Graphene-based multifunctional surface and structure gradients engineered by atmospheric plasma

Surface and structure gradients are ubiquitous in nature, where gradual changes in physical, chemical, mechanical and other properties of biomaterial are generated aiming to achieve intricate biological functions. These bioinspired gradients have been explored and fabricated using different materials in small scales (nm or µm) and usually with single function. Leveraging an outstanding property of graphene and their multi functionality, this paper presents a demonstration of the surface and structure gradients with gradual change of multiple functionalities such as structure, chemistry, wettability, charge, electrical and thermal conductivity in all three dimensions (1D, 2D and 3D). These multifunctional and multidimensional gradients with a gradual transition from graphene oxide (GO) to partially reduced GO (prGO) and reduced GO (rGO) were created by atmospheric plasma treatment of GO substrates (films and foams) using non-uniform exposure of plasma beams. The plasma beams with controllable energy, asymmetric contact exposure and timing were used to generate a gradual chemical reduction of oxygen groups and GO to prGO and rGO and fabricate the sizable (cm) range surface and 3D gradients with gradual change of multiple functionalities. This simple and low-cost method is a powerful and straightforward strategy to produce multifunctional gradients with 1D, 2D and 3D structures in different forms (films, foams) on variety of substrates (metals, semiconductors, glass,plastics, textile, etc.). Practical application of fabricated gradients is successfully demonstrated as absorbers for solar-driven water evaporation showing excellent water evaporation rates results for 1 sun irradiation of 1.782 kg•m−2•h−1 and the net solar to steam efficiencies of 97%. These gradients are very useful for diverse fundamental and practical applications, and one elegant example of efficient solar-driven water evaporation was demonstrated.