Nowadays, one of the main challenges in materials science concerns the synthesis of nanomaterials, since they exhibit interesting properties that can be different of those of bulk materials. Consequently, numerous processes of nanomaterial synthesis have been investigated aiming to control their size, morphology, structure, and chemical composition.
In laboratory thermal and chemical-heat treatment of materials, structural strength of materials is based on the thermal decomposition of metal precursor and allows to control directly the particle size and morphology with the process parameters: pressure, temperature, residence time, and precursor concentration [1, 2].
After a brief description of the experimental set-up and operating conditions, one example of obtained nanostructured materials is described in terms of chemical composition, crystal structure, morphology, and mean particle size. In the second part, a primary study of the material magnetic properties is presented.
Solvent used for experiments is a mixture of anhydrous ammonia (TC = 132.4∘C and PC = 11.29 MPa) and methanol in the molar proportion 70% NH3 / 30% methanol. Continuous flow experimental set-up is a tubular reactor (diameter of 1.6 mm, length of metricconverterProductID4.31 m4.31 m) with a flow rate of 11/h . Pressure was about 18 MPa and temperature was 280∘C in order to perform thermal decomposition of nickel precursor (nickel (II) hexafluroroacetylacetonate, Ni (hfac)2). Optimal decomposition temperature was determined by in situ UV-visible spectroscopy. Experimental data are reported in Table 1.
Physico-chemical characterization of materials was carried out by chemical analysis, thermogravimetric analysis (TGA) coupled with mass spectroscopy, conventional X-ray powder diffraction (XRD: CuKα radiation), and structural rectification . Powder morphology was observed by scanning electron microscopy (SEM, JEOL 840), and transmission electron microscopy in dark field (TEM, JEOL 2000FX). Size distributions were determined by manual counting.
Chemical analysis and structural rectification were used to determine the chemical composition and structure of the synthetized material. These characterizations show that a nickel oxinitride was synthesized with the Ni3NO0.18composition corresponding to the P6322 space group with a hexagonal unit cell (a = 4.6233Ǻ and c = 4.3084Ǻ).
Experimental and theoretical XRD patterns of Ni3NO0.18 are presented in Figure 1.
The structure rectification shows that oxygen atoms are inserted inside the nickel nitride structure (Ni3N) in position 2d with a proportion of 18%. Unit cell of nickel oxinitride is presented in Figure 2.
Powders are composed of shapeless aggregates, constituted by crystallized nanoparticles. Aggregates were observed by SEM and nanoparticles by TEM in dark field (Fig. 3). To summarize this part, homogeneous powders constituted of aggregated nanoparticles were obtained.
Magnetic measurements were performed with a SQUID at 0∘C. Two samples with studied: initial sample (4.4 nm); annealed sample (250 nm).
|Sample||Particle size (nm)||Unit cell Parameters (Ǻ)||Magnetic behavior at 0∘C|
|σ s (emu/g)||R||H c (Oe)|
|Initial||4.4 ± 2.5||a = 4.587||0.29||0.012||18|
|c = 4.334|
|Annealed||250 ± 50||a = 4.586||0.85||0.080||87|
|c = 4.331|
Annealed sample was obtained by thermal treatment of the initial sample under ammonia atmosphere (1 MPa) during 3 hours at 250∘C in order to increase crystal size without changing the structure. Thermal treatment modified sample particle size: starting with particles aggregated with an elementary size of 4.4 nm in average, isolated monodispersed crystals with a size distribution of 250 nm were obtained. TEM pictures of these samples were presented in Figure 3. We can notify that material structure did not change during the thermal treatment (the same unit cell parameters). The main characteristics of these samples and their magnetic properties are reported in Table 2 and Figure 5.
We have synthesized nanoparticles of nickel oxinitride by thermal decomposition of nickel hexafluroroacetylacetonate in supercritical ammonia, used here as solvent and reactant. Nickel oxinitride crystallized in the P6322 space group with a hexagonal unit cell (a = 4.6233Ǻ and c = 4.3084Ǻ) with the chemical composition Ni3NO0.18.
This preliminary study of magnetic properties of Ni3NO0.18 has shown that this material presents a ferromagnetic behavior. Moreover, these magnetic properties are influenced by the particle size and the transition of the ferromagnetic state to the superparamagnetic one is put in evidence.