Effect of solution concentration on surface morphology, optical properties and solar light response of ZnO thin films

The effects of zinc nitrate solution concentrations on the crystal structures, surface morphologies and optical properties of ZnO thin films were examined. X-ray diffraction patterns and scanning electron microscopy images indicated that all the samples exhibited the hexagonal structure and the morphologies of the thin films changed from 1D nanorods to 2D nanosheets with the increasing of solution concentration. The thin film grown in the 0.075 M solution presents the 3D network nanostructure, which was composed of many very thin 2D ZnO nanosheets, and possesses the largest first LO phonon mode, optical band gap, ratio for the intensity of deep level emission to that of UV emission. The results indicated that the optical properties of the ZnO thin films were affected by the solution concentration. Finally, the as-deposited ZnO thin film was used to generate a photocurrent and as a photocatalyst. Observations showed that the thin film grown in the 0.075 M solution exhibits the largest photocurrent density and degradation rate constant per unit area. The formation mechanism of the enhanced photoresponse and photocatalytic performance for the ZnO thin film has been investigated in detail.