Liquid-Phase Exfoliation of 3D Metal-Organic Frameworks into Nanosheets

Traditionally, the acquisition of 2D materials involved the exfoliation of layered crystals. However, the anisotropic bonding arrangements within 3D crystals indicate they are mechanically reminiscent of 2D counterparts and could also be exfoliated into nanosheets. This report delineates the preparation of 2D nanosheets from six representative 3D metal-organic frameworks (MOFs) through liquid-phase exfoliation. Notably, the cleavage planes of exfoliated nanosheets align perpendicular to the direction of the minimum elastic modulus (E_min) within the pristine 3D frameworks. The findings suggest that the in-plane and out-of-plane bonding forces of the exfoliated nanosheets can be correlated with the maximum elastic modulus (E_max) and E_min of the 3D frameworks, respectively. E_max influences the ease of cleaving adjacent layers, while E_min governs the ability to resist cracking of layers. Hence, a combination of large E_max and small E_min indicates an efficient exfoliation process, and vice versa. The ratio of E_max/E_min, denoted as A_max/min, is adopted as a universal index to quantify the ease of mechanical exfoliation for 3D MOFs. This ratio, readily accessible through mechanical experiments and computation, serves as a valuable metric for selecting appropriate exfoliation methods to produce surfactant-free 2D nanosheets from various 3D materials.