Ion percolation through annealed, supported graphene oxide films: Role of nanochannels and voids

Graphene oxide (GO), an ionic and molecular sieve, is an important material for wide-spectrum filtration, since its properties can be tuned by controlling the structure and dimensions of nanochannels between GO nanosheets. In the literature, mechanisms of ion percolation have been proposed assuming GO to be a uniform structure of vertically stacked graphene sheets decorated with functional groups, termed as lamellae. However, in practice, GO is known to have a hierarchical microstructure. In the present work, supported GO films annealed at different temperatures have been studied with the aim of discerning the relative contributions of nanochannels and voids, gaps between the lamellae, using electro-impedance spectroscopy, and the results were fitted to equivalent circuits. Monotonous decrease in the charge transfer resistance R_ct and an increase in the percolation resistance R_P were observed for GO films annealed up to 160 °C. Increase in R_P, taken in perspective with a gradual loss of ordering in nanosheets as observed from X-ray diffraction spectra, enables the conclusion that nanochannels are the dominant pathways of percolation. This was further confirmed by the response of GO films annealed at 180 °C and 200 °C, where a strong dynamic is observed. For these annealed GO films, charge transfer happens both in the conducting films and at the fluorine-doped tin oxide interface. The two processes of ion percolation and charge transfer are, however, interdependent, and are not separated in the impedance response.