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 this link for 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 impacted the material properties. Pristine CTF‑1 (DS) was semicrystalline with good visible light absorption and high thermal stability. In contrast, CTF‑1 (SS) was amorphous with embedded amide functionalities and limited visible light adsorption and thermal stability, which was attributed to a lower degree of polymerization/conjugation. Surprisingly, due to the embedded amide sites, the amorphous CTF‑1 (SS) exhibited higher photocatalytic activity than pristine CTF‑1 (DS). However, CTF‑1 (DS) exhibited the best photocatalytic properties after post-synthetic removal of residual triflic acid, with heat treatment allowing complete removal of residual acid, and ball-milling only achieving partial removal of the acid. This study, therefore, demonstrates that choosing appropriate methods for synthesis facilitates the enhancement of desired physical and light adsorption properties in CTF‑1-based photocatalysts and that nuanced characterisation techniques are required to fully understand the photocatalytic behaviour of different CTF-1 variants.