Aerosol-assisted chemical vapour deposition of MOF thin films

Lead Research Organisation: University of Southampton
Department Name: School of Chemistry

Abstract

Aerosol assisted chemical vapour deposition (AACVD) is a scalable and commonly used method for making thin films of diverse inorganic materials. On the other hand, many applications of high surface area crystalline microporous metal organic frameworks (MOFs) formed by the self-assembly of metal-ions and organic linkers also require a thin film configuration. Current methods for the generation of MOF thin films such as layer-by-layer deposition however, are often labour intensive or require long reaction times and are not generally scalable to cover large area substrates. AACVD offers an exciting alternative, and this project will systematically investigate the potential of AACVD methods to generate a range of MOF thin films onto diverse substrates and a subsequent investigation of their energy-related and photochemical applications and/or antimicrobial properties.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509747/1 01/10/2016 30/09/2021
1934793 Studentship EP/N509747/1 28/09/2017 30/09/2020 Iona Doig
 
Description As MOF applications are becoming ever more diverse, the need for an industry suited synthetic method for MOF thin films is becoming ever more pressing. Currently many methods used for the synthesis of MOF thin films are modifications of the solvothermal synthesis, and as such are not well suited for large scale synthesis. This project focuses on the use of aerosol assisted chemical vapour deposition (AACVD), a scalable technique used for thin film synthesis.
AACVD is a variant of CVD, in which the precursors are transported via aerosol droplets. The precursors are dissolved in a suitable solvent, one that has a low vapor pressure, low viscosity and a suitably high surface tension to endure the formation of an aerosol. This therefore means solvents commonly used to synthesise MOFs, such as water and DMF are unsuitable for the AACVD process, as they are unable to form suitable aerosols. Despite this, one major advantage of AACVD over CVD is that the precursors used do not have to be volatile, they are only required to be soluble in the suitable solvents.
In this project it has been demonstrated that AACVD can offer a simple and scalable synthetic rout to adherent and crystalline MOF thin films. The quality of the film was judged by its crystallinity, purity, adherence and coverage of substrates up to 5 x 15 cm. The synthesis consisted of a two-step process. This involved the dissolution of the precursors into methanol and placing both precursors into a flask. The solution was sonicated, producing an aerosol, which was carried into the reactor using argon gas. The thin films were then studied using X-Ray diffraction in order to gauge its crystallinity and composition. SEM images were also obtained to understand crystal growth and film thickness. Optimisation was required to ensure that the precursors reacted only once inside the reactor. To do this both the precursors, solvent, temperature and flow rate were all altered in order to produce a film. Thin film thickness could be controlled by altering the quantity of precursors.
In order to ensure that the formation of the MOF was occurring inside the reactor, a duel vessel synthesis was implemented. This involved placing each precursor in a different vessel, with an aerosol of each precursor formed. The separate aerosol flows converged just prior to entering the reactor, this ensured any thin film synthesised was not due to carrying preformed framework. As this synthesis produced a pure crystalline thin film, it could be concluded that the films formation was at least partially due to a reaction undergone in the reactor.
In order for the thin films to be suitable in applications such a gas storage and separation, it was important to maximise the films adherence on the substrate. In general, most MOF's had poor adherence on the glass substrate, therefore, zinc oxide was deposited onto glass substrate prior to the MOF deposition to investigate the effect on the MOFs adherence. The film adherence was shown to improve with the roughness of the substrate surface. This was further demonstrated as the adherence of the films was greatest on alumina.
In total, 5 MOFs have been synthesised using AACVD; ZIF-8, ZIF-67, MOF-74 (Zn), MOF-74 (Co) and HKUST-1 (Cu). Each MOF has shown to be pure and crystalline, with their adherence varying dependant on the metal used in the framework. All MOF were deposited on both glass substrates and alumina. The films adherence and coverage make these films promising for applications such as separation and catalysis. The technique itself shows much promise for synthesis on a larger scale.
Finally, MOF photocatalysis was looked at as a promising application for some of the frameworks, due to the ability to fine tune the band gap. For this the amino modified frameworks were synthesised in bulk, as these have been proven to reduce the size of the band gap. Photocatalysis was carried out on ZIF-8, NH2-MIL-53 (Al), NH2-MIL-101 (Fe) and NH2-UiO-66 (Zr). However, these frameworks showed very little evidence of photocatalytic capabilities, especially when compared to a conventional photocatalyst like TiO2.
Exploitation Route To apply thin films synthesised using AACVD in applications such as catalysis and separation, and to further understand the MOF growth method.
Sectors Chemicals,Environment