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2.1. Synthesis of cobalt ferriteCoFe2O4 (sample CFO) was synthesized by chemical coprecipitationby the following procedure. A 0.2 M solutionwas prepared by dissolving Co(NO3)26H2O (A.C.S.reagent, Aldrich) and Fe(NO3)39H2O (puriss, Fluka) in deionized water. The precipitating agent was a 5 M NaOH(reagent grade, Aldrich) solution with 100% molar excess. Both solutions were heated separately to 65 C and thenmixed under constant stirring. The temperature was raised to 80 C for 20 min, to completely crystallize the spinel phase. The precipitate was centrifuged and thoroughly washed with distilled water. The cobalt ferrite was kept in a water suspension. One small portion was dried at 80 C for characterization. 2.2. Preparation of a ferrofluid The cobalt ferrite/water suspension was heated to 80 C under gentle stirring. Oleic acid (reagent grade, Sigma– Aldrich) was added to the suspension at 30 wt.% with respect to the cobalt ferrite. The temperature was raised to 90 C for 30 min and then cooled to 60 C. Octane (reagent grade, Sigma–Aldrich) was added and the mixture was stirred for another 30 min. During this process, the nanoparticles were transferred from the aqueous phase to the organic phase, thus forming the ferrofluid. Prior to its collection, the ferrofluid was washed with distilled water in a separatory funnel to remove the non-modified particles. A gravimetric study of a 2.5 g aliquot was carried out to determine the content of inorganic material in the ferrofluid by weight difference before and after calcination at 700 C. A sample of the surface-modified ferrite was reserved before the addition of octane to determine the amount of oleic acid that reacted with the particles. The sample was dried at 80 C before performing a thermogravimetric analysis on a TGA Q500 by TA Instruments, from room temperature to 700 C at 5 min1. 2.3. Synthesis of cobalt ferrite–barium titanate core–shelltype nanostructures Core–shell-like nanostructures of CoFe2O4–BaTiO3 were synthesized by the sol–gel technique. Ba(C2H3O2)2 (A.C.S. reagent, Sigma–Aldrich) was mixed with titanium (IV) n-butoxide (reagent grade, Sigma–Aldrich), stearic acid (95%, Sigma–Aldrich) and glacial acetic acid (Aldrich) at 90 C to form a sol. The ferrofluid was added to the sol at an adequate amount so that the final nanostructures have a content of 20, 30, 40, 50 and 60 wt.% of cobalt ferrite with respect to the barium titanate, namely samples Co20, Co30, Co40, Co50 and Co60, respectively. Anhydrous 2-methoxyethanol (Aldrich) was added to promote the gel formation. The obtained gels were dried overnight at 80 C and sintered at 700 C for 2 h under the following conditions: (a) heating rate of 1.8 C min1 from room temperature to 350 C; (b) heating rate of 1.5 C min1 from 350 C to 700 C and (c) cooling rate of 4 C min1 from 700 C to room temperature, to avoid cracking of the barium titanate, which is the shell in the nanostructures and is expected to be subjected to high stresses. One additional sample without ferrofluid was also prepared as a reference (sample BTO). All samples were washed with a 0.1 M acetic acid solution to dissolve impurities such as barium carbonate [18]. We note that if the stearic acid, glacial acetic acid and 2-methoxyethanol were omitted from the preparation, the resulting material consisted of a simple mixture of individual cobalt ferrite and barium titanate nanoparticles. |
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