ZnO has received a lot of attention in for the degradation and complete mineralization of environmental pollutants. In this study, efficient ZnO-based visible-light photocatalysts are synthesized via a low-cost method by increasing the Co concentration at the outermost surface of ZnO: Co thin film under an annealing process. The simply adjusted Co concentration gradient (approximate to 1.5% increase) into the surface of ZnO: Co is driven by thermal diffusion. Co ions substitute Zn sites in the ZnO lattice. The band gap narrowed due to sp-d exchange interactions between the conduction band electrons and the d electrons of Co. The annealing process enhances the grain size and the degree of crystallinity of the diffused Co/ZnO:Co, which contributes to the absorption edge red-shift. Photocatalytic activities such as the photodegradation of methylene blue and the inactivation of bacteria are obviously enhanced. The proposed low-cost method significantly increases the efficiency of ZnO for the generation of the photocatalytic effect under visible-light exposure. (C) 2011 Elsevier B. V. All rights reserved.