Binder-free TiO₂ hydrophilic film covalently coated by microwave treatment

A binder-free attachment method for TiO₂ on a substrate has been sought to retain high active photocatalysis. Here, we report a binder-free covalent coating of phase-selectively disordered TiO₂ on a hydroxylated silicon oxide (SiO₂) substrate through rapid microwave treatment. We found that Ti–O–Si and Ti–O–Ti bonds were formed through a condensation reaction between the hydroxyl groups of the disordered TiO₂ and Si substrate, and the disordered TiO₂ nanoparticles themselves, respectively. This covalent coating approach can steadily hold the active photocatalytic materials on the substrates and provide long-term stability. The binder-free disordered TiO₂ coating film can have a thickness (above 38 μm) with high surface integrity with a strong adhesion force (15.2 N) against the SiO₂ substrate, which leads to the production of a rigid and stable TiO₂ film. This microwave treated TiO₂ coating film showed significant volatile organic compounds degradation abilities under visible light irradiation. The microwave coated selectively reduced TiO₂ realized around 75% acetaldehyde degradation within 12 h and almost 90% toluene degradation after 9 h, also retains stable photodegradation performance during the cycling test. Thus, the microwave coating approach allowed the preparation of the binder-free TiO₂ film as a scalable and cost-effective method to manufacture the TiO₂ film that shows an excellent coating quality and strengthens the application as a photocatalyst under severe conditions.