Photocatalytic degradation of methylene blue at nanostructured ZnO thin films

The photocatalytic degradation of the wastewater dye pollutant methylene blue (MB) at ZnO nanostructured porous thin films, deposited by direct current reactive magnetron sputtering on Si substrates, was studied. It was observed that over 4 photocatalytic cycles (0.3 mg center dot l(-1) MB solution, 540 minUV irradiation), the rate constant k of MB degradation decreased by similar to 50%, varying in the range (1.54 divided by 0.78) center dot 10(-9) (mol center dot l(-1)center dot min(-1)). For a deeper analysis of the photodegradation mechanism, detailed information on the nanostructured ZnO surface morphology and local surface and subsurface chemistry (nonstoichiometry) were obtained by using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) as complementary analytical methods. The SEM studies revealed that at the surface of the nanostructured ZnO thin films a coral reef structure containing polycrystalline coral dendrites is present, and that, after the photocatalytic experiments, the sizes of individual crystallites increased, varying in the range 43 divided by 76 nm for the longer axis, and in the range 28 divided by 58 nm for the shorter axis. In turn, the XPS studies showed a slight non-stoichiometry, mainly defined by the relative [O]/[Zn] concentration of ca. 1.4, whereas [C]/[Zn] was ca. 1.2, both before and after the photocatalytic experiments. This phenomenon was directly related to the presence of superficial ZnO lattice oxygen atoms that can participate in the oxidation of the adsorbed MB molecules, as well as to the presence of surface hydroxyl groups acting as hole-acceptors to produce OH center dot radicals, which can be responsible for the generation of superoxide ions. In addition, after experiments, the XPS measurements revealed the presence of carboxyl and carbonyl functional groups, ascribable to the oxidation by-products formed during the photodegradation of MB.