In situ infrared observation of a photo-decomposition process of organic contaminants on a TiO₂ nanotube film surface

We investigated a photocatalytic reaction on TiO₂ nanotube (NT) surfaces using infrared absorption spectroscopy with multiple-reflection geometry (MIR-IRAS). We used an anodization technique to form a film of well-aligned TiO₂ NTs on a Si prism used for MIR-IRAS measurements. The photocatalytic decomposition process of the endocrine disruptor, dioctyl phthalate (DOP), on the TiO₂ NT surface was monitored in-situ and in real time. We demonstrated that the photocatalytic decomposition of organic materials is promoted with the presence of molecular oxygen. It was observed that the amount of surface-adsorbed water molecules changed during the reaction. We proposed a simple reaction model that can reproduce the time-dependent change of the surface coverage of water and DOP. By comparing the photodecomposition of organic materials on TiO₂ NT films with that on TiO₂ nanoparticle (NP) films, we showed that TiO₂ NT films are superior in photocatalytic reactivity compared to NP films. We suggest that the NT structure provides wider and shorter paths for the transport of photo-generated radicals and byproducts, leading to a higher reactivity compared to TiO₂ NPs.