Article pubs.acs.org/cm
Superparamagnetic MFe2O4 (M = Fe, Co, Mn) Nanoparticles: Tuning the Particle Size and Magnetic Properties through a Novel One-Step Coprecipitation Route Clara Pereira,† André M. Pereira,‡ Carlos Fernandes,† Mariana Rocha,† Ricardo Mendes,† María Paz Fernández-García,‡ Alexandra Guedes,§ Pedro B. Tavares,∥ Jean-Marc Grenèche,⊥ Joaõ P. Araújo,*,‡ and Cristina Freire*,† †
REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal IFIMUP and IN − Institute of Nanoscience and Nanotechnology, Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal § Centro de Geologia e Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal ∥ Departamento de Química and CQ-VR, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal ⊥ LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, 72085 Le Mans Cedex 9, France ‡
S Supporting Information *
ABSTRACT: Superparamagnetic ferrite nanoparticles (MFe2O4, where M = Fe, Co, Mn) were synthesized through a novel one-step aqueous coprecipitation method based on the use of a new type of alkaline agent: the alkanolamines isopropanolamine and diisopropanolamine. The role played by the bases on the particles’ size, chemical composition, and magnetic properties was investigated and compared directly with the effect of the traditional inorganic base NaOH. The novel MFe2O4 nanomaterials exhibited high colloidal stability, particle sizes in the range of 4−12 nm, and superparamagnetic properties. More remarkably, they presented smaller particle sizes (up to 6 times) and enhanced saturation magnetization (up to 1.3 times) relative to those prepared with NaOH. Furthermore, the nanomaterials exhibited improved magnetic properties when compared with nanoferrites of similar size synthesized by coprecipitation with other bases or by other methods reported in the literature. The alkanolamines were responsible for these achievements by acting both as alkaline agents and as complexing agents that controlled the particle size during the synthesis process and improved the spin rearrangement at the surface (thinner magnetic “dead” layers). These results open new horizons for the design of waterdispersible MFe2O4 nanoparticles with tuned properties through a versatile and easily scalable coprecipitation route. KEYWORDS: coprecipitation, magnetic nanoparticles, iron oxide, cobalt ferrite, manganese ferrite, alkanolamines
1. INTRODUCTION Over the years, spinel-type ferrite nanoparticles [MFe2O4, where M(II) is a d-block transition metal] have been in the forefront of nanoscience and nanotechnology because of their outstanding properties such as nanometer size, large surface area to volume ratio, superparamagnetic behavior, and high saturation magnetization.1−3 This class of nanomaterials has been conquering new horizons in numerous research fields such as biomedicine,4−6 environmental remediation,7 catalysis,8 and high-density magnetic storage.9 Each type of application requires MFe2O4 nanoparticles with specific physicochemical and magnetic properties that can be engineered during the synthesis process.5,10 Several methods have been developed for the synthesis of magnetic nanoparticles (MNPs), including coprecipitation, thermal decomposition, microemulsion, and hydrothermal synthesis.2,11 Coprecipitation and thermal decomposition have © 2012 American Chemical Society
been the most common methods for synthesizing iron oxides and MFe2O4 MNPs. The former involves the simultaneous precipitation of M2+ and Fe3+ ions in aqueous solution induced by a base. In the thermal decomposition route, the MNPs are generated by decomposition of metal complex precursors [e.g., metal−cupferronate complexes (cup = N-nitrosophenylhydroxylamine), metal acetylacetonates, or metal carbonyls] at high temperatures in high-boiling organic solvents containing surfactants.2,11 The aqueous coprecipitation route continues to be one of the preferred choices for producing water-dispersible MNPs in high yields, since it is cost-effective, less time-consuming, and easily scalable for industrial applications. Additionally, it provides an Received: January 27, 2012 Revised: March 14, 2012 Published: March 20, 2012 1496
dx.doi.org/10.1021/cm300301c | Chem. Mater. 2012, 24, 1496−1504
Chemistry of Materials
Article
MFe2O4 samples are labeled as M_NaOH, M_MIPA, and M_DIPA, where M = Fe, Co, or Mn for Fe3O4, CoFe2O4, and MnFe2O4, respectively. 2.2.1. Synthesis of Fe3O4 MNPs. In the case of MIPA- and DIPAbased syntheses, 20 mmol of FeCl3·6H2O and 10 mmol of FeCl2·4H2O were dissolved in 25 cm3 of deoxygenated 0.5 M HCl solution. This solution was quickly added to 250 cm3 of a deoxygenated 3.0 M solution of MIPA or DIPA (pH 11−12) at room temperature with vigorous mechanical stirring under an argon atmosphere. A black precipitate formed immediately, and stirring was continued for 2 h at that temperature under an argon atmosphere. After that time, the precipitate was magnetically separated, washed with deoxygenated water several times, and stored in 0.1 M TMAOH for future use. A similar procedure was followed for the synthesis of Fe3O4 with the inorganic base NaOH (pH 14). 2.2.2. Synthesis of CoFe2O4 and MnFe2O4 MNPs. In the case of the syntheses with MIPA and DIPA, 10 mmol of CoCl2·6H2O or MnCl2·4H2O was dissolved in a solution of 1 cm3 of 37% HCl in 4 cm3 of water, and 20 mmol of FeCl3·6H2O was dissolved in 40 cm3 of water. The two solutions were heated at 50 °C, mixed, and then quickly added to 200 cm3 of 3.0 M MIPA or DIPA solution (pH 11− 12) at 100 °C with vigorous mechanical stirring. A black precipitate formed immediately, and stirring was continued for 2 h at 100 °C. After that time, the reaction mixture was cooled to room temperature, and the precipitate was magnetically separated, washed with water several times, and dispersed in 0.1 M TMAOH. For comparison, the nanomaterials were also prepared using aqueous NaOH solution (pH 14) under the same experimental conditions. 2.3. Physicochemical Characterization. Transmission electron microscopy (TEM) was performed at the Departamento de Engenharia Cerâmica e do Vidro, Universidade de Aveiro, Portugal, with a Hitachi H-9000NA microscope operating at an accelerating voltage of 200−300 kV. The samples were dispersed in high-purity ethanol under sonication, after which a carbon-coated 400 mesh copper grid was immersed in the suspension and then air-dried. A slight aggregation of the nanoparticles was observed due to magnetic interactions with the electron beam. The average particle sizes and size distributions were calculated from the diameters of at least 100 particles randomly selected from the TEM micrographs. The metal contents were determined by atomic absorption spectroscopy (AAS) using a Philips PU 9200X device with a hollow cathode lamp (S & J Juniper & Co). Powder X-ray diffraction (XRD) measurements were performed at room temperature over the range 2θ = 10−80° with a PW 3040/60 X’Pert Pro Röntgen diffractometer using Cu Kα radiation (λ = 1.5406 Å) and the Bragg−Brentano θ/2θ configuration. The system includes the ultrafast PW3015/20 X’Celerator detector and a secondary monochromator. The refinements and simulations of Bragg reflections of the XRD patterns were performed with FULLPROF software.23 Raman spectra were recorded at room temperature on a Jobin-Yvon LabRaman spectrometer equipped with a CCD camera using a He− Ne laser at an excitation wavelength of 632.8 nm and a power of 20 mW. An Olympus optical microscope with a 100× objective lens was used to focus the laser beam on the sample and collect the scattered radiation. The laser power was reduced 50% with a neutral density filter to avoid thermal decomposition of the samples. Fourier transform IR (FTIR) spectra were collected with a Jasco FT/IR-460 Plus spectrophotometer in the 400−4000 cm−1 range using a resolution of 4 cm−1 and 32 scans. The spectra of the samples were obtained in KBr pellets (Merck, spectroscopic grade) containing 1 wt % MNPs. Thermogravimetric analysis (TGA) was carried out at LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, Portugal, on a Netzsch STA 409 PC/PG thermobalance from 30 to 650 °C at a heating rate of 10 °C min−1 under a nitrogen flow (50 mL min−1). The magnetic properties of the dried MNPs were studied using a commercial Quantum Design superconducting quantum interference device (SQUID) magnetometer. The isothermal magnetization (M) versus applied magnetic field (H), zero-field-cooled (ZFC), and field-
eco-friendly route that avoids the use of hazardous solvents and reagents and high reaction temperature or pressure. Nevertheless, the control of the particle size, crystallinity, and magnetic properties through this route is still limited.2,11,12 On the other hand, thermal decomposition provides better tuning of the MNPs’ morphology, size, and monodispersion. However, the use of expensive, hazardous reagents/solvents and high reaction temperatures are still major drawbacks for greener technological applications.2,11 In this context, the quest for novel aqueous coprecipitation methodologies that allow tuning of the particle size and magnetic properties remains an ongoing challenge.12−14 These are critical issues for the design of functional magnetic probes for theranostics and imaging applications.1,11,15,16 For instance, in drug delivery, enhancement of the saturation magnetization provides more efficient transport of the magnetic drug carrier to the desired target.17 The combination of small particle size, superparamagnetism, and high saturation magnetization is also crucial for achieving higher signal sensitivity for better contrast in magnetic resonance imaging (MRI) and for improving the heating efficiency in magnetic hyperthermia.4,10,15,16 However, at the nanoscale level, a reduction in the particle size often implies a decrease in the saturation magnetization due to the surface spin-canting effect.18,19 This is especially noticed in MFe2O4 nanoparticles prepared by coprecipitation, since they exhibit greater structural disorder due to the increased contribution of the magnetic “dead” layer.20 In this work, we report the synthesis of superparamagnetic MFe2O4 (M Fe, Co, Mn) nanoparticles in high yields through a novel one-step coprecipitation methodology based on the use of a new generation of bases to induce the MNP coprecipitation. The alkanolamines isopropanolamine (MIPA) and diisopropanolamine (DIPA) were for the first time used as coprecipitation agents instead of the traditional bases reported in the literature (sodium hydroxide, ammonia, and tetraalkylammonium hydroxides).12−14,21,22 For comparison, MFe2O4 MNPs were also prepared under the same experimental conditions using the inorganic base NaOH. An in-depth evaluation of the influence of the novel bases on the morphology, structure, chemical composition, and magnetic properties of the nanomaterials is provided. Finally, we endeavor to unveil the role of MIPA and DIPA in the optimization of the MNPs’ physicochemical features, namely, on the tuning of the particle size and spin rearrangement at the surface. This work sheds new light on the design of MNPs with improved properties through the coprecipitation method.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Cobalt(II) chloride hexahydrate (≥99%), 1-amino-2-propanol (MIPA, 93%), bis(2-hydroxypropyl)amine (DIPA, ≥98%), and aqueous tetramethylammonium hydroxide (TMAOH, 25%) were purchased from Aldrich. Iron(II) chloride tetrahydrate, manganese(II) chloride tetrahydrate, ethanol, and sodium hydroxide (all of analytical grade) were supplied by Merck. Iron(III) chloride hexahydrate (analytical grade) was obtained from Riedel-de Haën, and hydrochloric acid (37%, analytical grade) was purchased from Panreac. Ultrapure water (Millipore, specific resistivity 18 MΩ cm) was used throughout the experiments. All reagents were used without further purification. 2.2. Synthesis of Nanomaterials. The MFe2O4 (M = Fe, Co, Mn) MNPs were prepared by the aqueous coprecipitation method under alkaline conditions using MIPA, DIPA, or NaOH as the base. The resulting aqueous MFe2O4 ferrofluids remained stable over extended periods of time (more than 1 year). For simplicity, the 1497
dx.doi.org/10.1021/cm300301c | Chem. Mater. 2012, 24, 1496−1504
Chemistry of Materials
Article
Figure 1. TEM micrographs and particle size distribution histograms of (A) Fe_MIPA, (B) Co_MIPA, and (C) Mn_MIPA nanomaterials. The solid line represents the log-normal distribution fit.
Table 1. Morphological and Magnetic Characterization of MFe2O4 Nanomaterials average particle size (nm) nanomaterial
dTEM (nm)a
Fe_NaOH Fe_MIPA Fe_DIPA Co_NaOH Co_MIPA Co_DIPA Mn_NaOH Mn_MIPA Mn_DIPA
8.6 6.3 4.9 18.6 4.8 4.2 59.5 9.3 11.7
magnetic characterization
σTEMb
⟨dXRD⟩ (nm)c
total weight loss (%)d
⟨TBZFC−FC⟩ (K)
⟨TBMvsH⟩ (K)
MS300K (emu g−1)
MS0 (emu g−1)
103Keff (J m−3)
DL (nm)
0.2 0.2 0.3 0.1 0.2 0.3 0.3 0.3 0.3
7.3 7.1 5.3 19.3 4.7 3.2 98.2 9.3 10.3
7.4 10.6 10.6 7.6 12.3 15.6 12.2 5.7 7.1
92.0 56.2 33.9 334.0 190.0 145.0 − − −
96.0 70.0 43.5 286.4 149.2 89.4 397.7 91.0 96.6
58.0 64.8 60.4 48.8 46.0 30.6 35.2 57.1 54.6
66.3 79.9 72.8 67.9 62.2 44.5 59.8 82.6 81.7
116 181 250 29.7 878 1810e 1.24 75.5 39.3
0.34 0.03 0.10 0.72 0.22 0.37e 4.40 0.34 0.42
a
Average particle size as estimated by TEM assuming a log-normal particle size distribution. bStandard deviation. cAverage particle size estimated by XRD. dTotal weight loss determined by TGA. eThese values were obtained using the ⟨dXRD⟩ value, since for this sample it was difficult to estimate the mean particle size by TEM with good accuracy because of the ultrasmall particle dimensions. cooled (FC) measurements were performed over the temperature range 5−370 K with H up to 30 kOe. Mössbauer spectra were recorded at 300 and 77 K with a conventional constant-acceleration spectrometer in transmission geometry using a 57Co/Rh source. The samples consisted of thinpowdered layers containing ∼5 mg of Fe cm−2. The experimental setup was calibrated using a standard Fe foil, while the isomer shift values were referenced to that of α-Fe at 300 K. The spectra were fitted by using the MOSFIT program.24
(from 59.5 to 9.3 or 11.7 nm). In the case of the MNPs synthesized with NaOH, the particle sizes increased in the order Fe_NaOH < Co_NaOH < Mn_NaOH, probably as a result of the progressive increase in the solubility product constant of the corresponding divalent metal hydroxides.28 XRD experiments were performed to identify the crystallographic structure and estimate the particle size (Figure 2a). All of the nanomaterials exhibited a typical ferrite diffractogram pattern, confirming the expected cubic spinel structure (Fd3m).29 The lattice parameters a of the cubic unit cells for CoFe2O4 and MnFe2O4, as determined by Le Bail refinement,23,29 were found to be 8.391 and 8.493 Å respectively, in accordance with the values for the bulk counterparts (8.391 Å for CoFe2O4, JCPDS no. 22-1086; 8.499 Å for MnFe2O4, JCPDS no. 100319). In the case of the iron oxide samples, the lattice parameter was determined to be 8.361 Å (considering a single phase), which is intermediate between those of magnetite (a = 8.396 Å, JCPDS no. 19-0629) and maghemite (a = 8.346 Å, JCPDS no. 39-1346), revealing the presence of both types of iron oxides. The proportions of magnetite (Fe3O4) and maghemite (γFe2O3) were estimated by a second refinement considering both phases. This adjustment showed the MNPs to be composed mainly of Fe3O4 (>75%), as sustained by their black color, with a small amount of γ-Fe2O3 (
