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Controllable synthesis, characterization, and magnetic properties of magnetic nanoparticles encapsulated in carbon nanocages and carbon nanotubes
Diamond & Related Materials 45 (2014) 12–19
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Controllable synthesis, characterization, and magnetic properties of magnetic nanoparticles encapsulated in carbon nanocages and carbon nanotubes Xiaosi Qi a,b, Jianle Xu a, Wei Zhong b,⁎, Chaktong Au c, Youwei Du b a b c
College of Science, Guizhou University, Guiyang 550025, People's Republic of China Nanjing National Laboratory of Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing 210093, People's Republic of China Chemistry Department, Hong Kong Baptist University, Hong Kong, China
a r t i c l e
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Article history: Received 24 December 2013 Received in revised form 20 February 2014 Accepted 24 February 2014 Available online 3 March 2014 Keywords: Nanostructures Chemical vapor deposition Morphology Magnetic properties
a b s t r a c t By controlling the pyrolysis temperature, core/shell materials with Fe3O4 encapsulated in carbon nanocages (Fe3O4@CNCs) and Fe encapsulated in carbon nanotubes (Fe@CNTs) were synthesized selectively from acetylene using Fe2O3 nanoparticles as catalyst in chemical vapor deposition. Scanning electron microscopic study showed that the efficiency of generating Fe3O4@CNCs and Fe@CNTs was high, exceeding 95%. Transmission electron microscopic study confirmed the high selectivity to Fe3O4@CNCs and Fe@CNTs, with the former having morphology similar to that of the catalyst particles. With Fe3O4 and Fe nanoparticles tightly wrapped in graphitic layers, the obtained Fe3O4@CNCs and Fe@CNTs materials show high stability and good magnetic properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Due to surface and/or quantum-size effects, metal nanoparticles especially those of magnetic materials exhibit unique physical properties and have potential applications in many areas [1–4]. However, metal nanoparticles are usually highly active and easily oxidized in air, resulting in deterioration of characteristics such as magnetism. The breakthrough in this regard was the encapsulation of metal nanoparticles in carbon layers. Since the discovery of fullerenes, there was much advance in the development of carbon nanomaterials (CNMs) and carbon-coated nanocapsules [5–7]. Compared with other inert materials such as silica, metal oxides and polymer shells [8–10], carbon shells exhibited much higher stability in chemical and physical environments. With the metal nanoparticles encapsulated in carbon shells, they are protected from oxidation. Also, with the encapsulation, there is reduction of magnetic coupling between magnetic phases, opening the possibility of optimizing the intrinsic magnetic properties of the metal nanoparticles [11–15]. In the past years, carbon nanocages (CNCs) and CNMs that were filled with ferromagnetic materials have
⁎ Corresponding author. Tel.: +86 25 83621200; fax: +86 25 83595535. E-mail address: [email protected] (W. Zhong).
http://dx.doi.org/10.1016/j.diamond.2014.02.013 0925-9635/© 2014 Elsevier B.V. All rights reserved.
attracted considerable attention due to their potential applications such as electromagnetic wave absorption [12], magnetic data storage and human tumor therapy [16,17]. There are methods for the synthesis of metal-filled CNMs or CNCs [18–22]. Nonetheless, most of the adopted procedures were complicated, and the efficiency of metal encapsulation needs improvement [23–25]. Based on knowledge revealed in the past [26–29], we worked on the controllable synthesis of CNCs-encapsulated Fe 3 O4 (Fe3 O 4@ CNCs) and carbon nanotubes (CNTs)-encapsulated Fe (Fe@CNTs). We selectively generated these core/shell structures with high efficiency over Fe2O3 nanoparticles by controlling the pyrolysis temperature of acetylene in chemical vapor deposition. To the best of our knowledge, our proposed route is very simple and has not been reported before.
2. Experimental 2.1. Synthesis of α-Fe2O3 catalyst All reagents were of analytical grade and used without further purification. Typically, 30 ml of aqueous NH4H2PO4 solution (0.05 M) was added to 30 ml of aqueous Fe(NO3)3 solution (0.3 M; yellowish in color) under vigorous stirring at room temperature (RT). After stirring for another five minutes, the resulted slurry was transferred into a Teflon-sealed autoclave and subject to hydrothermal treatment at
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220 °C for 48 h. With the setting cooled to RT, the as-obtained solid substance (red in color) was filtered out, washed three times with distilled water, and dried at 80 °C for 8 h.
2.2. Generation of Fe3O4@CNCs and Fe@CNTs core/shell materials First, the as-obtained red powder (0.1 g) was dispersed on a ceramic plate that was positioned inside a quartz tube. Then the temperature of the furnace was raised from RT to a designed temperature with Ar flowing (controlled by flowmeter at a flow rate of 40 sccm) through the reaction tube. After that, the gas supply was shifted from Ar to acetylene for the synthesis of carbon-encapsulated nanomaterials. The decomposition of acetylene (atmospheric pressure, 2 h) was conducted at 400, 500 or 600 °C (heated at a rate of 10 °C/min). The products generated at pyrolysis temperatures of 400, 500 and 600 °C are denoted hereinafter as C-400, C-500 and C-600, respectively.
2.3. Characterization of products The samples were examined on an X-ray powder diffractometer (XRD) at RT for phase identification using CuK α radiation (model D/Max-RA, Rigaku). Raman spectroscopic investigations were performed using a Jobin-Yvon Labram HR800 instrument with 514.5 nm Ar+ laser excitation. The morphology of samples was examined using a transmission electron microscope (TEM) (model JEM-2000EX, operated at an accelerating voltage of 80 kV), and field emission scanning electron microscope (FE-SEM) (model FEI Sirion 200, operated at accelerating voltages of 5 kV and equipped with a energy dispersive X-ray spectrometer (EDS)). The magnetic properties of samples were measured at 300 K using a Quantum Design MPMS SQUID magnetometer (Quantum Design MPMS-XL) equipped with a superconducting magnet capable of producing fields of up to 50 kOe. Fourier Transform Infrared (FTIR) spectra of samples (in KBr pellets) were recorded over a Nicolet 510P spectrometer.
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3. Results and discussion 3.1. FE-SEM, XRD and EDS characterizations of catalyst The FE-SEM images of the as-synthesized catalyst are shown in Fig. 1a and b. One can see a large amount of size-uniform hollow nanoparticles (as indicated by arrows), exceeding 94% of the entire content. The XRD pattern shows lines corresponding to α-Fe2 O 3 and P (as indexed in Fig. 1c). The EDS result shows the presence of Fe, O and P (Fig. 1d). Based on the results, it is deduced that the catalyst is mainly composed of hollow α-Fe2O3 nanoparticles with an average diameter of ca. 200 nm. 3.2. Microstructure and magnetic property of C-400 In each run, about 0.131 g of C-400 was harvested on the ceramic plate. The FE-SEM image (Fig. 2a) indicates that C-400 is mainly composed of hollow nanoparticles, with morphology similar to that of the α-Fe2O3 catalyst (Fig. 1a and b). The EDS result indicates the presence of C, Fe and O (Fig. 2b), and the Fe:O molar ratio of C-400 is much higher than that of the α-Fe2O3 catalyst (Fig. 1d). The XRD result of the obtained C-400 is shown in Fig. 2c. One can find that the diffraction peaks corresponding to α-Fe2O3 catalyst disappears completely after the reduction of α-Fe2O3 by C2H2 at 400 °C. And the diffraction peaks corresponding to phases of C, Fe3O4, Fe0.942O and Fe2C can be observed clearly (as indicated by the symbols in Fig. 2c), indicating that the obtained C-400 is a composite made up of C and Fe3O4. Fig. 2d shows the Raman spectrum of C-400; there are a total of four peaks. The one at ca. 1602 cm−1 (called G-band, indicative of graphitic layer of high crystallinity) is corresponding to the E2g mode of graphite that is related to the C\C vibration of carbon materials with sp2 orbital structure. The one at ca. 1336 cm−1 (called D-band) is a consequence of resonance scattering with the emission of phonon [30,31]. The intensity ratio of the G and D bands (IG/ID) is commonly used to characterize the crystallinity of carbon materials. In the study, an IG/ID of ca. 1.09 was recorded for C-400, implying high crystallinity. The peaks at 281 and
Fig. 1. (a,b) FE-SEM images, (c) XRD pattern, and (d) EDS result (collected from the area marked in 1a) of catalyst.
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Fig. 2. (a) FE-SEM image, (b) EDS result (collected from the area marked in panel a), (c) XRD pattern, (d) Raman spectrum, and (e) magnetization curves (inset is the enlarged part close to the origin) of C-400.
222 cm−1 can be ascribed to the Eg mode (symmetric bends of oxygen with respect to Fe) and T2g mode (translator movement of the whole FeO4 unit) of Fe3O4, respectively [32,33]. The Raman data confirmed further that C-400 is a composite made up of C and Fe3O4. According to the magnetization-coercivity (M–H) curves (Fig. 2e), the saturation magnetization (MS) and the coercivity (HC) of C-400 is 26.69 emu g−1 and 508 Oe, respectively, in good agreement with the ferromagnetic nature of Fe3O4 at 300 K. The FE-SEM and TEM images of C-400 are shown in Fig. 3. The FE-SEM and TEM results reveal that the majority of C-400 is Fe3O4@ CNCs of core/shell structure. As shown in Fig. 3a and b, Fe3 O 4@ CNCs is uniform in size, having an average diameter of 230 nm, slight higher than that of the α-Fe2O3 catalyst (200 nm). In the TEM and high-resolution TEM images, the core/shell structure can be clearly observed (Fig. 3c and d). The efficiency of Fe3O4 encapsulation inside
CNCs is high (at least 98%). The Fe3O4@CNCs shows an inner core that is tightly wrapped by a graphite layer of ca. 30 nm in thickness, giving an overall shape that resembles that of the α-Fe2O3 nanoparticles. Compared with the methods reported in the literature [18–20], the one adopted in this study is simple and efficient. With the shape of Fe3O4@CNCs resembling that of α-Fe2O3 nanoparticles, it is deduced that one can regulate the morphology of the former by controlling that of the latter. 3.3. The effect of pyrolysis temperature In order to investigate the effect of decomposition temperature on the final products, acetylene decomposition was conducted at 500 °C over hollow α-Fe2O3 nanoparticles with other experimental conditions unchanged. After cooling to RT, we harvested ca. 0.112 g of C-500 in
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Fig. 3. (a, b) FE-SEM and (c, d) TEM images of C-400.
each run, slightly lower than that (ca. 0.131 g) of C-400. Fig. 4a shows the FE-SEM image of C-500. The morphology of C-500 is similar to that of the catalyst (Fig. 1a) and that of C-400 (Fig. 2a). According to the EDS results (Fig. 4b), the Fe:O molar ratio and carbon content of C-500 is higher than that of the catalyst (Fig. 1d) and C-400 (Fig. 2b). Fig. 4c shows the XRD result of the obtained C-500. Compared with that of C-400 (Fig. 2c), the diffraction peaks can be indexed to phases of C, Fe3O4, Fe0.942O and Fe2C (as indicated by the symbols in Fig. 4c). And the diffraction peaks corresponding to Fe0.942O can be observed much more clearly, which further proves that the Fe:O molar ratio of C-500 becomes much higher than that of C-400 after the reduction of Fe0.942O by C2H2 at higher temperature. The Raman spectrum of C-500 (Fig. 4d) also shows four peaks. Compared to those of C-400, the G (~1611 cm−1) and D (~1353 cm−1) bands of C-500 shift to the higher frequency. Also, the IG/ID (ca. 1.12) of C-500 is higher than that of C-400 (ca. 1.09). The peaks at 300 and 229 cm−1 are ascribable to the Eg mode and T2g mode of Fe3O4, respectively. Despite the Raman peaks of C-500 corresponding to the Fe3O4 phase are not as obvious as those of C-400, the Raman data of C-500 further suggest that C-500 is mainly composed of C and Fe3O4. Fig. 4e gives the magnetic property of the obtained C-500. The MS and HC value of C-500 is 25.47 emu g− 1 and 409 Oe, respectively, lower than that of C-400 (26.69 emu g− 1 and 508 Oe). Due to the existence of magnetic nanoparticles (Fe3O4), the obtained samples (C-400 and C-500) show good magnetic properties. Therefore, the lower MS value of C-500 should be related to the lower Fe3O4 content in the obtained sample, which has been proved by the EDS and XRD results of C-400 and C-500. Moreover, it was observed that both C-400 and C-500 showed no changes in magnetic properties after being kept in air for over one year. The high stability of the Fe3O4 nanoparticles is a result of encapsulation inside the graphite layers. The FE-SEM and TEM images of C-500 are shown in Fig. 5. One can see that Fe3O4@CNCs was synthesized in large amount, and the
selectivity to Fe3O4@CNCs is ca. 86%. As indicated by the white arrows in Fig. 5a and b, there is sighting of CNTs-encapsulated magnetic nanoparticles. The Fe3O4@CNCs is uniform in size, having an average diameter of ca. 400 nm. Considering the nanoparticles inside CNCs as well as those inside CNTs, the efficiency of encapsulation is high (at least 95%). The core/shell structures can be seen clearly in the TEM and high-resolution TEM images of C-500. Similar to the Fe3O4@CNCs of C-400, the Fe3O4@CNCs of C-500 show the morphology resembles that of the catalyst nanoparticles. It is deduced that the decomposition temperature of acetylene has an effect on the structure, yield, magnetic property, and composition of the core/shell materials. 3.4. Characterization and magnetic property of C-600 In each run, ca. 2.366 g of C-600 could be collected, higher than that of C-400 or C-500. The yield of carbon species (mass ratio of carbon materials to catalyst) is extremely high, up to ca. 23.7. Fig. 6 shows the XRD pattern of C-600. The diffraction peaks can be indexed to phases of graphite, Fe3C and α-Fe. It is apparent that with rise of pyrolysis temperature from 400 to 600 °C, there is reduction of the catalyst from Fe2O3 to Fe3O4 and then to α-Fe, in agreement with the rise of Fe:O molar ratio observed in EDS studies. Fig. 6b shows the Raman spectrum of C-600. Unlike those of C-400 and C-500, the Raman scattering bands of C-600 (at ca. 1601 and 1321 cm−1) shift towards smaller frequency. Furthermore, the IG/ID of C-600 is ca. 1.12, which is similar to that of C-500 and higher than that of C-400. Fig. 6c shows the FTIR spectrum of C-600. There is no IR signals that could be associate with \CH_CH\, \CH2\, and \CH3 entities, implying that C-600 is free of hydrogen and oxygen. All the results indicate that at a pyrolysis temperature of 600 °C, the catalyst (Fe2O3) was reduced to α-Fe. Fig. 6d gives the M-H curves of C-600. The MS and HC value of C-600 is 1.95 emu g− 1 and 427 Oe, respectively. The values are much lower
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Fig. 4. (a) FE-SEM image, (b) EDS result (the area marked in panel a), (c) XRD pattern, (d) Raman spectrum, and (e) typical magnetization curves (inset: enlarged part close to the origin) of the obtained C-500.
than those of C-400 and C-500, plausibly due to the low content of αFe in C-600. Moreover, compared to that of Fe@CNTs reported by Grechnev et al. [34], the Ms of C-600 is low. Due to the tight encapsulation of Fe nanoparticles inside the graphite layers, there was no change in XRD features as well as magnetic property after a C-600 sample was exposed to air for a period of one year. The good stability of C-600 was proved further by treating C-600 with 37 wt.% HCl for 1 h (Fig. 6e). The Ms value of HCl-treated C-600 is 1.92 emu g−1, which is close to that of the untreated one. Shown in Fig. 7 are the FE-SEM and TEM images of C-600. One can see that Fe@CNTs is in majority, and the selectivity to Fe@CNTs is ca. 98%. As indicated by the arrows in Fig. 7c and d, the size of Fe@CNTs is uniform (average diameter: 50 nm). It is noted that there are Fe nanoparticles at the tips and nodes of CNTs, which is different from that reported by Grechnev et al. [34]. As shown in Table 1, a change in the decomposition temperature of acetylene has great effect on the yield, structure, magnetic property and composition of the core/shell
materials. By controlling the pyrolysis temperature, one can synthesize Fe3O4@CNCs or Fe@CNTs selectively. 4. Conclusions Hollow Fe2O3 nanoparticles were fabricated by a hydrothermal method. Over the Fe2O3 nanoparticles, one can synthesize a large quantity of Fe3O4@CNCs in acetylene decomposition at 400 °C, with the morphology being similar to that of the Fe2O3 nanoparticles. With the pyrolysis temperature raised from 400 to 600 °C, the product changed from Fe3O4@CNCs to Fe@CNTs. It was observed that the efficiency of encapsulating the Fe3 O 4 nanoparticles in CNCs or Fe nanoparticles in CNTs exceeds 95%. With the Fe2O3 and Fe nanoparticles tightly wrapped inside graphite layers, the Fe3O4@CNCs and Fe@CNTs show high stability and good magnetic properties. The adopted procedure is simple, and can be applied for the synthesis of core/shell materials of similar kinds.
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Fig. 5. (a) FE-SEM and (b–d) TEM images of C-500.
Prime novelty statement In this paper, over the hollow Fe2O3 nanoparticles fabricated by a hydrothermal method, a simple route for the generation of core/shell materials with Fe3O4 encapsulated in carbon nanocages (Fe3O4@CNCs) Fe3O4@CNCs has been proposed. It was observed that the efficiency of encapsulating the Fe3 O 4 nanoparticles in CNCs or Fe nanoparticles in CNTs is very high (exceeds 95%). Because of the Fe2 O 3 and Fe nanoparticles tightly wrapped inside graphite layers, the Fe3 O 4 @CNCs and Fe@CNTs show high stability and good magnetic properties. Therefore, the approach is very new, simple, inexpensive, and environment-benign, and can be adopted for the synthesis of core/shell materials of similar kinds. Acknowledgments This work was supported by the Doctorial Start-up Fund of Guizhou University, the International Cooperation Project of Guizhou Province (2012–7002), the Foundation of the National Natural Science Foundation of China (Grant Nos. 11364005 and 11174132), and the Foundation of the National Key Project for Basic Research (Grant Nos. 2010CB923402 and 2011CB922102), People's Republic of China, for financial support. References [1] G.A. Held, G. Gringstein, Quantum limit of magnetic recording density, Appl. Phys. Lett. 79 (2001) 1501–1503. [2] R.H. Kodama, Magnetic nanoparticles, J. Magn. Magn. Mater. 200 (1999) 359–372. [3] G. Reiss, A. Hutten, Magnetic nanoparticles—applications beyond data storage, Nat. Mater. 4 (2005) 725–726. [4] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, Applications of magnetic nanoparticles in biomedicine, J. Phys. D. Appl. Phys. 36 (2003) 16–81. [5] S. Tomita, S. Hayashi, Y. Tsukuda, M. Fujii, Ultraviolet–visible absorption spectroscopy of carbon onions, Phys. Solid State 44 (2002) 450–453. [6] C.H. Liang, G.W. Meng, L.D. Zhang, N.F. Shen, X.Y. Zhang, Carbon nanotubes filled partially or completely with nickel, J. Cryst. Growth 218 (2000) 136–139.
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Fig. 7. (a,b) FE-SEM and (c,d) TEM images of C-600.
Table 1 Effects of pyrolysis temperature of acetylene on products. Temperature (°C)
Product
Yield of product
Species as revealed by TEM studied
Efficiency of encapsulation
400 500 600
C-400 C-500 C-600
1.3 1.1 23.7
Fe3O4@CNCs Fe3O4@CNCs Fe@CNTs
98% 96% 98%
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Controllable synthesis, characterization, and magnetic properties of magnetic nanoparticles encapsulated in carbon nanocages and carbon nanotubes Xiaosi Qi a,b, Jianle Xu a, Wei Zhong b,⁎, Chaktong Au c, Youwei Du b a b c
College of Science, Guizhou University, Guiyang 550025, People's Republic of China Nanjing National Laboratory of Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing 210093, People's Republic of China Chemistry Department, Hong Kong Baptist University, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 24 December 2013 Received in revised form 20 February 2014 Accepted 24 February 2014 Available online 3 March 2014 Keywords: Nanostructures Chemical vapor deposition Morphology Magnetic properties
a b s t r a c t By controlling the pyrolysis temperature, core/shell materials with Fe3O4 encapsulated in carbon nanocages (Fe3O4@CNCs) and Fe encapsulated in carbon nanotubes (Fe@CNTs) were synthesized selectively from acetylene using Fe2O3 nanoparticles as catalyst in chemical vapor deposition. Scanning electron microscopic study showed that the efficiency of generating Fe3O4@CNCs and Fe@CNTs was high, exceeding 95%. Transmission electron microscopic study confirmed the high selectivity to Fe3O4@CNCs and Fe@CNTs, with the former having morphology similar to that of the catalyst particles. With Fe3O4 and Fe nanoparticles tightly wrapped in graphitic layers, the obtained Fe3O4@CNCs and Fe@CNTs materials show high stability and good magnetic properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Due to surface and/or quantum-size effects, metal nanoparticles especially those of magnetic materials exhibit unique physical properties and have potential applications in many areas [1–4]. However, metal nanoparticles are usually highly active and easily oxidized in air, resulting in deterioration of characteristics such as magnetism. The breakthrough in this regard was the encapsulation of metal nanoparticles in carbon layers. Since the discovery of fullerenes, there was much advance in the development of carbon nanomaterials (CNMs) and carbon-coated nanocapsules [5–7]. Compared with other inert materials such as silica, metal oxides and polymer shells [8–10], carbon shells exhibited much higher stability in chemical and physical environments. With the metal nanoparticles encapsulated in carbon shells, they are protected from oxidation. Also, with the encapsulation, there is reduction of magnetic coupling between magnetic phases, opening the possibility of optimizing the intrinsic magnetic properties of the metal nanoparticles [11–15]. In the past years, carbon nanocages (CNCs) and CNMs that were filled with ferromagnetic materials have
⁎ Corresponding author. Tel.: +86 25 83621200; fax: +86 25 83595535. E-mail address: [email protected] (W. Zhong).
http://dx.doi.org/10.1016/j.diamond.2014.02.013 0925-9635/© 2014 Elsevier B.V. All rights reserved.
attracted considerable attention due to their potential applications such as electromagnetic wave absorption [12], magnetic data storage and human tumor therapy [16,17]. There are methods for the synthesis of metal-filled CNMs or CNCs [18–22]. Nonetheless, most of the adopted procedures were complicated, and the efficiency of metal encapsulation needs improvement [23–25]. Based on knowledge revealed in the past [26–29], we worked on the controllable synthesis of CNCs-encapsulated Fe 3 O4 (Fe3 O 4@ CNCs) and carbon nanotubes (CNTs)-encapsulated Fe (Fe@CNTs). We selectively generated these core/shell structures with high efficiency over Fe2O3 nanoparticles by controlling the pyrolysis temperature of acetylene in chemical vapor deposition. To the best of our knowledge, our proposed route is very simple and has not been reported before.
2. Experimental 2.1. Synthesis of α-Fe2O3 catalyst All reagents were of analytical grade and used without further purification. Typically, 30 ml of aqueous NH4H2PO4 solution (0.05 M) was added to 30 ml of aqueous Fe(NO3)3 solution (0.3 M; yellowish in color) under vigorous stirring at room temperature (RT). After stirring for another five minutes, the resulted slurry was transferred into a Teflon-sealed autoclave and subject to hydrothermal treatment at
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220 °C for 48 h. With the setting cooled to RT, the as-obtained solid substance (red in color) was filtered out, washed three times with distilled water, and dried at 80 °C for 8 h.
2.2. Generation of Fe3O4@CNCs and Fe@CNTs core/shell materials First, the as-obtained red powder (0.1 g) was dispersed on a ceramic plate that was positioned inside a quartz tube. Then the temperature of the furnace was raised from RT to a designed temperature with Ar flowing (controlled by flowmeter at a flow rate of 40 sccm) through the reaction tube. After that, the gas supply was shifted from Ar to acetylene for the synthesis of carbon-encapsulated nanomaterials. The decomposition of acetylene (atmospheric pressure, 2 h) was conducted at 400, 500 or 600 °C (heated at a rate of 10 °C/min). The products generated at pyrolysis temperatures of 400, 500 and 600 °C are denoted hereinafter as C-400, C-500 and C-600, respectively.
2.3. Characterization of products The samples were examined on an X-ray powder diffractometer (XRD) at RT for phase identification using CuK α radiation (model D/Max-RA, Rigaku). Raman spectroscopic investigations were performed using a Jobin-Yvon Labram HR800 instrument with 514.5 nm Ar+ laser excitation. The morphology of samples was examined using a transmission electron microscope (TEM) (model JEM-2000EX, operated at an accelerating voltage of 80 kV), and field emission scanning electron microscope (FE-SEM) (model FEI Sirion 200, operated at accelerating voltages of 5 kV and equipped with a energy dispersive X-ray spectrometer (EDS)). The magnetic properties of samples were measured at 300 K using a Quantum Design MPMS SQUID magnetometer (Quantum Design MPMS-XL) equipped with a superconducting magnet capable of producing fields of up to 50 kOe. Fourier Transform Infrared (FTIR) spectra of samples (in KBr pellets) were recorded over a Nicolet 510P spectrometer.
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3. Results and discussion 3.1. FE-SEM, XRD and EDS characterizations of catalyst The FE-SEM images of the as-synthesized catalyst are shown in Fig. 1a and b. One can see a large amount of size-uniform hollow nanoparticles (as indicated by arrows), exceeding 94% of the entire content. The XRD pattern shows lines corresponding to α-Fe2 O 3 and P (as indexed in Fig. 1c). The EDS result shows the presence of Fe, O and P (Fig. 1d). Based on the results, it is deduced that the catalyst is mainly composed of hollow α-Fe2O3 nanoparticles with an average diameter of ca. 200 nm. 3.2. Microstructure and magnetic property of C-400 In each run, about 0.131 g of C-400 was harvested on the ceramic plate. The FE-SEM image (Fig. 2a) indicates that C-400 is mainly composed of hollow nanoparticles, with morphology similar to that of the α-Fe2O3 catalyst (Fig. 1a and b). The EDS result indicates the presence of C, Fe and O (Fig. 2b), and the Fe:O molar ratio of C-400 is much higher than that of the α-Fe2O3 catalyst (Fig. 1d). The XRD result of the obtained C-400 is shown in Fig. 2c. One can find that the diffraction peaks corresponding to α-Fe2O3 catalyst disappears completely after the reduction of α-Fe2O3 by C2H2 at 400 °C. And the diffraction peaks corresponding to phases of C, Fe3O4, Fe0.942O and Fe2C can be observed clearly (as indicated by the symbols in Fig. 2c), indicating that the obtained C-400 is a composite made up of C and Fe3O4. Fig. 2d shows the Raman spectrum of C-400; there are a total of four peaks. The one at ca. 1602 cm−1 (called G-band, indicative of graphitic layer of high crystallinity) is corresponding to the E2g mode of graphite that is related to the C\C vibration of carbon materials with sp2 orbital structure. The one at ca. 1336 cm−1 (called D-band) is a consequence of resonance scattering with the emission of phonon [30,31]. The intensity ratio of the G and D bands (IG/ID) is commonly used to characterize the crystallinity of carbon materials. In the study, an IG/ID of ca. 1.09 was recorded for C-400, implying high crystallinity. The peaks at 281 and
Fig. 1. (a,b) FE-SEM images, (c) XRD pattern, and (d) EDS result (collected from the area marked in 1a) of catalyst.
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Fig. 2. (a) FE-SEM image, (b) EDS result (collected from the area marked in panel a), (c) XRD pattern, (d) Raman spectrum, and (e) magnetization curves (inset is the enlarged part close to the origin) of C-400.
222 cm−1 can be ascribed to the Eg mode (symmetric bends of oxygen with respect to Fe) and T2g mode (translator movement of the whole FeO4 unit) of Fe3O4, respectively [32,33]. The Raman data confirmed further that C-400 is a composite made up of C and Fe3O4. According to the magnetization-coercivity (M–H) curves (Fig. 2e), the saturation magnetization (MS) and the coercivity (HC) of C-400 is 26.69 emu g−1 and 508 Oe, respectively, in good agreement with the ferromagnetic nature of Fe3O4 at 300 K. The FE-SEM and TEM images of C-400 are shown in Fig. 3. The FE-SEM and TEM results reveal that the majority of C-400 is Fe3O4@ CNCs of core/shell structure. As shown in Fig. 3a and b, Fe3 O 4@ CNCs is uniform in size, having an average diameter of 230 nm, slight higher than that of the α-Fe2O3 catalyst (200 nm). In the TEM and high-resolution TEM images, the core/shell structure can be clearly observed (Fig. 3c and d). The efficiency of Fe3O4 encapsulation inside
CNCs is high (at least 98%). The Fe3O4@CNCs shows an inner core that is tightly wrapped by a graphite layer of ca. 30 nm in thickness, giving an overall shape that resembles that of the α-Fe2O3 nanoparticles. Compared with the methods reported in the literature [18–20], the one adopted in this study is simple and efficient. With the shape of Fe3O4@CNCs resembling that of α-Fe2O3 nanoparticles, it is deduced that one can regulate the morphology of the former by controlling that of the latter. 3.3. The effect of pyrolysis temperature In order to investigate the effect of decomposition temperature on the final products, acetylene decomposition was conducted at 500 °C over hollow α-Fe2O3 nanoparticles with other experimental conditions unchanged. After cooling to RT, we harvested ca. 0.112 g of C-500 in
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Fig. 3. (a, b) FE-SEM and (c, d) TEM images of C-400.
each run, slightly lower than that (ca. 0.131 g) of C-400. Fig. 4a shows the FE-SEM image of C-500. The morphology of C-500 is similar to that of the catalyst (Fig. 1a) and that of C-400 (Fig. 2a). According to the EDS results (Fig. 4b), the Fe:O molar ratio and carbon content of C-500 is higher than that of the catalyst (Fig. 1d) and C-400 (Fig. 2b). Fig. 4c shows the XRD result of the obtained C-500. Compared with that of C-400 (Fig. 2c), the diffraction peaks can be indexed to phases of C, Fe3O4, Fe0.942O and Fe2C (as indicated by the symbols in Fig. 4c). And the diffraction peaks corresponding to Fe0.942O can be observed much more clearly, which further proves that the Fe:O molar ratio of C-500 becomes much higher than that of C-400 after the reduction of Fe0.942O by C2H2 at higher temperature. The Raman spectrum of C-500 (Fig. 4d) also shows four peaks. Compared to those of C-400, the G (~1611 cm−1) and D (~1353 cm−1) bands of C-500 shift to the higher frequency. Also, the IG/ID (ca. 1.12) of C-500 is higher than that of C-400 (ca. 1.09). The peaks at 300 and 229 cm−1 are ascribable to the Eg mode and T2g mode of Fe3O4, respectively. Despite the Raman peaks of C-500 corresponding to the Fe3O4 phase are not as obvious as those of C-400, the Raman data of C-500 further suggest that C-500 is mainly composed of C and Fe3O4. Fig. 4e gives the magnetic property of the obtained C-500. The MS and HC value of C-500 is 25.47 emu g− 1 and 409 Oe, respectively, lower than that of C-400 (26.69 emu g− 1 and 508 Oe). Due to the existence of magnetic nanoparticles (Fe3O4), the obtained samples (C-400 and C-500) show good magnetic properties. Therefore, the lower MS value of C-500 should be related to the lower Fe3O4 content in the obtained sample, which has been proved by the EDS and XRD results of C-400 and C-500. Moreover, it was observed that both C-400 and C-500 showed no changes in magnetic properties after being kept in air for over one year. The high stability of the Fe3O4 nanoparticles is a result of encapsulation inside the graphite layers. The FE-SEM and TEM images of C-500 are shown in Fig. 5. One can see that Fe3O4@CNCs was synthesized in large amount, and the
selectivity to Fe3O4@CNCs is ca. 86%. As indicated by the white arrows in Fig. 5a and b, there is sighting of CNTs-encapsulated magnetic nanoparticles. The Fe3O4@CNCs is uniform in size, having an average diameter of ca. 400 nm. Considering the nanoparticles inside CNCs as well as those inside CNTs, the efficiency of encapsulation is high (at least 95%). The core/shell structures can be seen clearly in the TEM and high-resolution TEM images of C-500. Similar to the Fe3O4@CNCs of C-400, the Fe3O4@CNCs of C-500 show the morphology resembles that of the catalyst nanoparticles. It is deduced that the decomposition temperature of acetylene has an effect on the structure, yield, magnetic property, and composition of the core/shell materials. 3.4. Characterization and magnetic property of C-600 In each run, ca. 2.366 g of C-600 could be collected, higher than that of C-400 or C-500. The yield of carbon species (mass ratio of carbon materials to catalyst) is extremely high, up to ca. 23.7. Fig. 6 shows the XRD pattern of C-600. The diffraction peaks can be indexed to phases of graphite, Fe3C and α-Fe. It is apparent that with rise of pyrolysis temperature from 400 to 600 °C, there is reduction of the catalyst from Fe2O3 to Fe3O4 and then to α-Fe, in agreement with the rise of Fe:O molar ratio observed in EDS studies. Fig. 6b shows the Raman spectrum of C-600. Unlike those of C-400 and C-500, the Raman scattering bands of C-600 (at ca. 1601 and 1321 cm−1) shift towards smaller frequency. Furthermore, the IG/ID of C-600 is ca. 1.12, which is similar to that of C-500 and higher than that of C-400. Fig. 6c shows the FTIR spectrum of C-600. There is no IR signals that could be associate with \CH_CH\, \CH2\, and \CH3 entities, implying that C-600 is free of hydrogen and oxygen. All the results indicate that at a pyrolysis temperature of 600 °C, the catalyst (Fe2O3) was reduced to α-Fe. Fig. 6d gives the M-H curves of C-600. The MS and HC value of C-600 is 1.95 emu g− 1 and 427 Oe, respectively. The values are much lower
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Fig. 4. (a) FE-SEM image, (b) EDS result (the area marked in panel a), (c) XRD pattern, (d) Raman spectrum, and (e) typical magnetization curves (inset: enlarged part close to the origin) of the obtained C-500.
than those of C-400 and C-500, plausibly due to the low content of αFe in C-600. Moreover, compared to that of Fe@CNTs reported by Grechnev et al. [34], the Ms of C-600 is low. Due to the tight encapsulation of Fe nanoparticles inside the graphite layers, there was no change in XRD features as well as magnetic property after a C-600 sample was exposed to air for a period of one year. The good stability of C-600 was proved further by treating C-600 with 37 wt.% HCl for 1 h (Fig. 6e). The Ms value of HCl-treated C-600 is 1.92 emu g−1, which is close to that of the untreated one. Shown in Fig. 7 are the FE-SEM and TEM images of C-600. One can see that Fe@CNTs is in majority, and the selectivity to Fe@CNTs is ca. 98%. As indicated by the arrows in Fig. 7c and d, the size of Fe@CNTs is uniform (average diameter: 50 nm). It is noted that there are Fe nanoparticles at the tips and nodes of CNTs, which is different from that reported by Grechnev et al. [34]. As shown in Table 1, a change in the decomposition temperature of acetylene has great effect on the yield, structure, magnetic property and composition of the core/shell
materials. By controlling the pyrolysis temperature, one can synthesize Fe3O4@CNCs or Fe@CNTs selectively. 4. Conclusions Hollow Fe2O3 nanoparticles were fabricated by a hydrothermal method. Over the Fe2O3 nanoparticles, one can synthesize a large quantity of Fe3O4@CNCs in acetylene decomposition at 400 °C, with the morphology being similar to that of the Fe2O3 nanoparticles. With the pyrolysis temperature raised from 400 to 600 °C, the product changed from Fe3O4@CNCs to Fe@CNTs. It was observed that the efficiency of encapsulating the Fe3 O 4 nanoparticles in CNCs or Fe nanoparticles in CNTs exceeds 95%. With the Fe2O3 and Fe nanoparticles tightly wrapped inside graphite layers, the Fe3O4@CNCs and Fe@CNTs show high stability and good magnetic properties. The adopted procedure is simple, and can be applied for the synthesis of core/shell materials of similar kinds.
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Fig. 5. (a) FE-SEM and (b–d) TEM images of C-500.
Prime novelty statement In this paper, over the hollow Fe2O3 nanoparticles fabricated by a hydrothermal method, a simple route for the generation of core/shell materials with Fe3O4 encapsulated in carbon nanocages (Fe3O4@CNCs) Fe3O4@CNCs has been proposed. It was observed that the efficiency of encapsulating the Fe3 O 4 nanoparticles in CNCs or Fe nanoparticles in CNTs is very high (exceeds 95%). Because of the Fe2 O 3 and Fe nanoparticles tightly wrapped inside graphite layers, the Fe3 O 4 @CNCs and Fe@CNTs show high stability and good magnetic properties. Therefore, the approach is very new, simple, inexpensive, and environment-benign, and can be adopted for the synthesis of core/shell materials of similar kinds. Acknowledgments This work was supported by the Doctorial Start-up Fund of Guizhou University, the International Cooperation Project of Guizhou Province (2012–7002), the Foundation of the National Natural Science Foundation of China (Grant Nos. 11364005 and 11174132), and the Foundation of the National Key Project for Basic Research (Grant Nos. 2010CB923402 and 2011CB922102), People's Republic of China, for financial support. References [1] G.A. Held, G. Gringstein, Quantum limit of magnetic recording density, Appl. Phys. Lett. 79 (2001) 1501–1503. [2] R.H. Kodama, Magnetic nanoparticles, J. Magn. Magn. Mater. 200 (1999) 359–372. [3] G. Reiss, A. Hutten, Magnetic nanoparticles—applications beyond data storage, Nat. Mater. 4 (2005) 725–726. [4] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, Applications of magnetic nanoparticles in biomedicine, J. Phys. D. Appl. Phys. 36 (2003) 16–81. [5] S. Tomita, S. Hayashi, Y. Tsukuda, M. Fujii, Ultraviolet–visible absorption spectroscopy of carbon onions, Phys. Solid State 44 (2002) 450–453. [6] C.H. Liang, G.W. Meng, L.D. Zhang, N.F. Shen, X.Y. Zhang, Carbon nanotubes filled partially or completely with nickel, J. Cryst. Growth 218 (2000) 136–139.
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Fig. 7. (a,b) FE-SEM and (c,d) TEM images of C-600.
Table 1 Effects of pyrolysis temperature of acetylene on products. Temperature (°C)
Product
Yield of product
Species as revealed by TEM studied
Efficiency of encapsulation
400 500 600
C-400 C-500 C-600
1.3 1.1 23.7
Fe3O4@CNCs Fe3O4@CNCs Fe@CNTs
98% 96% 98%
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