Synthesis and Investigation of Polyaniline−Tin Dioxide Nanocomposite

A. А. Мatnishyana, T. L. Akhnazaryana, G. V. Abaghyanb, S. I. Petrosyanb, G. R. Badalyanb, and M. G. Eghikyana

aYerevan Scientific-Research Institute of Optical-Physical Measurements, Yerevan, Armenia bInstitute for Physical Research, NAS of Armenia, Ashtarak, Armenia

Received February 16, 2010

Abstract⎯We propose a method for obtaining highly conductive nanosize composite of polyaniline with tin dioxide (SnO2). Synthesis of SnO2 and polycondensation of aniline are combined in a single reactor, which allows one to control the size of SnO2 nanoparticles in the range from 10 to 300 nm and to change their content in the nanocomposite depending on the conditions of synthesis (temperature, pH and concentration of reagents). Morphology, composition, IR spectra, and electroconductivity of the obtained samples and films have been studied.

DOI: 10.3103/S1068337210050099 Key words: polyaniline, tin dioxide, nanocomposite, synthesis, electroconductivity

1. INTRODUCTION Composites of polyaniline (PAN) with inorganic filling material have, due to a number of useful

properties, wide applications in electronics and electrical engineering. Hybrid materials consisting of organic and inorganic nanocomponents are especially needed in microelectronics [1–3]. They have sensitivity to light and various gases, and catalytic activity [4–7]. In particular, nanocomposites of polyaniline with titanium dioxide are used for fabrication of photoelectric converters, sensors of different types, and piezoelectric materials [4, 5]. Materials containing tin dioxide are good catalysts of oxidation of methanol [6], sensors for gases [7] and they are employed in nonlinear optics [8] and electrochemistry [9]. Methods of synthesis of nanocomposites of polyaniline with tin dioxide (PAN/SnO2) are studied insufficiently and come mainly to polymerization of aniline in the suspension of target-size filler powders [10]. Nanomaterials whose particle size may be controlled during synthesis in the range of 10–100 nm are most promising.

In the present work we propose a new technique for obtaining high-conductivity nanosize PAN/SnO2 composites and study some properties of the obtained materials.

2. TECHNIQUES OF EXPERIMENT Synthesis of nanocomposites was carried out using the technique similar to that described in [11, 12].

In the glass flask of an ultrasonic homogenizer 5 ml of butanol and 1 ml (2.23 g) of SnCl4 (0.01 mole) were mixed under intense stirring, left for 15 minutes, and then 25 ml of ethanol and 25 ml of distilled water were added. The solution was cooled down to 0°C in an ice bath and under intense stirring 0.93 ml (0.01 mole) of aniline was replenished. In 10 minutes 7.2 ml of 7% aqueous solution of NH4OH was added dropwise into the obtained emulsion. In another 5–10 minutes 30 ml of water solution (2.5 g, 0.01 mole) of ammonium persulphate was added drop by drop. In 2 hours after the redox potential of the system reached 415 mV, the obtained suspension was filtered and rinsed, first 5 hours with 0.1N-solution of HCl and then 30 minutes with distilled water. The resulting paste was wrung out in vacuum and dried 24 hours at temperature 60°C. Thus we obtained 2.35 g of the composite in the form of emeraldine salt with the conductivity ∼0.05 Ω−1cm−1. For preparation of a nanocomposite in the form of emeraldine base, the fabricated paste was processed 24 hours by the 7% solution of ammonia, then washed 1 hour with distilled water, filtered, and dried 10 hours at 60°C. Finally 1.86 g of nanocomposite with very low conductivity, ∼10−10 Ω−1cm−1, was obtained.

ISSN 1068–3372, Journal of Contemporary Physics (Armenian Academy of Sciences), 2010, Vol. 45, No. 5, pp. 246–250. © Allerton Press, Inc., 2010. Original Russian Text © A.A. Matnishyan, T.L. Akhnazaryan, G.V. Abaghyan, S.I. Petrosyan, G.R. Badalyan, M.G. Eghikyan, 2010, published in Izvestiya NAN Armenii, Fizika, 2010, Vol. 45, No. 5, pp. 376–382.

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Films were fabricated by precipitation of makeup gels of the composite on a rotating substrate [13], as well as by the method of vacuum deposition of nanocomposite powder [14] on the arrangement VUP-2k at evaporator temperature from 250°C to 400°C.

The morphology of the surface of nanocomposite and its elemental composition were studied by means of scanning electron microscope (SEM) equipped with microanalytical system for energy-dispersive X-ray (EDX) microanalysis, INCA Energy 300, which allows one to detect all chemical elements starting from boron. The quantitative elemental analysis was performed by means of processing the X-ray spectra obtained from different regions of the sample surface.

The electric conductivity was measured by the four-contact method. The FTIR spectra of the synthesized products were recorded (for powders in pellets with KBr and for

films on KBr substrates) with the spectrometer from the firm Nexus<Thermo Nicolet>.

3. RESULTS AND DISCUSSION The essence of the method of obtaining high-conduction nanosize PAN/SnO2 composites, proposed in

the present work, consists in chemical polymerization of aniline in the presence of nanosize SnO2 particles produced in situ during the synthesis. Synthesis of nanosize SnO2 particles and polycondensation of aniline are combined in the same reactor. The technique of composite production enables us to control the size of SnO2 particles in the range from 10 to 300 nm and to vary its content in the nanocomposite depending on the parameters of synthesis – temperature, concentration of agents, and medium acidity. It is established that the rate of polycondensation of aniline does not depend on the filler concentration and size. The yield of composites decreases with the increase in the acidity and at pH ≤ 2 the composite is not formed. Formation of nanosize SnO2 begins with pH = 3–4 and in this range the highest yield of nanocomposite is observed. Depending on the number of nanoparticles in the reaction medium, we obtained composites with the content of SnO2 up to 90 weight%.

Proceeding from the fact that it is not succeeded to completely extract PAN from the composite, we assume that the growth of PAN begins on the surface of nanoparticles and is bound with them chemically. Interaction of polyaniline with particles of tin oxide, as well as with carbon-black, graphite, and other fillers, is seemingly realized at the first stage of the synthesis, in the process of formation of aniline ion-radical [15]. The electric conductivity of produced nanocomposite decreases from 1 to 0.005 Ω−1cm−1 when the content of tin dioxide increases up to 70 weight%. The electroconductivity of the composite is increased by means of doping PAN with Cl− and SO4

−2 ions for obtaining the emeraldine salt of polyaniline. The conductivity of composite increases in this case by approximately 9 orders of magnitude as compared with non-doped sample (conductivity of emeraldine salt samples pressed under pressure of 50 MPa was 0.05 Ω−1cm−1, while that of base form was 10−10 Ω−1cm−1) and with the increase in the doping degree it reaches a maximum at the content of dopant 24 weight% in PAN.

Study of the composition of samples by the technique of EDX microanalysis has shown (Fig. 1) that in the spectrum of characteristic X-ray radiation appearing at interaction of the electron probe with the sample surface, there are peaks of chemical elements entering the composition of polyaniline (only C and N, because this technique does not detect H) and tin dioxide (Sn and O), as well as the peaks of sulfur and chlorine.

Fig. 1. Spectrum of characteristic X-ray radiation of a sample of composite.

keV

Cl Sn Cl Sn Sn Sn N S Sn Cl

O S Sn

C

Sn

0 1 2 3 4 5

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Measurements were performed in different regions of the sample surface. Table 1 lists average values of concentrations of chemical elements in one of samples.

Table 1. Concentrations of chemical elements composing the sample

Element C N O S Cl Sn

EDX, at% 49.78 7.56 21.67 3.02 1.46 10.51

EDX, weight% 23.51 4.17 17.43 3.81 2.03 49.03

Chem. analysis, weight% – 5.4 – 2.8 2.1 42

Analysis of results of this table allow us to draw, in some approximation, a conclusion on the molecular composition of the sample: there are compounds C24H20N4, SnO2, S (probably bound with O and H), and Cl (probably bound with H). Table 2 gives concentrations of compounds entering the composite and the film produced by vacuum deposition.

Table 2. Concentrations of chemical compounds of the composite

Compound C24H20N4 SnO2 Compounds with S Cl

Composite, mol% 13.09 66.37 19.07 1.46

Composite, weight% 30.68 57.47 9.82 2.03

Film, weight% 25.3 60.7 11.8 2.2

Consideration of results of chemical and EDX elemental analysis allows us to conclude that the produced PAN/SnO2 composite contains practically the amount of nanosize filler introduced at synthesis (∼57 weight%) and 12 weight% of the dopant (as Cl− and SO4

−2 ions), which is confirmed also by a considerable increase in the electric conductivity of samples. It follows from the ratios C / N ≈ 6 and O / Sn ≈ 2 (Table 1) that the stoichiometry of PAN and SnO2 does not change in the conditions of synthesis. Besides, comparison of the content of components of the composite with that of films (Table 2) shows that at vacuum deposition the composition of the produced film is close to that of the composite.

Study of the morphology with use of scanning electron microscopy shows high homogeneity of the nanomaterial consisting mainly of globular particles of the size 20–30 nm. As seen in Fig. 2, in the process of synthesis a homogeneous composite is produced with inclusion of larger formations – PAN globules with sizes in the range of 30–50 nm, number of which depends on the reaction conditions. Formation of globules with sizes of 30–50 nm is typical also for nanosize polyaniline [15].

Fig. 2. SEM-image of the surface of a composite sample.

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In the FTIR spectrum of a nanocomposite (Fig. 3), the absorption bands in the range 1607.5 and 1490.8 cm−1 are typical for chinoid and benzene rings of polyaniline, while the peak at 616.7 cm−1 corresponds, according to [16], to the antisymmetric Sn-O-Sn mode.

Fig. 3. FTIR spectrum of absorption of the composite. By precipitation of makeup gels of the composite with use of spin-coating technique we obtained coats

on substrates of glass and KBr. Conductivity of films on KBr was 5×10−3 Ω−1cm−1. By the method of vacuum deposition of composites on substrates of glass and KBr at the evaporator

temperature of 250–400°C, we obtained transparent films of nanocomposite with the thickness up to 100 nm. Study of the morphology of the surfaces of these films (Fig. 4) showed their high homogeneity.

Fig. 4. SEM-image of the surface of a film obtained by the vacuum deposition technique. Thus, the method we developed allows one to obtain homogeneous conducting nanocomposites

consisting mainly of globular particles with sizes of 20–30 nm, and the electric conductivity of a nanocomposite depends essentially on the content of SnO2, which may reach 90 weight%, as well as on the degree of doping of PAN.

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