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

Abstract

Visible light photocatalytic activity follows the single-slope pseudo-first-order reaction kinetics in pristine ZnO nanorods, while for pure Ag 2 O, a two-slope paradigm is pursued with a higher slope at a later period. For the Ag 2 O-impregnated ZnO heterostructured nanorod photocatalyst, the two-step photocatalysis phenomena proceed with dye degradation rate constants emerging higher than those of individual ZnO and Ag 2 O, at both time zones. Improved performance of ZnO/Ag 2 O heterostructures arises initially from the reduced e − /h + recombination rate by the synergistic effect between ZnO and Ag 2 O. At a later phase, metallic Ag is produced, which traps the valence electrons of Ag 2 O nanoparticles and advances the e − /h + separation across the ZnO/Ag 0 / Ag 2 O heterojunction structures, rendering them promptly accessible for dye degradation. At an increased Ag 2 O 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 ZAO 25 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.

Photocorrosion Inactivation of Ag2O/TiO2 Photocatalysts and the Inhibition Effect of Alkaline Conditions on the Inactivation

“…After five cycles, a small amount of Ag 2 O (eq ) is reduced by the photogenerated electrons. It has been reported in the literature that a small amount of Ag does not lead to the inactivation of the photocatalyst but also enhances electron transport . All active radicals generated in this process, including h + , · OH, and · O 2 – , participate in the degradation of organic pollutants (eq ).…”

“…With the rapid development of modern industry, organic pollutants from both industrial and domestic sources, such as dyes and pesticides, are becoming increasingly concentrated in water, leading to environmental pollution and affecting human health. Semiconductor photocatalysis technology is considered a clean and effective means of water purification and has the ability to utilize solar energy to degrade organic pollutants and purify water. − When a semiconductor is irradiated by sunlight, it absorbs the energy of photons, generating electrons and holes in the conduction band (CB) and valence band (VB), respectively. These further generate various active free radicals that can oxidize organic pollutants and achieve the goal of water purification.…”

“…It has been reported in the literature that a small amount of Ag does not lead to the inactivation of the photocatalyst but also enhances electron transport. 3 All active radicals generated in this process, including h + , • OH, and • O 2 − , participate in the degradation of organic pollutants (eq 8). The inactivation of the photocatalyst can be primarily attributed to the dissolution of Ag 2 O and the destruction of the crystal structure.…”

Cd-doped Ag2O/BiVO4 visible light Z-scheme photocatalyst for efficient ciprofloxacin degradation

“…, photocatalysts. 22–25 But to date, we have not encountered any doped Ag 2 O-based composite photocatalyst study in the literature.…”

“…For instance, there are reports on the fabrication and evaluation of ZnO/Ag/Ag 2 O, Ag 2 O/TiO 2 , BiVO 4 /Ag/Ag 2 O, Ag 2 O/Bi 2 O 3, etc., photocatalysts. [22][23][24][25] But to date, we have not encountered any doped Ag 2 O-based composite photocatalyst study in the literature.…”