Abstract
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.
Highlights
Counterpart, the key to the function of Furthering the promise of graphene-based planar nanofluidic devices as the nanofluidic diode is its ability to recflexible, robust, low cost, and facile large-scale alternatives to conventional tify the current such that ion transport is 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
In structurally/geometrically symmetric graphene oxide (GO) films, it was recently found that ion current rectification arises—quite unexpectedly given the absence of any apparent asymmetry in the system—from counter-ion trapping within the film during the positive bias cycle that retards ion migration through the negatively-charged sheets, and their corresponding release during the negative bias cycle that leads to an enhanced electromigration flux through the sheets.[30]
We show here that it is possible to increase the degree of rectification in the GO film by introducing further asymmetry to the system in a variety of ways, which we examine in turn: the film geometry (Section 3.1), solution pH (Section 3.2), and solution concentration (Section 3.3)
Summary
In structurally/geometrically symmetric GO films, it was recently found that ion current rectification arises—quite unexpectedly given the absence of any apparent asymmetry in the system—from counter-ion trapping within the film during the positive bias cycle that retards ion migration through the negatively-charged sheets, and their corresponding release during the negative bias cycle that leads to an enhanced electromigration flux through the sheets.[30]. It is possible that the nonuniform electric field that arises within the sheets due to the tortuosity in the spaces between them contributes to the inherent asymmetry of the system, this appears to be a secondary effect. While swapping the working and counter electrodes with respect to the film did not lead to any differences for the geometrically symmetric film,[30] we observe in contrast in Figure 2 that effects of the geometric asymmetry in the film (i.e., the saturation in the current at lower bias voltages and the lower limiting current values at the tapered end, the reason for which we elucidate below) are preserved, as would be expected. We note that only the polarity of the current inverts; the magnitude of the current at a given voltage remains similar, which is
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