Doping oxidation/reduction

The oxidation and/or reduction reactions yield polymeric systems having an extended Jt-electron system along the chain. Doping to the conducting state, in the instance of polyacetylene by exposnre to iodine vapor (p-doping, oxidizing). 

Due to participation in oxidation-reduction reactions the reducing or inflammable gases affect the stoichiometricity of oxide and, consequently the concentration of stoichiometric defects which usually control the dope electric conductivity of adsorbent [26, 67, 85, 86, 90]... 

At greater degrees of reduction, all of the Pr ions are in the trivalent state, and the oxide is in essence an acceptor-doped oxide with oxygen vacancy compensation. Any further reduction must then be accomplished by the transformation of Ce4+ to Ce3+, repeating the previous cycle ... 

The proposed mechanism of the doping processes in conducting polymers implies oxidation (p-doping) or reduction (n-doping) of the polymer with... 

The blue-violet ( max = 670 nm) color of the doped (oxidized) polypyrrole changes to yellow-green ( max = 420 nm) upon electrochemical reduction [49]. Polypyrrole itself has two distinct disadvantages that prevent its use in devices... 

The unique properties of lanthanide-based materials, e.g., lanthanide-silicates and lanthanide-doped silicas, can be related to the special properties of the 4f" orbitals. Among lanthanide oxides, only Ce, Pr and Tb form dioxides, which crystallize in one simple structure with M4+ ions showing octahedral coordination [17]. For instance, cerium dioxide exhibits an 8 4 catiomanion coordination [18]. Its characteristic feature is the ability to undergo oxidation-reduction cycles in a reversible way [19], It was shown that the presence of Ce and La additives in mesoporous silicas, e.g., MCM-41 [10,11] and MSU-X [12], improves their thermal and hydrothermal stability. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

As approximate fits to spectra, oscillator models often miss essential details in the physics of the material response. Spectra of real samples reveal the consequences of composition, structure, doping, oxidation or reduction, multiplicity of phases, contaminant or introduced charges, etc., on electronic structure. These consequences from sample preparation can qualitatively affect intermolecular forces. To the extent possible, the best procedure is to use the best spectral data collected on the actual materials used in force measurement or materials designed for particular force properties. Given the present progress in spectroscopy, such coupling of spectra and forces may soon become routine. 

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

PPy film is blue-violet in doped (oxidized) stet. Electrochemical reduction yields the yellow-green undoped form. The schematic of the doping/dedoping process can be given as... 

In doped polymers, hole or electron transport occurs by the transfer of charge from states associated with the donor or acceptor molecules, respectively. This can be described as a one-electron oxidation-reduction or donor-acceptor process between molecules in their neutral charged states (Pfister, 1977 Mort and Pfister, 1979 Pai et al., 1983 Facci and Stolka, 1986). For hole transport, some dopant molecules are initially positively charged (cation radicals). Under an applied field, neutral molecules will transfer electrons to the cation radicals. This results in the motion of positive charge. For this to occur, the dopant molecules must be donor-like in their neutral state. For electron transport, electrons are displaced from the anion radicals to neutral molecules, which requires that the dopant molecule be acceptor-like in its neutral state. It is generally accepted tliat these processes occur by hopping. 

Pai et al. (1983) measured hole mobilities of a series of bis(diethylamino)-substituted triphenylmethane derivatives doped into a PC and poly(styrene) (PS). The mobilities varied by four orders of magnitude, while the field dependencies varied from linear to quadratic. In all materials, the field dependencies decreased with increasing temperature. The temperature dependencies were described by an Arrhenius relationship with activation energies that decrease with increasing field. Pai et al. described the transport process as a field-driven chain of oxidation-reduction reactions in which the rate of electron transfer is controlled by the molecular substituents of the hopping sites. 

The initial discovery of the ability to dope conjugated polymers involved charge transfer redox chemistry oxidation (p-type doping) or reduction (n-type doping) [1,2], as illustrated with the following examples ... 

An important additional feature is that all of these polymer structures are amenable to facile oxidation/reduction processes that can be initiated at moderate potentials. For polypyrroles and polythiophenes two oxidation states can be reversibly switched, as shown in Eqn. 2 (Z = NH or S). The doped oxidised forms exhibit good electrical conductivity (0= 1-100 S cm ), while the reduced forms have very low conductivity (ct 10 S cm ). This ability to conduct electrons is important in that information can be readily relayed within an Intelligent... 

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