Tunable three-dimensional graphene assembly architectures through controlled diffusion of aqueous solution from a micro-droplet

The two-dimensional (2D) graphene sheets show superior electrical, thermal and mechanical properties. The three-dimensional (3D) graphene assemblies have also recently garnered great attention because of their high surface area, free-standing configuration and facile fabrication of graphene composites with nanomaterials in practical applications. Herein, we demonstrate the synthesis of a variety of 3D graphene assemblies including spheres, twiddles and hemispheres using micro-droplet reactors. Such 3D reduced graphene oxide (rGO) structures can be manipulated by controlling the aggregation pattern of the GO sheet inside the micro-droplets. The aggregation pattern depends on the diffusion rate of the aqueous solution in the droplet. The pattern was simply tuned by the amount of oil phase in the droplet during evaporation. In addition, micro-porous structures were manufactured by incorporating silica beads in the rGO microparticles, followed by a wet etching process. The local plasmonic properties of the 3D porous hemispherical rGO were investigated by electron energy-loss spectroscopy mapping. Tweaking the ratio of graphene-oxide-filled microdroplets to an oil phase allows control of the shape of three-dimensional (3D) graphene. Recent efforts have shown that encapsulating graphene oxide precursors inside bubbles and droplets can produce 3D graphene spheres after thermal annealing. Tae Seok Seo from KAIST and Kyung Hee University in South Korea and colleagues have now produced 3D graphene twisted spheres, hollow hemispheres and other structures by depositing microfluidic-generated droplets and an oily hexadecane solution onto a coated silicon wafer. Heat treatments caused the droplets to shrink and compress in ways that depend on the oil layer thickness, leading to different 3D frameworks. Adding silicon beads during the initial microdroplet formation enabled production of porous 3D materials by wet etching after annealing. The small holes generated promising surface plasmon effects for possible spectroscopic or photocatalysis devices. Spherical, twiddle and hollow hemispherical reduced graphene oxide (rGO) structures could be fabricated by tuning the GO assembly in the diffusion-controlled micro-droplets, and their size and thickness could be adjusted by controlling the concentration of GO and pH of the droplets. Besides, the porous rGO architecture was produced via wet etching of rGO-SiO2 composites, and their plasamon effect was characterized.