Redox leveling

The prediction of which semiconductor materials are suitable as electrodes for a given photoelectrolysis reaction would be straightforward if the energy position of the bands at the electrode surface remained fixed with respect to the solution redox levels and independent of the redox species in the solution. Gerischer... 

Fig. 5.6 (Left) Comparison of band energy levels for different II-VI compounds. Note the high-energy levels of ZnSe. Representation is made here for electrodes in contact with 1 M HQO4. The reference is a saturated mercury-mercurous sulfate electrode, denoted as esm (0 V/esm = +0.65 V vs. SHE). (Right) Anodic and cathodic decomposition reactions for ZnSe at their respective potentials (fidp, Fdn) and water redox levels in the electrolytic medium of pH 0. (Adapted from [121])...

In the thermodynamically redox-stable resting state, CcOs all Cu ions are in the Cu state and all hemes are Fe . From this state, CcOs can be reduced by one to four electrons. One-electron reduced CcOs are aerobically stable with the electron delocalized over the Cua and heme a sites. The more reduced forms—mixed-valence (two-electron reduced), three-electron reduced, and fully (four-electron) reduced—bind O2 rapidly and reduce it to the redox level of oxide (—2 oxidation state) within <200 p-s [Wikstrom, 2004 Michel, 1999]. This rate is up to 100 times faster than the average rate of electron transfer through the mammalian respiratory chain under normal... 

V,/V-bis(2-hydroxy-di-3,5-/-butylphenyl)amine forms complexes of zinc which have ligand-based redox processes with four oxidation levels of the coordinated anion.864 2 1 and 1 1 complexes are formed in the presence of zinc with the 2 1 complex coordinated in an octahedral geometry and the 1 1 complex square planar with a triethylamine ligand completing the coordination sphere. The complexes, at the different redox levels, have been investigated by EPR, spectro-electrochemistry, l I NMR, and magnetochemistry, as appropriate. 

Spacers with energy levels or redox states in between those of the donor and acceptor may help energy or electron transfer (hopping mechanism). Spacers whose energy or redox levels can be manipulated by an external stimulus can play the role of switches for the energy- or electron-transfer processes.141... 

Figure 7.18 A comparison of the Ni K-edge X-ray absorption spectra for redox-poised samples of hydrogenases from different bacteria in the edge and XANES regions.Spectra are separated by redox level, with line types indicating the different bacterial sources (bold line, t roseopersicina light line, D. gigas dotted line, A vinosum dashed line, D. desulfuricans ATCC27774 dashed-dot line, coli). Reprinted with permission from Gu, et a/. (1996) and the American Chemical Society.

As the energy of the excited states and the redox levels of each metal-polypyridine unit depend on metal and ligands in a predictable way, the simultaneous presence of different metals in a dendritic structures gives rise to intramolecular energy transfer processes as well to different redox patterns with multielectron processes. In particular, the tetranuclear [Os(2,3-dpp)3 (2,3-dpp)Ru(bpy)2 3]8+ (OsRu3) shown in Fig. 5.3 has been designed to achieve an efficient antenna effect. This species can also be considered a first-generation mixed-metal dendrimers.31... 

Some comments need to be made concerning the data in Table 1. For some couples an extensive set of additional data are available in a variety of media, e.g. Fe3+/2+. For others, data are available for a series of structurally related analogs, e.g. Fe(C5H5)2+/0. For couples like Cr(bipy)3+/° (5, Table 1) the electron transfer process is ligand n (bipy) rather than metal based and in clusters like those in couples 19 and 26 (Table 1) the redox levels are almost surely delocalized over the cluster unit. The inclusion of some of the entries listed as inner-sphere cases is not based on product studies but, rather, mechanistic details have been inferred from rate comparisons, as illustrated in a later section of the chapter. 

In an isolated electrode, the VB, the GB and the Fermi level all have constant values throughout the depth of the electrode. If it is immersed in an electrolyte there will be a movement of charge at the interface so that the Fermi level coincides with the redox level of the species contained in the electrolyte. This is somewhat similar to the equalization of the levels of two liquids in contact, the equilibrium condition being that the pressures should be equal at the point of contact. In an energy diagram this is shown as a... 

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. 5.1 Schematic energy band bending for (A) large particle, (B) small particle, and (C) metal-deposited particle. R, radius of the particle Lsc, space charge layer E(red/ox), redox level in solution E , Fermi level in semiconductor

Nonetheless, equilibrium considerations can greatly aid attempts to understand in a general way the redox patterns observed or anticipated in natural waters. In all circumstances equilibrium calculations provide boundary conditions toward which the systems must be proceeding, however slowly. Moreover, partial equilibria (those involving some but not all redox couples) are approximated frequently, even though total equilibrium is not approached. In some instances active poising of particular redox couples allows one to predict significant oxidation-reduction levels or to estimate properties and reactions from computed redox levels. 

As this is less than the difference between the flatband potential of SrTi03 and the hydrogen redox level (2) it is quite possible that the SrTi03 itself has sufficient catalytic activity to account for the observed production. 

In the former case, the positions of the conduction and valence band edges at the interface (E and E ) are fixed with respect to the electrolyte redox levels, while in the latter case the positions of E and E with respect to the electrolyte redox levels vary with electrode potential. Thus, with unpinned band edges, redox couples lying outside the band gap under flat band conditions can lie within the band gap under band bending conditions or under illumination. 

Alternatively, redox levels of the semiconductor material may lie at energies such that the hole, directly or through a multi-step process described below, accepts an electron from a semiconductor surface atom, producing concomittant surface oxidation. 

The oxide redox energy levels for all elements except gold are cathodic to the redox level of the H2O/O2 couple. Gold, however, is an impractical component for compound semiconductors. All other compound semiconductors employed as electrolysis photoanodes will undergo surface oxidation in aqueous electrolytes to produce a surface oxide film which normally constitutes the stable surface of the photoelectrode. Our proposed mechanism indicates that the proton induced oxide dissolution reaction arises from product ir ion interactions with the oxide anion (0=). 

---