Two-Dimensional Nanochannel Arrays Based on Flexible Montmorillonite Membranes

Two-dimensional (2D) nanochannel arrays are constructed by bottom-up reassembly of montmorillonite monolayers that are obtained by liquid-phase exfoliation of its layered crystals, and the as-constructed interstitial space between these monolayers is uniform and provides ions with nanoscale transport channels. Surface-charge-controlled ion transport behavior is observed through these nanochannels as the electrolyte concentration reduces to 10^-4 M at room temperature. Furthermore, the nanochannel structure remains even after 400 °C heat treatment, and nanofluidic devices based on the annealed nanochannel arrays still exhibit surface-charge-governed ion transport at low electrolyte concentrations. In addition, a drift-diffusion experiment is conducted to investigate the mobility ratio of cations/anions through the nanochannels with asymmetric bulk electrolyte concentrations, and the results show that the mobility of cations is about eight to nine times that of anions, which is consistent with the fact that the montmorillonite monolayers are negatively charged and the nanochannels are permselective. Last, ionic current rectification is observed in the nanofluidic system of asymmetric geometric shape, and rectification factors of ∼2.6 and ∼3.5 can be obtained in KCl and HCl electrolytes, respectively, at a bias between -1 and +1 V because of the asymmetric electrostatic potential through the nanochannels.