Gravitational Level Effects on Coupling Phenomena Between Morphological Variations of Electrodeposited Film of ZnO and Mass Transfer Rates

     Coupling Phenomena between morphological or microstructural evolution rate of electrodeposited film and mass transfer rates must be well understood in order to tailor the unique physical properties of nanostructured films. It is well known microstructure of electrodeposited ZnO film is drastically influenced by the adsorption phenomena of surfactant molecule.  However, the mass transfer rate of surfactant has been seldom discussed, especially when natural convection accompanying oxide layer synthesis reaction is induced in an electrolytic cell. The surfactant concentration is significantly influenced at the liquid/soli. interface. Now it is discussed how gravitational level influences the nucleation and growth phenomena of electrodeposited ZnO.

     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)₄^2-. 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.