Low temperature synthesis of bismuth ferrite nanoparticles

by a ferrioxalate precursor method

Sushmita Ghosh, Subrata Dasgupta *, Amarnath Sen, Himadri Sekhar Maiti

Central Glass & Ceramic Research Institute, 196 Raja S.C. Mullick Road, Kolkata, West Bengal 700032, India

Received 28 April 2005; received in revised form 5 July 2005; accepted 8 July 2005

Available online 3 August 2005

Abstract

The synthesis of bismuth ferrite by solid-state reaction of Bi2O3 and Fe2O3 results in the formation of multiphase

products. Even coprecipitation followed by calcination leads to the formation of impurity phases. Here, we report

the synthesis of magnetoelectric bismuth ferrite by a ferrioxalate precursor method. In this process, bismuth ferrite,

synthesized through solutions of some specific salts led to the formation of phase pure (perovskite) nanocrystalline

powder (11–22 nm as evident from X-ray diffraction analysis) at a temperature of 600 8C. The synthesized powderswere characterized by X-ray diffractometry, thermogravimetry and differential thermal analysis, Fourier trans-

formation infrared spectroscopy and scanning electron microscopy. The synthesis route is simple, energy saving

and cost-effective. Such nanosized bismuth ferrite powder may have a potential application in making lead free

piezoelectric materials for actuators as well as magnetoelectric sensors.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; B. Chemical synthesis; C. Infrared spectroscopy

1. Introduction

Compounds exhibiting simultaneously electric and magnetic dipoles are available in nature or have

been synthesized in the laboratory [1]. BiFeO3 is one of the few materials, which exhibits simultaneously

ferroelectric (Tc � 1083 K) and antiferromagnetic ordering (Tn � 657 K) [2]. Because of this magneto-

electric coupling, BiFeO3 based systems may be used to develop novel applications [3–6] in the field of

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Materials Research Bulletin 40 (2005) 2073–2079

* Corresponding author. Tel.: +91 33 24733496; fax: +91 33 24730957.

E-mail address: sdasgupta@cgcri.res.in (S. Dasgupta).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.07.017

radio, television, microwave and satellite communication, bubble memory devices, audio-video and

digital recording, etc. Though BiFeO3 was discovered in 1960, it failed to make an impact in the

electronic industries due to leakage current problems arising out of nonstoichiometry. This is primarily

because of the difficulty in obtaining a stoichiometric single phase material. The solid-state route for the

synthesis of BiFeO3 results in the formation of Bi2O3/Bi2Fe4O9 phases which have to be removed by

washing with HNO3 [7]. This results in the formation of a coarser powder and the process suffers from

poor reproducibility. Simultaneous precipitation/coprecipitation [1] either generates an impurity phase or

needs higher calcination temperature (830 8C) which in turn produces coarser powder.

In the present communication, we report a method of synthesis of stoichiometric, nanosized,

magnetoelectric bismuth ferrite by a ferrioxalate precursor method. The phase pure powder can be

obtained by calcination at a temperature of 600 8C. The process is simple, energy saving and cost-

effective. We also show that this process is superior to the age old Pechini’s [8] autocombustion method

of synthesis, where the latter failed [9] to produce phase pure BiFeO3.

2. Experimental procedure

Bismuth ferrite powder was synthesized by a solution evaporation route. 0.1 M Bi(NO3)3 and 0.1 M

Fe(NO3)3, dissolved in 2N nitric acid, were mixed together in a beaker. The parent solutions were

standardized for iron and bismuth as stated earlier [10]. Oxalic acid (0.2 M) was then added to the

resulting solution under constant stirring. This solution was then heated on a hot plate under continuous

stirring condition to its boiling temperature until all the liquid evaporated. There was an immense

evolution of brown fumes towards the end of the reaction leaving a fluffy orange mass at the base of the

beaker. This fine powder was dried on a hot plate at 130 8C for 30 min. The powder obtained was calcined

at 600 8C for 2 h to get phase pure BiFeO3. The yield was 90%.

We also followed [9] Pechini’s method of autocombustion to make BiFeO3 using various citrate nitrate

ratio and the resultant powders were analyzed.

The powders were characterized using X-ray diffraction (Phillips, PW 1710) analysis using Cu Ka

radiation. The powder morphology was studied using SEM (Leo 430i). The thermal studies (TGA and

DTA) were carried out using Shimadzu (TGA-50) analyzer. FTIR spectra studies were madewith Nicolet

Model 5PC FTIR.

3. Results and discussion

The XRD patterns of the synthesized powder prepared via oxalate route and heated in air at different

temperatures is shown in Fig. 1. The synthesized powder (green powder) is amorphous in nature (Fig. 1a).

The XRD patterns shown in Fig. 1 reveal that as the calcination temperature increases, the peaks become

sharper, and the crystalline phase formation is completed at 600 8C. The XRD patterns are in excellent

accord with the powder data of JCPDS Card No. 20-169. The particle sizes (11–22 nm) of the powders

were calculated using Scherrer’s formula:

D ¼ 0:9l

b cos u

S. Ghosh et al. /Materials Research Bulletin 40 (2005) 2073–20792074

where D is the average grain size, l = 1.541 A (X-ray wavelength) and b is the width of the diffraction

peak at half maximum for the diffraction angle 2u.The typical SEMmicrographs of the BiFeO3 powder heat treated at 600 8C is presented in Fig. 2. Fig. 3

shows the TG curve of the uncalcined powder at a heating rate of 10 8C/min. The TG curve depicts a

weight loss up to 600 8C (10.83%), which is due to the presence of trapped nitrates, residual carboxylate,

unburnt carbon and entrapped water. The IR spectrum of the green synthesized powder (Fig. 4a) shows

[11] a broad band at around 3400 cm�1 due to the n(OH) of the lattice water molecule. Intense band at

1650 cm�1 corresponds to the nasym (C O) of coordinated oxalate. The bands in the zone of 1320–

1410 cm�1 is attributed to nsym (C O) of coordinated oxalate which is overlapped with intense nitrate

S. Ghosh et al. /Materials Research Bulletin 40 (2005) 2073–2079 2075

Fig. 1. X-ray diffractograms of bismuth ferrite powder: (a) uncalcined, (b) calcined at 500 8C and (c) calcined at 600 8C.

Fig. 2. Scanning electron micrograph of the BiFeO3 powder calcined at 600 8C.

peak [12] at 1384 cm�1. The peaks in the range of 400–600 cm�1 is difficult to predict but the presence of

Fe–O bonding at 560 cm�1 is observed in both uncalcined and calcined (Fig. 4b) samples. Indications of

some carbonaceous matter, viz., C–C and M–C–O ring is observed here which disappears after

calcination process. Differential thermal analysis (Fig. 5) up to 700 8C at a scan rate of 20 8C/min

shows a phase change at 385 8C which is the Neel temperature of BiFeO3 [6].

The Pechini method [8] of auto ignition also generated a number of impurity phases including Bi2O3,

Bi2Fe4O9 and Bi46Fe2O72 [9].

S. Ghosh et al. /Materials Research Bulletin 40 (2005) 2073–20792076

Fig. 3. TGA curve of the uncalcined powder.

Fig. 4. FTIR spectra of BiFeO3 powder: (a) uncalcined and (b) calcined at 600 8C.

The uniqueness of oxalic acid as a chelating agent in synthesizing BiFeO3 probably resides in the

formation of transition metal ferrioxalate [13] precursor (Fig. 6), Bi[Fe(ox)3] as was observed by

Randhawa [14] in the synthesis of ferrites by thermolysis of ferrioxalate precursors. In our case, the

ferrioxalate complex undergoes in situ decomposition which occurs towards the end of the reaction when

HNO3 concentration is high enough to oxidize the precursor resulting in the generation of NO2, CO2 and

water vapor along with product. That is the reason we could see only residual carboxylates attached to the

metal atom in the IR spectra. It may be that the reaction was favoured as the reacting metal atoms come in

close proximity in the precursor which on decomposition in presence of nitric acid resulted active local

heteroatomic units giving rise to the reaction product.

The failure of Pechini’s route of autocombustion in producing BiFeO3 in the present case may be due

to the following reason. In this case, the tridentate chelation [15] by citrate is available from X-ray

crystallographic reports. The complex of bismuth(III) with a tridentate tetra-anionic citrate ligand adopts

a well-defined dimeric arrangement imposed by chelate coordination of the pendant carboxylate

moiety to a neighboring bismuth centre (Fig. 7). Due to the dimeric nature of the complex (in contrast

to heteroatomic metal ferrioxalate for oxalic acid), the possibility of formation of Bi(III)–Fe(III)

S. Ghosh et al. /Materials Research Bulletin 40 (2005) 2073–2079 2077

Fig. 5. DTA curve of BiFeO3 powder (calcined at 600 8C).

Fig. 6. Bismuth–iron ferrioxalate precursor.

heteronuclear arrangement is less likely via Pechini’s route. This resulted in the formation of impurity

phases via citrate method. Also, it is conceivable that during auto ignition process, the higher temperature

and an excess of carbonaceous materials lead to the formation of impurity phases. Such impurity phases

are always observed during the solid-state synthesis of BiFeO3.

4. Conclusion

The present study describes a simple low temperature synthesis route for preparing BiFeO3

nanopowders through a solution evaporation method. Oxalic acid is used here as the chelating agent.

Compared to the conventional solid-state reaction process and coprecipitation method, BiFeO3 phase can

be formed at a much lower temperature through ferrioxalate precursor method. Further, the oxidation of

ferrioxalate precursor by HNO3 was accompanied by the evolution of various gases (such as CO2, NO2

and water vapour) and the gas evolution helped the product to obtain a fine grain structure. Oxalic acid

and nitric acid present in the solution play the key role for the synthesis of BiFeO3 at a low temperature.

Acknowledgement

The financial support of the CSIR Network project CMM 002 is gratefully acknowledged.

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