ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 272–276 (2004) 2357–2358 Structural stability study of cobalt ferrite-based nanoparticle using micro Raman spectroscopy M.A.G. Solera,*, T.F.O. Meloa, S.W. da Silvaa, E.C.D. Limab, A.C.M. Pimentab, V.K. Garga, A.C. Oliveiraa, P.C. Moraisa a ! Instituto de F!ısica, Universidade de Bras!ılia, Nucleo de F!ısica Aplicada, 70919-970 Bras!ılia-DF, Brazil b ! 74001-970 Goiania-GO, # Instituto de Qu!ımica, Universidade Federal de Goias, Brazil Abstract Micro Raman scattering was used to study the structural stability of cobalt ferrite-based (CoFe2O4) nanoparticles, under illumination with the 514 nm line, at 7 mW laser power. Different samples were investigated after performing the steps of the magnetic ﬂuid (MF) preparation. Raman spectra of samples peptized at 0.25 mol/l perchloric acid showed features similar to bulk maghemite. However, samples peptized at 0.75 mol/l perchloric acid showed features similar to the Fe3O4 phase. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Mm; 74.62.Bf; 63.22.+m Keywords: Magnetic ﬂuid; Raman scattering; Cobalt ferrite; Structural stability The chemical synthesis of spinel ferrite-based nanoparticles, passivated and peptized as stable ionic magnetic ﬂuids (MFs), represents a very important step towards the engineering of speciﬁc magnetic carriers. The structural stability of the MF nanoparticles is essential in all technical and biological applications [1]. Compared to magnetite (Fe3O4) nanoparticles, recent results observed in the visible range showed the higher structural stability of cobalt ferrite (CoFe2O4) nanoparticles upon laser illumination [2]. This observation indicates that cobalt ferrite nanoparticles are more reliable as magnetic drug carriers in biological applications, as for instance in the photodynamic therapy [3]. Also, it is well known that in initially stable ionic MFs the suspended nanoparticles could progressively be dissolved in low pH values, harming the long-term MF stability [4]. In this study, micro Raman spectroscopy was used to investigate the inﬂuence of the peptization condition on the structural stability of cobalt ferrite nanoparticles suspended as ionic MFs. *Corresponding author. Tel.: +55-61-3072900; fax: +55-613072363. E-mail address: [email protected] (M.A.G. Soler). Stable CoFe2O4-based ionic MF samples were obtained following the three-step procedure described in the literature [5]. In the ﬁrst step, cobalt ferrite nanoparticles (sample S1) were synthesized by coprecipitating Co(II) and Fe(III) ions in alkaline medium. In the second step, sample S1 was submitted to the passivation process using concentrated ferric nitrate solution under boiling condition (sample S2). Finally, in the third step, peptization of non-passivated (sample S1) and passivated (sample S2) nanoparticles were performed using different perchloric acid concentration (0.25 and 0.75 mol/l). Twenty-four hours after peptization the four MF samples were precipitated using acetone. The precipitates were dried in air at room temperature to produce four samples (S3, S4, PS1, and PS2). Samples S3 and S4 refer to sample S1 peptized at 0.25 and 0.75 mol/l, respectively. Samples PS1 and PS2 refer to sample S2 peptized at 0.25 and 0.75 mol/l, respectively. The average CoFe2O4 nanoparticle diameter was estimated by X-ray diffraction measurements as 8.6 nm. The Raman system used to record the roomtemperature spectra was a commercial triple spectrometer equipped with a CCD detector. The 514 nm line of a CW Argon ion laser was used for excitation of the 0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.582 ARTICLE IN PRESS M.A.G. Soler et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2357–2358 2358 samples, and the incident power was kept at 7 mW, elevating the temperature up to 200 C at the focus. It was observed that samples S1, S2, PS1, and S3 present similar Raman spectra (see Fig. 1(a)). The best curve ﬁtting of the spectra, using lorentzian bandshaped lines, showed the presence of ﬁve structures at 220, 310, 467, 624, and 690 cm1. This ﬁnding is in very good agreement with the ﬁve optical active Raman modes (A1g+Eg+3F2g), characteristic of the cubic inverse-spinel structure O7h ðF d 3% mÞ space group [6]. Further, except for the downshift observed in all Raman features, the spectrum showed in Fig. 1(a) is similar to the spectrum of bulk maghemite (g-Fe2O3) presented in the literature [7]. The observed downshift is due to the largest Co-atom mass compared to the Fe-atom mass. The similarities between the Raman features of the nominal CoFe2O4 samples and the Raman features of bulk maghemite suggests that samples S1, S2, PS1, and S3 present the crystal structure of the cobalt-modiﬁed gFe2O3 reported in the literature [8]. This is supported by the room-temperature values of the hyperﬁne ﬁelds of 463 and 414 kOe, attributed to Fe3+ at A and B sites [9], . as obtained by Mossbauer Spectroscopy. Finally, excitation of samples S1, S2, PS1, and S3 in a wide CoFe2O4 - passivated range of optical intensity (0.7–70 mW), do not induce any signiﬁcant change in the Raman features showed in Fig. 1(a). The Raman spectra of samples PS2 and S4 are similar to one another and quite different from the Raman spectra of samples S1, S2, PS1, and S3. The Raman spectrum of sample PS2 (see Fig. 1(b)) presents seven lines at 190, 300, 340, 475, 516, 610, and 680 cm1. Except for two extra Raman modes at 190 and 475 cm1, the ﬁve observed lines are typical of the cubic inverse-spinel structure, characteristic of the nominal Fe3O4 structure [7]. The extra Raman peak at 475 cm1 is related to the O-site mode that reﬂects the local lattice effect in the octahedral sublattice of CoFe2O4. Note that the Raman spectra recorded from samples PS2 and S4, at 0.7 mW laser power, are similar to the Raman spectra quoted in Fig. 1(a). However, differences between the spectra of samples PS1 and PS2, at 7 mW laser power, are not clear yet. Atomic absorption measurements indicated that samples peptized at 0.75 mol/l release more Co-atom to the aqueous medium than samples peptized at 0.25 mol/l. Further, there are evidences in the literature that Co- and Fe-atoms may self-rearrange in the crystalline structure when heated above room temperature [10]. Finally, the differences between the samples PS1 and PS2 could be attributed to the annealing process induced by the laser excitation. The authors acknowledge the ﬁnancial support of the Brazilian agencies FINEP/CTPETRO, CAPES, FINATEC, and CNPq. 514 nm 7 mW Raman Intensity References (a) 0.25 mol/L (b) 0.75 mol/L 200 300 400 500 Wavenumber 600 700 800 (cm-1) Fig. 1. Raman spectra of samples PS1 (peptized at 0.25 mol/l) and PS2 (peptized at 0.75 mol/l). . [1] T. Goetze, C. Gansau, N. Buske, M. Roeder, P. Gornert, M. Bahr, J. Magn. Magn. Mater. 252 (2002) 399. [2] S.W. da Silva, T.F.O. Melo, M.A.G. Soler, E.C.D. Lima, M.F. da Silva, P.C. Morais, IEEE Trans. Magn. 39 (2003) 2645. [3] A. Gusens, A. Derycke, L. Missiaen, D. De Vos, J. Huwyler, A. Eberle, P. de Witte, Int. J. Cancer 101 (2002) 78. [4] F.A. Tourinho, R. Franck, R. Massart, J. Mater. Sci. 25 (1990) 3249. [5] P.C. Morais, V.K. Garg, A.C. Oliveira, L.P. Silva, R.B. Azevedo, A.M.L. Silva, E.C.D. Lima, J. Magn. Magn. Mater. 225 (2001) 37. [6] B. Ammundsen, G.R. Burns, M.S. Islam, H. Kanoh, J. Rozi"ere, J. Phys. Chem. B 103 (1999) 5175. [7] D.L.A. de Faria, S. Ven#ancio Silva, M.T. de Oliveira, J. Raman Spectrosc. 28 (1997) 873. [8] R.A. McCurrie, Ferromagnetic Materials: Structure and Properties, Academic Press, London, 1994, p. 175. [9] Xinyong Li, C. Kutal, J. Alloys Compounds 249 (2003) 264. [10] T. Yu, Z.X. Shen, Y. Shi, J. Ding, J. Phys.: Condens. Matter 14 (2002) L613.