Surface chemical composition oxide films

Evaporated thin films of rare earth oxides have, to our knowledge, not yet been used in catalytic investigations. We feel that rare earth oxides in thin film form should be particularly useful in catalytic studies because they can be easily generated under controlled preparation conditions, and because their structure and surface chemical composition can be readily monitored by modem surface science techniques. We hope therefore that work in this area will be stimulated in the near future. Preparation and characterization of rare earth oxide thin films have been compiled recently in a comprehensive review by Gasgnier (1980). 

Surface-Chemical Composition of Rh Oxide Fflms by XPS. The XPS data on electrochemically formed Rh oxide films are limited to one paper (27). They indicate that the initial electro-oxidation of Rh involves an electroadsorbed OH group, Rh - OH, and the process takes place beyond 0.55 V, RHE. Upon extension of the polarization potential a new surface species, Rh(OH)3, is formed one monolayer of Rh(OH)3 is grown upon reaching 1.40 V, RHE. At potential higher than 1.40 V, RHE, formation of RhO(OH) commences on top of 3 ML of Rh(OH)3 (25). The existing XPS data indicate that extension of the polarization potential beyond some 0.8 - 0.9 V, RHE, results in oxide films with Rh in the +3 oxidation state. Finally, the experimental data and results presented in ref 27 indicate that the OCl oxide-reduction peak corresponds to the surface process shown in equation 3. 

Alloys having varying degrees of corrosion resistance have been developed in response to various environmental needs. At the lower end of the alloying scale are the low alloy steels. These are kon-base alloys containing from 0.5—3.0 wt % Ni, Cr, Mo, or Cu and controlled amounts of P, N, and S. The exact composition varies with the manufacturer. The corrosion resistance of the alloy is based on the protective nature of the surface film, which in turn is based on the physical and chemical properties of the oxide film. As a rule, this alloying reduces the rate of corrosion by 50% over the fkst few years of atmosphere exposure. Low alloy steels have been used outdoors with protection. 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

The metal oxides prepared by conventional baking or by the CVD method are, in general, chemically stable, crystalline materials, and show excellent mechanical, electrical, optical, and physical properties. Flexible porous gel films obtained by the surface sol-gel process are totally different. In this chapter, we described a new preparative method for ultrathin metal oxide films by stepwise adsorption of various metal alkoxides. We named this method the surface sol-gel process. Structural characterization of the gel films thus obtained, the electrical property, and formation of nano-composites with organic compounds, were also explained. The soft porous gel contains many active hydroxyl groups at the surface and interior of the film. This facilitates adsorption of organic compounds, and consequent preparation of ultrathin metal oxide/polymer nano-composite films and organization of functional small molecules. In the nano-composites, proper selection of polymer components leads to the design of new materials with unique electrical, optical, and chemi-... 

With respect to the first requirement, a polycrystalline metal as ordinarily used exposes at die surface many different types of crystal structure (different crystal faces, edges, corners, and boundaries between crystals). Each type of structure has its special chemical properties. Measurements made on ordinary polycrystalline material are a composite quantity which may be useful for technological purposes but which give little information for an understanding of the basic process of oxidation. It is not yet generally appreciated that for thin oxide films die differences in rates and structures on the different crystal faces are under many conditions quite large, as indicated below. 

A metal CMP process involves an electrochemical alteration of the metal surface and a mechanical removal of the modified film. More specifically, an oxidizer reacts with the metal surface to raise the oxidation state of the material, which may result in either the dissolution of the metal or the formation of a surface film that is more porous and can be removed more easily by the mechanical component of the process. The oxidizer, therefore, is one of the most important components of the CMP slurry. Electrochemical properties of the oxidizer and the metal involved can offer insights in terms of reaction tendency and products. For example, relative redox potentials and chemical composition of the modified surface film under thermodynamically equilibrium condition can be illustrated by a relevant Pourbaix diagram [1]. Because a CMP process rarely reaches a thermodynamically equilibrium state, many kinetic factors control the relative rates of the surface film formation and its removal. It is important to find the right balance between the formation of a modified film with the right property and the removal of such a film at the appropriate rate. 

Steigerwald et al. reported that the Cu-BTA passivation film was almost 20 nm thick after a 10-min immersion in a solution at pH 2 [22]. Cohen and coworkers also studied the stoichiometry, thickness, and chemical composition of the Cu-BTA using in situ ellipsometry and ex situ X-ray photoelectron spectroscopy [13]. The authors reported that film grown on CU2O and bare Cu under oxidizing conditions are on the order of 5 40 A thick and the chemical composition of this layer is mostly Cu -BTA. Similar to the schematic view portrayed in Fig. 8.3, Walsh et al. suggests that the BTA film is composed of a monolayer that is in direct contact with the copper film and a multilayer built on top of the monolayer [6]. They reveal that in the monolayer, BTA molecular plane is oriented within 15° of the surface normal. In the multilayer, the molecular plane is tilted by about 40° from the plane of copper surface. 

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