Photoreduction of amorphous and crystalline ZnO films

Ultraviolet illumination with photon energy above 4.9 eV resulted in photodarkening on sol-gel derived amorphous ZnO films. Photoreduction of ZnO to Zn metal was achieved by irradiation with energetic photons above 6.7 eV. These changes basically originated from the photo-induced charge transfer from O^2- to Zn²⁺ in the amorphous structure. The photodarkening is ascribed to an increase in absorption in the visible region due to intermediate ionic states between the lattice oxygen and the dissociated oxygen. The formation of Zn metal was accompanied by desorption of oxygen atoms from the ZnO lattice stimulated through two-electron excitation by the energetic photons. The desorption of oxygen atoms was also observed at the surface of crystalline ZnO films. The photo-induced defective sites were highly reactive and transformed into OH groups by subsequent reaction with water molecules in air.