Nanoscale Structuring in Au–Ni Films Grown by Electrochemical Underpotential Co‐deposition

Abstract

Nanoscale phase separation is a simple but powerful method for the self‐assembly of nanostructures, which enables properties to be tuned by tailoring the phase composition, size, and topology of the structures. We demonstrate, contrary to thermodynamic predictions, the spontaneous nanoscale phase separation of Au–Ni alloy films, grown by electrochemical underpotential deposition of Ni onto a surface constantly renewed by the ongoing reduction of Au at overpotential. Strain‐energy relaxation during film growth, in combination with the net attractive interaction between Au and Ni atoms, results in the dynamic formation of a nanoscale alloy structure, consisting of Au‐rich grains of about 5 nm in size surrounded by relatively thick (2–3 nm) Ni‐rich grain boundaries. The estimated cohesive energy of such alloy configurations is stronger than that of the phase‐separated mixture, suggesting the formation of an unexpected local equilibrium configuration for the Au–Ni binary system, stabilized by the grain nanosize and the high density of grain boundaries.

Electrochemical Surface Processes and Opportunities for Material Synthesis

“…Note how the regular solution approximation fails to predict the observed composition vs. deposition potential curve at the negative end of the potential range, and how such deviation becomes more pronounced by decreasing the Ni ion concentration. Similar results have been obtained in our laboratories in the context of Au-Ni electrodeposition from an electrolyte containing 0.5 mM HAuCl 4 , 5-500 mM NiSO 4 , and Na 2 SO 4 in quantities necessary to maintain the ionic strength constant, at pH 2.5 [97]. The composition vs. deposition potential data (Figure 3.14a) fit the trend predicted by Eq.…”

“…(3.13) with positive ΔH mix ), solid solution formation could only be achieved at overpotential (Figure 3.18c), while a phase-separated mixture could be achieved without any shift in onset potential. Experimental data however show that the onset of Ni reduction is depolarized when Ni is codeposited with Au ( Figure 3.19a); furthermore, the resulting microstructure is neither a solid solution nor a phase-separated mixture, but instead is heterogeneous and consists of nanoscale Au-rich grains (Au 80-83 at%) separated by Ni-rich (Au 30 at%) grain boundaries (Figure 3.19b) [97]. The observed depolarization of Ni deposition is driven by the attractive pairwise atomic interaction between Au and Ni, which dominates the growth process during atomic adsorption and incorporation at the surface.…”

“…24 SEM images of the surface morphology of Au-Cu film ((a) Cu 30 at%, (b) Cu 85 at%) grown from a solution of 0.5 mM Au(ethylendiamine) 2 , 20 mM CuSO 4 , and 0.2 M glycine[97].…”

Underpotential deposition of Cu on Au(111) from neutral chloride containing electrolyte

“…13,14 Providing simple means of surface modification, underpotential deposition (UPD) has also been harnessed for the formation of self-assembled monolayers, [15][16][17][18][19] and the intrinsic limitation to one or two atomic layers makes the UPD process attractive for processing on the nanoscale as exemplified by the growth of layers exploiting surface limited redox replacement (SLRR), [20][21][22] the formation of nanostructures, 23,24 the tailoring of electrocatalysts, 22,25,26 or the modification of nanoparticles. 27,28 Among the numerous UPD systems, 1 Cu on metals such as Au, Ag, Pt, and Pd has been a focus of studies [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] with new aspects in regard to the intricate relationship between structures and deposition conditions continuing to be uncovered.…”

Electrodeposition of Alloys and Compounds in the Era of Microelectronics and Energy Conversion Technology

“…A most interesting observation for example is the possibility to access metastable configurations in electrodeposited Au-Ni alloys; while bulk Au-Ni tends to phase separate in two elemental structure, electroplated films under UPCD conditions exhibit a two-phase configuration consisting of Au-rich grains and Ni-rich grain boundaries [59].…”