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1 2004

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Electrochemical Formation and Properties of (Ir+Pb) Oxide

Piekarska A., Elżanowska H., Birss V.

Polish Journal of Chemistry

2004

Vol. 78 / nr 9

1391-1412

The incorporation of Pb redox sites into nanoparticulate Ir oxide films has been realized by the initial electrodeposition of Pb metal on Au substrate, followed by Ir metal coating. The amount of Pb was varied according to the conditions of the deposition process, i.e. time, potential and solution used. Nanoparticulate metallic Ir films were obtained via sol-gel (SG) route and the oxidation of the surface was performed electrochemically in sulfuric acid solutions. The amount of Pb incorporated into the oxide film significantly changed the electrochemical response of the mixed oxide.With a relatively large amount of Pb deposit, consisting of under potential deposition (upd) Pb monolayer and Pb clusters, metallic Ir coating is more difficult than on Au and, consequently, only a small amount of Ir oxide is formed by potential cycling of the Au/(Pb+Ir) metallic electrode. The redox kinetics of Ir(III)/Ir(IV) sites, calculated from chronocoulometric data as a diffusional constant D1/2C, is diminished. In contrast, with a relatively small amount of Pb present only as clusters on the Au/Pb electrode after potential cycling in sulfuric acid solution, the diffusional kinetics of Ir(III)/Ir(IV) sites is enhanced (7×10-7 mol cm-2 s-1/2 vs. 4×10-7 mol cm-2 s-1/2) as compared to Ir oxide. Also, a small negative shift of the prepeak region of the (Ir+Pb) oxide suggests the presence of additional redox process due to incorporation of Pb sites into the oxide structure. When such mixed (Ir+Pb) oxide containing a small amount of Pb is transferred into 1MKOH solution, the redox processes of Ir and Pb are then more clearly separated and new redox peaks appear negative of Ir oxide peaks at -0.2 V, and also at +0.3 V and +0.9 V, proving the presence of Pb in the oxide. These redox potentials are comparable to the potentials of Pb2(PbxIr2-x)O7-y compound obtained thermally and to the redox potentials of Pb electrode in alkaline solutions. The structure of the Pb and Ir deposits and the mixed (Ir+Pb) oxide films was probed using STM imaging. The Pb monolayer (upd) and uniformly distributed Pb clusters ca. 100-150 nm in size were observed on the Au support. The size of metallic Ir nanoparticles is estimated as close to 3-4 nm, increasing to 10-15 nm and 20-30 nm after oxidation of IrOx and mixed (Ir+Pb) oxide, respectively.