Island oxide growth

Film rearrangement resulting in the formation of oxide subgrain and grain boundaries these paths of easy ion migration promote the formation of oxide islands and result in an increase in the growth rate of the oxide. 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

The transients given in Figure 3b are asymmetric. The area under the curves is almost two times higher than the product of imax and traax. In ttle initial segment of the curves, the current varies linearly with time. These features are consistent with the process controlled by the instantaneous nucleation and growth of a fixed number of oxide islands % in the CO monolayer. The transients are well described by the expression [16, 17] ... 

The changes in the transient shapes reflect Changes in the reaction mechanism. At low potentials, the reaction is controlled simultaneously by the rate of nucleation and the growth of the oxide islands, at high potentials the reaction is controlled by the rate of the island growth. 

Another type of present-day formation of Fe " oxides occurs on the ocean floor. Fe oxides are associated with Mn oxides and occur as crusts and nodules. The growth rate of these nodules is extremely low and has been estimated as being 2-15 10 mm/yr (Rana et al., 1983). Two values lying in this range, viz. 6.6-7.S 10 mm/yr for the last 150000 years were derived from the decrease of Th and U isotope ratios (234/238 230/232, respectively) in two Fe-Mn crusts in the Marshall Islands area (Chabaux et al. 1995,1997). 

Figure 6.2-12 Cyclic voltammogram of 0.1 - 1 mmol dm Geb on gold in dry [BMIMj PFg , starting at-500 mV towards cathodic (a) and anodic (b) regime. Two quasireversible (E, and E2) and two apparently irreversible (E4 and E5) diffusion-controlled processes are observed. E3 is correlated with the growth of two-dimensional islands on the surface, E4 and E5 with the electrodeposition of germanium, Ej with gold step oxidation, and E, probably with the iodine/iodide couple. Surface area 0.5 cm (picture from [59] - with permission of the Peep owner societes).

Let us assume that the growth of a new oxide phase is controlled by surface diffusion. The number of impacts of oxygen molecules on a unit surface is I, (cm 2 s 4) the number of impacts on one surface site is I A, where A is the area of one surface site. If one assumes that migration over the surface is random, the number of molecules reacting at the island boundary due to the surface diffusion is 4IAll2(Dsx)112. 

The topochemical model [112] suggests that an island can have n oxide layers. Apparently, this model can be applied in the case of the chemisorbed two-dimensional phase growth, as had been done by Boreskov et al. [116]. 

Deposition of Au onto this surface leads to the nucleation of Au islands at the intersection of clean Cu stripes thus leading to a square island lattice with a period of 50 A [83,86-88]. The N-covered Cu(100) surface has also been used for the growth of so far less well-ordered lattices of Fe and Cu [89], Co [90-92], Ag [93,94], and Ni [95], We note that square lattices can in principle also be created on Au(f4,f5,f5) since this miscut leads to 70 A step distance, which is equal to the reconstruction period. However, the steps are already far apart reaching the limit of the elastic step repulsions which may render global order difficult. Finally we note that another interesting alternative square template, although with smaller lattice constant, is presented by the (3 /3 x 5)-phase of V-oxide on Rh(lll) [96]. 

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