Redox energy levels

Further there are some other correlations between the characteristic energy terms E, the equilibrium redox energy levels °E and the reorganization energies L which read as follows ... 

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=). 

Figure 9. Three situations for an n-type semiconductor-electrolyte interface at equilibrium showing overlap of the redox energy levels with the semiconductor conduction band (a) with surface states (b) and with the semiconductor valence band (c). A discrete energy level is assumed for the surface states as a first approximation.

The energy level, E , and energy level, E,, can be related to the redox energy level, Sredox. by a quantity called the reorientation energy X = l/2( ox - J red), which is determined by the relaxation process involved in the regrouping and reorientation of the solvation shell after electron transfer between the oxidized and reduced states. The value of X can be experimentally determined and is on the order of 0.5 to 1 The... 

For all types of cells, the position of the valence and conduction bands of the semiconductor relative to the redox energy levels of the redox couple is an important parameter with regard to estimate the efficiency of the cell in energetic terms. 

Considerations of interfacial electron transfer require knowledge of the relative positions of the participating energy levels in the two (semiconductor and solution) phases. Models for redox energy levels in solution have been exhaustively treated elsewhere [27, 28]. Besides the Fermi level of the redox system (Eq. 6), the thermal fluctuation model [27, 28] leads to a Gaussian distribution of the energy levels for the occupied (reduced species) and the empty (oxidized species) states, respectively, as illustrated in Fig. 5(a). The distribution functions for the states are given by... 

Fig. 8 Isoenergetic charge transfer of hole from n-type semiconductor to occupied state of electrolyte redox couple. Redox energy levels have a distribution due to solvent-ion interactions.

Based on correlations between energy level positions and electrochemical redox potentials, it has been estabHshed that polymethine dyes with reduction potentials less than —1.0 V (vs SCE) can provide good spectral sensitization (95). On the other hand, dyes with oxidation potentials lower than +0.2 V ate strong desensitizets. 

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])...

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. 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])...

Let us choose, as an arbitrary reference level, the energy of an electron at rest in vacuum, e ) (cf. Section 3.1.2). This reference energy is obvious in studies of the solid phase, but for the liquid phase, the Trasatti s conception of absolute electrode potentials (Section 3.1.5) has to be adopted. The formal energy levels of the electrolyte redox systems, REDox, referred to o, are given by the relationship ... 

The energy levels of several redox couples as well as the positions of the band edges of typical semiconductors in equilibrium with aqueous electrolyte (pH 7) are compared in Fig. 5.59. 

The equilibration proceeds by electron transfer between the semiconductor and the electrolyte. The solution levels are almost intact ( REdox — redox)> since the number of transferred electrons is negligible relative to the number of the redox system molecules (cox and cred). On the other hand, the energy levels of the semiconductor phase may shift considerably. The region close to the interface is depleted of majority charge carriers and the energy bands are bent upwards or downwards as depicted in Fig. 5.60b. 

Figure 4 Operating principles and energy level diagram of a dye-sensitized solar cell. S/S+/S = Sensitizer in the ground, oxidized and excited state, respectively. R/R = redox mediator (I3 / I-).

Since the energy of electrons in a material is specified by the Fermi level, ep, the flow of electrons across an interface must likewise depend on the relative Fermi levels of the materials in contact. Redox properties are therefore predicted to be a function of the Fermi energy and one anticipates a simple relationship between the Fermi level and redox potential. In fact, the Fermi level is the same as the chemical potential of an electron. Clearly when dealing with charged particles, the local energy levels e are increased by qV, where q is the charge on the particle and V is the local electrostatic potential. The e, should therefore be replaced by e,- + qV and so... 

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... 

The band gap, determined as the onset of the absorption band in thin films is 2.95 eV (425 nm). Janietz et al. [252] used the onset of the redox waves in CV experiments to estimate the /P and Ea energies of the dialkyl-PFs (Figure 2.11). The gap between the obtained energy levels (5.8 eV for 7P and 2.12 eV for EA) IP—EA 3.8 eV is substantially higher than the optical band gap. Although optical absorption and electrochemistry test two physically different processes (vertical electron excitation and adiabatic ionization) and are not expected to be the same,... 

Formation of a space charge after electrostatic equilibration of a semiconductor with a solution containing a redox couple, O, R and redox interaction with the electrolyte as a consequence of irradiation. Energy level diagrams are given... 

Apply simple ideas regarding energy levels to a predictive understanding of the feasibility of semiconductor-photosensitised redox reactions, including water-splitting reactions. 

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