Surfaces chemical passivation

Loscutoff, P. W. and Bent, S. F. Reactivity of the germanium surface Chemical passivation and functionalization. Annual Review of Physical Chemistry 57, 467 (2006). 

G. J. Sandroff etal.. Dramatic Enhancement in the Gain of a GaAs/AlGaAs Heterostmcture Bipolar Transistor by Surface Chemical Passivation, Appl. Phys. Lett. 1987, 51(1), 33-35. 

C. J. Sandorff et al.. Enhanced Electronic Properties of GaAs Surfaces Chemically Passivated by Selenium Reactions, J. Appl. Phys. 1990, 67(1), 586-588. 

According to the prediction, the Y values for pure metallic (m = 1) nanoparticle always drop with size at T> 0.25 T. However, the surface chemical passivation, defects, and the artifacts in measurement could promote the measured values. For instance, surface adsorption alters the surface metallic bonds (m = 1) to new kinds of bonds with m > 1. Surface compound formation or surface alloying alters the m value from one to a value around 4. 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

Surface chemical characterization of the passivation layer on the Al surface has been performed mainly via XPS, and the interpretation of results generated by various researchers still remains controversial. Because salt anions with active fluorine (LiPFe, LiAsFe, and LiBF4) are able to form stable surface layers on Al and protect it from corrosion, early studies had suggested that fluoride species such as LiF and AIF3 are crucial to the protection. 

Particles produced in the gas phase must be trapped in condensed media, such as on solid substrates or in liquids, in order to accumulate, stock, and handle them. The surface of newly formed metallic fine particles is very active and is impossible to keep clean in an ambient condition, including gold. The surface must be stabilized by virtue of appropriate surface stabilizers or passivated with controlled surface chemical reaction or protected by inert materials. Low-temperature technique is also applied to depress surface activity. Many nanoparticles are stabilized in a solid matrix such as an inert gas at cryogenic temperature. At the laboratory scale, there are many reports on physical properties of nanometer-sized metallic particles measured at low temperature. However, we have difficulty in handling particles if they are in a solid matrix or on a solid substrate, especially at cryogenic temperature. On the other hand, a dispersion system in fluids is good for handling, characterization, and advanced treatment of particles if the particles are stabilized. 

Here we describe some of the results. In each of these studies, the compound semiconductor was first etched in either acid or base to remove the oxide. The specific surface groups following the etch are not well understood. However, Pluchery et al. have followed the acid etching of InP by in situ infrared spectroscopy [175] and observed the removal of the oxide. Unlike Si, for which an acid (HF) etch leaves the surface hydrogen-terminated and temporarily passivated, acid etching of InP does not produce a chemically passivated surface. Presumably, the surface is left unprotected, and quickly oxidizes if not passivated by another process. Similar results showing reduction or removal of the oxide are seen for GaAs [174,176,177]. 

Bodlaki, D., Yamamoto, H., Waldeck, D. H. and Borguet, E. Ambient stability of chemically passivated germanium interfaces. Surface Science 543, 63-74 (2003). 

Chemical passivation should be started as soon as possible after the cleaning of metal surfaces. Accumulation of new corrosion products can occur if it is not initiated soon after cleaning. It may be achieved by treating equipment... 

White rust and similar visible surface corrosion problems, resulting from a failure to provide a proper chemical passivation program. 

The surface chemical composition of InP as a function of thermal cleaning temperature was studied by Cheng, et al. (19), also using AES. They used an arsenic molecular beam and temperature of about 500 C to clean a freshly oxide passivated InP. The surface oxides are replaced by arsenic oxides which then vaporize at these temperatures. An atomically flat and carbon contamination free surface was obtained, as monitored in situ with AES and RHEED OJ). 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Chemical passivation was discovered about 200 years ago. A piece of iron placed in concentrated nitric acid was found to be passive, while the metal dissolved readily in dilute HNO, with copious evolution of hydrogen. This type of behavior can be demon.straled in a very simple, yet quite spectacular, experiment. Nitric acid of various concentrations, from 1 mM to 70%, is introduced into a series of test tubes, and an aluminum wire is placed in each solution. No reaction is observed in the most dilute solutions. As the concentration is increased, however, hydrogen evolution becomes visible. At even higher concentrations, reduction of the acid takes place, in addition to hydrogen evolution. This is evidenced by the liberation of a brown gas, NO, which is one of the reduction products. When the concentration has reached 35%, the reaction suddenly stops. There is no gas evolution and the surface of he metal is not attacked. Accurate measurements show no weight loss when aluminum is kept in these solutions for months. Aluminum is passivated in concentrated HNO. A thin oxide film is formed on the surface and further attack is prevented. 

Rates of platelet destruction varied from 1.1 x 10 to 5.6 x 10 platelets per cm of exposed surface per day. Since studies evaluating polyurethanes as well as acrylic and methacrylic polymers and copolymers showed that platelet destruction rates may exceed 20 x 10 platelets/cm -day, the nine plasma polymers evaluated were considered to be considerably less reactive. Since each polymer was evaluated only four or five times with average results in each case near the lower sensitivity limit for this test system (about 1 x 10 platelets/cm -day), further statistical interpretations of the data presented in Table 35.7 would be inappropriate. Thus, due to the passive nature of these materials, conclusions could not be drawn regarding the relative importance of specific surface chemical moieties, i.e., all plasma polymers investigated are relatively nonreactive regardless of type of monomer used. This might imply that all type A plasma polymers have the characteristic feature of imperturbable surface regardless of what kind of atoms and moieties are involved, and because of this feature all plasma polymers tested performed better than most conventional polymers. 

To what extent can our concepts of the coordination chemistry of the oxide-water interface and our knowledge of the factors that enhance and retard dissolution of Fe(III) oxides contribute toward an understanding of the properties of passive iron oxides A review of the corrosion literature yields much phenomenological information that could be accounted for by surface-chemical theory. However, present passivity theories appear, with few exceptions, to be rather oblivious to the concepts of chemical surface reactivity. Thus, some perhaps speculative chemical ideas on the factors that enhance or reduce iron oxide passivity may be exposed to examination and discussion. 

Figure 9.11 Depiction of an idealised chemical passivation of Shockley surface states. As the bonding strength between the surface and chemical group increases, the energies of the antibonding and bonding orbitals are shifted outside the bandgap.

Figure 71. Differences of successive snapshots of the surface during passivation phases of two successive current oscillations of a period-2 state of the potentiostatic dissolution of an iron ring electrode. (Reprinted with permission from J. C. Sayer and J. L. Hudson, Ind. Eng. Chem. Res. 34, 3246, 1995. Copyright 1995, American Chemical Society.)...

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