Zinc exhaustion in ZnO electrodeposition

• ZnO films are grown at various potentials using platinum (inert) and zinc (non-inert) counter electrodes.
• Potentiostatic deposition using inert electrodes causes a drop in solution pH.
• Lower pH is due to Zn ion depletion in the growth solution, yielding nitric acid.
• The effects of reduced pH on film growth and quality are studied.

Zinc oxide (ZnO) is electrochemically grown using Zn(NO3)2 with both platinum and zinc counter electrodes. The effect of the counter electrode at different potentials and varying durations on the quality of the resulting ZnO film is examined. Measurements included time-varying pH, surface morphology, deposition current, film thickness, and crystallinity. It was observed that films grown at high potentials or for long durations using an inert electrode caused a drop in solution pH due to zinc ion exhaustion in the growth medium. In the standard electrochemical reaction, zinc ions are consumed faster than nitrate ions, causing hydrogen ions resulting from the electrolysis of water at the counter electrode to persist in the growth solution. The resulting acidity when at a sufficiently low pH significantly inhibits further growth of ZnO and damages the existing film. The growth of high quality, thicker films using Zn(NO3)2 therefor requires mitigative measures such as using a zinc counter electrode, replacing lost zinc, or by diluting the acid in the growth solution.