Conventional versus lattice photocatalysed reactions: Implications of the lattice oxygen participation in the liquid phase photocatalytic oxidation with nanostructured ZnO thin films on reaction products and mechanism at both 254 nm and 340 nm

Abstract

This study aimed to investigate the relationship between surface morphologies of nanostructured ZnO thin films, the reaction intermediates and products produced, and the liquid phase and solid phase reaction mechanisms under both oxygen rich and oxygen limited conditions at two different wavelengths (254nm and 340nm). Four different ZnO morphologies were prepared by hydrothermal deposition on two different substrates – clean glass (CG) and direct current magnetron sputtered ZnO glass (MS). The two films grown on the MS templates were highly vertically aligned columnar structures and those grown without templates (CG) had little or no structural alignment. Methylene blue (MB) was used as the reactant, since its reaction intermediates and products are well defined and measurable, allowing predictable comparison between these catalysts.Results showed that there was a significant difference in MB degradation rates as well as reaction intermediate formation and destruction rates correlated to the morphologies and crystallinity at both UV wavelengths, with the highest reaction rates at 340nm. Reaction analysis indicates that there may be a competition between two different photocatalytic mechanisms: conventional photocatalysed radical oxidation and lattice oxygen driven oxidation. The dominant reaction mechanism present depended on the thin film morphology, crystallinity, availability of oxidant and the wavelength of the incident UV. The surface photocatalysed radical formation is thought to be predominant for more highly aligned and more crystalline morphologies, where there was plentiful oxygen and UV irradiation at 340nm. Lattice oxygen photodegradation is thought to be predominant for less aligned more amorphous morphologies and UV irradiation at 254nm. Based on these results a new MB ZnO photocatalysed oxidation mechanism is proposed.