Boron Nitride nanotubes

Boron nitride nanotubes (BNNTs) are known as structural analogs of carbon nanotubes (CNTs) but with superior properties. Although they have structural similarities, they significantly differ in their chemical and physical properties. In contrast to CNTs, their electrical properties are not dependent on their chirality and diameter since they have a large band gap of about 5.5 eV. BNNTs also have excellent radiation shielding properties when compared to CNTs. Since the BNNTs are composed of B and N atoms, their electronic structures are expected to be rather different from that of CNTs. The charge distribution is asymmetric in B–N bonds in BNNTs as compared to the C–C bonds in CNTs. The electron density of B is attracted to the N atoms due to its higher electronegativity. Thus, the B–N bonds have a partially ionic character, which causes a gap between the valence and conduction bands. Therefore, the B–N bonds behave as a wide band gap semiconductor. Some relevant properties of BNNTs are as follows: high hydrophobicity, resistance to oxidation and heat, high hydrogen storage capacity and radiation absorption. Their electrical insulation is indeed very high, despite a high thermal conductivity. Due to these properties, they can be used in a wide range of applications. BNNTs can resist oxidation in air up to 1000 °C while CNTs are resistant only up to 500 °C under the same conditions. This makes BNNTs useful additives to increase stability against the oxidation of surfaces. Due to their highly hydrophobic character, BNNTs were also used to prepare super hydrophobic surfaces. A hydrophobic surface was prepared by the synthesis of BNNTs on the surface of a stainless steel substrate where the contact angle was found to be more than 170℃. The origin of this super hydrophobicity was attributed to the surface morphology and adsorption capacity of BNNTs for airborne molecules.

BNNTs were also used to prepare composite materials to enhance their physical properties. Bansal et al. fabricated a glass composite by adding 4 wt % BNNTs and measured the strength and fracture toughness as 90% and 35%, respectively, which were greater than that of the constituents. BNNTs also have significant hydrogen storage capacity, which was measured as 0.85 wt % – two times larger than that of the commercial CNTs.

The use of BNNTs in medical and biomedical applications has also been increasingly investigated. Their hydrophobicity and toxicity concerns are the two factors that may limit their use in such applications. Due to their high hydrophobicity, BNNTs can only be used in biological applications after noncovalent or covalent modifications to increase their water dispersibility. Thus, they have been modified with several surface modifiers such as PEGylated phospholipids, and molecules of biological origin including DNA, proteins, and flavin mononucleotides (FMN).