Iron Nitride and Carbide: from Crystalline ... - ACS Publications

Jun 19, 2012 - Iron nitride and carbide nanoparticles were synthesized using iron oxide particles as template. They were furthermore dispersed in aque...
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Iron Nitride and Carbide: from Crystalline Nanoparticles to Stable Aqueous Dispersions Constanze Schliehe,† Jiayin Yuan,† Stefan Glatzel,† Konrad Siemensmeyer,‡ Klaus Kiefer,‡ and Cristina Giordano*,† †

Max Planck Institute of Colloids and Interfaces, 14476 Potsdam, Germany Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: Iron nitride and carbide nanoparticles were synthesized using iron oxide particles as template. They were furthermore dispersed in aqueous solution via stabilization with a poly(ionic liquid). They provide a great potential combining a high saturation magnetization with low toxicity compared to the iron based compounds that are currently used in several applications such as cell-sorting and hyperthermia or as contrast enhancers for magnetic resonance imaging. We here present a sustainable and green procedure to synthesize iron nitride and carbide by resorting to the variety of iron oxide template nanoparticles. In this way the shape and the size can be precisely controlled and tuned within the nanometer range. During calcination, urea enables to control the composition of the product material, whereas a biopolymer agar protects the particles from agglomeration. We dispersed the particles in water by using poly(1-ethyl-3-vinylimidazolium bromide) as stabilizing agent. Magnetic measurements of the converted particles show that particles with a diameter of 18 nm are located at the border of superparamagnetic and ferromagnetic behavior. As expected after conversion the saturation magnetization of the particles was notably increased. The herein presented synthetic approach can be applied to other metals and has thus the potential to be important for the synthesis of nitrides and carbides in general. KEYWORDS: iron nitride, iron carbide, superparamagnetism, aqueous dispersion, polyionic liquid, nanoparticles

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accurate control over size and shape is achieved.13,14 Further, the influence of parameters like the particle size or the organic ligand shell on the magnetic properties has been studied to optimize the applicability.9,15 In case of the former aspect an increasing particle size leads to an increase of the saturation magnetization.16 Further improvements in terms of higher saturation magnetizations can be achieved by directing the attention to other iron based compounds. Iron nitride and iron carbide, for example, exhibit a maximum bulk magnetization of 123 emu/g and 140 emu/g, which is 34% and 52%, respectively, larger than the value of 92 emu/g for iron oxide.17 But the syntheses of iron nitride and carbide nanoparticles are by far less investigated than the ones for iron oxide particles. However, some methods describe how to synthesize these materials by nanocluster decomposition,18 sonication of amorphous iron powder,19 or by a precursor technique using ultrafine oxide powder.20 But these approaches result in unstructured material, a high amount of impurities, or a low crystallinity. The past few years our group has been working on the syntheses of several metal nitrides and carbides,21 focusing on iron carbide and nitride nanostructures.22−25 For example, superparamagnetic iron carbide

INTRODUCTION In the past decades, inorganic nanoparticles became prominent in various fields of scientific research and applications.1,2 Because of the small size, the characteristics of nanocrystalline materials can differ drastically from the corresponding bulk material. For magnetic materials confinement effects become dominant once the particle size falls below a critical diameter where the domain-wall energy becomes larger than the magnetostatic energy. Consequently the total energy of the nanoparticles is reduced by the formation of a single magnetic domain over the whole particle. These single-domain nanoparticles show superparamagnetic behavior featured by a large constant magnetic moment and at the same time a fast response with negligible remanence.3 These characteristics offer magnetic nanoparticles as sources for various applications like magnetic fluids,4 catalysis,5 or magnetic energy storage.6 Beside these, the use of magnetic nanoparticles is arising in fields of biology and medicine with methods like cell-sorting,7 hyperthermia,8 or magnet resonance tomography.9 By variation of the organic shell surrounding the inorganic particles a precise targeting of the nanoparticles in biological tissue can be achieved.10 Currently iron oxide, because of its low toxicity, is one of the commonly used materials in terms of biological or medical applications of magnetic nanoparticles.11,12 The synthesis of iron oxide nanoparticles is already well-investigated, and an © 2012 American Chemical Society

Received: March 7, 2012 Revised: June 18, 2012 Published: June 19, 2012 2716

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described here is demonstrated for iron based materials via oxide template particles, we believe that it has the potential to be transferable to a wide range of metal nitrides and carbides using appropriate template particles, as their similar chemical behavior has been shown before.21,32 We thus see this chemical procedure as a general approach in the synthesis of metal nitrides and carbides regarding the size and shape control.

nanoparticles of an average size of 5−10 nm were synthesized via a soft urea pathway. Through this simple procedure, the metal precursor, ethanol, and urea formed a glassy phase which was then calcined under inert atmosphere.24 We were also able to synthesize particles of a larger diameter via a biopolymer route.25 In this one-pot procedure, large iron carbide nanoparticles of an average diameter of ∼25 nm formed. These particles showed ferromagnetic behavior, which could be used for applications.26 It is our interest to take the next step in the direction of particle control and synthesize superparamagnetic particles with a tunable size within a desired nanometer range. A closer look at the mechanism of the biotemplating method showed that in a first step iron oxide particles were formed from the homogeneous, iron containing biopolymer mixture, which in a second in situ step, transformed into iron carbide nanostructures. The transcription of metal oxide into the corresponding nitrides has already been studied for various materials by using melamine, urea, or cyanamide as agents27,28 in which the control over the material composition was the focus of the investigations. But along with the required reaction temperatures of 800 °C−1000 °C and the crystallographic properties, an agglomeration and fusion of the nanostructured material took place because of the proposed dissolution-recrystallization mechanism. In this way it was not possible to conserve the shape and size of the particles during the transcription reaction. In terms of applications in the fields of biology or medicine a further encapsulation of the inorganic material with an organic shell is required. This should allow the formation of particle dispersions in aqueous media on the one hand and the stabilization of the inorganic material against dissolution and thus the release of toxic ions into the aqueous media on the other hand.29 Various methods have already been described in literature, especially for solution-processed particles. The presence of stabilizing molecules on the particles' surface allows different methods like ligand exchange, encapsulation, or micelle formation to achieve an effective stabilization.30 For solid state processed iron carbide particles the use of ionic liquids (ILs) is described as stabilizing agent. Here iron is removed from the surface, which allows stabilization of the particles by an IL-carbon interaction.31 Nevertheless the efficiency of the stabilization depends sensitively on the precise system, meaning the inorganic material and the functional groups of the stabilizing agent. In the present paper we focused on iron based materials and introduce a sustainable and green method to synthesize both, iron nitride and iron carbide nanoparticles. Using iron oxide nanoparticles as a template during the reaction, we want to conserve the variety of sizes and shapes of iron oxide template particles. Our procedure enables us to overcome the difficulties of recrystallization despite using moderate temperatures of 800 °C−1000 °C. Here, urea resumes the role as source for nitrogen and carbon respectively, whereas agar is responsible for the separation of the particles during the reaction by embedding the nanoparticles into a rigid carbon matrix. The particles synthesized in this way are coated with a shell of ordered carbon and can thus be isolated from amorphous carbon matrix by treatment with hydrogen peroxide. The remaining carbon shell can stabilize and protect the particles from dissolution and oxidation. To form a dispersion of the synthesized particles in water, we show how to treat the particles using a poly(IL) polymer as stabilizing agent, utilizing their interaction with the carbon shell. Although the procedure

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EXPERIMENTAL SECTION

Iron oxide template nanoparticles with a diameter of approximately 15−20 nm were synthesized following the procedure reported in literature by Park et al.33 For transmission electron microscopy (TEM) image of the iron oxide particles see the Supporting Information, Figure S1. For a typical synthesis of iron nitride and carbide nanoparticles, iron oxide particles (10 mg) were mixed with urea (for iron nitride 5−20 mg; for iron carbide >20−200 mg) and agar (10 mg). This mixture was ground in a mortar to form a homogeneous powder and combined with a small amount of water. The resulting slurry was then transferred into lidded crucibles and calcined in a box furnace under nitrogen atmosphere with the following program: 0.5 h nitrogen flow; in 10 K/ min up to 800 °C; 1 h at 800 °C. The resultant black powder contained the converted nanoparticles embedded in a carbon environment. The synthesized nanoparticles were isolated from the matrix by treatment with 30% H2O2. For this the powder was combined with an excess of H2O2 solution and stirred for several hours. The particles were separated from the solution with a magnet and washed thrice with water. X-ray diffraction (XRD) analysis of the particles before and after the H2O2 treatment showed that the washing process does not affect the particles, indicating the high stability of the prepared nanoparticles (see XRD in the Supporting Information, Figure S11). The total amount of oxygen is small (3−4%) and therefore is not expected to alter the magnetic properties. The isolated nanoparticles were dispersed in water by using poly(1ethyl-3-vinylimidazolium bromide) (PEVImBr) as ionic stabilizing agent. For this the particles (30 mg) were added to a solution of 100 mg of PEVImBr in 10 mL of water. The dispersion was then sonicated in an ultrasonic bath. After separation by ultracentrifugation the particles were washed several times to remove the excess of stabilizing agent. They were redispersed in water forming a stable dispersion for several weeks.

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RESULTS AND DISCUSSION We established a procedure that allows to synthesize iron nitride and iron carbide nanoparticles by using iron oxide as template material. The important point here is that within the conversion of the material we were able to conserve shape and size of the template particles. As can be seen in Figure 1, depending on the features of the template material (A−C), the size and shape of the product particles (D−F) were determined. Starting with big, spherical iron oxide particles results in the formation of big, spherical carbide particles, whereas cuboidal particles are formed if cuboidal particles are used as templates. Analogous is the behavior for iron nitride particles. The statistic size distribution was determined for two systems with different sized particles. The particle diameter of 21 nm (or 19 nm respectively) of the starting material is 5− 10% larger than the one of the product particles that have a diameter of 18 nm (or 16 nm respectively; see histogram in the Supporting Information, Figure S2). This can be explained with the higher density of the product material and thus a contraction of the particle volume during the reaction. The challenge of the synthetic route to iron nitride and iron carbide nanoparticles by using iron oxide template particles arises from the material specific properties. In most of the cases, 2717

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decomposes at higher temperatures with the formation of nitrogen and amorphous carbon. Urea can thus be used as a reducing agent and serve as nitrogen and carbon source.34 We discovered that by varying the amount of urea added to the reaction mixture the final material composition is tunable over a range including iron oxide, Fe0, iron nitride, and carbide (for XRDs see the Supporting Information, Figure S4 ). In detail, a weight ratio of iron oxide/urea of 1:1 leads to the formation of iron oxide particles, whereas a ratio of 1:5 results in Fe0 particles. With a further increase of the ratio to 1:10 iron nitride is synthesized as the main product and at 1:15 the main product is iron carbide. Finally, at a ratio of 1:20, pure iron carbide is formed. We followed the structural appearance of the particles at different reaction temperatures via TEM. At 700 °C the template particles are embedded in a thick organic shell of 10−20 nm (for TEM images see the Supporting Information, Figure S5). Because of the decomposition of the organic material, for particles treated at 800 °C the thickness of the organic shell is reduced to 5−10 nm (for TEM images see the Supporting Information, Figure S6). The corresponding XRD spectra show that the conversion of the material starts at 700 °C. Below this temperature the material still consists of iron oxide. At 700 °C a mixture of different phases of oxide, nitride, and carbide is present, whereas above this temperature the final material composition is achieved (see the Supporting Information, Figure S7). Although the particles are covered by an organic surrounding, the particles are not prevented from agglomeration and fusion because of its soft and flexible nature. We now introduced a second agent, agar, to the reaction mixture. The polymeric molecule melts at a temperature of 85 °C, and in combination with water it forms a gel.35 We used this gel to embed the template particles during the whole reaction and thus keep them separated. To determine the influence of agar, experiments excluding urea from the reaction mixture were conducted. The comparison of the XRDs of particles treated exclusively with urea and agar respectively shows a different broadening of the peaks, indicative of a successful separation of the particles. Calculations of the diameter of the crystalline parts with the Scherrer equation using the FWHM of the peaks suggest a size of 50 nm for the particles synthesized with urea and 15 nm for those synthesized with agar (see the Supporting Information, Figure S8). Treated at higher temperatures, the agar-gel decomposes and forms a rigid matrix of carbon. The TEM images confirm this: the synthesized particles are embedded in a carbon matrix and are thus kept separated from each other. Expectably because of the oxygen rich structure of agar, we observed that a variation of the amount of agar could influence the composition of the particles between iron oxide and Fe0, but did not result in the formation of iron carbide particles even for high amounts of agar (for XRD see the Supporting Information, Figure S9). But the more agar was added to the reaction mixture the denser the matrix made of carbon became (for TEM images see the Supporting Information, Figure S10). For further experiments we thus used small amounts of agar that led to an effective separation of the particles but not to a superfluous amount of carbon. To combine the control over the composition induced by urea and the control over the separation of the particles induced by agar, both agents were added to the reaction mixture at the same time. For further experiments we used a constant ratio of agar/iron oxide of 1:1. By using this mixture in addition with different ratios of urea/iron oxide from 1:2 to

Figure 1. TEM images of different iron oxide template particles (A− C) and of the corresponding particles after the conversion into iron carbide (D−F).

and likewise for iron, the reduction of the oxides requires a harsh treatment with temperatures beyond 1000 °C. For iron, the transcription of oxide into nitride or carbide has to go along with the reduction of the Fe2+/3+ ions to Fe0. Furthermore, the crystal structures of the template and the product material differ greatly: iron oxide is a cubic crystal system with an inverse spinel structure. In contrast, iron nitride and carbide crystallize in an orthorhombic and hexagonal structure, respectively. That means, during the conversion the crystal structure has to undergo a rearrangement. Hence the challenge is to minimize the side effects of this recrystallization to conserve the structural features, here the shape and size of the nanoparticles. In an initial trial iron oxide nanoparticles were calcined utilizing the temperature program described in the Experimental Section without any separating agents. Here it was demonstrated that agglomeration and fusion of the particles affects the formation of aggregates. During calcination the nanoparticles lose their structural features, and large unstructured crystals are formed (for TEM image see the Supporting Information, Figure S3). We reduced this agglomeration and fusion of the template particles by introducing urea and agar, suitable agents in terms of being cheap, green, and sustainable. We found that urea influences the material composition and agar keeps the particles separated during the reaction. The simple structure of urea decomposes at around 133 °C with the release of ammonia and carbon dioxide and the formation of melamine as a condensation product. This further 2718

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factor in conserving the shape and size of the particles during synthesis, is not desired in some applications. Especially for applications in the fields of biology and medicine dispersibility of the nanoparticles in aqueous media is required. Consequently, for further processing it is necessary to isolate the particles from the carbon matrix. A closer look on the carbon rich matrix structure (HR-TEM in Figure 3B) indicates a difference between the carbon in the vicinity of the nanoparticles and the carbon which is further away. Directly on the surface of the particles the lattices of ordered carbon are visible, whereas the carbon which is not in contact with the particles is amorphous. This agrees with the fact that iron rich materials catalyze the formation of ordered carbon.36 The lack of a peak at 26° in the XRD (Figure 3C) supports the assumption that the majority of the carbon in the overall sample consists of amorphous carbon. To isolate the particles from the amorphous carbon a treatment with hydrogen peroxide was conducted. Hydrogen peroxide reacts selectively with amorphous carbon and does not interact with the ordered carbon.37 The resulting isolated particles are thus covered with a remaining shell of ordered carbon. XRD analysis showed no change in the composition of the particles. Thus the carbon shell protects particles from oxidation during the reaction with hydrogen peroxide (for XRDs see the Supporting Information, Figure S11). But also during the further treatment or even during the application, the carbon shell can protect the particles from oxidation or dissolution and is thus an advantage for the stability of the system. For further functionalization and stabilization of the particles the carbon shell can be used as an advantage. The similarity of the surface behavior of the carbon coated particles and carbon nanostructures like nanotubes or fullerenes permits the use of similar stabilizing systems. For example, a treatment of the carbon nanostructures with acids allows a further reaction of the introduced defect sites with a stabilizing agent by forming a covalent bond.38 Stabilization of carbon nanostructures without damage to the structure can be achieved, for example, with ILs and poly(IL)s (PILs), because of a cation-π interaction with the carbon structure.39−41 To disperse the iron carbide particles (accordingly with ICDD 00-035-0775) we used the PIL poly(1ethyl-3-vinylimidazolium bromide) (PEVImBr) as the stabilizing agent. By sonication of an aqueous solution of the isolated particles in the presence of PEVImBr, the PIL macromolecules bind firmly to the carbon surface of the magnetic particles. After washing away the excess of stabilizing agent and redissolving the particles in water, a dark dispersion was formed which was stable for several weeks without any precipitation. Herein the particles are individualized from each other (for TEM images see the Supporting Information, Figure S12). Figure 3E shows an image of the dispersion of the particles stabilized with PEVImBr in water. Even in the presence of a magnet for a short period, about 5 min (Figure 3E, right side), the particles do not precipitate from the solution. Without this stabilizer, the particles however started to precipitate after sonication was switched off. In the presence of a magnet the particles are attracted immediately resulting in a clear solution (see the Supporting Information, Figure S13). Finally the magnetic properties of the synthesized particles were investigated with a superconducting quantum interference device (SQUID) at 300 K to determine the saturation magnetization. The magnetization of the particles was measured before and after the conversion of the material. In this way, a direct comparison of the properties could be

20:1, it was still possible to tune the material composition from iron oxide through Fe0 and iron nitride to iron carbide. The corresponding XRDs are shown in Figure 2.

Figure 2. XRD pattern of the particles synthesized in the presence of agar (ratio of agar/Fe3O4 is 1:1) and different ratios of urea/Fe3O4: (a) 1:2, (b) 1:1, (c) 2:1, and (d) 20:1. Fe3O4 (▲, ICDD 00-0190629), FeO (■, ICDD 04-005-9718), Fe0 (●, ICDD 04-007-9753), FeN (▼, ICDD 00-003-1197), Fe3C (□, ICDD 00-035-0775), FeN0.05 (○, ICDD 01-075-2129).

Because of the synthetic procedure the nanoparticles described here are all embedded in a carbon matrix, as is illustrated in Figure 3A. This matrix structure, which was a key

Figure 3. (A) TEM image of the particles embedded in the carbon matrix. (B) HR-TEM image of particles and the carbon rich surrounding. (C) XRD pattern of the particles that shows the absence of ordered carbon. (D) TEM image of particles after treatment with hydrogen peroxide indicating the residual carbon shell. (E) Image of the particles dispersed in water without (left) and in the presence of (right) a magnet. 2719

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this way the dependency of the saturation magnetization on the particles size is illustrated. As mentioned before, beside the size dependency of the saturation magnetization, the magnetic character also changes with the particles size. Below a certain diameter the particles show superparamagnetic behavior, as we detected for the particles with a diameter of 5−10 nm,24 whereas the particles with an average diameter of 25 nm showed ferromagnetic behavior.25 A closer look on the SQUID plots (see insets in Figure 4) shows negligible values for the remnant magnetization and the coercivity in the case of the iron oxide particles. In the case of the iron carbide particles these values are slightly bigger than the range of measurement errors, and hence a small hysteresis is detected. The iron carbide particles with an average diameter of 18 nm are thus at the border of superparamagnetic and ferromagnetic behavior.

achieved. Representatively we here show the results for the spherical template iron oxide particles (accordingly with ICDD 00-019-0629) with an average diameter of 21 nm and the corresponding iron carbide particles (accordingly with ICDD 00-035-0775) with an average diameter of 18 nm. For this reason the iron carbide particles were measured as synthesized in the carbon matrix. The magnetic moment was measured by applying a magnetic field of ±5 T. For the iron oxide results, a small discrepancy in the negative field is detectable. This is probably due to the behavior of the nanosized powder material in a magnetic field. The measurements are shown in Figure 4.

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SUMMARY We presented a simple method to synthesize iron nitride and carbide nanoparticles. They hold a great potential to be applied in fields of biology and medicine because of their high magnetization and expected low toxicity at the same time. By resorting to the variety of iron oxide template particles we were able to synthesize iron nitride and iron carbide nanoparticles of tunable size and shape. The use of urea and agar as two sustainable and cheap agents made the retention of the features of the template particles possible. Here, the amount of urea determines the composition of the transcribed particles, whereas agar prevents the particles from agglomerating and fusing during the reaction by embedding them into a matrix. The as-synthesized particles are covered with ordered carbon, whereas the majority of the matrix is amorphous carbon. The particles were separated from the amorphous carbon matrix by treatment with hydrogen peroxide, resulting in particles covered with a shell of ordered carbon. On the one hand, this shell protects the particles from oxidation or ion release and on the other hand, enables stabilization. For this, the isolated particles were treated with PEVImBr as stabilizing agent to form dispersions in water, which are stable for a long period of time even in the presence of a magnetic field. Finally, the saturation magnetization of iron carbide particles of an average diameter of 18 nm was determined to be 88 emu/gFe3C, which is 20% higher than the one of the template particles. Because of their size, they are located at the border of superparamagnetic and ferromagnetic behavior.

Figure 4. SQUID measurements at 300 K of the iron oxide particles (solid line) and the corresponding iron carbide particles (dotted line).

In both cases of iron oxide (solid curve) and of iron carbide (dotted curve) particles, the saturation magnetization is approached within the available magnetic field range of ±5 T. From Figure 4 it can be noted that iron carbide particles reach the saturation magnetization faster than the iron oxide particles. Because the final saturation magnetization is not reached in the presented measurements, we describe and discuss the magnetization values at ±5T. The maximum magnetization reached here for the as-synthesized particles is 46 emu/g for the iron oxide and 64 emu/g for the iron carbide. Because of the different organic environments of the educt (oleic acid stabilizers) and the product (carbon matrix) particles, the values of the saturation magnetization were calculated for the pure iron oxide and iron carbide respectively. Thus, the values of the saturation magnetization are determined to be 72 ± 2 emu/gFe3O4 for iron oxide and 88 ± 2 emu/gFe3C for iron carbide. By transcribing the iron oxide into the iron carbide it was thus possible to increase the saturation magnetization per mass by 20%. The values for the saturation magnetization of the iron carbide particles synthesized in the presented way compared well with the values determined for the iron carbide particles synthesized by our group in the past. We thus believe that the presence of the carbon shell does not influence surface structure of the inorganic particles and thus the magnetic properties. For the small iron carbide particles of an average diameter of 5−10 nm synthesized with the soft-urea pathway a saturation magnetization of 46.7 emu/gFe3C was detected,24 whereas for the big iron carbide particles synthesized with a biotemplating method of an average diameter of 25 nm a saturation magnetization of 104 emu/gFe3C was observed.25 In

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ASSOCIATED CONTENT

* Supporting Information S

TEM image of the iron oxide template particles, size distribution of the particles, TEM of raw product, XRDs correlated to amount of urea, TEM at 700 °C, TEM at 800 °C, XRDs at different temperatures, XRD comparison urea and agar, XRDs correlated to amount of agar, TEM with variation of agar, XRDs of particles before and after treatment with hydrogen peroxide, TEM of the particles stabilized with PEVImBr, and an image of the iron oxide dispersion. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 2720

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Notes

H.; Parak, W. J.; Förster, S.; Beisiegel, U.; Adam, G.; Weller, H. Nano Lett. 2007, 7, 2422. (31) Khare, V.; Kraupner, A.; Mantion, A.; Jelicic, A.; Thünemann, A. F.; Giordano, C.; Taubert, A. Langmuir 2010, 26, 10600. (32) Li, P. G.; Lei, M.; Tang, W. L. Mater. Res. Bull. 2008, 43, 3621. (33) Park, J.; An, K.; Hwang, Y.; Park, J.; Noh, H.; Kim, J.; Park, J.; Hwang, N.; Hyeon, T. Nat. Mater. 2004, 3, 891. (34) Mitoraj, D.; Kisch, H. Chem.Eur. J. 2010, 16, 261. (35) Ayyad, O.; Munoz-Rojas, D.; Oro-Sole, J.; Gomez-Romero, P. J. Nanopart. Res. 2010, 12, 337. (36) Nasibulin, A. G.; Moisala, A.; Jiang, H.; Kauppinen, E. I. J. Nano Res. 2006, 8, 465. (37) Wang, Y.; Bai, X. D.; Liang, J. New Carbon Mater. 2005, 20, 103. (38) Marshall, M. W.; Popa-Nita, S.; Shapter, J. G. Carbon 2006, 44, 1137. (39) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2074. (40) Aida, T.; Fukushima, T. Phil. Trans. R. Soc. A 2007, 365, 1539. (41) Hong, S. H.; Tung, T. T.; Trang, L. K. H.; Kim, T. Y.; Suh, K. S. Colloid Polym. Sci. 2010, 288, 1018.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Fritz-Haber-Institute of the MPG for HR-TEM, the Laboratory for Magnetic Measurements at the Helmholtz-Zentrum Berlin for SQUID measurements, and the Max-Planck Society for funding.

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