On May 15th, the internationally renowned cell journal "Matter" published the latest research results of Professor Cheng Qunfeng of the School of Chemistry, Beijing University of Aeronautics and Astronautics, Academician Jiang Lei and his collaborators. "Strong graphene films" (English translation: Ultrastrong Graphene Films via Long-Chain π-Bridging), Cheng Qunfeng is the corresponding author, 2014 straight Bo Sheng Wan Sijie is the first author, Beihang is the only communication unit.
It is of great research significance to assemble rich, inexpensive natural graphite into high-performance graphene films at room temperature. At present, low-temperature oxidation can effectively strip graphite into high-quality graphene oxide (GO) nanosheets; and hydroiodic acid can efficiently reduce graphene oxide at room temperature. It has been reported that large-area single-layer graphene films can be synthesized by CVD, but how to assemble low-cost GO nanosheets into macroscopic and high-performance graphene film materials is still a technical difficulty.
Natural abalone shells have excellent fracture toughness and have the following characteristics: (1) growth at room temperature; (2) ordered organic-inorganic alternating layered structure; (3) rich interface interactions. Inspired by this, Cheng Qunfeng's research group has proposed in recent years to improve the physical and chemical properties of graphene films by constructing different interface types to improve the interfacial interaction between graphene layers. The surface of graphene nanosheets has a large area of sp2 structure, which can provide abundant cross-linking sites for π-π stacking, which is conducive to improving interface strength. In addition, compared to other interface effects, π-π stacking can be maintained The conjugated skeleton structure of graphene nanosheets, therefore, the π-π stacking effect can simultaneously improve the tensile strength and conductivity of graphene films. However, because the cross-linking agent used for π-π stacking is usually a small molecule, it greatly limits the slippage of the graphene nanosheets during the stretching process, so it is difficult to greatly improve the performance of the graphene film.
Based on this, based on the previous research, Cheng Qunfeng's group recently designed a long-chain π-π stacking cross-linking agent to cross-link the reduced GO nanosheets into a super strong, super tough and highly conductive graphene film. The long-chain π-π stacking crosslinker is composed of 10,12-docosadienedioic acid dimethyl ester monomer polymerization; it can not only cross adjacent graphene nanosheets through fluorenyl groups at both ends. It can also grow long chain molecules by 1,4-addition of diyne group. The tensile strength and toughness of long-chain π-π stacked cross-linked graphene (π-bridged graphene (πBG) films reach 1054 MPa and 36 MJ / m3, respectively, which are the highest values reported in the literature; the electrical conductivity is 1192 S / cm, which is equivalent to graphene film treated at high temperature. Because the long-chain π-π stacking effect can improve the regularity of the graphene nanosheets, the πBG film has an efficient electromagnetic shielding effect. In addition, under cyclic stretching and folding deformation, the πBG film also has ultra-high fatigue resistance and excellent performance stability. More importantly, this work revealed the strong and tough mechanism of long-chain π-π stacking through in-situ Raman characterization and molecular dynamics simulation, and provided important theoretical guidance for the assembly of nano-primary materials.
The preparation process of πBG thin film is shown in Figure 1A. First, the GO aqueous solution is vacuum-filtered into a GO thin film; then, the GO thin film is converted into an rGO thin film by hydroiodic acid (HI) reduction; finally, the rGO thin film is immersed in 10, 12 -A πBG thin film in a solution of dioxanyl docosadienoate and UV light. The πBG film has excellent flexibility (Figure 1B) and an ordered layered structure (Figures 1C and D). Furthermore, wide-angle laser scattering (WAXS) results show that the πBG film (Figure 1F) has a more regular lamellar orientation than the rGO film (Figure 1E).
The tensile stress-strain curve of the πBG film is shown in Figure 2A, and its mechanical properties can be optimized by changing the content of the crosslinking agent. As shown in Figure 2B, the tensile strength, toughness, and electrical conductivity of the optimized πBG film reached 1054 MPa, 36 MJ / m3, and 1192 S / cm, respectively, which were 2.9, 4.6, and 1.3 times that of rGO films. This kind of performance is better than other room temperature cross-linked graphene films reported in the literature (Figure 2C). Due to its excellent conductive properties and an ordered layered structure, the πBG film has higher electromagnetic shielding effectiveness than that of rGO film (Figure 2D), and the electromagnetic shielding effectiveness in the frequency range of 0.3 to 18 GHz is 36.5 dB. As shown in Figure 2E and F, the main shielding mechanism of the πBG film is absorption. In addition, compared with other solid solid shielding materials, the πBG film has a lower density and therefore has a higher specific shielding effectiveness (Figure 2G).
Compared with rGO film, this πBG film has higher tensile fatigue resistance (Figure 3A), and can be stretched 264811 times under a stress of 780 ~ 860 MPa (Figure 3B). Due to the excellent fatigue crack suppression ability, πBG films have higher stability under cyclic stretching (Figure 3C) and bending (Figure 3D). The retention rates of tensile strength, electrical conductivity, and electromagnetic shielding effectiveness of πBG films were 93.4%, 85.3%, and 89.3%, respectively. After being folded 1,000 times in a 360 ° cycle, the tensile strength, electrical conductivity, and electromagnetic shielding effectiveness retention of πBG films were 81.2%, 78.4%, and 84.1%, respectively.
In situ Raman test results show that compared with rGO films (Figure 4A), πBG films (Figure 4B) have a larger G-peak displacement when fractured, indicating that long-chain π-π stacking has an efficient stress transfer efficiency. In addition, the molecular dynamics simulation tensile stress-strain curve is consistent with experimental test results (Figure 4C). As shown in FIG. 4D, during simulated tensile, the graphene sheet layer of the πBG film is first straightened, and then the long-chain π-π crosslinker is gradually straightened, thereby providing larger graphene nanosheets. At last, the cross-linking agent slips away from the graphene nanosheets, and the film breaks. Fig. 4E is a cartoon diagram of the corresponding fracture process of the πBG thin film, showing that the strong mechanism of the long-chain π-π stacking effect is an efficient stress transfer efficiency and a large slip of the graphene nanosheets, which is in line with the in-situ Raman test The results match. Compared with the rGO film (Fig. 4F), the cross-sectional morphology of the πBG film (Fig. 4G) exhibits more pronounced edge curl, which further proves the efficient stress transfer efficiency of long-chain π-π stacking.
Cheng Qunfeng's group is a high-performance multifunctional graphene film crosslinked by long-chain π-π stacking. It is expected to replace commercial carbon fiber fabric composites in the future and be used in aerospace and flexible electronic devices. Combined with advanced large-scale preparation technology, the long-chain π-π stacking cross-linking strategy proposed in this paper opens up new ideas for the preparation of high-performance graphene nanocomposites.