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Vast Magnetic Monolayer Film with Surfactant-Stabilized Fe3O4 Nanoparticles Using Langmuir-Blodgett Technique Don Keun Lee,† Young Hwan Kim,‡ Chang Woo Kim,‡ Hyun Gil Cha,‡ and Young Soo Kang*,‡ Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138, and Department of Chemistry, Pukyong National UniVersity, 599-1 Daeyeon-3-dong, Nam-gu, Busan 608-737, South Korea ReceiVed: April 3, 2007; In Final Form: May 31, 2007
Although several methods (e.g., self-assembly, spin coating, etc.) have been explored for making a monolayer film of nanoparticles, the monolayer on a substrate is typically smaller than 1 µm × 1 µm in certain regions. The approach is not ideally suitable for generating a highly ordered and close-packed homogeneous vast monolayer of nanoparticles, which is potentially important for applications. In this report, the preparation of the vast monolayer films of Fe3O4 nanoparticles with a wide range such as that over 3.25 µm × 3.95 µm is reported. Their TEM images showed a two-dimensional assembly of Fe3O4 nanoparticles, demonstrating the uniformity of these nanoparticles. The formation of a Langmuir monolayer of the oleic acid-coated Fe3O4 nanoparticles mixed with stearic acid molecules at the air/water interface and its stability were studied with a pressure-area isotherm curve. TEM and BAM studies demonstrated that increasing surface pressure resulted in a transition from well-separated domains of nanoparticles complex to well-compressed, monoparticulate layers.
Introduction Two-dimensional (2D) nanomaterials and their assembly are of scientific significance and industrial application because of their unique size- and dimension-dependent mechanical and optical properties and their promising applications as a superlattice in sensors, biomedical applications, magnetic storage media, nano-optoelectronics, and nanoelectronics.1-3 Moreover, the development of 2D nanostructures is the key point for that of three-dimensional (3D) nanostructures in relation to controlled chemical composition, morphology, dimensionality, phase purity, and their assembly. The advantages over the thermal decomposition method are based on artificial manipulation via changing the ligand molecules used in the synthesis to allow the design of 2D nanomaterials on the molecular scale and the scientific explanation of the fundamental mechanism in which ligand molecules such as surfactant control the nucleation and the growth of nanocrystallites.4-13 In recent years, the magnetic nanoparticles of iron oxides have been exploited extensively as the materials of choice for labeling and sorting of cells, ferrofluids, separation of biochemical products, and various biomedical applications due to their unique magnetism and low toxicity. Most of these promising applications require iron oxide nanoparticles to possess good chemical stability, narrow particle-size distribution, and uniform morphology. As a result of anisotropic dipolar attraction, the pristine Fe3O4 nanoparticles tend to aggregate into large clusters and thus lose the specific properties associated with single-domain and magnetic nanostructures. Using polymers or surfactants as the ligands or stabilizing agents was proven to be very powerful in the specific control of growth and in the prevention of * Corresponding author. E-mail:
[email protected]. † Harvard University. ‡ Pukyong National University.
agglomeration of the nanoparticles, which provides us with a promising vehicle for synthesizing Fe3O4 nanocrystals with uniform size and shape.14-18 In a point of view, our group has already reported on the chemical synthesis of monodisperse Co- and Fe-based nanoparticles as a soft magnetic phase with controlled size and shape by thermal decomposition of M2+-oleate2,19,20 and chemical reduction.21 Nanocomposites of organic materials and inorganic nanoparticles are of great promise as composites for utilization in high-speed and high-capacity optical and magnetic information storage media.22 It becomes more and more significant to explore epoch-making strategies to prepare the magnetic nanoparticles into a long-range monolayer ultrathin films to research into their collective properties and the potential applications like biomedical devices and the hyper-density magnetic storage media with high perfomance.22-27 The self-assembly of monodisperse magnetic nanoparticles can only provide the long-range ordering with the particlepacking density, the magnetic domain structure, and so forth. Various techniques have also been introduced to immobilize biomaterials on solid matrix surfaces, such as self-assembly,28 sol-gel process,29 Langmuir-Blodgett (LB) techniques,30 and so forth. Out of the various techniques, the Langmuir-Blodgett (LB) film deposition technique is a very useful tool in controlling the formation of an ultrathin film. In addition, various works have been done to create the interfacial films of phospholipids and to study the surface morphology of phospholipid monolayers and bilayers as well as the incorporation of protein on the various lipid surfaces.31,32 Among the techniques for the deposition of thin films of the magnetic nanoparticles on solid substrates, layer-by-layer (LbL) deposition and Langmuir-Blodgett (LB, vertical lift) techniques are some of the most promising methods because they enable fine control of the thickness and homogeneity of the monolayer, and ease
10.1021/jp072612c CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
LB Film of Fe3O4 Magnetic Nanoparticles
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Figure 1. (a) Procedure for the synthesis of nanocrystallites using Fe-oleate complex and (b) schematic drawing of experimental setup of LangmuirBlodgett deposition with surface stabilized Fe3O4 nanoparticles monolayer at the air/water interface.
for multilayer deposition.33,34 Yang et al. have already succeeded in making patterned Langmuir-Blodgett films of γ-Fe2O3 using soft lithography.29 Werts et al. have showed that it was possible to pattern LB films of gold nanoparticles.35 Although several methods have been explored in making a monolayer film of nanoparticles, the monolayer on a substrate is typically smaller than 1 µm × 1 µm in certain regions. The approach is not ideally suitable for generating highly ordered and close-packed homogeneous monolayer of nanoparticles, which is potentially important for applications.36 On the basis of the above consideration, if one could combine the advantages over Langmuir-Blodgett film to fabricate Fe3O4 magnetic nanoparticles, a novel vast monolayer film with a long range of Fe3O4 magnetic nanoparticles by adding stearic acid molecules with good stability and homogeneous dispersibility could be available. Here, we used Fe3O4 nanoparticles synthesized by a new low-pressure thermal decomposition method with the Pyrex tube. We report on a one-step, surfactant-mediated synthesis of shape-, size-, and phase-controlled Fe3O4 nanostructures that assemble spontaneously into ordered as well as into randomly distributed spherical particles. The as-synthesized Fe3O4 nanocrystals exhibit distinguishable size-dependent magnetic properties. The interfacial behavior of the nanoparticles at the air/water interface when forming Langmuir and Langmuir-Blodgett (LB) films are reported. Highly ordered 2D arrays of Fe3O4 nanocrystals are shown to be useful for bottomup fabrication of ultrahigh-density magnetic storage media. Experimental Synthesis of Fe3O4 Magnetic Nanoparticles. FeCl2‚4H2O (99+%) and sodium oleate (98%) were obtained from Aldrich Chemical Co. and used without further purification. To prepare the iron-oleate complex, 2 g of FeCl2‚4H2O (10 mmol) was dissolved in deoxygenated water (300 mL, 18 MΩ, nitrogen gas bubbling for 30 min), and the resulting solution was added into 6.09 g of sodium oleate (20 mmol) under vigorous stirring
for 2 h. The precipitate was separated by filtration and washed with doubly deionized water to be free of sodium and chlorine ions. After drying, the iron-oleate complex was transferred into the Pyrex tube. The complex was first flushed with nitrogen, and the tube was sealed at 0.3 Torr. The sample was slowly heated from room temperature to 300 °C at 1 °C/min. After reaching the desired temperature, it was held at 300 °C for 2 h and then cooled to room temperature. The complex color was changed to black, indicating that Fe3O4 nanoparticles were being formed. Preparation of Langmuir-Blodgett Films of Fe3O4 Magnetic Nanoparticles. The pressure-area isotherms of the surface stabilized Fe3O4 nanoparticles at the air/water interface were obtained with a KSV minitrough (45 cm × 15 cm, KSV model 2200). Figure 1 shows schematic drawings of a pressure-area isotherm experiment and LB deposition of the surface stabilized Fe3O4 Langmuir monolayer at the air/water interface. The setup included a surface pressure microbalance with a Wilhelmy plate. The trough system was controlled by a computer and KSV Film Control System Software. Isotherm compression and data collection were automatically achieved through the use of computer software. The Langmuir-Blodgett film of the surface stabilized Fe3O4 nanoparticles was prepared by transferring a monolayer by using the Langmuir-Blodgett technique. The trough must be carefully placed to ensure that it was completely leveled and that there was a good seal between trough and barrier; this is essential to the monolayer compression. A stock solution was prepared by dissolving 10 mg of the surface stabilized Fe3O4 in 10 mL of chloroform. The solution was added by 1 mL of 0.01 mg/mL stearic acid stock solution. The resulting solution was mixed by ultrasonic agitation for 3 h. The sub-phase temperature was controlled with a Jeio Tech Co. Ltd. refrigerated circulator, model RBC 20. Experimental Technique. To identify properties of the synthesized Fe3O4 nanoparticles, we conducted various experiments. The crystal structure of the synthesized nanoparticles
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Figure 2. X-ray diffraction pattern of surface stabilized Fe3O4 nanoparticles.
was identified by using X-ray powder diffraction (XRD) with a Philips X’Pert-MPD System with a Cu KR radiation source (λ ) 0.154056 mm). Mo¨ssbauer spectra were recorded using a conventional Mo¨ssbauer spectrometer of the electromechanical type with a 20 mCi 57Co source in a Rh matrix. To produce uniform thickness over the area of the Mo¨ssbauer absorber, a sample was mixed with boron nitride powder. The area density of Fe for the flattened sample was 10 mg/cm2. In the experimental setup of the Brewster Angle Microscope (BAM) (NanoFilm Tech., Germany) used for the characterization of monolayers and interfacial processes, the light beam of a pulsed laser (λ ) 532 nm, beam diameter of 1 mm) passed through a polarizer (set for p-polarization) was incident on the air/water interface at the Brewster angle (53.15°). The reflected beam was detected using a CCD camera. An image of the interface was formed through a microscope coupled to the CCD inclined at the Brewster angle, which collected the reflected light and a part of the light scattered by the interface. BAM images were recorded in situ with the NanoFilm Technology BAM 2 Plus on the KSV 2000 trough. The size and shape of nanoparticles were obtained by transmission electron microscopy (TEM). TEM measurements were carried out on a Hitachi H-7500 (low resolution) and JEOL JEM2010 (high resolution) TEM. The size distribution of the particles was measured from enlarged photographs of TEM images. TEM samples were prepared on the 300 mesh copper grid coated with Formvar film by LB technique. The magnetic properties such as saturation magnetization and coercivity were measured with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS XL 7) in a maximum field of 7 T.
Figure 3. 57Fe Mo¨ssbauer spectrum of surface stabilized Fe3O4 nanoparticles at room temperature.
Results and Discussion Structure Analyses of Fe3O4 Magnetic Nanoparticles. Figure 1 shows a procedure for the nontoxic synthesis of functional nanoparticles without any solvent. The precursors were prepared from the reaction of iron chloride and sodium oleate in water. The thermal decomposition of the complex took place at low pressure at about 300 °C without any solvent. The iron-oleate complex serves as the molecular precursor and provides the capping ligand to control particle growth. In the solventless reaction environment, interparticle collisions rarely occur and particle growth proceeds primarily by monomer addition to the particle surface leading to monodisperse size and shape distributions.37 The advantages of this method is that one does not need to make a separation nor to evaporate solvent. This method can easily be scaled up for industrial purposes. We produced 1 kg of Fe3O4 nanoparticles in one reaction by using a large stainless steel vessel instead of a Pyrex tube.
Figure 4. The pressure-area isotherm of the surface layer with stabilized Fe3O4 nanoparticles on the air/water interface at 25 °C. A, B, C, and D are target pressures for LB deposition, respectively.
Figure 2 shows an XRD spectrum of the as-synthesized Fe3O4 magnetic nanoparticles prepared by thermal decomposition using a Pyrex tube. The diffraction peaks are clearly broadened, which can be the result of the reduced particle size in Figure 2. The discernible peaks can be indexed to (220), (311), (222), (400), (422), (333), and (440) planes of a cubic unit cell, which corresponds to that of magnetite structure (JCPDS card no.790418). The crystal size determined by the Debye-Scherre equation with XRD data is 11.8 nm, which is close to the particle sizes calculated from TEM images (10.6 nm for Fe3O4). It indicates that the as-synthesized particles are all Fe3O4 nanoparticles.
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Figure 5. TEM images of Fe3O4 LB films deposited at different surface pressures of 2 mN/m (A), 10 mN/m (B), 20 mN/m (C), and 35 mN/m (D).
Figure 3 presents the 57Fe Mo¨ssbauer spectrum of Fe3O4 nanoparticles at room temperature. The solid line is a computerfitted Mo¨ssbauer spectrum, and the solid circles are experimental Mo¨ssbauer spectrum at room temperature. The spectrum was convoluted to two sextets that correspond to iron ions in tetrahedral (Td) A and octahedral (Oh) B sites of magnetite. Fe3O4 can be written as Fe3+(Fe2+ Fe3+)O4. A fast electrontransfer process (electron hopping) between Fe2+ and Fe3+ ions on the Oh B site takes place above 110-120 K. The spectrum shows clearly two hyperfine magnetic splittings, which are clear evidence for Fe3O4 nanoparticles.
Pressure-Area (π-A) Isotherm Study. Research on the LB monolayer to study the air/water interfacial phenomena of the surface on the surfactant-stabilized Fe3O4 nanoparticles is very significant to investigate the arrangement of each particle, particle-packing density and particle-packing order, magnetic phase behavior, and so forth. It is well-known that the Langmuir trough is composed with a tensiometer (Wilhelmy balance) to analyze the change in sub-phase surface tension (i.e., the surface pressure) as the insoluble amphiphilic compounds are spread and compressed at the air/water interface on the surfactantstabilized Fe3O4 nanoparticles.38 When molecules in a solution
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Figure 6. High-resolution TEM image (a) and Brewster angle microscope (BAM) images ((b) 2 mN/m; (c) 20 mN/m; (d) 35 mN/m; and (e) after collapse)).
are subject to the attractive forces, it is equal in the bulk of the solution. But, these attractive forces are unequal at a surface or interface with each other and the net effect of these forces is to attract the surrounding molecules into the bulk of the solution. The surface tension which this net effect causes can be defined as the work required expanding the surface isothermally by unit area.38 The tendency of surface-active molecules to accumulate at interfaces favors expansion of the interface and hence lowers the surface tension.39 Such behavior makes it possible to monitor the surface tension.39 Figure 4 shows the pressure-area isotherm of the surface layer with stabilized Fe3O4 nanoparticles mixed with stearic acid molecules at the air/water interface. There is no film collapse evident upon compression up to 50 mN/m. This demonstrates a successful formation of the Langmuir monolayer of the surface stabilized Fe3O4. As the area is reduced, at first there is coexistence between the gas and the liquid particle phases at very low surface pressures. The isotherm curves shows a typical continuous transition from a liquid tilted particle phase to an untilted phase.12,13 But in case of only oleic acid-coated Fe3O4 nanoparticles, we could not observe a transition point between the liquid phase and the crystal phase. It indicates that soft oleic acid molecules between Fe3O4 nanoparticles in the Langmuir monolayer at the air/water interface prevent phase transition from the liquid phase to the solid phase. TEM and BAM Study on Fe3O4 LB Monolayers. Figure 5 shows TEM images of Fe3O4 LB films deposited at surface pressures of 2 mN/m (A), 10 mN/m (B), 20 mN/m (C), and 35 mN/m (D) in the pure water sub-phase at 25 °C. They demonstrate that Fe3O4 nanoparticles form a long-range ordered monolayer above a surface pressure of 10 mN/m. A monolayer of nanoparticles was observed from the image with almost no double or multilayers in it. It shows an example of an extended area where particles are packed in a highly organized manner, exhibiting a remarkable degree of long-range order such as over 3.25 µm × 3.95 µm. There is a crevice in TEM images of Figure 5 (B, C, D). It is due to stearic acid molecules mixed with Fe3O4 nanoparticles and does not appear in any other monolayer film prepared by the self-assembly method. This crevice is expected to give us a clue of controlling a space between nanoparticles. The lattice image (Figure 5B (inset)) indicates that the Fe3O4 single domain was perfectly synthesized by the thermal decomposition method without any defect. Figure 6a shows highmagnification TEM images of the LB film. It demonstrates that
Figure 7. Hysteresis loop of the surface stabilized Fe3O4 nanoparticles.
monodispersed Fe3O4 nanoparticles were arranged in a 2-dimensional hexagonal closed packed (hcp) way, demonstrating the uniformity of the particle size. Especially the interparticle spacing is very even. Here, as in most cases, adjacent Fe3O4 nanoparticles were separated by a region of approximately 2.8 nm which did not exhibit any diffraction contrast. This distance is considerably less than twice the expected oleate length (1.75 nm),14 and interdigitation of the alkyl chains from nearestneighbor Fe3O4 particles can be inferred. The phase behavior of the Langmuir monolayer at the air/water interface was observed with BAM, as shown in Figure 6. Figure 6b shows the image of an uncompressed film. Some domains such as those shown in Figure 6c have been observed in simple surfactant monolayers. They have, for example, been viewed by BAM in monolayers formed from stearic acid, pentadecanoic acid, and myristic acid.15 Interpretation of the images of a monolayer of oleic acid-coated Fe3O4 nanoparticles mixed with stearic acid is complicated by the existence of the three components. The contrast in the images possibly is derived from the phase structure of an entirely uniform film or from an area rich in either Fe3O4 nanoparticles or surfactant. An area of closely packed Fe3O4 nanoparticles will necessarily reflect the incident light more strongly than areas of surfactant and this appears as brighter. Compression to the surface pressure of 35 mN/m produced a very uniform and dense film (Figure 6e). Figure 6e shows the collapse of the Langmuir monolayer of Fe3O4 nanoparticles. Magnetic Property Study on Fe3O4 Nanoparticles. Fe3O4 nanocrystallite was measured by SQUID in a maximum field of 7 T at room temperature. Figure 7 shows that Ms of Fe3O4 nanocrystallite was nearly saturated with an applied field of 1 T. As can be seen in Figure 7, there is almost negligible
LB Film of Fe3O4 Magnetic Nanoparticles coercivity (
