Self-Assembly and Deposits of Reduced Graphene Oxide and Graphene Oxide Colloids from Desiccating Aqueous Sessile Droplets.

An experimental investigation on the dried colloidal deposits of educed raphene xide (rGO) and raphene xide (GO) resulting from aqueous sessile droplet desiccation has been reported herein. Observations revealed a distinct crossover from coffee-ring patterns to uniform deposits as we changed the suspended species in the aqueous medium from rGO to GO. The desiccation of rGO-laden aqueous sessile droplets yields a conventional coffee ring pattern driven by the typical advection with radially outward replenishment flows. Conversely, GO-laden droplet desiccation results in a uniform, "saucer-shaped" deposit. The terminology "from coffee rings to saucer-shaped deposits" has been coined herein to define this changeover. Time-resolved polarizing optical microscopic visualization of the suspended species conclusively showed that the GO suspensions, owing to their amphiphilic nature and dense oxygen functionalities, are collected at the liquid-vapor interface and form self-assembled domains mediated by hydrogen-bonding interactions. The self-assembled domains of the GO sheets are subsequently guided by the descending interface of the evaporating droplet, forming a uniform deposit. The dominance of the above-mentioned mechanism has been verified by experimental characterizations and available theories involving advection-sedimentation, DLVO interactions, and the free energy associated with interface capture and self-assembly. A secondary investigation includes green synthesis of rGO via successful reduction of exfoliated GO by plant seed extracts, which has not been explored earlier to the authors' knowledge. The green synthesized rGO possesses physicochemical properties similar to those of the chemically synthesized rGO obtained by employing hydrazine hydrate as a reducing agent. Finally, the experiments conclusively proved that the observed coffee ring to saucer-shaped transitions in deposit patterns by switching from rGO to GO are universal, i.e., independent of the synthesis route of rGO. This study discloses a new avenue to control colloidal deposits of graphene-based materials by varying oxygen functionality with flexible synthesis techniques.