Two-dimensional (2D) materials have attracted wide interest for their unique electronic structure and abundant physiochemical properties. Due to the weak interlayer interactions, the van der Waals (vdW) materials can be exfoliated into monolayers through various processes while retaining the similar composition and structure to those of the bulk material. However, it remains a challenge to prepare the non-vdW layered materials with properties similar to the parent bulk material because of its stronger interlayer interactions.

Recently, the research team led by Prof. WU Changzheng and Prof. WU Xiaojun from the University of Science and Technology of China (USTC), achieved the exfoliation of the non- vdW solid of AgCrS₂ into 2D sheets without changing the properties. The as-exfoliated AgCrS2 nanosheets showed an excellent ionic conductivity at room temperature. This study was published in Nature Chemistry.

The research team synthesized the stoichiometric 2D non-vdW AMX₂ chalcogenide AgCrS₂，where A is a monovalent metal, M is a trivalent metal and X is a chalcogen, through a redox-controlled method. Utilizing controllable electrochemical intercalation, researchers selected the suitable redox potential difference between the mental A and tetraalkylammonium cations (TAA⁺). As a result, they successfully exfoliated AgCrS₂ into 2D sheets that consisted of similar composition and structure to those in the bulk material, adding a new member to the 2D material family. The as-exfoliated AgCrS₂ nanosheets were composed of an Ag layer sandwiched between two CrS₂ layers, and conformed to the formula Ag_nCr_n+1S_2(n+1), where n is the number of Ag layers.

Besides, the team also observed that the ionic conductivity of AgCrS₂ nanosheets increased dramatically along with the decrease of layer thickness. And the monolayers displayed a superionic conductivity behavior at room temperature which was three orders of magnitude higher than that of the bulk. Theoretical calculations showed that by exfoliating the bulk AgCrS₂ into nanosheets, the migration energy barriers of Ag⁺ ions along the tetrahedral voids significantly reduced, thus stabilizing the high-temperature superionic conductivity phase in the bulk (only observed above 673K) to room temperature.

This study identified a new member for the two-dimensional material family, and provided new approaches to the synthesis and investigation of the 2D non-vdW materials.

Two-dimensional structure of AgCrS₂ nanosheets and ionic transport performance in AgCrS₂ nanosheets. (Image by PENG Jing et al.)

(Written by YANG Min, edited by WANG Zhaokun, USTC News Center)