Rice University researchers create thin films of covalent organic frameworks through vapor deposition. Materials scientists have created an efficient, cost-effective, and scalable production technology for covalent organic frameworks (COFs). The salient features of these crystalline polymers are their tunable molecular structure, large surface area, and porous nature, making them potentially valuable in areas such as energy applications, semiconductor devices, sensors, filtration systems, and drug delivery.
"What makes these structures so special is that, although they are polymers, they are arranged in orderly repeating structures, making them a kind of crystal," said Jeremy Daum, a doctoral student at Rice and lead author of a study published in ACS Nano. "These structures look a little like chicken wire -- they are hexagonal lattices that repeat in a two-dimensional plane, and then they stack on top of themselves, and that's how you get layered two-dimensional materials."
Alec Ajnsztajn, a Rice Ph.D. alumnus and another lead author of the study, said this synthesis technique makes it possible to produce ordered two-dimensional crystalline COFs in record time using vapor deposition techniques.
"A lot of times when you make COFs through solution processing, there's no alignment on the film," Ajnsztajn said. "This synthesis technique allows us to control the orientation of the film and ensure that the pores are aligned, which is what's needed to make the film."
The ability to control pore size would be useful in separators, COFs could be used as membranes for desalination, and could potentially help replace energy-intensive processes like distillation. In the electronics field, COFs are used as battery separators and organic transistors.
"COFs have the potential to play a role in a variety of catalytic processes -- for example, you could use COFs to break down carbon dioxide into useful chemicals like ethylene and formic acid," Daum said.
Challenges and innovations in COF production
One of the obstacles preventing wider adoption of COFs is that production methods involving solution processing are time-consuming and more difficult to adapt in industrial settings.
"Producing the solution powder required for COF can require a reaction time of three to five days," Ajnsztajn said. "Our method is much faster. After several months of optimization, we can produce high-quality films in 20 minutes or less."
Analysis and verification of COF films
To ensure that their films exhibited the correct molecular structure, Daum and Ajnsztajn worked 71-hour shifts at Argonne National Laboratory where the samples were analyzed using the Advanced Photon Source.
"We knew it was time to 'go,' but we were very happy with the results," Daum said. "We had to go to the national lab because this technique was the only way to measure the quality of the film and make sure we were taking the right steps to optimize it."
Microscopy studies shed light on the growth process of COF crystals and helped demonstrate that temperatures as high as 340 degrees Celsius (about 644 degrees Fahrenheit) can be used to synthesize organic molecules.
"While working on this project, we heard from many people that heating organic molecules to such high temperatures would prevent the correct reactions from occurring, but we found that chemical vapor deposition is actually a viable method for making organic materials," Ajnsztajn said.
Low-cost DIY method for COF production
To make the COF, Daum and Ajnsztajn built a makeshift reactor out of discarded lab equipment parts and other cheap and readily available materials.
"The assembly costs for the entire process are very low," Daum said. "Establishing a robust and scalable process for producing various COF films is expected to enable better applications of COFs in catalysis, energy storage, membranes and other fields."
Compiled source: ScitechDaily