We discuss the effects of surface energy and seed layer annealing temperature (T-annealing) on seed layer growth and hydrothermally-grown zinc oxide (ZnO) nanowires (NWs). In this work, by varying the ultraviolet ozone (UVO) treatment times on a silicon surface, the surface energy conditions for the seed layer formation changed and the seed layer was annealed under different T-annealing conditions. Under a lower surface energy condition of the substrate, with increasing T-annealing, the coverage density and the average thickness of the seed layer increased, but island-like growth was observed. This case was inevitably accompanied by an increase in surface roughness, which resulted in agglomerated low density growth of ZnO NWs. After sufficient UVO treatment, hydroxyl groups on the silicon surface activated the ZnO seed layer formation in the chemical reaction and increased the bonding energy between the active nucleation sites of the seed layer and the substrate surface. This ensured higher coverage density of the seed layer with lower surface roughness under the same T annealing condition, thereby providing the ZnO NW growth with an enhanced density and aspect ratio as well as good crystallinity. (C) 2015 Elsevier B.V. All rights reserved.