Two-dimensional covalent organic frameworks (2D COFs) are a new class of organic semiconductors and have recently demonstrated great potential for solar fuel production. They are generally formed from the ordered π-π stack of molecular layers and usually possess periodic columnar π-arrays that can facilitate charge transfer between the layers.
In addition, the ordered assembly of organic building units in 2D COFs also gives rise to one-dimensional (1D) micro/meso channels that can promote mass transport and expose reactive sites. Despite major structural advantages, 2D COFs tend to suffer from low activity compared to their inorganic competitors.
This is mainly due to the high binding energy of the photoexcitons in organic substances and consequently the problematic dissociation of excitons. In addition, the hydrophobic nature of their -conjugated aromatic backbone often causes their large inner porosity to be inaccessible to water. In order to exploit the full potential of 2D COFs, synthetic and structural modifications are thereby desired to enhance their structural crystallinity and water wettability.
Recently, Professor Yanguang Li of Soochow University and collaborators reported a benzobisthiazole-based covalent organic framework (COF-BBT). The catalyst showed an excellent photocatalytic hydrogen evolution rate as high as 48.7 mmol h-1 g-1 in the presence of ascorbic acid as the sacrificial electron donor – one of the highest values ever reported for COF-based photocatalysts. It was found that the benzobisthiazole (BBT) units had a profound effect on its electronic properties and catalytic performance.
On the one hand, the BAT groups had rigid planar molecular configurations and were believed to promote structural crystallinity through π-π interactions between individual aromatic blocks. Such an orderly stacking of π units not only improved the delocalization of electrons between layers, but also provided the necessary channels for the migration of photogenerated charges to the surface.
This hypothesis was validated by a series of spectroscopic measurements. Transmission electron microscopy (TEM) analysis first confirmed the high structural crystallinity of COF-BBT. The 1D mesoporous channels and their hexagonal arrangement could be clearly observed.
Photophysical analysis showed that crystalline COF-BBT had weaker fluorescence emission and longer excited state lifetime compared to the amorphous counterpart of the same chemical structure, indicating that the charge recombination was suppressed in COF-BBT.
On the other hand, BAT was an aromatic heterocyclic ring rich in nitrogen and sulfur (>50 wt%). The introduction of BAT units would lead to an increase in the structural polarity and therefore to a greater water affinity of COF-BAT. This was confirmed by the much smaller water contact angle (21°) and greater water vapor absorption capacity compared to one sample without heteroatoms. Both the increased crystallinity and water wettability have contributed to the excellent photocatalytic performance of COF-BBT.
For practical applications, the authors showed that COF-BBT can grow directly on macroporous melamine foams by in-situ polymerization. The resulting composite not only enabled the stable hydrogen production, but could also be easily recovered by removing the foam from the solution. Furthermore, the authors also showed that photocatalytic hydrogen production can be linked to the oxidation of furfuryl alcohol to 2-furaldehyde at stoichiometry.
The research was published in National Scientific Review.
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Wei Huang et al, Highly crystalline and water-wettable benzobisthiazole-based covalent organic frameworks for enhanced photocatalytic hydrogen production, National Scientific Review (2022). DOI: 10.1093/nsr/nwac171
Quote: New covalent organic framework material accelerates solar fuel generation (2022, October 21) retrieved October 21, 2022 from https://phys.org/news/2022-10-covalent-framework-material-solar-fuel.html
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