Vapor-Based Synthesis of ZIF-8 Thin Films and Gas Adsorption Properties
Authors: Perrot Virginie , Roussey Arthur , Licitra Christophe , Ricoul Florence , Quadrelli Elsje Alessandra , Jousseaume Vincent 
Published: December 2020

On-site and real-time measurement of gas concentrations are crucial for both the understanding and the monitoring of industrial and environmental processes. In recent years, there is an increasing need to develop portable multi-gas analysis tools allowing in situ detection of complex gas mixtures mainly due to safety, process and environmental considerations. A promising approach is based on the integration of the different parts of the analytical system (i.e. pre-concentrator, gas chromatography column, gravimetric sensors) in a silicon die by using standard microelectronic technologies. Each of these devices need to be coated by an appropriate functional layer.
Metal Organic Frameworks (MOF), hybrid microporous crystalline materials with tuneable properties, are attractive for this type of application regarding their high specific surface area and chemical stability. However, these materials are usually synthetized via solvothermal techniques, which complicates the growth of continuous thin films and their integration in micro-devices. Recently, vapor phase-based routes were reported for the synthesis of MOF thin films [1-2]. These solvent-free synthesis methods hold great economic and environmental perspectives since they can not only avoid the use of solvents and metallic salts, but also reduce contamination and impurity incorporated into the crystals during the synthesis step. Moreover, vapour-phase process are usually preferred to obtain conformal coatings in the high aspect ratio features of devices and they present a good compatibility with the Si technology. These breakthroughs have paved the way for the use of MOF in micro- and nanotechnologies but much still remains to be done to control the growth and understand the potential and the limits of these growth methods.
In this work, pinhole-free and crystalline zeolitic imidazolate framework-8 (ZIF-8) layers were grown on different types of substrates and devices that can be used for gas analysis. Si wafers, Quartz Crystal Microbalance, Si micro-pillars arrays and 3D foam were envisaged. First, ZnO films were deposited by Atomic Layer Deposition (ALD). Then, the formation of ZIF-8 was realized through a vapor-solid reaction with a vaporized linker (2-methylimidazole). Continuous ZIF-8 films were obtained and the impact of the process parameters on the MOF growth and on the material properties was studied. Additionally, the composition, roughness and structure of the films were studied by FTIR, EDX, XPS, XRD and AFM experiments. Finally, the as-synthesized films were thermally activated and the porosity was assessed using ellipsometric-porosimetry. The adsorption properties of the films were also investigated using gravimetric gas sensors. Several gases were tested (methanol, toluene, formaldehyde) in order to determine the benefit of ZIF-8 thin films in sensors. It is demonstrated that the synthesis approach based on an ALD deposition followed by a vapor-solid treatment in appropriate conditions is compatible with different kinds of substrates, which allows the use of ZIF-8 as sensitive layer in devices for gas sensing.
[1] I. Stassen et al., Nat. Mat., 15, 304–310 (2016)
[2] E. Ahvenniemi et al., Chem. Com., 52, 1139-1142 (2016)