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

Effect of Reaction Temperature and Catalyst Type on the Formationof Boron Nitride Nanotubes by Chemical Vapor Deposition andMeasurement of Their Hydrogen Storage CapacityBurcu Saner Okan,† Zuleyha Ozlem Kocabas,‡ Asli Nalbant Ergun,‡ Mustafa Baysal,‡ Ilse Letofsky-Papst,§

and Yuda Yurum*,‡

†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 ofmatter no. 41) and Fe2O3/MCM-41 complex catalyst systems at relatively low temperatures for 1 h by the chemical vapordeposition technique in large quantities. The formation of BNNT was tailored at different reaction temperatures by changingcatalyst type. The use of Fe3+-MCM-41 and Fe2O3 as a complex catalyst system led to thin and thick tube formations. Thediameters 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 formationoriginated from the growth of BNNT over Fe3+-MCM-41 due to its average pore size of 4 nm. Higher reaction temperaturescaused both BNNT and iron-based side product formations. The hydrogen uptake capacity measurements by the IntelligentGravimetric Analyzer at room temperature showed that BNNTs could adsorb 0.85 wt % hydrogen which was two times largerthan that for commercial carbon nanotubes.

1. INTRODUCTION

The recent studies on the development of hydrogen storagematerials have been with the nanostructured materials whichadsorb large amounts of hydrogen by physisorption.1 Accordingto the commercial standards presented by the US Departmentof Energy, hydrogen storage capacity of CNTs is comparablylower than boron nitride nanotubes (BNNTs) due to highlycomplex electronic properties of CNTs.2 Therefore, BNNTsare promising hydrogen storage materials since their electronicproperties are independent of helicity, diameter, and number ofwalls compared to CNTs.3

There are numerous attempts about the synthesis and char-acterization of BNNTs due to their high mechanical strength,good resistance to corrosion, low density, and excellent thermaland electrical properties.4−6 Multiwalled-BNNTs7 and singlewalled-BNNTs8 were first synthesized by an adapted arc dischargetechnique. In the previous experimental studies, Tang et al.9

demonstrated the synthesis of multiwalled-BNNTs in tubularform, iron oxide-assisted chemical vapor transport at 1350 °Cunder ammonia atmosphere. Bando et al.10 produced nanotubularBN materials via a chemical vapor deposition (CVD) methodusing B−N−O precursors at a high temperature of 1700 °C. Caiet al.11 reported a convenient synthesis route to BNNT by thereaction of boron powder, iron oxide, and ammonium chlorideat 600 °C for 12 h. In addition, the morphologies of BNNTscrystallize in single- and multiwalled structures by changing thereactants and tailoring the reaction conditions.12,13 Wang et al.14

prepared BNNTs, BN-bamboos, and BN-fibers from borazineoligomer under the confinement of alumina anodic membraneas a template. Furthermore, Li et al.4 produced BNNTs with auniform diameter of about 7 nm using BCl3 and NH3 at relativelylow temperatures (650−850 °C) within the channels of

mesoporous silica SBA-15. So far, the growth of BNNTs overmesoporous templates by CVD carries a significant importanceto produce high-quality and high-yield BNNTs. Therefore,mesoporous MCM-41 (mobil composition of matter no. 41) asa template is a good candidate in BNNT synthesis since it has aregular hexagonal array of uniform pore openings with diametersbetween 2 and 10 nm.15

In the present work, a simple and shorter synthesis techniquefor the production of BNNT was conducted over iron impreg-nated mesoporous silica MCM-41 at a relatively low reactiontemperature of 600 °C by the CVD method. In addition, thestructural changes of BNNTs were tailored at different reactiontemperatures and using different catalyst systems. To the best ofour knowledge, this is the first comprehensive work in the litera-ture about the controllable synthesis of BNNTs over MCM-41templates. Hydrogen storage properties of BNNTs were inves-tigated by an Intelligent Gravimetric Analyzer at room tempera-ture in the pressure range of 1000−9000 mbar.

2. EXPERIMENTAL SECTION

2.1. Materials. Boron powder (Sigma-Aldrich, 99%), carbonnanotubes (CNT, Baytubes, purity >95%), hexadecyltrimethyl-ammonium bromide (HDTMABr, Merck, 99%), sodiumsilicate (Na2SiO3, Aldrich, 27 wt % SiO2), iron(III) nitratehexahydrate [Fe(NO3)3·6H2O, Merck, 99%], nitric acid (HNO3,Merck, 65%), hydrochloric acid (HCl, Merck, 37%), argon gas

Received: June 19, 2012Revised: August 7, 2012Accepted: August 17, 2012Published: August 17, 2012

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(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 gof 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 formthe final mixture including 25:75 mol ratio of Si/Fe. The pHof solution was adjusted to 11 using 1 M HCl, and the wholemixture was stirred for 2 h at 40 °C. Then, the resultant120 mL volume, homogeneous reaction mixture was placed in aTeflon autoclave. The autoclave was then placed in a domesticmicrowave oven. The microwave synthesis was performed withthe irradiation under reflex conditions at 120 W for 30 min.The product (Fe3+-MCM-41) was filtered, washed thoroughlywith distilled water, dried at 100 °C for 12 h, and finally calcinedin 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 synthesizedby CVD using argon and ammonia gases at different reactiontemperatures using Fe3+ impregnated mesoporous silicaMCM-41, Fe2O3, and their combinations as catalyst. The reactionconditions for the production of BN nanostructures were given inTable 1. The reaction time, boron source, and nitrogen source

were kept the same for each reaction. Reaction was performed ina conventional furnace with a horizontal quartz tube. In a typicalprocedure, an appropriate amount of boron powder and catalyst(Fe2O3, Fe

3+-MCM-41, Fe2O3/Fe3+-MCM-41) were mixed in

the weight ratio of 2:1 and put into the alumina crucible whichwas placed at the center of the furnace. The sample was heatedto the defined temperature under an argon flow (0.8 L/min).Afterward, argon flow was stopped, and NH3 with a flow rate of0.8 L/min was started to pass over the alumina crucible in thequartz tube; the process was maintained at an adjusted tem-perature for 1 h. Then, the sample was cooled at room temperatureunder the argon flow.2.4. Purification Procedure. For the purification process,

the sample obtained from the CVD treatment was mixed withabout 50 mL of 4 M HCl solution and kept for 4 h at roomtemperature. After HCl treatment, 50 mL of 1 M HNO3solution was poured to the reaction mixture and stirred for 24 hat 50 °C. After the HCl treatment, the color of the mixtureturned to green which indicates the dissolution of Fe ions.16

After the addition of HNO3, the solution became dark yellowdue to the dissolution of boron. At the end of the purificationprocess, the solution was filtered through Whatman No. 40filter paper with a pore size of 0.45 μm and washed severaltimes by distilled water. The filtrate was evaporated at 100 °C,and product fibers were observed inside the saturated solution.

The fibers were separated from the solution and kept in anoven overnight at 80 °C to dry.

2.5. Measurement of Hydrogen Storage Capacity.Intelligent Gravimetric Analyzer (IGA) is specifically designedas a versatile gravimetric analysis system to accurately measuregas sorption isoterms from vacuum to high pressure. The IGAsystem includes a conventional microbalance head (sensitivity,± 1.0 μg) mounted in a stainless steel vacuum-pressure reactor.Hydrogen adsorption studies were carried out for BNNTsand Baytubes CNTs by Hiden Isochema 001-IGA at roomtemperature in the pressure range of 1000−9000 mbar. Allsamples were outgassed for 24 h at room temperature. Bulkdensities of BNNTs and CNTs were estimated as 0.11 and0.16 g/cm3, respectively.

2.6. Characterization. The surface morphologies of sampleswere analyzed by a Leo Supra 35VP field emission scanningelectron microscope (SEM). Imaging was generally performedat 2−15 keV accelerating voltage, using the secondary electronand inlens imaging technique. Elemental analysis was conductedusing an energy-dispersive X-ray (EDX) analyzing system. Highresolution transmission electron microscopy (TEM) analysis wasperformed by JEOL 2100 Lab6 HRTEM. Fourier transforminfrared spectroscopy (FTIR) measurements of the sampleswere conducted using a Nicolet iS10 spectrometer ranging from525 to 4000 cm−1. X-ray diffraction (XRD) measurementsof all samples were done with a Bruker axs advance powderdiffractometer fitted with a Siemens X-ray gun and equippedwith Bruker axs Diffrac PLUS software. The sample was sweptfrom 2θ = 5° to 2θ = 90°. The X-ray generator was set to 40 kVat 40 mA. Thermogravimetric analyses (TGA) were performedwith a NETZSCH 449C thermogravimetric analyzer fromroom temperature to 1000 °C at a heating rate of 10 °C/minunder air flow. Surface area was measured by a QuantachromeNOVA 2200e series surface analyzer. The adsorption isothermsof nitrogen at 77 K were investigated using the Brunauer−Emmett−Teller (BET) method in the P/P0 range of0.05−0.3.17 All samples were outgassed for 24 h at 150 °C.The pore size distribution (PSD) was obtained from thedesorption isotherms using the Barrett−Joyner−Halenda(BJH) method.18

3. RESULT AND DISCUSSION

3.1. Effect of Catalyst Type on the Production of BNNanostructures. Figure 1 showed the XRD pattern of Fe3+-MCM-41 obtained by microwave assisted direct synthesisof about 30 min. Fe3+ ions were impregnated into MCM-41template at a silica/Fe mole ratio of 25:75. A high intensity(001) peak near 2θ = 2.34° was observed in the XRD pattern ofMCM-41, Figure 1.The effect of catalyst type on the production of BN nano-

structures was considered using Fe3+-MCM-41/Fe2O3, Fe3+-

MCM-41, and Fe2O3 catalyst systems. Figure 2 showed theXRD patterns of BN nanostructures produced using pureboron powder at 600 °C over three different catalyst systems.The dominant diffraction peaks assigned to (002) h-BN wereobserved at 2θ = 27.7°, 28°, and 28.4° for BNNT-1, BNNT-2,and BNNT-3, respectively.19 In BNNT-1, the B2O3 formationwas observed due to the complex catalyst system, but therewas no unreacted boron species in this sample, unlike BNNT-2and BNNT-3. This indicated that one type of catalyst used inBNNT reactions was more advantageous compared to complexcatalyst systems.

Table 1. Experimental Conditions for BNNT Productiona

experimentno. boron source

nitrogensource catalyst

temperature(oC)

BNNT-1 boron powder NH3 Fe3+-MCM-41/Fe2O3 600BNNT-2 boron powder NH3 Fe3+-MCM-41 600BNNT-3 boron powder NH3 Fe2O3 600BNNT-4 boron powder NH3 Fe2O3 750BNNT-5 boron powder NH3 Fe3+-MCM-41/Fe2O3 750BNNT-6 boron powder NH3 Fe2O3 800

aThe 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. Allexperiments were conducted for 1 h.

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The possible BNNT formation reactions and intermediatereactions occurred during the synthesis of BNNT:

→ +NH N 3/2H3 2 (1)

+ →B(s) N(g) BN(s) (nanotubes) (2)

+ +

→ + +

6B(s) Fe O (s) 2NH (g)

2BN(s) 2FeB(s) B H (s)2 3 3

2 6 (3)

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

+ → + +B O (g) 2NH (g) 2BN(s) 2H O(g) H (g)2 2 3 2 2(5)

+ → +2B(s) Fe O (s) B O (s) 2Fe(s)2 3 2 3 (6)

+ → +Fe N(s) 3B(s) 2Fe B(s) BN(s)4 2 (7)

+ → + +Fe B(s) NH (g) BN(s) 2Fe(s) 3/2H (g)2 3 2 (8)

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

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

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

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

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

Figure 5. FTIR spectra of BNNT-1 and BNNT-5.

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



reaction and provides oxygen atmosphere for the formation ofB2O2.

20 During reaction, one could observe B2O3 formationand Fe could also remain in the sample as seen in eq 6. Afterthe purification process by HNO3 and HCl, Fe can be removedfrom the media. In addition, Fe−B complexes were formedduring the reaction as shown in eqs 7 and 8, but thesecomplexes reacted with NH3 gas at higher temperatures;

21 thenBNNT formation occurred. Therefore, there were several sideproducts 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 twostrong characteristic absorption bands near 1402−1336 and803−812 cm−1 attributed to the in-plane B−N stretchingvibrations of the sp2-bonded h-BN and the B−N−B bendingvibrations out of the plane, respectively.22,23 The FTIRspectrum 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 couldbe attributed to tetrahedral B units25 of boron oxynitride(B−N−O) species.26

3.2. Effect of Reaction Temperature on the Produc-tion of BN Nanostructures. The formation of BNnanostructures at 600, 750, and 800 °C were investigated byXRD and FTIR techniques. First, the effect of reactiontemperature on BNNT-1 and BNNT-5 was investigated usingFe2O3/Fe

3+-MCM-41 as a complex catalyst system. Figure 4represented XRD patterns of BNNT-1 and BNNT-5. Thecharacteristic 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 asFe−B complexes were obtained due to the reduction of Fe2O3by boron particles and the reaction of Fe particles with boron,27

and the intensity of h-BN peak decreased in the XRD pattern ofBNNT-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 containeda 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 peakobserved near 1180 cm−1 was due to B−O vibrations in thestructure of BNNT-1.24 Both BNNT-1 and BNNT-5 includedmain diffraction peaks of h-BN, but BNNT-5 containeddifferent functional groups due to the side reactions by anincrease of reaction temperature.Only one-type catalyst was used in order to observe the

tendency of BN nanostructure formation as a function ofreaction temperature. Figure 6 exhibited XRD patterns ofBNNT-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 sideproducts. At higher temperatures, boron reduces Fe2O3particles to form metallic Fe particles, and then, metallic Feparticles 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 ofBNNT; (b) TEM image of Fe3+-MCM-41.

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

sample IDSi/metal (moleratio) EDX

BET surface area(m2/g)

BJH des. pore volume(cm3/g)

BJH des. porediameter (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



BNNT-6. All these samples included the main peak at around1380 cm−1 which belongs to h-BN stretching.

3.3. Surface Morphology Analysis. A SEM image ofcalcined Fe3+-MCM-41, which contained a Si/Fe mole ratio of25:75, was shown in Figure 8a. In the TEM image of Fe3+-MCM-41, size distribution of metal particles was less than5 nm, Figure 8b. The grains of metal salts seemed to bedistributed over the MCM-41 particles. The choice of a catalystsupport for the synthesis of BNNTs largely relies on thesurface area. A high surface area provides a high population ofthe active sites, and thus, maximum catalyst dispersion isachieved.29 Nitrogen adsorption isotherms showed thatBET surface area of Fe3+-MCM-41 was 1229 m2/g and porediameter of Fe3+-MCM-41 was evaluated as 4 nm from BJHdesorption, Table 2.After the catalytic CVD and purification processes, SEM

images of BNNT-1, BNNT-3, BNTT-4, and BNNT-5 wereshown in Figure 9. BNNT-1 and BNNT-3 were synthesized atthe same reaction temperature using different catalysts. Theimages revealed entangled fibrous structures grown on thecatalyst systems. SEM images showed the formation of 3Dfibrous networks structures by interconnecting nanosized fibrils.Two different BNNT formations were observed in electronmicrocope 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.

Figure 10. TEM image of BNNT-5.

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



in the range of 20−60 nm. This change stemmed from adifferent amount of Fe3+ ions in catalyst systems and differentreaction temperatures. In Figure 9a, the average diameter of BNfibrils (BNNT-1) was about 30 nm lower than BNNT-3, havingthe average diameter of 35 nm, Figure 9b, due to the presence ofMCM-41 in the catalyst system of BNNT-1. BNNT-3 was alsodenser than BNNT-1. Moreover, BNNT-3 and BNNT-4 weresynthesized at different reaction temperatures using the samecatalyst as Fe2O3. SEM images indicated that, as the reactiontemperature increased, the diameter of BN fibrils increased from35 to 50 nm and the length BN fibrils decreased; a denserstructure was obtained seen in Figure 9c. When comparingBNNT-1 and BNNT-5 produced using the complex catalystsystem at different reaction temperatures, the diameters of BNfibrils were almost the same, but at a higher temperature, adenser 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 therange of 2.5−4.0 nm since the pore diameter of Fe3+-MCM-41was evaluated as 4 nm.3.4. Thermogravimetric Analysis. TGA curves of

commercial CNT, Fe3+-MCM-41, and BNNT-5 were repre-sented in Figure 11. BNNT exhibited several oxidative reactionsas the temperature was raised from room temperature to1000 °C. At 150 °C, moisture present in the sample was lost.As the temperature increased up to 350 and 500 °C, two sets ofoxidative reactions occurred. At both of these temperatures,mass loss was around 5%. BNNTs (80%) were still available at1000 °C. On the other hand, commercial CNT showed thermalstability up to 550 °C and then started to lose weight andcompletely decomposed at 875 °C. Therefore, BNNTs showedgreater thermal stability at higher temperatures.3.5. Hydrogen Storage Measurements via IGA. The

Intelligent Gravimetric Analyzer (IGA) system uses thegravimetric technique to measure sorption isotherms. Figure 12represented hydrogen adsorption and desorption isothermsof BNNT-3, BNNT-4, BNNT-5 and commercial CNTs in therange of 1000−9000 mbar pressure at room temperature.The hydrogen uptake capacity measurements by IGA showedthat BNNT-3 and BNNT-4 adsorbed 0.87 and 0.75 wt %,respectively. The differences in hydrogen uptake capacities ofBNNT-3 and BNNT-4 stemmed from the diameters of BNNT.As the reaction temperature increased from 600 to 750 °C, thediameter of BNNTs decreased from 55 down to 35 nm; thespecific surface area of BNNTs also increased,30 and thus, thehydrogen uptake capacity was enhanced. In addition, BNNT-5grown over Fe2O3/Fe

3+-MCM-41 as complex catalyst systemadsorbed 0.85 wt % hydrogen at room temperature. Thehydrogen uptake capacity of the synthesized BNNTs wascompared with commercial CNTs having hydrogen uptakecapacity as 0.42 wt %. These differences in hydrogen uptakevalues stemmed from the dipolar nature of B−N bonds inBNNT which lead to stronger adsorption of hydrogen.31

Moreover, the diversity of CNTs in diameter and helicityresults in the change of their electronic properties which affecthydrogen storage capacity.32 On the other hand, the electronicproperties of BNNTs are independent of helicity, diameter,and number of walls.33 Furthermore, the main advantage ofIGA on hydrogen adsorption experiments provides highoutgassing rates under ultrahigh vacuum with approximately10−7 mbar pressure and thus enhances hydrogen adsorption onthe surface of BNNTs.

4. CONCLUSIONThe influence of reaction temperature and catalyst type onthe formation of BNNTs by a catalytic CVD technique wasinvestigated in the present work. Fe3+-MCM-41 and Fe2O3were used as catalyst with different variations. BNNT growth

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



was achieved both in the presence of Fe2O3 and in the absenceof Fe2O3. In addition, BNNTs were successfully grown overiron impregnated MCM-41 at a relatively low temperatureof 600 °C for 1 h by CVD technique. SEM and TEM char-acterization revealed thin and thick tube formations wereobserved as Fe3+-MCM-41 and Fe2O3 were used as a complexcatalyst system. The diameters of BNNTs were in the range of2.5−4.0 nm for thin tubes and 20−60 nm for thick tubes. Thethin tube formation stemmed from the growth of BNNT overFe3+-MCM-41 since the pore diameter of Fe3+-MCM-41wasevaluated as 4 nm. As the reaction temperature increased, theintensity of the (002) h-BN peak decreased and iron-based sideproduct formation was observed because of the reduction ofFe2O3 by boron particles at high temperatures and the reaction ofFe particles with boron. Oxidative TGA results indicated thatBNNTs were thermally stable at temperatures higher than 550 °C.Hydrogen storage measurements via IGA showed that BNNTscould adsorb 0.85 wt % hydrogen which was two times larger thanthat for commercial CNTs. Therefore, BNNTs can be a goodcandidate for hydrogen storage applications at room temperature.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Tel: +90 216 4839512. Fax:+90 216 4839550.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by Turkish National BoronInstitute (BOREN) under Project No: 2009.C.230.

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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