Two-Step Visible Light Photocatalytic Dye Degradation Phenomena in Ag2O-Impregnated ZnO Nanorods via Growth of Metallic Ag and Formation of ZnO/Ag0/Ag2O Heterojunction Structures

Visible light photocatalytic activity follows the single-slope pseudo-first-order reaction kinetics in pristine ZnO nanorods, while for pure Ag₂O, a two-slope paradigm is pursued with a higher slope at a later period. For the Ag₂O-impregnated ZnO heterostructured nanorod photocatalyst, the two-step photocatalysis phenomena proceed with dye degradation rate constants emerging higher than those of individual ZnO and Ag₂O, at both time zones. Improved performance of ZnO/Ag₂O heterostructures arises initially from the reduced e^-/h⁺ recombination rate by the synergistic effect between ZnO and Ag₂O. At a later phase, metallic Ag is produced, which traps the valence electrons of Ag₂O nanoparticles and advances the e^-/h⁺ separation across the ZnO/Ag⁰/Ag₂O heterojunction structures, rendering them promptly accessible for dye degradation. At an increased Ag₂O loading, the photodegradation rate constants boost up in both time zones, and the corresponding crossover time (t_C) between the two phases steadily diminishes, leading toward a unique photocatalytic phenomenon that prevails with a superior rate constant. The optimized ZAO₂₅ heterostructure photocatalyst demonstrates ∼96.24% photodegradation of methylene blue (MB) dye within 30 min of visible light exposure, and its degradation rate constant is ∼0.24848 min^-1, which is ∼26.75 times superior than that of pristine ZnO samples. The metal-induced biphasic photocatalysis phenomena have never been reported earlier.