Covalent organic framework (COF) is a highly porous crystalline polymer composed of light elements through strong covalent bonds between organic linkers. Because of their structural diversity, permanent porosity, order, and the ability to incorporate organic backbones, COF can be used in a variety of applications. However, traditional COF synthesis methods often require harsh conditions. More importantly, the obtained COF is usually formed in powder form, which is insoluble and insoluble, and therefore difficult to process. Recently, a similar COF has been synthesized by using polystyrene balls as a template. However, these COFs are also available in powder form. Therefore, for many practical applications, it is still a huge challenge to directly manufacture COF into a controllable and stable 3D structure with several length ranges.
Graphene oxide (GO) is considered to be an ideal material for assembling extended structures due to its hydrophilic surface and large surface area, which can realize multifunctional composite structures with a variety of emerging material categories. In these composites, not only the excellent properties of a single compound are retained, but due to the presence of graphene, they usually show excellent electrical conductivity and mechanical properties. COF has the characteristics of low density, strong chemical stability, and large surface area. Its skeleton function can be customized by using appropriate monomers. 2D COF with π-conjugated structure should be very suitable for forming composite materials with 2D graphene.
Results
Based on the above problems, a COF/reduced graphene oxide (rGO) aerogel was synthesized by the team of Professor Arne Thomas from the Technical University of Berlin, Germany through the hydrothermal method. COF/rGO aerogels show excellent absorption capacity, with an absorption capacity of more than 200 g/g for organic solvents, and can be used to remove various organic liquids from water. In addition, the aerogel can also be used as an active material for supercapacitor devices, providing a high capacitance of 269 F g-1 at 0.5 A g-1, and has good cycle stability during 5000 cycles. Relevant results were published on "NATURE COMMUNICATIONS" with the title "Ultralight covalent organic framework/grapheme aerogels with hierarchical porosity".
1. Material synthesis
In this study, the researchers prepared the COF/rGO composite by hydrothermal method, which has a 3D, layered porous, ultra-light and complete structure. First, the organic linker 1,3,5-triformylphloroglucinol (Tp) and diaminoanthraquinone (Dq) are reacted in situ in the presence of GO to obtain COF/rGO hydrogel. Then, under hydrothermal reaction conditions, GO is reduced to rGO, and TpDq-COF grows uniformly along the surface of the rGO nanosheet, so that the two phases are intimately mixed. After freeze-drying the obtained hydrogel, COF/rGO aerogel is finally formed, showing a layered porous structure.
2. The structural characteristics of the material
Next, the researchers further studied the structural characteristics of aerogels. COF/rGO aerogel has a low density, about 7.0 mg cm-3, so it can be easily fixed by leaves. In order to further understand the origin of low density, the morphology and structure of COF, rGO and COF/rGO aerogels were further studied by SEM and TEM. TpDq COF has a hollow tubular structure. For COF/rGO composites, this morphology has been completely changed. It is observed that the expanded and interconnected nanosheets form a 3D sponge-like structure. The pore size of these networks is in the range of a few microns, which is much smaller than that of pure rGO aerogels. Moreover, no isolated COFs particles were detected on the graphene nanosheets, indicating that COFs grow uniformly along the graphene surface. The TEM image of the COF/rGO flakes confirmed that they were very thin and partially wrinkled, indicating good flexibility.
3. Adsorption performance
Due to its highly porous structure, high surface area, low density and good mechanical stability, COF/rGO aerogel should be a promising absorbent for absorbing oil and other organic pollutants. In order to analyze the absorption selectivity, COF/rGO aerogel was placed on the surface of a mixture of water and silicone oil to produce selective absorption of floating silicone oil (dyed with oil red) within a few seconds. Similarly, when the aerogel was in contact with underwater chloroform (stained with oil red again), rapid absorption of chloroform was observed within one second. After this process, the oil or organic liquid can be completely separated, leaving clean water. The absorption capacity of pure rGO aerogel without COF is 66–93 times its own weight, and the absorption capacity of mixed aerogel (1:1/monomer: GO) for different solvents is 98 to 240 times its own weight. It is higher than the absorption capacity of many reported adsorbents. The recyclability of COF/rGO aerogel is measured by repeatedly absorbing ethanol and then drying it in an oven. It was found that the absorption capacity remained above 87% after 20 cycles. These results prove the potential of COF/rGO aerogel for efficient and recyclable use in oil purification.
4. Electrochemical performance
In addition to good electrical conductivity and mechanical strength of graphene, the quinone part of the COFs framework can also act as a redox active unit, providing a reversible Faraday reaction in electrochemical energy storage. The electrochemical capacitance of pure COF is very poor, without any charge and discharge capacity due to its insulating properties. Compared with pure COF, COF/C and pure rGO electrodes, 3D COF/rGO electrodes show obvious redox peaks and a significant increase in specific capacity. The COF/rGO hybrid has a triangular shape with partial deformation. Its extra capacity is attributable to the pseudo-capacity induced by the redox active anthraquinone and the extra electric double layer capacity generated by the increase in specific surface area.
In terms of specific capacitance, COF/rGO aerogel produces the highest specific capacitance of 269 F g-1 in the 1.5 V potential window at a current density of 1.5 Vg-1. As the current density increases, COF/rGO can still provide a specific capacitance of 222 F g-1 and retain 83% of the capacitance. The high specific capacity and rate capability of the COF/rGO electrode is attributed to the synergistic effect of rGO providing electrical conductivity and COF providing high surface area and redox sites, thereby increasing the double layer and pseudocapacitance respectively. In addition, the formed 3D network facilitates rapid charge transfer and ion diffusion to redox active sites. The cycle performance test of the COF/rGO device shows that after 5000 cycles, its retention force is 96%, indicating excellent cycle stability. Therefore, it can be concluded that the 3D structure of COF/rGO materials facilitates rapid charge transfer and ion diffusion to redox active sites.
In conclusion
In short, the COF/rGO aerogel is self-assembled through the green synthesis pathway at low temperature. Due to its layered porous structure, ultra-low density, good mechanical strength and enhanced electrical conductivity, 3D aerogel has the ability to absorb organic solvents and excellent capacitance performance. Considering the simple preparation method and excellent performance, 3D COF/GO aerogel is a promising material for environmental and energy applications.