Hydrogen peroxide oxygen precursor for zinc oxide electrodeposition II -: Mechanistic aspects

Zinc oxide thin films have been deposited cathodically at 70 degreesC from a chloride aqueous solution containing dissolved hydrogen peroxide and a zinc(II) chloride. The electrodeposition process has been studied by cyclic voltammetry (CV), chronoamperometry, and quartz crystal microbalance techniques. A parametric study has been carried out by varying the solution composition and the substrate. It is shown that the film growth is under kinetic control since, the catalytic activity of ZnO surfaces for the reduction of hydrogen peroxide species is low under these conditions. The nature of the substrates (tin oxide, gold) and their treatments prior to the deposition experiments are shown to have a marked influence on the electrochemical behavior of the system. Besides, we show that it is possible to normalize the film growth behavior by taking the mean current density as the key parameter. For values below the limiting diffusion current of zinc ions, the faradaic efficiency is close to 100%, whereas, above this value the faradaic efficiency decreases markedly. The results have been modeled with the help of a simple competition mechanism in which we assume that the hydroxide ions produced in excess at the surface diffuse towards the solution and react partly with zinc ions diffusing towards the surface. That reaction in solution reduces the availability of zinc ions for the heterogeneous deposition reaction at the surface. We have proposed an empirical law to describe this phenomenon. (C) 2001 Elsevier Science B.V. All rights reserved.