Modulation of cation trans-membrane transport in GO-MoS₂ membranes through simultaneous control of interlayer spacing...

The interlayer spacing and ion-nanochannel interactions of graphene oxide membranes (GOMs) were simultaneously modulated by thermal reduction and mixing with MoS₂ flakes for realizing selective ion separation, which was evaluated by the ratio of ion trans-membrane penetration rates (IPR). The results showed that the ratio of IPR_Cu2+ to IPR_Na+ increased to 1.90 in GOM after thermal reduction for 5 h, which was ∼9.56 times higher than that without thermal reduction, indicating the increase of selectivity of Cu²⁺ over Na⁺. This was because the reduction of oxygen-containing groups narrowed the interlayer spacing and moderated the coordination between Cu²⁺ and sp³ clusters in GO, leading to an enhancement of the size-sieving effect but a decrease in the Cu (II)-nanochannel interaction. Meanwhile, the value of IPR_Cu2+/IPR _Na+ was 0.374 after intercalating MoS₂ into GO laminates (GO-MoS₂ membrane, GMM), which was ∼1.87 times higher in comparison with that in GOM. This might be because the intercalation of MoS₂ narrowed the interlayer spacing, enhanced the size-sieving effect, and strengthened the Na⁺ ion-nanochannel interactions (cation-π and ion-MoS₂ chemical interactions) according to density functional theory calculations. Furthermore, IPR_Cu2+/IPR _Na+ was ∼5.09 in GMM under thermal reduction for 5 h, which was ∼25.5 times higher in comparison with that in GOM without thermal reduction, exhibiting a great enhancement in selectivity for Cu²⁺. This indicated that thermal reduction and MoS₂ intercalation could work in concert to control the size-sieving effect and ion-nanochannel interactions to achieve fine separation of heavy metal ions from main group metal ions.