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Polymers can be prepared using chemical and/or electrochemical methods of polymerization (see Chap. 2), although most redox polymers have been synthesized by chemical polymerization. Electrochemically active groups are either incorporated into the polymer structure inside the chain or included as a pendant group (prefunctionalized polymers), added to the polymer phase during polymerization, or fixed into the polymer network in an additional step after the coating procedure (postcoating functionalization) in the case of polymer film electrodes. The latter approach is typical of ion-exchange polymers. Several other synthetic approaches exist; in fact, virtually the whole arsenal of synthetic polymer chemistry methods has been exploited. Polyacetylene—now commonly known as the prototype conducting polymer—was prepared from acetylene using a Ziegler–Natta catalyst [1–7]. Despite its historical role and theoretical importance, polyacetylene has not been commercialized because it is easily oxidized by the oxygen in air and is also sensitive to humidity. From the point of view of applications, the oxidative electrochemical polymerization of cheap, simple, aromatic (mostly amines) benzoid (e.g., aniline [8–64], diphenylamine [65], o-phenylenediamine) or nonbenzoid (e.g., 1,8-diaminonaphthalene, 1-aminoanthracene, see Chap. 2) and heterocyclic compounds (e.g., pyrroles [13, 66–85], thiophenes [86–103], indoles, azines [104–110]) is of the utmost interest. Chemical oxidation can also be applied (e.g., the oxidation of aniline [111–117] or pyrrole [118] by Fe(ClO4)3 or peroxydisulfate in acid media leads to the respective conducting polymers), or less frequently reductive polymerization is also possible [119, 120]. The electrochemical polymerization is preferable, especially if the polymeric product is intended for use as a polymer film electrode, thin-layer sensor, in microtechnology, etc., because potential control is a prerequisite for the production of good-quality material and the formation of the polymer film at the desired spot in order to serve as an anode during synthesis. A chemical route is recommended if large amounts of polymer are needed. The morphology of the polymer strongly depends on the conditions of electropolymerization. For instance, the effects of pH on the oxidative electropolymerization of aniline and on the morphology and other properties of the resulting polymer have been reviewed recently [53]. It has been demonstrated that the formation of supramolecular structures such as nanoglobules, nanofibers, nanotubes, and microspheres strongly depends on the conditions of polymerization [53]. The polymers are obtained in an oxidized, high conductivity state containing counterions incorporated from the solution used in the preparation procedure. However, it is easy to change the oxidation state of the polymer electrochemically, e.g., by potential cycling between the oxidized conducting state and the neutral insulating state, or by using suitable redox compounds. The structure and conductivity can be altered through further chemical reactions [29].

The mechanism and the kinetics of the electropolymerization—especially in the cases of polyaniline [8–64] (see Fig. 4.1) and polypyrrole [13, 66–85]—have been investigated by many researchers. Two points have been addressed: the chemical reaction mechanism and kinetics of the growth on a conducting surface. Owing to the chemical diversity of the compounds studied, a general scheme cannot be provided. However, it has been shown that the first step is the formation of cation radicals. The subsequent fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable cases, the next step is a dimerization reaction, and then stepwise chain growth proceeds via the association of radical ions (RR route) or the association of a cation radical with a neutral monomer (RS route) [8–124]. There may even be parallel dimerization reactions leading to different products or to a polymer with a disordered structure.

Fig. 4.1
figure 1

The reaction scheme for the electropolymerization of aniline (Reproduced from [61] with the permission of Elsevier Ltd.)

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The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction of the oligomer helps the polymerization reaction. This might be the case if the RS route is preferred and the monomer carries a charge, e.g., a protonated aniline molecule (PANI can only be prepared in acidic media; at higher pH values, other compounds such as p-aminophenol, azobenzene, and 4-aminodiphenylamine are formed.). A relatively high concentration of cation radicals should be maintained in the vicinity of the electrode. The radical cation and the dimers can diffuse away from the electrode. Intensive stirring of the solution usually decreases the yield of the polymer produced. The radical cations can react with the electrode or take part in side reactions with the nucleophilic reactants (e.g., solvent molecules) present in the solution. Usually, the oxidation of the monomer is an irreversible process and takes place at higher positive potentials than that of the reversible redox reaction of the polymer. However, in the case of azines (e.g., 1-hydroxy-phenazine [125–127], methylene blue [104, 107, 109], neutral red [105, 107, 108]), reversible redox reactions of the monomers occur at less positive potentials, and this redox activity can be retained in the polymer, i.e., the polymerization reaction that takes place at higher potentials does not substantially alter the redox behavior of the monomer. For instance, the catalytic activity of methylene blue towards the oxidation of biological molecules (e.g., hemoglobin) is preserved in the polymer [104].

A knowledge of the kinetics of the electrodeposition process is also of the utmost importance. It depends on the same factors mentioned above, although the role of the material and the actual properties of the electrode surface are evidently more pronounced. For example, the oxidation of aniline at Pt is an autocatalytic process. The specific interactions and the wetting may determine the nucleation and the dimensionality of the growth process. Two or more stages of the polymerization process can be distinguished. In the case of PANI, it has been found that initially, a compact layer (L ~ 200 nm) is formed on the electrode surface via potential-independent nucleation and the two-dimensional (2-D, lateral) growth of PANI islands. At the advanced stage, 1-D growth of the polymer chains takes place with continuous branching, leading to an open structure [11, 15]. It is established that—in accordance with theory [128]—the density of the polymer layer decreases with film thickness, i.e., from the metal surface to the polymer–solution interface.

The very first stages of the electropolymerization were investigated using in situ FTIR, attenuated total reflection (ATR), and IR reflection absorption spectroscopy (IRRAS), which revealed that the mechanism of PANI formation is influenced by the deposition of oligomers, and the highest growth rate in cyclic electropolymerization occurs during the cathodic potential scan [129]. The film morphology (compactness, swelling) is strongly dependent on the composition of the solution, notably on the type of counterions present in the solution, and the plasticizing ability of the solvent molecules [34, 36, 44, 48, 64]. The effect of the counterions is illustrated in Fig. 4.2. The order of the growth rate depends on the nature of the anions (at the same positive potential limit and acidity) as follows: 4-toluenesulfonic acid (HTSA) > 5-sulfosalicylic acid (HSSA) > HClO4. This may be assigned to the stabilizing effect of the larger anions, i.e., lesser cationic oligomers formed at the surface diffuse into the solutions due to the lower solubility of the salts (ion pairs). It has been established that BF4 −, ClO4 −, and CF3COO− ions promote the formation of a more compact structure, while the use of Cl−, HSO4 −, NO3 −, TSA−, and SSA− results in a more open structure during electropolymerization [34, 36, 44, 48, 64]. Another finding is that certain anions (Cl−, HSO4 −, ClO4 −) also affect the apparent dissociation constant of PANI in its reduced form [130–132].

Fig. 4.2
figure 2figure 2

The cyclic voltammograms and the simultaneously detected EQCN frequency changes during the electropolymerization of aniline at a platinum electrode. Sweep rate: 100 mV s−1. Solution composition: 0.2 mol dm−3 aniline in 1 mol dm−3 (From [48], reproduced with the permission of Elsevier Ltd.) (a) HClO4, (b) 4-toluenesulfonic acid, (c) 5-sulfosalicylic acid

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The formation of the polymer involves about 2 mol electrons, associated with 1 mol of aniline [18, 26, 122, 123]. The growth rate is proportional—except for during the early induction period—to the square root of the film volume, and it is first order with respect to aniline concentration [55, 56]. Due to the autocatalytic nature of the electropolymerization, the positive potential limit of cycling can be decreased after 2–10 cycles, which is a common practice used to avoid the degradation of the polymer due to the hydrolysis of the oxidized PANI (pernigraniline form) [26, 35, 48, 55] (see Fig. 4.2). Although it is still debated, the appearance of the “middle peak” most likely reflects the occurrence of oxidative hydrolysis and degradation, and it can be assigned to the redox reaction of benzoquinone [61]. As well as the head-to-tail coupling that results in the formation of p-aminodiphenylamine, tail-to-tail dimerization (benzidine) also occurs; however, the latter is considered to be a minor dimer intermediate because the rate constant of dimerization for RR coupling that produces the former product (k is ca. 108 dm3 mol−1 s−1) is about 2.5 times higher than that for the tail-to-tail dimer [61]. The degradation process should be considered for other polymer films, but it can also be controlled electrochemically [79]. If the conditions are not carefully optimized, a mixed material containing electrochemically active and conducting as well as inactive and insulating parts is generally deposited on the surface [79]. It has been demonstrated that the current density is a crucial parameter in the synthesis of polypyrrole (PP) [68, 80, 84]. The structure of PP is dominated by one-dimensional chains at low current densities, while two-dimensional microscopic structures of the polymer are formed at high current densities [68, 80]. The structure substantially affects the conductivity of the polymer phase, the conductivity of the 2-D form is higher, and its temperature dependence is lower, which is of importance when this polymer is used for practical purposes. Detailed studies have shown that the more conductive 2-D islands are interconnected by short 1-D chain segments which act as tunneling barriers [80]. As described in Sect. 3.2.7.2, during the electropolymerization of polythionine films, structural changes occur during film thickening [133]. The results of ellipsometric measurements attest that during the growth of PEDOT films (up to a thickness about 200 nm), similar phenomenon occurs [134]. The factors affecting the morphology of growing thin films such as surface diffusion, growth, step-edge barrier, etc., have been analyzed by molecular simulation techniques [135]. Albeit mostly the deposition by electropolymerization has been studied, the chemical synthesis and deposition of polyaniline on a metal substrate were also evaluated by using the measurement of the variation of the open-circuit potential, mass, and impedance [135]. Nucleation and growth mechanism during cyclic voltammetric and potential step electropolymerization of methylene blue (MB) in a basic medium were studied in detail. The effect of preparation potential on structure and morphology of the poly(methylene blue) film formed was investigated by using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and UV–Vis absorption spectroscopy techniques. The results indicate that the deposition process starts with a progressive layer-by-layer nucleation and growth besides random adsorption. It is followed by a process involving both progressive layer-by-layer and 3D instantaneous mechanism resulting in highly oriented poly(methylene blue) nanofibers with increasing film thickness. Films prepared between the potential values of 900 and 950 mV show a well-ordered, smooth surface, but at the potential values higher than 1,000 mV, rough polymer surface arises as overoxidation takes place [106]. The effect of the conditions of electropolymerization and the properties of the resulting films in the case of polypyrrole [83] including the formation of nanostructures and nanocomposites [67] were analyzed recently. The growth mechanism of PP under potentiostatic conditions was studied in detail; the substrate effect was emphasized [75].

It has also been demonstrated by scanning microscopies that film growth at submicrometer or micrometer structured substrates is not restricted to conductive substrate domains. Instead, after the film thickness has risen to the level of the surrounding insulator, lateral outward growth on the nonconductive part also occurs [127]. This is a phenomenon that should be taken into account in microtechnical applications.

Although the region close to the electrode surface exhibits a more or less well-defined structure, in general the polymer layer can be considered to be an amorphous material [11, 15, 16, 82]. However, there are rare reports of crystalline structures too.

For instance, poly(p-phenylene) films obtained by the electrooxidation of benzene in concentrated H2SO4 emulsion show a highly crystalline structure [136, 137].

The conditions for polymerization were also found to be crucial in relation to polythiophene and polybithiophene films [86–103]. The relatively high potential required for the oxidation prevents the use of many metallic substrates. The electrochemical oxidation of substituted thiophenes and thiophene oligomers yields conducting polymers, and these compounds can be electropolymerized at less positive potentials, so it is a good strategy to use these derivatives instead of thiophene (see Sect. 2.2.2.4). Another approach is the deposition of a thin polypyrrole layer that ensures the deposition of polythiophene on these substrates (e.g., Ti, Au) [92]. Many other polymers as well as copolymers and composites (see Chap. 2) can also be synthesized.

Although deaerated solutions are usually used during electropolymerization, it has been proven that the presence of oxygen increases the amount of poly(neutral red) deposited on the electrode [138].

The choice of the supporting electrolyte is important not only in relation to the morphology and properties of the polymer; in several cases, the formation and deposition of the polymer can only be achieved using special electrolytes.

For instance, poly(9-fluorenone) can be electropolymerized in boron trifluoride diethyl etherate (BFEE) media, while the polymerization takes place in CH2Cl2–Bu4NBF4, albeit with a much smaller rate, and polymer formation cannot be observed in acetonitrile–Bu4NBF4, as seen in Fig. 4.3 [139].

Fig. 4.3
figure 3

Cyclic voltammograms of 3 × 10−2 mol dm−3 9-fluorenone in (a) BFEE, (b) acetonitrile + 0.1 mol dm−3 Bu4NBF4, and (c) CH2Cl2 + 0.1 mol dm−3 Bu4NBF4, respectively. Scan rate: 50 mV s−1 (Reproduced from [139] with the permission of Elsevier Ltd.)

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This effect has been explained by the interactions between the BFEE, which is a midstrength Lewis acid, and the aromatic monomers. The interactions lower the oxidation potential of the monomers, and the catalytic effect of BFEE facilitates the formation of high-quality polymer films.

As well as the nature and concentration of the supporting electrolytes (monomer concentration, temperature, etc.), organic additives also influence the morphology of the polymer film. Figure 4.4 shows SEM pictures of PANI prepared by the electropolymerization of aniline in the absence and presence of methanol, respectively.

Fig. 4.4
figure 4

Scanning electron microscopic pictures showing the effect of alcohols on the morphology of PANI films. The films were synthesized under potentiostatic conditions at 0.8 V vs. SCE from solutions containing 1 mol dm−3 HCl and 0.2 mol dm−3 aniline without (a) and with (b) 0.5 mol dm−3 methanol (Reproduced from [63] with the permission of Elsevier Ltd.)

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When alcohols were added to the electrolyte used in the electropolymerization, PANI nanofibers were formed with diameters of approximately 150 nm, which agglomerate into interconnected networks. This effect has been explained in terms of interactions between the methanol molecules and the polyaniline chains; i.e., the PANI chains are wrapped by alcohol molecules due to intermolecular H-bonding, which is advantageous to the one-dimensional growth of the polymer [63].

Rotation of the electrode during electrochemical polymerization has been shown to have a strong influence on the rate of formation of electrochemically polymerized films, and it affects the morphology and conductivity of the polymer. For instance, it has been demonstrated that Δ4,4′-di-cyclopenta [2.1-b; 3′,4′-b′]-dithiophene grows faster at higher rotation rates, and the morphology changes from fibrillar to globular structures. Both the electronic and ionic conductivities of the polymer increased by two orders of magnitude [140]. It is thought that the main effect of electrode rotation, when high monomer concentrations are used, is the removal of oligomers from the vicinity of the electrode, minimizing their precipitation. Consequently, only the polymerization of the species grafted on the electrode surface takes place, which results in a better-quality polymer film. It should be mentioned that in other cases a drop in the deposition rate has been reported [141].

Ultrathin functional films can be prepared with finely adjusted film thickness and properties by a layer-by-layer (LbL) method. Such multilayers are fabricated by the alternated adsorption of anionic and cationic polyelectrolytes. These polyelectrolyte multilayers are self-compensated in terms of the charge; however, the introduction of redox ions such as Fe(CN)6 4− or Os(bpy) 3+3 is also possible [142].

Higher electronic conductivity has been achieved by template synthesis using polycarbonate membranes [40], and this method has also been exploited to obtain nanostructures [46, 78].

Figure 4.5 shows a transmission electron micrograph of PANI nanotubes obtained by chemical oxidative polymerization and separated from a polycarbonate membrane. The polycarbonate template was removed by dissolving the samples in chloroform and then by filtering the green precipitate. The rest of the polycarbonate was removed by extraction using H2SO4 when the PANI nanotubes precipitate at the chloroform–acid interface [46].

Fig. 4.5
figure 5

Transmission electron micrograph of PANI nanotubes (Reproduced from [46] with the permission of Elsevier Ltd.)

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Spectacular fractal patterns can be obtained by utilizing a needle-to-circle electrode configuration [74].

It is also possible to modify the deposited conducting polymer in order to change its electrical, optical, and other properties. For instance, polyaniline film was modified by subsequent electrodeposition of diaminomethylbenzoate (Fig. 4.6) [29, 143]. As a comparison of the spectrum of PANI—where the absorbance related to the delocalized electrons at λ > 600 nm is clearly apparent—with the spectrum of the modified PANI shown in Fig. 4.7 reveals, the electronically conductive parent polymer can be transformed into a redox polymer. However, the electrochemical behavior, the color [29], and the conductivity [143] of the polymer during the modification procedure can easily be regulated, and so the required properties can be finely turned [29, 143].

Fig. 4.6
figure 6

Cyclic voltammograms (a) and the simultaneously obtained EQCN frequency changes (b) during the deposition of 3,5-diaminomethylbenzoate onto PANI film on Au. Solution composition: 0.13 mol dm−3 diaminomethylbenzoate and 2 mol dm−3 H2SO4. Scan rate: 100 mV s−1 (Reproduced from [29] with the permission of Elsevier Ltd.)

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Fig. 4.7
figure 7

The subtracted UV–Vis–NIR spectra of PANI (a) and modified PANI (b), respectively, obtained in situ at different potentials: (1) −0.35 V; (2) −0.25; (3) −0.15; (4) −0.05; (5) 0.05; (6) 0.15; (7) 0.25; (8) 0.35; (9) 0.45; (10) 0.55; and (11) 0.65 V. Solution: 1 mol dm−3 H2SO4. ΔA = Abs(E) − Abs(E = −0.35 V) (Reproduced from [29] with the permission of Elsevier Ltd.)

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Electropolymerization can be executed using droplets and particles immobilized on the surfaces of inert electrodes [144]. Water-insoluble monomers can be used for this purpose, and the electropolymerization is carried out in aqueous electrolytes. Microcrystals can be attached to platinum, gold, or paraffin-impregnated graphite (PIGE) by wiping the electrode with a cotton swab or filter paper containing the material. Alternatively, the electrodes can be covered with the monomer using an evaporation technique; i.e., the microcrystals are dissolved in appropriate solvents (e.g., tetrahydrofuran), and some drops of the solution are placed onto the electrode surface. After the evaporation of the solvent, a stable monomer layer remains on the surface. The attachment of microdroplets requires more skill. A 1–2-μl drop of monomer is placed on the electrode surface using a micropipette or syringe. If this electrode is carefully immersed into the aqueous solution, the droplet remains on the electrode. The surface tension of water, which is much higher than that of most organic liquids, plays an important role, but the difference in densities can also be controlled by varying the concentration of the electrolyte. A small “spoon” made from Pt plate can also be fabricated, which can be used to place the organic droplet in this small vessel. Figure 4.8 shows the electropolymerization of 3-methylthiophene droplets attached to a PIGE in the presence of an aqueous solution containing 0.5 mol dm−3 LiClO4 [91].

Fig. 4.8
figure 8

Consecutive cyclic voltammetric curves obtained for 3-methylthiophene droplets attached to a paraffin-impregnated graphite electrode in the presence of an aqueous solution containing 0.5 mol dm−3 LiClO4. Cycles: (a) 1–4 (curves 2–5); (b) 5–14; (c) 15–24 and 25–34. Curve 1 shows the background current of the uncoated PIGE. Scan rate: 100 mV s−1 (Reproduced from [91] with the permission of Elsevier Ltd.)

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The cyclic voltammograms and the changes that occur to them during repetitive cycling are similar to those of 3-methylthiophene oxidation in acetonitrile. When a platinum electrode is used, the color change (red-blue) due to the redox transformation of poly(3-methylthiophene) is easily visible. A visual inspection also reveals that the electropolymerization reaction starts at the three-phase junction, as theoretically expected, since in this region, the electron transfer between the metal and the monomer, as well as the interfacial transfer of the charge-compensating counterions between the droplet and the contacting electrolyte solution, can proceed simultaneously.

Electropolymerization using carbazole [145] and diphenylamine [65, 146] microcrystals has also been described. Figure 4.9 shows the cyclic voltammograms and the simultaneously detected EQCN frequency curves obtained during the electropolymerization of carbazole deposited by an evaporation method on gold. Due to the small amount of carbazole, the electropolymerization was completed during a single cycle (curve 2, Fig. 4.9). The amount of counterions and solvent molecules incorporated during the oxidation process can be calculated from the mass change since in this case, the polymer deposition does not contribute to the mass change. The next two cycles (Fig. 4.10) show the redox response of polycarbazole and the accompanying mass change. The high anodic current peak, which is due to the formation of cation radicals, dimers, the further oxidation of dimers, as well as the formation of the oxidized polymer, did not appear.

Fig. 4.9
figure 9

The cyclic voltammetry curves and the simultaneously detected frequency changes obtained for the gold substrate (1) and the virgin carbazole layer deposited on a gold electrode (2), respectively. Solution: 9 mol dm−3 HClO4. Scan rate: 50 mV s−1 (Reproduced from [65] with the permission of Elsevier Ltd.)

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Fig. 4.10
figure 10

The continuation of the experiment shown in Fig. 4.9 (the second and third cycles) (Reproduced from [65] with the permission of Elsevier Ltd.)

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Consecutive cyclic voltammetric curves obtained for diphenylamine microcrystals attached to a platinum electrode in the presence of aqueous solution containing 1 mol dm−3 H2SO4 are shown in Fig. 4.11 [65].

Fig. 4.11
figure 11

Consecutive cyclic voltammetric curves obtained for diphenylamine microcrystals attached to a platinum electrode (A = 1 cm2) in the presence of aqueous solution containing 1 mol dm−3 H2SO4 (Reproduced from [146] with the permission of Elsevier Ltd.). Scan rate: 100 mV s−1. Cycles: (a) 1–5, (b) 11–30 (started after a 3 min delay at −0.2 V), cycles: (c) 31–50 and (d) 71–90 (started after a 3 min delay at −0.2 V)

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The high oxidation peak at ca. E = 0.73 V vs. SCE is caused by the formation of diphenylamine cation radicals (DPAH•), the C–C para-coupled dimerization of these cation radicals to diphenylbenzidine (DPBH2), and the further oxidation of DPBH2. The progressively developing waves (E pa  ≈ 0.52 V, E pc = 0.43 V) belong to the reversible redox process of the dimer or of the polymer. The redox transformation of the polymer is accompanied by a color change from colorless (reduced) to a bright blue (oxidized) form. The reaction starts at the three-phase boundary since diphenylamine is an insulator; however, the formation of electronically conducting polymer wires provides an opportunity to enhance electron transport within the microcrystal bulk [65].

Copolymers are usually prepared by copolymerizing the two monomers. Different concentration (feed) ratios of the monomers are used to vary the composition of the resulting copolymer (See also Sect. 2.4 and the citations therein.) These efforts have mainly been directed at improving the mechanical properties and processability as well as altering the conductivity and optical and other properties of the polymeric material for special practical purposes.

As an illustrative example, the cyclic voltammograms obtained during the electrochemical copolymerization of aniline (ANI) and o-aminophenol (OAP) are shown in Fig. 4.12 [147].

Fig. 4.12
figure 12

The cyclic voltammograms obtained during the copolymerization of aniline (ANI) and o-aminophenol (OAP) at different concentration ratios: (a) 1 mM OAP + 20 mM ANI; (b) 2 mM OAP + 20 mM ANI; (c) 3 mM OAP + 20 mM ANI; and (d) 4 mM OAP + 20 mM ANI. Supporting electrolyte: 0.5 mol dm−3 H2SO4. Scan rate: 50 mV s−1 (Reproduced from [147] with the permission of Springer-Verlag)

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The oxidation of the hydroxyl group of OAP occurs at 0.7 V, while the oxidations of the amino groups of both monomers occur at ca. 1 V. The cyclic voltammograms are different from those of PANI and POAP at all concentration ratios.

According to Holze [147], the redox pair (E pa = 0.32 V and E pc = 0.28 V) that can be seen in Fig. 4.12a is related to the copolymer, as neither PANI nor POAP shows such voltammetric peaks. The brownish-blue color of the polymer film obtained at a concentration ratio of 1:10 also differs from that of the monopolymers. The color of the films formed at other concentration ratios was yellow. The synthesized poly(aniline-co-o-aminophenol) was found to be electroactive, even at pH 10, and its conductivity was decreased by three orders of magnitude compared to PANI.

Composites have been prepared by rather different methods due to the great variety of inorganic and organic materials used (See also Sect. 2.5 and Chap. 7).

Lamellar nanocomposites consisting of layered inorganic compounds and conducting polymers display novel properties which result from the molecular-level interactions of two dissimilar chemical components. The intercalative polymerization of aniline in an α-RuCl3 host has recently been reported. The insertion of aniline into α-RuCl3 has been executed by soaking the α-RuCl3 crystals in aniline or aniline/acetonitrile solution. It has been proven that polyaniline is formed between the RuCl3 layers, which are composed of hexagonal sheets of Ru atoms sandwiched between two hexagonal sheets of Cl atoms with ABC stacking.

The RuCl3 is a strongly oxidizing host which can take up the electrons from the aniline, leading to the formation of polyaniline (PANI). Simultaneously, a fraction of the Ru3+ atoms are reduced to Ru2+, resulting in a mixed-valence compound. The host material will have a negative charge, and RuCl −3 sites can act as counterions for anilinium cations and charged PANI in the nanocomposite \( ({\hbox{PANI}})_x^{{z + }} \) \( ({\hbox{RuC}}{{\hbox{l}}_3})_y^{{z - }} \). The X-ray diffraction patterns of the samples revealed that the structure of the inorganic host was preserved; however, the separation of the RuCl3 layers increases by Δd = 0.62 nm.

It has been established that the charge transport—which occurs by electron hopping between the ruthenium ions in the mixed-valence compound—is substantially enhanced by the presence of the conductive polymer. The results of the thermopower study indicate a bulk-metal-like conductivity which is controlled by the conductive polymer. (PANI) x (RuCl3) y shows a room temperature conductivity of ca. 1 S cm−1. It was suggested that the combination of the high conductivity of the polyaniline with the wide-ranging catalytic properties of RuCl3 could provide new materials with valuable electrocatalytic properties [148].

Figure 4.13 shows the cyclic voltammograms obtained for RuCl3 and (PANI) x (RuCl3) y samples attached to a gold electrode and studied in the presence of 0.5 mol dm−3 HCl.

Fig. 4.13
figure 13

Cyclic voltammograms obtained at different scan rates, (a) v = 50 mV s−1 and (b) v = 5 mV s−1, for Au–RuCl3 (curve 1) and for Au–(PANI) x (RuCl3) y (curves 2, 3, 4). Electrolyte: 0.5 mol dm−3 HCl (Reproduced from [148] with the permission of Springer-Verlag)

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In these experiments, first pure α-RuCl3 and then (PANI) x (RuCl3) y , prepared by 1-week-long soaking of α-RuCl3 microcrystals in aniline, were immobilized at the gold surface. The nanocomposite was washed with 0.5 mol dm−3 HCl before use. A comparison of the cyclic voltammograms displayed in Fig. 4.13a reveals that the oxidation of Ru2+ to Ru3+ becomes easier since wave II moves in the direction of smaller potentials while the reduction process remains unaltered. This is related to the presence of polyaniline, which conducts in this potential region and probably enhances the charge transfer processes. The waves belonging to the leucoemeraldine (LE) ⇆ emeraldine (E) transition are clearly seen in Fig. 4.13a (waves III and IV). Figure 4.13b shows the cyclic voltammograms obtained at a slow scan rate over the whole potential region and where the redox transformations of RuCl3 play no role (i.e., the response of the PANI can be seen), separately.

The electrochemical activity of PANI decreases with increasing pH, and at pH > 5 (except in the case of self-doped films), no redox response can be observed. Figure 4.14 shows the voltammograms of the nanocomposite in the presence of 0.5 M HCl and 0.5 M NaCl.

Fig. 4.14
figure 14

Cyclic voltammetric curves obtained for a Au–(PANI) x (RuCl3) y electrode in the presence of (1) 0.5 mol dm−3 HCl and (2) 0.5 mol dm−3 NaCl, respectively. Scan rate: 5 mV s−1 (Reproduced from [148] with the permission of Springer-Verlag)

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Although both of the waves belonging to the Ru3+ → Ru2+ and LE → E transitions, respectively, move in the direction of higher potential, it is clearly apparent that the electrochemical activity (see waves III and IV) of PANI was preserved (The sharp pair of waves at low potentials is a typical response of α-RuCl3 in neutral salt solutions).

Another strategy is the sol–gel preparation technique. Nanocomposites of V2O5 xerogel and polypyrrole were prepared from vanadyl tris(isopropyloxide) (VC9H21O4) precursor and pyrrole monomer by in situ oxidative polymerization of the pyrrole in the sol stage by gelation. Unlike other sol–gel nanocomposite synthetic routes, in this case—due to the stability of the solution—a thin homogeneous film could easily be deposited on various substrates. After casting on the given substrate, the system was heated at 100°C for 2 h. X-ray diffraction revealed that the PP chains are intercalated within the interlayer region of the V2O5, leading to an increase in the d-spacing from 1.185 nm for V2O5 to 1.38 nm for the nanocomposite [149]. This nanocomposite shows higher specific capacity, faster Li+ ion diffusion, and higher electronic conductivity than the parent oxide. A detailed literature survey of V2O5 conducting polymer nanocomposites can also be found in [149].

A sandwich-type composite film consisting of PP and CoFe2O4 nanoparticles has been prepared by a three-stage procedure; i.e., electropolymerization of pyrrole, then a second layer was deposited on the graphite–PP electrode by electropolymerization from a solution containing pyrrole and oxide nanoparticles, and finally, a top layer of PP was also created using electropolymerization [150]. This composite electrode exhibits electrocatalytic activity (see Chap. 7) towards oxygen reduction.

The application of combined electrochemical and nonelectrochemical techniques, such as piezoelectric nanogravimetry at EQCN [29, 48, 52, 65, 91, 93, 109, 138, 148, 151–153], radiotracing [26, 27], various spectroscopies [29, 87, 135, 152–157] and microscopies [87, 94, 127, 133], ellipsometry [82, 134, 158], conductivity [100, 143, 151], probe beam deflection [42, 89], and surface plasmon resonance [159], has allowed us to gain very detailed insights into the nature of electropolymerization and deposition processes, and so the production of conducting polymers, polymeric films, and composites with desired properties is now a well-established area of the electrochemical and material sciences.