Anion effects on the interfacial alloying in successively electrodeposited Cu and Au ultrathin films

Highlights: • The electrodeposition of Au on Cu films from Cl⁻ and S₂O₃²⁻ electrolytes is studied. • Alloying between Au and Cu is found unavoidable in these simple electrolyte systems. • Samples grown in Cl⁻ electrolyte exhibit large faceted crystallites after de-alloying. • Samples grown in S₂O₃²⁻ electrolyte feature interconnected porosity after de-alloying. The alloying between successively deposited ultrathin (∼10 nm) films of Cu and Au is explored. Au is deposited on Cu films via conventional overpotential deposition (OPD) and mediated methods in chloride containing electrolyte (CCE), and via OPD in thiosulfate containing electrolyte (TCE). It is determined via electrochemical means that Au deposition on Cu results in alloy formation in all cases, justified by the exothermic enthalpy of Au-Cu mixing. Deposition in TCE results in more homogeneous, multi-phase alloy formation evidenced by positively shifted Cu stripping behavior. This is attributed to the thiosulfate adsorption limiting Au and Cu mobility thus yielding the formation of continuous and smaller-grained over-layers. In contrast, the enhanced mobility of Au ad-atoms in CCE facilitates 3D growth, evidenced by SEM to yield larger grains with well-defined facets and limited alloying. It was shown via XPS that alloys deposited from TCE exhibit a concentration gradient from the center of films to the exteriors and that S is present in the as-deposited films. In addition, following Cu stripping, it is shown that a variety of nanoporous and S-free Au structures can be formed with variable ligament and facet size, ultimately providing opportunity for catalyst design on the nanoscale.