Covalent organic frameworks (COFs) are a group of crystalline materials that are structurally supported by covalent bonds mainly formed between light atoms. The most important and intriguing feature of COFs, crystallinity, is usually achieved by solvothermal synthesis, which is a thermodynamic-dominant process. But, the thermodynamics in this synthesis methodology also afford the COFs in a powdered state including innumerous tiny particles with single or multi-crystallinity at the microscopic scale. These discrete particles cannot be rebuilt to form a large bulk or a piece with structural consistency for intrinsic physiochemical investigations or practical application exploration1. Thus, to obtain a COF with structural consistency at the macroscopic scale, for instance a large area film, a bottom-up strategy must be applied to the COF formation process.
Upon enormous efforts, the research in making 2D COF films has gotten delightful achievements. Many successful cases exist, with reported 2D COF films having a size reaching wafer scale2 and even with single crystallinity3. However, the development of 3D COF films is way behind. This because making 3D COF films is a more challenging. Unlike 2D COFs with intrinsic 2D structures in molecular layers, 3D COFs have covalent networks extending in full 3D space. This means the structure build-up has no preferable orientation in a homogenous condition. For achieving a film state of a 3D COF, an extra control needs to be added to induce a planar growth of the material. Although difficult, the purely covalent bond supported 3D COFs, as an organic sibling of covalent crystals such as diamond and quartz4, would be very interesting to be seen and explored in a thin film state.
Our group has been dedicating to develop strategies for making 3D COF films in the past few years. In previous work, we have realized a controlled synthesis of all-carbon linked 3D COF films on the surface of a quartz crystal microbalance chip, using a continuous flow system5-8. Nevertheless, the size of the film was limited. Also, the widespread use of method was constrained by the particular reaction type and the special infrastructure. Thus, we have been eager to find a simpler method with enhanced compatibility to the major reaction type for COFs.
Now, in Nature Communications, we report on a method to fabricate 3D COF films having a large area using the most popular chemistry in the COF community, imine bond formation. From the physical-organic consideration, a liquid-liquid interfacial approach was applied to induce a two-dimensional growth of the 3D COF. In this method, the different reactants were designed to separate in the different phases of two immiscible solvents. Accordingly, the reaction rate distribution along the z direction decides that the COF networks can only extends at the liquid-liquid interface. The network propagation deviated from the interface are eliminated hastily with increase distance to the interface. As a result, the 3D COF was constrained at the interface and formed a 2D shape.
Benefitting from the molecular-smooth interface as a template, the 3D COF film is close to molecularly smooth, have a uniform thickness of 13 nm, and an area of several cm squared. Regularly aligned lattices in the film was observed by TEM, indicating a highly crystalline state. This covalent crystal film shows an anisotropic chemical environment in the different facets that exposed to different phases, as detected by XPS. Comprehensive nanomechanical research by AFM reveals that the 3D COF film shows considerable mechanical robustness, as well as unusual flexibility and elasticity for an 3D covalent crystal. More details about this work can be found in Nature Communications ‘A self-standing three-dimensional covalent organic framework film’ via link Nat Commun 14, 220 (2023).
Figure caption. Schematics of the kinetics of the reaction between DPP-CHO and TAPM in the liquid-liquid system. The top phase is a mixture of cyclohexane and dioxane. The bottom phase is a mixture of aqueous acetic acid and dioxane. When equilibrium is reached, the concentration of TAPM in the top phase is close to zero and the concentration of DPP-CHO in the bottom phase is close to zero. Taken from Nat Commun 14, 220 (2023).
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