Graphene-Based Planar Nanofluidic Rectifiers

Abstract

Structurally symmetric two-dimensional multilayered graphene oxide films, which facilitate ion transport through “nanochannels” comprising the interstitial spaces between each stacked sheet within the film, are for the first time shown to exhibit peculiar ion current rectification and nonlinear current–voltage characteristics below a critical electrolyte concentration when the interstitial spacing becomes comparable to the Debye screening length such that the film becomes permselective. We attribute the unexpected rectification behavior to the fore–aft asymmetry that arises in the diffusion boundary layer on both sides of the millimeter long film upon reversal between the high resistance positive bias state and the low resistance negative bias state, the asymmetry being primarily a consequence of the trapping and release of counterions within the film, compounded by the nonuniform electric field that occurs in the tortuous nanochannels within the film. In addition to elucidating the influence of the electrolyte concentration and applied bias voltage, we demonstrate the possibility of tuning the ion selectivity and hence the rectification behavior through the solution pH. These first graphene-based nanofluidic rectifiers, which are easily synthesized, therefore offer a flexible, robust, low cost, and facile large-scale alternative to conventional nanochannels that require elaborate and sophisticated nanofabrication.

Enhanced Ion Current Rectification in 2D Graphene‐Based Nanofluidic Devices

“…Figure a, in fact, shows that the rectification factor F (the ratio of the currents measured for applied voltages with the same amplitude but opposite polarities | I (− V ) / I (+ V ) |, wherein I (− V ) and I (+ V ) are the reverse and forward currents measured at negative and positive bias voltages applied to the working electrode (WE), respectively) approximately doubles with the tapered geometry (the dependence of F on the degree of tapering will be investigated subsequently), at least initially for voltages up to ±2 V. Beyond this voltage, however, we observe the rectification behavior of the asymmetric film to decrease despite the monotonic increase in the rectification with applied voltage in its symmetric counterpart, and hence the rectification enhancement to diminish considerably. As an aside, while swapping the working and counter electrodes with respect to the film did not lead to any differences for the geometrically symmetric film, we observe in contrast in Figure that effects of the geometric asymmetry in the film (i.e., the saturation in the current at lower bias voltages and hence the lower limiting current values at the tapered end, the reason for which we elucidate below) are preserved, as would be expected. Nevertheless, we note that only the polarity of the current inverts; the magnitude of the current at a given voltage however remains similar, which is consistent with that shown previously for geometrically similar films wherein the rectification, and hence the underlying charge trapping mechanism responsible for giving rise to it, is insensitive to reversal of the electrodes …”

“…As an aside, while swapping the working and counter electrodes with respect to the film did not lead to any differences for the geometrically symmetric film, we observe in contrast in Figure that effects of the geometric asymmetry in the film (i.e., the saturation in the current at lower bias voltages and hence the lower limiting current values at the tapered end, the reason for which we elucidate below) are preserved, as would be expected. Nevertheless, we note that only the polarity of the current inverts; the magnitude of the current at a given voltage however remains similar, which is consistent with that shown previously for geometrically similar films wherein the rectification, and hence the underlying charge trapping mechanism responsible for giving rise to it, is insensitive to reversal of the electrodes …”

“…From Figure b, it can be seen that both the symmetric and asymmetric GO films exhibit such typical nonlinear I – V characteristics although we note that the asymmetric film not only possesses a lower limiting current I lim but also a shorter voltage range over which the limiting current behavior is observed, i.e., a lower critical gating voltage of around 2 V exists for the asymmetric film above which the current increases sharply (in contrast to the critical gating voltage of approximately 6 V for the symmetric case, as observed in Figure b, and the inset of Figure a); this is possibly due to the stronger electromigration flux in the film arising from the the increased intensity of the convective vortices in the secondary polarization layer as a consequence of the field focusing effect at the narrow end of the asymmetric film. Given that the primary mechanism for the symmetry breaking and hence the observed rectification behavior in the GO film lies in the existence of the asymmetric diffusion boundary layers at both ends of the film as a consequence of the counter‐ion trapping and release, it then becomes clear that the disruption in the diffusion boundary layer to form the secondary double layer above the critical gating voltage of 2 V is the reason for the observed decrease in the rectification above this voltage in Figure a. A corresponding decrease in the rectification is not observed for the symmetric GO film in Figure a over the scan range of ±6 V because both its limiting current and critical gating voltage are considerably higher than its asymmetric counterpart, as seen in Figure b.…”

“…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 . It is this counter‐ion trapping and release that disrupts the electrokinetic fore–aft symmetry inherent in the structurally symmetric film, particularly in the diffusion boundary layer, thus endowing it with the rectification characteristics observed.…”

“…Recently, we observed that rectification is also displayed in ion transport across 2D carbon lattice (e.g., graphene‐based) materials, in particular, the “nanochannels” that make up the interstitial spaces between stacked sheets of graphene oxide (GO) films . In some ways, it was quite surprising that these GO films were observed to exhibit ion current rectification behavior, given their structural (or geometric) symmetry.…”