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Low-temperature synthesis of redispersible iron oxide nanoparticles under atmospheric pressure and ultradense reagent concentration
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Powder Technology 181 (2008) 45 – 50 www.elsevier.com/locate/powtec
Low-temperature synthesis of redispersible iron oxide nanoparticles under atmospheric pressure and ultradense reagent concentration Motoyuki Iijima a,⁎, Kimitoshi Sato b , Keiji Kurashima c , Takamasa Ishigaki b , Hidehiro Kamiya a a
Graduate School of Bio-Applications and Systems Engineering, BASE, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan b National Institute for Materials Science, Nano Ceramics Center, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan c National Institute for Materials Science, Materials Analysis Station, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan Received 17 August 2006; received in revised form 30 May 2007; accepted 20 June 2007 Available online 29 June 2007
Abstract Highly dispersed α-Fe2O3 nanoparticles ca. 3 to 8 nm in diameter were prepared at atmospheric pressure, low temperature, and at an ultradense reagent concentration by titrating an aqueous ammonia solution into a dense iron oleate/toluene mixture. A transparent suspension was obtained by redispersing the prepared particles in nonpolar solvents since they were redispersible to primary particles without aggregate formations. The prepared particles were characterized by TEM, XRD, and FT-IR, and their dispersion stability in organic solvents was determined by dynamic light scattering (DLS) and viscosity measurements. In order to analyze the formation process of the highly dispersed α-Fe2O3 nanoparticles, timecourse measurements of DLS and viscosity during the nanoparticle synthesis in toluene were carried out. A significant increase in the suspension viscosity and the formation of an aggregated structure were observed as soon as the titration of the aqueous ammonia solution. The suspension viscosity and aggregated particle size gradually reduced with continuous vigorous stirring; finally, α-Fe2O3 nanoparticles that were completely redispersible in nonpolar solvents were obtained after ca. 24 h. The particle size could be controlled by the synthesis temperature, and such redispersible α-Fe2O3 nanoparticles were obtained even when the reagent concentration was increased to 2.8 mol/L. © 2007 Elsevier B.V. All rights reserved. Keywords: Iron oxide; Nanoparticle; Iron oleate; Dispersion stability; Dense concentration
1. Introduction Considerable research has been focused on nanostructured materials because of their wide range of important applications. For example, nanoparticles possess novel size-dependent properties, such as electrical, magnetic, mechanical, optical, and chemical properties, which largely differ from those of their bulk materials [1–3].These nanoparticles have been used in bioprocessing [4], information storage media [5], color imaging [6], photochemical reactions [7], magnetic refrigeration [8], and catalysts [9]. α-Fe2O3 nanoparticles are one of such important materials which can be widely used as red pigments [10], catalysts [11], and gas sensors [12]. With regard to the handling of nanoparticles for the fabrication of nanocomposite materials or nanoparticle assembly, one of the major concerns is the prevention of nanoparticle aggregation by ⁎ Corresponding author. Tel./fax: +81 42 388 7068. E-mail address: [email protected] (M. Iijima). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.06.019
modifying the particle surface. There are two types of surface modification methods. One method involves adsorbing or grafting organic groups on nanoparticles by using various surfactants or silane alkoxides after nanoparticle synthesis [13– 17]. In this case, since a multistep procedure involving particle synthesis, drying, and redispersing is required, nanoparticles tend to form aggregates during the surface-modification procedure. Another method is the in situ modification process in which organic groups are introduced on the nanoparticle surface during the particle synthesis procedure [22–38]. Since the organic ligand in the synthesis solution controls particle growth and attaches to the particle surface during the procedure, redispersible nanoparticles can be obtained. Several different methods have been reported to obtain organic-capped nanoparticles by the in situ modification process, including reactions in reversed micelles [18,19], sol– gel reactions of metal alkoxides involving capping agents [20,21], nonhydrolytic sol–gel reactions of metal halides with capping agents [22–27], and thermal decomposition of metal
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complexes [28–31]. Although these methods produce nanoparticles that are redispersible in organic solvents, particle synthesis in a dilute solution and under high temperatures makes these methods unsuitable for industrial applications. A modified reverse micelle method in which the nanoparticle synthesis reaction occurs in the reverse micelle produced by metal–surfactant complexes, such as copper bis(2-ethylhexyl) sulfosuccinate, also produces redispersible nanoparticles [32– 38]. Although most studies on this modified reverse micelle method were performed using extremely dilute solutions to produce the reverse micelle structure, some reports show the possibility of particle synthesis under a dense reagent concentration of 2.7 × 10− 1 mol/L [32]. Wet chemical reactions using metal–surfactant complexes may provide a breakthrough with regard to increasing the reagent concentration during particle synthesis. In the present study, redispersible α-Fe2O3 nanoparticles were prepared by a wet chemical reaction using iron oleate as a reagent at atmospheric pressure, low temperature, and at an extremely dense reagent concentration. The synthesis temperature was between 303 K and 373 K, and the reagent concentration was 2.8 × 10− 1 mol/L of solvent or 2.8 mol/L of solvent, which was ca. 10 to 100 times greater than that in previously reported methods [18–39]. The prepared α-Fe2O3
nanoparticles were characterized by TEM, XRD, and FT-IR, and their dispersion stability in hexane was determined by dynamic light scattering (DLS) measurements. Furthermore, the action mechanism of iron oleate during nanoparticle synthesis was investigated by time-course DLS and viscosity measurements of the raw synthesis solution. 2. Experimental procedure 2.1. Materials All the materials were used without further purification. Iron chloride hexahydrate (FeCl3·6H2O, N99.0%), sodium oleate (N 90%), and ethanol (95%) were from Kanto Chemical Co., Ltd. (Tokyo, Japan), and toluene (N 99.5%) and 28 wt.% aqueous ammonia solution were obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). 2.2. Preparation of iron oleate/toluene mixture First, a mixture of iron oleate and toluene was prepared in a manner similar to that described by Park et al. [31]. For the synthesis of 2.8 × 10− 1 mol/L iron oleate in toluene, 20 mmol of FeCl3·6H2O and 60 mmol of sodium oleate were dissolved in a
Fig. 1. TEM observations of nanoparticles prepared under (a) 2.8 × 10− 1 mol/L, 303 K; (b) 2.8 × 10− 1 mol/L, 343 K; (c) 2.8 × 10− 1 mol/L, 373 K; and (d) 2.8 mol/L, 373 K.
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stirring. The additive content of the aqueous ammonia solution was 3.64 g and 9.10 g for the 2.8 × 10− 1 mol/L and 2.8 mol/L iron oleate solutions. After the titration, the synthesis solution was heated to 303 K, 343 K, or 373 K and maintained at that temperature for 24 h with stirring. The prepared nanoparticles were precipitated by adding an excess amount of ethanol and separated by centrifugation. 2.4. Characterization
Fig. 2. A photograph of the as-prepared α-Fe2O3 nanoparticles (Fig. 1(d)) redispersed in a solution of hexane and water. The content of α-Fe2O3 nanoparticles in hexane is 0.5 wt.%.
mixture of 30 mL distilled water, 40 mL ethanol, and 70 mL toluene. For the synthesis of 2.8 mol/L iron oleate dissolved in toluene, 50 mmol of FeCl3·6H2O, 150 mmol of sodium oleate, 30 mL of distilled water, 40 mL of ethanol, and 20 mL of toluene were used. The solution was heated at 343 K for 4 h with mild mixing. After the mixing procedure, the dark upper organic layer was washed three times with 15 mL distilled water and was subsequently collected by using a separatory funnel. 2.3. Synthesis of α-Fe2O3 nanoparticles For the synthesis of α-Fe2O3 nanoparticles, a 28-wt.% aqueous ammonia solution was slowly titrated into the asprepared mixture of iron oleate and toluene at 303 K with
To evaluate the present phase of the prepared nanoparticles, XRD diagrams of all the samples were measured on a Rigaku Rint 2000 diffractometer (CuKα radiation). The morphology of the prepared particles was characterized by TEM performed on a JEM-1010 microscope. The samples for the TEM observations were prepared by adding several drops of a diluted hexane/ iron oxide suspension onto carbon grids. The surface of the prepared particles was investigated by FT-IR performed on Avatar 360 (Thermo Electron Co.). The dispersion stability of the prepared particles in organic solvents was determined by DLS measurement carried out on Malvern HPP5001. The suspension (1 wt.%) of the prepared nanoparticles that were redispersed in toluene or hexane was used for the DLS measurements. For studying the time-course DLS measurements, the synthesis solution that was ten-folded by toluene was used. A viscometer (RheoStress, Haake) was also used to determine the dispersion stability of the prepared particles in organic solvents. For the viscosity measurements, a raw synthesis solution was used. 3. Results and discussion 3.1. Synthesis of α-Fe2O3 nanoparticles and their characterizations The conversion of iron oleate to α-Fe2O3 nanoparticles were more than 90% under every experimental conditions. The TEM measurements of the as-synthesized particles are presented in Fig. 1. Fig. 1(a)–(c) shows the TEM images of particles prepared under different temperatures, where the reagent concentration was 2.8 × 10− 1 mol/L. The sizes of the synthesized particles measured
Fig. 3. Particle size distribution of the as-synthesized nanoparticles in hexane measured by the DLS method. (a) The effect of synthesis temperature. Particles were prepared at a reagent concentration of 2.8 × 10− 1 mol/L. (b) The effect of reagent concentration. Particles were prepared at 373 K.
Fig. 4. XRD spectrum of the as-synthesized nanoparticles.
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Fig. 5. FT-IR spectrums of sodium oleate and α-Fe2O3 nanoparticles prepared at a reagent concentration of 2.8 × 10− 1 mol/L at 373 K.
by the image processing of each TEM pictures were ca. 3 nm, 6 nm, and 8 nm when the synthesis temperature was 303 K, 343 K, and 373 K, respectively. Nanoparticles that were ca. 6 nm in diameter were also obtained without any formation of large particulates when the reagent concentration was increased to 2.8 mol/L at 373 K (Fig. 1(d)). Fig. 2 shows the solution of hexane and water in which the as-prepared α-Fe2O3 nanoparticles were redispersed. The as-prepared particles were redispersible only in the organic layer and it remained transparent even after the redispersion of the particles. The particle-size distribution of the assynthesized particles redispersed in hexane was measured by the DLS method and is presented in Fig. 3. The mean particle size of every sample was almost identical to the particle size observed under the TEM and shown in Fig. 1. From these results, we can state that all the as-synthesized particles were redispersible in organic solvents, such as hexane, to their primary particle size. Even the particles synthesized under extremely dense concentra-
tions were redispersible in organic solvents without any agglomerate formations. Decreases in the particle size due to the decrease in the synthesis temperature (Fig. 3(a)) and the increase in the reagent concentration (Fig. 3(b)) were observed. The reason why the particle size changed by the synthesis temperature and reagent concentration is not well known and needs further investigation which will be one of our future works. One estimation is that iron oleate acted as reagent and nuclei of αFe2O3 nanoparticles. By increasing the amount of iron oleate in the synthesis system, the iron oleate which acted as nuclei of nanoparticles increased and resulted in decrease of particle size. Fig. 4 presents powder XRD patterns of the synthesized particles. The reflection pattern, which is attributed to α-Fe2O3, can be observed in particles prepared at 373 K and with a reagent concentration of 2.8 × 10− 1 mol/L. The reflection peaks become small and broad as the synthesized particle size decreases.
Fig. 6. Time-course measurements of particle-size distribution in the synthesis solution by the DLS method. α-Fe2O3 nanoparticles were prepared at a reagent concentration of 2.8 × 10− 1 mol/L at 373 K.
Fig. 7. Time-course viscosity measurements of the synthesis solution. α-Fe2O3 nanoparticles were prepared at a reagent concentration of 2.8 mol/L at 373 K.
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In Fig. 5, the FT-IR spectra of sodium oleate and α-Fe2O3 particles prepared at 373 K and with a reagent concentration of 2.8 × 10− 1 mol/L are presented. In Fig. 5(a), strong peaks at 3006 cm− 1, 2962 cm− 1, 2924 cm− 1, and 2853 cm− 1 that correspond to CH stretching modes of the olefin group, asymmetric vibration of –CH3, asymmetric vibration of –CH2 −, and symmetric vibration of –CH2−, respectively, are observed in both spectrums. Bands attributed to CH2 scissoring modes are also observed at 1441 cm− 1 and 1465 cm− 1 in Fig. 5(b) [40,41]. From these results, it is obvious that oleate exists on the surface of the as-prepared particles. While the asymmetric vibration of COO– is observed at 1420 cm− 1 due to both sodium oleate and the as-prepared α-Fe2O3 particles, the symmetric vibration of COO– is observed at 1555 cm− 1 and 1520 cm− 1 due to sodium oleate and the as-prepared α-Fe2O3 particles, respectively, as shown in Fig. 5(b). Since it is known that the coordinating complex of carboxylic acid can be predicted by changes in Δν (νas − νs) [41], it is believed that the carboxylic group of oleate is forming a complex between the Fe atoms on the surface of the asprepared particles. 3.2. Changes in the dispersion stability of the synthesis solution with stirring time
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viscosity of the synthesis solution and forms aggregated structures in the initial stage of the stirring procedure. As stirring continues, the hydrolysis reaction of the Fe(OH)3 gel takes place and an αFe2O3 phase appears. Simultaneously, ammonium oleate or oleic acid works as a capping agent and reorganizes the nanoparticle structure. Thus, the viscosity of the synthesis solution decreases due to the disappearance of the Fe(OH)3 gel networks and their reorganization into α-Fe2O3 nanoparticles; finally, completely redispersible α-Fe2O3 nanoparticles are obtained. 4. Conclusion Oleate-capped α-Fe2O3 nanoparticles were prepared by the titration of an aqueous ammonia solution into a dense mixture of iron oleate and toluene at room temperature, atmospheric pressure, and at an extremely dense reagent concentration. The particle size was controlled by the synthesis temperature. Although the synthesis solution gelated at the early stage of the mixing procedure, nanoparticles that were completely redispersible in nonpolar solvents were obtained after stirring for 24 h. A transparent suspension was obtained by redispersing the as-prepared α-Fe2O3 particles in hexane. Acknowledgment
In order to analyze and discuss the synthesis mechanism of the highly dispersed α-Fe2O3 nanoparticles, time-course measurements of the particle-size distribution in the synthesis solution measured by the DLS method are shown in Fig. 6. The synthesis condition for Fig. 6 corresponded to a synthesis temperature of 373 K and a reagent concentration of 2.8 × 10− 1 mol/L. The time at which the titration of the ammonia solution was complete was defined as 0 min. An extremely large aggregated structure was formed as soon as the titration of ammonia aqueous solution was completed. On continuing the stirring, the size of the aggregated structure slowly decreased. Finally, after stirring for 24 h, redispersible nanoparticles ca. 8 nm in diameter were obtained. Similar results were obtained in the case of time-course viscosity measurements of the synthesis solution, as shown in Fig. 7; here, the synthesis temperature was 373 K and reagent concentration was 2.8 mol/L. Immediately after the titration of the aqueous ammonia solution, the viscosity of the synthesis solution increased due to the formation of a large aggregate structure. On continuing the stirring of the synthesis solution, a gradual decrease in its viscosity was observed. 3.3. Inferring the action mechanism of iron oleate during the nanoparticle synthesis Based on the above results, the action mechanism of iron oleate in the nanoparticle synthesis can be inferred. During the titration of the aqueous ammonia solution, it is expected that the OH– ion is consumed for nanoparticle synthesis to form Fe(OH)3 gel, which is widely known as an α-Fe2O3 precursor [42,43]; the NH4+ion forms a complex with the oleate group. Note that ammonium oleate is not a stable compound; therefore, it may decompose to ammonia gas and oleic acid during the stirring procedure [44,45]. The gelated Fe(OH)3 network increases the
This study was supported by a Grant-in-Aid for Scientific Research (B) from the Japanese Ministry of Education, Culture, Sports, Science and Technology and the Structurization of Material Technology Project in the Japan METI Nanotechnology Program. One of the authors (M. I.) thanks support from Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. References [1] J.L. Marignier, J. Belloni, M.O. Delcourt, J.P. Chevalier, Nature 317 (1985) 344. [2] A.P. Alivisatos, Science 271 (1996) 933. [3] G.A. Ozin, Adv. Mater. 4 (1992) 612. [4] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078. [5] E. Katz, I. Willner, Chem. Commun. 45 (2005) 5641. [6] K. Naoi, Y. Ohko, T. Tatsuma, Chem. Commun. 10 (2005) 1288. [7] A. Hagfeldt, M. Grätzel, Chem. Rev. 95 (1995) 49. [8] Q. Chen, Z.J. Zhang, Appl. Phys. Lett. 73 (1998) 3156. [9] B.H. Hong, J.Y. Lee, T. Beetz, Y. Zhu, P. Kim, K.S. Kim, J. Am. Chem. Soc. 127 (2005) 15336. [10] C. Feldmann, Adv. Mater. 13 (2001) 1301. [11] Y. Zheng, Y. Cheng, Y. Wang, F. Bao, L. Zhou, X. Wei, Y. Zhang, Q. Zheng, J. Phys. Chem., B 110 (2006) 3093. [12] J.Z. Jiang, R. Lin, W. Lin, K. Nielsen, S. Moerup, K. Dam-Johanssen, R. Clasen, J. Phys. D: Appl. Phys. 30 (1997) 1459. [13] K. Askan, V.H. Perez-Luna, Langmuir 18 (2002) 6059. [14] C. Li, B.C. Benicewicz, Macromolecules 38 (2005) 5929. [15] X. Fan, C. Xia, R.C. Advincula, Langmuir 19 (2003) 4381. [16] A.B. Bourlinos, S.R. Chowdhury, R. Herrera, D.D. Jiang, Q. Zhang, L.A. Archer, E.P. Giannelis, Adv. Funct. Mater. 15 (2005) 1285. [17] A.B. Bourlinos, R. Herrera, N. Chalkias, D.D. Jiang, Q. Zhang, L.A. Archer, E.P. Giannelis, Adv. Mater. 17 (2005) 234. [18] I. Capek, Adv. Colloid Interface Sci. 110 (2004) 49. [19] M.A. Lopez-Quintela, Curr. Opin. Colloid Interface Sci. 8 (2003) 137.
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[20] S. O’Brien, L. Brus, C.B. Murray, J. Am. Chem. Soc. 123 (2001) 12085. [21] P.D. Cozzoli, A. Kornowski, H. Weller, J. Am. Chem. Soc. 125 (2003) 14539. [22] T.J. Trentler, T.E. Denler, J.F. Bertone, A. Agrawal, V.K. Colvin, J. Am. Chem. Soc. 121 (1999) 1613. [23] Y. Jun, M.F. Casula, J.-H. Sim, S.Y. Kim, J. Cheon, A.P. Alivisatos, J. Am. Chem. Soc. 125 (2003) 15981. [24] M. Niederberger, M.H. Bartl, G.D. Stucky, J. Am. Chem. Soc. 124 (2002) 13642. [25] M. Niederberger, G. Garnweitner, F. Krumeich, R. Nesper, H. Cölfen, M. Antonietti, Chem. Mater. 16 (2004) 1202. [26] M. Niederberger, M.H. Bartl, G.D. Stucky, Chem. Mater. 14 (2002) 4364. [27] J. Polleux, N. Pinna, M. Aetonietti, M. Niederberger, Adv. Mater. 16 (2004) 436. [28] J. Rockenberger, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 121 (1999) 11595. [29] N.R. Jana, Y. Chen, X. Peng, Chem. Mater. 16 (2004) 3931. [30] J. Park, E. Lee, N.-M. Hwang, M. Kang, S.C. Kim, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, T. Hyeon, Angew. Chem. Int. Ed. 44 (2005) 2872. [31] J. Park, K. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang, T. Hyeon, Nat. Mater. 3 (2004) 891.
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Powder Technology 181 (2008) 45 – 50 www.elsevier.com/locate/powtec
Low-temperature synthesis of redispersible iron oxide nanoparticles under atmospheric pressure and ultradense reagent concentration Motoyuki Iijima a,⁎, Kimitoshi Sato b , Keiji Kurashima c , Takamasa Ishigaki b , Hidehiro Kamiya a a
Graduate School of Bio-Applications and Systems Engineering, BASE, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan b National Institute for Materials Science, Nano Ceramics Center, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan c National Institute for Materials Science, Materials Analysis Station, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan Received 17 August 2006; received in revised form 30 May 2007; accepted 20 June 2007 Available online 29 June 2007
Abstract Highly dispersed α-Fe2O3 nanoparticles ca. 3 to 8 nm in diameter were prepared at atmospheric pressure, low temperature, and at an ultradense reagent concentration by titrating an aqueous ammonia solution into a dense iron oleate/toluene mixture. A transparent suspension was obtained by redispersing the prepared particles in nonpolar solvents since they were redispersible to primary particles without aggregate formations. The prepared particles were characterized by TEM, XRD, and FT-IR, and their dispersion stability in organic solvents was determined by dynamic light scattering (DLS) and viscosity measurements. In order to analyze the formation process of the highly dispersed α-Fe2O3 nanoparticles, timecourse measurements of DLS and viscosity during the nanoparticle synthesis in toluene were carried out. A significant increase in the suspension viscosity and the formation of an aggregated structure were observed as soon as the titration of the aqueous ammonia solution. The suspension viscosity and aggregated particle size gradually reduced with continuous vigorous stirring; finally, α-Fe2O3 nanoparticles that were completely redispersible in nonpolar solvents were obtained after ca. 24 h. The particle size could be controlled by the synthesis temperature, and such redispersible α-Fe2O3 nanoparticles were obtained even when the reagent concentration was increased to 2.8 mol/L. © 2007 Elsevier B.V. All rights reserved. Keywords: Iron oxide; Nanoparticle; Iron oleate; Dispersion stability; Dense concentration
1. Introduction Considerable research has been focused on nanostructured materials because of their wide range of important applications. For example, nanoparticles possess novel size-dependent properties, such as electrical, magnetic, mechanical, optical, and chemical properties, which largely differ from those of their bulk materials [1–3].These nanoparticles have been used in bioprocessing [4], information storage media [5], color imaging [6], photochemical reactions [7], magnetic refrigeration [8], and catalysts [9]. α-Fe2O3 nanoparticles are one of such important materials which can be widely used as red pigments [10], catalysts [11], and gas sensors [12]. With regard to the handling of nanoparticles for the fabrication of nanocomposite materials or nanoparticle assembly, one of the major concerns is the prevention of nanoparticle aggregation by ⁎ Corresponding author. Tel./fax: +81 42 388 7068. E-mail address: [email protected] (M. Iijima). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.06.019
modifying the particle surface. There are two types of surface modification methods. One method involves adsorbing or grafting organic groups on nanoparticles by using various surfactants or silane alkoxides after nanoparticle synthesis [13– 17]. In this case, since a multistep procedure involving particle synthesis, drying, and redispersing is required, nanoparticles tend to form aggregates during the surface-modification procedure. Another method is the in situ modification process in which organic groups are introduced on the nanoparticle surface during the particle synthesis procedure [22–38]. Since the organic ligand in the synthesis solution controls particle growth and attaches to the particle surface during the procedure, redispersible nanoparticles can be obtained. Several different methods have been reported to obtain organic-capped nanoparticles by the in situ modification process, including reactions in reversed micelles [18,19], sol– gel reactions of metal alkoxides involving capping agents [20,21], nonhydrolytic sol–gel reactions of metal halides with capping agents [22–27], and thermal decomposition of metal
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M. Iijima et al. / Powder Technology 181 (2008) 45–50
complexes [28–31]. Although these methods produce nanoparticles that are redispersible in organic solvents, particle synthesis in a dilute solution and under high temperatures makes these methods unsuitable for industrial applications. A modified reverse micelle method in which the nanoparticle synthesis reaction occurs in the reverse micelle produced by metal–surfactant complexes, such as copper bis(2-ethylhexyl) sulfosuccinate, also produces redispersible nanoparticles [32– 38]. Although most studies on this modified reverse micelle method were performed using extremely dilute solutions to produce the reverse micelle structure, some reports show the possibility of particle synthesis under a dense reagent concentration of 2.7 × 10− 1 mol/L [32]. Wet chemical reactions using metal–surfactant complexes may provide a breakthrough with regard to increasing the reagent concentration during particle synthesis. In the present study, redispersible α-Fe2O3 nanoparticles were prepared by a wet chemical reaction using iron oleate as a reagent at atmospheric pressure, low temperature, and at an extremely dense reagent concentration. The synthesis temperature was between 303 K and 373 K, and the reagent concentration was 2.8 × 10− 1 mol/L of solvent or 2.8 mol/L of solvent, which was ca. 10 to 100 times greater than that in previously reported methods [18–39]. The prepared α-Fe2O3
nanoparticles were characterized by TEM, XRD, and FT-IR, and their dispersion stability in hexane was determined by dynamic light scattering (DLS) measurements. Furthermore, the action mechanism of iron oleate during nanoparticle synthesis was investigated by time-course DLS and viscosity measurements of the raw synthesis solution. 2. Experimental procedure 2.1. Materials All the materials were used without further purification. Iron chloride hexahydrate (FeCl3·6H2O, N99.0%), sodium oleate (N 90%), and ethanol (95%) were from Kanto Chemical Co., Ltd. (Tokyo, Japan), and toluene (N 99.5%) and 28 wt.% aqueous ammonia solution were obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). 2.2. Preparation of iron oleate/toluene mixture First, a mixture of iron oleate and toluene was prepared in a manner similar to that described by Park et al. [31]. For the synthesis of 2.8 × 10− 1 mol/L iron oleate in toluene, 20 mmol of FeCl3·6H2O and 60 mmol of sodium oleate were dissolved in a
Fig. 1. TEM observations of nanoparticles prepared under (a) 2.8 × 10− 1 mol/L, 303 K; (b) 2.8 × 10− 1 mol/L, 343 K; (c) 2.8 × 10− 1 mol/L, 373 K; and (d) 2.8 mol/L, 373 K.
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stirring. The additive content of the aqueous ammonia solution was 3.64 g and 9.10 g for the 2.8 × 10− 1 mol/L and 2.8 mol/L iron oleate solutions. After the titration, the synthesis solution was heated to 303 K, 343 K, or 373 K and maintained at that temperature for 24 h with stirring. The prepared nanoparticles were precipitated by adding an excess amount of ethanol and separated by centrifugation. 2.4. Characterization
Fig. 2. A photograph of the as-prepared α-Fe2O3 nanoparticles (Fig. 1(d)) redispersed in a solution of hexane and water. The content of α-Fe2O3 nanoparticles in hexane is 0.5 wt.%.
mixture of 30 mL distilled water, 40 mL ethanol, and 70 mL toluene. For the synthesis of 2.8 mol/L iron oleate dissolved in toluene, 50 mmol of FeCl3·6H2O, 150 mmol of sodium oleate, 30 mL of distilled water, 40 mL of ethanol, and 20 mL of toluene were used. The solution was heated at 343 K for 4 h with mild mixing. After the mixing procedure, the dark upper organic layer was washed three times with 15 mL distilled water and was subsequently collected by using a separatory funnel. 2.3. Synthesis of α-Fe2O3 nanoparticles For the synthesis of α-Fe2O3 nanoparticles, a 28-wt.% aqueous ammonia solution was slowly titrated into the asprepared mixture of iron oleate and toluene at 303 K with
To evaluate the present phase of the prepared nanoparticles, XRD diagrams of all the samples were measured on a Rigaku Rint 2000 diffractometer (CuKα radiation). The morphology of the prepared particles was characterized by TEM performed on a JEM-1010 microscope. The samples for the TEM observations were prepared by adding several drops of a diluted hexane/ iron oxide suspension onto carbon grids. The surface of the prepared particles was investigated by FT-IR performed on Avatar 360 (Thermo Electron Co.). The dispersion stability of the prepared particles in organic solvents was determined by DLS measurement carried out on Malvern HPP5001. The suspension (1 wt.%) of the prepared nanoparticles that were redispersed in toluene or hexane was used for the DLS measurements. For studying the time-course DLS measurements, the synthesis solution that was ten-folded by toluene was used. A viscometer (RheoStress, Haake) was also used to determine the dispersion stability of the prepared particles in organic solvents. For the viscosity measurements, a raw synthesis solution was used. 3. Results and discussion 3.1. Synthesis of α-Fe2O3 nanoparticles and their characterizations The conversion of iron oleate to α-Fe2O3 nanoparticles were more than 90% under every experimental conditions. The TEM measurements of the as-synthesized particles are presented in Fig. 1. Fig. 1(a)–(c) shows the TEM images of particles prepared under different temperatures, where the reagent concentration was 2.8 × 10− 1 mol/L. The sizes of the synthesized particles measured
Fig. 3. Particle size distribution of the as-synthesized nanoparticles in hexane measured by the DLS method. (a) The effect of synthesis temperature. Particles were prepared at a reagent concentration of 2.8 × 10− 1 mol/L. (b) The effect of reagent concentration. Particles were prepared at 373 K.
Fig. 4. XRD spectrum of the as-synthesized nanoparticles.
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Fig. 5. FT-IR spectrums of sodium oleate and α-Fe2O3 nanoparticles prepared at a reagent concentration of 2.8 × 10− 1 mol/L at 373 K.
by the image processing of each TEM pictures were ca. 3 nm, 6 nm, and 8 nm when the synthesis temperature was 303 K, 343 K, and 373 K, respectively. Nanoparticles that were ca. 6 nm in diameter were also obtained without any formation of large particulates when the reagent concentration was increased to 2.8 mol/L at 373 K (Fig. 1(d)). Fig. 2 shows the solution of hexane and water in which the as-prepared α-Fe2O3 nanoparticles were redispersed. The as-prepared particles were redispersible only in the organic layer and it remained transparent even after the redispersion of the particles. The particle-size distribution of the assynthesized particles redispersed in hexane was measured by the DLS method and is presented in Fig. 3. The mean particle size of every sample was almost identical to the particle size observed under the TEM and shown in Fig. 1. From these results, we can state that all the as-synthesized particles were redispersible in organic solvents, such as hexane, to their primary particle size. Even the particles synthesized under extremely dense concentra-
tions were redispersible in organic solvents without any agglomerate formations. Decreases in the particle size due to the decrease in the synthesis temperature (Fig. 3(a)) and the increase in the reagent concentration (Fig. 3(b)) were observed. The reason why the particle size changed by the synthesis temperature and reagent concentration is not well known and needs further investigation which will be one of our future works. One estimation is that iron oleate acted as reagent and nuclei of αFe2O3 nanoparticles. By increasing the amount of iron oleate in the synthesis system, the iron oleate which acted as nuclei of nanoparticles increased and resulted in decrease of particle size. Fig. 4 presents powder XRD patterns of the synthesized particles. The reflection pattern, which is attributed to α-Fe2O3, can be observed in particles prepared at 373 K and with a reagent concentration of 2.8 × 10− 1 mol/L. The reflection peaks become small and broad as the synthesized particle size decreases.
Fig. 6. Time-course measurements of particle-size distribution in the synthesis solution by the DLS method. α-Fe2O3 nanoparticles were prepared at a reagent concentration of 2.8 × 10− 1 mol/L at 373 K.
Fig. 7. Time-course viscosity measurements of the synthesis solution. α-Fe2O3 nanoparticles were prepared at a reagent concentration of 2.8 mol/L at 373 K.
M. Iijima et al. / Powder Technology 181 (2008) 45–50
In Fig. 5, the FT-IR spectra of sodium oleate and α-Fe2O3 particles prepared at 373 K and with a reagent concentration of 2.8 × 10− 1 mol/L are presented. In Fig. 5(a), strong peaks at 3006 cm− 1, 2962 cm− 1, 2924 cm− 1, and 2853 cm− 1 that correspond to CH stretching modes of the olefin group, asymmetric vibration of –CH3, asymmetric vibration of –CH2 −, and symmetric vibration of –CH2−, respectively, are observed in both spectrums. Bands attributed to CH2 scissoring modes are also observed at 1441 cm− 1 and 1465 cm− 1 in Fig. 5(b) [40,41]. From these results, it is obvious that oleate exists on the surface of the as-prepared particles. While the asymmetric vibration of COO– is observed at 1420 cm− 1 due to both sodium oleate and the as-prepared α-Fe2O3 particles, the symmetric vibration of COO– is observed at 1555 cm− 1 and 1520 cm− 1 due to sodium oleate and the as-prepared α-Fe2O3 particles, respectively, as shown in Fig. 5(b). Since it is known that the coordinating complex of carboxylic acid can be predicted by changes in Δν (νas − νs) [41], it is believed that the carboxylic group of oleate is forming a complex between the Fe atoms on the surface of the asprepared particles. 3.2. Changes in the dispersion stability of the synthesis solution with stirring time
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viscosity of the synthesis solution and forms aggregated structures in the initial stage of the stirring procedure. As stirring continues, the hydrolysis reaction of the Fe(OH)3 gel takes place and an αFe2O3 phase appears. Simultaneously, ammonium oleate or oleic acid works as a capping agent and reorganizes the nanoparticle structure. Thus, the viscosity of the synthesis solution decreases due to the disappearance of the Fe(OH)3 gel networks and their reorganization into α-Fe2O3 nanoparticles; finally, completely redispersible α-Fe2O3 nanoparticles are obtained. 4. Conclusion Oleate-capped α-Fe2O3 nanoparticles were prepared by the titration of an aqueous ammonia solution into a dense mixture of iron oleate and toluene at room temperature, atmospheric pressure, and at an extremely dense reagent concentration. The particle size was controlled by the synthesis temperature. Although the synthesis solution gelated at the early stage of the mixing procedure, nanoparticles that were completely redispersible in nonpolar solvents were obtained after stirring for 24 h. A transparent suspension was obtained by redispersing the as-prepared α-Fe2O3 particles in hexane. Acknowledgment
In order to analyze and discuss the synthesis mechanism of the highly dispersed α-Fe2O3 nanoparticles, time-course measurements of the particle-size distribution in the synthesis solution measured by the DLS method are shown in Fig. 6. The synthesis condition for Fig. 6 corresponded to a synthesis temperature of 373 K and a reagent concentration of 2.8 × 10− 1 mol/L. The time at which the titration of the ammonia solution was complete was defined as 0 min. An extremely large aggregated structure was formed as soon as the titration of ammonia aqueous solution was completed. On continuing the stirring, the size of the aggregated structure slowly decreased. Finally, after stirring for 24 h, redispersible nanoparticles ca. 8 nm in diameter were obtained. Similar results were obtained in the case of time-course viscosity measurements of the synthesis solution, as shown in Fig. 7; here, the synthesis temperature was 373 K and reagent concentration was 2.8 mol/L. Immediately after the titration of the aqueous ammonia solution, the viscosity of the synthesis solution increased due to the formation of a large aggregate structure. On continuing the stirring of the synthesis solution, a gradual decrease in its viscosity was observed. 3.3. Inferring the action mechanism of iron oleate during the nanoparticle synthesis Based on the above results, the action mechanism of iron oleate in the nanoparticle synthesis can be inferred. During the titration of the aqueous ammonia solution, it is expected that the OH– ion is consumed for nanoparticle synthesis to form Fe(OH)3 gel, which is widely known as an α-Fe2O3 precursor [42,43]; the NH4+ion forms a complex with the oleate group. Note that ammonium oleate is not a stable compound; therefore, it may decompose to ammonia gas and oleic acid during the stirring procedure [44,45]. The gelated Fe(OH)3 network increases the
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