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 (tC) 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.