pubs.acs.org/Langmuir © 2010 American Chemical Society
Synthesis of Necklace-like Magnetic Nanorings Hui Wang,† Qian-Wang Chen,*,† Yu-Bing Sun,† Ming-Sheng Wang,† Li-Xia Sun,† and Wen-Sheng Yan‡ †
Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, University of Science and Technology of China, Hefei 230026, China, and ‡National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China Received October 31, 2009. Revised Manuscript Received March 12, 2010 Necklace-like magnetite and maghemite nanorings, composed of magnetic nanoparticles (NPs) with average size about 40 nm, have been prepared via a solvothermal process in a colloidal solution by a self-assembly process. The composition, phase, and morphology of these nanorings have been characterized by X-ray diffraction, X-ray absorption, and transmission electron microscopy. In this paper, we discuss the influence of reaction conditions on the formation of nanorings structure including the amount of PVP in starting materials, reaction time, and temperature. On the basis of experimental observation, we supposed that magnetite NPs may first assemble into chains by magnetic dipole-dipole interactions. These dipolar chains, which are metastable structures relative to necklace-like nanorings, then produced the rings. So, the stability of chains may determine the yield, size, and morphologies of necklace-like nanorings.
1. Introduction Necklace-like nanorings composed of dozens of magnetic particles have attracted much attention among researchers because of their special properties and potential applications in magnetoresistive random-access memories (MRAM).1-4 In the past several years, the synthesis and properties of magnetic nanorings have been the focus of continuously experimental and theoretical research since Monte Carlo simulations predicted the formation of dipolar chains and necklace-like rings.5-9 Hitherto various approaches have been developed to obtain 1D structured magnetic nanorings. For example, both Philipse and Klokkenburg et al. observed NPs ring structures in magnetite colloidal dispersions.10,11 First, Wei and co-workers prepared bracelet-like cobalt NPs rings dipolar self-assembly.12 Pyun et al. investigated self-assembly of polymer-coated cobalt NPs in solutions cast onto supporting substrates, where a large number of nanorings could be observed under certain conditions.13 The *Corresponding author: Fax þ81 551 3607292; e-mail
[email protected].
(1) Tripp, S. L.; Dunin-Borkowski, R. E.; Wei, A. Angew. Chem., Int. Ed. 2003, 42, 5591–5593. (2) Kasama, T.; Dunin-Borkowski, R. E.; Scheinfein, M. R.; Tripp, S. L.; Liu, J.; Wei, A. Adv. Mater. 2008, 20, 4248–4252. (3) Li, S. P.; Peyrade, D.; Natali, M.; Lebib, A.; Chen, Y.; Ebels, U.; Buda, L. D.; Ounadjela, K. Phys. Rev. Lett. 2001, 86, 1102–1105. (4) Rothman, J.; Kl€aui, M.; Lopez-Diaz, L.; Vaz, C. A. F.; Bleloch, A.; Bland, J. A. C.; Cui, Z.; Speaks, R. Phys. Rev. Lett. 2001, 86, 1098–1101. (5) Jia, C. J.; Sun, L. D.; Luo, F.; Han, X. D.; Heyderman, L. J.; Yan, Z. G.; Yan, C. H.; Zheng, K.; Zhang, Z.; Takano, M.; Hayashi, N.; Eltschka, M.; Kl€aui, M.; R€udiger, U.; Kasama, T.; Cervera-Gontard, L.; Dunin-Borkowski, R. E.; Tzvetkov, G.; Raabe, J. J. Am. Chem. Soc. 2008, 130, 16968–16977. (6) Weis, J. J. Mol. Phys. 1998, 93, 361–364. (7) Tavares, J. M.; Weis, J. J.; Telo da Gama, M. M. Phys. Rev. E 2002, 65, 061201-1–061201-11. (8) Weis, J. J. J. Phys.: Condens. Matter 2003, 15, 1471–1495. (9) Chantrell, R. W.; Bradbury, A.; Popplewell, J.; Charles, S. W. J. Phys. D: Appl. Phys. 1980, 13, 119–122. (10) Philipse, A. P.; Maas, D. Langmuir 2002, 18, 9977–9984. (11) Klokkenburg, M.; Vonk, C.; Claesson, E. M.; Meeldijk, J. D.; Erne, B. H.; Philipse, A. P. J. Am. Chem. Soc. 2004, 126, 16706–16707. (12) Tripp, S. L.; Pusztay, S. V.; Ribbe, A. E.; Wei, A. J. Am. Chem. Soc. 2002, 124, 7914–7915. (13) Keng, P. Y.; Shim, I.; Korth, B. D.; Douglas, J. F.; Pyun, J. ACS Nano 2007, 1, 279–292.
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synthesis of magnetite nanorings via a simple solvothermal method was first reported by our group.14 Soon after, Yu and co-workers reported high yield synthesis of Ni-Co magnetic alloy nanorings via a solvothermal method.15 However, so far, the cognition for fabrication and formation mechanism of nanorings is still inadequate, which may decrease the possibility for preparing magnetic nanorings with high yield and different size.15 According to previous reports, it is understood that the formation of nanorings may result from degradation of dipolar chains. The length and stability of chains, therefore, might determine the yield and size of rings. For example, long chains produced by assembly of more NPs tend to form larger rings by dipolar assembly. Therefore, a deeper understanding of the evolution and changes of dipolar chains in a dispersion system will be very useful for the high yield and controlled synthesis of nanorings. The purpose of this article is to investigate the mechanism of ring formation and realize the controlled synthesis of magnetite nanorings. Using ferrocene ((C5H5)2Fe), polyvinylpyrrolidone (PVP), and hydrogen peroxide (H2O2) as starting materials, we have successfully prepared high-yield magnetite NPs (∼40 nm) rings via a simple solvothermal process at 230 °C. The composition, phase, and morphology of these ordered nanostructures were characterized by X-ray diffraction, X-ray absorption, and transmission electron microscopy.
2. Experimental Section 2.1. Materials. Ferrocene (Fe(C5H5)2, g98%), polyvinylpyrrolidone (PVP, K30), hydrogen peroxide (H2O2, 30%), and ethanol (CH3CH2OH, g99%) were purchased from Shanghai Chemical Factory, China. All chemicals were used as received without further purification. 2.2. Synthesis of Magnetite Nanorings. In a typical synthesis, 1.0 g of ferrocene and 5.0 g (8.0 g) of PVP were dissolved in a mixed solution of distilled water (28.0 mL) and alcohol (7.0 mL). (14) Xiong, Y.; Ye, J.; Gu, X. Y.; Chen, Q. W. J. Phys. Chem. C 2007, 111, 6998– 7003. (15) Hu, M. J.; Lu, Y.; Zhang, S.; Guo, S. R.; Lin, B.; Zhang, M.; Yu, S. H. J. Am. Chem. Soc. 2008, 130, 11606–11607.
Published on Web 03/19/2010
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Figure 2. (a, c) TEM images of S1. (b) SEM image of S1. (d) TEM image of an necklace-like nanoring. (e, f) Lattice fringe and electron diffraction (ED) pattern of NP, respectively.
Figure 1. (a) X-ray diffraction patterns of the as-prepared samples (S1 and S2). (b) XAS spectra at the Fe L-edge of the as-prepared samples (S1 and S2). The inset shows the respective expanded plots for fields between 705 and 710 eV. After intense sonication with a ultrasonic cleaner (S10200A, Automatic Science Instrument CO., LTD) for 15 min with a frequency of 40 kHz and a power density of 0.4 W/cm2, the mixture solution was vigorously stirred for 15 min with a magnetic stirring apparatus. Then, 1.0 mL of hydrogen peroxide was slowly added into the above mixture solution, which was vigorously stirred for 15 min again. After that, the precursor solution was transferred onto the Teflon-lined stainless autoclave with the total volume of 50.0 mL and then heated to and maintained at 230 °C. After 48 h, the autoclave was cooled naturally to room temperature. In order to observe the morphology of products in the autoclave, solution from the autoclave was directly transferred onto carbon-coated copper grids for SEM and TEM observation. Two typical samples are defined as Sample 1 (S1) and Sample 2 (S2), which correspond to different amounts of PVP used in the nanoparticle synthesis (5.0 and 8.0 g, respectively). The products were collected for further characterization by applying a magnet with 0.10 T, and the supernatant was discarded under a magnetic field. The precipitates were then washed by ethanol four times to remove excess ferrocene and PVP. Finally, the black products were dried under vacuum. 2.3. Sample Characterization. The powder X-ray diffraction (XRD) patterns were collected on a Japan Rigaku D/MAX-γA 5958 DOI: 10.1021/la9041343
Figure 3. (a, c) TEM images of S2. (b) SEM image of S2. (d) TEM image of an necklace-like nanoring. (e, f) Electron diffraction (ED) pattern and lattice fringe of NP, respectively. X-ray diffractometer equipped with Cu KR radiation (λ = 1.541 78 A˚) over the 2θ range of 10°-70°. X-ray absorption spectroscopy (XAS) were performed at beamline U19 of National Synchrotron Radiation Laboratory (NSRL, China) in the total electron yield detection mode at 300 K. The estimated photonenergy resolution at the Fe L3 edge was better than 0.2 eV, and the magnetic field applied was 0.1 T. Transmission electron microscopy (TEM) images were obtained on Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 transmission electron microscope operating at 200 kV. Field emission scanning electron microscopy (FE-SEM) images were performed on a JEOL JSM-6700 M scanning electron microscope. Magnetic studies were carried out with a vibrating sample magnetometer (VSM, BHV- 55) on a physical properties measurement system at room temperature. Langmuir 2010, 26(8), 5957–5962
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Figure 4. TEM images of S1 with different reaction time: (a) 3, (b) 6, (c) 12, (d) 24, and (e, f) 48 h.
3. Results and Discussion Typical X-ray diffraction (XRD) patterns of the as-prepared samples (S1 and S2) are shown in Figure 1a. All of the reflection peaks have been indexed, which correspond to iron oxides (JCPDS file 19-0629, magnetite, or JCPDS file 39-1346, maghemite). However, it is hard to identify the iron oxides as Fe3O4 or γ-Fe2O3 simply by XRD due to their same spinel structure and similar lattice parameter a (0.8346 nm for γ-Fe2O3 and 0.8396 nm for Fe3O4).16 Therefore, XAS was used to identify the crystal phase of the products. Figure 1b shows the Fe L2,3-edge X-ray absorption of nanorings. The splitting of the L3 peak (705710 eV) is an important characteristic to distinguish magnetite and maghemite. We observe that the splitting of A, B peak of S1 and S2 are 1.1 and 1.4 eV, respectively, which is corresponding to film sample reported (1.2 eV for Fe3O4, 1.4 eV for γ-Fe2O3).17,18 The largest magnetic field strength the equipment could supply is 1000 Oe. However, our samples are unable to reach magnetic saturation under such weak magnetic field, which could result in the weak intensity of peak A. The morphology of the as-obtained products was characterized by TEM and SEM. Representative TEM images of S1 (Figure 2a, c) clearly show the shape of necklace-like nanorings, and the high yield of nanorings in S1 is also confirmed by SEM image (Figure 2b). A high-magnification TEM image (Figure 2d) shows that the ring is necklace-like and has one-particle (∼40 nm) annular thickness. Also, Figure 2a-d reveals that each ring contains of dozens of cubic magnetite NPs. The clear 2D lattice fringes are shown in Figure 2e. The interplanar distance is 0.4975 nm, which corresponds to the (111) lattice planes of magnetite. A typical SAED pattern of single nanoparticle is shown in Figure 2f, and the sharp diffraction spot can be found, which (16) Thewlis, J. Philos. Mag. 1931, 12, 1089. (17) Krasnikov, S. A.; Vinogradov, A. S.; Hallmeier, K. H.; Hohne, R.; Ziese, M.; Esquinazi, P.; Chass, T.; Szargan, R. Mater. Sci. Eng., B 2004, 109, 207–212. (18) Ge, J. P.; Hu, Y. X.; Biasini, M.; Beyermann, W. P.; Yin, Y. D. Angew. Chem., Int. Ed. 2007, 46, 4342–4345.
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indicates the single-crystalline nature of the NPs. Similar morphology has also been found for S2, which is shown in Figure 3a-d. The clear 2D lattice fringes are shown in Figure 3f. The interplanar distance is about 0.432 nm, which corresponds to the (111) lattice planes of maghemite. Like S1, the typical SAED pattern of single NP in Figure 3e confirms the single-crystalline nature of the NPs. As described above, we successfully synthesized magnetite and maghemite necklace-like nanorings in large scale, which has rarely been reported. However, no clear evidence have been shown to confirm the mechanism of the formation of these nanorings in the literature. The magnetic properties of nanoparticles and other kinetic factors such as assembly time, PVP, and temperature may have great influence on the formation of nanorings.13,15 Therefore, we first confirmed that under our experimental conditions whether the magnetic dipolar interactions between NPs plays a key role when self-assembly process of nanoring occurs (see the next paragraph). Crystallinity and size of nanoparticles are two major factors that determine the magnetic dipole.19,20 By changing reaction time, we achieve a series of samples with different crystallinity but similar size (Figure 4). It is clear that although the shapes of NPs are not regular, when reaction time is only 3 h (Figure 4a), ring structure has already formed and dominated. Since the possibility of evaporation-driven hole formation was excluded in our previous report,14 the formation process of nanorings is completed when they are still in the solution. The low yield of nanorings obtained in Figure 4a can be attributed to few ferrocene attending to reaction (the solution had almost the same color as starting ferrocene solution). The existence of nanorings indicated that although longer reaction time leads to better crystallinity, it is not the factor determining the formation of nanorings. (19) Murad, E.; Bowen, L. H.; Long, G. J.; Quin, T. G. Clay Miner. 1988, 23, 161–173. (20) Chatterjee, J.; Haik, Y.; Chen, C. J. J. Magn. Magn. Mater. 2003, 257(1), 113–118.
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Figure 5. TEM and SEM images reveal the influence of the reaction temperature on the formation of necklace-like nanorings in S1 (a-c) and S2 (d-f): (a, d) 210, (b, e) 230, and (c, f) 250 °C.
Figure 6. TEM images reveal the influence of the amount of PVP in starting materials on the formation of necklace-like nanorings in S1: (a, b) 3.0, (c, d) 5.0, and (e, f) 8.0 g.
It has been suggested that high temperature may decrease the energy difference between chains and rings, leading to increasing amount of chains.21 However, as displayed in Figure 5, with the temperature increased from 210 °C (Figure 5d) to 250 °C (21) Borrmann, P.; Stamerjohanns, H.; Hilf, E. R.; Jund, P.; Kim, S. G.; David Tomanek, D. J. Chem. Phys. 1999, 23, 10689–10693.
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(Figure 5f), both the amount and the diameter of nanorings became larger. One possible explanation to this result could be that the chains formed initially by the assembly of nanoparticles. When the temperature is higher, more NPs were absorbed by PVP and formed longer nanoparticle chains. In addition, higher temperature can improve the mobility of chains, which can enhance the possibility of intra- and interchains interplay. Therefore, Langmuir 2010, 26(8), 5957–5962
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Figure 7. Necklace-like nanorings with different size in S1.
Figure 8. Byproducts with different morphology in S1, showing the evolution and changes of dipolar chains in a dispersion system.
larger nanorings can be formed when the temperature is higher. It should be noted that larger magnetic dipolar interaction make little contribution to this enhancement, based on our observation of nanorings after 3 h reaction mentioned above. That is because the crystallinity of nanoparticles in Figure 4a is even worse than what we got at lowest temperature (210 °C). By changing the amount of PVP only, we find that it has significant influence on the formation of nanorings. As is shown in Figure 6, when the amount (3 g) of PVP is low, only several chains or unclosed rings can be found (Figure 6a). The ring started to form when the amount (5 g) of PVP is higher and almost remained the same with higher PVP concentration (8 g). This can also be reinforced by comparing Figure 6c to 6f and Figure 6b to 6e, respectively. PVP could change viscosity of solutions and at Langmuir 2010, 26(8), 5957–5962
the same time serve as reactant so it is hard to tell which effect promoted the formation of nanorings. More works about this point are in progress. In the process of sample preparation, necklace-like nanorings with different sizes have been obtained, which are shown in Figure 7. More interestingly, byproducts of nanorings with different morphologies also have been observed, as shown in Figure 8. According to our findings, self-assembly dynamics play a key role in the formation of nanorings and the stability of chains may determine the yield, size, and morphologies of necklace-like nanorings. Therefore, when the reaction condition was changed for tuning self-assembly dynamics of nanoring, the controlled synthesis of nanorings will be realized. Except reaction temperature, TEM images (Figure 9) were obtained at the same reaction DOI: 10.1021/la9041343
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Figure 9. TEM images (same magnification) of nanorings under different reaction temperatures: (a) 220 °C; (b) 240 °C.
conditions: PVP (6.0 g), hydrogen peroxide (1 mL), reaction time (48 h). Obviously, the size of the nanoring in Figure 9a is smaller than the nanoring in Figure 9b, which indicates that necklace-like nanorings with different size can be synthesized.
and morphologies of necklace-like nanorings. And, by varying the reaction parameters to control the kinetics of self-assembly, we readily realize the synthesis of nanoring with different size and high yield.
4. Conclusions
Acknowledgment. We acknowledge the financial support by the National Natural Science Foundation of China under Grant 10774138.
In summary, we successful synthesized magnetite and maghemite necklace-like nanorings in large scale via a onestep solvothermal process. By changing the reaction conditions, we found that higher temperature and more PVP favored the formation of magnetic nanorings. Studies show that the dynamics of self-assembly determine the yield, size,
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Supporting Information Available: M-H loop of the asprepared S1 (Figure SI). This material is available free of charge via the Internet at http://pubs.acs.org.
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