Covalent Organic Framework Films through Electrophoretic Deposition - Creating Efficient Morphologies for Catalysis

The ability to grow covalent organic framework (COF) films allows for studying their properties as solid layers and enables the incorporation of these materials into a variety of functional devices. Here, we report on the fabrication of COF films and coatings by electrophoretic deposition (EPD). We demonstrate that the EPD technique is suitable for depositing COFs featuring two- and three-dimensional structures linked by imine or boronate ester bonds, namely, BDT-ETTA COF, COF-300, and COF-5. For the deposition, COF nanoparticle suspensions are prepared by dispersing the as-synthesized bulk materials in solvents with low dielectric constants. Subsequently, two electrodes are immersed into the COF particle suspensions, and upon inducing electric fields ranging from 100 to 900 V cm^-1, COFs are deposited as films on the positively charged electrode. Through EPD, within 2 min, large-area films of up to 25 cm² are obtained on smooth or corrugated surfaces. COF films prepared by EPD feature an inherent textural porosity and tunable thickness, demonstrated from 400 nm to 24 μm. By controlling the deposition parameters such as duration, particle concentration, and applied potential, deposits of precise thickness can be produced. Furthermore, codepositions of different COFs as well as COF/Pt nanoparticles from mixed suspensions are demonstrated. The film morphologies obtained by EPD are shown to be advantageous for catalysis, as demonstrated for sacrificial agent-free photoelectrochemical water reduction. Here, BDT-ETTA COF photocathodes show a strongly increased photocurrent density compared to the respective dense and oriented films. Typical BDT-ETTA COF/Pt nanoparticle hybrid films exhibit photocurrent densities of over 100 μA cm^-2. The rapid and scalable deposition of COF particles as films and coatings through EPD is a versatile addition to the toolbox of COF film fabrication techniques, allowing for tailoring COF film architectures for desired functionalities.