Solution-based exfoliation of non-layered, non-van der Waals materials and applications: current status and future directions

Abstract

The exfoliation of non-layered and non-van der Waals (NvdW) materials is of recent research interest due to their exciting properties. Unlike layered vdW solids, which are easy to exfoliate due to weak interlayer interactions, these materials possess strong bonding networks in three spatial dimensions and high exfoliation energies. Initially, it was believed that these materials were unexfoliable, and therefore, the early research to synthesize these nanosheets primarily relied on bottom-up techniques. Later, various studies indicated that non-layered materials also possess structural anisotropy or preferential cleavage planes that can be exploited for exfoliation. To date, many such bulk materials have been thinned down into nanosheets using solution-based exfoliation techniques and have shown great potential in a wide range of applications, including optoelectronics, energy storage and harvesting, catalysis, magnetism, and biomedical applications. Therefore, it is essential to summarize the recent progress in the exfoliation of such materials. In this review, we highlight recent advances in solution-based exfoliation techniques, the underlying exfoliation mechanism, and discuss the critical aspects such as solvent selection, dispersion stability, and chemical stability. Finally, a summary of the challenges and prospects of these NvdW materials via solution processing is provided.