Aligned Proton-Conducting Graphene Sheets via Block Copolymer Supramolecular Assembly and Their Application for Highly Transparent Moisture-Sensing Conductive Coating

Here, we have demonstrated a well-defined strategy to prepare highly sulphonated reduced graphene oxide (S-rGO) sheets via non-covalent modification of rGO with water soluble rod-coil conjugated block copolymer poly(3-hexylthiophene)-block-poly(4-styrenesulfonic acid) (P3HT-b-PSSA) carrying a long PSSA block. S-rGO sheets are highly water soluble and its aqueous solution can be used to fabricate highly transparent conductive thin film coating on versatile smooth substrate surfaces like glass, indium tin oxide (ITO), quartz and flexible PET. The successful anchoring of sulfonic acid group on rGO surface via non-covalent modification by P3HT-b-PSSA was confirmed and analyzed by FTIR and XRD study. The bulk morphology of S-rGO reveals sheet like morphology where individual sheets are aligned with each other in a parallel arrangement through intercalation of PSSA chains driven by block copolymer self- assembly. AFM image of the thin film also supports nice parallel alignment of S-rGO sheets of average thickness similar to 100 nm on substrate surface. S-rGO sample shows very high water uptake (similar to 91% in comparison to its initial weight) and proton conductivity 0.5 S/cm after water vapor exposure for 1 hour. Such high proton conductivity is due to the synergy of alignment of graphene sheets with a continuous network of proton conducting nanochannels created by block copolymer microphase separation on the rGO surface. Nyquist plot with two semicircles suggested the presence of grain boundaries in the sample. I-V measurement of transparent thin film device fabricated from S-rGO sheets shows linear behavior with systematic increase of current on increasing water vapor exposure time. The block copolymer device shows well correlated, systematic and reversible resistance change with relative humidity (RH) confirming its efficient sensing capability towards moisture. We believe that high proton conductivity and interesting, reversible moisture sensitive electrical property of this material will be useful in fabricating transparent and flexible moisture sensors, flexible electronics, moisture induced energy storage, fuel cell, biological applications and others.