In a groundbreaking study, researchers have unveiled the dynamic growth processes of three-dimensional covalent organic frameworks (COFs), specifically COF-300 and LZU-79, utilizing advanced in situ dark-field optical microscopy. This innovative technique has allowed for real-time visual observations of crystal growth mechanisms, revealing two distinct pathways that crystals can follow during development.
The findings, which illuminate the simultaneous operation of classical crystal growth and non-classical oriented attachment mechanisms, provide vital insights into how these multifunctional materials evolve. COFs are recognized for their extensive surface areas and varied applications in catalysis, adsorption, and drug delivery, yet their growth pathways have remained relatively unexplored until now.
"Our dark-field optical microscopy imaging results reveal that two crystal-growth pathways are simultaneously operative during the liquid growth of COF-300 and LZU-79 microcrystals, including classical crystal growth modes and non-classical oriented attachment mechanisms," wrote the authors of the article published in Nature Communications. The ability to track these pathways sheds light on the intricate processes of crystal formation, which could enhance the design and performance of these materials.
The study showcases how COF-300 microcrystals can grow from an initial longitudinal length of approximately 1.61 µm to about 5.04 µm over a span of 150 minutes. This growth is facilitated by the introduction of trifluoroacetic acid as a catalyst and aniline as a modulator, demonstrating the potential for rapid crystallization under optimized conditions.
Further analysis identified five distinct stages in the oriented attachment process of COF-300 microcrystals: approach, alignment at (021) facets, tip-to-tip attachment, fusion, and shaping. Theoretical simulations indicated that these (021) facets have lower repulsive energy barriers due to steric solvation forces from the surrounding solvent, making them energetically more favorable for facilitating crystal interactions. "Theoretical simulation results show that (021) facets of COF-300 microcrystals... are energetically more favorable than (010) facets, inducing the oriented attachment between adjacent facets," wrote the authors of the article.
As the researchers captured these interactions through sequential imaging, they not only visualized the classical growth mode but also documented how aligned COF-300 crystals could fuse together, further contributing to larger microcrystal formations. Importantly, while oriented attachment has been commonly associated with nanoscale materials, this study represents a significant step in observing such processes within microcrystals.
The implications of these findings extend beyond just COF-300, as similar growth dynamics were observed in LZU-79, suggesting that this dual growth mechanism may be applicable to various other three-dimensional COFs. This observation signifies a leap in understanding the crystallization processes at the micro level and opens avenues for future research focused on enhancing the performance and versatility of COF materials.
By advancing our understanding of COF crystal growth, this study establishes a foundation for the rational design of high-performance porous materials that can serve a wide range of functions. The integration of real-time imaging technology has revealed critical aspects of crystal-growth pathways, paving the way for innovative applications in fields such as environmental remediation, energy storage, and biomedical technologies.
In conclusion, as researchers continue to explore the fundamental mechanisms that govern the formation of COFs, the findings from this study highlight the significant interplay between classical and non-classical growth pathways. The insights gained could lead to improved strategies in the synthesis of diverse COF materials, ultimately propelling forward the potential applications of these versatile frameworks.
