Polyaniline-coated silver nanowires Patrycja Bober a, * , Jaroslav Stejskal a , Miroslava Trchová a , Jir ˇina Hromádková a , Jan Prokeš b 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 8 February 2010 Received in revised form 4 May 2010 Accepted 23 May 2010 Available online 1 June 2010 Keywords: Conducting polymer Conductivity Nanowire Polyaniline Silver abstract Two non-conducting chemicals, aniline and silver nitrate, dissolved in formic acid solutions, yielded a com- posite of two conducting products, polyaniline and silver. As the concentration of formic acid increased, an alternative reaction, the reduction of silver nitrate with formic acid to silver became dominant, and the con- tent of silver in the composites increased. The formation of polyaniline was confirmed by UV–visible, FTIR, and Raman spectroscopies. The typical conductivity of composites was 43 S cm 1 at 84 wt.% of silver. Silver nanowires coated with polyaniline nanobrushes are produced at low concentrations of formic acid, the granular silver particles covered with polyaniline dominate at high acid concentrations. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The nanocomposites composed of conducting polymers [1–4], such as polyaniline (PANI) or polypyrrole, and noble metals, such as silver, are of interest in producing new materials with high con- ductivity and exploiting mechanical properties typical of polymers. The development of PANI–silver composites is important for the design of conducting patterns using ink-jet printing techniques in electronics [5,6]. The incorporation of metals is the most promising way to increase the conductivity of PANI, which is currently at the level of units S cm 1 [7]. The same principle applies to the con- struction of sensors based on conducting polymers [8]. Direct oxidation of aniline with silver nitrate is the most straightforward way of composite preparation [9,10]. Unless the oxidation of aniline was promoted by external stimuli, such as in- crease in reaction temperature [11], UV-irradiation [5,12–14], c-irradiation or sonication [15,16], the reaction was slow and ex- tended to several weeks or even months [9]. The ability of insolu- ble silver salts, such as silver cyanoferrate(III), to oxidize aniline to PANI was also reported [17]. The unpronounced absorption maxi- mum in the optical spectra in the region 600–800 nm or its ab- sence [9,15,16,18,19] indicate that many oxidation products have been composed mainly by non-conducting aniline oligomers. The oxidation of pyrrole with silver nitrate similarly required UV-irra- diation [20] or the presence of a porphyrin accelerator [21]. The improvement of the syntheses is sought, which would lead to: (1) highly conducting and (2) macroscopically uniform materi- als, (3) produced at reasonable reaction time and (4) in high yield. It is not easy to satisfy these requirements simultaneously. The composites prepared by the oxidation of aniline with silver nitrate in the solutions of nitric acid had the conductivity as high as 2250 S cm 1 at 52 wt.% silver content [9]. The homogeneity, how- ever, was poor and macroscopic silver flakes were present in the samples along with ca 50 nm silver nanoparticles. The yields were at the same time low, <30% of theory, even after several weeks of reaction time. Similar experiments in the solutions of acetic acid have resulted in the product having the highest conductivity 8000 S cm 1 and they were macroscopically homogeneous [10]. In spite of high conductivity, the polymer matrix was not produced in many cases, and materials contained mainly non-conducting aniline oligomers. Formic acid solutions have been selected as reaction media for the oxidation of aniline in the present series of experiments. It has earlier been demonstrated that aniline can be oxidized in the solutions of formic acid to PANI with classical oxidants, such as ammonium peroxydisulfate [22]. In the contrast to acetic acid, however, formic acid is able to reduce silver nitrate to metallic sil- ver, similarly like aniline does. Some synergistic effect thus may be anticipated, which would be of benefit in the synthesis. 2. Experimental 2.1. Preparation of PANI–Ag composites Aniline (0.2 M; Fluka, Switzerland) was oxidized with silver ni- trate (0.5 M; Lach-Ner, Czech Republic) in the 0.1–5 M aqueous 1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.05.009 * Corresponding author. E-mail address: bober@imc.cas.cz (P. Bober). Reactive & Functional Polymers 70 (2010) 656–662 Contents lists available at ScienceDirect Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react