  Films of titanate nanosheets (approx. 1.8-nm layer thickness and 200-nm size)
having a lamellar structure can form electrolytefilled semi-permeable channels
containing tetrabutylammonium cations. By evaporation of a colloidal solution,
persistent deposits are readily formed with approx. 10 micrometer thickness on
a 6-micrometer-thick poly(ethylene-terephthalate) (PET) substrate with a 20
micrometer diameter microhole. When immersed in aqueous solution, the titanate
nanosheets exhibit a p.z.c. of -37 mV, consistent with the formation of a
cation conducting (semi-permeable) deposit. With a sufficiently low ionic
strength in the aqueous electrolyte, ionic current rectification is observed
(cationic diode behaviour). Currents can be dissected into (i) electrolyte
cation transport, (ii) electrolyte anion transport and (iii) water heterolysis
causing additional proton transport. For all types of electrolyte cations, a
water heterolysis mechanism is observed. For Ca2+ and Mg2+ ions, water
heterolysis causes ion current blocking, presumably due to localised
hydroxide-induced precipitation processes. Aqueous NBu4+ is shown to invert the
diode effect (from cationic to anionic diode). Potential for applications in
desalination and/or ion sensing are discussed
