In-Flight Plasma Functionalization of Boron Nitride Nanotubes with Ammonia for Composite Applications

Surface functionalization is an essential step to successfully harness the properties of boron nitride nanotubes (BNNTs) in several applications. Currently available functionalization methods are prohibitively costly for commercial use and have significant environmental impacts. Here, we show that a low-pressure, capacitively coupled radio-frequency (RF, 13.56 MHz) glow discharge plasma reactor can be used to effectively carry out surface chemical functionalization of BNNTs. This low-pressure vacuum and gas handling system incorporates a solenoid valve to periodically fluidize the free-standing BNNTs, which provides for functionalizing BNNTs in-flight. Ammonia, which generates ·H, ·NH, and ·NH2 radicals by energetic electron impact, was employed as the functionalization gas. The functionalization begins by anchoring the ·H, ·NH, and ·NH2 radicals to the surface of BNNTs, which is promoted by the presence of free electrons forming reduced BNNTs. On the basis of density functional theory (DFT) calculations, we propose a mechanism wherein BNNTs are functionalized to the final product of BNNT–NH3, a weakly bonded complex, through subsequent cascade reactions with H radicals abundant in the plasma reactor. BNNT surface functionalization with ammonia was confirmed by tandem thermogravimetric analysis–infrared spectroscopy (TGA-IR), Fourier transform infrared (FTIR) spectroscopy, dynamic vapor sorption (DVS), our recently reported BNNT quality assessment method based on the formation of regiorandom poly(3-hexylthiophene) (rra-P3HT) aggregates on BNNTs, and solubility in water. This solvent-free and environmentally friendly approach can be applied by using other functionalizing gases, and the in-flight functionalization of fluidized BNNT powder is also advantageous in terms of scalability. These features offer the potential to dramatically advance BNNT composites research efforts and the development of BNNT applications.