Hierarchical ZnO nanoporous/nanowire architectures immobilized on a substrate were prepared by a facile two-step electrochemical technique. The layer optical properties have been characterized by absorption spectroscopy and photoluminescence measurements. The structures strongly absorbed UV light and had a high structural quality. The porosity of the hierarchical layers could be tuned by changing the duration of the second growth step. The photocatalytic activity of ZnO films was higher than that of arrayed ZnO nanowire layers and mesoporous ZnO films. The photocatalytic activity for 4-chlorophenol degradation could be enhanced by developing hierarchical structures with a high percentage exposure of polar (0001) facets, high specific surface area and good accessibility of the pollutant to the oxide surface. The modeling by density functional theory (DFT) of the degradation of 4-chlorophenol molecules by OH radicals lead to several important results, namely that the hydroxylation of the aromatic ring and its opening can occur in parallel releasing hydroperoxyl radical and hydroxyl radical, respectively. The restored OH radical can either further oxidize the primary ring opening product or attack another molecule of 4-chlorophenol. These computational results are in good agreement with the photocatalytic degradation observations made using both ZnO and TiO2. photocatalysts. (C) 2014 Elsevier B.V. All rights reserved.