Control of crystal growth for ZnO nanowhisker films in aqueous solution

Porous, highly oriented ZnO nanowhisker films were prepared on substrate in a zinc nitrate-hexamethylenetetramine-polyethylenimine aqueous solution at low temperature via a simple two-step method. The effects of temperature and solution concentration on the morphology and transmittance of ZnO nanowhisker films were investigated in situ with the aid of pH changes. During the growth process, the pH value increased slowly with prolonged reaction time at 75 °C and 88 °C after reaching the lowest value. However, a constant pH was maintained for a long time at 65 °C. Porous and transparent ZnO nanowhisker films were produced at temperatures over 80 °C in the 0.025-0.05 M of solution. Concentrated solution resulted in ZnO nanowhisker films with a dense middle layer between the FTO and whiskers, and with bad transmittances in the visible range even at over 80 °C.