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Materials Science and Engineering B 157 (2009) 81–86

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

ow temperature synthesis of Fe3O4 nanocrystals by hydrothermalecomposition of a metallorganic molecular precursor

iao Wua, Jingyuan Tangb, Yongcai Zhangb, Hao Wanga,∗

The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR ChinaCollege of Chemistry and Chemical Engineering, Shouxi Lake Campus, Yangzhou University, Yangzhou City 225002, PR China

r t i c l e i n f o

rticle history:eceived 7 July 2008

a b s t r a c t

Pure Fe3O4 nanocrystals with different sizes were synthesized via hydrothermal processing of an easilyobtained, air-stable metallorganic molecular precursor (ferric acetylacetonate: Fe-(ACAC)3) in 20 vol.%

eceived in revised form 6 December 2008ccepted 17 December 2008

eywords:e3O4

anomaterials

hydrazine hydrate aqueous solution at 80–160 ◦C for 0.5–12 h. The phase, purity and size of the resultantproducts were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transforminfrared spectroscopy (FTIR) and transmission electronic microscope (TEM), and the possible formationmechanism of Fe3O4 was tentatively proposed. The magnetic properties of the products have also beenstudied.

hemical synthesisagnetic properties

. Introduction

The powder of Fe3O4 is increasingly important for appli-ations as magnetic material, catalyst, biomaterial, and so on1–19]. Especially in the nanoscaled region, the particles of Fe3O4ften demonstrate unique size- and shape-dependent physical andhemical properties that are of technological importance and sci-ntific research interest [1–19]. Consequently, so far, considerableffort has been devoted to designing novel methods for the prepa-ation of Fe3O4 nanomaterials with different characteristics andurposes [1–19].

Among the various methods developed for synthesizing metalxide nanomaterials, the metallorganic molecular precursor routeas been regarded as one of the most convenient and practical tech-iques, because it not only enables to avoid special instrumentsnd complicated processes and severe preparation conditions, butlso provides good control over purity, homogeneity, composition,hase and microstructure of the resultant products [16–19]. Byhoosing a proper metallorganic molecular precursor, and cou-led with a rational calcining procedure or other thermolysisrocesses such as solvothermal processing [19], nanosized and crys-alline products could be obtained usually under the conditions

ignificantly milder than those employed in the conventional solid-tate synthesis. Because of its low cost, easy preparation and mildecomposition, Fe-(ACAC)3 has already been employed as an idealrecursor for preparing Fe3O4 nanoparticles [16–19]. For exam-

∗ Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392445.E-mail address: haowang@bjut.edu.cn (H. Wang).

921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2008.12.021

© 2009 Elsevier B.V. All rights reserved.

ple, Sun et al. have realized the size-controlled synthesis of Fe3O4nanoparticles through the high temperature (265–300 ◦C) reactionof Fe-(ACAC)3 in the presence of alcohol, oleic acid and oleylamine[16,17]. Li et al. prepared 5-nm magnetite particles via the reactionof Fe-(ACAC)3 with 2-pyrrolidone [18]. Pinna et al. obtained mag-netite nanocrystals through solvothermal reactions of Fe-(ACAC)3and benzyl alcohol or benzylamine at 175–200 ◦C [19]. Although theexisting nonaqueous approaches have demonstrated the effective-ness for the production of well-calibrated Fe3O4 nanoparticles, theywere not exempt of several drawbacks: the use of massive amountsof surfactants as well as toxic and expensive organic solvents maycause some environmental problems, and the resultant nanoparti-cles were often coated with large quantities of organic molecules[16–19] or composed of mixed phases [19].

In a continuing effort, we report here the preparation of pureFe3O4 nanocrystals via hydrothermal processing of Fe-(ACAC)3 inmainly aqueous solutions (i.e., 20 vol.% hydrazine hydrate aqueoussolutions) at low temperatures (80–160 ◦C). The effects of solvent,reaction temperature and reaction time on the phase and size of theresultant products were investigated, and the possible formationmechanism of Fe3O4 nanocrystals in the present system was alsoproposed. Powder XRD, Raman spectroscopy, FTIR, TEM, and vibrat-ing sample magnetometer (VSM) were used to characterize thephase, size, purity and magnetic property of the resultant products.

2. Experimental

All the chemical reagents used in our experiments are of analyt-ical grade without further purification.

8 and Engineering B 157 (2009) 81–86

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increasing reaction temperature from 80 to 160 ◦C (Fig. 3c–e), theXRD peaks of the resultant Fe3O4 became more intense and sharperwhich indicated a better crystallization of the products. The influ-ence of the reaction time on the formation of the products was also

2 X. Wu et al. / Materials Science

.1. Preparation of Fe-(ACAC)3

0.05 mol of FeCl3·6H2O powder and 0.15 mol of NaAc·3H2O pow-er were firstly dissolved in 140 ml of distilled water, respectively.hen the two solutions were mixed in a 500 ml beaker, and 17.5 mlabout 0.16 mol) of acetylacetone was added to it with stirring, afterontinuous stirring for 2 h, the reaction solution was kept station-ry in air for 3 h. The as-formed vermeil precipitate was filtered,ashed with distilled water, and dried in air at 50 ◦C.

.2. Preparation of nanocrystalline Fe3O4

0.005 mol of Fe-(ACAC)3 powder was placed into Teflon-linedtainless-steel autoclaves of 50 ml capacity, to which 40 ml of0 vol.% hydrazine hydrate aqueous solution was added. The auto-laves were sealed and maintained at 80–160 ◦C for 0.5–12 h, thenllowed to cool to room temperature naturally. The as-formed blacke3O4 precipitates were filtered or centrifuged, washed with dis-illed water and ethanol for several times, and dried in air at 60 ◦C.

.3. Characterization of the products

Powder XRD patterns were measured on a German Bruker AXS8 ADVANCE X-ray diffractometer. SEM images were taken on ahilips XL-30ESEM equipped with an energy dispersive X-ray spec-roscopy attachment. TEM images were taken on a Philips Tecnai-12

icroscope operated at an accelerating voltage of 120 KV. Ramanpectra were measured using a Britain Renishaw Invia Raman spec-rometer with a solid-state laser (excitation at 532 nm, 2 mW) atoom temperature in the range of 200–1000 cm−1. The Raman spec-rometer is equipped with an optical microscope and a CCD camerahat can provide a good laser beam. FTIR spectra were recorded onBruker Tensor 27 FT-IR Spectrometer at room temperature with

amples in a KBr wafer. The magnetic properties of the productsere determined using an American ADE EV7 vibrating sampleagnetometer at room temperature.

. Results and discussion

Fig. 1 shows the XRD pattern (Fig. 1a) and the morphologyFig. 1b) of our precursor. All its diffraction peaks can be assignedo orthorhombic structure (space group: Pcab [61]) Fe-(ACAC)3ith the calculated lattice constants of a = 15.333 Å, b = 16.441 Å and= 13.566 Å, which conform to the JCPDF card no. 30-1763. Fromhe SEM image shown in Fig. 1b, it can be seen that our precursoromprises mainly stave-like crystallites with the length of about.2–83 �m and width of about 0.5–3.5 �m.

Figs. 2–4 show the XRD patterns of the products derived fromhe reactions of Fe-(ACAC)3 and 5–20 vol.% hydrazine hydrate aque-us solution in autoclaves under different conditions. It can be seenrom Fig. 2 that the solvent played important roles in the phase(s)f the resultant products. When 5 vol.% hydrazine hydrate was useds solvent, no pure phase Fe3O4 but only a mixed phase of Fe3O4nd Fe2O3 could be obtained even at 120 ◦C (Fig. 2b). While whenhe synthesis was carried out in 20 vol.% hydrazine hydrate aqueousolution, pure Fe3O4 (JCPDS card no. 19-0629) could be obtained atnly 80 ◦C (Fig. 2c). It has been reported that hydrazine hydrate hasreducing property and can be used as a protecting agent to restrain

he oxidization of the products. However, when the concentrationf hydrazine hydrate is rather low (5 vol.%), this property is so weak

hat the Fe3O4 nanoparticles are partly oxidized to Fe2O3. There-ore, we choose the 20 vol.% N2H4 as the solvent. Fig. 3 shows theRD patterns of the products synthesized at the temperature rang-

ng from 10 to 160 ◦C for 12 h in 20 vol.% hydrazine hydrate aqueousolution. As can be seen from the figure, only pure FeO(OH) particles

Fig. 1. XRD pattern (a) and SEM image (b) of our precursor.

were synthesized at 10 ◦C for 12 h (Fig. 3a), and when the temper-ature increased to 40 ◦C, a mixed phase of FeO(OH) and Fe3O4 wasobtained (Fig. 3b). The pure phase Fe3O4 particles were not syn-thesized until the temperature increased to 80 ◦C (Fig. 3c) and with

Fig. 2. XRD patterns of the products derived from the reactions of Fe-(ACAC)3 and5 vol.% hydrazine hydrate aqueous solution in autoclaves at (a) 40 ◦C for 12 h, and(b) 120 ◦C 12 h; and those derived from the reactions of Fe-(ACAC)3 and 20 vol.%hydrazine hydrate aqueous solution in autoclaves at (c) 80 ◦C for 12 h, (d)120 ◦C for12 h, and (e) 160 ◦C for 12 h.

X. Wu et al. / Materials Science and Engineering B 157 (2009) 81–86 83

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Table 1The products derived from the reactions of Fe-(ACAC)3 and 5–20 vol.% hydrazinehydrate (HH) aqueous solution in autoclaves at 10–160 ◦C for 0.5–12 h.

No. Solvent Temperature (◦C) Time (h) Products

1 5% HH 10 12 FeO(OH)2 5% HH 40 12 Fe3O4

1, FeO(OH)3 5% HH 100 3 Fe3O4

1, FeO(OH), Fe2O3

4 5% HH 120 3 Fe3O41, Fe2O3

5 5% HH 120 12 Fe3O41, Fe2O3

6 20% HH 10 0.5 FeO(OH)7 20% HH 10 12 FeO(OH)8 20% HH 40 0.5 Fe3O4, FeO(OH)1

9 20% HH 40 12 Fe3O41, FeO(OH)

10 20% HH 80 12 Fe3O4

11 20% HH 100 12 Fe3O4

12 20% HH 120 0.5 Fe3O4

13 20% HH 120 3 Fe3O4

14 20% HH 120 12 Fe3O4

15 20% HH 140 12 Fe3O4

16 20% HH 160 0.5 Fe3O4

17 20% HH 160 12 Fe3O4

Note: the superscript 1 represent the major phase in the products.

ig. 3. XRD patterns of the products derived from the reactions of Fe-(ACAC)3 and0 vol.% hydrazine hydrate aqueous solution in autoclaves at (a) 10 ◦C, (b) 40 ◦C, (c)0 ◦C, (d) 120 ◦C and (e)160 ◦C for 12 h.

nvestigated and the results were shown in Fig. 4. Pure Fe3O4 coulde obtained when the reaction time was only 0.5 h (Fig. 4a) and withrolonged reaction time, the XRD peaks grew more intense andharper which revealed a better crystallization and a larger crys-al size (Fig. 4b–c). To sum up, with increasing the concentrationsf hydrazine hydrate aqueous solutions, reaction temperatures asell as reaction times, the relative contents of Fe3O4 in the resultantroducts can be increased (see Table 1), and the XRD peaks of theesultant Fe3O4 generally became more intense and sharper mean-ng that the products grew larger at higher reaction temperaturesr for longer reaction times.

The purity of the as-prepared Fe3O4 powders was further exam-ned by Raman and FTIR spectroscopy. The typical Raman spectrumf the obtained products shown in Fig. 5 displays one strong peak atround 667 cm−1 and two weak peaks at around 306 and 534 cm−1,hich can be assigned to the A1g, Eg and T2g modes of Fe3O4

20–22], respectively. In addition, Fig. 6 shows the FTIR spectrum

f the as-obtained Fe3O4 nanoparticles (Fig. 6a) and the precursorFig. 6b). Compared to Fig. 6b, only four peaks at about 584, 1388,622 and 3420 cm−1 are observed in Fig. 6a, which can be assignedo Fe3O4, CO2 and H2O [23]. The FTIR analysis revealed that the as-repared Fe3O4 nanocrystals are free of the organic contaminants,

ig. 4. XRD patterns of the products derived from the reactions of Fe-(ACAC)3 and0 vol.% hydrazine hydrate aqueous solution in autoclaves at 120 ◦C for (a) 0.5 h, (b)h and (c) 12 h.

Fig. 5. Typical Raman spectrum of the as-obtained Fe3O4 nanocrystals.

Fig. 6. FTIR spectrum of the as obtained Fe3O4 nanocrystals (a) and the precursor(b).

84 X. Wu et al. / Materials Science and Engineering B 157 (2009) 81–86

Fig. 7. TEM images of the products prepared by hydrothermal processing of Fe-(ACAC)3 in 20 vol.% hydrazine hydrate aqueous solution under the conditions of (a) 10 ◦C, 12 h;(b) 80 ◦C, 12 h; (c) 120 ◦C, 0.5 h; (d)120 ◦C, 12 h, and (e) 160 ◦C, 12 h. Histograms of individual particle sizes are shown as inset of (b–e).

X. Wu et al. / Materials Science and Engineering B 157 (2009) 81–86 85

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uch as the solvent, Fe-(ACAC)3 and possible organic byproductsecomposed from Fe-(ACAC)3.

Based on the XRD results we have known that the purity andizes of the as-prepared Fe3O4 powders were dependent on theemperature and the time of the reaction. Fig. 7 shows the TEMmage of some products synthesized under different conditions. Asan be seen from Fig. 7a, the intermediate phase FeO(OH) crys-als prepared by hydrothermal processing of Fe-(ACAC)3 in 20 vol.%ydrazine hydrate aqueous solution at 10 ◦C for 12 h consist ofanorods with some amorphous substance which might be non-rystalline FeO(OH) around them. The average diameters of the rodsre about 10 nm and the lengths are mostly less than 200 nm. Thee3O4 powders prepared at 80 ◦C comprise irregular-shaped Fe3O4anoparticles with minimum size of about 8 nm and maximum sizef 16 nm (Fig. 7b), the Fe3O4 powders prepared at 120 and 160 ◦Cor 12 h comprise, in turn, irregular-shaped Fe3O4 nanoparticlesith the sizes range of 14–27 nm (Fig. 7d) and 20–90 nm (Fig. 7e);hile those derived from hydrothermal processing of Fe-(ACAC)3

n 20 vol.% hydrazine hydrate aqueous solution at 120 ◦C for 0.5 honsist of irregular-shaped nanoparticles with the size rage from 6o 21 nm (Fig. 7c). The TEM images of the as-prepared Fe3O4 are alsoonsistent with their XRD patterns in Fig. 3. It can be seen from Fig. 7hat the Fe3O4 powders obtained at 160 ◦C 12 h have a wide size dis-ribution, while the Fe3O4 powders obtained at lower temperaturesave a narrower size distribution. In hydrothermal reaction, theemperature was intensified gradually. When the reaction temper-ture is 80 and 120 ◦C, the acquired energy of the reaction systems only sufficient for partial ions to nucleate and grow. But whenhe reaction temperature increases to 160 ◦C, the acquired energyf the reaction system is sufficient to make the residual ions nucle-

te to Fe3O4 nuclei, and at the same time, the earlier formed Fe3O4rystals are still growing to form Fe3O4 powders with bigger size. Sohen the reaction reaches to equilibrium, the as-obtained powders

t 160 ◦C have a wide size distribution. From the above discussion

ig. 8. Magnetic hysteresis curves of the Fe3O4 nanocrystals prepared by hydrother-al processing of Fe-(ACAC)3 in 20 vol.% hydrazine hydrate aqueous solution at 80 ◦C

denoted by square) and 160 ◦C (denoted by triangle) for 12 h.

rmation process for Fe3O4 nanocrystals.

and Fig. 7, it can be deduced that lower temperature such as 80 ◦C,120 ◦C might be favorable for the synthesis of products with narrowsize distribution.

It is well known that the precipitation or hydrothermal synthe-sis involves chemical reactions between ions or molecular speciesin aqueous solution and one or more solid phases, the chemicalcomposition as well as the properties of the solid phases and thephase morphology can be controlled by changing the physical andchemical variables of the given system [24]. On the basis of Table 1,the possible formation mechanism of Fe3O4 nanocrystals in thepresent system was tentatively proposed as the following equations(1)–(3):

N2H4 + H2O → N2H5+ + OH− (1)

Fe-(ACAC)3+2OH−+2N2H5+ → FeO(OH) + 2N2H5ACAC + HACAC

(2)

12FeO(OH) + N2H4 → 4Fe3O4 + N2 + 8H2O (3)

Firstly, Fe-(ACAC)3 hydrolyzed in the alkaline aqueous solution of20 vol.% hydrazine hydrate (Kb = 1.3 × 10−6 for Eq. (1) at room tem-perature [25] to generate FeO(OH), as described in Eq. (2); then, theintermediate FeO(OH) was further reduced to Fe3O4 by hydrazinehydrate ((E�(N2H4 + 4OH− → N2 + 4H2O + 4e−) = −1.16 V [25] duringsubsequent high pressure hydrothermal processing at 80–160 ◦C, asdepicted in Eq. (3).

In addition, based on the above discussion and the experimen-tal results (XRD patterns and TEM images), we propose that thetransformation from FeO(OH) to Fe3O4 nanocrystals in the properalkaline condition was a dynamic process, which occurs underthe influence of the different factors, such as the concentrationof hydrate hydrazine, the reaction temperature and time, etc. Theprobable conversion process from FeO(OH) to Fe3O4 nanocrystalswas elucidated as the two-step stages: formation of Fe3O4 nucleusfrom FeO(OH) according to the “dissolution-recrystallization”mechanism, and growth of stable Fe3O4 nanocrystals through an“Ostwald ripening” process.

As we known, the solubility plays an important role in determin-ing the growth of nanocrystals [24,26]. In the present hydrothermalprocess, initially, during the mixing of the reactants, massive pre-cipitation of FeO(OH) nuclei formed quickly, followed by the growthof the nuclei into FeO(OH) crystals. It is known that with the tem-perature and pressure increasing steadily, the solubility of manyoxides increases in water [27]. Thereby, due to the large solu-bility and metastability of FeO(OH) compared with Fe3O4, thisparticular phase is more sensitive to secondary growth, whichleads to crystallographically more stable phases, essentially byhetero-nucleation. Herein, the FeO(OH) intermediate phase decom-posed and re-crystallized to Fe3O4 nanoparticles according to the“dissolution-recrystallization” mechanism. Then, the formation of

tiny crystalline nuclei in a supersaturated medium occurs first, fol-lowed by the growth of larger crystals from smaller crystals dueto the fact that the smaller particles have larger solubility thanthe larger ones. Thereby, the classic Ostwald ripening process wasaccompanied.

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On the basis of the foregoing discussion, we believed thathe transformation process shown in Scheme 1 described theypothesis for the formation of Fe3O4 nanocrystals. Neverthe-

ess, much work is yet needed to discover the exact mechanism,nd the detailed characterization of the organic byproducts is inrogress.Fig. 8a and b show the magnetic hysteresis curves of thee3O4 nanocrystals prepared by hydrothermal processing of Fe-ACAC)3 in 20 vol.% hydrazine hydrate aqueous solution at 80 and60 ◦C for 12 h, respectively. Both products show a ferromagneticehavior at room temperature. The saturation magnetization (Ms),emanences (Mr) and coercive forces (Hc) for the Fe3O4 nanocrys-als prepared at 80 ◦C for 12 h are determined to be 11.2 emu/g,.9 emu/g and 34.0 Oe, respectively; while the corresponding val-es for the Fe3O4 nanocrystals prepared at 160 ◦C for 12 h are4.5 emu/g, 3.0 emu/g and 131.2 Oe, respectively. It is obvious thathe Fe3O4 nanocrystals prepared at 80 ◦C for 12 h have lower valuesf Ms, Mr and Hc than those prepared at 160 ◦C for 12 h, which maye attributed to their smaller particle size (or larger surface effect)11–14].

. Conclusions

Adopting Fe-(ACAC)3 as the precursor, pure Fe3O4 nanocrys-als were synthesized via subsequent hydrothermal processingn 20 vol.% hydrazine hydrate aqueous solution at 80–160 ◦C for.5–12 h. The phase, purity, size and magnetic properties of thebtained products were characterized by means of XRD, Raman,TIR, TEM and VSM. Furthermore, the possible formation mech-nism of Fe3O4 in this system was tentatively proposed. Theroposed method may be extended to prepare nanomaterials of

any other important metal oxides, because many other metal ions

uch as Cu2+, Mg2+, Zn2+, Cd2+, Ni2+, Co2+, Mn2+, TiO2+, ZrO2+, Al3+,a3+, In3+, Y3+, etc. can also easily form solid metal acetylacetonateompounds, which are inclined to decompose into metal oxidespon high pressure hydrothermal processing.

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gineering B 157 (2009) 81–86

Acknowledgement

This work is supported by the Science and TechnologicalDevelopment Project of the Beijing Education Committee (No.KM200710005029).

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