Valence band edge potential

FIGURE 22.5 Electrode potential consisting of interfacial potential A(f>H and space charge potential Ac()sc for an intrinsic semiconductor = flat band potential, v = valence band edge potential, and... 

Fig. 5.8 The energy levels of n-type M0S2 at the flat band potential relative to the positions of various redox couples in CH3CN/[n-Bu4N]C104 solution. The valence band edge of the semiconductor as revealed by accurate flat band potential measurement is at ca. +1.9 V vs. SCE implying that photooxrdations workable at Ti02 are thermodynamically possible at illuminated M0S2 as well. (Reproduced with permission from [137], Copyright 2010, American Chemical Society)...

Fig. S-41. Band edge levels and Fermi level of semiconductor electrode (A) band edge level pinning, (a) flat band electrode, (b) under cathodic polarization, (c) under anodic polarization (B) Fermi level pinning, (d) initial electrode, (e) under cathodic polarization, (f) imder anodic polarization, ep = Fermi level = conduction band edge level at an interface Ev = valence band edge level at an interface e = surface state level = potential across a compact layer.

Fig. 10-13. Anodic transfer of pho-toexdted boles (minority charge carrier) at an n>type semiconductor electrode E( -e 9o/e) = electrode potential E% (= — c /e) = potential of the valence band edge B02 (= - = equilibrium...

For metal electrodes, the anodic 03Q n reaction proceeds at electrode potentials more anodic than the equilibrium potential Bo of the reaction as shown in Fig. 10-14. For n-type semiconductor electrodes, the anodic photoexdted oxygen reaction proceeds at electrode potentials where the potential E of the valence band edge (predsely, the potential pEp of the quasi-Fermi level of interfadal holes, pCp = — CpEp) is more anodic than the equilibrium oxygen potential Eq, even i/the observed electrode potential E is less anodic than the equilibrium oxygen potential E03. The anodic hole transfer of the o Qgen reaction, hence, occurs at photoexdted n-type semiconductor electrodes even in the range of potential less anodic than the equilibriiun potential Eq of the reaction as shown in Fig. 10-14. 

Fig. 11-11. Potential at a film/solution interface and potential dfp in a passive film as a fimction of anodic potential of a passive metal electrode in the stationary state the interface is in the state of band edge level pinning to the extent that the Fermi level e, is within the band gap, but the interface changes to the state of Fermi level pinning as e, coincides with the valence band edge Cy.

In the case of nickel electrodes on which the passive film is a p-f pe nickel oxide (NiO), the energy gap ( 0.2 eV) between the valence band edge and the Fermi level at the flat band potential is small so that the transpassivation potential Etp is relatively close to the flat band potential as in Fig. 11-13. 

For heavily doped n-type semiconductors, the flat band is nearly coincident with the conduction band, while for heavily doped p-type semiconductors the flat band lies very close to the valence band edge. A necessary thermodynamic condition for the photoproduction of hydrogen and oxygen is that the p-type conduction band must be at or above the H7H2 half cell potential, while n-type valence band must lie below the 02/0H half cell potential. 

Fig. 7.1 Position of band edges and photodecomposition Fermi energies levels of various non-oxide semiconductors. E(e,d) represents decomposition energy level by electrons, while E(h,d) represents the decomposition energy level for holes vs normal hydrogen electrode (NHE). E(VB) denotes the valence band edge, E(CB) denotes the conduction band edge. E(H2/H20) denotes the reduction potential of water, and (H2O/O2) the oxidation potential of water, both with reference to NHE.

Figure 6 A schematic band diagram (electrical potential energy versus distance) of a conventional p-n homojunction solar cell at equilibrium (left) and at short circuit under spatially uniform illumination (right). The energies of the conduction- and valence-band edges are Ecb and Evb. respectively. EF is the Fermi level at equilibrium and EFn and EFp are the quasi-Fermi levels of electrons and holes, respectively, under illumination.

If oxygen can serve the role as acceptor, the role of the organic adsorbate as donor can be tested by choosing a substrate whose oxidation potential lies positive of the valence band edge of the chosen semiconductor and by seeking evidence for formation of the oxidized radical cation. The photoelectrochemical oxidation of many substrates can be rationalized on this basis We cite only a few illustrative examples here and discuss the observed chemistry in more detail in the following section. 

Chemoselecti vity could potentially be achieved if the oxidation potential of a desired donor adsorbate lies between the valence band edges of two possible semiconductor photocatalysts. Since TiOj has a more positive valence band edge than does CdS, it should be the more active photocatalyst. Consistent with this idea, decarboxylation of organic acids, Eq. (5), is much more efficient on irradiated suspensions of rutile than of CdS... 

The course of hydrocarbon photocatalyzed oxidations seems to depend significantly on the relative positions of the valence band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the clean oxidation of toluene described above, lower activity was observed in neat benzene, a substrate whose oxidation potential lies at or slightly below the valence band edge This observation implies the importance of radical cation formation (via photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If beitzene-saturated aqueous semiconductor suspensions are... 

The valence band edge of TiOz exhibits in aqueous suspensions a potential of more than + 2V vs. SCE and is capable of oxidizing hydroxide anions or water molecules adsorbed to the semiconductors surface (Eq. 62 and 63) producing hydroxyl radicals [90], Hydroxyl radicals, when desorbed from the surface, may react with organic substrate [91] by hydrogen abstraction (Eq. 64) or by electron transfer (Eq. 65), thus initiating oxidative degradation... 

If this is true, then the oxidation of carboxylic acids should be the preferred process on SrTi03 photoanodes, in the absence of such defect surface states. We see from Figure 5 that the range of potentials reported for the normal Kolbe reaction (at platinum) actually crosses the valence band levels of both SrTiC>3 and Ti02 in the neutral pH region. It may well be that at high pH, the photo-Kolbe potential lies at or below the valence band edge for these semiconductors, consistent with the observation that photo-Kolbe products are not observed under these conditions. 

---