Epitaxial Electrodeposition of Nickel on Pt(111) Electrode

Artificial nickel thin films, potentially useful as magnetic materials and electrocatalysts, have been prepared by electrodeposition on noble transition metal electrodes. This study employed scanning tunneling microscopy (STM) and cyclic voltammetry to study electrodeposition of Ni on Pt(111) from 0.1 M KClO₄ + 1 mM HCl + 0.06 M NiCl₂. Deposition of Ni was noted at potentials more positive than its Nernst potential, as proton discharge and hydrogen evolution occurred concomitantly. Bulk deposition of Ni commenced at potentials more negative than −0.6 V (vs Ag/AgCl), where reduction of water to hydrogen was imminent. The reduction reaction of Ni²⁺ ion to Ni metal was a slow process under the present experimental conditions, and not all Ni deposit was removed from the Pt electrode, as indicated by irreversible changes in the voltammetric profiles. In-situ STM provided direct views of the growth process and the atomic structures of the Ni thin film. The first Ni adlayer deposited at <i>E</i> > −0.525 V or the underpotential deposited (UPD) layer was disordered but was transformed into an ordered structure supporting the subsequently deposited Ni adlayers. From the second all the way up to the tenth Ni adlayers, STM imaging revealed prominent moiré patterns exhibiting long-ranged intensity modulations undulating along the ⟨110⟩ direction of the Pt(111) substrate. These moiré patterns are proposed to arise from a stack of Ni(111)-like planes on the Pt(111) electrode. The periodicities of the moiré patterns decreased from 3.0 to 2.5 nm as the Ni deposit thickened from the second to the fourth layer, suggesting that the spacing between Ni adatoms decreased from 0.254 to 0.25 nm