Semiconductors, polyaniline

Hu H., Trejo M., Nicho M. E., Saniger J. M., and Garcia-Valenzuela A., Adsorption kinetics of optochemical NH3 gas sensing with semiconductor polyaniline films. Sens. Actuators B, 82, 4-23, 2002. 

ARE SEMICONDUCTING POLYMERS POLYMERIC SEMICONDUCTORS POLYANILINE AS AN EXAMPLE OF "CONDUCTING POLYMERS ... 

In the context of this chapter, we focus on the undoped or lightly doped 7i-conjugated systems that are commonly referred to as organic semiconductors. Conducting polymers, such as PEDOT PSS, plexcore, polyaniline, polypyrrole, and others are not addressed here as their charge transfer mechanisms are rather different and would warrant an article in its own right. 

Besides CdS, many other semiconductors have been deposited by the SILAR technique as well as organic conducting polymers, such as polypyrrole and polyaniline. For representative references and to locate some groups working in this field, see Refs. 188-195. 

Many of the earliest studies focused on the use of polymer-coated semiconductor materials for the reduction of C02. An example was the study of Aurian-Blajeni et al., who electropolymerized polyaniline onto p-Si [116]. In an aqueous C02-saturated solution, a total faradaic efficiency for formic acid and formaldehyde of 28% was achieved, but at a potential of—1.9 V (versus SCE). Likewise, Cabrera and Abruna electropolymerized [Re(CO)3(v-bpy)Cl], where v-bpy is 4-vinyl-4 -methyl-2,2 -bipyridine [117]. For CO production, TONs of about 450 were observed, while the faradaic efficiencies approached 100%. Upon illumination in acetonitrile solution, the onset potential for reduction was -0.65 V (versus SCE). 

The signal travels through a thick, or even molecularly thin, semiconductor that connects these electrodes it could be an inorganic semiconductor (doped Si, doped Ge), an organic conducting polymer (polyaniline, polythiophene, polyacetylene), a carbon nanotube, or an organic semiconductor (sexithiophene). 

A highly p-doped CP (or a degenerate p-doped semiconductor) should also be able to serve as anode. This has been achieved with polyaniline, in what is an almost all-organic, flexible LED [267]. The work function of PAni is at least as uncertain as that of ITO, but it should be comparable, since the diodes using either material as anode have very similar properties. 

Several polymer conductors are commercially available, and have been used in the demonstration of printed transistors. These include PEDOT PSS, which is a commercially available polymer conductor, as well as various versions of polyaniline. The latter is typically doped with an acid or salt to increase conductivity. Both of these material systems are water soluble and easily printable. They also typically form good interfaces to organic semiconductors, making them attractive for use in printed transistors. As with polymer dielectrics, however, it is important to note that their usability with inorganic semiconductors is questionable, of course. 

Polyaniline provides the prototypical example of a chemically distinct doping mechanism [33,34], Protonation by acid-base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base) to metal (the emeraldine salt). The doping mechanism is shown schematically in Fig. II-2. The chemical structure of the semiconducting emeraldine base form of polyaniline is that of an alternating copolymer, denoted as [(1A)(2A)] , with... 

Properties of representative conducting polymers. Doped conjugated polymers have generated substantial interest in view of possible applications such as lightweight batteries, antistatic equipment, and microelectronics to speculative concepts such as molecular electronic devices.37-38 These polymers include doped polyacetylene, polyaniline, polypyrrole, and other polyheterocycles (Figure 5). While the conduction mechanism of metals and inorganic semiconductors is well understood and utilized in microelectronics, this is not true to the same... 

Polyanilines (Scheme 36) are conjugated polymers whose it electrons are delocalized over the whole molecule. They are important conducting polymers that also act as semiconductors, in a similar manner to inorganic semiconductors121 m. They are made by chemical or electrochemical (anodic) oxidation of aniline. The product, a poor textile colorant, dates from the 1860s, and is still known by the name given at that time, emeraldine. In the electrochemical process, it is possible to produce thin films directly on conductive substrates. Polyanilines have been used in photoelectrochemical devices124-126. 

Thin films of conductive polymers like polypyrrole, polyaniline, etc are also used for the surface modification of the semiconductor electrodes [26, 27]. Their performance mechanism has not yet been deciphered. Most likely, the coating is a combination of a protective film and the charge carrier, the more so that reversible redox couples are used to be introduced into films [28], as well as catalytically-active admixtures like Ru02 [29]. Such films were used to stabilize a promising "solar" electrode material, amorphous silicon [30]. 

By 1995 one finds that the main actors are different polymers, a change which can be partly attributed to the recent interest in semiconductor properties, especially light emission. In the field of conduction, polyacetylene has given its place to polyaniline, a polymer with a fascinating rich chemistry and a promise of good processability. In the semiconductor arena, the most studied polymer of the past decade is polythiophene, or rather its soluble derivatives which lend themselves very well to various fabrication processes. Another polymer, one which was only mentioned briefly in the 1986 Handbook, is competing with polythiophene for application in semiconductor devices poly(pura-phenylene vinylene). These three polymers received very little interest before 1985 no crystallographic studies had been published at that time. 

Twenty years ago, a new class of organic polymer materials was discovered with the new property of electronic conductivity comparable to metallic conductors [55]. The first representative was the polyacetylene followed in the subsequent years by several other polymers, such as polyaniline, polypyrrole, and polythiophene (see Fig. 7). The neutral structure is shown in Fig. 7. This structure has properties comparable to a semiconductor. The metal like conductance is obtained by chemical or electrochemical oxidation (shown for polypyrrole in Fig. 8). In this example, up to 30% of the pyrrole rings can be oxidized. The positive charge of the heterocyclic ring... 

The combination of the properties of nano-TiO and polyaniline enables to solve successfully the problems of the chemistry, physics and electronics. Specific electronic structures of the nano-TiO (as the n-type semiconductor) and polyaniline (as the electron s conductor in majority of the cases and as a p-type semiconductor under certain conditions) give the possibility to design the systems for different applications. For example, today such materials are equipped in the photocatalytic conversions of the different pollutants especially [7 4. The modification of the surface of TiO particles by polyanilines layers raises the catalytic activity of titanium (IV) oxide [5, 79]. Composite materials, which have integrated properties of 5-doped nano-TiO and polyaniline layers can be effective in the photo-catalytic processes especially. 

Results of conductivity studies of s mthesized samples are shown on Fig. 5. As it expected, the highest conductivity observes the polyaniline sample - 17.0x10 S cm (Fig. 5, dot A). The introduction and further increasing of semiconductor TiO -iS nanoparticles content in the composites leads to the exponential decreasing of the conductivity of samples. 

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