Redox band structure

The electronic properties of CNTs, and especially their band structure, in terms of DOS, is very important for the interfacial electron transfer between a redox system in solution and the carbon electrode. There should be a correlation between the density of electronic states and electron-transfer reactivity. As expected, the electron-transfer kinetics is faster when there is a high density of electronic states with energy values in the range of donor and acceptor levels in the redox system [2]. Conventional metals (Pt, Au, etc.) have a large DOS in the electrochemical potential... 

A metallic band structure is realized when the CT solids have a partial CT state and molecules form uniform segregated columns or layers. Figure 1 shows electrical conductivity data for 1 1 low-dimensional TTF TCNQ system, as a function of redox potentials [82]. The two lines a and b are related to the equation expressing the relationship between 7d, Ea, and the Madelung energy M 5) (5 = degree of CT) between partially charged component molecules (eq. 2) [83], where and Ea are... 

An important prerequisite for reaction (I) to proceed is that both the electrons and the ions have the chance to be conducted within the PCM. For ions, it is sufficient that a system of channels exists in the solid material and that these channels need to have a sufficiently wide diameter for ions to diffuse through. For the electrons, there are two pathways that depend on the electronic structure of the PCM. Provided the PCM is a semiconductor and the conduction band is accessible at room temperature, this conduction band can support the electron conduction. When the redox centers, that is, the metal ions interact less strongly so that the band structure of a... 

Interfacial electron transfer across a solid-liquid junction can be driven by photoexcitation of doped semiconductors as single crystals, as polycrystalline masses, as powders, or as colloids. The band structure in semiconductors (281) makes them useful in photoelectrochemical cells. The principles involved in rendering such materials effective redox catalysts have been discussed extensively (282), and will be treated here only briefly. 

Figure 1. Band Structure in a n-type Semiconductor A. Solid State. B. In contact with a liquid phase redox couple (0/R). IL=energy of the conduction band. Vertical line indicates solid-liquid interface. CB= conduction band VB = valence band.

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Fig. 46. Band structure model of Cu/Cu20/electrolyte with the space charge layer SCL, the valence band VB, the conduction band CB, and the sub-band SB formed by interband states, mediating electron transfer between the metal to the redox states of the Co(III) complex within the electrolyte. The formation of electron hole pairs by photoexcitation and the transfer of electrons to the empty states of the redox system via surface states SS is also indicated.

Fig. 16.29 Band structure of N-doped Ti02 with the oxidation potentials of a few redox couples [Reprinted with permission from Nakamura et al. (2004). Copyright (2004) American Chemical Society]...

It appears from the description of radical ions in Sects. 1 and 3 that redox reactions can significantly change the chemical and physical properties of conjugated 7r-systems. Whether the extended jc-species are treated within molecular orbital theory or within band-structure theory, the inherent assumption in these concepts is that an electron transfer is reversible and does not promote subsequent chemical reactions. While inspection of cyclic voltammetric waves and the spectroscopic characterization of the redox species provide reliable criteria for the reversibility of an electron transfer and the maintenance of an intact (T-frame, it is generally accepted that electron transfer, depending on the nature of the substrate and on the experimental conditions, can also initiate chemical reactions under formation or cleavage of er-bonds [244, 245],... 

Electronic conductivity is favored by electron transfer through the polymer delocalized band structure, via redox conductivity by site—site hopping. Redox conductivity occurs at electron energies centered around the formal equilibrium potential for the redox polymers. 

Most studies indicate that the metal has the greatest effect on the properties of the polymer when it is part of the backbone, rather than a sidegroup. The direct coupling of the metal orbitals with the Tr-orbitals of the conjugated polymer may allow redox-matching to enhance the conductivity and perturb the band structure of the polymer. However, it is often synthetically easier to add the metal center as a side chain of a known polymer (e.g., on PPV) than to develop new polymerization procedures for the metal complex. 

Figure 2.11 Strategies for stabilization of photoelectrodes against corrosion, (a) Chemical stabilization, (b) Stabilization by specific properties of the band structure as for group VIb transition metal dichalcogenides the five metal d-band contributions, the indirect gap, and the direct excitonic gap are shown also indicated is the thermalization process from the highly absorbing excitonic states to the indirect band edges, (c) Kinetic stabilization via reaction rate relations (1) redox reaction of...

SRPES measurements have been performed for three conditions along the polarization curve in Figure 2.90, indicated by Ci, C2, and at OV (SCE). First, the energetic situation at the Si-electrolyte contact is reviewed (Figure 2.84) as mentioned above for aUgnment between electrolyte levels and the Si band structure, the measured flatband potential of -0.48 V vs. SCE is used. The Pt deposition occurs in two major steps, for example, Pt(IV)/(II) and Pt(II)/(0). The redox levels are indicated in the figure and the reaction scheme is... 

According to Figure 6.34, electron exchange between the semiconductor and the ions in solution is facilitated when the distance between the conduction band and the Fermi level of the redox system is small. This is because the charge transfer rate depends on Dox and Generally, the rate of electron exchange will be more or less rapid, depending on the reversible potential of the dissolved species and on the band structure of the oxide. 

To obtain information about the energetics of semiconductor electrodes using CV, it is not necessary to have a series of redox couples whose are distributed in the entire band structure. In fact, one can pinpoint the flat-band position by recording CVs for just one redox couple having two-electron systems, AVA and A VA with respective redox potentials o(i) and f o(2) such that these potentials lie above and below Eq. The simation is sketched in Figure 9.22. 

Figure 30.3 Band structure of the redox-free Z-scheme SrTiOsla/Rh - Ir/CoOx/TasNs. (Reprinted with permission from Ref. [58]. Copyright 2014 American Chemical Society.)...

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