Photo-catalytic activity of hydrophilic-modified TiO₂ for the decomposition of methylene blue and phenol

We applied hydrophilic surface modification to P-25 TiO₂ nanoparticles via thermal deposition of polydimethylsiloxane (PDMS), followed by annealing at 800 °C under vacuum conditions. This process yielded a thin layer consisting of hydrophilic organic functional groups (e.g., -C=O) as well as SiO_x structures on the surface of TiO₂. Compared to bare TiO₂, the surface-modified TiO₂ showed a higher adsorption capacity and photo-catalytic decomposition rate toward methylene blue. However, regarding the photo-catalytic decomposition of phenol, total mineralization of phenol was more complete when bare TiO₂ was used as a photo-catalyst, whereas partial oxidation of phenol (i.e., into hydroquinone) was more dominant and total oxidation of phenol was supressed in the presence of surface-modified TiO₂. Overall, one cannot simply say that hydrophilic modification of TiO₂ leads to a higher affinity to H₂O molecules with a higher yield of strongly-oxidizing agents (OH radicals) to increase the photo-catalytic activity in aqueous solutions. Our results imply that a subtle balance of the adsorption energy or rates of H₂O, reactants, and reaction intermediates can be important factors for determining the photo-catalytic reaction rate. Different types of photo-catalyst surface modification can be beneficial or detrimental depending on the reaction. It is highlighted that one should be wary of evaluating the overall photo-catalytic activities of dissimilar catalysts based on the results of only a couple of reactions.