Invited Feature Article Preparation of magnetic nanoparticles via chemically induced transition: Dependence of components and magnetization on the concentration of treating solution used Nanomaterials and Nanotechnology Volume 7: 1–9 ª The Author(s) 2017 DOI: 10.1177/1847980416687164 journals.sagepub.com/home/nax Yanshuang Chen, Qin Chen, Hong Mao, Ting Zhang, Xiaoyan Qiu, Yueqiang Lin, and Jian Li Abstract Using an FeOOH/Mg(OH)2 precursor, maghemite-based magnetic nanoparticles can be prepared by a chemically induced transition in an Iron(II) chloride (FeCl2) treating solution FeCl2 solutions of various concentrations were used to investigate the dependence of sample components and magnetization on the treating solution The bulk chemical species, crystal structures, surface chemical components, morphologies, and specific magnetizations of the samples were characterized When the concentration of FeCl2 solution was in a moderate range of 0.060–0.250 M, maghemite nanoparticles coated by hydromolysite, that is, maghemite/hydromolysite nanoparticles, could be prepared At lower concentrations, below 0.030 M, the samples contained maghemite/hydromolysite and magnesium oxide nanoparticles, and at higher concentrations, up to 1.000 M, the samples contained maghemite/hydromolysite and hydromolysite nanoparticles The molar and mass percentages of each phase were estimated for each sample The apparent magnetization behavior of the samples, which exhibited a non-monotonic variation with increasing concentration of FeCl2 solution, is explained from the variation of mass percentage of the maghemite phase with concentration Keywords Magnetic nanoparticles, FeCl2 solution, concentration, component, magnetization Date received: 29 July 2016; accepted: 24 November 2016 Topic: Nanoparticles Topic Editor: Raphael Schneider Introduction Nanoscale materials having dimensions of roughly 1–100 nm can display different physical and chemical properties from those of their bulk counterparts Nanoparticles are typically defined as solids measuring less than 100 nm in all three dimensions Magnetic nanoparticles have attracted increasing interest as particles in this size range and may allow investigation of fundamental aspects of magnetic ordering phenomena in magnetic materials with reduced dimensions, leading to new technological applications 2–4 Studies of magnetic nanoparticles have focused on the development of novel synthesis methods.1 The synthesis of nanoparticles is a complex process, and hence there is a wide range of techniques available for School of Physical Science and Technology, Southwest University, Chongqing, China Corresponding Author: Jian Li, School of Physical Science and Technology, Southwest University, Chongqing 400715, China Email: aizhong@swu.edu.cn Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 3.0 License (http://www.creativecommons.org/licenses/by/3.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/ open-access-at-sage) 2 Nanomaterials and Nanotechnology Figure Typical (a) EDS spectra and (b) zone probed for the samples EDS: energy disperse X-ray spectroscopy producing different kinds of nanoparticles Liquidphase synthesis remains one of the most common methods to obtain inorganic nanoparticles The synthesis of many oxide nanoparticles, including ferrite particles, can be achieved by the co-precipitation method Reactions for the synthesis of oxide nanoparticles can be classified into two categories: oxide nanoparticles produced directly and production of a precursor that is then subjected to further processing (drying, calcination, and so on).6 Iron (Fe) oxides, including maghemite (g-Fe O 3), magnetite, hematite, akaganetite, and goethite, have attracted enormous attention due to their interesting electrical,7 magnetic,8 and catalytic9 properties They show a wide variety of potential applications in various fields as electro-optical materials, 10 sorbents, 11 ion exchanges,12 magnetic resonance imaging,13 catalysis,14 biotechnology,15 magneto-optical effects,16 the removal of toxic metals ions,17 and so on In the past decade, the synthesis of magnetic Fe oxide nanoparticles has been intensively developed not only because of its fundamental scientific interest but also for its many technological applications.18 Many methods have been presented for the preparation of g-Fe2O3 nanoparticles, such as coprecipitation, gas-phase reaction, thermal decomposition, sonochemical synthesis, microemulsion techniques, hydrothermal synthesis,19–25 and so on Generally, the preparation of g-Fe2O3 particles by FeOOH transformation is a complex process, including dehydration, reduction, and oxidation.26 Recently, we proposed a method of liquid-phase synthesis to produce g-Fe2O3-based magnetic nanoparticles, which involved preparing an FeOOH/Mg(OH)2 precursor having loose aggregation without regular edges and treating the hydroxide precursors in an Iron(II) chloride (FeCl2) solution at 100 C.27 For the preparation, FeOOH species dehydrated to transform it into the g-Fe2O3 nanoparticles and Mg(OH)2 dissolved to act as a precipitation agent This method is referred to as chemically induced transition (CIT), 28 which is a novel reaction on a nanoscale and may provide a new route for the preparation of oxide nanoparticles In the work presented here, we investigate the components and magnetization of the nanoparticles as a function of the concentration of FeCl2 treating solution used Chen et al Table Atomic percentages of the samples from EDS O Fe Cl Mg Cl/Fe( 10À2) 60.00 63.31 69.36 64.17 60.48 52.49 35.53 33.63 29.80 34.67 37.93 37.27 0.50 0.54 0.84 1.16 1.59 10.24 3.97 2.52 1.14 1.61 2.82 3.35 4.19 27.47 Samples EDS: energy disperse X-ray spectroscopy; O: oxygen; Fe: iron; Mg: magnesium; Cl: chlorine Experiments Chemicals FeCl 2.6H 2O(AR), Mg(NO 3) 6H 2O(AR), NaOH(AR), FeCl2.4H2O(AR), and acetone(AR) were all purchased from China National Medicines Corporation Ltd (Shanghai, China), and they were used as received without further purification Distilled water was used throughout the experiments Preparation The preparation of the nanoparticles by a CIT was divided into two steps First, a precursor based on FeOOH wrapped with Mg(OH) was synthesized using the well-known co-precipitation method, which has been described in detail elsewhere.24 Second, g of the dried precursor was added to various FeCl2 solutions using the concentrations of 0.010, 0.030, 0.060, 0.125, 0.250, and 1.000 M to obtain 400 mL of solution (samples 1–6, respectively) The mixing solutions, whose pH values were about 6, were then heated to boiling for 30 in atmosphere, and the nanoparticles gradually precipitated out after the heating was completed Finally, these samples were washed with acetone and allowed to dry at laboratory atmosphere It was noted that sample was significantly more hydrophilic than the other samples Characterization The bulk chemical species were analyzed by energy disperse X-ray spectroscopy (EDS) using an EDS spectrometer on a scanning electron microscope (Quanta-200, Hillsboro, Oregon State, USA) at 25 kV The crystal structures were measured by X-ray diffractometry (XRD; D/Max-RC, Japan) with Cu Ka radiation Sample’s morphologies and microstructure were observed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM; JEM-2100F, Japan) Surface chemical compositions were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi; Boston, Massachusetts, USA) with Al Ka radiation The specific magnetization curves of the precursor and a series of samples were measured by a vibrating sample magnetometer (HH-15; China) using go-and-return magnetic field cycles at room temperature Figure XRD patterns for the samples with (hkl), (hkl)Mg, and (hkl)Cl corresponding to g-Fe2O3, MgO, and FeCl3.6H2O, respectively (?)Mg means that the index corresponding the peak of MgO is undetermined in JCPDS file 30-0794 XRD: X-ray diffractometry; g-Fe2O3: maghemite; MgO: magnesium oxide; FeCl3.6H2O: hydromolysite 4 Nanomaterials and Nanotechnology Figure TEM images for the samples TEM: transmission electron microscopy Results and analysis EDS analysis indicated that all the samples contained oxygen (O), Fe, and chlorine (Cl) elements In addition, there was magnesium (Mg) in samples and For quantitative analysis, many zones in each sample were probed to average the content of each element Typical EDS spectra and the zone probed are shown in Figure The quantitative results are shown in Table The ratio of Cl to Fe increased monotonically with increasing concentration of FeCl2 solution, which means the Cl-containing phase increased with increasing concentration The XRD results (Figure 2) showed that these samples contained mainly g-Fe O (JCPDS file 39-1346) In Chen et al addition, there could be trace of hydromolysite (FeCl3Á6H2O; JCPDS file 33-0645), whose diffraction peaks of (201) (2θ ¼ 19.429) and (202) (2θ ¼ 36.774) overlapped with that of (111) (2θ ¼ 18.384) and (222) (2θ ¼ 37.249) of the g-Fe2O3 due to the diffraction peaks broadening, respectively Consequently, the peaks of both ( 201) and (202) planes of FeCl Á6H O could appear through the strength are very weak Samples and also had magnesium oxide (MgO; JCPDS file 30-0794) The intensity of diffraction peaks corresponding to the g-Fe2O3 phase varies non-monotonically with increasing concentration of FeCl2 treating solution, and sample showed the strongest response Typical TEM images of the samples are shown in Figure Particles in samples 1–5 are mostly spherical, with size ranging from nm to 20 nm Samples and show a mixture of large (arrow A) and small (arrow B) particles Sample shows both rod-shaped (inset) and spherical particles HRTEM indicates that the spherical and rod-shaped particles in sample are g-Fe O and FeCl3Á6H2O single crystalline grains, respectively, as shown in Figure Also, as shown in Figure 4, it can be seen from HRTEM images that the both g-Fe2O3 and FeCl3Á6H2O particles contained a coating layer, whose thickness is about 0.8 nm The fast Fourier transform pattern of the spherical particles (inset, Figure 4(a)) justifies the crystalline plans displayed on the XRD pattern Combining the results from EDS, XRD, and HRTEM, we conclude there could be two Fe-containing phases, g-Fe2O3 and FeCl3Á6H2O in all samples and an additional MgO phase in samples and To study the surface characteristics of the particles, XPS analysis was carried out, which indicated the existence of the same chemical species for each samples as EDS It is known that XPS spectra can be used to determine the oxidation state and the environment of Fe.29–31 For samples 1–5, the measured binding energy of O1s was approximately 530 eV, which agreed with binding energy of O1s line for ferric oxide.31 The binding energy of O1s for sample was 531.7 eV, which was significantly larger than that of the other samples and meant having two components The Fe 2p3/2 peaks for all the samples were approximately 710.5 eV, which could be assigned to Fe3ỵ oxidation sample Accordingly, the Fe 2p3/2 peaks were resolved into two peaks for each of the samples, and the O1s peak was resolved into two peaks for sample 6, as shown in Figure 5, in which the solid lines are experimental results and dashed lines are resolved results Table summarizes the results extracted from the XPS analysis These results confirmed the presence of g-Fe2O3 and FeCl3Á6H2O in all samples, MgO in samples and 2, and molecular water in sample In addition, the O1s peak corresponding to crystal water (P1 peak) in sample is very strong, whereas that in other samples is difficult to distinguish This means that the FeCl3Á6H2O phase in sample could be far more than that in other samples The signal collection depth in the XPS experiments is 3l, where Figure HRTEM image of (a) spherical particle and (b) rodshaped particles in sample (6) The corresponding spacing of 0.250 nm is indexed to the (311) plane of g-Fe2O3 and that of 0.443 nm is indexed to the (11) plane of FeCl3Á6H2O, respectively Inset exhibits the FFT pattern of the image HRTEM: high resolution transmission electron microscopy; FFT: fast Fourier transform; g-Fe2O3: maghemite; FeCl3.6H2O: hydromolysite l ¼ 1.12 nm for Fe 2p electrons.32 Thus, combining the results of HRTEM, it can be judged that for spherical particles contain g-Fe2O3 and FeCl3Á6H2O, g-Fe2O3 is the core, and FeCl3Á6H2O is the coating due to the depth of the Fe 2p electrons measured in XPS (approximately 3.4 nm) being larger than the thickness of the coating layer (approximately 0.8 nm) Figure shows the specific magnetization curves of the precursor and the samples The precursor was paramagnetic, whereas the samples prepared using the FeCl2 treating solution exhibited a superparamagnetic-like transition having no clear remanence or coercivity The specific magnetization of the samples varied non-monotonically, similar to the g-Fe2O3 diffraction peak intensity, with increasing concentration of FeCl2 treating solution, that is, magnetization increased as the concentration of the treating solution increased from 0.010 M to 0.060 M and then decreased with increasing concentration from 0.060 M to 1.000 M The specific saturation magnetization (σ s) of as-prepared Nanomaterials and Nanotechnology Figure XPS spectra of the (a) O 1s and (b) Fe 2p3/2 XPS: X-ray photoelectron spectroscopy; O: oxygen; Fe: iron Table Binding energies from XPS (electronvolt) for the elements in the samples.a O 1s Samples Fe2O3 FeCl3 MgO H2O P1 Fe 2p3/2 P2 530.14 530.04 529.88 530.17 530.08 532.00 530.50 529.5 P1 P2 Cl 2p3/2 Mg 1s 711.30 711.30 711.20 711.30 711.20 711.30 709.90 709.90 710.00 709.90 710.00 709.90 709.9 199.23 198.23 198.79 198.42 198.24 198.90 1303.62 1303.87 711.3 530.2 199.0 1303.9 532.8 a Data for Fe2O3, FeCl3, MgO, and H2O are sourced from the NIST online database for XPS (www.nist.gov) XPS: X-ray photoelectron spectroscopy; O: oxygen; Fe: iron; Mg: magnesium; Cl: chlorine samples was obtained from σ versus 1/H data at high field.33 The values of σ s are listed in Table Discussion According to the experimental results and analysis, it was determined that when the precursor based on FeOOH/ Mg(OH)2 was treated in the FeCl2 solution, the FeOOH species was transformed into g-Fe O crystallites by dehydration, and the Mg(OH)2 species was dissolved to either assist the precipitation of nanoparticles or to form MgO particles During the precipitation, both Fe 3ỵ resulted from the oxidized Fe2ỵ and Cl were absorbed on the g-Fe O crystallites to form g-Fe O -coated FeCl Á6H O (g-Fe 2O /FeCl 3Á6H 2O) nanoparticles, and single FeCl3Á6H2O nanorods could also be formed The characteristics of the as-prepared sample components depended on the concentration of the FeCl2 treating solution used In a moderate concentration range from 0.060 M to 0.250 M, the samples were pure g-Fe2O3/ FeCl3Á6H2O nanoparticles, in which g-Fe2O3 crystallites were coated by FeCl3Á6H2O.28 With concentrations less than 0.060 M, the dissolved Mg partially formed MgO, such that the sample contained spherical g-Fe O / FeCl3Á6H2O nanoparticles and a few MgO nanoparticles For high concentrations, the samples contained spherical g-Fe O /FeCl Á6H O nanoparticles and rod-shaped FeCl3Á6H2O nanoparticles, which shown that in addition to Fe3ỵ and Cl adsorbing on the g-Fe2O3 crystallites to form FeCl36H2O coating, excessive Fe3ỵ and Cl would crystallize to form FeCl3Á6H2O nanorod On the surface of the rod-shaped particles, there exists a thin amorphous layer (see HTRM image), which could result from crystal symmetry breaking at the surface.1,34 Furthermore, for concentrations exceeding 0.060 M, the samples contained no MgO species This indicates that the acidity of the treating solution, which increased with increasing FeCl2 concentration, could make MgO formation difficult Chen et al Table Specific saturation magnetization, σ s, molar percentage, yi, and mass percentage, zi, of the phase for the samples Molar percentage (y) Mass percentage (z) g-Fe2O3/FeCl3Á6H2O/ g-Fe2O3/FeCl3Á6H2O/ σs MgO MgO Samples (emu/g) 31.1 37.0 66.9 59.9 50.5 24.7 81.04/0.76/18.20 86.10/0.93/12.97 98.14/1.86 97.79/2.21 97.24/2.76 83.22/16.78 93.23/1.48/5.29 94.67/1.73/3.60 96.89/3.10 96.31/3.68 95.41/4.51 74.56/25.44 g-Fe2 O3 : maghemite; FeCl 3Á6H 2O: hydromolysite; MgO: magnesium oxide and a Mg y Mg ¼ a Fe Àa Cl  100 (1) 2ỵa Cl 3ỵa Mg Samples 36 have g-Fe2O3 and FeCl3Á6H2O phases Using the measured atomic percentages of Fe and Cl (aFe and aCl, respectively), the molar percentages of yg and yCl are estimated by a F e Àa C l yg ¼ a Fe aCl 100 2ỵaCl and Figure Specific magnetization curves for the precursor and samples a Cl y Cl ¼ aFe ÀaCl For magnetic nanoparticle systems having many phases, the magnetization of the system is related to the ratios among the phases.26,35 Samples and included g-Fe2O3, FeCl3Á6H2O, and MgO phases Using the measured atomic percentages of Fe, Cl, and Mg (aFe, aCl, and aMg, respectively), the molar percentages of the g-Fe2O3 phase, yg, FeCl3Á6H2O phase, yCl, and MgO phase, yMg, could be estimated by a Fe Àa Cl yg ¼ a Fe Àa Cl  100; 2ỵa Cl 3ỵa Mg a Cl y Cl ẳ a Fe a Cl 2ỵa Cl 3ỵa Mg 100; 100 (2) 2ỵaCl Thus, the molar percentages of yi in the respective samples are estimated from the atomic percentages (aFe, aCl, and aMg) measured by EDS (see Table 1) These values are listed in Table The mass percentage of each phase in a sample, zi, is yA zi ¼ Pi i  100 yi Ai (3) where Ai is the molar mass of the ith phase The calculated zi values are also listed in Table The g-Fe2O3 content changed non-monotonically with increasing concentration of FeCl2 solution, similar to the XRD results Also, it can be seen from Table that the FeCl3Á6H2O content increased monotonically with increasing concentration of FeCl2 treating solution, which confirms that the FeCl3 in the samples resulted from the treatment solution From Table 3, the FeCl3Á6H2O content in sample is significantly larger than that in the other samples The additional oxide has a Nanomaterials and Nanotechnology binding energy approximately 532.00 eV (Table 2), which could be attributed to crystal water in the FeCl3Á6H2O, which are hydrophilic As a consequence, sample was significantly more hydrophilic than the other samples since the FeCl3Á6H2O content of the former was far more than that of the latter Additionally, the FeCl3Á6H2O content in samples 1–5 is much less than the g-Fe2O3 content, and so the additional oxide is difficult to distinguish from the oxide of g-Fe2O3 from the O spectrum of the XPS For the systems of particles containing many phases, the magnetization should be given by the weighted sum of the contribution from every phase For samples and 2, the specific magnetization, σ, is σ ¼ φ m;g g ỵ m; Cl Cl ỵ φ m; Mg σ Mg (4) whereas for samples 3–6, it is ẳ m;g g ỵ m; Cl σ Cl (5) where σ g, σ Cl, and σ Mg are the specific magnetization of g-Fe2O3, FeCl3Á6H2O, and MgO phases, respectively, and φ m,i¼(zi/100) is the mass fraction Since FeCl3Á6H2O and MgO are non-magnetic and zCl, zMg is much less than zg, equations (4) and (5) can be simplified as σ % φ m;g σ g surface helped improve their chemical stability and prevented aggregation In the concentration range of 0.060– 0.250 M, the samples contained single g-Fe2O3/FeCl3Á6H2O nanoparticles The ratio of the g-Fe2O3 to FeCl3Á6H2O phase and hence the magnetization can therefore be modified by changing the concentration of FeCl2 solution used Such g-Fe2O3-based nanopaticles having hydrophilic FeCl3Á6H2O could be very suitable for the synthesis of the ionic ferrofluids.36 Obviously, for the preparation of g-Fe2O3-based magnetic nanoparticles by the CIT method, the dehydration of FeOOH into g-Fe2O3 (2FeOOH!g-Fe2O3 ỵ H2O) was induced by the Fe2ỵ in the FeCl2 solution transforming into Fe3ỵ The relation between the acting energy of alkalioxide dehydration and oxidation of ferrous ions may be very interesting as it indicates an unknown reaction process and a new route for preparing oxide nanoparticles This mechanism will be further investigated Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article (6) Thus, the specific magnetization of the samples is determined by the mass fraction of the g-Fe2O3 phase From Table 3, φ m,g varies non-monotonically with increasing concentration of FeCl2 solution, that is, φ m,g increases with increasing concentration from 0.010 M [sample (1)] to 0.060 M [sample (3)], then decreases as the concentration increases to 1.000 M [sample (6)], which corresponds with the measured σ s from samples (1)–(6) Conclusions Using a CIT to prepare g-Fe2O3-based magnetic nanoparticles, the components of the as-prepared products were found to depend on the concentration of the FeCl2 treatment solution When the FeCl2 concentration was less than 0.060 M, the samples consisted of g-Fe2O3/FeCl3Á6H2O and spherical MgO nanoparticles When the FeCl2 concentration reached 1.000 M, the sample consisted of spherical g-Fe2O3 and rod-shaped FeCl3Á6H2O nanoparticles In an appropriate range of FeCl2 concentration in the treating solution, 0.060–0.250 M, the samples contained only spherical g-Fe2O3/FeCl3Á6H2O nanoparticles The apparent magnetization was determined by the content of the gFe2O3 phase presented in the sample As the concentration of FeCl2 solution increased, the consequent concentration of FeCl3Á6H2O increased, whereas that of MgO decreased, such that the mass fraction of g-Fe O varied nonmonotonically Thus, the specific magnetization of the samples also exhibited non-monotonic variation with increasing concentration of FeCl2 solution For the g-Fe2O3/ FeCl3Á6H2O magnetic nanoparticles, the FeCl3Á6H2O inert Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Financial support for this work was provided by the Innovation Foundation Project of Southwest University, China (grant no 1318001), and the National Science Foundation of China (grant no 11274257) References Willard MA, Kurihara LK, Carpenter EE, et al Chemically prepared magnetic nanoparticles Int Mater Rev 2004; 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