3551
2007, 111, 3551-3554 Published on Web 02/13/2007
Synthesis and Magnetic Properties of Uniform Hematite Nanocubes Shang-Bing Wang, Yu-Lin Min, and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, the School of Chemistry & Materials, UniVersity of Science and Technology of China, Hefei 230026, P R China ReceiVed: December 16, 2006; In Final Form: January 27, 2007
Hematite nanocubes with a broad size distribution have been synthesized by the decomposition of an ironoleate complex under hydrothermal conditions, which was prepared from the reaction of iron chloride and sodium oleate. Well-defined assembly of uniform nanocubes with an average size of 15 nm can be obtained after a simple size-selection process. The molar ratio of iron chloride to sodium oleate and hydrothermal temperature had a crucial influence on the morphology of hematite nanopaticles in the synthesis. The blocking temperature of hematite nanocubes was ca. 80 K with an external field of 100 Oe, and they were superparamagnetic at room temperature. A cyclohexane-dispersed colloidal solution of hematite nanocubes showed a strong shape-dependent adsorption peak at 230 nm in the UV-vis spectrum. This synthesis approach is expected to become a general method for the preparation of other uniform nanoparticles with unique shapes such as cobalt and nickel oxides and so on.
Introduction Magnetic nanoparticles have extensive applications in many fields including magnetic storage,1a magnetic sealing,2 ferrofluid technology and magnetocaloric refrigeration,3 biomedicine and biotechnology,4 and catalysis.5c Meanwhile, self-organization of uniform magnetic nanoparticles in electronic devices appears as a bottom-up alternative to present lithography techniques for the fabrication of nanodevices in microelectronics and for the magnetic-storage industry.1c Furthermore, monodisperse magnetic nanoparticles with various morphologies are of importance for fundamental studies of light scattering, particle interaction, and electrophoresis, and so on.6 In recent years, high-temperature solution-phase synthesis has been well developed and great achievements have been made in the preparation of high-quality magnetic nanopaticles.1,5,7-15 Generally, these syntheses were usually carried out at high temperatures (150-400 °C) by decomposition of organometallic precursors in organic media (oleic acid) with or without an inert atmosphere, and various monodisperse and highly uniform magnetic nanocrystals such as Fe,5b,10 γ-Fe2O3,5b,e,f,7a,13,14 Fe3O4,1f,5b,f,14 Ni/NiO,5c,12 FePt,1b,c,7b Pt@Fe2O3 core-shell nanoparticles,7b and CoFe2O4 15 have been prepared. Considering the significant effect of shape on the properties of nanocrystals and their assemblies, synthesis of nanocrystals with well-defined shape and uniform size has been a challenging issue. Until now, only a few reports concern the synthesis of hematite nanoparticles;16-20 however, the uniformity of hematite nanocrystal assemblies obtained in those reports was relatively poor in terms of shape and size. To the best of our knowledge, this is the first report on the synthesis of uniform hematite nanocubes and their assemblies. Herein, we report a simple * Corresponding author. Fax: + 86 551 3603040. E-mail: shyu@ ustc.edu.cn.
10.1021/jp068647e CCC: $37.00
Figure 1. XRD pattern of the as-synthesized product.
hydrothermal method to prepare uniform oleic acid-capped hematite nanocubes by the use of a cheap and nontoxic reagent, FeCl3‚6H2O as an Fe source, and oleic acid as a capping agent. Iron chloride (analytical grade purity, AR), sodium oleate (chemical grade purirty, CP), and oleic acid (CP) were purchased from Shanghai Chemical Reagent Company and used without further purification. Deionized water was used (18.2 Ωcm-1, Millipore simplicity 185 types). In a typical synthetic procedure for hematite nanocubes, 1.6 g of sodium oleate and a mixture solution composed of 8 mL of ethanol and 1 mL of oleic acid were added subsequently to an aqueous solution (16 mL) with 0.48 g of iron chloride (FeCl3‚6H2O) in a Teflon-lined stainless autoclave with a capacity of 30 mL under stirring at room temperature. After 2 h, a rufous organic layer appeared on the upper part of the reaction solution. The autoclave was sealed and treated at 180 °C for 12 h and then cooled to room temperature. The precipitate was collected by centrifugation and washed with ethanol. The precipitate (ca. 0.11 g) was obtained after drying at room temperature, which mainly contained © 2007 American Chemical Society
3552 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Letters
Figure 2. (A) General view of hematite nanocubes. The inset shows the sample photograph; (B) a typical self-assembled monolayer of hematite nanocubes. Inset: HRTEM image of single nanocube, bar 8 nm.
hematite cubes with a broad-size distribution (see Supporting Information Figure S1). To obtain uniform hematite nanocubes with a narrow size distribution, a simple size-selecting process was performed. The as-synthesized product was dispersed in a certain amount of cyclohexane (10 mL) and centrifugated at a high speed (8000 rpm) to remove any insoluble precipitate, the red and transparent colloidal solution of hematite nanocubes was collected, and about 5 mg of uniform hematite nanocubes can be obtained. These nanocubes could be redeposited by adding ethanol. Also, they could be redispersed in various nonpolar solvents such as chloroform and hexane. The powder X-ray diffraction (XRD) was performed on a Philips X’pert PRO X-ray diffractometer (Cu KR radiation, λ ) 1.54178 Å). TEM (transmission electron microscopy) and high-resolution TEM (HRTEM) analyses were carried out with JEOL-2010 TEM equipment operating at 200 kV. The samples for TEM analysis were prepared by dropping one or two droplets of cyclohexane dispersion of hematite nanoparticles on the carbon-coated copper grids and drying at room temperature. UV-vis absorption spectra were obtained using a UV-2550 spectrometer (Shimadzu). FTIR spectra were conducted on an EQUINOX55 Vector 22 spectrometer (bruker) at room temperature. The magnetic properties of samples were studied using a superconducting quantum interface device (SQUID) magnetometer (Quantum Design MPMS XL). The samples were obtained by dropping the cyclohexane dispersion of hematite nanocubes on a 4 × 4 mm2 Si(100) wafer and dried at 60 °C for 4 h. The temperature-dependent magnetization was studied in an applied magnetic field of 100 Oe from 10 to 300 K using a zero-field cooling (ZFC) and field cooling (FC) procedure. The XRD pattern of the as-synthesized product is in good agreement with that of hematite in literature (JCPDS card no. 33-664), confirming that the nanocubes are hematite (Figure 1). The nanocubes are highly uniform in terms of size and shape, and their cyclohexane-dispersed colloidal solution is red (the color turns deeper with the concentration of nanoparticles increasing) and usually stable for months under ambient conditions, as shown in the inset of Figure 2A. These nanocubes tend to form a superlattice structure if the experimental conditions such as the solvent evaporation rate and the concentration of nanocubes could be well controlled (Figure 2B). The edges of the nanocubes have a mean size of 14.7 nm (see Supporting Information Figure S2) and an interparticle distance estimated at ca. 4.3 nm, which is close to that calculated for two times the chain length of oleic acid (for each oleic acid molecule, ∼2.2 nm). The single nanocube has a well-defined shape and highly crystalline nature (Figure 2B inset, see
Figure 3. TEM image of hematite nanoparticles synthesized with a molar ratio of sodium oleate/FeCl3 ) 3.5:1, showing that nonspherical hematite nanoparticles were obtained.
Supporting Information Figure S3). Tilted TEM observation confirmed its feature of cubic shape (see Supporting Information Figure S4). To better understand the process of formation and growth of hematite nanocubes, the effect of various experimental conditions on the synthesis has been investigated. The temperature and reaction time of hydrothermal treatment, the molar ratio of sodium oleate and iron chloride, and the amount of oleic acid has been varied to investigate systematically the parameters affecting the synthesis of hematite nanocubes. The molar ratio of sodium oleate/FeCl3 was a crucial factor for the production of hematite nanocubes. To obtain hematite nanocubes with uniform shapes and sizes, the ratio needed to be controlled at around 3.0 with the other parameters fixed. In case of a lower ratio such as 1.0, the bulky precipitate was produced, which could not be dispersed in cyclohexane, and no nanoparticles were obtained. When the ratio reached 3.5 or more, the as-synthesized product could be totally dissolved in nonpolar solvents to form a colloidal solution of hematite nanoparticles without any size-selective procedure. The excess amount of oleate can prevent the possible abnormal growth of any nucleus and aggregation of nanoparticles. However, the uniformity of nanoparticles became poor and irregular particles were produced (Figure 3).
Letters
J. Phys. Chem. C, Vol. 111, No. 9, 2007 3553
Figure 4. TEM images of nanocubes obtained after a size-selection process from the samples synthesized at different reaction temperatures. Images A-D correspond to the samples obtained at 120, 140, 160, and 180 °C, respectively.
Figure 5. (A) UV-vis spectrum of hematite nanocubes in cyclohexane; (B) Temperature-dependent magnetization of the assembly of hematite nanocubes formed on a 4 × 4 mm2 Si(100) wafer. The direction of an external magnetic field is perpendicular to the Si(100) surface. 4, zero-field cooling (ZFC); 3, field-cooling (FC).
The influence of temperature on the synthesis of hematite nanocubes was studied. The results showed that hematite nanocubes could not be produced until the temperature reached 140 °C. With the temperature increasing, well-defined nanocubes can be produced (Figure 4). Meanwhile, the crystallinity of nanoparticles also became better as confirmed by XRD patterns (see Supporting Information Figure S5). When the temperature was up to 180 °C, all nanocrystals were cube-shaped. In addition, changing the amount of oleic acid from 1 to 5 mL does not have any obvious influence on the formation of hematite nanocubes. Combined with the above experimental results, the following mechanism for hematite nanopaticles was proposed. First, the Fe(oleate)3 complex was produced by the reaction between iron chloride and sodium oleate at a certain temperature (80 °C) and
two phases occurred (the upper henna organic layer containing Fe(oleate)3 complex, and the bottom aqueous solution) in the initial reaction system (see Supporting Information, inset in Figure S6). The FT-IR spectrum shows a CdO stretching peak at ca. 1700 cm-1, which is a characteristic peak for a metaloleate complex5b (see Supporting Information Figure S6). With the temperature increasing, the Fe(oleate)3 complex began to decompose. The decomposition temperature of Fe(oleate)3 under the conditions of hydrothermal treatment is much lower than that required under high-temperature solution-phase conditions. As soon as the temperature reached ∼120 °C, Fe(oleate)3 could be decomposed to form an iron oxide nucleus. Oleic acid and sodium oleate acted as capping reagents and allowed the nucleus to grow steadily but protected them from aggregation, and thus resulted in the formation of uniform oleic acid/oleate-capped
3554 J. Phys. Chem. C, Vol. 111, No. 9, 2007 hematite nanoparticles. The nanoparticles are easily deposited in the reaction solution because of their weight and hydrophobic surface, which made them relatively easy to collect. Hematite nanomaterials have been used widely as ultravioletray absorbants for their broad adsorption in ultraviolet region from the electron transmission of Fe-O.21 Here, the UV-vis adsorption of hematite nanoparticles obtained mainly located in the far-ultraviolet region (200-300 nm), which could be related to the morphologies of hematite nanoparticles (see Supporting Information Figure S7). For nanocubes, their UVvis absorption spectrum shows a strong and narrow absorption peak at 230 nm and a shoulder peak at 267 nm (Figure 5A), which was different from other morphologies of hematite nanocrystals. Between the Ne´el temperature (TN) of ca. 961 K and the Morin transition temperature (TM) of ca. 265 K, conventional bulk hematite has a weakly ferromagnetic behavior, and below TM, an antiferromagnetic behavior; it has no net magnetization.22 However, the hematite nanocubes have a different magnetic behavior from bulk hematite because of their finite-size and surface effects. Figure 5B shows the temperature-dependent magnetization measured from 10 to 300 K with an external magnetic field of 100 Oe. The blocking temperature (TB) of the assembly of hematite nanocubes is ca. 80 K, above which the hematite nanocubes display superparamagnetic performance. In summary, uniform hematite nanocubes with an average size of 15 nm can be prepared by a simple, cheap, and environmentally friendly hydrothermal method. The blocking temperature of hematite nanocubes is ca. 80 K with an external field of 100 Oe, and above which they display superparamagnetic performance at room temperature. Further work on surface modification is ongoing to make these hematite nanocubes dissolvable in water, which is important from the viewpoint of application. This synthesis approach is expected to become a general method for preparation of other uniform nanoparticles with unique shapes, such as cobalt, nickel oxides, and so on. Acknowledgment. S.H.Y. acknowledges the special funding support from the National Natural Science Foundation of China (NSFC, Nos. 20325104, 20621061, 20671085, 50372065), the 973 project (2005CB623601), the Centurial Program of the Chinese Academy of Sciences, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the Partner-Group of the Chinese Academy of Sciences-the Max Planck Society. Supporting Information Available: TEM images, HRTRM images, FTIR spectra, and UV-vis spectra of the samples and
Letters other materials. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. Nature 2002, 420, 395. (b) Chen, M.; Kim, J.; Liu, J. P.; Fan, H. Y.; Sun, S. J. Am. Chem. Soc. 2006, 128, 7132. (c) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (d) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. Nano lett. 2004, 4, 187. (e) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (f) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (g) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. Nano Lett. 2004, 4, 187. (h) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. J. Am. Chem. Soc. 2004, 126, 11458. (2) Rai, K.; Moskowitz, R. J. Magn. Magn. Mater. 1990, 85, 233. (3) McMichael, R. D.; Shull, R. D.; Swartzendruber, L. J.; Bennett, L. H. J. Magn. Magn. Mater. 1992, 111, 29. (4) (a) Jordan, A.; Scholz, R.; Maier-Hauff, K.; Johannsen, M.; West, P.; Nadobny, J.; Schirra, H.; Schmidt, H.; Deger, S.; Loening, S.; Felix, W. R. J. Magn. Magn. Mater. 2001, 225, 118. (b) Nam, J. M.; Thaxton, C. S.; MirKin, C. A. Science 2003, 301, 1884. (5) (a) Park, S. J.; Kim, S.; Lee, S.; Khim, Z. G.; Char. K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581. (b) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (c) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Li, M. K.; Kim, J.; Kim, K. W.; Noh, H.-J.; Park, J. H.; Bae, C. J.; Park, J. G.; Hyeon, T. AdV. Mater. 2005, 17, 429. (d) Park, J.; Koo, B.; Hwang, Y.; Bae, C. J.; An, K., Park, J. G.; Park, H. M.; Hyeon, T. Angew. Chem., Int. Ed. 2004, 43, 2282. (e) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (f) Park, J.; Lee, E.; Hwang, N. M.; Kang, N. M.; Kim, S. C.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 2872. (6) Sugimoto, T. AdV. Colloid. Interface. Sci. 1987, 28, 65. (7) (a) Teng, X.; Yang, H. J. Mater. Chem. 2004, 14, 774. (b) Teng, X.; Yang, H. J. Am. Chem. Soc. 2003, 125, 14559. (8) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O ¨ .; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090. (9) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. (10) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821. (11) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (12) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M. J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (13) Casula, M. F.; Jun, Y.-w.; Zaziski, D. J.; Chan, E. M.; Corrias, A.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 1675. (14) Redl, F. X.; Black, C. T.; Papefthymious, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583. (15) Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164. (16) Fan, H. L; Song, B. Z,; Liu, J. H.; Yang, Z. Q.; Li, Q. X. Mater. Chem. Phys. 2005, 89, 321. (17) Rath, C.; Sahu, K. K.; Kulkarni, S. D.; Anand, S.; Date, S. K.; Das, R. P.; Mishra, N. C. Appl. Phys. Lett. 1999, 75, 4171. (18) Dong, W.; Zhu, C. S. J. Mater. Chem. 2002, 12, 1676. (19) Jing, Z. H.; Han, D. Z.; Wu, S. H. Mater. Lett. 2005, 59, 80. (20) Liang, X.; Wang, X.; Zhuang, J.; Chen, Y.; Wang, D.; Li , Y. D. AdV. Funct. Mater. 2006, 16, 1805. (21) Zhou, H. S.; Mito, A.; Kundu, D.; Honma, I. J. Sol-Gel Sci. Technol. 2000, 19, 539. (22) Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468.
