Fundamental kinetic modeling of dye sensitization photocatalysis by oxygen vacancy enriched ZnO for the quantification of degradation by catalyst or dye sensitizer

Dye sensitization enhances catalyst reactivity using dye molecules to amplify light absorption. Researchers debate using dyes as model compounds due to uncertainty about whether degradation is driven by dye or the catalyst. Instead of avoiding this question, our approach involves understanding and quantifying the distinct roles of each component in the degradation process. This study investigated methylene blue (MB) sensitization of ZnO using a binary dye mixture of MB and methyl orange. A comprehensive kinetic model was developed to account for the function of MB sensitizer by simultaneously tracking the interaction of dyes across the catalyst surface and solution in pH range of 3 to 11. The effects of pH on dye adsorption were incorporated using dye speciation (pKa). The dye removal was kinetically modeled considering reversible adsorption/desorption and surface/aqueous degradation by the interaction between active radicals (in the solution) and electron-holes (on the catalyst surface). Additionally, the source of the generated electron hole (i.e., ZnO or MB sensitizer) was considered separately. Oxygen vacancy effects on dye adsorption and sensitization were explored through hydrogen reduction of ZnO at 500 °C. Characterization techniques assessed oxygen vacancy presence, while quantum yield calculations quantified MB sensitization's contribution to dye degradation versus the catalyst.