ZJU researchers create strong graphene films with nanoscale fold locks

Researchers at Zhejiang University have developed a novel method for assembling graphene into exceptionally strong films by utilizing nanoscale fold locks. This breakthrough, detailed in a study published in Nature on October 14, was led by Professor Gao Chao and Associate Professor Xu Zhen from the Department of Polymer Science and Engineering at Zhejiang University, in collaboration with Professor Xu Zhiping’s team at Tsinghua University.

The innovation involves creating tiny folds on the surfaces of multilayer graphene oxide flakes, which allow the layers to interlock much like plastic bricks, thereby overcoming a common weakness in graphene-based materials. This approach marks a significant advancement in the design of two-dimensional materials, enabling researchers to engineer three-dimensional structures to enhance performance.

Graphene, known for its strength, stiffness, and electrical and thermal conductivity, is composed of a single layer of carbon atoms in a honeycomb lattice. However, its smoothness can be a disadvantage when sheets are stacked, as they are primarily held together by weak van der Waals forces. This can lead to sliding under stress, causing the structure to fracture despite the strength of individual sheets.

Professor Gao Cao’s team has been exploring ways to address this issue for nearly two decades. They proposed using delicate fold structures as locks to prevent sliding between layers. The challenge was creating these folds reliably. By lowering the reaction temperature of a graphene oxide dispersion to below freezing, the researchers produced films with remarkable strength.

High-resolution electron microscopy revealed that the films contained sandwich-like flakes with flat, crystalline layers in the center and folded, roughened surfaces on the outside. These folds could be generated simply by controlling the reaction temperature.

“During coating, these surface-folded multilayer graphene oxide flakes spontaneously assemble and clasp together, forming a stable macroscopic film that combines high strength, toughness, and elasticity,” explained Wang Lidan, a postdoctoral researcher at Zhejiang University and co-first author of the study.

The resulting film achieved a tensile strength of 2.9 gigapascals, approximately 50 percent higher than conventional graphene films and surpassing many conventional steels, while remaining lighter and thinner. Xu Zhen highlighted the material's remarkable combination of high strength, elasticity, and toughness.

Achieving these three properties simultaneously has been a longstanding challenge in materials science, as enhancing strength often compromises a material's ability to stretch or absorb energy. The new graphene structure appears to circumvent this trade-off.

The key is the nanoscale folds that function as hooks between adjacent graphene layers. In conventional films, smooth layers slip apart when pulled. In the new material, the folds act as hooks, converting pulling forces into compressive forces between layers as the film stretches.

This “pull-to-push” mechanism improves force transfer between layers, allowing the material to endure greater stress without losing toughness or elasticity. Nature offers a parallel in seashells, which have tiny protrusions between layers that strengthen the shell.

“But this kind of architecture has not previously been demonstrated in artificially assembled two-dimensional materials at the atomic scale,” Gao Cao stated. “That is one of the most important innovations of our work.”

The implications extend beyond graphene, as this method of engineering atomically thin sheets could lead to other lightweight materials that combine exceptional strength with toughness and elasticity.