Facile synthesis of super-hydrophobic, electrically conductive and mechanically flexible functionalized graphene nanoribbon/polyurethane sponge for efficient oil/water separation at static and dynamic states
Graphical abstract
Introduction
Assembly of two-dimensional (2D) graphene sheets into three-dimensional (3D) macrostructures has attracted considerable interest due to their unique structures and superior properties [1]. The 3D graphene-based materials developed to date have demonstrated promising applications in various fields, such as electronics, photonics, composite materials, energy generation and storage, sensors and metrology, and environmental remediation [2], [3], [4], [5], [6], [7]. Especially, the high porosity, robustness and flexibility, along with super-hydrophobicity of these porous graphene-based composites make them attractive candidates for the separation of various oils and organic solvents from water [8], [9], [10], [11], especially for oil spill remediation in aqueous environments [12], [13], [14].
Graphene nanoribbons (GNRs), as thin strips of 2D graphene sheets and unrolled one-dimensional (1D) carbon nanotubes (CNTs) [15], combine the outstanding structure and physical properties of graphene and CNTs [16], [17], [18]. Unlike irregular graphene sheets [19], [20], [21], [22], [23], [24], GNRs have flat crystal surfaces with high aspect ratio, as well as outstanding electrical and thermal properties [25], [26], [27], [28]. Integration of graphene oxide nanoribbons (GONRs) into macroscopic 3D porous structures with low-density and versatility is one of the effective routes to realize their practical application [27], [29], [30]. A solution based covalent bonding of individual 2D GONRs with glutaraldehyde was developed to assembly of cross-linked 3D GNRs with porous and well interconnected structures, and such 3D GNR based electrodes showed enhanced surface area and faster electron transfer kinetics compared with bare 2D GNRs [30]. Recently, introduction of GONRs into commercial polyurethane (PU) sponges produced 3D porous GNR composite sponge through an electrophoresis method. The 3D sponge with high content of GNRs (29 wt%) exhibited good mechanical, electronic and hydrophobic properties, showing potential application in flexible supercapacitors, strain sensors, and environmental cleanup [29]. However, these 3D GNR composites exhibited poor structural stability and hydrophobic properties, e.g. brittleness and low water contact angle (<150°), limiting their practical applications in strain sensors and oil/water separation.
As is well known, the ideal porous materials for oil/water separation should comply with the following criteria: (i) clean-up oil spills at high speed to prevent the oil spreading, (ii) have excellent oil/water separation efficiency for repeated use, (iii) work effectively under different environmental conditions, and (iv) be compatible with scaled-up production and easily manipulated [31]. Recently, modified polymer sponges with excellent hydrophobicity/oleophilicity and mechanical reliability have shown promise to realize oil remediation because they meet some of the above criteria [12], [13], [14], [31]. For example, graphene derivatives and organic silane molecules modified PU sponges display many favourable features, such as easy production, good hydrophobicity, high oil/water separation efficiency, and environment-friendly characteristics [9], [12], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. However, there are several crucial issues that hamper the practical application of these advanced absorbent materials. For example, the presence of a graphene-based coating usually blocks the pores of the PU sponge [33], [34], [35], and silane molecules produce modified sponges with discrete water contact angles from 90 to 160° [9], [36], [37], [38], [39], [40], [41], [42], [43], [44], thus leading to relatively poor oil/water separation efficiency. More importantly, frequent oil spills in oceans and along coastlines are difficult to clean up because of intense wave action. The oil/water separation of these modified sponge materials under different environmental conditions, e.g., slight water movement in a lake versus intense wave action in an ocean, still need to be clarified.
GONRs with high aspect ratio (<200 nm in width) and functional groups can be easily manipulated to avoid blocking the pores of sponges. Meanwhile, organic silane coupling agent (SCA) molecules with bifunctional groups, i.e. alkoxy and hydrophobic groups [45], are expected to covalently bond with GONR sheets through the reaction between hydroxyl groups in GONRs and silanol groups of SCAs after hydrolysis. Based on this inspiration, here we describe a facile silane functionalizing and dip-coating process to fabricate silane modified rGONR based PU sponges. Two SCA molecules with different hydrophobic end-groups were used to tailor the microstructure and surface property of the GNR based porous PU sponge composites with artificial self-cleaning surfaces. The microstructure, mechanical and electrical properties were investigated. Moreover, in potential oil/water separation, special feature and performance of the constructed 3D GNR porous materials are highly needed to meet the requirements in different environmental conditions, e.g., almost still water in a lake or intense waves in the ocean. Therefore, the surface hydrophobic behavior and oil/water separation efficiency of these silane modified porous GNR-based sponge composites under different conditions (static and dynamic states) were evaluated.
Section snippets
Materials
Multi-walled carbon nanotubes (MWCNTs) were supplied by Chengdu Organic Chemicals Co., Ltd., China. PU sponges with density of ∼0.024 g/cm3 were provided by Hangzhou Guangsheng Sponge Plastic Co., Ltd., China. Various chemical reagents including concentrated sulfuric acid (H2SO4), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 5%), acetone, ethanol, ammonium hydroxide and hydrazine hydrate were all of analytical grades and purchased from Sinopharm Chemical Reagent Co., Ltd., China.
Structural characterizations of silane-f-GONR
The fabrication and silane surface functionalization of GONR sheets are crucial in determining the final performance of rGONR-coated polymer sponge composites. So, a series of measurements were performed to characterize the structures and morphologies of GONR and silane-f-GONR sheets. Fig. 2a–c show representative TEM images of as-synthesized GONRs and f-GONRs. It can be clearly seen that the GONR sheets synthesized by longitudinal unzipping MWCNTs show rough morphologies with wide ribbon
Conclusions
Three-dimensional porous rGONR/PU sponge composites with super-hydrophobicity, flexible elasticity and electrical conductivity were fabricated by a facile silane functionalization and dip-coating method. Two types of silane molecules with different hydrophobic end-groups were used to functionalize the GONR sheets through the hydrolysis reaction of silane molecules and the condensation between the silanol groups and the hydroxyl groups of GONR. This approach offered the ability to tune the
Acknowledgements
We acknowledge the funding support from the National Natural Science Foundation of China (51403047 and 51203038), the Zhejiang Provincial Natural Science Foundation (LY15E030015, LQ13E030009 and LY18E030005), the Science and Technology Project of Zhejiang Province (2016C31019) and the Project for the Innovation of High Level Returned Overseas Scholars in Hangzhou.
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These authors contributed equally to this work.