크기분리과정 없이 균일한 자성체 산화철 나노결정의 제조

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

현택환, 이수성, 박종남

서울대학교 공과대학 응용화학부

Fabrication of Monodisperse Iron Oxide (gamma-Fe2O3) Nanocrystallites

Taeghwan Hyeon,* Su Seong Lee, and Jongnam Park

School of Chemical Engineering, Seoul National University, Seoul, Korea

서론

The development of uniform nanometer sized particles has been intensively pursued because of their technological and fundamental scientific importance. These nanoparticular materials often exhibit very interesting electrical, optical, magnetic, and chemical properties, which cannot be achieved by their bulk counterparts. The fabrication of patterned media arrays of discrete single domain magnetic nanoparticles is very important for their potential applications in multi-terabit/in2 magnetic memory devices [1]. The syntheses of several uniform-sized magnetic metal nanoparticles have been reported [2]. However, relatively little work has been done on the fabrication of monodispersed and crystalline magnetic oxide nanoparticles. Several magnetic oxide nanoparticles including □-Fe2O3 and magnetite have been synthesized using microemulsion and other methods. However, particle size uniformity and the crystallinity of these nanoparticles are comparatively poor. Although the syntheses of relatively uniform maghemite and magnetite nanoparticles have been recently reported, exhaustive size selection procedures were necessary [3]. Here we report upon a novel non-hydrolytic synthetic method of fabricating highly crystalline and monodisperse □-Fe2O3 nanocrystalline particles without a size selection process, which allows the production of selected particle sizes from 4 nm to 16 nm [4].

본론

The technique normally used to synthesize uniform nanoparticles, involves the thermal decomposition of organometallic compounds in the presence of suitable stabilizers, followed by a size selection process. The main obstacle to produce the monodisperse nanoparticles in large-scale (multi-kilogram) is the size selection process (Scheme 1). In the current work, we have developed new synthetic approach for the production of monodisperse nanocrystallites that does not require a time-consuming size selection process. Two different approaches were applied in the synthesis. In the first method, monodisperse iron nanoparticles were first generated, and were further oxidized to iron oxide. In the second approach, Fe(CO)5 was directly injected into a solution containing both the surfactant and trimethylamine oxide. In both approaches, monodisperse iron oxide nanocrystallites were directly obtained without a further size selection process; however, the first method allowed better control of particle size and reproducibility. By varying the experimental conditions, we were able to get □-Fe2O3 nanocrystallites with different particle sizes. The nanoparticles obtained were characterized by low-resolution and high-resolution TEM, electron diffraction, X-ray powder diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy.

In the following, a typical fabrication of iron oxide nanocrystallites with particle diameter of 11 nm through the first synthetic method is described. At first, the iron oleate complex was prepared by reacting Fe(CO)5 and oleic acid with a 1:3 molar ratio at 100 oC. The UV-Visible absorption spectrum of the metal complex revealed a single intense peak at 330 nm (Supporting Information), and FT-IR spectrum of the complex exhibited no CO stretching peak. These spectroscopic data and the M□ssbauer spectroscopic results on the decomposition of iron pentacarbonyl at 120 oC reported by Wonterghem and coworkers demonstrate that iron (II) complex seemed to be produced. However, the exact structure of the complex was not elucidated. Iron nanoparticles were then generated by aging iron complex at 300 oC. The TEM image of the iron nanoparticles (Supporting Information) revealed that nanoparticles are monodisperse. Electron diffraction pattern showed that the nanoparticles are nearly amorphous. The XRD pattern of the sample after heat treated under argon atmosphere at 500 oC revealed bcc -iron structure. These iron nanoparticles were then transformed into □-Fe2O3 nanocrystallites by oxidizing them at 300 oC using a mild oxidant, trimethylamine oxide ((CH3)3NO). The TEM image (Figure 1) showed that monodisperse nanoparticles of 11 nm diameter were arranged in a 2-dimensional hexagonal closed packed way, demonstrating the uniformity of the particle size. The high-resolution transmission electron micrograph (HRTEM) shown in Fig. 2 illustrates the highly crystalline nature of the nanoparticles. The electron diffraction pattern exhibited a maghemite (□-Fe2O3) structure.

The X-ray powder diffraction pattern of the material also proved its highly crystalline nature and the peaks matched well with standard □-Fe2O3 reflections. The XRD peaks of the nanocrystallites were compared with those of standard maghemite and magnetite data (Supporting Information). In the X-ray photoelectron spectrum, the positions of the Fe(2p3/2) and Fe(2p1/2) peaks are 711.3 and 724.4 eV, which are in good agreement with the values reported for □-Fe2O3 in the literature. The Raman spectrum, which was often applied as a tool to differentiate between maghemite and magnetite, matched well with the reported maghemite spectrum. By manipulating the TEM sample preparation condition, we could obtain a 3-dimensional close-packed superlattice assembly.

When the starting reaction mixture containing 1:1 and 1:2 molar ratio of Fe(CO)5 and oleic acid were applied in the synthesis, □-Fe2O3 nanocrystallites with particle sizes of 4 nm and 7 nm were obtained, respectively. In order to produce bigger nanocrystallites of > 11 nm, a reaction mixture with a 1:4 molar ratio of Fe(CO)5 and oleic acid was applied in the synthesis. The particle size of the resulting nanocrystallites was, however, still around 11 nm. However, we could get iron nanocrystallites with particle sizes bigger than 11 nm by adding more iron oleate complex into the previously prepared 11 nm sized iron nanocrystallites, followed by aging at 300 oC. The resulting bigger iron nanocrystallites were later oxidized to get monodisperse □-Fe2O3 nanocrystallites through the similar procedure as described above. Through this synthetic procedure, we could tune the particle size of the nanocrystallites from 11 nm to 16 nm.

Preliminary magnetic measurements were performed on the □-Fe2O3 nanocrystallites using a superconducting quantum interference device (SQUID). The temperature dependence of the magnetization was measured using zero-field-cooling (ZFC) and field-cooling (FC) procedures in an applied magnetic field of 100 Oe between 5K and 300K. The plot of temperature versus magnetization for 4 nm, 13 nm, and 16 nm

결론

The highly crystalline and monodisperse □-Fe2O3 nanocrystallites were fabricated from the controlled oxidation of uniform iron nanoparticles which were generated from the thermal decomposition of iron complex. The synthetic procedures developed in the present study offer several very important advantageous features for the fabrication of oxide nanoparticles. First, they allow highly crystalline and monodisperse nanoparticles to be obtained directly without a further size-selection process. Second, particle size can be easily and reproducibly be altered by changing the experimental parameters. Third, the yield of the current process is over 80 % and the indications are that scale-up can be achieved relatively easily.

References

(1) Speliotis, D. E. J. Magn. Magn. Mater. 1999, 193, 29.

(2) (a) Park, S.-J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581. (b) Puntes, A. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (d) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989.

(3) (a) Rockenberger, J.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 1999, 121, 11595. (b) Fried, T.; Shemer, G.; Markovich, G. Adv. Mater. 2001, 13, 1158.

(4) T. Hyeon, S. Lee, J. Park, Y. Chung, and H. Na, J. Am. Chem. Soc. 2001, 123, 12798.

화학공학의 이론과 응용 제8권 제1호 2002년

Scheme 1. General Colloidal Chemical Synthesis of Uniform Nanoparticles

Figure 1. TEM image of a two-dimensional hexagonal assembly of 11 nm □-Fe2O3 nanocrystallites.

Figure 2. High-resolution TEM image of 2D hexagonally close-packed 11 nm □-Fe2O3 nanocrystallites.