Shan Fanab,
Wen Wang*a,
Hua Ke*a,
Jian-Cun Rao*a and
Yu Zhoua
aInstitute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, P. R. China. E-mail: wangwen@hit.edu.cn
bSchool of Material Science and Engineering, Qiqihar University, Qiqihar 161006, P. R. China
First published on 26th September 2016
A heterostructured Co3O4/CoFe2O4 nanocomposite material is synthetised via the solid–solid reaction of an iron precursor with urchin-like Co3O4 which was prepared via hydrothermal treatment. The coupling of Co3O4 to the hard ferrimagnet CoFe2O4 leads to reinforcement for the magnetic interaction of the interfaces and the mismatched surface spin in the Co3O4/CoFe2O4 nanocomposite is greatly reduced. The magnetic performance of the exchange-coupled anti-ferromagnetic/ferrimagnetic (AFM/FM) system is investigated with superconducting quantum interference device magnetometry. In an applied magnetic field, the low-temperature magnetization loops of the nanocomposite heterostructure exchange bias under cooling. The antiferromagnetic ordering temperature of Co3O4 is increased owing to the adjacent hard magnetic CoFe2O4 phase. The Co3O4/CoFe2O4 nanocomposite behaves as an exchange coupled system with cooperative magnetic switching.
As one of the most important and widely utilized magnetic materials, CoFe2O4 consists of both magnetically hard materials with high magnetocrystalline anisotropy (Keff = 1.8 × 107 erg cm−3), high coercivity (Hc = 5.4 kOe), and moderate saturation magnetization (80 emu g−1).17 The magnetic moment of bulk Co3O4 is derived from the tetrahedrally coordinated Co2+ ions, meanwhile the Co3+ ions are diamagnetic. For bulk materials, a switching process from paramagnetic to antiferromagnetic occurs at around 40 K.18 For nano-materials their magnetic behavior is different from the bulk. Unusually, nanostructured Co3O4 shows singular magnetic properties due to its uncompensated surface spins, which are exchange-coupled to the antiferromagnetic core.19–21 The close contact two different magnetic phases can generate curious and exceptional magnetic properties. It is worth noting that the magnetism of magnetic materials is not only dependent on the intrinsic magnetic material themselves, but also is affected by the interaction between magnetic nanoparticles. The intrinsic magnetic performance can be revealed from uniform nanoparticles. While for nanocomposites, magnetic interactions are difficult to interactions from the magnetism for the uncontrollable interparticles distance.22 A direct comparative study of the two nanoparticle systems provides a better basic understanding of the correlations between magnetic nanoparticles and the magnetic couplings of their lattice sites. Magnetic nanomaterial composites with controllable microstructures are a great challenge for the synthetic technology available. Jiao et al. provided an alternative hard template strategy for the synthesis of shape and size controlled cubic ordered Fe3O4 by the reduction of α-Fe2O3. After that, they transform the ordered Fe3O4 to γ-Fe2O3 by oxidation reaction with the same micromechanism.23 Schüth et al. reported the fabrication of nanocrystal mixed CoFe2O4 via a one-step-doping of nano Co3O4 as a hard template, and measured its magnetic exchange bias effect at a low temperature.24 Magnetic microstructures with a ferrimagnetic core and antiferromagnetic shell (or contrary) display the exchange bias phenomenon and show hysteresis loops along the magnetic field axis displacement.
Herein, we report the nanofabrication route for a magnetic 3D heterostructure composed of Co3O4 and CoFe2O4. The microstructural characteristics and magnetic properties of the nanocomposite are investigated. Magnetism is studied according to magnetic hysteresis loops and zero field cooling–field cooling (ZFC–FC) curves. The results exhibit the enhancement of effective anisotropy relative to single phase Co3O4 nanoparticles. The influence of the interface magnetic interactions on the magnetism of the CoFe2O4/Co3O4 nano-composites is discussed.
First, urchin-like Co3O4 nanocrystals were synthesized. 1.0 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 4.0 mmol of urea were dispersed in 40 mL distilled water and stirred for 0.5 h at room temperature. Afterwards, the mixture was transferred to a 50 mL Teflon-lined autoclave. The autoclave was heated at 90 °C for 6 h. After the autoclave cooled to room temperature naturally, black precipitates (the precursors) were collected, washed with deionized water and absolute ethanol, followed by calcination at 350 °C for 2 h. In this step, Co3O4/CoFe2O4 was compounded via the solid–solid reaction of the iron(III) precursor with the urchin-like Co3O4. 0.4 mL of 0.4 M Fe(NO3)3·9H2O (in ethanol) was added to 0.2 g of urchin-like Co3O4 ((Fe)/(Fe + Co) = 0.16), with stirring for 1 h at room temperature. After that, the solvent was evaporated at 60 °C for 12 h, and subsequently the product was heated to 700 °C at the rate of 1 °C min−1 and kept at this temperature for 2 h. The concentration of the iron solution (3 M) was increased to obtain an Fe-enriched sample.
The morphology and structure of the urchin-like Co3O4/CoFe2O4 were characterized using a Tecnai G2 F30 TEM, and high resolution TEM and selected area electron diffraction (SAED). X-ray diffraction (XRD) patterns for all the samples were obtained on a Stoe STADI P diffractometer with Cu Kα radiation. Field emission scanning electron microscopy (FE-SEM) was performed with a HELIOS NanoLab 600i microscope. Nitrogen absorption–desorption isotherms were measured on an ASAP-2020. Magnetic properties were determined on a superconducting quantum interference device (SQUID) magneto-meter (MPMS-7 Quantum Design) in the temperature range of 10–320 K with a 100 kOe maximum field.
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Fig. 1 XRD patterns of urchin-like Co3O4 and Co3O4/CoFe2O4 nano-composites. (a) Co3O4, (b) Co3O4/CoFe2O4 ((Fe)/(Fe + Co) = 0.16), and (c) Co3O4/CoFe2O4 ((Fe)/(Fe + Co) = 0.60). |
The morphology of Co3O4 and Co3O4/CoFe2O4 were further investigated by FESEM and TEM. Fig. 2 displays FESEM images of the as-fabricated samples at different reaction steps, which represent the panoramic and locally enlarged images of the Co3O4 (Fig. 2a and b), Co3O4/CoFe2O4 (Fe)/(Fe + Co) = 0.16 (Fig. 2c and d), and Co3O4/CoFe2O4 (Fe)/(Fe + Co) = 0.60 (Fig. 2e and f) urchin-like samples. It is found that the Co3O4 sample is composed of 3D urchin-like structures, which are constructed of many rough nanorods with lengths ranging from 2 to 3 μm and widths ranging from 50 to 80 nm. It can be clearly observed that some nanorods align together to form the same central spheroidal morphology. From Fig. 2c and d, it is noticed that after iron(III)nitrate impregnated is into Co3O4 ((Fe)/(Fe + Co) = 0.16) and calcined at 700 °C the morphology of the corresponding samples remains unchanged. Although, the diameter of the urchins and the nanorods grow thicker (from 80 nm to 100 nm), due to the fusion of the nanocrystals at a higher annealing temperature and the decomposition of CoFe2O4 at this temperature.25,27 However, with an increase in the iron content ((Fe)/(Fe + Co) = 0.60), the morphology of Co3O4/CoFe2O4 becomes agglomerated clusters of the disorderly nanorods (Fig. 2e and f), and the morphology of the urchin-like structures change gradually.
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Fig. 2 HR-SEM for the panoramic and locally enlarged images of the (a and b) Co3O4, (c and d) Co3O4/CoFe2O4 ((Fe)/(Fe + Co) = 0.16) and (e and f) Co3O4/CoFe2O4 ((Fe)/(Fe + Co) = 0.60). |
Fig. 3 displays the FE-TEM, HR-TEM images and the corresponding SAED patterns of Co3O4, Co3O4/CoFe2O4 nanocomposite. Fig. 3a shows the FE-TEM image of the designed single Co3O4 urchin-like nanoparticles. The individual Co3O4 nanorod is composed of interconnected nanoparticles with a diameter of 10–20 nm, which confirms the XRD observations and shows a porous structure ranging from 2–6 nm in length (inset Fig. 3a). The measured lattice fringe with the interplane spacing of 0.24 nm corresponds well to the (311) plane of Co3O4 (Fig. 3b). Fig. 3c shows a low magnification TEM image of THE urchin-like Co3O4/CoFe2O4 ((Fe)/(Fe + Co) = 0.16) nanocomposite after impregnation of the iron(III) precursor and calcination. Compared with the pure Co3O4, the Co3O4/CoFe2O4 nanorod is composed of interconnected nanoparticles with a diameter of 20–30 nm and shows a porous structure ranging from 3–11 nm in length (inset Fig. 3c). The high-magnification TEM (HRTEM) images (Fig. 3d) of the Co3O4@CoFe2O4 nanorods unambiguously exhibit that the Co3O4 nanoparticles (5–10 nm) are highly crystalline and encapsulated by crystalline CoFe2O4 shells, although there are some plane stacking faults. The interlayer distance in the HRTEM image is calculated to be 0.30 nm for the shell lattice fringes and 0.24 nm for the core lattice fringes, which agree well with that of the (220) lattice plane of CoFe2O4 and the (311) lattice plane of Co3O4, and thus confirm the XRD analysis. Fig. 3e shows a TEM image of the Co3O4/CoFe2O4 ((Fe)/(Fe + Co) = 0.60) sample. The increase in the concentration of the iron solution increased the diameter of the Co3O4/CoFe2O4 nanorod, and the diameter of the Co3O4/CoFe2O4 nanorods is not uniformly distributed and there are no pores on the nanorods (inset Fig. 3e). The ranges of particle sizes are the same as the FE-SEM observation. In order to obtain structural information, HR-TEM imaging was also done on the edges of the nanorods (Fig. 3f), and the obtained neighboring interplanar distances are consistent with the (311) planes for the CoFe2O4 phase. SAED patterns of the urchin-like Co3O4 and urchin-like Co3O4/CoFe2O4 nanocomposite samples are shown in Fig. 3 (inset in panels (b), (d) and (f)). The ring patterns are consistent with the standard diffraction of the Fm3m Co3O4 and Fd3m CoFe2O4 phases. The planes match with that of the XRD patterns very well. The elemental mapping images (EDS) in Fig. 4 reveal the distribution of a randomly selected single urchin-like of the Co3O4/CoFe2O4 nanocomposite ((Fe)/(Fe + Co) = 0.16). The distribution of the Fe element covers the location of the Co elements, which displays the uniform distribution of CoFe2O4 on the surface of the Co3O4 nanorods.
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Fig. 4 Energy-dispersive X-ray spectrometry (EDS) mapping analysis for the Co3O4/CoFe2O4 nanoparticles ((Fe)/(Fe + Co) = 0.16). (a) TEM, (b) Fe and (c) Co. |
The N2 adsorption–desorption isotherms of Co3O4 and the Co3O4/CoFe2O4 nanocomposite are exhibited in Fig. 5. The isotherms of the Co3O4 and Co3O4/CoFe2O4 powders can be classified as type IV, which show the mesoporous characteristics of the sample.26 The H-3 hysteresis loop proves that asymmetric, interconnected, slit-like mesoporosity exists in the samples. The BET specific surface area of the urchin-like Co3O4/CoFe2O4 is measured to be about ∼112.3 m2 g−1, which is higher than that of the urchin-like Co3O4 (∼55.8 m2 g−1). The higher surface area of the urchin-like Co3O4/CoFe2O4 originates from the presence of the CoFe2O4 layer around the Co3O4 nanoparticles, except for the intrinsic pores of the Co3O4 phase. With an increase in CoFe2O4 particles, the particle growth increases the partial closure of the relatively large pores and the generation of many small pores. Thus, the numerous micropores in the products result in a higher BET surface area rather than the mere presence of mesopores. The pore-size-distribution of Co3O4 (Fig. 5b) displays the fact that most of the pores are ∼4 nm. In comparison, the pore size distribution of the Co3O4/CoFe2O4 sample (3 nm and 8 nm) does not changed significantly. This may be attributed to the low doping of iron(III) and similar volume density of Co3O4 (6.1 g cm−3) and CoFe2O4 (5.3 g cm−3).17,25
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Fig. 5 (a) N2 adsorption–desorption isotherms of Co3O4 and the Co3O4/CoFe2O4 nanocomposite, and (b) pore size. |
It is found that the hydrothermal time plays a key role in the formation of the perfect cobalt hydroxy carbonate (precursor) urchin-like nanostructures. Fig. 6 shows the FE-SEM nano-structures of the prepared samples obtained by the hydrothermal method at 90 °C for (a) 2 h, (b) 4 h, (c) 6 h and (d) 8 h. From Fig. 6a, only 1D nanorods are obtained for 2 h. The diameter of the nanorods is ∼50 nm. When increased to 4 h, the sample is a mixture of nanorods and deficient urchin-like shapes with a loose center (Fig. 6b). The above results may imply that the deficient urchin-like precursor could be self-assembled with these nanorods. With an increase in the reaction time (6 h and 8 h), the flawless urchin-like morphology can be obtained. Subsequently, the nanorods with the urchin-like structure become thick and uneven (Fig. 6c and d). In comparison with Fig. 2a, it is found that the morphology of the samples remains unchanged after calcination at 350 °C.
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Fig. 6 HR-SEM images for the starting precursor at different hydrothermal times. (a) 2 h, (b) 4 h, (c) 6 h and (d) 8 h. |
An illustration of the synthetic process of the Co3O4/CoFe2O4 heterocomposite is depicted in Scheme 1. In this study, the starting urchin-like precursor was first prepared via the traditional hydrothermal treatment. Under the mild hydrothermal reaction conditions at 90 °C, urea hydrolyzes gradually and releases NH3 and CO2. Soon afterwards, they become OH− and CO32− ions. Compared with the release of NH3, the solubility of CO2 is relatively low in aqueous solution, thus the solution is weak alkaline.27,28 The alkaline medium is believed to create heterogeneous nucleation of the Co species. The Co2− of the cobalt salt cooperates with the CO32− and OH− anions to form the nuclei of cobalt hydroxyl carbonate. The Co2+ exposed facets prior adsorb OH− ions. Consequently, dipolar interactions dominated by 3D self-assembly is achieved via hydrogen bonding and electrostatic force.29 In this system the reactions involved are listed as follows:
CO(NH2)2 + xH2O → 2NH3 + CO2 + (x − 1)H2O | (1) |
NH3 + H2O → NH4+ + OH− | (2) |
CO2 + H2O → CO32− + 2H+ | (3) |
Co2+ + OH− + 0.5CO32− + 0.11H2O → Co(CO3)0.5(OH)·0.11H2O | (4) |
Initially NH3 and CO2 are present in the form of bubbles in the solution, which act as soft templates.28 Then, the cobalt hydroxyl carbonate nanoparticles surround the bubbles to form a loosely packed hierarchical structure. By Ostwald ripening, the precursor particles grow along a certain crystallographic axis forming 1D nanorods. Then, by prolonging the hydrothermal time (6–8 h), the imperfect nanorods assemble to form an urchin-like structure. Subsequently, the samples are calcined in laboratory air at 350 °C for 2 h to generate the intermediate product, which consists of urchin-like Co3O4.
6Co(CO3)0.5(OH)·0.11H2O → 2Co3O4 + 3CO2 + 1.66H2O | (5) |
Finally, Fe(NO3)3·9H2O is directly impregnated into the urchin-like Co3O4 to generate the Co3O4/CoFe2O4 composite. Under the conditions of the calcination process, the iron source and cobaltosic oxide compound CoFe2O4 by a solid–solid reaction. The ferrimagnetic CoFe2O4 probably wraps around the antiferromagnetic Co3O4 matrix. Early studies on the pseudobinary Co3O4/CoFe2O4 system show that there is a miscibility gap in the spinel-structure region.30,31 In the report by Takahashi et al., the single or two phase regions are attributed to the calcination temperature and the atomic ratio of iron and cobalt.32 Thus, at a temperature greater than 400 °C but less than 900 °C, the single Co3O4 spinel phase is observed when the (Fe)/(Co + Fe) ratio is below 0.1. As the iron(III) concentration increases, the inverse spinel CoFe2O4 phase is formed in addition to Co3O4. In this paper, we chose the atomic ratio of 0.16 (Fe/(Co + Fe)) and calcination temperature of 700 °C. According to the conditions given by the phase diagram, the product components can be preliminarily controlled. It is suggested that a small amount of CoFe2O4 is gained along the Co3O4 matrix with the calcination process and iron species impregnation into the cobalt oxide.
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Fig. 7 Magnetic hysteresis loops of the urchin-like (a) Co3O4 and (b) CoFe2O4/Co3O4 nanocomposites. The insets are enlarged regions at low field. |
The magnetic susceptibility as a function of temperature curves of both Co3O4 and CoFe2O4/Co3O4 for ZFC–FC are displayed in Fig. 8a and b, respectively. The samples for the ZFC curve were registered by cooling in the zero field to 10 K, then increasing the temperature to 320 K in a field of 100 Oe. Due to the antiferromagnetic transition, a distinctive characteristic broad peak in both ZFC curves appears around 37 K. This TN is slightly less than Co3O4 in the bulk state (TN-bulk = 40 K).29–31 The principle of the TN reduction has been argued to be due to a variety of factors, such as dimension change,21,26 and surface and interface exchange coupling effects.30–32 For ferrite nanoparticles, their lower TN values are attributed to the increase of the super exchange interaction.32,33 The M of Co3O4 in the FC curve increases with the decrease temperature and presents a corner near T ∼ 33 K in Fig. 7a. This inflection is due to surface and interface exchange coupling effects between adjacent nanoparticles due to the small interparticle distance.14 The FC curve for the urchin-like CoFe2O4/Co3O4 nanocomposite is very different with that of the urchin-like Co3O4 mesoporous nanoparticles (in Fig. 8b), which show that the magnetism of the CoFe2O4/Co3O4 composite changes a lot at low temperature. The CoFe2O4 nanoparticles in the pores of the urchin-like Co3O4 reduce the distance of surface spin coupling, and the more mismatched spin of the surface of the Co3O4 nanoparticles can recouple the CoFe2O4 nanoparticles. Thus, the mismatched surface spin of the CoFe2O4/Co3O4 nanocomposite decreases, and the influence of the surface effect is attenuated for the magnetism. Consequently, the kink in the FC curve of the urchin-like Co3O4 is present in the CoFe2O4/Co3O4 nanocomposite. With a decrease in temperature, the magnetization (M) of the FC curve decreases at a very low temperature. At the same temperature, the M of CoFe2O4/Co3O4 nanocomposite is larger than that of Co3O4 in the ZFC–FC curves.
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Fig. 8 Temperature dependence of the zero field cooling (ZFC) and field cooling (FC) magnetization curves. (a) Co3O4 and (b) CoFe2O4/Co3O4. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18846j |
This journal is © The Royal Society of Chemistry 2016 |