"Solutions Prove to be the Predicament for This Emerging Class of Materials"

Materials scientists from Rice University have devised an effective, cost-friendly, and easily scalable process for creating covalent organic frameworks (COFs). These are crystalline polymers known for their modifiable molecular structure, vast surface area, and porous nature, which makes them potentially useful in areas such as energy applications, semiconductor devices, sensors, filtering systems, and drug delivery.

Jeremy Daum, a Rice doctoral student and prime author of the study published in ACS Nano, explains that the exceptional feature of these structures is their ordered, repeated crystal structure despite being polymers. These structures resemble chicken wire as they have a hexagonal lattice that repeats on a two-dimensional plane, with multiple layers stacking on top, creating a layered 2D material.

Alec Ajnsztajn, another lead author of the study and a Rice doctoral alumnus, says the synthesis process facilitates developing ordered 2D crystalline COFs quickly using vapor deposition.

According to Ajnsztajn, creating COFs through solution processing usually results in disordered films. The new synthesis technique allows for better control of the sheet orientation, which is crucial for membrane creation.

The ability to control pore size makes COFs useful in separators where they can act as desalination membranes and possibly replace energy-intensive processes like distillation. In the field of electronics, COFs may be used as battery separators and organic transistors.

Daum mentions that COFs could be beneficial for various catalytic processes, such as converting carbon dioxide into useful chemicals like ethylene and formic acid.

Despite their many advantages, COFs aren't popular because solution processing-based production methods are time-consuming and more challenging to industrialize.

Ajnsztajn explains the traditional solution production process could take up to five days. Their method, however, is significantly faster, allowing high-quality films to be produced in less than twenty minutes after several months of optimization.

To confirm that their films maintained the correct molecular structure, Daum and Ajnsztajn visited the Argonne National Laboratory. Using the Advanced Photon Source, they evaluated their samples over continuous 71-hour shifts.

Daum declared their success symbols are great joy. A visit to a national lab was mandatory because it was the only way to assess the film quality and confirm that their optimization measures were correct.

Microscopy studies revealed the growth process of COF crystals and demonstrated that temperatures as high as 340 degrees Celsius (~644 Fahrenheit) could be utilized for synthesizing organic molecules.

Ajnsztajn posits that high temperatures are often presumed to hinder the right reactions, but their work reveals that chemical vapor deposition is an effective method to create organic materials.

To create COFs, Daum and Ajnsztajn constructed an impromptu reactor using discarded lab equipment parts and other cheap, accessible materials.

Daum concludes that the whole procedure was very affordable to set up. Establishing a robust, scalable production process for different COF films may lead to better application of COFs in catalysis, energy storage, membranes, and other areas.

Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering, professor and chair of materials science and nanoengineering and professor of chemistry and of chemical and biomolecular engineering, and Rafael Verduzco, professor of chemical and biomolecular engineering and of materials science and nanoengineering, are corresponding authors on the study.