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 Zn(2+). 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 (< or =5 or > or =11); (b) 1mM ethylenediaminetetraacetic acid (EDTA) solution; (c) UV (lambda=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 24h of exposure. Minimal dissolution was observed on ZnO films in alkalised 1mM 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 (Na(2)SO(3)) 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.