Researchers from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences, in collaboration with partners from the Shenzhen International Graduate School of Tsinghua University, Institute of Metal Research of CAS, have successfully synthesized Bulk van der Waals (vdW) materials at near-room temperature (ranging from room temperature to 60°C), significantly reducing the energy consumption required for their fabrication by at least one order of magnitude. 

The results were published in Nature Materials. 

Previously, Bulk van der Waals (vdW) materials, such as graphite and hexagonal boron nitride, could only be synthesized at very high temperatures (>1000°C). Instead of directly sintering graphite or boron nitride particulates at such high temperatures, the particulates were exfoliated into two-dimensional (2D) nanosheets with a very low energy cost. Subsequently, a molding process at 45°C (or even at room temperature) was employed to transform these nanosheets into mechanically robust bulk vdW materials. 

The method applies to a wide range of 2D materials, including MXene and transition metal dichalcogenides. Its low fabrication temperature also allows for surface imprinting and in-situ shaping, which are challenging with high-temperature sintering due to thermal-induced shrinkage and expansion. Additionally, the additive-free vdW materials facilitate high-temperature applications where 2D material-based polymeric composites fail. 

This result mainly stems from the formation of vdW interaction, providing the manufactured bulk materials with high mechanical strength. Activating the vdW interaction doesn't require high temperatures but rather nanometer or sub-nanometer contact between adjacent nanosheets. The thinness and flexibility of the 2D nanosheets make them easily movable and deformable, facilitating intimate contact formation. 

The team also found that water adsorbed on the nanosheets is a powerful "sintering aid" which lubricates the nanosheets to give good alignment, and once this is established, the confined water desorbs from the nanosheets and escapes from the material due to the nano-confinement effects, which closes the capillary, activating the vdW interaction and resulting in a densified, strong bulk vdW material. 

"This process simplifies fabrication and reduces the high energy costs associated with bulk vdW materials production, offering scalability," said Prof. SU Yang, the corresponding author from the Shenzhen International Graduate School of Tsinghua University, "and it could also introduce innovative approaches to vdW material design, such as hybridizing various 2D materials, particularly those unstable at high processing temperatures.” 

Looking ahead, Prof. CHENG Huiming from SIAT stated that it suggests enhancements to traditional material processing methods through nanomaterial utilization. 

Fig. 1 (a) Schematic showing the exfoliation of nanosheets and their subsequent molding to form bulk vdW materials, (b) the mechanical strength and (c) the porosity analysis and (d) the microstructure of the bulk vdW hexagonal BN materials. (Image by SIAT) 

Fig. 2 (a) Mechanism of the near-room-temperature fabrication of vdW materials and (b-d) their applications. (Image by SIAT) 

Media Contact:
ZHANG Xiaomin
Email:xm.zhang@siat.ac.cn