Preparation and optical characteristics of ZnO films by chelating sol-gel method

The effect of different annealing temperatures on the structure, morphology, and optical properties of ZnO thin films prepared by the chelating sol-gel method was investigated. Zinc-oxide thin films were coated on quartz glass substrates by dip coating. Zinc nitrate, absolute ethanol, and citric acid were used as precursor, solvent, and chelating agent, respectively. The results show that ZnO films derived from zinc-citrate have lower crystallization temperature (below 400degreesC), and that the crystal structure is wurtzite. The films, treated over 500degreesC, consist of nano-particles and show to be porous at 600degreesC. The particle size of the film increases with the increase of the annealing temperature. The largest particle size is 60 nm at 600degreesC. The optical transmittances related to the annealing temperatures become 90% higher in the visible range. The film shows a starting absorption at 380 nm, and the optical band-gap of the thin film (fired at 500degreesC is 3.25 eV and close to the intrinsic band-gap of ZnO (3.2 eV).