Solution-Phase Synthesis and Characterization of Perovskite LaCoO3 Nanocrystals via A Co-Precipitation Route

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Solution-Phase Synthesis and Chmterization of Pemvskite LaCoO, Nanocrystals via A Co-Precipitation Route

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Zhu Junwu (&#if&), Sun Xiaojie ( 4 ~ . ~ J ~ , ? t tWang ), Yanping (3%,.$), Wang Xin (Z E )*, Yang Xujie Lu Lude)&&%t( (Materids Chemistly Laboratory, Nanjing University of Science and Technology, Nmjing 210094, China) Received 11 January 2007; revised 15 March 2007

Abstract: A facile co-precipitation route for the synthesis of well-dispersed LaCoO, nanocrystals was developed. The as-

prepared products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), and laser Raman spectroscopy (LRS). The results showed that modulating the growth parameters, such as the addition of surfactants as well as the adding manner of the precipitator had a significant effect on the overall shape and size of the obtained nanocrystals. The nanorods with the diameter of 20 nm and spherical LaCoO, nanocrystals with the size of about 25 nm could be obtained at a relatively low calcining temperature of 600 "c . Furthermore, the Raman properties of LaCoO, products obtained at different calcining temperatures were investigted. Key words: perovskite; LaCoO,; co-precipitation; nanocrystals; rare earths Article ID: 1002 -0721(2007)05 -0601 -04 cLx3 number: 0614 Document code: A

Perovskite-type complex oxides are of great interest and have been widely studied for their various properties and applications in the recent decades"-41. Among the perovskite oxides, lanthanum cobaltite (LaCoO,) is extremely promising for different technological app lications, such as superconductors, electrode materials, magnetic materials, and catalystsL5 -'I. Nanometre-sized LaCoO, is expected to possess improved properties and more applications than its bulk one. Traditionally, the LaCoO, was prepared via solid state reaction at high temperature (above 1000 "c) with prolonged reaction and often resulted in the formation of coarse aggregation, which was difficult to disperse. Moreover, since the property of such material is strongly dependent on its microstructure and synthetic procedure, increasingly more attention is devoted to the development of suitable strate*

gies to obtain nanostructure LaCoO,. However, recent successful synthetic studies of LaCoO, nanocrystals have mostly focused on spherical products"1-131,and few articles concerning the preparation of LaCoO, nanorods are available. In this paper, a simple co-precipitation procedure to prepare LaCoO, nanorods in aqueous solution at relatively low temperature was presented. The crystalline phase with perovskite structure can be obtained by calcining the precursor at 600 "c. The roles of parameters critical to the sizeand shape-guiding processes were presented. It is proved that the addition of a small amount of surfactants and the adding manner of precipitator have great influence on the particle size and shape of LaCoO, products .

Companding author(E-mail:[email protected])

Foundstion item: Project supported by the Postdoctoral Foundation of China (20060390284) and Jiangw Planned Projects for Postdoctoral Research Funds Biogaphy: Zhu Junwu (1976- ), Male, Postdoctor; Research field: synthesis and application of nanomaterials Copyright 02007, by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B.V. All rights reserved.

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1 Experimental 1.1 Prepamtion of Lacoo, nanocrystals La(NO,),, Co(NO,),, and NaOH of analytical purity were used as the starting materials without further purification. In a typical procedure, 300 ml of aqueous solution was prepared from La(NO,), and CO(NO,)~, which was 0.02 mol L - ’ in both lanthanum and cobalt. The solution was added to a round-bottom flask equipped with a refluxing device and was heated to 60 “c with vigorous agitation. Then, appropriate amounts of NaOH solution with 1.0 mol L - ’ concentration was added dropwise into the above solution until the pH value of the mixture reached 10 11. A large amount of precipitate was formed immediately, which was maintained at this temperature for 4 h. After the reactions were completed, the resulting products were centrifuged, washed with water and ethanol several times, dried at 100 “c, and calcined at different temperatures to obtain the final black products.

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1.2 Charactelization Crystal structure identification was done by Xray diffraction (XRD) using a Bruker D8 Advance Xray diffiactometer with Cu K a (A =0.15405 nm) radiation. Transmission electron microscopy (TEM ) observation was performed on JEM-2100 and JEOL 100s microscope. Energy-dispersive X-ray spectroscopy (EDX) analysis was obtained on a JEOL JSM6380LV scanning electron microscope. Laser Raman Spectroscopy (LRS) studies were carried out using a Renishaw Invia Raman spectrometer ranging from 100 to 1000 c m - ’ at room temperature.

that no sharp crystal phase peaks existed after calcining at 500 T for 2 h. Several sharp peaks are observed after the products are calcined at 600 T for 2 h. These peaks are attributed to the rhombohedra1 phase of LaCoO, crystals (JCPDS 84-0848). This result indicates that LaCoO, with pure perovskite structure can be synthesized at 600 “c using this procedure. With the calcination temperature rising, the peaks intensify significantly and some peaks even split into half. This can be explained by the fact that the crystalline phase of LaCoO, becomes more precise with the calcination temp erature. Fig. 2(a) shows the TEM image of the LaCoO, products calcined at 600 “c, which indicates that the LaCoO, nanorods with diameters of 20 nm and lengths of 200 nm can be obtained. The HRTEM image of a part of a single,LaCoO, nanorod (Fig. 2(b)) exhibits clear lattice fkinges. Additionally, Fig. 2 (c) displays the electron diffraction pattern of these nanorods, which is consistent with the crystalline nature. The EDX analysis was performed to further confirm the composition of the obtained products. Fig. 3 shows that the products calcined at 600 “c for

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2 Results and Discussion Fig. 1 shows the XRD patterns of products calcined at different temperatures for 2 h . It was seen

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Fig. 1

XRD patterns of the products calcined at different temperatures for 2 h

Fig. 2 TEM image of LaCoO, products calcined at 600 “c: for 2 h (a); HRTEM image of a part of a single LaCoO, nanorod 01);Electron diffraction pattern of LaCoO, products calcined at 600 “c: for 2 h (c)

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Solution-PhaseSynthesk and Chmacierimtionof Pemvskite LaCoO,

2 h are composed of La, Co, and 0. Quantitative EDX analysis shows that the molar ratio of La:Co:O was 15.81:17.28:49.63, providing a nearly stoichiometric formula of LaCoO,. The C peak in the spectrum is attributed to the electric latex of the SEM sample holder. It is necessary to point out that the adding manner of NaOH solution has an important influence on the shape of the LaCoO, nanocrystals. For comparison, the LaCoO, products were prepared when the NaOH solution was poured into the mixture solution of La(NO,), and Co(NO,),. The obtained precipitate was calcined at 600 "c for 2 h, and the TEM analysis shows that the products are composed of spherical particles (Fig. 4). It is assumed that when the NaOH solution was added dropwise into the mixture solution, some small LaCoO, crystal nuclei was formed at the initial stage of the reaction. With the increase of the NaOH solution, the slowly produced LaCoO, crystallites grew along one growth direction on the basis of the initially formed LaCoO, crystal nuclei. The epitaxial growth of crystallites, the slow nucleation, and the growth rates result in the formation of nanorods. On the contrary, if NaOH solution was quickly

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Fig. 3

EDX spectrum of the obtained LaCoO, nanocrystals

Fig. 4 TEM image of LaCoO, products when the NaOH solution was poured into the mixture solution

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added into the mixture solution, the solute was consumed rapidly and the epitaxial growth of crystallites was effectively inhibited. Meanwhile, the quick addition of NaOH solution can provide a quick rate to the crystal growth. The faster growth rate results in the crystal growth being considerably less selective in directions and hence spherical LaCoO, nanocrystals are . pr~duced"~] The shape and size of LaCoO, nanocrystals can be further tuned by adding small amounts of surfactants. When 0.13 g of poly(viny1 pyrrolidone) (PVP, M , ~ 4 4 0 0 0 54000) was added into the mixture solution of La(NO,), and CO(NO,)~,spherical nanoparticles with an average diameter of -25 nm (Fig. 5 ) were obtained after the precursor was calcined at 600 "c for 2 h. The result demonstrates that the presence of small amounts of PVP leads to the shape change of LaCoO, products. The use of a surfactant to control the morphological evolution of nanocrystals has been extensively explored. It is generally accepted that the surfactant kinetically controls the growth rates of various faces of nanocrystals by selective adsorption and desorption on these surfaces"61. The PVP may adsorb into the fastest growth face of LaCOO, nanocrystals. Hence, the epitaxial growth of nanocry stals is effectively inhibited and spherical LaCOO, nanocrystals are produced. The Raman spectra of LaCoO, products calcined at different temperatures are illustrated in Fig. 6. For the rhombohedra1 LaCoO,, only the A ,g and E, modes are Raman active. The peaks at around 138, 305, and 675 c m - ' are similar to those reported in Refs.[ 14, 171. Podobedov et al."sl assigned the peak at 675 c m - ' to the second-order Raman scattering. The intensity of the Raman peaks increases with increased calcination temperatures, which may be caused by the crystal perfection and the increase of particle size with elevated temperatures.

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Fig. 5

TEM image of LaCoO, products prepared in the presence of 0.13 g of PVP

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Fig. 6 Raman spectra of LaCoO, calcined at different temperatures for 2 h

3 Conclusion In summary, the rod-like and spherical LaCoO, nanocrystals were synthesized via a simple co-precipitation method, The pure perovskite LaCoO, products were formed by heat treatment at 600 "c for 2 h. The shape of the nanocrystals was controllable by adjusting the crystal growth rate. This control route was easily manipulative and well repeatable. It was expected to deliberately create nanocrystals with more regular and complex shapes using this control method and provide considerable insight to the growth mechanism.

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