The preparation and characterization of hcp and fcc Ni and Ni/NiO nanoparticles is reported. Ni and Ni/NiO nanoparticles were obtained starting from a precursor material prepared using a citric assisted Pechini-type method and, then, followed by a calcination of the precursor in air at either 400 or 600°C for different times. The precursor was analyzed using thermogravimetric and differential thermal methods (TGA-DTA), and the resulting nanoparticles were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and vibrational sample magnetometry. Nanoparticles showed a phase transformation for Ni from hcp to fcc and/or to fcc NiO structure as the calcination time increased. The influence of the phase transition and the formation of NiO on the magnetic properties of the samples are discussed.
The study of different types of nanoparticles has been an extremely active research area in the last decade. Nanoparticles are very promising for new applications due to the fact that a similar chemical composition can lead to different properties compared to bulk, microcrystalline powders, or thin films. In particular, the synthesis of Ni and Ni/NiO nanoparticles has received considerable attention, based on their interesting properties for applications in ferrofluids, catalysis, magnetic materials, gas sensors, and biomedical applications [
To the date, different methods have been used to prepare Ni and Ni/NiO nanoparticles, which include thermal reduction process [
In the present work, we report the synthesis of hcp and fcc Ni and Ni/NiO nanoparticles using a citric acid assisted Pechini-type method. We show that this method allows precise control of the structure of the nanoparticles through some easily adjustable processing parameters. Also herein we assess the influence of the processing conditions on the structural, morphological, and magnetic properties of nanoparticles through the analysis of X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM), and VSM measurements results.
The chemicals used in this study were analytical grade purity: nickel chloride (NiCl2·6H2O, Aldrich Co.), citric acid (C6H8O7·H2O, Aldrich Co.) and ethylene glycol (C2H6O2, Aldrich Co.), and distilled water. All reagents were used as received. The precursor material was prepared as follows: first, citric acid was dissolved in distilled water, and then nickel chloride was added to the citric acid solution and stirred until a clear and homogeneous solution was obtained. Finally, ethylene glycol was added, and the resulting solution was stirred and then heated at 100°C to slowly evaporate the solution until a highly viscous resin was generated. The resin was then dried at 130°C, milled, and calcinated in air at 400 or 600°C for 15 min, 1 h, and 2 h, depending on the sample.
The thermal decomposition of the precursor material was evaluated by simultaneous TGA/DTA analysis (Shimadzu TGA-50/DTA-50 thermoanalyzer). The phase identification of the obtained nanoparticles was recorded by X-ray diffractometer (Siemens D5000) with CuK
The TGA and DTA curves of the precursor material are shown in Figure
TGA-DTA analyses of the precursor material.
The XRD patterns of the nanoparticles obtained after calcination at 400°C (a) and 600°C (b) at different times are shown in Figure
XRD patterns of nanoparticles obtained at 400°C (a) and 600°C (b) for 15 min, 1 h, and 2 h.
During the calcination process precursor material decomposition occurs. This decomposition produces a CO/CO2-rich atmosphere. Such atmosphere promotes the reduction of the metallic salt, resulting in Ni nanoparticles. This atmosphere can prevent Ni oxidation for calcination times shorter than 1 h. For calcination treatments longer than 1 h, Ni nanoparticles start to oxidize and form Ni/NiO nanoparticles. When the temperature is increased up to 600°C, only the sample annealed for 15 minutes shows this Ni/NiO structure. Clearly, the samples annealed for more than 1 h show only NiO phase (Figure
It is important to mention that the formation of hcp Ni structure was not expected. According to the literature this is a metastable phase and only can be obtained under specific conditions. Here, two main processes for the formation of the hcp phase have been suggested, (a) as a result of a fast thermal decomposition of the precursor materials [
For the samples prepared at 400°C, the phase percentages obtained by the whole-profile fitting program are shown in Table
Particle size, phase percentage, saturation magnetization, and coercivity of the samples obtained at 400°C for 15 min, 1 h, and 2 h.
Time | Particle size (nm) | Phase percentage (%) | Ms (emu/g) | Hc (Oe) | ||||
hcp Ni | fcc Ni | fcc NiO | hcp Ni | fcc Ni | fcc NiO | |||
15 min | 17.0 | 9.8 | — | 65.47 | 34.53 | — | 2.8 | 86 |
1 h | 22.0 | 30.0 | — | 53.53 | 43.46 | 3.01 | 3.9 | 60 |
2 h | — | 30.2 | 18.4 | — | 48.74 | 51.26 | 5.2 | 150 |
The sizes of Ni and Ni/NiO nanoparticles were calculated from the X-ray peak broadening of the most intense reflection using the Scherrer’s formula [
The particle sizes of the samples obtained at 400°C at 15 min, 1 h, and 2 h calculated by (
The magnetic properties of the samples obtained at 400°C for 15 min, 1 h, and 2 h were studied by recording the hysteresis (M-H) loops, as shown in Figure
Hysteresis curves of the samples obtained at 400°C for 15 min, 1 h, and 2 h.
The prepared nickel nanoparticles are dispersed in carbon and/or NiO, so it is considered as a solid solution of nickel atoms in carbon and/or NiO; therefore the specific magnetization is low. Additionally, the magnetization could be further reduced due to the existence of random Ni magnetic moments at the nanoparticles surface and/or nonmagnetic Ni (or NiO) atoms located at the interfaces between the Ni nanoparticles and the amorphous carbon matrix or the NiO phase [
Also from Figure
TEM image of the Ni and Ni/NiO nanoparticles obtained at 400°C for 2 h is shown in Figure
Low-magnification TEM image of the Ni/NiO nanoparticles obtained at 400°C for 2 h. (b) HRTEM image of the Ni/NiO nanoparticles.
In summary, we prepared hcp and fcc Ni and Ni/NiO nanoparticles by simply changing the processing conditions, specifically, the time and temperature of calcination of the precursor material. Ni nanoparticles showed a mixture of fcc and hcp phases. When the time and temperature of calcination increased, a phase transformation from hcp Ni to fcc Ni and the formation of Ni/NiO were observed. The magnetic properties of the samples, ferromagnetic in nature, reflected the proposed phase transition. Based on the versatility of the method, it can be extended to prepare other metal nanoparticles and its respective metal/metal oxide nanoparticles, such as those of Fe, Cu, and Co.
The authors are grateful for the financial support by CONACYT-México under Grant no. 133991. They are grateful for E. Saucedo, M. L. López, and E. Díaz for their technical assistance in the FESEM and TEM micrographs, M. A. Hernández-Landaverde for the calculations of the phase percentage by WPF, and G. Hurtado for the PPMS-VSM measurements.