosphere. 2.3. Synthesis of BN Nanostructures at Different Reaction Conditions. BN nanostructures were synthesized by CVD using argon and ammonia gases at different reaction temperatures using Fe3+ impregnated mesoporous silica MCM-41, Fe2O3, and their combinations as catalyst. The reaction conditions for the production of BN nanostructures were given in Table 1. The reaction time, boron source, and nitrogen source Table 1. Experimental Conditions for BNNT Productiona experiment no. boron source BNNT-1 BNNT-2 BNNT-3 BNNT-4 BNNT-5 BNNT-6

boron boron boron boron boron boron

powder powder powder powder powder powder

nitrogen source

catalyst

temperature (oC)

NH3 NH3 NH3 NH3 NH3 NH3

Fe3+-MCM-41/Fe2O3 Fe3+-MCM-41 Fe2O3 Fe2O3 Fe3+-MCM-41/Fe2O3 Fe2O3

600 600 600 750 750 800

a

The weight ratio of boron and catalyst for each experiment is 2:1. The flow rate of NH3 gas is 0.8 L/min for each reaction. All experiments were conducted for 1 h.

were kept the same for each reaction. Reaction was performed in a conventional furnace with a horizontal quartz tube. In a typical procedure, an appropriate amount of boron powder and catalyst (Fe2O3, Fe3+-MCM-41, Fe2O3/Fe3+-MCM-41) were mixed in the weight ratio of 2:1 and put into the alumina crucible which was placed at the center of the furnace. The sample was heated to the defined temperature under an argon flow (0.8 L/min). Afterward, argon flow was stopped, and NH3 with a flow rate of 0.8 L/min was started to pass over the alumina crucible in the quartz tube; the process was maintained at an adjusted temperature for 1 h. Then, the sample was cooled at room temperature under the argon flow. 2.4. Purification Procedure. For the purification process, the sample obtained from the CVD treatment was mixed with about 50 mL of 4 M HCl solution and kept for 4 h at room temperature. After HCl treatment, 50 mL of 1 M HNO3 solution was poured to the reaction mixture and stirred for 24 h at 50 °C. After the HCl treatment, the color of the mixture turned to green which indicates the dissolution of Fe ions.16 After the addition of HNO3, the solution became dark yellow due to the dissolution of boron. At the end of the purification process, the solution was filtered through Whatman No. 40 filter paper with a pore size of 0.45 μm and washed several times by distilled water. The filtrate was evaporated at 100 °C, and product fibers were observed inside the saturated solution.

3. RESULT AND DISCUSSION 3.1. Effect of Catalyst Type on the Production of BN Nanostructures. Figure 1 showed the XRD pattern of Fe3+MCM-41 obtained by microwave assisted direct synthesis of about 30 min. Fe3+ ions were impregnated into MCM-41 template at a silica/Fe mole ratio of 25:75. A high intensity (001) peak near 2θ = 2.34° was observed in the XRD pattern of MCM-41, Figure 1. The effect of catalyst type on the production of BN nanostructures was considered using Fe3+-MCM-41/Fe2O3, Fe3+MCM-41, and Fe2O3 catalyst systems. Figure 2 showed the XRD patterns of BN nanostructures produced using pure boron powder at 600 °C over three different catalyst systems. The dominant diffraction peaks assigned to (002) h-BN were observed at 2θ = 27.7°, 28°, and 28.4° for BNNT-1, BNNT-2, and BNNT-3, respectively.19 In BNNT-1, the B2O3 formation was observed due to the complex catalyst system, but there was no unreacted boron species in this sample, unlike BNNT-2 and BNNT-3. This indicated that one type of catalyst used in BNNT reactions was more advantageous compared to complex catalyst systems. 11342

dx.doi.org/10.1021/ie301605z | Ind. Eng. Chem. Res. 2012, 51, 11341−11347


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Figure 1. XRD pattern of Fe3+-MCM-41 synthesized by direct synthesis.

Figure 4. XRD patterns of BNNT-1 and BNNT-5.

Figure 5. FTIR spectra of BNNT-1 and BNNT-5. Figure 2. XRD patterns of BNNT-1, BNNT-2, and BNNT-3.

Figure 6. XRD patterns of BNNT-3, BNNT-4, and BNNT-6.

6B(s) + 2Fe2O3(s) → 3B2O2 (g) + 4Fe(s)

Figure 3. FTIR spectra of BNNT-1, BNNT-2, and BNNT-3.

B2O2 (g) + 2NH3(g) → 2BN(s) + 2H 2O(g) + H 2(g)

The possible BNNT formation reactions and intermediate reactions occurred during the synthesis of BNNT: NH3 → N + 3/2H 2 B(s) + N(g) → BN(s)

(5)

(1)

(nanotubes)

(2)

6B(s) + Fe2O3(s) + 2NH3(g) → 2BN(s) + 2FeB(s) + B2H6(s)

(4)

2B(s) + Fe2O3(s) → B2O3(s) + 2Fe(s)

(6)

Fe4N(s) + 3B(s) → 2Fe2B(s) + BN(s)

(7)

Fe2B(s) + NH3(g) → BN(s) + 2Fe(s) + 3/2H 2(g)

(8)

In situ-formed B2O2 (g) reacted with NH3 gas resulting in the formation of BNNTs by eqs 4 and 5. Fe2O3 fastens the

(3) 11343



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reaction and provides oxygen atmosphere for the formation of B2O2.20 During reaction, one could observe B2O3 formation and Fe could also remain in the sample as seen in eq 6. After the purification process by HNO3 and HCl, Fe can be removed from the media. In addition, Fe−B complexes were formed during the reaction as shown in eqs 7 and 8, but these complexes reacted with NH3 gas at higher temperatures;21 then BNNT formation occurred. Therefore, there were several side products during the intermediate steps of BNNT reactions. Figure 3 represented the FT-IR spectra of the samples BNNT-1, BNNT-2, and BNNT-3. These samples included two strong characteristic absorption bands near 1402−1336 and 803−812 cm−1 attributed to the in-plane B−N stretching vibrations of the sp2-bonded h-BN and the B−N−B bending vibrations out of the plane, respectively.22,23 The FTIR spectrum of BNNT-1 contained a sharp peak near 1191 cm−1 due to the formation of B2O324 In the FTIR spectra of BNNT-2 and BNNT-3, the bands at 1100 and 940 cm−1 could be attributed to tetrahedral B units25 of boron oxynitride (B−N−O) species.26 3.2. Effect of Reaction Temperature on the Production of BN Nanostructures. The formation of BN nanostructures at 600, 750, and 800 °C were investigated by XRD and FTIR techniques. First, the effect of reaction temperature on BNNT-1 and BNNT-5 was investigated using Fe2O3/Fe3+-MCM-41 as a complex catalyst system. Figure 4 represented XRD patterns of BNNT-1 and BNNT-5. The characteristic peaks of h-BN were observed at about 2θ = 27.3° (002) and 2θ = 41.2° (100) in the XRD pattern of BNNT-1.16 As the reaction temperature increased, side products such as Fe−B complexes were obtained due to the reduction of Fe2O3 by boron particles and the reaction of Fe particles with boron,27 and the intensity of h-BN peak decreased in the XRD pattern of BNNT-5. FTIR spectra of BNNT-1 and BNNT-5 were exhibited in Figure 5. These spectra are dominated by 1383 and 813 cm−1 bands due to h-BN structures.12 The FTIR spectrum contained a strong and broad peak near 1400 cm−1 due to in-plane sp2 bonded B−N stretching vibrations and a peak near 850 cm−1 assigned to the B−N−B out-of-plane bending vibration28 encountered in h-BN formations. Also, a strong and sharp peak observed near 1180 cm−1 was due to B−O vibrations in the structure of BNNT-1.24 Both BNNT-1 and BNNT-5 included main diffraction peaks of h-BN, but BNNT-5 contained different functional groups due to the side reactions by an increase of reaction temperature. Only one-type catalyst was used in order to observe the tendency of BN nanostructure formation as a function of reaction temperature. Figure 6 exhibited XRD patterns of BNNT-3, BNNT-4, and BNNT-6. The results indicated that, as the reaction temperature increased, the intensity of (002) h-BN peak decreased due to the formation of iron-based side products. At higher temperatures, boron reduces Fe2O3 particles to form metallic Fe particles, and then, metallic Fe particles react with boron to produce Fe−B compounds.27 Figure 7 exhibited FTIR spectra of BNNT-3, BNNT-4, and

Figure 7. FTIR spectra of BNNT-3, BNNT-4, and BNNT-6.

Figure 8. (a) SEM image of Fe3+-MCM-41 prior to the synthesis of BNNT; (b) TEM image of Fe3+-MCM-41.

Table 2. Structural and Textural Properties of Fe3+-MCM-41 sample ID

Si/metal (mole ratio) EDX

BET surface area (m2/g)

BJH des. pore volume (cm3/g)

BJH des. pore diameter (nm)

d100 (nm)

lattice parameter, a (nm)

pore wall thickness, δ (nm)

Fe3+‑MCM-41

21

1229

0.66

4.00

3.78

4.36

0.56

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Figure 10. TEM image of BNNT-5.

Figure 11. TGA curves of CNTs, Fe3+-MCM-41, and BNNT-5.

3.3. Surface Morphology Analysis. A SEM image of calcined Fe3+-MCM-41, which contained a Si/Fe mole ratio of 25:75, was shown in Figure 8a. In the TEM image of Fe3+MCM-41, size distribution of metal particles was less than 5 nm, Figure 8b. The grains of metal salts seemed to be distributed over the MCM-41 particles. The choice of a catalyst support for the synthesis of BNNTs largely relies on the surface area. A high surface area provides a high population of the active sites, and thus, maximum catalyst dispersion is achieved.29 Nitrogen adsorption isotherms showed that BET surface area of Fe3+-MCM-41 was 1229 m2/g and pore diameter of Fe3+-MCM-41 was evaluated as 4 nm from BJH desorption, Table 2. After the catalytic CVD and purification processes, SEM images of BNNT-1, BNNT-3, BNTT-4, and BNNT-5 were shown in Figure 9. BNNT-1 and BNNT-3 were synthesized at the same reaction temperature using different catalysts. The images revealed entangled fibrous structures grown on the catalyst systems. SEM images showed the formation of 3D fibrous networks structures by interconnecting nanosized fibrils. Two different BNNT formations were observed in electron microcope images. The diameters of thick BNNTs were changed

Figure 9. SEM images of (a) BNNT-1, (b) BNNT-3, (c) BNNT-4, and (d) BNNT-5.

BNNT-6. All these samples included the main peak at around 1380 cm−1 which belongs to h-BN stretching. 11345



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in the range of 20−60 nm. This change stemmed from a different amount of Fe3+ ions in catalyst systems and different reaction temperatures. In Figure 9a, the average diameter of BN fibrils (BNNT-1) was about 30 nm lower than BNNT-3, having the average diameter of 35 nm, Figure 9b, due to the presence of MCM-41 in the catalyst system of BNNT-1. BNNT-3 was also denser than BNNT-1. Moreover, BNNT-3 and BNNT-4 were synthesized at different reaction temperatures using the same catalyst as Fe2O3. SEM images indicated that, as the reaction temperature increased, the diameter of BN fibrils increased from 35 to 50 nm and the length BN fibrils decreased; a denser structure was obtained seen in Figure 9c. When comparing BNNT-1 and BNNT-5 produced using the complex catalyst system at different reaction temperatures, the diameters of BN fibrils were almost the same, but at a higher temperature, a denser structure of BN fibrils was observed clearly in Figure 9d. Figure 10 exhibited TEM images of BNNTs grown over Fe3+-MCM-41. The diameter of thick BNNTs were in the range of 2.5−4.0 nm since the pore diameter of Fe3+-MCM-41 was evaluated as 4 nm. 3.4. Thermogravimetric Analysis. TGA curves of commercial CNT, Fe3+-MCM-41, and BNNT-5 were represented in Figure 11. BNNT exhibited several oxidative reactions as the temperature was raised from room temperature to 1000 °C. At 150 °C, moisture present in the sample was lost. As the temperature increased up to 350 and 500 °C, two sets of oxidative reactions occurred. At both of these temperatures, mass loss was around 5%. BNNTs (80%) were still available at 1000 °C. On the other hand, commercial CNT showed thermal stability up to 550 °C and then started to lose weight and completely decomposed at 875 °C. Therefore, BNNTs showed greater thermal stability at higher temperatures. 3.5. Hydrogen Storage Measurements via IGA. The Intelligent Gravimetric Analyzer (IGA) system uses the gravimetric technique to measure sorption isotherms. Figure 12 represented hydrogen adsorption and desorption isotherms of BNNT-3, BNNT-4, BNNT-5 and commercial CNTs in the range of 1000−9000 mbar pressure at room temperature. The hydrogen uptake capacity measurements by IGA showed that BNNT-3 and BNNT-4 adsorbed 0.87 and 0.75 wt %, respectively. The differences in hydrogen uptake capacities of BNNT-3 and BNNT-4 stemmed from the diameters of BNNT. As the reaction temperature increased from 600 to 750 °C, the diameter of BNNTs decreased from 55 down to 35 nm; the specific surface area of BNNTs also increased,30 and thus, the hydrogen uptake capacity was enhanced. In addition, BNNT-5 grown over Fe2O3/Fe3+-MCM-41 as complex catalyst system adsorbed 0.85 wt % hydrogen at room temperature. The hydrogen uptake capacity of the synthesized BNNTs was compared with commercial CNTs having hydrogen uptake capacity as 0.42 wt %. These differences in hydrogen uptake values stemmed from the dipolar nature of B−N bonds in BNNT which lead to stronger adsorption of hydrogen.31 Moreover, the diversity of CNTs in diameter and helicity results in the change of their electronic properties which affect hydrogen storage capacity.32 On the other hand, the electronic properties of BNNTs are independent of helicity, diameter, and number of walls.33 Furthermore, the main advantage of IGA on hydrogen adsorption experiments provides high outgassing rates under ultrahigh vacuum with approximately 10−7 mbar pressure and thus enhances hydrogen adsorption on the surface of BNNTs.

Figure 12. Hydrogen adsorption and desorption isotherms of (a) BNNT-3, (b) BNNT-4, (c) BNNT-5, and (d) commercial CNTs.

4. CONCLUSION The influence of reaction temperature and catalyst type on the formation of BNNTs by a catalytic CVD technique was investigated in the present work. Fe3+-MCM-41 and Fe2O3 were used as catalyst with different variations. BNNT growth 11346



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was achieved both in the presence of Fe2O3 and in the absence of Fe2O3. In addition, BNNTs were successfully grown over iron impregnated MCM-41 at a relatively low temperature of 600 °C for 1 h by CVD technique. SEM and TEM characterization revealed thin and thick tube formations were observed as Fe3+-MCM-41 and Fe2O3 were used as a complex catalyst system. The diameters of BNNTs were in the range of 2.5−4.0 nm for thin tubes and 20−60 nm for thick tubes. The thin tube formation stemmed from the growth of BNNT over Fe3+-MCM-41 since the pore diameter of Fe3+-MCM-41was evaluated as 4 nm. As the reaction temperature increased, the intensity of the (002) h-BN peak decreased and iron-based side product formation was observed because of the reduction of Fe2O3 by boron particles at high temperatures and the reaction of Fe particles with boron. Oxidative TGA results indicated that BNNTs were thermally stable at temperatures higher than 550 °C. Hydrogen storage measurements via IGA showed that BNNTs could adsorb 0.85 wt % hydrogen which was two times larger than that for commercial CNTs. Therefore, BNNTs can be a good candidate for hydrogen storage applications at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +90 216 4839512. Fax: +90 216 4839550. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Turkish National Boron Institute (BOREN) under Project No: 2009.C.230. REFERENCES

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