Morphology and fine structure of zinc electrodeposits

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Abstract
A 'nanolaminated' structure has been observed in electrodeposited Zn by using high-resolution transmission electron microscopy. It has been found that the individual Zn crystallites in the deposits are all bounded by an ultra-thin zinc oxide (ZnO) layer about 3·5 nm in thickness. The crystallites are predominantly hexagonal platelet shaped, typically 300 nm in diameter and 50 nm in thickness. The presence of grains with a specific hexagonal geometry in the deposit results from a layered texture, which is made by a repeated sandwich-like epitaxial growth between Zn (hexagonal) and ZnO (wurtzite structure). Such a structural growth pattern is discussed in terms of the kinetics of the electrodeposition process. It is proposed that Zn deposition occurs from zinc hydroxide (Zn(OH)2) which forms close to the metal-electrolyte interface as a result of H2 evolution and the local increase in electrolyte pH. The interaction between the depletion of the Zn(OH)2 colloidal layer due to Zn deposition and its re-formation due to the H2 evolution reaction could lead to cyclic changes near the cathode. This could control the formation of a layered texture in the Zn deposit, as ZnO which forms intermittently from the dehydration of Zn(OH)2 at the electrode-colloid interface. The structural observations also explains why superior corrosion resistance is often shown by electrodeposited Zn, that is because of controlled distribution of the ZnO in the deposit.