Selective Fluoride Transport in Subnanometer TiO2 Pores
XC Zhou and M Heiranian and MQ Yang and R Epsztein and K Gong and CE White and S Hu and JH Kim and M Elimelech, ACS NANO, 15, 16828-16838 (2021).
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 angstrom < d < 12 angstrom, with two distinct peaks at 5.5 and 6.5 angstrom), created by the hindered diffusion of ALD precursors (d = 7 angstrom), 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.
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