Vapor-Phase Processing of Metal–Organic Frameworks

Porous metal–organic frameworks (MOFs), formed from organic linkers and metal nodes, have attracted intense research attention. Because of their high specific surface areas, uniform and adjustable pore sizes, and versatile physicochemical properties, MOFs have shown disruptive potential in adsorption, catalysis, separation, etc. For many of these applications, MOFs are synthesized solvothermally as bulk powders and subsequently shaped as pellets or extrudates. Other applications, such as membrane separations and (opto)electronics, require the implementation of MOFs as (patterned) thin films. Most thin-film formation methods are adapted from liquid-phase synthesis protocols. Precursor transport and nucleation are difficult to control in these cases, often leading to particle formation in solution. Moreover, the use of solvents gives rise to environmental and safety challenges, incompatibility issues with some substrates, and corrosion issues in the case of dissolved metal salts. In contrast, vapor-phase processing methods have the merits of environmental friendliness, control over thickness and conformality, scalability in production, and high compatibility with other workflows.

In this Account, we outline some of our efforts and related studies in the development and application of vapor-phase processing of crystalline MOF materials (MOF-VPP). We first highlight the advances and mechanisms in the vapor-phase deposition of MOFs (MOF-VPD), mainly focusing on the reactions between a linker vapor and a metal-containing precursor layer. The characteristics of the obtained MOFs (thickness, porosity, crystallographic phase, orientation, etc.) and the correlation of these properties with the deposition parameters (precursors, temperatures, humidity, post-treatments, etc.) are discussed. Some in situ characterization methods that contributed to a fundamental understanding of the involved mechanisms are included in the discussion. Second, four vapor-phase postsynthetic functionalization (PSF) methods are summarized: linker exchange, guest loading, linker grafting, and metalation. These approaches eliminate potential solubility issues and enable fast diffusion of reactants and guests as well as a high loading or degree of exchange. Vapor-phase PSF provides a platform to modify the MOF porosity or even introduce new functionalities (e.g., luminescence photoswitching and catalytic activity). Third, since vapor-phase processing methods enable the integration of MOF film deposition into a (micro)fabrication workflow, they facilitate a range of applications with improved performance (low-k dielectrics, sensors, membrane separations, etc.). Finally, we provide a discussion on the limitations, challenges, and further opportunities for MOF-VPP. Through the discussion and analysis of the vapor-phase processing strategies as well as the underlying mechanisms in this Account, we hope to contribute to the development of the controllable synthesis, functionalization, and application of MOFs and related materials.