Multiscale design of ZnO nanostructured photocatalysts

A systematic investigation of the photocatalytic activity (PCA) of nanostructured ZnO films showed howthis is directly affected by the films’ morphology at different scales, from the macroscale morphology offilms (e.g. thickness and surface area), to the microscale feature arrangement (e.g. aligned vs. randomlyoriented structures or interpenetrated ones), to the nanoscale structure (e.g. crystal size and orientation).The interest in immobilizing photocatalysts in water treatment stems from concerns about the potentialtoxicity of their slurry form, which requires expensive downstream removal. Immobilisation, though,leads to a reduction in PCA, generally attributed to a lower surface area. By reducing the films’ featuresize to the nanoscale, an immobilized photocatalyst with high surface area can be achieved. At thisscale, however, feature structuring and morphology become important as they determine theinteraction between light and the photocatalytic material. In this work, nanostructured ZnO films withdifferent morphology, arrangement and structure were produced by electrochemical anodization of zincand were tested using the degradation of phenol in a batch reactor as a model system. Results showthat the PCA for immobilized catalysts can be optimised by controlling microscale arrangement (lightabsorbance capacity) and nanoscale structure (crystal size and orientation) rather than macroscalemorphology (surface area). These results provide a clear direction to maximising the photocatalyticactivity of immobilised photocatalysts for the removal of organic pollutants from water.