Surface band position

Figure 4.2(d) shows that an energy barrier forms at the semiconductor/redox electrolyte interface, similar to the Schottky barrier at a metal/semiconductor interface. The most important quantity is the barrier height (q ) or the flat band potential U, which essentially determines the surface band positions of the semiconductor with respect to the energy levels of solution species. The q B is given for an n-type semiconductor by... 

We assumed in Fig. 4.2 that no surface charge or surface dipole is present in the semiconductor. In general, however, both surface charges and surface dipoles are present in the semiconductor owing to adsorption equilibria for various ions between the electrolyte and the semiconductor surface as well as formation of polar bonds at the semiconductor surface. Such surface charges and surface dipoles change the potential difference in the (outer) Helmholtz layer and thus cause shifts in the surface band positions, as shown schematically in Fig. 4.3. The shifts can be expressed as changes in 0(0) or in the above equations, with the... 

Figure 4.4 shows the surface band positions of some typical semiconductors in aqueous electrolytes (pH 7), calculated from the experimentally determined C/ft, compared with the redox levels of some important redox reactions. It is known that the U for most semiconductors, such as n- and p-GaAs, n- and p-GaP, n- and p-InP, n-ZnO, n-Ti02, and n-Sn02, in aqueous electrolytes is solely determined by the solution pH and shifts in proportion to pH with a slope of -0.059 V/pH.3,4) This is explained by the adsorption equilibrium for H+ or OH- between the semiconductor surface and the solution, for example,... 

Fig. 4.4 Surface band positions of some semiconductors in an aqueous electrolyte (pH 7), calculated from experimentally determined U. ...

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])...

The variation of the electrostatic potential surface region entails a bending of the bands, since the potential contributes a term —eo4>(x) to the electronic energy. Consider the case of an n-type semiconductor if the value s of the potential at the surface is positive, the bands band downwards. We set 4> = 0 in the bulk of the semiconductor and the concentration of electrons in the conduction band is enhanced (see Fig. 7.4). This is called an enrichment layer. If cj)s < 0, the bands bend upward, and the concentration of electrons at the sur-... 

Since attainable redox potentials for adsorbates at the surface of an illuminated semiconductor particle are governed by the band positions of the semiconductor chosen, selectivity in activating a specific functional group in a multifunctional molecule or in activating one species from a mixture of adsorbates can be attained by judicious choice of the semiconductor. A great deal is known about the band positions ofcommon semiconductors and how they shift with changes in electrolyte. Band posi-... 

We can infer that the band positions of the irradiated semiconductor are greatly influential in controlling the observed redox chemistry and that formation of radical ions produced by photocatalyzed single electron transfer across the semiconductor-electrolyte interface should be a primary mechanistic step in most such photocatalyzed reactions. Whether oxygenation, rearrangement, isomerization, or other consequences follow the initial electron transfer seem to be controlled, however, by surface effects. 

We turn next to the information provided by the band position data. Table III shows that the surface methyl group can be characterized by bands in six regions three of them are always observed in on-specular VEELS (and in principle in RAIRS) and three virtually always in off-specular and sometimes also on-specular VEELS, as discussed earlier. Their values exhibit a metal dependence, as shown in Table IV, where average values are quoted, except that in the case of pCH3 s on Pt(lll), we have made use of a particularly accurate and consistent RAIR value. 

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