Cobalt ferrite nanoparticles have scientific and technological interest due to their magnetic properties, good chemical stability and low cost, combined with catalytic properties. These factors allow their use in various applications, such as ferrofluid technology [1], catalysts [2] and gas sensors [3].  One way to improve the catalytic properties of the cobalt ferrite is to control the size and the morphology of the nanoparticles. Studies show that crystallites which expose only a particular family of crystallographic planes have enhanced catalytic activity [4].We already obtained nanooctahedron exposing only {111} facets, 20 nm in size, by  a solvothermal method [5]. Another way to enhance catalytic properties is to control the composition of cobalt ferrites; previous studies showed that high amount of cobalt favors the catalytic conversion of methane [2]. The aim of this study is to synthesize cobalt ferrite nanoparticles with different compositions (CoFe₂O₄ and Co_1.8Fe_1.2O₄) and morphologies, in order to control and optimize the catalytic properties.

The powders were obtained by solvothermal synthesis using different solvents and precursors. The control of the shape was realised using different surfactants. For cobalt ferrite Co_1.8Fe_1.2O₄ cobalt nitrate and iron nitrate were used as precursors,water and ethylene glycol as solvents. The experiments showed that the solvent has a significant influence on the powder composition.  When water or a mixture of water and ethylene glycol were used as solvents, two phases were identified in the final product: Co(OH)₂ and Co_1.5Fe_1.5O₄ (fig. 1). When only ethylene glycol was used, a pure phase with homogeneous composition was obtained: Co_1.8Fe_1.2O₄ with the spinel structure and size around 8 nm (fig. 2).

In order to obtain different morphologies of CoFe₂O₄, cobalt acetylacetonate and iron acetylacetonate with benzyl alcohol as solvent were used with different amount of oleic acid and oleylamine as surfactants. The cobalt ferrite powder produced by both oleic acid and oleylamine has a heterogeneous composition containing some crystals with cubic shape (fig. 3). Using only acid oleic as surfactant led to a CoFe₂O₄ powder with homogeneous composition. The shapes of the particles are nearly cubic or octahedral (fig.4). These preliminary results indicate that shape of the nanoparticles is controled by the amount and nature of surfactant.

[1]      J. Li, D. Dai, X. Liu, Y. Lin, Y. Huang, L. Bai, J. Mater. Res. 22 (2007) 886-892.

[2]      L. Ajroudi, S. Villain, V. Madigou, N. Mliki, Ch. Leroux, J. Cryst. Growth 312 (2010) 2465–2471.

[3]      C. Xiangfeng, J. Dongli, G. Yu, Z. Chenmou, Sensors Actuators B Chem. 120 (2006) 177–181.

[4]      N. Ballarini, F. Cavani, S. Passeri, L. Pesaresi, A.F. Lee, K. Wilson, 366 (2009) 184–192.

[5]      A.L. Lopes-Moriyama, V. Madigou, C.P. de Souza, Ch. Leroux, Powder Technol. 256 (2014) 482–489.

Acknowledgments

This work was done in the general framework of the CAPES COFECUB Ph-C 777-13 french – brazilian cooperation project.

Figures:

Fig. 1: TEM image of Co1.5Fe1.5O4 nanoparticles and a Co(OH)2 platelet.

Fig. 2: TEM image of Co1.8Fe1.2O4 nanoparticles, along with the size distribution.

Fig. 3 TEM image of a CoFe2O4 nanocube synthesized with oleic acid and oleylamine as surfactant.

Fig. 4: TEM image of CoFe2O4 nanoparticles synthesized with only oleic acid as surfactant.

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