Epitaxial ZnO nanowires were grown on a- and c-plane sapphire substrates by metalorganic chemical vapor deposition without metal catalysts or templates. Nanowires with monodisperse diameters grow in dense arrays perpendicular to a-plane sapphire and with in-plane rotational alignment due to [0 0 0 1](ZnO)parallel to[1 1 (2) over bar 0](sapphire), [1 1 (2) over bar 0](ZnO)[0 0 0 1](sapphire) epitaxy. On c-plane sapphire, multiple possible epitaxial relations give a Mixture of nanowire orientations. The majority of the nanowires grow in one of the three directions all at an angle of 51.8degrees off the substrate plane with [0 0 0 1]ZnOparallel to[1 0 (1) over bar 4](sapphire), [1 0 (1) over bar 0]ZnOparallel to[1 (2) over bar 1 0](sapphire) epitaxy. A small fraction of the nanowires grow perpendicular to the substrate with [0 0 0 1](ZnO)parallel to011[0 0 0 1](sapphire). (C) 2004 Elsevier B.V. All rights reserved.