Filtration behavior of graphene oxide membrane with size-differentiated nanoflake and desalination performance enhancement by atomic layer deposition

Abstract

Multi-layered GO membrane (GOM) that is composed of atomic thick (0.34nm) graphene oxide (GO) flakes are mostly studied by researchers due to its unique water transport system, high chemical and mechanical stability. However, their low salt rejection efficiency and significant decrease of water permeability limit their wide application in water desalination. This research aims to understand the water transport mechanism and enhancing GOM performance. To investigate GOM behavior during filtration, GOMs composed with different sized GO flakes were applied. Moreover, on the purpose of enhancing water purification performance, a novel surface nano-functionalization method, atomic layer deposition (ALD) was applied. It was observed that water molecule transportation through GOM is limited by enveloped region where edge and basal plane of each GO meets. Interlayer space of wet state GOMs by X-ray diffractometer revealed that smaller GO stacked membrane possesses more enveloped region. Due to the narrowed nanochannel of GOM, water filtration with larger GO stacked GOM showed two folds higher permeability than GOM containing smaller GO nanoflakes. This study suggests that any modification of GOM that could affect the microstructure of membrane should consider such invariance of structural defects of GOM. ALD allowed for the formation of an ultra-thin Al2O3 layer (1.44nm) on the surface of GOM, due to the strong chemisorption of Al2O3 precursor, trimethylaluminum to GO. Atomically thin coating of Al2O3 layer enhanced water permeability (from 40LMH/bar to 75LMH/bar) and NaCl rejection up to 67.4%, overcoming the trade-off between permeability and selectivity. This enhanced filtration performance is mainly attributed to the changes of surface electrochemical properties without affecting the GOM microstructure. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and water contact angle were performed to investigate the effect of ALD on the physico-chemical properties of GOM. The distinctive features of ALD to the GO membrane may contribute towards enhancing its wider application in water desalination.