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

Journal Article · Thu Sep 16 04:00:00 EDT 1999 · Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical

DOI:https://doi.org/10.1021/jp9911088· OSTI ID:696659

Zuili, D; Maurice, V; Marcus, P ^[1]

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1. Ecole Nationale Superieure de Chimie de Paris (France)

The structure of hydroxylated oxide films (passive films) formed on Cr(110) in 0.5 M H{sub 2}SO{sub 4} at +0.35, +0.55, and +0.75 V/SHE has been investigated by in situ scanning tunneling microscopy (STM). Cathodic reduction pretreatments at {minus}0.54, {minus}0.64, and {minus}0.74 V/SHE destroy the well-defined topography of the single-crystal electrode and they have been excluded from the passivation procedure. Two different passive film structures have been observed, depending on the potential and time of passivation. At low potential (+0.35 V/SHE), the passive film, consisting mostly of chromium hydroxide, has a noncrystalline and granular structure whose roughness suggests local variations of thickness of ca. {+-} 0.5 nm. A similar structure is observed at higher potential (+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 crystalline structure is formed; the higher the potential, the faster the crystallization. It corresponds to the growth of a chromium oxide layer in the passive film. This chromium oxide layer is (0001) oriented. A structural model of the passive film is proposed, with termination of this oxide layer by a monolayer of hydroxyl groups or of chromium hydroxide in (1 {times} 1) epitaxy with the underlying oxide, and with surface steps resulting from the emergence of stacking faults of the Cr{sup 3+} planes in the oxide layer. Energy band models of the electronic structure of the semiconductive passive films show that the tunneling mechanism of the STM imaging involves empty electronic states 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){sub 3} (film) + Cr (metal) {yields} Cr{sub 2}O{sub 3} (film) + 3 H{sup +} + 3 e{sup {minus}}.

• OSTI ID: 696659
• Journal Information: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Journal Name: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical Journal Issue: 37 Vol. 103; ISSN 1089-5647; ISSN JPCBFK
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CHROMIUM
CHROMIUM HYDROXIDES
CRYSTAL STRUCTURE
EPITAXY
MICROSCOPY
OXIDES
PASSIVATION
TUNNELING