Facile Microwave Process in Water for the Fabrication of Magnetic

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Facile Microwave Process in Water for the Fabrication of Magnetic Nanorods Irena Milosevic,† Hicham Jouni,† Catalina David,† Fabienne Warmont,‡ Dominique Bonnin,§ and Laurence Motte*,† †

Laboratoire CSPBAT, UMR 7244 CNRS/University Paris 13, 74 Rue Marcel Cachin, 93017 Bobigny, France CRMD, UMR 6619 CNRS, 1b Rue de la Ferollerie 45 071 Orleans Cedex 2, France § LPEM, UMR 8213 CNRS, ESPCI ParisTech-UPMC, 10 rue Vauquelin, 75231 Paris, France ‡

bS Supporting Information ABSTRACT: Magnetic nanorods were successfully prepared in water by a simple and fast method through microwave (MW) assisted reduction using akaganeite βFeOOH nanorods and hydrazine as precursors and reductor, respectively. Elongated paramagnetic akaganeite precursors are synthesized for the first time using dopamine as green chemical shape-control agent. The nature and the growth mechanism are identified by XRD, Raman spectroscopy and HRTEM analysis. Fast MW reduction process induces a structural and magnetic change depending on MW irradiation cycles. After 2 min, MW reduction leads to iron oxide nanorods with aspect ratio 3.2. XRD and Raman spectroscopy indicate a heterogeneous composition with high proportion of maghemite. The FMR spectrum is characteristic of shape anisotropy and weak ferromagnetic behavior is observed from SQUID measurements.

’ INTRODUCTION In the field of biomedical applications, magnetic nanoparticles have received considerable attention because they offer unique advantages over other types of nanoparticles. The most commonly utilized forms are iron oxide γ-Fe2O3 (maghemite) and Fe3O4 (magnetite). At room temperature and below approximately 20 nm in size, such particles exhibit superparamagnetism.1 The intrinsic interaction of magnetic nanoparticles with applied magnetic field gradients makes these particles attractive for a large panel of biomedical applications: magnetic separation systems, magnetic resonance imaging, therapy like hyperthermia and drug delivery, as well as for multidetection systems based on biosensors.2 7 Magnetic iron oxide nanoparticles, such as magnetite (Fe3O4), with magnetic moment of 300 400 emu/cm3, is currently used for this purpose, and in order to improve their performance and reduce the required doses, there is significant interest in developing high magnetic moment materials close to metallic α-Fe (∼1000 emu/cm3).8 11 However, compared to the extensive research with the spherical nanoparticles, the use of nonspherical particles with anisotropic configuration has been less well demonstrated. This is especially related to the challenge for the preparation of one-dimensional (1-D) nanostructured ferrites. The high symmetry of the spinel structure unfavorably induces the 1-D growth without extra restriction.12 Although many studies exist on the synthesis of hematite and oxyhydroxides elongated structures,13 a few examples of single crystalline 1D Fe3O4 and γ-Fe2O3 nanostructures have been r 2011 American Chemical Society

reported in the literature. Fe3O4 nanorods are obtained by means of long (>10 h) hydrothermal condition in stainless steel autoclaves at high temperature (>100 °C) in presence or not of a magnetic field.14 18 The coprecipitation process in the presence of polyvinylpyrolidone (PVP) and sonochemical oxidation have been also reported.19,20 The literature concerning γ-Fe2O3 nanorods is limited. Two synthetic procedures are described using sol gel technique21 23 and thermal decomposition in ionic liquid.2 Here, we report the preparation of magnetic nanorods by a fast and simple microwave-assisted solution method. Figure 1A illustrates our approach. Akaganeite β-FeOOH nanorods are obtained by forced hydrolysis of aqueous FeCl3/HCl solutions in the presence of dopamine as the chemical shape-control agent. The magnetic nanorods are produced through microwave assisted reduction using β-FeOOH nanorods and hydrazine as precursors and reductor, respectively.

’ EXPERIMENTAL SECTION Preparation of Precursor Material. A 10 mL FeCl3.6H2O (0.5 mol L 1) solution was prepared and mixed with 10 mL of 0.04 mol L 1 HCl solution. One mL of dopamine (1.6 mg mL 1) was added in the mixture and stirred magnetically at 500 rpm for 10 min. After that 180 mL of water at 80 90 °C was poured into the flask. The solution was maintained at this temperature for 2 h Received: June 7, 2011 Revised: July 7, 2011 Published: August 16, 2011 18999

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Figure 1. (A) Schematic representation of the two steps synthetic route. (B and C) Initial iron hydroxide (orange) and final iron oxide (black) colloid. (B) Nanoparticles are well dispersed and stable at pH 2. (C) the solutions are at pH 7 and final iron oxide nanoparticles (in aggregated state) are attracted by a neodymium magnet.

under reflux and continuous stirring. The suspension was cooled to room temperature and the pH increased to 7 8 by addition of NaOH (1 mol L 1). The precipitate was separated by centrifugation and washed with deionized water. This washing procedure was repeated several times, and the product is dispersed in water at neutral pH. An orange colloidal suspension is obtained. Preparation of Final Nanorods. A total of 2 mL of precursors ([Fe] = 0.06 mol L 1) suspension was mixed with 40 μL of hydrazine aqueous solution (3.9  10 2 mol L 1). The tube was sealed and placed in the microwave oven. The suspension was irradiated at 200 W under specific time and temperature of 100 °C. The black product is collected magnetically, washed with deioinized water at pH 7, and peptized with HNO3 solution at pH 2. Characterization Techniques. All experiments under microwave irradiations were run in benchmate (close vessel) in a 5 mL microwave glass tube equipped with a silicon cap, using a CEMDiscover monomode microwave apparatus. Transmission electron microscopy (TEM) measurements were carried out using a Philips CM10. High resolution transmission electron microscopy observations were performed with a transmission electron microscope CM20 (Philips) operating at 200 kV. XRD was performed under the same conditions for all of the samples. An X-ray powder diffractometer (model X’Pert PRO MPD, PANalytical, Almelo, The Netherlands, Co Kα beam) in Bragg Brentano geometry (y/y) was used combined with a fast detector based on real time multiple strip technology (X’Celerator). Phase identifications were performed using EVA software (version 13, Bruker-AXS, Karlsruhe, Germany, 1996 2007) and JCPDS-International Centre for Diffraction Data Powder Diffraction File (PDF-2, JCPDS-ICDD, Newtown Square, PA). MAUD software was used for the Rietveld refinement. Structural data of magnetite, maghemite and akaganeite were imported as “crystallographic information files” (cif). The weight fraction for each phase was obtained from the refinement method, and refine lattice parameters were extracted parameters.24 Atomic coordinates, site occupancies, and temperature factors were held constant. The Raman measurements were performed on a Jobin-Yvon micro-Raman spectrophotometer (Labram 300) with a 632.8 nm

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excitation wavelength (He Ne laser). The sample in powder state, deposited on a microscope glass, was illuminated in normal incidence with collimated white light. The spectra were measured using a long work distance 80 magnification objective (NA = 0.75) in back scattering geometry, with a spectral resolution of 3 cm 1 and a spatial resolution about 1 μm. The position of the band maxima was reproducible within (1 cm 1. The laser power used was 1 mW. Magnetic properties were measured using a superconducting quantum interference device (Quantum Design Squid Magnetometer MPMS-5T). Ferromagnetic resonance (FMR) was performed with a conventional X-band spectrometer at 9.48 GHz. All of the FMR experiments were carried out at room temperature in liquid state. The sample-holding capillaries were transferred into the FMR cavity of the FMR spectrometer. No signal emerging from the baseline was detected. For FMR experiments, the magnetic signature of nanorods is compared with spherical nanoparticles of 10 nm diameter. The synthesis procedure and characteristics for the nanoparticles is reported elsewhere.

’ RESULTS AND DISCUSSION Size and Shape Control. The presence of various cations and anions, as well as organic molecules, may influence the subsequent β-FeOOH growth mechanism. Additives can show an affinity for a specific crystal faces thus limiting crystal growth in oriented direction, which gives the possibility to tailor-shape nanoparticles.13,25 In our work, we use a complexing agent from the family of catecholamine, named dopamine (DOPA or H3L+). The DOPA is a natural organic molecule composed of a catechol part which has a very strong affinity to iron(III), Figure 1. Acid solutions of dopamine react with iron(III) to give the green [Fe(H2L)]3+ complexes.26 TEM pictures of iron oxyhydroxides samples in the absence and presence of small amount of DOPA (nFe/nDOPA = 5000) are shown in Figure 2. The electron micrograph of the sample without additive, Figure 2A, depicts it to contain mainly hexagonal faceted nanoparticles (average diameter, AD 60 nm) in coexistence with some spherical nanoparticles (AD 6 nm). In the presence of DOPA nearly monodisperse elongated nanoparticles are observed (Figure 2, panels B and C). The morphology of the particles depends strongly on time aging. After 2 h of reaction, particles are rod-like shape (Figure 2B), whereas after 4 h aging, particles are cigar-shaped (Figure 2C). The average lengths were 46 nm (standard deviation SD 17 60 nm) and 82 nm (SD 30 100 nm) and the average breadths 8 nm (SD 3 15 nm) and 18 nm (SD 9 30 nm) for rod-like shape and cigars, respectively. Hence, the average aspect ratio of rods is decreased to 4.6 compared to that of 5.8 observed after 2 h. High resolution TEM image of the tip portion of one nanorod showing the clear fringes is displayed in Figure 2D. The interlayer spacing of 0.53 nm corresponds to the plane of β-FeOOH.23 Therefore, it is inferred that the long axis direction of the nanorod is parallel to the [001] direction, which is the preferential growth direction of the β-FeOOH elongated particles. This matches well with the general orientation of growth in onedimensional nanostructures. 1D nanostructures such as nanotubes,27 hollow spindles,28 and nanowires29,30 have been synthesized by using some shape-control agents including carbonate, phosphate, and sulfate ions which can be selectively adsorbed on the surfaces of β-FeOOH except 19000

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Figure 2. TEM micrographs of β-FeOOH particles in absence (A) and in presence of DOPA after 2 (B) and 4 h (C) of time aging. (D) HRTEM micrograph of one β-FeOOH nanorod precursor. (E) Schematic representation of the tetragonal β-FeOOH crystal structure: iron (red) and oxygen (blue) atoms. Hydrogen atoms are not shown.

for (001) plane. The large channels associated with the principal growth c axis (Figure 2E) are considered also to be energetically favorable for both growth and dissolution.31 In this study, the DOPA would play the same role like phosphate ions and sulfate ions in terms of the above-mentioned knowledge. In a second step, a microwave assisted reduction process in water at 100 °C (MW, 200W) was used on rod-like particles. Microwave (MW) heating is fundamentally different from conventional heating processes. In the MW process, heat is generated internally within the material, instead of originating from external heating sources.32 35 The optimal temperature is reached in 30s. The reduction process is performed varying the number of cycles between 1 (30 s) and 8 (240 s). After each cycle, the solution was cooled to room temperature. First qualitative observations tend to prove the modification of the structure and magnetic properties. After 4 MW cycles, the color of the colloidal suspension change from orange to black and the final particles are easily attracted by a permanent Neodymium magnet (Figure 1, panels B and C). These two qualitative modifications suggest the formation of Fe3O4 or γ-Fe2O3 in our sample. The shape of the final particles was found to be strongly dependent on the reaction conditions. After 4 MW cycles, the transformation reaction did not affect the shape of the particles. A typical TEM image of the as-prepared sample is shown in Figure 3A. When it is compared with β-FeOOH precursor, Figure 2B, the aspect ratio of final nanorod decrease to 3.2, Table 1. This corresponds to an increase of the nanorod width and concurrently to a decrease in average length. This result suggests that MW reduction process occurs via a dissolution-recrytallization mechanism.36 Increasing MW cycles from 4 to 6 or 8, the morphology of the nanorods change to hexagon-like particles (Figure 3, panels C and E). The size of these particles increased with MW cycles indicating that Ostwald ripening occurred also through a dissolution-recrystallization process. Such hexagonal morphology was already observed elsewhere by using MW irradiation.37 Characterization of the Phase Transition. Diffraction patterns of the as-synthesized precursor sample and after 4MW reduction are displayed in Figure 4.

Figure 3. Influence of microwave cycles on the final iron oxide shape. TEM (A, C, and E) and high magnification images (B, D, and F) of the selected zone after 4 (A), 6 (C), and 8 (E) MW cycles. (G) HRTEM micrograph of the nanorod after 4 MW reduction.

Table 1. Average Length (AL), Width (AW), Size Distribution (SD), and the Average Aspect Ratio (R = L/W) of the Elongated Nanoparticles rodlike akaganeite 2H

nanorods after 4MW cycles

AL (nm)

46

SD (nm)

17

38

AW (nm)

8

12

SD (nm)

3 15

4 17

R

5.8

3.2

60

12 80

Figure 4A showed the characteristic peaks of the β-FeOOH structure (JCPDS 34-1266). The intensity of the (211) peak was higher than that of the (310) peak, implying that the akaganeite was mainly present in rod-like shaped crystals.38 The broad diffraction peaks indicated that all of the samples were composed of small crystals with a crystalline size in nanometer scale. After 4 MW cycles reduction process, the XRD pattern (Figure 4B) indicates that Fe3O4/γ-Fe2O3 particles are formed in the solution in coexistence with the β-FeOOH phase. Indeed, all of the diffraction peaks can be indexed by tetragonal and cubic structures of β-FeOOH and Fe3O4/γ-Fe2O3, respectively. Rietveld refinement indicates 26.0% (in weight fractions) of the akaganeite β-FeOOH precursor, 71.3% of maghemite and 2.6% of 19001

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Figure 4. XRD patterns (A and B) and associated Raman spectra (C and D) of β-FeOOH precursors (A and C) and final nanorods (B and D). In B, the stars indicate the XRD peaks of the β-FeOOH phase. The inset in D is the Raman spectra after long exposure to laser beam. (E) Magnetic curves and enlarged view (inset) at room temperature for β-FeOOH (triangle) and final nanorods (circle; powders). (F) FMR spectra in water and at room temperature of nanorods (continuous line) compared to spherical particles (dash line).

magnetite (see Figure 1 of the Supporting Information). The calculated spinel lattice parameter is equal to 8.395 Å. This value is close to the magnetite value (8.397 Å, JCPDS file 19-629) and far from the maghemite one (8.346 Å, JCPDS file 39-1346). This suggest that the rod-like particles are nonstoichiometric in their whole volume.1 HRTEM image (Figure 3G) confirmed the single crystalline nature of the nanorods. The lattice fringes (∼0.48 nm) agree well with the separation distance between the (111) lattice planes of Fe3O4 and γ-Fe2O3 phases. Moreover, the growth direction is identified as the (110) axis, an intermediate easy magnetization axis reported for these two phases.11,39 The different structural phases of iron oxides can be easily distinguished by Raman spectroscopy (Supporting Information). Figure 4, panels C and D, shows Raman spectra of the assynthesized precursor sample and after 4MW reduction respectively. The as-synthesized precursor sample, Figure 4C, showed two main Raman bands at 315 and 390 cm 1 and two less intense bands at 550 and 730 cm 1. These low-wavenumber regions are related to Fe O stretching and Fe OH bending vibration bands for the β-FeOOH phase only.40 This spectrum confirms the results of the diffraction analysis. Interestingly, the final rodlike particles, Figure 4D, are characteristic of the γ-Fe2O3 phase with three main broad structures at 355, 510, and 705 cm 1 in coexistence with a small amount of the β-FeOOH phase. The Raman spectrum, Figure 4D, was fitted with Gaussian Lorentzian functions centered at 355, 520, and 672 cm 1 in coexistence with the characteristic β-FeOOH phase band at 730 cm 1 (Figure 4C).

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Taking into account Raman bands areas, this leads to 15% of β-FeOOH in the final nanorods. Upon exposure to a highintensity laser, four new bands appear at 220, 282, 500, and 611 cm 1 (inset Figure 4D) which can be attributed to hematite (α-Fe2O3). As expected, γ-Fe2O3 transforms to the thermodynamically more stable α-Fe2O3 under laser irradiation.41,42 Considering XRD, HRTEM, and Raman spectra, the MW reduction process of β-FeOOH nanorods induces mainly γ-Fe2O3 formation (between 70 and 85%). It is likely that subsequent dissolution of β-FeOOH nanorods occurs along the c axis (Figure 2E) and that the nucleation of γ-Fe2O3 particles probably takes place on the (200) surface of each β-FeOOH rod-like particle during the reduction process. γ-Fe2O3 regions may be developed from the randomly oriented nuclei into the interior of the β-FeOOH structure. Since the nanorods transformation process was very fast (2 min), it is quite conceivable that the reduction process is not complete, especially for the larger precursor nanorods (Table 1). Magnetic Properties of the Rod-Like Particles. The magnetic behavior of iron oxide nanorods was investigated by SQUID and Ferromagnetic resonance (FMR) experiments at room temperature. Figure 4E shows the magnetization curves for the β-FeOOH precursor nanorods (triangle). β-FeOOH exhibits usually an antiferromagnetic ordering with Neel temperature lying in the range 270 296 K for bulk material. For nanostructured particles, this temperature is much lower.43 Hence, at 300 K β-FeOOH nanorods exhibit a linear type loop, Figure 4E (triangle), which is in accordance with paramagnetic behavior. The transformed particles present weak ferromagnetic behavior with coercitive field Hc equal to 4.4 kA m 1, Figure 4E (circle). The saturation magnetization (Ms) is not reached and the maximum of magnetization is at roughly 18 emu g 1 at 3979 kA m 1 and 300 K. The magnetization value is smaller than the Ms reported for γ-Fe2O3 nanorods.2 Although the source of low Ms is not clear, the coexistence of β-FeOOH with γ-Fe2O3 phases and the shape anisotropy might be responsible for this effect. Magnetic anisotropy strongly affects the shape of hysteresis loops and controls the coercivity and remanence. The shape anisotropy of randomly oriented nanorods (aspect ratio 3.2) prevents them from magnetizing in directions other than along their easy magnetic axes. Similar observations were reported for both Fe3O4 nanorod and nanowire samples compared to analogous spherical nanoparticles.44 Higher HC values for 1D samples were attributed to the shape anisotropy of the nanocrystals. The reduced saturation magnetization could be also assigned to spin canting induced by vacancy disorder in spinel structure and the existence of a magnetically disordered surface layer.45,46 FMR can be used as a rapid technique for assessing the magnetic anisotropy (magnetocrystalline, shape) and interparticle interactions.39,47 49 Magnetocrystalline anisotropy is an intrinsic property of a ferrimagnet, independent of grain size and shape. In systems with reduced dimensions as nanostructures, the shape anisotropy might be even the dominating contribution to the overall magnetic anisotropy energy. The FMR spectra of γ-Fe2O3 nanorods (1.4%) was compared with the signature of spherical particles45 (2%), Figure 4F. The average size of spherical nanoparticle (∼10 nm) is similar to the nanorod width. The FMR spectra of spherical particles (dashed line) show a well-defined single broad signal with a peak to peak line width ΔHpp = 59 kA m 1 and an effective g value of about 2.5 (resonance field, Hres = 209 kA m 1). The high line width value suggests the existence of non-negligible dipole dipole 19002

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The Journal of Physical Chemistry C interactions between nanoparticles. Compared to spherical nanoparticles, the FMR spectra of maghemite nanorods (continuous line) present a dissymmetric shape and is shifted to lower resonance magnetic field Hres = 190 kA m 1, with a peak to peak line width ΔHpp = 57 kA m 1. This remarkable resonance field shift (ΔHres = 19 kA m 1) is the signature of shape anisotropy.47 The dissymmetric shape of the FMR line could be due to demagnetizing field effect.49

’ CONCLUSION In summary, we successfully demonstrated a facile microwave process for the synthesis of magnetic nanorods. 1D paramagnetic akaganeite nanorods precursors are synthesized with dopamine as the chemical shape-control agent. The nature and the growth mechanism of these precursors are identified by DRX, Raman spectroscopy, and HRTEM analysis. The MW reduction process induces a structural and magnetic change depending on MW irradiation cycles. The magnetic behavior of iron oxide nanorods was investigated with SQUID and FMR experiments at room temperature. Weakly ferromagnetic nanorods (aspect ratio 3.2) are synthesized after 4 MW cycles (2 min). XRD and Raman spectroscopy indicate a heterogeneous composition mainly composed of maghemite. ’ ASSOCIATED CONTENT

bS

Supporting Information. Rietveld refinement of XRD data for final nanorods and table of Raman shifts for various iron oxide and oxy(hydroxide) phases reported in the literature and experimental Raman shifts. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by grants of the French National Agency, ANR BiotecS 2008 and by grants from Region Ile-deFrance. We are grateful to N. Lievre for TEM observations and S. Tusseau-Nenez for XRD measurements. ’ REFERENCES (1) Santoyo Salazar, J.; Perez, L.; de Abril, O.; Truong Phuoc, L.; Ihiawakrim, D.; Vazquez, M.; Greneche, J.-M.; Begin-Colin, S.; Pourroy, G. Chem. Mater. 2011, 23, 1379. (2) Lee, C. M.; Jeong, H. J.; Lim, S. T.; Sohn, M. H.; Kim, D. W. ACS Appl. Mater. Interfaces 2010, 2, 756. (3) Motte, L.; Benyettou, F.; de Beaucorps, C.; Lecouvey, M.; Milosevic, I.; Lalatonne, Y. Faraday Discuss. 2010, 149, 211. (4) Tran, N.; Webster, T. J. J. Mater. Chem. 2010, 20, 8760. (5) Chen, S.; Wang, L. J.; Duce, S. L.; Brown, S.; Lee, S.; Melzer, A.; Cuschieri, S. A.; Andre, P. J. Am. Chem. Soc. 2010, 132, 15022. (6) Chou, L. Y. T.; Ming, K.; Chan, W. C. W. Chem. Soc. Rev. 2011, 40, 233. (7) Barreto, J. A.; O’Malley, W.; Kubeil, M.; Graham, B.; Stephan, H.; Spiccia, L. Adv. Mater. 2011, 23, H18. (8) Coulon, C.; Clerac, R.; Wernsdorfer, W.; Colin, T.; Saitoh, A.; Motokawa, N.; Miyasaka, H. Phys. Rev. B 2007, 76, 214422. (9) He, L.; Zheng, W.; Zhou, W.; Du, H.; Chen, C.; Guo, L. J. Phys.: Condens. Matter 2007, 19, 036216.

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