Controlling Interlayer Spacing of Graphene Oxide Membranes by External Pressure Regulation

Abstract

Graphene oxide (GO) membranes have been attracting numerous attention due to their impressive performance in various applications, especially in water purification. However, because the swelling in water and polar organic solvents causes the increase of interlayer channels, GO membranes usually possess inferior rejection for subnanometer-sized molecules. How to control the transport channels of GO membranes at angstrom level is a significantly scientific and practical issue. Herein, a concept of external pressure regulation (EPR) is reported for restraining GO swelling and controlling its interlayer spacing precisely. Since anisotropic GO films only swell at vertical direction, the interlayer channels can be manipulated by externally unidirectional reverse force. Based on this concept, an EPR system with GO membranes is designed for water desalination by adjusting the external pressure that has high resolution. In cross-flow filtration, the compressed GO membranes show high KCl, NaCl, and CaCl rejections of 94%, 97%, and 98%, respectively, accompanied by large water permeance up to 25 L m h under low feed pressure of 2 bar, despite the fact that the semi-free spatial swelling of ultrathin GO layer above the substrate pores can deteriorate salt rejection. Our work provides a straightforward physical strategy to adjust the interlayer spacing of the membranes fabricated by two-dimensional nanosheets for achieving desired filtration capacity.

2D Laminar Membranes for Selective Water and Ion Transport

“…f) Decrease of GO d ‐spacing in response to increasing pressure load. Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…Decreased channel size then imposes considerable transporting energy barrier to reduce overall membrane permeance. This external pressure regulation (EPR) effect has been employed by Li and co‐workers to limit ion penetration through GO laminar membranes . By mechanically compacting the membrane, the interlayer channels can be tuned with high precision.…”

“…It happens when d ‐spacing is wide enough to incorporate target species but small enough to block undesired ones, so their uptake can be differentiated to achieve selectivity for multiple purposes. A d ‐spacing between 2.8 Å (water) and 6.6 Å (K + , the smallest common hydrated cation) is ideal for seawater desalination ( Figure a), and that between 7.2 Å (Na + ) and ≈8.4 Å (Mg 2+ , Cu 2+ , and Cd 2+ ) can be used for water purification or ion recovery . Well‐tuned channel size is also crucial to laminar membranes working as battery separators.…”

Solvation‐Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

“…GO membranes, as a typical model of 2D‐NLMs, have been investigated intensively. Their interlayer spacing can be tuned by controlling the hydration through various strategies, including the oxidation level, the reduction level, covalent crosslinking, mechanical forces, and cationic insertion . For example, Nair and co‐workers devised a strategy in which GO membranes with different hydration degrees were glued into a shape‐fixed slot within an epoxy disk .…”

“…New Materials Design : So far, the vast majority of research on NLM‐based nanoionics has been focused on GO‐related materials, and membranes with homogeneous pores or surface chemistry. Methods for continuously tuning the channel size of NLMs based on other type of 2D nanomaterials have been rarely reported.…”