Simultaneous Photodeposition of an Ag–Co Cocatalyst in Different pH Conditions To Improve the Activity of Calcium Titanate Photocatalyst for Carbon Dioxide Reduction with Water

Photocatalytic CO2 reduction with water has been recognized as an emerging technology in view of atmospheric CO2 mitigation. Hence, further development of photocatalysts has been desired, where especially the suitable design of cocatalysts should be considered. In this work, it is revealed that solution pH adjustment in a simultaneous photodeposition (SPD) method to load Ag–Co cocatalysts on calcium titanate (CTO) photocatalyst surface provides various compositions and structures of cocatalysts, and both the optimal basic and acidic conditions provide highly efficient cocatalysts in contrasting ways compared to a neutral condition. In an optimal alkaline condition (pH = 10.5), redox chemical reaction between Ag and Co cations naturally provides Ag–Co3O4 composites on the CTO surface, and successive photoirradiation fabricates Ag–Co3O4 species with a unique core–shell structure on the reduction facet while it does not change the state of Ag–Co3O4 composites on the oxidation facet. The unique Co3O4 shell of the former structure assists the Ag nanoparticle (NP) core to catalyze the CO2 reduction with photoexcited electrons and protons while the latter species promoted the generation of oxygen compared to single Co3O4 and Co(OH)2 species. These two types of cocatalysts jointly worked to accelerate the photocatalytic CO2 reduction to form CO about 4 times. On the other hand, an acidic condition (pH = 1.0) of the SPD procedure prevents the Co cocatalyst from depositing on the CTO surface and provides larger spherical Ag NPs, which is responsible for the nearly 2 times enhancement of CO evolution rate. Overall, this study proposed the benefit of adjusting the pH conditions in the SPD method to form the corresponding structures of cocatalysts for the improvement of the efficiency of photocatalytic CO2 reduction.