Low band gap semiconducting covalent organic framework films with enhanced photocatalytic hydrogen evolution

Abstract

Semiconducting covalent organic frameworks (COFs) are emerging materials for the photocatalytic hydrogen (H₂) evolution, owing to their porous periodic network of π-conjugated scaffolds with tuneable band gap and optoelectronic properties. The optoelectronic properties, for example, band gap, band position and charge carrier mobility of the COF photocatalyst, play a pivotal role in their activity in photocatalytic conversions. However, it is challenging to control the band gap and band position of COFs simultaneously. In the present manuscript, we investigate the photocatalytic performance of low band gap semiconducting COFs constructed from anthracene containing π-conjugated building blocks linked with different hydroxy functionalized 1,3,5-triformylbenzene. Along with crystalline and porous COF powders, thin COF films were synthesized successfully. Such thin COF films offer several advantages in photocatalytic applications owing to their low light scattering, scalability and reusability. It is furthermore shown that the number of hydroxy groups on the 1,3,5-triformylbenzene linker largely influences the properties of the final COFs. The COF prepared from the linker with two hydroxy groups exhibits the lowest band gap of 1.8 eV and efficient exciton migration due to the formation of J-type aggregates resulting in the maximum hydrogen evolution rate (8.4 ± 0.5 mmol g⁻¹ h⁻¹) in the powder state. On the other hand, for COF films a steady increase in hydrogen evolution rate is observed with increasing hydroxy-functionalization and reaches a maximum for three hydroxy groups (1.6 ± 0.2 mmol m⁻² h⁻¹) due to the enhanced charge carrier mobility.