Semiconductors band edges position

Mapping of the Semiconductor Band-edge Positions Relative to Solution Redox Levels... 

Now consider the relative disposition of these solution energy levels with respect to the semiconductor band-edge positions at the interface. The total potential difference across this interface (Figure 6) is given by... 

Having located the semiconductor band-edge positions (relative to either the vacuum reference or a standard reference electrode), we can also place the Fermi level of the redox system, E f, redox, on the same diagram. Energy diagrams such as those... 

Figure 8. Relative dispositions of various semiconductor band-edge positions shown both on the vacuum scale and with respect to the standard hydrogen electrode (SHE) reference. These band-edge positions are for an aqueous medium of pH 3= 1.

The consequences of potential drop variations across the Helmholtz layer in the hole injection process have been examined by a variety of techniques [191, 192]. For example, chemical reaction of the GaAs surface with iodine results in a downward shift of the semiconductor band-edge positions such that the reduction of iodine is mediated by conduction band electrons [192]. When sufficient negative charge accumulates at the surface, the potential is redistributed between the semiconductor space-charge layer and the Helmholtz region. Now iodine is reduced by hole injection as gauged by EL and AC impedance measurements [192]. 

Figure 3. Semiconductor band edge positions vs NHE in pH = 1 aqueous solution.

Fig. 6. Band edge positions of several semiconductors ia contact with an aqueous electrolyte at pH 1 ia relation to the redox (electrode) potential regions (vs the standard hydrogen electrode) for the oxidation of organic functional groups (26,27).

Fig. 5.13 Energy level diagram comparing the surface band edge positions of SnS and the energies corresponding to selected redox couples and corrosion reactions involving the semiconductor. (Reproduced from [198])...

Fig. 3.11a Band edge positions of several oxide semiconductors in contact with a pH 1 aqueous electrolyte.

An appreciable space-charge layer also develops upon dispersion of a semiconductor into an electrochemically poised redox solution [21, 22], The valence and conduction band edges of a given semiconductor will be characteristic of the individual material. Shown in Table 1 is a summary of the band edge positions... 

We see therefore that photoactive semiconductor particles provide ideal environments for control of interfacial electron transfer. Photoinduced electron-hole pairs formed on irradiated semiconductor suspensions, as in photoelectrochemical cells, allow for reactivity control not available in homogeneous solution. This altered activity derives from controlled adsorption on a chemically manipula-ble surface, controlled potential afforded by the valence band edge positions, controlled kinetics by virtue of band bending effects, and controlled current flow by judicious choice of incident light intensity. 

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