Boron nitride nanomaterials : from fabrication to characterization

Abstract

Boron nitride nanotubes (BNNTs) and ultra thin BN sheets are two of the most important next generation nanomaterials. The synthesis and study of them are of both fundamental interest and technological importance. Improved methods of fabricating BNNTs and ultra thin BN sheets are presented in this thesis together with studies that characterize their properties and indicate potential applications. The production of high purity BNNTs in large quantities has been a major obstacle inhibiting the broad study and application of BNNTs. A new approach, involving a liquid boron (B) ink, has been developed to produce higher purity and larger quantities of BNNTs. In the new method, the ball milled B particles are homogeneously mixed with ethanol and metal nitrate in an ultrasonic bath. The ink-like liquid mixture, or B ink, is annealed in nitrogen containing gas to synthesize 0.5g of high purity BNNTs at one time. This significant invention has brought the practical application of BNNTs one step closer. As an example, bulk sized (not film shaped) BNNT reinforced polyurethane (PU) composites have been produced. The growth of nanotube films on various substrates and surfaces is vital for a range of applications. However, unlike the sophisticated carbon nanotube (CNT) film production, patterned growth of BNNTs over large areas had not previously been achieved. The new B ink method also overcomes this obstacle by providing a simple but powerful way to grow BNNT films on different substrates. It is demonstrated that the ink can be directly painted/brushed, sprayed, or even inkjet printed on different substrates with any desired pattern and the ink layer is converted to a film of high density BNNTs by the annealing. This is the first method capable of growing BNNTs on non-flat or complex surfaces. Because the coating method involves direct growth of nanotubes, the BNNT films have strong adhesion to the substrate, which is very desirable for practical applications and a number of potential applications of such coatings are demonstrated. The larger quantities and particularly the higher purity of the BNNTs grown have enabled new properties of BNNTs to be discovered. For example, for the first time, single deep ultraviolet (DUV) emission at 225nm (5.51eV) has been found from BNNTs with a cylindrical multiwall structure and diameters smaller than 10nm. This finding suggests that BNNTs can work as nanoscale DUV light emitters opening up potential applications in miniaturized optic and optoelectronic devices as well as in biomedical analysis and treatment. In addition, many intriguing anti-wetting properties have been demonstrated for these BNNT films. In addition to BNNTs, the ball milling method was modified to allow ultra thin BN sheets to be produced in large quantities. Investigations have shown that small sheets, as thin as 2nm, can be produced, and the in-plane structure of the sheets remains largely intact. Interestingly, the thin BN sheets exhibit better DUV luminescence than the un-milled hBN particles. Show more