Stable Core Shell Co3Fe7–CoFe2O4 Nanoparticles Synthesized via

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Stable Core Shell Co3Fe7−CoFe2O4 Nanoparticles Synthesized via Flame Spray Pyrolysis Approach Yunfeng Li,† Yanjie Hu,*,† Junchao Huo,† Hao Jiang,† Chunzhong Li,*,† and Guangjian Huang‡ †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China ‡ Department of Surgery, Huashan Hospital, Fudan University, Shanghai 200040, China S Supporting Information *

ABSTRACT: Air stable Co3Fe7−CoFe2O4 nanoparticles have been synthesized via one-step flame spray pyrolysis of a mixture of Fe/Co precursor solution under stronger reducing atmosphere. The as-synthesized nanoparticles with diameters of 20−80 nm showed a typical core shell structure and high stability for being one month in air, whose metallic Co3Fe7 cores were protected against oxidation by a surface shell of about 2−4 nm cobalt iron oxide (CoFe2O4). The ratio of metallic Fe/Co alloy nanoparticles was 7:3. The alloy nanoparticles exhibited enhanced saturation magnetization (126.1 emu/g), compared with flame sprayed iron nanoparticles with the same conditions. The formation process of metallic alloy nanoparticles with core−shell structure was investigated, which included three stages: flame combustion, reducing, and surface oxidation during the flame process. It is reckoned that such a continuous production approach is an effective way to produce the stable Co3Fe7 alloy nanoparticles with high saturation magnetization. synthesized by flame combustion and flame spray pyrolysis (FSP) of versatile precursor solutions consisting of metallic compounds and solvents, such as TiO2@SiO2 core/shell nanostructures, TiO2 ball in shell nanostructures, SnO2 nanorods and SiO2 nanowires, SnO2 nanowires,24−29 and even more complicated compositions including TCP, (Ce0.7Zr0.3O2)x(Al2O3)1−x, LiMn2O4,30−32 and so forth. Recently, the reducing flame technique, based on varying fuel to oxygen ratio, has been developed for the synthesis of nonoxidation compounds. For example, Strobel and Pratsinis33 reported the synthesis of maghemite, magnetite, and wustite nanoparticles by varying the fuel to air ratio and the valence state of the applied Fe precursor in the flame cooled rapidly via water spray. Benjamin et al.34 obtained magnetite nanoparticles using a new continuous, gas-phase, nonpremixed inverse flame method with hydrocarbon fuels and observed a trace of iron nanoparticles in a stronger reducing atmosphere. Nonoxidation compound such as metallic Fe, Bi, Co nanopowders, NiMo nanoalloy, and ZnS nanocompounds,6,35−38 can also be obtained by placing conventional FSP equipment in a glovebox fed with inert nitrogen and strictly controlling the supply of oxidation gas. However, reducing flame technology is new and still not well known for the development of flame synthesis, especially for the formation process of the corresponding nanomaterials. In this paper, we report a rapid, facile, and continuous synthesis of Co3Fe7 alloy nanoparticles via FSP of the corresponding bimetallic organo-precursors under reducing flame atmosphere. The as-synthesized nanoparticles showed a

1. INTRODUCTION Unique magnetic response properties of magnetic nanoparticles make them possess various promising applications, including magnetic fluids, recyclable catalysts, magnetic resonance imaging, data storage, and bionanotechnology.1−3 Iron oxide nanoparticles4,5 with suitable saturation magnetization and intriguing biocompatibility have been intensively investigated in the past decade. However, a relatively low saturation magnetization limited their further applications. Compared to metallic magnetic nanoparticles and nanoalloy with enhanced magnetic properties, such as Fe, Co, FePt, and CoPt,6−8 the soft magnetic FeCo alloy nanoparticles are of great interest for extensive applications because of their unique magnetic properties,9,10 such as large permeability, high saturation magnetization, and Curie temperature. As we know, alloying iron with 30 atomic % cobalt11,12 results in a higher room temperature saturation magnetization (δs = 245 emu/g), which make FeCo alloy nanoparticles show a huge potential in practical applications. So far, various techniques have been developed to realize the preparation of FeCo alloy nanoparticles with various morphologies and different molar ratios of Co and Fe atoms, including wet chemistry routes based on coreduction13−15 of solution Fe3+ and Co2+ in the presence of a reduction agent and gas phase methods such as chemical vapor deposition and thermolysis.16−19 However, these methods are multistep treatment processes and time-consuming, which limits their application at an industrial scale. It remains a great challenge to develop a facile and continuous strategy for producing FeCo alloy nanoparticles. Flame gas phase synthesis20−22 has been demonstrated to be a industrial scale way for producing high-quality and high-purity nanopowders, like TiO2, SiO2, and Al2O3.23 On the other hand, various nanomaterials with rich morphologies have been © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11157

April 26, 2012 August 1, 2012 August 9, 2012 August 9, 2012 dx.doi.org/10.1021/ie3010644 | Ind. Eng. Chem. Res. 2012, 51, 11157−11162

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particles. High-resolution transmission electron microscopy (HRTEM: JEM-2010, operated at 200 kv) was used to characterize the morphology microstructure. For the transmission electron microscopy (TEM) sample preparation, the particles were carefully scraped off the filter and suspended in ethanol. The suspension was dropped on a copper TEM grid with carbon film support. The particles were kept at the grid after evaporation of ethanol. Thermogravimetric analysis (TGA, SDT Q600) was recorded in the range of 25−800 °C at 10 °C/ min in air with a flow rate of 30 mL/min. Nitrogen adsorption/ desorption (ASAP 2010N) was determined by the Brunauer− Emmett−Teller (BET) method. Magnetization properties of samples were recorded by a vibrating sample magnetometer (VSM, Lakeshore 7407) under 20000 Gs.

typical core shell structure, in which the metallic Co3Fe7 alloy core is encapsulated by a cobalt iron oxide (CoFe2O4) shell with a thickness of 2−4 nm, high air stability, and enhanced magnetic properties. Three stages, that is, flame combustion, reducing, and surface oxidation, have been proposed to explain the formation process of metallic core shell structured nanoparticles in the flame.

2. EXPERIMENTAL SECTION 2.1. FSP Synthesis of Alloy Nanoparticles. The Co3Fe7 alloy nanopowders with core shell nanostructure were synthesized by a modified FSP technique. Briefly, liquid precursor was obtained by dissolving ferrocene (C10H10Fe, Sinopharm chemical reagent Co. Ltd., 98%) and cobalt(II) acetylacetonate (C10H14CoO4, Sinopharm chemical reagent Co. Ltd., 98%) in a mixing solvent composed of xylene (C8H10, Shanghai Ling Feng chemical reagent Co. Ltd., AR) and tetrahydrofuran (C4H8O, Shanghai Ling Feng chemical reagent Co., Ltd., AR) with an equal ratio. The total metal concentration with the certain Fe/Co molar ratio (7:3) was fixed at 0.4 M. The precursor solution was fed into the flame at a flow rate of 4 mL/min, dispersed with 2.5 L/min oxygen into fine droplets by a gas-assist nozzle.28,33 The spray was ignited by a supported inverse diffusion H2/Air flame (H2: 0.76 m3/h, Air 0.8 m3/h), in which H2 with enhanced flow rate was introduced through 8 symmetrical pores with a diameter about 1 mm (Figure 1c). The spray flame was cooled by 1.5 m3/h

3. RESULTS AND DISCUSSION Metallic Co3Fe7 alloy nanoparticles were prepared by one-step FSP of the corresponding precursor solution in an enclosed reactor. Figure 2 shows the XRD pattern of flame sprayed

Figure 2. XRD pattern of flame sprayed Co3Fe7 alloy nanoparticles. Figure 1. (a) Schematic setup for the FSP synthesis of the core shell nanoparticles; (b) The 3D model of the cooling ring;24 (c) The 3D model of the FSP nozzle.28,33.

nanopowders, indicating that the present samples with good crystalline nature are mainly composed of metallic Co3Fe7 (JCPDS NO. 48-1816) along with CFe15.1 (JCPDS NO. 520512) and CoFe2O4 (JCPDS NO. 03-0864). The formation of a reduced metallic compound can be attributed to the strong reducing atmosphere composed of larger amounts of reduced gas (i.e., H2, CO) owing to a high fuel to oxygen ratio.36 However, the chemical combination of the carbon species from incomplete combustion and primary metallic monomers in flame results in the formation of trace amounts of a CFe15.1 component. Furthermore, several peaks of cobalt iron oxide (CoFe2O4) can also be observed, which may be attributed to the surface oxidation of metallic Co3Fe7 nanocrystals contacting with air after being expelled from the reaction tube. With tailoring precursor concentration from 0.2 to 0.4 M, the obtained Co3Fe7 alloy nanoaparticles with the identical XRD patterns show the increasing crystallite sizes from 14.8 to 21.4 nm when taking the (110) plane into account (Supporting Information, Figure S1). This could be attributed to the increasing metal precursor in per unit volume and the high flame temperature when employing organic metal precursor with the high concentration.

nitrogen through a stainless steel metal torus pipe ring with 16 radial equispaced openings24 with 20° from up-centerline, 1 mm i.d. each (Figure 1b) positioned at 40 cm height away from nozzle. The FSP reactor was enclosed by stainless steel pipe (D = 5 cm, L = 80 cm). The products were collected on the filterbag26,27 with the aid of a vacuum pump. All gas flow rates were controlled by calibrated rotameters. The flame temperature at different distance from the burner was measured by a platinum−rhodium thermocouple to obtain the temperature distribution along the axial line of the flame. Figure 1 shows the experimental setup of the FSP reactor schematically. 2.2. Characterizations. The as-synthesized nanopowders were characterized by X-ray diffraction (XRD), performed on a Rigaku D/max 2550VB/PC diffractometer at room temperature. The patterns were recorded over the angular range 10− 80° (2θ), using Cu Kα radiation (λ = 0.154056 nm) with working voltage and current of 40 kV and 100 mA, respectively. Energy dispersive X-ray spectroscopy (EDS) was employed to obtain chemical element information of flame made nano11158

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TEM images, as shown in Figure 3, exhibit quasi-spherical nanoparticles of the as-prepared alloy products with diameters

Figure 3. (a) Low-magnification, (b) high-magnification, (c) highresolution TEM images, (d) EDS pattern, and (e) the corresponding model of flame sprayed metallic Co3Fe7 nanoparticles. The inset in panel b is a SAED pattern of a single nanoparticle.

Figure 4. TG curve of the corresponding metallic Co3Fe7 nanoparticles.

03-0864) components due to the total oxidation of metallic alloy species (Supporting Information, Figure S3). However, the trace of hematite (JCPDS NO. 33-0664) is detected because of the oxidation of the excess iron atoms in the alloy. From the TG data the metallic and oxide layer content of the nanoparticles could be estimated assuming oxidation conversion of Co3Fe7 to CoFe2O4 and Fe2O3 according to the measured mass gain of 26.2 wt %. This resulted in calculated content of 69.1 and 28.8 wt % for metal (Co3Fe7) and oxide (CoFe2O4), respectively. The estimated CoFe2O4 shell thickness is about 4−5 nm, supported by TEM images (Figure 3c) showing an oxide layer of about 3.5 nm thickness encapsulating the metallic alloy core. At the beginning of heating, a small weight loss of about 0.2 wt % can be seen in Figure 4 mainly due to desorption of the absorbed gases and moisture on the surface of samples. From 475 to 800 °C, a small weight loss of about 2.1 wt % was observed due to the oxidation loss of carbon from incomplete combustion. The results show that flame made Co3Fe7 alloy nanoparticles possess higher air stability. The nitrogen adsorption desorption isotherms and the corresponding pore size distribution of Co3Fe7 alloy nanoparticles with core shell structure (Supporting Information, Figure S4). The type IV isotherm with a hysteresis loop indicates the presence of mesopores. The BET surface area of the sample exhibits a higher value of 28.7 m2/g. Moreover, the pore size distribution measurement indicates the samples possess a mean pore diameter of about 45 nm, mainly resulting from the aggregation between interagglomerated primary particles and interaggregated secondary particles.26 To investigate the magnetic properties of the obtained samples, a vibrating sample magnetometer (VSM) was employed. As shown in Figure 5a (red line), the magnetization hysteresis loop at room temperature (300 K) indicates that flame-made Co3Fe7 alloy nanparticles are ferromagnetic with a saturation magnetization (δs) of 126.1 emu/g, which is much larger than that of magnetic metal oxide. For comparison, the magnetic properties of the Fe nanoparticles have also been investigated, as shown in Figure 5a (black line). Figure 5b shows theoretical and experimental saturation magnetization of

in the range of about 20−80 nm. It is obvious that TEM images in Figure 3 showed the typical core shell structures, in which metallic Co3Fe7 core is encapsulated by a cobalt iron oxide (CoFe2O4) shell with a thickness of 2−4 nm. For comparison, ferrocene was used as metal precursor with a concentration of 0.4 M instead of bimetallic precursor. The spherical iron nanoparticles coated by iron oxide shell with a thickness of about 5.5 nm were obtained (Supporting Information, Figure S2). The interplanar spacing of the lattice fringes in Figure 3c is 0.20 nm, which is in good agreement with the distance of the (110) plane of Co3Fe7 alloy nanoparticles. Furthermore, the spacing of the lattice fringes of the shell in the flame made Fe nanoparticles is about 0.26 nm, indicating that the shell is composed of Fe3O4 components. The observation of oxide shell layers (e.g., CoFe2O4 and Fe3O4) is consistent with the corresponding XRD results, and similar morphologies have been obtained in flame made iron6 and cobalt36 nanoparticles. The selected area electron diffraction (SAED) pattern is given in Figure 3b, which also indicates that flame made alloy nanoparticles have high crystalline structure. Furthermore, Figure 3d shows that the main element components of the samples are Fe and Co and the corresponding atom ratio is approximately 2.57, which is almost close to the theoretical value of 2.34. However, there exist a small amount of oxygen species (12.11 at %) as a result of surface oxidation of metallic alloy nanoparticles. The results indicate that mono- and bimetal coated by the corresponding oxide shell could be prepared via one-step FSP of the corresponding metallic precursor. Thermogravimetry (TG) analysis was applied to investigate the thermal stability properties of the as-synthesized sample from room temperature to 800 °C in air. High stability of assynthesized alloy nanoparticles in air up to 160 °C is observed in the mass profile (Figure 4), which could be attributed to the protection for passivation shell of oxide components. Subsequently, the core Co3Fe7 nanoparticles slowly oxidized at the temperature of 160 °C−475 °C with a weight increment of ∼26.2 wt %, indicating the existence of the metallic cores. The corresponding XRD pattern of the sample after TG measurement indicates the formation of CoFe2O4 (JCPDS NO. 11159

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formation of FSP made nanoparticles. In comparison to metal oxide, the formation mechanism of metallic alloy nanoparticles in flame can be attributed to the stronger reducing atmosphere characterized by the high fuel to oxygen ratio Φ,33−35 which is defined as Φ=

moles of oxygen required for complete combustion moles of oxygen available (1)

A conventional spray flame, with more oxide gas (Φ ≤ 1), can lead to the corresponding oxide nanoparticles. However, a spray flame operated at fuel rich condition (Φ ≥ 2) is reducing enough for the formation of metallic nanomaterials, which is attributed to the combustion products of reducing species such as H2 and CO. Meanwhile, a rough approximation of temperature profile along the centerline of flame related to the distance above the FSP burner was measured with a platinum−rhodium thermocouple. Flame temperature and residence time have been demonstrated that not only affect production speciation but also play an important role in the particles' morphology.26,27,37 The temperature gradient along the centerline of the flame is shown in Figure 6, suggesting that the spray flame

Figure 5. (a) H-M curves of flame sprayed metallic nanoparticles at room temperature (300 K) and (b) magnetization comparison between theoretical and experimental data.

FSP-made Co3Fe7 alloy and Fe nanoparticles, respectively. It can be observed that the saturation magnetization value of the Co3Fe7 alloy nanoparticles is significantly enhanced by an alloying of Fe and Co in flame with an increase of 68.4% compared to the value of Fe nanoparticles. The detailed magnetic properties of flame made Co3Fe7 alloy and Fe nanoparticles, including saturation magnetization, residual saturation magnetization and coercivity, is summarized (Supporting Information, Table S1). For the Co3Fe7 alloy nanoparticles, the saturation magnetization would be varied from 52.4 to 126.1 emu/g by increasing the precursor concentration from 0.2 to 0.4 M. The increasing saturation magnetization can be attributed to the increasing crystallite size, which is supported by the corresponding XRD analysis (Supporting Information, Figure S1). However, the value of Co3Fe7 alloy nanoparticles is lower than that of the theoretical value (about 245 emu/g), which may be attributed to the particle crystallite size,15 relatively low content of metallic Co3Fe7 species (69.1 wt %), or an interaction of the ferromagnetic alloy core with the corresponding oxide shell and the internal crystalline disorder.19 Such an intriguing saturation magnetization makes the as-synthesized powder have better magnetic response ability with an external field and be very promising candidate of magnetic nanomaterials. The typical liquid-fed aerosol flame synthesis (LAFS) process,20,21,23 including precursor evaporation/combustion, nucleation and surface growth, coagulation and sintering, aggregation and agglomeration, is employed to explain the

Figure 6. Illustration of the formation mechanism of the core shell structured Co3Fe7 nanoparticles.

temperature can quickly reach a maximum value of 2060 K and decrease slowly to 453 K with the increasing distance above the FSP burner (2 cm−100 cm). The rough temperature profile is a significant investigation in qualitative understanding of combustion aerosol formation. Figure 6 also shows an illustration of the formation process of the flame-made Co3Fe7 nanoparticles. First, the precursor solution was injected and sprayed into the combustion reaction room. The as-formed spray fine droplets as microreactors are ignited by the supported H2/Air diffusion flame, and then undergo the process of combustion and pyrolysis, leading to the formation of reduced and oxide species. During the flame process, quick shrinkage of droplets can be controlled by the combustion and evaporation of mixture solvent. The micrometer-size droplets explode and crush in the high temperature, which triggers the nucleation of Fe, Co, O atomic species and further growth in the crushed droplets, eventually leading to the formation of 11160

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nanoparticles. In the flame combustion zone (flame length: about 10 cm, temperature: 758−2060 K), primary nanoclusters are composed of reduced and oxide species due to the coexistence of oxide reaction and pyrolysis process. At the second stage, the incomplete combustion of precursor solution (Φ = 2.58) leads to the formation of stronger reducing atmosphere, in which all the clusters are reduced into metallic nanoclusters. Furthermore, metallic FeCo alloy phase nanoparticles are formed by further nucleation and growth in reducing gas atmosphere consisting of H2 and CO, which is different from the formation of oxide21,23 nanoparticles in conventional flame under rich oxygen. The in situ flame sprayed alloy nanoparticles pass the reducing zone of about 70 cm length with temperature gradient of 1060−453 K, and then are quickly quenched by the cooling N2 through a steel ring (above the burner 40 cm). The temperature can be cooled down to 453 K, which can effectively avoid the total oxidation of alloy nanoparticles. However, alloy nanoparticles with high activity are unstable and easily oxidized in air, which results in the formation of a thin passivation layer on the surface of the alloy nanoparticles. It is noted that the passivation layer can also protect from the further oxidation of the core, giving them a high air stability up to 160 °C (see TG analysis). Therefore, we concluded that the formation of metallic alloy nanoparticles undergoes three continuous stages: flame combustion, reducing, and surface oxidation. Finally, air stable Co3Fe7 alloy nanoparticles composed of metallic core and oxide protection shell have been successfully realized via one-step FSP approach.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Z.L.), [email protected] (Y.J.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (20925621, 20906027, 20906068, 21176068, 21176083, 21106038, 51173043, 21136006, 81071994), the Special Projects for Nanotechnology of Shanghai (11 nm0500800, 11 nm0500200), the Basic Research Program of Shanghai (10JC1403300, 10JC1403600, 11JC1403000), the Special Research Fund for the Doctoral Program of Higher Education of China (20110074110010), the Shanghai Shuguang Scholars Program (10SG31), and the Fundamental Research Funds for the Central Universities.

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4. CONCLUSIONS In summary, we have demonstrated a one-step, scalable, and continuous FSP method to realize the synthesis of metallic Co3Fe7 alloy nanoparticles with a typical core shell structure by controlling the fuel to oxygen ratio at the laboratory scale. The as-prepared quasi-spherical Co3Fe7 alloy nanoparticles have diameters of 20−80 nm, encapsulated by a layer of oxide (CoFe2O4) with a thickness of 2−4 nm. More significantly, the products possess an intriguing air stability up to 160 °C and high air stability for one month at room environment. The stronger reducing atmosphere, accurately obtained by tuning the fuel to oxygen ratios, plays a key role in the formation of metallic Co3Fe7 alloy nanoparticles in the flame, which can be explained by three stages including flame combustion, reducing, and surface oxidation. On the other hand, the as-synthesized Co3Fe7 alloy nanoparticles exhibit greatly enhanced saturation magnetization compared with that of flame sprayed Fe nanoparticles owing to the alloying of Fe and Co components, which makes them particularly attractive for magnetic nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns and corresponding crystal size of flame made Co3Fe7 alloy nanoparticles with different precursor concentration. TEM images of flame sprayed Fe nanoparticles. XRD pattern of sample formed after TG measurement. Nitrogen adsorption desorption isotherms. Magnetic properties of flame made metallic nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. 11161

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