Synthesis and characterization of nanosized ferrites ...

JOURNAL OF APPLIED PHYSICS 109, 07B510 (2011)

Synthesis and characterization of nanosized ferrites from the thermolysis of strontium and barium tris(succinato)ferrate(III) precursors Harpreet Kaur,a) Jashanpreet Singh, Karun Gandotra, and B. S. Randhawa Department of Chemistry, Guru Nanak Dev University, Amritsar, Punjab 143005 India

(Presented 15 November 2010; received 23 September 2010; accepted 2 November 2010; published online 23 March 2011) Thermal analysis of alkaline earth metal tris(succinato)ferrate(III) precursors, M3[Fe(C4H4O4)3]2.xH2O (M ¼ Sr, Ba) has been monitored isothermally and nonisothermally employing various physico-chemical techniques viz. simultaneous thermo gravimetry-derivative thermo gravimetry-differential scanning calorimetry (TG-DTG-DSC), XRD, Mo¨ssbauer spectroscopy, VSM, and TEM to characterize the intermediates/end products. Simultaneous TG-DTG-DSC depicts the sequence of the solid-state reactions in which dehydration is the first step followed by the decomposition of anhydrous precursors to yield iron(II) oxalate, Fe(II)C2O4, and respective metal oxalate intermediate species in the temperature range 150–210 C. A subsequent oxidative decomposition of iron(II) species leads to the formation of a-Fe2O3 and respective alkaline earth metal carbonate in the temperature range of 250–350 C. Finally, the alkaline earth metal carbonates decompose into their respective oxides in the temperature range of 590–680 C, followed by a solid-state reaction (>700 C) with a-Fe2O3 leading to the formation of ferrites of the stoichiometry, Sr2Fe2O5 and BaFe2O4. TEM studies reveal the formation of ferrite C 2011 American Institute of Physics. nanoparticles with average grain size of 25–30 nm. V [doi:10.1063/1.3537949]

Contemporary studies of magnetic nanoparticles are significantly motivated by their current and potential applications in magnetic separation and drug delivery and as contrast agents for magnetic resonance imaging1 and coloring pigments, colloidal mediators for cancer magnetic hyperthermia.2 A suitable approach to solve the outlined task is the use of complex magnetic oxides because their magnetic properties can be suitably tailored by modification of their intrinsic properties depending on their composition and structure. Owing to their large cationic size, strontium and barium cations present in respective ferrites exhibit electrical conductivity and interesting dielectric properties due to the simultaneous presence of Fe(III) and Fe(IV) oxidation states.3,4 These materials have been employed as electrode materials and heterogeneous catalysts.5 Specifically, BaFe2O4 has been known to be utilized for the low temperature preparation of high density ferrites and as suspension material for ferromagnetic fluids.6,7 With persisting high values for Curie temperature, intrinsic coercivity, saturated and remanent magnetizations, chemical stability, and resistance to corrosion, these magnetic materials can significantly be used in microwave devices. Keeping in view their remarkable applications, synthesis of nanosized strontium and barium ferrites from the thermolysis of strontium and barium tris(succinato)ferrate(III) precursors has been undertaken. Alkaline earth metal tris(succinato)ferrates(III), M3[Fe(C4H4O4)3]2.xH2O(M Sr, Ba) were prepared by mixing stoichiometric quantities of aqueous solutions of ferric nitrate, its respective alkaline earth metal succinate, and succinic acid. The reaction mixture was stirred vigorously and a)

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then concentrated on water bath until the formation of a brown product. The product was filtered, washed with cold water, dried, and stored in a vacuum desiccator. The identity of the precursors was established by chemical analysis: [C, % ¼ 22.85(Obs.); 23.06 (Calc.), H, % ¼ 3.48 (Obs.); 3.52 (Calc.), Fe,% ¼ 8.82 (Obs.); 8.96 (Calc.), Sr, % ¼ 21.05 (Obs.); 21.14 (Calc.)] and [C, % ¼ 19.92(Obs.); 20.07 (Calc.), H, % ¼ 3.25 (Obs.); 3.34 (Calc.), Fe,% ¼ 7.64 (Obs.); 7.80 (Calc.), Ba, % ¼ 28.72 (Obs.);28.85 (Calc.). for strontium ferrisuccinate decahydrate and barium ferrisuccinate dodecahydrate, respectively. The experimental details for recording thermo gravimetryderivative thermo gravimetry-differential scanning calorimetry (TG-DTG-DSC), Mo¨ssbauer, XRD, TEM, and magnetic data are reported elsewhere.8 Simultaneous TG-DTG-DSC curves of strontium tris (succinato)ferrate(III) decahydrate in flowing air atmosphere at a heating rate of 5 C min1 has been displayed in Fig. 1. The dehydration commences at 105 C and completes at 145 C as indicated by a mass loss of 14.8% (calc. loss ¼ 14.4%). This is accompanied by an endotherm at 125 C and a DTG peak at 120 C. The anhydrous precursor then undergoes an endothermic decomposition till a mass loss of 34% is reached at 190 C, suggesting the formation of strontium oxalate and iron(II) oxalate intermediates (calc. loss ¼ 34.9%). The respective DTG peak lies at 180 C, which is endotherm in DSC. The presence of iron(II) oxalate has been confirmed by recording the Mo¨ssbauer spectrum of the residue obtained by isothermal calcination of the precursor at 200 C for 30 min. As heating continues, the intermediate species undergoes an exothermic decomposition process to yield a-Fe2O3 and strontium carbonate as supported by a mass loss of 51% at 290 C (calc. loss ¼ 51.8%). Both DSC and DTG

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FIG. 1. Simultaneous TG-DTG-DSC curves of strontium tris(succinato)ferrate(III) decahydrate (a) and barium tris(succinato)ferrate(III) dodecahydrate (b) precursors.

show shoulders along with the respective peaks, indicating that two thermal processes [decompositions of strontium oxalate and iron(II) oxalate] are taking place simultaneously. The presence of a-Fe2O3 has been confirmed by Mo¨ssbauer parameters of the residue obtained by calcining the precursor at 300 C for 30 min (Table I). After remaining stable up to 625 C, strontium carbonate undergoes an endothermic decomposition into SrO as supported by a mass loss of 62% at 680 C (calc. loss ¼ 62.4%). At higher temperature, a solid-state reaction of SrO with a-Fe2O3 yields strontium ferrite, Sr2Fe2O5. Both of these thermal changes are reflected by the presence of an endotherm at 670 C (due to the evolution of CO2) followed by an exotherm at 740 C supporting the solid-state reaction. The existence of Sr2Fe2O5 has been confirmed by recording the Mo¨ssbauer spectrum [Fig. 2(a)] of the residue, which displays three overlapping sextets and a central doublet. The outermost sextet, with internal magnetic field value of 51.2 T, indicates the presence of a-Fe2O3 (unreacted).9,10 A central doublet with d and D values of 0.31 and 0.58 mm/s, respectively, is due to paramagnetic fraction of the ferrite. The Mo¨ssbauer parameters of the inner sextets listed in Table I resemble those reported11 for Sr2Fe2O5. The identity of the ferrite has also been confirmed by recording the XRD powder pattern [Fig. 3(a)] of the end product. A TEM micrograph [Fig. 4(a)] reveals the formation of Sr2Fe2O5 with an average grain size of 25 nm. Figure 2(b) shows the simultaneous TG-DTG-DSC curves of barium tris(succinato)ferrate(III) dodecahydrate in flowing air atmosphere at a heating rate of 5 C min1. The precursor undergoes simultaneous dehydration and decomposition proc-

esses until a mass loss of 33% is attained at 210 C as shown by the slope of TG curve, indicating the formation of barium oxalate and iron(II) oxalate intermediate species (calc. loss ¼ 32.9%). There exists a respective endotherm(doublet) with peak maxima at 185 and 195 C for this decomposition step. Corresponding to this step, DTG shows a broad peak centered at 195 C indicating that decomposition and dehydration steps are clubbed together. The presence of iron(II) oxalate has been confirmed by recording the Mo¨ssbauer data of the residue (Table I) obtained by isothermal calcination of the parent complex at 250 C for 30 min. As heating continues, barium oxalate and iron(II) oxalate undergo an oxidative decomposition to yield barium carbonate and a-Fe2O3, respectively, as supported by a mass loss of 47% (calc. loss ¼ 47.6%) at 310 C. A very sharp exotherm at 290 C with DH ¼ 2195.46 kJ mol1 confirms this oxidative decomposition step. DTG also shows the corresponding strong signal at 285 C. The presence of aFe2O3 has been confirmed by recording the Mo¨ssbauer data of the residue obtained by calcining the parent complex at 400 C for 30 min (Table I). After remaining stable up to 590 C, barium carbonate undergoes an endothermic decomposition process to yield BaO with a mass loss of 57% (calc. loss ¼ 56.8%) at 650 C. DSC shows a corresponding characteristic endothermic peak at 600 C due to the evolution of CO2. At higher temperature, a solid-state reaction occurs between oxides, that is, a-Fe2O3 and barium oxide to yield barium ferrite, BaFe2O4, as supported by an exotherm at 680 C. Formation of BaFe2O4 has been confirmed by recording the Mo¨ssbauer data and XRD powder pattern [Fig. 3(b)] of the final thermolysis residue. The room temperature Mo¨ssbauer spectrum [Fig. 2(b)] of the

TABLE I. Mo¨ssbauer parameters of thermolysis products obtained from strontium and barium precursors recorded at 300 K.

Precursor

Temp. of Calcination ( C)

da (mm=s)

D (mm=s)

Bb (T)

Cationic (Fe3þ) distribution (%)

Assignment

Strontium tris(succinato) ferrate(III) decahydrate

200 300 750

1.34 0.44 0.47 0.43 0.34 0.31

1.82 0.07 0.03 0.08 0.04 0.58

… 51.8 51.2(S) 50.6(S) 46.2(S) (CD)

… … 36 33(oct) 26(tet) 5

Fe(II)C2O4 a-Fe2O3 a-Fe2O3 Sr2Fe2O5

Barium tris(succinato) ferrate(III) dodecahydrate

250 400 750

1.30 0.43 0.43 0.24 0.38

1.88 0.08 0.03 0.06 0.84

… 51.6 49.3(S) 42.6(S) (CD)

… … 25.8(oct) 30.8(tet) 43.4

Fe(II)C2O4 a-Fe2O3 BaFe2O4

a

w.r.t. pure iron absorber. B: Internal magnetic field in Tesla (T); tet: tetrahedral site oct: octrahedral site; S ¼ Sextet; CD ¼ Central Doublet.

b


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FIG. 4. TEM micrographs of Sr2Fe2O5 (a) and BaFe2O4 (b) obtained.

and BaFe2O4, respectively, by treating them with 2N HNO3, followed by repeated washings with distilled water. Sr2Fe2O5 has been found to be antiferromagnetic,12 but BaFe2O4, with promising values of saturation magnetization (41.2 emu/g) and Curie temperature (456 C), exhibits ferrimagnetic behavior as supported by Mo¨ssbauer studies. Based on preceding discussion, the following conclusions may be drawn. •

•

FIG. 2. (Color online) Mo¨ssbauer spectra of the final thermolysis products obtained from the strontium precursor (a) and the barium precursor (b).

residue exhibits a complex Zeeman pattern showing two sextets along with a central doublet, suggesting the coexistence of ferrimagnetic and superparamagnetic domains of barium ferrite formed. TEM micrograph displayed in Fig. 4(b) reveals the formation of BaFe2O4 with an average particle size of 30 nm. The unreacted SrO and BaO contents present in the final thermolysis residues were removed to get the pure Sr2Fe2O5

•

The alkaline earth metal ferrisuccinate precursors yield different types of products, that is, BaFe2O4 and Sr2Fe2O5 during pyrolysis due to the different electrostatic properties (polarizability, cationic size, etc.) of alkaline earth metal cations. The precursor method adopted to obtain nanosized ferrites have certain advantages over the conventional ceramic method viz. (i) Ferrites are formed at lower temperature and in shorter time, (ii) An enormous amount of heat liberated during thermolysis of carbonaceous succinate precursors autocatalyze the further decomposition process, thus lowering the external temperature required for the preparation of ferrites, (iii) Ferrite nanoparticles of greater surface area are obtained, and (iv) No milling of starting materials is required (necessary in the ceramic method) that can introduce lattice defects in the ferrite obtained; which, in turn, affect its permanent magnetic properties. Thermal decomposition of ferrisuccinate precursor provides a low temperature route for the preparation of single phase ferrite nanoparticles. The obtention of ferrite (BaFe2O4) with the promising value of saturation magnetization (41.2 emu/g) as well as reduced particle size (30 nm) makes the precursor technique a novel method for the synthesis of nano-particles.

1

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FIG. 3. (Color online) XRD patterns of the final thermolysis product obtained from the strontium precursor (a) and the barium precursor (b).
