Band edge positions

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])...

Figure 2. Band edge positions obtained over a period of three weeks for p-and n-type WSe2 -CH3CN interfaces containing metallocene redox couples (ferrocene, FER decamethylferrocene, DFER and acetylferrocene, AFER) each at three concentrations (preceding letter refers to high.H medium,M and low, L). Two different electrodes were used to obtain the data for n-WSe2 with doping densities between 1016 -1017 cm-3.

The effects on the dynamics of photo-injected electrons where not systematically studied, despite scattered reports on the influence of amines, which induce surface deprotonation, and lower surface charge with a resulting negative shift in band edge position and an increase in the open circuit potential, Voc [103], The opposite effect is induced by Li+ ions, which intercalate in the oxide structure. Guanidinium ions increase Voc when used as counterions in place of Li+. Other adsorbing molecules that influence both Voc and short circuit current are polycar-boxylic acids, phosphonic acids, chenodeoxycholate and 4-guanidinobutyric acid. 

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

Fig. 4 Illustration of valence and conduction band edge positions of anatase at pH 0 and IM° reduction potential energy region for Au, Ag, Hg, Cr, Cu, and Fe. Energy scale takes vacuum as zero. See text for details.

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. 

Unpinning of band edges at the semiconductor/electrolyte interface is understood as a common phenomenon for n- and p-type materials. Thus, the band edge positions as obtained from Hatband potential measurements in the dark, cannot be taken as a fixed value for the interpretation of charge transfer processes. More investigations in this direction are necessary. 

As has been discussed in Section III.E, a photocatalytic reaction can proceed if the CB bottom and VB top are more cathodic and anodic than the standard electrode potentials of electron acceptors and donors, respectively. Therefore, band-edge position... 

The fixed potential of the band edges of a given semiconductor can be established independently by many physical or electrochemical methods. These band-edge positions define the limits of the attainable oxidation and reduction half reactions that can be achieved on this surface. As long as the oxidation potential of the adsorbed donor (hole trap) is lower than the valence band edge, and the reduction potential of the adsorbed acceptor (electron trap) is lower than the conduction-band edge. 

Table 1 provides a list of these values for the most commonly accessible semiconductor powders suspended in aqueous acid, along with a conversion of the band gap to an absorption onset wavelength A Eg). The band-edge positions [30] can also be adjusted by control of the particle size of the irradiated semiconductor. Quantization effects can shift these values by more than 100 nm this allows control of the onset wavelength and of the band positions of several common semiconductors. 

Chemical differentiation between two oxidizable sites in the same molecule can also be achieved in organic photocatalytic reactions by choice of a different semiconductor and thus adjustment of the electrochemical band-edge positions. Consistent with this idea, the photocatalytic oxidation of lactic acid on UV-irradiated plati-nized-Ti02 leads to decarboxylation, presumably through the singly oxidized carboxyl radical. In contrast, the same reagent on irradiated platinized CdS leads to pyruvic acid by oxidation of the alcohol group (Eq. 10) [96]. 

Figure 3. Band edge positions of some n-type semiconductors at pH 7. (a) Single crystal (b) powder.

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... 

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