Feb 10, 2021 (Nanowerk Spotlight) In order to exploit the remarkable mechanical properties of graphene for practical applications, nanoscale graphene sheets need to be assembled into much larger, macroscopic structures. However, there are two pivotal issues that make this task challenging: One is the inherently misaligned and wrinkled structure of graphene platelets; another is the weak interfacial interaction among graphene platelets. Both these problems greatly degrade the properties of macroscopic graphene assemblies such as sheets and fibers. While in-plane stretching can decrease the misalignment of graphene, it reappears when releasing the stretch. To address this issue, a research team in China used sequential inter-platelet bridging to permanently freeze the stretch-induced alignment, causing the simultaneous improvement in alignment, stacking compactness, and interfacial interactions of graphene nanoplatelets – thereby substantially increasing the mechanical and electrical properties of the resulting macroscopic graphene sheets. As the team reports in Nature Materials (“High-strength scalable graphene sheets by freezing stretch-induced alignment”), the graphene sheets obtained by this technique provide ultrahigh tensile strength (1.55 GPa), in combination with a high Young’s modulus, toughness, electrical conductivity and electromagnetic interference shielding capacity. “The stretch-bridged graphene sheets demonstrated in our work are scalable – using the simple process of doctor blade casting – and can be easily bonded together using a commercial resin without appreciably decreasing the performance, which establishes the potential for practical applications,” Qunfeng Cheng, a professor at Beihang University in Beijing who led this work, tells Nanowerk. “This novel strategy might also provide an avenue for fabricating high-performance assembled materials of other two-dimensional nanoplatelets.” Schematic illustration of the fabrication process and the structure of a sequentially bridged, biaxially stretched (BS) rGO sheet. A graphene oxide (GO) sheet made by filtration was biaxially stretched, infiltrated with PCO and then the PCO was polymerized using UV radiation. The BS-GO-PCO was reduced by hydrogen iodide (HI), and then PSE and AP were successively infiltrated into the sheet and reacted to form PSE–AP molecules while biaxial stretch was maintained. Scale bar: 1 cm. (Reprinted with permission by Nature Publishing Group) (click on image to enlarge) Cheng and his team report that the realized tensile strength and toughness of their graphene sheets are superior to those of carbon fiber fabric composites having isotropic in-plane mechanical properties that are currently used in various commercial products, such as car and aircraft frames. “We have obtained in-plane isotropic graphene sheets with a tensile strength that is 1.47, 2.50 and 1.41 times, respectively, that of the strongest previously described graphene composite, CNT composite, and carbon fiber fabric composite, having nearly isotropic in-plane properties,” Cheng summarizes the team’s results. He also points out that the graphene sheets are fabricated at near-room-temperature below 50 degrees Celsius using cheap graphite as raw material and are therefore much more cost-effective than commercial carbon fiber composites. According to the team, the higher gravimetric strength (that is extremely important in some fields requiring weight saving) combined with the lower-cost fabrication process of these graphene sheets might eventually allow them to replace the structural and electronic carbon fiber composites that currently are used for everything from aircraft and automobile bodies to windmill blades and sports equipment, as well as new applications in increasingly popular flexible electronics. We have reported on the team’s research efforts to improve the performance of graphene composite materials in several previous Nanowerk Spotlights: “High-performance graphene sheets for engineering high-tech materials”, “Stitching together ultrastrong graphene films” and “Improving the toughness of graphene films”. The researchers are now pursuing two major next steps in their research: On one hand, they will further develop the continuous fabrication process of the high-performance graphene sheets to make it more suitable for large-scale commercial applications. On the other hand, they will use the universal strategy of sequential inter-platelet bridging to assemble other two-dimensional nanoplatelets into high-performance materials for multifunctional applications. “Notwithstanding our findings, assembling 2D nanoplatelets into highly aligned and compact macroscopic assemblies, such as sheets and fibers with excellent mechanical properties, is a continuing challenge,” Cheng concludes. “An additional challenge is the commercial manufacturing of high-performance graphene nanocomposites for multiple functionality such as high electromagnetic interference shielding efficiency, smart separation, mechanical-adaptive, self-healing, etc, which will be highly desired for diverse applications. By

Michael
Berger
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Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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