Hydrothermal Growth of Fatty Acid Stabilized Iron Oxide Nanocrystals

Dec 30, 2008 - Seed nanocrystals formed by the coprecipitation of ferric and ferrous ions with oleate modification, at the molar ratios of sodium olea...
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J. Phys. Chem. C 2009, 113, 839–843

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Hydrothermal Growth of Fatty Acid Stabilized Iron Oxide Nanocrystals Takaaki Taniguchi,*,† Kazunori Nakagawa,† Tomoaki Watanabe,‡ Nobuhiro Matsushita,† and Masahiro Yoshimura† Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku Yokohama 226-8503, Japan, and Department of Applied Chemistry, School of Science and Technology, Meiji UniVersity, 1-1-1 Higashimita, Tama-ku, Kawasaki 214-8571, Japan ReceiVed: July 15, 2008; ReVised Manuscript ReceiVed: NoVember 24, 2008

Seed nanocrystals formed by the coprecipitation of ferric and ferrous ions with oleate modification, at the molar ratios of sodium oleate and Fe ion [SO]/[Fe] from 0.05 to 0.5, were subjected to a hydrothermal treatment at 100-230 °C to initiate the crystal growth. Fourier transform infrared spectroscopy revealed that these nanocrystals were stabilized by a surface oleate that formed a stable colloidal solution in various nonpolar solvents such as cyclohexane and benzene. An X-ray diffraction analysis showed that the higher hydrothermal temperatures and lower oleate concentration promoted the Ostwald ripening process to produce highly crystallized iron oxide nanocrystals based on a cation deficient magnetite polymorph. Transmission electron microscopy revealed that the oleate concentrations played an important role in controlling the nucleation and growth process; it was found that the lower oleate concentration was the preferable growth condition to produce uniformly sized and shaped nanoparticles with high saturation magnetization.

Ferromagnetic iron oxide nanocrystals, based on the magnetite, Fe3O4, and maghemite, γ-Fe2O3, phases, are important materials due to the advanced applications in the magnetic recording media1,2 and biomedical fields,3-5 and intensive investigations focused on their size- and shape-controlled synthesis have been conducted over the decade. For the synthesis of monodispersed iron oxide nanocrystals, the nonaqueous solution route was initially employed. For example, maghemite nanocrystals with size ranging from 4 to 16 nm have been produced by the decomposition of Fe(CO)5 in octyl ether and oleic acid or lauric acid and subsequent oxidation.6 The thermal decomposition of the Fe-oleate complex at 300 °C in 1-octadecence leads to 12 nm sized magnetite nanocrystals.7 These methods, in fact, can precisely control the size of the resultant iron oxide nanocrystals. However, unfortunately, toxic and expensive organic solvents are usually utilized at the high reaction temperatures around 300 °C to yield surfactant-stabilized nanocrystals via a nonaqueous approach. Therefore, interests in the preparation of iron oxide nanocrystals have been shifting toward the exploration of novel methods using lower reaction temperatures, less toxic solution, and inexpensive materials. It can be stated that the aqueous solution route, such as the precipitation method, is one of the most environmentally friendly and economically efficient approaches for the large-scale synthesis of metal oxide nanocrystals, due to the utilization of nontoxic water and cheap metal salts. Although several authors supporting the nonaqueous synthetic method often point out the difficulties of conventional aqueous approaches to control the size and surface property of the iron oxide nanocrystals,6,8 recent progress in aqueous chemistry has explored novel methods producing organic-passivated iron oxide nanocrystals with a

rather uniform size distribution.9-15 For example, Aslam et al. synthesized amine-stabilized iron oxide nanoparticles by the single-step reaction of a ferrous chloride aqueous solution with dodecyl amine under mild heating at 85 °C.9 Liang et al. reported the synthesis of nearly monodispersed magnetite and hematite nanocrystals based on an oleic acid/alcohol/water system.11 Nevertheless, most of the listed aqueous methods provide iron oxide nanocrystals with the tiny dimension of less than 10 nm and rather poor saturation magnetization, Ms, than the bulk due to the significant surface contributions, i.e., spin canting, surface disorder, and adsorbed species.16,17 There are still very few reports on the aqueous synthesis of highly magnetized and dispersed iron oxide nanocrystals.9,11 Recently, we reported a simple aqueous process to produce organophilic HfO2- or CeO2-based nanoparticles.18,19 In these studies, it was demonstrated that the hydrothermal treatment of oleate-stabilized nuclei produced by hydrolytic condensation of the metal-oleate complex with base led to the formation of the highly crystalline and nearly monodispersed nanocrystals. Furthermore, utilization of inexpensive and less toxic aqueous solutions during all the synthetic steps throughout the process provides the possible application of the proposed method in industry.In our earlier work,20 this attractive approach was initially employed for the fabrication of biomagnetic beads in which iron oxide nanocrystals are encapsulated in poly(glycidyl methacrylate), and we successfully obtained high-quality beads with a magnetization approximately 5 times higher than those of the commercially available ones. In the present study, we have continuously studied the proposed method in order to find the efficient synthetic condition providing highly crystallized and uniform iron oxide nanocrystals. The crystal growth mechanism depending on the hydrothermal temperatures and oleate concentration has been investigated.

* Corresponding author. Phone: +81-(45)-924-5310. Fax: +81-(45)-9245358. E-mail: [email protected]. † Tokyo Institute of Technology. ‡ Meiji University.

2. Experimental Section 2.1. Materials. FeCl2 (Wako, 99.5%), FeCl3 (Aldrich, 99.5%), sodium oleate, SO, C17H33COONa (Wako, Analytical grade),

1. Introduction

10.1021/jp8062433 CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

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Figure 1. Experimental flowchart of hydrothermal synthesis.

and an ammonia 25 wt % aqueous solution NH3(aq) (Wako) were used as received. Distilled water was utilized for preparation of the aqueous solutions and washing. 2.2. Syntheses of Iron Oxide Nanoparticles by the OleateModified Precipitation Method and Hydrothermal Growth. The overall synthetic procedure is presented in Figure 1. First, the sodium oleate dissolved in distilled water (15 mL) was added to 15 mL of an aqueous solution, containing a total of 3.5 mmol of ferric and ferrous chlorides at the molar ratio of 2:1. The molar ratio of sodium oleate and Fe ion, [SO]/[Fe], was varied from 0.05 to 0.5. Following this addition, a brown suspension based on the iron complex was immediately formed. Subsequently, 5 mL of the ammonia 25 wt % aqueous solution was added to this solution under vigorous stirring, resulting in the black suspension of magnetite-based seed nanoparticles. The aforementioned synthetic process was called the “oleatemodified precipitation method”, following our previous research.19 For subsequent hydrothermal growth, the reaction mixture, which contained the seed nanoparticles, was placed in a poly(tetrafluoroethylene) (PTFE) vessel (inner volume of 40 cm3). The vessel was sealed and placed inside a stainless steel autoclave, which was kept at the programmed hydrothermal temperature (100-230 °C) for 3 h under autogenous pressure. The products (the seed nanoparticles or the hydrothermal products) were collected by centrifugation at 5000 rpm for 1 h, subsequently washed twice with distilled water, and then dried at 60 °C for 12 h in air. For comparison, samples prepared in the absence of the sodium oleate were also prepared. The remaining experimental procedures were used for this synthesis. 2.3. Characterization. The products were characterized by powder X-ray diffraction (XRD) using a MAC Science MX 3VA diffractometer with Cu KR radiation (λ ) 1.54056 Å), operating at 40 mA and 40 kV. The grain size of the magnetite was calculated suing the Debye-Scherrer formula from the broadening of the (311) XRD reflection. The lattice parameter was calculated by the least-squares method using the peak position of the (311), (400), (511), and (440) reflections of the magnetite phase after Gaussian fitting. Transmission electron microscopy (TEM) was performed using a Hitachi HF-2000 operating at 200 kV. For the TEM measurements, the sample, poured into cyclohexane, was stirred for a few minutes and left at rest for 6 h. Subsequently, one drop of the stable colloidal solution was deposited on a holey carbon grid. The roomtemperature Fourier transform infrared (FTIR) spectra were recorded using a Jeol JIR-7000 spectrometer. The products (20

Taniguchi et al.

Figure 2. X-ray powder diffraction patterns of seed nanocrystals prepared by oleate-modified precipitation method using the solutions of [SO]/[Fe] ) (a) 0, (b) 0.05, (c) 0.25, and (d) 0.5.

mg) were thoroughly ground with (400 mg) potassium bromide powder (KBr for IR, Wako), and subjected to an IR analysis. The magnetization curve was recorded by a BHV-55 (vibrating sample magnetometer, VSM) at 300 K. 3. Results and Discussion 3.1. Seed Nanocrystals Prepared by the Oleate-Modified Precipitation Method. The reaction pathway to produce the oleate-passivated seed nanocrystals upon the oleate-modified precipitation method can be presented as follows. In the first step, the Fe-oleate complexes are formed by the addition of a C17H33COONa aqueous solution to the Fe2+ and Fe3+ containing solution (the molar ratio of the oleate-modified and unmodified Fe ions depends on the employed [SO]/[Fe]). Subsequently, these Fe species are condensed, and then dehydrated by the addition of base, which results in the formation of -Fe-O-Febonding (eq 1). Finally, the -Fe-O-Fe- network is extended to form the magnetite nucleus by coprecipitation of the ferric and ferrous ions.

-Fe - OH + -Fe - OH f -Fe - O - Fe - + H2O (1) During the overall reaction involving the condensation, nucleation, and crystallization, the products are dispersed by oleate located at the terminal metal centers. Figure 2 shows the XRD patterns of the seed nanocrystals prepared using precursor solutions with [SO]/[Fe] of 0, 0.05, 0.25, and 0.5. The patterns correspond well to the magnetite phase (JCPDS file 19-629) without any remarkable reflections from byproducts possibly formed by oleate modification of the ferric and ferrous ions. Also notable is that the reflection peaks generally got broader with an increase in the initial oleate concentration, presenting that decreases in the crystallite size with the increase in the organic amount. The crystallite sizes calculated by the Debye-Scherrer equation using the XRD patterns correspond to 12.0, 9.9, 7.5, and 8.7 nm for the samples obtained from the solution with the [SO]/[Fe] of 0, 0.05, 0.25, and 0.5, respectively. Similar relationships between the crystal size and amount of organic stabilizer have been reported in the literature presenting the synthesis of thiol-stabilized gold21 and amine-stabilized magnetite nanocrystals.9 In the present case, coordination of the Fe center with oleate likely suppressed the hydrolytic condensation making a -Fe-O-Fe- bonding, resulting in a smaller nucleus with the increased oleate concentration.

Hydrothermal Growth of Iron Oxide Nanocrystals

Figure 3. IR spectra of the seed nanocrystals prepared by using the solution of [SO]/[Fe] ) 0.25.

The IR spectroscopy was performed to reveal the interaction of the surfactant and the nanocrystalline surface. Figure 3 shows a typical IR spectrum for the iron nanocrystals prepared by the oleate-modified precipitation method. A series of bands in the 2840-2970 cm-1 region are attributable to the symmetric and asymmetric stretching of the CH2 groups and the terminal CH3 group. Two bands around 400 and 590 cm-1 correspond to the Fe-O stretching modes of the magnetite lattice.22 Bands in the 1000-2000 cm-1 region provide information about the terminal carboxyl group in the oleate/oleic acid.23 In this region, the bands at 1430 and 1540 cm-1, well corresponding to the symmetric and asymmetric COO- stretching on iron oxide surface, respectively,11 were clearly detected. This indicates that the seed nanocrystals were predominantly stabilized by the chemisorbed fatty acid. In fact, the seed nanocrystals could be dispersed in nonpolar solvents such as cyclohexane and benzene to form a stable colloidal solution, showing that the proposed hydrolytic sol-gel reaction could provide iron oxide nanocrystals with an effectively passivated surface by the oleate ligands. 3.2. Hydrothermal Growth. The effects of the hydrothermal temperatures on the phase and size evolution of the products were investigated. As the reaction temperature increased under hydrothermal conditions, the solubility of the iron oxide increased to eventually dissolve the smaller nanocrystals and reprecipitate the solute on the larger nanocrystals (Ostwald ripening). It is noted that the hydrothermal treatment was investigated up to 230 °C because of the softening point of the PTFE vessel of approximately 260 °C. Figure 4 shows the XRD patterns of the samples prepared by the hydrothermal treatment of the solution with the [SO]/ [Fe] of 0.25 at 100-230 °C for 3 h. As can be seen, the reflection peaks further developed as the hydrothermal temperature increased, which confirms that the higher reaction temperature promoted the Ostwald ripening process to increase the average crystalline diameter. Indeed, no reflections from both the impurity phases corresponding to hematite (R-Fe2O3) and goethite (R-FeOOH) were seen in the patterns. According to the literatures, the hydrothermal treatment of the precipitate based on the single precursor of Fe2+ or Fe3+ occasionally results in the formation of the aforementioned Fe3+-based phases.24-27 Thus, the absence of these compounds in the hydrothermal products was further indicative of the coprecipitation of Fe2+ and Fe3+ ions under the oleate-modified precipitation route. Two additional reflections corresponding to the maghemite phase (γFe2O3) (JCPDS file 39-1346), however, were weakly visible at

J. Phys. Chem. C, Vol. 113, No. 3, 2009 841

Figure 4. X-ray powder diffraction patterns of the iron oxide nanocrystals hydrothermally prepared at 100-230 °C for 3 h using the solution of [SO]/[Fe] ) 0.25. The reflections from the maghemite phase, (210) and (211), are marked by the asterisks (/).

Figure 5. X-ray powder diffraction patterns of nanocrystals at 230 °C for 3 h using the solution with [SO]/[Fe] of 0-0.5. The positions of the dotted lines correspond to the additional (210) and (211) reflections of the maghemite phase.

the 2θ values of ca. 23° and ca. 26° in the XRD pattern of the sample hydrothermally prepared at 230 °C. As is well-known, the maghemite phase is closely related to the magnetite phase. The chemical formula of the stoichiometric magnetite can be written as (8Fe3+)A[8Fe2+ + 8Fe3+]B32O2- where A and B denote the A site and B site in the inverse spinel structure, respectively. In the γ-Fe2O3 structure, 64/3 Fe atoms are statistically distributed in the 8 tetrahedral and 16 octahedral positions of the spinel unit cell, and the formula unit is (8Fe3+)A[(40/3)Fe3+ + (8/3)µ]B32O2- (µ denotes the cation vacancies). There is a series of intermediate compounds, the so-called cation deficient magnetite, Fe3-δO4, between Fe3O4 and γ-Fe2O3 whose chemical formula unit can be written as (Fe3+)A[Fe2+1-3δ + Fe3+1+δ + µδ]B32O2-.28 The lattice parameter of the hydrothermally prepared sample at 230 °C was calculated to be 0.8370 ((0.0008) nm by the least-squares method using the corresponding XRD pattern. This value is an intermediate between that of magnetite and maghemite, 0.8396 nm (JCPDS file 19-629) and 0.8346 nm (JCPDS file 39-1346), respectively, indicating that the products exhibit a remarkable nonstoichiometry from the chemical composition of magnetite, Fe3O4, or maghemite, γ-Fe2O3. On the basis of the relationship between the lattice parameter and δ investigated by Yang et al.,28 the chemical composition of the hydrothermal product approxi-

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Figure 6. Summary of calculated crystalline diameters before and after hydrothermal treatment at 230 °C for 3 h as a function of [SO]/[Fe] (they were present as before, D(BT), and after, D(HT), respectively), along with change in the crystalline size by the hydrothermal treatment (D(HT) - D(BT)).

mately corresponds to Fe3-δO4 with δ ) 0.15, which clearly shows the partial oxidation of Fe2+ ions during the synthesis. Oxidation is believed to occur through the outward diffusion of iron cations and, at the surface, the Fe reacts with dissolved O2 finally forming a thin layer of epitaxial maghemite.29 It is thus possible to speculate that the nanocrystals consisted of the magnetite-like core and maghemite-like surface. However, it is not possible to conclusively identify the real microstructure from the current XRD data; asymmetric reflections observed in several XRD patterns possibly indicate the presence of physical mixtures of Fe3-δO4 compounds with various δ values and lattice constants in the products. The effects of the oleate concentration on the crystal growth of the seed nanocrystals were subsequently investigated. As the results shown in the above section demonstrated that the higher hydrothermal temperature led to the higher crystallinity of the iron oxide nanocrystals with a ferromagnetic phase, the hydrothermal synthesis at 230 °C was studied in this investigation. Figure 5 shows the XRD patterns of the samples obtained by hydrothermal treatment of the solutions with [SO]/[Fe] of (a)

Taniguchi et al. 0, (b) 0.05, (c) 0.25, and (d) 0.5 at 230 °C for 3 h. These XRD patterns showed well-defined reflections and were generally common in the phase; the absence of refractions from the hematite/goethite phases, and the presence of those from the maghemite/magnetite phases, were detected. Thus, initial oleate concentrations had no remarkable influence on the crystalline phase of the hydrothermal products. Figure 6 displays a summary of the calculated crystallite sizes of the samples obtained before/after the hydrothermal treatment at 230 °C for 3 h. This figure indicates that the lower [SO]/[Fe] led to larger crystallites upon hydrothermal treatment and revealed that a change in the crystalline diameter after the hydrothermal treatment decreased with the increasing oleate amounts. Since the adsorption of oleate on the nanocrystalline surface decreased the surface energy, the reaction rate of dissolution/precipitation of Fe ions at the oxide surface in the growth solutions should be lowered. In addition, the larger amount of oleate in the initial solutions should increase the oleate density covering the nanocrystalline surface. Therefore, it is suggested that the crystal growth upon Ostwald ripening was more suppressed by increasing the oleate concentration in the starting solution. Individual nanocrystals were investigated by TEM. Figure 7a-c shows the low-magnification TEM image of the samples prepared by hydrothermal treatment at 230 °C for 3 h as well as the corresponding histograms of the particle size distributions obtained from the TEM images. As shown in the TEM images, the hydrothermal treatment based on the oleate precipitation methods resulted in the highly dispersed nanoparticles due to the oleate surface passivation. However, it was also observed that the morphological homogeneities were significantly influenced by the oleate concentration. When the highest [SO]/[Fe] of 0.5 was employed for the synthesis, various shapes and sizes of nanocrystals were observed (see Figure 7a). The histogram shows a bimodal size distribution with a relatively sharp band centered on ca. 7 nm and a broad band around 12-22 nm, whereas the lowest oleate concentration, [SO]/[Fe] of 0.05, produced rather uniform nanocrystals in both shape and size distribution (see Figure 7c). The particles shown in the latter micrograph generally correspond to monocrystals, as an average particle size determined from the corresponding histogram (14.5 nm) is close to the calculated grain size from the XRD pattern (16.4 nm). A high-resolution TEM observation (Figure 7d)

Figure 7. TEM data of the iron oxide nanocrystals prepared by hydrothermal treatment at 230 °C for 3 h: low-magnification images and the corresponding particle size distributions of the samples prepared from the solution of [SO]/[Fe] ) (a) 0.5, (b) 0.25, and (c) 0.05, and (d) a highresolution image of an isolated nanocrystals of the sample prepared from the solution with [SO]/[Fe] ) 0.05.

Hydrothermal Growth of Iron Oxide Nanocrystals

J. Phys. Chem. C, Vol. 113, No. 3, 2009 843 was revealed that the lowest oleate concentration provided rather uniformly sized and shaped nanocrystals, as this condition might allow to the homogeneous hydrolytic condensation of waterinsoluble and soluble iron species. On the basis of our earlier studies, we have considered that the proposed hydrothermal approach using oleate-modified precursor can be employed for the synthesis of oxidic nanocrystals with a wide range of metal components. The present investigation further supports this suggestion and also indicates the importance of the oleate concentration significantly impacting the overall nucleation growth process. Acknowledgment. Dr. C. S. Kuroda, Professor Y. Kitamoto, and Professor Y. Yamazaki are thanked for assistance with the TEM analysis, and Professor M. Abe is thanked for the VSM measurement.

Figure 8. Room-temperature magnetization curves of iron oxide nanoparticles of hydrothermally prepared nanocrystals at 230 °C.

confirmed this possibility; a representative nanoparticle is free of any grain boundaries and amorphous surface layers. The TEM observations revealed that a lower oleate concentration was preferable to form uniform iron oxide nanocrystals. Since the Fe-oleate species are hydrophobic, they tend to agglomerate in an aqueous solution. In an aqueous solution with the higher oleate concentration ([SO]/[Fe] of 0.5), the Fe-oleate species might be significantly agglomerated to provide two nucleation sites in which the Fe-oleate complex and watersoluble Fe species (hydrated Fe ions) were rather separately condensed, therefore producing the smaller and larger nanocrystals, respectively, as seen in the bimodal size distribution. In the solution with the relatively lower oleate concentration, both the oleate-modified and hydrated Fe species could be rather uniformly mixed, and therefore, they were homogeneously condensed to form nanocrystals with a narrow size distribution. The magnetization curves of the hydrothermally prepared samples at 230 °C for 3 h were investigated by VSM. Figure 8 shows the magnetization curves measured at room temperature for the samples prepared with the [SO]/[Fe] of 0-0.5. The saturation magnetization Ms of 71.7, 65.7, 59.3, and 58.9 emu/g were obtained for the 21.1, 16.4, 10.8, and 10.5 nm sized nanocrystals (these sizes were determined by XRD), respectively. The magnetizations smaller than their respective bulk values (73.5 emu/g for maghemite and 92 emu/g for magnetite) indicate that the organic/structurally defective layers at the nanocrystalline surface and nonstoichiometry are the contributable factors to reduce Ms values of the products from that of bulk magnetite. Nevertheless, the hydrothermally prepared nanocrystals generally showed a better magnetization than those of the organic-passivated iron oxide nanocrystals synthesized by conventional aqueous solution routes; for example, the Ms values are reported to be ca. 50 emu/g,11,12,15 which confirms the capability of the current approach toward the practical synthesis of highly crystallized, uniform, and magnetized iron oxide nanoparticles. 4. Conclusion This paper describes the growth mechanism of highly dispersed and crystallized iron oxide nanocrystals by a hydrothermal method using Fe-oleate precursors. The results showed that seed nanocrystals formed by the coprecipitation of ferric and ferrous ions with partial oleate modification were more efficiently grown at the higher hydrothermal temperatures. It

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