Current oscillations in the layer-by-layer electrochemical deposition of vertically aligned nanosheets of zinc hydroxide nitrate

Large scale, vertically aligned nanosheets of zinc hydroxide nitrate [Zn₅ (OH) ₈ (NO₃)₂ · 2H ₂O] were synthesized by electrochemical deposition in a methanol solution containing a small fraction of water. Structural analysis by X-ray diffraction and scanning electron microscopy reveal that the nanosheets are layered zinc hydroxide nitrate grown on top of a polycrystalline ZnO layer, forming a two layer structure. During the nanosheet growth, well defined current oscillations were observed at a constant voltage. It was determined that the current oscillations are associated with the layer-by-layer growth of the nanosheets. The alternative stacking of positive and negatively charged polar planes of zinc hydroxide nitrate causes the current fluctuation. Each current oscillation cycle represents one monolayer addition to the existing nanosheets. With a mild annealing treatment, the obtained nanosheets may be transformed to porous ZnO, a more appropriate and useful material for various technological applications. This study provides a new route to precisely control the growth of nanostructures in solution as well as the type of polar surface via current monitoring during deposition. Although layer-by-layer deposition has been extensively studied in vacuum such as atomic layer deposition, it has rarely been reported for solution growth which may have potential applications in materials research.