Our studies on crystal facet and reactivity relationship suggest that the (111) face of Cu₂O films electrodeposited from pH 12 cupric lactate solution is a preferred surface for photogeneration of H₂ from water, owing to its polar O^2--termination.  In order to prevent photodecomposition of this Cu₂O surface, which is initiated by interaction of the terminal O^2- and H⁺ (Cu₂O  + 2H⁺  + 2e^-  --> 2Cu⁰ + H₂O), while maintaining the attraction of H⁺ to the surface and potentially catalyzing H₂ generation, we have attempted to deposit a monolayer of nickel via underpotential deposition (UPD).  Underpotential deposition on oxides has not been explored much unlike the underpotential deposition on metals as a monolayer deposition method.

Nickel was deposited on Cu₂O films from a 0.08 M nickel acetate solution by applying various potentials for 1 min at room temperature.  The Cu₂O films with nickel deposited were dissolved in HNO₃, and then the quantities of the deposited nickel at the various potentials were determined with atomic absorption spectrometry (AAS).  The AAS results indicate that the underpotential deposition at -0.2 V vs. SCE (the calculated Nernst potential of the nickel solution is  -0.49 V vs. SCE) deposited nickel that is equivalent to a 23 % monolayer coverage, corresponding to 60 kJ stronger interaction of Ni²⁺(aq) with O^2- at Cu₂O surface than with bulk Ni surface, as predicted for the polar Cu₂O (111) face.    For the estimate of the 100 % coverage of Cu₂O surface, the atomic radius of Ni (124 pm) was used as well as the surface roughness of 1.09 obtained by atomic force microscopy measurements.  At -0.3 V vs. SCE, more than a monolayer of nickel (205 %) was deposited.

According to the x-ray photoelectron spectroscopy (XPS), the binding energy (BE) of Ni 2p_3/2 electron changed from 855 eV for -0.2 V vs. SCE UPD sample to 851 eV for -0.3 V vs. SCE UPD sample.  Since the BEs of 854.6 eV and 852.6 eV have been assigned for Ni²⁺ and Ni⁰, respectively, the XPS data indicate that the full reduction of nickel at the Cu₂O surface occurs at potentials more negative than -0.2 V vs. SCE. 

The effects of underpotentially deposited nickel on the activity and stability of Cu₂O films will be compared with respect to the deposition potential.