Electrochemical synthesis of Zn-Al layered double hydroxide (LDH) films.

A new electrodeposition condition to produce Zn-Al LDH films was developed using nitrate solutions containing Zn (2+) and Al (3+) ions. Deposition was achieved by reducing nitrate ions to generate hydroxide ions on the working electrode. This elevates the local pH on the working electrode, resulting in precipitation of Zn-Al LDH films. The effect of deposition potential, pH of the plating solution, and the Zn (2+) to Al (3+) ratio in the plating solution on the purity and crystallinity of the LDH films deposited was systematically studied using X-ray diffraction and energy dispersive spectroscopy (EDS). The optimum deposition potential to deposit pure and well-ordered Zn-Al LDH films was E = -1.65V versus a Ag/AgCl in 4 M KCl reference electrode at room temperature using a solution containing 12.5 mM Zn(NO 3) 2.6H 2O and 7.5 mM Al(NO 3) 3.9H 2O with pH adjusted to 3.8. The resulting film contained 39 atomic %Al (3+) ions replacing Zn (2+) ions, leading to a composition of Zn 0.61Al 0.39(OH) 2(NO 3) 0.39. xH 2O. Increasing or decreasing the aluminum concentration in the plating solution resulted in the formation of aluminum- or zinc-containing impurities, respectively, instead of varying aluminum content incorporated into the LDH phase. Choosing an optimum deposition potential was important to obtain LDH as a pure phase in the film. When the potential more negative than the optimum potential is used, zinc metal or zinc hydroxide was deposited as a side product, whereas making the potential less negative than the optimum potential resulted in the formation of zinc oxide as the major phase. The pH condition of the plating solution was also critical, as increasing pH destabilizes the formation of the LDH phase while decreasing pH promoted deposition of other impurities.