2016

Author: Hanan Maryil

[BORON NITRIDE NANOTUBES: THE FUTURE IS HERE] Abstract: Nanotechnology is one of the most futuristic fields of our time. The many applications of the different nanomaterials that are studied in this field show much promise. This research paper attempts to delve deeper into the realm of the unknown, where much promise is being shown. The science of tiny things is a vast area, where there are many things to be discovered. The boron nitride nanotube (BNNT) is a relatively novel discovery that is intended to be an alternative to the Carbon Nanotube (CNT). It is structurally similar to the CNT, but instead of hexagonal Carbon covalent bonding, the BNNT is composed of alternative hexagonal Boron and Nitrogen bonding (Figure 1). The BNNT was first successfully synthesised in 1995, but 14 years before this, one-dimensional Boron Nitride structures were discovered that possessed a bamboo-like structure. Many factors make the Boron Nitride Nanotube advantageous over the Carbon Nanotube for various applications, such as its physical properties. The potential applications of Boron Nitride Nanotubes are endless because of this various chemical and physical properties.

2 Table of Contents

INTRODUCTION ................................................................................................................................. 3

COMPARISION: BNNTS VS. CNTS ........................................................................................................ 4

APPLICATIONS OF BNNTS ................................................................................................................... 5

SYNTHESIS OF BNNT ........................................................................................................................... 6

CONCLUSION ..................................................................................................................................... 9

WORKS CITED .................................................................................................................................. 10

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Introduction

There are many differences between the older Carbon Nanotube and the Boron Nitride

Nanotube, some of which are discussed below. The CNT is was first theorised in the late 1950s,

therefore, it a relatively old nanomaterial. The BNNT on the other hand, is much newer, having been

first theorised in the 1980s. The CNT has some metallic properties and therefore is semiconducting

(small band gap), whereas the BNNT is an electrical insulator (wide band gap). The carbon nanotube is

hailed as one of the strongest materials, stronger than diamonds itself. But compared to the BNNT its

strength is relatively inferior.

The applications of Boron Nitride Nanotubes are endless. One of the most promising

applications of BNNTs is for space purposes. The carbon nanotube was hoped to be used for space

exploration, and research is still being continued on that front, but because of several reasons, the BNNT

is more ideal for this purpose. One reason is because of its increased tenacity and durability. One space

application that is being researched is that of a space elevator. For this, a "wire" of sorts is required, and

the BNNT is superb in strength abilities, so much so that it will be able to withstand the strength and

weight requirements. Another reason why the BNNT is optimal for space applications is because of its

neutron and UV radiation absorption abilities. This capability will protect crew and equipment from

unprotected radiation in space. This is

just one of the many applications of the

Boron Nitride Nanotube.

The processes of synthesising

BNNTs are still being developed, but

there are several different ways that

scientists are currently doing so. Two of

the broadest categories in the synthesis

of these nanotubes are low-

temperature synthesis and high-

temperature synthesis. Two of the most

common types of low-temperature

synthesis are ball-milling and chemical

vapour deposition. These types of low-

temperature synthesis can produce

hundreds of milligrammes, even

grammes of BNNTs. There are also two types of high-temperature synthesis, which are arc-discharge

and laser heating methods. This type of synthesis produces high-quality BNNTs that are much more

lacking in defects compared to low-temperature synthesis.

The Boron Nitride nanotube is one of the most promising advances in nanotechnology in awhile.

Its potential applications and those applications that are yet to be found show so much promise for the

future. As research techniques, progress and new discoveries are found, the applications of the BNNT

will be brought into consumer production.

Image of BNNT through a STM

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Comparison Boron Nitride Nanotubes vs. Carbon Nanotubes

Just as the possibilities of Carbon Nanotubes are highly promising, the potential of Boron Nitride

Nanotubes has many possibilities for real-world applications. The difference is, that BNNTs are an

enhanced version of the CNT, which gives it more benefits over the CNT. A major difference between

these two nanomaterials is its structure. Carbon Nanotubes are made up of only Carbon atoms, in a

hexagonal pattern. The Boron Nitride Nanotube is made up of alternating Boron and Nitride atoms, also

in a hexagonal pattern. The most

obvious difference between BNNTs and

CNTs is their appearance. Because of

the CNTs composition, it is entirely

black, while the BNNT is pure white or

sometimes slightly yellow. Although

white is the customary colour of BN

Nanotubes, it can be made transparent

or dyed different colours, while the

colour of the CNT cannot be changed.

Carbon Nanotubes have high electrical conductivity whereas the BN Nanotube has more

insulator properties. An electrical conductor is defined as an object or type of material that allow the

flow of electrical current in one or more directions. An electrical insulator is a material that resists the

flow of electric charge. Because of the differences in electrical conductivity, BNNTs have some

advantages over the CNT in applications that make use of its insulating capabilities, such as for hydrogen

storage. During the production and synthesis of a product, especially one in the nanoscale, its chemical

and thermal stability is crucial. Oxidation of CNTs starts at 500°C, which results in a steep weight loss

between ≈500°C and ≈700°C. On the other hand, the Boron Nitride Nanotube does not have much

weight loss and real change in its weight loss is only noticed once it reaches ≈950°C. Once BNNTs begin

to go above 950°C, Boron Trioxide begins to form, and as it continues above 1200°C, it begins to

vaporise. These properties of BN Nanotubes, show itself much better in its thermal and chemical

stabilities than the Carbon Nanotubes. Another difference between the CNT and BNNT is its

luminescence. Luminescence is the emission of light by a substance that is not a result of heat. Boron

Nitride Nanotubes have violet or ultraviolet luminescence, which is caused by the excitation of its

electrons or photons, and its wavelength is 220-460 nm. The CNT has an infrared luminescence, and its

wavelength is 800-1700 nm.

Carbon Nanotubes, when first being popularised, was touted as the next big thing, with

applications ranging from ultralight planes, to "super-batteries", to space elevators. These applications

of CNTs were so promising because it had proven to be more than a 100 times stronger than steel, while

one-sixth the weight. When the BN Nanotube began to gain popularity as a replacement to the CNT,

Canadian researchers took the forefront of the effort to magnify its potential capabilities. Because of its

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heat resistance, BNNTs are more effective in various applications, where flame-resistance or radiation

resistance is required. Using these nanotubes rather than CNTs in several applications would greatly

enhance its promise in several ways. For example, using BNNTs in the "ultralight planes", would not only

have the strength and lightness that CNTs have, but it would also provide greater heat resistance,

especially when the planes are going at such high speeds, where many factors such as friction, rapidly

increase the surface temperature of the plane. In space elevators also, BNNTs would be more practical,

because it would resist against the many radiations present in outer space, and would, therefore, last

much longer. The many possibilities for future applications of BN Nanotubes are endless, just like the

CNT, but for many applications, it is more intuitive to use the BN Nanotube.

Applications of Boron Nitride Nanotubes

When the Carbon Nanotube first entered the nanotechnology scene, its potential applications

were proclaimed as the next big thing. The limits were stretched, the possibilities seemed so endless,

and many could see its possibilities in many aspects, not only scientifically, but also in other industries. It

is true, the possibilities of the Carbon Nanotube, are boundless, but the discovery of the Boron Nitride

Nanotube, gave a new perspective to the endless possibilities of nanomaterials. Many scientists and

researchers who had given the Carbon Nanotube all this attention as a result of its surprising

capabilities, now viewed the Boron Nanotube as the reincarnation of one of the greatest discoveries in

Nanotechnology. As discussed in the previous section, several physical and chemical properties placed

the BNNT in a better position for many applications. Although the prospects of these Nanotubes are

interminable, there are possible safety concerns, such as their toxicity and how they interact and react

with humans that must be further researched before they are actually introduced into the marketplace.

Also, before they can be introduced, scientists must first come up with more effective synthesis

methods, or further advancements in the current methods that are being used. Two of the most

promising applications of Boron Nitride Nanotubes are for hydrogen storage and drug delivery.

One of the most researched applications of BNNTs is for hydrogen storage. Hydrogen is one of

the most promising clean energy sources that have high yields. Currently, there are many methods to

produce hydrogen as an energy source, but storage and transportation is a major problem. Because of

this obstacle to the use of hydrogen for energy, researchers began to study Boron Nitride Nanotubes as

a possible carrier. Investigations conducted showed that the bond length between Boron and Nitrogen

in BNNTs were larger than that of Carbon in CNTs. This indicated that hydrogen penetration into the

BNNT would occur much faster than that through the CNTs walls. Because of the higher

electronegativity of nitrogen, BNNTs absorbed or stored these hydrogen materials very well. BNNTs

have proven to be a very likely candidate for the storage of hydrogen.

In this day and age, the medical field is one of the vast frontiers still open for research

breakthroughs. Drug delivery is a very auspicious application for BNNTs, not only because of its very

small size but also because of their different capabilities. The magnetic properties of specially

synthesised BNNTs and its ability to bind molecules on its large surface area proved the potentiality of

BNNTs for drug delivery purposes. This type of BNNT was synthesised using Iron as a catalyst, which

increased its magnetic properties. The BNNTs were then coated with PEI and had QDs attached to them

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so that they could be tracked. Although there is much promise for BNNTs in this particular area, there is

still much more research to be done on the safety of BNNTs especially when coming into direct contact

with humans.

Synthesis of Boron Nitride Nanotubes

For Boron Nitride Nanotubes to be used in society today, a high yield synthesis process must be

created, so that it can be mass produced and used in various applications. There are several methods of

Boron Nitride Nanotube synthesis that are being used today. The processes of synthesising Boron

Nitride Nanotubes vary greatly, but currently, five of the most promising are arc-discharge, laser

ablation, template synthesis, ball milling, and Chemical Vapor Disposition (CVD).

Arc Discharge, although not the most common type of BNNT synthesis method, was first used to

fabricate these BNNTs. This process includes using arc discharge

between a Boron Nitride packed tungsten rod and a cooled copper

electrode. The product of this process was Multiwalled Boron Nitride

Nanotubes with a diameter between 1 and 3 nanometers. As the

development of arc discharge continued, different methods were

introduced that produced both single walled and multi-walled

BNNTs. The original arc-discharge experiments used BN as

electrodes, but these materials are insulating, therefore conductive

boron materials were used in following reiterations of this type of

synthesis. Some examples of boron materials that were used are

ZrB2 and YB6. Nitrogen gas was used as a nitrogen source for the

resultant compound. The high growth temperature of this particular

process, around 3000 Kelvin, provides the BNNTs with superb crystallisation properties.

A second synthesis process of Boron Nitride Nanotubes is laser ablation. In this process, Carbon

Dioxide lasers were aimed at single crystal cubic Boron Nitride targets to heat them up at a very high

temperature of 5000 Kelvin. In this melted layer that is created from this heating, multi-walled BNNTs

are harvested. As experimental procedures further developed, h-BN was used in place of the regular BN

targets, and this produced both SWBNNTs and MWBNNTs. Laser ablation was observed to produce near

perfect crystalline structures, and with the use of Cobalt or Nitrogen as catalysts, longer tubes were

created. A relatively recent modification of the laser ablation process called pressurised

vapour/condenser method, is proving to be a very possible to the solution to the inability to produce

quality high yield sets of NNTs. Boron vapour is created by heating hot or cold- pressed BN, amorphous

B powders and cast B to a temperature of over 4000oC. Compressed nitrogen gas can also be used as a

nitrogen source.

In the fabrication of Boron Nitride Nanotubes, there are two methods of template synthesis

used, CNT substitution reaction and porous filter membrane synthesis. In the CNT substitution reaction,

Carbon Nanotubes are combined with Boron Trioxide and nitrogen or ammonia is used as a nitrogen

source for the reaction. The product of this reaction is a BCN combination at varying chemical ratios.

Arc Discharge synthesis method

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Oxidation is used in an attempt to remove the excess Carbon, but the carbon in the lattice structure of

the nanotube is very difficult to remove, and thereby, on most occasions the final product is tainted with

Carbon. One way to increase the yield of Boron Nitride Nanotubes using this synthesis method was to

add metal oxides to the reaction.

Ball-milling as a synthesis method that is used in studies to obtain a high yield of BNNTs. This

ball-milling method increases the surface area and brings the catalyst, boron, and nitrogen into as much

contact as possible. This is done through the high transfers of mechanical energy to the boron powder.

Although this form of synthesis is known for its high yield produce, its purity is a major issue.

Experiments with this particular process have been ongoing, with attempts at increasing the overall

purity of the product fairly futile. Small advances have been made, but not enough to be of

groundbreaking significance. Using the boron powder and introducing Ammonia during the ball-milling

process. This optimisation of the synthesis process increased the yield of BNNTs while also reducing

their nanometers to around 10 nanometers. Despite the optimisation, the purity of the product

remained disappointingly low. Another improvement in the process was to use boron powder and Ferric

Nitrate nonahydrate in the ball milling process. This increases the yield of the end product but still does

not have a considerable improvement of its purity.

Ball-milling synthesis method

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Chemical Vapor Disposition (CVD) is currently the most commonly used synthesis process for

both Carbon Nanotubes and Boron Nitride Nanotubes. This process combines gaseous reactants in a

chamber set at ambient temperature. These gases, when coming into contact with the substrate within

the heated chamber, instigates a reaction that creates a film on the substrate surface. Using CVD to

deposit materials onto the substrate results in a much higher quality of the resulting substance. There

are many different substrings under this broad synthesis method, but two of the most widely and

modern methods are Low-Pressure Chemical Vapour Disposition (LPCVD) and Ultra-High Vacuum

Chemical Vapour Disposition (UHVCVD). As the name suggests, Low-Pressure CVD occurs in sub-

atmospheric pressured environments. This helps produce a uniform thickness of the substance on the

substrate while preventing unwanted reactions. UHVCVD occurs at very low pressures, between 10 - 6

Pascals. CVD growth of BNNTs began with the use of Borazine along with Cobalt, Nitrogen, Nickel

Boride, and Nickel Nitride as effective catalysts. A process that helps preserve BNNT purity is known as

Boron Oxide CVD, which consists of separating boron powder and metal oxide from the BNNTs during

growth.

CVD synthesis methods with multiple SEM images

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Conclusion

The properties of Boron Nitride Nanotubes make them superior to CNTs for many applications

that are instrumental in the advancement of nanotechnology in the market of the future. The Carbon

Nanotube is still very relevant in the nanotechnology field, but the BNNT has proven to be a very worthy

competitor to it. As BNNTs continue to be researched for the potential that it may have along with other

factors, it can only grow and will likely take over the Carbon Nanotube as the most versatile

nanomaterial. As synthesis methods of these nanomaterials continue to develop and advance through

research and testing, the Boron Nitride Nanotube will continue to become a bigger factor in many of the

applications that were discussed in this paper. The promise shown by the BNNT as a potential

replacement for CNTs for applications that require the many special properties that give superiority

over the CNT, allows the BNNT to become a greater factor in nanotechnology.

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