Formation of Cu₂O/titanate/titania heterojunctions from hydrothermally induced dual phase transitions

A microwave-assisted hydrothermal method has been developed as an efficient approach to readily induce phase transition of titanate assemblies in conjunction with decoration of Cu₂O clusters on the surface. The influence of Cu²⁺ ions on the hydrothermally induced structural evolution was examined, and the roles of heterojunctions in the resulting composites in charge separation for improved photocatalytic activity were clarified. Hierarchical titanate assemblies with high adsorption capacity for Cu²⁺ ions (95.7 mg/g) were prepared from a low alkaline condition. Microwave-assisted hydrothermal treatment was then used to transform the adsorbents into Cu₂O/titanate/titania photocatalysts in 20 min via inducing titanate-to-titania and Cu²⁺-to-Cu₂O dual transitions. While tubular architecture was maintained in the composites, the Cu₂O clusters highly dispersed on the surface. Adsorbed Cu²⁺ ions have been found to retard the titanate-to-titania transformation locally, thus leading to Cu₂O/titanate/titania heterojunctions. The multiheterojunctions enabled the composites to exhibit 1.7-5.1 times higher activity than the commercial product P25 (k_obs, 0.06 min^-1) for decomposition of bisphenol A due to charge separation. EPR results clearly reveal that the type II band alignment effectively drove electrons and holes to migrate toward the titania and the Cu₂O moieties, respectively, and the titanate moiety positioning in between prevented back recombination. The optimal Cu₂O loading to the highest activity (k_obs, 0.306 min^-1) was 3.7 wt %. Over the optimal amount, the lower reduction potential in the valence band of the Cu₂O clusters compensated for the positive effect from charge separation, thus causing the activity to decline in turn.