Superlattices and Microstructures 60 (2013) 231–239

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o ca t e / s u p e r l a t t i c es

Facile synthesis of porous magnetite microspheresand solvent-induced phase transition

0749-6036/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.05.001

⇑ Corresponding author at: Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, Anhui, PTel./fax: +86 551 5593360.

E-mail address: awzhao@iim.ac.cn (A. Zhao).

Zibao Gan, Aiwu Zhao ⇑Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, PR ChinaInstitute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, Anhui, PR China

a r t i c l e i n f o

Article history:Received 5 January 2013Received in revised form 27 April 2013Accepted 2 May 2013Available online 14 May 2013

Keywords:MagnetitePorous microspheresSolvothermal synthesisPhase transition

a b s t r a c t

Uniform porous magnetite microspheres have been synthesized ina large scale by a dodecyltrimethylammonium bromide (DTAB)-assisted solvothermal route. The as-obtained products werecharacterized by X-ray diffraction, scanning electron microscopy,transmission electron microscopy, thermal gravimetric analyzer,Brunauer–Emmett–Teller gas sorptometry and vibrating samplemagnetometer. The factors influencing the phase and morphologyof the products were investigated. It demonstrates that the ratioof water and ethylene glycol (EG) plays a crucial role for the phasestructure and morphology of the products. When the ratio of waterand EG does not exceed 1:3, the as-obtained products belong tomagnetite phase. Nevertheless, the as-obtained products begin toconvert to hematite phase with the ratio increase to 3:5. The reasoncould be ascribed by the increased water volume, which effectivelyweakens the reductive activity of EG. Followed by the change of theratio of water and EG, the morphologies of as-obtained productsundergo a series of evolution, such as from flower-like magnetitemicrospheres to polyhedral hematite nanoplates. On the basis ofexperimental investigation and analysis, a possible formationmechanism was proposed. The magnetic study demonstrated theporous magnetite microspheres with high saturation magnetization(Ms) and the decreased Ms caused by the phase transition.

� 2013 Elsevier Ltd. All rights reserved.

R China.

232 Z. Gan, A. Zhao / Superlattices and Microstructures 60 (2013) 231–239

1. Introduction

Magnetic nanostructured materials have received considerable interest because of their potentialapplications in ferrofluids, catalysts, colored pigments, chemical sensors, high-density magneticrecording media, medical diagnostics [1–6]. Especially, the cubic inverse spinel structured magnetite(Fe3O4) has been considered as an ideal candidate for biological applications such as a tag for sensingand imaging, a drug-delivery carrier for antitumor therapy, and an activity agent for medical diagnos-tics, due to its high saturation magnetization, low cytotoxicity and good biocompatibility [7–9]. Todate, various nanostructured magnetites, such as wires, tubes, octahedra, rings, hollow spheres, pyr-amids, triangles and spindle have been synthesized by a variety of methods, including reverse micelle,hydrothermal treatment, thermal decomposition of organometallics, coprecipitation, magnetic-fieldinduction, wet-chemical etching, chemical vapor deposition and polyol-mediated processes [10–18].Among the aforementioned methods, the polyol methods have been paid more attention in the latestdecade [19–21]. The attractive advantage of the polyol process is its capability of producing hydro-philic magnetite nanostructures or microstructures with high magnetization and controllable second-ary structures within one step. For example, Li et al. reported a typical polyol method to synthesizeuniform superparamagnetic magnetite microspheres in a large scale [19]. Gu’s group prepared differ-ent shaped magnetite nanocrystals in different poloys (ethylene glycol (EG) and 1,2-propylene glycol).Recently, our group also developed a surfactant-free route to synthesize ultrafine magnetite nanopar-ticles in triethylene glycol [22]. As far as the application in biological fields is concerned, the porousmagnetite micro/nano-structures with high magnetic saturation, large surface area, good dispersionin liquid media are of particular importance because they can act as promising drug delivery carriersand provide maximum signal owing to their porous interior structures and high magnetic saturation[8,9]. Several methods for generating the porous magnetite micro/nano-structures have been devel-oped by employing a few acceptable mechanisms, such as gas-bubble-assisted Ostwald ripening pro-cess, cooperation of oriented aggregation and Ostwald ripening mechanisms, nanoscale Kirkendalleffect and corrosion-based inside-out evacuation [23–27]. To the best of our knowledge, there are onlylimited reports concerning the synthesis of porous magnetite microspheres and their phase transitioncaused by tuning the ratio of water and EG.

In this paper, uniform porous magnetite microspheres have been synthesized in a large scale via adodecyltrimethylammonium bromide (DTAB)-assisted polyol process. The parallel experiments werecarried out and demonstrate that the ratio of water and EG plays a crucial role for the phase structureand morphology of the products. When the ratio of water and EG does not exceed 1:3, the as-obtainedproducts belong to magnetite phase. Nevertheless, the as-obtained products begin to convert to hema-tite phase with the ratio increase to 3:5. The reason could be ascribed by the increased water volume,which effectively weakens the reductive activity of EG. Followed by the change of the ratio of waterand EG, the morphologies of as-obtained products undergo a series of evolution, such as from flow-er-like magnetite microspheres to polyhedral hematite nanoplates. On the basis of experimentalinvestigation and analysis, a possible formation mechanism was given. The magnetic properties ofthe obtained products were characterized, which also indicates the porous magnetite microsphereswith high saturation magnetization (Ms) and the decreased Ms caused by the phase transition.

2. Experimental section

2.1. Sample preparation

All of the chemical reagents used in this experiment were of analytical grade and used withoutfurther purification. Ferric chloride hexahydrate (FeCl3�6H2O), sodium acetate trihydrate(CH3COONa�3H2O), EG, DTAB and ethanol were purchased from the Shanghai Chemical ReagentCompany. The porous magnetite microspheres were synthesized by a simple solvothermal route. Ina typical procedure, 2.4 mmol of DTAB were dispensed into 40 mL of EG and magnetically stirredfor 15 min. Next, 13.9 mmol of CH3COONa�3H2O was added. After intense stirring for 15 min,3.3 mmol of FeCl3�6H2O were added into the above solution. The resulting mixture was stirred for

Z. Gan, A. Zhao / Superlattices and Microstructures 60 (2013) 231–239 233

30 min, then sealed into a Teflon-lined autoclave, followed by solvothermal treatment at 200 �C for 8 hin an electric oven. After the treatment, the black products were collected, washed three times withethanol, and then vacuum-dried at 50 �C for 8 h.

2.2. Characterization

The morphologies of the products were observed with field-emission scanning electron microscopeimages (SEM) on a JEOL JSM-6300F SEM. Transmission electron microscope (TEM) and high resolutiontransmission electron microscope (HRTEM) were performed with a JEOL-2010 microscope operated atan accelerating voltage of 200 kV with a tungsten filament. The phase and composition of the productswere determined by a Rigaku D/Max-cA rotating-anode. The thermal gravimetric analysis (TGA) wereconducted on a DTG-60H thermal analyzer (Shimadzu, Japan) with a heating rate of 10 �C/min in aflowing air. N2 absorption–desorption isotherm was measured with a Micromeritics Coulter (USA)instrument. Magnetic properties of the products were measured using a vibrating sample magnetom-eter (VSM, Lake shore 7410) at room temperature.

3. Results and discussion

3.1. Morphology and microstructures

The morphologies and microstructures of the as-obtained products were examined by SEM, TEMand HRTEM. Fig. 1a shows the SEM image of the typical products, indicating that uniform magnetitemicrospheres with porous structures have been obtained in a large scale. The SEM image with a highresolution demonstrates that the magnetite microspheres have �200 nm in diameter and are con-structed by self-assembly of hundreds of nanoparticles (Fig. 1b). TEM image of the products alsoproves the magnetite microspheres are provided with the porous structures caused by the looselyassembly of nanoparticles (Fig. 1c). The HRTEM image (Inset in Fig. 1d) shows the lattice imagesobtained at the edge of microspheres. It clearly demonstrates well crystallization and the lattice

Fig. 1. (a) A typical SEM image, (b) high-resolution SEM image, (c) TEM image, and (d) high-resolution TEM image (the insetcorresponds to the HRTEM image of the boxed region) of the typical products.

0.0 0.2 0.4 0.6 0.8 1.0

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3 /g•n

m)

Pore Diameter (nm)

Adso

rbed

Vol

ume

(cm

3 /g)

Relative Pressure (P/Po)

Adsorption Desorption

Fig. 2. N2 absorption–desorption isotherm and the corresponding pore size distribution of the typical products.

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fringes with interplanar spacing of 4.95 Å, corresponding to the (111) plane of the crystallinemagnetite [15].

3.2. Surface area and porosity

The porous nature of the typical products was confirmed by measurement of the surface area andpore size distribution, which was obtained by the Brunauer–Emmett–Teller (BET) gas adsorption/desorption methods. Fig. 2 shows a typical adsorption/desorption isotherm and the pore size distribu-tion of the porous magnetite microspheres. The type-IV isotherm with a typical H4 hysteresis loop inthe range of 0.8–1.0 P/P0 is obtained. Accordingly, the BJH pore sizes range from several nanometers tomore than 50 nm. These data demonstrate that the typical products have porous structures resultedfrom the interspaces of the constituent nanoparticles, which conforms to the TEM observations. Quan-titative calculation shows that the BET surface area of the porous magnetite microspheres is 20.2 m2/g,which is much higher than the value of the reported magnetite hollow spheres with similar diameter[15]. Thus it can be seen that the porous magnetite microspheres may have potential application in thecatalysts and drug delivery.

3.3. Phase structures and composition

The phase structures and composition of the as-obtained products were characterized by XRD.Fig. 3 shows the XRD patterns of as-obtained products prepared under the different ratios of waterand EG, indicating that the phase structures of the products can be tuned by varying the ratio of waterand EG. As shown in Fig. 3b, the XRD patterns of the typical products match well with the cubic in-verse spinel structured magnetite [19]. No other impurities can be found in the XRD pattern. Whenthe reaction was completed without using any water, the obtained products also belong to the mag-netite phase (Fig. 3a). The magnetite phase of the obtained products are not well kept until the ratio ofwater and EG increases to 3:5 (Fig. 3c–e). However, it could be an acceptable hypothesis that the prod-ucts prepared with a 3:5 ratio of water and EG are not an absolute magnetite phase. The differences inintensity of diffraction peaks reflect the changed particle size and morphologies of the products(Fig. 3a–d). Following by the increased ratio of water and EG up to other higher values, the obtainedproducts display obvious hematite phase (Fig. 3f–g) [28]. Thus it can be seen that the increased watercontent effectively improve the phase transition from magnetite to hematite phase, which indicatesthe presence of water does depress the reductive activity of EG.

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.)

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JCPDS: 89-0597

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Fig. 3. (a) XRD patterns of as-obtained products prepared with Ferric chloride anhydrate and sodium acetate anhydrate asreagents (no any water, keep other conditions unchanged) and (b–g) XRD patterns of as-obtained products prepared withdifferent ratios of water and EG (b – 0, c – 1:7, d – 1:3, e – 3:5, f – 1:1, and g – 3:1).

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In order to further confirm the phase structures of the obtained products, the TGA measurementswere conducted in an air atmosphere. As shown in Fig. 4a, There are a slight weight loss and an obvi-ous weight increase from 25 to 300 �C, which could corresponds to the simultaneous evaporation ofmoisture and the oxidation of the porous magnetite microspheres [22,29], respectively. Nevertheless,the oxidation process of the porous magnetite microspheres are much longer due to high content ofFe2+. Inversely, it effectively slowers the weight loss caused by the decomposition of organic compo-nents from 300 to 600 �C [29]. In contrast, the oxidation process of the polyhedral nanoplates preparedwith a 3:5 ratio of water and EG is much shorter because of low content of Fe2+ so that the weight lossover 300 �C is faster than that of the porous magnetite microspheres (Fig. 4b). For the hematitepolyhedral nanoplates prepared with a 1:1 ratio of water and EG, the weight loss almost runs throughthe overall process (Fig. 4c). Therefore, the TGA experiments conducted on the products prepared withdifferent ratios of EG and water further confirm the phase transition.

3.4. Morphological evolution and possible growth mechanism

Following by the solvent-induced phase transition, various magnetic products with different mor-phologies were also obtained. As shown in Fig. 5a, a large number of compact magnetite microsphereswere prepared in the absence of any water. The particle size of the compact magnetite microspheres isobvious bigger than that of the porous magnetite microspheres. Nevertheless, flower-like magnetite

Fig. 4. TGA curves of the as-obtained products prepared with different ratios of water and EG (a) 0, (b) 3:5, and (c) 1:1.

Fig. 5. SEM image of as-obtained products prepared with Ferric chloride anhydrate and sodium acetate anhydrate as reagents(no any water, keep other conditions unchanged), (b–f) SEM images of as-obtained products prepared with different ratios ofwater and EG (b – 1:7, c – 1:3, d – 3:5, e – 1:1, and f – 3:1).

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microstructures were received and corresponding particle size displays an obvious decline when theratio of water and EG increased to 1:7 (Fig. 5b). Surprisingly, like-spindle nanostructures were ob-tained when the 1:3 ratio of water and EG was used. It should be concerned that the increased watercontent seems to have a positive effect on the decline in particle size of the obtained magnetite prod-ucts (Fig. 5c). As the ratio of EG and water was further increased to 3:5 or other higher values, poly-hedral hematite nanoplates began to come about (Fig. 5d–f). Herein, not only does the increased watercontent promote the phase transition, also effectively induce the morphological evolution of products.

On the basis of above experimental results, we propose a possible mechanical illustration for theformation of the porous magnetite microspheres (Scheme 1). The previous literature had demon-strated the necessity of the sodium acetate and EG for synthesis of magnetite particles [19]. DTAB,as a common and effective surfactant and stabilizer, have been widely used in shape-controlled syn-thesis of kinds of nanostructures [18]. Herein, a rapid stirring was used to accelerate the dissolutionand coordination of Fe3+ with EG molecules, following by formation of the iron alkoxide. Under thesolvothermal condition, the iron alkoxide was reduced to become the nuclei and quickly grew into

200 oC

DTAB-capped magneticnanoparticles

2 3 41

2. Self -assembly and growth 3. Ostwald ripening1. Solvothermal reaction 4. Vapor-based corrosion

Scheme 1. Schematic illustration of the formation mechanism of the porous magnetite microspheres.

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the primary magnetite nanoparticles capped by DTAB [30]. In the next growth stage, thousands ofDTAB-capped magnetite nanoparticles self-assembled into magnetite microspheres. It has been re-puted that the subunits assembly can be driven through the binding affinities between organic ligandsand inorganic nanoparticles or promoted by the interactions among surface-capped ligands [31]. Inthis context, the main driving force for the assembly of nanoparticles could be attributed to the re-duced high surface energy through the interactions of DTAB molecules attached on the surface ofthe nanoparticles. With the continued solvothermal reaction, the self-assembly magnetite micro-spheres began to become compact and smooth by a transient Ostwald ripening. However, the compactmagnetite microspheres with smooth surface are metastable due to the presence of vapor in the reac-tion system. According to the above discussion on the morphological changes caused by the increasedwater content (compact, porous, and like-flower magnetite microstructures), we concluded that thewater may have a positive corrosion effect on the obtained magnetite microstructures. Such a va-por-based corrosion behavior was also observed in recent reports [32,33]. Therefore, the special watercontent is very necessary for the synthesis of the porous magnetite microspheres. Furthermore, itshould be concerned that the continued increased water content directly results in the decompositionof the iron alkoxide rather than reduction, so the phase transition can be understood.

3.5. Magnetic properties

The magnetic properties of the obtained products were measured at room temperature using avibrating sample magnetometer, and their hysteresis loops are shown in Fig. 6. From the plots of Mversus H (Fig. 6a), the Ms of the porous magnetite microspheres is as high as 88.5 emu/g, which is veryclose to the value of the corresponding bulk magnetite [34]. Moreover, the remnant magnetization(Mr) and coercivity (Hc) of the porous magnetite microspheres are also close to zero, indicating thepresence of superparamagnetic state. Generally, magnetite microstructures display ferromagneticbehavior at room temperature, but the previous literatures have reported the superparamagneticmagnetite nanostructures or microstructures [21,22]. It is well-known that the particles change frommultidomain to single domain when the size of the magnetic particles decreases. If the single domainparticles become small enough, the magnetic moment in the domain fluctuates in direction because ofthermal agitation, which leads to superparamagnetism [34]. Herein, the superparamagnetic behaviorof the porous magnetite microspheres could be attributed to the fact that the porous magnetite micro-spheres are composed of primary nanoparticles, basically equal to the critical size of superparamag-netic magnetite nanoparticles [35]. In contrast, the Ms value of like-spindle nanostructures also

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Fig. 6. Magnetic hysteresis loops of as-obtained products prepared with different ratios of water and EG (a) 0, (b) 1:3, (c) 3:5,and (d) 1:1.

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reach up to 81.2 emu/g, no obvious remnant magnetization and coercivity appear (Fig. 6b). However,the Ms value of the obtained products prepared with a increased water content have a sharp decline,indicating the solvent-induced phase transition. For example, the Ms value of the polyhedral nano-plates prepared with a 3:5 ratio of water and EG drops to 61.3 emu/g, which further supports theabove viewpoint that the products prepared with a 3:5 ratio of water and EG are mixture, containingmagnetite and hematite nanoplates (Fig. 6c). The Ms value of the hematite polyhedral nanoplatesprepared with a 1:1 ratio of water and EG is as low as 28.8 emu/g (Fig. 6d), but both Mr and Hc appear.The result corresponds to the ferromagnetic hematite nanoparticles.

4. Conclusion

Uniform porous magnetite microspheres were obtained in a large scale via a DTAB-assisted solvo-thermal route. Based on the results of XRD, SEM and TG analysis, it was demonstrated that the ratio ofwater and EG plays a crucial role for the phase structure and morphology of the obtained products.When the ratio of water and EG does not exceed 1:3, the obtained products belong to magnetite phase.Nevertheless, the obtained products begin to convert to hematite phase with the ratio increase to 3:5.The reason could be ascribed by the increased water content, which effectively weakens the reductiveactivity of EG. Followed by the change of the ratio of water and EG, the morphologies of the productsundergo a series of evolution. A vapor-based corrosion activity was proposed to explain the formationmechanism of the porous magnetite microspheres. The magnetic study demonstrated the porous mag-netite microspheres are provided with a high Ms value, indicating they would have potential applica-tion in the separation and drug delivery.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20873153),the National Basic Research Program of China (2011CB302103) and the State Key Laboratories ofTransducer Technology (Skt0906).

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