Tuning nanochannel of graphene oxide membrane via crown ether intercalation for controlling cation permeation

Abstract

Desalination is one of the effective methods to deal with shortage of fresh water. Various ions and pollutants present in seawater can be effectively removed by membrane process. Commercialized membranes are generally based on polymers

however, their mechanical and chemical stability is inferior, so that membrane performance cannot be maintained over a long period of time. To deal with this problem, graphene oxide (GO) based membranes are considered as a promising candidate for desalination owing to excellent chemical and mechanical properties as well as a prospect of mass production. However, due to the hydrogen bonds between oxygen functional groups and water molecules, the channel size cannot be steadily retained, allowing small ionic species to penetrate the GO membrane. To solve this problem, the nanochannel of the GO membrane can be controlled via intercalation of a certain cation such as potassium between GO layers, which can efficiently inhibit the diffusion of other cations with comparable diameter. However, it is difficult to maintain the intercalated cations within the GO membrane during desalination where external pressure is applied. Here, we intercalated dibenzo 18-Crown-6, which can strongly bind with potassium ions, to prevent the leakage of potassium ions from GO nanochannels. We found that the complexes between potassium ions and DB18C6s make sub-nanochannels between the GO nanochannel, controlling the permeation of cations. Furthermore, we discovered that cations can be tightly retained in the GO membrane due to DB18C6s, and the salt rejection and water permeance can be regulated depending on the intercalated amount of crown ethers between GO nanosheets. It is expected that this finding could be contributed to GO membrane-based applications, such as water desalination, lithium-ion batteries and molecular sieving.