Spatially separated ZnO nanopillars were grown on Pt-thin-film-coated Si substrates by electrochemical deposition in Zn(NO3)2 aqueous solution. The effects of growth parameters, such as Zn(NO3)2 concentration, cathodic potential, bath temperature, and growth time, were studied with respect to the morphological, structural, and photoluminescence properties of the ZnO nanopillars. The isolated nanopillars fabricated at a relatively low concentration of Zn(NO3)2 (∼5 × 10-4 M) and high temperature (90 °C) had the best structural and optical qualities. The separated ZnO nanopillars were found to exhibit a single-crystal wurtzite structure and to grow along the c axis perpendicularly to the substrate. The as-grown nanopillars had a moderate UV-to-visible emission ratio of ∼4.2, but this value could be substantially improved by annealing. Annealing at 300 °C in all three atmospheres studied (air, 5% H2/95% N2, and vacuum) resulted in a considerable improvement in the UV-to-visible emission ratio (16.7−28.6), although a considerable number of defects were created by annealing at temperatures above 500 °C.