Type of presentation: Poster

MS-3-P-2855 CoxFe3-xO4 films for sensor applications

MADIGOU V.1,2, VILLAIN s.1,2, BENDAHAN M.1,3, ARAB M.1,2, BERNARDINI S.1,3, LEROUX C.1,2
1IM2NP UMR CNRS 7334 , 2Université de Toulon, BP20132, 83957 La Garde Cedex, France, 3Aix Marseille Université, Av.Escadrille Normandie Niemen, 13397 Marseille Cedex, France
madigou@univ-tln.fr

Spinel ferrites are important technological materials due to their magnetic properties [1]. During last decade spinel ferrites have been studied as semiconducting gas sensors [2-3]. The spinel structure is cubic with the general formula MxFe3-xO4 where M is a divalent metallic ion; depending on the composition of the metallic cation, ferrites exhibit n or p type conductivity. In this work, we have studied the cobalt ferrites CoxFe3-xO4. Previously, we have synthetized, by a new one pot solvo-thermal method, small and highly crystallized nanoparticles of cobalt ferrites [4]. We have showed that the semi-conducting behaviour of the particles changes with the cobalt content [5]. Hence, we were interested in studying cobalt ferrites as thin films for applications in gas sensors. Thin films of CoxFe3-xO4 were realized by spin-coating on Si substrate with Pt interdigitated electrodes. The precursor solution was the same as for the synthesis of nanoparticles (cobalt and iron acetylacetonates). Undecanoïc acid was added to the benzyl alcohol in order to improve the solubility of the acetylacetonates. After deposition, the films were annealed at 500°C in air during 2 hours. The obtained films are homogenous in morphology and in chemical composition; they show a nanostructuration of grains with a mean size of 9.5 nm for x=1 (Fig.1) and 8 nm for x=1.8 (Fig.2). The crystallographic structure was verified by electron diffraction, the pattern is indexed in the expected spinel structure (Fig.3). Figure 4 shows grains of a film for x=1.8, the mean grain size measured is about 6.5 nm which is consistent with the value deduced from the SEM observations. The electrical measurements were carried out under reducing gas (NH3) and oxidizing gas (NO2). Under NH3, for x=1 the film is n-type semi conductor and for x=1.8 is p-type semi conductor. Under NO2, the first results were obtained for x=1.8: the electrical resistance decreases which is a typical response of a p-type semi conductor. These results are very promising and particularly, the electrical response is meaningful under low concentration of gas (10 ppm of NO2).

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[3] Y.-L. Liu, Z.-M. Liu, Y. Yang, H.-F. Yang, G.-L. Shen, R.-Q. Yu, Sensors and Actuators B, 107 (2005) 600.

[4] L. Ajroudi, V. Madigou, S. Villain, N. Mliki, Ch. Leroux J. of Crystal Growth 312 (2010) 2465.

[5] Ch. Leroux, M. Bendahan, V. Madigou, L. Ajroudi, N. Mikli, Sensors and transducers in press (april 2014).

Fig. 1: SEM image of a CoFe2O4 film (mean size particles 9.5 nm)

Fig. 2: SEM image of a Co1.8Fe1.2O4 film(mean size particles 8 nm)

Fig. 3: Electron diffraction pattern of the particles of a Co1.8Fe1.2O4 film (spinel structure)

Fig. 4: TEM image of a Co1.8Fe1.2O4 film (nanoparticles are well crystallized)