Trace Amount CoFe₂O₄ Anchored on a TiO₂ Photocatalyst Efficiently Catalyzing O₂ Reduction and Phenol Oxidation

Semiconducting TiO₂ is the most studied photocatalyst for organic oxidation by O₂. To accelerate the reaction, a cocatalyst for O₂ reduction or for organic oxidation is often used, but the bifunctional one is rare. Herein we report a spinel CoFe₂O₄ (CF) efficiently catalyzing O₂ reduction and phenol oxidation on TiO₂ in aqueous suspensions at pH 3-11. The composite materials (CF/TiO₂) were made by depositing 0-5 wt % CF onto TiO₂ through a hydrothermal method. Solid characterization showed that CF nanoparticles (5 nm) homogeneously distributed in CF/TiO₂, whereas the TiO₂ phase remained unchanged in crystal structure and crystallite size. For phenol oxidation under UV light, CF was nearly not active, but 0.01 wt % CF/TiO₂ was more active than TiO₂, by approximately a factor of 3.6. Such trend in activity among the catalysts was also observed from the photocatalytic reduction of O₂ to H₂O₂, from the electrochemical reduction of O₂, and from the photoelectrochemical oxidation of phenol and H₂O. An open-circuit potential and photoluminescence measurement suggest that there is an interfacial electron transfer from TiO₂ to CF, followed by O₂ reduction. Accordingly, a possible mechanism is proposed, involving CF catalysis for O₂ reduction and phenol oxidation. Then the mutual promotion between electron and hole transfer results in great enhancement in the efficiency of charge separation and hence in the rate of chemical reaction. Because spinel compounds have rich composition and unique structures, they are worthy of being further investigated as cocatalysts of a semiconductor photocatalysis.