A simple solvothermal synthesis of MFe2O4 (M=Mn, Co and Ni) nanoparticles

A simple solvothermal synthesis of MFe2O4 (M=Mn, Co and Ni) nanoparticles S. Yáñez-Vilara, M. Sánchez-Andújara, C. Gómez-Aguirrea, J. Mirab, M.A. Señarís-Rodrígueza, S. Castro-Garcíaa, , a Departamento de Química Fundamental, Facultade de Ciencias, Universidade da Coruña, A Zapateira s/n, 15071 A Coruña, Spain b Departamento de Física Aplicada, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain Journal of Solid State Chemistry, Volume 182, Issue 10, October 2009, Pages 2685–2690 Received 10 June 2009, Revised 16 July 2009, Accepted 18 July 2009, Available online 25 July 2009 DOI: 10.1016/j.jssc.2009.07.028 Abstract Nanoparticles of MFe2O4 (M=Mn, Co and Ni), with diameters ranging from 5 to 10 nm, have been obtained through a solvothermal method. In this synthesis, an alcohol (benzyl alcohol or hexanol) is used as both a solvent and a ligand; it is not necessary, therefore, to add a surfactant, simplifying the preparation of the dispersed particles. We have studied the influence of the synthetic conditions (temperature, time of synthesis and nature of solvent) on the quality of the obtained ferrites and on their particle size. In this last aspect, we have to highlight that the solvent plays an important role on the particle size, obtaining the smallest diameters when hexanol was used as a solvent. In addition, the magnetic properties of the obtained compounds have been studied at room temperature (RT). These compounds show a superparamagnetic behaviour, as © Elsevier 2009. This manuscript version is made available under the CC-BY-NC-ND 4.0 license (https://creativecommons.org/licenses/by-nc-nd/4.0/) was expected for single domain nanoparticles, and good magnetization values. The maxima magnetization values of the MFe2O4 samples are quite high for such small nanoparticles; this is closely related to the high crystallinity of the particles obtained by the solvothermal method. Graphical abstract An adaptation of the solvothermal method allow us to obtain stable suspensions of monodispersed particles of MFe2O4 (M=Mn, Co and Ni), with diameters ranging from 5 to 10 nm, and with good crystallinity. Keywords Ferrite; Nanoparticle; Solvothermal synthesis; Magnetic particles 1. Introduction The synthesis of nanostructured magnetic materials has become an important area of research and is attracting growing interest, not only in answering basic research questions, but also in technological applications and in biosciences [1], [2], [3] and [4]. In particular, the nanometer-scale MFe2O4 (M=Mn, Fe, Co, Ni,…) spinel ferrites and their dispersions in various substances are among the most important magnetic materials, which have been widely used for studies of nanomagnetism and have shown great potential for important technological applications in many fields such as highdensity information storage, ferrofluids, colour imaging, catalysis, biomolecule separation, medical diagnosis, drug delivery and so forth[5], [6], [7], [8], [9], [10], [11], [12], [13] and [14]. It is well known that the magnetic and electrical properties of MFe2O4 nanoparticles can be varied by changing the identity of the divalent M2+ cation or by partial substitution, while maintaining the basic crystal structure. Additionally, the magnetic properties of the nanoparticles are strongly dependent on their shape, size and crystallinity. To use MFe2O4 ferrites for future magnetic nanodevices and biomedical applications, sizetuned ferrite particles with diameters ranging from the superparamagnetic threshold at room temperature of <10 nm to the critical single-domain size of 70 nm are needed [15], [16] and [17]. For many of the applications, such as magnetic carriers in bioscience, their size is limited to a very narrow margin of values (from 5 to 10 nm) in order to attain a compromise between magnetic moment and absence of magnetic memory: superparamagnetic particles must be smaller than 10 nm, but their magnetic moment decreases drastically below about 5 nm [18]. Many investigations have been focused until now, not only on the controlled synthesis of ferrite nanoparticles, but also on the correlation between their magnetic properties and either the particulate properties and/or the synthetic conditions [7], [11], [19], [20], [21], [22], [23], [24], [25], [26], [27] and [28]. Although significant progress has been made in this respect, systematic and profound understanding remains challenging, which justifies any effort to find a simple and costeffective way for the production of sized-tuned monodisperse nanoparticles [29]. Techniques such as sol–gel [30], coprecipitation [31], [32] and [33], mechanochemical processing [34] and [35], microemulsion [26], [36] and [37] and microwaves [38] have been commonly used to prepare ferrite nanoparticles. But, until now, the most economical ways for the production of large quantities of nanosized ferrite particles are chemical precipitation [39] and solvothermal synthesis [11], [19], [40],[41] and [42]. In general, solvothermal synthesis offers many advantages over other methods, such as its simplicity, the high crystallinity of the obtained products at relatively low temperature (T~180 °C), the capability to control crystal growth and its adequacy for the preparation of large quantities of samples. Hydrothermal synthesis—a specific solvothermal method where water is employed as a solvent—has been employed since the end of the 19th century for the synthesis of different ferrites [43], [44] and [45], but the experimental conditions for these syntheses are sometimes poorly defined [46]. Most of these preparations involve a combination of coprecipitation and hydrothermal synthesis [18]. An innovation to the hydrothermal method is the introduction of microwaves during the hydrothermal synthesis to increase the kinetics of the ferrite particles formation [47]. Another solvothermal synthesis method—in non-aqueous solvent—that has been used to prepare MFe2O4 (M=Fe, Co, Mg, Cu, Ni, Zn) [48] and [49] ferrites is based on the partial reduction of the reagents by ethylene glycol with the presence of NaAc and polyethylene glycol (PEG). In this so-called polyol process [50] the ethylene glycol serves both as a reducing agent and as a solvent, while NaAc and PEG were used for electrostatic stabilization to prevent particles from agglomeration and as a protective agent respectively. Pinna et al. [51] have recently developed a solvothermal method for the synthesis of metal oxide nanoparticles that employs benzyl alcohol as a solvent and a ligand at the same time, instead of the mixtures of solvents and ligands used before. Therefore, the use of surfactants to prevent agglomeration has been avoided. This solvothermal method simplifies even more the synthesis of the dispersed nanoparticles. In this aspect, we have to highlight the Niederberger works using the benzyl alcohol as a solvent system to prepare nanocrystals of different metal oxides, such as BaTiO3, CeO2, NaNbO3, etc. [52]. In the present work, we have employed an adaptation of the solvothermal synthesis of Fe3O4[50] to prepare single-phase nano-sized MFe2O4 (M=Mn, Co and Ni) ferrites, with particle sizes between 5 and 10 nm. We have explored the possibilities of further control of the size and agglomeration of the magnetic ferrite nanoparticles by this method, varying the temperature and time of synthesis, and using two different solvents/ligands: benzyl alcohol and hexanol. In order to improve the magnetic properties of the particles, we have investigated the effects of the synthesis conditions (solvents/ligands, temperature and reaction time) on the size of the nanoparticles and on their saturation magnetization. 2. Experimental Samples of MFe2O4 (M=Mn, Co and Ni) nanoparticles were prepared by the solvothermal method. All the reagents were of analytical grade and were used without any further purification. A typical preparation procedure was as follows: to obtain 0.1 g of ferrite, the required quantities of Mn(acac)2 (Aldrich, technical grade), Co(acac)2 (Fluka, 97%) or Ni(acac)2 (Aldrich, 90%) were mixed with Fe(acac)3 (Aldrich, 97%) and dissolved in benzyl alcohol (Panreac, 98%) or hexanol (Panreac, 98%). The resulting solutions were stirred thoroughly and then transferred into a 23 mL Teflonlined stainless-steel autoclave to a filling capacity of 40%. The crystallization was carried out under autogenous pressure at temperatures of 180 or 190 °C for 24 or 48 h. All the synthesis conditions (solvents, temperatures and reaction times) used for the preparation of the different MFe2O4 samples are summarized in Table 1. Then, the autoclave was cooled naturally to room temperature, after centrifugation at 5000 rpm for 30 min, the supernatant liquids were discarded and the remaining products were washed thoroughly with ethanol to remove the excess of ligands and air-dried at room temperature. Table 1. Synthesis conditions and structural and magnetic properties of the MFe2O4 samples. Sample Synthesis conditions: solvent temperature/time Crystalline phasesa MFe2O4 lattice parameter (Å) a Nanoparticle size (nm)b Magnetization at 10 kOe (emu/g) Mn-1 Benzyl alcohol 180 °C/24 h MnFe2O4 8.52(24) 7.4±0.8 53.5 Mn-2 Hexanol 180 °C/24 h MnFe2O4 8.52(52) 5.0±0.8 35.1 Co-1 Benzyl alcohol 180 °C/24 h CoFe2O4 8.39(20) 7.6±0.8 56.6 Co-2 Benzyl alcohol 180 °C/48 h CoFe2O4 8.39(03) 8.9±0.6 58.4 Co-3 Benzyl alcohol 190 °C/24 h CoFe2O4 8.39(27) 7.1±1.0 55.4 Co-4 Hexanol 180 °C/24 h CoFe2O4 8.39(18) 5.9±0.6 50.2 Ni-1 Benzyl alcohol 180 °C/24 h (Ni1−xFex)Fe2O4+Ni 8.35(45) − − Ni-2 Hexanol 180 °C/24 h NiFe2O4 8.34(07) 7.1±0.8 39.5 a Obtained by means of XRD. b Obtained by means of TEM. The crystal structure of the obtained materials was studied by X-ray powder diffraction (XRD) in a Siemens D-5000 diffractometer at room temperature and using CuKα radiation (λ=1.5418 Å). The X-ray diffraction patterns were obtained in the 2θ range of 20°–80° and then were inspected using Match software [53] to identify the present crystallographic phases. The morphology and the microstructure of the samples were tested by scanning electron microscopy (SEM) in a JEOL 6400 microscope, by transmission electron microscopy (TEM) in a JEOL 1010 microscope operating at 100 kV and by high-resolution transmission electron microscopy (HRTEM) in a JEOL 2010 microscope operating at 200 kV. For TEM observations we have used suspensions of the ferrites obtained after ethanol washes, which were deposited onto the copper grids. The elemental composition of the samples was tested by energydispersive X-ray spectroscopy (EDS) with an Oxford Inca Energy 200 attached to the electronic microscope. Magnetic properties were studied in a DMS-1660 vibrating sample magnetometer (VSM) at room temperature varying the magnetic field up to ±10 kOe. 3. Results and discussion XRD analysis shows that in all cases we have obtained crystalline MFe2O4 samples with the expected cubic spinel structure, identified as MnFe2O4 (JCPDS No. 01-0750035), CoFe2O4 (JCPDS No. 00-022-1086) and NiFe2O4 (JCPDS No. 01-086-2267). As a resume, Table 1 summarizes the crystalline phases and the cell parameters obtained by XRD for the samples prepared under different synthesis conditions. As an example, we show in Fig. 1 the XRD patterns of MFe2O4 (M=Mn, Co, Ni) samples obtained from the reactions at 180 °C over a period of 24 h in the autoclave. The lattice constants calculated from (311) reflections are 8.52, 8.39 and 8.34 Å, for MnFe2O4, CoFe2O4 and NiFe2O4, respectively. A detailed analysis of the XRD results reveals that all the samples are single phased, with the exception of the sample Ni-1 (nominal NiFe2O4, synthesized in benzyl alcohol). In the XRD pattern of that sample we observe the presence of extra peaks, marked with arrows in Fig. 1(c), which correspond to the diffraction of metallic Ni with cubic structure (JCPDS No. 00-065-2865). We attributed the presence of metallic nickel to the reducing power of benzyl alcohol, which is able to reduce part of the Ni2+ to the Ni0 form [54]. However, through substituting benzyl alcohol with hexanol, an alcohol with a lower reducing power, and maintaining the rest of the reaction conditions, we have obtained the desired NiFe2O4 phase (see Fig. 1(f)). Another interesting feature of the XRD patterns is the effective line broadening observed for all the samples, indicating the fine nature of the nanoparticles. Fig. 1. XRD patterns of: (a) MnFe2O4, (b) CoFe2O4 and (c) NiFe2O4, synthesized in benzyl alcohol at 180 °C/24 h; (d) MnFe2O4, (e) CoFe2O4 and (f) NiFe2O4, synthesized in hexanol at 180 °C/24 h. The morphology of the particles was studied in more detail by TEM, which reveals that we have obtained nanoparticles with spherical-like morphology and uniform sizes, ranging from 5 to 9 nm. In Fig. 2 and Fig. 3we show some representative TEM micrographs of the ferrite nanoparticles obtained under different synthesis conditions; in Table 1 we summarize the average grain size deduced from TEM results for all the samples. Comparing the ferrite samples that contain different metal cations, but prepared in similar synthetic conditions (see samples Mn-2, Co-4 and Ni-2 in Table 1 and in Fig. 2(a)–(c)), we observe a tendency of the particle size to increase when the ionic radii of the transition metal cation decreases. Another interesting observation is the increase of the particle size with the reaction time. For example, CoFe2O4 nanoparticles obtained after reaction times of 24 h (sample Co-1) and 48 h (sample Co-2)—while maintaining unchanged the rest of the synthesis conditions—exhibit uniform sizes of 7.6±0.8 and 8.9±0.6 nm, respectively. However, when the reaction temperature was increased from 180 to 190 °C, the size of the nanoparticles did not appreciably changed, as we observe for Co-1 and Co-3 samples (seeTable 1). Fig. 2. TEM micrographs of: (a) MnFe2O4, (b) CoFe2O4 and (c) NiFe2O4 synthesized in hexanol at 180 °C/24 h. Fig. 3. TEM micrographs of CoFe2O4 prepared in benzyl alcohol at: (a) 180 °C/24 h, (b) 190 °C/24 h and (c) 180 °C/48 h. One of the most interesting results concerning this synthesis is the influence of the solvent on the size and agglomeration of the particles. We have observed through TEM that hexanol leads, in general, to the formation of slightly smaller ferrite particles than does benzyl alcohol. For example, as we can see in Table 1 and Fig. 2 and Fig. 3, MnFe2O4 nanoparticles with a diameter of 5.0±0.8 nm were obtained using hexanol. However, using benzyl alcohol in similar conditions, we obtain particles of 7.4±0.8 nm. Similarly, we have prepared CoFe2O4 nanoparticles of 5.9±0.6 nm using hexanol, but nanoparticles of 7.6±0.8 nm using benzyl alcohol. The different broadening in XRD patterns confirms these TEM results: the line broadening in XRD patterns is higher for the ferrites prepared with hexanol (Fig. 1(d)–(f)) compared with the analogous samples prepared with benzyl alcohol (Fig. 1(a)–(c)). To explain these results, we have to consider the reaction mechanism in this solvothermal synthesis. This mechanism should be similar to that recently reported for the reaction of Fe(acac)3 in benzyl alcohol to yield magnetite nanocrystals [52]: the main reaction occurs upon solvothermal treatment, involving solvolysis of the acetylacetonate, followed by aldol condensation reactions. In the first step, benzyl alcohol nucleophilically attacks one carbonyl group of the acetylacetonate ligand. In our case, the higher Lewis basicity of hexanol— compared with benzyl alcohol—enhances the solvolysis of the transition metal acetylacetonate species. As a consequence, the nucleation-reaction is faster and favoured with respect to the growth-reaction, and leads to the formation of smaller nanoparticles when using hexanol rather than benzyl alcohol. Furthermore, we have observed appreciable differences in the stability of the ferrite suspensions prepared in hexanol compared to those prepared in benzyl alcohol. In general, the suspensions of the particles in hexanol remain stable for several days without precipitation, being more stable than the analogous in benzyl alcohol, which precipitate in minutes or hours. The clearest example is the case of the CoFe2O4nanoparticles, whose suspensions in hexanol are stable for more than one week, while the corresponding suspensions in benzyl alcohol precipitate in less than one day. The HRTEM images of the prepared particles indicate that the nanoparticles were structurally uniform and display good crystallinity. As an example, Fig. 4 shows a representative image of the CoFe2O4 particles (sample Co-1). The Fourier transform of the HRTEM image shows the electron diffraction pattern, which indicates that this particle is oriented along the [011] zone axis (see inset of Fig. 4). The interplanar distance of 8.3 Å indicated in Fig. 4 corresponds to the a-lattice parameter of the spinel structure, in agreement with the DRX results for CoFe2O4 particles. 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