Valence band coupling parameters

The redox level of the metallic couples related to the levels of the conduction and valence bands can be considered as the most important parameter to predict the feasibility of transformation of the species, and it is possible to take these values for a first approach. Figure 2 shows the redox levels of... 

All of the mixed-valence [(ttpy)Ru(tpy-ph,- y)Ru(ttpy)] conq>lexes, where n = 0, 1, or 2, showed IT bands in Ae 1150- to 158()-nm region [29]. From the electronic coupling parameter, it can be seen that the phenyl groups... 

Fig. 23a. Calculated and measured outer-level excitation spectra. In the uncoupled S(E,) (see fig. 19), the amplitude of the valence and conduction bands is chosen proportional to the hybridization parameters. In the calculation of the f-spectra with coupling, a Gaussian instrumental broadening of 0.4eV FWHM has been included. The experimental spectra have been taken from the following works. XPS Ba (HUfner and Steiner 1985) LaFj (Sato 1976) LajOs, La (Schneider et al, 1985) CeO (Wuilloud et al. 1984b). BIS Ba (Riehle 1978) LaF3 (Motais et al. 1984) LajOs, La (Schneider et al. 1985) CeOj (Wuilloud et al. 1984b). The origin of the total-energy scale is the Fermi energy in metals and the upper valence band edge in insulators. The f contribution corresponds to the hatched areas.

We have evaluated D (k,o)) numerically, assuming for simplicity that the coupling parameter A(k) is a constant A. We have chosen the bandwidth of the conduction band (12T ) and that of the valence band (12T ) to be 0.6... 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

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. 

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