Solar Desalination by Plasma-Made Graphene Nanostructures with Complete In-Water and In-Air Oil Rejection
Post date: October 4, 2020
Student: Shenghao Wu
Faculty: Timothy S. Fisher
Sponsor: China Scholarship Council: ZJU-UCLA Joint Doctoral Program
Summary: Water scarcity, driven by constantly increasing economic development and global environmental pollution, has elicited much demand for new, technology-driven solutions. Solar desalination that exploits interfacial evaporation represents a promising solution to the global water scarcity. Real-world feedstocks (e.g., natural seawater and contaminated water) include oil contamination issues, raising a compelling need for desalination systems that offer anti-oil-fouling capability; however, it is still challenging to prepare oil-repellent and meanwhile water-attracting surfaces.
We developed an in-water and simultaneously in-air oleophobic, hydrophilic surface by dispersing functional molecules on vertically oriented graphene nanosheets (VGs) to realize an anti-oil-fouling solar desalination system. As shown in Figure 1, VGs (black triangles) are perpendicularly oriented on a carbon cloth substrate (gray) by plasma-enhanced chemical vapor deposition (PECVD). The VGs are purposely introduced to enhance light absorptance and solid−liquid interfacial heat transfer, as well as to increase the surface roughness to improve the anti-oil-fouling behavior.
Figure 1. Design of an oleophobic, hydrophilic surface by dispersing fluorinated molecules on graphene nanostructures to achieve anti-oil-fouling solar desalination. (a) Schematic of dispersing fluorinated molecules on graphene surfaces. (b) Schematic of anti-oil-fouling solar desalination. The oil (rainbow-colored) is afloat on the seawater surface (dark green).
Fluorinated molecules (FMs, sodium perfluorooctanoate, C7F15COONa) that possess both oleophobic, hydrophobic −CFx and hydrophilic −COONa groups are uniformly dispersed on graphene nanosheets, thus achieving simultaneously in-air and in-water oleophobicity and hydrophilicity. Consequently, water wets the graphene structures, while oil is repelled from the surface, as shown in Figure 2. The oleophobic, hydrophilic graphene architecture is applied to solar-driven interfacial evaporation and exhibits a durable anti-oil-fouling ability with complete in-water and in-air oil rejection. Stable solar−vapor conversion efficiency of more than 85% is achieved regardless of whether the feedstock is pure or oil-contaminated water (e.g., a mixture of oil floating on water, an oil-in-water emulsion), leading to the long-term efficient production of clean water over several days.
Figure 2. Surface wettability characterization. (a) Optical images of an FM-VG/CC sample. (b) Optical images of a fluorinated-molecule-coated carbon cloth (FM-CC) sample. (c) Oil contact angle in the air. (d) Water contact angle in the air. (e) Oil contact angle under water.