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参考文献

1 Introduction▲
2 Experimental▲
2.1 Synthesis of Fe2O3 nanoparticles
2.2 Synthesis of Fe3O4 particles
2.3 Characterization
3 Results and discussion▲
4 Conclusion▲
图表▲

Fig.4 Room-temperature magnetic hysteresis loop of Fe3O4 particles prepared with and without Na2CO3 coating (inset:amplified curves around coercive value,samples of Fe3O4 particles prepared with and without Na2CO3 coating dispersed in ethanol and magnetically attracted samples)

Rare Metals2019年第8期

A facile synthesis of magnetite single-crystal particles by employing GO sheets as template for promising application in magnetic fluid

Zhen Qin Zhen-Hui Ma Jian-Kang Zhi Yong-Ling Fu

School of Mechanical Engineering and Automation,Beihang University

College of Materials Science and Engineering,Beijing University of Technology

作者简介：*Yong-Ling Fu e-mail:fuyongling@buaa.edu.cn;

收稿日期：8 October 2018

基金：financially supported by the National Natural Science Foundation of China(No.51701109);

A facile synthesis of magnetite single-crystal particles by employing GO sheets as template for promising application in magnetic fluid

Zhen Qin Zhen-Hui Ma Jian-Kang Zhi Yong-Ling Fu

School of Mechanical Engineering and Automation,Beihang University

College of Materials Science and Engineering,Beijing University of Technology

Abstract：

It was reported a facile strategy to fabricate magnetite(Fe₃ O₄)single-crystal particles with critical single-domain size by employing graphene oxide(GO)sheets as template.In this method,the small-sized Fe₂ O₃ nanoparticles were first synthesized,and then low-temperature annealing under H₂ would convert them into large-sized Fe₃ O₄ single-crystal particles.The synthetic particles with an average size of 100 nm exhibit high saturation magnetization(M_s)of 0.085 A·m²·g^-1,which is very close to theoretical value,being among the highest values in ever reported for Fe₃ O₄ made from chemical methods.On this basis,the small-sized Fe₃ O₄ particles(average size of 30 nm)were also fabricated by coating with Na₂ CO₃ shell.

Keyword：

Fe₃O₄; Critical single-domain size; Saturation magnetization; Chemical synthesis;

Received： 8 October 2018

1 Introduction

Magnetic nanoparticles have been extensively studied for their advanced applications such as magnetic storage [ 1, 2, 3] ,hard/soft magnetic nanocomposites [ 4, 5, 6, 7, 8] ,magnetic fluid [ 9] ,sensors and drug cariers in biomedical technology [ 10, 11, 12, 13, 14] .Especially for superparamagnetic nanoparticles,they can respond quickly to an external magnetic field,generating a secondary field around each nanoparticles and interfering with the proton nuclear spin relaxation,being applied for sensitive magnetic probes (contrast agents) in magnetic resonance lmaging (MRI) [ 15, 16, 17, 18] ,magnetic fluid seal,measurement of fluid film thickness [ 19, 20] ,as well as magnetic fluid hyperthermia and targeted drug delivery [ 21, 22, 23] .

Among available superparamagnetic materials,magnetite (Fe₃O₄) has attracted much attention for its high saturation magnetization (M_s) and good chemical stability.Fe₃O₄ (FeO.Fe₂O₃) has an inverse spinel structure,where the spins of two Fe³⁺are antiferromagnetic coupled and cancel each other and magnetizationvalues are determined by Fe²⁺ [ 24] .Such special structure poses quite a synthetic challenge to the conventional aqueous phase synthesis method.

Generally,thermal decomposition of iron organometallie compounds in a higher boiling point organic solvent could yield monodisperse iron oxide nanoparticles (NPs) [ 25, 26] .For example,Fe₃O₄ nanoparticles with controllable size (6-13 nm) have been obtained by thermal decomposition of carbonyl iron,which exhibits low magnetization(0.02 A·m2.g^-1) [ 27] .Also,ferric acetylacetonate has been employed as raw material to fabricate Fe₃O₄nanoparticles in a mixed organic solution,and the product with size of 16nm has a M_s of 0.07 A·m²·g^-1 [ 28] .Nevertheless,magnetite nanoparticles synthesized in organic solvent do not facilitate biomedical applications due to energy-intensive feature and the employ of toxic chemicals [ 29] .To avoid using organic chemicals,coprecipitation in aqueous solution has been developed to acquire Fe₃O₄nanoparticles [ 30, 31] .For instance,20-nm Fe₃O₄nanoparticles have been prepared by precipitating ferrous sulfate in water,showing a M_s of 0.058 A·m²·g^-1 [ 32] .

Despite these significant progresses in chemical synthesis,the magnetic performance of synthetic particles is still lower than theoretical value of 0.092 A m².g^-1 (bulk) [ 33] .This may be caused by the degree of crystallinity,size and shape of Fe₃O₄ nanoparticles [ 34] .In principle,magnetic single-crystal particles at critical size from multidomain to single-domain structure would own high performance.For Fe₃O₄,the critical single-domain size is about 128 nm [ 35] .Therefore,the preparation of Fe₃O₄single-crystal nanoparticles around critical single-domain size has become an effective approach to enhance magnetic performance.However,it is also challenging due to the difficulty in controlling size in chemical synthesis.

In this paper,a facile strategy is proposed to fabricate Fe₃O₄ single-crystal particles with critical single-domain size by employing graphene oxide (GO) sheets as template.Firstly,the small-sized Fe₂O₃ particles were synthesized,and then low-temperature annealing under H₂ will convert them into large-sized Fe₃O₄ (average size of 100 nm)single-crystal particles.In this process,the size of Fe₃O₄particles can be tuned by coating with Na₂CO₃ shell.The synthetic particles with 100 nm in size exhibit high M_s of85 0.092 A·m²·g^-1,which is very close to theoretical value,being among the highest values in ever reported for Fe₃O₄ade from chemical methods [ 25, 26, 27, 28, 29, 30, 31, 32, 35] .

2 Experimental

2.1 Synthesis of Fe2O3 nanoparticles

0.5 g Fe(NO₃)₃·9H₂O was dissolved in 80 ml distilled water and 20 ml ethanol in a 250-ml beaker.Then 10 ml graphite oxide dispersed aqueous solution (0.5 g·L^-1) was added in above solution.The graphite oxide sheets solution was bought from Nanjing XFNANO Materials Tech Co.,Ltd,which was prepared from graphite powder by a modified Hummers method.The solution was further homogenized under sonication for 1 h.After that,the beaker was put on a heating platform with strong magnetic stirring to heat to 367 K and this temperature was kept for1.5 h.After evaporating all of solution,the red powder was collected and put in alumina crucible.And then the alumina crucible was heated to 773 K at a rate of 10 K·min^-1in the air.After keeping at this temperature for 2 h to remove GO sheets,the red Fe₂O₃ powder wasobtained.The synthesis of Fe₂O₃@Na₂CO₃ particles is similar with the synthetic process of Fe₂O₃ particles.2.4 g sodium citrate was added in mixed solution (80 ml water and20 ml ethanol) under otherwise identical conditions.The same process was followed,and at last deep yellow Fe₂O₃@Na₂CO₃ powder was obtained.

2.2 Synthesis of Fe3O4 particles

As-prepared Fe₂O₃ or Fe₂O₃@Na₂CO₃ particles were placed in an alumina boat,and the boat was put in tube furnace which was degassed 3 times to remove air and moisture.The temperature was raised to 673 K at a heating rate of 8 K min^-1 and kept for 2 h under 95 vol%Ar+5vol%H₂ atmosphere.After cooling the sample down to room temperature,the black Fe₃O₄ or brown Fe₃O₄@-Na₂CO₃ powder was obtained.For the Fe₃O₄@Na₂CO₃particles,they were dispersed in distilled water under sonication to remove Na₂CO₃,and this washing was run 3times.

2.3 Characterization

The crystallographic structure was identified by X-ray diffractometer (XRD,D/MAX 2200 PC) with Cu Kαradiation (λ=0.15418 nm).The microstructure and morphology of the above samples were investigated using scanning electron microscope (SEM,ZEISS-SUPRA55)and transmission electron microscope (TEM,Tecnai G2F20).The magnetic properties were measured at room temperature using a vibrating sample magnetometer(VSM) under a maximum applied field of 3 T.

3 Results and discussion

The synthesis process is illustrated in Fig.1.First,a certain amount of Fe(NO₃)₃ and GO sheets was dispersed in aqueous soIution in a beaker.After evaporating all of solution,Fe(NO₃)₃ coated with GO sheets was obtaincd.Undergoing an annealing atlow temperature in air,the GO sheets were removed and the agglomcrative Fe₂O₃ particles with small size were synthesized.A further annealing under H₂ would convert Fe₂O₃ into Fe₃O₄ single-crystal particles,where the size of Fe₃O₄ particles is far larger than that of Fe₂O₃ due to the grain growth.To impede the grain growth,an improved route was designed,where sodium citrate was added in original solution to prepare Fe₂O₃@-Na₂CO₃ core@shell particles.Followed by H₂ annealing,Fe₃O₄@Na₂CO₃ particles would be fabricated,where Na₂CO₃ could prevent Fe₃O₄ particles from bonding with each other during annealing process.After washing Na₂CO₃,the small-sized Fe₃O₄ particles were achieved.Also,the size of final product can be tuned by controlling the amount of sodium citrate.

Figure 2a shows XRD patterns of as-prepared Fe₂O₃and Fe₃O₄particles.It can be seen from diffraction pattern of Fe₂O₃ particles that as-prepared Fe₂O₃ particles have two types of crystalline structures:one belongs to cubic Fe₂O₃ and another corresponds to hexagonal Fe₂O₃,which match well with the standard Fe₂O₃ patterns (JPCD No.39-1346 and No.33-0664),respectively.After undergoing an annealing under Ar+H₂ at 673 K,they were reduced into cubic Fe₃O₄ which can be well indexed to standard PDF card of No.19-0629.Additionally,the sharper diffraction peaks of Fe₃O₄ compared to Fe₂O₃ indicate that the grain growth occurs during annealing procedure.Figure 2b gives SEM image of Fe₂O₃ particles,which suggests that as-prepared Fe₂O₃ particles have a well distribution and uniform size of 30-40 nm.It also can be observed that there is an obvious agglomeration.TEM

Fig.1 Schematic illustration of synthesis of Fe₃O₄ nanoparticles with different sizes by using GO sheets as template

Fig.2 Characterization of as-prepared Fe₂O₃ and Fe₃O₄ particles:a XRD patterns of Fe₂O₃ and Fe₃O₄,b SEM image of as-prepared Fe₂O₃nanoparticles,c TEM image of Fe₂O₃ particles,d HRTEM image of Fe₂O₃ particles,e TEM image of Fe₃O₄ particles after annealing under H₂,and f HRTEM image of Fe₃O₄ particles

Fig.3 Characterization of as-prepared Fe₂O₃@Na₂CO₃ and corresponding Fe₃O₄ particles:a XRD patterns of Fe₂O₃@Na₂CO₃,Fe₃O₄@Na₂CO₃ and Fe₃O₄,b SEM image of as-prepared Fe₂O₃@Na₂CO₃ nanoparticles,c TEM image of Fe₂O₃@Na₂CO₃ nanoparticles,d elemental mappings of Fe₂O₃@Na₂CO₃ nanoparticles,e TEM image of Fe₃O₄ particles after annealing under H₂ and removing Na₂CO₃,and f HRTEM image of Fe₃O₄ particles

image of Fe₂O₃ is given in Fig.2c,where these bonding particles integrate with each other like GO sheets shape,implying the template effect of GO sheets.And there are many particles forming rod-like particles with length of about 100 nm and diameter of 10 nm.The high-resolution TEM (HRTEM) image in Fig.2d further confirms the formation of Fe₂O₃ nanorods.The lattice fringes possessing the same direction illustrate the single-crystal structure of this nanorod.The interplanar distances of 0.270 and0.251 nm,which can be well fitted to (104) plane in hexagonal Fe₂O₃ and (311) plane in cubic Fe₂O₃,respectively,further confirming the existence of two types of Fe₂O₃ phase structure,in agreement with XRD data.According to Fig.2e,the synthetic Fe₃O₄ particles exhibit regularly cubic shape and a narrow size distribution with size of 80-120 nm (average size of 100 nm),being close to the critical single-domain size of 128 nm.And HRTEM

image in Fig.2f reveals the single-crystal structure of Fe₃O₄ particle.The formation of Fe₃O₄ phase is confirmed by the interplanar distance of 0.253 nm,which corresponds to the (311) plane in cubic Fe₃O₄,being consistent with XRD results in Fig.2a.The large size and sound single crystal of Fe₃O₄ may be attributed to the grain growth and recrystallization of small-sized Fe₂O₃ particles during reductive annealing process.

Fig.4 Room-temperature magnetic hysteresis loop of Fe3O4 particles prepared with and without Na2CO3 coating (inset:amplified curves around coercive value,samples of Fe3O4 particles prepared with and without Na2CO3 coating dispersed in ethanol and magnetically attracted samples)

Figure 3 a shows XRD patterns of as-prepared Fe₂O₃@Na₂CO₃,Fe₃O₄@Na₂CO₃ and Fe₃O₄ particles.According to the diffraction pattern,the Fe₂O₃@Na₂CO₃composites are obtained after annealing in air,where the reaction occurs:2Na₃C₆H₅O₇+3O₂ 3Na₂CO₃+5H₂O+3CO_2.There are also two types of crystalline structures for Fe₂O₃ particles (hexagonal and cubic phases),which is in accordance with Fig.2a.After undergoing an annealing under Ar+H₂ at 673 K,Fe₃O₄@Na₂CO₃particles were fabricated,in which Fe₃O₄ phase possesses cubic structure,as same as Fe₃O₄ particles without Na₂CO₃coating.After removing Na₂CO₃ with water,the single Fe₃O₄ phase was achieved.Compared to Fe₃O₄ particles prepared without Na₂CO₃ coating,their diffracted peaks widen apparently,suggesting that smaller grain size is obtained and Na₂CO₃ could impede the grain growth.Figure 3b gives SEM image of Fe₂O₃@Na₂CO₃,demonstrating that the well-distribution composites have been synthesized.TEM result in Fig.3c further shows that small-sized Fe₂O₃ particles were well dispersed in Na₂CO₃matrix,where Na₂CO₃ particles can prevent Fe₂O₃ smallsized particles from contacting with each other.As shown in Fig.3d,the elemental mapping images give the distribution of Na and Fe,which further confirms that Fe₂O₃particles were embedded in Na₂CO_3.Fe₃O₄@Na₂CO₃composites were further prepared by H₂ reduction at673 K.Owing to the similar morphology with Fe₂O₃@-Na₂CO₃ composites,the morphology of Fe₃O₄@Na₂CO₃ is not presented here.After removing extra Na₂CO₃,the small-sized Fe₃O₄ nanoparticles with size of 10-50 nm(average size of 30 nm) were achieved,as shown in Fig.3e.Unlike Fe₃O₄ particles without Na₂CO₃ coating,these Fe₃O₄ nanoparticles possess irregular shape,which may be explained that the existence of Na₂CO₃ matrix would restrain the free growth of Fe₃O₄ particles.HRTEMimage in Fig.3f illustrates that these small particles are single crystal and have interplanar distance of 0.253 nm,matching well to the (311) plane in cubic Fe₃O_4.

Figure 4 gives the room-temperature magnetic hysteresis loop of Fe₃O₄ particles prepared with and without Na₂CO₃ coating.The large-sized Fe₃O₄ particles (average size of 100 nm) exhibit high M_s of 0.085 A·m²·g^-1,which is very close to the theoretical value of 0.092 A·m²·g^-1 and is among the highest values ever reported for Fe₃O₄ particles made from chemical methods.The small-sized Fe₃O₄particles (average size of 30 nm) show a suitable M_s of0.068 A·m².g^-1.As shown in inset of Fig.4,the coercivities (Hc) of 0.0170 and 0.0106 T are achieved in 100-and30-nm Fe₃O₄ particles,respectively.The high M_s and H_efor large-sized Fe₃O₄ particles are mainly attributed to the good crystallization and less surface defects,as well as the critical single-domain structure.As known,the 100-nmsized particle and 30-nm-sized particles have smaller size than critical single-domain size of Fe₃O₄ particles(128 nm) [ 34] .Also,two types of particles exhibit singlecrystal structure.Thus,it can be concluded that both types of particles are in single-domain state.The different coercivities may be attributed to the particle size and defects amount on the particles surface.Generally,when the particles size is smaller than critical single-domain size,the coercivity of magnetic particles will increase with the increase in particles size.On the other hand,the surface defect would reduce coercivity.In this paper,the 30-nmsized particles present rough surface,which increases the amount of surface defects and reduces their coercivity.According to the picture of real product,both of smallsized and large-sized Fe₃O₄ particles can be well dispersed in ethanol.Therefore,they can be applied in magnetic fluid hyperthermia.After being magnetically attracted by magnet,all of large-sized Fe₃O₄ particles could be moved quickly and the clear solution can be obtained,while smallsized Fe₃O₄ particles were moved slowly and some particles still existed in the solution.Thus,the large-sized Fe₃O₄ particles are suitable for application in magnetic fluid,sensitive magnetic probes and targeted drug delivery due to their quick response.

4 Conclusion

In summary,the large-sized Fe₃O₄ particles (average size of 100 nm) were synthesized by employing GO sheets as template.The high M_s of 0.085 A.m².g^-1 and coercivity of0.0170 T are achieved in Fe₃O₄ particles with critical single-domain size,which is very close to the theoretical value of 0.092 A·m²·g^-1.On this basis,the small-sized Fe₃O₄ particles (average size of 30 nm) were fabricated by embedding them in Na₂CO₃ matrix,which have a suitable M_s of 0.068 A·m².g^-1.For large-sized Fe₃O₄ particles,the high magnetic performance is due to good crystallization,less surface defects and critical single-domain structure.Therefore,large-sized Fe₃O₄ particles are suitable for application in magnetic fluid,sensitive magnetic probes and targeted drug delivery due to their quick response.This approach is easy to extend to the synthesis of other magnetic nanoparticles such as C.oO and CoFe₂O_4.