conducting polymer-silver composite (review)

Chemical Papers 67 (8) 814848 (2013)DOI: 10.2478/s11696-012-0304-6

REVIEW

Conducting polymersilver composites

Jaroslav Stejskal*

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

Received 10 July 2012; Revised 22 October 2012; Accepted 25 October 2012

Preparations of hybrid composites composed of two conducting components, a conducting polymerand silver, are reviewed. They are produced mainly by the oxidation of aniline or pyrrole withsilver ions. In another approach, polyaniline or polypyrrole are used for the reduction of silverions to metallic silver. Other synthetic approaches are also reviewed. Products of oxidation ofaniline derivatives, including phenylenediamines, are considered. Morphology of both the conductingpolymers and the silver in composites displays a rich variety. Conductivity of the composites seldomexceeds 1000 S cm1 and seems to be controlled by percolation. Interfacial eects are also discussed.Potential applications of hybrid composites are outlined; they are likely to extend especially toconducting inks, printed electronics, noble-metal recovery, antimicrobial materials, catalysts, andsensors.c 2012 Institute of Chemistry, Slovak Academy of Sciences

Keywords: polyaniline, polypyrrole, poly(o-phenylenediamine), poly(p-phenylenediamine), silver,silver nanoparticles, hybrid composites, conductivity

Introduction 815Polyanilinesilver composites 816Oxidation of aniline with silvercompounds 816Physical acceleration 817Chemical acceleration 818Morphology of composites 818

Reduction of silver ions with PANI 820Eect of counter-ions 821Silver complexes 821Morphology of PANI 821PANI lms and coatings 821

Preparation of PANI in the presence ofsilver particles 822Reduction of silver ions with externalreductants in the presence of PANI 823Mixing of PANI and silver particles 823More complex systems 823Two reductants of silver ions 823Two oxidants of aniline 824Ternary composites 824Colloids 824

Polypyrrolesilver composites 824

Oxidation of pyrrole with silvercompounds 824Morphology of composites 825

Reduction of silver ions with PPy 826Preparation of PPy in the presenceof silver particles 827Reduction of silver ions with externalreductants in the presence of PPy 827Mixing of PPy and silver particles 827More complex systems 827Multiple reactants 827Ternary composites 827Colloids 828

Related polymersilver composites 828Poly(p-phenylenediamine) 829Poly(m-phenylenediamine) 829Poly(o-phenylenediamine) 829Substituted polyanilines 830Polymethylanilines 830Poly(o-methoxyaniline) 830Poly(2,5-dimethoxyaniline) 830Poly(4-aminodiphenylamine) 830Sulfonated polyaniline 830

*Corresponding author, e-mail: [email protected]

J. Stejskal/Chemical Papers 67 (8) 814848 (2013) 815

Aniline oligomers 830Other related systems 831

Conductivity 831Conductivity of polyanilinesilvercomposites 832Direct preparation 832Reduction with polyaniline 833Other syntheses 833

Conductivity of polypyrrolesilvercomposites 834Direct preparation 834Reduction with polypyrrole 834

Conductivity of other systems 834

Temperature dependenceof conductivity 835Deprotonation phenomena 835

Applications 835Antimicrobial activity 835Bioapplications 836Catalysis and electrocatalysis 836Conducting composites 836Sensors 837Silver recovery and water-treatment 837Surface-enhanced Raman scattering 837Other uses 837

Conclusions 837References 838

Introduction

Composites of conducting polymers, such as poly-aniline (PANI) and polypyrrole (PPy), and silver con-tain two types of electric conductors. Conductingpolymers are organic semiconductors, silver is a metal-lic conductor. Such composites are called here as hy-brid because they combine dierent electrical featuresof the parent constituents. Both the conducting poly-mers and silver display a variety of morphologies onthe nanoscale. Their nature, distribution within thecomposites, and interfacial phenomena are expectedto control or aect their electrical properties. In addi-tion to electrical features, many potential applicationsrely on chemical, electrochemical, optical, and otherphysical properties.Early studies on polyacetylene have illustrated the

potential of conducting polymers to compete withmetals in the conduction of electric current, and con-ductivities comparable to those of metals have beenreported. Limited stability of polyacetylene under am-bient conditions shifted the research interest to otherconducting polymers. Most of the currently-used con-ducting polymers, such as PANI or PPy, have a typi-cal conductivity of the order of 1 S cm1 (Stejskal &Gilbert, 2002; Blinova et al., 2007b), i.e. at the semi-conductor level, four to ve orders of magnitude lowercompared with metals such as mercury, copper, sil-ver, or gold. Various ways to further enhance the con-ductivity have therefore been sought. Processing ofPANI in m-cresol in the presence of camphorsulfonicacid leads to the conductivity exceeding 1000 S cm1

(Cao et al., 1993; Pron & Rannou, 2002). Polyani-line produced in emulsion polymerization was also re-ported to reach a room-temperature conductivity ex-ceeding 1000 S cm1 (Lee et al., 2006); the follow-upstudies, however, are missing. Mechanical orientationof conducting-polymer chains is another way demon-strating the potential of conducting polymers as elec-tric conductors. Virtually all papers report the con-ductivity determined in the dry state or at ambienthumidity. It has recently been proposed that, when

immersed in acidic aqueous media, the eective con-ductivity of PANI may exceed 1000 S cm1 due tothe contribution of proton conduction (Stejskal et al.,2009a).Preparation of composites is an alternative way to

increase the conductivity of these polymers. For thatreason, organic conducting polymers have been com-bined with inorganic conducting materials, such ascarbon nanotubes, graphene, or graphite (Tchmutinet al., 2003; Konyushenko et al., 2006; Jimnez, et al.,2010; Spitalsky et al., 2010). Noble metals, however,are the most promising candidates for this task. Silverhas received most attention due to its highest conduc-tivity among metals, 5.6 105 S cm1 at room tem-perature, and relatively low cost compared to othernoble metals.Electrical properties of hybrid composites com-

posed of two conducting components are determined:(i) by composition, i.e. by the mutual proportions ofthe organic semiconductor and the metal; (ii) by themorphology of both components; and (iii) by inter-facial phenomena, such as the presence of insulat-ing barriers between both conducting phases (Efros& Shklovski, 1976; Baibarac et al., 1999; Tchmutinet al., 2003). Conducting polymers generally pro-duce globules, nanobres, nanotubes, microspheres,and various hierarchical nanostructures (Sapurina& Stejskal, 2008, 2010; Stejskal et al., 2010; Zu-jovic et al., 2011b). Silver may be present as micro-spheres, nanocubes, nanobres or nanowires, nanorodsor nanocables, and various two- and three-dimensionalobjects (Nadagouda & Varma, 2007; Chang et al.,2011; Chen & Liu, 2011a). It is of interest to notethat the richness of morphologies displayed is an in-herent feature of both the conducting polymers andsilver. The size of objects becomes important when itdiminishes to the submicrometre level and the interfa-cial area increases. Various interfacial phenomena maythen become important.It has to be stressed, however, that the conduc-

tivity of composites need not be the primary goalin the potential applications. Conducting polymers

816 J. Stejskal/Chemical Papers 67 (8) 814848 (2013)

Fig. 1. Oxidation of aniline with silver nitrate in an acidic aqueous medium yields a composite of PANI and silver. Nitric acid isa by-product (Blinova et al., 2009; Bober et al., 2011a).

are stimuli-responsive. They change their electricaland optical properties depending on external impulses,such as a change in acidity, humidity, temperature, thepresence of oxidants or reductants, etc. The respon-sivity of conducting polymers can be improved by thepresence of noble metals and this property may be-come useful in sensors. Conducting polymers manifestthemselves as oxidants or reductants and can thereforeparticipate in redox reactions. Corrosion protectionand its improvement may serve as such an example(Li et al., 2012b) although the cost of such compos-ites would be rather prohibitive. Widely used catalyticproperties of noble metals can be enhanced by the in-teraction with conducting polymers (Sapurina & Ste-jskal, 2009). Optical properties of metal nanoparticlesare well known since the times of Faraday (Kelly etal., 2003; Stamplecoskie & Scaiano, 2011) and may beaected by the interaction with conducting polymers.Synergetic eect of the two components may becomeimportant in pharmacology and medicine.There are four basic strategies for the prepara-

tion of the composites of conducting polymers andsilver: (i) a single-step preparation, where a corre-sponding monomer, typically aniline or pyrrole, is ox-idized with a silver cation to produce a composite ofconducting polymer and silver. This is the most im-portant approach; (ii) a conducting polymer is pre-pared by conventional oxidative polymerization andsubsequently used for the reduction of silver salts tosilver; (iii) in another two-step process, the compo-nents are prepared consecutively. Silver particles areprepared at rst, and a conducting polymer is syn-thesized afterwards in their presence. Alternatively,a conducting polymer is synthesized at rst and sil-ver nanoparticles subsequently with the help of ex-ternal reductants; (iv) nally, both components, theconducting polymer and silver particles, are preparedseparately and simply mixed or blended. Some spe-cial approaches may combine these four basic synthe-sis methods.

Polyanilinesilver composites

Oxidation of aniline with silver compounds

Oxidation of aniline with silver salts, such as silvernitrate, is the most direct way of preparation of PANIsilver composites (Blinova et al., 2009, 2010; Bober etal., 2011a) (Fig. 1). In this way, the reaction betweentwo non-conducting chemicals, aniline and silver ni-trate, yields a mixture of two conductors, PANI andsilver. The standard redox potential (E) of silver ionsin the conversion to metallic silver, Ag+ + e Ag, is0.80 V, which is much lower compared with that of thecommonly used ammonium peroxydisulfate (2.00 V).Nevertheless, this is a suciently high value becauseother oxidants such as iron(III) salt, Fe3+ + e Fe2+ (E = 0.77 V), are also able to oxidize aniline toPANI (Li et al., 2007; ednkov et al., 2011; Sapurina& Stejskal 2012).There are several requirements on the synthesis by

most potential applications: (i) the products shouldhave high conductivity; (ii) they should be macroscop-ically homogenous; (iii) they should not be contami-nated by other components, such as insoluble silversalt or aniline oligomers; (iv) or damaged by side re-actions, such as the nitration of PANI; and (v) theyshould be produced within a reasonable time interval.Simultaneous satisfaction of these conditions is nottrivial.The success of PANI preparation can be best con-

rmed by UV-VIS spectra, when the PANI base is dis-solved in N-methylpyrrolidone (Blinova et al., 2009) ordispersed in dimethylsulfoxide (Li et al., 2012b). Theobservation of the absorption maximum at approxi-mately 630 nm, corresponding to the transitionof quinoneimine rings, is an unambiguous proof thatPANI was produced (Du et al., 2005; Blinova et al.,2009; Karim et al., 2009; Bober et al., 2011b; Li et al.,2012b) (Fig. 2). In its absence (Gao & Xing, 2012), theformation of aniline oligomers is the most likely ex-


Fig. 2. UV-VIS absorption spectra of PANI base prepared bythe oxidation of aniline with silver nitrate (a) or am-monium peroxydisulfate (b) (adapted from Blinova etal., 2009).

planation (Stejskal et al., 2008b; Stejskal & Trchov,2012). Silver nanoparticles often display a separate ab-sorption band at 400500 nm caused by plasmon res-onance (Martins et al., 2006; Dallas et al., 2007), theposition of the absorption band being dependent onthe particle size. Aniline oligomers, however, also ab-sorb in this region (Stejskal & Trchov, 2012).pH of the reaction medium decreases in the course

of aniline oxidation. Two hydrogen atoms are ab-stracted from each aniline molecule that becomes in-corporated into PANI chains and they are released asprotons (Stejskal et al., 2008b). This applies also tothe oxidation with silver nitrate (AgNO3) (Fujii et al.,2010), where nitric acid is a by-product (Blinova et al.,2009, 2010) (Fig. 1). pH of the reaction mixture has tobe below pH 23 for the formation of PANI (Stejskalet al., 2010); otherwise, aniline oligomers are the ex-clusive product of the oxidation (Stejskal & Trchov,2012).In practice, however, the oxidation of aniline with

AgNO3 in strongly acidic media, such as nitric acidsolutions, is slow and it takes months (Blinova et al.,2009) to achieve an appreciable yield. Acidic reactionconditions are satised for the propagation of PANIchains but, obviously, the initiation of chain growthis restricted. Some papers reported the formation ofpolymer already after 8 h but the yield was not spec-ied (Neelgund et al., 2008). Physical or chemical ac-celeration is thus needed, as reviewed below. The ox-idation of aniline with AgNO3 at neutral conditionsyielded a composite of brown aniline oligomers andsilver (Gao & Xing, 2012).The content of silver in the composite as predicted

by the stoichiometry (Fig. 1) is 68.9 mass % of sil-ver (Blinova et al., 2009). This assumption was con-rmed experimentally by thermogravimetric analysis.This method is applicable only if the sample is ho-mogeneous and a few milligrams are representative ofthe whole sample. In case of heterogeneous samplescontaining silver particles visible to the naked eye,

standard determination of ash made on larger sam-ples provides more reliable results. It has to be stressedthat thermogravimetric analysis has to be made in air(Stejskal et al., 2009d) because when performed in ni-trogen atmosphere (Khanna et al., 2005; Afzal et al.,2009; Karim et al., 2009; Wang et al., 2009a; Huang etal., 2009; Grinou et al., 2012; Jia et al., 2012), PANIis carbonized leaving about a 40 mass % residue (Tr-chov et al., 2009; Rozlvkov et al., 2011) in additionto silver. The residue thus does not correspond to thesilver content in the composite and the results of suchthermogravimetric analyses are dicult to interpret.Silver nitrate dissolved in solutions of nitric acid

(Blinova et al., 2009), formic acid (Bober et al.,2010a), or sulfonic acids (Bober et al., 2011a) is typ-ically used for the oxidation of aniline. These acidsdo not precipitate silver ions and they are sucientlystrong to provide the high acidity, i.e. low pH, neededfor the formation of PANI. Silver salts of limited sol-ubility, such as silver acetate (Blinova et al., 2010),silver chloride (Karim et al., 2009), silver hexacyano-ferrate (III) (de Azevedo et al., 2008b; Kanwal et al.,2009), silver nitride (Ag3N) (Jia et al., 2010a, 2010b,2012), or silver vanadate (Mai et al., 2011) have beenused as heterogeneous oxidants. In such cases, insol-uble silver salts always constitute a part or even themajority of the product (Blinova et al., 2010).Interfacial version of the reaction, when aniline is

dissolved in chloroform and silver methanesulfonate inan aqueous medium, has been reported (Huang et al.,2009). The oxidation took place at the interface of theimmiscible phases. Nitrobenzene has also been used asthe organic phase (Gniadek et al., 2010a). A similarexperiment has been demonstrated for PPy (Gniadeket al., 2010b).An ionic liquid, a 1-methyl-3-butyl-imidazolium

derivative, has recently been used as the reactionmedium for the oxidation of aniline with a silver saltof this ionic liquid (Correa et al., 2012). A green pre-cipitate of PANIsilver composite was produced in afew minutes and the composite had the conductivityof 100 S cm1.The reaction between the aniline salt and silver

nitrate can proceed in the solid state, even in the ab-sence of solvents, by mechanical blending of the reac-tants (ednkov et al., 2011). The product, however,was composed mainly of aniline oligomers. In anothertype of the preparation performed in the solid state,in frozen reaction mixtures at 24C (Bober et al.,2011b), a PANIsilver composite was obtained in highyield, 74 mass %. The conductivity of such a prod-uct exceeded 1000 S cm1. Acceleration with a smallamount of p-phenylenediamine, which is discussed be-low, was needed.

Physical acceleration

The slow oxidation of aniline can be accelerated


Fig. 3. Typical morphologies of PANIsilver composites observed by transmission electron microscopy (Blinova et al., 2010).

by either physical or chemical means. It seems thatacceleration is associated with the creation of new ini-tiation centres of the chain growth rather than by theincrease in the rate of chain propagation. The formerway, a physical acceleration, is represented by irradi-ation with -rays (Pillalamarri et al., 2005; Kang etal., 2006; de Barros & de Azevedo, 2008; Karim et al.,2009; Huang et al., 2010; Kim et al., 2011, 2012) orUV-light (Khanna et al., 2005; de Barros, et al., 2005,2010; Li et al., 2007, 2009a, 2009b, 2012a, 2012b; deBarros & de Azevedo, 2008; de Azevedo et al., 2008a;Singh et al., 2011; Shukla et al., 2012). Irradiation withUV-light (de Barros et al., 2005) was also used to en-hance the oxidation of aniline with silver nitrate afterprinting both reactant solutions on paper to developthe conducting pattern. Daylight alone had no accel-eration eect on the oxidation of aniline with silvernitrate (Blinova et al., 2009).Temperature elevated to 100C was also used to in-

crease the rate of aniline oxidation (Han et al., 2012).The oxidation of aniline with silver nitrate in ethanoland in the absence of acid carried out in an auto-clave at 250C yielded micrometre-sized PANI micro-spheres (Du et al., 2005) decorated by silver nanopar-ticles. There is always a danger of polymer degrada-tion under such conditions but the emeraldine struc-ture of PANI (Stejskal et al., 1996b) was conrmedby UV-VIS spectra. A similar oxidation of aniline at90C (Yang et al., 2011), which took place under alka-line conditions, produced only silver nanoparticles ac-companied by aniline oligomers. Ultrasonic radiationwas also used to accelerate the oxidation of anilinewith silver nitrate (Li & Wang, 2010; de Barros & deAzevedo, 2010) but the associated increase in temper-ature might have contributed to the acceleration.

Chemical acceleration

The oxidation of aniline with ammonium peroxy-

disulfate is accelerated by the addition of smallamounts of p-phenylenediamine, typically a few per-cent relative to aniline (Stejskal et al., 1995; Tranet al., 2008; Shenashen et al., 2010, 2011; Zu-jovic et al., 2011b). Similar accelerating eect ofp-phenylenediamine has been observed when silver ni-trate was used as an oxidant (Bober et al., 2010b,2011b) and the practical and feasible way of the prepa-ration of PANIsilver composites became available.Also, small amounts of ammonium peroxydisulfate

eciently accelerate the oxidation of aniline with sil-ver nitrate (Karim et al., 2009; Bober et al., 2011a).The introduction of another oxidant, hydrogen perox-ide, had a similar eect (Li & Wang, 2010). The ad-dition of a small amount of the second oxidant alongwith silver nitrate is thus another way to promote theoxidation of aniline (Bober et al., 2011a).

Morphology of composites

The resulting composites are produced as powders.Concerning their morphology (Fig. 3), PANI is ob-tained as globules (Bedre et al., 2009; Li et al., 2009b;Blinova et al., 2010), nanobres (Pillalamarri et al.,2005; Li et al., 2007; de Barros & de Azevedo, 2008;de Azevedo et al., 2008a; Nadagouda & Varma, 2008;Zhou et al., 2009; Huang et al., 2009, 2010; Tamboliet al., 2012; Jia et al., 2012), nanotubes (Gao et al.,2009; Blinova et al., 2009, 2010; Yin & Yang, 2012),fused nanospheres (Karim et al., 2007), nanobrushes(Blinova et al., 2009; Bober et al., 2011c) (Fig. 4), orsheets (Blinova et al., 2010) depending on the con-centration of the reactants and the type of acid usedin the reaction medium and other reaction conditions.Transmission electron microscopy is better suited forthe morphology assessment because of the higher con-trast between the polymer phase and silver (Fig. 4). Amarble-like texture in the objects is a typical featureof composites (Blinova et al., 2010; Routh et al., 2010;


Fig. 4. Scanning (a) and transmission electron (b) micrographs of a PANIsilver composite (Blinova et al., 2009).

Li et al., 2012b) (Fig. 3b). This pattern seems to beassociated with PANI because it was observed also inthe absence of silver (Dispenza et al., 2012).Like PANI, silver also appears in the composites in

various morphologies (Sharma & Imae, 2009). Silvernanoparticles having a typical size of tens of nanome-tres are the most common (Kang et al., 2006; Li etal., 2007, 2009b, 2012b; Karim et al., 2007; Gao et al.,2009; Blinova et al., 2009; Zhou et al., 2009; Bedre etal., 2009; Afzal & Akhtar, 2010; Hosseini & Momeni,2010; Garai et al., 2010; Bober et al., 2011c; Correaet al., 2012; Jia et al., 2012) (Figs. 3b and 4). Othermorphologies are represented by dendrites (ednkovet al., 2009; Kovachuk et al., 2010) (Fig. 5), triangu-lar micrometre-sized plates (Fujii et al., 2010), fusedtriangles (Zhang et al., 2011a), hexagons (Kang etal., 2006), nanorods (Chen et al., 2006; Bober et al.,2010a; Yin & Yang, 2012) (Fig. 6), microrods (deBarros & de Azevedo, 2010), nanowires and nano-bres (Bober et al., 2010a; Xu et al., 2010c, 2010d),nanosnakes (Munoz-Rojas et al., 2008a, 2011; Yin &Yang, 2012), and nanobelts (Xia, 2011). The presenceof whiskers and lamellar morphologies has also beenreported (Gniadek et al., 2010a). Several types of mor-phologies are usually present simultaneously (Blinovaet al., 2009; Fujii et al., 2010) (Fig. 3a). Macroscopicsilver particles are often visible by the naked eye.Conducting polymers often produce a coating of

silver objects, i.e. coreshell morphology (Feng et al.,2007a; Munoz-Rojas et al., 2008a, 2011; Bober et al.,2010a; Fujii et al., 2010; Li et al., 2012b), or silverparticles decorate conducting polymer objects (Gaoet al., 2009; Blinova et al., 2009; Li et al., 2012a), orboth components form separate phases.Occasionally, the size of silver nanoparticles is be-

low 10 nm (Li et al., 2012a). An oxidation of anilinewith silver nitrate under alkaline conditions at 90Cled to silver nanoparticles of 9 nm size (Yang et al.,2011). Aniline oligomers have probably been producedsimultaneously under such conditions (Stejskal & Tr-

Fig. 5. Silver dendrites.

chov, 2012), PANI was not detected, as expected.Small silver nanocrystals of 5 nm in size have beenproduced by chronopotentiometric preparation in amicroemulsion (Zhou et al., 2009) and in other cases(Li et al., 2009b). Morphology of both the conductingpolymer and silver may be aected, rather than con-trolled, by the addition of surfactants (Thanjam et al.,2011) or organic acids to the reaction mixture.The oxidation of aniline with ammonium peroxy-

disulfate in the presence of water-soluble polymers,such as poly(vinyl alcohol) or poly(N-vinylpyrrolid-one), yields colloidal PANI dispersions (Stejskal et al.,1996a; Stejskal, 2001; Dispenza et al., 2012). Anilinewas similarly oxidized with silver nitrate in the pres-ence of poly(vinyl alcohol). The formation of PANIwas claimed (Fujii et al., 2010) but not proved be-yond doubt because the green color, typical of thePANI salt, has not been mentioned. Stable colloidal


Fig. 6. Silver nanorods produced by the oxidation of aniline with silver nitrate in the solutions of formic acid. Two magnications,scale-bar 500 nm (a) and 200 nm (b), obtained by transmission electron microscopy.

Fig. 7. Polyaniline (emeraldine nitrate) is able to reduce silver nitrate to metallic silver. It is oxidized from the emeraldine to thepernigraniline oxidation state at the same time (Stejskal et al., 2008c, 2009c).

dispersions having silver core and organic shell wereproduced but the particles were accompanied by largesilver crystals.

Reduction of silver ions with PANI

The use of PANI as the reductant of silver ions tometallic silver is the second method for the prepa-ration of hybrid composites. Early studies on elec-trochemically prepared PANI lms demonstrated theability of PANI to reduce silver ions (Zhang et al.,1995). This is called electroless deposition by elec-trochemists (Ivanov & Tsakova, 2005; Ocypa et al.,2006; Dimeska et al., 2006; Tsakova, 2008; Wang etal., 2009a), in contrast to the electrodriven process atelectrodes. The emeraldine form of PANI converts topernigraniline at the same time (Stejskal et al., 1996b,

2008b; He et al., 2012b; Patil et al., 2012) (Fig. 7).Polyaniline can enter the reaction in the protonatedstate (Zhang et al., 1995; Wang et al., 2007; Stejskalet al., 2008c, 2009c; Sim et al., 2009; Ghorbani et al.,2011; Yan et al., 2012; Qin et al., 2012), i.e. as a PANIsalt in acidic or neutral media, or as a PANI base un-der neutral or alkaline conditions (Wang et al., 2007;Stejskal et al., 2009c; Bouazza et al., 2009; Ayad etal., 2010; Lee et al., 2012). PANI was exceptionallyreduced with hydrazine to leucoemeraldine before thereaction with silver ions (He et al., 2012b).Nitric acid is a by-product of this reaction (Fig. 7).

This means that pH drops which can be used to moni-tor the progress of the reaction (Stejskal et al., 2009d)(Fig. 8). pH of the reaction medium is very important(Lyutov & Tsakova, 2011). The increase in the massof the PANI lm due to deposited silver in quartz mi-


Fig. 8. Decrease in pH during the reaction of PANI base withsilver nitrate (Stejskal et al., 2009d).

crobalance was also used for this purpose (Ayad etal., 2010). Progress or the reaction was also followedby potentiometric determination of the silver-ion con-centration (Bouazza et al., 2009). The reaction hasusually been completed in minutes (He et al., 2012a)or tens of minutes (Stejskal et al., 2009d).

Eect of counter-ions

Reduction of silver ions takes place in aqueoussolutions. The reduction has often been performedwith PANI base suspended in water (Bouazza et al.,2009; Ayad et al., 2010; Trchov & Stejskal, 2010)or in solutions of nitric acid (Stejskal et al., 2009c,2009d; Trchov & Stejskal, 2010; Ghorbani et al.,2011; Yan et al., 2012) to avoid the precipitation ofsilver cations. Also, silver ions do not precipitate insolutions of perchloric acid and sulfonic acids, suchas methanesulfonic, camphorsulfonic, or toluenesul-fonic acid. For that reason, sulfonate anions belongto the preferred counter-ions in the studies associatedwith hybrid composites (Sim et al., 2009; Bober et al.,2011b, 2011c; Yan et al., 2012). The reaction can pro-ceed even in the absence of solvents, in the solid state,by mechanical blending of reactants (ednkov et al.,2009).The role of the counter-ions in PANI salt enter-

ing the reaction has also been analyzed in literature(Zhang et al., 1995; Dimeska et al., 2006; Stejskal etal., 2009d; Xu et al., 2010a; Yan et al., 2012). Counter-ions in the PANI salt often produce insoluble saltswith silver ions. These are especially inorganic anions,such as chloride, iodide, sulfate, or phosphate (Stejskalet al., 2009d). Insoluble silver salts are also producedby most carboxylic acids, such as acetic (Blinova etal., 2010) or tartaric acid (Stejskal et al., 2009b). Insuch cases, the formation of insoluble silver-salt inter-mediates may aect the reaction. The use of reducingcounter-ions, such as citrate or formate, along withPANI makes the system even more complex (Xu etal., 2010a; Bober et al., 2010a).

The reaction between PANI and silver salts hasrarely been performed in solutions because of thelimited solubility of PANI in organic solvents. Onlyin a single case has the reduction been made in anon-aqueous medium, dimethylformamide (Sun et al.,2012). The electrospinning of mixed solutions of theemeraldine base in N-methylpyrrolidone and aque-ous solution of poly(vinyl alcohol) and silver nitrateyielded composite nanobres (Shahi et al., 2011).

Silver complexes

When ammonia solution is used as an alkaline re-action medium, it has to be kept in mind that a silverion forms a diamosilver complex cation, Ag(NH3)

+2

(Yao et al., 2009; Lee et al., 2012), which has a dif-ferent redox potential than the simple silver cation,Ag+. A complex anion is produced also with thiosul-fate, Ag(S2O3)

32 , or ethylenediaminetetraacetic acid

(EDTA), Ag(EDTA)3 (Ivanov & Tsakova, 2005), orexcess chlorides, AgCl2 (Vorotyntsev et al., 2011).The list can be substantially extended.

Morphology of PANI

The reaction between PANI powders and silverions has been used for the preparation of PANIsilver composites (Bouazza et al., 2009; Leyva et al.,2011; Manivel & Anandan, 2011; Ghorbani et al.,2011; Chang et al., 2012a). Polyaniline in the leu-coemeraldine oxidation state (Bouazza et al., 2009)converts to emeraldine, and the emeraldine form ofaniline is oxidized to pernigraniline at the same time(Stejskal et al., 1996b) (Fig. 7). The reaction wascompleted in tens of minutes (Bouazza et al., 2009).Polyaniline enters the reaction as globules (Stejskal etal., 2009b, 2009d; Ghorbani et al., 2011), nanobres(Chang et al., 2012a), or nanotubes (Stejskal et al.,2009b, 2009c). Silver is produced as globular nanopar-ticles of 2050 nm in size (Bouazza et al., 2009; Ste-jskal et al., 2009b; Ghorbani et al., 2011; Chang et al.,2012a) but it is often accompanied by micrometre-sized objects (Ivanov & Tsakova, 2005; Bouazza etal., 2009; Stejskal et al., 2009c). The particles weresmaller at low silver ion concentrations or when stir-ring or sonication was used (Bouazza et al., 2009).Silver has been also been occasionally produced in-side PANI nanotubes (Stejskal et al., 2009c; Trchov& Stejskal, 2010; Sun et al., 2012) (Fig. 9). This wasexplained by the dierent molecular structure of thenanotube interior (Stejskal et al., 2006), which is richin phenazine units, compared with the external sur-face.

PANI lms and coatings

Polyaniline often entered the reaction as thin lmsdeposited chemically or electrochemically on suitable


Fig. 9. When silver nitrate is reduced by PANI nanotubes, sil-ver is often found inside the nanotubes (Stejskal et al.,2009c).

Fig. 10. Silver nanoparticles deposited on PANI-coated cellu-lose bres (Stejskal et al., 2008c).

substrates (Zhang et al., 1995;Wang et al., 2007; Kellyet al., 2007; Shin & Kim, 2009; Trchov et al., 2012)or cast from solutions in N-methylpyrrolidone (Xu etal., 2010a, 2010b, 2010c, 2010d; Yan et al., 2012).PANI deposited in situ during the oxidation of ani-line on various fabrics, such as silk, polyamide, lycra,cotton, linen, acrylic, or polyester was tested with re-spect to silver recovery (Dimeska et al., 2006). PANI-coated cellulose bres have been used for the deposi-tion of silver nanoparticles from a silver nitrate solu-tion (Kelly et al., 2007; Stejskal et al., 2008c) (Fig. 10),as well as PANIlignin composite (He et al., 2012b)or PANI-coated poly(methyl methacrylate) micropar-ticles (Lee et al., 2012). Finally, PANI-coated carbonnanotubes have been used for the reduction of silvernitrate (Gronou et al., 2012).Silver nitrate has been usually used; silver dode-

canoate solution in xylene was an alternative (Shin &Kim, 2009). The morphology of silver was aected bythe acid which constituted a salt with PANI (Wang et

al., 2007; Xu et al., 2010a; Yan et al., 2012; He et al.,2012a, 2012b). Silver particles produced irregular frag-ments, globules (Shin & Kim, 2009; Xu et al., 2010a),wires (Xu et al., 2010a), hierarchical spheres (Xu etal., 2010c, 2010d), sheets (He et al., 2012a), or owers(Song et al., 2007; Xu et al., 2010b; Yan et al., 2012;He et al., 2012a) of micrometre size. The depositionof silver was often non-uniform (Xu et al., 2010a) andconcentrated on the most protruding polymer struc-tures (Lyutov & Tsakova, 2011). The non-uniformityin the size of silver objects was dependent on the pH ofthe reaction medium. The lms cast from PANI pro-tonated with citric acid, which alone is able to reducesilver ions, led to silver nanowires (Xu et al., 2010a),in contrast to other counter-ions.In producing more complex composites, the PANI

base was exposed to silver nitrate in 1 M nitric acid(Leyva et al., 2011). The resulting composite wasthen dispersed in epoxy resin. Ternary PANIsilverpoly(vinyl alcohol) composite nanobres have beenprepared by electrospinning (Shahi et al., 2011).

Preparation of PANI in the presence of silverparticles

The synthesis of PANI in a medium containing sil-ver nanoparticles is another way to prepare PANIsilver composites. Silver nanoparticles have been pre-pared at rst by a variety of methods based on thereduction of silver salts dissolved in organic or in-organic media with organic reductants, e.g., citrates,glucose, hydrazine, etc., or inorganic reductants, suchas sodium borohydride, or electrochemically. It hasto be stressed that both the monomers, such as ani-line, pyrrole, and their substituted derivatives, and thecorresponding polymers are also organic reductants ofsilver ions. These cases, however, are discussed sepa-rately.Aniline was oxidized with ammonium peroxydisul-

fate in the presence of silver nanoparticles (Choud-hury, 2009; Crespilho et al., 2009; Fuke et al., 2009a,2009b, 2010; Gupta et al., 2010; Alam et al., 2012),or silver colloids stabilized with dodecylbenzenesul-fonic acid (Yin & Yang, 2012), mercaptocarboxylicacid (Jing et al., 2007a), or poly(N-vinylpyrrolidone)(Kang et al., 2006). In the last two cases, the absorp-tion band located above 500 nm has not been ob-served in the visible spectra. This means that, due tothe relatively low acidity of the medium, only anilineoligomers have probably been produced. Aniline dis-solved in toluene containing dodecanethiol-stabilizedsilver nanoparticles was mixed with an aqueous so-lution of ammonium peroxydisulfate and compositenanoparticles were produced (Oliveira et al., 2004,2006). This approach is somewhat similar to a two-phase process reported later (Dawn et al., 2007). Sil-ver nanoparticles have also been combined with 4-thioaniline which was subsequently polymerized with


ammonium peroxydisulfate to a PANI-like derivative(Kovachuk et al., 2010).In another experiment, a polyurethane lm was

immersed in a solution containing silver nanoparti-cles and aniline, which was subsequently converted toPANI by the oxidation with peroxydisulfate (Prab-hakar et al., 2011). Similarly, graphite decorated withsilver nanoparticles was coated with a PANI lm dur-ing the oxidation of aniline hydrochloride with ammo-nium peroxydisulfate (Wu et al., 2010, 2012). Multi-wall carbon nanotubes were similarly decorated withsilver nanoparticles at rst and then coated with aPANI overlayer during the in-situ oxidation of aniline(Kim & Park, 2011; Nguyen & Shim, 2011). This is awell-established method for the surface modication ofcarbon nanotubes with a coaxial conducting-polymercoating (Konyushenko et al., 2006, 2008a).

Reduction of silver ions with external reduc-tants in the presence of PANI

The preparation of silver nanoparticles using clas-sical reductants in the presence of PANI has to bementioned for the sake of completeness. The fact thatPANI is the second and competitive reductant in thesystem makes the overall chemistry rather complex.Fly-ash microspheres were coated with PANI in

situ during the oxidation of aniline hydrochloridewith ammonium peroxydisulfate and subsequently im-mersed in an alkaline solution of silver nitrate andsodium tartarate (Wang et al., 2009a). Tartarate is ex-pected to reduce silver cations to metallic silver thatdeposits on PANI. Similarly, PANI-coated carbon nan-otubes were immersed in a medium containing silvernitrate and sodium citrate (Huang et al., 2009).It should be noted that the presence of aniline can

aect the morphology of silver nanoparticles and pro-mote the formation of nanorods (Tan et al., 2003).This was the case when a PANI salt was mixed withsilver nitrate in solutions of formic acid (Bober et al.,2010b; Garai et al., 2010) (Fig. 6).The role of the external reductant can be taken

over by the PANI electrode. The electrochemicalpreparation of PANI lms followed by the electrode-position of silver nanoparticles from silver nitrate so-lutions (Hosseini & Momeni, 2010) belongs to this cat-egory. A similar approach has also been used in caseof PPy (Vorotyntsev et al., 2011).

Mixing of PANI and silver particles

The most straightforward method of compositepreparation is based on simple mixing of components,PANI and silver (Afzal et al., 2009; Sezer et al., 2009;Afzal & Akhtar, 2012). Silver nanoparticles have beenadded to a solution of emeraldine base dissolved inN-methylpyrrolidone. Composite lms were also ob-tained after evaporation of the solvent (Afzal et al.,

2009) or by other deposition techniques (Sezer et al.,2009), and their dielectric (Afzal et al., 2009) or non-linear optical (Sezer et al., 2009) properties were an-alyzed. The mixtures of PANI camphorsulfonate withsilver powder were tested in electromagnetic interfer-ence shielding (Lee et al., 1999; Joo & Lee, 2000).Inkjet printing of silver nanoparticles and PANI in or-ganic solvents also belongs, formally, to this category(Ihalainen et al., 2012). The case when PANI was pre-pared by the oxidation of aniline with ammonium per-oxydisulfate followed by an addition of silver nanopar-ticles (Afzal & Akhtar, 2011; Barkade et al., 2011), canalso be regarded as a simple blending of both compo-nents. In a less typical case, a lm from the PANI basesolution in N-methylpyrrolidone was cast on a roughsilver surface. Interfacial interactions of both compo-nents were investigated by surface-enhanced Ramanspectroscopy (Baibarac et al., 1999). A more com-plex composite of silver-coated carbon nanobres andpolypropylenePANI laments has also been reported(Nasher et al., 2011).Electrochemical preparations of PANI lms fol-

lowed by electrochemical deposition of silver nanopar-ticles (Tsakova, 2008), vacuum deposition (Krishna etal., 2008), or simple adsorption of silver nanoparticles(Narang et al., 2012) belong to this section. A reverseprocess, when silver nanoparticles were deposited onsubstrate at rst and PANI subsequently, was also re-ported (Yi & Song, 2012).

More complex systems

Two reductants of silver ions

In electrochemical preparation, aniline and silverions in a nitric acid solution were electrochemicallytreated to produce nanobrillar PANI lm and sil-ver nanoparticles (Zhou et al., 2006). Chemistry ofthe process which combines two types of oxidation,chemical and electrochemical, may be quite com-plex. In a similar electrochemical preparation usingporous aluminium oxide template, composite PANIsilver nanobres were obtained (Drury et al., 2007).It should be realized that PANI or PPy produced

during the oxidation of aniline are also able to reducesilver ions to metallic silver. From this point of view,two silver reductants, a monomer and a polymer, arealways present in the reaction mixture. The similar re-duction ability of the reaction intermediates also con-tributes to the complexity of the system.A simple redox reaction between PANI and the

silver salt may become complicated when two reduc-tants of silver ions are present. This is illustrated forPANI citrate which was used for the reduction of sil-ver nitrate (Mack et al., 2011). Silver was produced asmicrometre-sized spheres composed of nanobres ornanosheets. The complexity of this system consists inthe fact that both components constituting the PANI


salt, a PANI base (Ayad et al., 2010; Trchov & Ste-jskal, 2010) and citric acid (Xu et al., 2010a), are inde-pendently able to reduce silver ions. The same situa-tion applies to PANI tartarate (Stejskal et al., 2009d).

Two oxidants of aniline

Another complex system is illustrated by the si-multaneous use of two aniline oxidants, typically am-monium peroxydisulfate and silver nitrate (Pillala-marri et al., 2005; Gao et al., 2009; Karim et al., 2009;Bedre et al., 2009; Bober et al., 2011a; Tamboli et al.,2012; Li et al., 2012a). It has been reported that thepresence of ammonium peroxydisulfate, even in smallamounts, accelerates the oxidation of aniline with sil-ver nitrate (Bober et al., 2011a). The addition of am-monium peroxydisulfate is thus an ecient way of ac-celerating the oxidation of aniline with silver nitrate.In addition, by varying the proportions between thetwo oxidants, the silver content in the composite canbe varied (Bober et al., 2011a). Interfacial modica-tion of the synthesis using a chloroformwater systemhas been described (Bedre et al., 2009). Oxidation ofaniline with hydrogen peroxide in combination withsilver nitrate was also reported (Li & Wang, 2010).An even more complex system is represented by thesimultaneous use of two reductants, aniline and cit-ric acid and two oxidants, ammonium peroxydisulfateand silver nitride (Ijeri et al., 2010; Jia et al., 2012).

Ternary composites

Ternary composites composed of three componentsconstitute a rather special group. In addition to a con-ducting polymer and silver they also contain: (i) an-other conducting component, such as graphite (Wuet al., 2010, 2012; Xu et al., 2010b, 2010d), carbonnanobres (Nesher et al., 2011), or carbon nanotubes(Reddy et al., 2009; Xu et al., 2010d; Nguyen & Shim,2011; Narang et al., 2012; Grinou et al., 2012); (ii) anon-conducting inorganic ller, such as silica (Kim etal., 2010b, 2011, 2012; Han et al., 2012) or silver vana-date (Mai et al., 2011); (iii) an organic component,such as deoxyribonucleic acid (Dawn & Nandi, 2006),ribonucleic acid (Route et al., 2010); (iv) polymersmeant to improve mechanical and processing proper-ties, such as cellulose (Kelly et al., 2007; Stejskal et al.,2008c), poly(vinyl chloride) (Afzal & Akhtar, 2010),poly(vinyl alcohol) (Fujii et al., 2010), or epoxy resin(Leyva et al., 2011). Especially the rst group is im-portant. The system complexity, however, increaseswith the number of components and their contribu-tion to the composite properties is dicult to assess.From the scientic point of view, multicomponent sys-tems should be avoided unless there is a concrete rea-son for their study or application. A special group ofternary composites is represented by colloidal disper-sions.

Colloids

Conducting polymer colloids have been preparedby oxidation of the respective monomers in the pres-ence of suitable water-soluble polymers which act asstabilizers. Probably the rst record can be tracedto 1935, when aniline was oxidized to PANI in thepresence of gelatin (Ptschelin, 1935). Preparations ofcolloidal PANI and PPy dispersions have been re-viewed (Stejskal, 2001) and many interesting novelapproaches have appeared in recent literature (Fujiiet al., 2010; Shi et al., 2010; Chen & Liu, 2011b; Auet al., 2011; Dispenza et al., 2012). The preparation ofnoble-metal colloids is much older and dates to syn-theses demonstrated by Faraday in 1847.Polyanilinesilver colloids are potentially useful as

conducting inks. Reports on their preparation, how-ever, are limited so far. In principle, they can be ob-tained by the oxidation of aniline with silver nitrate inthe presence of suitable water-soluble polymers. Intro-ductory experiments performed in acidic media, how-ever, have always led to colloidally unstable systems.The preparation of colloidal PANI particles of the

average size of 190 nm has been reported in systemswhere the acidity drifted from pH 6.4 to 5.0 during theoxidation of aniline (Fujii et al., 2010). The pH-driftconrms that aniline was oxidized, but rather to ani-line oligomers than to PANI at such a low acidity level;successful formation of PANI requires a much higheracidity, pH lower than 2.5 (Stejskal et al., 2008b; Sa-purina & Stejskal, 2008; Konyushenko et al., 2010).An alternative way, the reduction of silver salts

with separately prepared PANI colloidal dispersions,has been reported in a single paper (Li et al., 2006).Colloidal PANI nanoparticles stabilized with poly-acrylic acid and of the size of 100150 nm were pre-pared. Silver was produced as separate nanosheets incase of non-protonated PANI-base colloids and insidethe particles for the PANI salt.

Polypyrrolesilver composites

Oxidation of pyrrole with silver compounds

Chemistry of aniline and pyrrole oxidations to thecorresponding conducting polymers is similar (Blinovaet al., 2007b) (Fig. 11) but dierent stoichiometry hasalso been proposed (Chang et al., 2012b). In contrastto aniline oxidation, pyrrole is oxidized easily to PPyin an aqueous medium and no physical or chemical ac-celeration is needed (Chen et al., 2005a, 2006; Xing &Zhao, 2007; Dallas et al., 2007; Feng et al., 2008; Chat-terjee et al., 2011; Zhao & Nan, 2012a, 2012b; FirozBabu et al., 2012). An interesting modication of thisexperiment is represented by the counter-diusion ofsilver nitrate and pyrrole solutions through a polyte-trauoroethylene membrane (Shi et al., 2012); such anapproach using ammonium peroxydisulfate and ani-


Fig. 11. Oxidation of pyrrole with silver nitrate in an acidic aqueous medium yields a composite of polypyrrole and silver. Nitricacid is a by-product.

line hydrochloride solution has been reported earlier(Blinova et al., 2007a).Nevertheless, UV-irradiation has often been used

(Feng et al., 2007b; Zhao et al., 2007; Martins et al.,2006; Ijeri et al., 2010; Wei et al., 2010a) to increasethe reaction rate. An increase of the reaction temper-ature to 95 C was also used (Chang et al., 2012b).The catalytic role of silver in the synthesis was con-sidered (Della Pina et al., 2011; Kate et al., 2011). Thereaction exceptionally took place at elevated temper-atures (Kate et al., 2011), approximately 100C, orin an autoclave at 150C (Wang & Shi, 2007). Theoxidation of pyrrole has usually been carried out inaqueous media, exceptionally in methanol (Martins etal., 2006) or dimethylformamide (Zhao et al., 2007).Chemical acceleration with sulfonated porphyrin wasalso reported (Wei & Lu, 2009).When the oxidation of pyrrole with silver nitrate

takes place in the presence of silver nanorods, these be-come coated with a PPy overlayer (Chen et al., 2006).This trend applies generally and various silver objectsproduced during the reaction have been coated in situwith an overlayer of PPy.Interfacial polymerization of pyrrole has also been

demonstrated. Pyrrole was dissolved in chloroformand brought into contact with an aqueous solution ofsilver nitrate (Dallas et al., 2007). Tetrachloromethane(Feng et al., 2007a) or an ionic liquid, 1-butyl-3-methylimidazolium tetrauoroborate (Wei et al.,2010a) or hexauorophosphate (Zhou et al., 2009),have similarly been used as the organic phase immis-cible with water.In addition to AgNO3, silver nitrite (AgNO2) is

another silver salt used for the oxidation of pyrrole(Chen et al., 2005a). Although the well-soluble silvernitrate has been the oxidant of the choice, the use ofinsoluble silver oxide (Ag2O) was also successful inproducing PPysilver nanocomposites (Munoz-Rojaset al., 2008a, 2008b, 2011). The reaction has been car-

ried out at elevated temperatures, typically at 120C.The silver salt of methyl orange reacted with pyrroleto PPy nanotubes decorated with silver nanoparticleson the surface (Feng et al., 2007b).In more complex experiments, a polyacrylonitrile

solution containing silver nitrate was electrospun atrst and the resulting nanobres were exposed to pyr-role in boiling toluene. Polypyrrole nanobres werethus decorated with silver nanoparticles of 50150 nmin size (Chen et al., 2008). In another approach, silicaspheres were penetrated with a diamosilver complex,and subsequently exposed to pyrrole which producedPPy (Yao et al., 2009).Morphology of the composites was aected by their

preparation in the presence of cationic surfactants,such as alkyltrimethylammonium bromides (Dallas etal., 2007; Chatterjee et al., 2011), an anionic surfac-tant, sodium dodecylsulfate (Dallas et al., 2007; Mah-moudian et al., 2012), or their mixtures (Xing & Zhao,2007). In case of PPy, surfactants play an importantrole because the conductivity of the composites wassubstantially enhanced when the preparation was car-ried out in their presence (Omastov et al., 2003; Ste-jskal et al., 2003).The oxidation of pyrrole with silver nitrate was

typically carried out in neutral or acidic media. PPysilver composites were claimed to be produced alsounder strongly alkaline conditions (Li et al., 2009d),and the obtained composites were conducting. Un-der such conditions, however, the oxidation of anilineyields only aniline oligomers, not a polymer (Stejskal& Trchov, 2012). It still has to be checked if the sit-uation is dierent for pyrrole oxidation.

Morphology of composites

Concerning the composite morphology, PPy wasproduced as fused globules (Dallas et al., 2007), hol-low spheres (Gniadek et al., 2010b), or leaf-like ob-


Fig. 12. Polypyrrole reduces silver nitrate to metallic silver.

jects (Gniadek et al., 2010b). Silver was generated asglobular nanoparticles having the diameter of severalnanometres (Kate et al., 2011) but more often tens ofnanometres (Chen et al., 2005b, 2008; Dallas et al.,2007; Xing & Zhao, 2007; Feng et al., 2007a; Wang &Shi, 2007), small and large crystals (Gniadek et al.,2010b), nanorods (Feng et al., 2007a, Wei & Lu, 2009;Gniadek et al., 2010b; Chatterjee et al., 2011), nanoca-bles (Chen et al., 2005b), nanosheets (Chen et al.,2005b), nanosnakes (Munoz-Rojas et al., 2008b), ortriangular objects (Chen et al., 2005b; Munoz-Rojaset al., 2008b).

Reduction of silver ions with PPy

The ability of PPy to reduce silver ions to metal-lic silver has been experimentally proved many times.From the historical point of view, the paper by Pickupet al. (1998) is quite important. The authors illus-trated the ability of an electrochemically preparedPPy lm to reduce silver ions to silver, which wasconrmed by subsequent experiments (Ocypa et al.,2006; Tian et al., 2008). The course of the reaction,reduction of silver ions on the PPy lm, was conve-niently followed by a quartz microbalance (Ayad &Zaki, 2009).A formal reaction scheme can be proposed by anal-

ogy with the conversion of the emeraldine form tothe pernigraniline form of PANI (Fig. 12). Some au-thors proposed also the incorporation of oxygen atomsduring this process (Ansari & Delavar, 2008), as it isin case of PANI (Stejskal & Trchov, 2012), or othermodications (Pintr et al., 2005). Other studies as-sume the complexation of silver ions with nitrogenatoms (Choi & Jang, 2008) rather than a chemicalreaction between both species. The ability of PPy toreduce a silver cation to metallic silver nanoparticles

has been well documented (Qin et al., 2011). Whenthe reactants were heated to 90C, the silver nanopar-ticles had the size of tens of nanometres. The reaction,however, typically takes place at room temperature.Polypyrrole was used in the recovery of silver from

solutions of silver ions (Pickup et al., 1998) and thecounter-ions in PPy demonstrated a signicant eecton the silver yield (Dimeska et al., 2006). Polypyr-role was often deposited on supports, such as sawdust(Ansari & Delavar, 2008) or porous carbon (Choi &Jang, 2008), in recovery experiments. Polypyrrole wasused as-prepared; only in a single case it was reducedwith sodium borohydride at rst, and subsequentlyused for the reduction of silver nitrate to metallic sil-ver (Kelly et al., 2007).The morphology of PPy has also been consid-

ered, especially with respect to nanotubes. PPy nano-tubes were prepared by coating of vanadium(V) oxidenanowires with PPy and subsequent dissolution of thetemplate. When exposed to silver nitrate solutions, 48 nm silver nanoparticles were produced on their sur-face and also agglomerated inside the nanotube cav-ity (Zhang & Manohar, 2005). Nanotubes prepared inthe presence of methyl orange as the structure-guidingagent were immersed in a silver nitrate solution andthus decorated with silver nanoparticles (Yang et al.,2010a, 2010b, 2010c). Polypyrrole nanotubes preparedby using a hard templete, aluminium oxide, have alsobeen used for reduction of silver ions (Park et al.,2012). Analogous experiments have been made withPANI nanotubes (Stejskal et al., 2009c; Trchov &Stejskal, 2010).Depending on the reaction conditions, reduction of

silver ions with PPy yielded silver particles having thesize below 10 nm (Visy et al., 2005; Pintr et al., 2005),tens of nanometres (Kabir et al., 2008; Mahmoudian etal., 2012; Park et al., 2012), or micrometres (Pickup et


al., 1998). The occurrence of cubic or triangular silvercrystals (Ayad & Zaki, 2009), nanosheets (Wang &Zhang, 2009;Wang et al., 2009b), or nanocables (Chenet al., 2005b; Nadagouda & Varma, 2007) have beenreported. Nanoparticles deposited on PPy microbowlsbelong to spectacular composite structures (Wang etal., 2009b).

Preparation of PPy in the presence of silverparticles

Another strategy consists in the separate forma-tion of silver nanoparticles followed by the preparationof PPy in their presence. This approach suers fromthe low concentration of silver nanoparticles that canbe incorporated in the composite. Silver nanoparti-cles have usually been prepared using ethylene gly-col (Ye & Lu, 2008), citrate (Borthakur et al., 2011),or formaldehyde (Yang et al., 2012a) for the reduc-tion of the silver salt and PPy subsequently by theoxidation of pyrrole with iron(III) chloride. The over-all chemistry may be complicated by the reaction be-tween the citrate and the iron(III) ions. Otherwise, sil-ver particles were prepared electrochemically (Lee &Liu, 2005). In another approach, silver nanoparticleswere coated with a silica overlayer and, subsequently,with PPy (Wang et al., 2010c). After the dissolutionof the intermediate layer of silica, yolkshell particleswere obtained. The reduction of silver iodate nanopar-ticles with sodium borohydride has also served forthe preparation of silver nanoparticles followed by theoxidation of pyrrole with ammonium peroxydisulfate(Jing et al., 2007b). Silver nanoparticles were coatedwith PPy. A closely related scenario was applied alsoto the preparation of a silverPANI composite (Jinget al., 2007a).In other experiments, silver nanorods were pre-

pared at rst by the polyol method, i.e. by the re-duction of silver nitrate in ethylene glycol at elevatedtemperatures, and subsequently coated with PPy dur-ing the oxidation of pyrrole with silver nitrate (Chenet al., 2006) or copper(II) acetate (Qiu et al., 2011).Practically identical morphology can also be foundfor PANI although under dierent reaction conditions(Bober et al., 2010a) (Fig. 6). A similar approach, us-ing iron(III) chloride for the oxidation of pyrrole, hasalso been applied to spherical silver particles (Ye etal., 2009).

Reduction of silver ions with external reduc-tants in the presence of PPy

Polypyrrole lms were rst prepared electrochemi-cally, and silver nanoparticles were deposited from theammonia solution of complex silver ions with glucoseas the reductant (Wang & Zhang, 2009; Wang et al.,2010b). The resulting lms were tested as supports forsurface-enhanced Raman scattering. Electrochemical

reduction of silver complex ions to silver nanoparti-cles inside PPy-based lms belongs to this category(Vorotyntsev et al., 2011).In yet another approach, PPy nanotubes were

prepared at rst using metyl orange as a structure-guiding agent and functionalized with carboxyl groups(Peng et al., 2012). Silver nitrate was then reducedwith sodium borohydride in their presence and silvernanoparticles were produced on their surface. Also inthis case, the resulting composites were successfullytested for surface-enhanced Raman scattering detec-tion of rhodamine. The fact that PPy alone is ableto reduce silver ions to silver, however, has not beenconsidered.

Mixing of PPy and silver particles

Silver nanoparticles were deposited on grapheneand PPy was subsequently grown in situ during theoxidation of pyrrole with ammonium peroxydisulfate(Kim et al., 2010a). In the electrochemical preparationprocess, PPy was prepared at rst on indium tin oxide(Alqudami et al., 2007) or a glassy carbon electrode(Atmeh & Alcock-Earley, 2011) and the electrodepo-sition of silver nanoparticles followed. A ten-fold im-provement of the PPy lm conductivity after silverdeposition was reported (Alqudami et al., 2007).

More complex systems

Multiple reactants

More complex systems contain an oxidant in ad-dition to silver nitrate. Silver nitrate was added toan acetic acid solution at rst, forming insoluble sil-ver acetate (Yang & Lu, 2005). Similarly, silver bro-mide particles were produced from calcium bromideand silver nitrate (Cheng et al., 2006). Silver chlorideparticles were prepared by sodium chloride and silvernitrate (Mo et al., 2007). In all three cases, insolu-ble silver salts were used as templates for the depo-sition of PPy. For that purpose, pyrrole and iron(III)chloride were added to the reaction mixture, i.e. twooxidants of pyrrole, silver salts and iron(III) chloride,were present in the system. The template particleswere claimed to be coated with PPy but the possiblereaction of insoluble silver salts with pyrrole to PPywas not considered. The subsequent dissolution of thetemplate in thiocyanate or thiosulfate solutions led tohollow microparticles.

Ternary composites

Many composites reported in literature includean additional component which is intended to aectthe composite morphology, processing, or mechani-cal properties as in the case of PANI. Such a com-ponent has often been a polymer, such as cellulose


Fig. 13. Colloidal PPy nanoparticles prepared by the oxidation of pyrrole with silver nitrate in the presence of a water-solublepolymer (unpublished results).

(Kelly et al., 2007; Firoz Babu et al., 2012), chitosan(Cheng et al., 2006; Feng et al., 2008), polyacryloni-trile (Chen et al., 2008), poly[(4-styrenesulfonic acid)-co-(maleic acid)] (Jung et al., 2011), poly[styrene-co-(methyl acrylate)] latex (Borthakur et al., 2011),poly(N-vinylpyrrolidone) (Chen et al., 2005a; Feng etal., 2007a; Feng, 2010; Wang & Shi, 2007; Wei & Lu,2009; Shi et al., 2010; Wei et al., 2010b; Zhang etal., 2012; Jung et al., 2011; Zhao & Nan, 2012a),a low-molecular-mass organic component, such astripyridin-3-yl-benzene-1,3,5-tricarboxylate (Li et al.,2011), an ionic liquid (Wei et al., 2010b), or varioussurfactants (Dallas et al., 2007; Mo et al., 2007; Zhanget al., 2012; Zhao & Nan, 2012b). Inorganic compo-nents, such as iron(III) oxide (Zhang et al., 2012),magnetite (Zhao & Nan, 2012a), or silica (Yao et al.,2009), have also been used. Especially electrically ac-tive components, such as graphite (Mo et al., 2007;Yang et al., 2012a) and graphene (Kim et al., 2010a),should be mentioned as interesting but complex ma-terials.In other approaches, silver(I)-coordinated organo-

gel produced by (tripyridin-3-yl-benzene-1,3,5-tri-carboxylate) was exposed to pyrrole (Li et al., 2011).Composite PPy nanobres with incorporated silvernanoparticles were produced. Another composite wasobtained by UV-irradiation-initiated polymerizationof substituted dimethacrylate along with pyrrole andsilver nitrate (Ijeri et al., 2010). The silver nanoparti-cles had 200 nm size.

Colloids

In contrast to PANI, the preparation of compos-ite PPysilver dispersions was more successful. Theoxidation of pyrrole with silver nitrate in the pres-ence of suitable water-soluble polymers acting as

steric stabilizers is the rst way of composite PPysilver colloids preparation (Fig. 13). The prepara-tion of such colloids has been demonstrated in papersusing poly(N-vinylpyrrolidone) (Feng et al., 2007a;Feng, 2010; Wang & Shi, 2007; Jung et al., 2011),poly[4-styrenesulfonic acid-co-(maleic acid)] sodiumsalt (Jung et al., 2011), or starch (Chang et al., 2012b)for stabilization. The colloidal particles had a silvercore of 90300 nm in size and a PPy shell of compa-rable thickness. Closer inspection suggests that rathera gemini than a coreshell morphology is often pro-duced (Change et al., 2012b) (Fig. 13a). This meansthat PPy does not wrap silver particles completely,and both PPy and silver phases are in contact withthe surrounding aqueous medium. In a single case, 1012 nm silver nanoparticles prepared separately wereused as seeds (Feng, 2010) but it is not clear if theywere needed for the success of the synthesis.The second approach to the preparation of com-

posite colloids is based in principle on the reduction ofsilver cations to silver by colloidal PPy nanoparticles(Fig. 12). The preparation of template PPy colloidshas been reviewed (Stejskal, 2001). Although such asynthesis was claimed in literature (Qin et al., 2011),true colloidal PPy nanoparticles stabilized by water-soluble polymer have not been used.

Related polymersilver composites

The use of a ring-substituted aniline instead of ani-line itself is the logical extension of the experiments.Among substituted anilines, phenylenediamines rep-resent a special group. The reduction of silver ionswith o- and p-phenylenediamines is of great histor-ical importance as it constitutes the basis of thephotographic development process. Please note thatm-phenylenediamine has never been used for this pur-


pose. All the important developers have been basedon N,N -disubstituted phenylenediamines. Reactionsused in the photographic processes, however, have al-ways taken place under alkaline conditions while thesynthesis of conducting polymers proceeds in acidicmedia. The latter type is reviewed below even thoughthe resulting oxidation products have not always beenconducting. Two main cases: (i) direct synthesis by theoxidation of the respective monomer with silver saltsand (ii) reduction of silver salts with the respectivepolymers, are discussed separately.

Poly(p-phenylenediamine)

Recent studies have demonstrated that, in con-trast to aniline, p-phenylenediamine is easily oxidizedwith silver nitrate to the poly(p-phenylenediamine)silver composite (Bober et al., 2010b, 2011b; Ciric-Marjanovic et al., 2011). The chemistry of p-phenyl-enediamine oxidation was analyzed in detail on ba-sis of quantum-chemical calculations and conrmedby infrared and Raman spectra (Ciric-Marjanovic etal., 2011). Molecular weight of the p-phenylenediamineoxidation product (103104) is somewhat lower thanthat of PANI, and the organic component in the com-posite can be regarded as only oligomeric (Bober etal., 2011b). The oxidation of p-phenylenediamine pro-ceeds easily even at 24C in frozen aqueous reac-tion mixtures (Bober et al., 2011b). Conductivity ofthe composites has often reached impressive valuesof the order of 103 S cm1 and even exceeded thatof the PANIsilver analogues. This is unexpected be-cause poly(p-phenylenediamine) is rated as a non-conducting polymer (Sulimenko et al., 2001).p-Phenylenediamine was oxidized with silver ni-

trate and the formation of silver microowers (Song etal., 2007), dendrites (Sun & Hagner, 2007; Sun, 2010;Liao et al., 2012), nanowires (Liao et al., 2012), fusedtriangles (Zhang et al., 2011a), hexagonal nanoplates(Wang et al., 2012a), or globular nanoparticles (Boberet al., 2011b; Ciric-Marjanovic et al., 2011) was re-ported. Except for the last two papers, however, theformation of poly(p-phenylediamine) or other oxi-dation products has not been considered. Only re-cently, the possibility that the morphology can beproduced partly by the polymer, rather than exclu-sively by silver, has been admitted (Bober et al.,2011b; Ciric-Marjanovic et al., 2011; Zhang et al.,2011a).Composites of this type are of interest because,

rather surprisingly, they are conducting; often evenmore than the analogous PANIsilver composites. Thereduction of silver salts with poly(p-phenylenediamine)has not yet been investigated.

Poly(m-phenylenediamine)

Oxidation of m-phenylenediamine with silver ni-

trate in an aqueous solution led to the forma-tion of silverpoly(m-phenylenediamine) coreshellparticles (Zhang et al., 2011c). Polymeric charac-ter of the oxidation product, however, was notproved.In another type of experiments, poly(m-phenyl-

enediamine) was prepared by the chemical oxidationof m-phenylenediamine with ammonium peroxydisul-fate at rst (Zhang et al., 2011b; Tian et al., 2011).Then, it was used for the adsorption of silver ions(Zhang et al., 2011b), and the adsorption capacity of1.7 g of silver ions per 1 g of polymer was reported.In this case, however, the chemical reaction, a re-duction of silver ions with poly(m-phenylenediamine)to metallic silver (Tian et al., 2011), similar to thatshown in Fig. 7, is more likely than a mere adsorp-tion.

Poly(o-phenylenediamine)

Silver ions are easily reduced to metallic silverby o-phenylenediamine. Uniform micrometre-sized sil-ver spheres were obtained (Sun et al., 2005; Liao etal., 2011a, 2011b), accompanied by microbelts (Sunet al., 2005). The fate of o-phenylenediamine oxida-tion and its potential contribution to the obtainedmorphology, however, was not discussed. The for-mation of a polymer was suggested in the follow-up studies resulting in ower-like objects (Guo &Wang, 2008; Liao et al., 2011a, 2011b). The pres-ence of o-phenylenediamine oligomers has also beenconsidered as an alternative. Indeed, a similar oxi-dation of o-phenylenediamine with ammonium per-oxydisulfate in an aqueous medium at room tem-perature yielded only dimers or trimers (Sestrem etal., 2010), orange-colored 2,3-diaminophenazine be-ing the most important product (Yang & Wang,2011). The same pathway seems to apply for the ox-idation with iron(III) chloride (Tian et al., 2010).Only the oxidation carried out at above 100C inglacial acetic acid yielded a black non-conductingproduct which was believed to be a polymer (Li etal., 2009c).Poly(o-phenylenediamine) particles were used to

reduce silver ions to metallic silver and compos-ites containing up to 35 mass % of silver wereobtained (Li et al., 2009c). The conductivity in-creased by four orders of magnitude after the in-corporation of silver but still did not exceed theorder of 108 S cm1. Similarly, nanobres pro-duced from the o-phenylenediamine dimer were dec-orated with silver nanoparticles when exposed to sil-ver nitrate (Tian et al., 2010). Whether the oxida-tion of o-phenylenediamine leads to a polymer orto oligomers only is still open to discussion. Elec-trodeposited poly(o-phenylenediamine) lms under-went subsequent electrodeposition of silver nanopar-ticles (Wang et al., 2012b).


Substituted polyanilines

Polymethylanilines

Oxidation of N-methylaniline with silver nitratewas found to produce a silver mirror (Nadagouda &Varma, 2007) but the formation of the correspond-ing polymer was not proved. The composite of a con-ducting polymer was prepared by the oxidation of o-methylaniline (o-toluidine) with ammonium peroxy-disulfate in the presence of silver nanoparticles (Reddyet al., 2008).

Poly(o-methoxyaniline)

Poly(o-methoxyaniline) in chloroform was used toreduce silver nitrate in aqueous solutions at the inter-face of immiscible liquids (Dawn et al., 2007; Mukher-jee & Nandi, 2009). The size of the silver parti-cles was 922 nm and they were associated withpoly(o-methoxyaniline) and thus transferred to or-ganic phase. They were, however, non-conducting.The ability of poly(o-methoxyaniline) to reduce sil-ver ions to silver has also been demonstrated in aque-ous media (Dawn & Nandi, 2006; Routh et al., 2010).Globular silver particles had the size of 516 nm(Routh et al., 2010), i.e. they were smaller than incase of PANI. In another type of experiments, poly(o-methoxyaniline) was deposited electrochemically onthe electrode (Mazur et al., 2007). Hydrogen mi-crobubbles generated at the same time left circularholes in the polymer coating which were subsequentlylled with silver.

Poly(2,5-dimethoxyaniline)

Oxidation of 2,5-dimethoxyaniline with silver ni-trate was claimed to produce a polymer (Huang etal., 2006a, 2006b; Huang & Wen, 2007; Neelgund etal., 2008). Brown color of the products and the ab-sence of the absorption band with a maximum locatedabove 500 nm in the UV-VIS spectra, however, suggestthat only non-conducting oligomers were produced. Inall these papers, the presence of poly(styrenesulfonicacid) in the reaction medium prevented the aggrega-tion of silver nanoparticles having the size below 100nm. Colloids have probably been produced under suchconditions (Stejskal, 2001) but neither this fact nor thestability of the colloid were discussed.

Poly(4-aminodiphenylamine)

Oxidation of an aniline dimer, 4-aminodiphenyl-amine (p-semidine, N-phenyl-p-phenylenediamine),with silver nitrate led to poly(4-aminodiphenylamine)silver composites (Paulraj et al., 2011; Thanjam etal., 2011, 2012a, 2012b). The products were solublein the reaction medium and the polymeric charac-

ter of the formed product was not proved. UV-VISspectra, however, displayed a polaronic band above600 nm (Paulraj et al., 2011; Thanjam et al., 2011),which is typical of PANI salts. A similar oxidationof 4-aminodiphenylamine with ammonium peroxy-disulfate led only to semiconducting oligomers (Ciric-Marjanovic et al., 2008).

Sulfonated polyaniline

A composite of sulfonated PANI and silver wasprepared by the co-oxidation of aniline and orthanilicacid (o-aminobenzenesulfonic acid) with silver nitrate(Karim et al., 2009). Sulfonated PANI was used for thereduction of silver ions to globular silver nanoparticles(Krutyakov et al., 2010) or nanobelts (Xia, 2011). Inan earlier case, the emeraldine form of PANI was re-duced with sodium borohydride at rst. It is not quiteobvious if it was the leucoemeraldine or the borohy-dride which reduced the silver ions to silver. A statis-tical copolymer of aniline and 5-sulfo-2-anisidine (5-sulfo-2-methoxyaniline) was also used as the reductantof silver ions to metallic silver (Li et al., 2010).

Aniline oligomers

Aniline oligomers are closely related to PANI (Ste-jskal & Trchov, 2012). They may have various chem-ical structures, ranging from linear oligomers resem-bling PANI to phenazine-like heterocyclic molecules(Wudl et al., 1987; Stejskal et al., 2008b) andquinoneimine-containing structures (Silva et al., 2011;K et al., 2011; Stejskal & Trchov, 2012). Their pre-cise molecular structure has not yet been establishedand they are probably represented by complex mix-tures of molecules. Aniline oligomers are produced atthe early stages of aniline oxidation and especially un-der low-acidity conditions, where the growth of PANIchains is not feasible. This includes also oxidations un-der alkaline conditions but the structure of the anilineoligomers is then probably dierent from those pro-duced in acidic media.Aniline oligomers are non-conducting or their con-

ductivity is low. They are ecient reductants of sil-ver nitrate (Stejskal et al, 2009b; Trchov & Stejskal,2010) (Fig. 14). The deposited silver, however, hadno eect on the overall conductivity which remainednegligible at 1013 S cm1 (Stejskal et al., 2009b).In spite of their electrical inactivity, aniline oligomersand their composites with silver are promising mate-rials for application due to their chemical properties(Chao et al., 2009; Stejskal & Trchov, 2012). Anilineoligomers produce a variety of spectacular morpholo-gies represented by leaves, owers, and micromats un-der acidic conditions (Zujovic et al., 2011a; Yang etal., 2012b; Zhao et al., 2013), and compact or hollowmicrospheres in alkaline media (Stejskal et al., 2010;Luo et al., 2011; Cheng et al., 2011; Stejskal & Tr-


Fig. 14. Polyanilinesilver composites obtained by the reactionof oligoaniline microspheres with silver nitrate (Tr-chov & Stejskal, 2010).

chov, 2012). In contrast to PANI, the objects pro-duced by aniline oligomers are more crystalline andlarger, of micrometre size, due to their better abilityto self-organize.The oxidation of aniline always results in a prod-

uct which contains some fraction of aniline oligomers.The content of oligomers is low when the oxidationis carried out in acidic media. The opposite is truewhen the acidity is low and aniline oligomers arethe exclusive products as in an alkaline medium. Ox-idation potential of the oxidant is also important,and the fraction of aniline oligomers becomes largeras the oxidation potential (E) is reduced (Sapu-rina & Stejskal, 2012), e.g., when using silver ni-trate (E = 0.8 V) instead of ammonium peroxy-disulfate (E = 2.0 V). This is reected in the UV-VIS spectra which do not display the typical ab-sorption polaron band of a PANI salt at approxi-mately 810 nm, or that of the transition of thequinoneimine rings in the PANI base at 630 nm (Ste-jskal et al., 1993). The presence of aniline oligomersis also conrmed by increased absorption in the 300400 nm region. Aniline oligomers have dierent molec-ular structure than PANI and they contain quinoneor quinoneimine constitutional units (Stejskal & Tr-chov, 2012).Although many papers claim to have prepared

PANI, in fact, only aniline oligomers have been pro-duced (Kim et al., 2011; Gao & Xing, 2012). Theformation of aniline oligomers, instead of PANI, issuspected also in the cases when pH of the reactionmedium was higher than 23 (Fujii et al., 2010; Yanget al., 2011; Gao & Xing, 2012), i.e. it did not reachthe acidity level needed for the formation of PANI (Sa-purina & Stejskal, 2010; Konyushenko et al., 2010).

Other related systems

Congo red, a dye containing an aniline constitu-tional moiety, was oxidized with silver nitrate (Sinai& Avnir, 2011). The formation of a polymer has notbeen considered and the authors called the resultingmaterial as silver doped with Congo red. The reactionof N-(3-trimethoxysilyl)aniline with silver nitrate wasclaimed to produce a corresponding polymer (Maneshet al., 2010). The silyl moiety was then incorporatedinto the silica network.Electrochemically prepared poly(o-aminophenol)

was used for the reduction of silver ions to sil-ver (Zhang et al., 1996). Poly(m-aminophenol) wasclaimed to have been prepared by the oxidation ofm-aminophenol with ammonium peroxydisulfate in astrongly alkaline medium (Kar et al., 2011). Undersuch conditions, however, the formation of PANI-likestructures is unlikely and indeed, the absorption max-imum in the red region of the visible spectrum wasmissing. Nevertheless, the oxidation products wereable to reduce silver nitrate to metallic silver.A product of the joint oxidation of sodium di-

phenylamine-4-sulfonate and 1,8-diaminonaphthalene,probably a copolymer of these two compounds, wastested for the reactive adsorption of silver ions fromsolutions of various acidity (Li et al., 2005). The sameadsorption ability was reported for various polydi-aminonapththalenes (Huang et al., 2005).The reaction between benzidine and silver nitrate

was reported to produce a polybenzidinesilver com-posite (Manivel et al., 2012) and, similarly, electro-chemically prepared polybenzidine lms were foundto reduce silver ions to silver (DEramo et al., 2000).

Conductivity

Many researchers assume that the introduction ofsilver to conducting polymers leads to an increasein the conductivity. Practical experiments, however,show that this expectation is usually not fullled. Asit is shown below, most composites do not even reachthe conductivity of the conducting polymers alone butexceptions to this rule do exist.In PANIsilver composites, PANI is regarded as

the polymer matrix and silver particles as the ller. Inthe classical percolation concept, approximately 1617 vol. % of the conducting component in the non-conducting matrix are needed for the formation ofconducting pathways in the three-dimensional com-posite and for the material to become conducting.In the present case, the percolation threshold corre-sponds to at least 60 mass % of silver (Fig. 15). It canbe expected that, above this limit, the conductivityis controlled by silver and is therefore high, while be-low this threshold, the control is governed by PANI atthe semiconductor level. Practical experiments seemto suggest the location of the percolation threshold


Fig. 15. Volume vs. mass fraction of silver in the compositesof silver with PANI. Calculated by assuming the den-sities of silver and PANI equal to 10.5 g cm3 and 1.4g cm3, respectively.

Fig. 16. Compilation of the conductivities reported for PANIsilver composites in several recent papers (Blinova etal., 2009; Bober et al., 2010a, 2010b, 2011a, 2011c).

between 7080 mass % of silver (Fig. 16).Many results, however, suggest that the conductiv-

ity of composites falls below the conductivity of bothconstituents. This implies that additional factors arein operation. The conductivities reported in literatureare not easy to compare because they refer to widelydiering contents of silver.For the purpose of discussion, we shall distinguish:

(i) direct synthesis of composites by the oxidation ofaniline or pyrrole with silver salts; and (ii) reductionof silver salts with PANI and PPy. In the former case,reaction stoichiometry predicts the content of silverin the composites to be close to 70 mass % (Blinovaet al., 2009) and, in the latter, to about 37 mass %(Stejskal et al., 2009d). Thus, the former approachprovides better chances for the preparation of highlyconducting materials; the attractivity of the latter isin its simplicity.

Conductivity of polyanilinesilver composites

Direct preparation

Oxidation of aniline with silver nitrate in solutions

of nitric acid has led to PANIsilver composites hav-ing the conductivity of 492250 S cm1 depending onthe concentrations of aniline and nitric acid (Blinovaet al., 2009). The process, however, was slow and sev-eral months were needed to obtain reasonable yieldof the composite. The samples were not homogeneousand contained macroscopic silver particles. Also, un-desirable nitration of the PANI chains was observed.Physical acceleration with UV-light yielded a producthaving the conductivity of 3050 S cm1 (Khanna etal., 2005) or 93 S cm1 (Li et al., 2012b) within severaldays. When using -irradiation for the same purpose,the achieved conductivity was 0.050.20 S cm1 (Pil-lalamarri et al., 2005). Chemical acceleration of anilineoxidation with p-phenylenediamine made the synthe-sis feasible from the practical viewpoint (Bober et al.,2010b) but the homogeneity of the samples still re-mained a problem. The use of larger proportions ofp-phenylenediamine with respect to aniline led to adrop in the conductivity due to the loss of regularityof the PANI chains caused by the copolymerization ofaniline and p-phenylenediamine (Bober et al., 2011b).Oxidation of aniline in solutions of acetic acid

provided products widely diering in conductivity,0.0783550 S cm1 (Blinova et al., 2010). The sam-ples, however, contained a high fraction of insolublesilver acetate, which is undesirable. There was alsoa notable fraction of aniline oligomers in the sam-ples, obviously due to the relatively low acidity af-forded by acetic acid. The problem of the slow re-action rate persisted and weeks were still needed tocomplete the reaction. The reaction was acceleratedby p-phenylenediamine (Bober et al., 2010b) but otherproblems remained. Oxidation of aniline in solutions ofhydrochloric acid yielded composites having the con-ductivity of 5.9 1030.11 S cm1 (Karim et al.,2009). Also in this case, the composites probably con-tained insoluble silver chloride.Formic acid solutions are better suited as a medium

for the oxidation of aniline (Bober et al., 2010b). Theconductivity of PANIsilver composites was lower andranged between 0.8545.00 S cm1 and the reactionrate increased so that the products could be isolatedwithin a few days. Macroscopic homogeneity, however,was again poor. Chemistry of the reaction is compli-cated by the fact that silver nitrate is reduced, in ad-dition to aniline, also by formic acid. This reactionis fast and exothermic but, surprisingly, it was inhib-ited after the addition of aniline. Further studies areneeded to understand the course of aniline oxidationin this medium.So far, solutions of sulfonic acids seem to be the

most promising, as they do not precipitate silver ions,yet they provide sucient acidity of the medium forthe preparation of PANI (Bober et al., 2011c). Amongthem, methanesulfonic acid performed the best, thePANIsilver composite reaching the conductivity of880 S cm1. Conductivity of composites prepared in


solutions of camphorsulfonic or toluenesulfonic acidsremained at the level of PANI alone and no contri-bution of silver to the conductivity was observed. Onthe other hand, the reaction yield was high, over 77 %,and the homogeneity of samples was also good. Accel-eration with p-phenylenediamine was still necessary.Conductivity exceeding the value of 103 S cm1 wasalso determined for the samples prepared in frozen re-action mixtures at 24C (Bober et al, 2011a), whenthe polymerization took place in the solid state, in ice(Konyushenko et al., 2008b).In most literature sources, conductivity of PANI

silver composites has been mentioned only rarely.When it was reported, it usually did not exceed thatof the conducting polymer alone, units of S cm1.For example, oxidation of aniline with silver nitrate inethanol at 250C yielded a composite having the con-ductivity of 0.18 S cm1 (Du et al., 2005). The patternprinted by silver nitrate and aniline solution and ex-posed to UV-irradiation had the conductivity of 0.02S cm1 (de Barros et al., 2005); experimental details ofthe preparation have not been disclosed. The compos-ite prepared from PANIsilver nanoparticles stabilizedwith poly(vinyl alcohol) had the conductivity of 0.014S cm1 (Fujii et al., 2010) but the presence of ani-line oligomers rather than of PANI is suspected whennoting the low-acidity conditions. Similar compositesprepared in the presence of poly(styrenesulfonic acid)had the conductivity of 103104 S cm1 (Neelgundet al., 2008). Oxidation of aniline with a mixture ofoxidants, ammonium peroxydisulfate and silver ni-tride, in the presence of citric acid yielded compositeshaving the conductivity of 0.701.62 S cm1 (Jia etal., 2010a), and the values of 0.871.56 S cm1 wereachieved in solutions of nitric acid (Jia et al., 2010b).The conductivity of PANI prepared in the absence ofa silver salt was 0.13 S cm1. The reaction betweenaniline and silver salts can proceed even in the absenceof solvents, in the solid state, by mechanical blendingof the reactants (ednkov et al., 2011). The product,however, had a poor conductivity (1.5 103 S cm1)and the organic part was composed mainly of anilineoligomers.

Reduction with polyaniline

This strategy uses the ability of PANI or PANIcomposites to reduce silver salts to silver. Conduc-tivity of the PANIcellulose composite (6.0 104S cm1) was reduced to 3.6 109 S cm1 after thedeposition of silver nanoparticles (Kelly et al., 2007).It was correctly concluded that the conducting emeral-dine salt was converted to a non-conducting perni-graniline base (Kelly et al., 2007; Patil et al., 2012).The content of silver is too low to aect the overallconductivity. The same conclusion was made in an-other study (Stejskal et al., 2008c). The reduction ofsilver nitrate by PANI-coated multi-wall carbon nano-

tubes led to an increase in their conductivity from0.082 S cm1 to 1.680 S cm1 with the increasing sil-ver content (Grinou et al., 2012); however, these valuesare still relatively low.When PANI nanotubes were used as a reductant

of silver nitrate in 1 M nitric acid, the produced com-posites had the conductivity of 104102 S cm1 de-pending on the mole ratio of the reactants (Stejskalet al., 2009c). Preparations using globular PANI in1 M hydrochloric acid yielded products of the sameconductivity (Patil et al., 2012). A similar reactionin water, however, yielded a composite with the con-ductivity of 68 S cm1 (Stejskal et al., 2009c). Theacidity of the reaction medium thus has an importantrole. The composite obtained by the reduction of silvernitrate with PANI had the conductivity of 35 S cm1

(Leyva et al., 2011).Mechanochemical synthesis based on the blending

of reactants, PANI base and silver nitrate, in the solidstate yielded a composite with the conductivity of103102 S cm1 (ednkov et al., 2009). The pro-cess is interesting due to the fact that nitric acid isa by-product. The observed conductivity is thus a re-sult of the protonation of PANI base to PANI nitraterather than the contribution of generated silver.The most promising direction is represented by

the composites composed of poly(methyl methacry-late) microparticles coated with PANI and decoratedwith silver (Lee et al., 2012). Such a composite doesnot obey the simple percolation principle and the per-colation has to be reached only within the interstitialspace (Proke et al., 1997; Kivka et al., 1999). Un-der such conditions, even a relatively small fraction ofsilver in the composite may lead to high conductivity.

Other syntheses

When PANI was prepared in the presence of silvernanoparticles, conductivity of the resulting compos-ites increased from 0.03 S cm1 for neat PANI to 0.51S cm1 for the highest (but unspecied) content ofsilver (Gupta, et al., 2010). Similarly, when PANI wasprepared in the presence of carbon nanotubes deco-rated with silver nanoparticles, conductivity was twoorders of magnitude higher than in the absence of sil-ver (0.034 S cm1) (Kim & Park, 2011), still rela-tively low. The reverse process in which PANI-coatedcarbon nanotubes were decorated with silver nanopar-ticles obtained by the reduction of silver nitrate withsodium citrate also led to composites with relativelylow conductivity (2.5 1035.0 S cm1).Mixing of PANI with silver nanoparticles is not

an eective way to increase the conductivity. This iscaused by the low volume fractions of silver which canbe introduced in this way. Conductivity of PANI (0.4S cm1) was found to increase to 2.3 S cm1 afterthe incorporation of silver nanoparticles (Oliveira etal., 2006) but such an increase is hardly signicant.


After combining PANI methanesulfonate with silvernanoparticles, the level of conductivity stayed in theunits S cm1 (Sim et al., 2009). The composite ofPANI with dinonylnaphthalenedisulfonic acid and sil-ver nanoparticles had the conductivity of 7.10 1031.58 102 S cm1 at approximately 20 mass % ofthe silver content (Garai et al., 2010) and it displayedohmic behavior.

Conductivity of polypyrrolesilver composites

Direct preparation

Incorporation of silver in PPy led in most casesto an increase in conductivity. PPy prepared with thepotassium ferricyanide (K3[Fe(CN)6]) oxidant had theconductivity of 4 S cm1 (Feng et al., 2007b). Oxida-tion of pyrrole with the silver salt of methyl orangeproduced

conducting polymer-silver composite (review)

Documents

reduction of silver

morphology of composites

silver complexes

silver nanoparticles

metallic silver

reductants of silver

ternary composites

polypyrrolesilver composites