Anodic Electrochemical Deposition of Metal-Organic Framework Films

Metal-organic frameworks (MOFs) are highly porous materials, which consist of inorganic metal nodes coordinated by multitopic organic linkers. Thanks to their highly tunable chemical and structural properties, ordered pores, and high surface area, MOFs have attracted ever-growing interest in many applications. For many of these applications, such as membrane-based separations, sensors, electrocatalysis, and energy storage, it is required to integrate MOFs as films on a specific substrate. Nevertheless, most MOFs are prepared in powder form with low processibility due to their crystalline nature. Thus, there is an urgent need to develop robust and scalable MOF film synthesis methods to achieve their commercialization. Several techniques have been proposed for the in situ MOF deposition. Among them, anodic electrodeposition has been considered as one of the most promising synthesis approaches owing to the advantages of low cost, simple operation, strong repeatability, high efficiency, good film quality, real-time control, and the ability for roll-to-roll production.

The objective of this thesis is to advance the anodic technique from the applicability and controllability aspects by understanding the nucleation, crystallization, crystal growth, and film growth in the deposition process.

First, the nucleation of zeolitic imidazolate frameworks (ZIFs), an important subclass of MOFs, in anodic MOF film deposition was investigated. These materials are interesting for applications due to their stability in alkaline conditions and tunable pore size. However, the anodic deposition of ZIF films was considered challenging because of the long incubation time before nucleation. To solve this problem, an ammonia coordination strategy is demonstrated as a novel route for accelerating their nucleation kinetics. Theoretical calculations reveal that the zinc ammine complexes promote the formation of the key intermediates by reducing the energy barrier for the coordination of ligands and metal ions. As a result, the deposition of continuous ZIF-8 films can be achieved within 100 s. In addition, thanks to the ultra-fast nucleation kinetics, the utilization rate of the anodic dissolved zinc ions for film deposition can reach 77.5%, limiting the undesired nucleation in the solution. Several ZIFs with different metal nodes and organic linkers are successfully deposited as thin films via this strategy. These findings not only broaden the applicability of the anodic electrodeposition technique but also provide a new perspective for controlling the nucleation kinetics of MOFs under mild conditions.

Apart from nucleation, the controllability of crystal growth during anodic deposition is also important. In this thesis, inspired by the Kelvin equation, MOF films with specific exposed facets are electrodeposited anodically on various substrates by tuning the deposition parameters, including the pH of the solution, current density, concentration of linker, and solvent. The results demonstrate that precise control of the supersaturation during crystal growth processes is an effective method for depositing MOF film with well-defined facets.

In addition, the thickness and adhesion of the MOF film are crucial to their applications as well. Nevertheless, the anodically deposited MOF films suffer from typical detachment issues during the deposition process since after a closed film formation, the dissolution of the metal at the film-substrate interface weakens their contact. Here, a pre-anchored substrate modification strategy is demonstrated for improving the adhesion between MOF films and substrates. This strategy makes it possible to grow much thicker MOF films by anodic deposition (≥40 μm). Nano-scratch tests show that the MOF films on the modified substrate have a much higher adhesion strength than those prepared on the non-modified substrates.

The last part of this thesis discusses the sustainable synthesis of iron(III) carboxylate MOF films via anodic techniques. These MOFs are interesting due to their robust thermal and chemical stability, large pore size, and nontoxic metal sites with redox activity. However, because of the crystallization issues, the deposition of iron(III) carboxylate MOF films is challenging, and most of these methods require high temperature or corrosive acids, which increases the cost and raises environmental concerns. Here, a room temperature oxygen-assisted anodic deposition method is demonstrated for synthesizing crystalline iron(III) carboxylate MOF films. By tuning the initial oxidation state of iron as Fe2+, highly crystalline MIL-100(Fe) films are anodically deposited in oxygen-saturated aqueous electrolytes without corrosive acids. The crystallinity of the deposited MIL-100(Fe) film improves via the solid-solid aggregation of the semi-amorphous nanoparticles with the deposition time. X-ray photoelectron spectroscopy (XPS) measurements confirm the co-existence of ferrous ions and ferric ions in the MOFs, and the ferrous ions gradually transform to ferric ions with the deposition time.