14818
J. Phys. Chem. C 2009, 113, 14818–14822
Ferromagnetic Fe@CaS Nanopeapods with Protecting BN Tubular Sheaths Jing Lin,*,† Yang Huang,*,† Chengchun Tang,† Yoshio Bando,† Youguo Shi,†,‡ Eiji. Takayama-Muromachi,†,‡ and Dmitri Golberg† International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, Japan and Superconducting Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, 305-0044 Ibaraki, Japan ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: June 24, 2009
Novel one-dimensional nanohybrids made of Fe@CaS nanopeapods homogeneously coated with protecting boron nitride sheaths are synthesized through a solid-liquid-solid reaction at ∼1500 °C. Within the nanopeapods, discrete Fe nanoparticles with an average size of ∼90 nm are periodically embedded in CaS nanowires along the growth axes. The structures have diameters of 150-180 nm and lengths in the range of tens of micrometers. Detailed electron microscopy studies reveal a unique stacking sequence of nanopeapods and the protecting tubular BN sheaths on them with a uniform thickness of ∼3 nm. Magnetic measurements show that nanopeapods are ferromagnets at room temperature with a coercive force of 64 Oe. These novel nanostructures may not only enrich the existing bank of hybrid nanosystems but may also have high expectations for magnetic-optical and spintronic devices. 1. Introduction One-dimensional (1D) hybrid nanosystems such as core-shell nanowires and nanopeapods have been under active investigation because of their unique properties and potential applications as building blocks in nanodevices.1 In particular, a nanopeapod, which encapsulates a foreign nanoparticle chain inside a nanowire/nanotube, has attracted prime attention. For instance, nanopeapods consisting of C60 fullerene molecules encapsulated in carbon nanotubes (CNTs) have been widely investigated.2 Liu et al.3 have fabricated Pt@CoAl2O4 nanopeapods by electrodeposition of Co/Pt multilayered nanowires (NWs) into anodic aluminum oxide membranes followed by a solid-state reaction. Hu et al.4 have utilized a microreactor approach to produce photosensitive gold peapoded silica NWs. Hsieh et al.5 have reported on gold-in-Ga2O3 peapod nanowires synthesized through a vapor-liquid-solid (VLS) reaction. However, most of the studies have been focused on carbon-based or noble metal nanoparticle peapoded nanowires. It is important to develop new kinds of nanopeapods in order to realize a wider variety of functionalities. Magnetic nanoparticles, e.g. Fe, are promising candidates for applications in magnetic recording devices, catalysis, magnetic resonance imaging (MRI), environmental remediation, and biomedical fields.6 However, such nanoparticles are not stable to oxidation.7 Encapsulation of Fe nanoparticles within a foreign nanowire would be an effective way to protect them from oxidation and interaction with an atmosphere. The magnetic hybrid nanopeapods can be expected to display unique physical phenomena and find potential applications in novel magnetic nanodevices. However, it is still a challenging task to fabricate such complex nanostructures. Hexagonal boron nitride (h-BN) is a structural analogue of graphite in which alternating B and N atoms substitute for C atoms. h-BN has many attractive properties such as superb * To whom correspondence should be addressed. E-mail: Lin.Jing@ nims.go.jp (J. Lin);
[email protected] (Y. Huang). † International Center for Materials Nanoarchitectonics (MANA). ‡ Superconducting Materials Center.
chemical and thermal stabilities combined with perfect electrical insulation and thus has received considerable attention.8 On the other hand, alkaline earth sulfides (AES), group IIA-VI wide band gap semiconductors, have been known for decades as versatile phosphors.9 However, in comparison with common IIB-VI nanomaterials such as ZnS and CdS10 the studies related to AES-based nanomaterials have been limited due to their low chemical stabilities and difficulties in preparation.11 In this paper, we report a novel method to fabricate magnetic Fe nanoparticle chains encapsulated in CaS NWs, which are also uniformly protected with thin graphitic-like BN layers. The BN coated CaS nanowire as a “pod” can provide an effective physical barrier to protect the magnetic Fe nanoparticle “peas”. These novel hybrid nanopeapods exhibit unique ferromagnetic properties. The pioneering fabrication of BN-coated Fe@CaS nanopeapods not only enriches the existing bank of hybrid nanostructured morphologies but also deepens the general understanding of crystal growth at the nanoscale. Moreover, combined with the superior optical properties of CaS, the fabricated nanopeapods are envisaged to be of high promise for novel magneticoptical and spintronic nanoscale devices. 2. Experimental Section The BN-coated Fe@CaS peapod nanowires were synthesized in a vertical induction furnace. The furnace consisted of a fused quartz tube and an induction-heated cylinder made of high-purity graphite coated with a carbon fiber thermo-insulating layer. There were two inlets on its top and base and one outlet on its side. A graphite crucible, containing a mixture of CaSO4 (1 g), activated carbon (0.02 g), B2O3 (0.1 g), and Fe2O3 (0.01 g) powders was placed at the center cylinder zone. Another designed graphitic lid covered the top of the cylinder. There were many drilled holes in the lid to provide efficient gas/vapor exchange during the experiments. After evacuation of the quartz tube to 2 × 10-1 Torr, two pure N2 flows were introduced through the inlets at a flow rate of 1.5 L/min (top) and 1.0 L/min (base). Then, the furnace was rapidly heated to 1500 °C and kept at this temperature for 2 h. After the system was cooled to
10.1021/jp905388z CCC: $40.75 2009 American Chemical Society Published on Web 07/15/2009
Ferromagnetic Fe@CaS Nanopeapods
Figure 1. (a,b,c) Low- and high-magnification SEM images of a nanopeapod product. (d) XRD pattern of the product. Line spectra correspond to peak positions of CaS (JCPDS card 65-894) and Fe (JCPDS card 87-722).
room temperature, a wool-like product was collected from the top of a caked mass in the graphite crucible. An as-prepared product was characterized by a scanning electron microscope (SEM, JEOL, JSM-6700F), a powder X-ray diffractometer (XRD, RIGAKU, Ultima III, 50 V/40 mA with Cu KR radiation) and a 300 kV high-resolution field-emission transmission electron microscope (TEM, JEM-3000F) equipped with an energy-dispersive X-ray analysis detector (EDX) and electron energy loss spectrometer (EELS). The magnetic properties were measured in a superconducting quantum interference device (SQUID) (MPMS-5 T; Quantum Design). A plastic straw held a plastic capsule containing a 6.4 mg wool-like peapod sample that was prepared for the measurement.
J. Phys. Chem. C, Vol. 113, No. 33, 2009 14819
Figure 2. (a,b) Typical TEM images of a single peapod nanowire, indicating discrete Fe nanoparticles embedded in the nanowire. (c) EDS spectrum taken from the pea part framed in panel b. (d) EDS spectrum collected from the pod part framed in panel b. (e) EELS spectra of the pea part.
3. Results and Discussion A typical SEM image (Figure 1a) shows that the wool-like product consists of uniform, wire-like nanostructures with diameters of 150-180 nm and lengths of several tens of micrometers. A high-magnification image (Figure 1b) clearly reveals some bright spots on an individual nanowire, indicating Fe nanoparticle locations. Figure 1c confirms that all nanowires have a similar peapod-like morphology. The structures were further analyzed by XRD. A representative XRD pattern is shown in Figure 1d. The main peaks can be readily indexed to cubic CaS (JCPDS card 65-894) and cubic Fe (JCPDS card 87722). Figure 2a displays a typical TEM image of a single peapod nanowire. The nanowire has discrete Fe nanoparticles periodically embedded along the growth axis direction. Magnified images depict that the size and separation of Fe peas are averaged at 90 and 260 nm, respectively, and that the CaS pod exhibits a rough surface (Figure 2b). Each peapod nanocomposite actually consists of Fe nanoparticles, a core CaS nanowire, and a thin protecting BN sheath on its outer surface. Energy dispersive spectrometry (EDS) spectra were collected from two areas in the nanowire: the pea part (Figure 2c) and pod part (Figure 2d). The results show that the pea domains are composed of Fe (detected Ca, S, and N originate from the surrounding pod shells), whereas the pod part contains Ca, S, B, and N, where the B and N signals come from the protecting sheaths. The Ca/S molar ratio is close to 1:1, in accordance with the CaS stoichiometry. An electron energy loss (EEL) spectrum taken from the nanopeapod also indicates that the wire is made
Figure 3. (a) TEM images of typical peapod nanowires. (b,c) SAED patterns taken from the pea and pod portions of the nanowire in panel a. (d,e) TEM images of the CaS pod, indicating the jagged edge sheathed with a very thin and uniform BN layer. (f) TEM image of the Fe pea, indicating a hexagon-like shape. (g) HRTEM image clearly revealing the single-crystalline nature of the Fe pea and CaS pod. (h) HRTEM image taken from the surface region of the peapod, displaying BN sheath layers on the jagged edge.
of Ca, S, B, N, and Fe, further confirming the existence of a BN phase on the wire (Figure 2e). Detailed structural analysis of the nanopeapods was performed by high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). Figure 3a shows a TEM image of a single peapod nanowire. SAED patterns taken from the pea and pod portions of the nanowire are displayed in panels b and c of Figure 3. Both can be indexed to the [01-1] zone axis of cubic CaS. These two patterns display almost the same diffraction spots. This is due to the crystal lattice match
14820
J. Phys. Chem. C, Vol. 113, No. 33, 2009
Figure 4. (a,e) TEM and SEM images of a typical nanopeapod composed of compact stacked blocks, displaying a relatively smooth surface. (c,f) TEM and SEM images of a typical nanopeapod with sparse stacked blocks, displaying a rough surface. (b,d) Corresponding structural models. (g,h) TEM images of the tip end, displaying an attached particle with a uniform BN sheath. (i) EDS spectrum taken from the particle.
between the Fe pea and CaS pod as also indicated in the XRD spectrum. Besides, in Figure 3b, some weak reflections originating from BN sheaths are also visible (indicated by the arrow). Panels d and e of Figures 3 display a complex structure of a nanopeapod. This indicates that the whole CaS pod is composed of many fragments. They overlap with each other and display a jagged edge. Very thin and uniform protecting BN layer sheaths exist on all wire outer surfaces. Figure 3f shows that an embedded Fe particle has a relatively regular hexagon-like shape. This morphology is suggested to be due to the minimization of the total surface energy during the present peapod growth.5 HRTEM investigations reveal the single-crystalline nature of Fe peas and CaS pods as shown in Figure 3g. Another HRTEM image (Figure 3h) taken from the surface regions of a peapod clearly displays a multilayered BN sheath on the jagged edge with an interlayer spacing of 0.33 nm. The interplanar distance corresponds well to the (002) plane separation in h-BN as is also verified by the SAED patterns (indicated by the arrow in Figure 3b).
Figure 5. Schematic illustration of the nanopeapod growth mechanism.
Lin et al. In order to further clarify the regarded nanomaterial complex structure, we carefully checked numerous peapod nanowires through TEM and SEM examinations. As shown in panels a-f of Figure 4, each CaS nanowire is stacked with many ordered blocks. These blocks have uniform BN layer sheaths and connect with each other, forming a complex one-dimensional structure. The stacked density of the blocks determines the overall surface morphology of a nanowire. Compact stacked blocks always form a relatively smooth surface, while sparse stacking leads to a rough surface. In addition, the tip structure of the nanopeapods was also investigated. Figure 4g shows a particle attached to the tip end. Notably, the uniform layered BN sheath extends along the whole wire and also entirely covers the tip particle as displayed in Figure 4h. Corresponding EDS analysis on the particle indicates the presence of Ca, S, Fe, B, and N (Figure 4i). This morphology reveals that the dominant growth mechanism of the peapod nanowires may be the well-known vapor-liquid-solid (VLS) process.12 For a typical VLS mechanism, the catalyst acts as a liquid-forming agent, which reacts with a vapor phase and forms eutectic alloy droplets. With further absorption from the vapor phase, the droplets become supersaturated, resulting in precipitation of nanowires from the droplets. However, in the present case, the generated Cacontaining vapor flow is negligible at the synthesis temperature. This suggests that the standard VLS mechanism is probably unsuitable to account for the present nanowire growth. We thus conclude that the most plausible growth mechanism is a solid-liquid-solid (SLS) process,13 which is close to a VLS mechanism. The growth mechanism of nanopeapods is schematically illustrated in Figure 5. During a temperature increase, boron oxide powder first melts at 450 °C to form a bulky matrix. The CaSO4, activated carbon, and Fe2O3 reactants immerge in this matrix (step 1). Subsequently, a small amount of CaS precipitates in the matrix via the reaction between mixed CaSO4 and activated carbon: CaSO4 + 2C f CaS + 2CO2. These CaS precipitates could gradually form as a caked mass at the bottom of the matrix. Moreover, liquid droplets consisting of (Ca-SO)-Fe may be formed by incorporating superfluous CaSO4 with Fe2O3 in the matrix at a high reaction temperature (step 2). The superfluous CaSO4 continuously diffuses into the liquid droplets. When the droplets become supersaturated, a new liquid-solid interface forms, resulting in the formation of nanowires, including a main Ca-S-O phase and a small amount of Fe (step 3). During further growth of a nanowire, melted boron
Ferromagnetic Fe@CaS Nanopeapods
J. Phys. Chem. C, Vol. 113, No. 33, 2009 14821 netism) is about ∼20 nm.16 However, herein the size of Fe particles within the peapods is ∼100 nm, which means that there should be remaining domain walls in each particle. This determines the regarded ferromagnetic properties. Compared to a Hc value of bulk Fe (∼1 Oe), the Fe particles herein exhibit a distinctly enhanced coercive force, which is provisionally attributed to the unique peapod appearance of the structures, and it is also worth noting that a similar increasing trend in coercive force was previously observed in FeCo nanofragments (compared to that of a bulk FeCo alloy) encapsulated into CNTs.17 Needless to say, the detailed mechanism of such a dramatic increase needs further theoretical verifications. 4. Conclusions
Figure 6. (a) Hysteresis loops of nanopeapods measured in a magnetic field up to (5 T at 300 K (black line) and 5 K (red line). (Insets) Enlarged sections of hysteresis loops. (b) Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of a nanopeapod product.
oxide adheres to the nanowire surface due to its high viscosity and excellent wetting properties.14 However, in the reaction system, carbon (from the graphite crucible and susceptor) can deoxidize and transform the nanowire into a CaS-Fe phase. Because of a perfect crystal lattice match between cubic CaS and Fe, the CaS-Fe nanowire is formed as a solid solution. In addition, the adhered melted boron oxide can react with N2 gas and form BN sheaths on the nanowire surfaces (step 4). Moreover, it is worth noting that in our case, the whole synthesis process was performed in a vertical induction furnace, which provided more drastic thermal changes, resultant temperatures, and vapor pressure gradients compared to those of a conventional tube furnace.15 Rapid temperature change during the nucleation and growth process as well as the nanosized dimensions lead to the generation of high internal stresses within the CaS-Fe nanowires. Then, phase separation and shear strains may be invoked to release the stresses. The phase separation of a CaS-Fe solid solution can result in the formation of peapod structures, while the acting shear strains lead to the formation of unique block-stacked nanostructures (step 5). We then elucidated the magnetic properties of these novel peapod nanowires. Figure 6a shows the magnetic hysteresis loops measured in a magnetic field up to (5 T at 300 and 5 K, respectively. The peapod nanowires exhibit ferromagnetism, and the saturation magnetization shows a decreasing tendency from 5 K to room temperature. The enlarged sections of Figure 6a clearly demonstrate that the sample has hysteresis with a coercive force (Hc) of ∼64 Oe at 300 K and ∼77 Oe at 5 K. Figure 6b displays zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of a product, indicating a blocking temperature TB above 300 K. Therefore, it also suggests the ferromagnetic characteristics of the nanopeapods at room temperature, which is in agreement with the hysteresis loop measurements. It is known that nanoparticle size can influence magnetic behavior. It has been reported that the critical size for single-domain BCC R-Fe particles (to reveal superparamag-
Novel Fe@CaS peapod nanowires were synthesized via a solid-liquid-solid growth mechanism using Fe2O3 as catalysts. Nanopeapods have discrete Fe nanoparticles periodically embedded in CaS nanowires along the growth axis direction. SEM and TEM studies indicate a unique stacking sequence of nanopeapods and the chemically, thermally, and electrically protecting tubular BN sheaths on them with a uniform thickness of ∼3 nm. Ferromagnetic properties of nanopeapods were discovered at room temperature with a coercive force of 64 Oe. These novel magnetic hybrid nanopeapods are envisaged to find potential applications in novel magnetic-optical and spintronic nanodevices. Acknowledgment. The authors thank Drs. X. S. Fang, C. Li, T. Y. Zhai, and M. S. Wang for helpful discussions and suggestions. References and Notes (1) (a) Zhang, G.; Wang, W.; Li, X. AdV. Mater. 2008, 20, 3654. (b) Yan, J.; Fang, X.; Zhang, L.; Bando, Y.; Gautam, U. K.; Dierre, B.; Sekiguchi, T.; Golberg, D. Nano Lett. 2008, 8, 2794. (c) Li, L.; Yang, Y. W.; Li, G. H.; Zhang, L. D. Small 2006, 2, 548. (d) Shen, G.; Bando, Y.; Gao, Y.; Golberg, D. J. Phys. Chem. B 2006, 110, 14123. (2) (a) Yoon, Y. G.; Mazzoni, M. S. C.; Louie, S. G. Appl. Phys. Lett. 2003, 83, 5217. (b) Chiu, P. W. Appl. Phys. Lett. 2001, 70, 3845. (3) Liu, L.; Lee, W.; Scholz, R.; Pippel, E.; Go˝sele, U. Angew. Chem., Int. Ed. 2008, 47, 1. (4) Hu, M. S.; Chen, H. L.; Shen, C. H.; Hong, L. S.; Huang, B. R.; Chen, K. H.; Chen, L. C. Nat. Mater. 2006, 5, 102. (5) Hsieh, C. H.; Chou, L. J.; Lin, G. R.; Bando, Y.; Golberg, D. Nano Lett. 2008, 8, 3081. (6) (a) Gao, S. Y.; Shi, Y. G.; Zhang, S. X.; Jiang, K.; Yang, S. X.; Li, Z. D.; Takayama-Muromachi, E. J. Phys. Chem. C 2008, 112, 10398. (b) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 2287, 1989. (c) Zhang, W. X. J. Nanopart. Res. 2003, 5, 323. (d) Plank, C.; Schillinger, U.; Scherer, F.; Bergemann, C.; Remy, J. S.; Kroetz, F. J. Biol. Chem. 2003, 384, 737. (e) Parker, F. T.; Spada, F. F.; Cox, T. J.; Berkowitz, A. E. J. Magn. Magn. Mater. 1996, 162, 122. (f) Smits, J.; Wincheskib, B.; Namkungb, M.; Crooksa, R.; Louiec, R. Mater. Sci. Eng., A 2003, 358, 384. (g) Tiefenauer, L. X.; Tscgirky, A.; Kuhne, G.; Andres, R. Y. Magn. Reson. Imaging 1996, 14, 391. (h) Elliott, D. W.; Zhang, W. X. EnViron. Sci. Technol. 2001, 35, 4922. (7) Wang, Y.; Wei, W.; Maspoch, D.; Wu, J.; Dravid, V. P.; Mirkin, C. A. Nano Lett. 2008, 8, 3761. (8) (a) Golberg, D.; Bando, Y.; Tang, C. C.; Zhi, C. Y. AdV. Mater. 2007, 19, 2413. (b) Sichel, E. K.; Miller, R. E.; Abrahams, M. S.; Buiocchi, C. J. Phys. ReV. B 1976, 13, 4607. (c) Chaudhry, M. A. J. Mater. Sci. 1991, 26, 1106. (d) Watanabe, S.; Miyake, S.; Murakawa, M. Surf. Coat. Technol. 1991, 49, 406. (e) Buzhinskij, O. I.; Lopatin, V. V.; Sharupin, B. N. J. Nucl. Mater. 1992, 196, 1118. (9) (a) Wang, C.; Tang, K.; Yang, Q.; An, C.; Hai, B.; Shen, G.; Qian, Y. Chem. Phy. Lett. 2002, 351, 385. (b) Lehmann, W.; Ryan, F. M. J. Electrochem. Soc. 1971, 118, 477. (c) Pandey, R.; Kunz, A. B.; Vail, J. M. J. Mater. Res. 1988, 3, 1362. (d) Leskela¨, M. J. Alloys Comp. 1998, 275277, 702. (10) (a) Fang, X.; Bando, Y.; Shen, G.; Ye, C.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C.; Tang, C.; Golberg, D. AdV. Mater. 2007, 19, 2593. (b) Fang, X.; Gautam, U. K.; Bando, Y.; Dierre, B.; Sekiguchi, T.; Golberg, D. J. Phys. Chem. C 2008, 112, 4735. (c) Dukovic, G.; Merkle, M. G.;
14822
J. Phys. Chem. C, Vol. 113, No. 33, 2009
Nelson, J. H.; Hughes, S. M.; Alivisatos, A. P. AdV. Mater. 2008, 20, 4306. (d) Wang, C. Z.; E, Y. F.; Fan, L. Z.; Wang, Z. H.; Li, H. B.; Li, Y. L.; Yang, S. H.; Li, Y. L. AdV. Mater. 2007, 19, 3677. (11) Brightwell, J. W.; Ray, B.; Sephton, P.; Viney, I. V. F. J. Cryst. Growth 1990, 86, 634. (12) (a) Wanger, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (b) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (c) Duan, X.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (d) Duan, X.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (e) Yu, D. P.; Hang, Q. L.; Ding, Y. Appl. Phys. Lett. 1998, 73, 3076. (f) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (13) Yu, D. P.; Xing, Y. J.; Hang, Q. L.; Yan, H. F.; Xu, J.; Xi, Z. H.; Feng, S. Q. Physica E 2001, 9, 305.
Lin et al. (14) Pe´rez-Enciso, E.; Ramos, M. A.; Viera, S. Phys. ReV. B 1997, 56, 32. (15) Shen, G.; Bando, Y.; Liu, B.; Tang, C.; Golberg, D. J. Phys. Chem. B 2006, 110, 20129. (16) Chen, Z. G.; Zou, J.; Li, F.; Liu, G.; Tang, D. M.; Li, D.; Liu, C.; Ma, X.; Cheng, H. M.; Lu, G. Q.; Zhang, Z. AdV. Funct. Mater. 2007, 17, 3371. (17) Elias, A. L.; Rodrigues-Manzo, J. A.; McCarttney, M. R.; Golberg, D.; Zamudio, A.; Balatazar, S. E.; Lopes-Urias, F.; Munos-Sandoval, E.; Gu, L.; Tang, C. C.; Smith, D. J.; Bando, Y.; Terrones, H.; Terrones, M. Nano Lett. 2005, 5, 467.
JP905388Z
