A facile strategy for synthesis of spinel ferrite nano-granules and their potential applications

Abstract

We have demonstrated a facile method to synthesize a number of spinel ferrite nano-granules in dimethyl formamide (DMF) through a calcination process under air. There is no pH regulation, gas atmosphere, centrifugation, supplementary reagents, or other complicated and cumbersome procedures during the preparing process. The structures, morphologies, and magnetic properties of NiFe₂O₄, CoFe₂O₄, and NiZnFe₂O₄ nano-granules in various DMF concentrations and different calcination temperatures were investigated systematically, and other MFe₂O₄ nano-granules were also investigated by this method. As a result, it will cause an impurity phase at low DMF concentration or low calcination temperature, and large quantities of uniform nano-granules will be achieved at about 680 °C in pure DMF. The results show that the method realizes a simple, rapid and convenient route for assembling unitary and binary spinel ferrite nano-granules. In addition, the formation mechanism of the nano-granules was also studied in detail, and DMF plays a dispersing and covering role during the reaction. This synthetic approach also shows great potentiality in the synthesis of maghemite γ-Fe₂O₃ with high saturation magnetization, and rare nonmetal-doped CoFe₂O₄ is prepared using this strategy. Magnetic properties of doped CoFe₂O₄ nano-granules are also improved. These results provide a convenient way for the advancement of magnetic nanomaterials.

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1. Introduction

Magnetic nanomaterials have gradually attracted much interest due to their particular physicochemical properties, which show many potential applications in information storage, magnetic refrigeration, sensors, ferrofluids, photocatalysis and medical treatment, etc.^1–6 Recently, the research of magnetic nanomaterials is fascinating in consideration of its contributions on biomedicine and chemical engineering.^7–9 In particular, MFe₂O₄ (M = Co, Ni, Zn, Cu, Mn, etc.) with natural and stable properties, are well-known and outstanding magnetic nanomaterials, which are important for a broad range of applications.^1,10,11 By changing the identity of the divalent M²⁺, the magnetic properties of MFe₂O₄ can be molecularly engineered to provide a wide range of characteristics.^12–14 Thus, new strategies for fabrication of ferrites are of fundamental importance in the development of science and technology.^6,15–19 Previous efforts and methods have been devoted to prepare MFe₂O₄ nano-granules, such as thermal decomposition,^10,11,19,20 liquid solid solution,¹² co-precipitation,²¹ sol–gel,^1,22 thermal solvent,²³ hydrothermal,²⁴ high temperature reaction,²⁵ nonhydrolytic process,²⁶ template method,²⁷ and others,^28,29 etc. Those processes or methods are important for synthesis of new materials and composite materials. However, additives or surfactants (like citric acid, fatty acid, linoleate, ethanediol, and oleic acid, etc.) play a particular role in the improvement of stability and dispersity of MFe₂O₄ nano-granules during those synthetic processes. Suitable pH and long reaction time are the vital factors for the controllable morphology of nano-granule. Furthermore, centrifugation and purification are indispensable for the single and dispersive products. Those synthetic processes are complicated and cumbersome with the high preparation cost. Particularly, if the properties of nano-granules are further modified, the experimental process has to be changed. Those disadvantages of the methods mentioned above may limit the application of nano-granule. Therefore, a facile, green and inexpensive strategy for synthesis of MFe₂O₄ ferrite nano-granules and its composite material is worth being considered.

In the present investigation, we have been developing a facile-direct synthesis of a number of MFe₂O₄ nano-granules in DMF without other supplementary reagents by a calcination process under the air. It realizes a simple, rapid and convenient route for assembling MFe₂O₄ nano-granules. We systematically study the synthetic process, and the structure, morphology and magnetic properties of nano-granules under various calcination temperature and different DMF concentration are also investigated (more details are shown in the ESI†). The growth mechanism of MFe₂O₄ nanoparticles is presented clearly. In addition, this simple strategy also shows great potential for the synthesis of other nano-granules, i.e., maghemite γ-Fe₂O₃ and nonmetal-doped MFe₂O₄ nano-granules.

2. Experimental

2.1 Synthetic procedures

A unified methodology is provided by using a simple and convenient route for assembling materials. Ferric nitrate (0.4 mol L⁻¹), and M nitrate (i.e. cobalt nitrate, nickel nitrate, and zinc nitrate) (0.2 mol L⁻¹) were dissolved in dimethyl formamide (DMF, 15 mL). For the binary ferrite, we add two different metal nitrate salts (e.g., for NiZnFe₂O₄ nano-granules, M nitrate (0.2 mol L⁻¹) is nickel nitrate (0.1 mol L⁻¹) and zinc nitrate (0.1 mol L⁻¹)). The mixed solution was calcined at 680 °C for 2 hours in air with the heating rate of 1 °C min⁻¹. The obtained products are MFe₂O₄ nano-granules. The detailed synthesis process was shown in Part I of ESI.†

2.2 Characteristics

The crystal structure and morphology of all the nano-granules were analyzed at the nanoscale using a X-ray diffraction instrument (XRD, Philips X'pert Pro MPD), a field-emission scanning electron microscope (FESEM, Hitachi S-4800), high-resolution transmission electron microscopy (HRTEM, Tecnai G² F30, FEI) equipped with energy-dispersive X-ray analysis (EDX, Oxford Instrument), high-angle annular dark and scanning transmission electron microscope (HAADF-STEM). The X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics) was performed using a monochromatic Al-Kα irradiation and a charge neutralizer. All binding energies were referred to C 1s peak at 284.6 eV of the surface of adventitious carbon. The elemental compositions of the nano-granules were performed by using an inductively couple plasma-atomic emission spectrometry (ICP-AES). Magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM, Lakeshore 7304).

3. Results and discussion

3.1 Formation process and mechanism of MFe₂O₄ nano-granules

To understand the formation mechanism, including chemical reactions and phase transformations of MFe₂O₄ nano-granules, the morphologies and crystal structures of the nano-granules under different DMF concentration and calcination temperature were observed. The detailed descriptions and explanations are presented in Part II and III of ESI.† It provides a clear formation process of MFe₂O₄ nano-granules during the calcination. Here, we chose NiFe₂O₄ nano-granules as the representative sample to study. The results of other nano-granules are similar with NiFe₂O₄, which are also shown in ESI.†

XRD spectra of NiFe₂O₄ nano-granules shown in Fig. S2a† reveal that α-Fe₂O₃ phases are observed at the low DMF concentration (0–33%), and increases with the improvement of DMF concentration. The impurity phase (α-Fe₂O₃) decreases gradually until almost disappears when DMF concentration is improved continuously (53–100%). This suggests that higher DMF concentration can improve the purity of MFe₂O₄ nano-granules. Comparing with XRD results of CoFe₂O₄ nano-granules (Fig. S2b†), particularly, α-Fe₂O₃ phase is only observed at a concentration of 0–67% in CoFe₂O₄, and when the solvent is nearly pure DMF (>87%), α-Fe₂O₃ phase is disappeared. As discussed in Part III of ESI,† this is attributable to the higher crystalline temperature of NiFe₂O₄ and NiZnFe₂O₄ than that of CoFe₂O_4, and NiFe₂O₄ and NiZnFe₂O₄ cannot crystallize completely at 650 °C. SEM pictures of NiFe₂O₄ (Fig. S3†) nano-granules with different DMF concentration indicate the morphologies of the samples are changed from bulk and dispersive particles to the compact and uniform nano-granules with the improvement of DMF concentration. The nano-granules formed in distilled water (DMF concentration is 0%) are irregular and disorderly, and reunite bulk together, but DMF coated nano-granules are the smooth and flat near-sphere. In detail, when the samples are calcined under the low DMF concentration (13–33%), the nano-granules are composed with irregular bulk particles (which are formed by distilled water) and near-sphere smooth granules (which are formed by DMF), which can be seen clearly in Fig. S3b and c.† Afterwards, DMF concentration is further increased (53–67%), i.e., more DMF coated particles are formed, and the granules become ever more uniform. The results are shown in Fig. S3d and e.† Finally, when DMF concentration is close to saturation (87–100%), large area and smooth nano-granules with uniform size are achieved, especially in the pure DMF concentration (100%). As a result, distilled water and DMF are the competitive relation during the calcination process, and DMF plays a dispersing and covering role during the reaction. TEM morphologies of NiFe₂O₄ nano-granules (Fig. S6†) also change gradually from bulk and irregular particles to the compact and uniform nano-granules with the improvement of DMF concentration. In particular, the crystallinity of nano-granules in the pure DMF (100%) presents a better result when compared with low DMF samples. The results are consistent with the results of SEM. SEM and TEM results of CoFe₂O₄ and NiZnFe₂O₄ nano-granules (see Fig. S3–S8, Part II of ESI†) also support the above conclusion.

On the basis of the experiments and results of ESI† (Part II and III), a formation mechanism of the nano-granules is proposed in this work. Chose NiFe₂O₄ as an example (it is also suitable for other ferrites), the schematic diagrams are shown in Fig. 1, and it is suggested that the following reactions occur during the calcination process:

Fe(NO₃)₃·9H₂O → Fe(NO₃)₃ + 9H₂O

Ni(NO₃)₂·6H₂O → Ni(NO₃)₂ + 6H₂O

2Fe(NO₃)₃ → γ-Fe₂O₃ + NO_x

Ni(NO₃)₂ → NiO + NO_x

γ-Fe₂O₃ → α-Fe₂O₃

α-Fe₂O₃ + NiO → NiFe₂O₄

Fig. 1 The formation mechanism of NiFe₂O₄ nano-granules (it is also suitable for other ferrites).

As shown in Fig. 1a, the precursors are composed of DMF, iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O). The solvents are beginning the evaporation process when the heating temperature is low (Fig. 1b). DMF in precursors plays a role as the polar solvent, which facilitates the diffusion and contraction of the reactant molecules in the course of volatilization. When the temperature increases to 80–130 °C (Fig. 1c), the gel is formed gradually. DMF is exhausted, and Fe(NO₃)₃·9H₂O and Ni(NO₃)₂·6H₂O lose their water of hydration. The processes of Fig. 1a–c reveal that the surface morphology of sample changes to amorphous nature with the increased temperature. The corresponding SEM picture (Fig. S15a†) and XRD pattern (Fig. S10a,† 100 °C) shows bulk grains, suggesting an amorphous structure. The none-magnetic results (VSM loops in Fig. S13a†) also confirm the amorphous structure. When the specimen is calcined at a moderate temperature about 130–220 °C (Fig. 1d), Fe(NO₃)₃ and Ni(NO₃)₂ decompose into γ-Fe₂O₃ and NiO (see XRD spectra of Fig. 4a 200 °C). VSM loop of the sample (Fig. S13a†) at 200 °C shows a perfect magnetism because of the formation of magnetite γ-Fe₂O₃. When the heating temperature subsequently increases to 220–300 °C (Fig. 1e), γ-Fe₂O₃ is further crystallized, and part of γ-Fe₂O₃ changes to α-Fe₂O₃ at the same time. This can be supported by the XRD patterns of Fig. S10† of 300 °C. Due to the present of none-magnetic α-Fe₂O₃, M_s of the sample at 300 °C displays a small decrease in quantity (Fig. S13a†). While after, the heating temperature is improved to 300–400 °C (Fig. 1f), more γ-Fe₂O₃ transforms to α-Fe₂O₃, and little α-Fe₂O₃ and NiO are reacted to NiFe₂O₄ (Fig. S10a† 300–400 °C). M_s is further reduced because of the increased none-magnetic α-Fe₂O₃, and various kinds of reunited and irregular particles can be seen in Fig. S11a and b.† Combining with XRD results of Fig. S10a,† those variety classes of littery particles may represent γ-Fe₂O₃, α-Fe₂O₃, NiO, and NiFe₂O₄. As the temperature increases continuously to 400–600 °C (Fig. 1g and h), all γ-Fe₂O₃ almost transforms to α-Fe₂O₃, and α-Fe₂O₃ and NiO are produced more NiFe₂O₄. The structures of samples change from mixed phases to pure NiFe₂O₄ ferrites (see XRD spectra of Fig. S10a,† 400–600 °C), and the morphologies are mainly composed of regular NiFe₂O₄ particles (SEM image of Fig. S11d†). Due to the decreased α-Fe₂O₃ phase and improvement of NiFe₂O₄ nano-granules, M_s is also enhanced (Fig. S13a†). When the temperature reaches to about 600–700 °C (Fig. 1i), the impurities of samples disappear, and pure NiFe₂O₄ nano-granules are achieved (Fig. S10a,† 700 °C and 6e). When temperature exceeds 700 °C (Fig. 1j), there will cause increased grain size of NiFe₂O₄ nano-granules (Fig. S11f and g†). At the same time, the increased grain size also leads to the improvement of M_s (Fig. S13a†). M_s of sample at 200 °C (γ-Fe₂O₃ phase) is higher than that of 700 °C (NiFe₂O₄ phase), this is because that M_s of pure γ-Fe₂O₃ is intrinsically larger than pure NiFe₂O₄. Moreover, XRD (Fig. S10a and b†) and SEM results (Fig. S11 and S12†) indicate that the crystalline temperature of CoFe₂O₄ is lower than NiFe₂O₄ nano-granules. To sum up, DMF is a polar solvent and good dispersing agent, which is associated well with some cations (the ions may be covered or coated by DMF).^30,31 According to all results of spinel ferrites and γ-Fe₂O₃ nano-granules (Part II, III, and V), DMF may slower the reaction process of calcination, which enhances the crystallization temperature of nano-granules.

3.2 The detailed results of MFe₂O₄ nano-granules at 680 °C in DMF

Based on the understanding of the formation process and mechanism above, we have demonstrated that low DMF concentration and low calcination temperature are not beneficial to the crystallization of pure MFe₂O₄ nano-granules. It will cause the impurity phase at any of the low DMF concentration or low calcination temperature. As the general results, large quantities of uniform and pure MFe₂O₄ nano-granules can be achieved at about 680 °C for 2 h in pure DMF. The results are shown below.

As shown in Fig. 2, XRD analysis of the powder confirms that the results exhibit spinel ferrite crystal structure without other detectable phases. In addition, the peak positions and relative intensity of all diffraction peaks for the three ferrites match well with standard powder diffraction data NiFe₂O₄ (JCPDS 10-0325), CoFe₂O₄ (JCPDS 22-1086), and binary NiZnFe₂O₄ (JCPDS 08-0234), respectively. The average crystalline size estimated by Scherrer analysis of full width at half maximum is about 26 nm for NiFe₂O₄, 27 nm for CoFe₂O₄, and 25 nm for NiZnFe₂O₄, respectively.

Fig. 2 XRD patterns of NiFe₂O₄, CoFe₂O₄, and NiZnFe₂O₄ nano-granule.

Fig. 3 shows SEM images of MFe₂O₄ nano-granule, indicating a large quantity of uniform spherical nano-granules was achieved using this approach. The average particle sizes estimated from SEM of the histograms are about 36 nm for NiFe₂O₄, 76 nm for CoFe₂O₄, and 40 nm for NiZnFe₂O₄, respectively, which is a little larger than the crystallite sizes estimated from XRD line width analysis. This suggests that the particle may be composed with one more crystallites. In addition, the granular size of CoFe₂O₄ is larger than the others, and XRD pattern of Fig. S10† and SEM results of Fig. S11 and S12† indicate CoFe₂O₄ is easier to crystallize when compared with NiFe₂O₄ and NiZnFe₂O₄.

Fig. 3 SEM images and grain size distributions of MFe₂O₄ nano-granule, NiFe₂O₄ (a), (b), and (c); CoFe₂O₄ (d), (e), and (f); NiZnFe₂O₄ (g), (h), and (i).

Fig. 4 shows TEM images of typical samples of NiFe₂O₄, CoFe₂O₄, and binary NiZnFe₂O₄ nano-granules. Analysis of the TEM micrographs clearly indicates the formation of near-spherical NiFe₂O₄ (Fig. 4a and b), CoFe₂O₄ (Fig. 4g and h) and NiZnFe₂O₄ (Fig. 4m and n) nano-granules. TEM micrographs show that the granular size of CoFe₂O₄ is larger than others, which agrees with SEM pictures. In addition, the large and black nano-granules shown in Fig. 4g are the conglobation of CoFe₂O₄ granules. The particle size distributions, obtained from the TEM micrographs, are shown in Fig. 4f, l and r. The histograms show that all the samples are with uniform distributions. The mean particle sizes obtained from Gaussian fitting of the histograms are 38 ± 3 nm, 75 ± 4 nm, and 42 ± 2 nm, respectively, for NiFe₂O₄, CoFe₂O₄, and NiZnFe₂O₄ nano-granules, which is comparable with SEM results. HRTEM characterizations show the lattice fringes of the obtained ferrites, and the interfringe distance shown in Fig. 4c, i and o is 0.249 nm, 0.209 nm and 0.253 nm, which corresponds well with {311}, {400}, and {311} planar spaces of NiFe₂O₄, CoFe₂O₄, and NiZnFe₂O₄ nano-granules, respectively. Both the lattice fringes and SAED corresponding to a group of atomic planes within each particle are clearly visible, revealing the highly crystallization of these nano-granules. Meanwhile, EDX data of NiFe₂O₄ (Fig. 4e), CoFe₂O₄ (Fig. 4k), and NiZnFe₂O₄ (Fig. 4q) nano-granules indicate that atomic ratio of metallic element in MFe₂O₄ is about Ni : Fe = 3.7 : 7.3, Co : Fe = 6.1 : 11.9, and Ni : Zn : Fe = 1.3 : 1.3 : 5.3, which is very close to the stoichiometry of MFe₂O₄. ICP analysis of nano-granules also shows the element proportion of NiFe₂O₄ and NiZnFe₂O₄ is Ni : Fe = 30 : 6 and Ni : Zn : Fe = 15 : 12 : 65. This further confirms that the composition and structure are coincident with the chemical formulation of MFe₂O₄ nano-granules.

Fig. 4 TEM images, HRTEM image, EDX, SAED, and particle size distributions of MFe₂O₄ nano-granules: (a–f) NiFe₂O₄; (g–l) CoFe₂O₄; (m–r) NiZnFe₂O₄.

The room temperature hysteresis loops of the ferrites were measured using VSM. As shown in Fig. 5, these loops display typical ferromagnetic hysteresis loops. M_s and H_c of NiFe₂O₄, CoFe₂O₄, and NiZnFe₂O₄ nano-granules are 33 emu g⁻¹ and 240 Oe, 67 emu g⁻¹ and 1098 Oe, 51 emu g⁻¹ and 75 Oe, respectively. Those observed values are within the range reported in those literatures.^10,12,25,29

Fig. 5 Room temperature hysteresis loop of NiFe₂O₄, CoFe₂O₄, and NiZnFe₂O₄ nano-granules.

3.3 Synthesis of other MFe₂O₄ nano-granules

As the results, large quantities of uniform MFe₂O₄ nano-granules have been achieved at 680 °C for 2 h in pure DMF. Thus, we also synthesize the other MFe₂O₄ (ZnFe₂O₄, CuFe₂O₄, MnFe₂O₄) nano-granules under this condition, and the results are shown in Fig. 6. As shown in Fig. 6a–c, the positions and relative intensity of all diffraction peaks for the three ferrites match well with standard powder diffraction data ZnFe₂O₄ (JCPDS 22-1012), CuFe₂O₄ (JCPDS 34-0425), and MnFe₂O₄ (JCPDS 24-0507), respectively. SEM pictures in Fig. 6d–f also show a large quantity of spherical nano-granules with good uniformity using this approach.

Fig. 6 XRD patterns (a–c) and SEM images (d–f) of ZnFe₂O₄, CuFe₂O₄, and MnFe₂O₄ ferrites nano-granules. The inset of (d–f) is the amplifying SEM pictures.

3.4 Comparison with other typical methods or processes

From the above investigations, we have demonstrated a facile-direct synthesis of a number of uniform-sized and smooth MFe₂O₄ nano-granules by calcination process under the air. Significantly, we discuss some typical methods or process as comparative results with ours. The comparative data are shown in Table 1. Except for the main metal salt during the preparing process, Table 1 detailedly analyzes various methods through solvent, additive or surfactant, pH, reaction time, centrifugation, and other parts. These methods or process are not limited to the literatures we provided. As a result, although the dispersity and size of nano-granules in our results are incomparable with some of the earlier reports, this technique only needs one solvent without other complicated process. The process realizes a simple, rapid and convenient route for assembling MFe₂O₄ nano-granules when compared with others.

Table 1 Comparisons of fabrication condition of MFe₂O₄ using different method. These methods or processes are not limited to the literatures we provided

Method or process	Solvent or additive or surfactant	pH	Main reaction condition	Centrifugation or wash	Others
Hydrothermal^24	Deionized water, acetone, ammonia	Yes	180 °C, 12 h	Yes	(1) Need ultrasonic; (2) need high pressure
Reduction and oxidation^29	Tetrahydrofuran, deionized water, chloride/iodide, crown ether, CoI2, KNa, NaCl	—	Reaction at −30 °C for long time; H2O for long time; annealing for 4 h	Yes	All reactant was performed in N2 or vacuum
Nonhydrolytic process, seed-mediated growth^26	Phenyl ether, oleic acid, oleylamine, ethanol, 1,2-hexadecanediol, 1-octadecanol	—	(1) 140 °C for 30 min; (2) adding additive at 260 °C for 30 min; (3) repeat the process	Yes, using ethanol	(1) Repeat adding reactant; (2) more than 3 steps
Thermal decomposition^20	Benzyl ether, 1,2-hexadecandiol, oleic acid, oleylamine	Yes	Under N2, (1) 110 °C for 1 h; (2) 210 °C for 2 h; (3) 295 °C for 1 h; (4) cooled to RT	Yes, using ethanol and hexane	(1) High requirements of reactant; (2) need more surfactant (3) need N2
Coprecipitation and solvothermal^13	Ethylene glycol, NaAc, polyethylene glycol	Yes	(1) Coprecipitation; (2) 200 °C for 8–72	Yes, using ethanol	(1) High pressure; (2) dried at 60 °C for 6 h
Combustion^15	Deionized water, C2H6N4O2 (ODH), C4H16N6O2 (TFTA)/oxalic acid dihydrazide	—	(1) Keep at 350 °C; (2) ignite; (3) sintered 1050 °C for 12 h	—	Need post processing
High temperature reaction^25	NaDBS, N2H4, ethanol, deionized water, hydrazine, toluene/alkanes, hydrazine	—	(1) Stirring and adding reactant; (2) RT for 12 h; (3) 90 °C for 5 h under Ar	Yes	(1) Complicated reaction process; (2) need Ar
Liquid–solid-solution^12	Ethanol, linoleic acid, deionized water, sodium, linoleate	—	(1) Long time 90–120 °C; (2) repeat adding reactant during reaction	Yes	(1) Different samples need specific temperature; (2) complicated procedure
Co-precipitation, thermal decomposition, high-temperature hydrolysis^28	Benzyl ether, heptane, acetone, ethanol, oleic acid, oleylamine	—	(1) 230–300 °C, 1–5 h; (2) adding reactant, 200 °C for 2 h	Yes, using acetone and ethanol	Need nitrogen of all process
Thermolysis^10	Deionized water, ethanol, hexane, 1-octadecene, sodium oleate, oleic acid	—	(1) 60 °C for 4 h; (2) evaporation at 70 °C in ethanol and hexane; (3) evaporation at 110 °C in water; (4) stirring for 1 h under N2; (5) 300 °C for 30 min; (6) 500 °C for 1 h	Yes, using hexane and water	(1) Complicated solution proportioning; (2) need N2
Sol–gel^1	Ethyl alcohol, acetone, epichlorohydrin	—	(1) Stirred for 15 min; (2) gelation for 45 min; (3) aged for 16 h; (4) several days; (5) 10 °C under liquid CO2 for 20–36 min; (6) 300–400 °C for 20 h; (7) cooled Ar	Yes	(1) Long reaction time; (2) need CO2; (3) need Ar
Thermal solvent^23	N-Methyl 2-pyrrolidone, ethanol, 1,2-hexadecanediol, oleic acid, oleylamine	—	(1) Stirred under Ar; (2) 200 °C for 1 h; (3) stirred for ∼12 h; (4) dried at 50 °C in a vacuum	Yes, using ethanol, oleic acid and oleylamine	(1) Need Ar; (2) need vacuum; (3) complex process
Solution reaction^14	Oleylamine, dibenzyl ether, ethanol, sodium hydroxide solution	Yes	(1) Stirred for 1 h under Ar; (2) 120 °C for 1 h; (3) 240 °C for 30 min; (4) cooled to RT	Yes, using sodium hydroxide solution	(1) Complex temperature control; (2) need Ar
Co-precipitation^21	HCl, deionized water, ethanol, buffer, ammonium, hydroxide, ACN, TFA, Na2HPO4	11–12	(1) Stirring at RT for 30 min; (2) kept with ethanol in a 4 °C fridge; (3) other post processing	Yes, using water and ethanol	Need cold storage
Template method^27	Citrate, EDTA, ammonia	∼7	(1) 450 °C for 5 h; (2) 950 °C for 10 h	—	Evaporation of water
This work	DMF	No	Calcined at 680 °C for 2 h	No	No

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3.5 Synthesis of magnetite γ-Fe₂O₃ nano-granules

Magnetite (γ-Fe₂O₃) nano-granules are generated at different calcination temperature (see ESI Part V for detail†). XRD results (Fig. S15†), SEM images (Fig. S15†) and TEM images (Fig. 7c) indicate that when the calcination temperature increases to 200 °C, single and uniform-sized γ-Fe₂O₃ nano-granules with the cubic lattice (JCPDS 39-1346) is achieved. The obtained particles are γ-Fe₂O₃ rather than α-Fe₂O₃ due to the different XRD pattern of α-Fe₂O₃ (JCPDS 80-2377) from γ-Fe₂O₃.^32,33 Pure γ-Fe₂O₃ nanoparticles also show good crystallinity through TEM pictures (Fig. 7c), and the element ratio (EDX results in Fig. 7c) of Fe : O is calculated to be 17.9 : 30.8, which is very close to the stoichiometry of γ-Fe₂O₃. Moreover, characteristic XPS peaks (Fig. 7b) at 710.6 and 724.1 eV further confirm that the obtained nano-granules are pure γ-Fe₂O₃.^32,34 Finally, M_s (Fig. 7d and S18†) of γ-Fe₂O₃ nano-granules increases to 74 emu g⁻¹, which is comparable with bulk γ-Fe₂O₃ sample,^35,36 but much larger than other γ-Fe₂O₃ nanoparticles.^37–40

Fig. 7 The XRD, XPS, TEM and VSM results of γ-Fe₂O₃ nano-granules at 200 °C.

3.6 Synthesis of nonmetal-doped CoFe₂O₄ nano-granules

This method can be also used to modify the materials (e.g. nonmetal-doped CoFe₂O₄), and further improve the properties of samples (see ESI Part VI for detail†). We chose S-doped CoFe₂O₄ nano-granules as an example here. S-doped CoFe₂O₄ nano-granule with different S concentration is performed by various techniques. The diffraction (311) peaks of the samples (Fig. S18†) are shifted a little to lower angles due to the doping of S element. The typical results of S-doped CoFe₂O₄ nano-granule (S concentration is 0.3 mol L⁻¹) are shown in Fig. 8. The mapping (Fig. 8c and S20†) and XPS spectra (Fig. 8b and S21†) further confirm that S element is doped into CoFe₂O₄ nano-granule. Furthermore, M_s (Fig. 8d and S22†) of S-doped CoFe₂O₄ nano-granule is improved to 81 emu g⁻¹, which is quite different from the former reduced metal-doped CoFe₂O₄ nanoparticles.^41–47 Significantly, nonmetal C elements doped CoFe₂O₄ nano-granule is also fabricated by this method. Various characterization results determine that the nonmetal elements are efficiently doped in CoFe₂O₄, and the magnetic properties are also enhanced after doping. These results are detailedly shown in ESI Part VI.†

Fig. 8 The XRD, XPS, TEM mapping and VSM results of pure CoFe₂O₄ and S-doped CoFe₂O₄ nano-granules (S concentration is 0.3 mol L⁻¹).

4. Conclusion

In summary, we have demonstrated the synthesis of MFe₂O₄ ferrite nano-granule through a facile, rapid and convenient calcination process. There is no pH regulation, gas atmosphere, centrifugation and other post procedure or supplementary reagents during the preparing process. The obtained MFe₂O₄ nano-granules display good uniformity and shows good magnetic properties. Using this approach, maghemite γ-Fe₂O₃ nano-granule with high magnetization is obtained. In addition, various nonmetal-doped MFe₂O₄ is also prepared, and the properties of doped MFe₂O₄ nano-granules are enhanced. These results provide considerable proposal in the advancement of magnetic nanomaterial.

Acknowledgements

This work is supported by National Basic Research Program of China (2012CB933101), National Science Fund of China (11574121, 51371092).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13373h

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