Electrochimica Acta 56 (2011) 3580–3585 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Polyaniline–silver composites prepared by the oxidation of aniline with silver nitrate in solutions of sulfonic acids Patrycja Bober a,∗ , Miroslava Trchová a , Jan Prokeˇ s b , Martin Varga b , Jaroslav Stejskal a a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic b Faculty of Mathematics and Physics, Charles University Prague, 182 00 Prague 8, Czech Republic article info Article history: Received 31 May 2010 Received in revised form 10 August 2010 Accepted 11 August 2010 Available online 19 August 2010 Keywords: Conductivity Polyaniline Silver Polyaniline–silver composite Silver nanoparticles abstract Aniline was oxidized with silver nitrate in aqueous solutions of sulfonic acids: camphorsulfonic, methane- sulfonic, sulfamic, or toluenesulfonic acids. Polyaniline–silver composites were produced slowly in 4 weeks in good yield, except for the reaction, which took place in sulfamic acid solution, where the yield was low. Polyaniline in the emeraldine form was identified with UV–visible, FTIR, and Raman spectra. Thermogravimetric analysis was used to determine the silver content, which was close to the theoretical prediction of 68.9 wt.%. Transmission electron microscopy demonstrated the presence of silver nanoparti- cles of ca 50 nm average sizes as the dominating species, and hairy polyaniline nanorods having diameter 150–250 nm accompanied them. The highest conductivity of 880 S cm -1 was found with the composite prepared in methanesulfonic acid solution. Its conductivity decreased with temperature increasing in the 70–315 K range, which is typical of metals such as silver. The conductivity of composites prepared in solutions of other acids was lower and increased with increasing temperature. Such dependence is typical of semiconductors, reflecting the dominating role of polyaniline in the conductivity behaviour. It is proposed that interfaces between the polyaniline matrix and dispersed silver nanoparticles play a dominating role in macroscopic level of conductivity. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Conducting composites based on polyaniline (PANI) and silver can be prepared in several ways: (1) by a simple blending of both components [1–3], (2) by the reduction of silver compounds with PANI [4–11], or (3) by introducing silver compounds during the oxi- dation of aniline with ammonium peroxydisulfate [12,13]. Probably the most interesting approach consists in (4) the direct oxidation of aniline with silver nitrate [14–16], when the reaction between two non-conducting compounds yields a composite of two conducting components, PANI and silver. Such a procedure is analyzed in the present study. The goal consists in the preparation of materials hav- ing (a) a defined nanostructure with respect to both the polymer and the metal, (b) a homogeneous distribution of both components at the micrometer level, and (c) a good conductivity. In addition, the composites should be produced within a reasonable time and in high yield. Such materials may potentially be useful in appli- cations such as electrode materials in electrocatalysis, in energy conversions, sensors, flexible electronics, etc. The ability of silver(I) salts to oxidize aniline has recently been demonstrated in several papers [13–16]. The resulting ∗ Corresponding author. E-mail address: bober@imc.cas.cz (P. Bober). polyaniline–silver (PANI–Ag) composites are composed of a poly- mer semiconductor and metallic silver, the most conducting element among metals. The preparation of quantities sufficient for conductivity measurements have been reported for the oxidation of aniline in aqueous solutions of nitric acid. The conductivity of the products reached 2250 S cm -1 at 51.8 wt.% silver content [15]. The induction period extending over several weeks, which precedes the polymerization, is a drawback in the routine preparation of composites. UV irradiation was reported to promote the polymer- ization [4,16] but this technique is not efficiently applicable when the oxidation is not carried out in thin layers. The oxidation of aniline to PANI proceeds in an acidic medium [17]. Except for nitric acid solutions, the choice of inorganic acids is limited because most acids produce insoluble salts with silver ions. Carboxylic acids also form salts of limited solubility with sil- ver cations. This has recently been illustrated for the oxidation of aniline in solutions of acetic acid, where silver acetate was present in the products in significant amounts [18]. Moreover, ani- line oligomers have often been the only products or their major component but, despite this fact, the conductivity of the composites was as high as 8000 S cm -1 . The oxidation of aniline in solutions of sulfonic acids proved to proceed relatively easily and in good yields in exploratory tests. No precipitate was formed after mixing the reactants, aniline and silver nitrate, and the oxidation of aniline thus started in a homo- 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.041