Gravitational Level Effects on Coupling Phenomena Between Morphological Variations of Electrodeposited Film of ZnO and Mass Transfer Rates
ZnO electrodeposition onto FTO/ITO substrate is firstly performed in 0.1MZn(NO3)2-LiNO3 aqueous solution (pH=5.1) at 70°C without addition of surfactant molecule. The substrates were ultrasonically cleaned sequentially in acetone, ethanol and deionized water for 15 min., respectively. The counter electrode was a sheer of pure zinc (Nilaco Corp.). Effective surface of workin and counter electrodes were 10x10mm. They were embedded in each side of channel walls. Ag/AgCl was used as a reference electrode. The cell was arranged in two different configurations: (1) a horizontally installed cathode facing downward over a horizontal anode (C/A) and (2) an anode over a cathode (A/C). No natural convection is principally expected under the former configuration.
Current transients were measured at various electrode potential. No apparent differences were observed between -0.85 and -0.8V. Below -0.9V, they were categorized into four stages.
Stage 1: Double layer charging and start of NO3-reduction
Stage 2: Induction time for nucleation
Stage 3: ZnO nucleation and growth
Stage 4: Transport limited
The initial three stages can be described by a model including progressive nucleation and kinetically controlled 3D growth. Surface OH- concentration during the nucleation period was estimated by using a simple model. We assume ZnO nucleation can be caused by the production of precursor like Zn(OH)42-. Surface morphology and XRD spectrum are changed by the gravitational field direction. The effect may be attributed to natural convection, eventually, concentration of reactant near the substrate cathode surface during electrodeposition.