Rational Compositional Control of Electrodeposited Ag-Fe Films

Abstract

In the field of alloys, be they bulk of thin films, systematic control of composition is a prerequisite to understand details of microstructure evolution and phase transformation. Because of the complete immiscibility of Ag-Fe, electrodeposition is capable of forming metastable structures, enabling an economical alternative to synthesis from vacuum methods. However, due to the negative redox potential of Fe(II)/Fe, Fe cations must be complexed in order to prevent the formation of hydroxides in the deposit. On the other hand, due to the difference in redox potential, Ag(I) will be spontaneously reduced by the plain Fe(II) bath due to the reaction Fe(II)/Fe(III). Using Fe(III) could prevent the spontaneous reduction of Ag(I) in the solution, however, hydroxides could form due to the small stability constant of Fe(OH)3. Furthermore, the fast Ag(I) reduction rate could lead to the formation of dendrites, an unfavorable morphology for achieving uniform and smooth alloy films. In order to achieve films with high quality, dimethylhydantoin (DMH) was used as a surfactants and complexation agents for Ag(I).Herein we demonstrate the paradigm for designing this novel alkaline DMH-citrate for Ag-Fe deposition, compare the prediction from available thermodynamic data with the observed CV profiles of different baths, and rationalize its composition control at the limiting current range. Two aspects are highlighted: 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. Based on the mixed potential theory, Fe(III) is added to positively shift the redox potential of Fe(III)/Fe(II). By adding 1%at,Fe(II) of Fe(III), the open-circuit potential positively shifted for 0.07V and loss of Ag(I) in the solution was partially resolved.Prediction of composition: after stabilization of the bath, the film composition at the limiting current condition could be predicted using a limiting current mass-transfer ratio. Small deviations from this prediction were found, probably caused by solution instability and formation of Fe(II) hydroxide species due to high pH. In summary, we would like to draw attention to the electrodeposition process by highlighting the importance of complexation chemistry, that unusually here has the purpose of stabilizing the electrolyte with multivalent cations. 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