Morphological evolution of nanocluster aggregates and single crystals in alkaline zinc electrodeposition

The morphology of Zn electrodeposits is studied on carbon-coated transmission electron microscopy grids. At low overpotentials (η = -50 mV), the morphology develops by aggregation at two distinct length scales: ∼5 nm diameter monocrystalline nanoclusters form ∼50 nm diameter polycrystalline aggregates, and the aggregates form a branched network. Epitaxial (000̄2) growth above an overpotential of |η_c| > 125 mV leads to the formation of hexagonal single crystals up to 2 μm in diameter. Potentiostatic current transients were used to calculate the nucleation rate from Scharifker et al.'s model. The exp(η) dependence of the nucleation rates indicates that atomistic nucleation theory explains the nucleation process better than Volmer-Weber theory. A kinetic model is provided using the rate equations of vapor solidification to simulate the evolution of the different morphologies. On solving these equations, we show that aggregation is attributed to cluster impingement and cluster diffusion while single-crystal formation is attributed to direct attachment.