Reactivity of Boron Nitride Nanomaterials with Phosphoric Acid and Its Application in the Purification of Boron Nitride Nanotubes

Boron nitride nanotubes (BNNTs) offer a compelling promise in 21st century materials due to their high-tensile strength, thermal conductivity, chemical stability, and insulating properties. Current synthetic methods result in the formation of many non-nanotube byproduct structures with similar boron nitride (BN) connectivity, which makes their removal difficult. As a result, there is considerable focus on developing purification methods that produce neat BNNTs. Here, we present a new purification process that removes impurities to produce BNNTs with high purity. Employing the intercalating nature of phosphoric acid and taking advantage of the wet thermal etching of boron nitride, our strategy combines phosphoric and hydrochloric acids with samples containing a mixture of BN materials (such as BNNTs and BN nanocages) inside a constant volume reaction setup in a furnace. The reaction removes impurities such as nanocages, hexagonal boron nitride (hBN), and less ordered BN material. Overall, this method can result in mass yields up to 29% purified material, a result that is highly dependent on the concentration of impurities in the starting material. This result is confirmed using scanning electron microscopy (SEM). Analyses using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and infrared (IR) spectroscopy demonstrate that there are not significant chemical changes to the BNNTs after the reaction, which produces BNNTs that are highly enriched and purified.