Electrodeposition of Epitaxial Co(OH)₂ on Gold and Conversion to Epitaxial CoOOH and Co₃O₄

An electrodeposition method for growing epitaxial Co(OH) ₂ films on single crystalline gold (111), (100), and (110) substrates is described. The films were grown by electrochemical reduction of [Co(en) ₃ ] ³⁺ in an alkaline electrolyte. The Co(OH) ₂ grew with a [0001] out-of-plane orientation on all the gold crystal orientations. The in-plane orientation follows the symmetry of the gold (111), (100), and (110) substrates. The Co(OH) ₂ can be converted to CoOOH by electrochemical oxidation in 1 M KOH at 95 °C, and after conversion remains epitaxial with a [0001] out-of-plane orientation. The CoOOH film can be further converted to epitaxial Co ₃ O ₄ with a [111] out-of-plane orientation by decomposition of the CoOOH film in air at 300 °C. This synthesis method allows for a simple fabrication of epitaxial catalysts and could be useful to probe the catalytic activity of specific crystal planes.