Surface Structure of Nickel in Acid Solution Studied by In Situ Scanning Tunneling Microscopy

The structure of Ni(111) single‐crystal surfaces in (pH 3) has been investigated in situ by scanning tunneling microscopy after cathodic reduction and after passivation at potentials ranging from 0.45 to 1.15 V/SHE. The metallic surface has monoatomic and diatomic step edges and an unreconstructed Ni(111)‐(1 × 1) atomic lattice. The passivated surface has two different structures corresponding to the two chemical layers of the passive film. The hydroxide outer layer has a granular and amorphous structure. The lack of crystalline order is assigned to the incorporation of water molecules. Aging under polarization favors the hydration of this layer and the increase of the passivation potential favors its thickening. The oxide inner layer of the passive film has a crystalline structure with a stepped surface independent of the passivation potential. The average tilt angle between the terraces of the oxide layer and the substrate terraces is 3 ± 1°. The lattice parameters correspond to NiO(111) in parallel or antiparallel epitaxy with the substrate. The (111)‐(1 × 1) unreconstructed orientation of the oxide layer is stabilized by surface hydroxylation. The local orientation of the step edges of the oxide is assigned to its dissolution in the passive state. © 2000 The Electrochemical Society. All rights reserved.