Morphology transitions (nanorods-nanowalls and nanorods-nanotubes-layer) were induced in the growth of ZnO nanostructures by metal organic chemical vapor deposition (MOCVD) on c-sapphire, using helium as carrier gas, and dimethylzinc-triethylamine and nitrous oxide as zinc and oxygen sources, respectively. A systematic study of the influence of the VI/II ratio and precursor flow-rates on the morphology of ZnO nanorod arrays has been carried out, taking advantage of the ability of MOCVD to individually control the precursor partial pressures. Growth mechanisms are discussed to understand the evolution of the nanostructures morphology for different growth conditions. In particular, the influence of the gas phase supersaturation on the vertical alignment and aspect ratio of the nanorods is emphasized. (C) 2012 Elsevier B.V. All rights reserved.