Aniline electrodeposition

Fig. 13. Electrodeposition of PANi in N-F solution (NH4F, 2.35 HF) 0.02 M aniline, by potential sweeping 20 mV s 1 from — 0.2 V to 1.0 V. Reproduced from [182].

Using a different approach, the research gronps of Fabre and Freund have synthesized boronate-functionalized conjugated polymers, which serve as electrochemical sensors. For example, a conjugated redox-active film of polypyrrole (174) was electrodeposited onto a platinum electrode from acetonitrile solution. Addition of fluoride anions led to a new redox system that showed an anodic shift relative to polypyrrole itself, which was attributed to fluoride binding to the boronate group. A related poly(aniline boronic acid) (175) was also reported and studied for saccharide detection. " ... 

A remarkable recent example of the influence of electropolymerization temperature on the properties of PAn products is observed in the potentiostatic polymerization of aniline in the presence of chiral (+)-HCSA to give optically active PAn/(+)-HCSA films. The CD spectra of the polymers electrodeposited at < 25°C were inverted compared to the spectra of analogous emeraldine salts deposited at > 40°C, indicating an inversion of the preferred helical hand for PAn chains.38 The observations may be rationalized in terms of a temperature-induced interconversion between two initially (kinetically) formed diastereomeric PAn products. 

We have shown that such colloids are electroactive and can be electrodeposited.126 However, the electrodeposited hints are not coherent and redissolve once the negative potential is removed. Optically active colloidal PAn nanocomposites of the type PAn/HCSA/polyacrylic acid have also recently been synthesized using the chemical oxidation route, by the oxidation of aniline with (NH4)2S208 in the presence of (+)- or (-)-HCSA as the dopant acid and polyacrylic acid as the steric stabilizer.127... 

The formation of PANl-NFs during electrochemical polymerization has been attributed to the combined effect of an electrophilic substitution reaction mainly taking place at the para position of aniline or its oligomers, and aniline oligomers with 1-D structures [290]. It was discovered that the PANl film formed on the electrode exhibited a lamellar structure. Firstly, a thin compact 2-D PANl layer was formed during the initial stage of electrodeposition. Subsequently, microgranular, nanorod-shaped, and nanofibrillar PANl layers were formed from bottom to top, respectively. 

Figure 2.14 SEM image of microrings electrodeposited at current density of W mA cm for 45 min in aqueous solutions of 0.05 M aniline hydrochloride and 0.05 M o-ABSA. (Reprinted with permission from Nanotechnology, Direct electrochemical generation of conducting polymer micro-rings by J. Song, D. Han, L. Guo and L. Niu, 17, 824—829. Copyright (2006) Institute of Physics)...

Illustration of this effect in the case of a two-step process for aniline electropolymerization on mild steel and zinc from an aqueous electrolyte was reported by Lacroix et In acidic medium (the usual conditions for the electrodeposition of PANI) the direct electropolymerization of aniline on zinc or steel fails, because large amounts of metal dissolve before the aniline polymerizes. This can be avoided if the surface is first treated by depositing a thin polypyrrole film in neutral salicylate medium. This PPy layer behaves as a thin noble-metal layer and can be used for the electrodeposition of a PANI film of controllable thickness in an acidic medium. Using this pretreatment, no or very little metal dissolution occurs in this second step. The films exhibit very stable electroactivity in acidic electrolytes, similar to that of PANI deposited on platinum, which indicates that the underlying oxidizable metal is fully protected (Figure 16.1). 

Electrodeposition of PANI derivatives was achieved by Shah and Iroh [108] in the case of Al alloy (2024) and N-ethylaniline in aqueous oxalic acid solution. The coating was electrodeposited by cyclic voltammetry by application of an unusually large potential range (-1V to -I-3V/SCE). Corrosion current was measured in chloride solutions from Tafel plots and was found to be significantly lower (about one order of magnitude) than that of bare Al-2024. Multifunctional coatings (PPy/PANI) were also achieved on aluminium by coelectropolymerization of aniline and pyrrole (in a feed ratio of 3 7) in oxalic acid solution, but protection performances were not given [109,110]. 

Figure 7.7 Schematic representation for the production and electrodeposition of aniline monomer on graphene paper. Reprinted with permission from Ref. [85]. Copyright 2009, American Chemical Society.

There are no reports about PANI composites with oxides of silver and gold because of the high redox reactivity of these oxides (e.g., Ag O and Au O can oxidize PANI), while PANI composites with copper oxides are known. Electrodeposition of mesoporous bilayers of PANI supported Cu O semiconducting films from lyotropic liquid crystalline phase has recently been reported by Xue et al. [16]. The control of size, morphology, and conductivity of PANI nanofibers (PANI-NFs) in PANI-NFs/CuO nanocomposites (Figure 2.1) was achieved by systematic variation of CuO loadings during the oxidative polymerization of aniline with mixture of oxidants [ammonium peroxydisulfate (APS) and sodium hypochlorite] in an acidic aqueous solution [17]. 

NFs acted as an oxidant to oxidatively polymerize aniline. Consequently, Mn O was completely consumed while PANl/TiO -NFs were formed. Composite fiber film having a PANI nanowire-on-TiO nano/microfiber hierarchical nano/microstructure was fabricated by electrospinning, calcinations and in situ polymerization [121], The photocatalytic electrodeposition of PANI in nanotubular TiO host matrice was recently reported by Janaky ef a/. [ 108]. TiO films were initially electrosynthesized on titanium foil by anodization at different voltages in fluoride-containing aqueous media, and then the PANI was electrografted onto the entire surface of... 

Several ring-substituted anilines (o-toluidine, m-toluidine, o-anisidine, and o-chloroaniline) were electrodeposited on passivated Fe surfaces by cyclic voltammetry, potentiostatic, or galvanostatic techniques from aqueous oxalic acid solutions [180]. With the exception of o-chloroaniline, the films exhibited protective properties against corrosion of Fe in sulfuric acid solution by stabilizing the Fe passive state, though performance was slightly poorer than that of polyaniline. 

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