Advances in Materials Science and Engineering

Volume 2013 (2013), Article ID 926540, 5 pages

http://dx.doi.org/10.1155/2013/926540

Research Article

Microstructural Study of Neodmium Nickelate Doped with Strontium Synthesized by Gelatin Method

R. M. P. B. Oliveira,1,2 P. M. Pimentel,³ J. H. Araújo,⁴ D. M. A. Melo,¹ M. A. F. Melo,¹ and A. E. Martinelli¹

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.

Abstract.

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.

1. Introduction

In recent years, the search for new perovskite-type oxides has been intensified due to the wide range of properties such as dielectric [1], catalytic activity [2] and electrocatalytic [3], optical [4], and magnetic properties [5]. 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.

2. Experimental

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 [11]. 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) [12], oxides with A₂BO₄ structure (K₂NiF₄) are more stable then oxides with structure ABO₃ when is doped with strontium.

(a)

(b)

Figure 1: The Rietveld refinement of the system Nd2-xSrxNiO₄ to calcined at (a) 700°C and (b) 900°C.

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.

Table 1: Refinement parameters of system to and calcined at 700°C and 900°C.

Sample
Bragg
Phase
ICSD
Crystalline

system
Space

Group
Crystallite size (nm)

()

; 700°C
11.5
Nd₂O₃
32514
Trigonal
P-3m1
35.51
3.8302
3.8302
6.0046

NiO
9866
Cubic
Fm-3m
20
4.1791
4.1791
4.1791

NdNiO₃
67722
Orthorhombic
Pbnm:cab
87.13
5.4311
5.4049
7.7910

; 900°C
13.2
Nd₂O₃
32514
Trigonal
P-3m1
68.30
3.8301
3.8301
6.0036

NiO
9866
Cubic
Fm-3m
42.94
4.1793
4.1793
4.1793

NdNiO₃
67722
Orthorhombic
Pbnm:cab
43.78
5.5115
5.4280
7.6705

Nd₂NiO₄
71137
Orthorhombic
Bmab
100
3.8054
12.3934
7.7910

; 700°C
12.07
Nd1,6Sr0,4NiO₄
71138
Tetragonal
I4/mmm
59.61
3.7831
3.7831
12.4603

NiO
9866
Cubic
Fm-3m
23.91
4.1803
4.1803
4.1803

Nd₂O₃
32514
Trigonal
P-3m1
100
3.8272
3.8272
5.9910

; 900°C
10.5
Nd1,6Sr0,4NiO₄
71138
Tetragonal
I4/mmm
110
3.7840
3.7840
12.4584

NiO
9866
Cubic
P-3m1
64.79
4.1791
4.1791
4.1791

(a)

(b)

Figure 2: The Rietveld refinement of the system Nd1,6Sr0,4NiO₄ calcined at 700 and 900°C.

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 [13]. 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 [14]. The quantitative phase analysis of samples obtained by MAUD program is given in Table 2.

Table 2: Quantitative analysis of samples obtained by MAUD refinement.

Sample
Temperature (°C)
Phase (%)

()
700
Nd₂O₃
65

NiO
21

NdNiO₃
14

()
900
Nd₂O₃
58

NiO
19.5

NdNiO3
12

Nd₂NiO₄
10.5

()
700
Nd1.6Sr0.4NiO₄
77.5

NiO
20

Nd₂O₃
2.5

()
900
Nd1.6Sr0.4NiO₄
80

NiO
20

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.

(a)

(b)

(c)

(d)

Figure 3: (a) SEM image of system Nd2-xSrxNiO₄ to calcined at 700°C; (b) Nd1,6Sr0,4NiO₄ calcined at 700°C; (c) Nd2-xSrxNiO₄ to calcined at 900°C; (d) Nd1,6Sr0,4NiO₄ calcined at 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.

4. Conclusions

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.

Acknowledgments

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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