Enhanced Methylene Blue and Diclofenac Removal in Ho/Ga Codoped ZnO: Balancing Defect Chemistry, Surface Accessibility, and Interfacial Oxidation

Abstract

Developing ZnO-based photocatalysts for organic-pollutant removal requires materials in which structural modification, surface accessibility, and reactive oxygen species generation are effectively balanced. In this work, Ho/Ga codoped ZnO powders were evaluated as photocatalysts for the removal of methylene blue (MB) and diclofenac (DCF), selected as representative dye and pharmaceutical contaminants with different interfacial behaviors. Zn1–x–yHoxGayO powders with (x, y) = (0.00, 0.00), (0.005, 0.015), and (0.02, 0.03) were synthesized by coprecipitation and calcined at 450 °C. The structural, textural, morphological, and optical responses were correlated with photocatalytic performance, reactive species formation, and reuse stability to clarify how Ho/Ga addition influences the activity of ZnO. All samples preserved the wurtzite ZnO structure, while codoping refined the crystallite size and modified the accessible surface area and defect-related optical response. Among the investigated compositions, Zn0.98Ho0.005Ga0.015O showed the best photocatalytic response, with removal efficiencies of 86% for MB and 70% for DCF after 120 min, and apparent rate constants of 0.0161 and 0.00956 min–1, respectively. Scavenger tests showed that hydroxyl-radical-mediated oxidation was the most strongly affected pathway, while oxygen-reduction-related and hole-mediated steps made secondary but relevant contributions depending on the contaminant. The comparison between MB and DCF showed that pollutant-dependent interfacial behavior affects how the same catalyst promotes surface-mediated discoloration and solution-driven oxidative conversion. The improved performance of Zn0.98Ho0.005Ga0.015O is associated with its more favorable defect-related optical response and accessible surface configuration, which support oxidative conversion without the stronger lattice disorder observed at higher codoping levels. Reuse tests showed higher stability for MB than for DCF, with the activity decreasing from 86% to 83% after three cycles for MB and from 70% to 53% after four cycles for DCF, suggesting stronger interfacial deactivation during DCF removal.