Passive film crystallinity, epitaxy

Summary. Scanning tunneling microscopy (STM) provides new possibilities to explore the link between the structure and the properties of thin oxide overlayers (passive films) formed electrochemically on well-defined metal surfaces. Passive oxide films protect many metals and alloys against corrosion. A better understanding of the growth mechanisms, the stability, and the degradation of passive films requires precise structural data. Recently, new results on the atomic structure of passive films have been obtained by STM. The important questions of crystallinity, epitaxy and the nature of defects have been addressed. Data on the structure of passive films on Ni, Cr, Fe, Al, and Fe-Cr alloys are reviewed with enq>hasis on atomically resolved structures. Ihe perspectives of future developments are discussed. 

The thictoess of the hydroxide layer present in Ae outer part of the film was measured to be about 0.6 nm (5), and corresponds to the thickness of about one monolayer of p Ni(OH)2 (0001) (0.46 nm as calculated from lattice parameters). This hydroxide layer could not be observed in a distinct manner in the STM images. This results from the good agreement between the measured lattice parameters with both the lattice parameters of p-Ni(OH)2 (0001) andNiO(l 11), 0.32 nm and 0.295 nm respectively. A likely situation is that the crystalline passive film is NiO (111) oriented and terminated by a p-Ni(OH)2 (0001) hydroxide layer in (1x1) epitaxy on the oxide layer. These results on the cristallinity and orientation of the passive film have been confirmed by in situ STM for a different crystallographic orientation of the electrode and in a different electrolyte Ni(lOO) in IM NaOH (5). This shows that independently of the substrate orientation (Ni( 100) or Ni( 111))... 

AR-XPS has been extensively used to investigate the chemical composition, the chemical states and the thickness of thin anodic oxide overlayers (passive films) formed on well-defined metal and alloy single crystal surfaces. More recently direct imaging of the surface structure by STM with atomic resolution provided new data on the crystallinity, the epitaxy and the nature of the structural defects existing in the thin oxide layers. Such data are useful... 

As shown above, scanning probe microscopies (mostly STM) have provided many of the data on the structure of passive films. Prior to the advent of scanning probe microscopies, a few data were available, obtained by reflection high energy electron diffraction (RHEED). For example, the crystallinity and epitaxy of the passive film... 

The formation of passive oxide films on the (111) surfaces of Cu and Ni has also been studied in detail by SXS [94, 95]. Measurements of Cu(lll) in 0.1 M NaCl04 (at pH 4.5) showed that the oxide exhibited a crystalline cuprite structure (CU2O) that was epitaxially aligned with the underlying Cu substrate [94]. Although a similar oxide structure was observed for oxidation in air, there were some key differences in the structure of the aqueous oxide. In particular it was found that a preferred reversed orientation of the oxide film was formed, and this indicated that oxide growth occurs at the interface between the oxide and the Cu(lll) surface... 

---