Digital Deposition of Ultrathin Pd Films on Well-Defined Pt(111) Electrodes via Surface-Limited Redox Replacement Reaction: An Electron Spectroscopy-Electrochemistry Study
In this study, ultrathin (submonolayer to eight-monolayer) Pd films were deposited one layer at a time on well-defined Pt(111) surfaces via a process known as surface-limited redox replacement reaction (SLR^3). In this digital-deposition method, one monolayer of a nonnoble metal (Cu) is deposited on a noble metal (Pt) by underpotential deposition (UPD). When the UPD adlayer is exposed to cations of less reactive metals (Pd^2 ), it is oxidatively stripped and reductively displaced by the more inert metal. The positive difference between the equilibrium potential of the noble metal in contact with its solvated cations and the equilibrium potential of the UPD adlayer is the driving force behind SLR^3. The Pd films were characterized by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and electrochemistry. The LEED patterns indicated (1?1) surface structure of the deposited films. No residual Cu was detected by AES in the Pd films. The Pd ultrathin films on Pt(111) showed HUPD adsorption/desorption peaks which are not observed in bulk Pd. These peaks were observed even at 8 monolayer thick films. The interfacial structure and electrochemical properties of SLR^3-prepared films were compared with those prepared by controlledpotential deposition (CPD). There is a linear correlation between Cu deposition charge (i.e., Pd deposition charge) and I-catalyzed Pd dissolution charge. Electrochemical and LEED results suggest that SLR^3 prepared films are smooth (if not slightly smoother) compared to those prepared by CPD. SLR^3 thus appears to be capable of preparing atomically smooth ultrathin films on Pt(111) surfaces without any additional thermal or electrochemical annealing.