The oxidation of aniline with silver nitrate to polyaniline–silver composites
Graphical abstract
Introduction
The composites of conducting polymers, such as polyaniline (PANI), with noble metals find applications in electrocatalysis [1], [2], [3], catalysis [4], [5], design of fuel-cell electrodes [6], [7], [8], [9], [10], sensors [11], conducting printing inks [12], [13], recovery of noble metals [14], [15], etc. The oxidation of aniline with tetrachloroauric acid, resulting in the PANI–gold composite, has often been reported [16], [17], [18], [19], [20]. Also, the synthesis of PANI using hexachloroplatinate has been successful [21], [22]. The oxidation of the substituted aniline with palladium acetate similarly yielded composites of palladium as nanoparticles dispersed in the polymer matrix [23]. Silver is the next noble metal which can be combined with PANI.
The composites of PANI and noble metals can generally be prepared in three ways: (1) by the synthesis of PANI in the presence of metal particles, (2) by using PANI as a reductant of noble-metal salts or acids, or (3) by the oxidation of aniline with noble-metal compounds. Silver has recently received considerable attention in this respect. The first method has only been used exceptionally for modifying silver nanoparticles with PANI [24], [25] or substituted PANI [26]. Composites of silver and PANI have been prepared by the second technique, by the direct reduction of silver nitrate with PANI [14], [27], [28], [29] or PANI derivatives [30]. The latter chemical process may cause some reduction in the conductivity of the resulting materials due to the oxidation of emeraldine to pernigraniline. The use of colloidal PANI dispersions [31], instead of a PANI powder, gave rise to materials with unique morphologies [32], [33]. The reduction of silver nitrate with PANI nanotubes has recently been used to discuss the loci of silver in the composites [34].
The third approach, the oxidation of aniline with silver nitrate to PANI–silver composites, has been demonstrated by de Barros et al. [12], [13]. These authors have used a paper soaked with aniline dissolved in nitric acid and a pattern was printed on it with silver nitrate solution. A green image became visible after development with UV-light irradiation. Spectra typical of PANI have been recorded and the conductivity of the printed patterns was 0.02 S cm−1. Polyaniline nanowires were obtained in a similar reaction carried out in solution, again subject to UV-light irradiation [35]. Also, other authors have used UV light for a similar purpose [36]. The electropolymerization of a mixture of aniline and silver nitrate in 1 M nitric acid produced fibrous composites [37]. Huang and Wen [38] and Neelgund et al. [39] have recently reported the preparation of PANI and poly(2,5-dimethoxyaniline) by the oxidation of corresponding monomer with silver nitrate in the presence of poly(styrenesulfonic acid). The general absence of the characteristic absorption maximum at long wavelengths in the visible spectra, however, suggests that these products were not an analogy of a conducting PANI but rather non-conducting aniline oligomers composed of mixed ortho and para-linked aniline constitutional units [40], [41]. The oxidation of aniline with silver nitrate using ultrasonic waves or γ-irradiation [42] probably also yielded such oligomers as main product.
The present study reports the oxidation of aniline with silver nitrate in an aqueous solution of nitric acid yielding PANI–silver composites. The study was stimulated by several reasons: (1) composite materials containing silver particles embedded in a matrix of conducting polymer may exhibit good electrical and thermal conductivities. Electronic components are the obvious application target [43]. (2) The only by-product of the oxidation is nitric acid, which can easily be evaporated. There is no need to separate copious amounts of inorganic salts, like ammonium sulfate, produced in the oxidations with ammonium peroxydisulfate (APS) [44]. (3) Silver makes a good model for the deposition of more expensive noble metals, such as gold, platinum, palladium, and rhodium that are used in catalytic and electrocatalytic applications. (4) The antimicrobial activity of silver is well known and a similar ability has recently been reported for PANI [45], [46]. The combination of these two materials might have synergetic effects worthy of consideration [28]. (5) Moreover, PANI has been successfully tested for compatibility with animal tissues [47] and in experiments with cell cultures [48]. Applications of PANI–silver composites in neural tissue engineering, artificial muscles, and various monitoring devices can be anticipated [49], [50] if suitable materials were available.
Section snippets
Synthesis
Aniline (Fluka, Switzerland) was dissolved in 0.4 or 1 M nitric acid, as was silver nitrate (Fluka, Switzerland). The solutions were mixed to start the oxidation at room temperature, ∼20 °C. The concentration of aniline was 0.1–0.8 M, the silver nitrate-to-aniline molar ratio was 2.5 (Fig. 1). The reaction is slow, characterized by an induction period extending for weeks. The unstirred mixtures were left to stand in a laboratory, and occasionally briefly shaken. The access of daylight was not
Polymerization
The oxidation of aniline with silver nitrate (Fig. 1) is slow. An induction period, extending up to several weeks, where no obvious reaction takes place to the naked eye, is followed by a faster polymerization and dark green precipitate is then produced within a few days. This is probably the reason why only processes accelerated with the UV irradiation have been reported in the literature so far [13], [35].
The oxidation of 1 g of aniline with the stoichiometric amount of silver nitrate (Fig. 1
Conclusions
- (1)
Silver nitrate is able to oxidize aniline to polyaniline in the aqueous solutions of nitric acid. The oxidation products are composed of two conducting components, polyaniline and metallic silver. The theoretical composition calculated from stoichiometry is 70 wt.% of silver; 50–80 wt.% have been found in the experiment. The polymerization is slow and takes several weeks.
- (2)
The oxidation proceeds more easily at higher concentration of aniline but only when nitric acid is present in molar excess with
Acknowledgments
The authors thank the Czech Grant Agency (202/06/0419 and 203/08/0686), the Ministry of Education, Youth, and Sports of the Czech Republic (ME 847), and the Ministry of Science and Environmental Protection of Serbia (Contract No. 142047) for financial support. Thanks are also due to J. Kovářová from the Institute of Macromolecular Chemistry, and to J. Prokeš, from the Charles University in Prague, for the characterization of samples.
References (74)
- et al.
Electrochim Acta
(2007) - et al.
Appl Catal A Gen
(2007) - et al.
Tetrahedron Lett
(2007) - et al.
Carbon
(2005) - et al.
Compos Sci Technol
(2007) - et al.
J Power Sources
(2007) - et al.
Mater Charact
(2003) - et al.
Synth Met
(2005) - et al.
Polymer
(2006) - et al.
J Colloid Interface Sci
(2007)