Conducting Polymer-Based Electrode with Magnetic Behavior

1352

J. Phys. Chem. C 2009, 113, 1352–1358

Conducting Polymer-Based Electrode with Magnetic Behavior: Electrochemical Synthesis of Poly(3-thiophene-acetic-acid)/Magnetite Nanocomposite Thin Layers Csaba Jana´ky,† Csaba Visy,*,† Otto´ Berkesi,† and Etelka Tomba´cz‡ Institute of Physical Chemistry, UniVersity of Szeged, Rerrich square 1, Szeged H-6720, Hungary, and Department of Colloid Chemistry, UniVersity of Szeged, Aradi square 1, Szeged H-6720, Hungary ReceiVed: October 22, 2008; ReVised Manuscript ReceiVed: NoVember 21, 2008

Polythiophene-magnetite composite layers have been prepared through the electropolymerization of 3-thiophene-acetic-acid in the presence of Fe3O4 nanoparticles in nitrobenzene. Stabilization of magnetite in this organic medium could be achieved by the reaction between surface -OH groups of the nanoparticles and the -COOH function of the monomers. Fourier transformed infrared spectroscopic (FT-IR) measurements evidenced the chemisorption of the monomer on the surface of the nanoparticles. By modifying the amount of iron-oxide in the polymerization solution, the inorganic material content of the layer could be increased up to 80 m/m%. Electrochemical results, including data obtained by electrochemical quartz crystal microbalance (EQCM), proved that the presence of Fe3O4 did not influence the redox properties of the polymeric film. In the presence of magnetite, an extraordinary microstructure can be detected, where the self-assembling magnetic component strongly determines the morphology of the composite, leading to band formation of ∼1 µm width. This new modified electrode, incorporating such a large amount of Fe3O4, may be used in magnetic electrocatalysis. Introduction Organic-inorganic nanocomposites with an ordered structure provide a new functional hybrid between organic and inorganic materials. Incorporation of nanosized particles in organic polymeric materials has been extensively studied because they combine the advantages of the inorganic materials and the organic polymers. Moreover, due to synergetic effects, also new properties can show up, which can be hardly obtained from the individual components. Nanocomposites based on conjugated polymers and metallic oxides are getting growing interest due to their potential applications in corrosion protection, electrocatalysis, or as electrode materials for batteries. Combination of magnetic ironoxides with different conducting polymers is in the focus of research because materials having both high conductivity and a high magnetic susceptibility can be used in different applications, such as electrical and magnetic shielding, nonlinear optics, magnetic electrocatalysis, and as microwave absorbers.1-3 In the past decade, several papers have been published on the chemical synthesis of such nanocomposites. Polypyrrole,4,5 polyaniline,6-8 and different polythiophenes9-11 were combined with Fe3O4, γ-Fe2O3, and R-Fe2O3 through various chemical procedures. Depending on the method of preparation, the resultant product either was a bulk material or formed separate micro/nanoparticles. Synthesis of thin conducting polymer layers on electrode surfaces can act as modified electrodes with advanced properties. The encapsulation of magnetic nanoparticles into the polymer film may also lead to special behavior. For electrocatalytic applications, the electrochemical synthesis is the best way to prepare these composite films, directly on the electrode surface. * Corresponding author. Phone: +36 62544667. Fax: +36 62544652. E-mail address: [email protected]. † Institute of Physical Chemistry. ‡ Department of Colloid Chemistry.

Despite these facts, just a few papers report on the electrochemical preparation of intrinsically conducting polymermagnetic nanoparticle composites.12-15 The main reason for it is probably that while during the chemical synthesis the nanoparticles work as nuclei the forming oligomer chains are deposited onto them anyway, without any specific interaction between the monomer/oligomer and the particles. The case of the electrochemical synthesis is completely different: the oligomerization takes place in the close vicinity of the electrode, and after the deposition the growth continues on the surface.16 As a consequence, without this interaction the layer forms without or just occasionally incorporated particles. One successful attempt was described where functionalized nanoparticles with negative charge were incorporated during the doping of polypyrrole.14,15 To overcome the above-mentioned problem, in this work we followed another route: we used thiophene-acetic-acid (TAA) as monomer. The adsorption of different carboxyl group containing molecules on magnetite is well-known from the literature.17,18 Polythiophenes are known as both p- and ndopable polymers,19 and their conductivity can be tuned by composite formation.20 In spite of some special successful attempts in water,21 the electropolymerization of TAA can be carried out dominantly in organic solvents, applying various electrochemical methods, resulting in well-adhesive electroactive films.22 Due to the fact that magnetite nanoparticles are usually synthesized in water, they have to be transferred in this case into an organic medium to prepare PTAA-Fe3O4 nanocomposite layers. Our goal was to incorporate magnetite nanoparticles into PTAA layers during the electropolymerization. We report on the synthesis of magnetite containing nanocomposite layers with different composition, which are characterized by various methods. The work is novel because we have realized this aim through the stabilization of the nanoparticles by the monomer

10.1021/jp809345b CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

Polymer-Based Electrode with Magnetic Behavior

Figure 1. X-ray diffraction pattern of the prepared magnetite particles. The indicated reflections and Miller indexes refer to the magnetite phase.

itself. Moreover, the electrochemical synthesis results in thin layers, which can be used as modified electrodes with magnetic properties. Experimental Section Synthesis of Magnetite Nanoparticles. Magnetite (Fe3O4) nanoparticles were synthesized by alkaline hydrolysis of iron(II) and iron(III) salts (FeCl2 · 4H2O and FeCl3 · 6H2O, Reanal, Hungary).23,24 The concentrated solutions of iron(II) and iron(III) salts were mixed in the ratio of 1.1-2 and filtered into Millipore water using a 0.2 µm microfilter. The calculated amount of freshly prepared NaOH solution was added in 10% excess to the double diluted iron salt solution in two portions under rigorous stirring. The formed black suspension was stirred further for some minutes and then transferred into a larger amount of Millipore water. The suspension was washed several times with water to eliminate the alkaline impurities from synthesis and then acidified with HCl solution down to pH∼2, and it was washed again with water until peptization and finally

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1353 dialyzed against 0.001 M HCl. The stock suspension was stored in the dark at 4 °C. Preparation of Thin Nanocomposite Layers. The 3-thiophene-acetic-acid (TAA) monomer, Bu4NBF4, and nitrobenzene (NB) solvent were purchased from Sigma-Aldrich. The water content of nitrobenzene was determined by coulometric Karl Fischer titration, and it was kept below 30 ppm. All solid chemicals have been dried in a Bu¨chi Glass Oven B-580 vacuum system before use. Freeze-dried magnetite nanoparticles were redispersed in the polymerization solution by 60 min sonication resulting in a dark brown sol. For EQCM measurements, poly(3-thiophene-acetic-acid) and PTAA-magnetite composite thin films were deposited galvanostatically at a 3 mA/cm2 current density onto a quartz crystal electrode (10 MHz). The charge density was restricted to 20 mC/cm2, to avoid viscoelastic effects.25 All polymerization solution contained 0.1 M of the monomer 3-thiophene-aceticacid and 0.1 M Bu4NBF4 in nitrobenzene. The working electrode was a Au-coated quartz crystal (A ) 0.196 cm2). The reference electrode was a Ag/AgCl microelectrode, having a potential 0.200 V vs SHE. All the potential values in the paper are given with respect to the silver/silver chloride electrode. The EQCM measurements were carried out by using a quartz-crystal analyzer (QCA917, EG&G Seiko). The amount of magnetite particles varied between 0.1 and 5 g/dm3 in eight steps (0.1; 0.5; 0.75; 1; 2; 3; 4; and 5 g/dm3). For further voltammetric studies, the solution was changed after the polymerization to a monomer and magnetite-free solution of Bu4NBF4 in nitrobenzene. For the SEM and EDX analysis, thicker layers were prepared under identical conditions, with 100 mC/cm2 charge density. Characterization Techniques. The XRD pattern of freshly prepared samples was taken using a Philips PW 1830 X-ray diffractometer operating in the reflection mode with Cu KR radiation (λ ) 0.1542 nm). A copper sample holder was used. The scanning range was between 2Θ ) 20 and 80 degrees. Identification of the synthesized iron oxide was based on the position of characteristic peaks in the diffractograms using the JCPDS (Joint Committee on Powder Diffraction Standards) database.26,27 The XRD patterns were evaluated determining the

Figure 2. FT-IR spectra of magnetite, PTAA, and the composite material. The inset shows the enlargement of the marked section between 900 and 1900 cm-1.

1354 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Jana´ky et al.

SCHEME 1: Stabilization of Magnetite Nanoparticles by Chemisorption of 3-Thiophene-Acetic-Acid

lattice spacings (dhkl values) by the Bragg equation and the Miller (hkl) indices corresponding to the crystalline phases present in the samples. Studies on the adsorption of TAA monomers on the magnetite nanoparticles have been investigated by FTIR using a Bio-Rad Digilab Division FTS-65A/896 Fourier Transform Infrared spectrometer equipped with a Peltier cooled deuterated triglyceride-sulfate (DTGS) detector. The powder samples have been mixed with KBr, and their spectra were recorded in a Bio-Rad Universal Reflection Accessory in diffuse reflectance mode, between 4000 and 400 cm-1, at 4 cm-1 resolution by averaging 256 scans. Hitachi S-4700 scanning electron microscopy (SEM) coupled with EDX spectroscopy (Ro¨ntec QX2) was used to investigate the morphology of the samples, resulting also in complementary data for their elementary composition. Cyclic voltammograms of the thin films were registered in 0.1 M Bu4NBF4 monomer-free solutions at different sweep rates between 10 and 100 mV/s. The electrochemical measurements were performed on a PGSTAT 10 (Autolab) instrument. Results and Discussion Identification of the Prepared Iron-Oxide Nanoparticles. The XRD pattern of the synthesized nanoparticles is shown in Figure 1. The XRD spectra showed the diffraction peaks of Fe3O4(220), Fe3O4(311), Fe3O4(400), Fe3O4(422), Fe3O4(511),

and Fe3O4(440). In general, due to the overlapping peaks, identification of coexisting magnetite and maghemite phases is difficult. However, the analysis based on the JCPDS database clearly showed that the dominant iron-oxide in the sample is magnetite, because some typical peaks can be found, which preferably correspond to the magnetite (at 35.6°, 42.3°, 57.4°, and 63.1°) rather than the maghemite phase (at 31.8° and 49.2°). Considering the d values and Miller indexes, the peaks appearing at 35.6 and 42.3 2Θ degrees are attributed to the Fe3O4 phase. The average particle size was calculated from the broadening of the diffraction peak corresponding to the most intensive reflection. The Scherrer equation dav ) K × l /(B × cos Θ) was used,28,29 where dav is the average particle size; K is the Scherrer constant (shape factor, its value is 0.9 for magnetite and maghemite); l is the X-ray wavelength (λ ) 0.1542 nm); B is the broadening related to the particle (difference between the measured width at half-maximum of the diffraction peak (Bb) and the instrumental broadening of the single crystal form (Bs)); and Θ is the position of the diffraction peak maximum (B is given in radians). According to the B values of the Fe3O4(311) and Fe3O4(440) peaks, using the above-mentioned Scherrer equation, the average grain sizes of Fe3O4 were calculated to be 11.3 nm. Adsorption of TAA on Magnetite Nanoparticles. Previous studies evidenced that the surface of magnetite nanoparticles prepared by our procedure is covered by -OH groups.26 It can

Figure 3. Chrono-potentiometric E-t curves obtained during the galvanostatic polymerization (i ) 3 mA/cm2) of TAA: 1, in the absence; 2, in the presence of 5 g/dm3 of magnetite.

Polymer-Based Electrode with Magnetic Behavior

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1355

Figure 6. EDX spectra of a nanocomposite layer deposited on a gold electrode with a charge density of 100 mC/cm2. Figure 4. Comparison of the mass changes obtained under identical (polymerization) conditions, as in Figure 3.

Figure 5. Slope of the EQCM signals (∆m/t) during the polymerization at different magnetite concentrations. Each point was obtained as the average of three experiments.

be assumed that some adduct might be formed through the -OH and -COOH functional groups, as there is a possibility for the formation of a carboxylic salt. To prove the existence of such an adduct between the magnetite nanoparticles and the TAA monomers, FT-IR spectra were taken. In Figure 2, three spectra are compared: 1, TAA alone; 2, magnetite nanoparticles alone; 3, the TAA-Fe3O4 system obtained when TAA was let to adsorb on magnetite in NB solvent (after separation from the sol). The ν (CdO) vibration at 1690 cm-1 can be related to the free carboxylic group of the TAA monomer.30 In the spectrum of the pure magnetite, the broad band between 3500 and 3100 cm-1 can be assigned to the ν (O-H) vibration, while two characteristic peaks can be observed at 590 and 400 cm-1 which correspond to the Fe-O stretching modes of the magnetite lattice. Two shoulders at 632 and 440 cm-1, that can be assigned to maghemite,31 the oxidized form of magnetite, probably present at the nanoparticle surface (XRD spectra in Figure 1 did not show any peak that could correspond to the presence of maghemite). In contrast to the neat materials, in the sample obtained from the adsorption experiment, beyond the evident appearance of the previously mentioned peaks, two significant

Figure 7. Cyclic voltammograms of PTAA and the nanocomposite layers (as in Figure 3), obtained in 0.1 M Bu4NBF4 in nitrobenzene at a 25 mV/s sweep rate.

spectral modifications can be revealed. First, the broad band related to the surface -OH groups disappeared. Second, the signal of ν (CdO) vibration in the free-carboxylic group decreased considerably, while two new bands appeared between 1625 and 1500 cm-1 (νas CO2-) and between 1430 and 1340 cm-1 (νs CO2-), which can be assigned as the carboxylate stretching mode of the anion. It is not surprising since the neutralization reaction readily takes place between the surface metal-hydroxide and the carboxylic acid in nonaqueous media.30 All these modifications serve as a direct evidence for the chemisorption of TAA molecules on the magnetite surface, which can stabilize the nanoparticles in the organic solution. This way the adsorbed monomer itself is the stabilizer of the nanoparticles, functioning as a natural “bridge” during the polymerization, as is illustrated in Scheme 1. Chemical Composition. The comparison of the chemical composition of the pure polymer and the composite films should be performed with materials synthesized under totally identical conditions, except the presence of the nanoparticles. Chronopotentiometric curves registered during the galvanostatic polymerizationspresented in Figure 3sgive evidence that the polymerization process is not disturbed by iron oxide sol in the case of the composite formation.

1356 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Jana´ky et al.

Figure 8. SEM images at 5000 × magnification. (A) PTAA. (B) PTAA-Fe3O4 (prepared in the presence of 5 g/dm3 of magnetite).

Figure 9. SEM images at 10 000 × magnification. (A) PTAA. (B) PTAA-Fe3O4 (prepared in the presence of 5 g/dm3 of magnetite).

SCHEME 2: In Situ Formation of Composite Bands during the Electrochemical Polymerization of TAA in the Presence of the Stabilized Magnetic Nanoparticles

To get information about the composition of the electrochemically deposited film, EQCM measurements were performed. The mass increase of PTAA films with time (and with the transferred charge under galvanostatic conditions) is linear for all systems with or without nanoparticles as shown in Figure 4. As the film thickness remains smaller than 150 nm, the behavior of the film is ideally nonelastic,25 and we may apply the Sauerbrey equation.

∆m ) (4.47 × 1011 × ∆f) ⁄ (f0 × f)

(1)

These facts can be related to the monotonous growth of the films and to the assumingly uniform particle incorporation. The calculated mass change observed during identical galvanostatic polymerization conditions is shown for neat PTAA, one of the nanocomposites, and for a pure magnetite sol in Figure 4. The difference is striking: while in the case of pure magnetite sol almost no mass change can be seen, during the composite synthesis the incorporation of Fe3O4 particlesscompared to the formation of the neat polymersis clearly visible. By using the Sauerbrey equation and by taking into account the constant current applied during the galvanostatic synthesis, a slope of 0.811 µg/mC is obtained for the neat PTAA. As is known, such a result can be used to determine the virtual molar mass of the deposited layer,32 if there is a reliable guess for the oxidation (doping) level. For this calculation, the d ) 2Qox/

Qpol - Qox expression can be used33 if we assume that the oxidation level is the same at the anodic polarization potential of the film and at the end of the polymerization. Calculation gives a value of y ) 0.23. (The same value was obtained by using reduction charge of the film.) Using this calculated doping level, we can compare the measured value with the theoretical one, based on the following equation

j experimental ) M

mpol mpol · (2 + y)F ∆m ) ) × npol qpol i × ∆t

(2 + y) × F ) 0.811 µg ⁄ mC × (2 + y) × F ) 174.49 g ⁄ mol (2) j theoretical ) MTAA-2H+ + y × Manion ) M (140.18 + 0.23 × 86.81) g ⁄ mol ) 160.15 g ⁄ mol (3) It is clearly visible that the calculated result is in good agreement with the experimental data. The slightly larger value of the latter may indicate the incorporation of some solvent into the polymer layer during the polymerization.34 This calculation proves that we can rely on the EQCM measurements, even in the respect that they support the assumption applied in the calculation of the doping level, based on electrochemical charge data. When Fe3O4 nanoparticles are present during the polymerization, their incorporation into the polymeric film can be easily

Polymer-Based Electrode with Magnetic Behavior detected by EQCM measurements. In this part of the work, we varied the magnetite concentration in the range of 0-5 g/dm3 in eight steps. At each concentration, we performed three polymerizations and calculated the average of every three ∆m/ ∆Q slopes. These values are presented in Figure 5, which shows that the slope of the mass change/time curves gradually increases with the amount of the nanoparticles in the polymerization solution, and the curve tends to reach a saturation value. This limit/maximum amount seems to be at ∼4-5 g/dm3 in accordance with our experimental observation, namely, that above this concentration the sol becomes very unstable, exhibiting fast tendency for precipitation. If we compare the saturation value (∼12.4 mg · cm-2 · s-1) with the value measured in the case of the neat polymer (∼2.4 mg · cm-2 · s-1), we may calculate by subtraction the approximate composition of the layer. On this basis, the “concentration” of magnetite in the layer can be increased up to 80 m/m%. From this value, we can make a rough estimation on the percentage of iron-oxide in volume by using the average density of PTAA (∼1.3 g · cm-3) and the density of magnetite nanoparticles (∼5 g · cm-3). In the latter case, the 80 m/m% value means approximately 20 v/v%. The elementary composition could have been obtained also from EDX data, but the signal of the gold support was dominant (Figure 6). Moreover, the overlapping of the peaks related to gold and sulfur renders the quantitative analysis difficult. Nevertheless, the presence of magnetite is clearly evidenced by the iron and oxygen peaks. Basic electrochemical measurements with the composite electrode with different magnetite content have been performed by cyclic voltammetry at different sweep rates. Current-potential curves of the two extreme cases at a sweep rate of 25 mV/s are compared in Figure 7. They show a very similar pattern, although the dotted curve was obtained with the nanocomposite prepared in a 5 g/dm3 sol of magnetite. This observation clearly proves that by applying the same polymerization charge, we obtain the same PTAA, independently of the presence or absence of Fe3O4. Furthermore, it seems that the mixed oxide of iron remains electrochemically inactive in this composite in the studied potential range. So we may conclude that the relatively large amount of magnetite influences only slightly the basic redox behavior and the charge capacity of the film. Thus, although the polymeric film contains ∼80% semiconductor material, its presence does not disturb the electrochemical properties of the matrix. Surface Morphology. In contrast to the unmodified electrochemical properties, the structural difference in the two types of films is striking. According to Figures 8A and 9A, the neat PTAAs morphology is the generally obtained, randomly varying surface, consisting of globular, cauliflower-like units. In the case of the composite layer (at 5 g/dm3), the structure is totally different. Although the globular shape is preserved, there is a secondary structure, where the layer contains ordered, aligned bands resulting in a specific microstructure, with parallel strips of ∼1 µm width. It is obvious that this structure should be related to the presence of magnetic nanoparticles. It is assumed that in this structure Fe3O4 is ferromagnetic, and this behavior leads to some kind of chain formation. Since magnetite nanoparticles with a size of 11 nm are considered to be superparamagnetic, this band-type structure can be associated with partial aggregation during the redispersion in NB, resulting in larger, ferromagnetic particles. Chain formation of magnetic nanoparticles was reported many times,35,36 mostly in the presence

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1357 of a magnetic field. It should be emphasized that in our case this chainlike microstructure has been obtained for a conducting polymer composite without an external magnetic field. Thus, the structure is the consequence of self-assembling of the nanoparticles, and the deposition takes place on the surface of the chains (Scheme 2). Conclusions In this work, we give a report on the successful synthesis of PTAA-Fe3O4 nanocomposite layers. To achieve this task, magnetite nanoparticles have been redispersed in nonaqueous (NB) solution, which was performed so that the adsorbing TAA monomers themselves acted as the stabilizer. The adsorption proved to be a chemisorption, realized through a chemical interaction between the surface -OH groups of the magnetite nanoparticles and the -COOH groups of the monomers. The formation of the carboxylate anion was evidenced by FT-IR measurements. EQCM data acquired during the polymerization clearly showed that the incorporated amount of magnetite nanoparticles increases with the concentration of Fe3O4 in the polymerization sol, leading to a saturation pattern. This limiting value is equal to about 80 m/m % magnetite. Cyclic voltammetric results evidenced that the polymer formation depends only on the polymerization charge, and the electrochemical behavior of the film is not disturbed by the presence of the different amount of iron-oxide. The presence of the built-in nanoparticles leads to a novel morphology, where an aligned, bandlike microstructure shows up, forming about 1 µm wide stripes. Acknowledgment. The authors are grateful to professor Jouko Kankare (University of Turku, Finland) for ensuring the opportunity for the EQCM measurements in his laboratory. Financial support from the Hungarian National Office of Research and Technology (NKTH) and the Agency for Research Fund Management and Research Exploitation (KPI) no. DAMEC09/2006 as well as from the Hungarian National Research Fund (OTKA no. K72989) is gratefully acknowledged. References and Notes (1) Gomez-Romero, P. AdV. Mater. 2001, 13, 163. (2) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (3) Rajeshwar, K.; de Tacconi, N. R.; Chenthamarakshan, C. R. Chem. Mater. 2001, 13, 2765. (4) Gangopadhyay, R.; De, A. Eur. Polym. J. 1999, 35, 1985. (5) Turcu, R.; Pana, O.; Nan, A.; Giurgiu, L. M. Polymeric Nanostructures and Their Applications; Nalwa, H. S., Ed.; American Scientific Publishers, 2006; Vol 1, p 337. (6) Long, Y; Chen, Z; Duvail, J. L; Zhang, Z; Wan, M. Physica B. 2005, 370, 121. (7) Jacobo, S. E; Aphesteguy, J. C; Anton, R. L; Schegoleva, N. N.; Kurlyandskaya, G. V. Eur. Polym. J. 2007, 3, 1333. (8) Li, X.; Shen, J.; Wan, M.; Chen, Z.; Wei, Y. Synth. Met. 2007, 157, 575. (9) Silva, R. A.; Santos, M. J. L.; Rinaldi, A. W.; Zarbin, A. J. G.; Oliveira, M. M.; Santos, I. A.; Cotica, L. F.; Coellho, A. A.; Rubira, A. F.; Girotto, E. M. J. Solid State Chem. 2007, 180, 3545. (10) Zhang, X.; Lee, J.-S; Lee, G. S.; Cha, D.-K.; Kim, M. J.; Yang, D. J.; Manohar, S. K. Macromolecules 2006, 39, 470. (11) Jana´ky, C.; Visy, C. Synth. Met., in press 10.1016/j.synthmet.2008.07.014. (12) Garcia, B; Lamzoudi, A; Pillier, F; Nguyen, T. Le H; Deslouis, C. J. Electrochem. Soc. 2002, 149, B560. (13) Pailleret, A.; Hien, N. T. L.; Thanh, D. T. M.; Deslouis, C. J. Solid State Electrochem. 2007, 11, 1013. (14) Bidan, G.; Jarjayes, O.; Fruchart, F.; Hannecart, E. AdV. Mater. 1994, 6, 152. (15) Jarjayes, O.; Fries, P. H.; Bidan, G. Synth. Met. 1995, 69, 343.

1358 J. Phys. Chem. C, Vol. 113, No. 4, 2009 (16) Inzelt, G. Conducting Polymers; Springer-Verlag: Berlin-Heidelberg, 2008; Chapter 4. (17) Avdeev, M. V.; Bica, D.; Vekas, L.; Marinica, O.; Balasoiu, M.; Aksenov, V. L.; Rosta, L.; Garamus, V. M.; Schreyer, A. J. Magn. Magn. Mater. 2007, 311, 6. (18) Tomba´cz, E.; Bica, D.; Hajdu´, A.; Ille´s, E.; Majzik, A.; Ve´ka´s, L. J. Phys.: Condens. Matter 2008, 20, 6. (19) Visy, C.; Lukkari, J.; Kankare, J. Macromolecules 1993, 26, 3295. (20) Visy, C; Fekete, Z. A.; Pinter, E; Makra, P; Berkesi, O; Patzko, A. J. Phys. Chem. C 2007, 111, 11872. (21) Lagrost, C.; Lacroix, J.-C.; Chane-Ching, K. I.; Jouini, M.; Aeiyach, S.; Lacaze, P.-C. AdV. Mater. 1999, 11, 664. (22) Li, J.; Aoki, K. J. Electroanal. Chem. 1998, 458, 155. (23) Ille´s, E; Tomba´cz, E. Colloids Surf. A 2003, 230, 99. (24) Ille´s, E.; Tomba´cz, E. J. Colloid Interface Sci. 2006, 295, 115. (25) Skompska, M.; Jackson, A.; Hillman, A. R. PCCP 2000, 20, 4748. (26) Cornell, R. M; Schwertmann, U. The iron oxides; VCH: Weinheim, 1996; p 573 and p 207.

Jana´ky et al. (27) Tomba´cz, E.; Ille´s, E.; Majzik, A.; Hajdu´, A.; Rideg, N. Croat. Chem. Acta 2007, 80, 503. (28) Bartram, F. Handbook of X-rays; Kaelble, E. F., Ed; McGrawHill: New York, 1967; pp 17.1.-17.18. (29) Patterson, A. L. Phys. ReV. 1939, 56, 978. (30) Mehrotra, R. C; Bohra, R. Metal Carboxylates; Academic Press: London, 1983; p 58, p 19. (31) Roca, A. G.; Marco, J. F.; Morales, M. P.; Serna, C. J. J. Phys. Chem. C 2007, 111, 18577. (32) Inzelt, G.; Kerte´sz, V. Electrochim. Acta 1997, 42, 229. (33) Visy, C.; Lukkari, J.; Kankare, J. Synth. Met. 1989, 33, 289. (34) Kriva´n, E; Visy, C; Kankare, J. J. Phys. Chem. B. 2003, 107, 1302. (35) Novakova, A. A.; Smirnov, E. V.; Gendler, T. S. J. Magn. Magn. Mater. 2006, 300, e354. (36) Lalatonne, Y.; Richardi, J.; Pileni, M. P. Nat. Mater. 2004, 3, 121.

JP809345B