Study on photocatalytic activity of ZnO nanoneedles, nanorods, pyramids and hierarchical structures obtained by spray pyrolysis method

In this research, ZnO nanostructured layers with various morphology, i.e. nanoneedles, nanorods, pyramid shaped ZnO crystals, and hierarchical structures of ZnO deposited on nanorods and pyramid shaped crystals, were synthesised by pneumatic spray pyrolysis method. Hierarchical structures, a dense network of well-ordered platelets covering the side facets of the rod-like and pyramid shaped ZnO crystals, were obtained by spraying Zn(Ac)2 solution over the surface of pre-grown ZnO crystals at deposition temperature of 330 °C within 5 min. All ZnO layers have been characterised by scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), absorbance of ultraviolet and visible light (UV–vis), surface wettability measurements and photoluminescence (PL). Photocatalytic activity of the layers was evaluated by means of photocatalytic oxidation activity (PCO) of several organic pollutants, namely, methyl-tert-butyl ether (MTBE), humic acid (HA), N,N-dimethyl p-nitrosoaniline (RNO), and prednisolone (PNL); the results were compared to the performance of well-established commercial photocatalyst, P25 titanium dioxide (Evonik). MTBE was the most efficiently degradable compound by all objects, while RNO was effectively degraded by plain structures only. In HA photocatalysis, needle-like ZnO as the most efficient material; PNL was the least degradable by ZnO layers. In the degradation of MTBE, RNO and HA, ZnO objects have shown activity comparable to or highly exceeding that of P25 TiO2. The explanation of the differences in ZnO objects׳ photoactivity is proposed to be the different character of oxidising species produced by the ZnO objects, and different interactions of these with the degradation substrates, based on the relative amount of surface defects, surface morphology and hydroxylation of the ZnO structures׳ surface.