Free-Standing Halogen-Bonded Nanosheets Formed by Ultrasonic Liquid Exfoliation

Strong in-layer interactions are typically considered a key requirement for exfoliating layered materials to form nanosheets. Here, the formation of free-standing nanosheets held together by halogen bonding interactions is reported for the first time. Layered co-crystals are synthesized using either iodo- or bromo-functionalized halogen bond donors with pyridyl functionalized acceptors. Sonication resulted in dissolution in a range of polar organic solvents; however, stable suspensions showing Tyndall scattering are observed for the iodo-systems after exfoliation in water. Atomic force microscopy confirms the formation of micron-sized nanosheets approaching monolayer thickness. Powder X-ray diffraction of the nanosheets shows preferred orientation along the (1 1 0) plane, which confirms that the nanosheets are composed of halogen-bonded chains in one dimension, and π-π stacking interactions in the second. The iodo-nanosheets are stable in acidic and alkaline conditions and when heated at 80 °C for 48 h. Comparisons between the iodo- and bromo- analogues confirm that stronger halogen bonding is likely responsible for the difference in stability. The remarkable stability shown by these nanosheets held together by only supramolecular interactions calls for re-evaluation of the conventional understanding of the requirements for forming 2D materials and establishes halogen-bonded nanosheets as an exciting new class of materials.