In situ scanning tunneling microscopy study of the structure of the hydroxylated anodic oxide film formed on Cr(110) single-crystal surfaces

The structure of hydroxylated oxide films (passive films) formed on Cr(110) in 0.5 M H2SO4 at +0.35, +0.55, and +0.75 V/SHE has been investigated by i n situ scanning tunneling microscopy (STILI). Cathodic reduction pretreamen ts at -0.54, -0.64, and -0.74 V/SHE destroy the well-defined topography of the single crystal electrode and they have been excluded from the passivati on procedure. Two different passive film structures have been observed, dep ending on the potential and time of passivation. At low potential (+0.35 V/ SHE), the passive film, consisting mostly of chromium hydroxide, has a nonc rystalline and granular structure whose roughness suggests local variations of thickness of ca. rt 0.5 nm. A similar structure is observed at higher p otential (+0.55 V/SHE), but only for a short polarization time. For longer polarization at 0.55 V/SHE, and at higher potentials (+0.75 V/SHE), a cryst alline structure is formed; the higher the potential, the faster the crysta llization. It corresponds to the growth of a chromium oxide layer in the pa ssive film. This chromium oxide laver is (0001) oriented. A structural mode l of the passive film is proposed, with termination of this oxide layer by a monolayer of hydroxyl groups or of chromium hydroxide in (1 x 1) epitaxy with the underlying oxide, and with surface steps resulting from the emerge nce of stacking faults of the Cr3+ planes in the oxide layer. Energy band m odels of the electronic structure of the semiconductive passive films show that the tunneling mechanism of the STM imaging involves empty electronic s tates located in the band gap of the passive film. The growth of the oxide layer in the passive film is governed by a combined reaction of dehydration of chromium hydroxide and oxidation of chromium: Cr(OH)(3) (film) + Cr (me tal) --> Cr2O3 (film) + 3 H+ + 3 e(-).