Center for Functional Nanomaterials Seminar

Recent nanofluidic experiments with single or few nanopores in graphene, molybdenum disulfide and hexagonal boron nitride have shown unique fluidic transport properties and the potential for electrokinetic energy conversion with unprecedented power densities. In such nanopores, the high-surface charge makes possible a diffusio-osmotic mechanism for ion-selective transport, distinct from the Donnan exclusion that is typical of conventional membranes. However, no such macroscopic BNNT membranes have ever been fabricated, and their performance at large scales and pore densities is unknown. Thus, we seek to devise scalable means to manufacture such large-area nanotube membranes and field-assisted alignment and assembly methods of nanotubes into functional devices.

Here, we describe the fabrication of the first-ever macroscopic vertically aligned- (VA-) BNNT membranes, and our study of their ion-selectivity mechanisms and osmotic-power-generation performance. The membranes are fabricated with a unique solution-based technique in which BNNTs are aligned, assembled, and concentrated in a liquid oligomer with magnetic fields, and then locked in place by in-situ polymerization. With this scalable technique, we have demonstrated the fabrication of VA-BNNT membranes of size 1-10 cm2 and having tube densities of 108-109 BNNTs/cm2 and open pore densities on the order of 107 pores/cm2. To further elucidate the mechanism(s) for the ion selectivity and transport, we compare the power generation and transport rates of the VA-BNNT membranes for salts having different cation and anion diffusivities and thus diffusio-osmotic parameters. We also compare field-assisted assembly of BNNTs with their carbon-based counterparts, CNT-membranes, as well as boron-nitride-nanopore membranes (BNAAO)s fabricated with a less-scalable CVD technique.