Here a systematic study on diameter dependence of photocatalytic properties of ZnO nanowires is presented. The photocatalytic activity of ZnO nanowires of various diameters ranging from 30 to 300 nm were studied by degradation of Resazurin dye in aqueous phase under UV light. The photocatalytic activity of ZnO nanowires were found to increase anomalously for nanowires of diameters <90 nm. The increased photocatalytic activity of thinner nanostructures is due to higher surface to volume ratio. This resulted in increased surface activity because of lowering of activation energy (20-2 kJ for diameter 300 and 30 nm, respectively). (C) 2012 Elsevier B.V. All rights reserved.