Enhanced Photocatalytic Conversion of CO ₂ by H ₂ O over Ag@Cr Cocatalyst-Modified ZnTa ₂ O ₆

A zinc tantalate (ZnTa2O6) photocatalyst modified with an Ag cocatalyst via ultrasonic reduction (denoted as Ag(USR)/ZnTa2O6) exhibited higher activity for the photocatalytic conversion of CO2 by H2O than Ag/ZnTa2O6 prepared by other widely used methods. However, the CO formation rate was low. In this study, a photocatalyst was prepared by coating the Ag nanoparticles of Ag(USR)/ZnTa2O6 with Cr(OH)3·xH2O via the photodeposition (PD) method (denoted as Cr(PD)/Ag(USR)/ZnTa2O6) to increase the CO formation rate. Cr(PD)/Ag(USR)/ZnTa2O6 produced CO at the maximum formation rate of ∼600 μmol h–1, which was 50 and 8.5 times higher than those of bare ZnTa2O6 and Ag(USR)/ZnTa2O6, respectively. Furthermore, an induction period was observed during the time course of photocatalytic activity. The scanning electron microscopy images highlighted that Ag nanoparticles migrate to a specific site on the surface of ZnTa2O6 as photoirradiation proceeded, causing the rearrangement of Ag nanoparticles during this period. This rearrangement caused the separation of the photocatalytic reaction field, achieving high activity toward the photocatalytic conversion of CO2 because of the efficient reduction and oxidation reactions. Moreover, inductively coupled plasma mass spectrometry and ultraviolet–visible absorption spectroscopy revealed that Cr3+ in Cr(PD)/Ag(USR)/ZnTa2O6 is oxidized to dissolvable CrO42– during the induction period. These CrO42– anions in solution were found to play a crucial role in maintaining the Cr layer of Cr(PD)/Ag(USR)/ZnTa2O6.