Furthering the promise of graphene-based planar nanofluidic devices as flexible, robust, low cost, and facile large-scale alternatives to conventional nanochannels for ion transport, we show how the nonlinear current-voltage (I-V) characteristics and ion current rectification in these platforms can be enhanced by increasing the system asymmetry. Asymmetric cuts made to the 2D multilayered graphene oxide film, for example, introduces further asymmetry to that natively inherent in the structurally symmetric system, which was recently shown to be responsible for its rectification behavior due to diffusion boundary layer fore-aft asymmetry. Supported by good agreement with theory, we attribute the enhancement to the decrease in the limiting current in the positive bias state in which counter-ion trapping occurs within the negatively charged graphene oxide sheets due to increased film permselectivity as its cross-section and surface charge distribution is altered on one end; these effects being shown to be sensitive to the electrolyte pH. Further, we show that an imbalance in the pH or concentration in the microreservoirs flanking the film can also increase asymmetry and hence rectification, in addition to displaying a host of other phenomena associated with the I-V characteristics of typical nanochannel electrokinetic systems.
Keywords: asymmetry; graphene oxide; nanochannels; nanofluidics; rectification.