Size dependence of the magnetic properties of Ni nanoparticles prepared by thermal decomposition method
© He et al.; licensee Springer. 2013
Received: 13 July 2013
Accepted: 25 September 2013
Published: 28 October 2013
By means of thermal decomposition, we prepared single-phase spherical Ni nanoparticles (23 to 114 nm in diameter) that are face-centered cubic in structure. The magnetic properties of the Ni nanoparticles were experimentally as well as theoretically investigated as a function of particle size. By means of thermogravimetric/differential thermal analysis, the Curie temperature TC of the 23-, 45-, 80-, and 114-nm Ni particles was found to be 335°C, 346°C, 351°C, and 354°C, respectively. Based on the size-and-shape dependence model of cohesive energy, a theoretical model is proposed to explain the size dependence of TC. The measurement of magnetic hysteresis loop reveals that the saturation magnetization MS and remanent magnetization increase and the coercivity decreases monotonously with increasing particle size, indicating a distinct size effect. By adopting a simplified theoretical model, we obtained MS values that are in good agreement with the experimental ones. Furthermore, with increase of surface-to-volume ratio of Ni nanoparticles due to decrease of particle size, there is increase of the percentage of magnetically inactive layer.
KeywordsSize dependence Curie temperature Cohesive energy Magnetically inactive layer
The transition metal nickel shows distinct magnetic and catalytic properties [1, 2]. In nanostructure, Ni has great application potential in fields such as pharmaceutical synthesis , magnetic biocatalysis , biomolecular separation , and biosensor . In the literatures, there are reports on the preparation and properties of novel Ni nanomaterials such as sea urchin-like Ni nanoparticles , tetragonal Ni nanoparticles , hexagonal close-packed (hcp) Ni nanoparticles , conical Ni nanorods , triangular and hexagonal Ni nanosheets , and Ni nanochains . It is known that the performance of technological devices is greatly influenced by the purity, structure, shape, and size of Ni nanoparticles. Hence, it is of great significance to prepare high-quality Ni nanomaterials of specificity using convenient and low-cost methods.
For the fabrication of Ni nanoparticles, methods such as sputtering [13, 14], solution glow discharge , pulsed laser ablation , reversed micelles , thermal decomposition [17–20], and wet chemical reduction [7, 21, 22] are used. Among them, the ones based on thermal decomposition are preferred. The single-step process is facile, environment-benign, inexpensive, and reproducible, yielding high-quality Ni powders that can be controlled in terms of structure, morphology, size, and size distribution. It should be pointed out that pure Ni nanoparticles are difficult to prepare because they are easily oxidized. One of the ways to evade the formation of oxide or hydroxide is to carry out the pyrolysis process in organic media. For example, relatively large Ni nanoparticles were prepared through thermal decomposition of Ni(ac)2 4H2O in oleylamine in the presence of 1-adamantane carboxylic acid (ACA) and trioctylphosphine oxide (TOPO) . Furthermore, structure-controlled Ni nanoparticles were prepared via thermal decomposition of Ni(ac)2 4H2O in long-chain amines that acted both as solvent and reducing agent . More interestingly, trigonal Ni nanoparticles were prepared by reacting Ni(COD)2 in tetrahydrofuran with tetra-n-octylammonium carboxylates (as reductant and stabilizer) . Despite the synthesis of superior Ni nanomaterials through the pyrolysis of organometallic salts in organic media has been known for quite some time, the synthesis of Ni nanoparticles 20 to 100 nm in size has only been reported lately.
Bulk Ni exhibits a rock-salt structure and is ferromagnetic (Curie temperature of bulk Ni TCb = 358°C) and electroconductive . In contrast to the bulk counterparts, Ni nanoparticles show magnetic parameters (such as Curie temperature TC, saturation magnetization MS, and coercivity HC) that vary with particle size, usually in a non-linear fashion. Despite the endless number of reports on magnetic studies of Ni nanoparticles [1, 10, 18, 26–30], the influence of particle size on the magnetic properties has not been systematically studied. It is envisaged that the application of Ni nanoparticles can be widened once the intrigue relationship between magnetic properties and particle size of Ni nanomaterials can be delineated.
Herein, we report a facile and reproducible process for large-scale synthesis of face-centered cubic (fcc) Ni nanoparticles (spherical and 23 to 114 nm in diameter). We controlled the size of Ni nanoparticles by regulating the synthesis temperature. We studied the influence of particle size on Curie temperature, saturation magnetization, and coercivity. We establish the size dependence of magnetic properties based on experimental as well as theoretical results, and comment on the critical size of Ni nanoparticles and percentage of magnetically inactive layer.
Size and magnetic parameters for the Ni-particle samples obtained at different temperatures
X-ray diffraction (XRD) patterns of all the samples were obtained by using an X-ray diffractometer (Philips X’pert, Philips, Amsterdam, The Netherlands) with Cu Kα radiation. To examine the morphology and particle sizes, a field-emission scanning electron microscope (SEM) (Hitachi S-4800, Hitachi Ltd., Chiyoda-ku, Japan) was used. High-resolution transmission electron microscope (TEM) image and selected-area electron diffraction (SAED) pattern were obtained from a FEI Tecnai G2-F30 instrument (FEI, Hillsboro, OR, USA) operated at accelerating voltage of 300 kV. In the TEM experiment, the incident electron beam was along the direction perpendicular to the sample. The sample was transferred onto a copper grid by solution dripping with the sample powder under sonication in ethanol. The Curie temperature was measured using a thermogravimetric (TG)/differential thermal analysis (DTA) instrument (Scinco STA-1500, Scinco Co. Ltd., Seoul, South Korea) equipped with a piece of Nd2Fe14B permanent magnet during sample heating (up to 500°C) in argon at a rate of 10°C/min. For comparing, the TG/DTA measurement of a bulk Ni sample (nickel sphere) was conducted under the same conditions. Magnetic measurements were carried out with a vibrating sample magnetometer (VSM) (Lake Shore 7300, Lake Shore Cryotronics, Inc., Westerville, OH, USA). The hysteresis was recorded for powder samples in gelatin capsule, and the hysteresis loops were obtained in a magnetic field up to ±10 kOe. Magnetization versus temperature (M-T) curves were measured in the range 30°C to 400°C using an applied magnetic field of 5 kOe.
Results and discussion
The variation of TC as a function of particle size calculated using Equation (4) is compared with the TG/DTA results. Specifically, the absolute error between the two results has been given by the error bars in Figure 5, and the obtained relative errors are less than 1%. Through this, we can understand that TC decreases with decrease of particle size D, and there is a good match between theoretical and experimental results. In other words, the model represents well the size dependence of the Curie temperature of Ni nanoparticles.
where MSb denotes the saturation magnetization of the corresponding bulk materials, t is the thickness of magnetically inactive layer, and D is the diameter of nanoparticles. Considering our samples of spherical Ni nanoparticles (MSb = 54.4 emu/g) , the percentage of magnetically inactive layer t/D can be calculated using Equation (5). The t/D values for all samples are listed in Table 1. Clearly, as the surface-to-volume ratio of Ni nanoparticles increases with decreasing particle size, the percentage of magnetically inactive layer increases too. In fact, as the particle size reduced from 114 to 23 nm, the t/D value of Ni nanoparticles slightly increases from 2.03% to 4.26%. This slight change suggests that the dead layer theory cannot satisfactorily explain the size dependence of saturation magnetization [39–41].
Substituting the parameters mentioned above into Equation (10), the Dc value of spherical Ni nanoparticles is 0.90 nm. As shown in the final column of Table 1, the percentage of magnetically inactive layer is 4.26%, 3.09%, 2.43%, and 2.03% for spherical Ni nanoparticles with particle sizes of 23, 45, 80, and 114 nm, respectively. Using Equation (5), the corresponding thickness t of magnetically inactive layer is 0.98, 1.39, 1.94, and 2.31 nm for the four Ni-nanoparticle samples. Herein, the critical size (Dc = 0.90 nm) and t values are in the same order of magnitude, but the Dc value is significantly smaller than the four t values. Thus, the size-dependent cohesive energy model under critical condition is consistent with the magnetically dead layer theory.
We systematically studied the size-dependent magnetic properties of spherical Ni nanoparticles in terms of experimental measurement and theoretical calculation. Our results show that the Curie temperature, saturation magnetization, and remanent magnetization increase whereas the coercivity decreases monotonously with the increase of particle size. According to the size-dependent cohesive energy model, a simplified theoretical calculation can be applied to analyze the size dependence of Curie temperature and saturation magnetization. The results of calculation are in good agreement with the experimental results. Under critical condition of critical size Dc = 0.90 nm, the size dependence of magnetization obtained by cohesive energy model is consistent with the analysis of magnetically dead layer theory. With the particle size decreasing, the surface-to-volume ratio of Ni nanoparticles increases and the percentage of magnetically inactive layer increases as well.
The authors greatly acknowledge finical support of the National Natural Science Foundation of China (grant no. 11174132), the National Key Project for Basic Research (grant nos. 2011CB922102 and 2012CB932304), and PAPD, People's Republic of China.
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