Synthesis of ZnO/ PrO₂ + Pr(OH)₃ nanoparticles for solar photodegradation of anionic and cationic mixed dyes

A composite of ZnO and Pr (3% mol of Pr/Zn) was synthesized in one-step using ZnCl₂ and PrCl₃ as precursors and NaOH to precipitate the oxides, resulting in a composite of ZnO/PrO₂ + Pr(OH)₃. Composite of ZnO/PrO₂ + Pr(OH)₃ was characterized by scanning electron microscopy, thermogravimetric analysis, UV/Vis–NIR spectroscopy and X-ray diffraction. Composite consisted of micro-rods of 5 to 10 µm in length coated with agglomerated nanoparticles of PrO₂ + Pr(OH)₃. From the UV/Vis–NIR spectrum, the calculated band-gap energy values of composite were 3.05 and 2.73 eV for direct and indirect transitions, respectively. In addition, the characteristic f–f transitions of the Pr³⁺ ions were also identified due to photoactivity in both: the UV and visible regions of the electromagnetic spectrum. ZnO/PrO₂ + Pr(OH)₃ composite was tested in photodegradation of methylene blue (MB) and reactive red 120 (RR-120) dyes under solar irradiation (1.15 ± 0.15 W/m²) and 1 g/L of photocatalyst load. It was observed that for a MB dye solution 30 mg/L of initial concentration, 81% of photodegradation was achieved after 120 min of solar irradiation, this efficiency increased to 97% for initial MB dye concentration of 10 mg/L in 90 min. The photodegradation efficiency for RR-120 was 98% at 10 mg/L of RR-120 initial concentration in only 45 min, however in the competitive photodegradation of the mixed dyes at 20 mg/L of initial concentration, 99% and 95% of efficiency were achieved for MB and RR-120 after 120 min, respectively. The photocatalytic activity of composite was associated with two mechanisms: (1) light absorption in the UV/Vis region activating the CTB band (charge transfer band, O²⁻ → Zn²⁺, Pr³⁺) and the f–f bands of the Pr³⁺ ions. (2) The presence of the redox-pair Pr⁴⁺ + e⁻ → P³⁺ (ε⁰ = 3.2 V) that would capture the photogenerated e⁻_CB. Both mechanisms increase the lifetime of the exciton.