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Coating of zinc ferrite particles with a conducting polymer, polyaniline
Journal of Colloid and Interface Science 298 (2006) 87–93 www.elsevier.com/locate/jcis
Coating of zinc ferrite particles with a conducting polymer, polyaniline Jaroslav Stejskal a,∗ , Miroslava Trchová a , Jitka Brodinová b , Petr Kalenda b , Svetlana V. Fedorova c , Jan Prokeš d , Josef Zemek e a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic b Faculty of Chemical Technology, University of Pardubice, 532 10 Pardubice, Czech Republic c Lumex, St. Petersburg 198 005, Russia d Faculty of Mathematics and Physics, Charles University in Prague, 121 16 Prague 2, Czech Republic e Institute of Physics, Academy of Sciences of the Czech Republic, 162 53 Prague 6, Czech Republic
Received 3 June 2005; accepted 15 December 2005 Available online 18 January 2006
Abstract Particles of zinc ferrite, ZnO·Fe2 O3 , were coated with polyaniline (PANI) phosphate during the in situ polymerization of aniline in an aqueous solution of phosphoric acid. The PANI–ferrite composites were characterized by FTIR spectroscopy. X-ray photoelectron spectroscopy was used to determine the degree of coating with a conducting polymer. Even a low content of PANI, 1.4 wt%, resulted in the 45% coating of the particles’ surface. On the other hand, even at high PANI content, the coating of ferrite surface did not exceeded 90%. This is explained by the clustering of hydrophobic aniline oligomers at the hydrophilic ferrite surface and the consequent irregular PANI coating. The conductivity increased from 2 × 10−9 to 6.5 S cm−1 with increasing fraction of PANI phosphate in the composite. The percolation threshold was located at 3–4 vol% of the conducting component. In the absence of any acid, a conducting product, 1.4 × 10−2 S cm−1 , was also obtained. As the concentration of phosphoric acid increased to 3 M, the conductivity of the composites reached 1.8 S cm−1 at 10–14 wt% of PANI. The ferrite alone can act as an oxidant for aniline; a product having a conductivity 0.11 S cm−1 was obtained after a one-month immersion of ferrite in an acidic solution of aniline. © 2005 Elsevier Inc. All rights reserved. Keywords: Conductivity; Conducting polymer; Ferrite; FTIR spectra; Percolation; Polyaniline; Surface coating
1. Introduction Materials comprising a ferromagnetic component and a conducting polymer have recently been investigated in several studies that concentrated on the preparation of nano-colloidal ironoxides, γ -Fe2 O3 [1,2] and Fe3 O4 [3–11], and their subsequent modification with conducting polymers, specifically polyaniline (PANI) or polypyrrole. Most of the papers have reported the magnetic and electrical properties of the resulting nano-sized composite materials. Less attention has been paid to the coating of larger micrometer-sized magnetic particles. Several studies only concerned the surface modification of ferrites with conducting polymers. * Corresponding author. Fax: +420 296 809 410.
E-mail address: [email protected] (J. Stejskal). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.12.034
Ferrites have the general formula MO·Fe2 O3 , where M is an element in a bivalent state, e.g., M 2+ = Fe2+ , Co2+ , Ni2+ , Mn2+ , Zn2+ , Mg2+ , etc., or a combination of them. Nickel–cobalt ferrites have been coated electrochemically with polypyrrole and used as composite electrodes for hydrogen peroxide formation [12]. Manganese–zinc ferrite particles of micrometer size have recently been coated with PANI by using a chemical oxidation of aniline [13]. It was observed that the magnetic properties of a ferrite were influenced by a coating of a conducting polymer. This was explained as the result of an electronic interaction occurring at the interface between the conducting polymer and the ferromagnetic material. The structure of the interface is thus of interest, and is investigated in the present paper. Conducting polymers, viz. PANI, may improve the corrosion protection of metals [14]. Iron oxides, e.g., γ -Fe2 O3 , Fe3 O4 , or ferrites, have fundamental importance in the study of iron corro-
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sion. They are currently used as components in formulations of corrosion-protective materials. The coating of iron oxides with conducting polymers is a logical extension in the design of new anti-corrosion pigments. Inorganic oxides have often been coated with conducting polymers. Silica gel [15], titanium dioxide [16,17], aluminum oxide [18], cerium oxide [19], and copper oxide [20] may serve as examples. The coating is produced in situ during the polymerization of aniline [15], and this technique is also applicable to iron oxides and ferrites. In this study, the coating of zinc ferrite powder with PANI is reported and analyzed. The materials so formed are potential candidates in the corrosion protection of metals. More detailed information on the nature of the coating, an interface between ferrite and conducting polymer, is needed for the understanding of its performance. 2. Experimental 2.1. Preparation of ferrite An equimolar mixture of zinc and ferric oxides was thoroughly homogenized in an agate mortar and calcinated in unglazed corundum pot in an electric high-speed furnace operating at 1100 ◦ C for 2 h at ambient atmosphere. The calcinated zinc ferrite, ZnO·Fe2 O3 , was leached with water and wet-ground in a vibration ball-mill for 5 h at 390 rpm, followed by washing with water, and drying in a desiccator. 2.2. Coating of ferrite with polyaniline In the first series of experiments, the PANI–ferrite composites of various compositions were prepared. To portions of ferrite (2 g), various volumes (5, 10, 20, 30, 50, 75, 100, and 200 ml) of freshly prepared reaction mixture (0.1 M aniline, 0.125 M ammonium peroxydisulfate in 0.4 M phosphoric acid) were added at 20 ◦ C. The mixture was stirred during the polymerization of aniline, which was completed within 1 h. Next day, the ferrite coated with PANI was separated on a paper filter, rinsed with 0.4 M phosphoric acid and with acetone, and dried at 60 ◦ C in a vacuum oven. In the second series, the effect of the acidity of the reaction medium was assessed. The ferrite (2 g) was coated with PANI in 50 ml of reaction mixtures based on 0–3 M phosphoric acid. The precipitates were similarly separated, rinsed with the corresponding acid, then acetone, and similarly dried. 2.3. Characterization The content of PANI in the coated ferrites was determined as an ash. The weight fractions of PANI were converted into volume fractions by using densities of zinc ferrite df = 4.90 g cm−3 , and PANI phosphate, dPANI = 1.45 g cm−3 . Due to a large difference in the densities of the components, the volume fraction of PANI in the composite is considerably larger than the corresponding weight fraction. Infrared spectra in the range of 400–4000 cm−1 were recorded at 64 scans per spectrum at 2 cm−1 resolution using
a fully computerized Thermo Nicolet NEXUS 870 FTIR spectrometer (Nicolet, USA) with a DTGS TEC detector. Samples were dispersed in potassium bromide and compressed into pellets. Spectra were corrected for the moisture and carbon dioxide in the optical path. The X-ray photoelectron spectroscopic (XPS) measurements were carried out in an ADES-400 spectrometer (VG Scientific, UK) equipped with a hemispherical angle-resolved energy analyzer operating with pass energy of 100 eV and using MgKα (1253.6 eV) and AlKα (1486.6 eV) excitation sources. Widesurvey and narrow-scan spectra (C 1s, O 1s, Fe 2p, Zn 2p, N 1s, S 2p, and P 2p) were recorded for qualitative and quantitative analysis, respectively. All spectra were measured at the normal emission angle. To avoid the expected ion-beam-induced modifications of composition and bonding in the analyzed volume, the sample surfaces were analyzed without any cleaning. The energy positions were referenced to the Cu 2p peak at 932.6 eV and Au 4f peak at 84.0 eV binding energy. The overall energy resolution for the XPS measurements reached 0.8 eV. Atomic concentrations were determined from background-subtracted peak areas corrected for the photoelectron cross-sections [21], the inelastic mean free paths [22], and the experimentally determined transmission function of the hemispherical energyanalyzer [23]. Conductivity was determined by a four-point van der Pauw method using a Keithley 237 High-Voltage Source Measurements Unit and a Keithley 2010 Multimeter equipped with a 2000-SCAN 10 Channel Scanner Card on samples compressed into pellets, 13 mm in diameter and ca. 1 mm thick. The density of the PANI was evaluated by weighing the pellets in air and immersed in decane with a Sartorius R160P balance. 3. Results The oxidation of aniline in an acidic aqueous medium yields PANI (Fig. 1) [24,25]. When the reaction mixture contains a ferrite powder, composite materials of both components are obtained (Fig. 2). Phosphoric acid has been selected as a medium for the present study because a phosphate counter-ion in the PANI is beneficial in anti-corrosion applications [14]. The conductivity and electrical stability of PANI phosphate are also good [26]. The dependence of the composite density on the volume fraction of PANI in the composite is linear, as expected, confirming the internal integrity of the experimental data (Fig. 3). The FTIR spectra prove that PANI has been produced. The typical bands of protonated PANI phosphate located at 1561 cm−1 (with a second maximum at 1574 cm−1 and a shoulder at 1603 cm−1 ), 1483, 1304, 1242, 1130, 879, and 802 cm−1 are observed (Fig. 4). Strong bands located in ferrite spectra at 551 and 430 cm−1 decrease as the fraction of the PANI in the composites increases. The peaks are visible even at a high content of PANI, e.g., at 41 wt% PANI, because the penetration depth of the IR radiation is of the order of micrometer, i.e., it is much larger than the thickness of the PANI coating, which is expected to be of the order of 100 nm and lower [27,28]. This
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Fig. 1. Polyaniline (emeraldine) protonated with an acid, HA.
Fig. 2. SEM micrographs of (a) original ferrite and ferrite containing (b) 1.8 wt% (45% coating) and (c) 22.7 wt% (79% coating) of PANI at low (left) and large (right) magnifications.
also suggests that the coating of ferrite with PANI need not be complete. The shape of the quinone-ring-deformation mode in PANI phosphate changes from a single peak at 1583 cm−1 (with a shoulder at 1603 cm−1 ) for a low fraction of PANI phosphate (2 wt%) to typical two maxima at 1561 and 1574 cm−1 (again with a shoulder at 1603 cm−1 ) for a high content of PANI phosphate (41 wt%). The position of the benzene-ringdeformation mode in PANI phosphate is red-shifted from 1498 to 1483 cm−1 . A similar shift has been observed during the conversion of the non-conducting PANI base to a protonated conducting emeraldine form of PANI [29,30]. The degree of the surface coverage of ferrites with PANI was quantified by analysis of the Fe 2p peak shape and the shape of
extended background behind the peak. The method relies on the fact that the energy distribution of emitted electrons depends on the in-depth concentration profile and is able to differentiate among thin and thick homogeneous overlayers, buried layers, three-dimensional island structures on different substrates, etc. The theoretical framework of the technique is described in detail in the literature [31]. The validity of the technique has been established through systematic experimental investigations and comparison to measurements on the same samples by Rutherford backscattering spectrometry, ion scattering spectrometry, and atomic force microscopy [31–34]. The corresponding spectra processing is presently facilitated by the commercial software package QUASES-Tougaard [35]. The technique is de-
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Fig. 3. The densities of composites containing various volume fractions of PANI phosphate.
Fig. 5. The Fe 2p and Fe LMM electron spectra recorded for uncoated and partly PANI-coated (79% and 91%) ferrites.
Fig. 4. FTIR spectra of ferrite coated with PANI phosphate. The content of PANI phosphate (wt%) is denoted at each spectrum.
scribed in more detail elsewhere [36]. Typical Fe 2p spectra with an extended inelastic background are shown in Fig. 5. From the QUASES-Tougaard analysis it follows that (1) surfaces under study are partially covered by PANI, and (2) the PANI thickness exceeds the information depth of the method. For the Fe 2p photoelectrons with kinetic energy of 540 eV traveling through the PANI overlayer, their inelastic mean free path (IMFP) reaches 1.7 nm [37]. Therefore, the PANI overlayer thickness exceeds 5.1 nm. In other words, the Fe 2p photoelectrons originate from PANI uncovered surfaces. The results of QUASES-Tougaard analysis are shown in Fig. 5 together with the results of nitrogen atomic percentage calculated by quantitative analysis from photoelectron spectra described above. The results of conductivity measurement are summarized in Figs. 5 and 6, and discussed below.
Fig. 6. The dependence of the degree of ferrite-surface coating and nitrogen atomic content at the surface on the composition of the PANI–ferrite composite.
4. Discussion 4.1. The coating of ferrite with polyaniline Any surface in contact with the reaction mixture used for the polymerization of aniline becomes coated with a thin PANI film. It is assumed that aniline oligomers are produced at first [28]. Because of their reduced solubility in water, they adsorb at the available interfaces. The adsorbed species start the growth of PANI chains, and that is why the polymerization at the interface is preferred to the corresponding process in the whole volume of the reaction mixture [30,38]. The polymerization of aniline is auto-accelerated [39], i.e., PANI formation is
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preferred at the spots where some PANI has already been produced. This leads to a proliferation of the PANI film nucleated at the interface [28]. This result is consistent with the QUASESTougaard analysis of PANI coverage and also with the results of quantitative analysis of nitrogen content. The latter is proportional to the PANI coverage. When a powdered material is introduced into the reaction mixture, the surfaces of the individual particles serve as template areas for the growth of PANI. This is the case with ferrite particles. The degree of coating is suitably evaluated by XPS (Fig. 6) from the decrease of the signal assigned to iron atoms. A low amount of conducting component PANI, 1.8 wt% PANI (=4.5 vol% PANI) results in 45% coating of the ferrite surface. Obviously, the coating is thin and is not discernible in the SEM micrographs (Fig. 2). When the amount of PANI becomes greater, the degree of coating increases but is never complete (Fig. 6). A similar observation has been made when silica particles [36,40] or organic polymer microparticles [41– 43] have been used as templates for coating. It seems that hydrophilic objects are more difficult to coat completely with conducting polymers, than hydrophobic surfaces [44]. This is probably connected with the better and uniform adsorption of hydrophobic oligomers at the hydrophobic substrate surface. At hydrophilic surfaces, oligomers are likely to produce dropletlike clusters, resulting in the subsequent patchy coating of the surface with a conducting polymer. The granular PANI deposits are visible at the surface of ferrite particles at a larger content of PANI (Fig. 2c), probably because of the secondary nucleation of PANI growth on the already existing PANI [28]. Some aggregates of ferrite particles, held together with PANI, are also seen in SEM micrographs (Fig. 2c). The presence of PANI precipitate in the composites has not been found. This indicates that the growth of PANI at the surface of particles is preferred to a precipitation polymerization of aniline, in accordance with the earlier results obtained with the coating of silica particles [15]. 4.2. Conductivity of PANI–ferrite composites The conductivity of surface-modified ferrites is important for electric applications, and electromagnetic-radiation shielding by the composites [13], while for the corrosion protection the conductivity is of secondary interest. The zinc ferrite is non-conducting, its conductivity being 2 × 10−9 S cm−1 . The conductivity of a PANI-coated ferrite exhibits typical percolation behavior (Fig. 7). This means that the conductivity starts to grow exponentially, after the volume fraction of conducting component exceeds a so-called percolation threshold, up to the conductivity of neat PANI prepared in 0.4 M phosphoric acid, 6.5 S cm−1 . The percolation limit is estimated to be located at 3–4 vol% of PANI. A quantitative analysis could not be performed, because the samples of low PANI content, specially prepared for this purpose, could not be compressed into pellets, so their conductivity could only be estimated on powders. The percolation limit for polymer microparticles coated with a conducting polymer has often been found to be below 10 vol% of the conducting component [40,44,45]. The experimental values of the percolation parameters for the PANI–ferrite composite
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Fig. 7. The conductivity of PANI–ferrite composites containing various volume fractions of PANI phosphate. Table 1 The properties of ferrite coated with polyaniline in aqueous solutions of phosphoric acid of various molar concentrations, [H3 PO4 ]a [H3 PO4 ] (mol L−1 )
Composition (wt% PANI)
Density (g cm−3 )
Conductivity (S cm−1 )
Coating (%)
0 0.2 0.4 0.6 1.0 1.5 2.0 3.0
10.2 12.7 12.0 13.9 11.7 14.4 12.9 12.4
3.98 3.82 3.61 3.70 3.83 3.61 3.70 3.68
0.014 0.33 0.13 1.05 1.01 1.55 1.69 1.83
75.1 80.6 72.6 90.1 93.1 82.3 86.0 88.5
a 0.1 M aniline was oxidized at 20 ◦ C with 0.125 M ammonium peroxydisul-
fate. We refer to the PANI prepared in the presence of phosphoric acid as to PANI phosphate. This need not be precise, because PANI can be protonated with dihydrogen phosphate counter-ions, as many indications suggest.
found in the present case thus confirm the coating of the particles; the percolation threshold for mixtures of ferrite and PANI would be considerably higher [46]. 4.3. The coating of ferrite in media of various acidities The acidity of the reaction medium affects the polymerization of aniline; for this reason, this parameter has been taken into account in the coating of ferrites. Aqueous solutions of phosphoric acid of various concentrations were therefore used as reaction media. In the absence of acid, the polymerization proceeds surprisingly well, with good yield and coating level (Table 1). The conductivity of the composite was 1.4 × 10−2 S cm−1 , i.e., the same as that of neat PANI prepared under the same reaction conditions in the absence of ferrite [26]. When phosphoric acid was present in the reaction mixture, the effect of the acidity was relatively weak. The content of PANI in the composite and the degree of coating are comparable in all cases. The conductivity increased with increasing acid concentration to the level of 1–2 S cm−1 . This is a relatively high value, considering the 10–14 wt% content of PANI in the composite.
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The conductivity of neat PANI prepared in 1.2 M phosphoric acid was 4.8 S cm−1 [26].
and 202/06/0459), and the Ministry of Education, Youth and Sports of the Czech Republic (MSM 0021620834) for the financial support.
4.4. Coating without an external oxidant References Coating the ferrites with a conducting polymer can also be achieved also without using a conventional oxidant, like ammonium peroxydisulfate. Iron(III) compounds themselves act as oxidants, the oxidation of pyrrole with iron(III) chloride to polypyrrole being an example [10,47]. The self-induced polymerization of pyrrole on iron(III)-oxide particles, α-Fe2 O3 , has also been reported [48]. When a zinc ferrite powder was immersed in a solution of aniline in 1 M hydrochloric acid in the absence of ammonium peroxydisulfate, and the suspension was occasionally shaken, the ferrite powder darkened in a period of days. In the case of polypyrrole, Partch et al. [48] and later Chen et al. [10] concluded that the polymerization was initiated directly at the oxide surface. This seems to be also the case with PANI, where the polymerization at surfaces is preferred to the same process in the surrounding aqueous phase [38]. After one month, the coated ferrite was separated on a filter and dried. It had a good conductivity, 0.11 S cm−1 . Although the polymerization was slow, yet it is feasible. A green PANI film also developed on the glass walls of the reaction vessel, demonstrating that iron(III) ions were present in the liquid phase, i.e., the ferrite partly dissolved in the acid. The ferrite mass-loss was marginal, and was compensated by the deposited PANI. 5. Conclusions Even a low amount of PANI, 1.8 wt%, produced on the zinc ferrite particles during the in situ polymerization of aniline, results in a high degree of coating, 45%. On the other hand, the coverage was always incomplete, even at high PANI content >80 vol%. This is likely to be beneficial for the application of the surface-modified ferrite in corrosion protection, because of the potential synergism of ferrite and conducting polymer. The conductivity of composites exhibits percolation behavior, the threshold being located at ca. 3–4 vol% of the conducting component. Despite incomplete coating with PANI, the conductivity of PANI–ferrite composites was of the order of 10−4 –10−1 S cm−1 above 10 vol% PANI content. A composite having a conductivity of 1.4 × 10−2 S cm−1 was produced when aniline was oxidized in the absence of any acid. Increasing the concentration of phosphoric acid in the reaction mixture to 3 M increased the conductivity of the composite to 1.8 S cm−1 at 12 wt% content of PANI. The ferrite itself can act as an oxidant of aniline, and the PANI coating is slowly obtained on its surface in strongly acidic aniline solution in the absence of ammonium peroxydisulfate. Acknowledgments Authors thank the Grant Agency of the Academy of Sciences of the Czech Republic (A4050313, A400500504, and AVOZ 10100521), Grant Agency of the Czech Republic (202/06/0419
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Coating of zinc ferrite particles with a conducting polymer, polyaniline Jaroslav Stejskal a,∗ , Miroslava Trchová a , Jitka Brodinová b , Petr Kalenda b , Svetlana V. Fedorova c , Jan Prokeš d , Josef Zemek e a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic b Faculty of Chemical Technology, University of Pardubice, 532 10 Pardubice, Czech Republic c Lumex, St. Petersburg 198 005, Russia d Faculty of Mathematics and Physics, Charles University in Prague, 121 16 Prague 2, Czech Republic e Institute of Physics, Academy of Sciences of the Czech Republic, 162 53 Prague 6, Czech Republic
Received 3 June 2005; accepted 15 December 2005 Available online 18 January 2006
Abstract Particles of zinc ferrite, ZnO·Fe2 O3 , were coated with polyaniline (PANI) phosphate during the in situ polymerization of aniline in an aqueous solution of phosphoric acid. The PANI–ferrite composites were characterized by FTIR spectroscopy. X-ray photoelectron spectroscopy was used to determine the degree of coating with a conducting polymer. Even a low content of PANI, 1.4 wt%, resulted in the 45% coating of the particles’ surface. On the other hand, even at high PANI content, the coating of ferrite surface did not exceeded 90%. This is explained by the clustering of hydrophobic aniline oligomers at the hydrophilic ferrite surface and the consequent irregular PANI coating. The conductivity increased from 2 × 10−9 to 6.5 S cm−1 with increasing fraction of PANI phosphate in the composite. The percolation threshold was located at 3–4 vol% of the conducting component. In the absence of any acid, a conducting product, 1.4 × 10−2 S cm−1 , was also obtained. As the concentration of phosphoric acid increased to 3 M, the conductivity of the composites reached 1.8 S cm−1 at 10–14 wt% of PANI. The ferrite alone can act as an oxidant for aniline; a product having a conductivity 0.11 S cm−1 was obtained after a one-month immersion of ferrite in an acidic solution of aniline. © 2005 Elsevier Inc. All rights reserved. Keywords: Conductivity; Conducting polymer; Ferrite; FTIR spectra; Percolation; Polyaniline; Surface coating
1. Introduction Materials comprising a ferromagnetic component and a conducting polymer have recently been investigated in several studies that concentrated on the preparation of nano-colloidal ironoxides, γ -Fe2 O3 [1,2] and Fe3 O4 [3–11], and their subsequent modification with conducting polymers, specifically polyaniline (PANI) or polypyrrole. Most of the papers have reported the magnetic and electrical properties of the resulting nano-sized composite materials. Less attention has been paid to the coating of larger micrometer-sized magnetic particles. Several studies only concerned the surface modification of ferrites with conducting polymers. * Corresponding author. Fax: +420 296 809 410.
E-mail address: [email protected] (J. Stejskal). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.12.034
Ferrites have the general formula MO·Fe2 O3 , where M is an element in a bivalent state, e.g., M 2+ = Fe2+ , Co2+ , Ni2+ , Mn2+ , Zn2+ , Mg2+ , etc., or a combination of them. Nickel–cobalt ferrites have been coated electrochemically with polypyrrole and used as composite electrodes for hydrogen peroxide formation [12]. Manganese–zinc ferrite particles of micrometer size have recently been coated with PANI by using a chemical oxidation of aniline [13]. It was observed that the magnetic properties of a ferrite were influenced by a coating of a conducting polymer. This was explained as the result of an electronic interaction occurring at the interface between the conducting polymer and the ferromagnetic material. The structure of the interface is thus of interest, and is investigated in the present paper. Conducting polymers, viz. PANI, may improve the corrosion protection of metals [14]. Iron oxides, e.g., γ -Fe2 O3 , Fe3 O4 , or ferrites, have fundamental importance in the study of iron corro-
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sion. They are currently used as components in formulations of corrosion-protective materials. The coating of iron oxides with conducting polymers is a logical extension in the design of new anti-corrosion pigments. Inorganic oxides have often been coated with conducting polymers. Silica gel [15], titanium dioxide [16,17], aluminum oxide [18], cerium oxide [19], and copper oxide [20] may serve as examples. The coating is produced in situ during the polymerization of aniline [15], and this technique is also applicable to iron oxides and ferrites. In this study, the coating of zinc ferrite powder with PANI is reported and analyzed. The materials so formed are potential candidates in the corrosion protection of metals. More detailed information on the nature of the coating, an interface between ferrite and conducting polymer, is needed for the understanding of its performance. 2. Experimental 2.1. Preparation of ferrite An equimolar mixture of zinc and ferric oxides was thoroughly homogenized in an agate mortar and calcinated in unglazed corundum pot in an electric high-speed furnace operating at 1100 ◦ C for 2 h at ambient atmosphere. The calcinated zinc ferrite, ZnO·Fe2 O3 , was leached with water and wet-ground in a vibration ball-mill for 5 h at 390 rpm, followed by washing with water, and drying in a desiccator. 2.2. Coating of ferrite with polyaniline In the first series of experiments, the PANI–ferrite composites of various compositions were prepared. To portions of ferrite (2 g), various volumes (5, 10, 20, 30, 50, 75, 100, and 200 ml) of freshly prepared reaction mixture (0.1 M aniline, 0.125 M ammonium peroxydisulfate in 0.4 M phosphoric acid) were added at 20 ◦ C. The mixture was stirred during the polymerization of aniline, which was completed within 1 h. Next day, the ferrite coated with PANI was separated on a paper filter, rinsed with 0.4 M phosphoric acid and with acetone, and dried at 60 ◦ C in a vacuum oven. In the second series, the effect of the acidity of the reaction medium was assessed. The ferrite (2 g) was coated with PANI in 50 ml of reaction mixtures based on 0–3 M phosphoric acid. The precipitates were similarly separated, rinsed with the corresponding acid, then acetone, and similarly dried. 2.3. Characterization The content of PANI in the coated ferrites was determined as an ash. The weight fractions of PANI were converted into volume fractions by using densities of zinc ferrite df = 4.90 g cm−3 , and PANI phosphate, dPANI = 1.45 g cm−3 . Due to a large difference in the densities of the components, the volume fraction of PANI in the composite is considerably larger than the corresponding weight fraction. Infrared spectra in the range of 400–4000 cm−1 were recorded at 64 scans per spectrum at 2 cm−1 resolution using
a fully computerized Thermo Nicolet NEXUS 870 FTIR spectrometer (Nicolet, USA) with a DTGS TEC detector. Samples were dispersed in potassium bromide and compressed into pellets. Spectra were corrected for the moisture and carbon dioxide in the optical path. The X-ray photoelectron spectroscopic (XPS) measurements were carried out in an ADES-400 spectrometer (VG Scientific, UK) equipped with a hemispherical angle-resolved energy analyzer operating with pass energy of 100 eV and using MgKα (1253.6 eV) and AlKα (1486.6 eV) excitation sources. Widesurvey and narrow-scan spectra (C 1s, O 1s, Fe 2p, Zn 2p, N 1s, S 2p, and P 2p) were recorded for qualitative and quantitative analysis, respectively. All spectra were measured at the normal emission angle. To avoid the expected ion-beam-induced modifications of composition and bonding in the analyzed volume, the sample surfaces were analyzed without any cleaning. The energy positions were referenced to the Cu 2p peak at 932.6 eV and Au 4f peak at 84.0 eV binding energy. The overall energy resolution for the XPS measurements reached 0.8 eV. Atomic concentrations were determined from background-subtracted peak areas corrected for the photoelectron cross-sections [21], the inelastic mean free paths [22], and the experimentally determined transmission function of the hemispherical energyanalyzer [23]. Conductivity was determined by a four-point van der Pauw method using a Keithley 237 High-Voltage Source Measurements Unit and a Keithley 2010 Multimeter equipped with a 2000-SCAN 10 Channel Scanner Card on samples compressed into pellets, 13 mm in diameter and ca. 1 mm thick. The density of the PANI was evaluated by weighing the pellets in air and immersed in decane with a Sartorius R160P balance. 3. Results The oxidation of aniline in an acidic aqueous medium yields PANI (Fig. 1) [24,25]. When the reaction mixture contains a ferrite powder, composite materials of both components are obtained (Fig. 2). Phosphoric acid has been selected as a medium for the present study because a phosphate counter-ion in the PANI is beneficial in anti-corrosion applications [14]. The conductivity and electrical stability of PANI phosphate are also good [26]. The dependence of the composite density on the volume fraction of PANI in the composite is linear, as expected, confirming the internal integrity of the experimental data (Fig. 3). The FTIR spectra prove that PANI has been produced. The typical bands of protonated PANI phosphate located at 1561 cm−1 (with a second maximum at 1574 cm−1 and a shoulder at 1603 cm−1 ), 1483, 1304, 1242, 1130, 879, and 802 cm−1 are observed (Fig. 4). Strong bands located in ferrite spectra at 551 and 430 cm−1 decrease as the fraction of the PANI in the composites increases. The peaks are visible even at a high content of PANI, e.g., at 41 wt% PANI, because the penetration depth of the IR radiation is of the order of micrometer, i.e., it is much larger than the thickness of the PANI coating, which is expected to be of the order of 100 nm and lower [27,28]. This
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Fig. 1. Polyaniline (emeraldine) protonated with an acid, HA.
Fig. 2. SEM micrographs of (a) original ferrite and ferrite containing (b) 1.8 wt% (45% coating) and (c) 22.7 wt% (79% coating) of PANI at low (left) and large (right) magnifications.
also suggests that the coating of ferrite with PANI need not be complete. The shape of the quinone-ring-deformation mode in PANI phosphate changes from a single peak at 1583 cm−1 (with a shoulder at 1603 cm−1 ) for a low fraction of PANI phosphate (2 wt%) to typical two maxima at 1561 and 1574 cm−1 (again with a shoulder at 1603 cm−1 ) for a high content of PANI phosphate (41 wt%). The position of the benzene-ringdeformation mode in PANI phosphate is red-shifted from 1498 to 1483 cm−1 . A similar shift has been observed during the conversion of the non-conducting PANI base to a protonated conducting emeraldine form of PANI [29,30]. The degree of the surface coverage of ferrites with PANI was quantified by analysis of the Fe 2p peak shape and the shape of
extended background behind the peak. The method relies on the fact that the energy distribution of emitted electrons depends on the in-depth concentration profile and is able to differentiate among thin and thick homogeneous overlayers, buried layers, three-dimensional island structures on different substrates, etc. The theoretical framework of the technique is described in detail in the literature [31]. The validity of the technique has been established through systematic experimental investigations and comparison to measurements on the same samples by Rutherford backscattering spectrometry, ion scattering spectrometry, and atomic force microscopy [31–34]. The corresponding spectra processing is presently facilitated by the commercial software package QUASES-Tougaard [35]. The technique is de-
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Fig. 3. The densities of composites containing various volume fractions of PANI phosphate.
Fig. 5. The Fe 2p and Fe LMM electron spectra recorded for uncoated and partly PANI-coated (79% and 91%) ferrites.
Fig. 4. FTIR spectra of ferrite coated with PANI phosphate. The content of PANI phosphate (wt%) is denoted at each spectrum.
scribed in more detail elsewhere [36]. Typical Fe 2p spectra with an extended inelastic background are shown in Fig. 5. From the QUASES-Tougaard analysis it follows that (1) surfaces under study are partially covered by PANI, and (2) the PANI thickness exceeds the information depth of the method. For the Fe 2p photoelectrons with kinetic energy of 540 eV traveling through the PANI overlayer, their inelastic mean free path (IMFP) reaches 1.7 nm [37]. Therefore, the PANI overlayer thickness exceeds 5.1 nm. In other words, the Fe 2p photoelectrons originate from PANI uncovered surfaces. The results of QUASES-Tougaard analysis are shown in Fig. 5 together with the results of nitrogen atomic percentage calculated by quantitative analysis from photoelectron spectra described above. The results of conductivity measurement are summarized in Figs. 5 and 6, and discussed below.
Fig. 6. The dependence of the degree of ferrite-surface coating and nitrogen atomic content at the surface on the composition of the PANI–ferrite composite.
4. Discussion 4.1. The coating of ferrite with polyaniline Any surface in contact with the reaction mixture used for the polymerization of aniline becomes coated with a thin PANI film. It is assumed that aniline oligomers are produced at first [28]. Because of their reduced solubility in water, they adsorb at the available interfaces. The adsorbed species start the growth of PANI chains, and that is why the polymerization at the interface is preferred to the corresponding process in the whole volume of the reaction mixture [30,38]. The polymerization of aniline is auto-accelerated [39], i.e., PANI formation is
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preferred at the spots where some PANI has already been produced. This leads to a proliferation of the PANI film nucleated at the interface [28]. This result is consistent with the QUASESTougaard analysis of PANI coverage and also with the results of quantitative analysis of nitrogen content. The latter is proportional to the PANI coverage. When a powdered material is introduced into the reaction mixture, the surfaces of the individual particles serve as template areas for the growth of PANI. This is the case with ferrite particles. The degree of coating is suitably evaluated by XPS (Fig. 6) from the decrease of the signal assigned to iron atoms. A low amount of conducting component PANI, 1.8 wt% PANI (=4.5 vol% PANI) results in 45% coating of the ferrite surface. Obviously, the coating is thin and is not discernible in the SEM micrographs (Fig. 2). When the amount of PANI becomes greater, the degree of coating increases but is never complete (Fig. 6). A similar observation has been made when silica particles [36,40] or organic polymer microparticles [41– 43] have been used as templates for coating. It seems that hydrophilic objects are more difficult to coat completely with conducting polymers, than hydrophobic surfaces [44]. This is probably connected with the better and uniform adsorption of hydrophobic oligomers at the hydrophobic substrate surface. At hydrophilic surfaces, oligomers are likely to produce dropletlike clusters, resulting in the subsequent patchy coating of the surface with a conducting polymer. The granular PANI deposits are visible at the surface of ferrite particles at a larger content of PANI (Fig. 2c), probably because of the secondary nucleation of PANI growth on the already existing PANI [28]. Some aggregates of ferrite particles, held together with PANI, are also seen in SEM micrographs (Fig. 2c). The presence of PANI precipitate in the composites has not been found. This indicates that the growth of PANI at the surface of particles is preferred to a precipitation polymerization of aniline, in accordance with the earlier results obtained with the coating of silica particles [15]. 4.2. Conductivity of PANI–ferrite composites The conductivity of surface-modified ferrites is important for electric applications, and electromagnetic-radiation shielding by the composites [13], while for the corrosion protection the conductivity is of secondary interest. The zinc ferrite is non-conducting, its conductivity being 2 × 10−9 S cm−1 . The conductivity of a PANI-coated ferrite exhibits typical percolation behavior (Fig. 7). This means that the conductivity starts to grow exponentially, after the volume fraction of conducting component exceeds a so-called percolation threshold, up to the conductivity of neat PANI prepared in 0.4 M phosphoric acid, 6.5 S cm−1 . The percolation limit is estimated to be located at 3–4 vol% of PANI. A quantitative analysis could not be performed, because the samples of low PANI content, specially prepared for this purpose, could not be compressed into pellets, so their conductivity could only be estimated on powders. The percolation limit for polymer microparticles coated with a conducting polymer has often been found to be below 10 vol% of the conducting component [40,44,45]. The experimental values of the percolation parameters for the PANI–ferrite composite
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Fig. 7. The conductivity of PANI–ferrite composites containing various volume fractions of PANI phosphate. Table 1 The properties of ferrite coated with polyaniline in aqueous solutions of phosphoric acid of various molar concentrations, [H3 PO4 ]a [H3 PO4 ] (mol L−1 )
Composition (wt% PANI)
Density (g cm−3 )
Conductivity (S cm−1 )
Coating (%)
0 0.2 0.4 0.6 1.0 1.5 2.0 3.0
10.2 12.7 12.0 13.9 11.7 14.4 12.9 12.4
3.98 3.82 3.61 3.70 3.83 3.61 3.70 3.68
0.014 0.33 0.13 1.05 1.01 1.55 1.69 1.83
75.1 80.6 72.6 90.1 93.1 82.3 86.0 88.5
a 0.1 M aniline was oxidized at 20 ◦ C with 0.125 M ammonium peroxydisul-
fate. We refer to the PANI prepared in the presence of phosphoric acid as to PANI phosphate. This need not be precise, because PANI can be protonated with dihydrogen phosphate counter-ions, as many indications suggest.
found in the present case thus confirm the coating of the particles; the percolation threshold for mixtures of ferrite and PANI would be considerably higher [46]. 4.3. The coating of ferrite in media of various acidities The acidity of the reaction medium affects the polymerization of aniline; for this reason, this parameter has been taken into account in the coating of ferrites. Aqueous solutions of phosphoric acid of various concentrations were therefore used as reaction media. In the absence of acid, the polymerization proceeds surprisingly well, with good yield and coating level (Table 1). The conductivity of the composite was 1.4 × 10−2 S cm−1 , i.e., the same as that of neat PANI prepared under the same reaction conditions in the absence of ferrite [26]. When phosphoric acid was present in the reaction mixture, the effect of the acidity was relatively weak. The content of PANI in the composite and the degree of coating are comparable in all cases. The conductivity increased with increasing acid concentration to the level of 1–2 S cm−1 . This is a relatively high value, considering the 10–14 wt% content of PANI in the composite.
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The conductivity of neat PANI prepared in 1.2 M phosphoric acid was 4.8 S cm−1 [26].
and 202/06/0459), and the Ministry of Education, Youth and Sports of the Czech Republic (MSM 0021620834) for the financial support.
4.4. Coating without an external oxidant References Coating the ferrites with a conducting polymer can also be achieved also without using a conventional oxidant, like ammonium peroxydisulfate. Iron(III) compounds themselves act as oxidants, the oxidation of pyrrole with iron(III) chloride to polypyrrole being an example [10,47]. The self-induced polymerization of pyrrole on iron(III)-oxide particles, α-Fe2 O3 , has also been reported [48]. When a zinc ferrite powder was immersed in a solution of aniline in 1 M hydrochloric acid in the absence of ammonium peroxydisulfate, and the suspension was occasionally shaken, the ferrite powder darkened in a period of days. In the case of polypyrrole, Partch et al. [48] and later Chen et al. [10] concluded that the polymerization was initiated directly at the oxide surface. This seems to be also the case with PANI, where the polymerization at surfaces is preferred to the same process in the surrounding aqueous phase [38]. After one month, the coated ferrite was separated on a filter and dried. It had a good conductivity, 0.11 S cm−1 . Although the polymerization was slow, yet it is feasible. A green PANI film also developed on the glass walls of the reaction vessel, demonstrating that iron(III) ions were present in the liquid phase, i.e., the ferrite partly dissolved in the acid. The ferrite mass-loss was marginal, and was compensated by the deposited PANI. 5. Conclusions Even a low amount of PANI, 1.8 wt%, produced on the zinc ferrite particles during the in situ polymerization of aniline, results in a high degree of coating, 45%. On the other hand, the coverage was always incomplete, even at high PANI content >80 vol%. This is likely to be beneficial for the application of the surface-modified ferrite in corrosion protection, because of the potential synergism of ferrite and conducting polymer. The conductivity of composites exhibits percolation behavior, the threshold being located at ca. 3–4 vol% of the conducting component. Despite incomplete coating with PANI, the conductivity of PANI–ferrite composites was of the order of 10−4 –10−1 S cm−1 above 10 vol% PANI content. A composite having a conductivity of 1.4 × 10−2 S cm−1 was produced when aniline was oxidized in the absence of any acid. Increasing the concentration of phosphoric acid in the reaction mixture to 3 M increased the conductivity of the composite to 1.8 S cm−1 at 12 wt% content of PANI. The ferrite itself can act as an oxidant of aniline, and the PANI coating is slowly obtained on its surface in strongly acidic aniline solution in the absence of ammonium peroxydisulfate. Acknowledgments Authors thank the Grant Agency of the Academy of Sciences of the Czech Republic (A4050313, A400500504, and AVOZ 10100521), Grant Agency of the Czech Republic (202/06/0419
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