Unexpected Reduction of Silver(I) Cyanide Complex by Gold Yields a Surface-Limited Silver Monolayer

Galvanic replacement reactions (GRR) between gold and silver are widely explored in various coating applications and, in particular, for producing a wide range of nanostructures. In all cases, gold metal complexes had a higher standard potential than silver, and therefore, Au(I) and Au(III) reacted with Ag. In all previously reported GRRs, metallic gold was deposited on top of the silver surface, while silver was oxidised and dissolved into the solution. In this paper, we show a unique case where cyanide ligands flip the reaction direction and lead to a surface-limited reaction. Thus, cyanide ligands from [Ag(CN)₂]⁻ complex stabilize the Au(I) complex dramatically and lower its standard reduction potential. The standard reduction potential of the cyanide gold complex ([Au(CN)₂]⁻) is even lower than the standard reduction potential of the [Ag(CN)₂]⁻ complex (-0.6 and -0.31 V, respectively), so that the silver complex is able to oxidise metallic gold. In this GRR, ultrathin silver film (0.2–0.3 nm) was deposited on top of the gold surface, and the reaction stops after full coverage of the gold surface. Further investigation of this reaction showed it occurs both on Au nanoparticles and electrodeposited and sputtered gold films, with low dependency on the [Ag(CN)₂]⁻ concentration.