Ultrafast and selective ion transport in scalably fabricated boron-nitride nanopore and nanotube membranes
Creators: name: Cetindag, Semih
Publisher: name: No Publisher Supplied
Year: 2022

description: The selective transport of ions through nanoscale pores is central to biological systems, separation processes, and energy conversion and storage. Among various nanomaterials, boron nitride in crystalline h-BN nanotube form is of great interest due to its atomic-scale smoothness, high surface charge in solution, and the 1-D confinement it provides to ions. In particular, the ultra-narrow size and high surface charge of the BNNT pores make possible a diffusio-osmotic mechanism for ion-selective transport.Herein, we present the developed nanofabrication strategies and ion-transport characteristics for boron-nitride nanopore/nanotube membranes for osmotic power generation. We first discuss surface-charge effects on the electro-orientation of insulating boron-nitride nanotubes in aqueous solutions. As an alternative nanomanufacturing strategy, we then present BN-nanopore membranes fabricated by depositing a thin h-BN layer within the pores of anodized alumina substrates. These BN-AAO membranes indicated the possibility of osmotic power generation within the non-overlapping double-layer regime.For the solution-based, scalable fabrication, we present a unique technique using external magnetic fields to align BNNTs. After the functionalization of BNNTs with iron-oxide nanoparticles, BNNTs were aligned and concentrated in a liquid oligomer with an external magnetic field, and then locked in place by in-situ polymerization. The vertically aligned BNNT membranes were fabricated with 3 nm and 12 nm nanotubes and had tube densities of 10^8-10^9 BNNTs/cm2 and open-pore densities up to 7x10^6 pores/cm2 for membranes sizes of 1-10 cm2. We found that, due to the high surface charge in their pores, the BNNT membranes were highly cation-selective, exhibiting osmotic energy-conversion efficiencies of up to 40%, and osmotic power densities (based on open pore area) of 8,000, 11,000 and 15,000 W/m2 for 1M: 1mM KCl, NaCl, and LiCl, respectively, at pH 11. We further compared these highly charged BNNT arrays with CNT arrays, which showed negligible ion selectivity and electrokinetic power generation, despite being of the same diameter. Notably, diffusion through the BNNT pores exhibited: (i) ultrafast cation diffusion that exceeded bulk diffusion by more than an order of magnitude, and (ii) relative diffusion rates for K+, Na+, and Li+ that were opposite to their mobilities in bulk solutions. These diffusion anomalies were explained by continuum modeling and MD simulations which revealed (i) enhanced transport within concentrated double layers due to diffusio-osmosis, and (ii) the faster axial diffusion of smaller Li+ cations due to better contact neutralization of OH--group-charged BN walls. Finally, we describe experimental studies of ion and fluid transport through BN-SWCNT heterostructure nanotubes. We found that the hydrodynamic slip length was an order of magnitude lower and the surface charge in aqueous solution was an order of magnitude larger for BN-SWCNTs as compared to the underlying SWCNTs. Thus, the inner graphene surface of the BN-SWCNTs showed both hydrodynamic and charge-regulation (i.e., the development of surface charge in solution) translucency to the outer h-BN layer. These results are the first demonstrations, to our knowledge, of such translucencies in BN-SWCNT heterostructures.