Deposition of nickel on electrodeposited Cu 2 O films was investigated at potentials more positive than the estimated Nernst equilibrium potential of Ni 2+ /Ni 0 for the purpose of developing a monolayer deposition method that will act as a protective and catalytic overlayer on the oxide films. Cu 2 O films were subjected to 0.08 mM nickel acetate solutions for 60 s at potentials more positive than the estimated Nernst equilibrium potential of Ni 2+ /Ni 0 . The quantities of Ni recovered from the Cu 2 O films were determined by atomic absorption spectrometry. The oxidation state of Ni deposited on the Cu 2 O was determined to be +3 via X-ray photoelectron spectroscopy. The absence of evidence of reduction of Ni 2+ to Ni 0 suggests that the nature of the nickel deposition is adsorption, and it is not characterized as underpotential deposition. Photoelectrochemical measurements showed that deposited Ni protected Cu 2 O films from the photodecomposition to Cu 0 by interfering with H + binding to the O 2− surface sites, and indicated the possibility of catalytic activity of deposited nickel for H + reduction. The catalytic activity of deposited nickel was explained by the shift of the Cuprous oxide (Cu 2 O) has a relatively small bandgap (2.0 eV) and an approximately 0.7 V more negative conduction band-edge position than the reduction of hydrogen ion, 1 which are favorable characteristics in a photocathode for H 2 production. Some of us reported that the stability and reactivity of electrodeposited Cu 2 O films depends on the crystal orientation. UPD method was considered in order to limit the deposition of the Ni protective coating to a monolayer. UPD, which is deposition of metal at more positive potentials than the reversible Nernst potential, occurs if the bonding energies between the metal adatom and a foreign substrate are stronger than the metal itself.7 UPD results in the formation of a monolayer because energy necessary for the deposition of a successive layer is not available at underpotentials than the reversible * Electrochemical Society Student Member.* * Electrochemical Society Active Member. z E-mail: afillinger@ithaca.edu Nernst potential. Unlike rigorously studied UPD on a metal surface, there are only a few papers regarding UPD on a metal oxide surface. One such example is UPD of Cu on RuO surface. 8 Increased underpotential shift for Cu UPD on the oxide (170 mV) in comparison to Cu UPD on Ru surface (110 mV) was monitored by cyclic voltammetry and X-ray photoelectron spectroscopy. The other example is UPD of Cu on TiO 2 , which was detected at 200 mV more positive potential than Cu bulk deposition by cyclic voltammetry and electrochemical quartz crystal nanobalance techniques.9 The former paper suggests a possible application of Cu UPD to Ru-based platable seedless Cu diffusion barrier, while the latter proposes the use of Cu UPD for the determination of electroactive surface area of oxide electrodes. In this paper we report deposition of Ni on electrodeposited Cu 2 O films at potentials more posi...
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