Surface structure of nickel in acid solution studied by in situ scanning tunneling microscopy

The structure of Ni(111) single-crystal surfaces in 0.05 M H2SO4 + 0.095 M NaOH (pH 3) has been investigated iii situ by scanning tunneling microscopy alter cathodic reduction and after passivation at potentials ranging from 0.45 to 1.15 V/SHE. The metallic surface has monoatomic and diatomic step e dges and an unreconstructed Ni(111)-(1 x 1)atomic lattice. The passivated s urface has two different structures corresponding to the two chemical layer s of the passive film. The hydroxide outer layer has a granular and amorpho us structure. The lack of crystalline order is assigned to the incorporatio n 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 ti lt angle between the terraces of the oxide layer and the substrate terraces is 3 + 1 degrees. The lattice parameters correspond to NiO(111) in par all el or antiparallel epitaxy with the substrate. The (111)-(1 x 1) unreconstr ucted orientation of the oxide layer is stabilized by surface hydroxylation . The local orientation of the step edges of the oxide is assigned to its d issolution in the passive state, (C) 2000 The Electrochemical Society. S001 3-4651(99)08-091-X. All rights reserved.