Rational Compositional Control of Electrodeposited Ag-Fe Films
Authors: Sun Yunkai , Zangari Giovanni 
Published: December 2020

In the field of alloys, be they bulk or thin films, systematic control of composition is a prerequisite to understand details of microstructure evolution and phase transformation. Some alloys are very difficult to be electrodeposited, due to problems such as the large difference in the equilibrium potentials and/or deposition kinetics of the alloy components, as well as the bath instability, due to the spontaneous reactions in the bulk electrolyte. The Ag-Fe system is one of those. In this work, a novel alkaline citrate-dimethylhydantoin (DMH) Fe(II)-Ag(I) bath with 1% Fe(III) as the stabilizer has been used to synthesize thermodynamically immiscible Ag-Fe alloy films, in order to partially resolve the difficulties such as:

Using Fe(III)-only electrolyte could possibly lead to solution instability caused by the precipitation of Fe(OH)3 due to its small stability constant.

Using electrolyte with Fe(II) as the main Fe source, Fe(II) cations must be complexed in order to prevent the formation of hydroxide in the deposit, because the redox potential of Fe(II)/Fe, constrained by the stability constant of Fe(OH)2 at given pH, is always more negative than the hydrogen evolution reaction.

The potential difference between Fe(III)/Fe(II) and Ag(I)/Ag is very large. Based on the mixed potential theory, pairing with the anodic reaction of Fe(II)/Fe(III), Ag(I) will be spontaneously reduced by the Fe(II)-only bath.

The fast Ag(I) reduction rate could lead to the formation of Ag dendrite, which is unfavorable for achieving a uniform and smooth alloy film via electrodeposition.

In this presentation we will demonstrate the paradigm for designing this novel metastable alkaline DMH-citrate AgFe deposition bath, compare the prediction from available thermodynamic data with the observed CV profiles with different baths, rationalize its composition control at the limiting current range, and discuss the possible reasons for the non-linearity of this control process due to solution instability, mainly from following two aspects:

Stabilization of the deposition bath: the solution instability is dominated by the redox pairs of Ag(I)/Ag and Fe(II)/Fe(III). Using plain Fe(II)/Ag(I) citrate-DMH solution, we observed gradual degrading of deposition bath with high Fe(II) concentration. Therefore, based on the philosophy of the mixed potential theory, which has been extensively recognized in the fields of corrosion and electroless deposition but not alloy electrodeposition yet, Fe(III) need to be added in order to positively shift the redox potential of Fe(III)/Fe(II). By adding 1%at,Fe(II) of Fe(III), the open-circuit potential of the bath positively shifted for 0.07V and loss of Ag(I) in the solution was partially resolved. However, at high Fe(II) concentration, the bath with Fe(III) additives is still unstable, indicating the metastability of the bath, which is supported by the thermodynamic calculation regarding DMH-citrate complexed Ag(I)-Fe(II) bath with Fe(III) additive.

Prediction of composition at limiting current condition: after the bath has been stabilized, the composition of the films at the limiting current condition could be predicted using a limiting current mass-transfer ratio mAg:mFe≈0.9, suggesting a simple mass-transfer behavior of Fe(II) and Ag(I) at the limiting current electrodeposition condition. Few deviations with this prediction were found, which is considered to be caused by the solution instability and formation of Fe(II) hydroxide species due to the local high pH generated by the hydrogen evolution reaction near the surface of the electrode.

In summary, we would like to draw attention of the electrodeposition field into the importance of complexation chemistry, which is not usually explored with the purpose of stabilizing bulk electrolytes with multivalent ions, regarding a more qualitative explanation of the deposition behavior and solution stability, offer a reference for the composition control of alloy electrodeposition in aqueous or non-aqueous systems, and thus push the field of alloy electrodeposition into more quantitative research methods in the aspect of its macroscopic behaviors.

Graphical abstract:
Calculation results of redox potentials related to the DMH-citrate complexed Ag-Fe electrolyte based on the Nernst equation and thermodynamic data available, with the redox pair which influences the solution stability marked with arrows in the plot. The stabilized bath possesses a well-behaved linear trend between solution concentration ratio and film (deposited at limiting current condition) composition ratio.

Figure 1