Anion dopants

Fluoride is also used as an anionic dopant. An early study demonstrated that F substitutes surface OH species leading to an increase in the degradation of phenol at least three times faster than an undoped sample [56]. Other anions are also effective as dopants in reducing the Ti02 bandgap. Chloride, for example, was shown to give active photocatalysts in the visible range thanks to the red shift in the absorption spectra (Eg = 3 eV) but also increased surface acidity [57]. 

In general, doping tends to lead to a loss of x-ray order in polyacetylene and polyphenylene, suggesting that dopant ions may be distributed more or less at random. The structural models shown in Fig. 16 are clearly idealised as only limited order is seen even in cation-doped polymer. The anion dopants are much larger and apparently disrupt the structure too much for any sign of regularity to be seen, except in the case of iodine. 

Coating by a thin layer of PPy has been realized on multiwalled nanotubes (MWNT) [29,91,93], well-aligned MWNT [85] and single-wall nanotubes (SWNT) [88], When MWNT are oxidized, their surface is covered with oxygenated functionalities, which can be used as anionic dopant of a PPy film electrodeposited on the MWNT [94], These films are notably less brittle and more adhesive to the electrode than those formed using an aqueous electrolyte as source of counterion. 

Polyaniline is less tolerant of preparation conditions than polypyrrole, and the list of anion dopants used in the preparation is more limited. However, subsequent replacement of the anion used in preparation by a dodecylben-zene sulfonate makes polyaniline become soluble in solvents such as Af-methyl pyrrolidone (NMP), or zn-cresol, and it can be spin-coated or otherwise solution-processed. In a similar way derivatives of aniline, such as... 

Anion movement predominates in cases where a small mobile dopant, e.g., Cl-, is used. If large anion dopants such as polyelectrolytes are employed, then cation movement will predominate. 

An optically active PPy 17 has also been synthesized by the electropolymerization of pyrrole monomer bearing a homochiral sugar covalently attached to the pyrrole nitrogen.132 This chiral polymer discriminated between (+)- and (-)- cam-phorsulfonate ions as potential anionic dopants in cyclic voltammetry studies. 

Pyrocatechol violet (PV) [23], alizarin red (AR), and anthraquinone-2-sulfonic acid sodium salt (AQ) [24] had also been used as anionic dopants for the electrochemical fabrication of PPy/MWCTs composite films on stainless steel (SS) and Ni substrates. These anionic dyes could enhance the adhesion of PPy on the substrates, and adsorb on MWCNT and allows efficient dispersion, charging and controlled electrochemical incorporation into the PPy films during electro-polymerization. The composite PPy-MWCNT films showed higher SC, improved SC retention with increasing film mass and lower impedance, compared to pure PPy films. 

Usually the functional moiety is covalently linked to the conjugated backbone, but it can be sometimes added to ECP as a dopant when it is under an ionic form e.g., sulfonated P-cyclodextrins have been successfiilly incorporated in one step as anionic dopants in PPy by electropolymerization [230]. In some cases, electrochemistry can be combined to chemical reactions to derive functional ECPs functional PANI materials can be obtained from the reduction of the emeraldine form by alkylthiols [255] and the functionality degree on the PANI backbone can be monitored by successive oxidation-reduction cycles various functionalized PANIs can also be synthesized through electrophilic substitution or nucleophilic addition reactions [256]. For example. Figure 18.10 shows the similar electrochemical behaviors of sulfonated PANI made from reduction of emeraldine by sulfite salt followed by reoxidation, compared to thin films deposited from solution of highly sulfonated PANIs. 

Soluble conducting polymers can be solvent cast to form coatings. The addition of appropriate substituents to the polymer backbone or to the dopant ion can impart the necessary solubility to the polymer. For example, alkyl or alkoxy groups appended to the polymer backbone yield polypyrroles [117,118], polythiophenes [118], polyanilines [119,120], and poly(p-phenylenevinylenes) [97] that are soluble in common organic solvents. Alternatively, the attachment of ionizable functionalities (such as alkyl sulfonates or carboxylates) to the polymer backbone can impart water solubility to the polymer, and this approach has been used to form water-soluble polypyrroles [121], polythiophenes [122], and polyanilines [123]. These latter polymers are often referred to as self-doped polymers as the anionic dopant is covalently attached to the polymer backbone [9]. For use as a corrosion control coating, these water-soluble polymers must be cross-linked [124] or otherwise rendered insoluble. 

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