ИСТИНА

Graphene oxide (GO), structurally represented by graphene nanosheets modified with oxygencontaining groups, is a bright star among 2D materials owing to its highly tunable d-spacing, feasibility of adjusting carbon-to-oxygen (C:O) ratio coupled with diversity of surface modification. The presence of polar oxygen groups endows GO with high permeance towards water molecules making graphene oxide an advanced membrane material suitable for the variety of separation processes including dehumidification and nanofiltration. Today, despite an increasing number of studies devoted to structural and functional features of GO membranes, there is still a gap concerning two key points. The first one is a need for an in-depth understanding of the influence of C:O ratio on the transport properties of GO-based membranes both towards water vapors and permanent gases as the variation of oxygen-containing groups contributes to changes of GO interlayer spacing. It should be noted this type of study requires accurate observance of GO membranes chemical composition and structural characteristics. The second problem arises when GO membranes operate in pressure-driven nanofiltration and dehumidification processes: under elevated pressures, GO interlayer galleries become compacted accompanied with water molecules ousting from GO d-spacing which inevitably leads to significant drop of GO-based membranes water permeance. To address the first issue, we have prepared composite GO-based membranes with variable C:O ratio of GO nanosheets from 2.2 to 1.7 by simple adjusting of the graphite:oxidizer ratio during the synthesis by improved Hummers’ method. GO-based composite membranes were prepared by the formation of thin GO selective layers using spin-coating of GO suspensions on the surface of porous anodic alumina (AAO) supports. In accordance with measurement of permeation and sorption characteristics of the membranes, it was revealed, that with an increase in C:O ratio, only a slight decrease of GO permeance towards permanent gases is observed, accompanied by rise in water vapor permeance up to over 60 m3·m-2·atm-1·h-1. It was also elucidated, that the water vapor transport is mainly governed by water diffusivity not by H2O sorption capacity of graphene oxide. To improve water vapors stability, the microstructure of GO-based selective layers was modified using two different types of intercalants: flat-shaped graphene oxide nanoribbons and ball-shaped fullerenols with variable OH content. The pressure stability of the membranes was tested under stepwise pressure increase-decrease cycles with long-lasting exposure under transmembrane pressure of 0.1 MPa. It has been shown that graphene oxide nanoribbons create flexible pressureresistant nanochannels between GO nanosheets but still leading to an irreversible decline in water permeance up to ~35%. On the other hand, ball-shaped fullerenols endowed GO-selective layers with prominent pressure stability leading to decline of only 10% of water permeance under transmembrane pressure of 0.1 MPa and nearly complete permeance restoration after pressure release.