Structure–Activity Relationship in ZnO–CeO₂ Photocatalysts: Role of Surface Area, Oxygen Vacancies, S-Scheme Charge Transfer and Temperature Effect in 2,4-D Degradation

CeO₂–ZnO materials were synthesized via a modified sol–gel method, varying the ZnO loading to evaluate its effect on the photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) under UV irradiation. XRD analysis confirmed the cubic fluorite structure of CeO₂ and, at higher loadings, the coexistence with a hexagonal wurtzite phase. At low loadings, Zn²⁺ incorporation into the CeO₂ lattice was evidenced, leading to lattice parameter contraction and the generation of oxygen vacancies. BET and SEM analyses indicated that an intermediate ZnO content (CZ1.5) increased the surface area (~41%) without collapsing the mesoporosity. XPS revealed Ce³⁺ species, defects, and Zn–O–Ce bonds, consistent with the formation of an S-scheme heterojunction. UV–Vis spectra showed similar Eg values (~3.13 eV) across the series, indicating that the photocatalytic enhancement is driven by structural and interfacial effects rather than optical changes. The CZ1.5 catalyst achieved ≈80% degradation and 74% mineralization (TOC), approximately twice that of pure CeO₂, attributed to efficient charge separation and the generation of reactive species O₂•⁻ and •OH. Furthermore, a temperature treatment CZ1.5 calcined to 400°C was the most active above the material treated at 300 and 500 °C.