Magnetite nanoparticles: Electrochemical synthesis and characterization

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Electrochimica Acta 53 (2008) 3436–3441

Magnetite nanoparticles: Electrochemical synthesis and characterization L. Cabrera a,b , S. Gutierrez a , N. Menendez b , M.P. Morales c , P. Herrasti b,∗ a

b

IIC, Universidad de Guanajuato, Cerro de la Venada S/N, 36040 Guanajuato, Mexico Departamento de Qu´ımica F´ısica Aplicada, Universidad Aut´onoma de Madrid, 28049 Cantoblanco, Madrid, Spain c Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Cantoblanco, Madrid, Spain Received 26 September 2007; received in revised form 27 November 2007; accepted 2 December 2007 Available online 16 January 2008

Abstract Magnetite (Fe3 O4 ) nanoparticles (NP) with sizes between 20 and 30 nm have been obtained by Fe electrooxidation in the presence of an amine surfactant, which acted as a supporting electrolyte and coating agent, controlling particle size and aggregation during the synthesis. The effect of different parameters on the nature and size of the particles as well as the mechanism of formation of the particles have been studied by different techniques. It was concluded that, under the electrochemical conditions used in this work, the NP mean size was found to be constant at around 20 nm when the electrooxidation current density is increased from 10 to 200 mA cm−2 . However, when the potential is over 6 V, particle size decreases from 30 to 20 nm and metallic iron appears as an impurity. The mechanism of particles formation has being clarified and the critical effect of the distance between electrodes for obtaining magnetic iron oxide nanoparticles has been understood. Finally, the presence of an electrostatic adsorbed surfactant coating the particles allows the functionalization of the particles easily by exchange reaction with biomolecules of interest, which makes this material very promising for future application in biotechnology. © 2007 Elsevier Ltd. All rights reserved. Keywords: Magnetite nanoparticles; Electrochemical synthesis; Magnetic properties

1. Introduction Magnetic nanoparticles (NP), due to their size, exhibit electrical, chemical, magnetic and optical properties different from those presented in bulk size [1,2]. In the case of magnetite nanoparticles, their synthesis has been studied intensively because of their technological importance. Magnetite is widely used in numerous industrial processes (e.g., printing ink), environmental applications (e.g., magnetite carrier precipitation processes for metal ion removal and magnetic filtration) and also medical applications (biomolecule separation and contrast agents for NMR imaging), some of which are really exciting and are under development at the moment (drug targeting and hyperthermia) [3,4]. In some of those applications, the required particle size should be small enough to allow the dispersion of the particles in colloidal suspensions, stable in water at pH 7, but large enough to allow the particles to move quickly under

∗

Corresponding author. E-mail address: [email protected] (P. Herrasti).

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.006

a magnetic field gradient for more effective separations or drug targeting, or to produce a more effective heating under an alternating field for hyperthermia treatments [5]. Several methods are employed for the synthesis of magnetite [6]. The conventional method for obtaining Fe3 O4 is by coprecipitation [7]. This method consists mixing ferric and ferrous ions in a 1:2 molar ratio in highly basic solutions. In order to obtain well disperse NP, surfactants such as dextran or polyvinyl alcohol (PVA) can be added in the reaction media, or the particles can be coated in a subsequent step [8,9]. Surfactants act as protecting agent for controlling particle size and stabilizing the colloidal dispersions. However, this method generates particles with a wide particle size distribution, which requires secondary size selection methods. In addition, wastewaters with very basic pH values are also generated, which require subsequent treatments. Even when aqueous synthetic routes for magnetite nanoparticles preparation have been improved during the last decade, all of them present two important inconveniences. One is the lack of control in the particles size and size polydispersity, which should be as narrow as possible to avoid secondary effects.

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The second problem is the particles aggregation phenomenon, which should be avoided in order to have single particle behavior and the advantages of the nanometer size. In this sense, nanocomposites based on magnetic nanoparticles dispersed in a polymer [10] or inorganic matrix [11] have been developed but their use in biomedicine is limited by the difficulty in preparing colloidal suspensions from those powders. On the contrary, synthetic routes in organic media give rise to uniform and isolated magnetite nanoparticles but the hydrophobic characteristic of the particle surface generated by this method, again, makes this material difficult for its use in biotechnology as prepared [3,12]. In the search of alternative methods, the electrochemical synthesis has begun to emerge as an option among the conventional methods for the generation of magnetic nanoparticles [4,13–15]. Among the advantages that this method presents, it can be mentioned the control over the particle size by adjusting the imposed electrooxidation current density (i) or potential (E) to the system. Furthermore, if the synthesis is performed in the presence of a surfactant, it is possible to avoid the aggregation of particles. Few reports have been published in this field, and in this matter, the work here presented aims to aboard the generation of Fe3 O4 NP of size between 20 and 30 nm. This size is difficult to obtain using conventional methods. For example, by co-precipitation, smaller particles in the range of 5–12 nm are generated; while methods like sol–gel or hydrolysis allow the formation of bigger particles but never larger than 20 nm and, most of the time, they are difficult to suspend in an aqueous colloidal solution. This encourages the search of a method of synthesis in water where the size of the particles could be around 20 nm or larger, which has been proposed as ideal for hyperthermia and separation processes [5,16]. On the contrary, since the electrochemical synthesis of Fe3 O4 is performed in aqueous solution, the generated particles would be hydrophilic and, therefore, their surface could be easily modified by exchange reactions with biomolecules of interest. The generated NPs have been characterized by different techniques such as TEM, X-ray diffraction, infrared and M¨ossbauer spectroscopy and magnetization curves at room temperature. 2. Experimental A sacrificial iron anode (1 cm × 1 cm dimension, 0.2 mm thick) purchased from Goodfellow (purity 99.5%) and an iron cathode (1 cm × 4 cm) was used for the synthesis. The electrodes were cleaned by sonication and ethanol before being used. The distance between both electrodes was optimized to obtain magnetite and kept at 1 cm for all the experiments. The supporting electrolyte was 0.04 M aqueous solution of Me4 NCl (Merck) salt. The water used was Milli-Q with an 18.2 M resistance. The potential was varied between 1 and 15 V (samples named as V), and current density between 10 and 200 mA cm−2 (samples named as C) by means of a potentiostat/galvanostat VersaStatTM , EG&G Instruments Princeton Applied Research. The conditions used in the preparation of four representative samples are collected in Table 1. Reaction time was 1800 s in every case and the reaction temperature was kept at 60 ◦ C with a thermostatic bath. The obtained product was washed with distilled water and dried at 60 ◦ C for its subsequent characterization. The reaction

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Table 1 Experimental conditions and particle size and for magnetite nanoparticles electrochemically generated Sample

Voltage (V)

Mean size (nm)

V1 V2

3 5

33 ± 8 19 ± 6

Sample

Current (mA)

Mean size (nm)

C1 C2

50 150

25 ± 8 23 ± 5

Some constant parameters are: reaction time of 1800 s and 60 ◦ C for the reaction temperature.

mechanism was followed by the color change and aliquots taken every 60 s during the first 600 s and pH values of the reaction solution were recorded. Particle size and shape were analyzed by transmission electron microscopy (TEM) in a JEOL-2000 FXII electron microscope operated at 200 keV. For its study, the samples were prepared by placing one drop of a dilute suspension of Fe3 O4 NP in water onto a carbon coated copper grid and allowing the solvent to evaporate at room temperature. The mean particle size and distribution were calculated by measuring the internal dimension of at least 100 particles. The phase of the resulting iron oxide nanoparticles was investigated by X-ray diffraction and M¨ossbauer spectroscopy. X-ray diffractograms were recorded between 10◦ and 95◦ 2θ in an X’Pet PRO Panalytical diffractometer, with a θ–2θ geometry, equipped with a primary and secondary monochromators, and an ultrafast X’Celerator detector, with a CuK␣ radiation. M¨ossbauer spectra were registered at room temperature in a triangular mode using a conventional spectrometer with a 57 Co(Rh) source. The spectral analyses were performed with a non-linear adjustment, using the NORMOS [17] program. The calibration energy was performed with a ␣-Fe (6 ␮m) foil. Finally, the presence of surfactant molecules attached at the nanoparticles surface was studied by infrared spectroscopy in a Nicolet 20 SXC FTIR. Fe3 O4 NPs were dispersed in KBr at 2 wt% and pressed in a pellet. The IR spectra were registered between 4000 and 300 cm−1 . Magnetic characterization was carried out using a vibrating sample magnetometer (MLVSM9 MagLab 9T, Oxford Instrument). The magnetization curves were measured at room temperature after applying a maximum magnetic field of 1 T. From the magnetization curves, parameters such as the saturation magnetization (Ms ) and the coercitive field (Hc ) were calculated. 3. Results and discussion 3.1. Magnetic nanoparticles A black precipitate was obtained by electrooxidation under the conditions described in Section 2 and it was identified basically as magnetite by X-ray diffraction. The diffractograms for two representative samples (V2 and C2) are shown in Fig. 1. Fig. 1a shows the corresponding diffractogram of the Fe3 O4 NP

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L. Cabrera et al. / Electrochimica Acta 53 (2008) 3436–3441

Fig. 1. X-ray diffractogram of Fe3 O4 NP generated electrochemically under different conditions (see Table 1) (a) sample V2 and (b) sample C2.

synthesized at 5 V (sample V2). All peaks have been indexed as the corresponding ones to magnetite (reference code: JCPDS 01-088-0315) and no impurities were present. The crystal size was calculated from the broadening of the (3 1 1) reflection of the spinel structure and the obtained values were similar and within the margin error to the ones determined by TEM. Fig. 2a and b shows TEM images of this sample with two different magnifications. The particles are quasi-spherical with a mean particle size of 19 nm with a standard deviation of 6 nm. The good agreement between crystal size by X-ray diffraction and particle size from TEM studies indicates the high crystalline character of the particles as a result of a slow growth process during their generation [18]. When voltages higher than 6 V were used in the electrooxidation, a small peak was observed in the X-ray diffractogram at 44.7◦ (2θ), which corresponds to the (1 1 0) reflection of iron ˚ (Fig. 1b). metal (␣-Fe) with an interplanar distance of 2.02 A

Fig. 3. M¨ossbauer spectrum for Fe3 O4 NP generated electrochemically at different conditions (see Table 1) (a) sample V2 and (b) sample C2.

These results are confirmed by M¨ossbauer spectroscopy (Fig. 3). Fig. 3a shows the M¨ossbauer spectrum for sample V2, where the contribution of two magnetic subspectra corresponds to Fe3+ in the tetrahedral position and [Fe3+ /Fe2+ ] in octahedral coordination in the spinel structure (AB2 O4 ). The resonant lines of both sextets present an important broadening, the hyperfine

Fig. 2. TEM micrographs of Fe3 O4 NP obtained at E = 5 V, sample V2. (a) ×250 and (b) ×50.

L. Cabrera et al. / Electrochimica Acta 53 (2008) 3436–3441

magnetic fields, HA = 49.0 T (isomer shift = 0.32 mm s−1 ) and HB = 45.1 T (isomer shift = 0.52 mm s−1 ), are slightly inferior to the ones corresponding to the bulk material [19], and the deviation in the ideal ratio (1:2) of the iron in tetrahedral and octahedral position obtained from the subspectra area are in accordance to the small particle size. However, in the spectrum for sample C2 (Fig. 3b), it can be observed that, in addition to the Fe3 O4 spectrum, a third sextet with H = 32.7 T, corresponding to ␣-Fe at nanoscale, appeared. In all cases, this impurity represented less than 5% of total iron. The variation of the current density did not affect so much the mean particle size, but it does affect particle size distribution. Thus, when i was varied between 10 and 200 mA cm−2 , particle size was found to be around 20 nm but histograms show that particle size distribution becomes narrower as i increases (Fig. 4). These results do not totally agree with previous results on the electrochemical generation of ␥-Fe2 O3 nanoparticles by Pascal et al. [13]. Thus, particle size was observed to decrease as i increased, and the effect was attributed to an increase in the diffusion rate of the generated NPs and a decrease in the Helmoltz layer, where the reaction takes place. In our case, the effect is lower may be due to the fact that Pascal et al. imposed a maximum current of 25 mA cm−2 while the maximum current used in this work was 200 mA cm−2 . However, in both cases, lower

Fig. 4. Size distribution histograms for Fe3 O4 NP obtained at different current density (see Table 1) (a) sample C1, and (b) sample C2.

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Fig. 5. Infrared spectrum of Fe3 O4 NP obtained at E = 5 V, sample V2.

size dispersion was obtained when the applied i was increased (Fig. 4). Infrared spectroscopy was used to detect the presence of surfactants adsorbed at the nanoparticle surface. Fig. 5 shows the infrared spectrum for these nanoparticles, which can be divided in three regions. The first one goes from 4000 to 1500 cm−1 and two bands appear at 3600 and 1620 cm−1 due to O–H vibrations of water. The second region from 1500 to 900 cm−1 corresponds to the area where vibrations due to the surfactant molecules should appear. In this case, broad bands at 1394 and 1120 cm−1 (marked with arrows in the spectrum) were detected and assigned to C–N vibrations coming from the amines. Finally, between 850 and 250 cm−1 , two main bands can be observed at 584 and 442 cm−1 , which correspond to Fe3 O4 , and two shoulders at 629 and 387 cm−1 that can be assigned to maghemite [20], i.e., the oxidized form of magnetite, probably present at the nanoparticle surface (X-ray and M¨ossbauer spectra did not show any peak that could correspond to the presence of maghemite).

Fig. 6. Magnetization curve at RT for Fe4 O4 NP obtained at E = 5 V (sample V2).

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Fig. 6 shows the magnetization curve recorded for Fe3 O4 NP at RT. Ferromagnetic behavior was observed with a hysteresis loop typical of magnetite nanoparticles with sizes larger than 10 nm. The lack of magnetization saturation at high fields is a well-know effect due to the small particle size and the high surface area, which lead to some spin canting. A saturation magnetization (Ms ) of 70 emu g−1 and coercivity (Hc ) of 140 Oe were obtained from the curve. The Ms value is slightly lower than the reported value for bulk Fe3 O4 (92 emu g−1 at room temperature [21]) that can be assigned to surface effects, i.e., spin canting [22]. However, the coercivity is in agreement with the expected value for random oriented, uniaxial and non-interacting particles of magnetite [21]. 3.2. Mechanism of formation It was worthy to study the mechanism of formation of magnetite nanoparticles by this method in order to achieve a better control of particle size and distribution. The first thing to remark is that the reaction solution went through several color changes. At the beginning of the experience, when no i or E was imposed, the solution was colorless. As the reaction takes place, the solution color turns brown-red, and subsequently it changes to black indicating the formation of magnetite. The last change in color from brown to black only took place when the distance between anode and cathode was less than 5 cm, which means that such distance is critical for the formation of a magnetic iron oxide by electrooxidation in the electrolyte used. The mechanism of formation of the magnetite nanoparticles was followed by analysis of intermediate products and pH monitorization. The spectra were obtained from aliquots taken during the experience every 60 s during the first 600 s of reaction. The UV spectrum of the initial brown color solution corresponds to ferric hydroxide [4], while the spectrum for the final black solution corresponds to magnetite (data not shown). The data collected from the pH measurements show that at the beginning of the reaction pH is slightly acid, 5.1. As the reaction carries on, pH increases up to 9 and then drop to 8. From these data, the reactions that take place during iron electrooxidation can be deduced. At the beginning of the reaction, at the anode, iron was oxidized first to ferrous ions and then to ferric ions due to high i or E imposed ((1) and (2)): Fe ⇔ Fe2+ + 2e−

(1)

Fe2+ ⇔ Fe3+ + 1e−

(2)

Another reaction that takes place and provides protons to the proximities of the anode is the electrolysis of water according to reaction (3): H2 O ⇔ 2H+ + 2e− + 21 O2

(3)

At the cathode the reduction of water takes place (4) 2H2 O + 2e− ⇔ H2 + 2OH−

(4)

The solution’s initial pH was 5.1 as it was mentioned previously. Therefore, the amount of protons present in solution is

not very high. When reaction (4) occurs, pH increases up to 9. In such conditions, the precipitation of ferric ions as hydroxide is favoured [23] and precipitation of ferric hydroxide (brown solution) occurs during the first minutes of process following reaction (5): Fe3+ + OH− ⇔ Fe(OH)3 (s)

(5)

The OH− ions produced at the cathode (4) arrive to the anode’s surface (2) by diffusion, proportioning the basic medium necessary for the iron hydroxide to be formed. Ferric hydroxide can now react in two different ways. If the pH of the solution is not basic enough, it may dehydrate to generate a non-magnetic ferric oxide. However, if the pH is around 8 or 9, ferric hydroxide can be reduced at the cathode to form Fe3 O4 as represented in the following Eq. (6). 3Fe(OH)3 (s) + H+ + e− ⇔ Fe3 O4 (s) + 5H2 O

(6)

It can be concluded that the distance between anode and cathode seems to be critical for the formation of a magnetic iron oxide by electrooxidation, in agreement with our experimental results. Thus, if the distance between the anode and cathode is greatly increased, the interface close to the anode would not reach the pH value necessary to form the iron hydroxide. Even if the hydroxide is formed, it would not be reduced at the cathode, at least not in an appreciable amount. Hence, when the electrode distance was greater than 5 cm, electrogeneration of Fe3 O4 was not observed during the standard reaction time obtaining on the contrary a mixture of hematite and goethite, both non-magnetic. This mechanism could also explain the presence of metallic iron in the final product; when the potential is very high, the formation of H2 and O2 occurs at a very high rate (reactions (3) and (4)), then bubbles generated on the electrodes prevent direct oxidation of some iron atoms at the crystal lattice, making them part of the final product. 4. Conclusions Magnetite nanoparticles have been obtained by electrooxidation of iron in the presence of an amine surfactant, resulting in quite uniform and spherical particles with mean sizes between 30 and 20 nm depending on the experimental conditions. Characterization of the product by different techniques indicates the presence of pure Fe3 O4 when potentials lower than 6 V are imposed. Although the increase in current density or potential promotes higher size homogeneity of the nanoparticles, metallic iron is present as an impurity. Furthermore, the distance between anode and cathode seems to be important to obtain Fe3 O4 by electrooxidation. It should be noted that in this work, preliminary results are shown for the production of magnetic nanopowders using this technique as a possible synthetic method. The final product could be substantially improved by changing other experimental conditions such as the reaction temperature and the nature of the surfactant. Finally, it should be emphasized that the yield of the reaction product is always higher than 80%.

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Acknowledgements This work was supported by the Spanish M.E.C (CTQ200504469/BQU) and the Comunidad de Madrid through the project S-0505/MAT/019. The authors would like to acknowledge CONACyT (Mexico) for financial support. References [1] A. Hernando, J. de la Venta, B. Sampedro, E.F. Pinel, J.M. Merino, P. Crespo, M.L. Ruiz-Gonz´alez, M.A. Garc´ıa, J.M. Gonz´ales-Calbet, Magnetic properties of metallic nanoparticles, in: 2nd NanoSpain Worshop, Barcelona, Spain, 2005. [2] S. Guchhait, Magnetism in Nanostructure, University of Texas, Austin, Texas, 2005, p. 1. [3] P. Tartaj, M.P. Morales, S. Venimillas-Verdaguer, T. Gonz´alez-Carre˜no, C.J. Serna, J. Phys. D: Appl. Phys. 36 (2003) R182. [4] T.-Y. Ying, S. Yiacoumi, C. Tsouris, J. Dispersion Sci. Technol. 23 (2002) 569. [5] A. Jordan, R. Scholz, P. Wust, H. F¨ahling, R. Felix, J. Magn. Magn. Mater. 201 (1999) 413. [6] A.-H. Lu, E.L. Salabas, F. Schuth, Angew. Chem. Int. Ed. 46 (2007) 1222. [7] V.L. Calero D´ıaz del Castillo, Master of Science, University of Puerto Rico, Mayag¨uez Campus, 2005. [8] A.A. Novakova, V.Y. Lanchinskaya, A.V. Volkov, T.S. Gendler, T.Y. Kiseleva, M.A. Moskvina, S.B. Zezin, J. Magn. Magn. Mater. 258–259 (2003) 354.

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[9] P. Sipos, Roman. Rep. Phys. 58 (2006) 269. [10] P.C. Morais, R.B. Azevedo, D. Rabelo, E.C.D. Lima, Chem. Mater. 15 (2003) 2485. [11] M. Zayat, F.D. Monte, M.D.P. Morales, G. Rosa, H. Guerrero, C.J. Serna, D. Levy, Adv. Mater. 15 (2003) 1809. [12] K. Woo, J. Hong, S. Choi, H.-W. Lee, J.-P. Ahn, C.S. Kim, S.W. Lee, Chem. Mater. 16 (2004) 2814. [13] C. Pascal, J.L. Pascal, F. Favier, M.L.E. Moubtassim, C. Payen, Chem. Mater. 11 (1999) 141. [14] S. Franger, P. Berthet, J. Berthon, J. Solid State Electrochem. 8 (2004) 218. [15] C. Tsouris, D.W. DePaoli, J.T. Shor, US 6,179,987 B1 (2001). [16] L.M. Rossi, A.D. Quach, Z. Rosenzweig, Anal. Bioanal. Chem. 380 (2004) 606. [17] R.A. Brand, Nucl. Instrum. Methods Phys. Res. B 28 (1987) 398. [18] T. Sugimoto, Fine Particles, Surfactant sciences series, vol. 92, Marcel Dekker Inc., New York, 2000. [19] N.N. Greenwood, T.C. Gibb, M¨ossbauer Spectroscopy, Chapman and Hall Ltd., 1971. [20] A.G. Roca, M.P. Morales, C.J. Serna, IEEE Trans. Magn. 42 (2006) 3025. [21] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Publishing Company, USA, 1972. [22] G.F. Goya, T.S. Berqu´o, F.C. Fonseca, M.P. Morales, J. Appl. Phys. 94 (2003) 3520. [23] R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrence and Uses, VCH Verlagsgesellschaft, Weinheim, Federal Republic of Germany, 1996.