Journal of NanoScience, Nanoengineering & Applications Volume 1, Issue 2, Sep, 2011, Pages 52-58. _____________________________________________________________________________________________

Synthesis of Nanocrystalline Bismuth Ferrite by Solution Combustion Synthesis Method

V. Sesha Sai Kumar*, K. Venkateswara Rao, Ch. Shilpa Chakra, A. Shiva Kishore Goud & T. Krishnaveni Centre for Nano Science and Technology, Institute of Science and Technology, Jawaharlal Nehru Technological University Hyderabad, Hyderabad-500 085, Andhra Pradesh, India Abstract The present paper describes a simple method of nanosized BiFeO3 by the solution combustion synthesis using bismuth and iron nitrates as oxidizers and the combination fuel of citric acid and ammonium nitrate, with fuel to oxidizer ratio (Ψ = 1) one. The average crystallite size of BiFeO3 nanoparticles was estimated from the full width half maximum of the X-ray diffraction peaks of powders using Scherrer’s formula. The ferroelectric transition of the sample at 8310C was detected by differential thermal analysis. Elemental analysis was done using Energy Dispersive X-Ray Analysis (EDX). Fourier Transform Infrared spectroscopy confirms the ferrite bond formation. Thermal analysis was done by Thermal gravimetric-Differential thermal analyzer and obtained results were presented in this paper. Keywords: Solution Combustion Synthesis, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Thermal gravimetric-Differential thermal analysis (TG-DTA), Fourier Transform Infrared Spectroscopy (FTIR) *

Author for Correspondence E-mail: [email protected]

1. Introduction Multiferroic materials are known to exhibit magnetic and electric ordering simultaneously at room temperature [1]. Recent interest in these materials is driven by their potential applications in memory devices and sensors. Bismuth ferrite is one of the very few multi ferroics materials with a simultaneous coexistence of ferroelectric (TC ~ 1103K) and antiferromagnetic (TN ~ 643K) order parameters in perovskite structures. Several ways for the preparation of BiFeO3 were informed [2-10]. Bismuth ferrite obtained by solid state methods [2], rapid liquid phase sintering methods [3], mechanochemical methods [7], and soft chemical methods (such as sol-gel [4, 6], chemical solution deposition [9], etc.) need ISSN 2231 -1777 © STM Journals 2011. All Rights Reserved

high temperature treatments (>8000C) which in turn generates coarser particles. Solution combustion synthesis is becoming one of the most popular methods for the preparation of a wide variety of materials. The main advantage of using this technique is due to its simplicity, the broad applicability range, the self-purifying feature due to the temperatures involved, the possibility of obtaining products in the desired size and shape. Among the various wet chemical processes, the combustion route is found to be simple and cost effective for the synthesis of homogeneous, very fine, crystalline nanopowders without the intermediate decomposition and/or calcination steps which other conventional synthesis routes would require. This method is rapidly 52

Journal of NanoScience, Nanoengineering & Applications Volume 1, Issue 2, Sep, 2011, Pages 52-58. _____________________________________________________________________________________________

emerging as one of the most-convenient methods for the preparation of oxide materials [11-12]. In order to obtain nanosized oxides and to avoid bismuth volatization, a low temperature synthesis is required. Hence in this paper we have selected solution combustion synthesis. In this paper, we report the synthesis of nanosized bismuth ferrite by a solution combustion synthesis, using Bi (NO3)3.5H2O and Fe (NO3)3.9H2O as oxidizers and citric acid as a fuel with the combination of ammonium nitrate [13]. 2. Experimental Procedure Bismuth ferrite nanocrystalline powder was synthesized by a solution combustion synthesis. 0.1M Bi (NO3)3.5H2O and 0.1M Fe (NO3)3.9H2O were dissolved in 2N nitric

acid. Citric acid with the combination of ammonium nitrate is added to the above metal nitrates in 1:1 molar ratios. Usually any fuel (citric acid, glycine and urea) ends up in combustion products with the traces of carbon. So ammonium nitrate, which acts as an extra oxidizer was used along with citric acid. Ammonium nitrate is not self combustible, but as it is an oxidizing agent, it can assist other materials to burn, even if air is excluded [13]. This solution was then heated on a hot plate until all the liquid is evaporated. The solution containing the above redox mixture boils, foams, catches fire and burns with a smoldering flame leaving a fluffy brown mass at the base of the beaker. After the combustion the fine crystalline powder was further dried in an oven at 1200C for 30min.

2Bi (NO3)3(aq) + 2Fe (NO3)3(aq) + 5C6H8O7 (aq) + 15NH4NO3 (aq) (g) + 50H2O (l) + 30CO2 (g)

2BiFeO3 (s) + 15N2

The above reaction is balanced according to the rocket fuel chemistry. According to propellant chemistry, the valencies of the elements Carbon, Hydrogen, Nitrogen and Oxygen are +4, +1, 0 and -2 respectively. The valency of nitrogen is taken as zero because of its convertibility into molecular nitrogen during combustion. The valencies of metal depend upon metal ions in that compound. The valencies of the metals Bismuth and Iron are +3 and +3 respectively.

investigate the chemical composition of the sample. Thermo gravimetric / differential thermal analysis (A new Exstar TG/DTA 6000 series Thermogravimetry/Differential Thermal Analyser, from Sieko Instruments Incorporated Nano Technologies (SII NT), Japan) has been used to understand the thermodynamics of the BFO powders. FTIR spectrum of the samples (by KBr pellet method) was recorded on an FTIR Spectrometer from Schimadzu, Japan (Model FTIR-8400S).

An X-ray diffractometer (Xpert system from Philips) with a Cu Kα source was used to study the crystallographic phase of the sample. The surface morphology of BFO nanopowders was studied using scanning electron microscope (SEM, Hitachi S 3700N). Energy dispersive X-ray analysis (EDX, Hitachi S 3700 N) was used to

3. Results and Discussion

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Fig 1 shows the X-ray diffraction pattern of the as-prepared BFO sample synthesized by solution combustion synthesis. The XRD pattern is in accord with the powder data of JCPDS Card No: 72-2035 thus indicating the formation of single phase BiFeO3. Also the XRD pattern suggests the formation of 53

Journal of NanoScience, Nanoengineering & Applications Volume 1, Issue 2, Sep, 2011, Pages 52-58. _____________________________________________________________________________________________

highly crystalline BiFeO3 (R3m space group) with rhombohedral perovskite

structure with lattice parameters a= 5.5 Ao and c= 13.35 Ao.

Fig. 1: Room temperature X-ray diffraction pattern of nanocrystalline BiFeO3 sample. The average particle sizes of the powders were estimated using Debye Scherrer’s formula

D

0.9  cos 

where D is the average grain size, λ = 1.1541 Ao( X-ray wavelength) and β is the width of diffraction peak at half maximum for the diffraction angle 2θ. The average particle sizes of BFO particles are 33 nm obtained by Debye-Scherrer’s formula.

(a)

Even if the particles are very small (at nanometric scale) so that the average particle size cannot be estimated by SEM, however this method provides useful information about the aggregation mode of the powders. Fig 2. (a) and (b) exhibits a SEM micro structural image of the phase pure Bismuth ferrite (BFO) nanopowders. The SEM images showed in fig 2. (a) and (b) at different magnifications reveal the presence of BFO nanoparticles. SEM studies also indicated the spherical morphology of the particles with high tendency of agglomeration.

(b)

Fig. 2: (a) and (b) showing the SEM images of BFO nanopowders ISSN 2231 -1777 © STM Journals 2011. All Rights Reserved

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Journal of NanoScience, Nanoengineering & Applications Volume 1, Issue 2, Sep, 2011, Pages 52-58. _____________________________________________________________________________________________

(a)

(c)

(b)

(d)

Fig. 3: (a), (b), (c) and (d) shows EDX mapping of BFO nanopowders tracing O, Fe and Bi elements.

Fig. 4: EDX analysis of BFO nanopowders ISSN 2231 -1777 © STM Journals 2011. All Rights Reserved

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Fig 3. shows the distribution of Bi, Fe and O within an agglomerate of nanoparticles. It is understood that Bi, Fe and O (Fig 3) are well distributed in the sample. Fig 4 shows the EDX spectrum of BFO nanopowder. Energy Dispersive X-ray fluorescence analysis

indicated Bi/Fe atomic percent ratio equal to 0.842~1, [14] which is close to one. The atomic percentages of Bi, Fe and O are 19.44, 22.98 and 57.59 respectively. The percentages show the sample is almost chemically homogeneous

Fig. 5: TG-DTA Analysis of BFO nanopowders. Fig 5. shows the TG-DTA analysis of BFO nanopowder. On the DTA curve, an exothermic peak around 4300C can be assigned to the decomposition of trapped nitrates and carbonaceous matters [15]. DTA studies also indicated that there is an endothermic peak around 9790C which is most probably due to the melting point of BFO [15]. It also reveals an endothermic peak around 843oC which is attributed to

ferroelectric to paraelectric phase transformation in BFO. The Curie temperature of 8430C is in good agreement with a few references [16]. In TG analysis we observed no significant weight loss due to stability in the obtained ceramic BFO material. The weight loss we observed in the sample is 1.72%, this shows the sample is very pure.

Fig. 6: Shows FTIR spectrum of nanocrystalline BFO powder sample

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Journal of NanoScience, Nanoengineering & Applications Volume 1, Issue 2, Sep, 2011, Pages 52-58. _____________________________________________________________________________________________

Fig 6. shows the IR spectrum of nanocrystalline BFO powders. The broad band around 3400cm-1 indicates the presence of OH groups due to water molecule [17]. The intense band at 1381cm-1 is due to trapped nitrates in the sample. The band at 1650cm-1 is due to asymmetric C=O vibrations. The strong peak at 555 cm-1 is due to Fe-O interaction [18, 19].The Fe-O stretching and bending vibrations being characteristics of the octahedral FeO6 groups. The peaks in the range < 1000 cm-1 are difficult to assign. The IR spectrum shows curvature from 1000 - 4000 cm-1 which is a characteristic of semiconductors. 4. Conclusion In this present study, nanocrystalline BiFeO3 is prepared by solution combustion synthesis. Citric acid with the combination of ammonium nitrate and dilute nitric acid in the solution plays a key role in the synthesis of BiFeO3. The synthesis of nano sized BiFeO3 through solution combustion method resulted in the formation of highly crystalline nano sized BiFeO3 perovskite, with good homogeneous distribution. The X-ray Diffraction results indicated rhombohedral phase (R3m) with JCPDS data card no: 72-2035. Scanning electron microscopy with EDX analysis confirmed the chemical homogeneity of the BFO sample. DTA analysis indicated ferroelectric phase transition at 8310C which is Curie’s temperature of the BFO sample. Acknowledgement The author wants to thank Dr. D. Jaya Prakash, Director, Central Analytical Facility, Osmania University – Hyderabad for his help in doing Scanning Electron Microscopy. ISSN 2231 -1777 © STM Journals 2011. All Rights Reserved

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