Band gap tuning and optical properties of BiFeO3 nanoparticles

Band gap tuning and optical properties of BiFeO3 nanoparticles

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Materials Today: Proceedings xxx (xxxx) xxx

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Band gap tuning and optical properties of BiFeO3 nanoparticles Sheetal Sharma, Manoj Kumar ⇑ Department of Physics and Materials Science and Engineering, Jaypee Institute of Information and Technology, A-10, Sector, Noida 201307, India

a r t i c l e

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Article history: Received 5 January 2020 Received in revised form 23 January 2020 Accepted 27 January 2020 Available online xxxx Keywords: BiFeO3 XRD Optical properties Nanoparticles FTIR

a b s t r a c t BiFeO3 nanoparticles prepared by using sol-gel technique and sintering at different temperatures (500 °C, 600 °C and 700 °C) have been studied. The influence of different sintered temperature on crystal structure and energy band gap of BiFeO3 samples has been investigated. X-ray diffraction results exhibit distorted rhombohedral structure of BiFeO3 samples. However, a small fraction of impurity phase has been observed for sample sintered at 700 °C and therefore, sintering temperature of 600 °C has been found suitable to obtain phase pure BiFeO3 sample. From XRD analysis it has been observed that crystallite size increases from 18 nm to 43 nm with rise in sintering temperature. The presence of various Fe-O and Bi-O bands has been confirmed from infrared spectroscopy. UV–Visible spectroscopy measurements show the variation in optical band gap from 2.5 eV to 2.0 eV with increasing sintering temperature from 500 °C to 700 °C for BiFeO3 samples indicating the tuning of optical band gap of BiFeO3 system in visible region. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.

1. Introduction For the past decade, multiferroic materials have gained a lot of attention for various applications in field of photo-catalytic, synchronous multiferroics harmonic imaging, resistive switching, gas sensors, solar cell and many more [1–6]. Its unique properties such as coexistence of electric and magnetic ordering in single phase are very fascinating and interesting from fundamental physics point of view. Amongst multiferroics, BiFeO3 (BFO) has been extensively explored. Distorted rhombohedral perovskite structure of BFO with R3c symmetry possesses the unit parameters as ar = 5.63 Å, ar = 59.35 [6–9]. In spite of everything, difficulty in obtaining single phase, low magnetization and high leakage current are the challenges associated with BFO [10,11]. Recently, BFO nanoparticles have attracted researchers due to its interesting optical properties in visible range. It is predicted that optical and magnetic properties of BFO have been greatly affected by particle size in nano range [12–19]. There are many reports available on BiFeO3 synthesis by different routes; however, no detailed study is available on synthesis of BiFeO3 nanoparticles by sol-gel technique using tartaric acid route and effect of sintering temperature on optical properties. In this work, we report the synthesis of BiFeO3 samples sintered at different temperatures and the effect of sinter-

ing temperature on structural and optical properties of BiFeO3 samples. 2. Experimental Sol-gel technique was utilized to obtain pure BFO. To prepare BiFeO3 nanoparticles, nitrates of Bi and Fe in 1:1 M ratio were separately dissolved in deionized water. Clear and transparent solution of Bi(NO3)35H2O was obtained by adding few drops of dilute nitric acid with continuous stirring and then (Fe(NO3)39H2O solution was added to it. Tartaric acid was added to the obtained solution of nitrates and stirred at 60 °C for 12 h. The fluffy gel was obtained after drying the transparent solution at 120 °C for 2 days in an oven. The obtained fluffy gel was crushed into fine powder and sintered at 500 °C, 600 °C and 700 °C to get BiFeO3 nanoparticles. The structural analysis of sintered powders was done by X-ray diffractometer (Shimadzu 6000) using CuKa radiation. Fourier Transform infrared (FTIR) spectra of prepared samples were recorded on Perkin Elmer Spectrum BS-III spectrometer and optical band gap was studied by using UV–Visible spectroscopy. 3. Results and discussion 3.1. X-ray diffraction analysis

⇑ Corresponding author. E-mail address: [email protected] (M. Kumar).

Fig. 1(a) exhibits X-ray diffraction (XRD) patterns of BFO nanoparticles sintered at 500 °C, 600 °C and 700 °C. The XRD

https://doi.org/10.1016/j.matpr.2020.01.496 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.

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Fig. 1. (a-d) XRD pattern of BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C. Rietveld refined XRD patterns of BiFeO3 samples sintered at (e) 500 °C (f) 600 °C (g) 700 °C.

study, the XRD patterns of BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C were refined using Rietveld analysis. Fig. 1(e– g) shows the Rietveld refined patterns of BFO powders. The crystallite size of BiFeO3 samples sintered at different temperatures was estimated using Debye-Scherer’s formula: D = kk/(bhklcosh) where D is the crystallite size, k symbolizes the shape factor (0.9), bhkl represents the full width half maxima and k represents wavelength of CuKa radiation (1.5406 Å) [1]. The average crystallite size of 18 nm, 20 nm and 43 nm (Table-1) was estimated for BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C, respectively. The unit cell parameters obtain from refinement of XRD patterns are listed in Table 1.

pattern matches well with JCPDS card number 71-2494 confirming the presence of rhombohedral structure of BFO [10]. However, a small fraction of impurity phase (Bi24Fe2O39) has been observed in the XRD pattern of BiFeO3 sample sintered at 700 °C. This indicates that 600 °C is the optimum sintering temperature to obtain phase pure BiFeO3 nanoparticles. Fig. 1(b–d) shows the magnified view of doublets in the XRD patterns around 2h values of 21.2° and 23°; 30.8° and 32.5°; 38° and 39.8°. It can be seen that with increasing sintering temperature the diffraction patterns slightly shift towards lower 2h values and the peaks become sharper. The sharpening of diffraction peaks indicates the increase in crystallite size with increasing sintering temperature. For detailed structural

Table 1 Rietveld refined parameters of BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C. Sample

BiFeO3500 °C R3c BiFeO3 600 °C R3c BiFeO3 700 °C R3c

Lattice parameters

a = 5.576(Å) c = 13.899(Å) V = 377.332 (Å3) D = 18 nm a = 5.579(Å) c = 13.866(Å) V = 373.744 (Å3) D = 20 nm a = 5.647(Å) c = 14.0118(Å) V = 387.399 (Å3) D = 43 nm

Atoms Position

R-factors (%)

x

y

z

Bi Fe O

0.0 0.0 0.2400

0.0 0.0 0.3206

0.2281 0.0034 0.316

BBragg = 11.8 Rf = 10.8 Χ2 = 1.55

Bi Fe O

0.0 0.0 0.2107

0.0 0.0 0.3206

0.2881 0.0204 0.0833

BBragg = 8.13 Rf = 9.08 Χ2 = 0.77

Bi Fe O

0.0 0.0 0.2307

0.0 0.0 0.3581

0.2402 0.0190 0.1277

BBragg = 13.8 Rf = 11.7 Χ2 = 3.21

Please cite this article as: S. Sharma and M. Kumar, Band gap tuning and optical properties of BiFeO3 nanoparticles, Materials Today: Proceedings, https:// doi.org/10.1016/j.matpr.2020.01.496

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3.2. FTIR analysis Fig. 2 shows infrared spectra of BiFeO3 nanoparticles sintered at different temperature (500 °C, 600 °C and 700 °C). Two broad bands positioned around 450 and 550 cm 1 have been observed. The broad nature of these observed bands may be ascribed to close proximity Fe-O and Bi-O bands. BFO infrared spectra in the spectral range from 400 to 600 cm 1 are recognized by two bands. The absorption peaks corresponding to O-Fe-O bending vibration and Fe-O stretching vibration appear around 445 cm 1 and

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575 cm 1, respectively [1]. The absorption peak at 450 to 525 cm 1 indicate the Bi-O bond in the BiO6 octahedral structure unit [20]. From infrared spectra of BiFeO3 nanoparticles sintered at 500 °C, 600 °C and 700 °C show that the intensity of absorption bands becomes shaper which shows high degree of crystallization at higher temperature. Moreover, the broad peaks at 3450 cm 1 corresponds to stretching vibrations of the O–H group and two absorption bands around 2337 and 2362 cm 1 belong to CO2 modes. The impurity peaks on higher wavenumber may be attributed to the surface adsorbed organic precursors [1] and these impurities peak intensity become small at increasing sintered temperature.

3.3. UV–Visible spectroscopy analysis

Fig. 2. FTIR transmittance spectra of BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C. Inset shows the FTIR spectra in the wavenumber range 400–4000 cm 1.

The absorption spectra of BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C are shown in Fig. 3(a). The hybridization between 3d and 2p orbitals gives rise to the optical band gap of BiFeO3. The energy band gap of these samples has been estimated using classical Tau’s equation: ahm = A(hm-Eg)n, where ‘A’ is a constant, hm denotes the photon energy, Eg denotes the energy band gap, a denotes the absorption coefficient and n is a number having value ½ or 1. The value of n depends upon electronic transition and the value of n = 1/2 allows for direct transition. Fig. 3(b–d) illustrates (ahm)2 versus hm curves for BFO nanoparticles sintered at 500 °C, 600 °C and 700 °C. The value of the band gap is determined by extending the linear portion of the graph up to the X-axis. The estimated values of energy band gap are 2.50 eV, 2.46 eV and 2.00 eV for BFO nanoparticles sintered at 500 °C, 600 °C and 700 °C, respectively. It is worthy to note that the band gap of BiFeO3 sample decreases with increasing crystallite size (which increase with increasing sintering temperature). The increase in

Fig. 3. (a) UV–Vis absorbance spectra of BiFeO3 samples sintered at 500 °C, 600 °C and 700 °C. Plot of (ahm)2 versus energy (hm) for BiFeO3samples sintered at (b) 500 °C (c) 600 °C (d) 700 °C.

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band gap (blue shift) with decreasing crystallite size may be ascribed to quantum confinement due to nano size of crystallites in these samples. 4. Conclusion In summary, BiFeO3 nanoparticles sintered at 500 °C, 600 °C and 700 °C were prepared by sol-gel technique. The single phase formation (except sample sintered at 700 °C) of BiFeO3 nanoparticles has been confirmed by XRD studies. The 600 °C has been found to be optimum sintering temperature to obtain single phase BFO sample. The crystallite size increased from 18 nm to 43 nm with increasing sintering temperature from 500 °C to 700 °C. The infrared spectra indicated the existence of Fe-O and Bi-O bonds. The optical band gap decreased from 2.50 eV to 2.0 eV with increasing sintering temperature from 500 °C to 700 °C. Therefore, band gap in BiFeO3 samples may be tuned by controlling crystallite size and hence by controlling sintering temperature. The band gap tuning of BiFeO3 samples in visible region indicates its potential applications in photocatalytic applications. CRediT authorship contribution statement Sheetal Sharma: Writing - original draft. Manoj Kumar: Conceptualization, Validation, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements One of the authors, Sheetal Sharma, would like to acknowledge the research facilities and fellowship provided by JIIT-Noida.

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Please cite this article as: S. Sharma and M. Kumar, Band gap tuning and optical properties of BiFeO3 nanoparticles, Materials Today: Proceedings, https:// doi.org/10.1016/j.matpr.2020.01.496