Simply controlling the surface structure of graphene oxide films using multiple drop-casting

Abstract

Aqueous graphene oxide inks are promising to fabricate a large-area thin film which can be utilized in various applications such as sensors and energy storage and conversion. In this work, we experimentally demonstrate a facile way to controllably fabricate wrinkled structures on the surface of graphene oxide films by using a drop-casting method. Simple drop-casting of the graphene oxide ink enabled to fabricate randomly distributed wrinkles uniformly over the entire film deposited after water was totally evaporated. Deposition mechanism after a single drop-casting was clearly demonstrated, which involves graphene oxide flake transport, aggregation/deposition at the pinned contact edge of a sessile droplet during water solvent evaporation and also buckled deformation(wrinkle formation) of the deposited graphene oxide thin film due to compressive stress by the receding water droplet contact line. Multiple drop-casting on the initially formed wrinkled structures of the graphene oxide film allowed the wrinkles to grow in their width and height linearly by increasing the number of drop-casting, which resulted in the increase of the size polydispersity and area density of wrinkles. The wrinkle growth mechanism during multiple drop-casting was explained in the perspective of collision and accumulation of graphene oxide flakes dispersed within additional dropped droplet on the initially formed wrinkles during water evaporation and droplet contact line recession, which was supported by high-resolution imaging of the films using scanning electron microscopy (SEM) and atomic force microscopy (AFM). This work suggests that the behavior of the graphene oxide flakes dispersed within a sessile droplet can affect surface structures of deposited thin films, and it can be foundational to extend the simple multiple drop-casting strategy to controlled fabrication of various 2D nanomaterial thin films with wrinkled structures by appropriately controlling ink solution and evaporation conditions, which can be used in a variety of functional applications.