Elastic three-dimensional graphene sponge fabricated by the liquid crystals of controlled large graphene oxide sheets
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Three-dimensional graphene (3DG) sponge has attracted increasing attention because it combines the unique properties of cellular materials and the excellent performance of graphene. Preparation of 3DG sponge depends mainly on the self-assembly of graphene oxide sheets. In the case of using uniform large graphene oxide and ultralarge graphene oxide sheets, the nematic liquid crystals (LCs) phases are formed at low concentration. After chemical reduction, the LCs of GO solution are converted to 3DG sponges with a high degree of orientation, offering a new methodology to regulate the controlled large GO sheets. The orientation of GO solution can be inherited by 3DG sponge, making the sponge to have a large-scale ordered network structure. The 3D elastic graphene sponges have low density and good elasticity, promising for the applications in strain sensing, shock damping, and energy cushioning. Our work explores a novel strategy for organizing the ordered alignment of controlled large GO sheets and exploring the relationship between the microstructures and mechanical properties of 3DG sponge.
KeywordsElastic Three-dimensional Liquid crystals Graphene sponge Self-assembly
The Graphene, a single atomic plane of graphite, can be well used as the building block of graphene-based macroscopic materials [1, 2]. Recently, the graphene-based macroscopic materials such as one dimensional graphene (1DG) fiber, two dimensional graphene (2DG) paper , and three-dimensional graphene (3DG) sponge have attracted significant attention as a means of further expanding the significance of graphene [4, 5]. Because GO sheets are a precursor for the cost-effective and mass production of graphene-based materials, the lateral dimensions of GO sheets play an important role in determining the structures and properties of graphene-based macroscopic materials .
The large GO sheets are ideally suited for the preparation of ultrastrong 1DG fibers , highly aligned 2DG papers [8, 9]. In these cases, their better alignments are the main factors to improve the mechanical performance . The liquid crystals (LCs) of GO sheets with regular ordering provides the most viable fluid assembly approach. It is an important precursor for fabrication of high performance aligned graphene-based macroscopic materials. Therefore, the LCs suspensions of high aspect ratio GO are of strong practical interest. From a fundamental point of view, the LCs of GO sheets could be the closest experimental realization of theoretical models based on infinitely thin and high aspect ratio rigid platelets [6, 11, 12]. The 1DG fiber and 2DG paper prepared with the LCs of controlled large GO sheet s have excellent mechanical properties [7, 8]. Therefore, the 3DG sponge constructed with these systems have huge potential in further research.
Due to its high flexibility, size heterogeneity, random distribution of functional groups and disorderly stacking of the GO board, it is a great challenge to accurately arrange the controlled large GO board and control the restore process in 3D architecture. . It is known that fluid phase assembly or the formation of lyotropic LCs is one of the most viable approaches to obtain large-scale, ordered microstructures from nanoscale building blocks . The forming of LCs of GO sheets depends on the sizes distribution, concentration of GO solution and the viscosities of solvents . The liquid crystalline phase is produced easily with concentrated GO constructed using large size sheets [15, 16]. Nevertheless, the ordered 3DG sponge produced with LCs of concentrated large GO sheets will lose its advantage of low weight densities. An excellent work by Shi et al. addresses this issue to some extent. They successfully regulate the arrangement of GO sheets via increasing its pH value with potassium hydroxide (KOH) . The 3DG sponge constructed using the LCs of GO sheets  and large GO sheets [17, 18, 19] are reported recently. However, the ordered structure of 3DG sponge fabricated by controlled large GO sheets has been rarely discussed.
This paper reports a chemical reduction method to prepare elastic and low-density 3DG sponge fabricated by highly oriented LCs of large graphene oxide (LGO) and ultralarge graphene oxide (ULGO) sheets . The LGO and ULGO sheets with extremely high aspect ratios as building blocks reduce defective edges, and achieve highly ordered alignment of rGO sheets in 3DG sponge. This work provides a new method for the preparation of 3DG materials constructed using LCs of controlled large GO sheets and the mechanical properties of 3DG sponge are expected to be improved. The method may shed new light on the relationship between the microstructures and mechanical properties of 3D graphene assemblies.
2 Materials and method
2.1 Materials and reagents
Natural Graphite (AR), KOH(AR) was purchased from Aladdin Company. HCl, L-ascorbic acid are supplied from Sinopharm Chemical Reagent Co. All chemical reagents are used without further purification. Deionized water is used throughout the experiments.
2.2 Preparation of LCs of controlled large GO sheets
The SGO, LGO and ULGO solution were prepared by different natural graphite flakes with 32, 325 and 1200 mesh. The process included three steps: oxidization, size fractionation, and exfoliation. In the first step, the three kinds of GO sheets were oxidized by a modified Hummer method. In the second step, the mixture was repeatedly washed with HCl solution (1:10) and water, after the process of oxidation. The GO dispersion was centrifuged at 1100 rpm for 3 min to separate into two portions: large and small lateral dimensions. Larger GO (32 and 325 mesh) left the larger part, while the smaller portion was left by 1200 mesh. The low speed centrifugation process was repeated three times for narrowing the size distribution. Then, it was purified by dialysis for 1 week. In the third step, graphite oxide was exfoliated by the freeze–thaw method . The LCs of controlled large GO sheets were formed with LGO and ULGO solution (5 g 0.3%) with the adding of KOH (0.014 M) respectively.
2.3 Preparation of 3DG sponge and three-dimensional elastic graphene 3DEG sponge
These two kinds of LCs of large GO solution constructed with LGO and ULGO sheets were put in the glass bottle through freeze-drying process. The GO sponges were obtained. The 3DG sponges were prepared by mild chemical reduction. The ascorbic acid was added in the LCs of GO solution (0.3%) constructed using LGO and ULGO sheets respectively. The ratio of ascorbic acid to GO was double. Then, the bottles were maintained at 90 °C for 1 h. The 3DG sponges were obtained. In order to prepare the elastic materials, the network of 3DG sponge should be strengthened by one more step. The LCs of GO solution were freezing in dry ice before chemical reduction. Then, these samples were maintained at 90 °C for 1 h. The 3D EG sponges prepared by LGO and ULGO solution were obtained.
X-ray diffraction (XRD) patterns of the Pt-Cu@3DG were performed on a Bruker D8 diffractometer with Cu Kα radiation with a scan speed 10° min−1. Scanning electron microscopy (SEM) was performed on a Philips XL 30 microscope operating at 30 kV.
3 Results and discussion
3.1 The formation of LCs of GO solution at low concentration
The GO sheets with more oxygenated groups were more flexible, which formed more wrinkled morphology and influenced the formation of LCs phase . The treatment with KOH extended the rigid domains of GO sheets, enabling them to self-assemble into a highly ordered structure. However, the order degree of LCs solution was insufficient and the large-scale ordered arrangement could not be obtained in GO sponge with inadequate amount of base (Fig. S2a). The fewer oxygenated groups with excess base led to stacking behavior of sheets and influenced the ordered arrangement in 3DG sponge (Fig. S2b).
3.2 The ordered structure of GO sponge
There was another way to form GO sponge with the LCs of concentrated GO solution (7 mg g−1) after lyophilization (Fig. 3a, b). By contrast, the GO sponge prepared with LCs of GO solution (3 mg g−1) had larger scale network due to the ordered arrangement of LCs solution upon adding the base (Fig. 3c, d). The enhanced electrostatic repulsion between GO sheets by excess KOH prevented sheets from precipitation and stacking each other, enabling them to form an ordered structure in LCs. The enhanced electrostatic repulsion also increased the fluidity of a GO solution. Furthermore, the increased fluidity facilitated tuning the direction vector of LCs, realizing rational regulation of the arrangement of GO sheets .
3.3 The ordered structure of 3DG sponge
3.4 The elastic property of 3D elastic graphene (EG) sponge
The maximum stress of the 3D EG sponge fabricated by ULGO sheets decreased from 2.5 to 2.2 kPa for 10 cycles. The energy loss coefficient was measured to be ≈ 25% in the first cycle and 9% after 10 cycles. In the 3D EG sponge fabricated by LGO sheets, its maximum stress decreased from 3.4 to 2.7 kPa. The energy loss coefficient was measured to be ≈ 43% in the first cycle and 15% after 10 cycles (Fig. 6e). These results indicated that the 3D EG sponge constructed using ULGO solution had better elastic property. The preparation method extended the application of controlled large GO sheets, but it still had limitation. The technology did not take full advantage of the self-assembly of ULGO sheets. The forming of 3D EG sponge was more complex than fibers and papers. It was difficult to control the reduction process of ULGO sheets into 3D EG sponge with large scale network structure, because of their high flexibility, less functional groups and highly wrinkled topography  resulting in unordered stacking of ULGO sheets.
In summary, in the case of using uniform LGO and ULGO sheets, the nematic LCs phases are formed at low concentration. After chemical reduction, the LCs of GO solution are converted to 3DG sponges with a high degree of orientation, offering a new methodology to regulate the controlled large GO sheets. The orientation of GO solution can be inherited by 3DG sponge, making the sponge to have a large-scale ordered network structure. The ordered arrangement of rGO sheets play an important role in determining the mechanical property of 3DG sponge. The 3D EG sponges have low density and good elasticity, promising for the applications in strain sensing, shock damping, and energy cushioning. This work provides a new method for the preparation of 3DG materials constructed using the LCs of controlled large GO sheets and may shed new light on the relationship between the microstructures and mechanical properties of 3DG sponge.
This work was financially supported by the key project of Shanghai Science and Technology Committee (No. 14231200300), Shanghai Key Laboratory of Green Chemistry and Chemical Processes and SRF for ROCS, SEM.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
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