**2. Synthesis**

#### **2.1 Polypyrrole (PPy) films**

PPy films are obtained through electropolymerization of Py on suitable substrates/working electrodes by using different electrochemical techniques such as cyclic voltammetry (CV), potentiodynamic, galvanostatic, potentiostatic, reversal potential pulsing technique, etc. To carry out the electropolymerization of Py, a three-electrode one-compartment cell (50 or 100 ml capacity) with provision of passing an inert gas (N2/Ar) into the electrolyte before the start of the oxidation and above the surface of the electrolyte during the polymerization process is employed. Saturated calomel electrode (SCE) or Ag/AgCl (saturated KCl) is used as the reference electrode. PPy composites prepared were either in the form of sandwichtype or simple films.

Generally, the electroplymerization is carried out on noble metals or inert materials such as Pt, Au, glassy carbon (GC), or indium tin oxide (ITO). It is because of the fact that the standard oxidation potential of pyrrole is fairly high (Eox = 0.70 V vs. SCE) and so, when scanning the potential the dissolution of most of metal supports takes place prior to the oxidation of monomer is reached and, thus, the electropolymerization reaction is inhibited. However, when we are using supports which are oxidizable, for example, Fe, Zn, Al, etc, as the working electrode, it is necessary to find new electrochemical conditions for slowing the dissolution of the working electrode without preventing electropolymerization. Recently, a new electrosynthesis process of PPy films on oxidizable metals Al and Fe has been reported (Bazzaoui et al. 2005). In this process, the formation of homogeneous and strongly adherent

Polypyrrole Composites: Electrochemical Synthesis, Characterizations and Applications 135

However, such electrode materials are less stable than those in which the substituents are covalently attached to the polymeric skeleton, owing to some exchange of dopant anions

Several methods have been reported on modification of conducting PPy by metal catalysts

1. Composite layers of metal particles embedded in PPy are formed in a two-step process: electropolymerization of Py followed by an electrochemical deposition of the metal from a solution containing its own ions. Electrodeposition of the metal by this method leads to the formation of metal crystallites on the surface of PPy film, while the amount of the metal tends to zero inside the film. Using this technique PPy/metal composites prepared are: PPy/Pd (Mangold et al., 2004; Chen et al., 2006) PPy/Pt (Vork & Barenrecht, 1989; Bouzek et al., 2000; 2001), PPy/Pt or PPy/Pt+Pb (Del Valle et al.,

2. Insertion of colloidal metal particles in the course of electropolymerization of the PPy film. For the purpose, a solution of nanodispersed metal particles is separately prepared by suitable chemical reduction method. To this solution, the polymer monomer is added for the electropolymerization. Both PPy and metal particles are deposited simultaneously. Using this technique PPy/Pt electrocatalysts were obtained by Bose &

3. Insertion of a suitable metal comlex anion as a counterion in the PPy film during the electropolymerization process and subsequent electrochemical reduction of the complex metal anion into the metal. Using this technique PPy/metal composites prepared are: PPy/Pt (Vork &Barenrecht, 1989; Bouzek et al., 2000; 2001) Cu/functionalized PPy (Pournaghi-Azar et al., 1999), PPy/PtClO4-2 (Zhang et al., 2005a), PPy/[Fe(CN)6]3-

Methods, 2 and 3, results a homogeneous distribution of the metal within the whole

4. PPy-metal nanoparticle composites were electro-co-deposited from a solution containing Py, metal salt as source of metal ions, a suitable neutral salt as supporting electrolyte and small amount of a suitable additive to achieve stability of the metal complex salt. For examples, PPy-Au (Rapecki et al., 2010), PPy/Ni (Haseko et al., 2006), PPy/Fe (Chipara et al., 2007), PPy/Co (Ikeda et al., 1983) and Pd/PPy/foam-Ni (Sun et

Sandwich-type composite films of PPy and an oxide (Ox) having composition, PPy/PPy(Ox)/PPy, onto a GC electrode were obtained by a sequential electrodeposition method. For the purpose, two electrolyte solutions (A and B) were used. Solution A contained 0.10M Py and 0.05M K2SO4 and that solution B contained 0.10M Py, 0.05M K2SO4 and 8.33 g L-1 oxide powders. The first layer of PPy (~2.1 μm thick) was obtained onto graphite (G)/GC electrode in solution A by carrying out electrolysis at j (current density) = 5.0 mA cm-2 for 100 s under unstirred condition. After electrolysis, G/PPy electrode was removed from the cell, washed with distilled water, dried in air and then introduced into the cell containing solution B and electrolysis was again carried out at j = 20 mA cm-2 for 200

al., 2010) are some examples of nano composites obtained by this technique.

(Pournaghi-Azar et al., 2000), PPy/[Fe(CN)6]4- (Raoof et al., 2004).

(Juttner et al., 2004). They can be put into the following four main categories:

1998), Pt-based alloy/PPy (Becerik & Kadirgan, 2001).

Rajeshwar (1992) and Del Valle et al. (1998).

with counter-anions of the electrolyte.

**2.3 PPy-metal composite films** 

PPy film.

**2.4 PPy-Oxide composite films** 

PPy films is achieved in only one step using an aqueous medium containing saccharin and Py. Similarly, Bazzaoui et al. (2006b) obtained PPy coating on Fe by the use of an aqueous solution of 0.1M sodium saccharinate and 0.5M Py.

Various techniques have been employed to prepare composite films of PPy with pure metals, oxides (pure as well as mixed valence ones), anions (simple as well as complex ones), etc. It is difficult to to describe all of them here. However, efforts would be made to include as much as possible those techniques which have repeatedly been used in preparation of different composites.

#### **2.2 Functionalized PPy film**

The functionalized PPy films can be obtained in mainly two ways. The first involves the polymerization of a functionalized monomeric molecule. The second technique makes use of the anion exchange properties of the oxidized form of the polymer.

The functionalization of a monomer Py molecule can be carried out by introducing a substituent on the nitrogen atom or on its 3, 4 positions. N-substituted pyrroles are excellent electropolymerizable monomers for the preparation of polymer films containing active centres. A series of Py-substituted pyridine, 2,2'-bipyridine, l,l0-phenanthroline and 2,2':6-2" terpyridine ligands and their corresponding Ru(II), Fe(II), Re(I), Cu(I), Cu(II), Co(II), Ni(II), Rh(III), Mn(III), Pd(II), Zn(II), Ag(I) and Ir(III) complexes have been synthesized. In most cases, films of PPy bearing these complexes can be obtained by anodic electropolymerization of the monomer. There are also two types of Py-substituted metallic tetraphenylporphyrins (M=Ni+, Zn+, Co+, Cu2+, Mn+2); one involving the connection of the Py group directly on the phenyl group, the other through a flexible chain. The efficiency of electropolymerization seems to be lower for complexes in which the Py is connected directly to the macrocycle. With this configuration, steric hindrance and mesomeric effects limit the efficiency of the electrochemical polymerization process (Deronzier & Moutet, 1996). Recently, Diab and Schuhmann (2001) synthesized Mn+2-meso-tetracarboxyphenylporphyrin linked via a spacer chain to a Py unit using electropolymerization method. Abrantes et al. (2000) prepared substituted Mn and Fe metalloporphyrins with two Py groups bonded in lateral chains through the electropolymerization under potentiodynamic condition.

The literature suggests that 3-substituted Py molecules would require a less anodic potential for their oxidation and would be more conductive than the N-substituted PPy. However, Py monomers substituted at the 3-position with metallic complexes are limited in literature; only two examples are cited here, the first one involves ferrocene derivatives, and the second one an entwining phenanthroline Cu(I) complex (Deronzier & Moutet, 1996).

The functionalized PPy films can also be obtained by carrying out electropolymerization of Py in a medium containing an anionic species as the supporting electrolyte. Following this procedure PPy has been doped by several metallic tetraphenylporphyrin, tetrasulphonates or carboxylates (CoII, FeII, MnII) and metallic phthalocyanine, mono and tetrasulphonates (CoII, FeII, CuII). Some other complexes have also been incorporated by this technique. For instance, PPy films containing cobalt salts, thiomolybdate anions, nickel and palladium complexes of maleonitriledithiol, heteropolyanions, a chromium oxalate complex, an iron trisphenanthroline sulphonic acid complex or ferri/ferrocyanide and Prussian blue have been prepared by electropolymerization in a medium containing these anions as supporting electrolyte. All these works has been cited in (Deronzier & Moutet, 1996). It is expected that the polymer frame would be formed around the trapped anion and retains it strongly. However, such electrode materials are less stable than those in which the substituents are covalently attached to the polymeric skeleton, owing to some exchange of dopant anions with counter-anions of the electrolyte.
