α-Fe2O3 Nanocrystals: Controllable SSA-Assisted Hydrothermal

Wenyan Yin, Xing Chen, Minghua Cao*, Changwen Hu* and Bingqing Wei*. The Institute for Chemical Physics, and Department of Chemistry, Beijing Institut...
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J. Phys. Chem. C 2009, 113, 15897–15903

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r-Fe2O3 Nanocrystals: Controllable SSA-Assisted Hydrothermal Synthesis, Growth Mechanism, and Magnetic Properties Wenyan Yin,† Xing Chen,‡ Minghua Cao,*,† Changwen Hu,*,† and Bingqing Wei*,§ The Institute for Chemical Physics, and Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China, and Department of Physics and Astronomy and Department of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: July 23, 2009

R-Fe2O3 nanocrystals with different morphologies were successfully synthesized by a facile 5-sulfosalicylic acid dihydrate (SSA)-assisted hydrothermal process, in the absence of any template or surfactant in the system. The chelating ligand SSA, pH values of Fe-SSA complex solution, and alkaline reagents play important roles in the morphological control of the R-Fe2O3 nanostructures. Magnetic hysteresis measurements reveal that the size of the nanostructures has a remarkable effect on the magnetic properties at room temperature. A possible formation mechanism for controllable synthesis of these R-Fe2O3 nanocrystals was proposed. In addition, the successful synthesis of honeycomb-like hierarchical Ni(OH)2 structures and Co3O4 microspheres in our experiments gave further evidence for the wide applicability of the SSA-assisted hydrothermal synthesis process. The results suggest that this process may be a convenient and effective approach to controllable synthesis of a variety metal oxides and hydrates nanostructures. 1. Introduction It is well-known that the properties of inorganic nanomaterials extensively depend on their size and shape.1,2 As a consequence, the synthesis of inorganic nanomaterials with well-controlled shape and size has attracted considerable interest in recent years,3-5 while developing new controlled methodologies for the synthesis of inorganic nanomaterials, and researching the related novel properties has received particular focus. Hematite (R-Fe2O3), an important transition magnetic metal oxide with n-type semiconducting properties (Eg ) 2.1 eV) under ambient conditions, has received increasing attention due to its extensive applications, such as pollution treatment,6 catalysts,7 pigments,8 active materials in lithium ion batteries,9 gas sensors, and magnetism.10 Up to now, R-Fe2O3 with different morphologies and sizes such as nanowires,11,12 nanobelts,13 nanotubes14,15 nanoparticles,16 nanocubes,17,18 nanoflowers,19 hollow spheres,20 micropines,21 nano/microrings,22 nanorods,23 and so on have been prepared via vapor-phase methods (including physical and chemical processes), template methods (“soft” template: reverse micelles; “hard” template: substrate), thermal decomposition of organometallic compounds, and a series of wet chemical methods (including coprecipitation, sol-gel, solvothermal method, hydrothermal process, and so on). Among these methods, the vapor-phase methods generally require expensive and highly sophisticated equipment, rigorous vacuum condition, and high reaction temperatures (600-1100 °C). Sometimes the utilization of toxic metal-organic compounds and catalysts cannot be avoided. The disadvantage of template methods is that the presence of surfactant or substrate is difficult to remove from the product, easily resulting in the formation of byproducts or impurities, which may limit their applications. The wet chemical * To whom correspondence should be addressed. E-mail: [email protected], [email protected], and [email protected]. † Beijing Institute of Technology. ‡ Department of Physics and Astronomy, University of Delaware. § Department of Mechanical Engineering, University of Delaware.

methods mentioned above are attractive as they avoid highly sophisticated equipment and high reaction temperature. Nonetheless, the reported studies often employ environmentally harmful long-chain organic solvents or surfactants to control the size and morphology. With growing emphasis on green chemistry principle, it is becoming increasingly important to develop more environmentally friendly, simple, and effective methods for the synthesis of R-Fe2O3 nanostructures. The hydrothermal process in all the wet chemical methods mentioned above is often used and appears to have some advantages, including mild synthetic conditions, simple manipulation, and good crystallization of the products. For example, Yu’s group24 recently reported template-free hydrothermal synthesis of hematite microrings. In view of the above, our objective is to develop a facile hydrothermal synthesis strategy without any template or surfactant to synthesize R-Fe2O3 nanostructures with controllable morphologies and sizes. In this paper, we reported a facile, “green”, 5-sulfosalicylic acid dihydrate (SSA)-assisted hydrothermal process for controllable synthesis of R-Fe2O3 nanocrystals with different morphologies. SSA contains three potential coordinating groups: -COOH, -SO3H, and -OH (Figure 4a), one of the most powerful ligands with rich coordination chemistry.25 However, to our knowledge, there is no previous report on the dissociation of Fe-SSA complex precursor with suitable pH values for the synthesis of R-Fe2O3 nanocrystals in a hydrothermal process. The experimental results indicated that experimental parameters, such as chelating ligand SSA, pH values of Fe-SSA complex solution, and alkaline reagents (i.e., ammonia or sodium hydroxide aqueous solution), play important roles in the morphological control of the R-Fe2O3 nanostructures. A possible formation mechanism for R-Fe2O3 nanocrystals with different morphologies was proposed in detail. The magnetic properties of R-Fe2O3 nanocrystals (nanoaggregates, nanospheres, and nanorhombohedra), which show different magnetic behavior, were also reported. It is found that the particle size has a remarkable effect

10.1021/jp904413m CCC: $40.75  2009 American Chemical Society Published on Web 08/14/2009

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Figure 1. XRD patterns of as-prepared R-Fe2O3 samples: (a) nanoaggregates, (b) nanospheres, and (c) nanorhombohedra as described in Table 1.

on the magnetic properties. In addition, as reported in ref 26, SSA can coordinate with other metal ions, such as Al(III), Ni(II), Co(III), Fe(III), Cu(II), and so on. The successful synthesis of honeycomb-like hierarchical Ni(OH)2 and microsphere-like Co3O4 structures in our experiments gave further evidence for the wide applicability of the SSA-assisted hydrothermal route. Therefore, the SSA-assisted hydrothermal route is expected to have advantages for the synthesis of other metal oxides and hydrates with different morphologies and sizes, which is of paramount importance for future property exploration and applications. 2. Experimental Section All reagents used were of analytical purity and were used directly without further purification. Initially, 0.108 g (0.4 mmol) of ferric chloride hexahydrate (FeCl3 · 6H2O) was dissolved into 5 mL of distilled water to form a transparent yellow solution. Then 0.104 g (0.4 mmol) of 5-sulfosalicylic acid dihydrate (SSA, C7H6O6S · 2H2O) was added to the above solution. The color of the Fe3+ cations changed from yellow to modena. The phenomenon indicated that Fe3+ cations reacted with SSA to form Fe-SSA complex solution. The pH value of the solution was about 1.58. To investigate the effect of the pH value on the complex solution, 2.0 M ammonia (NH3 · H2O) was added dropwise to the above solution. Subsequently, distilled water was added to the solution to keep the total volume of the complex at 30 mL. The final pH value of the complex solution was controlled from 3.1 to 9.7 by using ammonia. After being stirred for 5 min, the solution was transferred and sealed in an 80 mL stainless Teflon-lined autoclave, heated at 180 °C for 10 h, and cooled to room temperature naturally. The modena color of the solution disappeared completely after the hydrothermal reaction. The as-prepared red precipitates were collected by centrifugation, washed several times with distilled water and absolute ethanol, and dried at 40 °C in a vacuum oven. The as-synthesized samples were characterized with X-ray powder diffraction (XRD) patterns, which were recorded on a PHILIPS XRG-3100 X-ray diffractometer with Cu KR radiation (λ ) 1.54056 Å) at 40 kV and 30 mA. Field emission scanning electron microscopy (FE-SEM) images were obtained on a JSM6700F field emission microscope. Transmission electron microscopy (TEM) images were captured by using a JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. The Fourier transform infrared spectra (FT-IR) were recorded with a Nicolet 170SXFT/IR spectrometer, using KBr pressed wafers to test the chemical bondings of the products. The XPS measurements were carried out on a VG ESCA-

3. Results and Discussion 3.1. XRD and XPS Analyses of As-Prepared Samples. Samples obtained via the dissociation of Fe-SSA complex aqueous solution with different pH values at 180 °C for 10 h under hydrothermal conditions have the same phase structure and are listed in Table 1. Figure 1 shows the XRD patterns of R-Fe2O3 nanocrystal representatives with three typical morphologies: (a) nanoaggregates (sample 1), (b) nanospheres (sample 2), and (c) nanorhombohedra (sample 3). All diffraction peaks can be readily indexed to the pure rhombohedral R-Fe2O3 phase with lattice parameters a ) 5.036 Å and c ) 13.74 Å, which are in good agreement with the literature values (JCPDS No. 33-0664). The half-width of the peaks in panel a of Figure 1a is wider than that in panels b and c, indicating that the mean crystalline size of the R-Fe2O3 nanoaggregates is obviously smaller than that of the other two morphologies. This is attributed to the fact that the R-Fe2O3 nanoaggregates consisted of nanoclusters with an average diameter of 10 nm, as evidenced from TEM image in Figure 3d, which is much smaller than that of the nanospheres and nanorhombohedra as shown in Table 1. To further ascertain the purity and the phase of the final products, XPS spectra of the three samples (a) nanoaggregates, (b) nanospheres, and (c) nanorhombohedra were measured (Figure 2). The survey spectra (Figure 2A) show the presence of Fe, O, and C. The existence of the C1s peak may be caused by the residual solvent absorbing on the surface of the samples. The centers of electron-binding energy of Fe(2p)3/2 and Fe(2p)1/2 core levels appeared at the binding energies of 711.0 and 725.2 eV (Figure 2B), consistent with the literature values of 710.8 and 724.8 eV for the bulk R-Fe2O3.27 Also, the corresponding satellite peak at 718.4 eV can be solely attributed to the presence of Fe3+ ions of R-Fe2O3, as the binding-energy values are too high to be other oxide species of iron.28,29 In addition, the O1s core levels show the dominant oxide peaks at around 529.0 eV, which are in good agreement with the literature values of R-Fe2O3.29 The Fe2p and O1s core levels indicate that the valence states of elements Fe and O are +3 and -2, respectively. So, combining this result with that of XRD we can make sure that the products are pure R-Fe2O3 and no other impurities such as Fe3O4 or γ- Fe2O3 were detected. 3.2. FE-SEM and TEM Images of As-Prepared r-Fe2O3 Samples. The morphology and size of the as-prepared R-Fe2O3 samples were characterized by FE-SEM and TEM. When ammonium hydroxide was used to adjust the pH values of the Fe-SSA complex aqueous solution, sample 1, 2, and 3 were obtained. Figure 3a shows the FE-SEM image of sample 1 obtained with the pH value of the Fe-SSA complex aqueous solution of 4.0. As can be seen from Figure 3a, a large amount of R-Fe2O3 nanoaggregates with diameters ranging from 50 to 100 nm is observed. To further study the fine structure of the above Fe2O3 nanoparticles, TEM was performed. As shown in Figure 3d, the nanoaggregates were formed by the self-assembly of many R-Fe2O3 nanoclusters with an average diameter of 10 nm. Figure 3e clearly shows a magnified TEM image of a single self-assembly nanoaggregate with a diameter of 95 nm. Figure 3f shows the HRTEM image of the white pane in Figure 3e, all particles attached and having identical lattice fringes with d spacing of 0.254 nm, which is in agreement

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TABLE 1: Summary of the Main Results on the Products Obtained under Different pH Values and Textural Characterization reaction conditionsa sample

phase structures

pH value

1

R-Fe2O3

2.77- 4.10

ammonia

2 3 4 5 6 7

R-Fe2O3 R-Fe2O3 R-Fe2O3 R-Fe2O3 R-Fe2O3 R-Fe2O3

5.86-6.55 7.50-9.00

ammonia ammonia none (30 mL of distilled water) NaOH NaOH NaOH

a

3.10 5.86 7.98

alkaline reagents

morphologies and sizes nanoaggregates with diameters 50-100 nm consisting of primary nanoparticles with a diameter of 10 nm nanospheres with diameters of 50-110 nm nanorhombohedra with diameters of 60-120 nm nanocubes with edge lengths of 50-70 nm irregular particles nanoparticles with diameters of 80-120 nm plate-like structures

Reaction conditions: t ) 10 h, T ) 180 °C.

Figure 2. (A) XPS survey scan spectra and (B) Fe2p spectra of the samples: (a) nanoaggregates, (b) nanospheres, and (c) nanorhombohedra.

with the {110} crystal facet of rhombohedral R-Fe2O3, indicating that these individual nanocluster units are single crystal structures. Also, fast Fourier transform (FFT) analysis of selected regions reveals details of the local R-Fe2O3 structure. The inset to Figure 3f is the corresponding FFT diffraction pattern of a single nanocluster in panel f, which can be indexed to single crystal structure of these individual R-Fe2O3 nanocluster units. However, when the pH value was increased to 5.86, R-Fe2O3 nanospheres with sizes ranging from 50 to 110 nm (sample 2) were formed (Figure 3b). The TEM image (Figure 3g) further disclosed that most of the particles are nanospheres, while a few are nanoellipses in the final product. However, the surface of the nanospheres is not so smooth with some hollow. This morphological feature is clearly demonstrated in the inset in Figure 3g by a magnified image of a representative single nanosphere, which is completely different from the surface of R-Fe2O3 nanoaggregates (Figures 3d-f). Figure 3h shows a HRTEM image of a typical nanosphere of Figure 3g. The lattice planes spacing is 0.254 nm, which corresponds to the {110} crystal planes of rhombohedral R-Fe2O3. Figure 3i shows the FFT diffraction pattern of Figure 3h, indicating the single crystalline nature of the R-Fe2O3 nanosphere. With the pH value further increasing to 7.98, a number of R-Fe2O3 nanorhombohedra with sizes ranging from 60 to 120 nm were obtained as shown in the FE-SEM image in Figure 3c (sample 3). The lowmagnification TEM image in Figure 3j was in good agreement with the morphology as presented in the SEM picture. Figure 3k is a magnified TEM image of a typical nanorhombohedron 60 nm in diameter, which reveals that the surface of the nanorhombohedron is rough. Figure 3l shows a HRTEM image of a quarter of an individual R-Fe2O3 nanorhombohedron. The lattice plane spacing between the adjacent lattice planes perpendicular to the preferential growth direction (marked with arrows) is 0.254 nm, consistent with the {110} d-spacing of rhombohedral R-Fe2O3. The structure of the nanorhombohedron exhibits well-defined lattice fringes and has a single crystalline

nature, and no amorphous layer is found on the surface. The experimental results indicated that R-Fe2O3 nanocrystals with different morphologies and sizes could be controllably synthesized by using SSA as a complex reagent with different pH values if ammonia was used to adjust the pH value. 3.3. Possible Formation Mechanism, Effects of Reaction Conditions on the Morphology, and Size of r-Fe2O3 Nanocrystals. To highlight the influence of SSA on the size and morphology of the R-Fe2O3 sample, a reference experiment was carried out by reacting FeCl3 aqueous solution under hydrothermal conditions without SSA and ammonia, and only R-Fe2O3 nanocubes with edge lengths ranging from 50 to 70 nm were observed (Figure 7a, sample 4). As described above, the SSA ligand is crucially important for the control of nucleation and growth as well as the crystal shape of the obtained nanocrystals. It has been reported that SSA, a strong coordinating reagent, could form three kinds of Fe-SSA complexes by coordinating with Fe3+ cations with different pH values, accompanied by a color change: modena for [Fe(SSA)]+, brown for [Fe(SSA)2]-, and red for [Fe(SSA)3]3-.30 The above process can be generally formulated as follows:

It can be seen from reaction 1 that the amount of n is mainly dependent on the pH value of the aqueous solution. In our experiments, when SSA was added to yellow FeCl3 aqueous solution, a color change from yellow to modena was observed (pH 1.27), indicating the formation of [Fe(SSA)]+, as shown in Figure 4, panels b and c. The modena solution could be maintained until its pH value was increased to 4.10 by using 2.0 M ammonia. If this modena solution is used as precursor, an optimum pH range from 2.77 to 4.10 was obtained for the

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Figure 3. Typical FE-SEM and TEM images of three R-Fe2O3 nanocrystals prepared at different pH values of the Fe-SSA complex precursor as described in Table 1: (a, d, e, f) nanoaggregates (pH 4.0), (b, g, h) nanospheres (pH 5.86), (c, j, k, l) nanorhombohedra (pH 7.98). The inset in panel f is the corresponding FFT diffraction pattern of panel f. (i) The corresponding FFT diffraction pattern of panel h.

Figure 4. (a) Chemical structure of the SSA used; vials b-e show the photographs of the color change of the Fe-SSA complex solution obtained with different pH values with the addition of 2.0 M ammonia: (b) the yellow-colored of FeCl3 solution; (c) the modena [Fe(SSA)]+ complex (pH 1.27-3.16); (d) the brown [Fe(SSA)3]3- (pH 4.10-6.55); and (e) the red [Fe(SSA)2]- (pH 7.50-9.00).

controllable synthesis of R-Fe2O3 nanoaggregates (Figure 3a, sample 1), while no precipitates were formed when the pH value of the modena solution was lower than 2.77. When the pH value of the complex solution was increased to higher than 4.10 still with 2.0 M ammonia, the color of the complex solution changed from modena to brown (Figure 4d), indicating the formation of [Fe(SSA)2]- (reaction 1, n ) 2). This brown solution could be kept constant with a pH value between 4.10 and 6.55. In the pH value range of 5.86-6.55, R-Fe2O3 nanospheres can be

obtained (Figure 3b, sample 2). With the pH value further increasing to 7.50, red [Fe(SSA)3]3- complex (reaction 1, n ) 3) was obtained (Figure 4e). The optimum pH value range to obtain R-Fe2O3 nanorhombohedra is from 7.50 to 9.00 (Figure 3c, sample 3). When the 2.0 M ammonia was further added dropwise to the red complex solution, the Fe(OH)3 precipitate was observed and the transition point of the pH value from solution to precipitate is near 10.0. The transparent Fe-SSA complex solution provides a facile and “green” environment for the growth of high-quality R-Fe2O3 nanocrystals. In principle, the crystal growth process in solution mainly consists of nucleation and growth, which are affected by the intrinsic crystal structure and the external conditions including the kinetic energy barrier, temperature, time, capping molecules, and so forth. The formation of nanostructures after nucleation in solution relates to two primary mechanisms: the aggregation growth process and the Ostwald ripening process. Crystal growth by aggregation can occur by random aggregation and oriented aggregation, and the Ostwald ripening process involves the growth of larger crystals at the expense of smaller ones.31 In our experiments, R-Fe2O3 formation proceeds probably through two steps: First, the Fe-SSA complex precursor and

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Figure 5. (a) XRD pattern and (b) FE-SEM image of the sample synthesized at 180 °C for 5 min at pH 4.10 via SSA-assisted hydrothermal processes.

Figure 6. Schematic illustration of the possible mechanism for the formation of the R-Fe2O3 nanocrystals: (1-2-3) nucleation-aggregation-Ostwald ripening growth for nanoaggregates; (1-2-3′-4) nucleation-aggregation-recrystallization-aggregation growth (coarsening aggregation and oriented aggregation) for nanospheres, and nanorhombohedra.

ammonia slowly dissociate into Fe3+ cations and OH- ions under hydrothermal conditions (reactions 2 and 3), respectively. Second, Fe3+ cations and OH- ions immediately react to form R-Fe2O3, not FeOOH or Fe(OH)3 (reaction 4). This whole growth process might be controlled by reaction 2 because of its weak dissociation tendency. This conclusion and reaction 4 have been proved by an experiment of reaction time of as short as 5 min. When the reaction time was decreased to as short as 5 min while the other reaction conditions are kept the same as those for sample 1 in Table 1, only a little R-Fe2O3 (Figure 5a) precipitate with a spherical diameter of 10-50 nm (Figure 5b) was obtained when the reaction is stopped, and no other phases, such as FeOOH or Fe(OH)3, were observed. Moreover, the color of the final solution was still modena, further confirming the slow dissociation process of Fe-SSA complex precursor. While in our system we have used three Fe-SSA complex precursors with a thermal stability sequence of [Fe(SSA)]+ > [Fe(SSA)2]> [Fe(SSA)3]3-, which can be proved by the stability constants,32,33 and three different R-Fe2O3 nanostructures were obtained, this result indicates once again that the Fe-SSA complex precursor is responsible for the formation of final nanostructures.

On the basis of the above investigation, a feasible formation mechanism of different R-Fe2O3 nanostructures was proposed as shown in Figure 6. At a lower pH value (pH 4.10), the relatively stable Fe-SSA complex [Fe(SSA)]+ led to the slow release of Fe3+ cations in aqueous solution; the nucleation rate is also relatively slow and the as-obtained R-Fe2O3 nuclei

Figure 7. FE-SEM images of R-Fe2O3 samples (a) obtained by only FeCl3 aqueous solution dissociating without the addition of SSA and ammonia (sample 4 in Table 1) and (b-d) obtained in the presence of 1.0 M NaOH aqueous solution used to adjust the pH value of Fe-SSA complex to 3.10, 5.86, and 7.98 as described in Table 1 from samples 5, 6, and 7, respectively.

are small. Once the small R-Fe2O3 nuclei are formed, they are active because of their high surface energy and tend to aggregate growth (Figure 6, step 2, primary aggregation), leading to the formation of larger aggregates to minimize the surface energies, which is similar to the formation of SnO2 nanorods.34 After the primary aggregation, the larger R-Fe2O3 self-aggregates were formed by aggregation of the smaller primary R-Fe2O3 particles at lower pH value, which were probably driven by the Ostwald ripening process (Figure 6, step 3). On the basis of the Ostwald ripening mechanism, the surfaces of these spheres are not particularly smooth. This also implies that the growth rates of

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various directions are almost the same at lower pH values. With the increase of the pH value (i.e., pH 5.86, 7.98), the poorer stability of the Fe-SSA complex leads to the relatively rapid release speed of Fe3+ ions in the solution, as a result of which nucleation occurred rapidly and large quantities of R-Fe2O3 primary nuclei were formed in the solution. In this case, SSA acts as not only a chelating ligand to kinetically control the reaction rates but also a surface capping reagent interfering with the nanocrystal growth. The capping role of SSA on R-Fe2O3 surfaces was confirmed by FT-IR spectra of the R-Fe2O3 sample (see Figure S1 in the Supporting Information). The formed R-Fe2O3 primary nanoparticles rotated and rearranged to find a place to minimize the surface energy (ordered primary aggregation, Figure 6, step 2), followed by further recrystallization to form compact bulk crystals through fast simultaneous coarsening aggregation growth for the nanosphere R-Fe2O3, and oriented attachment aggregation growth for the R-Fe2O3 nanorhombohedra because the SSA may coordinate selectively to the R-Fe2O3 crystal, hindering the growth of certain crystal surfaces (Figure 6, step 3′ and 4). Especially, the process for the formation of nanorhombohedra involves arrangement of primary nanoparticles into an isooriented crystal via oriented attachment, which can form a single crystal upon fusion of the nanoparticles as reported in ref 35. Also, the anisotropy growth mechanism of the R-Fe2O3 crystals at a higher pH value (pH 7.98) is similar with that of ref 36. The whole process is illustrated in Figure 6. In our experiments, ammonia was used to provide OH- ions through gradual thermohydrolysis as shown in reaction 3. To further investigate the influence of alkaline source on the morphology and size of the final product, ammonia was replaced by NaOH aqueous solution while other reaction conditions were kept constant as described in Table 1 from samples 5 to 7. When 1.0 M NaOH was used to adjust the pH value of the Fe-SSA complex solution to 3.10, irregular R-Fe2O3 particles instead of regular sphere-like aggregates were generated as revealed by the SEM image in Figure 7b (sample 5). When the pH value was increased to 5.86, the as-obtained R-Fe2O3 sample consisted of “loose” nanoparticles with diameters ranging from 80 to 120 nm instead of the “compact” nanospheres shown in Figure 3b. The surface of the nanoparticles is rough (Figure 7c, sample 6). With the pH value further increasing to 7.98, R-Fe2O3 platelike structures with diameters ranging from 100 to 130 nm were obtained (Figure 7d, sample 7). This result showed that the ammonia, which has a slower release rate to generate NH4+ and OH- compared with that of the NaOH aqueous solution, plays an important role in the Fe-SSA complex system. Therefore, the existence of the ammonia or NaOH as an alkaline source to control the pH value of the reaction system will influence the growth rate of different crystal directions and finally influence the morphologies of products. 3.5. Synthesis of Ni(OH)2 and Co3O4 via the SSA-Assisted Hydrothermal Route. The SSA-assisted hydrothermal route is expected to have advantages for the preparation of other metal oxides and hydrates at a suitable pH value. To prove the wide applicability of this route, two additional experiments were performed for the synthesis of Ni(OH)2 and Co3O4. The process is similar with the synthesis of R-Fe2O3 nanocrystals shown in the Experimental Section. The R-Ni(OH)2 sample with two sizes is well-dispersed and has a honeycomb-like hierarchical structure, which was assembled by nanoplates (see Figure S2a,b in the Supporting Information). The Co3O4 sample has a microsphere structure with a rough surface consisting of nanoparticles

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Figure 8. Magnetic hysteresis loops of the R-Fe2O3: (a) nanoaggregates, (b) nanospheres, and (c) nanorhombohedra at room temperature; The inset in panel c shows the expended view of the panel.

with an average size of 50 nm (see Figure S2c-e in the Supporting Information). 3.6. Magnetic Properties. It is well-known that the magnetic properties of materials have been believed to be highly dependent on many factors, such as the morphology, size, and crystal structure (including impurities or substitutions) of the as-synthesized sample.23,37,38 Nanoscaled R-Fe2O3 often exhibits unusual magnetic behaviors, which are different from those of the bulk sample, owing to the shape- and size-dependent effects.39 Therefore, the magnetic property measurements of the as-synthesized R-Fe2O3 nanocrystals were investigated. Figure 8 shows the magnetic hysteresis loops of these samples at room temperature in the applied magnetic field sweeping from -10 to 10 kOe. It can be seen that saturation magnetization for all three samples is not observed in the magnetic field, which is similar to the reported cases in the literature, indicating the disordered surface spin.40 The magnetization values of the three samples are almost the same and are 0.48 emu/g for nanoaggregates, 0.51 emu/g for nanospheres, and 0.49 emu/g for nanorhombohedra, respectively. In addition, the three nanoscaled R-Fe2O3 samples have significantly different magnetic behaviors, although they all exhibit a slight hysteresis feature. For R-Fe2O3 nanoaggregates (Figure 8a), it displays a relatively bigger hysteresis with coercivity (Hc) of 203.76 Oe, which is higher than that of nanospheres (141.59 Oe, Figure 8b). The remnant magnetization (Mr) of nanoaggregates is 0.09 emu/g, which is almost near the Mr value of nanospheres (0.085 emu/g). However, for nanorhombohedra, it exhibits an extremely small coercivity and weak hysteresis loop behavior (Figure 8c and inset) compared with the other two samples. The Hc is 26.1 Oe and Mr is 0.05 emu/g. The difference in Hc values among the three samples in the present study must be related to the differences in shape, size, and amount of defects of the samples. Moreover, the values of the Mr and Hc are also different from those reported for R-Fe2O3 nanostructures.19,23,41,42 According to the XRD pattern (Figure 1) and FE-SEM and TEM images (Figure 3), it is clearly seen that the diameter of nanoaggregates was smaller than that of two other samples (nanospheres and nanorhombohedra). The evident increases of the diameters from (a) nanoaggregates, (b) to nanospheres, to (c) nanorhombohedra can be clearly seen from the FE-SEM and TEM images. The detail diameter data from nanoaggregates, to nanospheres, to

R-Fe2O3 Nanocrystals nanorhombohedra are summarized as shown in Table 1. The assembled nanoaggregates in fact consisted of many primary nanoparticles with a diameter of 10 nm, which makes the number of subparticles increase, resulting in the enhancement in Hc. Therefore, it can be concluded that the Hc value decreased as the size of the corresponding R-Fe2O3 nanostructures increased. This is because the reduced size of the nanostructures increases surface disorder spins, leading to the higher Hc values. For the R-Fe2O3 nanorhombohedra, as reported in the literature, it exhibits an extremely small coercivity and weak hysteresis loop, which may be the result of its larger size.42 Comparing the Mr and Hc values in our case with those in ref 42, the Mr is somewhat higher while the Hc is much lower than those of other R-Fe2O3 nanorhombohedra. The shape anisotropy combining with the surface capping reagents SSA on the surface of the nanoscaled R-Fe2O3 rhombohedra may be another reason for the extremely small coercivity and weak hysteresis loop. Further work should be done to clarify the difference of the magnetic properties and influencing factors in these R-Fe2O3 nanocrystals. 4. Conclusions R-Fe2O3 nanocrystals with different morphologies were successfully synthesized by using a mild SSA-assisted hydrothermal process. Various experimental parameters, such as chelating ligand SSA, pH values of the Fe-SSA complex solution, and the alkaline reagents (i.e., ammonia or sodium hydroxide), played important roles in the morphological control of the R-Fe2O3 nanostructures. A possible growth mechanism for the different R-Fe2O3 nanocrystals was proposed. In addition, honeycomb-like hierarchical R-Ni(OH)2, microsphere Co3O4 structures were also synthesized by using the SSA-assisted hydrothermal process. The results suggest that this process may be a convenient and effective approach to controllable synthesis of other metal oxides and hydrates nanostructures. In addition, the influence of size on the magnetic hysteresis performances of three different R-Fe2O3 nanocrystals was also studied. Acknowledgment. This work was supported by the State Scholarship Fund of China Scholarship Council (CSC, File No. 2008603051), the 111 Project (B07012), the Natural Science Foundation of China (NSFC, Nos. 20671011, 20731002, 10876002, 20771022, and 20871016), Key Laboratory of Structural Chemistry Foundation (KLSCF, No. 060017), Excellent Young Scholars Research Fund of the Beijing Institute of Technology (No. 2006Y0715), Basic Research Fund of the Beijing Institute of Technology (Nos. 20060742022 and 20070742010), and the Program for New Century Excellent Talents in University. We also thank the Fengyun Cui for FTIR measurements and Prof. Xiao John and Qiang Gao for fruitful discussions of the formation mechanism and magnetic properties. Supporting Information Available: FT-IR spectra of R-Fe2O3 samples and experiment details, XRD, and SEM of as-synthesized R-Ni(OH)2 and Co3O4 samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lim, B.; Jiang, M. J.; Tao, J.; Pedro, H. C.; Camargo; Zhu, Y. M.; Xia, Y. N. AdV. Funct. Mater. 2009, 18, 1.

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