Vertical zinc oxide nanowires, used as electron emitters, were synthesized on ZnO:Ga/glass substrate by a vapor-liquid-solid process without a catalyst. The nanowires, 5 mu m long and 50-250 nm in diameter, were grown with a preferred (0 0 2) orientation and a small XRD full-width at half-maximum intensity. Illumination of ZnO nanowires by UV improved their field emission attributes, reducing their turn-on electric field to 2.1 V/mu m at a current density of 1 mu A/cm(2). This phenomenon is attributed to the generation of a large number of hole-electron pairs by UV illumination. (C) 2010 Elsevier B.V. All rights reserved.