Solvothermal Synthesis and Characterization of Fe3O4 and γ-Fe2O3 Nanoplates

Jian Lu, Xiuling Jiao, Dairong Chen,* and Wei LiKey Laboratory for Special Functional Aggregate Materials, Education Ministry, School of Chemistry andChemical Engineering, Shandong UniVersity, Jinan 250100, People’s Republic of China

ReceiVed: December 02, 2008; ReVised Manuscript ReceiVed: January 10, 2009

γ-Fe2O3 nanoplates were prepared by a simple solution process and could be transformed to Fe3O4 nanoplateswhile keeping the size and morphology unchanged after reduction in hydrazine solution. The crystal structureand surface organic groups of the nanoplates were characterized in detail. The nanoplates exhibited gooddispersibility in polar solvents except water and strong dipolar interactions, which favored the formation ofone-dimensional chainlike structure. The poly(vinylpyrrolidone) molecules in this system played a key rolefor the growth of nanoplates, and it was supposed that the formation of nanoplates was a kinetically controlledprocess. The γ-Fe2O3 and Fe3O4 nanoplates with strong dipolar interactions are ferromagnetic at roomtemperature.

1. Introduction

Magnetic nanoparticles have attracted much attention becauseof their potential applications in information storage, colorimaging, bioprocessing, magnetic refrigeration, gas sensors,ferro-fluids, and so on.1 Among these nanomaterials, iron oxide(γ-Fe2O3 and Fe3O4) nanocrystals have been widely studied fortheir excellent physical and chemical properties, but theseproperties are greatly affected by the synthesis route andnanocrystals’ size and shape.2 For this reason, various ap-proaches have been employed to prepare γ-Fe2O3 and Fe3O4

nanocrystals with different sizes and shapes in order to obtainexpected properties. Among the synthetic methods, the solutionprocesses were superior to others in controlling size and shapeof nanocrystals, as well as adjusting the dispersibility in polaror nonpolar solvents.3 For the Fe3O4 nanocrystals, the generallyused solution synthetic methods included the hydrothermalreaction of Fe2+ compounds,4 coprecipitation of Fe2+ and Fe3+

ions,5 and thermal decomposition of Fe3+ compounds in thepresence of reducing agent or organic salts of Fe2+.6 But forthe γ-Fe2O3 nanocrystals, the solution synthesis usually involvedthe controlled oxidation of Fe3O4 and direct mineralization ofFe3+ ions. For instance, the inorganic salts (e.g.,Fe(NO3)3 · 9H2O) were applied as precursors to prepare pureγ-Fe2O3 nanoparticles in some aqueous systems.7 Other non-aqueous solution methods have also been developed to synthe-size highly crystalline, monodispersed, and shape-controlledγ-Fe2O3 nanoparticles, in which organometallic compounds werealways used as the precursors.8 In these syntheses, many effortshave been devoted to the monodispersed nanoparticles withsmall size and surface modifications.

For the anisotropic magnetic nanocrystals, there is interestfrom the point of view of understanding not only nanocrystalgrowth9 but also the interesting magnetic properties derived fromtheir shape anisotropy10 and potential applications in ultrahigh-density magnetic storage devices.11 Otherwise, assembly ofanisotropic nanocrystals with specific structure is a prerequisitefor the generation of nanodevices.12 However, it is difficult toform the anisotropic γ-Fe2O3 or Fe3O4 nanocrystals due to their

cubic spinel structure. Up to now, although γ-Fe2O3 or Fe3O4

nanoparticles with different sizes have been prepared, little workis related to their single-crystalline anisotropic nanostructuressuch as the one- or two-dimensional nanostructures.13 Notably,Wee’s group synthesized Fe3O4 nanosheets at low temperatureby oxidizing pure Fe substrates in acidic solution on a hot platemaintained at 70 °C,14 Zhu et al. reported a precursor-templatedconversion method for the controlled synthesis of hierarchicalhollow spheres assembled by Fe3O4 or γ-Fe2O3 nanosheets,15

and Ge and co-workers prepared γ-Fe2O3 nanoplates in anaqueous system by a two-step reduction-oxidation method: Fe2+

is reduced into metallic Fe by γ-ray irradiation in N2 atmospherefollowed by the oxidation into γ-Fe2O3.16 Herein we present afacile solvothermal process to prepare the nearly monodispersedγ-Fe2O3 nanoplates coated by a layer of hydrophilic poly(vi-nylpyrrolidone) (PVP) molecules, which could be well-dispersedin several polar solvents except water. Furthermore, the γ-Fe2O3

nanoplates could be transformed to Fe3O4 nanocrystals withshape and size unchanged by reducing in hydrazine solution.These nanoplates showed strong dipolar interactions due to thelarge size and anisotropic shape.

2. Experimental Section

Synthesis. Absolute ethanol was dehydrated by Mg powderand I2 before use, and all the other reagents were of analyticalgrade and used without further purification.

In a typical synthesis, into 13.0 mL (0.223 mol) of absoluteethanol 0.121 g (0.3 mmol) of Fe(NO3)3 ·9H2O was dissolvedto form a dark red solution, and into it 1.3 g (0.024 mmol) ofPVP (Mw ) 58 000) was added under magnetic stirring. Withthe PVP dissolving, the solution gradually transformed into aviscous sol, which was poured into a Teflon-lined autoclaveand heated at 240 °C for 4.0 h. After that the autoclave wascooled to room temperature, the as-obtained black suspensionwas mixed with an equal volume of ethanol, and the formedprecipitate (γ-Fe2O3) was separated by centrifugation at 9500rpm and washed with ethanol. Then it was redispersed into 13.0mL (0.082 mol) of n-octanol to form a solution, and into it 0.5 g(8.6 × 10-3 mmol) of PVP and 0.2 mL (3.2 mmol) of hydrazinehydrate (80%) were added under stirring to form a homogeneoussol, which was transferred to an autoclave and heated at

* Corresponding author. E-mail: cdr@sdu.edu.cn. Tel: 86-0531-88364280.Fax: 86-0531-88364281.

J. Phys. Chem. C 2009, 113, 4012–40174012

10.1021/jp810583e CCC: $40.75 2009 American Chemical SocietyPublished on Web 02/16/2009

180 °C for 6.0 h. After that the product (Fe3O4) was collectedby centrifugation and washed with ethanol.

Characterization. The morphology and microstructure werecharacterized by using transmission electron microscopy (TEM,JEM 100-CXII) with an accelerating voltage of 80 kV, high-resolution TEM (HR-TEM, GEOL-2010) with an acceleratingvoltage of 200 kV, and field emission scanning electronmicroscopy (FE-SEM, JEOL, JSM 6700F). X-ray diffraction(XRD) patterns were collected on a Rigaku D/Max 2200PCdiffractometer with a graphite monochromator and Cu KRradiation (λ ) 0.154 18 nm). Thermogravimetric (TG) analysiswas carried out on a Mettler Toledo SDTA851e thermalgravimetric analyzer at a heating rate of 10.0 °C ·min-1 fromroom temperature to 1000 °C under air or N2 flow. Infrared(IR) spectra were recorded on a Nicolet 5DX FT-IR spectrom-eter using the KBr pellet technique. X-ray photoelectron spectra(XPS) were recorded on a PHI-5300 ESCA spectrometer(Perkin-Elmer) to characterize the particles’ surface with itsenergy analyzer working in the pass energy mode at 35.75 eV,and the Al KR line was applied as the excitation source. Themagnetic properties were measured on a SQUID magnetometer(MPMSXL-7) as a function of temperature (10 K < T < 310 K)and magnetic field (Hmax ) 10 kOe).

3. Results and Discussion

TEM and SEM images (Figure 1a, b) show that the γ-Fe2O3

nanocrystals exhibit a platelike morphology with a thicknessof 10-13 nm and a side length of 30-40 nm, and their aspectratio (side length/thickness) is ca. 3. Most of the nanoplateshave triangle or truncated triangle shapes, and a few are irregularpolygons. TEM and SEM images (Figure 1c, d) show that thesize and morphology of the formed Fe3O4 nanoplates remainedunchanged after these γ-Fe2O3 nanocrystals were reduced byhydrazine hydrate in n-octanol. The platelike morphology couldbe further proved by assembling these nanoplates into super-structures, which is still under investigation (Figure S1).

Although the XRD patterns of the samples (Figure S2) clearlyshow the spinel structure, it is difficult to distinguish the γ-Fe2O3

and Fe3O4 phases only from the XRD patterns due to theirsimilarity. XPS patterns (Figure 2) were applied to determinethe product because XPS is very sensitive to Fe2+ and Fe3+

cations.17 The levels of Fe2p3/2 and Fe2p1/2 are, respectively,710.35 and 724.0 eV for γ-Fe2O3 and 711.29 and 724.82 eVfor Fe3O4. It is in agreement with the literature that the peaksshift to high binding energy and broaden for Fe3O4 due to theappearance of Fe2+(2p3/2) and Fe2+(2p1/2).18 A shoulder around

711 eV seems to appear in Figure 2b, which further evidencesthat the nanoplates have been reduced to Fe3O4.19 In addition,the presence of the satellite peak at ∼719.0 eV (indicated byan arrow) is characteristic of γ-Fe2O3, while the satellite peakin Fe3O4 is covered and becomes less resolved.20 The XPSpatterns reveal that the as-synthesized nanoplates are γ-Fe2O3,and Fe3O4 is obtained after reducing. As the N atoms coordi-nated to metals, the N1s peak would shift to lower bindingenergy by 1∼3.0 eV due to a transfer of electron density fromN to metals.21 Herein the N1s level at 399.75 eV does not shiftto lower energy, suggesting that the PVP molecules coordinatedwith γ-Fe2O3 via their CdO group rather than the N atoms(Figure S3).22

The corresponding HR-TEM images (Figure 3) indicate thatthe nanoplates have a single-crystal structure, and a latticespacing of ca. 0.48 nm corresponds to the (111) planes of aspinel-structured iron oxide. It is deduced that the growth ofnanoplates along the (111) direction would be hindered to makethe (111) planes as the basal planes of nanoplates. The latticespacings of 0.30 and 0.247 nm from the flattening nanoplatesare assigned to the (220) and (311) planes of γ-Fe2O3.

For the γ-Fe2O3 nanoplates, their FT-IR spectrum (Figure S4)shows two strong absorption peaks at 560 and 441 cm-1

assigned to the Fe-O vibrations of γ-Fe2O3 and weak absorp-tions around 3300 and 2800-3000 cm-1 attributed to thehydroxyl groups and C-H stretching vibrations. The absorptionsat 1643 and 1008 cm-1 correspond to the vibrations of CdOand C-N groups of the PVP molecules absorbed on the

Figure 1. TEM and SEM images of γ-Fe2O3 (a, b) and Fe3O4

nanoplates (c, d).

Figure 2. Fe2p XPS patterns of γ-Fe2O3 (a) and Fe3O4 (b) nanoplates.

Figure 3. HR-TEM images of γ-Fe2O3 (a, b) and Fe3O4 (c) nanoplates.Scale bar: 10 nm.

Fe3O4 and γ-Fe2O3 Nanoplates J. Phys. Chem. C, Vol. 113, No. 10, 2009 4013

nanocrystals’ surface. But for the pure PVP molecules, theabsorption of CdO appears at 1664 cm-1; the obvious red shiftmight be due to the CdO bond weakening via partial donationof oxygen loan pair electrons of PVP to the vacant orbital ofFe atoms.23 The band at 1375 cm-1 ascribed to the NO3

- groupsindicates that they have not decomposed completely during thesolvothermal process.24 The IR spectrum of Fe3O4 is similar tothat of γ-Fe2O3, but the vibration absorptions of C-N andhydroxyl groups become much weaker. The FT-IR spectraindicate the prepared nanoparticles’ surface is coated by a layerof PVP molecules with a small amount of absorbed water.

The TG curves (Figure 4) show that both γ-Fe2O3 and Fe3O4

nanoplates have two weight loss steps from room temperatureto 800 °C under air atmosphere. For the γ-Fe2O3 nanoplates,the drop of 1.7% below 150 °C results from the removal ofabsorbed water, and the second one of 8.3% from 150 to450 °C is assigned to the decomposition of PVP molecules.25

For the Fe3O4 nanoplates, a weight gain at 150∼250 °C is dueto the oxidation of Fe3O4 to γ-Fe2O3, which is overlapped withthe decomposition of PVP. So the TG analysis in N2 flow isalso conducted, and the weight loss of 6.75% from 150 to 700°C resulting from the elimination of organic species is observed.By subtracting 3.71% (the weight loss under air atmosphere),the weight gain should be 2.99%, so the reduced product iscalculated to be Fe2.98O4.

PVP-coated γ-Fe2O3 nanoplates exhibited good hydrophilicityand could be dispersed in various polar solvents such asdimethylsulfoxide (DMSO), formamide, N,N-dimethylforma-mide (DMF), n-butanol, ethanol, and methanol, forming stableferro-fluids (Figure 5). No precipitate could be observed forseveral weeks. Phase separation did not occur even when theywere attracted by a magnet (Figure S5a). However, if they weredispersed in water or the mixture of water and ethanol (1:1 v/v),the γ-Fe2O3 nanoplates would aggregate and precipitate on thebottom instantly. TG analysis indicates that the amount of PVPon the nanoplates’ surface was reduced to 4.7% after they werewashed by water (Figure 4a). It was speculated that only partof PVP molecules coordinate to the nanoplates, and the othersjust were absorbed on the surface. As they were dispersed inwater, the PVP molecules absorbed on the surface were releasedin water, and the aggregation of the nanoplates occurred.

However, the experiments demonstrate that as the γ-Fe2O3

nanoplates were reduced to Fe3O4, they could not be dispersedin polar solvent even at 6.75 wt % of PVP molecules stillabsorbed on the surface (Figure S5b).

Macroscopic dispersibility is due to the integration ofinteractions between the nanoparticles. The colloidal stabilitycould be described by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and four kinds of forces contributeto the interparticle potential: van der Waals force, the electro-static force, dipole force, and steric repulsion force. Becausethe van der Waals attractive force or the electrostatic repulsiveforce is isotropic, when the dipole force between magneticparticles becomes the primary interaction, the nanocrystals couldself-assemble into one-dimensional (1D) dipole chains or fluxrings on the substrate spontaneously.26 There is strong dipolarinteraction between these platelike nanoparticles, which favorsthe self-assembly of nanoplates into 1D chainlike structures ona Cu grid (Figure 6). But the chain formation is influenced bysubstrate, concentration, and solvent evaporation (Figure 1a).Compared to the γ-Fe2O3 nanocrystals, the Fe3O4 nanoplatesmore easily form 1D chainlike structures, and some of thenanoparticles tend to stand on the Cu grid and stack face-to-face (the inset in Figure 6b).

The magnetic dipole moment of a single-domain sphere is µ) 4πr3Ms/3 (r is the sphere’s radius and Ms is the bulk saturationmagnetization), and the dipole-dipole interaction between twoadjacent particles scales as µ2/σ3 (σ is the distance between twoparticles, consisting of the core diameter and the thickness ofsurface organic layer). In brief, this interaction is proportionalto the saturation magnetization of the bulk materials and particlevolume, and inversely proportional to the distance between two

Figure 4. TG curves of γ-Fe2O3 (a) and Fe3O4 (b) nanoplates.

Figure 5. γ-Fe2O3 nanoplates dispersed in different solvents.

Figure 6. TEM images of γ-Fe2O3 (a) and Fe3O4 (b) assembledchainlike structures.

Figure 7. TEM images of the products for the reactions conductedfor 2.0 h (a), 2.25 h (b), 3.0 h (c), and 3.5 h (d).

4014 J. Phys. Chem. C, Vol. 113, No. 10, 2009 Lu et al.

particles. Based on the upper equations, the formation of 1Ddipolar chains or flux rings on the supporting surfaces is usuallyobserved in the follow situations: (1) strongly magnetic materi-als, such as metallic Fe,27 Co,28 and Ni-Co alloy nanoparticles,29

(2) relatively weak magnetic nanoparticles but with much largersizes, such as chains formed by 21 nm Fe3O4 spheres,30 fluxrings by 50 nm Fe3O4 cubes,31 and 1D chains by Ni micro-spheres32 or Ni/Ni3C particles,33 and (3) low-dimensionalnanocrystals, as the shape anisotropy enhances the interparticleinteractions to assemble nanorods34 and nanoplates12 into ribbonswith particles parallel to each other. As reported,30 the criticalsize required for dipole chain formation for Fe3O4 spheres is16 nm in diameter. Due to the larger size of synthesizednanoplates than that of Fe3O4 spheres, these nanoplates showstronger dipolar interaction, which promotes the assembled 1Dchainlike structure on the substrate. Also, their anisotropic shapeshould be considered to have an additional contribution, as someof the plates tend to stand on the TEM grid to increase theirdipolar interactions. Otherwise, the chains of nanoplates stackedface-to-face are along their (111) direction, which is the easymagnetization axis of magnetic iron oxide.35 Fe3O4 nanoplateshave stronger dipolar interactions than γ-Fe2O3 due to its largersaturation magnetization (73.5 emu/g for γ-Fe2O3 and 92.0emu/g for Fe3O4), which causes them to assemble in thechainlike structure even at a concentrated suspension.

A time-dependent experiment was conducted to investigatethe solvothermal formation process of γ-Fe2O3 nanoplates(Figure 7). Nanoparticles with a size of 5 nm appeared as thereaction was conducted for 2.0 h, and a few large particles ofca. 12 nm diameter showed triangular shape. The average sizeof nanoparticles increased to 20 nm after reacting for 2.25 h,and some particles showed rodlike projections, indicating thatthe plates formed. With the reaction time prolonged to 3.0 h,1D chainlike structure assembled by particles could be observed.After reacting for 3.5 h, the triangle and truncated triangleparticles with a side length of ca. 30 nm were obtained.

In the formation of polyhedra nanocrystals, the facets withhigher energy grow faster and tend to disappear, leading to thenanocrystals bounded by the low-energy facets. The growth ratecould be tuned by bounding to the surfactants on the faces ofnanocrystals. Shape control of nanocrystals is generally achievedby selective adhesion of surfactant to a particular crystal facetand slowing its growth rate along this direction.36 The facetsfree energy of a fcc (face-centered cubic) crystal is on the orderof γ{110} > γ{100} > γ{111}.37 In the spinel structure ofγ-Fe2O3, the {111} plane has the strongest polarity due to thealternated octahedral coordinated Fe3+ and O2- layers (FigureS6).38 In the Fe-terminated {111} surfaces of γ-Fe2O3 nano-particles, Fe3+ ions are coordinated to only three oxygen atoms,which promotes the interactions between the surface Fe3+ andCdO groups of adsorbing PVP molecules. So PVP moleculesare selectively adhered to the {111} facets, which greatly

reduced the growth rate along the (111) direction, resulting innanoplates bounded by the most stable {111} facets.

Now that PVP does not change the order of facet free energy,octahedron bounded by eight (111) facets is the most stablemorphology, which has been prepared in the presence of strongcoordination agent, such as ethylenediamine tetraacetic acid(EDTA).39 In our system, the nucleation and growth rate is veryfast due to the high reaction temperature, and the formation ofnanoplates bounded by (111) is the result of a kineticallycontrolled process. Our group reported the synthesis of Co3O4

nanocrystals by the solvothermal decomposition of nitrate, whichusually reacted below 200 °C.40 In this experiment, the reactionwas conducted at 240 °C, and nitrate decomposed very fastwhich facilitated the nucleation of nanocrystals. The additionof PVP increased the viscosity of the system, which hinderedthe diffusion of monomers and the growth of nanocrystals fora diffusion-limited growth model. When the ratio of nucleationrate to growth rate is proper, γ-Fe2O3 nanoplates could beobtained. Further experiments showed that as the amount ofFe(NO3)3 ·9H2O and PVP changed to 0.4 mmol and 0.9 g andthe reaction time decreased to 3.5 h, the chainlike structuresassembled by nanoplates were obtained due to increase of boththe nucleation and growth rates (Figure 8a). Similar productwas obtained by decreasing the amount of Fe(NO3)3 ·9H2O to0.2 mmol, increasing that of PVP to 1.6 g, and prolonging thereaction time to 4.5 h (Figure 8b). However, when thesolvothermal reaction was conducted at 180 °C, the product wasmonodispersed R-Fe2O3 nanoparticles with a size of 40 nm(Figure 8c). This indicates that the reaction temperature has agreat effect on the crystal phase of the product.

Magnetic property investigation of the γ-Fe2O3 and Fe3O4

nanoplates shows that both of them are ferromagnetic at roomtemperature (Figure 9). The saturation magnetization (Ms) andcoercivity (Hc) values are 57.7 emu/g and 60 Oe for γ-Fe2O3

and 71.0 emu/g and 110 Oe for Fe3O4, respectively, which aresmaller than those of bulk materials. The reduced value mightbe due to the existence of nonmagnetic surfactant on the surfaceand the spin canting of surface Fe atoms.41 The reduced remnantmagnetization (Mr/Ms) value is 0.14 and 0.13 for γ-Fe2O3 andFe3O4, respectively, which is smaller than that of reported ironoxide nanoplates.42 Standard zero-field-cooling (ZFC) and field-cooling (FC) procedure was followed at the applying field of100 Oe. The ZFC curves of γ-Fe2O3 and Fe3O4 nanoplates showa constantly increasing trend with the temperature up to 310 K,further evidencing their ferromagnetic property at room tem-perature. FC magnetization measurements could provide infor-mation about magnetic interactions between nanoparticles. Inmany types of magnetic systems, the FC magnetization increaseswith decreasing temperature due to the decreased thermalfluctuations and spins aligning with the applied field.43 Dipolarinteraction between particles suppresses these fluctuations, andthus the increase in magnetization during a FC experiment is

Figure 8. TEM images of the products prepared under different reaction conditions: (a) Fe(NO3)3 · 9H2O 0.4 mmol, PVP 0.9 g, temperature240 °C, time 3.5 h; (b) Fe(NO3)3 ·9H2O 0.2 mmol, PVP 1.6 g, temperature 240 °C, time 4.5 h; and (c) Fe(NO3)3 ·9H2O 0.3 mmol, PVP 1.2 g,temperature 180 °C, time 3.5 h.

Fe3O4 and γ-Fe2O3 Nanoplates J. Phys. Chem. C, Vol. 113, No. 10, 2009 4015

inverse to the amount of interaction.44 The FC curve of γ-Fe2O3

nanoplates shows a decrease with lowering the temperature until160 K, followed by a plateau at lower temperature. Thisbehavior is in accordance with the presence of strong interpar-ticle interaction in assemblies.45 Strong dipolar interaction blocksthe rotation of spins to align with the field, which results in thedecreasing of FC magnetization. Also, a collective magneticfreezing with disordered particle spins is observed below 160K. The FC curve of Fe3O4 is similar to that of γ-Fe2O3, but thedecreasing rate of magnetization from room temperature to 160K is much reduced.

4. Conclusions

γ-Fe2O3 nanoplates coated by hydrophilic PVP are synthe-sized in a simple system for the first time, which could bedispersed in various dipolar solvents except water. Fe3O4

nanoplates with the same size are obtained by reducing theγ-Fe2O3 nanoplates in hydrazine solution. Strong dipolarinteractions between nanoplates, which derive from their largesize and anisotropic shape, lead to their self-assembly into 1Dchainlike structure on substrate. The formation mechanisminvestigation reveals that PVP has two important effects: (1)selective coordination with {111} facets of γ-Fe2O3 nanocrystalsto reduce the growth rate along (111) direction and (2) increasedviscosity of the system to tune the growth rate of γ-Fe2O3. Theγ-Fe2O3 nanoplates are obtained when the nucleation and growthrate is at a proper proportion. The γ-Fe2O3 and Fe3O4 nanoplatesare ferromagnetic at room temperature.

Acknowledgment. This work was supported by the Programfor New Century Excellent Talents in University, People’sRepublic of China.

Supporting Information Available: TEM images of nano-plates, XRD patterns of nanoplates, XPS pattern of N1s inγ-Fe2O3 nanoplates, FT-IR spectra of γ-Fe2O3 nanoplates, andschematic structure of maghemite. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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