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Electrodeposition of composite iron oxide–poly(allylamine hydrochloride) films
Materials Chemistry and Physics 96 (2006) 289–295
Electrodeposition of composite iron oxide–poly (allylamine hydrochloride) films J. Cao, I. Zhitomirsky ∗ , M. Niewczas Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S 4L7 Received 24 February 2005; received in revised form 7 June 2005; accepted 15 July 2005
Abstract New method has been developed for the fabrication of ␥-Fe2 O3 films and composites containing ␥-Fe2 O3 particles in the poly(allylamine hydrochloride) (PAH) matrix. The proposed method is based on the electrosynthesis of ␥-Fe2 O3 and electrodeposition of cationic PAH macromolecules. Cathodic deposits of various thicknesses in the range of up to several microns were obtained on Pt and graphite substrates. Obtained films were studied by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential thermal analysis and atomic force microscopy. dc and ac magnetic measurements performed in the temperature range of 2–298 K revealed the superparamagneric behavior of the films. Magnetic properties can be varied by the variation of film composition. © 2005 Elsevier B.V. All rights reserved. Keywords: Oxides; Precipitation; Electrochemical techniques; Magnetic properties
1. Introduction Nanostructured superparamagnetic materials are now being extensively studied for integrated circuits, color imaging, magnetic refrigerators and biomedical applications [1–7]. The size-controlled synthesis of magnetic nanoparticles is of great technological and scientific importance. Below a critical size, nanocrystalline single-domain magnetic particles show superparamagnetic behavior [1–3]. A critical obstacle in assembling and maintaining a nanoscale magnetic material is its tendency to aggregate. To overcome this, nanoparticles of magnetic materials were isolated in a polymer or ceramic matrix and advanced composite materials were developed [4–10]. Electrodeposition of magnetic thin films is an area of intense interest [11–14]. Electrodeposition is unique in that it can be used for the deposition of various magnetic materials, including metals, oxides, alloys and composites. Powders and thin films of oxide materials can be prepared by cathodic electrosynthesis [15–20] from the solutions of metal salts. ∗
Corresponding author. Tel.: +1 905 5259140; fax: +1 905 5289295. E-mail address: [email protected] (I. Zhitomirsky).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.015
In this method metal ions or complexes are hydrolyzed by electrogenerated base to form oxide or hydroxide deposits on cathodic substrates. Hydroxide deposits can be converted to corresponding oxides by thermal treatment. An important discovery was the feasibility of cathodic electrodeposition of nanostructured composite materials based on metal oxides or hydroxides and polyelectrolytes [20,21]. The proposed method is based on electrosynthesis of an organic phase and electrophoretic deposition of charged polymers. New electrochemical strategies are currently under development, which are based on the use of strong and weak polyelectrolytes [22–24]. Electrodeposition has been utilized for the fabrication of composite films containing Fe3 O4 nanoparticles and poly(diallyldimethylammonium chloride) (PDDA) [25]. Obtained films exhibited superparamagnetic behavior. Cathodic electrodeposition enables the formation of oxide particles in situ in a polymer matrix, preventing particle agglomeration. It was shown that magnetic properties, composition, nanostructure and morphology of the composite films could be tailored by variation of bath composition, deposition parameters and mass-transport conditions for organic and inorganic components.
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J. Cao et al. / Materials Chemistry and Physics 96 (2006) 289–295
In a previous investigation [25], the formation of superparamagnetic Fe3 O4 –PDDA composite films was achieved from mixed solutions of Fe3+ and Fe2+ salts in a deaerated methanol–water solvent. Deposition and film drying was performed in a forming gas (93% Ar–7% H2 ) in order to prevent oxidation of Fe2+ species. In our new work, we report a simple method for the fabrication of magnetic composite films. In the new electrochemical strategy, composite films containing a cationic polyelectrolyte and iron oxide were obtained using only Fe3+ salt instead of a mixture of Fe2+ and Fe3+ salts used in the previous investigation [25]. Moreover, electrodeposition was performed in air, thus eliminating the need of using the forming gas during the solution preparation, electrodeposition and film drying. Therefore, new method offers important processing advantages compared to the method described in the previous investigation [25]. We report magnetic properties of the films and discuss the deposition mechanism.
2. Experimental procedures Ferric chloride hexahydrate (FeCl3 ·6H2 O), and poly(allylamine hydrochloride) (PAH) were obtained from Aldrich. Electrodeposition was performed from 5 mM FeCl3 solutions containing 0–0.5 g l−1 PAH in a mixed ethanol–water (30% water) solvent. The electrochemical cell for deposition included a cathodic substrate centered between two parallel platinum counter electrodes. The films were deposited on Pt foil and graphite (50 mm × 20 mm × 0.1 mm) cathodes at a current density of 1–5 mA cm−2 . After drying the deposits were scraped from the Pt electrodes for X-ray diffraction analysis (XRD), thermogravimetric (TG) and differential thermal (DTA) analysis. The thermoanalyzer (Netzsch STH-409) was operated in air between room temperature and 1473 K at a heating rate of 5 K min−1 . The phase content was determined by XRD analysis with a diffractometer (Nicolet I2) using monochromatized Cu K␣ radiation at a scanning speed of 0.5 ◦ C min−1 . The structure of the deposited films was studied using a Philips 515 scanning electron microscope (SEM). The surface topography of films deposited on polished stainless steel substrates was imaged using atomic force microscopy (AFM). An atomic force microscope (NanoScopeIIIa, Digital Instruments) was used in tapping mode. Magnetic properties were studied using a Quantum Design PPMS-9 system. dc Magnetization studies were performed using the extraction magnetometer option. Magnetization hysteresis loops were measured in the field range of −90 kOe < H < +90 kOe at temperatures ranging from 2 to 298 K. The external magnetic field was changed in the sweep mode at the sweep rate of 10–100 Oe s−1 . The temperature dependence of the magnetization was studied by both zerofield-cooled (ZFC) and field-cooled (FC) procedures in the field range of 200–500 Oe. The sample was cooled down to 1.9 K in the zero external field (ZFC) and then magnetization
was measured during heating to 298 K under the applied field. The sample was subsequently cooled back to 1.9 K under an applied field (FC) and the measurements of magnetization were carried out during heating to 298 K. ac Magnetic susceptibility was studied at temperatures 2–298 K.
3. Experimental results Electrodeposition performed from the 5 mM FeCl3 solutions containing 0–0.5 g l−1 PAH resulted in the formation of cathodic deposits. The films prepared from pure FeCl3 solutions exhibited low adhesion. The use of PAH additive enabled the formation of thick and adherent films. It is suggested that the polymer acts as a binder, which improves deposit adhesion and enables the formation of thick films. The deposits were analyzed by XRD before and after annealing in air at different temperatures. Fig. 1 shows Xray diffraction pattern of the deposits prepared from pure FeCl3 solutions. The fresh deposits and those annealed at 473 K, exhibited peaks, which can be attributed to ␥-Fe2 O3 or Fe3 O4 . However, it is difficult to distinguish between ␥Fe2 O3 and Fe3 O4 due to the similar spinel structure of the both phases and peak broadening, which is attributed to low particle size. Moreover, the two phases can form solid solutions. Small peaks of ␣-Fe2 O3 appear on the X-ray diffraction patterns at 673 K in addition to the peaks of the spinel phase. After heat treatment at 773 K the X-ray diffraction pattern shows peaks of ␣-Fe2 O3 . Fig. 2 shows X-ray diffraction patterns of the deposits prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH. The fresh deposits and those annealed at 473 K showed small peak around 2Θ = 36◦ . At 573 K, peaks of spinel iron
Fig. 1. X-ray diffraction patterns of the deposits prepared from 5 mM FeCl3 solutions: as-prepared (a) and after annealing at 473 K (b), 673 K (c) and 773 K (d); () ␥-Fe2 O3 (JCPDS file 39-1346), () ␣-Fe2 O3 (JCPDS file 33-664).
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Fig. 4. TG (a) and DTA (b) data for the deposits prepared from 5 mM FeCl3 solutions, containing 0.5 g l−1 PAH.
Fig. 2. X-ray diffraction patterns of the deposits prepared from 5 mM FeCl3 solutions, containing 0.5 g l−1 PAH: as-prepared (a) and after annealing at 473 K (b), 573 K (c) and 673 K (d); () ␥-Fe2 O3 (JCPDS file 39-1346), () ␣-Fe2 O3 (JCPDS file 33-664).
oxide phase were observed. The XRD pattern of the sample annealed at 673 K exhibits small peaks of the spinel phase in addition to the major peaks of ␣-Fe2 O3 . Fig. 3 shows the TG and DTA data for the deposits prepared from the FeCl3 solutions without PAH. A sharp reduction of sample weight was recorded below 423 K, followed by a more gradual weight change. No appreciable weight loss was observed at temperatures exceeding 673 K. The total weight loss at 1473 K was found to be 10.9 wt%. The observed weight loss can be attributed to the liberation of the adsorbed water. On the basis of the TG data it can be suggested that the deposition process resulted in the formation of ␥-Fe2 O3 . Indeed, no weight gain was recorded, which can be attributed to the Fe3 O4 → ␣-Fe2 O3 transformation. It is important to note that in this work electrodeposition was performed in air using FeCl3 solutions. In contrast, chemical and electrochemical precipitation of Fe3 O4 from mixed
Fig. 3. TG (a) and DTA (b) data for the deposits prepared from 5 mM FeCl3 solutions.
solutions containing Fe2+ and Fe3+ species was performed in the atmosphere of nitrogen or forming gas [25,26] in order to prevent oxidation of Fe2+ species. DTA data showed a broad endotherm at ∼373 K and a broad exotherm started at ∼773 K with a peak value around 823 K. The observed endotherm is associated with weight loss. The exotherm can be attributed to the ␥-Fe2 O3 → ␣-Fe2 O3 transformation. It is important to note that the DTA data was recorded at a heating rate of 5 ◦ C min−1 , whereas the X-ray data were taken from the samples annealed at different temperatures during 1 h. This can explain the difference in the transition temperature obtained from DTA and XRD data. TG data for the deposits prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH showed several steps in the weight loss below 823 K (Fig. 4). The total weight loss at 1473 K was found to be 61.1 wt%. The total weight loss at 1473 K for this deposit (Fig. 4) exceeds significantly the weight loss for the deposit prepared from pure FeCl3 solution (Fig. 3). These results indicate co-deposition of iron oxide and PAH. The additional weight loss is attributed to burning out of an organic phase. The DTA data for the deposits prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH (Fig. 4) showed a broad endotherm in the range below 473 K and exotherm in the range 623–823 K. The endotherm is associated with weight loss. The exotherm can be attributed to burning out of PAH and the ␥-Fe2 O3 → ␣-Fe2 O3 transformation. The results of the thermogravimetric analysis were used to calculate the amount of an organic phase in the composite films. When considering the composite material as a mixture of organic and inorganic components, the weight ratio of Fe/polymer in the film was found to be 0.48. This value is close to the weight ratio of Fe/polymer = 0.56 in the solutions used for deposition. However, it is important to note that some composite materials cannot be considered as a simple mixture of inorganic and organic phases. Composite films of various thicknesses in the range of up to several micrometers were obtained on Pt and graphite substrates by the variation of the deposition time and current
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Fig. 7. Magnetization vs. applied field for the deposits prepared from 5 mM FeCl3 solutions. Fig. 5. SEM picture of a sectioned film (F) obtained from a 5 mM FeCl3 solution, containing 0.5 g l−1 PAH on a graphite substrate (S).
density. Fig. 5 shows a SEM picture of a composite iron oxide–PAH film on a graphite substrate. It was observed that electrodeposition results in uniform films. Surface roughness of the films was studied using AFM. Fig. 6 shows the AFM image of the film prepared at a current density of 2 mA cm−2 . The root mean square (rms) surface roughness of the film was found to be 1.6 nm. Fig. 7 shows typical magnetization versus magnetic field dependencies for the deposits prepared from FeCl3 solutions without PAH. The magnetization data obtained in the low field range indicated a hysteresis behavior at 5 K. In contrast no magnetic hysteresis was observed at room temperature. The magnetization curve recorded at room temperature showed zero remanence and zero coercivity. These data are consistent with superparamagnetic behavior of the nanopar-
Fig. 6. AFM image of the film prepared from 5 mM FeCl3 solution containing 0.5 g l−1 PAH at a current density of 2 mA cm−2 .
ticles. Plots of magnetizations versus magnetic field for the composite films show hysteresis behavior at 5 K (Fig. 8). However, nearly linear behavior was observed at 298 K. Fig. 9 compares the magnetization curves at 2 K obtained in field range up to 90 kOe for the iron oxide deposits and composite films. The saturation magnetization was found to be ∼60 emu g−1 for the deposits prepared from 5 mM FeCl3 solutions. The composite films prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH showed lower magnetization. However, annealing of the films at temperatures above 473 K resulted in a significant increase of the saturation magnetization, which was found to be ∼25 emu g−1 after heat treatment at 573 K. Fig. 10 shows dc magnetization of the deposits prepared from 5 mM FeCl3 solutions. A separation of the ZFC and FC curves was observed at low temperatures. ZFC curves exhibited broad cusps. The ZFC and FC curves obtained at 200 and 500 Oe superimposed at temperatures above 250 and 175 K, respectively.
Fig. 8. Magnetization vs. applied field for the deposits prepared from 5 mM FeCl3 solution containing 0.5 g l−1 PAH.
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Fig. 9. Magnetization vs. applied field for the deposits prepared from 5 mM FeCl3 solutions without PAH (a) and containing 0.5 g l−1 PAH (b–d): asprepared (a and b) and after annealing at 473 K (c) and 573 K (d).
Real part of ac susceptibility of the deposits prepared from 5 mM FeCl3 solutions exhibited a broad maximum around Tm ∼260 K, which could be attributed to the superparamagnetic relaxation (Fig. 11). In contrast, composite films, prepared from the solutions containing different amounts of PAH exhibited maximum of the real part of ac susceptibility at lower temperatures. The experimental results for as-prepared composite films (Figs. 11 and 12) indicate that Tm decreases with increasing PAH concentration in the solutions. The annealing of the films at temperatures of 473 and 573 K resulted in increasing Tm as shown in Fig. 12.
4. Discussion The experimental results presented above indicate the possibility of cathodic electrodeposition of iron oxide and composite iron oxide–PAH films. The important point to be
Fig. 10. dc magnetization for the deposit prepared from the 5 mM FeCl3 solutions: field cooling (a and c) and zero field cooling (b and d).
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Fig. 11. Real part χ of ac magnetic susceptibility vs. temperature at a frequency of 10 Hz for the deposits prepared from the 5 mM FeCl3 solutions without PAH (a) and containing 0.2 g l−1 (b) and 0.3 g l−1 (c) PAH.
discussed is the mechanism of electrochemical deposition. We suggest that the mechanism of deposition is based on the cathodic electrosynthesis of iron oxide particles from the FeCl3 solutions and electrodeposition of cationic PAH macromolecules. Various cationic species exist in FeCl3 solutions, including monomers FeOH2+ , Fe(OH)2 + , dimers Fe2 (OH)2 4+ and other polynuclear species with general formula Fep III Or (OH)s [3p−(2r+s)]+ [27]. The structure of the species is influenced by pH, temperature, concentration of metal cations, solvent composition and nature of additives. In the cathodic electrosynthesis method, the high pH of the cathodic region [20,28] brings about the formation of colloidal particles, which precipitate at the electrode. Reduction of water is the cathodic reaction that generates OH− : 2H2 O + 2e− → H2 + 2OH−
(1)
Fig. 12. Real part χ of ac magnetic susceptibility vs. temperature at a frequency of 10 Hz for the deposits prepared from the 5 mM FeCl3 solutions containing 0.5 g l−1 PAH: as-prepared (a) and after annealing at 473 K (b) and 573 K (c) during 1 h.
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Cathodic electrosynthesis of iron oxide particles and electrophoresis of PAH from mixed FeCl3 solutions containing PAH resulted in accumulation of the particles and polymer macromolecules at the electrode surface. Deposit formation was achieved via the heterocoagulation of the colloidal particles and polymer macromolecules. The interaction of iron oxide and PAH can be electrostatic, non-electrostatic or a combination of both. It is important to note that isoelectric point ␥-Fe2 O3 was found to be 6.7 [29]. Therefore, the colloidal particles formed in the high pH region at the electrode surface can be negatively charged and interact electrostatically with the cationic PAH macromolecules. However, the charge of PAH decreases with increasing pH [30]. Therefore, non-electrostatic interactions, which are influenced by solvent, surface properties of the particles and other factors [20], must be considered. In this work electrodeposition was performed using mixed ethanol–water solvent. The influence of solvent on the deposition of oxide/hydroxide films and composites was discussed in previous investigations [20]. The use of non-aqueous solutions enables decreasing porosity, which results from gas evolution at the cathodic substrate. However, the deposition process requires certain amount of water for base generation and prevention of the formation of non-stoichiometric oxides. It is important to note that the polymer can be adsorbed on the surface of colloidal particles when its solubility in the dispersion medium is low. Therefore, the use of ethanol enables enhanced adsorption of PAH on iron oxide particles and promotes deposit formation. The polymer matrix, within which the inorganic phase is formed, plays an important role in the synthesis of the inorganic particles and determining their physical properties in addition to providing a means of particle dispersion [1]. Polyallylamine molecules produce polymer–metal ion complexes in solutions of metal salts [31]. In a previous investigation it was shown that polymer–metal ion complexes behave as polyelectrolytes and contribute to the deposition process [23]. The influence of PAH on the hydrolysis [32] of inorganic species was explained by the formation of the unstable PAH–metal ion complexes, which then readily decomposed with the formation of hydroxides or oxides. It was shown that magnetic oxides can be obtained in PAH capsules without additional annealing [32]. The presence of PAH dissolved in the capsule interior resulted in increasing pH and promoted the formation of the magnetic oxides. In contrast, in this work room temperature electrosynthesis of crystalline iron oxide was performed in the high pH region at the electrode substrate from pure FeCl3 solutions (Fig. 1). The electrosynthesis of iron oxide combined with electrodeposition of PAH enabled the dispersion of small particles in the polymer matrix. The variation of PAH concentration in the solutions resulted in composite materials with different magnetic properties. The deposition method resulted in the formation of iron oxide particles in the matrix of electrically insulating polymer. Heat treatment at temperatures exceeding 300 ◦ C resulted in burning out of PAH and the formation of oxide films. Therefore,
burning out of polymer can result in changes in the electrical properties of the films. The isothermal magnetization data at 5 K for pure iron oxide exhibited hysteresis in agreement with the ferrimagnetic nature of ␥-Fe2 O3 [27]. The saturation magnetization measured at 2 K (∼60 emu g−1 ) was lower than that of bulk ␥-Fe2 O3 (74 emu g−1 ) [33]. The lower magnetization can be attributed to the small particle size. It is known that magnetization of nanoparticles decreases with decreasing particle size [33]. Another possible explanation is the presence of adsorbed water, revealed in the TG experiments (Fig. 3). The investigations of iron oxide deposits showed no magnetic hysteresis at room temperature (Fig. 7). The absence of magnetic hysteresis can be attributed to the superparamagnetic behavior. It is known that superparamagnetic materials behave ferrimagnetically below the blocking temperature. Above the blocking temperature the magnetization is unstable and the materials show no hysteresis in magnetization versus field measurements. The results of dc measurements are consistent with the superparamagnetic behavior. The ZFC and FC curves clearly diverge from each other at low temperatures (Fig. 10). The observed cusps in the ZFC curves are very broad. Such behavior can be attributed to the broad particle size distribution and particle interactions in the iron oxide deposits. The difference in the dc data obtained in different fields (Fig. 10) can be attributed to the influence of magnetic field on the blocking temperature [1]. It is known that the blocking temperature decreases with increasing measuring field [1]. Therefore, in higher fields, the cusp in ZFC curve and separation of the FC and ZFC curves can be observed at lower temperatures. As expected, magnetic studies of composite films showed lower magnetization, compared to the magnetization of pure iron oxide deposits. It is suggested that magnetic properties of composite materials depend on the concentration of magnetic particles in a polymer matrix and particles size. Heat treatment of the composite deposits resulted in increasing dc magnetization (Fig. 9) due to the burning out of polymer, increasing concentration of the iron oxide phase and growth of the crystallites. These results were supported by the ac investigations. ac Data showed maxima in the temperature dependencies of magnetic susceptibility, as usually observed for the blocking process of superparamagnetic particles. The lower blocking temperature of the composite films compared to the pure iron oxide can be attributed to the lower relaxation time. The lower relaxation time can result from the lower particle volume V. The relaxation time of the superparamagnetic particles is given by the equation: τ = τ0 exp
KV κB T
(2)
where K is the anisotropy constant, V the particle volume, κB the Boltzmann constant, T the temperature and τ 0 is a time constant characteristic of the material.
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We suggest that the increase in PAH concentration in solutions resulted in increasing PAH concentration in the deposits and lower size of the iron oxide nanoparticles. Indeed, the experimental data shown in Figs. 11 and 12 for as-prepared composite films indicate that blocking temperature decreased with increasing PAH concentration in solutions, indicating decreasing relaxation time and decreasing particle volume. Heat treatment of as-prepared films resulted in increasing blocking temperature due to the growth of iron oxide crystallites (Fig. 12). Moreover, heat treatment resulted in broader χ(T) peaks. It is suggested that burning out of the polymer and particle growth resulted in broader particle size distribution, lower interparticle distances and increasing particle interactions. The results of this work can be utilized for the fabrication of composite films, containing iron oxide nanoparticles in a polymer matrix. The particle size and magnetic properties of the composite films can be modified by the variation PAH composition in solutions. Moreover, the use of small PAH additive can be used for the fabrication of thick and adherent iron oxide films. However, more detailed investigations, supported by the transmission electron microscopy data, are necessary to establish correlation between deposition conditions, deposit composition and properties of the films.
5. Conclusions New method has been developed for the fabrication of thin films of ␥-Fe2 O3 and ␥-Fe2 O3 –PAH nanocomposites. The deposition mechanism is based on cathodic electrosynthesis of ␥-Fe2 O3 and electrodeposition of cationic PAH macromolecules. TG/DTA results coupled with the data of X-ray diffraction analysis indicate co-deposition of ␥-Fe2 O3 and PAH. The ␥-Fe2 O3 –␣-Fe2 O3 transformation was observed at temperatures above 673 K. The deposition method enables the formation of adherent and uniform composite films on conductive substrates. Films of various thicknesses in the range of up to several microns were obtained. The pure ␥Fe2 O3 films and ␥-Fe2 O3 –PAH composites exhibited superparamagnetic behavior. Magnetic properties of the composites can be varied by the variation of PAH concentration in solutions. Blocking temperature of the prepared films decreased with increasing PAH concentration in the solutions used for electrodeposition. Heat treatment of the composite films resulted in increasing magnetization and increasing blocking temperature. Obtained films exhibited magnetic hysteresis below the blocking temperature. The proposed
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method can be practiced with other polymer materials for the development of novel magnetic composites.
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Electrodeposition of composite iron oxide–poly (allylamine hydrochloride) films J. Cao, I. Zhitomirsky ∗ , M. Niewczas Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S 4L7 Received 24 February 2005; received in revised form 7 June 2005; accepted 15 July 2005
Abstract New method has been developed for the fabrication of ␥-Fe2 O3 films and composites containing ␥-Fe2 O3 particles in the poly(allylamine hydrochloride) (PAH) matrix. The proposed method is based on the electrosynthesis of ␥-Fe2 O3 and electrodeposition of cationic PAH macromolecules. Cathodic deposits of various thicknesses in the range of up to several microns were obtained on Pt and graphite substrates. Obtained films were studied by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential thermal analysis and atomic force microscopy. dc and ac magnetic measurements performed in the temperature range of 2–298 K revealed the superparamagneric behavior of the films. Magnetic properties can be varied by the variation of film composition. © 2005 Elsevier B.V. All rights reserved. Keywords: Oxides; Precipitation; Electrochemical techniques; Magnetic properties
1. Introduction Nanostructured superparamagnetic materials are now being extensively studied for integrated circuits, color imaging, magnetic refrigerators and biomedical applications [1–7]. The size-controlled synthesis of magnetic nanoparticles is of great technological and scientific importance. Below a critical size, nanocrystalline single-domain magnetic particles show superparamagnetic behavior [1–3]. A critical obstacle in assembling and maintaining a nanoscale magnetic material is its tendency to aggregate. To overcome this, nanoparticles of magnetic materials were isolated in a polymer or ceramic matrix and advanced composite materials were developed [4–10]. Electrodeposition of magnetic thin films is an area of intense interest [11–14]. Electrodeposition is unique in that it can be used for the deposition of various magnetic materials, including metals, oxides, alloys and composites. Powders and thin films of oxide materials can be prepared by cathodic electrosynthesis [15–20] from the solutions of metal salts. ∗
Corresponding author. Tel.: +1 905 5259140; fax: +1 905 5289295. E-mail address: [email protected] (I. Zhitomirsky).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.015
In this method metal ions or complexes are hydrolyzed by electrogenerated base to form oxide or hydroxide deposits on cathodic substrates. Hydroxide deposits can be converted to corresponding oxides by thermal treatment. An important discovery was the feasibility of cathodic electrodeposition of nanostructured composite materials based on metal oxides or hydroxides and polyelectrolytes [20,21]. The proposed method is based on electrosynthesis of an organic phase and electrophoretic deposition of charged polymers. New electrochemical strategies are currently under development, which are based on the use of strong and weak polyelectrolytes [22–24]. Electrodeposition has been utilized for the fabrication of composite films containing Fe3 O4 nanoparticles and poly(diallyldimethylammonium chloride) (PDDA) [25]. Obtained films exhibited superparamagnetic behavior. Cathodic electrodeposition enables the formation of oxide particles in situ in a polymer matrix, preventing particle agglomeration. It was shown that magnetic properties, composition, nanostructure and morphology of the composite films could be tailored by variation of bath composition, deposition parameters and mass-transport conditions for organic and inorganic components.
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In a previous investigation [25], the formation of superparamagnetic Fe3 O4 –PDDA composite films was achieved from mixed solutions of Fe3+ and Fe2+ salts in a deaerated methanol–water solvent. Deposition and film drying was performed in a forming gas (93% Ar–7% H2 ) in order to prevent oxidation of Fe2+ species. In our new work, we report a simple method for the fabrication of magnetic composite films. In the new electrochemical strategy, composite films containing a cationic polyelectrolyte and iron oxide were obtained using only Fe3+ salt instead of a mixture of Fe2+ and Fe3+ salts used in the previous investigation [25]. Moreover, electrodeposition was performed in air, thus eliminating the need of using the forming gas during the solution preparation, electrodeposition and film drying. Therefore, new method offers important processing advantages compared to the method described in the previous investigation [25]. We report magnetic properties of the films and discuss the deposition mechanism.
2. Experimental procedures Ferric chloride hexahydrate (FeCl3 ·6H2 O), and poly(allylamine hydrochloride) (PAH) were obtained from Aldrich. Electrodeposition was performed from 5 mM FeCl3 solutions containing 0–0.5 g l−1 PAH in a mixed ethanol–water (30% water) solvent. The electrochemical cell for deposition included a cathodic substrate centered between two parallel platinum counter electrodes. The films were deposited on Pt foil and graphite (50 mm × 20 mm × 0.1 mm) cathodes at a current density of 1–5 mA cm−2 . After drying the deposits were scraped from the Pt electrodes for X-ray diffraction analysis (XRD), thermogravimetric (TG) and differential thermal (DTA) analysis. The thermoanalyzer (Netzsch STH-409) was operated in air between room temperature and 1473 K at a heating rate of 5 K min−1 . The phase content was determined by XRD analysis with a diffractometer (Nicolet I2) using monochromatized Cu K␣ radiation at a scanning speed of 0.5 ◦ C min−1 . The structure of the deposited films was studied using a Philips 515 scanning electron microscope (SEM). The surface topography of films deposited on polished stainless steel substrates was imaged using atomic force microscopy (AFM). An atomic force microscope (NanoScopeIIIa, Digital Instruments) was used in tapping mode. Magnetic properties were studied using a Quantum Design PPMS-9 system. dc Magnetization studies were performed using the extraction magnetometer option. Magnetization hysteresis loops were measured in the field range of −90 kOe < H < +90 kOe at temperatures ranging from 2 to 298 K. The external magnetic field was changed in the sweep mode at the sweep rate of 10–100 Oe s−1 . The temperature dependence of the magnetization was studied by both zerofield-cooled (ZFC) and field-cooled (FC) procedures in the field range of 200–500 Oe. The sample was cooled down to 1.9 K in the zero external field (ZFC) and then magnetization
was measured during heating to 298 K under the applied field. The sample was subsequently cooled back to 1.9 K under an applied field (FC) and the measurements of magnetization were carried out during heating to 298 K. ac Magnetic susceptibility was studied at temperatures 2–298 K.
3. Experimental results Electrodeposition performed from the 5 mM FeCl3 solutions containing 0–0.5 g l−1 PAH resulted in the formation of cathodic deposits. The films prepared from pure FeCl3 solutions exhibited low adhesion. The use of PAH additive enabled the formation of thick and adherent films. It is suggested that the polymer acts as a binder, which improves deposit adhesion and enables the formation of thick films. The deposits were analyzed by XRD before and after annealing in air at different temperatures. Fig. 1 shows Xray diffraction pattern of the deposits prepared from pure FeCl3 solutions. The fresh deposits and those annealed at 473 K, exhibited peaks, which can be attributed to ␥-Fe2 O3 or Fe3 O4 . However, it is difficult to distinguish between ␥Fe2 O3 and Fe3 O4 due to the similar spinel structure of the both phases and peak broadening, which is attributed to low particle size. Moreover, the two phases can form solid solutions. Small peaks of ␣-Fe2 O3 appear on the X-ray diffraction patterns at 673 K in addition to the peaks of the spinel phase. After heat treatment at 773 K the X-ray diffraction pattern shows peaks of ␣-Fe2 O3 . Fig. 2 shows X-ray diffraction patterns of the deposits prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH. The fresh deposits and those annealed at 473 K showed small peak around 2Θ = 36◦ . At 573 K, peaks of spinel iron
Fig. 1. X-ray diffraction patterns of the deposits prepared from 5 mM FeCl3 solutions: as-prepared (a) and after annealing at 473 K (b), 673 K (c) and 773 K (d); () ␥-Fe2 O3 (JCPDS file 39-1346), () ␣-Fe2 O3 (JCPDS file 33-664).
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Fig. 4. TG (a) and DTA (b) data for the deposits prepared from 5 mM FeCl3 solutions, containing 0.5 g l−1 PAH.
Fig. 2. X-ray diffraction patterns of the deposits prepared from 5 mM FeCl3 solutions, containing 0.5 g l−1 PAH: as-prepared (a) and after annealing at 473 K (b), 573 K (c) and 673 K (d); () ␥-Fe2 O3 (JCPDS file 39-1346), () ␣-Fe2 O3 (JCPDS file 33-664).
oxide phase were observed. The XRD pattern of the sample annealed at 673 K exhibits small peaks of the spinel phase in addition to the major peaks of ␣-Fe2 O3 . Fig. 3 shows the TG and DTA data for the deposits prepared from the FeCl3 solutions without PAH. A sharp reduction of sample weight was recorded below 423 K, followed by a more gradual weight change. No appreciable weight loss was observed at temperatures exceeding 673 K. The total weight loss at 1473 K was found to be 10.9 wt%. The observed weight loss can be attributed to the liberation of the adsorbed water. On the basis of the TG data it can be suggested that the deposition process resulted in the formation of ␥-Fe2 O3 . Indeed, no weight gain was recorded, which can be attributed to the Fe3 O4 → ␣-Fe2 O3 transformation. It is important to note that in this work electrodeposition was performed in air using FeCl3 solutions. In contrast, chemical and electrochemical precipitation of Fe3 O4 from mixed
Fig. 3. TG (a) and DTA (b) data for the deposits prepared from 5 mM FeCl3 solutions.
solutions containing Fe2+ and Fe3+ species was performed in the atmosphere of nitrogen or forming gas [25,26] in order to prevent oxidation of Fe2+ species. DTA data showed a broad endotherm at ∼373 K and a broad exotherm started at ∼773 K with a peak value around 823 K. The observed endotherm is associated with weight loss. The exotherm can be attributed to the ␥-Fe2 O3 → ␣-Fe2 O3 transformation. It is important to note that the DTA data was recorded at a heating rate of 5 ◦ C min−1 , whereas the X-ray data were taken from the samples annealed at different temperatures during 1 h. This can explain the difference in the transition temperature obtained from DTA and XRD data. TG data for the deposits prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH showed several steps in the weight loss below 823 K (Fig. 4). The total weight loss at 1473 K was found to be 61.1 wt%. The total weight loss at 1473 K for this deposit (Fig. 4) exceeds significantly the weight loss for the deposit prepared from pure FeCl3 solution (Fig. 3). These results indicate co-deposition of iron oxide and PAH. The additional weight loss is attributed to burning out of an organic phase. The DTA data for the deposits prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH (Fig. 4) showed a broad endotherm in the range below 473 K and exotherm in the range 623–823 K. The endotherm is associated with weight loss. The exotherm can be attributed to burning out of PAH and the ␥-Fe2 O3 → ␣-Fe2 O3 transformation. The results of the thermogravimetric analysis were used to calculate the amount of an organic phase in the composite films. When considering the composite material as a mixture of organic and inorganic components, the weight ratio of Fe/polymer in the film was found to be 0.48. This value is close to the weight ratio of Fe/polymer = 0.56 in the solutions used for deposition. However, it is important to note that some composite materials cannot be considered as a simple mixture of inorganic and organic phases. Composite films of various thicknesses in the range of up to several micrometers were obtained on Pt and graphite substrates by the variation of the deposition time and current
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Fig. 7. Magnetization vs. applied field for the deposits prepared from 5 mM FeCl3 solutions. Fig. 5. SEM picture of a sectioned film (F) obtained from a 5 mM FeCl3 solution, containing 0.5 g l−1 PAH on a graphite substrate (S).
density. Fig. 5 shows a SEM picture of a composite iron oxide–PAH film on a graphite substrate. It was observed that electrodeposition results in uniform films. Surface roughness of the films was studied using AFM. Fig. 6 shows the AFM image of the film prepared at a current density of 2 mA cm−2 . The root mean square (rms) surface roughness of the film was found to be 1.6 nm. Fig. 7 shows typical magnetization versus magnetic field dependencies for the deposits prepared from FeCl3 solutions without PAH. The magnetization data obtained in the low field range indicated a hysteresis behavior at 5 K. In contrast no magnetic hysteresis was observed at room temperature. The magnetization curve recorded at room temperature showed zero remanence and zero coercivity. These data are consistent with superparamagnetic behavior of the nanopar-
Fig. 6. AFM image of the film prepared from 5 mM FeCl3 solution containing 0.5 g l−1 PAH at a current density of 2 mA cm−2 .
ticles. Plots of magnetizations versus magnetic field for the composite films show hysteresis behavior at 5 K (Fig. 8). However, nearly linear behavior was observed at 298 K. Fig. 9 compares the magnetization curves at 2 K obtained in field range up to 90 kOe for the iron oxide deposits and composite films. The saturation magnetization was found to be ∼60 emu g−1 for the deposits prepared from 5 mM FeCl3 solutions. The composite films prepared from 5 mM FeCl3 solutions containing 0.5 g l−1 PAH showed lower magnetization. However, annealing of the films at temperatures above 473 K resulted in a significant increase of the saturation magnetization, which was found to be ∼25 emu g−1 after heat treatment at 573 K. Fig. 10 shows dc magnetization of the deposits prepared from 5 mM FeCl3 solutions. A separation of the ZFC and FC curves was observed at low temperatures. ZFC curves exhibited broad cusps. The ZFC and FC curves obtained at 200 and 500 Oe superimposed at temperatures above 250 and 175 K, respectively.
Fig. 8. Magnetization vs. applied field for the deposits prepared from 5 mM FeCl3 solution containing 0.5 g l−1 PAH.
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Fig. 9. Magnetization vs. applied field for the deposits prepared from 5 mM FeCl3 solutions without PAH (a) and containing 0.5 g l−1 PAH (b–d): asprepared (a and b) and after annealing at 473 K (c) and 573 K (d).
Real part of ac susceptibility of the deposits prepared from 5 mM FeCl3 solutions exhibited a broad maximum around Tm ∼260 K, which could be attributed to the superparamagnetic relaxation (Fig. 11). In contrast, composite films, prepared from the solutions containing different amounts of PAH exhibited maximum of the real part of ac susceptibility at lower temperatures. The experimental results for as-prepared composite films (Figs. 11 and 12) indicate that Tm decreases with increasing PAH concentration in the solutions. The annealing of the films at temperatures of 473 and 573 K resulted in increasing Tm as shown in Fig. 12.
4. Discussion The experimental results presented above indicate the possibility of cathodic electrodeposition of iron oxide and composite iron oxide–PAH films. The important point to be
Fig. 10. dc magnetization for the deposit prepared from the 5 mM FeCl3 solutions: field cooling (a and c) and zero field cooling (b and d).
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Fig. 11. Real part χ of ac magnetic susceptibility vs. temperature at a frequency of 10 Hz for the deposits prepared from the 5 mM FeCl3 solutions without PAH (a) and containing 0.2 g l−1 (b) and 0.3 g l−1 (c) PAH.
discussed is the mechanism of electrochemical deposition. We suggest that the mechanism of deposition is based on the cathodic electrosynthesis of iron oxide particles from the FeCl3 solutions and electrodeposition of cationic PAH macromolecules. Various cationic species exist in FeCl3 solutions, including monomers FeOH2+ , Fe(OH)2 + , dimers Fe2 (OH)2 4+ and other polynuclear species with general formula Fep III Or (OH)s [3p−(2r+s)]+ [27]. The structure of the species is influenced by pH, temperature, concentration of metal cations, solvent composition and nature of additives. In the cathodic electrosynthesis method, the high pH of the cathodic region [20,28] brings about the formation of colloidal particles, which precipitate at the electrode. Reduction of water is the cathodic reaction that generates OH− : 2H2 O + 2e− → H2 + 2OH−
(1)
Fig. 12. Real part χ of ac magnetic susceptibility vs. temperature at a frequency of 10 Hz for the deposits prepared from the 5 mM FeCl3 solutions containing 0.5 g l−1 PAH: as-prepared (a) and after annealing at 473 K (b) and 573 K (c) during 1 h.
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Cathodic electrosynthesis of iron oxide particles and electrophoresis of PAH from mixed FeCl3 solutions containing PAH resulted in accumulation of the particles and polymer macromolecules at the electrode surface. Deposit formation was achieved via the heterocoagulation of the colloidal particles and polymer macromolecules. The interaction of iron oxide and PAH can be electrostatic, non-electrostatic or a combination of both. It is important to note that isoelectric point ␥-Fe2 O3 was found to be 6.7 [29]. Therefore, the colloidal particles formed in the high pH region at the electrode surface can be negatively charged and interact electrostatically with the cationic PAH macromolecules. However, the charge of PAH decreases with increasing pH [30]. Therefore, non-electrostatic interactions, which are influenced by solvent, surface properties of the particles and other factors [20], must be considered. In this work electrodeposition was performed using mixed ethanol–water solvent. The influence of solvent on the deposition of oxide/hydroxide films and composites was discussed in previous investigations [20]. The use of non-aqueous solutions enables decreasing porosity, which results from gas evolution at the cathodic substrate. However, the deposition process requires certain amount of water for base generation and prevention of the formation of non-stoichiometric oxides. It is important to note that the polymer can be adsorbed on the surface of colloidal particles when its solubility in the dispersion medium is low. Therefore, the use of ethanol enables enhanced adsorption of PAH on iron oxide particles and promotes deposit formation. The polymer matrix, within which the inorganic phase is formed, plays an important role in the synthesis of the inorganic particles and determining their physical properties in addition to providing a means of particle dispersion [1]. Polyallylamine molecules produce polymer–metal ion complexes in solutions of metal salts [31]. In a previous investigation it was shown that polymer–metal ion complexes behave as polyelectrolytes and contribute to the deposition process [23]. The influence of PAH on the hydrolysis [32] of inorganic species was explained by the formation of the unstable PAH–metal ion complexes, which then readily decomposed with the formation of hydroxides or oxides. It was shown that magnetic oxides can be obtained in PAH capsules without additional annealing [32]. The presence of PAH dissolved in the capsule interior resulted in increasing pH and promoted the formation of the magnetic oxides. In contrast, in this work room temperature electrosynthesis of crystalline iron oxide was performed in the high pH region at the electrode substrate from pure FeCl3 solutions (Fig. 1). The electrosynthesis of iron oxide combined with electrodeposition of PAH enabled the dispersion of small particles in the polymer matrix. The variation of PAH concentration in the solutions resulted in composite materials with different magnetic properties. The deposition method resulted in the formation of iron oxide particles in the matrix of electrically insulating polymer. Heat treatment at temperatures exceeding 300 ◦ C resulted in burning out of PAH and the formation of oxide films. Therefore,
burning out of polymer can result in changes in the electrical properties of the films. The isothermal magnetization data at 5 K for pure iron oxide exhibited hysteresis in agreement with the ferrimagnetic nature of ␥-Fe2 O3 [27]. The saturation magnetization measured at 2 K (∼60 emu g−1 ) was lower than that of bulk ␥-Fe2 O3 (74 emu g−1 ) [33]. The lower magnetization can be attributed to the small particle size. It is known that magnetization of nanoparticles decreases with decreasing particle size [33]. Another possible explanation is the presence of adsorbed water, revealed in the TG experiments (Fig. 3). The investigations of iron oxide deposits showed no magnetic hysteresis at room temperature (Fig. 7). The absence of magnetic hysteresis can be attributed to the superparamagnetic behavior. It is known that superparamagnetic materials behave ferrimagnetically below the blocking temperature. Above the blocking temperature the magnetization is unstable and the materials show no hysteresis in magnetization versus field measurements. The results of dc measurements are consistent with the superparamagnetic behavior. The ZFC and FC curves clearly diverge from each other at low temperatures (Fig. 10). The observed cusps in the ZFC curves are very broad. Such behavior can be attributed to the broad particle size distribution and particle interactions in the iron oxide deposits. The difference in the dc data obtained in different fields (Fig. 10) can be attributed to the influence of magnetic field on the blocking temperature [1]. It is known that the blocking temperature decreases with increasing measuring field [1]. Therefore, in higher fields, the cusp in ZFC curve and separation of the FC and ZFC curves can be observed at lower temperatures. As expected, magnetic studies of composite films showed lower magnetization, compared to the magnetization of pure iron oxide deposits. It is suggested that magnetic properties of composite materials depend on the concentration of magnetic particles in a polymer matrix and particles size. Heat treatment of the composite deposits resulted in increasing dc magnetization (Fig. 9) due to the burning out of polymer, increasing concentration of the iron oxide phase and growth of the crystallites. These results were supported by the ac investigations. ac Data showed maxima in the temperature dependencies of magnetic susceptibility, as usually observed for the blocking process of superparamagnetic particles. The lower blocking temperature of the composite films compared to the pure iron oxide can be attributed to the lower relaxation time. The lower relaxation time can result from the lower particle volume V. The relaxation time of the superparamagnetic particles is given by the equation: τ = τ0 exp
KV κB T
(2)
where K is the anisotropy constant, V the particle volume, κB the Boltzmann constant, T the temperature and τ 0 is a time constant characteristic of the material.
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We suggest that the increase in PAH concentration in solutions resulted in increasing PAH concentration in the deposits and lower size of the iron oxide nanoparticles. Indeed, the experimental data shown in Figs. 11 and 12 for as-prepared composite films indicate that blocking temperature decreased with increasing PAH concentration in solutions, indicating decreasing relaxation time and decreasing particle volume. Heat treatment of as-prepared films resulted in increasing blocking temperature due to the growth of iron oxide crystallites (Fig. 12). Moreover, heat treatment resulted in broader χ(T) peaks. It is suggested that burning out of the polymer and particle growth resulted in broader particle size distribution, lower interparticle distances and increasing particle interactions. The results of this work can be utilized for the fabrication of composite films, containing iron oxide nanoparticles in a polymer matrix. The particle size and magnetic properties of the composite films can be modified by the variation PAH composition in solutions. Moreover, the use of small PAH additive can be used for the fabrication of thick and adherent iron oxide films. However, more detailed investigations, supported by the transmission electron microscopy data, are necessary to establish correlation between deposition conditions, deposit composition and properties of the films.
5. Conclusions New method has been developed for the fabrication of thin films of ␥-Fe2 O3 and ␥-Fe2 O3 –PAH nanocomposites. The deposition mechanism is based on cathodic electrosynthesis of ␥-Fe2 O3 and electrodeposition of cationic PAH macromolecules. TG/DTA results coupled with the data of X-ray diffraction analysis indicate co-deposition of ␥-Fe2 O3 and PAH. The ␥-Fe2 O3 –␣-Fe2 O3 transformation was observed at temperatures above 673 K. The deposition method enables the formation of adherent and uniform composite films on conductive substrates. Films of various thicknesses in the range of up to several microns were obtained. The pure ␥Fe2 O3 films and ␥-Fe2 O3 –PAH composites exhibited superparamagnetic behavior. Magnetic properties of the composites can be varied by the variation of PAH concentration in solutions. Blocking temperature of the prepared films decreased with increasing PAH concentration in the solutions used for electrodeposition. Heat treatment of the composite films resulted in increasing magnetization and increasing blocking temperature. Obtained films exhibited magnetic hysteresis below the blocking temperature. The proposed
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method can be practiced with other polymer materials for the development of novel magnetic composites.
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