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Synthesis of nanosized BiFeO3 powdersby co-precipitation methodM. Muneeswaran a , P. Jegatheesan a & N. V Giridharan aa Department of Physics , National Institute of Technology ,Tiruchirappalli 620015 , IndiaPublished online: 15 Aug 2012.

To cite this article: M. Muneeswaran , P. Jegatheesan & N. V Giridharan (2013) Synthesis ofnanosized BiFeO3 powders by co-precipitation method, Journal of Experimental Nanoscience, 8:3,341-346, DOI: 10.1080/17458080.2012.685954

To link to this article: http://dx.doi.org/10.1080/17458080.2012.685954

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Synthesis of nanosized BiFeO3 powders by co-precipitation method

M. Muneeswaran, P. Jegatheesan and N.V. Giridharan*

Department of Physics, National Institute of Technology, Tiruchirappalli 620015, India

(Received 30 November 2011; final version received 10 April 2012)

A soft chemical co-precipitation method is proposed for the synthesis of nanosized multiferroicBiFeO3 (BFO) powders. Pure phase (BFO) powder was obtained by controlling the chemical co-precipitation process, pH level and calcination temperature. The evolutions of phase constitutionand structural characteristics showed that the BFO powders had R3c crystal structure underoptimised conditions. Crystallite sizes deduced from X-ray diffraction line width analysis werefound to be within 27–31 nm. Fourier transform infrared spectra showed that strong bandcorresponds to the Fe–O stretching and O–Fe–O bending vibrations. Dielectric and leakagebehaviours were also measured at room temperature.

Keywords: BiFeO3; multiferroic; co-precipitation

1. Introduction

In the past few years much attention has been paid to multiferroics, because they exhibits bothferroelectricity and ferromagnetism at room temperature. These materials also exhibitmagnetoelectric effect by virtue of electric polarisation; this can be induced in the material bythe application of a magnetic field, and magnetisation can be induced by the application of anelectric field. This is a rare phenomenon, since ferroelectricity and ferromagnetism normally aremutually exclusive [1]. The main multiferroic perovskite oxides studied so far include BiFeO3

(BFO), BiMnO3 and ReMnO3 (Re¼Y,Tb,Ho-Lu). Among them, BFO has a high Curietemperature (TC¼ 830�C) and a high Neel temperature (TN¼ 370�C), and has more desirablepotential technological applications in memory devices, sensors and optical filter sand smartdevices [2]. Among the multiferroic materials, BFO has rhombohedrally distorted perovskitestructure with space group R3c at room temperature. The ceramic phase pure BFO compound isdifficult to achieve, as secondary phases such as Bi2O3, Bi2Fe4O3 and Bi25FeO39 are reported tosystematically appear due to the kinetics of phase formation.

In recent years, the synthesis of BFO nanoparticles by several methods such as solid-state [3],ferrite precursor [4], hydrothermal [5], sol–gel [6] and co-precipitation [7] were reported. In solid-state route, nitric acid leaching is applied to eliminate the impurity phases after the calcinationof mixed bismuth and iron oxides [8]. Similar leaching method is also adopted for the BFOceramics prepared by the sol–gel synthesis [9]. Morphologies tunable synthesis of bismuthferrites using the hydrothermal method were also reported [10]. The resulting size of BFOnanoparticles was sometimes large (up to several 100 nanometres) although no impurities werefound in the final products. Using the same method, Chen et al. [11] prepared pure phase BFO

*Corresponding author. Email: giri@nitt.edu

© 2013 Taylor & Francis

Journal of Experimental Nanoscience, 2013Vol. 8, No. 3, 341–346, http://dx.doi.org/10.1080/17458080.2012.685954

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nanocrystallites at 200�C using KOH concentration of 4M. But again, most of the literaturereports on the hydrothermal method involve acids. Similarly, using nitric acid as an oxidisingagent to synthesise nanosized bismuth ferrite using a soft chemical route with tartaric acid as atemplate material were also reported [12].

In this study, the BFO nanopowders were synthesised using a soft chemical approach inwhich distilled water is used as a solvent. Furthermore, the pH values of the solutions werealtered by ammonia hydroxide used as a precipitating agent.

2. Experimental

All chemicals reagents used in these experiments are of high purity without any furtherpurification. Typical synthesis procedures are as follows. First Bi(NO3)3 � 5H2O andFe(NO3)3 � 9H2O were dissolved in 200mL of double distilled water and stirred for about20min to form a clear solution. By a precisely controlled chemical precipitation process, variouspH levels were tried through synchronised dropping of mixture of ammonia (2.5M) and distilledwater (10M) solution to get the reaction product. These precipitates were kept at roomtemperature for about 24 h and were washed several times with double distilled water to removeunreactant products and then filtered. Final products were dried in hot air oven at 100�C forabout 5 h. These powders were annealed at 600�C for 2 h. The phase identification of thesamples was examined on a Rigaku (D/Max ultima III) X-ray diffractometer using Cu-Karadiation. Fourier transform infrared (FT-IR) spectroscopy was performed to identify the localstructure of the particles by using Thermo Scientific Nicolet 10. The morphology of the preparedparticles was observed using scanning electron microscope (SEM S-3000N, HITACHI, Japan).The dielectric and leakage studies were performed with HIOKI 3532-50 LCR meter andKiethley 6517, respectively.

3. Results and discussion

Figure 1(a) shows the X-ray diffraction (XRD) pattern of BFO powders synthesised at differentpH values and annealed at 600�C for 2 h. BFO powders synthesised at pH 9.8 had secondaryphases such as Bi25FeO39 and Bi2Fe4O9. This is because of the kinetics of formation, someimpurity phases are always obtained along with BFO as the major phase during synthesis. Theexistence of Bi2Fe4O9 as impurity phases has been reported by several authors [13].

BFO powders synthesised at pH 10.4 and 10.8 had lower percentage of the secondary phase.The percentage of impurity phase calculated from XRD patterns was found to be 9.5%, 1.7%and 1.2% for the powders prepared at pH value of 9.8, 10.4 and 10.8, respectively. In contrast, aslight shift in peaks towards lower angle was observed with increase in pH value, as shown inFigure 1(b). This may be due to the lattice distortion [14].

Further, crystallite size was evaluated using the Debye–Scherrer formula [15], D ¼ 0:9�� cos �

where D is the average crystalline size, � the wavelength of incident X-ray (1.5406 A), � thediffraction angle in degree and � the full width at maximum in radians. The calculated crystallitesizes were found to be �31, 28 and 27 nm for pH 9.8, 10.4 and 10.8, respectively, as shown inFigure 1(c) including the error analysis.

Figure 2(a) shows the FT-IR spectrum of the as-prepared and annealed BFO powders. Thebroad absorption band in the range 3500–3200 cm�1 is assigned to N–H stretching [16]. This maybe overlapped with O–H stretching. The bands at 800–850 and 350–1410 cm�1 are due tothe existence of the trapped nitrates on the surface [12]. Moreover, the band at around1011–1305 cm�1 is attributed to NO�

3 stretching vibration [17]. The expanded spectrum in therange 400–800 cm�1 is shown in Figure 2(b). In the as-prepared powders, broad bands werelocated between 600 and 400 cm�1, which were generated due to the vibration of Fe–O and Bi–O.

M. Muneeswaran

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nanocrystallites at 200�C using KOH concentration of 4M. But again, most of the literaturereports on the hydrothermal method involve acids. Similarly, using nitric acid as an oxidisingagent to synthesise nanosized bismuth ferrite using a soft chemical route with tartaric acid as atemplate material were also reported [12].

In this study, the BFO nanopowders were synthesised using a soft chemical approach inwhich distilled water is used as a solvent. Furthermore, the pH values of the solutions werealtered by ammonia hydroxide used as a precipitating agent.

2. Experimental

All chemicals reagents used in these experiments are of high purity without any furtherpurification. Typical synthesis procedures are as follows. First Bi(NO3)3 � 5H2O andFe(NO3)3 � 9H2O were dissolved in 200mL of double distilled water and stirred for about20min to form a clear solution. By a precisely controlled chemical precipitation process, variouspH levels were tried through synchronised dropping of mixture of ammonia (2.5M) and distilledwater (10M) solution to get the reaction product. These precipitates were kept at roomtemperature for about 24 h and were washed several times with double distilled water to removeunreactant products and then filtered. Final products were dried in hot air oven at 100�C forabout 5 h. These powders were annealed at 600�C for 2 h. The phase identification of thesamples was examined on a Rigaku (D/Max ultima III) X-ray diffractometer using Cu-Karadiation. Fourier transform infrared (FT-IR) spectroscopy was performed to identify the localstructure of the particles by using Thermo Scientific Nicolet 10. The morphology of the preparedparticles was observed using scanning electron microscope (SEM S-3000N, HITACHI, Japan).The dielectric and leakage studies were performed with HIOKI 3532-50 LCR meter andKiethley 6517, respectively.

3. Results and discussion

Figure 1(a) shows the X-ray diffraction (XRD) pattern of BFO powders synthesised at differentpH values and annealed at 600�C for 2 h. BFO powders synthesised at pH 9.8 had secondaryphases such as Bi25FeO39 and Bi2Fe4O9. This is because of the kinetics of formation, someimpurity phases are always obtained along with BFO as the major phase during synthesis. Theexistence of Bi2Fe4O9 as impurity phases has been reported by several authors [13].

BFO powders synthesised at pH 10.4 and 10.8 had lower percentage of the secondary phase.The percentage of impurity phase calculated from XRD patterns was found to be 9.5%, 1.7%and 1.2% for the powders prepared at pH value of 9.8, 10.4 and 10.8, respectively. In contrast, aslight shift in peaks towards lower angle was observed with increase in pH value, as shown inFigure 1(b). This may be due to the lattice distortion [14].

Further, crystallite size was evaluated using the Debye–Scherrer formula [15], D ¼ 0:9�� cos �

where D is the average crystalline size, � the wavelength of incident X-ray (1.5406 A), � thediffraction angle in degree and � the full width at maximum in radians. The calculated crystallitesizes were found to be �31, 28 and 27 nm for pH 9.8, 10.4 and 10.8, respectively, as shown inFigure 1(c) including the error analysis.

Figure 2(a) shows the FT-IR spectrum of the as-prepared and annealed BFO powders. Thebroad absorption band in the range 3500–3200 cm�1 is assigned to N–H stretching [16]. This maybe overlapped with O–H stretching. The bands at 800–850 and 350–1410 cm�1 are due tothe existence of the trapped nitrates on the surface [12]. Moreover, the band at around1011–1305 cm�1 is attributed to NO�

3 stretching vibration [17]. The expanded spectrum in therange 400–800 cm�1 is shown in Figure 2(b). In the as-prepared powders, broad bands werelocated between 600 and 400 cm�1, which were generated due to the vibration of Fe–O and Bi–O.

Figure 1. (Colour online) (a) XRD pattern of BFO powders prepared with various pH values annealed at600�C; (b) shifting of diffraction peak (024) with respect to the pH values and (c) variation of crystallite sizewith pH value.

Figure 2. (Colour online) (a) FT-IR spectrum of the as-prepared and annealed BFO powder and (b)expanded FT-IR spectrum in the range between 400 and 800 cm�1.

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But strong bands appeared at 555 and 445 cm�1 in the annealed powders prepared at different pHvalues. The peaks at 555 and 445 cm�1 are attributed to the modes of Fe–O stretching vibrationsand O–Fe–O bending vibration, respectively. This is the characteristic of the FeO6 octahedra inthe perovskites [18].

Figure 3(a) and (b) shows the SEM image of the prepared BFO powders at pH 9.8 and 10.4.The powders prepared at pH 9.8 exhibit plate-like nonuniform morphology. Interestingly, whenthe pH level was increased from 9.8 to 10.4, the particle size was reduced. It should be carefullynoted that at pH 9.8, large rectangular particles were observed which may correspond to theimpurity phase such as Bi2Fe4O9, and as the pH value increases to 10.4 rectangular structuresslowly disappear and form small spherical particles.

The frequency dependence of dielectric constant and dielectric loss at room temperature inthe frequency range from 100Hz to 1MHz at different pH values measured are as shown inFigure 4. It was observed that powders prepared at pH 10.8 had higher dielectric constant valuescompared to powders prepared at pH 10.4 and 9.8. This may be attributed to the lowerpercentage of impurity phase in the powders prepared at pH 10.8. Also dielectric constant and

Figure 4. Frequency-dependent dielectric constant and dielectric loss of BFO ceramics prepared withdifferent pH values.

Figure 3. SEM image of BFO powders prepared at pH: (a) 9.8 and (b) 10.4.

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But strong bands appeared at 555 and 445 cm�1 in the annealed powders prepared at different pHvalues. The peaks at 555 and 445 cm�1 are attributed to the modes of Fe–O stretching vibrationsand O–Fe–O bending vibration, respectively. This is the characteristic of the FeO6 octahedra inthe perovskites [18].

Figure 3(a) and (b) shows the SEM image of the prepared BFO powders at pH 9.8 and 10.4.The powders prepared at pH 9.8 exhibit plate-like nonuniform morphology. Interestingly, whenthe pH level was increased from 9.8 to 10.4, the particle size was reduced. It should be carefullynoted that at pH 9.8, large rectangular particles were observed which may correspond to theimpurity phase such as Bi2Fe4O9, and as the pH value increases to 10.4 rectangular structuresslowly disappear and form small spherical particles.

The frequency dependence of dielectric constant and dielectric loss at room temperature inthe frequency range from 100Hz to 1MHz at different pH values measured are as shown inFigure 4. It was observed that powders prepared at pH 10.8 had higher dielectric constant valuescompared to powders prepared at pH 10.4 and 9.8. This may be attributed to the lowerpercentage of impurity phase in the powders prepared at pH 10.8. Also dielectric constant and

Figure 4. Frequency-dependent dielectric constant and dielectric loss of BFO ceramics prepared withdifferent pH values.

Figure 3. SEM image of BFO powders prepared at pH: (a) 9.8 and (b) 10.4.

dielectric loss values decreased gradually as the frequency increased from 100Hz to 80 kHz andthen become almost constant up to 1MHz for all pH values. These observations may beexplained by the phenomenon of dielectric dispersion evidently. Such a strong dispersion seemsto be a common feature in ferroelectric materials concerned with ionic conductivity, which isreferred to as low-frequency dielectric dispersion [19]. When the frequency increases, the relativeeffect of ionic conductivity becomes small and as a result, the frequency dependence of dielectricconstant becomes weak [20].

Generally, the high leakage current in BFO-based materials is attributed to the space charges(such as oxygen vacancies) induced mainly by bivolatilisation [21]. The oxygen vacancies aretrapping centres for electrons, and the electrons trapped there can be readily activated forconduction by the applied electric field and thus increase the leakage current density of theceramics [22]. Figure 5 shows the electric field dependence of leakage current density (J–E) forthe powders prepared at pH 9.8, 10.4 and 10.8. As pH value increased, decrease in the leakagecurrent density was observed. The leakage current measured at pH 10.8 had lower valuecompared to pH 9.8 due to the presence of impurity phases at lower pH value.

4. Conclusion

Multiferroic BFO powders were synthesised through a soft chemical co-precipitation route. Purephase (BFO) powder was obtained at a desired pH level, and a calcination temperature of 600�C.Crystallite sizes deduced from XRD line width analysis were found to be within 27–31 nm. FT-IRspectra reveal that absorption bands at 555 and 445 cm�1 are due to the stretching vibration ofFe–O and the bending vibration of O–Fe–O bond, respectively. Higher dielectric and low leakagebehaviours observed for the powders prepared at pH 10.8 confirms better phase purity.

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