Single Crystalline FeNi3 Dendrites: Large Scale Synthesis, Formation

Dec 15, 2008 - Single Crystalline FeNi3 Dendrites: Large Scale Synthesis, ... Auhui Key Laboratory of Functional Molecular Solids, Anhui Normal Univer...
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Single Crystalline FeNi3 Dendrites: Large Scale Synthesis, Formation Mechanism, and Magnetic Properties Xian-Min Zhou and Xian-Wen Wei* College of Chemistry and Materials Science, Auhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 7–12

ReceiVed January 27, 2008; ReVised Manuscript ReceiVed NoVember 21, 2008

ABSTRACT: Single crystalline FeNi3 dendrites were successfully synthesized in high yield by a simple and facile hydrothermal method without the presence of surfactants and were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and vibrating sample magnetometer (VSM). The individual FeNi3 dendrite consists of a long central trunk with secondary, tertiary, and even quaternary branches. On the dendritic hierarchical structures, several leaves with different lengths of 0.2-2.5 µm and widths of 100 nm to 1 µm are connected to the trunk with a length of 1-5 µm. The reaction parameters such as temperature, the reaction time, the concentration of NaOH, and the initial concentration of Fe3+ ions that affected the FeNi3 morphology were investigated systematically, based on which possible formation mechanism for the dendrites was proposed. Compared with FeNi3 particles, FeNi3 dendritic structures exhibited a decreased saturation magnetization (Ms, 61.6 emu/g) but an enhanced coercivity (Hc, 145.6 Oe) due to their peculiar morphology. To our best knowledge, this is the first report on making crystalline metal alloy dendrites with such a full control. Magnetic micro/nanostructures have attracted intensive interest because of their excellent physical, catalytic, and magnetic properties and their crucial applications in diverse fields, including highdensity magnetic storage devices, magnetic fluids, magnetic sensors, catalytic applications; they address some basic issues about magnetic phenomena in low dimensional systems and are favored for various biological applications.1 Soft magnetic materials, with high saturation magnetization (Ms), high permeability, high Curie temperature, and low energy losses, were widely used in AC electrical and electronic devices such as inductors, transformers, electromagnetic wave absorbers, antennas, magnetic sensors, and magnetic recording heads.2 Among these, FeNi alloys especially FeNi3 present interesting magnetic properties and wide applications. FeNi alloy nanoparticles,3-5 FeNi3 nanoparticles,6,7 FeNi nanorods and nanotubes8 have been successfully synthesized by different methods, including nanotemplate approach,3 mechanical alloying,4 hydrogenation of Ni[(COD)2] and Fe[(SiMe3)2]2] at 150 °C using stearic acid and hexadecyl-amine as stabilizing ligands,5 chemical reduction,6 hydrothermal treatment of Fe2+ and Ni2+ by using hydrazine as reductant and SDS as surfactant,7 electrochemical reduction,8 thermal decomposition of organometallic precursor,9 and the microwave plasma method.10 It is still a challenge to fully control the morphology of FeNi alloys with dendritic fractals, although we have synthesized the FeNi nanostructures with tunable shape and size by solution phase reduction,11 since dendritic fractals structures not only can provide a framework for the study of disordered systems12 but also can be used as catalysts whose activity and selectivity are strongly dependent on the morphology of the structures.13 Up to now, only Co,14 Ni,15 Au,16 Ag,17 Pd,18 CuNi,19 R-Fe2O3,20 copper sulfide,21 ZnO,22 PbS,23 PbMoO4,24 silicates,25 etc. with dendritic structures have been fabricated; the threedimensional self-assembly of FeNi alloy dendritic structures has not been reported. Herein, we describe a surfactant-free hydrothermal route to prepare single-crystal hierarchical FeNi3 dendrites in high yield within a short time by using a cheap and nontoxic reagent, FeCl3 · 6H2O as the iron source. The formation of such dendrites occurs in a single process, carried out by directly reducing the Fe(III) and Ni(II) salts with hydrazine hydrate in alkaline condition without any shape modifier and controller. In a typical procedure, 0.4 g of sodium hydroxide was added to 20 mL of the aqueous * Corresponding author. E-mail: [email protected]. Phone: 86-5533937138. Fax: 86-553-3869303.

mixture solution of FeCl3 (0.0075 M) and NiCl2 (0.0225 M) with stirring. Subsequently, 5 mL of aqueous hydrazine (N2H4 · H2O, 85 wt %) as reductant was added dropwise into the above solution, and after being vigorously stirred for several minutes, the mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 30 mL. The autoclave was sealed and put into a furnace, which was heated to and maintained at 180 °C for 30 min, and then it was taken out and cooled naturally to room temperature. The final product was collected through magnetic separation by a magnet with 105 mT, washed with distilled water and ethanol several times under ultrasonic, and then was dried in a vacuum oven at 60 °C for 4 h. All the reagents used were analytical grade and used without further purification. The crystal structure and purity of the resulting products were examined by X-ray powder diffraction (XRD, Shimadzu XRD-6000 X-ray diffractometer with Cu KR radiation, λ ) 1.5406 Å, V ) 40 kV, I ) 30 mA). The typical XRD patterns of the as-synthesized products are shown in Figure 1a. The characteristic peaks at 2θ values of 44.02°, 51.75°, and 75.62° correspond to the crystal planes of (111), (200), and (220), respectively, of the crystalline facecentered cubic FeNi3 alloy (JCPDS 38-0419) with a cell parameter a ) 3.549 Å. No impurity phases such as iron-nickel oxides and hydroxides were detected. The size and morphology of the as-prepared products were further examined by scanning electron microscopy (SEM, Hitachi S-4800 microscope) and transmission electron microscopy (TEM, JEM JEOL-2010 microscope operated at 200 kV). The overall morphology of the samples, as shown in Figure 1b, indicated the obtained product consists of a great deal of novel three-dimensional (3D) dendritic superstructures with lengths ranging from 1 to 5 µm, and also indicated high yield and good uniformity were achieved with this approach. The high-magnification image of a single dendrite shown as the inset in Figure 1b revealed a clear and well-defined dendritic fractal structure with a pronounced trunk consisting of highly ordered branches distributed on both sides of the trunk. It is interesting that the secondary branches are parallel to each other and emerge at 60° angles with respect to the central trunk. Similar phenomena are also found toward the tertiary branches. A representative TEM image of an individual hierarchical dendrite is shown in Figure 1c, which is in accordance with the results from SEM. The corresponding selected-area electron diffraction (SAED, JEM JEOL-2010 microscope) pattern shown in Figure 1d was taken from the trunk tip or branch tip. The diffraction spots imply that the

10.1021/cg8000976 CCC: $40.75  2009 American Chemical Society Published on Web 12/15/2008

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Figure 2. SEM images of the products obtained at different alkali concentrations of (a) 80 mM, (b) 0.4 M, (c) 1.0 M, and (d) 2.0 M with the typical procedure.

Figure 1. Integrated characterization of the FeNi3 dendrites obtained by reduction of 0.0075 M Fe3+ and 0.0225 M Ni2+ with hydrazine at 180 °C for 30 min. (a) XRD pattern, (b) SEM image, (c) TEM image of a single dendrite, (d) SEAD pattern taken from the tip of the trunk or the branch in panel c, (e) size distribution of dendrites evaluated from the corresponding SEM images shown in panel b, and (f) EDX spectrum taken from the tip of dendritic FeNi3 superstructures circled in panel c.

individual trunk or branch is single crystalline. The average length of the dendrites over the whole sample was 2.0-2.5 µm, which is in accordance with the size distribution of the dendrites (Figure 1e). The chemical composition (atomic percent) of the as-prepared dendritic superstructures was determined by energy-dispersive X-ray (EDX) spectrometry as shown in Figure 1f. The only detectable elements are iron, nickel, copper, and chromium while Cu is from the copper grid and Cr is from the thin macromolecule film covered on the copper grid, and the corresponding elementary analysis reveals that the atomic ratio of iron to nickel is 24.35:75.65, which is very close to the initial set ratio of Fe3+/Ni2+ ) 1:3. This supports the XRD and SAED results that the as-prepared products are pure FeNi3 crystals. The morphology, size, and structure of the final products can be effectively controlled by adjusting experimental parameters such as concentration of NaOH and metal ions, reaction time and temperature etc. It was found that the concentration of sodium hydroxide was an important factor that affected the reaction kinetics through turning the dissolution-deposition equation of Fe(OH)3 and Ni(OH)2 and further affected the morphology and structure of the products. SEM images in Figure 2 show the typical morphologies of four samples obtained with different alkali concentrations while keeping the other reaction conditions constant. At a low NaOH concentration of 80 mM, there were undeveloped dendrites (Figure 2a) with unclear morphology. When the alkali concentration was increased to 0.4 M, there were perfect dendrites (Figure 2b). With the NaOH concentration being further improved to 1.0 M, the products became a mixture of spheres and dendrites with a rough surface (Figure 2c). Similar results could be seen at a NaOH concentration of 2.0 M (Figure 2d), and there were more spheres than dendrites in the products. It was known that a basic medium could improve the reducing power of hydrazine. Consequently, the

Figure 3. SEM images of the product obtained at different Fe3+ concentrations of (a) 0.005, (b) 0.01, (c) 0.015, and (d) 0.03 M while keeping the molar ratio Fe3+/Ni2+ ) 1:3.

metal crystal growth rate was increased correspondingly by increasing the alkali concentration.26 However, the trend for Fe(OH)3 and Ni(OH)2 to release Fe3+ and Ni2+ decreases if the concentration of NaOH is exorbitant, making the formation of Fe3+ and Ni2+ in the solution difficult and decreasing the iron-nickel crystal growth rate and which further causes less FeNi3 to be formed. This can also be seen in the formation of Co dendrites.14c The observed shape variation with alkali concentration clearly showed that a suitable alkali concentration (0.4 M) can effectively control the redox rate, and thus kinetically control the growth rate of the iron-nickel dendrites. It was also found that the concentration of metal salts is of great importance for controlling the morphology of iron-nickel dendrites. Figure 3a-d shows the SEM images of the typical morphologies of the as synthesized samples with a concentration of iron (III) chloride from 0.005 to 0.03 M while keeping the molar ratio Fe3+/ Ni2+ ) 1: 3. It can be seen that at lower metal salts concentration (0.005 M Fe3+), a mixture of imperfect dendrites and pagodas was obtained (Figure 3a). Well-defined dendrites were obtained when the concentration of Fe3+ was increased to 0.0075 M (Figure 1b) and 0.01 M (Figure 3b). Figure 3c shows that there are plenty of microdendrites with some micropagodas obtained at 0.015 M Fe3+. Only micropagodas were obtained when the concentration of Fe3+ was increased to 0.03 M (Figure 3d). It was demonstrated that pure and high yield iron-nickel dendrites can be obtained only at an optimal concentration range of metal salts (0.0075-0.01 M Fe3+). The effect of temperature on the formation of the dendrites was also studied. XRD patterns (Figure S1,Supporting Information) show that all the final products separated from the mixtures obtained at different temperatures with a 30 min reaction are FeNi3 crystals. SEM images of them are shown in Figure 4a-d. The products obtained at 120 °C are a mixture of FeNi3 and iron-nickel

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Figure 4. SEM images of the FeNi3 crystals formed at different temperatures of (a) 120, (b) 140, (c) 160, (d) 200 °C with the same reaction time of 30 min. Figure 6. SEM images of the FeNi3 crystals obtained at 180 °C with different reaction times of (a) 1, (b) 5, (c) 10, (d) 20, (e) 30, and (f) 60 min, respectively.

Figure 5. TEM images and the corresponding SAED patterns of FeNi3 with structures of (a, b) microspheres, and (c, d) micropagodas obtained at 120 and 140 °C, respectively.

hydroxides. FeNi3 in 38% yield, which was separated from the mixture by a magnet, was composed of monodispersed microspheres with an average diameter of 1.5-2.0 µm as shown by the SEM image in Figure 4a. The SEM image (not shown here) indicates iron-nickel hydroxides are irregular microsheets. Similar phenomenon is observed at 140 °C, but FeNi3 multilayer micropagodas with an average height of 1.5 µm are observed (Figure 4b), and the clear structure of an individual micropagoda is also shown in Figure 4b (inset). At 160 °C, the products have grown into dendrites but with imperfect dendritic structures (Figure 4c). By increasing the reaction temperature to 180 °C, the products are perfect dendrites (Figure 1b) and in a yield of 93%. When temperature was further increased to 200 °C, the morphology of the products was dendrites too, but the tip of the trunk and branches consisted of hexagonal nanoplatelets (Figure 4d). TEM image (inset of Figure 4d) shows the big hexagonal platelets with a side length of about 60-100 nm. EDX spectra of microspheres and pagodas (Figure S2, Supporting Information), obtained from the Hitachi S-4800 microscope, showed the detectable elements are iron and nickel, and the corresponding elementary analysis reveals that the atomic ratio of nickel to iron for spheres and pagodas are 26.61:73.69 and 23.63: 76.37, respectively, which are very close to the initial set ratio of nickel to iron ions. These results support the XRD results. Figure 5 shows the TEM images and the corresponding SAED patterns of the different FeNi3 structures obtained at 120 and 140 °C, respectively, which are in agreement with the SEM results. SAED results show that spheres were polycrystallite (Figure 5b), and pagodas were single crystallites (Figure 5d), but the spots of the pagodas had appreciably elongated which may be due to a slight defect on the single crystal orientation. It can be concluded that the low temperature is not favorable for the growth of FeNi3 dendritic structures with a reaction time of 30 min. The morphology

of the sample changed obviously from individual microspheres to micropagodas and then to microdendrites with increasing reaction temperature; meanwhile the yield increased. Higher temperature is required to overcome the kinetic difficulty in reducing Fe(OH)3 because the standard electrode potential of Fe(OH)3 to Fe (-0.317 V) is higher than that of Fe(OH)2 to Fe (-0.877 V).6 Therefore, 180 °C is the optimal temperature in our reaction system, at which perfect dendritic structures with relatively high yields can be obtained. In order to reveal the formation process of the 3D binary metallic iron-nickel dendritic superstructures in more detail, time-dependent experiments were carried out at 180 °C with changing reaction times from 1, 5, 10, 20, 30, to 60 min while keeping all the other reaction conditions constant. The products separated from the mixtures by magnet were pure FeNi3 crystals indicated by the XRD patterns (Figure S3 in the Supporting Information); their chemical composition measured on SEM confirmed that the atomic ratio of iron to nickel (Table S1,Supporting Information) was very close to the initial set ratio of Fe3+ to Ni2+. The representative SEM images of the products obtained at certain reaction time intervals are shown in Figure 6. It can be clearly seen that when the shorter reaction time (1 min) is employed, there are only particles with a diameter of about 1-2 µm (Figure 6a). And we are surprised to find that nanoscale pagoda-like structures have formed on some microspheres at this time (inset of Figure 6a). However, Fe-Ni hydroxides and FeNi3 coexist in that case because the Fe-Ni resource is supplied by slow-release of iron-nickel hydroxides and the reduction reaction cannot proceed sufficiently within such a short time. After 5 min, there are nanopagodas (Figure 6b) with an average height of about 500 nm. From the inset image it can be clearly seen that the pagodas are layer by layer structures with a decreasing length from the bottom to the top, and each layer is not in the same plane but rather in a three-dimensional structure, which is different from the reported layered In2O3 nanotowers27 and ZnO towers.28 Then 10 min later, some layers of the pagodas began to grow into dendritic structures at a relative fast growth rate as shown by the marked arrow and form underdeveloped dendrites (Figure 6c). If the reaction time is prolonged to 20 min, the underdeveloped 3D superstructures as well as dendrites with hierarchical assemblies exist as shown in Figure 6d, indicating that self-assembly is still underway. Seen from the inset image, 6-fold snow-like dendrites are formed at the bottom, and at the top there are several layers of smaller dendrites formed with a slower growth rate, and the other layers have been dissolved under Ostwald ripening.14d When the reaction time is prolonged to 30 min, as shown in Figure 6e, the products are composed entirely of the well-assembled 3D dendritic superstructures. It can be concluded that the average size of the

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Figure 7. (a) TEM image and SAED pattern (inset) of the spheres obtained after 1 min, (b) TEM image and SAED pattern (inset), (c) HRTEM image of the pagodas obtained with 5 min, (d) TEM image, (e) HRTEM image of the tip of the dendrite (labeled e in panel d), (f) HRTEM image of the branch (labeled f in panel d) of the smaller dendrite selected from the products obtained 30 min later. The different FeNi3 structures were prepared at 180 °C.

products was increased and dendritic structures were gradually formed by prolonging the reaction time. However, after the reaction time is further prolonged to 60 min, there is a mixture of dendrites and multilayer flower-like superstructures (Figure 6f), which indicates a proper reaction time is very important for forming perfect dendritic structures. This is in accordance to the situation of snowflake-like cobalt microcrystals.14c To further understand the growth mechanism of the FeNi3 dendrites accurately, the TEM, HRTEM images and corresponding SAED spots of the three different structures obtained in the above time-dependent experiments (1, 5, and 30 min, respectively) are shown in Figure 7. Figure 7a shows the FeNi3 microspheres with tine-like structures on the surface, which was in agreement with the SEM image (Figure 6a), and the corresponding SAED spots revealed the obtained microspheres were polycrystallites (Figure 7a). Figure 7b showed the TEM image of the pagoda-like structure of FeNi3 crystals. The SAED spots (inset Figure 7b) projected along the [011j] zone axis were taken from the tip of the pagoda marked by the circle, and the elongated reciprocal lattice points observed clearly was probably due to a slight defect on the single crystal orientation as in the above-mentioned FeNi3 structures obtained at 140 °C and the silver nanodisks,29 and it can be further confirmed by the HRTEM observations (Figure 7c). From the HRTEM image (inset of Figure 7c, enlarged by the marked rectangle area) taken from the thinner tip as circled in Figure 7b, the stacking faults were clearly observed, which was the (111) stacking faults lying parallel to the (111) surface.30 The SAED spots (right-up inset of Figure 7d) were obtained from the trunk (marked by the black circle). The diffraction pattern exhibited six bright and sharp spots in a 6-fold symmetry corresponding to the {220} reflections of the fcc FeNi3 single crystals orientated to the [1j11] direction. The additional

Communications spots with a weak intensity are due to the 1/3(422) reflections, as observed in thin Au, Ag, or Pd crystals,31 which was similar to the branch’s SAED spots (left-down inset of Figure 7d) obtained from the area marked by the white circle. These indicated that the dendrite was a single crystal grown in a direction parallel to the {110} lattice planes, which was also supported by the HRTEM observations, in which the clear lattice fringes shown in Figure 7e,f, taken from the regions labeled e and f in Figure 7d, respectively, demonstrated the lattice structures of the trunk and the branch are same, and the entire dendrite is with a single-crystallites structure. The lattice spacing of the trunk and the branch is about 0.22 nm, which matches well with the interplanar spacing of 1/3{422) plane (Figure 7e,f). This result further proves that the preferential growth occurred parallel to the {110} lattice planes which is in good agreement with SAED results. Usually, the [111] HRTEM image of the fcc crystal is built by three sets of the {220} spacing with a 6-fold symmetry. For FeNi3 alloy, the {220} d spacing is 0.125 nm. Because this interfringe distance is beyond the resolution of the microscope, the [111] HRTEM image of the FeNi3 crystal will be not observable. However, for this sample, HRTEM images built by three sets of 3 × {422} spacing were frequently observed, and this indicated that there is a superlattice caused by a bigger periodicity than a regular [111] unit cell, which is similar to that in the Ag nanodisks.30 It is well known that usage of templates and/or shape modifier and controller32 can induce the formation of hierarchical and complex micro/nanostructures. In this work, FeNi3 3D superstructures are synthesized without using any template and surfactant; thus, it is supposed that the concentration of sodium hydroxide and metal salts coaffect the formation of the self-assembled 3D FeNi3 dendrites, which is confirmed by the experimental results in which perfect dendritic superstructures cannot be obtained at lower or higher concentrations of sodium hydroxide and metal salts. When Fe(III) and Ni(II) are treated with hydrazine hydrate in alkalescent medium, Fe(OH)3 are easily formed under the basic conditions. Ni(II) is readily reduced to Ni (0) particles for cooperation with hydrazine hydrate, which in turn reduces Fe(OH)3 to Fe (0) particles as surface catalyst.33 In our experiment, the molar ratio of Fe3+ to Ni2+ is equal to 1:3, and Fe3+ and Ni2+ can be completely reduced into Fe and Ni, resulting in the formation of fcc FeNi3 phase with AuCu3 type structure.6 Based on a precipitate slow-release controlled process,14c Fe3+ and Ni2+ can be slow-released from Fe(OH)3 and Ni(OH)2 by high temperature decomposition in hydrothermal conditions. As a result, the concentrations of Fe3+ and Ni2+ are maintained at a stable level, which is in favor of anisotropic growth.14a,25 Fractal growth is a nonequilibrium, diffusioncontrolled kinetic process, which has been demonstrated by the diffusion-limited aggregation (DLA)14,34 or oriented aggregation17c,23,24 mechanism in the literatures. On the basis of experimental results mentioned above, the possible formation processes for the 3D FeNi3 dendritic structure in our system can be illustrated by the DLA mechanism14 as shown in Figure 8. At an early reaction stage a lot of FeNi3 nuclei will be first formed and then fast nucleation of primary particles occurs under high temperature. These nuclei self-assembled into pagoda-like structures (Figure 6a)

Figure 8. Schematic illustration of the formation process of the 3D hierarchical FeNi3 dendritic superstructures.

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Figure 9. Room temperature magnetic hysteresis loops of different FeNi3 structures obtained at (a) 120 °C, (b) 140 °C, (c) 180 °C for 30 min. The up-left insets are the magnified hysteresis loops at low applied field (-200 Oe < H < 200 Oe) and the down-right insets are the corresponding images for each sample. Table 1. Magnetic Data of Obtained FeNi3 Structures at Room Temperature FeNi3 structures

Ms (emu/g)

Mr (emu/g)

Mr/Ms

Hc (Oe)

spheres pagodas dendrites

73.2 94.9 61.6

2.1 7.4 11.3

0.0286 0.0779 0.1834

41.8 94.4 145.6

followed by slow, controlled growth through diffusion of molecularscale species to the surfaces of existing nuclei because of the relatively strong stochastic diffusive force and then the pagodas continued to grow in the {111} direction (Figures 6b and 7b). Similar to larger particles growing at the expense of smaller particles by the diffusion of molecular-scale species through solution,35 the pagoda-like structures grow to form undeveloped dendritic hierarchical structures (Figure 6c) through coarsening, also known as Ostwald ripening.14d At this stage, the height of pagodas is not further increased, but their branches enlarge as observed from Figure 6d-f, which means the growth in the {111} directions is limited and that in the {110} directions predominantly occurs, resulting in the formation of dendritic structures. Finally, the underdeveloped dendrites are further assembled to form perfect 3D FeNi3 dendritic superstructures in certain directions. With the formation of the trunk, the secondary branches branch off parallel to each other and emerged at 60 angles with respect to the central trunk. With further growth, the tertiary family of branches grow along the directions parallel to trunk, emerging at 60 angles with respect to the secondary branches (Figure 6f-h), which is the same as the HgS dendrites.36 The whole formation process finished in a short time (about 30 min) without any shape modifier and controller. The magnetic properties of the as-synthesized FeNi3 structures, namely, spheres with a diameter of 1.5-2 µm, pagodas with a length of 2-2.5 µm (Figure S4a,b,Supporting Information), and dendrites with a length of 2.0-2.5 µm (Figure 1e), were investigated at room temperature using a vibrating sample magnetometer (VSM, BHV-55) with an applied field of -10 kOe < H < 10 kOe. The samples were first quantified carefully and then sealed into plastic tubes for the magnetization measurements. The hysteresis loop of the samples shown in Figure 9 revealed that the all of FeNi3 structures are ferromagnetic at room temperature. The up-left insets are the corresponding magnified hysteresis loops at low applied field and the down-right insets are the corresponding images of the different structures. The saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values for the each FeNi3 structure are shown in Table 1. The saturation magnetization value of the FeNi3 dendrites (61.6 emu/g) is lower than that of the spheres (73.2 emu/g), pagodas (94.9 emu/g), and our previous work of Fe-Ni alloy spheres (99 emu/g).11a Surface oxidation at grain boundaries and the large surface to-volume ratio may be the reason to cause a decrease in saturation magnetization.11a,37 On the other hand, compared to the coercivity value of FeNi3 spheres (41.8 Oe) and pagodas (94.4 Oe), the 3D FeNi3 dendritic structures exhibit an enhanced value (145.6 Oe) that is also higher than the previous reported value of FeNi3 particles (100 Oe6 and 87.12 Oe7). The

larger coercivity of the FeNi3 dendrites is considered to be attributed to the intrinsic large magnetocrystalline anisotropy of ordered intermetallic compound (compared with the disordered FeNi3 permalloy) and also mainly due to the effect of size, shape, and structure (compared with the FeNi3 particles).38 Certainly, these are not the only parameters that we have to consider. The composition, internal stress, and defects also influence this value. Our experimental results give a hint that the magnetic properties of FeNi3 dendrites may be explained considering crystalline anisotropy and shape anisotropy, as similar to the cobalt dendrites,14,39 which need to be studied further. In summary, we have successfully synthesized highly ordered metallic alloy FeNi3 dendritic superstructures via a facile and surfactant-free hydrothermal route in which soluble iron(III) chloride and nickel(II) chloride was employed to supply Fe and Ni sources. The results demonstrated that it is possible to control the morphology and structure of iron-nickel alloy FeNi3 dendrites by adjusting process parameters such as hydrothermal temperature, reaction time, concentrations of NaOH, and initial Fe3+. The formation mechanism of dendritic microcrystals was proposed. FeNi3 dendritic structures exhibited an enhanced coercivity compared with FeNi3 particles. The highly ordered hierarchical FeNi3 dendrites might have potential applications in electromagnetic wave absorbers and/or catalysts.

Acknowledgment. This work is supported by Science and Technological Fund of Anhui Province for Outstanding Youth (No. 08040106906), the National Natural Science Foundation (Nos. 20671002, 20490217) of P R China, the State Education Ministry (EYTP, SRF for ROCS, SRFDP 20070370001), and the Education Department (Nos. 2006KJ006TD, 2001KJ115ZD) of Anhui Province. Supporting Information Available: XRD patterns, size distributions, and EDX spectra of the products obtained with different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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