Nanoporous crystalline materials, specifically metal-organic frameworks (MOFs), have been extensively studied due to their remarkable properties. Typically, MOF films consist of organic linkers and metal ion clusters and should be fabricated on various substrates in a facile manner for practical applications. They are commonly fabricated via liquid-phase epitaxy (LPE) and in-situ solvothermal growth methods. LPE process involves immersing self-assembled monolayer substrates in the organic linker and metal ion solutions, a process suitable for fabricating homogenous and flat films. On the other hand, solvothermal growth involves growing the films on substrate immersed in the mother MOF solution. This approach is particularly appropriate for the selective growth of MOFs, a process that requires substrate surface modifications. These, together with other approaches like thermal gradient synthesis, are limited and not viable for large-scale and commercial production of MOF films.
The spray-assisted process has been identified as a promising technique for the commercial production of MOF films. This technique has several advantages, including scalability, process simplicity and reduced fabrication time. Previous studies have demonstrated the feasibility of the spray-assisted technique in preparing HKUST-1 and MOF thin films on various substrates. However, despite the good progress, the existing studies fail to provide adequate insights into the underlying process mechanism behind the formation of MOF thin films. Additionally, the fabrication of MOF films based on facile incorporation of different nanomaterials is sparsely explored.
To address the mentioned challenges, Professor Masaru Kubo, Dr. Takuya Sugahara and Professor Manabu Shimada from Hiroshima University developed a new simple spray-assisted method for facile fabrication of HKUST-1 thin films. The films were fabricated on different substrates like porous alumina and Si(100) wafer. Specifically, the effects of spraying time and temperature on the film morphology and thickness were investigated through electron and optical microscopes to elucidate the HKUST-1 films formation process. The work is currently published in the journal, Microporous and Mesoporous Materials.
In their approach, a two-fluid nozzle was used to spray the HKUST-1 precursor solutions on the heated substrates. The fabrication process consisted of two critical steps. In the first step during the initial spraying, nanoparticles were formed from the droplets on the substrate surface. The second step was characterized by the growth of the seed particles to form thin films by attaching more droplets. Lastly, the authors demonstrated the possibility of obtaining freestanding HKUST-1/MWCNT films by incorporating the multi-walled carbon nanotubes (MWCNTs) into the fabricated films.
Results demonstrated the successful synthesis of the HKUST-1 films on various substrates within 3 minutes by adding MWCNTs to the spraying solution. Consequently, a freestanding film was facilely fabricated by peeling off the HKUST-1/MWCNT films. The thickness of the film was controlled by altering the volume of the sprayed solution. It was worth noting that the porosities of the resulting films agreed well with those of the HKUST-1 powders prepared via both conventional spray-assisted and batch synthesis processes.
In a nutshell, a facile and one-step spray-assisted method was reported for the fabrication of HKUST-1 and freestanding MWCNT/HKUST-1 films. The fabrication process proved advantageous in terms of simplicity and fabrication time. Besides, the spray-assisted technique is versatile and exhibited potential for synthesizing various nanomaterial-incorporated metal-organic frameworks on large surface areas. This would expand further its applications across various fields. First author Professor Masaru Kubo mentioned to Advances in Engineering that improving the spray method would enable the fabrication of various MOF films for various applications such as nanofiltration.
Kubo, M., Sugahara, T., & Shimada, M. (2021). Facile fabrication of HKUST-1 thin films and freestanding MWCNT/HKUST-1 film using a spray-assisted method. Microporous And Mesoporous Materials, 312, 110771.