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Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property
Author's Accepted Manuscript
Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property Chunju Xu, Dan Nie, Huiyu Chen, Yujie Wang, Yaqing Liu
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02196-X http://dx.doi.org/10.1016/j.matlet.2014.12.030 MLBLUE18181
To appear in:
Materials Letters
Received date: 1 October 2014 Accepted date: 3 December 2014 Cite this article as: Chunju Xu, Dan Nie, Huiyu Chen, Yujie Wang, Yaqing Liu, Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.12.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property Chunju Xu*, Dan Nie, Huiyu Chen, Yujie Wang, Yaqing Liu School of Materials Science and Engineering, North University of China, Taiyuan 030051, China *
Corresponding author. Tel. /fax: +86-351-3559669, E-mail: [email protected] (C. Xu),
[email protected] (H. Chen), [email protected] (Y. Liu)
Abstract: Novel cauliflower-like CoNi microstructures assembled by many thick petals were synthesized via a solvothermal process at 120 oC with the assistance of citric acid as complex agent. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM) techniques. It was found that the amount of reducing agent and solvothermal reaction time played important roles in the formation of cauliflower-like CoNi alloy assemblies. Magnetic hysteresis measurement at room temperature revealed that CoNi cauliflowers exhibited a ferromagnetic property with a coercivity of 178.3 Oe, which was much higher than those of CoNi micro/nanostructures reported in recent literatures. Our work may shed light on the design and preparation of other nanomaterials with similar complex morphology.
Keywords: Metals and alloys; Microstructures; CoNi; Solvothermal synthesis. 1. Introduction It is well know that the chemical and physical properties of nanomaterials are highly dependent on their size, morphology and dimensionality. Self-assembly of inorganic building blocks generally yields collective properties, depending on their spacing and high-order structures [1, 2]. Considerable attention has been paid in recent years to synthesize hierarchical micro/nanostructure, 1
which was constructed by primary building blocks such as nanoparticles, nanorods, and nanosheets. For example, cobalt microtrees assembled by many dendrites were synthesized via a hydrothermal method [3]. ɑ-Fe2O3 hierarchically hollow microspheres assembled with nanosheets were obtained via a surfactant-free solvothermal combined with precursor thermal transformation process [4]. Cu2S nano hollow-cactus arrays composed of micrometer-sized hollow-cactuses with nano-sized Cu2S “thorn” arrays over each cactus were self grown out over the electrodeposited Cu semi-sphere arrays film during a simple solid–gas reaction at room temperature [5]. Several methods have been developed to prepare hierarchical nanostructures, among which the hydrothermal/solvothermal method is regarded to be simple, low-cost, and large-scale production. CoNi is an important alloy with interesting properties, and its magnetic and catalytic properties can be tailored by changing the molar ratios [6]. Hence, it has received increasing attention in the past decade due to their potential applications in magnetic resonance imaging, biomedical micro-devices, catalysis, microwave absorber, ultra-high-density magnetic recording, and microelectromechanical systems (MEMS) [7-9]. CoNi alloy nanostructures with various morphologies such as nanoparticles [10], fibers [11], wires [12], and chains [13], have been achieved via different chemical synthesis. In addition, three-dimensional (3D) CoNi dendrites and microflowers were obtained in autoclave using deionized water and absolute ethanol as solvent, respectively [14, 15]. Surfactant or complex reagent is usually employed to tune the ordered alignment of primary nanobuilding blocks, and the self-assembly process is considered to be complicated. Therefore, it still remains challenging for rational design and controllable synthesis of CoNi alloy nanostructures with complex shapes. Herein, novel cauliflower-like CoNi microstructures were prepared through a solvothermal
2
method at 120 oC with the assistance of citric acid as complex agent. To our knowledge, this novel shape was seldom reported previously, and as for CoNi alloy is concerned, this is the first work about the synthesis of CoNi cauliflowers via a simple and inexpensive route. The cauliflower-like CoNi alloy may find applications in the field of catalysis, microwave absorbing, and so on. The parameters such as the amount of reducing agent and reaction time were investigated in details to reveal their influence on the final CoNi shapes, and the magnetic property was also simply studied.
2. Experimental procedure Materials and method: All reagents were of analytical grade and used without further purification. In a typical synthesis, 0.48 g of CoCl2·6H2O, 0.48 g of NiCl2·6H2O, and 0.6 g of citric acid were dissolved in 24 mL ethanol and water mixed solution (v/v=1:1) with stirring. Then 10 mL (6 M) of sodium hydroxide solution and 3 mL hydrazine hydrate (N2H4·H2O) were introduced orderly. The mixture was transferred into a Teflon-lined stainless steel autoclave, sealed, and maintained at 120 oC for 10 h. After reaction, the sample was collected, rinsed, and dried for characterization. Characterizations: XRD pattern was recorded on a Bruker D8 focus diffractometer with Cu Kα radiation (λ=1.5406 Å). SEM images were taken on a Hitachi SU-1500 scanning electron microscope. TEM image and SAED pattern were taken on a JEOL JEM2010 transmission electron microscope performed at an accelerating voltage of 200 kV. Room temperature magnetic measurement was conducted by a vibrating sample magnetometer (VSM, Lakeshore 7404) with a maximum magnetic field of 10 kOe.
3
3. Results and discussion The phase purity of the sample was determined by XRD technique, and the XRD pattern was shown in Fig. 1a. All the observed peaks could be assigned to face-centered cubic CoNi [13]. No peaks of impurities such as Ni(OH)2 and Co(OH)2 were detected, indicating that the reduction reaction should be rather completed to form CoNi alloy. The morphology of the obtained sample was investigated by SEM and TEM techniques. Fig. 1b displayed the typical SEM image with a panoramic view, from which a large amount of cauliflower-like structures with size of 2-3 µm were observed. The magnified view (Fig. 1c) clearly revealed that each CoNi cauliflower was constructed by many thick petals closely packed together. These petals were very stable, even ultrasonication with long time could not disassemble them into discrete particles, suggesting that the structures were actually integrated instead of aggregations of petals. TEM image revealed that the petals possessed thorn-like tip ends, and the SAED pattern captured from the tip position indicated the single crystalline nature (Fig. 1d). To synthesize micro-/nano-materials, the reducing reagents that were normally used included polyols, potassium borohydride, sodium hypophosphite, hydrazine hydrate, and etc. Among them, the hydrazine hydrate with high concentration possesses a very strong reducing capability, and was usually employed to synthesize metal or alloy nanostructures. In this work, it was found that the volume of N2H4·H2O had great influence on the final shape of CoNi alloy. A lot of urchin-like CoNi microstructures were produced when 1 mL of N2H4·H2O was used (Fig. 2a). The length of the assembled petals is about 1-2 µm, and such long petals grew into near platelets when the amount of reducing agent was increased to 2 mL (Fig.2b). At this time, the rudiment of
4
cauliflower-like shape could be seen. The reaction rate was relatively slow when less volume of N2H4·H2O was used, and slow reaction was beneficial for the anisotropic growth of nanomaterials. However, as the dosage was further increased to 3 mL, the reaction rate was greatly accelerated and perfect CoNi cauliflowers formed. In the synthetic procedure we used concentrated alkali solution. It can be seen from Equation (1) that high concentrations of hydrazine hydrate and hydroxyl ions would result in fast reaction rate. Interestingly, huge cauliflowers with size of ca. 5 µm aggregated by smaller ones were produced when we continuously increased the volume to 4 or 5 mL (Fig. 2c and d). Co2+ + Ni2+ + N2H4·H2O + OH- → CoNi +N2 + H2O
(1)
Time-dependent experiments were carried out in order to investigate the growth mechanism of such novel CoNi cauliflowers, and Fig. 3 displayed the SEM images of CoNi samples synthesized with 6, 8, 12, and 24 h. Only CoNi particles were generated when the reaction time was less than 6 h (Fig. 3a), and these particles showed a broad size distribution with diameter ranging from 0.1 to 0.8 µm. Cauliflower-like shape with petals growing on the surface formed when the solvothermal duration was extended to 8 or 10 h (Fig. 3b and 1b). Simultaneously, smaller particles could not be detected and it was believed that the growth process of CoNi cauliflowers was governed by traditional Ostwald ripening mechanism. CoNi particles within nanoscale were initially generated, and these crystals could serve as seeds for the growth of large microspheres at the expense of smaller CoNi alloy nanoparticles in the subsequent step. Assembled petals were produced gradually with the assistance of magnetic dipole reaction and the elimination of large surface energy between the CoNi particles. As the reaction proceeded for 12 h, we could see from Fig. 3c that platelet-like petals with high density were closely packed. It was very difficult to observe the
5
voids between the adjacent petals. The size of CoNi cauliflowers would continuously grow larger to 6 µm after 24 h of solvothermal treatment (Fig. 3d). Close observation would find that such huge cauliflower was composed of many smaller ones. When the Co2+ and Ni2+ cations were reduced by hydrazine hydrate, large quantities of nitrogen gas bubbles were produced as byproduct during this process. The released N2 gas may also have some influence on the movement of initially generated CoNi nuclei and subsequently formed particles in solution. Driven by the reduction of total energy, smaller cauliflowers had strong tendency to aggregate in different directions with the assistance of both N2 gas bubbles and magnetic dipolar reaction. The magnetic property of the cauliflower-like CoNi microstructures was investigated at room temperature, and the hysteresis loop shown in Fig.4 demonstrated the ferromagnetic nature. The saturation magnetization (Ms) of the sample is 110.6 emu/g, which is very close to that of bulk CoNi alloy (112 emu/g). The Ms value of magnetic microstructure is generally lower than that of bulk counterpart due to the inevitable surface oxidation. The coercivity (Hc) reached 178.3 Oe, which was remarkably higher than that of Co50Ni50 nanoparticles with average size of 25 nm (0 Oe) [10], handkerchief-like Co48Ni52 nanostructure (31.2 Oe) [16], and prickly CoNi microwires (119.1 Oe) [12]. It is well known that the coercivity of magnetic nanomaterial depends on their size, composition, and shape. Both magnetocrystalline anisotropy and shape anisotropy favor high coercivity. The CoNi cauliflowers obtained in this work possessed obvious shape anisotropy, which was expected to show enhanced coercivity.
4. Conclusions In summary, 3D cauliflower-like CoNi mictrostructures constructed by many petals were fabricated by a facile solvothermal method at 120 oC for 10 h. No surfactant or template was
6
employed during the overall synthesis. The morphology could be adjusted by the amount of hydrazine hydrate. Larger CoNi cauliflowers assembled by smaller ones were obtained with reaction proceeding on. Such novel CoNi cauliflowers displayed ferromagnetic behaviors at room temperature with an enhanced coercivity of 178.3 Oe.
Acknowledgments This work was supported by Youthful Science Foundation of North University of China (NUC), and Shanxi Scholarship Council of China.
References [1] Kim B, Tripp SL, Wei A. J Am Chem Soc 2001; 123: 7955-7956. [2] Pileni MP. J Phys Chem B 2001; 105: 3358-3371. [3] Chen HY, Zhou RH, Xu CJ, Liu YQ, Zhao GZ. Mater Lett 2013; 99: 1-4. [4] Xu JS, Zhu YJ. CrystEngComm 2011; 13: 5162-5169. [5] Li LQ, Yuan Y, Chen ZY, Liu ZF, Li M, Hong L, et al. Mater Lett 2013; 108: 300-303. [6] Talapatra S, Tang X, Padi M, Kim T, Vajtai R, Sastry GVS, et al. J Mater Sci 2009; 44: 2271-2275. [7] Fernando R, Jellinek J, Johnston RL. Chem Rev 2008; 108: 845-910. [8] Liu Q, Guo X, Wang T, Li Y, Shen W. Mater Lett 2010; 64: 1271-1274. [9] Kurlyandskaya GV, Bhagat SM. J Appl Phys 2006; 99: 104308. [10] Sharma S, Gajbhiye NS, Ningthoujam RS. J Colloid Interface Sci 2010; 351: 323-329. [11] Barakat NAM, Khalil KA, Mahmoud IH, Kanjwal MA, Sheikh FA, Kim HY. J Phys Chem C 2010; 114: 15589-15593. [12] Pan SL, An ZG, Zhang JJ, Song GZ. Mater Lett 2010; 64: 453-456. [13] Nie D, Xu CJ, Chen HY, Wang YJ, Li JW, Liu YQ. Mater Lett 2014; 131: 306-309. [14] Li H, Liao JY, Feng YF, Yu SW, Zhang XB, Jin Z. CrystEngComm 2012; 14: 2974-2980. [15] Wei XW, Zhou XM, Wu KL, Chen Y. CrystEngComm 2011; 13: 1328-1332.
7
[16] Wen M, Wang YF, Zhang F, Wu QS. J Phys Chem C 2009; 113: 5960-5966.
Figure captions Fig. 1. (a) XRD pattern, (b-c) SEM images, and (d) TEM image of the cauliflower-like CoNi microstructures. Inset of (d) is the SAED pattern. Fig. 2. SEM images of the CoNi alloy synthesized with different amount of hydrazine hydrate: (a) 1, (b) 2, (c) 4, and (d) 5 mL. Fig. 3. SEM images of the CoNi samples synthesized with solvothermal time of (a) 6, (b) 8, (c) 12, and (d) 24 h. Fig.4. Hysteresis loop of the cauliflower-like CoNi microstructures measured at room temperature (a) and the hysteresis loop at low field (b).
Fig. 1
8
Fig. 2
Fig. 3
Fig. 4
9
Highlights
Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property
·Cauliflower-like CoNi microstructures were synthesized via a solvothermal method ·No template or surfactant was used ·Citric acid served as complex agent ·Such CoNi microstructures showed significantly enhanced coercivity of 178.3 Oe Graphical Abstract
Novel cauliflower-like CoNi microstructures assembled by many thick petals were synthesized via a solvothermal process at 120 oC with the assistance of citric acid as complex agent. Such CoNi cauliflowers displayed ferromagnetic behaviors at room temperature with an enhanced coercivity of 178.3 Oe.
10
Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property Chunju Xu, Dan Nie, Huiyu Chen, Yujie Wang, Yaqing Liu
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02196-X http://dx.doi.org/10.1016/j.matlet.2014.12.030 MLBLUE18181
To appear in:
Materials Letters
Received date: 1 October 2014 Accepted date: 3 December 2014 Cite this article as: Chunju Xu, Dan Nie, Huiyu Chen, Yujie Wang, Yaqing Liu, Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.12.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property Chunju Xu*, Dan Nie, Huiyu Chen, Yujie Wang, Yaqing Liu School of Materials Science and Engineering, North University of China, Taiyuan 030051, China *
Corresponding author. Tel. /fax: +86-351-3559669, E-mail: [email protected] (C. Xu),
[email protected] (H. Chen), [email protected] (Y. Liu)
Abstract: Novel cauliflower-like CoNi microstructures assembled by many thick petals were synthesized via a solvothermal process at 120 oC with the assistance of citric acid as complex agent. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM) techniques. It was found that the amount of reducing agent and solvothermal reaction time played important roles in the formation of cauliflower-like CoNi alloy assemblies. Magnetic hysteresis measurement at room temperature revealed that CoNi cauliflowers exhibited a ferromagnetic property with a coercivity of 178.3 Oe, which was much higher than those of CoNi micro/nanostructures reported in recent literatures. Our work may shed light on the design and preparation of other nanomaterials with similar complex morphology.
Keywords: Metals and alloys; Microstructures; CoNi; Solvothermal synthesis. 1. Introduction It is well know that the chemical and physical properties of nanomaterials are highly dependent on their size, morphology and dimensionality. Self-assembly of inorganic building blocks generally yields collective properties, depending on their spacing and high-order structures [1, 2]. Considerable attention has been paid in recent years to synthesize hierarchical micro/nanostructure, 1
which was constructed by primary building blocks such as nanoparticles, nanorods, and nanosheets. For example, cobalt microtrees assembled by many dendrites were synthesized via a hydrothermal method [3]. ɑ-Fe2O3 hierarchically hollow microspheres assembled with nanosheets were obtained via a surfactant-free solvothermal combined with precursor thermal transformation process [4]. Cu2S nano hollow-cactus arrays composed of micrometer-sized hollow-cactuses with nano-sized Cu2S “thorn” arrays over each cactus were self grown out over the electrodeposited Cu semi-sphere arrays film during a simple solid–gas reaction at room temperature [5]. Several methods have been developed to prepare hierarchical nanostructures, among which the hydrothermal/solvothermal method is regarded to be simple, low-cost, and large-scale production. CoNi is an important alloy with interesting properties, and its magnetic and catalytic properties can be tailored by changing the molar ratios [6]. Hence, it has received increasing attention in the past decade due to their potential applications in magnetic resonance imaging, biomedical micro-devices, catalysis, microwave absorber, ultra-high-density magnetic recording, and microelectromechanical systems (MEMS) [7-9]. CoNi alloy nanostructures with various morphologies such as nanoparticles [10], fibers [11], wires [12], and chains [13], have been achieved via different chemical synthesis. In addition, three-dimensional (3D) CoNi dendrites and microflowers were obtained in autoclave using deionized water and absolute ethanol as solvent, respectively [14, 15]. Surfactant or complex reagent is usually employed to tune the ordered alignment of primary nanobuilding blocks, and the self-assembly process is considered to be complicated. Therefore, it still remains challenging for rational design and controllable synthesis of CoNi alloy nanostructures with complex shapes. Herein, novel cauliflower-like CoNi microstructures were prepared through a solvothermal
2
method at 120 oC with the assistance of citric acid as complex agent. To our knowledge, this novel shape was seldom reported previously, and as for CoNi alloy is concerned, this is the first work about the synthesis of CoNi cauliflowers via a simple and inexpensive route. The cauliflower-like CoNi alloy may find applications in the field of catalysis, microwave absorbing, and so on. The parameters such as the amount of reducing agent and reaction time were investigated in details to reveal their influence on the final CoNi shapes, and the magnetic property was also simply studied.
2. Experimental procedure Materials and method: All reagents were of analytical grade and used without further purification. In a typical synthesis, 0.48 g of CoCl2·6H2O, 0.48 g of NiCl2·6H2O, and 0.6 g of citric acid were dissolved in 24 mL ethanol and water mixed solution (v/v=1:1) with stirring. Then 10 mL (6 M) of sodium hydroxide solution and 3 mL hydrazine hydrate (N2H4·H2O) were introduced orderly. The mixture was transferred into a Teflon-lined stainless steel autoclave, sealed, and maintained at 120 oC for 10 h. After reaction, the sample was collected, rinsed, and dried for characterization. Characterizations: XRD pattern was recorded on a Bruker D8 focus diffractometer with Cu Kα radiation (λ=1.5406 Å). SEM images were taken on a Hitachi SU-1500 scanning electron microscope. TEM image and SAED pattern were taken on a JEOL JEM2010 transmission electron microscope performed at an accelerating voltage of 200 kV. Room temperature magnetic measurement was conducted by a vibrating sample magnetometer (VSM, Lakeshore 7404) with a maximum magnetic field of 10 kOe.
3
3. Results and discussion The phase purity of the sample was determined by XRD technique, and the XRD pattern was shown in Fig. 1a. All the observed peaks could be assigned to face-centered cubic CoNi [13]. No peaks of impurities such as Ni(OH)2 and Co(OH)2 were detected, indicating that the reduction reaction should be rather completed to form CoNi alloy. The morphology of the obtained sample was investigated by SEM and TEM techniques. Fig. 1b displayed the typical SEM image with a panoramic view, from which a large amount of cauliflower-like structures with size of 2-3 µm were observed. The magnified view (Fig. 1c) clearly revealed that each CoNi cauliflower was constructed by many thick petals closely packed together. These petals were very stable, even ultrasonication with long time could not disassemble them into discrete particles, suggesting that the structures were actually integrated instead of aggregations of petals. TEM image revealed that the petals possessed thorn-like tip ends, and the SAED pattern captured from the tip position indicated the single crystalline nature (Fig. 1d). To synthesize micro-/nano-materials, the reducing reagents that were normally used included polyols, potassium borohydride, sodium hypophosphite, hydrazine hydrate, and etc. Among them, the hydrazine hydrate with high concentration possesses a very strong reducing capability, and was usually employed to synthesize metal or alloy nanostructures. In this work, it was found that the volume of N2H4·H2O had great influence on the final shape of CoNi alloy. A lot of urchin-like CoNi microstructures were produced when 1 mL of N2H4·H2O was used (Fig. 2a). The length of the assembled petals is about 1-2 µm, and such long petals grew into near platelets when the amount of reducing agent was increased to 2 mL (Fig.2b). At this time, the rudiment of
4
cauliflower-like shape could be seen. The reaction rate was relatively slow when less volume of N2H4·H2O was used, and slow reaction was beneficial for the anisotropic growth of nanomaterials. However, as the dosage was further increased to 3 mL, the reaction rate was greatly accelerated and perfect CoNi cauliflowers formed. In the synthetic procedure we used concentrated alkali solution. It can be seen from Equation (1) that high concentrations of hydrazine hydrate and hydroxyl ions would result in fast reaction rate. Interestingly, huge cauliflowers with size of ca. 5 µm aggregated by smaller ones were produced when we continuously increased the volume to 4 or 5 mL (Fig. 2c and d). Co2+ + Ni2+ + N2H4·H2O + OH- → CoNi +N2 + H2O
(1)
Time-dependent experiments were carried out in order to investigate the growth mechanism of such novel CoNi cauliflowers, and Fig. 3 displayed the SEM images of CoNi samples synthesized with 6, 8, 12, and 24 h. Only CoNi particles were generated when the reaction time was less than 6 h (Fig. 3a), and these particles showed a broad size distribution with diameter ranging from 0.1 to 0.8 µm. Cauliflower-like shape with petals growing on the surface formed when the solvothermal duration was extended to 8 or 10 h (Fig. 3b and 1b). Simultaneously, smaller particles could not be detected and it was believed that the growth process of CoNi cauliflowers was governed by traditional Ostwald ripening mechanism. CoNi particles within nanoscale were initially generated, and these crystals could serve as seeds for the growth of large microspheres at the expense of smaller CoNi alloy nanoparticles in the subsequent step. Assembled petals were produced gradually with the assistance of magnetic dipole reaction and the elimination of large surface energy between the CoNi particles. As the reaction proceeded for 12 h, we could see from Fig. 3c that platelet-like petals with high density were closely packed. It was very difficult to observe the
5
voids between the adjacent petals. The size of CoNi cauliflowers would continuously grow larger to 6 µm after 24 h of solvothermal treatment (Fig. 3d). Close observation would find that such huge cauliflower was composed of many smaller ones. When the Co2+ and Ni2+ cations were reduced by hydrazine hydrate, large quantities of nitrogen gas bubbles were produced as byproduct during this process. The released N2 gas may also have some influence on the movement of initially generated CoNi nuclei and subsequently formed particles in solution. Driven by the reduction of total energy, smaller cauliflowers had strong tendency to aggregate in different directions with the assistance of both N2 gas bubbles and magnetic dipolar reaction. The magnetic property of the cauliflower-like CoNi microstructures was investigated at room temperature, and the hysteresis loop shown in Fig.4 demonstrated the ferromagnetic nature. The saturation magnetization (Ms) of the sample is 110.6 emu/g, which is very close to that of bulk CoNi alloy (112 emu/g). The Ms value of magnetic microstructure is generally lower than that of bulk counterpart due to the inevitable surface oxidation. The coercivity (Hc) reached 178.3 Oe, which was remarkably higher than that of Co50Ni50 nanoparticles with average size of 25 nm (0 Oe) [10], handkerchief-like Co48Ni52 nanostructure (31.2 Oe) [16], and prickly CoNi microwires (119.1 Oe) [12]. It is well known that the coercivity of magnetic nanomaterial depends on their size, composition, and shape. Both magnetocrystalline anisotropy and shape anisotropy favor high coercivity. The CoNi cauliflowers obtained in this work possessed obvious shape anisotropy, which was expected to show enhanced coercivity.
4. Conclusions In summary, 3D cauliflower-like CoNi mictrostructures constructed by many petals were fabricated by a facile solvothermal method at 120 oC for 10 h. No surfactant or template was
6
employed during the overall synthesis. The morphology could be adjusted by the amount of hydrazine hydrate. Larger CoNi cauliflowers assembled by smaller ones were obtained with reaction proceeding on. Such novel CoNi cauliflowers displayed ferromagnetic behaviors at room temperature with an enhanced coercivity of 178.3 Oe.
Acknowledgments This work was supported by Youthful Science Foundation of North University of China (NUC), and Shanxi Scholarship Council of China.
References [1] Kim B, Tripp SL, Wei A. J Am Chem Soc 2001; 123: 7955-7956. [2] Pileni MP. J Phys Chem B 2001; 105: 3358-3371. [3] Chen HY, Zhou RH, Xu CJ, Liu YQ, Zhao GZ. Mater Lett 2013; 99: 1-4. [4] Xu JS, Zhu YJ. CrystEngComm 2011; 13: 5162-5169. [5] Li LQ, Yuan Y, Chen ZY, Liu ZF, Li M, Hong L, et al. Mater Lett 2013; 108: 300-303. [6] Talapatra S, Tang X, Padi M, Kim T, Vajtai R, Sastry GVS, et al. J Mater Sci 2009; 44: 2271-2275. [7] Fernando R, Jellinek J, Johnston RL. Chem Rev 2008; 108: 845-910. [8] Liu Q, Guo X, Wang T, Li Y, Shen W. Mater Lett 2010; 64: 1271-1274. [9] Kurlyandskaya GV, Bhagat SM. J Appl Phys 2006; 99: 104308. [10] Sharma S, Gajbhiye NS, Ningthoujam RS. J Colloid Interface Sci 2010; 351: 323-329. [11] Barakat NAM, Khalil KA, Mahmoud IH, Kanjwal MA, Sheikh FA, Kim HY. J Phys Chem C 2010; 114: 15589-15593. [12] Pan SL, An ZG, Zhang JJ, Song GZ. Mater Lett 2010; 64: 453-456. [13] Nie D, Xu CJ, Chen HY, Wang YJ, Li JW, Liu YQ. Mater Lett 2014; 131: 306-309. [14] Li H, Liao JY, Feng YF, Yu SW, Zhang XB, Jin Z. CrystEngComm 2012; 14: 2974-2980. [15] Wei XW, Zhou XM, Wu KL, Chen Y. CrystEngComm 2011; 13: 1328-1332.
7
[16] Wen M, Wang YF, Zhang F, Wu QS. J Phys Chem C 2009; 113: 5960-5966.
Figure captions Fig. 1. (a) XRD pattern, (b-c) SEM images, and (d) TEM image of the cauliflower-like CoNi microstructures. Inset of (d) is the SAED pattern. Fig. 2. SEM images of the CoNi alloy synthesized with different amount of hydrazine hydrate: (a) 1, (b) 2, (c) 4, and (d) 5 mL. Fig. 3. SEM images of the CoNi samples synthesized with solvothermal time of (a) 6, (b) 8, (c) 12, and (d) 24 h. Fig.4. Hysteresis loop of the cauliflower-like CoNi microstructures measured at room temperature (a) and the hysteresis loop at low field (b).
Fig. 1
8
Fig. 2
Fig. 3
Fig. 4
9
Highlights
Solvothermal synthesis of cauliflower-like CoNi microstructures with enhanced magnetic property
·Cauliflower-like CoNi microstructures were synthesized via a solvothermal method ·No template or surfactant was used ·Citric acid served as complex agent ·Such CoNi microstructures showed significantly enhanced coercivity of 178.3 Oe Graphical Abstract
Novel cauliflower-like CoNi microstructures assembled by many thick petals were synthesized via a solvothermal process at 120 oC with the assistance of citric acid as complex agent. Such CoNi cauliflowers displayed ferromagnetic behaviors at room temperature with an enhanced coercivity of 178.3 Oe.
10