Electro-deposition & dissolution behavior of zinc-based alloys in additive-free chloride baths and formation of doped ZnO nanostructures by their hydrothermal oxidation

Abstract

Zn-Ni and Zn-Co alloy coatings with 5-18 at% Ni and 8-25 at% Co have been prepared by DC plating in additive free chloride baths. Effects of bath composition on the alloy composition, texture, grain size, morphology and hardness were investigated. Potentiodynamic anodic stripping, reverse chronopotentiometry were employed in combination with XRD and EDS to correctly determine the electro-dissolution (dezincification) behavior of alloy electrodeposits. Potentiodynamic cyclic stripping was also performed to prepare compact Zn-Co electrodeposits. Zn-rich alloy deposits are predominantly formed by DC plating in these baths due to anomalous codeposition. With the help of careful cyclic voltammetry, chronopotentiometry, chronoamperometry, and (potentiodynamic) cyclic voltammetry, it has been established for the first time in this work that it is primarily the electrochemical potential that determines the deposition mode. Between the window of normal codeposition where nickel or cobalt rich phases are deposited and anomalous codeposition where zinc-rich phases are formed, a range on electrochemical potential exists where the formation of zinc hydroxide hinders the electrodeposition and cathodic current mostly becomes insignificant. A shift from this region to the cathodic direction allows anomalous codeposition of zinc and nobler alloy constituent. A shift in the anodic direction may again allow cathodic deposition of nobler constituent with under-potential deposition of zinc. The transition potentials depend on bath composition and temperature. Hydrothermal oxidation of Zn, Zn-Ni and Zn-Co electrodeposited on conducting substrates resulted in wide variety of nanostructures depending on the oxidation temperature and alloy content. In case of pure electrodeposited zinc, nanorods with diameter ranging from 300-800nm are seen at oxidation temperature of 100oC. The size of nanorods becomes coarser with rise in oxidation temperature. Hydrothermal oxidation of Zn-Ni alloys resulted in the doped ZnO nanostructures with quantity of dopant ranging from 2 at% to 11 at%. Not only nanorods and nanowires are synthesized by this technique, but also novel structures like nanotulips, hollow nanocones, faceted nanotubes and electronically translucent nanosheets arranged are obtained. Hydrothermal oxidation of Zn-Co alloys resulted in hollow and tubular ZnO nanostructures with doping of cobalt around 2at%. The doped ZnO nanostructures become finer with a rise in synthesis temperature. Hence, dopant and temperature exhibit synergistic effects in determining the morphology of the ZnO nanostructures grown by hydrothermal oxidation of electrodeposited nanocrystalline alloys.