Potential dissolution and photo-dissolution of ZnO thin films

Potential dissolution and photo-dissolution are important concerns for zinc oxide (ZnO) photocatalysts due to the possible results of catalyst inactivation and secondary pollution from free Zn2+. In this study, magnetron-sputtered ZnO thin films were prepared and exposed under a series of corrosive conditions. ZnO films suffered rapid dissolution under: (a) extreme pH levels (≤5 or ≥11); (b) 1 mM ethylenediaminetetraacetic acid (EDTA) solution; (c) UV (λ = 254 nm). The dissolution rate of ZnO films was moderate at pH = 6 and decreased markedly as pH increased to 7. It continued to decrease as pH increased from 7 to 10, then the trend quickly reversed as pH increased further. The lowest dissolution rate was obtained at pH = 10, with only 1.2% ZnO dissolved after 24 h of exposure. Minimal dissolution was observed on ZnO films in alkalised 1 mM oxalate and acetate solutions. Pitting corrosion was observed on ZnO films after UV irradiation, which was ascribed to photo-generated holes on surface defect sites. The presence of hole scavengers (Na2SO3) caused significant suppression on ZnO photo-dissolution. This suppression effect remained in place until hole scavengers were completely consumed, from where the photo-dissolution rates accelerated.