Deposition of Metal and Alloy Thin Films By Surface Limited Redox Replacement in a Galvanic Cell Configuration (SLRR-GC)
Authors: Dimitrov Nikolay , Ambrozik Stephen John 
Published: February 2020

The drive towards development of efficient and low cost catalysts for fuel cell applications relies mainly on Pt-rich surfaces. To keep the Pt loading low such catalysts have been developed by depositing Pt-rich ultra thin coatings on inexpensive substrates. To that end, maintaining minimum amount of Pt loading and achieving maximum activity constitutes a challenge that cannot be easily addressed and ultimately solved. One way of dealing with this challenge is to apply the approach of deposition by surface limited redox replacement (SLRR) that enables high level of control in the growth of epitaxial metals at sub-monolayer to monolayer level. SLRR typically involves the formation of a sacrificial underpotentially deposited (UPD) layer via a potential step, followed by the release of potential control that triggers redox replacement of the sacrificial metal by the more noble growing metal of interest at open circuit.1 Upon repetition of the SLRR cycle thin films can be grown to desired thicknesses.2-7

In SLRR the UPD layer formation is typically performed by a potentiostat that ensures the application of appropriate potential. However, with thin film deposition aimed at the industrial scale this will result in the necessity to employ an industrial scale potentiostat, rendering the deposition energy intensive and setup sophisticated. In order to counteract these hindrances, our group has introduced the all-electroless approach of carrying out SLRR in a galvanic cell configuration (SLRR-GC). This configuration entails the working electrode to be ON and OFF coupled with an “executive electrode” in a galvanic couple for application of the UPD layer forming potential and initialization of the replacement process, respectively. This simple sequence of event completely replaces the use of potentiostat for controlling the SLRR deposition. This approach can then be repeated multiple times thus guaranteeing the thickness and quality control of the deposit.
This presentation will illustrate with a variety of examples the ability of SLRR-GC to reproduce results previously reported by conventional SLRR. Details along these lines will be demonstrated throughout all stages of the growth of Ag on Au by replacement of Pb UPD at a proof of concept level, and will be extended to current work which will be reported on in the deposition of Pt via Pb and Cu UPD sacrificial layers. The fundamentals of elementary steps constructing a building-block SLRR-GC cycle will be thoroughly discussed. Constraint and limitations of the proposed approach will be also critically considered. Finally, comparison with other deposition approaches will be presented as well.
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