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Synthesis and magnetic properties of Fe3C–C core–shell nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 085601 (http://iopscience.iop.org/0957-4484/26/8/085601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 21/02/2015 at 03:40
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 085601 (7pp)
doi:10.1088/0957-4484/26/8/085601
Synthesis and magnetic properties of Fe3C– C core–shell nanoparticles Jun Liu1, Bowen Yu1, Qiankun Zhang1, Lizhen Hou2,3, Qiulai Huang1, Chunrui Song1, Shiliang Wang1,3, Yueqin Wu3, Yuehui He1, Jin Zou3 and Han Huang3 1
School of Physics and Electronics, State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, People’s Republic of China 2 School of Physics and Information Science, Hunan Normal University, Changsha, 410081, People’s Republic of China 3 School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD, 4072, Australia E-mail: [email protected], [email protected] and [email protected] Received 4 November 2014, revised 14 January 2015 Accepted for publication 15 January 2015 Published 3 February 2015 Abstract
Fe3C–C core–shell nanoparticles were fabricated on a large scale by metal-organic chemical vapor deposition at 700 °C with ferric acetylacetonate as the precursor. Analysis results of x-ray diffraction, transmission electron microscope and Raman spectroscope showed that the Fe3C cores with an average diameter of ∼35 nm were capsulated by the graphite-like C layers with the thickness of 2–5 nm. The comparative experiments revealed that considerable Fe3O4–Fe3C core– shell nanoparticles and C nanotubes were generated simultaneously at 600 and 800 °C, respectively. A formation mechanism was proposed for the as-synthesized core–shell nanostructures, based on the temperature-dependent catalytic activity of Fe3C nanoclusters and the coalescence process of Fe3C–C nanoclusters. The Fe3C–C core–shell nanoparticles exhibited a saturation magnetization of 23.6 emu g−1 and a coercivity of 550 Oe at room temperature. Keywords: core–shell nanoparticles, iron carbide, carbon, metal-organic chemical vapor deposition (Some figures may appear in colour only in the online journal) 1. Introduction
doped Fe3C–C core–shell nanoparticles exhibited a high capacity of ∼500 and ∼750 mAh g−1, respectively [6, 7]. Nenriched Fe/Fe3C–C core–shell nanorods could significantly improve activities and advanced kinetics for the oxygen reduction reaction in the neutral phosphate buffer solution compared to the expensive commercial Pt/C catalysts [5]. In addition, due to the low toxicity and excellent magnetic property of the Fe3C cores as well as the non-toxicity of the C shells, Fe3C–C core–shell nanoparticles are expected to be a strong candidate as the carrier for drug delivery [8]. Fe3C–C core–shell nanoparticles were generally prepared by solution-based method, such as supercritical fluid reaction [9], hydrothermal reaction [6, 10, 11], sol–gel reaction [12], and electric plasma discharge in liquid ethanol [13]. In addition, carbothermal reduction methods were also widely used for the synthesis of the core–shell nanoparticles [2–
Iron carbide (Fe3C) nanoparticles have excellent magnetic and catalytic properties, and thus exhibit potential applications as magnetic recording media and high-performance catalyst [1–3]. For example, Fe3C nanoparticles exhibit superior catalytic activity and high stability in ammonia decomposition with a conversion of 95% at 700 °C [2]. Fe3C can serve as the catalyst for hydrogen generation at 300 °C, which is significantly lower than the temperature (>500 °C) required in the classic steam-iron method [3]. Recently, Fe3Cbased nanocomposites, especially Fe3C–C core–shell nanomaterials, have drawn much attention due to their novel properties and applications as compared to pure Fe3C [4, 5]. It was found that the theoretical capacity of Fe3C in lithium ion batteries was only of ∼26 mAh g−1, while Fe/Fe3C–C and N0957-4484/15/085601+07$33.00
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Nanotechnology 26 (2015) 085601
5, 14–16]. Nevertheless, the simple, low-cost, high-purity and large-scale synthesis of Fe3C–C core–shell nanoparticles remains a significant challenge. Chemical vapor deposition (CVD) is generally considered as a powerful method for the controlled and large-scale synthesis of high-purity nanomaterials [17], and also has been used for preparing the Fe3C–C core–shell nanoparticles [18, 19]. However, iron oxide nanoparticles were usually formed simultaneously in the assynthesized products. In our previous study, we demonstrated that the common metal-organic CVD was a simple route for the large-scale synthesis of core–shell nanostructures by using metal acetylacetonates as precursors, such as Cu(acac)2 [20– 22] and Ni(acac)2 [23]. In this study, we report the synthesis of pure Fe3C–C core–shell nanoparticles, using ferric acetylacetonate, Fe(acac)3, as the precursor. We also discuss the growth mechanism and characterize the magnetic properties of the as-synthesized products.
2. Experimental details 2.1. Growth
The Fe3C–C core–shell nanostructures were synthesized in a horizontal tube furnace with a vacuum pump system [24]. In the experiment, the reaction precursor, Fe(acac)3 (Aldrich Chemical Co., 99%), were firstly loaded in a quartz boat settled in the evaporation zone of the furnace, and a SiO2 wafer with a size of 4 × 15 cm was laid on the deposition zone for collecting the products. Then, the air-tightness of the furnace tube was tested, and the high-purity H2 (99.99%) was filled into the tube with a constant flow of 200 standard cubic centimeters per minute (sccm) and the pressure in the furnace was controlled to be 50 Pa, which was kept during the whole process of experiment. After that, the evaporation zone and deposition zone of the furnace were heated to 180 and 700 °C at the rates of 5 and 10 °C min−1, respectively. After the synthesis, which lasted for 2 h, the furnace was naturally cooled down to the room temperature with the protection of H2. To understand the effects of the deposition temperature on the morphologies and structures of synthesized products, two parallel experiments were carried out, using deposition temperatures of 600 and 800 °C, respectively.
Figure 1. (a) XRD diffractograms and (b) Raman spectra of the
products fabricated at different temperatures.
3. Results and discussion Figure 1(a) shows the XRD patterns of the synthesized products. The products obtained at 700 and 800 °C are composed of the orthorhombic structured Fe3C (JCPDS 76-1877; a = 0.4517 nm, b = 0.5070, c = 0.6730 nm) and hexagonal structured graphite (JCPDS 34-0567; a = 0. 2463 nm, c = 0.6714 nm). However, considerable cubic structured Fe3O4 (JCPDS 88-0315; a = 0.8375 nm) and orthorhombic structured Fe(acac)3 (JCPDS 30-1763; a = 1.5467 nm, b = 1.6560, c = 1.3574 nm) were identified in the products collected at 600 °C. The board diffraction peaks were used to estimate the grain size of the synthesized particles by the Scherrer’s relation [25]. The average crystallite sizes of the products synthesized at 600, 700 and 800 °C were approximately 53, 31 and 15 nm, respectively. Figure 1(b) is the Raman spectra taken from the three samples. Two peaks located at 1345 and 1580 cm−1 were indexed as the D band and G band of C, respectively [26, 27]. It has been well documented that the G band originates from the stretching motion of sp2 C pairs in both rings and chains, while the D band arises from defects in the hexagonal sp2 C network or
2.2. Characterization
The phase and size features of synthesized products were investigated by x-ray diffraction (XRD, Rigaku D max 2500 VB) and Raman Spectrometer (LabRAM HR 800). The morphological, structural and compositional characteristics of the as-synthesized products were examined by transmission electron microscope (TEM, FEI Tecnai F20 and JEOL JEM —2100F, both operated at 200 kV) equipped with an energy dispersive spectrometer (EDS, Phoenix EDAX 2000). Their magnetic properties were characterized using a physical property measurement system (PPMS, 1076–100A, Quantum Design, USA). 2
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Nanotechnology 26 (2015) 085601
Figure 2(a) shows a typical TEM image of the product obtained at 700 °C, which consists of nanoparticles with the diameter far below 100 nm. In our TEM examination, we measured more than 300 nanoparticles and found that the diameters of the particles varied from 10 to 70 nm, as can be seen from the particle size distribution shown in the inset of figure 2(a). Obviously, the average diameter of ∼30 nm examined by TEM was well consistent with the value of 31 nm estimated by the Scherrer’s relation. Figure 2(b) is the EDS spectrum collected from the synthesized product, showing that the nanoparticles were composed of Fe and C. Note that the Cu peaks should come from the TEM Cu grid. Figure 2(c) displays the high-resolution TEM (HRTEM) image of a nanoparticle, which exhibits a typical core–shell structure, i.e. a nanocore encapsulated by a ‘layered’ material or a shell. The lattice spacings of the nanocores, 0.20 and 0.22 nm, correspond to the {103} and {120} crystal planes of the orthorhombic structured Fe3C. In the corresponding fast Fourier transformation (FFT) pattern (the inset in figure 2(c)), the angle between the corresponding diffraction spots is ∼78°, the same as the angle between (103) and (120) planes of Fe3C structure. In addition, the lattice spacing of the shell is 0.35 nm, which can be indexed to the {0002} planes of graphite. Therefore, the nanostructures obtained at 700 °C are Fe3C nanocores capsulated by graphite-like C shells. To understand the effects of the deposition temperature on the morphologies and structures of the products, intensive TEM studies were also performed on the samples obtained at 600 and 800 °C. It was found that the products obtained at 600 °C mainly consisted of core–shell nanoparticles with a size of ∼100 nm and a thin shell of several nm (figure 3(a)). Figure 3(b) shows the HRTEM image taken from the edge of a nanoparticle and the corresponding selected area electron diffraction (SAED) pattern. In the HRTEM image, the lattice spacings of 0.24 nm within the shell and 0.21 nm within the core can be indexed to the {112} crystal planes of the orthorhombic-structured Fe3C and the {400} crystal planes of cubic-structured Fe3O4, respectively. This matches well with the diffraction rings from poly-crystal Fe3C and the diffraction spots from single-crystal Fe3O4 in the corresponding SAED pattern (the insert in figure 3(b)). As a result, the nanoparticles obtained at 600 °C are single-crystal Fe3O4 nanocores capsulated by poly-crystal Fe3C shells. Besides the Fe3O4–Fe3C core–shell nanoparticles, there are considerable Fe3C–C core–shell nanoparticles co-existed in the synthesized product, as shown in figure 3(c), where Fe3C–C nanoparticles have the particle size ranged from several to 100 nm and the shell thickness up to 20 nm. Figure 4(a) displays the typical TEM images of the products synthesized at 800 °C. The Fe3C nanoparticles have an average crystallite size of 15 nm, which is smaller than those synthesized at 600 and 700 °C. It should be noted that most of these small Fe3C nanoparticles are located at the tips of the short curly CNTs. This highly suggests that the growth of the CNTs should be induced the Fe3C nanoparticles, although the validity of the carbide-assisted growth mechanism for CNTs still remains in dispute [23, 28–30]. Besides the
Figure 2. (a) Low-magnification TEM image and the corresponding
size distribution of the synthesized core–shell nanoparticles. (b) EDS pattern of the nanoparticles. (c) HRTEM image and the corresponding FFT pattern of a core–shell nanoparticle.
the finite particle-size effect [26, 27]. The ratios of the integrated intensities of the G–D bands, IG/ID, were found to be 0.6, 1.8 and 1.9 for the products synthesized at 600, 700 and 800 °C, respectively. This suggests that the C generated at 700 and 800 °C should have a reasonable crystallinity, while the C formed at 600 °C might be mainly amorphous [26, 27]. 3
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Based on the structural analysis and characterization of the products synthesized at different temperatures, the formation process of the core–shell Fe3C–C nanoparticles can be summarized in figure 5. Firstly, Fe(acac)3 was evaporated from the upstream at 180 °C and then carried by H2 gas to the deposition zone for reaction. Secondly, gaseous Fe(acac)3 was thermally decomposed into Fe atoms, water vapor and Ccontained gases (such as CO2, acetone, and CH4) in the deposition zone [31]. Thirdly, the resultant Fe atoms reacted with the C-contained gases to form Fe3C molecule clusters, which further induced the catalytic decomposition of the Ccontained gases to generate C atoms, and condensed with the C atoms into small Fe3C–C nanoclusters. Finally, the Fe3C–C nanoclusters aggregated to form nanoparticles due to their high surface activity and high surface adsorption during their collision process in the reaction atmosphere, forming Fe3C–C core–shell nanostructures through the coalescence mechanism [21, 22]. Since the catalytic decomposition of C-contained gases preferred to take place at high temperature such as 800 °C in our case, more C atoms were expected to be generated and then adhered onto the Fe3C molecule clusters. This resulted in a high ratio of C atoms in the subsequent Fe3C–C nanoclusters. Our previous work revealed that the high ratio of C atoms in the composite nanoclusters formed at high temperature were unfavorable for the coalescence process [22]. Therefore, the Fe3C particles generated at 800 °C exhibited a smaller size than those obtained at 700 °C. In addition, a large amount of CNTs were generated at the relatively high temperature of 800 °C in our study. As the high temperature favored both the carbonization of Fe atoms and the thermal decomposition of the metastable Fe3C, the CNTs formed in the synthesis were likely initiated by the thermal decomposition of the metastable Fe3C nanocores, as suggested in the recent works on the carbide-induced growth of CNTs [23, 28–30]. On the contrary, at a relatively low temperature, such as 600 °C, the carbonization reaction was slow, so that only a part of the resultant Fe atoms was transferred to Fe3C molecules clusters through the carbonization [32]. At the same time, considerable Fe atoms might be transferred to Fe3O4 molecule clusters, because Fe could react with the trace of H2O (less than 0.2%) in H2–H2O atmosphere at a temperature below 600 °C, according to the Baur– Glaessner diagram of Fe and H2–H2O system [32]. It should be noted that Fe3O4 can be easily reduced to Fe by H2 or be carbonized to Fe3C by the C-contained vapor at temperatures higher than 650 °C, even though the content of H2O in the atmosphere is relatively high [32]. That is the reason why Fe3O4 could not be detected in the samples obtained at 700 and 800 °C. Therefore, during the subsequent deposition process at ∼600 °C, the clusters of Fe3O4 and Fe3C molecule might be condensed into small Fe3O4–Fe3C nanoclusters, which then further aggregated to form Fe3O4–Fe3C core–shell nanoparticles through the coalescence mechanism. The magnetic properties of the synthesized Fe3C–C core– shell nanoparticles were characterized by PPMS at room
Figure 3. TEM image of the products obtained at 600 °C. (a) Fe3C–C core–shell nanoparticles, (b) HRTEM image taken from the edge of Fe3O4–Fe3C core–shell nanoparticle, (c) low-magnification TEM image of the Fe3C–C core–shell nanoparticle. The insets in (a) and (b) show the high-magnification TEM of a typical core–shell nanoparticle and the SAED pattern, respectively.
short CNTs, there are also considerable long CNTs appearing in the sample (figure 4(b)). The CNTs have a length up to several hundred micrometers, a diameter of 50–100 nm and a reasonable crystallinity, as shown in the inset of figure 4(b).
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Nanotechnology 26 (2015) 085601
Figure 4. TEM images of the product obtained at 800 °C. (a) Fe3C nanoparticles and short CNTs with small diameters, (b) long CNTs with large diameters. The inset in (b) shows the tube wall of the CNT.
Figure 5. Growth model for the nanostructures obtained at different temperatures.
temperature and the obtained magnetization hysteresis loop is shown in figure 6. For the sample synthesized at 700 °C, the saturation magnetization (Ms), remanence magnetization (Mr), coercivity (Hc) and remanence squareness (Mr/Ms) are 23.6 emu g−1, 5.3 emu g−1, 550 Oe and 0.23, respectively. The measured value of Ms is comparable to the reported values of 20–55 emu g−1 for the similar core–shell nanoparticles [10, 11, 16, 33, 34], but smaller than the reported value of ∼140 emu g−1 for bulk Fe3C [35]. The existence of nonmagnetic C shells should be one of the major factors for the decrease in Ms. Moreover, the size effect is believed to be another main reason, because Ms may decrease significantly with the decrease of the particle size [10, 11, 33, 34]. The measured coercivity (Hc = 550 Oe) is higher than the reported values of 50–280 Oe for the Fe3C–C core–shell nanoparticles [10, 11, 33, 34], but quite close to that of 560 Oe for bulk Fe3C [35]. For comparison purpose, the magnetization hysteresis loop of the sample obtained at 600 °C was also
Figure 6. The magnetization hysteresis loops of the products
obtained at 600 and 700 °C.
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Nanotechnology 26 (2015) 085601
presented in figure 6. Obviously, the measured Ms of 46.2 emu g−1 was twice of the value for Fe3C–C core–shell nanoparticles. It was believed that the high Ms could be attributed to the Fe3O4 nanocores in the sample, because the typical Ms value for Fe3O4 nanoparticles was greater than 50 emu g−1 [36].
[8]
[9] [10]
4. Conclusions A simple MOCVD process was developed for the large-scale fabrication of pure Fe3C–C core–shell nanoparticles. The resultant core–shell nanoparticles had an average size of ∼35 nm and a shell thickness of 2–5 nm, and were presumed to be generated by the coalescence mechanism. The saturation magnetization, coercivity and remanence squareness of the Fe3C–C core–shell nanoparticles are 23.6 emu g−1, 550 Oe and 0.23, respectively. The obtained Fe3C–C core–shell nanostructures exhibited high coercivity and remanence, and their C shells could prevent the magnetic cores from the direct contact with the outer substances. They were expected to be a good candidate for applications in magnetic data storage, such as magnetic toners in xerography and magnetic inks or ferrofluids, and in bio-engineering applications, such as drug delivery and magnetic resonance imaging.
[11]
[12] [13]
[14]
[15]
Acknowledgments
[16]
This study is supported by the National Natural Science Foundation of China (Grant Nos. 50804057 and 51074188), and the Australian Research Council (Grant No. DP130101828).
[17]
[18]
References [19] [1] Zhao X Q, Liang Y, Hu Z Q and Liu B X 1996 Oxidation characteristics and magnetic properties of iron carbide and iron ultrafine particles J. Appl. Phys. 80 5857–60 [2] Kraupner A, Antonietti M, Palkovits R, Schlicht K and Giordano C 2010 Mesoporous Fe3C sponges as magnetic supports and as heterogeneous catalyst J. Mater. Chem. 20 6019–22 [3] Akiyama T, Miyazaki A, Nakanishi H, Hisa M and Tsutsumi A 2004 Thermal and gas analyses of the reaction between iron carbide and steam with hydrogen generation at 573 K Int. J. Hydrog. Energy 29 721–4 [4] Giordano C, Kraupner A, Fleischer I, Henrich C, Klingelhofer G and Antonietti M 2011 Non-conventional Fe3C-based nanostructures J. Mater. Chem. 21 16963–7 [5] Wen Z H, Ci S Q, Zhang F, Feng X L, Cui S M, Mao S, Luo S L, He Z and Chen J H 2012 Nitrogen-enriched core– shell structured Fe/Fe3C–C nanorods as advanced electrocatalysts for oxygen reduction reaction Adv. Mater. 24 1399–404 [6] Su L W, Zhou Z and Shen P W 2013 Core–shell Fe@Fe3C/C nanocomposites as anode materials for Li ion batteries Electrochim. Acta 87 180–5 [7] Zhao X, Xia D, Yue J and Liu S 2014 In situ generated nanoFe3C embedded into nitrogen-doped carbon for high
[20]
[21]
[22]
[23] [24]
[25]
6
performance anode in lithium ion battery Electrochim. Acta 116 292–9 Reddy L H, Arias J L, Nicolas J and Couvreur P 2012 Magnetic nanoparticles: design and characterization, toxicity and biocompatibility, pharmaceutical and biomedical applications Chem. Rev. 112 5818–78 Song H and Chen X 2003 Large-scale synthesis of carbonencapsulated iron carbide nanoparticles by co-carbonization of durene with ferrocene Chem. Phys. Lett. 374 400–4 Fan N, Ma X, Ju Z and Li J 2008 Formation, characterization and magnetic properties of carbon-encapsulated iron carbide nanoparticles Mater. Res. Bull. 43 1549–54 Luo N, Li X J, Wang X H, Yan H H, Zhang C J and Wang H T 2010 Synthesis and characterization of carbon-encapsulated iron/iron carbide nanoparticles by a detonation method Carbon 48 3858–63 Wang X, Zhang P, Gao J, Chen X and Yang H 2015 Facile synthesis and magnetic properties of Fe3C/C nanoparticles via a sol–gel process Dyes Pigm. 112 305–10 Kim S, Shibata E, Sergiienko R and Nakamura T 2008 Purification and separation of carbon nanocapsules as a magnetic carrier for drug delivery systems Carbon 46 1523–9 Zhou J, Song H, Chen X, Zhi L, Yang S, Huo J and Yang W 2009 Carbon-encapsulated metal oxide hollow nanoparticles and metal oxide hollow nanoparticles: a general synthesis strategy and its application to lithium-ion batteries Chem. Mater. 21 2935–40 Zhang H, Song P, Han D and Wang Q 2014 Synthesis and formaldehyde sensing performance of LaFeO3 hollow nanospheres Physica E: Low-Dimensional Syst. Nanostructures 63 21–6 Bo T, Hu G X and Xia M 2012 An easy method for the preparation of core–shell structural Fe3C@graphite-like carbon hollow spheres Mater. Lett. 68 104–7 Xia Y N, Yang P D, Sun Y G, Wu Y Y, Mayers B, Gates B, Yin Y D, Kim F and Yan Y Q 2003 One-dimensional nanostructures: synthesis, characterization, and applications Adv. Mater. 15 353–89 Sajitha E P, Prasad V, Subramanyam S V, Eto S, Takai K and Enoki T 2004 Synthesis and characteristics of iron nanoparticles in a carbon matrix along with the catalytic graphitization of amorphous carbon Carbon 42 2815–20 Qiu J S, Li Q X, Wang Z Y, Sun Y F and Zhang H Z 2006 CVD synthesis of coal-gas-derived carbon nanotubes and nanocapsules containing magnetic iron carbide and oxide Carbon 44 2565–8 Wang S, He Y, Liu X, Huang H, Zou J, Song M, Huang B and Liu C T 2011 Novel C/Cu sheath/core nanostructures synthesized via low-temperature MOCVD Nanotechnology 22 405704 Wang S, Huang X, He Y, Huang H, Wu Y, Hou L, Liu X, Yang T, Zou J and Huang B 2012 Synthesis, growth mechanism and thermal stability of copper nanoparticles encapsulated by multi-layer graphene Carbon 50 2119–25 Ma L, Yu B, Wang S, Su G, Huang H, Chen H, He Y and Zou J 2014 Controlled synthesis and optical properties of Cu/C core/shell nanoparticles J. Nanopart. Res. 16 2545 Yu B, Wang S, Zhang Q, He Y, Huang H and Zou J 2014 Ni3C-assisted growth of carbon nanofibres 300 °C by thermal CVD Nanotechnology 25 325602 Huang X L, Hou L Z, Yu B W, Chen G L, Wang S L, Ma L, Liu X L and He Y H 2013 Preparation, formation mechanism and optical properties of C/Cu shell/core nanostructures Acta Phys. Sin. 62 108102 Patterson A L 1939 The Scherrer formula for x-ray particle size determination Phys. Rev. 56 978–82
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[31] Hoene J V, Charles R G and Hickam W M 1958 Thermal decomposition of metal acetylacetonates: mass spectrometer studies J. Phys. Chem. 62 1098–101 [32] Nemec M, Wacker L and Gaggeler H 2010 Optimization of the graphitization process at AGE-1 Radiocarbon 52 1380–93 [33] Kim J H, Kim J, Park J H, Kim C K, Yoon C S and Shon Y 2007 Synthesis of carbon-encapsulated iron carbide nanoparticles on a polyimide thin film Nanotechnology 18 115609 [34] Geng J, Jefferson D A and Johnson B F G 2004 Direct conversion of iron stearate into magnetic Fe and Fe3C nanocrystals encapsulated in polyhedral graphite cages Chem. Commun. 2442–3 [35] Hofer L J E and Cohn E M 1959 Saturation magnetizations of iron carbides1 J. Am. Chem. Soc. 81 1576–82 [36] Xuan S, Wang Y-X J, Yu J C and Cham-Fai L K 2009 Tuning the grain size and particle size of superparamagnetic Fe3O4 microparticles Chem. Mater. 21 5079–87
[26] Ferrari A C and Robertson J 2001 Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon Phys. Rev. B 64 075414 [27] Pimenta M A, Dresselhaus G, Dresselhaus M S, Cancado L G, Jorio A and Saito R 2007 Studying disorder in graphitebased systems by Raman spectroscopy Phys. Chem. Chem. Phys. 9 1276–90 [28] Yoshida H, Takeda S, Uchiyama T, Kohno H and Homma Y 2008 Atomic-scale in-situ observation of carbon nanotube growth from solid state iron carbide nanoparticles Nano Lett. 8 2082–6 [29] Ni L, Kuroda K, Zhou L P, Ohta K, Matsuishi K and Nakamura J 2009 Decomposition of metal carbides as an elementary step of carbon nanotube synthesis Carbon 47 3054–62 [30] He Z, Maurice J-L, Gohier A, Lee C S, Pribat D and Cojocaru C S 2011 Iron catalysts for the growth of carbon nanofibers: Fe, Fe3C or both? Chem. Mater. 23 5379–87
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Synthesis and magnetic properties of Fe3C–C core–shell nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 085601 (http://iopscience.iop.org/0957-4484/26/8/085601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 21/02/2015 at 03:40
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 085601 (7pp)
doi:10.1088/0957-4484/26/8/085601
Synthesis and magnetic properties of Fe3C– C core–shell nanoparticles Jun Liu1, Bowen Yu1, Qiankun Zhang1, Lizhen Hou2,3, Qiulai Huang1, Chunrui Song1, Shiliang Wang1,3, Yueqin Wu3, Yuehui He1, Jin Zou3 and Han Huang3 1
School of Physics and Electronics, State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, People’s Republic of China 2 School of Physics and Information Science, Hunan Normal University, Changsha, 410081, People’s Republic of China 3 School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD, 4072, Australia E-mail: [email protected], [email protected] and [email protected] Received 4 November 2014, revised 14 January 2015 Accepted for publication 15 January 2015 Published 3 February 2015 Abstract
Fe3C–C core–shell nanoparticles were fabricated on a large scale by metal-organic chemical vapor deposition at 700 °C with ferric acetylacetonate as the precursor. Analysis results of x-ray diffraction, transmission electron microscope and Raman spectroscope showed that the Fe3C cores with an average diameter of ∼35 nm were capsulated by the graphite-like C layers with the thickness of 2–5 nm. The comparative experiments revealed that considerable Fe3O4–Fe3C core– shell nanoparticles and C nanotubes were generated simultaneously at 600 and 800 °C, respectively. A formation mechanism was proposed for the as-synthesized core–shell nanostructures, based on the temperature-dependent catalytic activity of Fe3C nanoclusters and the coalescence process of Fe3C–C nanoclusters. The Fe3C–C core–shell nanoparticles exhibited a saturation magnetization of 23.6 emu g−1 and a coercivity of 550 Oe at room temperature. Keywords: core–shell nanoparticles, iron carbide, carbon, metal-organic chemical vapor deposition (Some figures may appear in colour only in the online journal) 1. Introduction
doped Fe3C–C core–shell nanoparticles exhibited a high capacity of ∼500 and ∼750 mAh g−1, respectively [6, 7]. Nenriched Fe/Fe3C–C core–shell nanorods could significantly improve activities and advanced kinetics for the oxygen reduction reaction in the neutral phosphate buffer solution compared to the expensive commercial Pt/C catalysts [5]. In addition, due to the low toxicity and excellent magnetic property of the Fe3C cores as well as the non-toxicity of the C shells, Fe3C–C core–shell nanoparticles are expected to be a strong candidate as the carrier for drug delivery [8]. Fe3C–C core–shell nanoparticles were generally prepared by solution-based method, such as supercritical fluid reaction [9], hydrothermal reaction [6, 10, 11], sol–gel reaction [12], and electric plasma discharge in liquid ethanol [13]. In addition, carbothermal reduction methods were also widely used for the synthesis of the core–shell nanoparticles [2–
Iron carbide (Fe3C) nanoparticles have excellent magnetic and catalytic properties, and thus exhibit potential applications as magnetic recording media and high-performance catalyst [1–3]. For example, Fe3C nanoparticles exhibit superior catalytic activity and high stability in ammonia decomposition with a conversion of 95% at 700 °C [2]. Fe3C can serve as the catalyst for hydrogen generation at 300 °C, which is significantly lower than the temperature (>500 °C) required in the classic steam-iron method [3]. Recently, Fe3Cbased nanocomposites, especially Fe3C–C core–shell nanomaterials, have drawn much attention due to their novel properties and applications as compared to pure Fe3C [4, 5]. It was found that the theoretical capacity of Fe3C in lithium ion batteries was only of ∼26 mAh g−1, while Fe/Fe3C–C and N0957-4484/15/085601+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Nanotechnology 26 (2015) 085601
5, 14–16]. Nevertheless, the simple, low-cost, high-purity and large-scale synthesis of Fe3C–C core–shell nanoparticles remains a significant challenge. Chemical vapor deposition (CVD) is generally considered as a powerful method for the controlled and large-scale synthesis of high-purity nanomaterials [17], and also has been used for preparing the Fe3C–C core–shell nanoparticles [18, 19]. However, iron oxide nanoparticles were usually formed simultaneously in the assynthesized products. In our previous study, we demonstrated that the common metal-organic CVD was a simple route for the large-scale synthesis of core–shell nanostructures by using metal acetylacetonates as precursors, such as Cu(acac)2 [20– 22] and Ni(acac)2 [23]. In this study, we report the synthesis of pure Fe3C–C core–shell nanoparticles, using ferric acetylacetonate, Fe(acac)3, as the precursor. We also discuss the growth mechanism and characterize the magnetic properties of the as-synthesized products.
2. Experimental details 2.1. Growth
The Fe3C–C core–shell nanostructures were synthesized in a horizontal tube furnace with a vacuum pump system [24]. In the experiment, the reaction precursor, Fe(acac)3 (Aldrich Chemical Co., 99%), were firstly loaded in a quartz boat settled in the evaporation zone of the furnace, and a SiO2 wafer with a size of 4 × 15 cm was laid on the deposition zone for collecting the products. Then, the air-tightness of the furnace tube was tested, and the high-purity H2 (99.99%) was filled into the tube with a constant flow of 200 standard cubic centimeters per minute (sccm) and the pressure in the furnace was controlled to be 50 Pa, which was kept during the whole process of experiment. After that, the evaporation zone and deposition zone of the furnace were heated to 180 and 700 °C at the rates of 5 and 10 °C min−1, respectively. After the synthesis, which lasted for 2 h, the furnace was naturally cooled down to the room temperature with the protection of H2. To understand the effects of the deposition temperature on the morphologies and structures of synthesized products, two parallel experiments were carried out, using deposition temperatures of 600 and 800 °C, respectively.
Figure 1. (a) XRD diffractograms and (b) Raman spectra of the
products fabricated at different temperatures.
3. Results and discussion Figure 1(a) shows the XRD patterns of the synthesized products. The products obtained at 700 and 800 °C are composed of the orthorhombic structured Fe3C (JCPDS 76-1877; a = 0.4517 nm, b = 0.5070, c = 0.6730 nm) and hexagonal structured graphite (JCPDS 34-0567; a = 0. 2463 nm, c = 0.6714 nm). However, considerable cubic structured Fe3O4 (JCPDS 88-0315; a = 0.8375 nm) and orthorhombic structured Fe(acac)3 (JCPDS 30-1763; a = 1.5467 nm, b = 1.6560, c = 1.3574 nm) were identified in the products collected at 600 °C. The board diffraction peaks were used to estimate the grain size of the synthesized particles by the Scherrer’s relation [25]. The average crystallite sizes of the products synthesized at 600, 700 and 800 °C were approximately 53, 31 and 15 nm, respectively. Figure 1(b) is the Raman spectra taken from the three samples. Two peaks located at 1345 and 1580 cm−1 were indexed as the D band and G band of C, respectively [26, 27]. It has been well documented that the G band originates from the stretching motion of sp2 C pairs in both rings and chains, while the D band arises from defects in the hexagonal sp2 C network or
2.2. Characterization
The phase and size features of synthesized products were investigated by x-ray diffraction (XRD, Rigaku D max 2500 VB) and Raman Spectrometer (LabRAM HR 800). The morphological, structural and compositional characteristics of the as-synthesized products were examined by transmission electron microscope (TEM, FEI Tecnai F20 and JEOL JEM —2100F, both operated at 200 kV) equipped with an energy dispersive spectrometer (EDS, Phoenix EDAX 2000). Their magnetic properties were characterized using a physical property measurement system (PPMS, 1076–100A, Quantum Design, USA). 2
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Figure 2(a) shows a typical TEM image of the product obtained at 700 °C, which consists of nanoparticles with the diameter far below 100 nm. In our TEM examination, we measured more than 300 nanoparticles and found that the diameters of the particles varied from 10 to 70 nm, as can be seen from the particle size distribution shown in the inset of figure 2(a). Obviously, the average diameter of ∼30 nm examined by TEM was well consistent with the value of 31 nm estimated by the Scherrer’s relation. Figure 2(b) is the EDS spectrum collected from the synthesized product, showing that the nanoparticles were composed of Fe and C. Note that the Cu peaks should come from the TEM Cu grid. Figure 2(c) displays the high-resolution TEM (HRTEM) image of a nanoparticle, which exhibits a typical core–shell structure, i.e. a nanocore encapsulated by a ‘layered’ material or a shell. The lattice spacings of the nanocores, 0.20 and 0.22 nm, correspond to the {103} and {120} crystal planes of the orthorhombic structured Fe3C. In the corresponding fast Fourier transformation (FFT) pattern (the inset in figure 2(c)), the angle between the corresponding diffraction spots is ∼78°, the same as the angle between (103) and (120) planes of Fe3C structure. In addition, the lattice spacing of the shell is 0.35 nm, which can be indexed to the {0002} planes of graphite. Therefore, the nanostructures obtained at 700 °C are Fe3C nanocores capsulated by graphite-like C shells. To understand the effects of the deposition temperature on the morphologies and structures of the products, intensive TEM studies were also performed on the samples obtained at 600 and 800 °C. It was found that the products obtained at 600 °C mainly consisted of core–shell nanoparticles with a size of ∼100 nm and a thin shell of several nm (figure 3(a)). Figure 3(b) shows the HRTEM image taken from the edge of a nanoparticle and the corresponding selected area electron diffraction (SAED) pattern. In the HRTEM image, the lattice spacings of 0.24 nm within the shell and 0.21 nm within the core can be indexed to the {112} crystal planes of the orthorhombic-structured Fe3C and the {400} crystal planes of cubic-structured Fe3O4, respectively. This matches well with the diffraction rings from poly-crystal Fe3C and the diffraction spots from single-crystal Fe3O4 in the corresponding SAED pattern (the insert in figure 3(b)). As a result, the nanoparticles obtained at 600 °C are single-crystal Fe3O4 nanocores capsulated by poly-crystal Fe3C shells. Besides the Fe3O4–Fe3C core–shell nanoparticles, there are considerable Fe3C–C core–shell nanoparticles co-existed in the synthesized product, as shown in figure 3(c), where Fe3C–C nanoparticles have the particle size ranged from several to 100 nm and the shell thickness up to 20 nm. Figure 4(a) displays the typical TEM images of the products synthesized at 800 °C. The Fe3C nanoparticles have an average crystallite size of 15 nm, which is smaller than those synthesized at 600 and 700 °C. It should be noted that most of these small Fe3C nanoparticles are located at the tips of the short curly CNTs. This highly suggests that the growth of the CNTs should be induced the Fe3C nanoparticles, although the validity of the carbide-assisted growth mechanism for CNTs still remains in dispute [23, 28–30]. Besides the
Figure 2. (a) Low-magnification TEM image and the corresponding
size distribution of the synthesized core–shell nanoparticles. (b) EDS pattern of the nanoparticles. (c) HRTEM image and the corresponding FFT pattern of a core–shell nanoparticle.
the finite particle-size effect [26, 27]. The ratios of the integrated intensities of the G–D bands, IG/ID, were found to be 0.6, 1.8 and 1.9 for the products synthesized at 600, 700 and 800 °C, respectively. This suggests that the C generated at 700 and 800 °C should have a reasonable crystallinity, while the C formed at 600 °C might be mainly amorphous [26, 27]. 3
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Based on the structural analysis and characterization of the products synthesized at different temperatures, the formation process of the core–shell Fe3C–C nanoparticles can be summarized in figure 5. Firstly, Fe(acac)3 was evaporated from the upstream at 180 °C and then carried by H2 gas to the deposition zone for reaction. Secondly, gaseous Fe(acac)3 was thermally decomposed into Fe atoms, water vapor and Ccontained gases (such as CO2, acetone, and CH4) in the deposition zone [31]. Thirdly, the resultant Fe atoms reacted with the C-contained gases to form Fe3C molecule clusters, which further induced the catalytic decomposition of the Ccontained gases to generate C atoms, and condensed with the C atoms into small Fe3C–C nanoclusters. Finally, the Fe3C–C nanoclusters aggregated to form nanoparticles due to their high surface activity and high surface adsorption during their collision process in the reaction atmosphere, forming Fe3C–C core–shell nanostructures through the coalescence mechanism [21, 22]. Since the catalytic decomposition of C-contained gases preferred to take place at high temperature such as 800 °C in our case, more C atoms were expected to be generated and then adhered onto the Fe3C molecule clusters. This resulted in a high ratio of C atoms in the subsequent Fe3C–C nanoclusters. Our previous work revealed that the high ratio of C atoms in the composite nanoclusters formed at high temperature were unfavorable for the coalescence process [22]. Therefore, the Fe3C particles generated at 800 °C exhibited a smaller size than those obtained at 700 °C. In addition, a large amount of CNTs were generated at the relatively high temperature of 800 °C in our study. As the high temperature favored both the carbonization of Fe atoms and the thermal decomposition of the metastable Fe3C, the CNTs formed in the synthesis were likely initiated by the thermal decomposition of the metastable Fe3C nanocores, as suggested in the recent works on the carbide-induced growth of CNTs [23, 28–30]. On the contrary, at a relatively low temperature, such as 600 °C, the carbonization reaction was slow, so that only a part of the resultant Fe atoms was transferred to Fe3C molecules clusters through the carbonization [32]. At the same time, considerable Fe atoms might be transferred to Fe3O4 molecule clusters, because Fe could react with the trace of H2O (less than 0.2%) in H2–H2O atmosphere at a temperature below 600 °C, according to the Baur– Glaessner diagram of Fe and H2–H2O system [32]. It should be noted that Fe3O4 can be easily reduced to Fe by H2 or be carbonized to Fe3C by the C-contained vapor at temperatures higher than 650 °C, even though the content of H2O in the atmosphere is relatively high [32]. That is the reason why Fe3O4 could not be detected in the samples obtained at 700 and 800 °C. Therefore, during the subsequent deposition process at ∼600 °C, the clusters of Fe3O4 and Fe3C molecule might be condensed into small Fe3O4–Fe3C nanoclusters, which then further aggregated to form Fe3O4–Fe3C core–shell nanoparticles through the coalescence mechanism. The magnetic properties of the synthesized Fe3C–C core– shell nanoparticles were characterized by PPMS at room
Figure 3. TEM image of the products obtained at 600 °C. (a) Fe3C–C core–shell nanoparticles, (b) HRTEM image taken from the edge of Fe3O4–Fe3C core–shell nanoparticle, (c) low-magnification TEM image of the Fe3C–C core–shell nanoparticle. The insets in (a) and (b) show the high-magnification TEM of a typical core–shell nanoparticle and the SAED pattern, respectively.
short CNTs, there are also considerable long CNTs appearing in the sample (figure 4(b)). The CNTs have a length up to several hundred micrometers, a diameter of 50–100 nm and a reasonable crystallinity, as shown in the inset of figure 4(b).
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Figure 4. TEM images of the product obtained at 800 °C. (a) Fe3C nanoparticles and short CNTs with small diameters, (b) long CNTs with large diameters. The inset in (b) shows the tube wall of the CNT.
Figure 5. Growth model for the nanostructures obtained at different temperatures.
temperature and the obtained magnetization hysteresis loop is shown in figure 6. For the sample synthesized at 700 °C, the saturation magnetization (Ms), remanence magnetization (Mr), coercivity (Hc) and remanence squareness (Mr/Ms) are 23.6 emu g−1, 5.3 emu g−1, 550 Oe and 0.23, respectively. The measured value of Ms is comparable to the reported values of 20–55 emu g−1 for the similar core–shell nanoparticles [10, 11, 16, 33, 34], but smaller than the reported value of ∼140 emu g−1 for bulk Fe3C [35]. The existence of nonmagnetic C shells should be one of the major factors for the decrease in Ms. Moreover, the size effect is believed to be another main reason, because Ms may decrease significantly with the decrease of the particle size [10, 11, 33, 34]. The measured coercivity (Hc = 550 Oe) is higher than the reported values of 50–280 Oe for the Fe3C–C core–shell nanoparticles [10, 11, 33, 34], but quite close to that of 560 Oe for bulk Fe3C [35]. For comparison purpose, the magnetization hysteresis loop of the sample obtained at 600 °C was also
Figure 6. The magnetization hysteresis loops of the products
obtained at 600 and 700 °C.
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presented in figure 6. Obviously, the measured Ms of 46.2 emu g−1 was twice of the value for Fe3C–C core–shell nanoparticles. It was believed that the high Ms could be attributed to the Fe3O4 nanocores in the sample, because the typical Ms value for Fe3O4 nanoparticles was greater than 50 emu g−1 [36].
[8]
[9] [10]
4. Conclusions A simple MOCVD process was developed for the large-scale fabrication of pure Fe3C–C core–shell nanoparticles. The resultant core–shell nanoparticles had an average size of ∼35 nm and a shell thickness of 2–5 nm, and were presumed to be generated by the coalescence mechanism. The saturation magnetization, coercivity and remanence squareness of the Fe3C–C core–shell nanoparticles are 23.6 emu g−1, 550 Oe and 0.23, respectively. The obtained Fe3C–C core–shell nanostructures exhibited high coercivity and remanence, and their C shells could prevent the magnetic cores from the direct contact with the outer substances. They were expected to be a good candidate for applications in magnetic data storage, such as magnetic toners in xerography and magnetic inks or ferrofluids, and in bio-engineering applications, such as drug delivery and magnetic resonance imaging.
[11]
[12] [13]
[14]
[15]
Acknowledgments
[16]
This study is supported by the National Natural Science Foundation of China (Grant Nos. 50804057 and 51074188), and the Australian Research Council (Grant No. DP130101828).
[17]
[18]
References [19] [1] Zhao X Q, Liang Y, Hu Z Q and Liu B X 1996 Oxidation characteristics and magnetic properties of iron carbide and iron ultrafine particles J. Appl. Phys. 80 5857–60 [2] Kraupner A, Antonietti M, Palkovits R, Schlicht K and Giordano C 2010 Mesoporous Fe3C sponges as magnetic supports and as heterogeneous catalyst J. Mater. Chem. 20 6019–22 [3] Akiyama T, Miyazaki A, Nakanishi H, Hisa M and Tsutsumi A 2004 Thermal and gas analyses of the reaction between iron carbide and steam with hydrogen generation at 573 K Int. J. Hydrog. Energy 29 721–4 [4] Giordano C, Kraupner A, Fleischer I, Henrich C, Klingelhofer G and Antonietti M 2011 Non-conventional Fe3C-based nanostructures J. Mater. Chem. 21 16963–7 [5] Wen Z H, Ci S Q, Zhang F, Feng X L, Cui S M, Mao S, Luo S L, He Z and Chen J H 2012 Nitrogen-enriched core– shell structured Fe/Fe3C–C nanorods as advanced electrocatalysts for oxygen reduction reaction Adv. Mater. 24 1399–404 [6] Su L W, Zhou Z and Shen P W 2013 Core–shell Fe@Fe3C/C nanocomposites as anode materials for Li ion batteries Electrochim. Acta 87 180–5 [7] Zhao X, Xia D, Yue J and Liu S 2014 In situ generated nanoFe3C embedded into nitrogen-doped carbon for high
[20]
[21]
[22]
[23] [24]
[25]
6
performance anode in lithium ion battery Electrochim. Acta 116 292–9 Reddy L H, Arias J L, Nicolas J and Couvreur P 2012 Magnetic nanoparticles: design and characterization, toxicity and biocompatibility, pharmaceutical and biomedical applications Chem. Rev. 112 5818–78 Song H and Chen X 2003 Large-scale synthesis of carbonencapsulated iron carbide nanoparticles by co-carbonization of durene with ferrocene Chem. Phys. Lett. 374 400–4 Fan N, Ma X, Ju Z and Li J 2008 Formation, characterization and magnetic properties of carbon-encapsulated iron carbide nanoparticles Mater. Res. Bull. 43 1549–54 Luo N, Li X J, Wang X H, Yan H H, Zhang C J and Wang H T 2010 Synthesis and characterization of carbon-encapsulated iron/iron carbide nanoparticles by a detonation method Carbon 48 3858–63 Wang X, Zhang P, Gao J, Chen X and Yang H 2015 Facile synthesis and magnetic properties of Fe3C/C nanoparticles via a sol–gel process Dyes Pigm. 112 305–10 Kim S, Shibata E, Sergiienko R and Nakamura T 2008 Purification and separation of carbon nanocapsules as a magnetic carrier for drug delivery systems Carbon 46 1523–9 Zhou J, Song H, Chen X, Zhi L, Yang S, Huo J and Yang W 2009 Carbon-encapsulated metal oxide hollow nanoparticles and metal oxide hollow nanoparticles: a general synthesis strategy and its application to lithium-ion batteries Chem. Mater. 21 2935–40 Zhang H, Song P, Han D and Wang Q 2014 Synthesis and formaldehyde sensing performance of LaFeO3 hollow nanospheres Physica E: Low-Dimensional Syst. Nanostructures 63 21–6 Bo T, Hu G X and Xia M 2012 An easy method for the preparation of core–shell structural Fe3C@graphite-like carbon hollow spheres Mater. Lett. 68 104–7 Xia Y N, Yang P D, Sun Y G, Wu Y Y, Mayers B, Gates B, Yin Y D, Kim F and Yan Y Q 2003 One-dimensional nanostructures: synthesis, characterization, and applications Adv. Mater. 15 353–89 Sajitha E P, Prasad V, Subramanyam S V, Eto S, Takai K and Enoki T 2004 Synthesis and characteristics of iron nanoparticles in a carbon matrix along with the catalytic graphitization of amorphous carbon Carbon 42 2815–20 Qiu J S, Li Q X, Wang Z Y, Sun Y F and Zhang H Z 2006 CVD synthesis of coal-gas-derived carbon nanotubes and nanocapsules containing magnetic iron carbide and oxide Carbon 44 2565–8 Wang S, He Y, Liu X, Huang H, Zou J, Song M, Huang B and Liu C T 2011 Novel C/Cu sheath/core nanostructures synthesized via low-temperature MOCVD Nanotechnology 22 405704 Wang S, Huang X, He Y, Huang H, Wu Y, Hou L, Liu X, Yang T, Zou J and Huang B 2012 Synthesis, growth mechanism and thermal stability of copper nanoparticles encapsulated by multi-layer graphene Carbon 50 2119–25 Ma L, Yu B, Wang S, Su G, Huang H, Chen H, He Y and Zou J 2014 Controlled synthesis and optical properties of Cu/C core/shell nanoparticles J. Nanopart. Res. 16 2545 Yu B, Wang S, Zhang Q, He Y, Huang H and Zou J 2014 Ni3C-assisted growth of carbon nanofibres 300 °C by thermal CVD Nanotechnology 25 325602 Huang X L, Hou L Z, Yu B W, Chen G L, Wang S L, Ma L, Liu X L and He Y H 2013 Preparation, formation mechanism and optical properties of C/Cu shell/core nanostructures Acta Phys. Sin. 62 108102 Patterson A L 1939 The Scherrer formula for x-ray particle size determination Phys. Rev. 56 978–82
J Liu et al
Nanotechnology 26 (2015) 085601
[31] Hoene J V, Charles R G and Hickam W M 1958 Thermal decomposition of metal acetylacetonates: mass spectrometer studies J. Phys. Chem. 62 1098–101 [32] Nemec M, Wacker L and Gaggeler H 2010 Optimization of the graphitization process at AGE-1 Radiocarbon 52 1380–93 [33] Kim J H, Kim J, Park J H, Kim C K, Yoon C S and Shon Y 2007 Synthesis of carbon-encapsulated iron carbide nanoparticles on a polyimide thin film Nanotechnology 18 115609 [34] Geng J, Jefferson D A and Johnson B F G 2004 Direct conversion of iron stearate into magnetic Fe and Fe3C nanocrystals encapsulated in polyhedral graphite cages Chem. Commun. 2442–3 [35] Hofer L J E and Cohn E M 1959 Saturation magnetizations of iron carbides1 J. Am. Chem. Soc. 81 1576–82 [36] Xuan S, Wang Y-X J, Yu J C and Cham-Fai L K 2009 Tuning the grain size and particle size of superparamagnetic Fe3O4 microparticles Chem. Mater. 21 5079–87
[26] Ferrari A C and Robertson J 2001 Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon Phys. Rev. B 64 075414 [27] Pimenta M A, Dresselhaus G, Dresselhaus M S, Cancado L G, Jorio A and Saito R 2007 Studying disorder in graphitebased systems by Raman spectroscopy Phys. Chem. Chem. Phys. 9 1276–90 [28] Yoshida H, Takeda S, Uchiyama T, Kohno H and Homma Y 2008 Atomic-scale in-situ observation of carbon nanotube growth from solid state iron carbide nanoparticles Nano Lett. 8 2082–6 [29] Ni L, Kuroda K, Zhou L P, Ohta K, Matsuishi K and Nakamura J 2009 Decomposition of metal carbides as an elementary step of carbon nanotube synthesis Carbon 47 3054–62 [30] He Z, Maurice J-L, Gohier A, Lee C S, Pribat D and Cojocaru C S 2011 Iron catalysts for the growth of carbon nanofibers: Fe, Fe3C or both? Chem. Mater. 23 5379–87
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