Synthesis, Characterization, and Magnetic Properties - American

J. Phys. Chem. C 2008, 112, 18793–18797

18793

Monodispersed Nickel Nanoparticles with Tunable Phase and Size: Synthesis, Characterization, and Magnetic Properties Hanyu Wang, Xiuling Jiao,* and Dairong Chen Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100 People’s Republic of China ReceiVed: June 25, 2008; ReVised Manuscript ReceiVed: September 17, 2008

A facile thermal-decomposition route to the monodispersed Ni nanoparticles was introduced, in which the cheap and low toxic nickel acetate was used as precursor and the surfactant hexadecylamine was used as solvent, reducing agent, and stabilizing agent. The product was characterized in detail by using X-ray diffraction, transmission electron microscope (TEM), high-resolution TEM, thermal- gravimetric, and Fourier transform infrared techniques. The effects of reaction parameters such as reaction temperature, time, and the molar ratio of reagents on the phase and size of Ni nanoparticles were investigated, and the transformation of facecentered cubic- (fcc) to hexagonal close-packed-phase was also discussed. Furthermore, the magnetic properties of fcc-Ni nanoparticles with tunable size were measured and compared by superconducting quantum interference device magnetometer. 1. Introduction Magnetic nanomaterials exhibit a series of interesting properties such as optical,1a photoelectrical,1c giant magnetoresistance,1b superparamagnetism, large coercivities,1d high Curie temperature, and low saturation magnetization compared with the corresponding bulk materials. Thus, the magnetic nanostructures have attracted much attention for their potential applications in sensors, data storage, ferro-fluids, biomedicine, and so on.2 In particular, the one-dimensional nickel (Ni) nanostructures have further expanded to the applicable fields such as the hydrogen storage of Ni nanotubes array and memory devices.3 For the magnetic nanoparticles, their properties are sensitive to synthetic conditions, particle size, and shape, so the development of a facile synthetic method toward high quality nanocrystals with uniform size and shape appears to be of key importance for the exploration of new research and application fields. To prepare magnetic nanostructures with characteristic nanoscale dimensions, many fabrication methods including sonochemical reaction,4 sol-gel technique,5 electrochemical process,6 host template,7 microemulsion procedure,8 thermal decomposition of organic complexes (precursor techniques),9 microwave,10 and hydrothermal or solvothermal route have been developed.11 For the synthesis of Ni nanostructures, the thermal decomposition of organic complexes exhibited some superior advantages over other methods for its control in the uniform morphology and size, in which the stabilizing agent was extensively applied to modulate the growth dynamics of nanocrystals and prevent the nanoparticles from aggregating. For example, Cordente et al. synthesized Ni nanorods in THF solution in the presence of hexadecylamine (HDA) or trio-ctylphosphine oxide (TOPO),2c and Hyeon’s group prepared the monodispersed Ni nanoparticles with sizes of 2, 5, and 7 nm from the thermal decomposition of Ni-alkylamine complexes.6f A typical synthesis system for this approach consists of three components: precursor, surfactant, and solvent. The precursor might be organometallic compound, * Corresponding author. E-mail: [email protected]. Phone: 86-053188364280. Fax: 86-0531-88364281.

metal complex, or inorganic compound, and the use of expensive organometallic compounds made the synthesis relatively complicated and noxious. The size of Ni nanoparticles shows significant effect on their catalytic, optical, and magnetic properties,12 and many researches have focused on the control synthesis of Ni nanoparticles with different sizes. Although great success has been achieved in the fabrication of monodispersed Ni nanoparticles,13 the process is usually complicated, and the fine control of nanocrystal size still remains challenging. Herein, the thermal decomposition of nickel acetate in hexadecylamine at N2 atmosphere to monodispersed Ni nanoparticles with tunable phase and size has been developed, in which the low-cost and safe nickel acetate was used as precursor, and hexadecylamine was used as both solvent and reducing reagent. In the present work, the particle size of face-centered cubic (fcc) Ni nanocrystals could be controlled by simply adjusting the reaction time or reactant ratio. In addition, the phase transformation of fcc- to haxagonal closepacked (hcp) Ni was investigated, and the magnetic properties of fcc-Ni nanoparticles with tunable size were measured in which the effects of Ni nanoparticles’ size on their magnetic properties were also discussed. 2. Experimental Section 2.1. Chemicals. Ni(CH3COO)2 · xH2O (Sinopharm Chemical Reagent Co., Ltd.) and hexadecylamine (Alfa Aesar) were used as raw materials after a dehydration process. The hydrated Ni(CH3COO)2 (30.0 g) was dissolved in acetic anhydride (120.0 mL), and the solution was heated to 170 °C and held at that temperature for 3 h under stirring; the superfluous acetic anhydride was removed by distillation to give a light green powder. The powder, whose chemical formula was Ni(CH3COO)2 · 3.15H2O based on the thermal-gravimetric (TG) and elemental analyses, was dried under vacuum at 80 °C for 6 h and then stored in air atmosphere. Zeolite and hexadecylamine were added into the flask at the weight ratio of 1:1, heated up to 80 °C, maintained at that temperature for 48 h, and then cooled to room temperature.

10.1021/jp805591y CCC: $40.75  2008 American Chemical Society Published on Web 11/08/2008

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Wang et al.

Figure 1. XRD pattern (a) and TEM (b) and HR-TEM (c) images of product.

Figure 2. IR spectrum (a) and TG curve (b) of product.

2.2. Synthesis. In a typical synthesis, under N2 atmosphere 30.0 mmol (7.244 g) hexadecylamine was melted under 100 °C to form the homogeneous liquid. Then, 5.0 mmol (0.884 g) Ni(CH3COO)2 · 3.15H2O was rapidly added into hexadecylamine to form a light green solution. With the temperature increasing to 200 °C, a brown dark solution was formed, which was cooled quickly to room temperature. The formed precipitate was obtained by centrifugation, washed with hexanaphthene alkyl and ethanol for several times, and dried under vacuum at 60 °C for 2 h. According to this manipulation, the samples derived from the solution which reacted at 200 °C for the designed periods of time were obtained. 2.3. Characterization. The X-ray diffraction (XRD) patterns of the powder samples were measured at room temperature on a Rigaku D/MAX 2200PC diffractometer with Cu KR (λ ) 0.15418 nm) radiation and graphite monochramator. The morphology and microstructure were characterized using a transmission electron microscope (TEM, JEOL JEM-1230) and a high-resolution TEM (HR-TEM, JEOL JEM-2100); before observation, the products were dispersed in cyclohexane. TG analysis was carried out on a Mettler Toledo SDTA851e thermal gravimetric analyzer at a heating rate of 10.0 °C · min-1 under air atmosphere. The infrared (IR) spectra were examined on a Nicolet 5DX Fourier transform infrared (FT-IR) spectrometer using the KBr pellet technique. The magnetic properties of the samples were determined on a superconducting quantum interference device (SQUID, MPMSXL-7) magnetometer. 3. Results and Discussion Figure 1 shows the XRD pattern and TEM image of the product prepared at 200 °C as soon as the reaction solution changed to black, at which the reaction time was defined as 0 min. It demonstrated that all of the reflections on the XRD pattern could be indexed to fcc-Ni (JCPDS, No.04-0850), indicating its fcc phase-pure nature and that it is stable enough to prevent it being oxidized to NiO in air atmosphere. On the

basis of the width of (111) reflection, the mean size of Ni nanoparticles was 3 nm calculated by using the Scherrer equation (D ) 0.89 λ/β cos θ, where D, λ, β, and θ are respectively the average crystalline size of the particles, the wavelength of the X-ray, the breadth of the corresponding reflection, and the bragg angle of the reflection.). TEM image (Figure 1b) indicates that the present Ni nanoparticles exhibit sphere-like morphology with the size of approximately 7 ( 1 nm, which is not in accord with that calculated by using Scherrer equation. In addition, the Ni nanoparticles also show monodispersibility in cyclohexane. The excellent dispersibility and stability of the present Ni nanoparticles might be due to the organic molecules coated on the particle surface. The corresponding HR-TEM image (Figure 1c) shows that the direction of the lattice fringes is different at various regions for a single nanoparticle, indicating the polycrystalline nature of the nanoparticles. The size of the region with the same lattice frings’ direction is approximately 3∼4 nm, which is consistent with the result calculated by Scherrer equation. The fringe spacing of approximately 2.01 Å corresponded to the (111) lattice plane of fcc-Ni (2.03 Å), further confirming the cubic phase structure of Ni nanoparticles. Furthermore, although the random aggregation was observed in the particles, all of the observed surface exhibited the (111) plane of fcc-Ni, indicating the orientation of the nanoparticles along the [111] direction. From the HR-TEM images, it could be concluded that the polycrystalline Ni nanoparticles with the size of approximately 7 ( 1 nm were formed through the aggregation of 3∼4 nm Ni nanoparticles as indicated by the white circles. IR and TG techniques are applied to detect the surface state of Ni nanoparticles. As shown in Figure 2a, the band around 3400 cm-1 is attributed to the vibrations of NsH and OsH bonds, and those from 2800 cm-1 to 3000 cm-1 are ascribed to CsH vibrations. The absorption at approximately 1620 cm-1 is due to the CdO and NsH vibrations, and those from 700 cm-1 to 900 cm-1 result from the NsH vibrations.14 The

Monodispersed Nickel Nanoparticles

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Figure 3. XRD patterns of Ni nanoparticles prepared at different temperatures (a) and TEM (b) and HR-TEM (c) images of hcp-Ni nanoparticles.

Figure 4. TEM and HR-TEM images of fcc-Ni nanoparticles with different molar ratio of hexadecylamine to nickel acetate: (a,d) 24:1, (b,e) 18:1, and (c,f) 12:1.

absorption at 1048 cm-1 is ascribed to the CsN stretching vibration, while those at approximately 1560 and 1420 cm-1 are attributed to the νas(OCO) and νs(OCO) of coordinated acetate species.15 The absorptions below 600 cm-1 are contributed to the vibrations of NisO bonds.16 On the basis of the IR spectrum, it could be concluded that there are hexadecylamine and acetate species on the surface of Ni nanoparticles, which results in the excellent dispersibility of Ni nanoparticles and protects the Ni nanoparticles from being oxidized in air atmosphere. Furthermore, the TG curve of Ni nanoparticles (Figure 2b) indicates that there are two weight losses and one weight gain. The first drop of approximately 1.5% from room temperature to 100 °C is attributed to the removal of absorbed organics, and the second one above 100 °C is overlapped with a weight gain, which is due to the removal of the surface organics and the oxidation of Ni to NiO. As the temperature went up to 550 °C, the Ni nanoparticles were completely oxidized; the weight was approximately 94.5% of that of initial Ni nanoparticles. From the weight of NiO and weight-loss of sample, it can be calculated that the Ni content is approximately 74.3% in the as-prepared Ni nanoparticles. It is well-known that the metal nickel has fcc and hcp phases which exhibit similar properties, but fcc-Ni is the stable phase, and hcp-Ni exists under 673 K.17 Because of its metastable

Figure 5. XRD patterns of Ni nanoparticles.

nature, there are few reports on the nanoparticles prepared by fast growth,18 However, for some crystal structures (fcc, hcp, epsilon18), their phase transformation often could be conducted with the change of reactive temperature because of the energy of one crystal structure is close to the other.19 Herein, the control of Ni phase is realized by adjusting the reaction temperature. In the present experiment, no crystalline product was formed with the reaction temperature lower than 200 °C. Figure 3a shows the XRD patterns of the products obtained at different temperatures ranging from 200 to 290 °C. As the reaction temperature was 200 °C, the phase-pure fcc-Ni nanoparticles were obtained. With the temperature increasing to 250 °C, the

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Figure 6. TEM images of Ni nanoparticles prepared as the reaction is maintained at 200 °C for (a) t ) 0 min, (b) t ) 15 min, (c) t ) 45 min, and (d) t ) 60 min. The molar ratio of hexadecylamine to nickel acetate is 6.0.

fcc- and hcp-Ni coexisted in the product. Further increasing the reaction temperature to 290 °C, the phase-pure hcp-Ni nanoparticles were formed based on the XRD analysis, in which all of the reflections could be indexed to the hexagonal close packing of Ni (JCPDS No. 45-1027). Accompanying the phase transformation from fcc-Ni to hcp-Ni, the XRD peaks significantly changed sharper, implying the increasing of particle size with the temperature raising. TEM image (Figure 3b) revealed the similar morphology of hcp-Ni nanoparticles with the size of approximately 35 nm. HR-TEM image (Figure 3c) shows continuous lattice fringes in a single particle, indicating the single crystalline nature of the prepared hcp-Ni nanoparticles. With the reaction time increasing to 15 min at the reaction temperature of 290 °C, the Ostwald ripening occurred, and the size of nanoparticles became inhomogeneous. On the basis of the experiments, it could be concluded that higher reaction temperature favored the formation of hcp-Ni, which was formed through the phase transformation from fcc-Ni. Figure 4 shows the TEM and HR-TEM images of Ni nanoparticles with different reactant ratio while the reaction temperature was held at 200 °C. As shown in the TEM images, those Ni nanoparticles monodispersed in hexanaphthene alkyl

Wang et al. exhibited uniform size and morphology. The TEM images clearly indicated that the nanoparticle size increased with the molar ratio of hexadecylamine to nickel acetate decreasing. By simply adjusting the molar ratio from 24:1 to 18:1, 12:1, and 6:1, the size of fcc-Ni nanoparticles changed from 13 nm to 11, 10, and 7 nm. HR-TEM images indicated that all of the prepared fcc-Ni nanoparticles were composed of the subunits of 3-4 nm as indicated by the white circles. Thus, it can be concluded that the reactant ratio affected the nanoparticle size by changing the aggregation degree of initial Ni clusters. Herein, hexadecylamine acted as both solvent and reductant. In the present experiment, the hexadecylamine content was much more than the required amount as the reductant. Because the formation of Ni nanoparticles went through an aggregated growth mode, the Ni clusters formed at the initial stage were unstable because of their small size and high surface energy. Then they aggregated rapidly to lower the surface energy. When the particle size increased to the value that the nanoparticles could exist stably, the aggregation stopped. On the basis of the experiments, it is proposed that the presence of CH3COO- anions on the particle surface might stablize the particles and decrease the size of Ni nanoparticles. The size of fcc-Ni nanoparticles could also be controlled by adjusting the reaction time while other reaction conditions were held constant. The intensity of the reflections in XRD patterns significantly increased with the peaks sharpening as the reaction time prolonging (Figure 5), indicating the crystallinity and particle size increasing. This phenomenon was also found by Williams and his co-workers in synthesizing the gold nanoparticles.20 The corresponding TEM images (Figure 6) indicate that the nanocrystals exhibit a similar sphere-like morphology with average sizes of 16, 24, and 32 nm for the reaction times of 15, 45, and 60 min, respectively. However, with the reaction time prolonging, the size distribution broadened. This broadening of the nanoparticle size resulted from the Ostwald ripening, in which the smaller particles dissolved accompanying with the growth of larger ones. Thus, the preferable method to control the particle size in the present experiment is to adjust the reactant ratio. For the monodispersed metals, metal oxides, and metal sulfur nanocrystals, the sizes and shapes usually have important influences on their magnetic properties.21 Thus, the magnetic properties of fcc-Ni nanoparticles with different sizes are investigated, and the temperature-dependent magnetization (MT) and hysteresis (M-H) curves of fcc-Ni nanoparticles with different sizes were obtained (Figure 7). The zero-field-cooled (ZFC) curves indicate that the blocking temperatures (TB, the temperture at which the ZFC and FC curves start to diverge) of fcc-Ni nanoparticles were estimated to be 71, 177, and 338 K

Figure 7. (I) Zero-field-cooling (ZFC, solid line) and field-cooling (FC, open line) magnetization curves of Ni nanoparticles under 100 Oe applied magnetic field. (II) Hysteretic loops of Ni nanoparticles at 300 K. The inset in II is the magnified figure of the central part of II. The sizes of Ni nanoparticles are (a) 7 nm, (b) 11 nm, and (c) 13 nm, respectively.

Monodispersed Nickel Nanoparticles for samples of 7, 11, and 13 nm, respectively. The blocking temperatures increased with the nanoparticles’ size increasing,13b,22 which was consistent with the previous reports.18a,22a,23 Meanwhile, the FC magnetization grandually increased with the temperature decreasing from 350 to 5 K, and deviated from the ZFC curve near Tmax (The temperature at which the ZFC magnetization maximum is observed.). The small difference between the TB and Tmax as well as the narrow cusps on the ZFC curves of samples a and b indicated a narrow size distribution, which is deduced from the narrow energy barrier distribution.24 The field dependence of the magnetization at 300 K has been followed for all three samples. Field-dependent magnetization measurements demonstrate the superparamagnetic characteristics of the as-prepared nanoparticles except for the sample c, and the hysteretic behavior in the M-H curves clearly shows that the fcc-Ni nanoparticles are ferromagnetic for T < TB and superparamagnetic for T > TB.25 From the M-H curves, it was estimated that the coercivity values (Hc) was 0, 0 and 27.3 Oe at 300 K for samples a, b, and c, respectively. The magnetization rose rapidly as the applied field increased and reached a saturation point at approximately 2500 Oe. At 300 K, the saturation magnetization value of the samples with the sizes of 7, 11, and 13 nm are respectively 7.9, 19.0, and 27.9 emu g-1, which was far from the saturation value for bulk nickel (∼50 emu g-1). This result is consistent with the previous reports, in which the saturation magnetization increases with the particle size increasing. Such a behavior is attributed to high surface/volume ratio and the corresponding surface effects (spin canting).26 4. Conclusions In summary, a facile solution method to monodisperse Ni nanocrystals through thermal decomposition of nickel acetate has been developed in which the surfactant hexadecylamine was used as solvent, reducing agent, and stabilizing agent. The size of nanoparticles could be adjusted by changing the reactant ratio and reaction time, and the Ni nanoparticles would transform from fcc-phase to hcp-phase as the reaction temperature increases from 200 to 290 °C. The measurements of magnetic properties indicate that the Tmax in ZFC magnetization curves and the saturation magnetization increased significantly from 71 to 338 K with the increasing Ni nanoparticle size. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant 20671057), the Program for New Century Excellent Talents in the University (People’s Republic of China), and Doctoral Foundation of Shandong Province (2007BS04042). References and Notes (1) (a) Stamm, C.; Marty, F.; Vaterlaus, A.; Weich, V.; Egger, S.; Maier, U.; Ramsperger, U.; Fuhrmann, H.; Pescia, D. Science 1998, 282, 449. (b) Prinz, G. A. Science 1998, 282, 1660. (c) Lu, L.; Sui, M. L.; Lu, K. Science 2000, 287, 1463. (d) Kumar, D.; Narayan, J.; Kvit, A. V.; Sharma, A. K.; Sankar, J. J. Magn. Magn. Mater. 2001, 232, 161. (2) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Cordente, N.; Reapaud, M.; Senocq, F.; Casanove, M. J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (d) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002,

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