Photostability challenges of catechol-derived LMCT complexes on TiO2 for visible-light-driven photocatalytic air purification

Abstract

Ligand-to-metal charge transfer (LMCT) complexes formed between catechol derivatives and the TiO₂ surface have attracted considerable attention due to their visible-light sensitisation effects. However, their stability under continuous light irradiation remains a critical issue for practical applications. In this study, we systematically investigated the stability of catechol-derived LMCT complexes on TiO₂ under blue, green, and red light irradiation. The complexes were stable against water washing but underwent significant degradation under blue and green light in dry air, leading to a complete loss of sensitisation. In contrast, they remained stable under red light, although the photocatalytic activity was limited due to the low photon energy. This finding highlights the importance of improving photostability under higher-energy visible light (e.g., blue light) to achieve effective visible-light-driven photocatalytic toluene removal. Increasing the catechol concentration during complex formation enhanced both visible-light sensitisation and photostability under blue light. However, even the complexes prepared with concentrated catechol solutions experienced a complete loss of sensitisation due to their degradation after overnight exposure to blue light. This degradation could not be prevented by polymerisation of the LMCT complexes. Photostability tests under various atmospheric conditions (dry air, dry N₂, humid air, and humid N₂ atmospheres) revealed that oxygen plays a critical role in the photodegradation of LMCT complexes, while water vapour further accelerates degradation in the presence of oxygen. Given the challenges associated with using sacrificial agents under practical photocatalytic air purification conditions, our findings suggest that a thin protective layer inhibiting oxygen-related photodegradation may improve the stability of catechol-derived LMCT complexes.