Photocatalysis with nanostructured zinc oxide thin films: The relationship between morphology and photocatalytic activity under oxygen limited and oxygen rich conditions and evidence for a Mars Van Krevelen mechanism

The aim of this study was to evaluate the effectiveness of using a range of innovative nanostructured high surface area zinc oxide (ZnO) thin films as photocatalysts. and thereafter to systematically relate initial and reacted surface morphology and irradiated surface area to photocatalytic activity under both limited and rich oxygen conditions The thin films were produced using an innovative combination of magnetron sputtered surfaces and hydrothermal solution deposition that allows the morphology, porosity and thickness to be controlled by varying the composition and processing conditions. Methylene Blue (MB) was chosen as the model compound and the reaction was performed with ultra violet light (UV) at 254 nm. The thin film morphology and surface area before and after reaction was determined by scanning election microscopy (SEM) The photocatalytic activity (measured as the rate and extent of MB degradation) was determined for seven different ZnO nanostructurecl thin films. three different ZnO hydrothermal solution depositions on bare glass slides (S1-CG. S2-CG and S3-CG films), the same three ZnO hydrothermal solution depositions but on glass slides coated with a magnetron sputtered ZnO film (S1-MS, S2-MS and S3-MS films), and glass slides coated with just a magnetron sputtered ZnO film (MS films) A clear relationship between surface morphology (and the related thin film preparation method) and photocatalytic activity was observed for ZnO thin film supported catalysts. the tallest, most aligned structure had the highest photocatalytic activity, whilst the smallest, least aligned structure had the lowest photocatalytic activity. Thus. MB degradation rate was the fastest for the 1 mu m thick ZnO thin film with a uniform arrayed structure from the S2-MS deposition technique The degradation rates of the ZnO thin films were comparable to commercially available ZnO powder on a surface area basis Photocatalytic degradation of MB under oxygen rich conditions increased for all other films except one film (S1-CG) This was most effective for thin film structure S2-MS, whose reaction rate was increased by 15% Adding oxygen made the films more stable in oxygen limited conditions. SEM and atomic absorption spectroscopy indicated zinc leaching had occurred However, with additional oxygen the zinc leaching was minimised under the same reaction conditions It is thought that this additional oxygen is either minimising the release of or replacing lost ZnO lattice oxygens. indicating that this ZnO photocatalytic oxidation could be occuring via a Mars Van Krevelen type redox mechanism (C) 2010 Elsevier B V. All rights reserved.