Boron Nitride Nanotube Impurity Detection and Purity Verification

Interest in the boron nitride nanotube (BNNT) composites has grown exponentially because of BNNTs’ unique properties. The vast majority of papers have used a BNNT material without identifying its purity. Impurities are known to misrepresent the BNNT concentration effect on performance properties and can have their own property effect on performance. A recent paper reported a major improvement in a biological application after purification. This demonstrates the need for a discussion of the optimum procedures to detect each impurity and quantify its degree of removal after a purification process. Here we evaluate the ability of thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction, and nonresonant Raman to detect BNNT’s boron, boron oxide, and varying forms of boron nitride impurities. We discover that unstacked boron nitride impurities are present and detected in BNNT using nonresonant Raman. Stacked crystalline hexagonal boron nitride (h-BN) is separately detected using X-ray diffraction. We show that, in contrast to aggressive high temperature purification methods that attack the boron nitride tubes, a low temperature hydrocarbon purification procedure removes both unstacked boron nitride impurities as well as stacked h-BN. We demonstrated the ability of nonresonant Raman spectra to detect and quantify the degree of removal of both forms of unstacked and stacked boron nitride impurities, of the ability of TGA to detect and quantitively verify the absence of boron, and of FTIR techniques to detect and quantitavely verify the degree of removal of boron oxide.