Surfactant-Assisted Hydrothermal Synthesis of Dendritic Magnetite

Dec 10, 2008 - Each dendrite was mainly composed of one trunk and four groups of branches. The trunk grew along [110] with two pairs of ⟨111⟩-orie...
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Surfactant-Assisted Hydrothermal Synthesis of Dendritic Magnetite Microcrystals Ming Hu, Ji-Sen Jiang,* and Xiaodong Li Department of Physics, Center of Functional Nanomaterials and DeVices, East China Normal UniVersity, Shanghai 200062, China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 820–824

ReceiVed April 16, 2008; ReVised Manuscript ReceiVed October 10, 2008

ABSTRACT: Dendritic magnetite crystals were fabricated by a surfactant-assisted hydrothermal method. The phase purity and composition of the dendrites were characterized by Mo¨ssbauer spectroscopy and X-ray diffraction (XRD), respectively. The morphology of the dendrites was measured by scanning electron microscopy (SEM) images. The oriented growth direction of the dendrites was determined by transmission electron microscopy (TEM) and selection area electron diffraction (SAED). The results showed that the products were pure magnetite crystals consisting of dendritic structures. Each dendrite was mainly composed of one trunk and four groups of branches. The trunk grew along [110] with two pairs of 〈111〉-oriented branches grown on the trunk perpendicularly. The shape of the crystals strongly depended on the concentration of K3[Fe(CN)6], concentration of cetyltrimethylammonium bromide (CTAB), hydrothermal temperature, and type of surfactant. With the use of mixed surfactant chain-like particles could be obtained. Fourier transform infrared spectrum (FT-IR) showed CTAB adsorbed on the surface of dendritc magnetite strongly. A slow oriented growth mechanism was put forward to explain formation of dendrites. The slow dissociation of K3[Fe(CN)6] should be responsible for the slow growth rate of the crystals. Adsorption of CTAB on the surface of the magnetite crystals led to oriented growth of the dendritic microcrystals.

1. Introduction Dendritic crystal growth patterns formed under nonequilibrium conditions have fascinated scientists for several years.1 With the development of nanotechnology, architectural control of nanocrystals with well-defined shapes has been a key issue in materials chemistry.2 In order to seek new properties of materials, several branched materials have been fabricated.3,4 As a hyperbranched structure, the possibility of forming dendritic crystals opens a door to promising candidates for design and fabrication of new functional materials.4 In recent years, numerous nanodendritic or microdendritic crystals have been synthesized containing several structures. However, most of the reports focused on the cubic copper structure,5 cubic rock salt structure,6 hexagonal cobalt structure,7 wurtzite structure,8 covellite structure,9 and tetragonal stolzite structure, etc.10 Only a few examples of crystals of complex structures such as the corundum-type crystal11 and spinel-type crystal12 have been reported. As one of the most important spinel-type materials, magnetite (Fe3O4) has gained more and more attention in materials science because of its potential application in various fields such as information storage,13 hyperthermic treatment,14 magnetic resonance imaging,15 and cancer cells targeting and imaging.16 Therefore, fabrication of dendritic magnetite crystals has become a very important aspect of materials science. To date, Qian et al. reported the preparation of two-dimensional (2D) fractal magnetite nanocrystals through a solvothermal process.17 The branches of the nanocrystals could be destroyed by ultrasonication. When porous silicon was used as a substrate, dendritelike self-assembly of Fe3O4 nanoparticles was reported.12 However, there is no report about the fabrication of threedimensional (3D) dendritic magnetite crystals. Herein, we report for the first time the surfactant-assisted synthesis of 3D dendritic spinel-type Fe3O4 microcrystals * To whom correspondence should be addressed. E-mail: jsjiang@ phy.ecnu.edu.cn.

through a hydrothermal route. In contrast to the reported fabrication of fractal-like17 or dendrite-like12 self-assembly Fe3O4 nanocrystals, our approach does not require expensive acetone or porous substrate and the morphology of the dendritic magnetite crystal is absolutely different. Moreover, a growth mechanism is proposed to explain formation of dendritic Fe3O4.

2. Experimental Section In a typical synthesis, 315.1 mg of K3[Fe(CN)6] was dissolved in 40 mL of distilled water to form a transparent solution. Then, 43.7 mg of cetyltrimethylammonium bromide (CTAB) was dissolved in 65 mL of distilled water to form a transparent solution, adding into the K3[Fe(CN)6] solution subsequently. The mixture was stirred for 30 min. Afterward, 15 mL of 85% hydrazine was added into the mixture solution under continuous stirring for 30 min, and then the mixture was transferred into a Teflon-lined stainless-steel autoclave, sealed, and maintained at 160 °C for 48 h. After the solution was cooled to room temperature, the obtained black solid was collected by centrifugation, washed several times with water and ethanol, and then dried in a vacuum oven at 60 °C for 10 h. The transmission 57Fe Mo¨ssbauer spectrum was collected using a Mo¨ssbauer spectrometer in a constant acceleration mode with a 57Co(Pd) source. Measurements were performed at room temperature. Hyperfine interaction parameters were derived from the Mo¨ssbauer spectrum using a least-squares method. The spectrometer was calibrated using a standard 25 µm R-Fe foil. The phase composition of samples was monitored by XRD using an X’Pert-Pro MPD diffractometer with Cu KR radiation and conventional θ-2θ geometry. SEM and energy dispersion spectroscopy (EDS) measurements were made on a JSM6460 microscope. A JEOL JEM-2100F TEM operating at 200 kV accelerating voltage was used for TEM analysis. FT-IR was obtained using a Nicolet Nexus 670 FT-IR spectrometer with a resolution of 0.09 cm-1. The room-temperature magnetic hysteresis loop of the dendritic magnetite crystals was measured by a vibrating sample magnetometer (HH-15, China).

3. Results and Discussion It is well known that Mo¨ssbauer spectrum is more accurate to identify the composition of samples containing Fe elements than the other methods. Figure 1a shows the room-temperature

10.1021/cg8003933 CCC: $40.75  2009 American Chemical Society Published on Web 12/10/2008

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Figure 1. Structure characterization and SEM images of dendritic Fe3O4 prepared with a K3[Fe(CN)6] concentration of 0.008 mol/L, a CTAB concentration of 0.001 mol/L, and 15 mL of 85% hydrazine at 160 °C for 48 h. (a) Mo¨ssbauer spectrum of the product. (b) General view of dendritic Fe3O4 crystals. (c) SEM image of a mutiple-symmetric structure. (d) High-magnification SEM image of a single dendrite.

Mo¨ssbauer spectrum of the product obtained with a K3[Fe(CN)6] concentration of 0.008 mol/L and CTAB concentration of 0.001 mol/L at 160 °C for 48 h in the typical experiment. Two hyperfine magnetic sextets are presented in the spectrum. The sextet with an isomer shift of 0.29 mm s-1, quadrupole splitting of -0.01 mm s-1, and hyperfine field of 494.2 kOe represents the Fe3+ on the tetrahedral A sites of magnetic Fe3O4. The other sextet with an isomer shift of 0.68 mm s-1, quadrupole splitting of -0.02 mm s-1, and hyperfine field of 462.3 kOe represents the Fe2+ and Fe3+ on the octahedral B sites. The spectrum matches very well with the reported one for Fe3O4,18 which illustrates that the product is pure spinel Fe3O4. This result is further supported by the XRD pattern of the product (Figure S1, Supporting Information). All peaks can be indexed as facecentered cubic (fcc) spinel structure, which is close to that of Fe3O4 (JCPDS 85-1436). No diffraction peaks of any other phases were detected, indicating the high purity of the main product. The SEM image shown in Figure 1b clearly shows that the majority of the particles are in the form of exquisite dendrites. Some of the dendrites show flower-like morphology with symmetrical trunks as demonstrated by Figure 1c, and some of the dendrites contain single trunks. Both of the two kinds of dendrites are considered to be one type dendrite intrinsically.6a,11 Each of the Fe3O4 dendrites has a 3D structure with one trunk (long axis) and four branches (short axes). The length of the trunks is 10-15 µm, and that of the branch trunks ranges from 500 nm to 2 µm. An individual dendrite can be observed in Figure 1d. The higher magnification SEM image clearly shows that each branch is composed of small dendritic structures. These small dendrites are parallel to each other and perpendicular to the trunk. More interestingly, the small dendrites present a similar structure of the whole dendrites. Such a dendritic magnetite is different from the fractal Fe3O4 reported in ref 17. The fractal Fe3O4 shows a 2D structure with all the branches in the same plane, and the branches are not perpendicular to the trunk. In our result, two pairs of branches grow on the trunk symmetrically, which shows a 3D dendritic structure. The corresponding EDS spectrum of the dendrites is shown in Figure S2, Supporting Information, revealing the dendrites only contain

Figure 2. (a) TEM image of the trunk of the dendritic Fe3O4 microcrystal. (Inset) SAED of the trunk tip. (b) HRTEM image of the bottom of the trunk. (c) TEM image of the branches of the dendritic Fe3O4 microcrystal. (Inset) SAED of the branch. (d) HRTEM image of the branch tip.

Fe and O elements. The intense peak of C comes from the conductive adhesive. To determine the crystal orientation of the obtained dendritic microcrystals, TEM and high-resolution transmission electron microscope (HRTEM) observations were carried out. Figure 2a and 2c represents the TEM images of the dendritic microcrystals along different projections. In Figure 2a the gray axis in the center of the crystal represents the trunk and the black region represents the branches. In Figure 2c the gray area represents the branches perpendicular to the projection direction while the black area represents the branches parallel to the projection

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Figure 3. SEM images of Fe3O4 particles obtained under different conditions. (a and b) Products synthesized at 160 °C with a concentration of CTAB of 0.001 mol/L and concentrations of K3[Fe(CN)6] of 0.008 and 0.02 mol/L, respectively. (c) Product synthesized with a concentration of K3[Fe(CN)6] of 0.008 mol/L and a concentration of CTAB of 0.001 mol/L at 200 °C. (d, e, and f) Products synthesized at 160 °C with a concentration of K3[Fe(CN)6] of 0.008 mol/L and concentrations of CTAB of 0, 0.02, and 0.04 mol/L, respectively.

direction, the trunk, are covered by the branches. The inset in Figure 2a shows the selected area electron diffraction (SAED) pattern of the trunk tip. The diffraction pattern indicates the trunk of the dendrite oriented along [110]. Figure 2b shows the HRTEM of the bottom area of the trunk. The lattice spacing of 2.9 Å between adjacent lattice planes corresponds to the (220) lattice spacing of magnetite, indicating that crystal growth of the trunk is along [110] as the SEAD shows. The SAED pattern of the branches tip (inset in Figure 2c) indicates the branches of the dendrite are oriented along [11j1j]. Considering the symmetrical structure of the branches, the orientation direction should be 〈111〉. The HRTEM image of the branch tip further confirms that the preferential growth direction of branch should be 〈111〉 because the lattice spacing of 4.9 Å is very close to the (111) lattice spacing of magnetite. The reaction parameters affect the morphology of the product significantly. When the concentration of K3[Fe(CN)6] varied from 0.008 to 0.02 mol/L while maintaining the other reaction conditions (160 °C, 48 h, 0.001 mol/L CTAB, 15 mL of 85% hydrazine), the sophisticated branches of the dendritic crystals (Figure 3a) vanished as shown in Figure 3b. Only the smooth trunk could be observed. When the reaction temperature was raised to 200 °C while keeping the other synthetic parameters the same as the product shown in Figure 1 (48 h, 0.008 mol/L K3[Fe(CN)6], 0.001 mol/L CTAB, 15 mL of 85% hydrazine), the length of the dendritic crystals was reduced to about 5 µm and more irregular particles appeared (Figure 3c). Figure 3d-f shows the morphology variation, while the concentration of CTAB was altered separately (160 °C, 48 h, 0.008 mol/L K3[Fe(CN)6], 15 mL of 85% hydrazine). When no CTAB was used, the product was composed of irregular particles (Figure 3d). When 0.02 mol/L CTAB was introduced, the major products were dendritic crystals while a few spheres existed (Figure 3e). Until the concentration of CTAB was increased to 0.04 mol/L, more spheric particles were generated (Figure 3f). Besides CTAB, hydrazine plays an important role in formation of Fe3O4. Without the use of hydrazine, we cannot obtain any Fe3O4, only dendritic micropine hematite can be observed (Figure S3, Supporting Information).11

Figure 4. SEM images of product synthesized at 160 °C with surfactant instead of CTAB: (a) 0.01 mol/L SDS and (b) 120 mg of F127.

As discussed above (Figure 3e and 3f), CTAB plays an important role in formation of dendritic crystals. To facilitate a comparative study, CTAB was replaced by anion surfactant sodium dodecyl sulfate (SDS) and nonionic surfactant F127, respectively (see the Supporting Information for experimental details). SEM images (Figure 4a and 4b) show that the products are composed of irregular particles; no dendrites are observed. This phenomenon indicates that neither anion surfactant nor nonionic surfactant can guide formation of dendritic Fe3O4 crystals. Because no positive group could be provided by SDS or F127, the CTA+ may be responsible for formation of dendrites. If negative groups are introduced into the reaction solution, shall we obtain the dendrites? Due to the negative group containing sodium citrate, sodium citrate was chosen to add into the solution (see the Supporting Information for experimental details). No dendritic Fe3O4 crystals could be found in the SEM images as shown in Figure 5. More interestingly, when the concentrations of sodium citrate and CTAB were 0.01 mol/L, the product was composed of both spheres and chainlike irregular particles (Figure 5a). While the concentration of CTAB was increased to 0.02 mol/L independently, the product became chain-like spheres (Figure 5b). This result proved that the existence of a negative group could inhibit formation of dendritic crystals significantly even when CTAB is present, which indicated that CTA+ should be a crucial factor for dendritic growth of magnetite.

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Figure 5. SEM images of product synthesized at 160 °C with a concentration of sodium citrate of 0.01mol/L and concentrations of CTAB of 0.01 (a) and 0.02 mol/L (b). Figure 7. Magnetization curve of dendritic magnetite crystals at room temperature.

Figure 6. SEM images of magnetite grown at different growth stages while keeping the initial concentration of K3[Fe(CN)6] at 0.008 mol/L, initial concentration of CTAB at 0.001 mol/L, and reaction temperature at 160 °C: (a) 2, (b) 5, (c) 12, and (d) 24 h.

In order to investigate the formation process of the dendritic Fe3O4 microcrystals, a time-dependent experiment was performed. Figure 6 shows the SEM images of the particles obtained at different stages while keeping the other conditions the same as the dendritic crystals shown in Figure 1. When the reaction time was 2 h, irregular polyhedral particles were obtained with sizes of several hundreds nanometers (Figure 6a). When the reaction time was increased to 5 h (Figure 6b), larger microparticles with rugged surfaces formed. The high-magnification SEM image (Figure S4, Supporting Information) shows that the microparticles are formed by assembling of small particles which are several hundreds of nanometers. The shape and size indicate the small particles should be the polyhedral particles formed at 2 h. When the reaction time increased to 12 h, no obvious change of the morphology of the microparticles could be observed (Figure 6c). However, the diameter of the particles increased from 3-4 to 4-5 µm. Until the reaction time reached 24 h (Figure 6d), dendritic crystals emerged and the microparticles with rugged surfaces still existed. The dendrites are 4-5 µm long, not as long as the dendrites illustrated in Figure 1, and the microparticles are larger than the particles obtained at 12 h. Finally, when the reaction time reached 48 h, perfect dendritic crystals were generated and the rugged microparticles vanished. Such a time-consuming crystal growth process should be related to the slow reaction process. Due to the large stability constant (Ks ) 1.0 × 1042),11 [Fe(CN)6]3- dissociates slowly to release Fe3+ ions into solution under a hydrothermal environment. The Fe3+ ions are partially reduced to Fe2+ ions by hydrazine. Then, Fe3+ and Fe2+ can react to form Fe3O4. As a result, Fe3O4 crystals can only grow slowly.

On the basis of the above investigation, formation of the dendritic microcrystals can be suggested as follows: First, the irregular polyhedral nanoparticles with sizes of several hundreds nanometers are generated. Because the surface energy of the naonoparticles is generally high, the nanoparticles prefer to selfassemble with each other to form the rugged microcrystals to reduce the high surface energy in the presence of surfactant.19 Then, the rugged microcrystals are formed in the presence of CTAB. In the following stage, the microcrystals are growing slowly because of the low production rate of Fe3O4 crystal grains. This is another indirect reason for the rugged microcrystals formed by attachment of preformed nanoparticles as there is not sufficient time for production of enough new crystal grains to support formation of microcrystals in only 3 h. Finally, oriented growth of crystals happens, and the dendritic crystals successfully are generated. To clarify the reason for the oriented growth, we start from the crystal structure of magnetite. As an inverse spinel structure, the O2- ions are arranged parallel to form a close-packed structure.18 Therefore, the negative charge density of different facets could be different as some facets are higher than the other facets. Because it is a cation surfactant, CTAB prefers to adsorb on the polar facets of the magnetite crystals. The FT-IR spectrum of the dendritic magnetite microcrystals shown in Figure S5 of the Supporting Information confirms adsorption of CTAB on the dendritic magnetite crystals. The band at 590 cm-1 is assigned to the stretching vibration of Fe-O in magnetite.20 The two weak bands at 2930 and 2852 cm-1 are assigned to the asymmetric and symmetric stretching vibrations of C-CH2 from CTAB, respectivly.21 The broad band between 3300 and 3600 cm-1 and the band centered at 1620 cm-1 found on all samples are assigned to O-H stretching and deformation vibrations of weakly bound water.21b On the basis of the analysis of the FT-IR spectrum, the strong adsorption of CTAB on the surface of the dendritic magnetite can be concluded because the sample was washed by water and ethanol several times, and CTAB is soluble in water and ethanol both, no free CTAB can exist. When the negative polar facets are covered by CTAB, the new nucleus could only attach on other facets, which may result in oriented growth. Due to the slow growth rate, there is enough time for CTAB to impact the newly formed surface. Finally, the dendritic structure is generated. The slow oriented growth mechanism of crystal is similar to the mechanism proposed by Cao’s group17 but different from diffusion-limited aggregation (DLA).1a,b,22 As illustrated in Cao’s report, both the low dissociation rate of K3[Fe(CN)6] and the polar facets induced oriented growth are crucial factors for formation of dendritic hematite crystal. The two factors could be found in the formation process of the dendritic magnetite microcrystal also.

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The magnetic hysteresis curve of the dendritic Fe3O4 was measured at room temperature as shown in Figure 7. The hysteresis loop of the dendritic Fe3O4 exhibits ferromagnetic behavior with high saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values of ca. 83 emu/ g, 20 emu/g, and 135 Oe, respectively. The saturation magnetization is higher than the Ms value of 2D fractal magnetite (78.75 emu/g).17 The higher value of Ms value may be caused by the larger size of the 3D dendritic structure.

4. Conclusions Novel 3D dendritic magnetite crystals have been fabricated through a simple hydrothermal process in the presence of CTAB as a surfactant. A slow oriented growth mechanism has been put forward to explain formation of dendrites. The morphologies of the magnetite crystals can be controlled conveniently by varying the concentration of reactants, reaction temperature, and type of surfactant. Our experimental method could be a general route for fabrication of dendritic spinel-type materials. Acknowledgment. This work was supported by the PhD Program Scholarship Fund of ECNU 2008 (Grant 20080045) and Shanghai Nanotechnology Promotion Center (0852nm03200). Supporting Information Available: XRD pattern of the dendritic magnetite microcrystals, EDS spectrum of the dendritic magnetite microcrystals, SEM of the micropine hematite, high-magnification SEM image of the rugged microparticles, FT-IR spectrum of the dendritic crystals, and details of the sample preparation. This material is available free of charge via the Internet at http://pubs.acs.org.

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