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 TiO 2 film using the atomic layer deposition (ALD) technique. The subnanometer voids within the fabricated TiO 2 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 TiO 2 pore further support our proposed mechanisms for the selective F – transport and enhanced Na + permeation in the TiO 2 film. Overall, our work provides insights toward the design of ion-selective nanopores using the ALD technique.