Low-temperature synthesis of BiFeO3 nanopowders via a sol–gel method Jia-Huan Xua, Hua Kea, De-Chang Jia , a, , Wen Wanga and Yu Zhoua

aInstitute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology,

Harbin 150001, China

Received 29 February 2008; 

revised 28 April 2008; 

accepted 29 April 2008. 

Available online 20 June 2008.

Abstract

Bismuth ferrite (BiFeO3) nanopowders were synthesized by a sol–gel method at the temperature as low as 450 °C.

The obtained sol was transparent and homogenous when the mixture ionic concentration was properly controlled with

the help of ethylene alcohol. The preparation process of crystalline BiFeO3 could be divided into three stages: (i) the

evaporation of organics and decomposition of nitrogen-containing organics below 200 °C; (ii) the collapse of Bi–Fe

gel network in the temperature range of 200–300 °C and (iii) the formation of BiFeO3 nanopowders by the solid-state

reaction between Bi2CO3O2 and Fe2O3.

Keywords: BiFeO3; Sol–gel processes; Thermal analysis

Article Outline

1.

Introduction

2.

Experimental2.1. Synthesis of BFO by a sol–gel method

2.2. Characterization

3.

Results3.1. The decomposition process of dried gels

3.2. Structural characterization

4.

Discussion

5.

Conclusions

Acknowledgements

References

1. Introduction

Multiferroic materials, which exhibit both ferroelectric and magnetic ordering, have attracted broad interests

due to their potential applications for novel magnetoelectric devices and for exploring fundamental science

in the coupling mechanism between electronic and magnetic order parameters. As one of the

representative single-phase multiferroics, BiFeO3 (BFO) is most extensively studied because of its high-

Curie temperature at 1043 K and G-type antiferromagnetic ordering temperature at 655 K [1].

Conventionally, single-phase or doped BFO ceramics with improved resistivity have been mainly

synthesized through solid-state reaction [2], [3] and [4]. In the solid-state route, nitric acid leaching is

required to eliminate the impurity phases, such as Bi2Fe4O9 and Bi25FeO40. The crystallization temperature

of BFO for this method is too high to avoid bismuth loss. Potential applications of BFO in the memory

devices, sensors, satellite communications, optical filters and smart devices are greatly limited due to its

leakage current, which is usually caused by defects and nonstoichiometry. In order to overcome these

disadvantages, various wet chemical methods are applied to prepare pure-phase BFO powders, such as

co-precipitation [5], hydrothermal synthesis [6], ferrioxalate precursor method [7] and microemulsion

techniques [8]. Sol–gel process is also widely used for preparing pure-phase powders and thin films. In the

sol–gel synthesis of BFO, the sol is usually prepared based on citric acid route [9], [10] and [11]. In order to

compensate the evaporation loss of bismuth during postannealing process, the excess bismuth source is

usually added to the solution.

In the present paper, a simple sol–gel method based on ethylene alcohol is used to prepare BFO

nanopowders. The process from the sol to the final inorganic materials is investigated in detail. A

micromechanism for crystallization of BFO gels is proposed.

2. Experimental

2.1. Synthesis of BFO by a sol–gel method

The synthetic procedure of BFO powders by the sol–gel method is outlined in Fig. 1. Bismuth subnitrate

(99.99%, Shanghai Kechang Chemical Reagent) and iron(III) nitrate nonahydrate (98.5%, Tianjin Guangfu

Fine Chemical Research Institute) were separately dissolved into glacial acetic acid with the stoichiometric

ratio Bi/Fe = 1/1. Ethylene alcohol was added under constant stirring after the solution became transparent.

After stirring for half an hour, bismuth solution and iron solution were mixed together. The resultant solution

was transparent, blackish red, and clear. A stable sol was obtained when the final concentration of the

precursor was adjusted in the range of 0.05–0.2 M by adding acetic acid and ethylene alcohol. The

precursor solution was dried at 40 °C for about 1 week to obtain the BFO dried gel. The dried gel was

ground in an agate crucible. Ground precursor powders were calcined in air with a soaking time of 2 h at

temperatures from 200 to 500 °C.

Full-size image (18K)

Fig. 1. Flow diagram for the preparation procedure of the BiFeO3 powder.

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2.2. Characterization

Thermogravimetric analysis and differential scanning calorimeter analysis (TG–DSC, Netzsch, STA 449C)

were performed in air from 35 to 600 °C at a heating rate of 10 °C/min to determine the thermal behaviors of

dried gel. The decomposition products of the dried gel were characterized by gas chromatography–mass

spectrometry (GC–MS, Agilent5975/6890N, cds5000 pyrolyzer). The Fourier transform infrared

spectroscopy (FT-IR, Nicolet Avatar 370 FT-IR spectrophotometer) using pressed-disk technique was used

to monitor gel → crystal transformations. The structure and morphology of the powders calcined at various

temperatures were characterized by an X-ray diffraction (XRD, Rigaku, D/Max 2200VPC) with Cu Kα

radiation and a high-resolution transmission electron microscopy (HRTEM, Philips Tecnai F30).

3. Results

3.1. The decomposition process of dried gels

Pyrolysates of samples were investigated by pyrolytic-GC/MS. Identified compounds by GC/MS were listed

in Table 1. Below 100 °C, the identified products were mainly attributed to the vaporization of starting

solvents and esterification reaction of ethylene glycol and acetic acid. Decomposition products between 100

and 200 °C were mainly nitrogen-containing molecules. N2O is mainly assigned to the decomposition of

non-carbonized anions NO3− in the precursor and the other nitrogen-containing molecules [12]. Among

pyrolysates in the temperature range from 200 to 300 °C, carbon dioxide was assigned to the combustion of

organic substances. Linear organic molecules possibly corresponded to the decomposition products of gel

network or esterification products. With the temperature increasing to 350 °C, the decomposition product

was only carbon dioxide.

Table 1. Pyrolysis poducts identified by GC/MS

Temperature (°C)

Identified compounds

Molecular structure

Accumulated intensities (%)

Possibly originate from

Below Ethylene glycol 98.5 1,2-Ethanediol source

Acetic acid 85.3 Acetic acid source

Ethylene glycol monoacetate

95Reaction products from 1,2-ethanediol and acetic acid

100–200

Nitrous oxide N N O 98.2Decomposition of nitrogen-containing materials

Nitrogen-containing organic pieces

Temperature (°C)

Identified compounds

Molecular structure

Accumulated intensities (%)

Possibly originate from

200–300

Carbon dioxide O C O 89.5Combustion product of organic material

Ethylene glycol diacetate

89Decomposition products after gel network collapsed or esterification products

Diethylene glycol diacetate

90.3

2-(2-(2-Methoxyethoxy)ethoxy)ethyl acetate

79.8

2-Hydroxyethyl formate

89.8

300–350

Carbon dioxide O C O 100Decomposition product of carbonates

Full-size table

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Fig. 2 shows the DSC–TG curve of the dried gel. The two exothermic peaks at 113 and 170  °C were

attributed to decomposition of non-carbonized anions NO3− and other nitrogen-containing molecules. The

exothermic reaction with a largest weight loss (25%) in the temperature range between 200 and 300 °C

could be assigned to the collapse of gel network and combustions of most of the organic materials. A tiny

weight loss occurring between 300 and 450 °C corresponded to the release of carbon dioxide. The peak at

430 °C shown in inset indicated the end of carbon dioxide release. No more weight loss was observed in

the temperature range from 450 to 600 °C corresponding to the phase-crystallization step.

Full-size image (7K)Fig. 2. DSC–TG curve of the dried gel.

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Fig. 3 shows the infrared spectra of thermal-treated powders. The broad band between 3500 and 3000 cm−1

was attributed to O–H stretching [13] originated from ethylene glycol or condensation products, which

decreased in intensity as thermal-treatment temperature increased, but persisted to around 300 °C. The

bands in the frequency range 1600–1400 cm−1 were assigned to the asymmetric and symmetric COO−

stretching vibrations of the acetate ligands [14]. The bands around 1655 and 1300 cm−1 possibly correspond

to the formation of esters during the condensation reaction between ethylene glycol and acetic acid in the

sol-formation process. The bands located at 1440–1350 cm−1 and 1070–1030 cm−1 indicated the existence

of nitrate ions [13], which decreased in intensity with increasing thermal-treatment temperature. The peak at

800 cm−1 of the low-temperature (50 and 140 °C) treated samples was related to NH2 wagging vibration.

Evidence of carbonate groups appeared in the temperature range between 200 and 400 °C, as signified by

the band at 1500–1300 cm−1, as well as at peaks at 1080–1030 cm−1 and 845 cm−1 [13]. The sharp band

around 2335 cm−1 suggested that atmospheric carbon dioxide exist in the sample treated at 400 °C. The

bands between 700 and 400 cm−1 were mainly attributed to the formation of metal oxides. The 560 and

440 cm−1 peaks in 500 °C sample were respectively, assigned to the mode of stretching vibrations along the

Fe–O axis and the mode of the Fe–O bending vibration, being characteristics of the octahedral FeO 6 groups

in the perovskite compounds [15] and [16].

Full-size image (9K)Fig. 3. FT-IR spectrum of powders calcined at different temperatures.

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3.2. Structural characterization

X-ray diffraction patterns of BFO gel powder annealed at different temperatures were shown in Fig. 4. When

thermal treated at and below 200 °C, the powders were still in the amorphous state. The amorphous

precursor gel was fired at 300 °C to make organics burn and decompose. It was observed that crystalline

phases, including bismutite (Bi2O2CO3) and bismuth oxide (Bi2O3), appeared. The phase transformation of

the BFO precursor to perovskite was started at a temperature as low as 400 °C. BFO was completely

crystallized into perovskite phase at 450 °C. The XRD patterns of BFO were consistent with the data of

JCPDS Card No. 71-2494.

Full-size image (31K)Fig. 4. XRD patterns of powders calcined at different temperatures.

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Further identification of the crystalline feature was characterized by scanning transmission electron

microscopy (STEM) and high-resolution TEM. The results are presented in Fig. 5. Crystallites appearing as

tiny dark spots in the amorphous matrix with sizes less than 10 nm could be observed (Fig. 5(a)). The

corresponding HRTEM images were given in Fig. 4(b)–(f). The lattice fringes of the nanocrystals were

observed in Fig. 5(b) and (c). It is clearly seen that at the initial crystallization stage of BFO precursor gels,

some ordered atomic clusters precipitate first from the amorphous matrix, distributed randomly. The lattice

fringes in HRTEM photographs shown in Fig. 5(d) are identified to be ( ) and (1 3 0) planes of Fe2O3

phases across all the domains. Fig. 5(e) illustrates HRTEM image of single-crystalline Bi2CO3O2. The

interplanar spacings of about 2.54 and 2.44 Å corresponded to (2 4 0) and (1 1 2), respectively. Fig. 5(f)

shows a clearly resolved crystalline domain with a uniform interplanar spacings of about 3.95 and 2.79  Å,

which correspond to the (0 1 2) and ( ) planes of a rhombohedral phase BFO crystal.

Full-size image (142K)Fig. 5. (a) STEM image; (b and c) the corresponding HRTEM images of powders calcined at 400 °C.

HRTEM images and the corresponding reduced FFT images (inset) of (d) Fe2O3; (e) Bi2CO3O2; (f)

BiFeO3.

View Within Article

4. Discussion

For BiFeO3 preparation process, different electronegativities (Bi 2.02 and Fe 1.83) of two metal elements

caused different hydrolysis rates. Bismuth element also easily hydrolyzed in the solution. This makes it

difficult to obtain high-quality sol. Ethylene glycol has been proved to be a very good solvent to prepare

metal oxides (i.e. SrBi2Ta2O9 [17] and Ba0.5Sr0.5TiO3 [14]) because the presence of two terminal hydroxyl

groups in the molecule makes it easy for ethylene glycol to readily keep heterometallic units during

hydrolysis reaction. It is reported that the linearly structured molecule of ethylene glycol is favorable for

obtaining stable sol and acts as the template for a dense and linearly aligned structure of the precursor

molecules [14]. The decomposition products between 200 and 300 °C are mainly linear organic molecules.

Ethylene glycol as solvent can keep the different electronegativities of bismuth and iron during hydrolysis

and its linearly structured molecule makes it easy to obtain a stable precursor. For sol–gel process, metal

alkoxides were usually used as the starting materials. Most of the metal alkoxides suffer from high cost,

unavailability, toxicity, and fast hydrolysis rate (thus difficult in controlling the homogeneity of different

components during experimental processes). Cost-effective nitrates as starting materials facilitate to lower

the crystallization temperature of BFO due to their lower decomposition temperature compared with other

metal-salt (i.e., carbonates, chloride). Acetic acid as a catalyst in the sol system can control the hydrolysis

speed and adjust the solution concentration. In addition, the extraneous factors need to be strictly

controlled, such as temperature, humidity, stirring speed. A stable sol is obtained once the hydrolytic–

polymeric reaction reaches a dynamic balance. The sol is kept in a closed container in air for about 5

months without evidence of precipitation or gel formation. The stable sol makes it suitable for being treated

in air for a long time.

The formation of crystalline BiFeO3 undergoes organic removal, crystallization, solid-state reaction. Below

200 °C, the starting reagents and esterification products firstly volatilize and nitrogen-containing materials

decompose as observed by GC/MS (Table 1) and IR results (Fig. 3). With the temperature increasing from

200 to 300 °C, the metal–carbonyl links begin to break and most of organic molecules combust until

polymeric network collapses. GC/MS detected carbon dioxide and linear organic pieces. Carbon dioxide is

mainly from combustion of organic materials. The linear organics are originated from decomposition of the

linearly structured gel network. The linearly structured molecule of ethylene glycol will easily react with the

nitrates to form a stable precursor molecule and will also act as the template for a dense and linearly

aligned structure of the precursor molecules [14]. Based on a micromechanism for crystallization of

amorphous alloys proposed by Lu and Wang [18], a micromechanism for crystallization of BFO gels is

proposed. At the initial crystallization stage, the nucleus can be formed directly from the amorphous matrix

with increasing nucleation rate. The circle in Fig. 5(a) shows that there exist intercrosses between crystal

grains. This behavior implies that the crystal growth process of BFO gels during crystallization mainly

proceeds through the combination or deposition of crystal grains. The randomly oriented and nanometer-

sized crystallites uniformly dispersed in amorphous matrix (Fig. 5(a)), suggesting that the rate of nucleation

is relatively high, and the transformation of amorphous phase to crystalline phase is completed with a

minimum of time. Single atom jumps from amorphous matrix to the crystal nucleus. Then, ordered atomic

clusters were formed with increasing the temperature (Fig. 5(c)), which may be act as nucleation sites,

facilitating the subsequent crystallization process. The grain boundaries are further reconstructed to regular

shape, as is evident in the STEM (Fig. 5(a)). The XRD (Fig. 4) and HRTEM (Fig. 5) show that the structure

of the samples is formed by nanocrystallined grains of Bi2CO3O2 and Fe2O3 embedded in the amorphous

matrix. The absence of well-defined crystalline peaks for Fe2O3 in Fig. 4 is probably because these particles

are too small to be detected by XRD [19] and [20] or percentage composition of iron oxide is relatively

small. Carbon dioxide detected by IR results (Fig. 3) and Bi2O3 phase observed by XRD (Fig. 4) should be

the decomposition products of Bi2CO3O2. Considering the BFO nanocrystalline formation reaction during gel

decomposition, the results suggest that BFO principally forms by the reaction:

Bi2CO3O2 + Fe2O3 → 2BiFeO3 + CO2. The weight loss of carbon dioxide obtained by TG results well agrees

with that calculated by the above reaction equation. With the evaporation of carbon dioxide, BFO single

phase with perovskite structure started to form at a temperature as low as 400 °C and BFO single phase

fully generated at 450 °C (Fig. 3 and Fig. 4).

5. Conclusions

A simple sol–gel method based on ethylene alcohol to synthesize multiferroic BFO is proposed. The

ethylene alcohol plays an important role in the formation process of the transparent and homogenous BFO

sol. The phase-pure BFO powders are obtained at the temperature as low as 450 °C.

Acknowledgements

This research was supported by Program for New Century Excellent Talents in University (NCET-04-0327),

Program of Excellent Team in Harbin Institute of Technology and National Natural Science Foundation of

China (no. 50502013).