Several photocatalysts with different architectures were synthesized via a facile hydrothermal approach in the presence of diverse structure-directing reagents at varied concentrations. It was found that the concentration of the directing reagents played a crucial role in the architecture of the obtained products. Flower-like ZnO architectures were synthesized at a high concentration of PVP, while pencil-like mesocrystals (MCs) at a low concentration. The flower shaped assembly was composed of leaf-like ZnO MCs. More interestingly, the MCs were assembled with ZnO nanocrystals aligned along [111] direction. The photocatalytic activities of the samples were examined by photocatalytic degradation of methylene blue, and the results indicated that the flower-like architectures exhibited highest performance, compared with those of ZnO nanocrystals, pencil shaped ZnO mesocrystal, and the sheet-like ZnO MCs. This could be attributed to not only the hierarchical architecture of the assembly, high crystallinity, and large surface area, but also the unique mesoscopic structure of ZnO MCs. (C) 2016 Elsevier B.V. All rights reserved.