A tale of two syntheses: a comparative study of the physical and photocatalytic properties of covalent triazine frameworks (CTFs)

Abstract

Covalent triazine frameworks (CTFs) have exhibited promising photocatalytic capabilities for organic transformations, CO₂ reduction and water treatment, yet the links between synthesis procedure and structural- and catalytic properties are essentially unexplored. This study investigates CTF-1 type materials prepared by two distinct synthesis procedures: One variant of CTF-1 was synthesised via a solvent-assisted route over three days at 30 °C (CTF-1(SS)), while the other form was synthesised at 250 °C over 12 hours (CTF-1(DS)) followed by post-synthetic modification through either heat- or mechanical treatment for removal of excess triflic acid catalyst. All synthesised materials could be identified as different CTF-1 variants; however, the synthesis choice profoundly impacts the material properties. Pristine CTF-1(DS) is semicrystalline with good visible light absorption and high thermal stability. In contrast, CTF-1(SS) is amorphous with embedded amide functionalities and limited visible light absorption and thermal stability, which is attributed to a lower degree of polymerization/conjugation. Surprisingly, due to the embedded amide sites, the amorphous CTF-1(SS) exhibits higher photocatalytic activity than pristine CTF-1(DS) under UV light. However, CTF-1(DS) shows the best photocatalytic properties after post-synthetic removal of residual triflic acid. Heat treatment allows complete removal of residual acid, and ball-milling only achieves partial acid removal. This study, therefore, demonstrates that choosing appropriate methods for synthesis facilitates the enhancement of desired physical- and light absorption properties in CTF-1-based photocatalysts and that nuanced characterisation techniques are required to fully understand the photocatalytic behaviour of different CTF-1 variants.