A systematic study of the influence and mechanism of chloride ions on the morphology of ZnO nanostructures in nitrate electrolyte by electrochemical deposition was performed. The concentration of Zn(NO3)(2) remained constant, and the additional KNO3 and KCl were added to modulate the effect of chloride ions. Scanning electron microscopy graphs revealed that the effect of chloride ions on ZnO nanostructure morphology was not only dependent on the concentration of chloride ions, but also relied on the concentration of hydroxide ions. When the concentration of hydroxide ions was low, the adsorption of chloride ions on the ZnO (0001) surface played a dominant role and promoted two dimensional growth of ZnO. When the concentration of hydroxide was increased to high level, enough hydroxide ions gave rise to the faster deposition rate of ZnO under the same concentration of chloride ions, especially growth rate along c-axis due to polar nature of ZnO. The high density one dimensional nanostructure was achieved. In this case, the adsorption of chloride ions caused flat top surface of nanorods. Morphology evolution from dense film to nanorods or nanotowers would be achieved by altering the concentration KCl and additional KNO3, which is a simple method to control the morphology of ZnO nanostructures for abundant application. (C) 2013 Elsevier Ltd. All rights reserved.