Solvothermal Synthesis and Characterization of Fe3O4 and γ-Fe2O3

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Solvothermal Synthesis and Characterization of Fe3O4 and γ-Fe2O3 Nanoplates Jian Lu, Xiuling Jiao, Dairong Chen,* and Wei Li Key Laboratory for Special Functional Aggregate Materials, Education Ministry, School of Chemistry and Chemical 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 nanoplates while keeping the size and morphology unchanged after reduction in hydrazine solution. The crystal structure and surface organic groups of the nanoplates were characterized in detail. The nanoplates exhibited good dispersibility in polar solvents except water and strong dipolar interactions, which favored the formation of one-dimensional chainlike structure. The poly(vinylpyrrolidone) molecules in this system played a key role for the growth of nanoplates, and it was supposed that the formation of nanoplates was a kinetically controlled process. The γ-Fe2O3 and Fe3O4 nanoplates with strong dipolar interactions are ferromagnetic at room temperature. 1. Introduction Magnetic nanoparticles have attracted much attention because of their potential applications in information storage, color imaging, 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 for their excellent physical and chemical properties, but these properties are greatly affected by the synthesis route and nanocrystals’ size and shape.2 For this reason, various approaches have been employed to prepare γ-Fe2O3 and Fe3O4 nanocrystals with different sizes and shapes in order to obtain expected properties. Among the synthetic methods, the solution processes were superior to others in controlling size and shape of nanocrystals, as well as adjusting the dispersibility in polar or nonpolar solvents.3 For the Fe3O4 nanocrystals, the generally used solution synthetic methods included the hydrothermal reaction of Fe2+ compounds,4 coprecipitation of Fe2+ and Fe3+ ions,5 and thermal decomposition of Fe3+ compounds in the presence of reducing agent or organic salts of Fe2+.6 But for the γ-Fe2O3 nanocrystals, the solution synthesis usually involved the controlled oxidation of Fe3O4 and direct mineralization of Fe3+ ions. For instance, the inorganic salts (e.g., Fe(NO3)3 · 9H2O) were applied as precursors to prepare pure γ-Fe2O3 nanoparticles in some aqueous systems.7 Other nonaqueous solution methods have also been developed to synthesize highly crystalline, monodispersed, and shape-controlled γ-Fe2O3 nanoparticles, in which organometallic compounds were always used as the precursors.8 In these syntheses, many efforts have been devoted to the monodispersed nanoparticles with small size and surface modifications. For the anisotropic magnetic nanocrystals, there is interest from the point of view of understanding not only nanocrystal growth9 but also the interesting magnetic properties derived from their shape anisotropy10 and potential applications in ultrahighdensity magnetic storage devices.11 Otherwise, assembly of anisotropic nanocrystals with specific structure is a prerequisite for the generation of nanodevices.12 However, it is difficult to form the anisotropic γ-Fe2O3 or Fe3O4 nanocrystals due to their * Corresponding author. E-mail: [email protected]. Tel: 86-0531-88364280. Fax: 86-0531-88364281.

cubic spinel structure. Up to now, although γ-Fe2O3 or Fe3O4 nanoparticles with different sizes have been prepared, little work is related to their single-crystalline anisotropic nanostructures such as the one- or two-dimensional nanostructures.13 Notably, Wee’s group synthesized Fe3O4 nanosheets at low temperature by oxidizing pure Fe substrates in acidic solution on a hot plate maintained at 70 °C,14 Zhu et al. reported a precursor-templated conversion method for the controlled synthesis of hierarchical hollow spheres assembled by Fe3O4 or γ-Fe2O3 nanosheets,15 and Ge and co-workers prepared γ-Fe2O3 nanoplates in an aqueous system by a two-step reduction-oxidation method: Fe2+ is reduced into metallic Fe by γ-ray irradiation in N2 atmosphere followed by the oxidation into γ-Fe2O3.16 Herein we present a facile solvothermal process to prepare the nearly monodispersed γ-Fe2O3 nanoplates coated by a layer of hydrophilic poly(vinylpyrrolidone) (PVP) molecules, which could be well-dispersed in several polar solvents except water. Furthermore, the γ-Fe2O3 nanoplates could be transformed to Fe3O4 nanocrystals with shape and size unchanged by reducing in hydrazine solution. These nanoplates showed strong dipolar interactions due to the large size and anisotropic shape. 2. Experimental Section Synthesis. Absolute ethanol was dehydrated by Mg powder and I2 before use, and all the other reagents were of analytical grade and used without further purification. In a typical synthesis, into 13.0 mL (0.223 mol) of absolute ethanol 0.121 g (0.3 mmol) of Fe(NO3)3 · 9H2O was dissolved to form a dark red solution, and into it 1.3 g (0.024 mmol) of PVP (Mw ) 58 000) was added under magnetic stirring. With the PVP dissolving, the solution gradually transformed into a viscous sol, which was poured into a Teflon-lined autoclave and heated at 240 °C for 4.0 h. After that the autoclave was cooled to room temperature, the as-obtained black suspension was mixed with an equal volume of ethanol, and the formed precipitate (γ-Fe2O3) was separated by centrifugation at 9500 rpm and washed with ethanol. Then it was redispersed into 13.0 mL (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 hydrazine hydrate (80%) were added under stirring to form a homogeneous sol, which was transferred to an autoclave and heated at

10.1021/jp810583e CCC: $40.75  2009 American Chemical Society Published on Web 02/16/2009

Fe3O4 and γ-Fe2O3 Nanoplates

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

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

180 °C for 6.0 h. After that the product (Fe3O4) was collected by centrifugation and washed with ethanol. Characterization. The morphology and microstructure were characterized by using transmission electron microscopy (TEM, JEM 100-CXII) with an accelerating voltage of 80 kV, highresolution TEM (HR-TEM, GEOL-2010) with an accelerating voltage of 200 kV, and field emission scanning electron microscopy (FE-SEM, JEOL, JSM 6700F). X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max 2200PC diffractometer with a graphite monochromator and Cu KR radiation (λ ) 0.154 18 nm). Thermogravimetric (TG) analysis was carried out on a Mettler Toledo SDTA851e thermal gravimetric analyzer at a heating rate of 10.0 °C · min-1 from room temperature to 1000 °C under air or N2 flow. Infrared (IR) spectra were recorded on a Nicolet 5DX FT-IR spectrometer 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 its energy analyzer working in the pass energy mode at 35.75 eV, and the Al KR line was applied as the excitation source. The magnetic 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 thickness of 10-13 nm and a side length of 30-40 nm, and their aspect ratio (side length/thickness) is ca. 3. Most of the nanoplates have triangle or truncated triangle shapes, and a few are irregular polygons. TEM and SEM images (Figure 1c, d) show that the size and morphology of the formed Fe3O4 nanoplates remained unchanged after these γ-Fe2O3 nanocrystals were reduced by hydrazine hydrate in n-octanol. The platelike morphology could be further proved by assembling these nanoplates into superstructures, which is still under investigation (Figure S1). Although the XRD patterns of the samples (Figure S2) clearly show the spinel structure, it is difficult to distinguish the γ-Fe2O3 and Fe3O4 phases only from the XRD patterns due to their similarity. XPS patterns (Figure 2) were applied to determine the 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 eV for Fe3O4. It is in agreement with the literature that the peaks shift to high binding energy and broaden for Fe3O4 due to the appearance of Fe2+(2p3/2) and Fe2+(2p1/2).18 A shoulder around

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

711 eV seems to appear in Figure 2b, which further evidences that the nanoplates have been reduced to Fe3O4.19 In addition, the presence of the satellite peak at ∼719.0 eV (indicated by an arrow) is characteristic of γ-Fe2O3, while the satellite peak in Fe3O4 is covered and becomes less resolved.20 The XPS patterns reveal that the as-synthesized nanoplates are γ-Fe2O3, and Fe3O4 is obtained after reducing. As the N atoms coordinated to metals, the N1s peak would shift to lower binding energy by 1∼3.0 eV due to a transfer of electron density from N to metals.21 Herein the N1s level at 399.75 eV does not shift to lower energy, suggesting that the PVP molecules coordinated with γ-Fe2O3 via their CdO group rather than the N atoms (Figure S3).22 The corresponding HR-TEM images (Figure 3) indicate that the nanoplates have a single-crystal structure, and a lattice spacing of ca. 0.48 nm corresponds to the (111) planes of a spinel-structured iron oxide. It is deduced that the growth of nanoplates along the (111) direction would be hindered to make the (111) planes as the basal planes of nanoplates. The lattice spacings of 0.30 and 0.247 nm from the flattening nanoplates are 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 absorptions around 3300 and 2800-3000 cm-1 attributed to the hydroxyl groups and C-H stretching vibrations. The absorptions at 1643 and 1008 cm-1 correspond to the vibrations of CdO and C-N groups of the PVP molecules absorbed on the

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

Lu et al.

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

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

nanocrystals’ surface. But for the pure PVP molecules, the absorption of CdO appears at 1664 cm-1; the obvious red shift might be due to the CdO bond weakening via partial donation of oxygen loan pair electrons of PVP to the vacant orbital of Fe atoms.23 The band at 1375 cm-1 ascribed to the NO3- groups indicates that they have not decomposed completely during the solvothermal process.24 The IR spectrum of Fe3O4 is similar to that of γ-Fe2O3, but the vibration absorptions of C-N and hydroxyl groups become much weaker. The FT-IR spectra indicate the prepared nanoparticles’ surface is coated by a layer of 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 temperature to 800 °C under air atmosphere. For the γ-Fe2O3 nanoplates, the drop of 1.7% below 150 °C results from the removal of absorbed water, and the second one of 8.3% from 150 to 450 °C is assigned to the decomposition of PVP molecules.25 For the Fe3O4 nanoplates, a weight gain at 150∼250 °C is due to the oxidation of Fe3O4 to γ-Fe2O3, which is overlapped with the decomposition of PVP. So the TG analysis in N2 flow is also 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 is calculated to be Fe2.98O4. PVP-coated γ-Fe2O3 nanoplates exhibited good hydrophilicity and could be dispersed in various polar solvents such as dimethylsulfoxide (DMSO), formamide, N,N-dimethylformamide (DMF), n-butanol, ethanol, and methanol, forming stable ferro-fluids (Figure 5). No precipitate could be observed for several weeks. Phase separation did not occur even when they were attracted by a magnet (Figure S5a). However, if they were dispersed in water or the mixture of water and ethanol (1:1 v/v), the γ-Fe2O3 nanoplates would aggregate and precipitate on the bottom instantly. TG analysis indicates that the amount of PVP on the nanoplates’ surface was reduced to 4.7% after they were washed by water (Figure 4a). It was speculated that only part of PVP molecules coordinate to the nanoplates, and the others just were absorbed on the surface. As they were dispersed in water, the PVP molecules absorbed on the surface were released in water, and the aggregation of the nanoplates occurred.

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

However, the experiments demonstrate that as the γ-Fe2O3 nanoplates were reduced to Fe3O4, they could not be dispersed in polar solvent even at 6.75 wt % of PVP molecules still absorbed on the surface (Figure S5b). Macroscopic dispersibility is due to the integration of interactions between the nanoparticles. The colloidal stability could be described by using the Derjaguin-Landau-VerweyOverbeek (DLVO) theory, and four kinds of forces contribute to the interparticle potential: van der Waals force, the electrostatic force, dipole force, and steric repulsion force. Because the van der Waals attractive force or the electrostatic repulsive force is isotropic, when the dipole force between magnetic particles becomes the primary interaction, the nanocrystals could self-assemble into one-dimensional (1D) dipole chains or flux rings on the substrate spontaneously.26 There is strong dipolar interaction between these platelike nanoparticles, which favors the self-assembly of nanoplates into 1D chainlike structures on a Cu grid (Figure 6). But the chain formation is influenced by substrate, concentration, and solvent evaporation (Figure 1a). Compared to the γ-Fe2O3 nanocrystals, the Fe3O4 nanoplates more easily form 1D chainlike structures, and some of the nanoparticles tend to stand on the Cu grid and stack face-toface (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 saturation magnetization), and the dipole-dipole interaction between two adjacent particles scales as µ2/σ3 (σ is the distance between two particles, consisting of the core diameter and the thickness of surface organic layer). In brief, this interaction is proportional to the saturation magnetization of the bulk materials and particle volume, and inversely proportional to the distance between two

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Figure 8. TEM images of the products prepared under different reaction conditions: (a) Fe(NO3)3 · 9H2O 0.4 mmol, PVP 0.9 g, temperature 240 °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.

particles. Based on the upper equations, the formation of 1D dipolar chains or flux rings on the supporting surfaces is usually observed in the follow situations: (1) strongly magnetic materials, such as metallic Fe,27 Co,28 and Ni-Co alloy nanoparticles,29 (2) relatively weak magnetic nanoparticles but with much larger sizes, such as chains formed by 21 nm Fe3O4 spheres,30 flux rings by 50 nm Fe3O4 cubes,31 and 1D chains by Ni microspheres32 or Ni/Ni3C particles,33 and (3) low-dimensional nanocrystals, as the shape anisotropy enhances the interparticle interactions to assemble nanorods34 and nanoplates12 into ribbons with particles parallel to each other. As reported,30 the critical size required for dipole chain formation for Fe3O4 spheres is 16 nm in diameter. Due to the larger size of synthesized nanoplates than that of Fe3O4 spheres, these nanoplates show stronger dipolar interaction, which promotes the assembled 1D chainlike structure on the substrate. Also, their anisotropic shape should be considered to have an additional contribution, as some of the plates tend to stand on the TEM grid to increase their dipolar interactions. Otherwise, the chains of nanoplates stacked face-to-face are along their (111) direction, which is the easy magnetization axis of magnetic iron oxide.35 Fe3O4 nanoplates have stronger dipolar interactions than γ-Fe2O3 due to its larger saturation magnetization (73.5 emu/g for γ-Fe2O3 and 92.0 emu/g for Fe3O4), which causes them to assemble in the chainlike structure even at a concentrated suspension. A time-dependent experiment was conducted to investigate the solvothermal formation process of γ-Fe2O3 nanoplates (Figure 7). Nanoparticles with a size of 5 nm appeared as the reaction was conducted for 2.0 h, and a few large particles of ca. 12 nm diameter showed triangular shape. The average size of nanoparticles increased to 20 nm after reacting for 2.25 h, and some particles showed rodlike projections, indicating that the 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 triangle particles with a side length of ca. 30 nm were obtained. In the formation of polyhedra nanocrystals, the facets with higher energy grow faster and tend to disappear, leading to the nanocrystals bounded by the low-energy facets. The growth rate could be tuned by bounding to the surfactants on the faces of nanocrystals. Shape control of nanocrystals is generally achieved by selective adhesion of surfactant to a particular crystal facet and slowing its growth rate along this direction.36 The facets free energy of a fcc (face-centered cubic) crystal is on the order of γ{110} > γ{100} > γ{111}.37 In the spinel structure of γ-Fe2O3, the {111} plane has the strongest polarity due to the alternated octahedral coordinated Fe3+ and O2- layers (Figure S6).38 In the Fe-terminated {111} surfaces of γ-Fe2O3 nanoparticles, Fe3+ ions are coordinated to only three oxygen atoms, which promotes the interactions between the surface Fe3+ and CdO groups of adsorbing PVP molecules. So PVP molecules are selectively adhered to the {111} facets, which greatly

reduced the growth rate along the (111) direction, resulting in nanoplates 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 stable morphology, which has been prepared in the presence of strong coordination agent, such as ethylenediamine tetraacetic acid (EDTA).39 In our system, the nucleation and growth rate is very fast due to the high reaction temperature, and the formation of nanoplates bounded by (111) is the result of a kinetically controlled process. Our group reported the synthesis of Co3O4 nanocrystals by the solvothermal decomposition of nitrate, which usually reacted below 200 °C.40 In this experiment, the reaction was conducted at 240 °C, and nitrate decomposed very fast which facilitated the nucleation of nanocrystals. The addition of PVP increased the viscosity of the system, which hindered the diffusion of monomers and the growth of nanocrystals for a diffusion-limited growth model. When the ratio of nucleation rate to growth rate is proper, γ-Fe2O3 nanoplates could be obtained. Further experiments showed that as the amount of Fe(NO3)3 · 9H2O and PVP changed to 0.4 mmol and 0.9 g and the reaction time decreased to 3.5 h, the chainlike structures assembled by nanoplates were obtained due to increase of both the nucleation and growth rates (Figure 8a). Similar product was obtained by decreasing the amount of Fe(NO3)3 · 9H2O to 0.2 mmol, increasing that of PVP to 1.6 g, and prolonging the reaction time to 4.5 h (Figure 8b). However, when the solvothermal reaction was conducted at 180 °C, the product was monodispersed R-Fe2O3 nanoparticles with a size of 40 nm (Figure 8c). This indicates that the reaction temperature has a great 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 room temperature (Figure 9). The saturation magnetization (Ms) and coercivity (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 are smaller than those of bulk materials. The reduced value might be due to the existence of nonmagnetic surfactant on the surface and the spin canting of surface Fe atoms.41 The reduced remnant magnetization (Mr/Ms) value is 0.14 and 0.13 for γ-Fe2O3 and Fe3O4, respectively, which is smaller than that of reported iron oxide nanoplates.42 Standard zero-field-cooling (ZFC) and fieldcooling (FC) procedure was followed at the applying field of 100 Oe. The ZFC curves of γ-Fe2O3 and Fe3O4 nanoplates show a constantly increasing trend with the temperature up to 310 K, further evidencing their ferromagnetic property at room temperature. FC magnetization measurements could provide information about magnetic interactions between nanoparticles. In many types of magnetic systems, the FC magnetization increases with decreasing temperature due to the decreased thermal fluctuations and spins aligning with the applied field.43 Dipolar interaction between particles suppresses these fluctuations, and thus the increase in magnetization during a FC experiment is

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Figure 9. Magnetic hysteresis loops (a) of γ-Fe2O3 and Fe3O4 nanoplates (inset: magnified view); ZFC/FC curves of γ-Fe2O3 (b) and Fe3O4 nanoplates (c).

inverse to the amount of interaction.44 The FC curve of γ-Fe2O3 nanoplates shows a decrease with lowering the temperature until 160 K, followed by a plateau at lower temperature. This behavior is in accordance with the presence of strong interparticle interaction in assemblies.45 Strong dipolar interaction blocks the rotation of spins to align with the field, which results in the decreasing of FC magnetization. Also, a collective magnetic freezing with disordered particle spins is observed below 160 K. The FC curve of Fe3O4 is similar to that of γ-Fe2O3, but the decreasing rate of magnetization from room temperature to 160 K is much reduced. 4. Conclusions γ-Fe2O3 nanoplates coated by hydrophilic PVP are synthesized in a simple system for the first time, which could be dispersed in various dipolar solvents except water. Fe3O4 nanoplates with the same size are obtained by reducing the γ-Fe2O3 nanoplates in hydrazine solution. Strong dipolar interactions between nanoplates, which derive from their large size and anisotropic shape, lead to their self-assembly into 1D chainlike structure on substrate. The formation mechanism investigation reveals that PVP has two important effects: (1) selective coordination with {111} facets of γ-Fe2O3 nanocrystals to reduce the growth rate along (111) direction and (2) increased viscosity of the system to tune the growth rate of γ-Fe2O3. The γ-Fe2O3 nanoplates are obtained when the nucleation and growth rate is at a proper proportion. The γ-Fe2O3 and Fe3O4 nanoplates are ferromagnetic at room temperature. Acknowledgment. This work was supported by the Program for New Century Excellent Talents in University, People’s Republic of China. Supporting Information Available: TEM images of nanoplates, XRD patterns of nanoplates, XPS pattern of N1s in γ-Fe2O3 nanoplates, FT-IR spectra of γ-Fe2O3 nanoplates, and schematic structure of maghemite. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Lu, A.; Salabas, E. L.; Schu¨th, F. Angew. Chem., Int. Ed. Engl. 2007, 46, 1222. (b) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. AdV. Mater. 2007, 19, 33. (c) Zeng, H.; Sun, S. AdV. Funct. Mater. 2008, 18,

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