Growth of Pd Ultrathin Films on Au in One-Cell Configuration By Surface Limited Redox Replacement of H and Cu UPD Monolayers
Authors: Dimitrov Nikolay , Achari Innocent , Ambrozik Stephen 
Published: February 2020

In light of dealing with the mitigation of global warming effects and minimization of growing pressure on limited crude oil resources, green renewable energy sources have been the focus of today’s materials science research. Palladium has been used to store hydrogen gas and to, in part, facilitate catalytic reactions in fuel cells (1, 2). However, Platinum is still the major catalytic material that has been used for fuel cells; due to its high cost and limited supply, research is now intensifying to replace Pt with Pd (3, 4). Techniques of growing Pd on conductive materials include a variety of ultrahigh vacuum approaches that very efficient but are also expensive, complicated and operate at high temperatures. A feasible alternative is electrodeposition. Recent research and development has shown that ultra-thin films of Pd can be grown on polycrystalline Au under optimum conditions using automated flow cell (5, 6).
Our lab has been working recently on devising optimum conditions to efficiently grow epitaxial Pd on Au using one-cell configuration instead, of expensive automated flow cells (7). This is to embrace nanotechnology that relies on nanoscale atomic layers of Pd that have a large surface area. The growth uses surface limited reduction reaction (SLRR) of Cu or H underpotentially deposited (UPD) sacrificial layers. After the growth, the as-deposited Pd films are tested for surface roughness using HUPD and CuUPD and the efficiency of the deposition technique is measured by stripping charge determination. Consequently, the resulting Pd-Au, deposited via optimum conditions, will be tested for catalytic ability and durability in formic acid oxidation, methanol oxidation and oxygen reduction (8).
In this presentation results of this work will be presented in detail and critically discussed. The emphasis is on the illustration of a facile deposition approach that takes place in one cell and in the case of using HUPD mediation requires nothing else than acidified solution of [PdCl4]2- complex. It is demonstrated that this approach provides for maximum efficiency and again in the case of HUPD driven process, eliminates potential contamination thereby rendering “green” the Pd deposition process. The CV results indicate the deposition of smooth Pd films taking place for up to 30 and 20 SLRR cycles for HUPD and CuUPD sacrificial layers, respectively. This is followed by a rapid transition to dendritic growth at higher thickness. The quasi-2D growth resulting in smooth and uniform Pd-film morphology has also been confirmed up to 20 SLRR cycles by in-situ STM. Analysis of results from Pd-film stripping experiments corroborate these findings. The comparison of charges obtained by stripping of Pd films of different thickness with generic growth models suggests that in the case of H not only adsorbed but also absorbed H UPD participates in redox exchange with [PdCl4]2- complex in 2:1 stoichiometric ratio. The applicability of the proposed presented facile SLRR approach has been compared for the use of H and Cu UPD sacrificial layers and the obtained results have been also critically discussed vis-à-vis existing literature results emphasizing the use of flow cell approach.

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