Magnetic alloy micro/nanostructures with controllable size and morphology have drawn intensive attentiondue to their interesting physicochemical properties and potential applications in micro/nanodevices. In thisletter, CoNi hollow submicrospheres consisting of CoNi nanoplatelets with a thickness of ca. 10 nm havebeen successfully synthesized via a facile wet-chemical approach free of any template or surfactant. Scanningelectron microscope (SEM) and transmission electron microscope (TEM) images indicate that the diameterand shell thickness of the as-prepared hollow submicrosphere are ca. 600 nm and 200 nm, respectively.Elemental maps demonstrate that Co and Ni elements are distributed homogeneously in the CoNi hollowsubmicrosphere. Magnetic measurement reveals that the hollow submicrospheres display ferromagneticbehaviour with a coercivity of 109.5 Oe.
Recently, CoNi bimetals have received increasing attention due totheir potential applications in ultra-high-density magnetic recordingand magnetic resonance imaging [1], biomedical microdevices [2],catalysis [3], microwave absorber [4] and microelectromechanicalsystems (MEMS) [5]. In contrast to elemental Co or Ni nanomaterials,CoNi nanoalloys aremore interesting in that some physical and chemicalproperties, such as magnetic properties and catalytic properties, can betailored by changing their molar ratios [6,7]. Moreover, recent investiga-tions revealed CoNi nanoalloys exhibited better magnetic properties andcatalytic performance than themonometallic counterparts inmany cases[3,8]. Owing to their size-, morphology- and composition-dependentproperties, CoNi nanomaterials with controllable sizes, shapes, dimen-sions and different molar ratios, are highly desirable for differentapplications. Over the past several years, many approaches have beendeveloped to fabricate differently-shaped CoNi micro/nanostructureswith various dimensionality, including zero-dimensional (0-D) CoNinanoparticles [9,10], one-dimensional (1-D) CoNi nanowires [3,11],CoNi nanofiber [8], chain-like CoNi assemblies [12,13], and CoNi nanor-ings [14], two-dimensional (2-D) CoNi film [2] and handkerchief-likeCoNi nanostructure [15]. Although fabrication of three-dimensional(3-D) metallic Co and Ni micro/nanostructures has been documentedin the literature, preparation of 3-D CoNi alloy micro/nanostructureswith regular morphology is still a challenge and has been rarelyreported. This is partly due to the different crystal growth characteristic
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of Co and Ni, which will result in difficulty in the formation of wellcontrolled 3-D architecture with homogeneous element distribution.
In this study, we have presented a wet-chemical synthesis of hollowCoNi (47:53 in atomic ratio) alloy submicrospheres with homogeneouselement distribution consisting of CoNi nanoplatelets by a facilemethodfree of any template or surfactant. To the best of our knowledge, suchCoNi alloy microstructure has not been previously reported in theliterature.
2. Experimental section
The CoNi submicrospheres were synthesized as follows: 2 mmolCoCl2·6H2O and 2 mmol NiCl2·6H2O were firstly dissolved in 34 mLof deionized water, and then 20 mL NaOH solution (15 M) and 25 mLNaH2PO2 (2 M) solutionwere dropped in sequence into the above solu-tion under strong stirring. Afterwards, the obtained suspension wastransferred into a Teflon-lined stainless steel autoclave, which waskept in an air oven at 180 °C for 6 h. After the reaction, the resultingproducts were collected and rinsed with deionized water and acetone,and finally dried in a vacuum oven at 60 °C for 8 h.
X-ray diffraction (XRD) patterns were recorded using a ShimadzuXD-3A X-ray diffractometer with CuKa radiation (1.5406 Å). Themorphology and elements of the sample were studied using a HitachiS-4800 field emission scanning electron microscope (FE-SEM) equippedwithX-ray energy-dispersive spectrometer (EDS). Transmission electronmicroscopy (TEM) measurement was performed using a Teccai G2S-TWIN transmission electron microscope. X-ray photoelectronspectrum (XPS)was obtained on a VG ESCALAB 210 X-ray photoelectronspectrometer with Al Kα radiation. The magnetic hysteresis loop was
Fig. 1. SEM images (a, b, c) and TEM image (d) of the CoNi submicrospheres.
347H. Li et al. / Materials Letters 67 (2012) 346–348
recorded with a Quantum Design MPMSXL-7 quantum interferencedevice magnetometer.
3. Results and discussion
The low-magnification SEM image, as shown in Fig. 1a, indicatesthat the obtained products are submicrospheres with a uniformdiameter of ca. 600 nm. Fig. 1b displays the morphology of a singleCoNi submicrosphere with beautiful geometric configuration, whichis composed of plentiful nanoplatelets with radiating arrangementfrom the centre of the submicrosphere. Fig. 1c clearly reveals thesenanoplatelets with a uniform thickness of ca. 10 nm aggregate toform CoNi submicrosphere. The morphology of the product wasfurther determined by TEM and the typical TEM image (Fig. 1d) indi-cates that the as-prepared submicospheres are hollow structure withthe shell thickness of ca. 200 nm.
The crystal structure of the sample has been determined by XRDand the result is shown in Fig. 2a. Three peaks at 2θ=44.3°, 51.6°and 76.2° can be observed in the XRD pattern, corresponding to the(111), (200) and (220) planes of face-centered cubic (fcc) phaseCoNi alloy [5,12]. Fig. 2b is the XPS survey spectrum of the sample,
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Fig. 2. XRD pattern (a) and XPS spectrum (b) of the CoNi submicrospheres. The
showing the characteristic peaks of O, Co, Ni and C. The high-resolution XPS spectra of Co2p3/2 and Ni2p3/2 region inserted inFig. 2b show two main peaks centered at ca. 780.3 eV and 855.7 eV,respectively, which can be indexed to the Co2+ and Ni2+ oxidationstate [16,17], suggesting the surface oxidation of the sample.
To illuminate the distribution of Co and Ni elements in the as-prepared submicrospheres, the SEM image and the correspondingelemental maps of a single submicrosphere are shown in Fig. 3a, band c. The results demonstrate a very homogeneous element distributionin the CoNi submicrosphere, which implies that the sample is alloy ratherthan the mixture of Co and Ni submicrospheres. Typical EDS pattern ofthe submicrosphere in Fig. 3d indicates that the atomic ratio of Co/Ni isca. 47:53, which is very close to the ratio of corresponding cationsin the starting solution. In addition, a tiny amount of oxygen (ca.0.93 wt.%) is detectedwith EDS,which further verifies the slight surfaceoxidation of the sample.
Considering that the reactions take place in strongly basic solutionin our study, Co(OH)2 and Ni(OH)2 are supposed to be produced atearly stage of the synthesis process. Due to their intrinsic lamellarstructures, Co(OH)2 and Ni(OH)2 tend to grow into Co(OH)2 and Ni(OH)2 nanoplatelets, which will separately change into Co and Ni
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Co2p3/2: 780.3eV
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insets show the high-resolution XPS spectra of Co2p3/2 and Ni2p3/2 region.
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Fig. 3. SEM image (a), corresponding element maps (b, c) and the typical EDS pattern(d) of a single CoNi submicrosphere.
348 H. Li et al. / Materials Letters 67 (2012) 346–348
nanoplatelets during reduction process under hydrothermal condition[18]. Thus, it is believed that the co-reduction of Co(OH)2 and Ni(OH)2by sodiumhypophosphite will lead to the formation of CoNi alloy nano-platelets, which will therefore self-assemble into 3-D submicrospheresto lower the energy of the system, resulting from the high surface energyof the nanoplatelets and the strong magnetic interaction among them.Note in Fig. 1c that the nanoplatelets in the submicrosphere are randomlyarranged and their typical diameter is above 100 nm, therefore, it willbe difficult for these nanoplatelets with relatively large diameters toaggregate to form a solid core due to their steric hindrance. So, toachieve a balance between the magnetic interaction and the sterichindrance of nanoplatelets, these nanoplatelets gather together toform an outer shell with some empty space inside the submicrospheres,resulting in the formation of hollow microstructures.
To determine themagnetic properties of the CoNi submicrospheres,magnetic measurement was carried out at room temperature (300 K)and the result is shown in Fig. 4. The saturation magnetization (Ms) ofthe sample is 77.8 emu/g. The Ms of bulk Co50Ni50 is 997 emu/cm3
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Fig. 4. Magnetic hysteresis loop of the CoNi submicrospheres measured at roomtemperature. The inset shows the low field part of the hysteresis loop.
[19], which amounts to ca. 112 emu/g considering the density of bulkCo50Ni50 (8.9 g/cm3[20]). Ms of the sample is remarkably lower thanthat of the bulk CoNi alloy, which may be attributed to the surfaceoxidation and the large surface to volume ratio of the nanostructures[21]. The coercivity of the sample is 109.5 Oe, which is higher than thatof Co50Ni50 nanoparticles with an average size of 25 nm (0 Oe [22]),handkerchief-like Co48Ni52 nanostructure (31.18 Oe [15]), flower-likeNi48Co52 spheres (84.27 Oe [21]) and hierarchical spheres (52.71 Oe[21]). It is known that a nonspherical specimen can possess shapeanisotropy and a small departure from sphericity in shape would resultin the significant increase in coercivity [23]. As far as our sampleconcerned, the CoNi nanoplatelets in the submicrospheres are predictedto produce high coercivity due to their remarkable shape anisotropy.Besides, the high coercivity can also be attributed to the hollowstructure of the submicrospheres [24]. Since large aspect ratio andinterior-cavity size are favourable for high coercivity, further work cantherefore be done to improve the coercivity of CoNi submicrospheresby increasing the diameter/thickness ratio of the CoNi nanoplateletsand decreasing the shell thickness of the submicrospheres duringsynthesis process.
4. Conclusions
In summary, CoNi hollow submicrospheres composed of CoNinanoplatelets have been successfully synthesized by a facile wet-chemical method. Elemental maps demonstrate the uniform distribu-tion of Co and Ni elements in the as-prepared products. Magneticmeasurement reveals that the hollow CoNi alloy submicrospheresare ferromagnetic with coercivity of 109.5 Oe. Due to the novel con-figuration and relatively high coercivity, these submicrospheres areexpected to exhibit some interesting physicochemical propertiesand have important applications in micro/nanodevices.
Acknowledgements
This work was supported by the National Natural Science Foundationof China (No. 51001052), the Natural Science Foundation of GuangdongProvince (Nos. 10451601501005315 and 10451601501006188) andThe Science & Technology project of Huizhou City (2010B020008017).
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