Materials Letters 64 (2010) 486–488

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Materials Letters

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Magnetic and optical properties of multiferroic bismuth ferrite nanoparticles bytartaric acid-assisted sol–gel strategy

Xiong Wang a,⁎, Yan'ge Zhang b, Zhibin Wu a

a Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, Chinab Institute of Surface Micro and Nano Materials, Xuchang University, Xuchang, 461000, China

⁎ Corresponding author. Tel./fax: +86 25 84313349.E-mail address: wx@ustc.edu (X. Wang).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.11.059

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 November 2009Accepted 24 November 2009Available online 2 December 2009

Keywords:Bismuth ferriteMultiferroicSol–gelMagnetismUV absorption

Pure BiFeO3 nanoparticles have been successfully synthesized through the tartaric acid-assisted sol–gelmethod at relatively low temperature. The as-prepared nanoparticles were characterized by a variety oftechniques. The success in preparing pure BiFeO3 may be attributed to the formation of heterometallicpolynuclear complexes in the tartaric acid system. The ferroelectric phase transition (TC=851 °C) wasdetermined, revealing the ferroelectric nature of the as-prepared BiFeO3 nanoparticles. The result ofmagnetic measurement indicates the weak ferromagnetic order of BiFeO3 nanoparticles at roomtemperature, which may be ascribed to the size confinement effect. The observed strong absorption in theUV region will enable BiFeO3 nanoparticles to be potentially used as promising photocatalytic decompositionmaterial.

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1. Introduction

Multiferroic materials, as multifunctional oxides, have beeninvestigated intensively during the last decade, including BiFeO3,BiCrO3, BiMnO3, etc. [1–4]. Due to the simultaneous presence offerroelectricity and antiferromagnetic order with weak ferromagne-tism at room temperature, multiferroics have potential applications innewmagnetic and ferroelectric devices design as well as fundamentalphysics studies [5–7]. Among them, bismuth ferrite (BiFeO3) belongsto the perovskite class of materials with a rhombohedrally distortedcell having the polar R3c space group. It is an antiferromagnetic,ferroelectric, and ferroelastic material with electrical, magnetic andstructural ordering temperatures well above room temperature [8,9].It possesses simultaneous ferroelectric (TC ∼1103 K) and G-typeantiferromagnetic properties above room temperature (TN ∼643 K).However, its multiferroic properties and potential applications weregreat hampered by the leakage current arising from impurities,defects or nonstoichiometry, which is mainly due to the difficulty inpreparing pure BiFeO3 phase [10,11].

Conventionally, BiFeO3 have been synthesized by the traditionalsolid-state calcination at high temperature (above 800 °C) with thehelp of nitric acid leaching out the unavoidable impurities, such asBi2Fe4O9 and Bi25FeO40 [12,13]. Herein, we report a tartaric acid-assisted sol–gel strategy to synthesize pure BiFeO3 nanoparticles byrapid sintering at relatively low temperature. The results of thermalanalysis reveal the ferroelectric nature of the as-prepared BiFeO3

nanoparticles. The magnetism and optical absorption property of theBiFeO3 nanoparticles were also investigated.

2. Experimental

Typically, 5 mmol bismuth nitrate pentahydrate [Bi(NO3)3·5H2O]and ferric nitrate nonahydrate [Fe(NO3)3·9H2O] in stoichiometricproportion are separately dissolved in the diluted nitric acid (20%HNO3), and then mixed together in a beaker. Tartaric acid (C4H6O6,10 mmol) and ethylene glycol (HOCH2CH2OH, 5 mL) were succes-sively added to the resulting transparent solution under constantmagnetic stirring. The obtained solution was kept at 140 °C until afluffy dried gel was obtained. The resultant dried gel was transferredto a crucible, and preheated to 400 °C at a heating rate of 5 °C/min toremove organic compounds and NO3

−. Then the finely groundpowders were loaded into the furnace maintained at 500 °C for anhour before quenching to room temperature in air. Finally, the as-prepared sample was washed with distilled water for several times,centrifuged, and dried at 80 °C. As a comparison, the same procedurewas repeated using citric acid (C6H8O7) as chelating agent instead oftartaric acid.

The crystalline structure was identified by X-ray diffraction (XRD)using a Philips X'Pert Pro Super diffractometer with graphitemonochromatized Cu Kα radiation (λ=1.54178 Å). The morphologyof the sample was observed by scanning electron microscopy (SEM,Hitachi X-650) and transmission electron microscopy (TEM, Hitachi,Model H-800) with an accelerating voltage 200 kV. Thermogravi-metric analysis and differential thermal analysis (TGA-DTA, ShimadzuTA-50) were performed to determine the ferroelectric phasetransition temperature (TC). The magnetic property was measured

Fig. 1. XRD patterns of the obtained powders calcined at 500 °C for an hour, using(a) tartaric acid and (b) citric acid as chelating agent, respectively.

Fig. 3. TGA and DTA curves of the resulting BiFeO3 nanoparticles in the range from 600to 900 °C.

487X. Wang et al. / Materials Letters 64 (2010) 486–488

using a vibrating sample magnetometer (VSM) at room temperature.The optical absorption spectrum was recorded on a Shimadzu UV-2401PC UV–Vis recording spectrophotometer.

3. Results and discussion

Fig. 1 represents the XRD patterns of the powders calcined at500 °C by the sol–gel method. The pure perovskite BiFeO3 can beobtained by the rapid sintering at 500 °Cwith tartaric acid as chelatingagent (shown in Fig. 1a). All the reflection peaks can be readilyindexed to the pure rhombohedral structure of BiFeO3 with latticeparameters of a=b=5.576 Å and c=13.867 Å [space group: R3c(161)], which are in good agreement with the reported data (JCPDSno.: 86-1518).When the same synthesis procedurewas repeatedwithcitric acid as chelating agent instead of tartaric acid, the impuritiesBi2Fe4O9 and Bi25FeO40 (markedwith * and∇) appear unavoidably (asshown in Fig. 1b). The success in preparing pure BiFeO3 might beattributed to the formation of heterometallic polynuclear complexesin the tartaric acid system [14]. Firstly, tartaric acid forms bonds tometal ions through two carboxyl and two hydroxyl groups leading toform a stable polynuclear complex. Then in the presence of ethyleneglycol, esterification occurs at 140 °C. Finally, the resulting gelnetwork breaks on sintering at 500 °C to form the pure bismuthferrite. In contrast to tartaric acid, the dimeric nature of the citratecomplex and the higher temperature during auto ignition process aswell as the excess of carbonaceous materials inevitably give rise to theformation of impurity phases [14,15].

Fig. 2. (a) SEM and (b) TEM images of the as-prepared BiFeO3 sample by tartaric acid-assisted sol–gel method.

Fig. 2 exhibits the SEM and TEM images revealing the morphologyof the as-obtained pure BiFeO3 powders. From Fig. 2a, the uniformnanoparticles with fairly narrow size distribution were observed. Themean particle size is about 60–90 nm with polyhedron morphology(Fig. 2b). The unique tartaric acid-assisted sol–gel system facilitate themolecular level mixing of the metal ions, leading to the formation ofpure BiFeO3 nanoparticles with homogeneous morphology at rela-tively low temperature.

Thermogravimetric and differential thermal analysis (TG-DTA)was performed to determine the ferroelectric phase transitiontemperature (TC). The TGA and DTA curves ranging from 600 to900 °C of the BiFeO3 nanoparticles are shown in Fig. 3. Over the wholetemperature range, the weight of the sample remains constant with adistinct endothermic peak at around 851 °C, which is attributed to theferroelectric-to-paraelectric phase transition. The Curie temperature(TC) of 851 °C for the nanoparticles is similar to that reported byRoginskaya et al. [16], revealing the ferroelectric nature of the as-prepared BiFeO3 nanoparticles. There is no agreement on the values ofthe Curie point for BiFeO3 reported by various research groups [17–20]. It seems that the Curie temperature of bismuth ferrite could varyslightly depending on the processing conditions.

The room temperature magnetic property was also measured byVSM. Fig. 4 shows the variation of magnetization (M) with an appliedfield (H) of the BiFeO3 nanoparticles. A typical magnetic hysteresisloop was observed, indicating that the BiFeO3 nanoparticles show aweak ferromagnetic order at room temperature, which is quite

Fig. 4. Room temperature M–H hysteresis loop of the BiFeO3 nanoparticles.

Fig. 5. (a) UV–vis spectrum of BiFeO3 nanoparticles and (b) (αEphoton)2 versus Ephotoncurve based on the spectrum data of (a).

488 X. Wang et al. / Materials Letters 64 (2010) 486–488

different from the linear M–H relationship in the bulk [20,21]. Thesimilar ferromagnetic phenomenon was also observed in BiFeO3 films[22], nanotubes[20], and nanocrystallites [11,23]. The origin of theweak magnetic property in our sample may be attributed to the sizeconfinement effect of the nanostructures. Further research is inprogress.

Fig. 5 exhibits the optical absorption spectrum of the BiFeO3

nanoparticles. A strong absorption in the UV region was foundassociated with the band gap absorption (Fig. 5a). The absorptionband gap Eg can be determined by the equation (αhν)2=B(hν−Eg),where hν is the photo energy Ephoton, α is the absorption coefficient,and B is a constant relative to the material [24,25]. The absorptionedge at around 470 nm was observed, and its energy band gap Egdetermined from Fig. 5b is about 2.6 eV, consistent with previousreports [18,26].

4. Conclusions

The pure BiFeO3 nanoparticles with mean particle size about 60–90 nmhave been successfully synthesized by the tartaric acid-assistedsol–gel method at relatively low temperature. The weak roomtemperature ferromagnetism of the as-prepared BiFeO3 nanoparticlesmay be ascribed to the size confinement effect of the nanostructures.The strong absorption in the UV region will enable BiFeO3 nanopar-

ticles to be used potentially as promising photocatalytic decomposi-tion material.

Acknowledgements

The authors are grateful to the financial support from theOutstanding Scholars Supporting Programme of NUST (AB39114).Prof. Zhang thanks the aids from the Education Department of HenanProvince (2008A150023) and the Natural Science Foundation ofHenan Province, China (082300440120).

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