Ultrathin membranes formed by stacking two-dimensional (2D) nanomaterials (e.g. graphene, graphene oxide, and MoS2) have been widely studied for applications such as waste water treatment, desalination, and industrial product purification. The interlayer-spacing of the stacked-2D nanomaterial, significantly affecting the molecular sieving, ion diffusion, and mechanical properties is considered to be critical to the performance of the separation layer. However, the current manipulation strategies on the interlayer-spacing including covalent cross-linking and implanting nano-spacers typically involve complicated procedures and have not yet achieved a precise control in the sub-nanometer scale. Inspired by the unique interfacial interactions between two different 2D nanomaterials, we propose a new strategy by stacking multiple 2D nanomaterials into heterostructure membranes. The heterostructure membranes take advantage of the intrinsic properties of the 2D nanomaterials and thus require less post-treatment. The interlayer-spacing of the heterostructure, subjecting to the attractive and repulsive interactions between two adjacent layers, was found to have unique behaviors depending on the polarity of the material, surface charge, pH and ionic strength of the aqueous environment. An integrated system of quartz crystal microbalance with dissipation (QCM-D), which measures the areal mass of the membrane and the absorbed solvent, and ellipsometry, which optically measures the thickness of the membrane was employed to accurately characterize the interlayer-spacing of the heterostructure in both gaseous and aqueous phase. Additionally, the integrated system can perform real-time monitoring, providing valuable information about the evolution of the interlayer-spacing in the aqueous solutions. Molecular simulations were also conducted to fundamentally understand the heterostructure and the mobility of water in such interlayer channel. The separation performance of the heterostructure membranes were experimentally tested in a pressurized filtration system, demonstrating fine tuning of the interlayer-spacing.

Heterostructure membranes made from stacked-two-dimensional nanomaterials: Tuning the interlayer-spacing for aqueous phase separation

255th National Meeting (2018)

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