Growth of Boron Nitride Nanotubes Having Large Surface Area Using Mechanothermal Process

doi:10.4236/wjnse.2011.14018

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World Journal of Nano Science and Engineering, 2011, 1, 119-128

doi:10.4236/wjnse.2011.14018 Published Online December 2011 (http://www.SciRP.org/journal/wjnse)

Growth of Boron Nitride Nanotubes Having Large

Surface Area Using Mechanothermal Process

Sunil K. Singhal1, Avanish K. Srivastava2, Rakesh B. Mathur1

1Physics & Engineering of Carbon, National Physical Laboratory, Council of Scientific & Industrial

Research, New Delhi, India

2Ion & Electron Microscopy, National Physical Laboratory, Council of Scientific & Industrial

Research, New Delhi, India

E-mail: [email protected]

Received September 22, 2011; revised October 5, 2011; accepted October 29, 2011

Abstract

Nanostructures of boron nitride (BN) including nanotubes, nanofibers and nanosheets having a large surface

area are very useful in storing hydrogen and other gases. In the present paper we report the synthesis and

characterization of these nanostructures of BN using mechanothermal process. Under this process elemental

boron powder is first ball milled to about 50 h in an inert atmosphere and then annealed at 1100˚C - 1250˚C

for 6 h in the presence of NH3 gas. By this treatment nanotubes and other nanostructures of BN were synthe-

sized. The diameter of BN nanotubes varied from 20 to 50 nm and most of them exhibited spindle or bamboo

like morphology. Because of the large surface area, these nanotubes may be explored for a better hydrogen

gas storage device as compared to the crystallized nanotubes. The main advantage of this technique is that

the nanotubes can be grown in large quantity. A possible growth mechanism towards the evolution of such

fascinating nano-objects of boron nitride has been discussed employing high-resolution transmission electron

microscopy and photoluminescence.

Keywords: Boron Nitride Nanotubes, Mechanothermal Process, Morphology, Photo Luminescence

1. Introduction

Boron nitride (BN) nanotubes have received consider-

able attention in the recent years due to their outstanding

properties such as an electrical insulator with a band gap

of 5.9 eV [1], resistance to oxidation at high temperature

(above 800˚C) [2] and excellent thermal conductivity [3].

Recently the use of BN nanotubes in the fabrication of

polymer based composites for photovoltaic packaging [4]

and in the reinforcement of glass to be used as a sealing

material in a solid oxide fuel cell [5] is being reported.

However, as the synthesis of nanostructure materials ma-

tures, there is a growing interest to study the hydrogen

storage of new nanostructure materials. Nanostructure ma-

terials are one of most promising classes of advanced

materials for hydrogen (H2) storage owing to their uni-

que chemical, physical, thermodynamic and transport

properties as compared to their bulk forms. In this class,

carbon nanotubes (CNTs) and nanofibers (NF) are being

considered as promising candidates for H2 storage be-

cause of their fine powdered morphology and highly

porous structure. However, the hydrogen storage capac-

ity of boron nitride nanostructures is found to be better

than CNTs. The H2 storage capacity of pure multi walled

CNTs at room temperature (RT) was measured to be <

0.01 wt% [6] and for single walled CNTs bundles ~0.6

wt% under 10 MPa at RT [7].

It has been found that the nanostructures of BN,

namely boron nitride nanotubes, exhibit reproducible hy-

drogen uptake of 1.8 - 2.2 wt% at 6 MPa [8] and nano-

fibers could absorb 2.9 wt% under 10 MPa at RT [9].

Theoretical calculations by Zhou et al [10] have shown

that a well-crystallized, perfect boron nitride nanotubes

is infect not a good candidate for H2 storage because of

either physical or chemical absorption mechanisms. There-

fore, novel nanostructures in BN have been found to be

of prime importance, and an increase in surface area for

these nanostructure materials may result in an increased

H2 storage capacity. Among the various nanostructures of

BN, collapsed or surface-modified boron nitride nano-

tubes are found to have superb H2 storage capacity [11]

due to their highest surface area of these materials. A

S. K. SINGHAL ET AL.

120

collapsed boron nitride nanotube consists of numerous

curved BN fragments, has a large surface area and con-

tains numerous dangling bonds and may favor the ad-

sorption of H2.

Thus, although, BN nanotubes are synthesized using a

number of methods including arc-discharge [12], laser

ablation [13], carbon substitution reactions [14], chemi-

cal vapor deposition [15] and ball milling and anneal- ing

methods [16-23], the BN nanotubes produced in these

methods were mostly smooth surface having bam-

boo-like or cylindrical morphology and therefore, may

have lower surface area. In this direction, very recently

Terao et al [24] have reported a carbon free CVD process

for the synthesis of surface modified BN nanostructures,

however, the yield of these nanostructures was not very

high and it is suggested that the addition of some metal

oxide may improve the yield of these nanostructures [25].

In the present study we report the synthesis of nanos-

tructures of BN (nanotubes, nanosheets and nanofibers)

having large surface area by mechanically milled boron

powder in the presence of ammonia gas followed by an-

nealing at a very controlled rate. This process has an ad-

vantage over the other methods reported earlier as one

can synthesize these nanostructures in a large quantity. A

possible growth mechanism for the growth of these

nanostructures is also described.

2. Experimental Details

The starting material used in the present investigation

was B powder supplied by Sigma Aldrich having a purity

of about 95% - 97%. It contained a small amount of O

(1.7%) and Mg (0.5%) as impurities and its average par-

ticle size was reported to be 900 nm. The high-energy

ball milling (HEBM) experiments were carried out in a

Planetary Ball mill (M/s Insmart Systems, Hyderabad,

India), in which four stainless steel containers of size 250

ml or 500 ml can be used. In the present investigation,

two containers of 250 ml were used. A small quantity (7

g) of elemental B powder was loaded in each cell con-

taining 40 Nos. of hardened steel balls of diameter 12

mm. The stainless containers were filled with ammonia

gas at a pressure of 300 KPa and closed tightly. Although

we have not studied the effect of gas pressure on milling,

we believe that at milling is very effective at high pres-

sure environment of the cell and also it reduces the mill-

ing time. The balls to powder weight ratio was 40:1. The

erosion of steel balls during ball milling provides suffi-

cient Fe nanoparticles that may acts as catalyst for the

nucleation of BN nanotubes during subsequent annealing.

The powders were milled at a speed of 500 rpm and the

milling time was varied from 10 to 50 h. A small quan-

tity (70 mg) of the milled powder was taken out after an

interval of 10 h and characterized by X-ray diffraction

(XRD) in order to observe the degree of amorphization,

particle size reduction and metal contamination. High-re-

solution transmission electron microscopy (HRTEM) was

performed to study the growth pattern, lattice scale fea-

tures and a qualitative evolution of BN nanotubes having

large surface area. The X-ray characterization of the BN

nanostructures was carried out by means of Rigaku D/MAX-

2400 X-ray diffraction analysis (Rigaku Corp. Tokyo,

Japan) using CuKα radiation (λ = 0.15418 nm) at room

temperature. The morphology of the nanoparticles and

nanotubes was observed by a scanning electron micro-

scope (SEM, model LEO 440) and a high resolution trans-

mission electron microscope (HRTEM, model FEI Tecnai

G2 F30 STWIN with FEG source at the electron accelera-

tion voltage of 300 kV).

Annealing experiments were carried out in a horizon-

tal tubular furnace. For annealing experiments, B powder

milled for 40 h - 50 h was used. The annealing was car-

ried out at the optimized conditions namely 1100˚C -

1300˚C in nitrogen/ammonia atmosphere for 6 h as re-

ported by Chen et al. [17]. The milled samples were taken

in an alumina boat and placed at the center of the quartz

tube and heated first in Ar atmosphere at the heating rate

of about 15˚C/min up to 1300˚C. When the desired an-

nealing temperature (1100˚C - 1300˚C) was achieved, Ar

was replaced by NH3 and the reaction was carried out for

about 6 h. After the reaction was over, the sample was

cooled to room temperature in NH3 atmosphere slowly

(10˚C/min).

3. Results and Discussion

3.1. Phase Formation and Chemistry of the

Materials at Different Milling Conditions

Figure 1(a) shows an X-ray diffraction pattern of the star-

ting B powder used in the present work. This powder was

found to be highly crystalline and all peaks of B powder

were indexed with “d” values of 5.0610 A, 4.84546 A,

4.67290 A and 4.41290 A etc.

Figures 1(b)-(e) shows XRD patterns of B powders

milled for 10, 20, 30 and 50 h respectively. From Figure

1(b) it was seen that although all major peaks of elemen-

tal B powder were indexed as mentioned above, the peak

height or intensities of all peaks observed in Figure 1(a)

decreased to about 30% indicating the partial transfor-

mation of crystalline to disordered structure. However, a

sharp change in the crystallinity was observed in the

XRD patterns of B powder milled for 20 and 30 h re-

spectively as shown in Figures 1(c) and (d). The 20 h

milled sample still showed partly conversion from crys-

talline to amorphous phase of elemental B powder. The

121

S. K. SINGHAL ET AL.

intensity of almost all XRD peaks of B decreased sharply.

However, one broad peak appeared at around 2θ = 44˚

with a “d” value of 2.0769 A. This peak was indexed to

be due to α-Fe, mixed in the milled powder as a small

contamination mainly due to the abrasion from the sur-

face of stainless steel container and hardened steel balls

during high-energy ball milling. An XRD pattern of a 30

h milled B powder shown in Figure 1(d) indicated an

almost complete amorphization of B powder as no peak

due to elemental B could be indexed in this XRD pattern.

Again in this XRD pattern also an XRD peak appeared at

around 2θ = 44˚ indicating the contamination of Fe na-

noparticles also observed in a 20 h milled sample. Figure

1(e) shows an XRD pattern of B powder milled for 50 h.

This pattern shows complete amorphization of B.

Although the quantitative measurement of N and Fe

concentration in the milled B powders could not be done

because of the presence of O present in the starting B

powder, we could get some qualitative information about

these elements present in the milled powder using EDS

analysis. Absorption of oxygen could not be avoided

because of large surface area of the milled powder. A

comparative qualitative analysis showed that the nitride-

tion of B powder was higher in a 30 h milled powder as

compared to the nitridation observed in a 20 h milled B

powder.

Figures 2(a) and (b) show EDS (attached with SEM)

patterns of B powders milled for 20 h and 30 h respect-

tively. From these patterns it is clear that the concentra-

tion of N in a 30 h milled sample was relatively higher to

that observed in a 20 h milled sample. It is also clear

from these two patterns that the density of Fe contamina-

tion also increased when the milling time was increased

from 20 h to 30 h. These results are in agreement with

the results obtained by Chen et al. [16-19]. As already

pointed out these nitridation reactions also results in the

formation of nanosized BN phases which may serve as

Figure 1. XRD pattern of B powder; (a) Pure and milled for;

(b) 10 h; (c) 20 h; (d) 30 h; and (e) 40 h respectively.

(a)

(b)

Figure 2. EDS spectrum of B powders milled for (a) 20 h

and (b) 30 h.

nuclei for the growth of BN nanotubes during the subse-

quent annealing. As the milled powder produced after 40

- 50 h was completely amorphous, this powder was used

to study the growth of BN nanotubes during the subse-

quent annealing.

Figures 3(a)-(c) shows XRD patterns of BN nano-

tubes synthesized from 50 h milled B powder followed

by annealing at 1100˚C - 1250˚C under NH3 gas for 6 h.

Figure 4 represents a XRD pattern of BN nanotubes

grown from 50 h milled B powder after annealing at

1300˚C. From Figures 3 and 4 it is clear that although B

samples milled for 40 and 50 h were completely amor-

phized, the yield of BN nanotubes obtained at 1250˚C was

relatively higher than that obtained at 1300˚C, as evi-

denced by the peak height of (002) reflection of hBN.

From these results it is observed that the annealing tem-

perature has a pronounced effect on the yield of BN

nanotubes. The yield is lower at higher annealing tem-

S. K. SINGHAL ET AL.

122

perature (1300˚C and higher). The growth of BN nano-

tubes is presumed to takes place via the dissolution of

nanosized B/BN species in the catalyst nanoparticles of

Fe. The catalyst (Fe-nanoparticles) should be in a quasi-

liquid state in order to dissolve maximum concentration

of B/BN phased to a state of supersaturation. BN nano-

tubes are grown from the supersaturated solution layer

by layer along the (002) planes. It was also found by Yu

et al. [26] that a much higher annealing temperature (T >

1300˚C) is not favored for the growth of BN nanotubes

as at this temperature, the catalyst nanoparticles (Fe) would

be in the liquid state and they may merged together to

form large size Fe crystals and then may impede the

growth of BN nanotubes. Also the B particles may react

directly with N to form BN particles instead of nanotubes

and used up the B source.

ticles (Cr, Ni etc.) were introduced into the milled B po-

wder during ball milling from the surface of stainless

steel balls and container. The milling contamination is

almost unavoidable by the mechanical milling process,

especially under high milling intensity, and this conta-

mination increases with milling time as observed by the

increase in peak intensity of Fe shown in XRD patterns

of the milled B powders. In general, such milling con-

tamination is undesirable but, in the present case (Fe, Ni,

Cr from the stainless steel container) have been found to

have positive catalytic role in promoting BN nanotube

At temperatures lower than 1100˚C, the low diffusion

rate of B and N in Fe would slow down the precipitation

rate of BN layers (i.e. the growth rate of BN nanotubes).

Figure 5 shows an XRD pattern of the products pro-

duced from un-milled B powder and annealed at 1300˚C

in the presence of ammonia gas. No BN nanotube growth

was found from this powder under the same annealing

conditions that produced nanotubes from the milled sam-

ples. This suggests that the milling process has an essen-

tial role to play in BN nanotube formation during subse-

quent annealing, which appears to be associated with the

creation of suitable nucleation sites for nanotube growth.

Under the high milling intensity used in the present work,

elemental B was transformed into disordered amorphous

phase. The amorphous phase is metastable and recrystal-

lized during the annealing process. Figure 3. XRD patterns of BN nanotubes synthesized from

B powder milled for 50 h and annealed at (a) 1100˚C; (b)

1150˚C and (c) 1250˚C under NH3 gas for 6 h.

It has been observed that Fe and other metal nanopar-

Figure 4. XRD pattern pf BN nanotubes synthesized at 1300˚C.

123

S. K. SINGHAL ET AL.

Figure 5. XRD pattern of the products produced from un-milled B powder.

formation. Actually, a high yield of BN nanotubes is

reported to be formed above a certain level of metal con-

tent (1 wt%). Such catalytic effects have been reported

extensively in the literature for nanotube formation by

other methods [27].

The XRD results shown in Figures 3 and 4 have re-

vealed that the amorphous phase of B recrystallizes to

form dominant (002) phases during annealing process.

As the growth of BN nanotubes was observed in the

temperature range of about 1100˚C - 1300˚C, which is

much below the boiling point of B (2550˚C) and subli-

mation temperature of BN (3000˚C), we speculate that

the vapor phases of B or BN are not possible during BN

nanotube growth under the experimental conditions used.

In the present case, it is expected that the nucleation

and growth of BN nanotube takes place in the solid state,

and during the annealing conditions surface diffusion is

the most likely mechanism for nanotube growth as ob-

served by Chadderton and Chen [28], although in our

opinion solid-liquid-solid (SLS) mechanism is also ex-

pected to be operative in the present case. It is likely that

some of the milled B/BN phases dissolve in nanoscale Fe

particles to a state of super saturation during the anneal-

ing conditions, and the BN nanotubes are crystallizes

from this supersaturated solution as the temperature is de-

creased following a solid-liquid-solid mechanism. Thus, the

nucleation and growth of BN nanotubes and other nanos-

tructures is expected to takes place by a combination of

VLS and SLS mechanisms.

3.2. Surface and Internal Morphologies of BN

Nanostructures

Scanning electron microscopy (SEM) analysis was used

to examine the surface morphology of BN nanostructures

produced in the present work. Figure 6 shows a SEM mic-

rograph of high-density BN nanostructures including na-

notubes, nanoparticles and nanofibers etc. produced from

a 50 h milled B powder followed by annealing at 1250˚C

under NH3. The micrograph shows that the nanotubes are

grown inside the cluster suggesting that the powder-like

cluster actually consists of large number of BN nanotubes

as well as nanoparticles. The diameter of the BN tubes

was around 20 nm - 50 nm and length several microns.

X-Ray dispersive analysis (EDS) was carried out to

measure elemental B, nitrogen, Fe and other elements.

To increase the measurement accuracy, the powdered

samples were pressed into pellets of diameter 6.00 mm

with a smooth surface before the measurement. The EDS

spectra (Figure 7) were collected from a large area of the

pallet as well as from several places on each pellet and

thus an average content of each element were obtained

Figure 6. SEM micrograph of BN nanotubes produc ed from

B powder ball milled for 50 h in NH3 and annealed at

1250˚C in NH3 for 6 hours.

S. K. SINGHAL ET AL.

124

for comparison between different samples. The EDS ana-

lysis confirmed that the nanotube consists of elemental B

and N, as indicated by the pronounced B and N peaks

shown in Figure 7.

The peaks of O and Fe are due to exposure to air and

contamination during milling from steel balls and con-

tainer. The peaks of Cr could not be seen in Figure 7. Fe

nanoparticles act as a catalyst and enhance BN nanotube

formation during the subsequent annealing. Therefore,

high-energy ball milling creates a new disordered struc-

ture mixed with metal catalysts for subsequent BN na-

notube growth. The peak of Au in the EDS spectra is

mainly due to gold thin film deposited on the samples

before SEM characterization study.

A detailed characterization employing HR-TEM equip-

ed with EDS, delineated many useful information of BN

nano-objects. Figures 8(a)-(e) shows HR-TEM images of

Figure 7. EDS spectra of BN nanotubes showing the pres-

ence of B and N and low levels of O and Fe.

(a) (b) (c)

(d) (e)

Figure 8. HR-TEM images of (a) BN nanostructures; (b) Bamboo-like BN nanotubes; (c) Enlarged view of nanotube, the inset

shows a complete filling of a compartment; (d) Multi walled structure of a nanotube; and (e) EDS spectrum.

S. K. SINGHAL ET AL. 125

entional bright field elec-

tro

) represents an individual BN nanotube,

he transmission elec-

tro

ure 9(a). Figure 9(c) reprnts the corresponding EDS

N nanostructures (nanotubes, nanofibers, nanoparticles B

etc) produced from B powder milled for 50 h and annea-

led at 1250˚C in NH3 for 6 h.

Figure 8(a) exhibits a conv

n micrograph revealing the presence of BN nano-

tubes of diameter 20 to 50 nm. These nanotubes are aggre-

gated with ultrafine-sized nanoparticles of size less than

100 nm. Figure 8(b) shows an individual BN nano- tube

having bamboo-like morphology with different com-

partments (~50 nm - 70 nm). It has been understood that

the compartments that could have formed by the expul-

sion of the catalyst or the detachment of tubular structure

from the catalyst [29]. The molten catalyst is occasion-

ally partially trapped inside the bamboo-like nanotubes

as shown in Figure 8(a), and may be responsible for the

formation of these nanostructures via an improved

stress-induced sequential growth mechanism. The overall

diameter of these nanotubes was found to be almost uni-

form throughout the length, and are different from spin-

dle-like morphology observed by Huo et al. [30] from the

nitridation of Fe-B nanoparticles at 1100˚C in the pres-

ence of a mixture of nitrogen and ammonia where the

tube diameter was found to decrease along the growth

direction, although the growth mechanism for the forma-

tion of these defective BN nanostructures was al- most

similar and follow the stress-induced sequential growth

mechanism.

Figure 8(c

amined at higher magnification. Fe nanoparticles were

found to entrap between the walls of nanotubes as con-

firmed by EDS analysis. An inset in Figure 8(c) further

elucidated a complete filling of a compartment by Fe.

Figure 8(d) shows the enlarged view of the bamboo-like

BN nanotube, revealing the structure of the tip of the

tube in detail. The image shows that the nanotube is well

crystallized with the multiwalled structure on both sides

of tube with lattice spacing between two neighboring

fringes of ~0.34 nm, corresponding to d0002 spacing in

bulk hexagonal BN (0.333 nm) [31]. The tip of the cor-

responding nanotube was found to be rectangular in

shape (Figure 8(d)). Figure 8(e) represents the EDS

analysis of the tube nanostructures (as observed in Fig-

ure 8(c) such as BN nanotubes filled with catalyst nano-

particles, BN nanofibers, catalyst nanoparticles etc. The

spectrum shows some additional peaks of Fe, Cr etc in

addition to the peaks observed for B, N, C and O. The

weak signal at ~530 eV is due to the oxygen adsorption

during sample transfer through air.

Figures 9(a) an d (b) represents t

n micrographs of surface modified BN nanotubes pro-

duced in the present work. Figure 9(b) shows the enlar-

ge view of the one of the nanotubes as displayed in Fig-

ese

(a)

(b)

(c)

Figure 9. HR-TEM image of (a) Surface modified BN nano-

tubes; (b) Enlarged view; and (c) EDS spectrum of surface

modified BN nanotubes.

S. K. SINGHAL ET AL.

126

Bigated by several investiga-

spectrum of the surface modified BN nanotubes. These

nanostructures consist of nanotubes, nanofibers and to some

extent nanosheets exhibiting large surface area. The nano-

fibers attached to the surface of the tubes are different

from tubes in that they do not have any hollow cores and

thus, results an extraordinary increased surface area. The

Brunauer, Emmett and Teller (BET) measurements have

shown that BN nanofibers have a specific surface area

~260 m2/g in comparison with ~210 and ~150 m2/g in

bamboo-like nanotubes and in multi walled nanotubes,

respectively. The nanofibers as shown in Figure 9(b)

usually exhibited a defective structure and rough surface.

The surface feature is somewhat to that of bamboo-like

nanotubes. However, for BN nanofibers as shown in Fig-

ures 9(a) and (b) all the BN layers may be considered to

be open on the external surface. This feature increases

the surface area of the material and also decreases the

energy barrier for trapping H2 due to the existence of

“wedge” channels onto the exterior surface. The BN lay-

ers were separated by 0.34 nm as shown in Figure 8(d),

which is slightly larger than the kinetic diameter, 0.29

nm, of H2 molecules. Under high pressures, the open-e

nded BN layers may be assumed to be major channels

for encapsulating H2; a mechanism similar to the path-

ways proposed for H2 storage in C nanofibers [32]. Un-

like CNTs, whose non-polar C-C bonds form a uniform

and smooth sidewall structures, boron nitride nanotubes

contain hetropolar B-N bonding [33] with ionic character.

Here it is mentioned that such surface modified or col-

lapsed BN nanotubes have also been synthesized earlier

by Terao et al. [24] using carbon-free CVD method us-

ing a mixture of B, MgO, SnO and ZnS powders and

heating them at 1450˚C in the presence of ammonia for 2

h, the overall yield of these nanotubes obtained was still

very low. However, in the present process, we have been

able to achieve a high yield of surface modified BN

nanotubes for the first time using mechanothermal proc-

ess. In our subsequent studies, we are in the process of

optimizing synthesis conditions to obtain single phase of

these nanotubes in throughout the product.

3.3. Influence of Luminescence Due to Different

BN Nanostructures

In addition to the studies of hydrogen storage capacity of

nanostructures as investN

tors [8-11], the BN nanostructures were also analyzed

employing photoluminescence (PL) spectroscopy in the

present work. Figure 10 shows the PL spectra of one

typical sample of BN nanotubes produced in the present

study. The spectra has one main emission peak centered

around 340 nm, and two small emission peaks around

515 and 525 nm respectively which are the characteris-

Figure 10. PL spectrum of BN nanotubes.

tics PL emissions associated with hexagonal bori-

tride strnd 515

nd 535 nm are presumably due to small band gap of BN

The present method demonstrates the synthesis of sur-

nanotubes coexisting with conven-

tional nanotubes, from the mechanical alloying of ele-

The authors are grateful to the Director, National Physi-

or his permission to publish

thresults reported in the present work. Sincere thanks

on n

ucture [34]. The emission peaks at arou

nanotubes produced in this work. It has been shown by

Yu et al. [35] that the band gap of BN nanotubes be-

comes smaller when the tube diameter decreases. It is

likely that at lower milling time used in the present work,

nanotubes with small diameter (band gap 2.32 eV) pre-

dominate. The small diameter of the nanotubes produced

in the present study has also been confirmed by SEM and

HRTEM results shown in Figures 6 and 8 respectively.

4. Conclusions

face modified BN

mental boron powder in the presence of a controlled at-

mosphere followed by annealing in NH3 for 6 h. These

nanotubes have been found to exhibit with large surface

area than the well crystalline BN nanotubes and could

have many applications including the hydrogen storage.

Although these surface modified BN nanotubes could

also be produced from CVD in- volving various chemi-

cal systems, the yield produced using these methods is

not satisfactory. However, using the present method one

can synthesize a very high yield of surface modified BN

nanotubes. Photoluminescence measurements led to pos-

sible band gap engineering of BN nanostructures based

on the morphology and diameter of the product phase.

5. Acknowledgements

cal Laboratory, New Delhi f

are due to Mr. K.N. Sood and Ms Arpita Vajpayee for

S. K. SINGHAL ET AL. 127

] C. H. Lee, J. Wang, V. K. Kayatsha, J. Y. Huang and Y.

of Boron Nitride Nanotubes

Deposition,” Nanotechnol-

their help in SEM and XRD characterization of BN na-

notubes produced in this work.

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