Enhanced Photocatalytic Conversion of CO2 by H2O over Ag@Cr Cocatalyst-Modified ZnTa2O6

Abstract

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 CrO₄^2– during the induction period. These CrO₄^2– anions in solution were found to play a crucial role in maintaining the Cr layer of Cr(PD)/Ag(USR)/ZnTa₂O₆.