Electrochemical Atomic Layer Deposition of Pd Ultrathin Films by Surface Limited Redox Replacement of Underpotentially Deposited H in a Single Cell

This work illustrates the application of a surface limited redox replacement (SLRR) protocol for the electrochemical atomic layer deposition (E-ALD) of Pd ultrathin films on Au in a single cell using underpotentially deposited H (H-UPD) as the sacrificial mediator. The facile deposition approach presented herein requires neither the use of sophisticated instrumentation nor the presence of metal ions other than the Pd(11) chloride complex. This provides for maximum growth efficiency and eliminates potential contamination of the deposit thereby rendering the Pd deposition a greener process. The growth progress is monitored by open circuit chronopotentiometry while the roughness evolution of accordingly deposited Pd films is assessed by cyclic voltammetry (CV) of H-UPD and Cu-UPD. The CV results indicate the deposition of smooth Pd films occurring for up to 30 SLRR cycles (20 equiv monolayers) 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 scanning tunneling microscopy. Analysis of results from Pd-film stripping experiments corroborate these findings. The comparison of charges obtained by the stripping of Pd films of different thickness with generic growth models suggests that not only adsorbed but also absorbed Hupp participates in redox exchange with [PdCl4](2-) complex in 2:1 stoichiometric ratio. The presented facile SLRR approach for contamination-free Pd deposition offers unsurpassed simplicity and can also be extended to other conductive substrates featuring strong H affinity.