Controllable ZnO architectures with different morphologies and defect states play an important role in the adjustment of the surface area and chemical/physical properties in metal oxide-based photocatalysts. However, the rapid recombination of the photo-induced electron–hole pairs limits their practical application for the purification of organic pollutions. In the present study, we synthesized 1-D ZnO nanorods (NRs) with a tubular top facet using a facile hydrothermal method, and investigated the effect of growth time on the morphology, chemical bonding state, and photocatalytic activity of the NRs. As the growth time increased, the size (diameter and length) of the NRs slightly increased and the morphology of the top facet varied from a flat to a tubular shape due to the more dominant reaction of dissolution than that of growth. The photocatalytic activity was evaluated based on the degradation of methylene blue in aqueous solution under UV irradiation. The photocatalytic activity of the ZnO NRs increased with increasing growth time, which was attributed to the increased surface area and the reduced defect states (oxygen vacancies) in the NRs. These results suggest that controlling the hydrothermal reaction time is an effective way to tailor the morphology, electronic state, and photocatalytic activity of ZnO nanostructures.