A two-dimensional ion-pump of a vanadium pentoxide nanofluidic membrane

Abstract

The reactive surface and layered crystal structure of vanadium pentoxide (V₂O₅) are exploited here to prepare a two-dimensional (2D) ion pump that transports ions against their concentration gradient. The exfoliated layers of V₂O₅ were assembled into membrane form to create ion-channels with excellent nanofluidic transport characteristics. At the surface-charge-governed regime, the flexible and freestanding membrane of V₂O₅ showed a remarkable proton conductivity (∼0.01 S cm⁻¹). The activation energy of proton conductivity (0.066 eV) suggests that the exceptional mobility of H⁺ ions (5.2 × 10⁻³ cm² V⁻¹ s⁻¹) inside V₂O₅ ion-channels originates from the coordinated hopping of charges between the two-dimensionally arranged water molecules. The transport characteristics of V₂O₅ ion-channels can also be tuned just by tailor-cutting its lamellar membranes into different shapes. While rectangular devices of V₂O₅ membranes exhibit linear I–V curves, the triangularly cut membranes display a diode-like non-linear I–V curve. The ionic current rectification in the V₂O₅ triangle was found to originate from a combination of the unipolar conductivity of counter-ions inside the 2D nanochannels and geometrical asymmetry. The 2D ion rectifier of V₂O₅ also pumps ions at the rate of 3.32 × 10⁻⁸ amp s⁻¹ against a 1000-fold concentration gradient under a fluctuating external potential with zero mean.