Effect of starting properties and annealing on photocatalytic activity of ZnO nanoparticles

Eight commercial ZnO samples were investigated, as-received and after annealing in the flow of oxygen or forming gas. We found that ZnO nanoparticles with higher photoluminescence (PL) intensity (fewer nonradiative defects) exhibited a high photocatalytic activity. Samples exhibiting high PL intensity presented lower concentrations of surface adsorbates, which is consistent with lower native defect density in those samples. It was observed that annealing did not result in an improvement in photocatalytic activity of the samples with low defect density (high PL intensity). For the samples with low PL intensity, the same annealing condition could result in an improvement or worsening of photocatalytic activity, depending on the starting properties of the samples. Variable effects of annealing on samples with different properties indicate that the effect of a post-growth treatment cannot be generalized for all ZnO samples. The improvements in the photocatalytic activity were observed for a cationic dye methylene blue, while for an anionic dye acid red 27 annealing resulted in the changes in the amount of adsorbed dye, but no improvement in the photocatalytic activity for dye degradation.