Controlled deposition of graphene oxide on an anodic aluminum oxide substrate via coffee-ring effect

Abstract

The suppression of the coffee-ring effect is important for obtaining uniform colloid deposition. In this study, the controlled deposition of graphene oxide on anodic aluminum oxide (AAO) through the suppression of the coffee-ring effect was investigated. The effect of factors such as the AAO’s thickness and pore diameter, the external temperature, and the applied electric potential on downward capillary flow through pores in the presence of outward capillary flow associated with the coffee-ring effect was studied. When graphene oxide (GO) suspension was drop-casted on an AAO substrate, an increasing AAO thickness led to enhanced downward capillary flow, which suppressed the coffee-ring effect. An increase in the pore diameter resulted in greater suppression the coffee-ring effect since GO flakes could not completely block the pore entrance. Furthermore, an elevated temperature suppressed the coffee-ring effect on the AAO substrate through the rapid evaporation of the droplet, which reduced the outward capillary flow. When an electric potential was applied to the droplet, the downward capillary flow improved because of electrowetting and the outward capillary flow was reduced by the Coulomb force. In particular, the effect of the varied electric potential was adjusted by the AAO thickness. These observations indicated that the coffee-ring effect could be suppressed by appropriately setting the outward and downward capillary flows on the AAO substrate. Furthermore, the pore diameter had a stronger influence than the AAO thickness on the drop-casted GO film’s thickness. Notably, optimized GO film manipulation and GO film thickness control could be achieved by manipulating the hydrodynamic flow. The findings of this study have the potential to be used in applications such as filtration, gas separation, and wearable sensors, where a uniform thin film is required.