Layered dissolution

Using the above concept of compositional boundary layer, dissolution rate of a falling or rising crystal may be written as... 

Iodine adlayers can provide a protective layer against oxidation and contamination while single-crystal surfaces are handled in ambient atmospheres. Also, the adsorbed iodine can be replaced by CO, which in turn can be electrochemically oxidized in solution to yield a clean metal surface. Anodic dissolution of various metals occurs at step edges under carefully adjusted electrochemical conditions, and this is a promising method for in-situ preparation of atomically flat terraces. Such layer-by-layer dissolution has been demonstrated for Ni, Ag, Co, and iodine-modified Pd and Cu surfaces. 

Precipitate layer and leached layer dissolution mechanisms can also cause nonlinear dissolution rates. The precipitate layer hypothesis proposes that a more or less coherent, contiguous layer of secondary product precipitates on the reactant surfaces of the mineral that is dissolving. Consequently, reactants released from the surface must diffuse through the precipitate layer to reach the bulk solution. As the layer thickens, the diffusional path lengthens, and the integrated rate of diffusion decreases. If the integrated rate of diffusion of reaction products from the surface is lower than the rate of reaction at the surface of the primary mineral, then the dissolution reaction becomes diffusion controlled. 

At low values oij (j < 5 pA cm ) silver electrodissolution proceeds at step edges (Fig. 4) without roughening (Fig. 5). In this case, the dynamic scaling analysis of STM profiles results in = 0, a figure which is consistent with a layer-by-layer dissolution mechanism as established by the FM model [3]. 

Figure 2 shows photographs of LEED patterns for Pd(lll)-( 3 x V3)R30°-I prior to and after removal of about 30 monolayers of Pd siuface atoms. In this experiment, the potential was held close to but not past the anodic dissolution peak that is, the dissolution was carried out at a fairly high rate. The LEED data clearly show that the post corrosion I-coated Pd surfaces remained as well-ordered as they were prior to the dissolution reaction. A layer-by-layer dissolution process is thus suggested. The LEED results, however, do not provide information on whether the corrosion transpires at steps or via place-exchange between the I and Pd atoms. For such information, in-situ STM experiments were invoked. 

For KJKd = 0 (i.e., no lateral surface processes) we get random dissolution with a monotonic decrease in the reflectivity vs. the number of dissolved layers (bold line, Fig. 29). If KJKd 1, we find a periodic variation of the reflectivity, with the reflectivity minima (maxima) corresponding to the presence of half (fully) occupied surface layers (open circles, Fig. 29B). This oscillatory pattern corresponds to layer-by-layer dissolution in which the dissolution of a given layer is essentially complete before dissolution of the subsequent layer begins. Finally, if Ks and Kd have comparable sizes (e g., for KJKd =... 

From these considerations, we conclude that the data in Figure 27C reveal that dissolution at pH 12.9 is fully stoichiometric and dominated by lateral dissolution processes producing ideal layer-by-layer dissolution. In contrast, dissolution at pH 1.1 results in a strongly damped oscillatory pattern, indicative of a more random dissolution process in which the orthoclase surface is substantially disrupted and roughened. Separate CTR measurements of reacted surfaces provided a more detailed picture of the dissolving surface after 1 to 15 layers were dissolved at pH 1.1 and 12.9 (Teng et al. 

The importance of water to the sulphation was shown by the fact that <10% conversion of gibbsite was observed on passing gaseous sulphur dioxide or trioxide over the gibbsite, apparently as a result of tlie formation of a protective sulphate layer. Dissolution of some of the protective layer of sulphate accounts for more extensive conversion in aqueous solutions. 

E > Ec, 2D layer-by-layer dissolution was replaced by the 3D growth of voids and it was hypothesized that this is related to the formation of highly mobile Au(Cl )n complexes [144,145]. 

The second-generation point defect model (PDM-II) [39] addressed the deficiencies of the previous model by incorporating a bilayer structure of the film consisting of a defective oxide layer on the metal surface and an outer layer that is formed by precipitation of products firom the reaction of transmitted cations firom the underlying metal with species in the environment. PDM-II assumed that the barrier layer controls the passive current and recognized the barrier layer dissolution and the need to distinguish whether the reactions are lattice conservative or nonconservative. The model also introduced the metal interstitials to the suite of defects. The model is in agreement with experimental results. Model PDM-III extends the apphcation of the PDM model and addresses the formation of multiple passive layers at the outer layer [40]. 

Recently, in addition to the in situ STM/AFM, many other surface-analysis techniques such as surface X-ray scattering (SXS) [19, 20] and electrochemical quartz crystal microbalance (EQCM) [21, 22] have also been employed to investigate the electrochemical deposition and dissolution processes at atomic resolution. Atomically controlled electrochemical epitaxial growth and layer-by-layer dissolution... 

The step-selective layer-by-layer dissolution behavior was also observed on an I-Pd(l 11) surface. However, the anisotropic-dissolution features are not as obvious as that observed on an I-Pd(lOO) surface. The pit formation on the I-modified Pd(llO) precludes layer-by-layer dissolution and leads to progressive disorder [117, 118]. 

The anistropic layer-by-layer dissolution of palladium and gold are enhanced hy an adlayer of iodine and chloride on the substrate surfaces. 

The reader will note that there has been a steady increase in the level of sophistication of the PDM, with the introduction of barrier layer dissolution, and the relaxation therein, interstitial cations, hydride barrier layers, and bilayer structures. Current work in the author s laboratory is focused on extending the PDM to consider vacancies in the substrate metal and their annihilation on grain boundaries, precipitates, and dislocations, and the incorporation of the porous outer layer in oxide/oxide bilayer structures. 

Pensado et al. [2001] Lithium. Cation and anion vacancies, LiH hydride barrier layer, LiOH outer layer Irreversible reactions with kinetic effects, barrier layer and outer layer dissolution Concentrations of cation and anion vacancies and barrier layer thickness First impedance analysis of bilayer structure and of hydride barrier layer. Cathodic reaction included in the model... 

Au(Cr)-covered crystals were used in nonaqueous solutions (e.g., butanol [111] or acetonitrile [158]), anodic oxidation of chromium was not observed. When Ti was used instead of Cr as the adhesion-promoting layer, dissolution of this metal through gold was very slow. Long-term use of Au(Ti) coatings shows that Ti also disappears, but the adhesion of gold to the resonator surface remains as before. 

In layer structure, it usually occurs medium-thick layer dissolution argillaceous limestone thin interlayer and medium-thick layer containing thin layer dissolution argillaceous limestone alternately, partly mega-thick layer argillaceous. 

When a negative solubility gradient exists from the metal surface to the outside of the salt layer, dissolution of the NiO film covering the metal is followed by reprecipitation of NiO at some distance away from the surface. The oxide precipitate is highly porous and therefore non-protective [23]. Figure 9.40 schematically shows such a situation. 

Russo L, Colangelo F, Cioffi R, Rea I, De Stefano L (2011) A mechano-chemical approach to porous silicon nanoparticles fabrication. Materials 4 1023-1033 Stephen RG, Riley FL (1989) Oxidation of silicon by water. J Eur Ceram Soc 5 219-222 Stevulova N, Suzuki T, Senna M, Balintova M, Sepelak V, Tkacova K (1997) Mechanochemical oxidation of silicon and selectivity of oxide superficial layer dissolution in aqueous solutions of HF and KOH. Solid State Ion 101103(2) 681-686 Verdoni LP, Fink MJ, Mitchell BS (2011) A fractionation process of mechanochemically synthesized blue-green luminescent alkyl-passivated silicon nanoparticles. Chem Eng J 172 591-600... 

Gravimetric analysis Microporous or mesoporous layers from anodization Weight losses from etching and complete layer dissolution (Lowell and Shields 1991 Lowell et al. 2004) (Brumhead et al. 1993)... 

If a simple layer-by layer-dissolution of a pol3mier takes place, the film thickness will decrease, the measured film capacitance will increase. 

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