Phase Separation in Electrodeposited Ag-Pd Alloy Films from Acidic Nitrate Bath

Abstract

The electrochemical deposition of Ag-Pd from acidic nitrate solutions has been studied. Under suitable conditions, phase separation in the form of solid solutions with different compositions is observed at the micron scale. The phase separation is not generated by electrochemical instabilities, but instead is due to a wide difference in the intrinsic reduction kinetics and the distinct nucleation characteristics. At UPD conditions Ag-Pd films could be grown via underpotential codeposition, in close agreement to thermodynamic predictions. At intermediate overpotentials, phase separation occurs due to local concentration gradients generated by mass transfer limitation for Ag and large nucleation barrier for Pd. Under mass transfer limitations, the growth is uniform and the composition corresponds to the prediction based on the ion diffusivities.

Electrodeposition of Ag-Pd Alloy at Ru Substrate from Simple Acidic Nitrate Bath

“…In this work, the behavior of the Ag-Pd electrodeposition at Ru substrates from simple acidic nitrate baths was investigated. We find that, similarly to the results obtained at an Au substrate, 10 the film composition could be precisely predicted using a model involving the mass-transfer rate of different species at limiting current conditions. In contrast, the nucleation kinetics, morphology, phase composition and the mechanical stability of the films are shown to be different: both Ag and Pd show a large nucleation overpotential at Ru substrates.…”

“…are experimentally determined, based on the mass-transfer coefficient ratio of = m m : 0.479 Pd Ag derived from previous work. 10 Considering the instability of Pd(II) in the simple acidic nitrate bath, the deposition bath was immediately used after preparation. All electrochemical experiments, including cyclic voltammetry and potentiostatic depositions, were performed using a EG&G PAR Model 273 A potentiostat in a three-electrodes setup cell, with Pt mesh as the counter electrode, saturated Hg/Hg 2 SO 4 (MSE, 0.65V SHE ) as the reference electrode and polycrystalline (0001)-textured Ru (80 nm)/ Ta (5 nm)/Si wafer as the working electrodes.…”

“…In addition to the difficulty in solution preparation and the incompatible deposition kinetics, according to the CALPHAD database, the AgPd alloy system exhibits a miscibility gap within the compositional range (63, 99) % at,Pd . 9,10 The limited solubility of AgPd is also suggested by a significant difference in the metallic radius and the electronegativity, using for example the Darken-Gurry plot (visualization of Hume-Rothery criteria). 11 However, since both evidences are not definitive for assessing alloy solubility, especially at room temperature, the formation of this miscibility gap in AgPd is still uncertain.…”

“…[13][14][15] Recently, we have published the behavior of Ag-Pd deposition from simple acidic nitrate bath, achieving a satisfactory metastability of the electrolyte. 10 In particular, it was shown that despite Ag-Pd alloy has a high mutual solubility, significant phase separation of Ag-rich and Pd-rich phases could be observed, originated from the large difference in their reduction kinetics rather than alloy thermodynamics.…”

Commentary and Notes on the Original Derivations of the Scharifker-Hills Model

“…On the other hand, when there are side reactions or nucleation incubation times (e.g. in our Ag-Pd work 29 ), the magnitude of the side reactions needs to be evaluated before the fitting with the S-H model in peak J and t , peak which is also very subjective. Therefore, despite seemingly experimentally difficult, it is the slope in the log-log plot (e.g.…”