JACS: Micrometer-Level Precision—Aerosol Jet Printing Enables Controlled Patterning and Compositing of COF Thin Films
Introduction
In the field of materials science, covalent organic frameworks (COFs) are regarded as promising candidates for next-generation high-performance materials due to their high specific surface area, tunable pore structures, and outstanding functional properties. However, conventional synthesis methods for COFs often encounter significant challenges in processing, shaping, and patterning: COFs are typically insoluble in common solvents, making it difficult to fabricate them into thin films or complex architectures using standard techniques.
Recently, a significant study published in the Journal of the American Chemical Society proposed an innovative strategy: first synthesizing a soluble polyimine precursor and then converting it into a highly crystalline COF material via dynamic covalent bond exchange. In this process, aerosol jet printing (AJP), as a high-precision, patternable microfabrication technique, played a pivotal role, opening new avenues for the “customized fabrication” of COFs.
Main text
Why is COF difficult to process?
COFs are crystalline porous materials composed of organic building blocks linked by strong covalent bonds, featuring well-defined pore channels and exceptional stability. However, their highly ordered crystalline structure renders them insoluble in most solvents and non-meltable, thereby posing significant challenges for film formation or fabrication of complex geometries using conventional techniques such as spin coating, spray coating, and printing.
Previous studies have attempted to fabricate COFs using colloidal inks, microfluidic devices, and even custom inkjet printers; however, these approaches are typically complex, costly, and ill-suited for achieving high-resolution patterning.
Aerosol Jet Printing Technology
This technology offers the following advantages:
• High resolution: capable of producing features on the order of tens of micrometers;
• Wide applicability: supports printing with a variety of functional inks;
• Operation at ambient temperature and pressure: suitable for heat-sensitive materials;
• Multi-layer printing: suitable for manufacturing complex structures.
When Aerosol Jet Printing Meets COF Precursors
The key breakthrough of this study lies in the fact that, instead of directly printing the inherently difficult-to-handle COF itself, we print its soluble polyimine precursor.
Researchers synthesized two soluble linear polyimides: 4MMCA-PDA and 4MMCA-F4PDA. These polymers exhibit excellent solubility in common organic solvents such as THF and chloroform, making them suitable for formulation into print-ready inks.
The printing process is as follows:
• Preparation of the ink: Dissolve the polyimide precursor and a crosslinking diamine monomer (such as TAPB) in a chloroform/terpineol mixed solvent;
• Aerosol jet printing: A 300 μm nozzle is used to print the desired pattern (e.g., a 1 cm × 1 cm square) onto a substrate heated to 60°C;
• Gas-phase annealing conversion: The printed precursor film is placed in a mixed vapor of 1,4-dioxane, mesitylene, and acetic acid and heated at 70°C, where dynamic imine bond exchange reactions occur in situ, converting the film into a highly crystalline COF film.
Powder X-ray diffraction analysis confirmed that the printed and post-converted material exhibits a crystal structure consistent with the bulk COF, with high crystallinity and a well-preserved porous framework.
Technical Advantages
• Avoiding rapid curing: In conventional COF synthesis, amines and aldehydes react rapidly to form insoluble polymers, leading to nozzle clogging. In contrast, the polyimine precursor remains stable during printing and undergoes chemical conversion only after printing is complete, thereby perfectly circumventing the curing issue.
• Realization of complex shapes and patterning: This technology not only enables the printing of simple geometries but also, through multi-layer and composite printing approaches, facilitates the fabrication of COF components with intricate three-dimensional structures or functional gradients, thereby opening up possibilities for the fabrication of devices such as sensors and microelectrodes.
• Applicable to composite material fabrication: The study also demonstrates that carbon nanotubes can be incorporated into the precursor ink to print COF/CNT composites. While preserving the porous structure of the COF, these composites exhibit a substantial enhancement in electrical conductivity, opening new avenues for the development of electronic devices and energy-storage materials.
Why is COF difficult to process?
The integration of aerosol jet printing with convertible precursors offers unprecedented flexibility for the micro- and nanofabrication, patterned integration, and functional compositing of COF materials. In the future, this technology is expected to play an even greater role in the following areas:
• Flexible electronics: printing high-surface-area COF electrodes for supercapacitors or sensors;
• Separation membranes: Preparation of COF composite membranes with aligned pores for gas separation or water treatment;
• Catalytic devices: COFs patterned with catalytically active sites for load-bearing applications, enabling the fabrication of microreactors;
• Optoelectronic materials: Composed with luminescent or conductive materials for use in display or photovoltaic devices.
Illustrated Guide
Figure 1. Synthesis of COFs via dynamic bond exchange of polyimides.
Figure 2. (a) Synthesis and characterization of the newly reported polyimides 4MMCA-PDA and 4MMCA-F4PDA in this study. (b) GPC analysis and summary of the thermal properties of 4MMCA-PDA and 4MMCA-F4PDA.
Figure 3. Mechanical property characterization of 4 mmca-PDA and 4 mmca-f4pda.
Figure 4. (a) Vertex-amine structures used in this study: TAPB, TAPT, and ETTA. (b) Synthesis and structure of TAPB-PDA COF, TAPTPDA COF, and ETTA-PDA COF synthesized from 4MMCA-PDA. (c) TAPB-F4PDA COF synthesized from 4MMCA-F4PDA and its structure. (d) Powder X-ray diffraction.
Figure 5. (a) TAPB–PDA thin films fabricated by casting from TAPB and 4MMCA-PDA. (b) SEM images of the 4MMCA-PDA/TAPB film on the blade mold (top) and the TAPB–PDA COF film (bottom). (c) GIWAXS patterns of the TAPB–PDA COF film at different reaction times. (d) One-dimensional projections of the GIWAXS scattering patterns.
Figure 6. Process for fabricating COFs with desired shapes. (a) B2- and aircraft-shaped COFs prepared by blade casting and laser cutting. (b) SEM images of the top layer consisting of a two-layer precursor film—4 mm CA–PDA as the bottom layer and 4 mm CA–PDA/TAPB as the top layer—and the bottom layer of TAPB–PDA COF. (c) Square-shaped COF fabricated via aerosol jet printing of the precursor mixture. (d) PXRD patterns of TAPB–PDA COFs with various shapes obtained through thin-film processing and aerosol jet printing.
Figure 7. (a) PXRD patterns of the ttap–pda COF composite films COF/CNT-0.9 and COF/CNT-4.5. (b) SEM cross-sectional images of the ttap–pda COF/CNT composites COF/CNT-0.9 (top) and COF/CNT-4.5 (bottom). (c) Resistivity of the 4MMCA-PDA/TAPB/CNT film and the TAPB-pda COF/CNT composite film at initial carbon nanotube loadings of 0.9 wt % and 4.5 wt %.
Conclusion
From “difficult to process” to “printable manufacturing,” research on COFs is advancing from synthetic chemistry toward the integration of functional devices. Aerosol jet printing, with its high precision, broad process adaptability, and patterning capability, is emerging as a critical bridge that connects COF materials science with micro- and nanomanufacturing technologies.
In the future, as printing processes are further optimized and material systems continue to expand, we can expect to see more high-performance, customized, and multifunctional COF devices transition from the laboratory to real-world applications, thereby truly realizing the vision of “materials as devices.”
Original article link: https://doi.org/10.1021/jacs.5c03079
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