New photocatalytic membrane that can be cleaned with light energy

An international collaboration led by researchers from Kobe University has successfully developed a nanosheet-laminated photocatalytic membrane that exhibits both excellent water permeability and photocatalytic activity. The photocatalytic properties of the membrane make it easier to clean, as irradiating the membrane with light successfully reduces fouling. They developed this membrane by laminating 2D nanomaterials (nanosheets) on a porous support.

This revolutionary membrane technology can be applied to: water purificationand thus has the potential to contribute to addressing both the global environmental and energy problems by helping to ensure safe drinking water supply and clean energy. It is hoped that this will accelerate the transition to carbon neutral, sustainable societies.

This development was made by a research group from the Graduate School of Science, Technology and Innovation/Research Center for Membrane and Film Technology of Kobe University (Associate Professor NAKAGAWA Keizo, Professor YOSHIOKA Tomohisa and Professor MATSUYAMA Hideto) in collaboration with Professor TACHIKAWA Takashi of Kobe University Molecular Photoscience Research Center, Associate Professor Chechia Hu of National Taiwan University of Science & Technology and Professor Shik Chi Edman Tsang of Oxford University.

The results were first published in the Chemical Engineering Magazine on Apr 7, 2022.

Adequate access to water in many regions of the world is becoming an increasing problem in light of global climate change and strong population and economic growth in developing countries. It is estimated that two-thirds of the world’s population will experience water shortages by 2025. For this serious water shortagesthe widespread adoption of water recycling and purification technologies, as well as efficient use of water production technologies (eg seawater desalination), are of crucial importance.

The membrane filtration method is currently used in 900 water treatment plants because it provides continuous and stable water of good quality. However, there is the problem of membrane fouling where the membrane, which separates and removes contaminants from the water, becomes clogged. When membrane fouling occurs, it is no longer possible to obtain the required amount of treated water. Therefore, it is necessary to wash or replace the membrane. To tackle this problem, a lot of research has been done into different methods to prevent fouling, but a satisfactory solution has not yet been found.

A method has been proposed that consumes less energy and has a low environmental impact. This includes introducing a photocatalytic material (such as titania) in the membrane and the removal of contaminants via photocatalysis. However, such a membrane must not only be able to treat water, but also show a responsiveness to visible light and a high photocatalytic activity. This requires the designer to look at the membrane design from multiple perspectives, including the membrane’s material and structure.

This research group previously developed a nanofiltration membrane, which works by using 2D channels between the layers of nanosheets. They developed this membrane by laminating niobate nanosheets (a type of metal oxide nanosheet, where each sheet is about a nanometer thick and a few hundred nanometers wide) onto a porous supporting membrane, which created the 2D channels between the nanosheets.

In this study, they found that adding carbon nitride nanosheets (which react to visible light) to the niobate nanosheet-layered membrane gave the membrane improved water permeability while greatly increasing the photocatalytic activity. In addition, the photocatalytic properties of the membrane have completely solved the problem of the reduced permeance of the membrane due to fouling.

Nanoplate-laminated membranes can be formed by simple vacuum filtration of nanoplate materials (colloidal solutions) on polymer support membranes. In this study, the research group produced an ultra-thin nanosheet-laminated membrane about 100 nanometers thick (Figure 1a). X-ray diffraction and molecular weight fractionation measurements revealed that introducing carbon nitride nanosheets into a niobate nanosheet-laminated membrane could control the diameter of nanochannels between the layers.

In terms of membrane functionality, the laminated nanofiltration membrane with a ratio of 74:25 of niobate (HNB₃O₈) nanosheet to carbon nitride (gC₃N₄) nanosheet maintained its separation performance while demonstrating an 8-fold increase in water permeability (Figure 1b). In terms of photocatalytic performance, the integration of carbon nitride nanosheets caused visible light to be absorbed. In addition, this combination of nanosheets significantly enhanced the membrane’s ability to photodegrade cationic dyes (rhodamine B) (Figure 1c).

When the developed composite membrane is used as a separation membrane, the niobate nanosheets give the laminated membrane its structure, while the carbon nitride is introduced between these layers and acts as a spacer. As a result, the channels in the laminated membrane expand, significantly increasing the rate of water permeation (left side of Figure 2a). By controlling the channel structure in this way, 90% of a dye (with a molecular weight of about 1000) can be separated from the water.

The photocatalytic functionality of the membrane is as follows: the carbon nitride nanosheets function as photocatalysts that absorb visible light and the niobate nanosheets act as catalytic promoters. In addition, the research group revealed that proper control of the band structure allowed the electrons to move efficiently, resulting in a dramatic increase in the photocatalytic activity (right side of Figure 2a). Based on these results, the researchers applied the membrane to water purification and performed a membrane fouling experiment using Bovine Serum Albumin (BSA) as fouling. BSA fouling reduced the water permeation rate of the membrane to 1/5 of its normal performance. However, the researchers managed to fully restore the permeability by irradiating the composite nanosheet membrane (Figure 2b).

By interweaving different types of nanosheets to form 2D nanochannels, the researchers have successfully developed a membrane that exhibits both excellent water permeability and photocatalytic activity. It is expected that further improvements can be made to membrane functionality and photocatalytic performance by the type nanosheet to more precisely control 2D nanochannel formation and band structure. Next, the researchers hope to membrane field and developing the photocatalytic process, aiming at industrial and practical application.