A Facile and Scalable Method of Fabrication of Large-Area Ultrathin Graphene Oxide Nanofiltration Membrane

Wastewater treatment and seawater desalination technologies have been rapidly developed to solve the current severe global water crisis that has been attributed to industrial pollution and overexploitation of natural resources. Most of these technologies are based on separation membranes. Unlike the traditional separation membranes, graphene oxide (GO)-based membranes have extraordinary features such as excellent molecular sieving capability and ultrafast water transport, making them ideal candidates for developing excellent membrane materials.

To date, many methods for fabricating GO membranes, such as vacuum filtration, spray-coating and drop-casting, have been developed. However, the available methods are limited by their inability to realize large-scale production of uniform GO membranes due to the limited size of most processing devices. Although the doctor-blade technique can be used to produce large-scale GO membranes because they allow uniform coating through shear flow, it requires extra complex steps to synthesize viscous GO ink. Other methods like using printing machines are also limited. Therefore, developing scalable and continuous methods for the direct production of uniform GO membranes remains a big challenge that requires an urgent solution.

Controlling the interlayer spacing of the GO membrane is a common way of enhancing its separation property. This involves using reduction methods such as chemical reduction, which are usually toxic and environmentally unfriendly. Though other reduction treatments like photoreduction are generally environmentally friendly, they occur in solutions that affect the uniformity of GO membranes. These limitations compromise their industrial applications. Thus, it is also necessary to develop rapid, efficient and large-area reduction treatment strategies for GO membrane fabrication.

To address these issues, a group of researchers from The Hong Kong Polytechnic University: Dr. Zhiyu Liu, Mr. Zhong Ma, Dr. Baitai Qian, Mr. Anson Chan, Dr Xiaowen Wang and led by Professor John Xin in collaboration with Professor Yang Liu from Sun Yatsen University developed a facile, scalable and continuous method for fabricating large-area ultrathin GO nanofiltration membrane. This method was based on a combination of Mayer rod-coating and high-power UV reduction carried out at normal pressure and temperature. This approach is environmentally friendly as it requires no additional chemicals, no heating energy and generates no wastewater. The work is currently published in the journal, ACS Nano.

The authors reported an excellent and uniform and facile fabrication of GO membranes with a large area of approximately 80 x 30 cm² and ultrathin thickness of up to 3nm. The membrane’s interlayer spacing could be scaled and regulated effectively to a suitable degree in several seconds to enhance salt rejection rate. The resulting membrane showed a high separation efficiency and superior water permeability of more than 96.0% and 60.0 kg m²h^-1, respectively for sodium sulfate solution. It also exhibited remarkable and robust mechanical stability under various conditions, including harsh crossflow. Furthermore, this fabrication technique is facile, controllable, green, and allows roll-to-roll scalability for large-scale and cost-effective fabrication of GO membranes.

In summary, the research team successfully developed a scalable ultrathin uniform GO nanofiltration membrane fabrication method based on a combination of Mayer rod-coating and UV radiation. This approach allows uniform nanomaterial deposition and is thus suitable for large-scale GO membrane production. In a statement to Advances in Engineering, Professor John Xin, the lead and corresponding author said that their method would overcome the challenge of scalability previously reported to hinder the production of GO membranes and would facilitate industrial production of GO membranes for applications in large-scale water purification. He also commented that GO membrane filtration technology is particular suitable for removing salts commonly used in dyehouses, such as Sodium sulfate.