Surface states, electric charge

Fig. 6-53. Interfadal charges, electron levels and electrostatic potential profile across an electric double layer with contact adsorption of dehydrated ions on semiconductor electrodes ogc = space charge o = charge of surface states = ionic charge due to contact adsorption dsc = thickness of space charge layer da = thickness of compact la3rer.

Physical Particle size and size distribution True density Bulk and tapped densities Surface area Electric charge of surface Stability of solid state Porosity Hygroscopicity Compactibility Intrinsic dissolution... 

Figure 42. Scheme comparing expected potential-independent charge-transfer rates from Marcus-Gerischer theory of interfacia) electron transfer (left) with possible mechanisms for explaining the experimental observation of potential-dependent electron-transfer rates (right) a potential-dependent concentration of surface states, or a charge-transfer rate that depends on the thermodynamic force (electric potential difference) in the interface. 

Electrode polarization is associated with a change in EDL charge density at the electrode surface. Other changes in surface state of the electrode are possible, too (e.g., the adsorption or desorption of different components, which also involve a consumption of electric charge). By convention, we describe this set of nonfaradaic processes as charging of the electrode surface. 

We also address the models of adsorption change in electrophysical characteristics of semiconductor adsorbent caused both by diemisorbed charging of the surface due to the charge transition between surface states and volume bands of adsorbent and by local diemical interaction of adsorbate with electrically active defects of semiconductor. 

The predicted state of the sorbing surface in the two calculations differs considerably. At pH 4, the surface carries a positive surface charge and potential. The electrical charge arises largely lfom the predominance of the protonated surface species > (w)FeOH, which occupies about two thirds of the weakly binding sites. At pH 8, however, the surface charge and potential nearly vanish because of the predominance of the uncomplexed species >(w)FeOH, which is electrically neutral. 

Although a family of OgS - Jig8 values are allowed under Equation 7 the actual equilibrium state of the oxide/solution interface will be determined by the dissociation of the surface groups and the properties of the electrolyte or the diffuse double layer near the surface. For surfaces that develop surface charges by different mechanisms such as for semiconductor, there will be an equation of state or charge-potential relationship that is analogous to Equation 7 which characterizes the electrical response of the surface. 

Fig. 6-99. An interfacial electric double layer on semiconductor electrodes a = charge of surface states 0.1 = interfadal charge of adsorbed ions IHP = inner Helmholtz plane.

Fig. 5-60. Equivalent circuit for an interfacial electric double layer comprising a space charge layer, a surface state and a compact la3 er at semiconductor electrodes Csc = capacity of a space charge layer C = capacity of a surface state Ch = capacity of a compact layer An = resistance of charging and discharging the surface state.

The situation where the excess electric charge in the bulk of the semiconductor is zero has a particular importance because this can often be obtained experimentally. This state is called flat band situation and the respective electrode potential, flat band potential because in the absence of electric fields inside the semiconductor the position of the band edge energies runs flat from the interior to the surface 20>. This energy pattern at the semiconductor-electrolyte contact is shown in Fig. 10 for the flat band situation, i. e. an anodic and a cathodic... 

Fig. 108a-c. Proposed equivalent circuits for. a an empty and b a semiconductor-particle-coated BLM. Porous structure of the semiconductor particles allowed c the simplification of the equivalent circuit. Rm, RH, and Rsol are resistances due to the membrane, to the Helmholtz electrical double layer, and to the electrolyte solutions, while C and CH are the corresponding capacitances Rf and Cf are the resistance and capacitance due to the particulate semiconductor film R m and Cm are the resistance and capacitance of the parts of the BLM which remained unaltered by the incorporation of the semiconductor particles R and Csc are the space charge resistance and capacitance at the semiconductor particle-BLM interface and Rss and C are the resistance and capacitance due to surface-state on the semiconductor particles in the BLM [652]... 

The existence of surface states leads to two most important effects. First, electrons and holes may be trapped at the surface to form surface electric charge. This influences the equilibrium properties of semiconductors. Second, the surface levels may change significantly the kinetics of processes, in which electrons and holes are involved. On the one hand, they create additional... 

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