Tuning Water Nanofiltration Performance of Few-Layered, Reduced Graphene Oxide Membranes by Oxygen Plasma

Ultrathin, graphene oxide (GO) membranes have shown great potential for water nanofiltration applications. However, due to the difficulties in controlling the intrinsic oxygen-containing functional groups on GO and the swelling of GO interlayers in aqueous solution, it is highly challenging to tune GO membranes' nanofiltration performance by adjusting their hydrophilicity and interlayer nanochannel size. In this study, oxygen plasma was reported as an effective technique to tune water nanofiltration performance of few-layered, reduced graphene oxide (rGO) membranes, by improving rGO membranes' hydrophilicity and/or adjusting effective permeation nanochannel/pore sizes. The 5 nm thick GO membranes were fabricated through a layer-by-layer deposition method and subsequently reduced at 220 °C under vacuum. No detectable water permeance (<1.0 L/(m²·h·bar)) under 5 bar pressure drop suggests the high quality of the rGO membranes possibly with very small hydrophobic nanochannels/pores. Exposure to oxygen plasma for different time intervals (0-30 s) gradually improved the hydrophilicity of the rGO membranes, with the water contact angle decreasing from 74 to 31°. This is consistent with X-ray photoelectron microscopy results showing gradual increase of oxygen-containing functional groups with the increase of oxygen plasma etching time. By tuning membrane properties upon different oxygen plasma exposure times, the 5 nm thick rGO membranes showed the optimum nanofiltration performance with water permeance of 44 L/(m²·h·bar) and 98% rejection for methylene blue. Oxygen plasma treatment, therefore, may provide a viable way for tuning few-layered, graphene-based membranes for highly efficient water nanofiltration applications.