Doped, reversibly oxidation

The stoichiometry of the redox reactions of conducting polymers (n and m in reactions 1 and 2) is quite variable. Under the most widely used conditions, polypyrroles and polythiophenes can be reversibly oxidized to a level of one hole per ca. 3 monomer units (i.e., a degree of oxidation, n, of ca. 0.3).7 However, this limit is dictated by the stability of the oxidized film under the conditions employed (Section V). With particularly dry and unreactive solvents, degrees of oxidation of 0.5 can be reversibly attained,37 and for poly-(4,4 -dimethoxybithiophene), a value of n = 1 has been reported.38 Although much fewer data are available for n-doping, it appears to involve similar stoichiometries [i.e., m in Eq. (2) is typically ca. 0.3].34,39"41 Polyanilines can in principle be reversibly p-doped to one... 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

The ratio of the integrated currents for the first reduction wave of V2+ and the oxidation wave of the polythiophene from 0.4 V to 1.0 V vs. Ag+/Ag is about 4. This value means that upon oxidation of poly(I) one electron is withdrawn from four repeat units in the backbone of the polymer upon scanning to +1.0 V vs. Ag+/Ag. At this potential, the polythiophene achieves its maximum conductivity (vide infra). The level of oxidation to achieve maximum conductivity is consistent with the result reported by Gamier and co-workers (31-33) that the doping level of oxidized polythiophene is about 25%, but the Garnier work did not establish that the 25% doping level corresponds to maximum conductivity. Scheme III illustrates the electrochemical processes of poly(I) showing reversible oxidation of the polythiophene backbone and reversible reduction of the pendant V2+ centers. 

Further research on the substitution of the thiophene 3-position with phenyl groups containing electron-withdrawing or electron-donating groups (such as methyl, methoxy, fluoro, chloro, bromo, trifluoromethyl, sulfoxy) in the para position have lead to polymers with unique features [57]. The electron-withdraw-ing groups allow the formation of a radical anion and thus stabilize the n-doped state. As a result, such conducting polymers can be reversibly oxidized and reduced and electrochromic devices can be built with identical anode and cathode materials [58]. 

The electrochemical behavior of poly(ferrocenylsilanes) has been studied at three levels—in solution by cyclic voltammetry, as films deposited on electrodes, and in the solid state via iodine doping. Solution cyclic voltammetric oxidation and reduction has shown that the polymer, where R/R is Me/Me, reversibly oxidizes in methylene chloride in two stages, apparently with the first oxidation being on alternating iron atoms along the chain.29 Films cast on electrodes behave in a similar way and also show an electrochromic response to oxidation and reduction.30... 

Pale yellow cerium dioxide (ceria, ceric oxide) has the fluorite structure and is used in catalysis" ", solid oxide fuel cells (SOFC)", thin film optical waveguides" , reversible oxygen storage materials for automobile catalysts" and for doping copper oxide superconductors". The diverse cerium enolate precursors and deposition methods used in the formation of cerium oxide thin films are summarized in Table 6, whereby the most common precursor for ceria is Ce(thd)4. 

Electrochemical (cyclovoltametric) investigations of the ladder-type poly-(para-phenylene) species 71 support the results of the chemical oxidation (doping) experiments both in solution and in the solid state (film). A reversible oxidation takes place and it is well-separated into two waves especially in the solid-state experiment. These are assigned to the formation of radical cationic (79) and dicationic species (80), respectively. The halfwave potential (E1/2) for the first oxidation wave lies between 0.75 V (solution experiment) and 0.95 V (solid state - film) - versus a standard calomel electrode SCE) [106]. Consequently, one has to search for an alternative synthetic process to generate the ladder-type poly(phenylenemethide)s 77 or polymers containing extended segments of the fully unsaturated structure desired. The oxidation of polymeric carbanions appeared suitable, but it proved necessary to work under conditions which completely exclude water and air. 

The first reports on a reversible DET between redox proteins and electrodes were published in 1977 showing that cytochrome c is reversibly oxidized and reduced at tin-doped indium oxide [30] and gold in the presence of 4,4 -bipyridyl [31]. Only shortly after these publications appeared, papers were published describing the DET between electrode and enzyme for laccase and peroxidase [32,33]. It was observed that the overpotential for oxygen reduction at a carbon electrode was reduced by several hundred millivolts compared to the uncatalyzed reduction when laccase was adsorbed. This reaction could be inhibited by azide. The term bioelectrocatalysis was introduced for such an acceleration of the electrode process by... 

First realization of reversible ET of cytochrome c employing tin-doped indium oxide electrodes... 

In 1983 the application of doped tin oxide films to silicon solar cells has been reported [5]. lida and coworkers realized a setup with the following characteristics 7sc = 14 mAcm", Kic = 800 mV, efficiency = 7.5 % and fill factor = 0.67. Vishwakarma et al. [166, 167] prepared arsenic-doped tin oxide films for silicon solar cells and investigated the diode properties of Sn02 As/Si02/n-Si and Sn02 As/n-Si cells. The barrier height 0 was 0.78-0.89 eV and 0.68-0.69 eV, respectively, and the reverse saturation current density 7u was 2-45 pAcm and 0.07-9.2 pAcm", respectively, with diode quality factors of 2.2-2.9 and 1.7-1.9. The optimized results for solar cell applications are given below... 

The electrochemical behavior of poly(RCOT)s has also been examined [40]. As expected from the electrochemical properties of unsubstituted polyacetylene, films of poly(RCOT)s coated on an electrode and immersed in an acetonitrile electrolyte solution (in which the polymers are not soluble) are found to undergo reversible oxidative and reductive doping. Unlike unsubstituted polyacetylene, these films may be prepared readily by casting from solution, or, in the case of poly(scc-butylCOT), by electrodecomposition from a THF solution. In contrast to the voltammetry of polymer films, cyclic voltammograms of methylene chloride... 

Yeh and Kuwana " were the first to report on the electrochemistry of cytochrome c at doped metal oxide semiconductor electrodes. A nearly reversible electrode reaction was indicated by the cyclic voltammetry and differential pulse voltammetry of cytochrome c at tin-doped indium oxide electrodes. Except for the calculated diffusion coefficient, all of the characteristics of the electrochemistry of cytochrome c at this electrode indicated that the electrode reaction was well-behaved. A value of 0.5 x 10" cmVs was determined for the diffusion coefficient which, like previously determined values at mercury, is lower than the value obtained by nonelectrochemical methods (i.e., 1.1 X 10 cm /s " " ). The electrochemical response of cytochrome c at tin oxide semiconductor electrodes was reported to be quasi-reversible, although no details were given. " ... 

The global reaction requires 2 moles of electron per mole of monomer plus an excess of charge 8 to dope (or oxidize) the polymer (Fig. 2). The thicknesses of the films were estimated from the total anodic charge Qs Qs =n 2 + 8)F where F is the faraday) passed during electrodeposition and consequently depends on experiment time. The electropolymerized film is obtained in its oxidized state and is accompanied by the incorporation of counter anions to assure the electroneutrality of the film. This p-doping reaction is reversible since the neutral polymer can be obtained by reduction. 

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