Surfaces depletion

Constraints on the degree of surface depletion of 7Li along the plateau are provided by (i) the relative smoothness of the plateau revealed by more recent... 

Thus, the polarisation data, cyclic voltammetric results and the a.c. impedance measurements all suggest that, when an Ru02/TiC>2 anode exhibits a high overpotential, this is a direct consequence of the surface depletion of Ru. This is also consistent with the estimated Re values of approximately 20 Q for the failed electrodes, in contrast to the known, much higher specific resistivity of Ti02 of... 

Another option to lower the Ru losses is to dope the anode coatings with IrO2, which has been shown [53, 54] to markedly decrease the Ru corrosion rate during electrolysis in NaCl solutions. This would minimise the surface depletion of Ru, as shown by the SIMS analysis of the TiC>2 + RuC>2 + IrCU coatings (Fig. 5.19), and thus extend the operating life of the anodes. 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

According to the macropore formation mechanisms, as discussed in Section 9.1, the pore wall thickness of PS films formed on p-type substrates is always less than twice the SCR width. The conductivity of such a macroporous silicon film is therefore sensitive to the width of the surface depletion layer, which itself depends on the type and density of the surface charges present. For n-type substrates the pore spacing may become much more than twice the SCR width. In the latter case and for macro PS films that have been heavily doped after electrochemical formation, the effect of the surface depletion layer becomes negligible and the conductivity is determined by the geometry of the sample only. The conductivity parallel to the pores is then the bulk conductivity of the substrate times 1 -p, where p is the porosity. 

Figures 4 and 5 show an n-type semiconductor with a surface depletion layer, the energy bands bending upwards as the surface is approached from the interior. Under illumination (Figure 5) the band bending is diminished because of the photo-generation...

Figure 5. n-Type semiconductor—electrolyte solution interface with a surface depletion layer, in the dark and with two intensities of illumination. Symbols as in Figure 3 and 4 with Ec and E the band edges of the conduction and valence bands, respectively, under illumination, and Ef(H2) Ef(Om) abbreviations for Ef(H20/h2) and Ep(02/H20)y respectively. The quasi-Fermi levels Ei> and pEp are at different positions in the surface region than in the bulk as a result of the limited penetration of light into the interior. Fermi levels in solution as in Figures 3 and 4(13). 

Band bending at the semiconductor surface causes a depletion of the majority carriers (electrons for n-type CdSe) underneath the surface (depletion layer). Formation of a depletion layer gives rise to a system equivalent to a Schottky barrier between a metal and a semiconductor. 

Surface tensions for the interface between air and aqueous solutions generally display one of the three forms indicated schematically in Figure 7.14. The type of behavior indicated by curves 1 and 3 indicates positive adsorption of the solute. Since dy/dc and therefore dy/d In c are negative, E must be positive. On the other hand, the positive slope for curve 2 indicates a negative surface excess, or a surface depletion of the solute. Note that the magnitude of negative adsorption is also less than that of positive adsorption. 

The concentrations of Mg, Ca and Sr in seawater are very nearly constant at constant salinity. However, in contrast to the Group 1 elements, when Ca and Sr vertical distributions are examined in detail, small but significant surface depletions are observed. In the case of Ca, this depletion is attributable to the... 

Distributions of the platinum group elements (PGEs) are shown in Fig. 12.8. The distribution of ruthenium (Ru) is unknown, but concentrations are estimated to be less than 50 fmol kg-1. The distributions of rhodium (Rh) and palladium (Pd) (Period 5) show significant surface depletions relative to deep waters and overall concentrations are somewhat less than 1 pmol kg 1. 

When the interface is charged, the Poisson-Boltzmann equation predicts stronger fields and higher ionic concentrations in the vicinity of the surface, therefore a dielectric constant smaller than in the bulk, and the local Born energy indicates that the ions are repelled by the interface. However, a neutral surface depleted of ions should have a dielectric constant larger than the bulk and the ions should be attracted, not repelled by such an interface. Therefore, other interactions should be included to explain the ion depletion near a neutral surface. 

Fig. 2. Possible dependence of the surface tension on electrolyte concentration, for various ion-hydration potentials for the anions (Ref. [5]). When the surface excess of the SB ions exceeds the surface depletion of the SM ions, the surface tension decreases with ionic strength.

To date, no photoenhanced wet etching technique has been reported for p-type GaN. The difficulty in photo-etching p-type semiconductors arises from the nature of the surface depletion region exhibited by p-type materials [30], The depletion of holes from p-type surfaces due to surface states results in a near-surface electric field, which sweeps photogenerated holes away from the surface into the bulk. A variety of techniques have been utilised to electrochemically etch other p-type semiconductors, including the application of biases to supply holes from the p-type bulk and the use of reductive etch chemistries, but these remain to be demonstrated for p-GaN. 

In a fuel cell, as the cell current becomes high, which indicates the electrochemical reaction rate on the electrode surface is fast, the mass transfer rate of the reactants is not fast enough to provide enough reactants to the electrode surface. Depletion of reactants at the electrode surface leads to a drop in cell voltage. The calculation of the cell voltage drop in this part is difficult, and a semi-empirical equation is usually used to estimate the mass transfer drop. The most popular expression for the mass transfer drop is... 

The coins made before 135 B.C. are almost all leaded bronzes (Figure 10). However, the two earliest coins, made in the late third century B.C., have low lead contents. The lead contents increased rapidly with passing time, until exceptionally high lead contents were found in the coins of three denominations semisses, asses, and a triens, all of the middle of the second century B.C. The reported lead contents of some of these coins are probably lower than the actual contents because of the segregation and surface depletion of lead. Appreciable amounts of lead were not frequently added to coins after roughly 100 B.C. A marked change in the composition of coins occurred both before 135 B.C. and after about 100 B.C. (Only one... 

An early attempt to identify a functional group by deriv-tization and ESCA study, was published by Riggs and Dwight in 1974 (Fig. 2) (2). Treatment of polytetrafluoroethylene (PTFE) with sodium in ammonia produced a surface depleted in fluorine. To... 

Negative adsorption is in most cases very small compared to positive adsorption and therefore not easily detected directly. A sophisticated optical technique (evanescent-wave-induced fluorescence, EWIF) has been used to prove the reduction in segment concentration close to a non-adsorbing surface Depletion also has an effect on the flux of polymer solutions through pores since the viscosity of the liquid near the non-adsorbing surface is lower than that of the polymer solution, the flux is then higher than would be expected on the basis of the bulk viscosity. Negative adsorption at a liquid-air interface leads to a measurable increase In surface tenslon ... 

Figure 10.7 Schematic representation of the change in plasma pol5merization of TMS caused by fluorine-containing moiety emanating from the substrate surface (depletion of Si moieties in plasma polymer).

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