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The chemical synthesis of SmCo5 single-crystal particles with small size and high performance
Chemical Engineering Journal 304 (2016) 993–999
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
The chemical synthesis of SmCo5 single-crystal particles with small size and high performance Zhenhui Ma, Shengxue Yang, Tianli Zhang ⇑, Chengbao Jiang ⇑ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
h i g h l i g h t s SmCo5 particles have been fabricated by an assistance of graphite oxide sheets. The size and magnetic performance can be controlled by tuning surfactants. SmCo5 nanoparticles with size of 20–100 nm exhibit a high coercivity of 24.4 kOe. Graphite oxide sheets can impede the growth and sintering of SmCo5 particles.
a r t i c l e
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a b s t r a c t
Article history: Received 1 May 2016 Received in revised form 5 July 2016 Accepted 6 July 2016 Available online 7 July 2016
SmCo5 nanoparticles have promising applications in high-density magnetic storage and magnetic nanocomposites. However, conventional calciothermic reduction process yields agglomerated SmCo5 magnets for high temperature annealing, limiting the application and development of SmCo5 nanoparticles. In this work, we exploited a novel strategy to fabricate SmCo5 single-crystal nanoparticles by an assistance of graphite oxide sheets. Sm2O3-Co particles coated with graphite oxide sheets were synthesized by solvothermal route, and the following reductive annealing generated SmCo5 particles. The size and magnetic performance of SmCo5 particles can be tuned by manipulating surfactants. The high coercivity of 24.4 kOe and ideal size of 20–100 nm can be achieved in SmCo5 single-crystal nanoparticles. In this technology, graphite oxide sheets can prevent the particles from sintering effectively and sustain the generation of single crystal. Ó 2016 Elsevier B.V. All rights reserved.
Keywords: SmCo5 Nanoparticles Coercivity GO sheets
1. Introduction Magnetic particles at nanoscale have promising perspective for advanced applications such as high-density magnetic storage, high-energy product magnets, sensors and drug carriers in biomedical technology [1–11]. Among all the materials, SmCo5 (Sm2Co17) single crystal nanoparticles have the highest exceptionally-large magnetocrystalline anisotropy (Ku = 2 108 erg/cm3) as well as a high Curie temperature (Tc = 1020 K), which contributes high coercivity and high operating temperature respectively, being considered as an irreplaceable candidate in aerospace, motors, and automotive industries [12–17]. Unfortunately, rare-earth element (Sm) is very active and prone to oxidation under ambient conditions, which poses significant preparation
⇑ Corresponding author. E-mail (C. Jiang).
addresses:
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(T.
http://dx.doi.org/10.1016/j.cej.2016.07.024 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
Zhang),
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challenges for the fabrication of SmCo5 nanoparticles with high magnetic performance [18,19]. Many research efforts have been spent on the fabrication of SmCo5 nanoparticles with desired coercivity adopting physical or chemical method [19–28]. Conventional techniques like high energy ball-milling have been explored to fabricate SmCo5 particles [20,21]. However, a large particle size (usually submicrometre) is obtained since the crushed fine particles can be cold welded again during ball milling [22]. Alternatively, direct solution phase approach have been employed to yield SmCo5 nanoparticles, but it failed to obtain a high performance due to electronegativity and instability of Sm element [23–28]. Recently, researchers exploited a novel method combined solution-phase synthesis with calciothermic reduction [29–32]. Nevertheless, the synthetic particles showed agglomeration and sintering for a high temperature annealing. An improved method of using CaO-coated Sm-Co oxide nanoparticles followed by Ca-reduction at 960 °C yields 6 nm SmCo5 [33]. But these magnetic particles only exhibited low coercivity of 7.2 kOe. Recently, graphite oxide (GO) sheets have been
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also applied to prepare SmCo/Co magnets by calciothermic reduction of Sm[Co(CN)6]4H2O@GO/Co(acac)2, where GO play a role in the generation of the core-shell structure in composite magnets by obstructing interphase diffusion of Co [34]. In this technique, although a high coercivity of 20.7 kOe could be obtained, it was committed to a large size SmCo magnet with 200 nm, rather than nanoparticles. Therefore, the synthesis of desired SmCo5 particles possessing nanometer size and high coercivity simultaneously is still an arduous task due to the obstacles in controlling synthesis of single crystal nanoparticles. Herein, we have fabricated SmCo5 single crystal nanoparticles with size of 20–100 nm and high coercivity of 24.4 kOe by a novel strategy. We first synthesized SmCo-O@GO particles by solvothermal route, and the following reductive annealing generates SmCo5 particles. Although calciothermic reduction process has been adopted in previous literatures [29–33], the precursors in this paper are entirely different from these reports. The Sm2O3-Co coated with GO sheets can be tuned by surfactants; therefore, the particle size and magnetic performance of SmCo5 can also be controlled, which is unavailable in previous studies. Additionally, the employment of GO sheets can prevent particles from sintering effectively and promote formation of single crystal. Finally, the coercivity of small-sized SmCo5 is up to 24.4 kOe, which is higher than SmCo5 particles in previous reports. The high coercivity may be caused by a sound single crystal structure formed by the addition of GO sheets. This route offers a novel approach for the preparation of high performance permanent magnetic nanoparticles.
2. Material and methods 2.1. Synthesis of precursor GO sheets were bought from Xianfeng Nano, which is prepared from graphite powder by a modified Hummers method. A certain amount of GO sheets was dispersed in 100 ml triglycol with 0.30 g CTAB under ultrasonic concussion for 4 h. 0.4837 g (1 mmol) samarium acetylacetonate (Sm(acac)3xH2O) and 1.0688 g (3 mmol) cobalt(II) acetylacetonate (Co(acac)2) were dried at 110 °C for 2 h. Then, dried Sm(acac)3 and Co(acac)2 were dispersed into the GO solution by mechanical stirring. Following that, the mixture was heated to 120 °C for 1 h to move moisture. 0.5 ml oleylamine and 0.5 mL oleic acid were added into this solution. Then, the mixture solution was transferred into teflon vessel for solvothermal reaction at 265 °C for 3 h. Finally, the resultant was centrifuged at 12,000 r/min for 5 min to obtain brown powder. The powder was further washed with ethanol for 5 times and dried at room temperature.
2.2. Synthesis of SmCo@graphene The prepared precursor was mixed with 2.0 g potassium chloride (KCl), 2.0 g calcium oxide (CaO) and 4.0 g calcium (Ca). The mixture was transferred in a tungsten crucible with cap. The tungsten crucible was then moved to a corundum tube. After that, the tube was degassed to remove air and moisture. Subsequently, the tube was flushed with Ar and heated to 860 °C at a rate of 8 °C/ min. The reaction was held for 90 min before being cooled down to room temperature. The resultant was washed with degassed distilled water and 0.5% hydrochloric acid to dissolve CaO, KCl, and extra Ca. A black powder was obtained by centrifuging at 12,000 r/min for 3 min. The powder was further washed with degassed distilled water and ethanol and dried under vacuum for further usage.
2.3. Characterization The crystallographic structure was identified by X-ray diffraction (XRD, D/MAX 2500 PC) with Cu-Ka radiation (k = 0.15418 nm) and scanning speed of 6°/min. The microstructure and morphology of the particles were investigated using transmission electron microscopy (TEM, JEM-2100F). For TEM observations, the samples were dispersed in hexane with 1–2 drops of ethanol in it. The drops of the well dispersed nanoparticles were placed over the carbon coated microscopic copper grids (200 mesh size) and were subsequently dried. The particle composition was determined by energy dispersive spectrometer (EDS) analysis with JEOL JSM-6700F. The magnetic properties were measured at room temperature using a Physical Property Measurement System (PPMS) under a maximum applied field of 90 kOe.
3. Results and discussions The overall schematic illustration for fabrication of SmCo5 particles is displayed in Fig. 1. First, Sm(acac)3 and Co(acac)2 were dispersed in triglycol solvent. Then, Sm2O3 and Co particles wrapped in GO sheets were prepared by solvothermal process. At last, the precursors were reduced by Ca at 860 °C for 1.5 h to yield SmCo5@graphene single-crystal particles. Here, GO sheets will effectively prevent the growth and agglomeration of SmCo5 particles. As comparison, polycrystalline SmCo5 particles without addition of GO sheets were also obtained, which shows serious agglomeration. Fig. 2a shows the XRD patterns of the precursor prepared with different surfactants and 10 mg GO sheets. All samples possess similar diffraction peaks, indicating the surfactants have little effect on the phases of precursor. There are broad diffraction peaks at 44.2°, indexed to the (1 1 1) planes in cubic Co, demonstrating that Co particles are reduced by polyalcohol during solvothermal process. And obvious amorphous peaks are observed at 20–30°, which is attributed to uncrystallized Sm2O3 according to previous report [29]. The precursor prepared with CTAB was heated to 860 °C for 1 h under Ar atmosphere and it was converted to the mixture of crystalline Sm2O3 and Co as shown in Fig. S1 (in Supplementary material), confirming that the precursor was comprise of Co and amorphous Sm2O3. The precursor was further characterized by TEM to observe the morphology, which is outlined in Fig. 2b–d. It can be seen that the precursor prepared without any surfactants (Fig. 2b) exhibits irregular morphology and serious aggregation. But mostly, the precursor prepared only with CTAB (Fig. 2c) exhibits more regular morphology with size of about 100 nm, where Sm2O3-Co particles can be well wrapped in GO sheets. Moreover, the GO sheets are unfolded and the particles are well dispersed on GO sheets, suggesting that the addition of CTAB can stretch the GO sheets and prevent the aggregation of precursor to some degree. In addition, for the precursor prepared with mixture of CTAB, oleylamine and oleic acid (Fig. 2d), particles exhibit well distribution with size of 40 nm but the Sm2O3ACo particles are loaded on the surface of GO sheets. It indicates that oleylamine and oleic acid can only effectively decrease the size of precursor and are not beneficial to wrap precursor particles. Consequently, this addition of appropriate surfactants provides a good basis for following preparation of uniform SmCo5 particles. The mechanisms of the chemical reactions can be explained as follows. In solvothermal process, Co (acac)2 and Sm(acac)3 converted into Co and Sm2O3, as shown in Eqs. (1)–(4). At some reaction temperature, the dehydration of ACH2AOH generates ACHO, which can reduce Co(acac)2 into Co. Meanwhile, the hydrolysis of Sm(acac)3 yields Sm(OH)3, which further converts into Sm2O3 by dehydration. Due to the interaction
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Fig. 1. Schematic illustration of the synthetic strategy of SmCo5 particles.
Fig. 2. (a) XRD patterns and (b)–(d) TEM images of precursor prepared with different surfactants with addition of 10 mg GO sheets: (b) without any surfactant; (c) CTAB; (d) mixture of CTAB, oleylamine and oleic acid.
between AOH (ACOOH) in GO sheets and AO in Sm2O3 nanoparticles, Sm2O3 nanoparticles and GO sheets were joined by AOAH bridge, which results in the formation of precursor particles coated with GO sheets.
2HOCH2 ðCH2 OCH2 Þ2 CH2 OH ! 2CH3 ðCH2 OCH2 Þ2 CHO þ 2H2 O
ð1Þ
2CH3 ðCH2 OCH2 Þ2 CHO þ CoðacacÞ2 ! Co þ CH3 ðCH2 OCH2 Þ2 COCOðCH2 OCH2 Þ2 CH3 þ 2acac-H SmðacacÞ3 þ 3H2 O ! SmðOHÞ3 þ 3acac-H
ð2Þ ð3Þ
2SmðOHÞ3 ! Sm2 O3 þ 3H2 O
ð4Þ
These precursors were thermally reduced by Ca at 860 °C to fabricate SmCo5@graphene particles. Fig. 3 gives the XRD patterns of SmCo5@graphene particles by reductive annealing of precursors prepared with different surfactants. All diffraction peaks for three samples can be indexed to the hexagonal SmCo5 (JPCD No. 271122), demonstrating that the thermal reduction of Sm2O3-Co@GO yields SmCo5 alloy. Broaden diffraction peaks are also found in the existing of surfactant, indicating that the addition of surfactant decrease the grain size of synthetic SmCo5@graphene particles. However, there are unknown peaks at 40° in Fig. 3d, which may be impurity (oxide or carbide) caused by the thermolysis of
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Z. Ma et al. / Chemical Engineering Journal 304 (2016) 993–999 Table 1 EDS data of samples with different surfactants (a) without surfactants, (b) CTAB, (c) CTAB, oleylamine and oleic acid.
Fig. 3. XRD patterns of SmCo5 particles prepared with different surfactants with addition of 10 mg GO: (a) the standard diffraction pattern of SmCo5 (JCPDS No. 271122); (b) without any surfactant; (c) CTAB; (d) mixture of CTAB, oleylamine and oleic acid.
residual oleylamine and oleic acid [31,32,35]. We dispersed SmCo5 particles prepared with mixture of CTAB, oleylamine and oleic acid in ethanol under ultrasonic concussion, and used magnet to extract these SmCo5 particles. It was only found that this solution remained black though there were also magnetic particles at the
Samples
Sm atomic%
Co atomic%
O atomic%
Sm:Co (atom)
a b c
20.06 19.92 19.18
79.94 80.08 76.68
– – 4.14
1:3.98 1:4.02 1:3.99
bottom of the centrifuge tube as shown in Fig. S2, which also proved the existence of nonmagnetic impurity in SmCo5 particles. The composition of the obtained nanoparticles was measured by EDS and the data is given in Table 1 (corresponding energy spectrum is shown in Fig. S3). From the Table 1, it can be observed that all samples comprise of Sm and Co elements, and the Sm/Co is 1:3.98, 1:4.02 and 1:3.99 in samples prepared with different surfactants, respectively. However, O element can be apparently observed in the sample prepared with CTAB, oleylamine and oleic acid, further confirming that the addition of oleylamine and oleic acid can introduce purity. And the energy peaks of C element mainly come from the substrate (carbon film), where the minority of C element may be caused by the thermolysis of organic compound. Fig. 4 shows the TEM images of SmCo5@graphene prepared with different surfactants. Large-size (200–500 nm) SmCo5 particles prepared without surfactants are wrapped in graphene (Fig. 4a)
Fig. 4. TEM and HRTEM images of SmCo5 particles prepared with different surfactants with addition of 10 mg GO: (a) and (b) without any surfactant; (c) and (d) CTAB; (e) and (f) mixture of CTAB, oleylamine and oleic acid.
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The mechanisms of calciothermic reduction process can be explained in Eqs. (5) and (6). In reductive annealing process, the collapse of AOAH bridges generates Sm atoms, and following fusion process yields SmCo5 alloy. In the isolated space separated by graphene, the rearrangement of Sm and Co atoms resulted in the formation of SmCo5 single-crystal particles.
Fig. 5. Room temperature magnetic hysteresis loop of SmCo5 particles prepared with different surfactants with addition of 10 mg GO: (a) without any surfactant; (b) CTAB; (c) mixture of CTAB, oleylamine and oleic acid.
and show a certain extent of aggregation, which may be arouse by the aggregation of precursor. The high resolution TEM (HRTEM) image of Fig. 4b illustrates that the large particles are singlecrystal and have lattice at 0.293 nm, matching well to the (1 0 1) plane in hexagonal SmCo5. The SmCo5 particles prepared with CTAB have size of 20–100 nm and are coated well with graphene, as shown in Fig. 4c. Fig. 4d shows that a single particle is wrapped in graphene and has lattice at 0.211 nm, which corresponds to the (1 1 1) plane in SmCo5. The size of SmCo5 particles with mixture of CTAB, oleylamine and oleic acid further decreases, as shown in Fig. 4e. These particles are loaded on the surface of graphene and exhibit slight agglomeration. The lattice spacing value of 0.198 nm in Fig. 4f fits well to the (0 0 2) planes in SmCo5 (JCPDS No. 27-1122). These results demonstrate that the size and morphology of SmCo5 particles can be manipulated by controlling the size and morphology of precursor with different surfactants.
Sm2 O3 þ 3Ca ! 2Sm þ 3CaO
ð5Þ
Sm þ 5Co ! SmCo5
ð6Þ
Fig. 5 presents the room temperature magnetic hysteresis loop of SmCo5@graphene prepared with different surfactants. The loops show that all samples are ferromagnetic. The coercivity of 29.2 kOe, 24.4 kOe and 15.9 kOe are achieved in SmCo5@graphene prepared without CTAB, only with CTAB and the mixture of CTAB, oleylamine and oleic acid, respectively. The high coercivity for SmCo5@graphene prepared without CTAB is owing to the large size of single-crystal particles [34,36]. SmCo5@graphene particles prepared with only CTAB possess a smooth loop and appropriate coercivity. However, the loop of SmCo5@graphene prepared with the mixture of CTAB, oleylamine and oleic acid has obvious kink in the second quadrant and exhibits poor magnetic performance, which may be caused by the introduction of impurity [32,35]. According to Fig. 5, the saturation magnetization of particles prepared with addition of oleylamine and oleic acid exhibits a little lower than the sample with CTAB, which is attributed to the existence of purity since there are so many non-magnetic particles in sample. Meanwhile, Fig. S2 also confirms the existence of purity. Due to the existence of purity, the samples will not exhibit single hard magnetic performance, resulting in the kink, in consistent with previous report [35]. The coercivity of prepared SmCo5 particles is dependent on the size and morphology. With the decrease of size, the coercivity reduces obviously, which is attributed to the increasing defects and low magnetocrystalline anisotropy [18]. On the one hand, the addition of GO sheets can tune the size of SmCo5 particles,
Fig. 6. (a) The standard diffraction pattern of SmCo5 (JCPDS No. 27-1122) and XRD patterns of SmCo5 particles prepared with CTAB and different additive amount of GO sheets; (b)–(d) TEM images of SmCo5 particles prepared with CTAB and different additive amount of GO sheets: (b) 0; (c) 5 mg; (d) 15 mg.
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Fig. 7. Room temperature magnetic hysteresis loop of SmCo5 particles prepared with CTAB and different additive amount of GO sheets: (a) 0; (b) 5 mg; (c) 15 mg.
which can further control the magnetic performance; therefore, the addition of GO sheets in this system would reduce the coercivity of SmCo5 particles. On the other hand, there are strong interactions between nanoparticles when the size of SmCo5 particles reduces to nanoscale, which also lowers coercivity, as reported in the literature [34]. And the wrapping of GO sheets can increase the inter-particle distance and decrease exchange interactions, which results in an improvement of coercivity. Therefore, the SmCo5 particles with size of 20–100 nm have a high room coercivity of 24.4 kOe, which is far higher than SmCo5 particles with similar size in previous report. To investigate the function of GO sheets on SmCo5 particles, the SmCo5@graphene particles were further synthesized from the precursors with addition of 0, 5 mg and 15 mg GO sheets, and using CTAB as a surfactant. And the XRD patterns are outlined in Fig. 6a. The diffraction peaks of particles with addition of 0, 5 mg and 10 mg GO (in Figs. 6a and 3c) match well with the SmCo5. But the alloy particles prepared with addition of 15 mg GO sheets contain Sm2Co17 phase. Meanwhile, with the amount of GO sheets increase, the diffraction peaks gradually become broad, suggesting that the GO sheets can effectively prevent the growth of particles. The phenomenon can be explained that GO sheets with moderate amount prevent the growth of SmCo5 grain size, and excessive GO sheets will impede the interface diffusion of Sm and Co atoms, where redundant Sm without alloying was removed in washing process. The phenomenon agrees with the function of CaO in our previous report [30]. The corresponding TEM images are shown in Fig. 6b–d. According to Fig. 6b, SmCo5 particles prepared without GO sheets exhibit serious agglomeration and sintering, where the size is 200 nm– 1.5 lm. Compared with Sm2O3-Co precursor without GO sheets which possesses size of 40–200 nm (in Fig. S4), SmCo5 particles grow obviously under the absence of GO sheets. And partially agglomerated SmCo5 particles prepared with 5 mg GO sheets can be observed in Fig. 6c, which cannot be fully wrapped by GO sheets due to low additive amount (5 mg). For SmCo5 particles prepared with 10 mg GO sheets, the particles well wrapped in graphene exhibit uniform size of 20–100 nm (Fig. 4c). The SmCo nanoparticles with addition of 15 mg GO sheets have broad size distribution, as shown in Fig. 6d, which is smaller than SmCo with 0, 5 mg and 10 mg GO sheets. The HRTEM image of SmCo5 particle without GO sheets is displayed in Fig. S5. The random-orientated SmCo5 grains suggest the SmCo5 particles without GO sheets are polycrystalline, which is quite different from SmCo5 single-crystal particles with 10 mg GO sheets (in Fig. 4). It can be concluded that GO sheets can help to maintain SmCo5 particles as sound single crystal, which
agrees with previous report [32]. Moreover, the addition of GO sheets can also prevent SmCo5 particles from sintering. They exhibit small size and have a well distribution, which is favourable for further preparation of exchange-coupled nanocomposites. Finally, with ideal amount of GO sheets, SmCo5 particles are well wrapped in graphene and have unique suitable size, which is conductive to their magnetic properties. Fig. 7 gives the room temperature magnetic hysteresis loop of SmCo5 particles prepared with 0, 5 mg and 15 mg GO. The room temperature coercivity of 46.6 kOe and 32.9 kOe is achieved in SmCo5 particles with 0 and 5 mg GO sheets, respectively. Compared with the coercivity of 24.4 kOe with 10 mg GO sheets, the large coercivity may be attributed to the large size of particles [20]. Meanwhile, SmCo5 particles with 15 mg GO have relative low coercivity of 6.4 kOe and obvious kink in the second quadrant, which is caused by the generation of Sm2Co17 phase. Different from the sample prepared with addition of oleylamine and oleic acid, the saturation magnetization of particles prepared with addition of 15 mg GO sheets is 60% higher than the products with 10 mg GO. The high saturation magnetization is caused by the formation of 2:17 SmCo, which results in kink due to poor exchangecoupling. These results imply that we can control the SmCo particles size and magnetic property by tuning GO additive amount, where 10 mg GO addition can obtain an ideal size and appropriate coercivity. Thus, GO sheets have at least three indispensable contributions to the formation of SmCo5 single-crystal particles. Firstly, during solvothermal process, GO sheets will disperse the precursor particles and reduce agglomeration, which can be seen from comparison between the precursor with 10 mg GO (in Fig. 2c) and the precursor without GO sheets (in Fig. S4). Secondly, during high temperature annealing process, GO sheets will obstruct the growth and sintering of SmCo5 particles and facilitate the generation of isolated particles by impeding inter-particle fusion. Finally, due to the wrapping of GO sheets, SmCo5 particles are prone to forming single crystals during high temperature fusion process.
4. Conclusions In summary, we have developed a novel procedure to fabricate SmCo5@graphene single-crystal particles. In this route, the Sm2O3Co precursor wrapped in GO sheets was first obtained by solvothermal process. And then SmCo5@graphene particles were yielded by reductive annealing of that precursor, where the size and magnetic performance could be tuned by changing surfactants. The high coercivity of 24.4 kOe was obtained in 20–100 nm SmCo5 particles by using CTAB as a surfactant. We found that the GO sheets played a vitally important role in the particles size reduction of SmCo5 and generation of single crystal. This work can be extended to the synthesis of other rare-earth permanent magnetic particles with highly ferromagnetic performance.
Acknowledgements This work was supported by National Natural Science Foundations of China (NSFC) under Grant No. 51471016 and No. 51520105002, and the Key Natural Science Foundation of Beijing No. 2151002.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2016.07.024.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
The chemical synthesis of SmCo5 single-crystal particles with small size and high performance Zhenhui Ma, Shengxue Yang, Tianli Zhang ⇑, Chengbao Jiang ⇑ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
h i g h l i g h t s SmCo5 particles have been fabricated by an assistance of graphite oxide sheets. The size and magnetic performance can be controlled by tuning surfactants. SmCo5 nanoparticles with size of 20–100 nm exhibit a high coercivity of 24.4 kOe. Graphite oxide sheets can impede the growth and sintering of SmCo5 particles.
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 May 2016 Received in revised form 5 July 2016 Accepted 6 July 2016 Available online 7 July 2016
SmCo5 nanoparticles have promising applications in high-density magnetic storage and magnetic nanocomposites. However, conventional calciothermic reduction process yields agglomerated SmCo5 magnets for high temperature annealing, limiting the application and development of SmCo5 nanoparticles. In this work, we exploited a novel strategy to fabricate SmCo5 single-crystal nanoparticles by an assistance of graphite oxide sheets. Sm2O3-Co particles coated with graphite oxide sheets were synthesized by solvothermal route, and the following reductive annealing generated SmCo5 particles. The size and magnetic performance of SmCo5 particles can be tuned by manipulating surfactants. The high coercivity of 24.4 kOe and ideal size of 20–100 nm can be achieved in SmCo5 single-crystal nanoparticles. In this technology, graphite oxide sheets can prevent the particles from sintering effectively and sustain the generation of single crystal. Ó 2016 Elsevier B.V. All rights reserved.
Keywords: SmCo5 Nanoparticles Coercivity GO sheets
1. Introduction Magnetic particles at nanoscale have promising perspective for advanced applications such as high-density magnetic storage, high-energy product magnets, sensors and drug carriers in biomedical technology [1–11]. Among all the materials, SmCo5 (Sm2Co17) single crystal nanoparticles have the highest exceptionally-large magnetocrystalline anisotropy (Ku = 2 108 erg/cm3) as well as a high Curie temperature (Tc = 1020 K), which contributes high coercivity and high operating temperature respectively, being considered as an irreplaceable candidate in aerospace, motors, and automotive industries [12–17]. Unfortunately, rare-earth element (Sm) is very active and prone to oxidation under ambient conditions, which poses significant preparation
⇑ Corresponding author. E-mail (C. Jiang).
addresses:
[email protected]
(T.
http://dx.doi.org/10.1016/j.cej.2016.07.024 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.
Zhang),
[email protected]
challenges for the fabrication of SmCo5 nanoparticles with high magnetic performance [18,19]. Many research efforts have been spent on the fabrication of SmCo5 nanoparticles with desired coercivity adopting physical or chemical method [19–28]. Conventional techniques like high energy ball-milling have been explored to fabricate SmCo5 particles [20,21]. However, a large particle size (usually submicrometre) is obtained since the crushed fine particles can be cold welded again during ball milling [22]. Alternatively, direct solution phase approach have been employed to yield SmCo5 nanoparticles, but it failed to obtain a high performance due to electronegativity and instability of Sm element [23–28]. Recently, researchers exploited a novel method combined solution-phase synthesis with calciothermic reduction [29–32]. Nevertheless, the synthetic particles showed agglomeration and sintering for a high temperature annealing. An improved method of using CaO-coated Sm-Co oxide nanoparticles followed by Ca-reduction at 960 °C yields 6 nm SmCo5 [33]. But these magnetic particles only exhibited low coercivity of 7.2 kOe. Recently, graphite oxide (GO) sheets have been
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also applied to prepare SmCo/Co magnets by calciothermic reduction of Sm[Co(CN)6]4H2O@GO/Co(acac)2, where GO play a role in the generation of the core-shell structure in composite magnets by obstructing interphase diffusion of Co [34]. In this technique, although a high coercivity of 20.7 kOe could be obtained, it was committed to a large size SmCo magnet with 200 nm, rather than nanoparticles. Therefore, the synthesis of desired SmCo5 particles possessing nanometer size and high coercivity simultaneously is still an arduous task due to the obstacles in controlling synthesis of single crystal nanoparticles. Herein, we have fabricated SmCo5 single crystal nanoparticles with size of 20–100 nm and high coercivity of 24.4 kOe by a novel strategy. We first synthesized SmCo-O@GO particles by solvothermal route, and the following reductive annealing generates SmCo5 particles. Although calciothermic reduction process has been adopted in previous literatures [29–33], the precursors in this paper are entirely different from these reports. The Sm2O3-Co coated with GO sheets can be tuned by surfactants; therefore, the particle size and magnetic performance of SmCo5 can also be controlled, which is unavailable in previous studies. Additionally, the employment of GO sheets can prevent particles from sintering effectively and promote formation of single crystal. Finally, the coercivity of small-sized SmCo5 is up to 24.4 kOe, which is higher than SmCo5 particles in previous reports. The high coercivity may be caused by a sound single crystal structure formed by the addition of GO sheets. This route offers a novel approach for the preparation of high performance permanent magnetic nanoparticles.
2. Material and methods 2.1. Synthesis of precursor GO sheets were bought from Xianfeng Nano, which is prepared from graphite powder by a modified Hummers method. A certain amount of GO sheets was dispersed in 100 ml triglycol with 0.30 g CTAB under ultrasonic concussion for 4 h. 0.4837 g (1 mmol) samarium acetylacetonate (Sm(acac)3xH2O) and 1.0688 g (3 mmol) cobalt(II) acetylacetonate (Co(acac)2) were dried at 110 °C for 2 h. Then, dried Sm(acac)3 and Co(acac)2 were dispersed into the GO solution by mechanical stirring. Following that, the mixture was heated to 120 °C for 1 h to move moisture. 0.5 ml oleylamine and 0.5 mL oleic acid were added into this solution. Then, the mixture solution was transferred into teflon vessel for solvothermal reaction at 265 °C for 3 h. Finally, the resultant was centrifuged at 12,000 r/min for 5 min to obtain brown powder. The powder was further washed with ethanol for 5 times and dried at room temperature.
2.2. Synthesis of SmCo@graphene The prepared precursor was mixed with 2.0 g potassium chloride (KCl), 2.0 g calcium oxide (CaO) and 4.0 g calcium (Ca). The mixture was transferred in a tungsten crucible with cap. The tungsten crucible was then moved to a corundum tube. After that, the tube was degassed to remove air and moisture. Subsequently, the tube was flushed with Ar and heated to 860 °C at a rate of 8 °C/ min. The reaction was held for 90 min before being cooled down to room temperature. The resultant was washed with degassed distilled water and 0.5% hydrochloric acid to dissolve CaO, KCl, and extra Ca. A black powder was obtained by centrifuging at 12,000 r/min for 3 min. The powder was further washed with degassed distilled water and ethanol and dried under vacuum for further usage.
2.3. Characterization The crystallographic structure was identified by X-ray diffraction (XRD, D/MAX 2500 PC) with Cu-Ka radiation (k = 0.15418 nm) and scanning speed of 6°/min. The microstructure and morphology of the particles were investigated using transmission electron microscopy (TEM, JEM-2100F). For TEM observations, the samples were dispersed in hexane with 1–2 drops of ethanol in it. The drops of the well dispersed nanoparticles were placed over the carbon coated microscopic copper grids (200 mesh size) and were subsequently dried. The particle composition was determined by energy dispersive spectrometer (EDS) analysis with JEOL JSM-6700F. The magnetic properties were measured at room temperature using a Physical Property Measurement System (PPMS) under a maximum applied field of 90 kOe.
3. Results and discussions The overall schematic illustration for fabrication of SmCo5 particles is displayed in Fig. 1. First, Sm(acac)3 and Co(acac)2 were dispersed in triglycol solvent. Then, Sm2O3 and Co particles wrapped in GO sheets were prepared by solvothermal process. At last, the precursors were reduced by Ca at 860 °C for 1.5 h to yield SmCo5@graphene single-crystal particles. Here, GO sheets will effectively prevent the growth and agglomeration of SmCo5 particles. As comparison, polycrystalline SmCo5 particles without addition of GO sheets were also obtained, which shows serious agglomeration. Fig. 2a shows the XRD patterns of the precursor prepared with different surfactants and 10 mg GO sheets. All samples possess similar diffraction peaks, indicating the surfactants have little effect on the phases of precursor. There are broad diffraction peaks at 44.2°, indexed to the (1 1 1) planes in cubic Co, demonstrating that Co particles are reduced by polyalcohol during solvothermal process. And obvious amorphous peaks are observed at 20–30°, which is attributed to uncrystallized Sm2O3 according to previous report [29]. The precursor prepared with CTAB was heated to 860 °C for 1 h under Ar atmosphere and it was converted to the mixture of crystalline Sm2O3 and Co as shown in Fig. S1 (in Supplementary material), confirming that the precursor was comprise of Co and amorphous Sm2O3. The precursor was further characterized by TEM to observe the morphology, which is outlined in Fig. 2b–d. It can be seen that the precursor prepared without any surfactants (Fig. 2b) exhibits irregular morphology and serious aggregation. But mostly, the precursor prepared only with CTAB (Fig. 2c) exhibits more regular morphology with size of about 100 nm, where Sm2O3-Co particles can be well wrapped in GO sheets. Moreover, the GO sheets are unfolded and the particles are well dispersed on GO sheets, suggesting that the addition of CTAB can stretch the GO sheets and prevent the aggregation of precursor to some degree. In addition, for the precursor prepared with mixture of CTAB, oleylamine and oleic acid (Fig. 2d), particles exhibit well distribution with size of 40 nm but the Sm2O3ACo particles are loaded on the surface of GO sheets. It indicates that oleylamine and oleic acid can only effectively decrease the size of precursor and are not beneficial to wrap precursor particles. Consequently, this addition of appropriate surfactants provides a good basis for following preparation of uniform SmCo5 particles. The mechanisms of the chemical reactions can be explained as follows. In solvothermal process, Co (acac)2 and Sm(acac)3 converted into Co and Sm2O3, as shown in Eqs. (1)–(4). At some reaction temperature, the dehydration of ACH2AOH generates ACHO, which can reduce Co(acac)2 into Co. Meanwhile, the hydrolysis of Sm(acac)3 yields Sm(OH)3, which further converts into Sm2O3 by dehydration. Due to the interaction
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Fig. 1. Schematic illustration of the synthetic strategy of SmCo5 particles.
Fig. 2. (a) XRD patterns and (b)–(d) TEM images of precursor prepared with different surfactants with addition of 10 mg GO sheets: (b) without any surfactant; (c) CTAB; (d) mixture of CTAB, oleylamine and oleic acid.
between AOH (ACOOH) in GO sheets and AO in Sm2O3 nanoparticles, Sm2O3 nanoparticles and GO sheets were joined by AOAH bridge, which results in the formation of precursor particles coated with GO sheets.
2HOCH2 ðCH2 OCH2 Þ2 CH2 OH ! 2CH3 ðCH2 OCH2 Þ2 CHO þ 2H2 O
ð1Þ
2CH3 ðCH2 OCH2 Þ2 CHO þ CoðacacÞ2 ! Co þ CH3 ðCH2 OCH2 Þ2 COCOðCH2 OCH2 Þ2 CH3 þ 2acac-H SmðacacÞ3 þ 3H2 O ! SmðOHÞ3 þ 3acac-H
ð2Þ ð3Þ
2SmðOHÞ3 ! Sm2 O3 þ 3H2 O
ð4Þ
These precursors were thermally reduced by Ca at 860 °C to fabricate SmCo5@graphene particles. Fig. 3 gives the XRD patterns of SmCo5@graphene particles by reductive annealing of precursors prepared with different surfactants. All diffraction peaks for three samples can be indexed to the hexagonal SmCo5 (JPCD No. 271122), demonstrating that the thermal reduction of Sm2O3-Co@GO yields SmCo5 alloy. Broaden diffraction peaks are also found in the existing of surfactant, indicating that the addition of surfactant decrease the grain size of synthetic SmCo5@graphene particles. However, there are unknown peaks at 40° in Fig. 3d, which may be impurity (oxide or carbide) caused by the thermolysis of
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Z. Ma et al. / Chemical Engineering Journal 304 (2016) 993–999 Table 1 EDS data of samples with different surfactants (a) without surfactants, (b) CTAB, (c) CTAB, oleylamine and oleic acid.
Fig. 3. XRD patterns of SmCo5 particles prepared with different surfactants with addition of 10 mg GO: (a) the standard diffraction pattern of SmCo5 (JCPDS No. 271122); (b) without any surfactant; (c) CTAB; (d) mixture of CTAB, oleylamine and oleic acid.
residual oleylamine and oleic acid [31,32,35]. We dispersed SmCo5 particles prepared with mixture of CTAB, oleylamine and oleic acid in ethanol under ultrasonic concussion, and used magnet to extract these SmCo5 particles. It was only found that this solution remained black though there were also magnetic particles at the
Samples
Sm atomic%
Co atomic%
O atomic%
Sm:Co (atom)
a b c
20.06 19.92 19.18
79.94 80.08 76.68
– – 4.14
1:3.98 1:4.02 1:3.99
bottom of the centrifuge tube as shown in Fig. S2, which also proved the existence of nonmagnetic impurity in SmCo5 particles. The composition of the obtained nanoparticles was measured by EDS and the data is given in Table 1 (corresponding energy spectrum is shown in Fig. S3). From the Table 1, it can be observed that all samples comprise of Sm and Co elements, and the Sm/Co is 1:3.98, 1:4.02 and 1:3.99 in samples prepared with different surfactants, respectively. However, O element can be apparently observed in the sample prepared with CTAB, oleylamine and oleic acid, further confirming that the addition of oleylamine and oleic acid can introduce purity. And the energy peaks of C element mainly come from the substrate (carbon film), where the minority of C element may be caused by the thermolysis of organic compound. Fig. 4 shows the TEM images of SmCo5@graphene prepared with different surfactants. Large-size (200–500 nm) SmCo5 particles prepared without surfactants are wrapped in graphene (Fig. 4a)
Fig. 4. TEM and HRTEM images of SmCo5 particles prepared with different surfactants with addition of 10 mg GO: (a) and (b) without any surfactant; (c) and (d) CTAB; (e) and (f) mixture of CTAB, oleylamine and oleic acid.
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The mechanisms of calciothermic reduction process can be explained in Eqs. (5) and (6). In reductive annealing process, the collapse of AOAH bridges generates Sm atoms, and following fusion process yields SmCo5 alloy. In the isolated space separated by graphene, the rearrangement of Sm and Co atoms resulted in the formation of SmCo5 single-crystal particles.
Fig. 5. Room temperature magnetic hysteresis loop of SmCo5 particles prepared with different surfactants with addition of 10 mg GO: (a) without any surfactant; (b) CTAB; (c) mixture of CTAB, oleylamine and oleic acid.
and show a certain extent of aggregation, which may be arouse by the aggregation of precursor. The high resolution TEM (HRTEM) image of Fig. 4b illustrates that the large particles are singlecrystal and have lattice at 0.293 nm, matching well to the (1 0 1) plane in hexagonal SmCo5. The SmCo5 particles prepared with CTAB have size of 20–100 nm and are coated well with graphene, as shown in Fig. 4c. Fig. 4d shows that a single particle is wrapped in graphene and has lattice at 0.211 nm, which corresponds to the (1 1 1) plane in SmCo5. The size of SmCo5 particles with mixture of CTAB, oleylamine and oleic acid further decreases, as shown in Fig. 4e. These particles are loaded on the surface of graphene and exhibit slight agglomeration. The lattice spacing value of 0.198 nm in Fig. 4f fits well to the (0 0 2) planes in SmCo5 (JCPDS No. 27-1122). These results demonstrate that the size and morphology of SmCo5 particles can be manipulated by controlling the size and morphology of precursor with different surfactants.
Sm2 O3 þ 3Ca ! 2Sm þ 3CaO
ð5Þ
Sm þ 5Co ! SmCo5
ð6Þ
Fig. 5 presents the room temperature magnetic hysteresis loop of SmCo5@graphene prepared with different surfactants. The loops show that all samples are ferromagnetic. The coercivity of 29.2 kOe, 24.4 kOe and 15.9 kOe are achieved in SmCo5@graphene prepared without CTAB, only with CTAB and the mixture of CTAB, oleylamine and oleic acid, respectively. The high coercivity for SmCo5@graphene prepared without CTAB is owing to the large size of single-crystal particles [34,36]. SmCo5@graphene particles prepared with only CTAB possess a smooth loop and appropriate coercivity. However, the loop of SmCo5@graphene prepared with the mixture of CTAB, oleylamine and oleic acid has obvious kink in the second quadrant and exhibits poor magnetic performance, which may be caused by the introduction of impurity [32,35]. According to Fig. 5, the saturation magnetization of particles prepared with addition of oleylamine and oleic acid exhibits a little lower than the sample with CTAB, which is attributed to the existence of purity since there are so many non-magnetic particles in sample. Meanwhile, Fig. S2 also confirms the existence of purity. Due to the existence of purity, the samples will not exhibit single hard magnetic performance, resulting in the kink, in consistent with previous report [35]. The coercivity of prepared SmCo5 particles is dependent on the size and morphology. With the decrease of size, the coercivity reduces obviously, which is attributed to the increasing defects and low magnetocrystalline anisotropy [18]. On the one hand, the addition of GO sheets can tune the size of SmCo5 particles,
Fig. 6. (a) The standard diffraction pattern of SmCo5 (JCPDS No. 27-1122) and XRD patterns of SmCo5 particles prepared with CTAB and different additive amount of GO sheets; (b)–(d) TEM images of SmCo5 particles prepared with CTAB and different additive amount of GO sheets: (b) 0; (c) 5 mg; (d) 15 mg.
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Fig. 7. Room temperature magnetic hysteresis loop of SmCo5 particles prepared with CTAB and different additive amount of GO sheets: (a) 0; (b) 5 mg; (c) 15 mg.
which can further control the magnetic performance; therefore, the addition of GO sheets in this system would reduce the coercivity of SmCo5 particles. On the other hand, there are strong interactions between nanoparticles when the size of SmCo5 particles reduces to nanoscale, which also lowers coercivity, as reported in the literature [34]. And the wrapping of GO sheets can increase the inter-particle distance and decrease exchange interactions, which results in an improvement of coercivity. Therefore, the SmCo5 particles with size of 20–100 nm have a high room coercivity of 24.4 kOe, which is far higher than SmCo5 particles with similar size in previous report. To investigate the function of GO sheets on SmCo5 particles, the SmCo5@graphene particles were further synthesized from the precursors with addition of 0, 5 mg and 15 mg GO sheets, and using CTAB as a surfactant. And the XRD patterns are outlined in Fig. 6a. The diffraction peaks of particles with addition of 0, 5 mg and 10 mg GO (in Figs. 6a and 3c) match well with the SmCo5. But the alloy particles prepared with addition of 15 mg GO sheets contain Sm2Co17 phase. Meanwhile, with the amount of GO sheets increase, the diffraction peaks gradually become broad, suggesting that the GO sheets can effectively prevent the growth of particles. The phenomenon can be explained that GO sheets with moderate amount prevent the growth of SmCo5 grain size, and excessive GO sheets will impede the interface diffusion of Sm and Co atoms, where redundant Sm without alloying was removed in washing process. The phenomenon agrees with the function of CaO in our previous report [30]. The corresponding TEM images are shown in Fig. 6b–d. According to Fig. 6b, SmCo5 particles prepared without GO sheets exhibit serious agglomeration and sintering, where the size is 200 nm– 1.5 lm. Compared with Sm2O3-Co precursor without GO sheets which possesses size of 40–200 nm (in Fig. S4), SmCo5 particles grow obviously under the absence of GO sheets. And partially agglomerated SmCo5 particles prepared with 5 mg GO sheets can be observed in Fig. 6c, which cannot be fully wrapped by GO sheets due to low additive amount (5 mg). For SmCo5 particles prepared with 10 mg GO sheets, the particles well wrapped in graphene exhibit uniform size of 20–100 nm (Fig. 4c). The SmCo nanoparticles with addition of 15 mg GO sheets have broad size distribution, as shown in Fig. 6d, which is smaller than SmCo with 0, 5 mg and 10 mg GO sheets. The HRTEM image of SmCo5 particle without GO sheets is displayed in Fig. S5. The random-orientated SmCo5 grains suggest the SmCo5 particles without GO sheets are polycrystalline, which is quite different from SmCo5 single-crystal particles with 10 mg GO sheets (in Fig. 4). It can be concluded that GO sheets can help to maintain SmCo5 particles as sound single crystal, which
agrees with previous report [32]. Moreover, the addition of GO sheets can also prevent SmCo5 particles from sintering. They exhibit small size and have a well distribution, which is favourable for further preparation of exchange-coupled nanocomposites. Finally, with ideal amount of GO sheets, SmCo5 particles are well wrapped in graphene and have unique suitable size, which is conductive to their magnetic properties. Fig. 7 gives the room temperature magnetic hysteresis loop of SmCo5 particles prepared with 0, 5 mg and 15 mg GO. The room temperature coercivity of 46.6 kOe and 32.9 kOe is achieved in SmCo5 particles with 0 and 5 mg GO sheets, respectively. Compared with the coercivity of 24.4 kOe with 10 mg GO sheets, the large coercivity may be attributed to the large size of particles [20]. Meanwhile, SmCo5 particles with 15 mg GO have relative low coercivity of 6.4 kOe and obvious kink in the second quadrant, which is caused by the generation of Sm2Co17 phase. Different from the sample prepared with addition of oleylamine and oleic acid, the saturation magnetization of particles prepared with addition of 15 mg GO sheets is 60% higher than the products with 10 mg GO. The high saturation magnetization is caused by the formation of 2:17 SmCo, which results in kink due to poor exchangecoupling. These results imply that we can control the SmCo particles size and magnetic property by tuning GO additive amount, where 10 mg GO addition can obtain an ideal size and appropriate coercivity. Thus, GO sheets have at least three indispensable contributions to the formation of SmCo5 single-crystal particles. Firstly, during solvothermal process, GO sheets will disperse the precursor particles and reduce agglomeration, which can be seen from comparison between the precursor with 10 mg GO (in Fig. 2c) and the precursor without GO sheets (in Fig. S4). Secondly, during high temperature annealing process, GO sheets will obstruct the growth and sintering of SmCo5 particles and facilitate the generation of isolated particles by impeding inter-particle fusion. Finally, due to the wrapping of GO sheets, SmCo5 particles are prone to forming single crystals during high temperature fusion process.
4. Conclusions In summary, we have developed a novel procedure to fabricate SmCo5@graphene single-crystal particles. In this route, the Sm2O3Co precursor wrapped in GO sheets was first obtained by solvothermal process. And then SmCo5@graphene particles were yielded by reductive annealing of that precursor, where the size and magnetic performance could be tuned by changing surfactants. The high coercivity of 24.4 kOe was obtained in 20–100 nm SmCo5 particles by using CTAB as a surfactant. We found that the GO sheets played a vitally important role in the particles size reduction of SmCo5 and generation of single crystal. This work can be extended to the synthesis of other rare-earth permanent magnetic particles with highly ferromagnetic performance.
Acknowledgements This work was supported by National Natural Science Foundations of China (NSFC) under Grant No. 51471016 and No. 51520105002, and the Key Natural Science Foundation of Beijing No. 2151002.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2016.07.024.
Z. Ma et al. / Chemical Engineering Journal 304 (2016) 993–999
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