Crystallographic structure of gold films electrodeposited at low current densities

Abstract

In this paper we report on the electrodeposition of Au from dicyanoaurate electrolytes at low current densities. If electrodeposition is carried out at current densities lower than 0.25 mA cm⁻² a new hexagonal crystalline structure can be observed. This structure is generally codeposited with fcc Au. The new structure dominates for short deposition times. This structure is stabilised by the addition to the bath of metal ions giving rise to UPD behaviour on Au, such as Cu²⁺ and Tl⁺. This structure was tentatively assigned to the space group 184 (p6cc). This structure can be explained in terms of the incorporation of extraneous, probably cyanide-related, material. This interpretation is also supported by in situ Raman spectroscopy and by XPS analyses showing that the bulk deposits contain C, N and K. The electrochemical behaviour of the electrodeposition system was studied by cyclic voltammetry. Cyanoaurate baths display a significant cathodic passivation. This phenomenon is reduced by the addition of Cu²⁺ or Tl⁺. Voltammograms of baths containing these additives show clear UPD features but no stripping peaks. This behaviour is suggestive of an assisted Au electrodeposition mechanism. Tl is deposited at UPD and oxidatively desorbed by Au(I). Cu is codeposited at UPD with Au forming an alloy.