Pre-annealing of seed layers before the growth of ZnO nanorods (NRs), at various temperatures (non-annealing -800 degrees C) and in various atmospheres (vacuum, N-2, or O-2), was systematically studied to investigate the effects of pre-annealing on the material properties of ZnO NRs as well as the rectifying behaviour of ZnO NRs/p-Si heterojunction diodes (HJDs). A seed layer was initially prepared on the Si substrate through hydrothermal (HT) method and subsequently pre-annealed; finally, the ZnO NRs were grown through the same HT method. We found that without the annealed seed layer, the ZnO NRs cannot be grown on the Si template and increase in the pre-annealing temperature led to better crystallization and fewer defect-centres in ZnO NRs. However, at a high pre-annealing temperature, the characteristics of ZnO NRs degraded due to the evaporation of oxygen atoms, resulting in more oxygen-vacancy-related defects. The smallest diameter and shortest length of ZnO NRs were observed on the samples pre-annealed at 450 degrees C. The short length of ZnO NRs implies a slow growth rate, because of which the NRs have sufficient time to align normal to the surface of the substrate. When the seed layer is pre-annealed in an O-2 atmosphere, the oxygen atoms fill the oxygen-vacancy-related defects, which lead to a higher nucleation density and improved characteristics of ZnO NRs. This leads to an extremely high rectification ratio of 1.8 x 10(5) in ZnO NR/p-Si HJDs. The related mechanisms were explored in this study. (C) 2012 Elsevier B.V. All rights reserved.