Diffusive transport of two charge equivalent and structurally similar ruthenium complex ions through graphene oxide membranes

Here, we report a study of ion transport across graphene oxide (GO) membranes of various thicknesses, made by vacuum filtration of GO aqueous solutions. The diffusive transport rates of two charge-equivalent ruthenium complex ions Ru(bpy)₃ ²⁺ and Ru(phen)₃ ²⁺, with a sub-angstrom size difference, are distinguishable through GO membranes and their ratio can be a unique tool for probing the transport-relevant pore structures. Pore and slit-dominant hindered diffusion models are presented and correlated to experimental results. Our analysis suggests that ion transport is mostly facilitated by large pores (larger than 1.75 nm in diameter) in the relatively thin GO membranes, while slits formed by GO stacking (less than 1.42 nm in width) become dominant only in thick membranes. By grafting PEG molecules to the lateral plane of GO sheets, membranes with enlarged interlayer spacing were engineered, which showed drastically increased ion transport rates and lower distinction among the two ruthenium complex ions, consistent with the prediction by the slit-dominant steric hindered diffusion model.