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Advances in Materials Science and Engineering
Volume 2013 (2013), Article ID 926540, 5 pages
Microstructural Study of Neodmium Nickelate Doped with Strontium Synthesized by Gelatin Method
1Programa de Pós Graduação em Ciência e Engenharia de Materiais, Universidade Federal do Rio Grande do Norte, 59078-970 Natal, RN, Brazil
2Faculdade de Engenharia de Materiais, 68505-080 Marabá, PA, Brazil
3Universidade Federal Rural do Semi-Árido, 59515-000 Angicos, RN, Brazil
4Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, 59078-970 Natal, RN, Brazil
Received 14 December 2012; Revised 22 February 2013; Accepted 24 March 2013
Academic Editor: Steven Suib
Copyright © 2013 R. M. P. B. Oliveira et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this work nickelate nanoparticles were synthesized using a simple and fast new route, which makes use of gelatin as an organic precursor in order to evaluate the performance of the synthesis method to obtain materials and the influence of the strontium replacement on the structural parameters of the powders. Samples of Nd2-xSrxNiO4 ( and 0.4) were calcined at 700 and at a heating of /min for 4 hours. X-ray powder diffraction patterns were obtained for all the samples, and the Rietveld Method was applied in order to determine the crystallite size using Scherrer's equation, the lattice parameters, and phase concentration. The results obtained using these techniques confirmed that the main crystal structure consists of the distorted K2NiF4-type tetragonal I4/mmm. In addition, scanning electron microscopy images revealed the formation of nanosized particles.
In recent years, the search for new perovskite-type oxides has been intensified due to the wide range of properties such as dielectric , catalytic activity  and electrocatalytic , optical , and magnetic properties . The synthesis of these materials requires methods that provide a high surface area and high homogeneity. There are several methods for synthesis of perovskites, which directly affect the properties of these systems, specifically in its texture and specific surface area, oxidation states of cations, and oxygen stoichiometry. Traditional methods of preparation of perovskite-like materials adopted usually mixtures of constituents of oxides, hydroxides, or carbonates. However, as these materials generally have large particle size, these steps often require repeated and mixtures prolonged heating at high temperatures to produce a homogenous material and comprising a single phase. To overcome the disadvantages of low specific surface and limited control of the microstructure inherent in high temperature processes, the precursors are generally obtained by processes such as sol-gel or coprecipitation of metal ions by precipitating agents such as hydroxides, cyanide, oxalate, and citrate ion, among others. Among the synthesis methods, the ceramic is the most widely used due to its simplicity but has the disadvantage of low specific area [6, 7].
Recently, a route has been developed for obtaining nanoscale materials using gelatin as the organic precursor that has the advantage of producing homogeneous and nanometric powders [8–10]. The procedure is the formation of colloidal dispersions between the gelatin and metal ions, which are connected gelatin coordinates groups such as NH2 and COOH. The dispersion is subjected to heating to reduce volume and, consequently, formation of a gel which is subsequently calcined at a predetermined temperature for the decomposition of gelatin and formation of inorganic oxides. This study is focused on the preparation of solid solutions of neodymium nickelates doped with strontium by a method that makes use of gelatin as an organic precursor in order to evaluate the structural properties of these oxides in function of increasing content of strontium and the heat treatment.
Perovskites were prepared using gelatin as an organic precursor and metal nitrates as starting reagents. Gelatin was added to a beaker containing deionized water and stirred for 30 minutes at 50°C. Ni(NO3)2·6H2O (99.9%, Sigma-Aldrich) and Nd(NO3)3·6H2O (99.9%, Sigma-Aldrich) were added to the solution at 70°C for several minutes. Sr(NO3) P.A. (≤99%, Sigma-Aldrich) was added for further 30 minutes. The temperature was slowly increased to 90°C, and the solution was stirred on a hot plate until a gel formed. The gel was then calcined at 350°C for 2 hours with a heating rate of 5°C min−1. This resulted in a precursor powder, which was calcined at 700 and 900°C for 4 hours and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques.
X-ray patterns were obtained from samples calcined at different Sr concentrations and temperatures. Measurements were recorded on a Shimadzu XRD-6000 diffractometer with polychromatic radiation of CuKα1,2 ( Å, Å). A 2θ angular range was used between 10 and 90° with a scan speed of 2° min−1 and step scan of 0.02°. Crystalline phases were identified using the International Center for Diffraction Data (ICDD) database. Crystallite sizes were obtained with Scherrer’s equation. The Rietveld method was used to refine the XRD data using the MAUD program (version 2.044). The instrumental broadening has been used following the procedure as adopted by Lutterotti and Scardi . The analyses were carried out by observing the plot of calculated and observed patterns. The morphology of the ceramic powders Nd2-xSrxNiO4 ( and ) calcined at 700 and 900°C was observed by SEM images obtained in a scanning electron microscope model SSX-550 from Shimadzu.
3. Results and Discussions
Figure 1 shows the observed and calculated X-ray powder patterns of crystallization products of the Nd2-xSrxNiO4 () powders calcined at 700°C and 900°C. According to a refinement, there is a good agreement between experimental and refined diffraction, indicating that the results are highly reproducible and reliable; that is, the model parameters of crystal structure (low R Bragg). The identified phases in the powder with were Nd2O3 hexagonal (JCPDS 41-1089), NiO cubic (JCPDS 73-1523), and NdNiO3 rhombohedral. In samples calcined at 900°C appears to Nd2NiO4 orthorhombic (JCPDS 21-1274). According to Zhao et al. (1996) , oxides with A2BO4 structure (K2NiF4) are more stable then oxides with structure ABO3 when is doped with strontium.
Figure 2 shows the Rietveld refinement data for Nd2-xSrxNiO4 () powder calcined at 700 and 900°C. The result shows a good agreement between experimental and refined XRD patterns. Table 1 shows the refined parameters for Nd2-xSrxNiO4 ( and 0.4) powders calcined at 700 and 900°C, respectively. The inclusion of strontium in the structure on favors the formation of Nd2-xSrxNiO4 tetragonal (JCPDS 80-2324) and NiO cubic (JCPDS 73-1523). For calcination at 900°C, Figure 2(b), for , shows Nd2-xSrxNiO4 tetragonal (JCPDS 80-2324) and NiO cubic (JCPDS 73-1523). It is known that the substitution in the perovskite-type oxide of a trivalent metal ion in the A site for a bivalent or tetravalent metal cation () is accompanied by a change in the oxidation state of the site B metal cation by modifying the activity catalyst. However, the change in oxidation state of the B site cation in the insertion of is accompanied by the formation of structural defects.
It is observed that the lattice parameter for the NdNiO3 decreases due to oxidation of Ni2+ to Ni3+ compared with system Nd2NiO4 when it is calcined at 900°C. It was reported that the substitution of Nd with Sr in Nd2NiO4 might induce a structural phases transition from orthorhombic to tetragonal symmetry leading to a mixed valence (Ni2+/Ni3) for the transition metal ion, which would in turn induce interesting electrical and magnetic properties in this system . Moreover, increasing the lattice parameter is due to replacement of ion Nd3+ with Sr2+ ion, promoting the removal of the layers in the structure of perovskite . The quantitative phase analysis of samples obtained by MAUD program is given in Table 2.
The morphology of the ceramic powders synthesized was determined by SEM images shown in Figure 3. By means of these images, it is observed that the particles have a rounded shape and uniform distribution of particle size. The particles are very small (less than 100 nm), and this is evident in all images corresponding to the Nd2-xSrxNiO4 ( and ) powders calcined at 700 and 900°C.
The porous material has been generated during the evolution of gases formed from the decomposition of the gelatin during the calcination step. As shown in Figure 3(d), the quantity and pore size decrease with increasing temperature of calcination; this is a consequence of the phenomenon which leads to sintering of the agglomerated particles.
The synthesis route using gelatin was feasible for the synthesis of nanosized and porous ceramic powders, since it is a polymeric material containing groups which can coordinate with the metal ions as well as being a material of low cost and nontoxic.
The ceramic powders calcined under the conditions formed a solid solution whose main phase was perovskite-like for powders with partial replacement of Nd+2 metal ion with Sr+3. In addition there were no significance differences in the structures of these powders calcinated at °C and 900°C.
The authors are grateful to the Brazilian agencies Capes and CNPq for the financial support and GELITA for supplying the gelatin.
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