In the past few decades, membrane-based separation technology has attracted considerable attention in many separation fields due to its advantages of easy-operation, energy-efficiency, and environmental friendliness [1
]. Advanced membranes with superior selectivity and permeability are essential to the development of membrane-based separation technology. Currently, polymeric membrane has governed the entire membrane market, including real-world application and academic research, owing to its advantages of energy-efficiency, easy-operation, low-cost, and inherent simplicity. Nevertheless, restrictions of polymeric membranes still exist for most practical applications, because most of them tend to foul, have low resistance to chlorine, strong acids/alkaline, high temperature and organic solvents, and suffer from aperture shrinkage under high pressure [2
]. The strong trade-off relation between membrane selectivity and permeability is a common challenge for all of polymeric membranes [3
]. These restrictions have urged membrane scientists to constantly seek new membrane materials and develop novel membrane structures with superior chemical stability, thermal stability, water permeability, as well as high selectivity [4
]. Recently, carbon-based materials like carbon nanotubes (CNTs), graphene, and its derivative graphene oxide (GO), have shown notable potential in membrane-based separation fields because of their strong mechanical strength, high resistance to strong acids/alkaline and organic solvents, and easy accessibility [5
]. Among them, GO was served as one of the emerging nano-building materials for the fabrication of novel separation membrane owing to its distinct two-dimensional (2D) and single-atomic-thick structure, high mechanical strength, high chemical inertness, nearly frictionless surface, and good flexibility combined with large-scale and cost-effective production in solution [9
GO was first synthesized by Brodie [12
] in 1859. Subsequently, Staudenmaier [13
] and Hummers [14
] improved the preparation method in 1898 and 1958, respectively. Afterwards, several modified Hummers’ methods as well as some other new methods were successively developed [15
]. In order to identify the surface morphology and chemical structure of the resultant GO, several characterization techniques are widely employed, such as atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and Zeta potential [15
]. The resultant GO contains plentiful of oxygenated functional groups—such as hydroxyl, epoxy, and carboxyl—on its basal plane and at its edge [20
], as shown in Figure 1
. These functional groups endow GO good hydrophilicity and favorable water solubility, which enables a convenient and cost-effective solution process for the preparation of GO-based membrane [21
]. Additionally, these oxygenated functional groups make GO nanosheets readily to be further surface-modified and the correspondingly functional GO-based composite membranes with preferable separation performance can be obtained.
Based on these advantages as well as high surface-to-volume ratio structure of GO nanosheets, various GO-based membranes have been widely developed and exhibited great promise in many membrane separation fields such as gas separation [24
], water purification [26
], desalination [27
], and pervaporation (PV) [28
]. In recent decades, patents and papers (including research articles and review papers) focusing on GO-based membranes are growing exponentially, as shown in Figure 2
. Among them, several review papers focused on summarizing the structure, physicochemical property, application, and separation mechanism of GO-based membranes appeared [9
]. Based on these research articles, we learned that the structure, mechanical strength, and structural stability of GO-based membrane have significant influence on membrane separation performance.
In this review paper, the latest research progress in GO-based membranes centered on improving membrane structure, mechanical strength as well as structural stability in aqueous solution is highlighted and discussed in more detail. First, we briefly reviewed the preparation and characterization of GO. Then, the preparation method, characterization, and type of GO-based membrane are summarized. Finally, the advancements of GO-based membrane in adjusting membrane structure and enhancing their mechanical strength as well as structural stability in aqueous environment are particularly discussed, in order to promote the development of GO-based membranes in real-world applications.
5. Advanced Aqueous Stability and Mechanical Strength of GO Membranes
At present, despite significant advancements in GO-based membranes have been achieved, a few critical challenges in realizing real-world application of GO-based membranes still exist. Specifically, the instability of the inter-layer spacing between adjacent GO nanosheets is a great challenge for utilizing laminar GO membranes as selective aqueous separation barriers, especially for water-related treatment. This is because GO membrane easily disintegrated and redispersed in water over time due to the highly hydrophilic nature of the GO sheets and electrostatic repulsion between the negatively charged GO sheets on hydration , and then the integrity of the laminar GO membranes and inter-layer nanochannels formed by stacking GO sheets would be damaged during aqueous separation process [118
]. Therefore, it is very much desirable to enhance the structural stability of GO membrane by forming stable bonding between GO nanosheets to realize real-world applications of GO membranes in aqueous environment. Currently, it has been reported that stable GO-based membranes suitable for aqueous system application could be obtained by introducing various cross-linking interactive forces, including electrostatic interactions and covalent bonds between adjacent GO nanosheets or by reducing GO membranes [6
Park et al. [122
] first developed a chemically cross-linking GO membrane using divalent ions and polyallylamine (PAA), respectively. In comparison to the original GO membrane, the modified GO membrane showed significantly enhanced mechanical strength. Using LbL deposition, Mi et al. [26
] fabricated cross-linked GO membranes using TMC (Figure 10
) and positively charged PAH as cross-linkers (Figure 4
d), respectively. Results showed that the cross-linked GO membrane exhibited excellent stability for water treatment compared to the pristine GO membrane.
A highly stable GO-based ultrathin hybrid membrane was developed by Zhao et al. [121
], who utilized gelatin (GE) as the cross-linker to interconnect adjacent GO nanosheets by electrostatic interaction, hydrophobic interaction, and hydrogen bond, as shown in Figure 11
a. Enhanced operation stability was obtained for the GE cross-linked GO hybrid membrane used for water/ethanol separation. Recently, a covalently cross-linked GO membrane was developed by Lim et al. [124
], who used tannic-acid (TA)-functionalized GO as the membrane material and PEI as cross-linker, as shown in Figure 11
b. The resultant cross-linked TA–GO membrane exhibited excellent structural stability in an aqueous environment due to the stable layered structure formed by the cross-linking reaction between TA-GO and PEI. Nguyen et al. [49
] fabricated ultra-stiff GO thin films cross-linked GO with borate. They reported that the mechanical strength of the cross-linked GO films obtained by adding 0.94 wt % boron to the GO suspensions was remarkably increased (up to 255% and 20%, respectively) compared to that of the unmodified films. Such significant enhancement was attributed to the strong bonded force between neighboring GO sheets because of the formation of covalent bonds between the hydroxyl groups on GO nanosheets surface and the borate ions (Figure 12
). Recently, Liu et al. [127
] fabricated highly-aqueous-stable GO membrane by incorporating triethanolamine (TEOA) modified titanate nanowires (TNWs) in GO membrane. They reported that the GO/TEOA–TNWs composite membrane showed significantly improved aqueous stability within even for one month usage due to the strong covalent bonds between the epoxy groups and carboxyl groups on the surface of GO and the N+
groups in TEOA. Furthermore, the water flux of the composite membranes was significantly increased due to the intercalation of TNWs between GO sheets, which could introduce plentiful of nanochannels inside the membranes and simultaneously improved surface hydrophilicity of membranes. These results demonstrated that the GO/TEOA–TNWs composite membranes had great potential in the long-term practical water treatment applications. Zhang et al. [128
] designed novel GOF membranes using isophorone diisocyanate (IPDI) as a chemical crosslinker for covalent crosslinking of GO nanosheets by a facile vacuum-assisted filtration method. The resultant IPDI–GOF membranes not only presented enhanced structural stability but also showed improved water permeation due to the enlarged nanochannels among GO sheets. The IPDI-GOF membranes exhibited a high water flux of 80 L/m2
/h under an extremely low pressure (1.0 bar) and excellent removal efficiency for organic dyes molecules (up to 96%). This study provided an approach for enhancing the stability and water permeability of GO membrane which could be applied to real-world water treatment.
Additionally, some studies also showed that reducing GO sheets might also increase the stability of GO membrane by enhancing the π-π interactions between the GO nanosheets. Nevertheless, this would also reduce the water permeation of the membrane because of the shrunken channel distance [6
]. Yang et al. [129
] fabricated PDA-coated reduced GO (PDA–rGO) membranes by chemically reducing GO laminates and then introducing a hydrophilic adhesive PDA layer onto the rGO laminates and used for FO desalination. Study results showed that the resultant PDA–rGO membranes presented excellent aqueous stability and outstanding water flux (36.6 L/m2
/h) with a high salt rejection rate (92.0%) in FO desalination due to the compacted nanochannels and improved surface hydrophilicity of rGO laminates. They pointed out that the chemical reduction of GO laminates could remarkably increase the salt rejection rate of the membranes by forming highly stable and compacted nanochannels between GO sheets. Moreover, the introduction of a hydrophilic PDA coating onto the rGO laminate surface could further improve the water flux by facilitating the water absorption speed into rGO nanochannels.
Yeh et al. [118
] found that the neat GO membranes readily disintegrated in water, but the membranes became stable once they were crosslinked by multivalent cationic metal contaminats (e.g., Al3+
), which were introduced unintentionally during the synthesis and processing of GO (Supplementary Figures S1 and S2
). They contributed remarkably improved membrane stability in water to the unexpected contaminants (i.e., the Al3+
in the resultant GO membrane), which acted as crosslinkers and then effectively strengthened the final membrane (Supplementary Figure S3
). Meanwhile, they pointed out that significant variability existed for GO membrane stability in water between different modified methods. For example, an around 10% increase in overall stiffness could be observed for GO membrane crosslinked with divalent metal ions [105
]. In contrast, in this study even partial Al3+
contamination could lead to a more than 340% enhancement in the membrane stiffness. The remarkable variability was attributed to the fact that the ‘unmodified’ GO papers were probably already crosslinked by unintentionally introduced multivalent cationic metal contaminats (e.g., Al3+
), thus only a modest stiffness difference between unmodified and “crosslinked” GO membrane. That is, for the variability in reported stiffness and stability of GO membranes obtained with different modified methods might be, at least partially, attributed to different degrees of crosslinking by unintentionally introduced contaminants. In order to further identify this point, they removed Al3+
from GO (AAO) membranes through ionic exchange with HCl or other monovalent cations such as Na+
, after which the membranes readily disintegrated in water (Supplementary Figure S4a,b
). XPS detected no Al after the ionic exchange (Supplementary Figure S4c
). In addition, they intentionally treated a clean GO (Teflon as the filter disc) membrane with Al3+
by utilizing this crosslinking effect, which effectively strengthened the water stability of GO (Teflon) membranes (Supplementary Figure S5
). Based on this study, we learned that it is essential for researchers in the field to provide thorough and necessary characterization data for GO (e.g., XPS, XRD in this work) to further identify the potential mechanisms of such phenomena. This finding is very helpful to understand the intrinsic mechanical properties of GO membranes and strengthened mechanism of GO membrane stability in water.
Although the mechanical integrity and structural stability of GO-based membranes could be enhanced in different strategies, more efforts should be taken to prepare highly-efficient GO-based membranes with enhanced separation performance and long-term operation stability for practical applications. At the same time, the intrinsic mechanical properties of GO membranes and strengthened mechanism of GO membrane stability in water should be explored and better understood in more detail.
In summary, based on the unique single-atomic-thick and two-dimensional structure, together with excellent physicochemical property, GO as an emerging star nano-building material has attracted great interest in the membrane-based separation field. In this review paper, the preparation and characterization of GO were simply summarized. Then we focused on reviewing the preparation method, characterization as well as type of GO-based membrane. Special attention has been paid to the latest advancements of GO-based membrane with respect to the adjustment of membrane structure as well as the enhancement of mechanical strength and structural stability in aqueous environment. An approach which is highly oxidized, low-cost, safe, simple, and environmentally friendly will provide the possibility for massive production of GO. The structure and separation performance of GO membrane significantly depend on the fabrication method and corresponding fabrication conditions. So in a specific practical application, a desired GO membrane can be obtained by employing appropriate preparation method and optimized the fabrication conditions. Despite many characterization techniques having been extensively utilized for analyzing the structure and performance of GO membrane, there still remain several challenges for the accurate and deep characterization of GO membrane. The separation performance of GO membranes could be effectively and successfully improved by different approaches, including physical approach, chemical approach, and some other novel approaches. The mechanical strength and structural stability of GO membrane could be enhanced by different strategies, such as cross-linked GO membrane using different cross-linkers through covalent bonding or electrostatic interaction, or reduced GO membrane through thermal or chemical process to enhance the π-π interactions between the adjacent GO nanosheets. However, several challenges still remain for these strategies. So in order to facilitate the development of GO-based membrane in real-world application, continuous efforts are still required to improve the separation performance and structural stability of GO-based membranes, especially for water-related separation applications.