Browsing by Subject "galvanic displacement"
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Item Digital Deposition of Ultrathin Pd Films on Well-Defined Pt(111) Electrodes via Surface-Limited Redox Replacement Reaction: An Electron Spectroscopy-Electrochemistry Study(2010-12-04) Hossain, MohammadIn 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.Item Interfacial Properties of Ultrathin- Film Metal Electrodes: Studies by Combined Electron Spectroscopy and Electrochemistry(2012-07-16) Cummins, KyleA pair of studies investigating the deposition and surface chemical properties of ultrathin metal films were pursued: (i) Pt-Co alloys on Mo(110); and (ii) Pd on Pt(111). Experimental measurement was based on a combination of electron spectroscopy (low energy ion scattering spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low energy electron diffraction) and electrochemistry (voltage efficiency, voltammetry, and coulometry). Mixed-metal preparation of Pt-Co films by thermal vapor deposition (TVD) resulted in a thin-film binary alloy. Careful analysis revealed a substantial divergence between the composition at the interface and that in the interior. This outcome was observed for all compositions and allowed for the construction of a ?surface phase diagram?. The proclivities of the alloys of pre-selected compositions towards enhanced catalysis of the oxygen-reduction reaction were assessed in terms of their voltage efficiencies, as manifested by the open-circuit potential (OCP) in O2-saturated dilute sulfuric acid electrolyte. The particular alloy surface, Pt3Co (XPt=3,XCo=1), whether from the thin film or a bulk single crystal, exhibited the highest OCP, a significant improvement over pure Pt but still appreciably lower than the thermodynamic limit. Under test conditions, the degradation of thusly-prepared films was primarily due to Co corrosion. Ultrathin Pd films on well-defined Pt(111) surfaces, with coverages from 0.5 to 8 monolayers (ML), were prepared by surface-limited redox replacement reaction (galvanic exchange) of underpotentially deposited Cu. Spectroscopic data revealed that films prepared in this manner are elementally pure, pseudomorphic to the substrate, and stable, independent of the surface coverage (?) of palladium. Analysis of the voltammetric profiles in the hydrogen evolution region revealed unique properties of hydrogen adsorption unseen in bulk electrodes. Notably, at 1 ML coverage, a step-free film was produced that did not exhibit hydrogen absorption. At higher coverages, digital (layer-by-layer) deposition gave way to 3D islands in a Stranski- Krastanov growth mode; under these conditions, onset of bulk-like behavior was observed. This method makes possible the synthesis of well-ordered noble-metal films in the absence of high-temperature treatment