UV photodegradation of 4-nitrophenol over Ag-decorated ZnO: interfacial charge separation and hydroxyl-radical-dominant oxidation

Clarifying whether Ag decoration enhances ZnO photocatalysis mainly through changes in oxygen-related paramagnetic centers or through interfacial charge separation is important for rational catalyst design. Here, Ag-decorated ZnO nanoparticles with systematically varied AgNO3 contents were used as a model system for UV-driven degradation of 4-nitrophenol (4-NP). Quantitative EPR analysis shows that Ag decoration does not increase oxygen-related paramagnetic centers (g ≈ 2.004–2.006); instead, the corresponding signal remains comparable to or lower than that of pristine ZnO. In contrast, DMPO spin-trapping EPR reveals markedly enhanced UV-induced hydroxyl radical formation, reaching a maximum 10.58-fold increase at high Ag loading, whereas the DMPO–O2•–-related adduct signal remained comparatively weak. Under fixed UV-LED irradiation conditions (8 W, λ≈ 365 nm), the intermediate-loading sample (AgNO3 = 1.18 × 10–4 mol) showed the highest activity among the investigated Ag-loading series, achieving 98.02% 4-NP removal within 60 min with an apparent rate constant of 0.050 min-1, compared with 51.93% removal and k = 0.009 min-1 for pristine ZnO. Although the strongest DMPO–OH signal is observed at high Ag loading, the best photocatalytic performance occurs at intermediate Ag loading, indicating that overall activity is determined by a balance between ROS promotion and detrimental high-loading effects. Overall, these results suggest that, under the present UV-driven conditions, the enhanced UV photodegradation performance of Ag/ZnO is more closely associated with a literature-supported Ag-assisted interfacial charge-separation pathway and •OH-dominated oxidation than with an increase in EPR-detectable oxygen-related paramagnetic centers.