Transformation of Nanocubes into Nanoframes

Metal–organic frameworks (MOFs) are crystalline, porous materials composed of metal centers and organic linkers. They have applications, e.g., in catalysis or gas separation. Investigating the crystallization and growth of MOFs can be useful, and different nucleation mechanisms and crystal growth models have been proposed. However, understanding crystal transformations between MOF structures that have the same building blocks has often remained challenging.

Zeolitic imidazole frameworks (ZIFs) are widely used examples of MOFs with tetrahedral metal ion nodes and imidazolate ligands. ZIF-67, for example, has a three-dimensional structure that relies on Co–N coordination. Another crystalline structure, named ZIF-L, can be obtained from the same building blocks as ZIF-67. ZIF-L has a two-dimensional, layered structure.

According to Junwu Zhu, Yongsheng Fu, Nanjing University of Science and Technology, China, and colleagues, the spontaneous transformation from ZIF-67 to ZIF-L should theoretically be almost impossible due to thermodynamic barriers. Nevertheless, the team achieved such a transformation from 3D ZIF-67 to 2D ZIF-L by utilizing the Marangoni effect. The Marangoni effect is a mass transfer along an interface between two phases due to a gradient of the surface tension.

The team first synthesized ZIF-67 nanocubes from cobalt(II) nitrate hexahydrate and 2-methylimidazole (2-Hmim), using cetyltrimethylammonium bromide (CTAB) in deionized water. The desired nanocubes then precipitate, and after removal of most of the supernatant, a droplet of the remaining mother liquid was transferred onto a silicon plate. ZIF-L starts to form under these conditions and coats the edges of the ZIF-67 nanocubes. As the solvent evaporates, ZIF-L continues to grow and ZIF-67 disintegrates, until the ZIF-67 nanocubes are gone and replaced by cube-shaped ZIF-L “nanoframes”.

The team states that the Marangoni effect inside the droplet regulates the transport of matter and provides the driving force for the formation of ZIF-L. They propose that employing this effect could be useful for the preparation of other novel materials.