Deposition of ZnO thin films by ultrasonic spray pyrolysis technique. Effect of the milling speed and time and its application in photocatalysis

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by the ultrasonic spray pyrolysis technique (USP), starting from zinc acetylacetonate. A novelty process was implemented to the Zn precursor, previous to the solution preparation, namely, the Zn precursor was milled in a planetary ball milling equipment at different speed and time conditions, in order to generate new chemical species. The starting solutions were prepared by dissolving the milled precursor in a mix of methanol, acetic acid, and water. ZnO thin films were deposited at different substrate temperatures, 375, 400 and 450 °C and constant deposition time, 6 min. Sprayed ZnO films were characterized by X-ray diffraction, XRD, UV-Vis spectrophotometry and scanning electron microscopy, SEM. All films presented a wurtzite hexagonal phase confirmed from XRD, and corroborated by SEM analysis. Films exhibited a maximum optical transmittance oscillating between 80 and 85%. A band gap varying between 3.3 and 3.4 eV was estimated from the transmittance spectra. The photocatalytic response of ZnO thin films was tested by monitoring the bleaching of methylene blue (MB) dye at different time periods, under UV light irradiation. It was observed an increase in the degradation performance of the dye in all the films deposited from milled precursor, as compared with those deposited from unmilled precursor, since the MB degradation increased from 40 to 99% in 3 h. Novel mechanochemical process implemented to conventional chemical techniques enhanced the photocatalytic response of ZnO films.