Semiconductor standard redox potential

This means that the photoelectron is transferred to an electron acceptor concomitantly with trapping of the photohole by an electron donor (Fig. 10.1). Semiconductor materials have been tested as photocatalysts for the photodissociation of water. Fig. 10.4 shows the energetics in terms of standard redox potential of some semiconductors as compared to the standard redox potential of H2/H+ and H20/02 at pH 0. 

Standard redox potential at pH 0 of the valence band and of the conduction band of various semiconductors as compared to the standard redox potential of the redox couples H20/02 and H2/H+. (From Darwent, 1982)... 

Fig. 5-64. Band edge levels of compound semiconductor electrodes in aqueous solutions at different pH values hydrated redox partides and their standard redox potentials are on the right hand side. [From Gleria-Memming, 1975.]...

Figure 5-64 shows the band edge potential for compound semiconductor electrodes in aqueous solutions, in which the standard redox potentials (the Fermi levels) of some hydrated redox particles are also shown on the right hand side. In studying reaction kinetics of redox electron transfer at semiconductor electrodes, it is important to find the relationship between the band edge level (the band edge potential) and the Fermi level of redox electrons (the redox potential) as is described in Chap. 8. 

TABLE 8-1. Preference for the conduction band mechanism (CB) and the valence band mechanism (VB) in outer sphere electron transfer reactions of hydrated redox particles at semiconductor electrodes (SC) Eo = standard redox potential referred to NHE c, = band gap of semiconductors. [From Memming, 1983.]... 

Figure 8-30 shows the normalized cathodic transfer current of redox electrons for several redox reactions as a function of the standard redox potential sbdox on n- semiconductor electrodes of zinc oxide in aqueous solutions. The bell-like curve observed in Fig. 8-30 is in agreement with the forgoing conclusion that the maximum current occurs at the electrode potential at whidi tox equals e. ... 

The electrochemical potential of the solution and semiconductor, see Fig. 3.6, are determined hy the standard redox potential of the electrolyte solution (or its equivalent the standard redox Fermi level, Ep,redo, and the semiconductor Fermi energy level. If these two levels do not lie at the same energy then movement of charge across the semiconductor - solution interface continues until the two phases equilibrate with a corresponding energy band bending, see Fig. 3.8. 

In the discussions by many authors of the energy conversion efficiency of semiconductor photoelectrochemical systems, it has been tacitly assumed that the maximum theoretical photovoltages produced is the difference between E (in units of eV) and E(0x/R). The best conversion efficiency should then be obtained with a redox couple whose standard redox potential is as low as possible, with a reasonable margin x, say 0.3 V, above E (Fig. 11). From this it follows that the maximum photovoltage obtainable is equal to the band gap, Eg, in an eV unit, minus a small margin x plus A. 

It has been pointed out by some authors CL, 2) that for a semiconductor having a thermodynamic decomposition potential, E in between Ec and E , a redox couple with a standard redox potential, E°, more negative than E is needed in order to operate the photoanode without decomposition. Then, the maximum photovoltage attainable is Us - Ej, which is often much lower than Eg -A-x. For GaP, this is only 0.8 V (4) (Fig. 11). S... 

Figure 19.2 Band gaps, together with valence and conduction band edges of common semiconductors, placed alongside the standard redox potentials (versus normal hydrogen electrode (NHE)) of the OijOY and OH/ OH redox couple.

Fig. 7.11 Energy diagram for the semiconductor-electrolyte interface for two different standard redox potentials (syst. I and II) left side n-type semiconductor right side p-typc semiconductor...

Fig. 2 Correlation between the yield of methanol and the conduction bands of semiconductor catalysts. The dashed line denotes the standard redox potential of the CH3OH/H2CO3 couple vs NPfE. (Reprinted from [61] with permission by the Nature Publishing Group)...

This equation was derived at first by Heyrovsky and llkovic [10] and is usually called the Heyrovsky-llkovic equation. In most cases, the diffusion coefficients of the Ox and Red species are not very different, so that essentially the standard redox potential. A theoretical current-potential curve in terms of versus — Uy2 shown in Figure 7.7. A semilogarithmic plot would yield a straight line (see Section 7.1.3). It should be mentioned here that Eq. (7.32) can also be applied to majority carrier processes at semiconductor electrodes (see, e.g.. Section 7.3.4). 

The photoelectrolysis of H2O can be performed in cells being very similar to those applied for the production of electricity. They differ only insofar as no additional redox couple is used in a photoelectrolysis cell. The energy scheme of corresponding systems, semiconductor/liquid/Pt, is illustrated in Fig. 9, the upper scheme for an n-type, the lower for a p-type electrode. In the case of an n-type electrode the hole created by light excitation must react with H2O resulting in 02-formation whereas at the counter electrode H2 is produced. The electrolyte can be described by two redox potentials, E°(H20/H2) and E (H20/02) which differ by 1.23 eV. At equilibrium (left side of Fig. 9) the electrochemical potential (Fermi level) is constant in the whole system and it occurs in the electrolyte somewhere between the two standard energies E°(H20/H2) and E°(H20/02). The exact position depends on the relative concentrations of H2 and O2. Illuminating the n-type electrode the electrons are driven toward the bulk of the semiconductor and reach the counter electrode via the external circuit at which they are consumed for Hj-evolution whereas the holes are dir tly... 

As for semiconductor/metal contacts, a change in the Fermi level of the liquid phase should result in a different amount of charge transferred across the semicondnctor/liqnid junction. For semiconductor/liquid junctions, the important energetic trends for a series of different liqnid contacts can thns be determined by measuring the solntion redox potential relative to a standard reference electrode system. Within this model, solutions with more positive redox potentials shonld indnce greater charge transfer in contact with n-type semicondnctors. 

The semiconductor solid-state physics community has adopted the electron energy in vacuum as a reference whereas electrochemists have traditionally used the standard hydrogen electrode (SHE) scale. Although estimates vary [23-25, SHE appears to lie at —4.5 eV with respect to the vacuum level. We are now in a position to relate the redox potential, Eredox (as defined with reference to SHE), with the Fermi level, Ep,redox, expressed versus the vacuum reference (Figure 5a) ... 

Figure 11. Principle of operation of the dye-sensitized nanocrystalUne solar cell. Photoexcitation of the sensitizer (S) is followed hy electron injection into the conduction band of an oxide semiconductor film. The dye molecule is regenerated by the redox system, which itself is regenerated at the counter-electrode by electrons passed through the load. Potentials are referred to the normal hydrogen electrode (NHE). The energy levels drawn match the redox potentials of the standard N3 sensitizer ground state and the iodide/triiodide couple. (Redrawn from Gratzel [187] with permission from publisher, Elsevier. License Number 2627070632803).

Fig. 7.12 Bandgap position of several common PEC semiconductors relative to the water-splitting redox potentials measured vs. the standard hydrogen electrode (SHE)...

To make the connection between the energy levels of the electrolyte and the semiconductor it is necessary to introduce the flat-band potential, Ufi, as a critical parameter characterizing the semiconductor electrode. The flat-band potential is the electrode potential at which the semiconductor bands are flat (zero space charge in the semiconductor) it is measured with respect to a reference electrode, usually either the standard normal H /H2 redox potential (n.h.e.) or the standard calomel electrode (s.c.e.). 

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