Efficient charge separation and photooxidation on cobalt phosphate-loaded TiO2 mesocrystal superstructures

Development of efficient photocatalysts based on semiconductor materials for organic synthesis, fuel generation, and environmental purification is a central theme in current research and various industries. In this study, we propose a novel strategy for improving the photooxidation activity of photocatalysts by combining metal oxide superstructures and oxygen/hydrogen-evolving co-catalysts. Cobalt phosphate (CoPi) and Pt nanoparticles were selected as model co-catalysts and photochemically deposited on anatase TiO2 mesocrystals. The structures and reaction dynamics of the composites were thoroughly studied by ensemble-averaged and single-particle spectro-microscopies. Time-resolved diffuse reflectance and electron spin resonance spectroscopy measurements revealed that photogenerated holes in TiO2 are transferred to the Co species in CoPi upon UV light irradiation. The photooxidation properties of the composites were tested using fluorescence dye probes. It was found that CoPi-loaded TiO2 mesocrystals had higher activity than standard TiO2 photocatalysts, and their activity was further enhanced by introducing Pt nanoparticles on specific surfaces. In situ fluorescence imaging on a single crystal provides information on the location of reactive sites and the diffusion of product molecules. Consequently, the site-specific modification of co-catalysts tailored by anisotropic electron flow in the mesocrystal superstructures significantly retarded the charge recombination between the holes and electrons, thereby resulting in enhanced (up to approximately 300 times) photooxidation activity.