Electrochemical synthesis of nanocrystalline zinc oxide and phase transformations of zinc hydroxides

Nanoscaled ZnO was synthesized by an electrochemical process using zinc or Al-alloyed zinc electrodes in an aqueous system with acetic acid as a conductive salt. Depending on the synthetical parameters, the precipitated precursor solids were found to consist of various compounds such as zincite, presumably delta-Zn(OH)(2), beta(1)-Zn(OH)(2), epsilon-Zn(OH)(2), Zn-5(CO3)(2)(OH)(6) and, in case an Al-alloyed electrode was used for the synthesis, Zn0.71Al0.29(OH)(2) (CO3)(0.145)center dot xH(2)O. The intermediate solids served as precursors for the subsequent thermal treatment resulting in zincite powders with various morphologies. Depending on the processing conditions, zincite was formed between 100 and 260 A degrees C with a mean crystallite size between 6 and 25 nm. Selected zincite powders were pressed to pellets, sintered at temperatures between 900 and 1,100 A degrees C and characterized by measurements of the electrical bulk conductivities, yielding values up to 1.69 S cm(-1) in samples with 1.24 wt% Al. Comparison with samples prepared by precipitation methods showed that the latter had values of up to 44 S cm(-1).