Distance Dependence of the Photocatalytic Efficiency of TiO2 Revealed by in Situ Ellipsometry

Spectroscopic ellipsometry was utilized to follow in situ photodegradation of organic species in the vicinity of TiO2 nanoparticles during UV irradiation. Stacked layers composed of TiO2, mesoporous SiO2, and mixed mesoporous SiO2/TiO2 nanocomposites with controlled thickness and porosity were used as model materials. Lauric acid molecules and poly(vinyl chloride) (PVC) layers were used as model mobile and immobile pollutants, respectively. The local photocatalytic activity was deduced by monitoring the variation of the thickness and refractive index of each independent layer. We show that the photocatalyzed degradation of an organic pollutant takes place only when the latter is located in close vicinity to the TiO2 nanoparticle surface or can naturally diffuse toward it. As a result, the reaction efficiency is directly related to the organic pollutant diffusion. We also show that the distance of photocatalysis efficiency (d(s)) at which radical intermediates are still present and active is <10 nm from the TiO2 surface under the conditions of the experiments. This was confirmed by the fact that an immobile condensed organic phase such as PVC was protected from the photocatalytic degradation when separated from the TiO2 by a 20 nm layer of mesoporous silica.