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.