Selective Fluoride Transport in Subnanometer TiO₂ Pores

Synthesizing nanopores which mimic the functionality of ion-selective biological channels has been a challenging yet promising approach to advance technologies for precise ion-ion separations. Inspired by the facilitated fluoride (F^-) permeation in the biological fluoride channel, we designed a highly fluoride-selective TiO₂ film using the atomic layer deposition (ALD) technique. The subnanometer voids within the fabricated TiO₂ film (4 Å < d < 12 Å, with two distinct peaks at 5.5 and 6.5 Å), created by the hindered diffusion of ALD precursors (d = 7 Å), resulted in more than eight times faster permeation of sodium fluoride compared to other sodium halides. We show that the specific Ti-F interactions compensate for the energy penalty of F^- dehydration during the partitioning of F^- ions into the pore and allow for an intrapore accumulation of F^- ions. Concomitantly, the accumulation of F^- ions on the pore walls also enhances the transport of sodium (Na⁺) cations due to electrostatic interactions. Molecular dynamics simulations probing the ion concentration and mobility within the TiO₂ pore further support our proposed mechanisms for the selective F^- transport and enhanced Na⁺ permeation in the TiO₂ film. Overall, our work provides insights toward the design of ion-selective nanopores using the ALD technique.