Tailoring permeation channels of graphene oxide membranes for precise ion separation

Abstract

The design and tailoring of membrane permeation channels is crucial for precise separation of ions or molecules. Herein, a method for tuning the permeation channels of graphene oxide (GO) membranes is developed, in which the basal planes and edges of GO flakes are simultaneously crosslinked by dicarboxylic acid and diamine. By altering the chain length of the crosslinkers, the size, structure and properties of the permeation channels are successfully tuned. In the permeation of single metal salt solution, the fluxes of metal ions are related to the swelling degree of GO membranes. In the separation of mixed salt solutions, the fluxes are determined by the radii of hydrated cations, and the crosslinked GO membranes display outstanding size-selectivity. This is the first report about the distinct orders of permeation fluxes of single salt solution and mixed salt solution. With the increasing hydrophobicity of diamines, the permeation fluxes of aqueous solution decrease. The optimized crosslinked GO membranes display excellent fluxes and separation factor, which are more than 2 times and 3 times that of the pristine membranes, respectively. The elastic modulus of the crosslinked GO membranes is double that of the pristine one, and the swelling degree in water is 75 times lower.

Introduction

Materials with nanopores and nanochannels such as carbon nanotubes [1], [2], nanoporous graphene [3], graphene oxide (GO) [4], [5], have attracted significant research interest in recent years due to their potential applications in separation [6]. GO, one of the most important derivatives of graphene, contains hydroxyl and epoxy functional groups on the basal planes, along with carbonyl and carboxyl groups at the flake edges [7]. GO can be exfoliated into aqueous colloidal suspensions of flakes and then assembled into free-standing membranes, thin films or paper-like materials by means of drop-casting, spin-coating [8], vacuum or pressure driven filtration [9], gas–liquid interfacial self-assembly [10], L–B [11], etc. Water permeation is expected to proceed through the whole interlayer space including oxidized areas which occupy a major part of the graphene oxide surface [12] and the nonoxidized regions [13], [14]. This endows the GO membranes excellent performance in water purification [15], [16], liquid pervaporation [17], [18] and fuel cells [19].

Although single layered GO flake exhibits outstanding mechanical strength, the weak interaction between GO flakes results in unsatisfactory mechanical performance of GO membranes [20]. The hydrophilic functional groups also lead to unstability and easily damage of GO membranes especially in aqueous solutions [21]. The integral mechanical strength of GO membranes is considered as the critical challenge for the practical separation. To solve these problems, chemical crosslinking of GO membranes with metal ions [22], borate [23], dopamine [24], amino acids, acyl chloride, isocyanates, dicarboxylic acids, diols, polyols [25], silsesquioxane [26], polyallylamine [27], polyetheramine [28], etc., has been reported to produce membranes with improved mechanical properties and stability [29]. Apparently, the molecular structure of the crosslinkers determines the reaction locations (basal planes and/or edges of GO flakes), structure and properties (hydrophilicity, charges, coordination capability, etc.) of GO membranes. Nevertheless, to our best knowledge, there are few reports related to the design and tailoring of micro-structure of GO membranes. Crosslinking both of the basal planes and edges of GO flakes is also seldom addressed. Moreover, the GO membranes are usually applied in the dialysis of single salt solution [30] while the application in the separation of mixed salts solutions is very limited.

Herein, we employed dicarboxylic acids and diamines successively to bond both of the basal planes and edges of GO flakes. By adjusting the molecular structure of diamines and dicarboxylic acids, the size, structure and properties of permeation channels are tuned, giving rise to the adjustable permeation, hydrophilicity, mechanical strength and stability of GO membranes. This is critical for precise separation of ions or molecules in practice [31], [32]. The permeation performance of GO membranes were conducted in the single salt solution and mixed salts solution (KCl, NaCl, MgCl₂ and NiCl₂), and distinct orders of permeation fluxes in the two cases were observed for the first time.