The ITO/TiO2 film was immersed in 0.1 M ZnCl2 complexed with ammonium hydroxide and hydrogen peroxide for a period of 25 s. Excess ammonium hydroxide solution was used to
control the pH of the ZnCl2 solution to 14. Addition of the ammonium hydroxide solution to the ZnCl2 resulted in the formation of a Zn(OH)2 precipitate, but this redissolved in excess ammonium hydroxide to form the tetra-amine zinc complex. Then, the substrate was immersed in distilled water at 90 °C. High temperatures promoted the adherence of the ZnO film. In the first preparative stage, the ammonia-zinc ions complex was adsorbed onto the substrate surface, and in the second preparative stage, the hydroxide of the anion precursor solution was adsorbed onto the surface and reacted with the adsorbed zinc species to yield zinc hydroxide, which in turn was converted into ZnO. There were several insoluble zinc species present in a reaction bath containing OH-, NH3, and Zn2+ ions.