The detection function of a sensing material is dependant of a high surface to volume ratio, but also to the exposed crystallographic facets. It should then be possible to tailor the reactivity and sensitivity of the sensing materials by controlling their shape and size, for a given composition [1]. We already showed that the composition of the cobalt ferrite Co_xFe_3-xO₄ influences their catalytic properties [2]. In order to understand and control the gas sensing properties as well as the catalytic properties, we synthesized cobalt ferrites as nanoparticles with various shapes and sizes. Nanoparticles with specific shapes allow to study the influence of the cristallographic facets, hence the cation distribution at the surface, in the gas interaction with the particles.

            By solvothermal methods, we synthesised CoFe₂O₄ nanooctahedra (Fig. 1) and nanocubes (Fig. 2). Conventional TEM coupled with EDS, high resolution TEM, environmental TEM, were carried out in order to understand the mechanisms involved in the growth of the grains and their reaction under gas. Octahedron-like nanoparticles of CoFe₂O₄ were submitted to H₂ -O₂ cycles, at ambient temperature, under 1mbar gas pressure in an TEM 300 kV. The {100} facets extended which led to truncated octahedra and the {111} facets became more rounded under oxygen. The phenomenon was reversible and rounded particles under O₂ became facetted under H₂ (Figure 3). These CoFe₂O₄ nanooctahedra exhibit a high sensibility to oxidative gases like NO₂  at low gas concentration, and the shape effect on sensibility was clearly demonstrated.The study of nanocubes under oxydo reduction cycles is under progress.

[1] C. Wang, L. Yin , L. Zhang, D. Xiang and R. Gao, Sensors 2010, 10, 2088-2106

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

[3] A. L. Lopes-Moriyama, V. Madigou, C. Pereira de Souza, Ch. Leroux Powder Tech., 256,482-489 , 2014

Acknowledgments :

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

Figures:

Figure 1 : Cobalt ferrite nanooctahedron

Figure 2 : Cobalt ferrite nanocube

Figure 3 : Shape changes of a cobalt ferrite ferrite nanoparticle during a oxydo-reduction cycle

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