Theory-Guided Low-Speed Centrifugation for High-Purity Boron Nitride Nanotubes

Boron nitride nanotubes (BNNTs) are promising building blocks for next-generation technologies, yet synthesis-derived impurities and structural damage from conventional purification methods (e.g., harsh acid treatments, high-speed centrifugation) hinder their practical applications. Here, we report a theory-guided low-speed centrifugation strategy for high-purity BNNT enrichment without detectable structural damage. Leveraging fundamental sedimentation principles, this approach capitalizes on the distinct sedimentation contrast between compact impurities and high-aspect-ratio BNNTs. This mechanism enables highly selective separation, where impurities sediment rapidly while BNNTs remain suspended. Comprehensive experimental validations confirm that estimated BNNT purity approaching ∼95% is achievable, providing a favorable purity-recovery balance while preserving nanotube morphology and dispersibility under the tested conditions. The strategy exhibits versatility across tested surfactant systems in environmentally benign aqueous media, eliminating the need for strong acids or organic solvents while maintaining compatibility with ionic and nonionic stabilizers. Importantly, the purified BNNT dispersions retain their capacity for Onsager-type anisotropic assembly to form uniformly aligned architectures during subsequent gravity filtration. This offers direct opportunities for advanced anisotropic applications, including thermal management, mechanical reinforcement, and directional transport. By unifying sedimentation theory with practical processing, this work establishes low-speed centrifugation as a viable and sustainable strategy for high-aspect-ratio one-dimensional nanomaterials.