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Optimisation and the Effect of Addition of Extra Bismuth on the Dielectric and Optical Properties of Bismuth Ferrite (BFO)
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ScienceDirect Materials Today: Proceedings 5 (2018) 2064–2073
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ICMS 2017
Optimisation and the Effect of Addition of Extra Bismuth on the Dielectric and Optical Properties of Bismuth Ferrite (BFO) Ibetombi Soibam, Angom Devadatta Mani* NIT Manipur, Langol, Imphal 795004, India
Abstract Bi1+xFeO3 (BFO) crystallites with x = 0.00, 0.05 and 0.10 were prepared by sol gel route using molarity method stoichiometry control. Optimisation for the formation of pure phase sample was done by adding extra bismuth to compensate the bismuth loss during synthesis. XRD studies confirm the pure phase formation for 5% (0.05) Bi added leached BFO sample. FTIR studies revealed the presence of stretching and bending vibrations of the various bonds present in the samples. Dielectric response of 0.00, 0.05 (leached), and 0.10 Bi added BFO samples were studied as a function of frequency in the range 20 Hz – 2 MHz. With the increase in the concentration of Bi the dielectric constant showed a decreasing trend. The plot of tanδ with frequency for all the samples follows the same behaviour as the dielectric constant. The energy band gap was calculated by studying the optical absorbance spectra of all the samples. The energy band gap values of all the samples were found to be in the range of that of semiconductor making it suitable for photocatalysis and photovoltaic applications. All the mechanisms contributing to the above results and the various possible ways on how to improve the ferroelectric and optical properties of the pure phase BFO were being discussed. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Materials Science (ICMS2017).
Keywords: Bismuth ferrite; sol gel; XRD; dielectric properties; optical properties.
* Corresponding author. Tel.:+919089886311. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Materials Science (ICMS2017).
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1. Introduction Pure phase BiFeO3 (BFO) is the only single phase multiferroic material which shows the simultaneous coexistence of both weak ferromagnetism and strong ferroelectricity at room temperature. BFO possess a rhombohedrally distorted perovskite structure with the space group R3c at room temperature. It possesses strong ferroelectricity with Curie temperature around 1100 K and antiferromagnetism (G type ordering with Neel temperature, 643K) [1 – 6]. The weak magnetoelectric coupling between the ferroelectric and ferromagnetic domains has made pure phase BFO a novel candidate in the charge storage and memory devices. Strong optical absorption characteristics in the range of visible spectrum corresponding to energy band gap less than 3 eV [26,32] has made pure phase BFO a novel candidate in photocatalytic applications such as degradation of organic pollutants and removal of dye using BFO as the catalysts. A smaller band gap (< 3 eV) reveals a tremendous potential in photovoltaic applications [32]. It is advantageous over other ferroelectrics because it is nontoxic and environment friendly. Further, synthesis of pure phase BFO is cost effective and doesn’t require high temperature [25]. Ferroelectricity is caused by the stereochemical activity of 6s2 lone pair of the Bi ions. It causes the energy of the empty 6porbital comparable to that of 2p orbital causing hybridisation between the two orbitals [1 – 6][10]. Hence, spontaneous polarisation is developed due to the relative non-centrosymmetric shift of cations (Bi3+ and Fe3+) and anions (O2-) along the three fold pseudocubic [111] direction [7]. In BFO, each Fe3+ is surrounded by six nearest antiparallel Fe spins in such a way that it follows G-type antiferromagnetic ordering due to Dzyaloshinky-Moriya interaction [3,20]. The Fe magnetic moments of BFO are coupled ferromagnetically within the pseudocubic (111) planes and antiferromagnetically between adjacent planes. This canted antiferromagnetism results in the production of inherent macroscopic net magnetisation. However, the BFO spin structure is modified by a long range (620 A periodicity) cycloid type spatial spin modulation which when superimposed onto G-type antiferromagnetic ordering leads to zero net magnetisation [20]. The optimisation of the synthesis of pure phase BFO is extremely difficult. Various workers are using different strategies and methods for obtaining pure phase BFO. J Dhahri et.al. synthesised BFO by mixing the oxide precursor using solid state reaction and heated the mixture at 1023K for 12h [10]. However, the formation pure phase BFO has not been achieved yet. Also, S. Godara et.al. have synthesised BFO by sol gel method using nitrate precursor. The heat treatment was done at 500oC for 3h but it still shows the presence of some impurity phases. Besides the formation of secondary phases, there are various problems of BFO that needs to be tailored properly like weak magnetisation, improper stoichiometry control and high oxygen vacancies which lead to the production of large amount of leakage charge [13]. Due to this leakage charge, the conductivity (resistivity) of BFO is high (low) and hence the dielectric response is greatly reduced [10,14,15,18]. This makes BFO a semiconductor not a perfect dielectric. The current investigation aims at synthesising the pure phase BFO by using novel strategies which has not been reported earlier while at the same time ensuring energy conservation. The properties of pure phase BFO is reviewed so that various aspects on how to tailor its properties according to the desired applications can be studied. 2. Experimental Bismuth ferrite (BFO) powder is synthesized by sol gel route using Bi(NO3)35H2O, Fe(NO3)35H2O and 2 methoxyethanol as starting materials. Stoichiometric amounts of the solutes are calculated by molarity method to make 1M 100 ml final solution. Precursor solutions of bismuth nitrate [Bi(NO3)35H2O] and ferric nitrate [Fe(NO3)35H2O] are made in two separate beakers by taking some amount of 2 methoxyethanol. Both the solutions are mixed and stirred. The mixture is then added with few drops of 3M HNO3 acid followed by the addition of stoichiometric amount of citric acid and extra 2 methoxyethanol is added to make the final solution of 100 ml. The solution is heated at 120oC for 2 hours and a fluffy gel is obtained. The gel so formed is dried and ground to form powder. The powder so formed is calcined at 550oC for 30 minutes. A part of 0.05 Bi added calcined BFO powder is leached with 1M HNO3 acid and dried. All the powder so obtained was pressed into pellets of diameter 10 mm and thickness 1mm and sintered at 600oC for 30 min. Finally, various experiments were performed to investigate their structural, dielectric and optical properties.
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Phase identification of the prepared samples were carried out using XRD (Pan analytical X Pert Pro) with Cu Kα (1.54 A) as the source. Lattice parameter of the samples was obtained from Rietveld refinement using Fulprof software. The topographical analyses and microstructural features were carried out using SEM. The stretching and bending behaviour of the various bonds present in the samples were analysed using FTIR (Perkin Elmer, Spectrum Two). The capacitance of the prepared samples were measured using LCR meter (Agilent 6E 4980) and the dielectric constant is calculated by using the formula, ɛ′ = Cd/ɛoA where ɛ′ is the dielectric constant, C is the measured capacitance, d is the thickness, A is the cross sectional area of the sample and ɛo is the permittivity of free space. Optical absorbances were studied using UV-vis spectrophotometer (Perkin Elmer, Lambda 35) and energy band gaps were obtained from Tauc’s plot 3. Results and Discussions 3.1 XRD analysis
Fig 1: XRD pattern of BFO samples at different bismuth concentrations (a) 0% Bi addition without leaching (b) 5% Bi addition without leaching (c) 5% Bi addition with leaching (d) 10% Bi addition without leaching.
Fig 1 shows the XRD pattern of 0%, 5%, 5% (leached) and 10% Bi added BFO samples. Various parasitic phases were seen as in case of 0%, 5% and 10% Bi added samples without leaching. These were probably due to Bi-deficient (Fe-rich) and Bi-rich (Fe-deficient) phases [9]. The Bi deficient and Bi rich impurity phases such as Bi2Fe4O9 and Bi25FeO4 were labelled as ● and ■ respectively as shown in Fig. 1. The Bi deficient (Fe-rich) phases were due to the volatilisation of Bi by heat treatment during synthesis. On the other hand, Birich (Fe-deficient) phase arises due to the addition of extra Bi more than the stoichiometric amount to get pure phase. In the present study, 5% Bi added BFO sample showed least parasitic phases which were again removed by leaching the calcined powder with 1M HNO3 acid. Fig 1(c) shows the formation of pure phase BFO sample and all the peaks were indicated as ♦. The hkl values of all the peaks were indexed to rhombohedral BFO with R3c space group in accordance to the available literature values [8,11]. The average crystallite sizes of all the samples as estimated by Debye Scherrer’s formula, . D= where D is the crystallite size, β is half width full maxima, θ is angle between θ1 and θ2. The crystallite sizes of 0%, 5% (leached) and 10% Bi added samples were obtained as 60 nm, 48 nm and 45 nm respectively. Since the ionic radius of Bi is smaller than that of Fe, the addition of extra bismuth decreases the crystallite size and hence follows a decreasing trend. The crystalline properties of pure phase BFO sample were obtained from Rietveld refinement using Fullprof software. It has been observed that the XRD pattern of 0.05 Bi added leached BFO sample directly matches with the peaks of the model pure phase BFO [13]. The Rietveld refined lattice constants,
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volume and density were obtained as a = b = 5.578 A, c = 13.868 A; 373.602 A3 and 8.343 gcm-3 respectively and these values were comparable to the observed available literature values as obtained in case of bulk BFO [8,11]. Hence, XRD analysis confirms the formation of pure phase BFO for 0.05 Bi added leached sample. 3.2 SEM studies Fig 2 shows the SEM micrographs of 0%, 5% (leached) and 10% Bi added BFO samples. The study of the morphological features is extremely important to ascertain their effects on the properties of the sample. It shows that the particles exist as agglomerated grains with continuous growth in all directions. This confirms the crystallinity of the samples which was earlier evident from XRD studies. The grain size of 0%, 5% (leached) and 10% Bi added BFO samples were obtained as 250 nm, 205 nm and 150 nm respectively by using imageJ software. Thus, SEM studies also confirm the decreasing trend in the values of grain size with the addition of Bi which were earlier predicted by XRD studies.
(a)
(b)
(c)
Fig 2: SEM micrographs of (a) 0% (b) 5% (leached) (c) 10 % Bi added BFO samples.
3.3 FTIR studies Fig 3 shows the FTIR spectra of 0%, 5% Bi (leached) and 10% Bi added BFO samples. The peak around 530 cm-1 – 570 cm-1 in all the samples were probably due to the stretching vibration of Fe-O bond which signifies the presence of FeO3 octahedral in the structure. The peak in the range 810 cm-1 – 840 cm-1 can be attributed to the presence of carbonates. The peaks around 1035 cm-1 and 2890 cm-1 were probably due to the presence of trapped nitrate ions and nitrile formation respectively. The stretching vibrations of Fe-O bond in 0%, 5% (leached), 10% Bi were observed at 539 cm-1 , 554 cm-1, 569 cm-1 respectively. The positions of all the observed peaks of 0.05 Bi added BFO sample were comparable to the available literature values of pure phase BFO [17, 12]. The large variation of vibration wavenumber in 0% and 10% Bi added sample might possibly due to the formation of secondary phases created by improper stoichiometry and optimisation. Hence, the formation of pure phase BFO was also confirmed by FTIR studies.
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(a)
(b)
(c) Fig 3: FTIR spectra of (a) 0% (b) 5% (leached) and (c) 10% Bi added BFO samples.
3.4 Dielectric studies Fig 4 shows the frequency dependence of dielectric constant on the addition of excess Bi in the range 20 Hz – 2 MHz at room temperature. It shows the dispersive behaviour of all the samples where the values of dielectric constant is high at low frequencies and decreases as we increase the frequency and beyond a particular high frequency, it remains constant. Dielectric response in ferrites is explained in accordance with the presence of mixed valence states of B site cations (Fe2+ and Fe3+) and Maxwell Wagner space charge polarisation in connection with Koop’s phenomenological two layer model [14,19,20]. The mixing of valence states is attributed to the hopping mechanism of electrons between Fe2+ and Fe3+ cations at the adjacent B sites [19]. The conduction mechanism in ferrite is mainly due to B-B hopping mechanism since the distance between the two metal ions in the B site is smaller as compared to the distance between the metal ions at A and B sites and the cations capable for changing the valence states occupy the B site only [20]. Improper stoichiometry control and presence of secondary phases results in the creation of oxygen vacancies in the sample. This leads to the increase in leakage current in the sample and reduces the dielectric response of the sample.
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Fig 4: Frequency dependence of dielectric constant of 0%, 5% and 10% Bi added BFO samples. At higher frequencies, the dielectric constant of any material is contributed by the electronic and ionic polarisations. However, at lower frequencies the dipolar and interfacial polarisations (space charge polarisation) play an important role to the behaviour of dielectric response. Koop’s two layer model suggests that every dielectric medium is characterised by two layers, well and poor conducting layers constituted by crystallite grains and boundaries respectively. At lower frequencies, the contribution from the grain boundaries is significant or space charge polarisation dominates [16]. Therefore, resistivity of the sample enhances and hence the dielectric response is greatly enhanced raising the value of dielectric constant to its maximum. As the frequencies of the applied field increases the dipolar and interfacial polarisation becomes insignificant and the effect of electronic polarisation becomes effective. The value of ɛ′ decreases as the frequency of the applied field increases. This explanation is attributed to the dipole relaxation phenomena of the dipoles or the dependence of hopping frequency of electrons on the applied field. At lower frequencies the time period is large and hence the dipoles or the hopping frequency of electrons have enough time to follow the pace of the applied electric field but at higher frequencies the time period is small and the dipoles or hopping frequency couldn’t follow the field resulting in lowering the values of dielectric constant [14]. Beyond a critical frequency of the applied field the dielectric response of the sample is independent of the hopping frequency and space charge polarisation and remains constant thereafter [19,20].
Fig 5: Frequency dependence of dielectric loss (tan δ) of 0%, 5% and 10% Bi added BFO samples. Fig 5 shows the frequency dependence of dielectric loss (tan δ) of 0%, 5% and 10% Bi added BFO samples. Fig 6 shows the variation of dielectric constant and loss as a function of concentration of Bi addition at 1 kHz. The values of dielectric constant and dielectric loss of 5% (leached) Bi added BFO sample are obtained as 42 and 0.10
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respectively. In the current investigation, the large value of dielectric constant for 0% Bi added BFO sample is attributed to the increase in polarisation due to the predominance of Fe2+ species in the sample. This increases the probability of hopping which eventually leads to the piling up of more electrons at the grain boundaries. As the concentration of excess Bi increases as 5% and 10%, the amount of Fe2+ content decreases and hence the amount of electrons available for hopping decreases leading to the decrease in polarisation hence reducing the dielectric constant.
Fig 6: Variation of dielectric constant (ɛ′) and dielectric loss (tan δ) as a function of concentration of Bi addition.
It has also been observed that the dielectric loss follows the same behaviour as that of the dielectric constant. The dielectric loss in the sample is due to the energy consumed by the leakage charge for its mobility in the sample. At lower frequencies, the resistive grain boundaries contribute and hence a large amount of energy is consumed in hopping process. Hence the dielectric loss is high at lower frequencies. However, at higher frequencies the conducting grains contributes and hence comparatively smaller amount of energy is required in hopping process resulting in the decrease in dielectric loss. In the present investigation, with the increase in Bi concentration the hopping process decreases due to the decrease in Fe2+ content and hence lesser amount of energy is being spent in the hopping process. Thus, the value of dielectric losses decreases with the increase in Bi concentration. 3.5 Optical studies
Fig 7: Optical absorbance as a function of wavelength of 0%, 5% (leached) and 10% Bi added BFO samples.
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Fig 7 shows the wavelength dependence of UV-vis absorption spectra of 0%, 5% (leached) and 10% Bi added BFO samples. Strong optical absorption was shown in the UV-visible region 325 – 550 nm whereas weak optical absorption in 550 – 595 nm. The absorption cut-off wavelength was about 595 nm suggesting the material can absorb visible light in the wavelength range 325 – 595 nm. The intensity of absorption follows a reducing trend with the increase in the concentration of excess Bi addition. This is due to the decrease in Fe2+ content in the sample with the increase in the concentration of Bi addition. Therefore, the availability of electrons in the valence band for transition to the conduction band reduces. Since the absorption coefficient of any sample directly depends on the density of states in the valence band, the absorption is greater for samples having larger amount of Fe2+ content and then reduces on decreasing the amount of Fe2+ content. The intensity of pure phase (5% Bi added leached) BFO is reported comparable to the literature values [26 – 28]. This absorption characteristic might bring tremendous potential in photocatalytic applications. Moreover, pure phase BFO can also be used in the environmental purification by degrading the organic pollutants [26-32].
Fig 8: Variation of (hνlnT)2 as a function of hν of 0%, 5%(leached) and 10% Bi added BFO samples.
The energy bandgap is calculated with the help of Tauc relation given by αhν = B(hν – Eg)n where α, B, hν are the absorption coefficient, constant and energy of the incident photon respectively [8,26,29,31]. At room temperature the value of n for allowed direct transition is ½. A = 2 – log10T T = Antilog (2-A) where T is the percentage transmittance and A is the absorbance. The transmittance and absorbance characteristics are related by T = e-αL
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α=-
T L
where L is the thickness of the sample. Using this relation in the above relation, it can be concluded as -
T L
hν = B(hν – Eg)1/2
(hνlnT)2 = BL2 hν - EgBL2 The energy band gap Eg is estimated by extrapolating the graph of hν Vs (hνlnT)2 on the hν axis as shown in Fig 8. The energy band gap Eg of 0%, 5% (leached) and 10% Bi added BFO samples are obtained as 2.3 eV, 2.4 eV and 2.5 eV respectively. A slight variation in the energy band gap is observed with the increase in excess Bi concentration. This is due to the variation in optical absorption with the increase in Bi content. However, no shifting in the position of fundamental absorption is observed. It has been observed that the energy band gap, Eg of 5% leached Bi added BFO sample is less than 3 eV. This characteristic has made pure phase BFO a potential candidate for semiconductor applications like photocatalysis and photovoltaic devices. 4 Conclusions In summary, Bi1+xFeO3 where x= 0.0 ≤ x ≤ 0.10 in steps of 0.05 powders have been prepared by sol gel route using molarity method stoichiometry control. XRD and FTIR studies confirmed the formation of pure phase BFO for 5% (leached) Bi added BFO sample. This optimisation is done to account the volatility of bismuth by heat treatment during synthesis. The dielectric response of the BFO samples follows a reducing trend with the increase in the concentration of Bi addition. Reduced optical absorption is observed for samples with higher concentration of Bi content. This is attributed to the decrease in the amount of Fe2+ content in the sample. The energy band gap of the pure phase BFO is 2.4 eV which shows its possible application in photocatalysis and photovoltaic devices. Further improvement in the ferroelectric and optical properties of pure phase BFO can be done by making composites or doping A and B sites with suitable materials. This will make BFO material a potential candidate in ferroelectric as well as optoelectronic applications. References [1] G. Catalan and J.F Scott, Adv. Matter, 21 (2009) 2463 – 2485. [2] W. Eerenstein, N.D. Mathur and J.F. Scott, Nature, 442 (2006) 759 – 765. [3] Seva V.V. Khikhlovskyi, The Renaissance of multiferroics : bismuth ferrite (BiFeO3), 3 (2010) 7. [4] Khomskii D.I., J. Magn. Magn. Matter, 306 (2006) 1 – 13. [5] S.W. Cheong, M. Mostovoy, Nature Materials, 6 (2007) 13 – 14. [6] Ramesh R. and Spaldin N.A., Nature Materials, 6 (2007) 21 – 29. [7] S. Chakraborty, Soumya M., Siddarth M., J. Aus. Ceram. Soc. 51[1] (2015) 45 – 53. [8] S. Godara, Nidhi Sinha, Geeta Ray, Binay Kumar, J. Asian. Ceram. Soc., 2 (2014) 416 – 421. [9] S. Gupta, Monika T., V. Gupta, J. Applied Phy., 115 (2014) 234105(1 – 8). [10] J. Dhahri, M. Boudard, S. Zemni, H. Roussel, M. Ouemezzine, J. Solid State Chem, 181 (2008) 802 – 811. [11] S. Chakraborty, Soumya M., Siddartha M., Inter. J. of Semiconductor Sc. And Tech. (IJSST), 3[1] (2013)1 – 10. [12] A.K. Ghosh, H. Kevin, B. Chatterjee, S. State Comm., 152 (2012) 557 – 560. [13] Shengzhen Cai, Bismuth-containing multiferroics, Chalmers University of Technology, Sweden (2013) [14] A. Kumar, K.L. Yadav, J. Phy and Chem., 72, (2011) 1189 – 1194. [15] A. K. Ghosh, G.D. Dwivedi, B. Chatterjee et. al, S. State Comm., 166 (2013) 22 – 26. [16] H.B. Sharma, K. Nomita, S. Bobby Singh, J. Alloys and Compounds, 599 (2014) 32 – 39. [17] Soumya M., S. Chakraborty, Inter. J. Nano Dimen. 5[1] (2014) 41 – 46. [18] D. Varshney, A. Kumar, K. Verma, J. Alloys and Comm., 509 (2011) 8421 – 8426. [19] Mamata M., Sumitra P., Solid State Comm., 152 (2012) 320 – 323. [20] Ibetombi S., Sumitra P, Indian J. Phys., 83(3) (2009) 285 – 290. [21] K. Sarkar, Soumya M., Siddartha M., Process and Applic. of Ceram. 9[1] (2015) 53 – 60.
Ibetombi Soibam et al./ Materials Today: Proceedings 5 (2018) 2064–2073 [22] Y.K. Jun, S.H. Hong, Solid State Comm., 144 (2007) 329 – 333. [23] W. Cai, Chunlin Fu, Wenguang Hu, J. Alloys and Compounds, 554 (2013) 64 – 71. [24] Manoj K., K.L. Yadav, G.D. Varma, Material Letters, 62 (2008) 1159 – 1161. [25] Muthafar F. Al-Hilli, Iraqi journal of Physics, 9[14] (2011) 6 – 12. [26] S.K. Srivastava, N.S. Gajbhiye, J. Am. Ceram. Soc., 95(11) (2012) 3678-3682. [27] T. Gao, Z. Chen et.al. Adv. Mater. Sc., 40 (2015) 97-109. [28] W. Wang, N. Li et.al., Ceram. Inter., 29 (2013) 3511-3518. [29] E. Kim, Z. Tao Jiang et.al., J. Appl. Phys. 39 (2000) 4820-4828. [30] BI Wen’gang et.al., Chinese Phys. Lett. 9(1) (2000) 53. [31] N. Gao, W. Chen et.al., Computational and Theoretical Chemistry 1084 (2016) 36-42. [32] Z. Zhang, P. Wu et.al., Appl. Phys. Lett. 96, (2010) 0129905.
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Optimisation and the Effect of Addition of Extra Bismuth on the Dielectric and Optical Properties of Bismuth Ferrite (BFO) Ibetombi Soibam, Angom Devadatta Mani* NIT Manipur, Langol, Imphal 795004, India
Abstract Bi1+xFeO3 (BFO) crystallites with x = 0.00, 0.05 and 0.10 were prepared by sol gel route using molarity method stoichiometry control. Optimisation for the formation of pure phase sample was done by adding extra bismuth to compensate the bismuth loss during synthesis. XRD studies confirm the pure phase formation for 5% (0.05) Bi added leached BFO sample. FTIR studies revealed the presence of stretching and bending vibrations of the various bonds present in the samples. Dielectric response of 0.00, 0.05 (leached), and 0.10 Bi added BFO samples were studied as a function of frequency in the range 20 Hz – 2 MHz. With the increase in the concentration of Bi the dielectric constant showed a decreasing trend. The plot of tanδ with frequency for all the samples follows the same behaviour as the dielectric constant. The energy band gap was calculated by studying the optical absorbance spectra of all the samples. The energy band gap values of all the samples were found to be in the range of that of semiconductor making it suitable for photocatalysis and photovoltaic applications. All the mechanisms contributing to the above results and the various possible ways on how to improve the ferroelectric and optical properties of the pure phase BFO were being discussed. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Materials Science (ICMS2017).
Keywords: Bismuth ferrite; sol gel; XRD; dielectric properties; optical properties.
* Corresponding author. Tel.:+919089886311. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Materials Science (ICMS2017).
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1. Introduction Pure phase BiFeO3 (BFO) is the only single phase multiferroic material which shows the simultaneous coexistence of both weak ferromagnetism and strong ferroelectricity at room temperature. BFO possess a rhombohedrally distorted perovskite structure with the space group R3c at room temperature. It possesses strong ferroelectricity with Curie temperature around 1100 K and antiferromagnetism (G type ordering with Neel temperature, 643K) [1 – 6]. The weak magnetoelectric coupling between the ferroelectric and ferromagnetic domains has made pure phase BFO a novel candidate in the charge storage and memory devices. Strong optical absorption characteristics in the range of visible spectrum corresponding to energy band gap less than 3 eV [26,32] has made pure phase BFO a novel candidate in photocatalytic applications such as degradation of organic pollutants and removal of dye using BFO as the catalysts. A smaller band gap (< 3 eV) reveals a tremendous potential in photovoltaic applications [32]. It is advantageous over other ferroelectrics because it is nontoxic and environment friendly. Further, synthesis of pure phase BFO is cost effective and doesn’t require high temperature [25]. Ferroelectricity is caused by the stereochemical activity of 6s2 lone pair of the Bi ions. It causes the energy of the empty 6porbital comparable to that of 2p orbital causing hybridisation between the two orbitals [1 – 6][10]. Hence, spontaneous polarisation is developed due to the relative non-centrosymmetric shift of cations (Bi3+ and Fe3+) and anions (O2-) along the three fold pseudocubic [111] direction [7]. In BFO, each Fe3+ is surrounded by six nearest antiparallel Fe spins in such a way that it follows G-type antiferromagnetic ordering due to Dzyaloshinky-Moriya interaction [3,20]. The Fe magnetic moments of BFO are coupled ferromagnetically within the pseudocubic (111) planes and antiferromagnetically between adjacent planes. This canted antiferromagnetism results in the production of inherent macroscopic net magnetisation. However, the BFO spin structure is modified by a long range (620 A periodicity) cycloid type spatial spin modulation which when superimposed onto G-type antiferromagnetic ordering leads to zero net magnetisation [20]. The optimisation of the synthesis of pure phase BFO is extremely difficult. Various workers are using different strategies and methods for obtaining pure phase BFO. J Dhahri et.al. synthesised BFO by mixing the oxide precursor using solid state reaction and heated the mixture at 1023K for 12h [10]. However, the formation pure phase BFO has not been achieved yet. Also, S. Godara et.al. have synthesised BFO by sol gel method using nitrate precursor. The heat treatment was done at 500oC for 3h but it still shows the presence of some impurity phases. Besides the formation of secondary phases, there are various problems of BFO that needs to be tailored properly like weak magnetisation, improper stoichiometry control and high oxygen vacancies which lead to the production of large amount of leakage charge [13]. Due to this leakage charge, the conductivity (resistivity) of BFO is high (low) and hence the dielectric response is greatly reduced [10,14,15,18]. This makes BFO a semiconductor not a perfect dielectric. The current investigation aims at synthesising the pure phase BFO by using novel strategies which has not been reported earlier while at the same time ensuring energy conservation. The properties of pure phase BFO is reviewed so that various aspects on how to tailor its properties according to the desired applications can be studied. 2. Experimental Bismuth ferrite (BFO) powder is synthesized by sol gel route using Bi(NO3)35H2O, Fe(NO3)35H2O and 2 methoxyethanol as starting materials. Stoichiometric amounts of the solutes are calculated by molarity method to make 1M 100 ml final solution. Precursor solutions of bismuth nitrate [Bi(NO3)35H2O] and ferric nitrate [Fe(NO3)35H2O] are made in two separate beakers by taking some amount of 2 methoxyethanol. Both the solutions are mixed and stirred. The mixture is then added with few drops of 3M HNO3 acid followed by the addition of stoichiometric amount of citric acid and extra 2 methoxyethanol is added to make the final solution of 100 ml. The solution is heated at 120oC for 2 hours and a fluffy gel is obtained. The gel so formed is dried and ground to form powder. The powder so formed is calcined at 550oC for 30 minutes. A part of 0.05 Bi added calcined BFO powder is leached with 1M HNO3 acid and dried. All the powder so obtained was pressed into pellets of diameter 10 mm and thickness 1mm and sintered at 600oC for 30 min. Finally, various experiments were performed to investigate their structural, dielectric and optical properties.
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Phase identification of the prepared samples were carried out using XRD (Pan analytical X Pert Pro) with Cu Kα (1.54 A) as the source. Lattice parameter of the samples was obtained from Rietveld refinement using Fulprof software. The topographical analyses and microstructural features were carried out using SEM. The stretching and bending behaviour of the various bonds present in the samples were analysed using FTIR (Perkin Elmer, Spectrum Two). The capacitance of the prepared samples were measured using LCR meter (Agilent 6E 4980) and the dielectric constant is calculated by using the formula, ɛ′ = Cd/ɛoA where ɛ′ is the dielectric constant, C is the measured capacitance, d is the thickness, A is the cross sectional area of the sample and ɛo is the permittivity of free space. Optical absorbances were studied using UV-vis spectrophotometer (Perkin Elmer, Lambda 35) and energy band gaps were obtained from Tauc’s plot 3. Results and Discussions 3.1 XRD analysis
Fig 1: XRD pattern of BFO samples at different bismuth concentrations (a) 0% Bi addition without leaching (b) 5% Bi addition without leaching (c) 5% Bi addition with leaching (d) 10% Bi addition without leaching.
Fig 1 shows the XRD pattern of 0%, 5%, 5% (leached) and 10% Bi added BFO samples. Various parasitic phases were seen as in case of 0%, 5% and 10% Bi added samples without leaching. These were probably due to Bi-deficient (Fe-rich) and Bi-rich (Fe-deficient) phases [9]. The Bi deficient and Bi rich impurity phases such as Bi2Fe4O9 and Bi25FeO4 were labelled as ● and ■ respectively as shown in Fig. 1. The Bi deficient (Fe-rich) phases were due to the volatilisation of Bi by heat treatment during synthesis. On the other hand, Birich (Fe-deficient) phase arises due to the addition of extra Bi more than the stoichiometric amount to get pure phase. In the present study, 5% Bi added BFO sample showed least parasitic phases which were again removed by leaching the calcined powder with 1M HNO3 acid. Fig 1(c) shows the formation of pure phase BFO sample and all the peaks were indicated as ♦. The hkl values of all the peaks were indexed to rhombohedral BFO with R3c space group in accordance to the available literature values [8,11]. The average crystallite sizes of all the samples as estimated by Debye Scherrer’s formula, . D= where D is the crystallite size, β is half width full maxima, θ is angle between θ1 and θ2. The crystallite sizes of 0%, 5% (leached) and 10% Bi added samples were obtained as 60 nm, 48 nm and 45 nm respectively. Since the ionic radius of Bi is smaller than that of Fe, the addition of extra bismuth decreases the crystallite size and hence follows a decreasing trend. The crystalline properties of pure phase BFO sample were obtained from Rietveld refinement using Fullprof software. It has been observed that the XRD pattern of 0.05 Bi added leached BFO sample directly matches with the peaks of the model pure phase BFO [13]. The Rietveld refined lattice constants,
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volume and density were obtained as a = b = 5.578 A, c = 13.868 A; 373.602 A3 and 8.343 gcm-3 respectively and these values were comparable to the observed available literature values as obtained in case of bulk BFO [8,11]. Hence, XRD analysis confirms the formation of pure phase BFO for 0.05 Bi added leached sample. 3.2 SEM studies Fig 2 shows the SEM micrographs of 0%, 5% (leached) and 10% Bi added BFO samples. The study of the morphological features is extremely important to ascertain their effects on the properties of the sample. It shows that the particles exist as agglomerated grains with continuous growth in all directions. This confirms the crystallinity of the samples which was earlier evident from XRD studies. The grain size of 0%, 5% (leached) and 10% Bi added BFO samples were obtained as 250 nm, 205 nm and 150 nm respectively by using imageJ software. Thus, SEM studies also confirm the decreasing trend in the values of grain size with the addition of Bi which were earlier predicted by XRD studies.
(a)
(b)
(c)
Fig 2: SEM micrographs of (a) 0% (b) 5% (leached) (c) 10 % Bi added BFO samples.
3.3 FTIR studies Fig 3 shows the FTIR spectra of 0%, 5% Bi (leached) and 10% Bi added BFO samples. The peak around 530 cm-1 – 570 cm-1 in all the samples were probably due to the stretching vibration of Fe-O bond which signifies the presence of FeO3 octahedral in the structure. The peak in the range 810 cm-1 – 840 cm-1 can be attributed to the presence of carbonates. The peaks around 1035 cm-1 and 2890 cm-1 were probably due to the presence of trapped nitrate ions and nitrile formation respectively. The stretching vibrations of Fe-O bond in 0%, 5% (leached), 10% Bi were observed at 539 cm-1 , 554 cm-1, 569 cm-1 respectively. The positions of all the observed peaks of 0.05 Bi added BFO sample were comparable to the available literature values of pure phase BFO [17, 12]. The large variation of vibration wavenumber in 0% and 10% Bi added sample might possibly due to the formation of secondary phases created by improper stoichiometry and optimisation. Hence, the formation of pure phase BFO was also confirmed by FTIR studies.
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(a)
(b)
(c) Fig 3: FTIR spectra of (a) 0% (b) 5% (leached) and (c) 10% Bi added BFO samples.
3.4 Dielectric studies Fig 4 shows the frequency dependence of dielectric constant on the addition of excess Bi in the range 20 Hz – 2 MHz at room temperature. It shows the dispersive behaviour of all the samples where the values of dielectric constant is high at low frequencies and decreases as we increase the frequency and beyond a particular high frequency, it remains constant. Dielectric response in ferrites is explained in accordance with the presence of mixed valence states of B site cations (Fe2+ and Fe3+) and Maxwell Wagner space charge polarisation in connection with Koop’s phenomenological two layer model [14,19,20]. The mixing of valence states is attributed to the hopping mechanism of electrons between Fe2+ and Fe3+ cations at the adjacent B sites [19]. The conduction mechanism in ferrite is mainly due to B-B hopping mechanism since the distance between the two metal ions in the B site is smaller as compared to the distance between the metal ions at A and B sites and the cations capable for changing the valence states occupy the B site only [20]. Improper stoichiometry control and presence of secondary phases results in the creation of oxygen vacancies in the sample. This leads to the increase in leakage current in the sample and reduces the dielectric response of the sample.
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Fig 4: Frequency dependence of dielectric constant of 0%, 5% and 10% Bi added BFO samples. At higher frequencies, the dielectric constant of any material is contributed by the electronic and ionic polarisations. However, at lower frequencies the dipolar and interfacial polarisations (space charge polarisation) play an important role to the behaviour of dielectric response. Koop’s two layer model suggests that every dielectric medium is characterised by two layers, well and poor conducting layers constituted by crystallite grains and boundaries respectively. At lower frequencies, the contribution from the grain boundaries is significant or space charge polarisation dominates [16]. Therefore, resistivity of the sample enhances and hence the dielectric response is greatly enhanced raising the value of dielectric constant to its maximum. As the frequencies of the applied field increases the dipolar and interfacial polarisation becomes insignificant and the effect of electronic polarisation becomes effective. The value of ɛ′ decreases as the frequency of the applied field increases. This explanation is attributed to the dipole relaxation phenomena of the dipoles or the dependence of hopping frequency of electrons on the applied field. At lower frequencies the time period is large and hence the dipoles or the hopping frequency of electrons have enough time to follow the pace of the applied electric field but at higher frequencies the time period is small and the dipoles or hopping frequency couldn’t follow the field resulting in lowering the values of dielectric constant [14]. Beyond a critical frequency of the applied field the dielectric response of the sample is independent of the hopping frequency and space charge polarisation and remains constant thereafter [19,20].
Fig 5: Frequency dependence of dielectric loss (tan δ) of 0%, 5% and 10% Bi added BFO samples. Fig 5 shows the frequency dependence of dielectric loss (tan δ) of 0%, 5% and 10% Bi added BFO samples. Fig 6 shows the variation of dielectric constant and loss as a function of concentration of Bi addition at 1 kHz. The values of dielectric constant and dielectric loss of 5% (leached) Bi added BFO sample are obtained as 42 and 0.10
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respectively. In the current investigation, the large value of dielectric constant for 0% Bi added BFO sample is attributed to the increase in polarisation due to the predominance of Fe2+ species in the sample. This increases the probability of hopping which eventually leads to the piling up of more electrons at the grain boundaries. As the concentration of excess Bi increases as 5% and 10%, the amount of Fe2+ content decreases and hence the amount of electrons available for hopping decreases leading to the decrease in polarisation hence reducing the dielectric constant.
Fig 6: Variation of dielectric constant (ɛ′) and dielectric loss (tan δ) as a function of concentration of Bi addition.
It has also been observed that the dielectric loss follows the same behaviour as that of the dielectric constant. The dielectric loss in the sample is due to the energy consumed by the leakage charge for its mobility in the sample. At lower frequencies, the resistive grain boundaries contribute and hence a large amount of energy is consumed in hopping process. Hence the dielectric loss is high at lower frequencies. However, at higher frequencies the conducting grains contributes and hence comparatively smaller amount of energy is required in hopping process resulting in the decrease in dielectric loss. In the present investigation, with the increase in Bi concentration the hopping process decreases due to the decrease in Fe2+ content and hence lesser amount of energy is being spent in the hopping process. Thus, the value of dielectric losses decreases with the increase in Bi concentration. 3.5 Optical studies
Fig 7: Optical absorbance as a function of wavelength of 0%, 5% (leached) and 10% Bi added BFO samples.
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Fig 7 shows the wavelength dependence of UV-vis absorption spectra of 0%, 5% (leached) and 10% Bi added BFO samples. Strong optical absorption was shown in the UV-visible region 325 – 550 nm whereas weak optical absorption in 550 – 595 nm. The absorption cut-off wavelength was about 595 nm suggesting the material can absorb visible light in the wavelength range 325 – 595 nm. The intensity of absorption follows a reducing trend with the increase in the concentration of excess Bi addition. This is due to the decrease in Fe2+ content in the sample with the increase in the concentration of Bi addition. Therefore, the availability of electrons in the valence band for transition to the conduction band reduces. Since the absorption coefficient of any sample directly depends on the density of states in the valence band, the absorption is greater for samples having larger amount of Fe2+ content and then reduces on decreasing the amount of Fe2+ content. The intensity of pure phase (5% Bi added leached) BFO is reported comparable to the literature values [26 – 28]. This absorption characteristic might bring tremendous potential in photocatalytic applications. Moreover, pure phase BFO can also be used in the environmental purification by degrading the organic pollutants [26-32].
Fig 8: Variation of (hνlnT)2 as a function of hν of 0%, 5%(leached) and 10% Bi added BFO samples.
The energy bandgap is calculated with the help of Tauc relation given by αhν = B(hν – Eg)n where α, B, hν are the absorption coefficient, constant and energy of the incident photon respectively [8,26,29,31]. At room temperature the value of n for allowed direct transition is ½. A = 2 – log10T T = Antilog (2-A) where T is the percentage transmittance and A is the absorbance. The transmittance and absorbance characteristics are related by T = e-αL
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α=-
T L
where L is the thickness of the sample. Using this relation in the above relation, it can be concluded as -
T L
hν = B(hν – Eg)1/2
(hνlnT)2 = BL2 hν - EgBL2 The energy band gap Eg is estimated by extrapolating the graph of hν Vs (hνlnT)2 on the hν axis as shown in Fig 8. The energy band gap Eg of 0%, 5% (leached) and 10% Bi added BFO samples are obtained as 2.3 eV, 2.4 eV and 2.5 eV respectively. A slight variation in the energy band gap is observed with the increase in excess Bi concentration. This is due to the variation in optical absorption with the increase in Bi content. However, no shifting in the position of fundamental absorption is observed. It has been observed that the energy band gap, Eg of 5% leached Bi added BFO sample is less than 3 eV. This characteristic has made pure phase BFO a potential candidate for semiconductor applications like photocatalysis and photovoltaic devices. 4 Conclusions In summary, Bi1+xFeO3 where x= 0.0 ≤ x ≤ 0.10 in steps of 0.05 powders have been prepared by sol gel route using molarity method stoichiometry control. XRD and FTIR studies confirmed the formation of pure phase BFO for 5% (leached) Bi added BFO sample. This optimisation is done to account the volatility of bismuth by heat treatment during synthesis. The dielectric response of the BFO samples follows a reducing trend with the increase in the concentration of Bi addition. Reduced optical absorption is observed for samples with higher concentration of Bi content. This is attributed to the decrease in the amount of Fe2+ content in the sample. The energy band gap of the pure phase BFO is 2.4 eV which shows its possible application in photocatalysis and photovoltaic devices. Further improvement in the ferroelectric and optical properties of pure phase BFO can be done by making composites or doping A and B sites with suitable materials. This will make BFO material a potential candidate in ferroelectric as well as optoelectronic applications. References [1] G. Catalan and J.F Scott, Adv. Matter, 21 (2009) 2463 – 2485. [2] W. Eerenstein, N.D. Mathur and J.F. Scott, Nature, 442 (2006) 759 – 765. [3] Seva V.V. Khikhlovskyi, The Renaissance of multiferroics : bismuth ferrite (BiFeO3), 3 (2010) 7. [4] Khomskii D.I., J. Magn. Magn. Matter, 306 (2006) 1 – 13. [5] S.W. Cheong, M. Mostovoy, Nature Materials, 6 (2007) 13 – 14. [6] Ramesh R. and Spaldin N.A., Nature Materials, 6 (2007) 21 – 29. [7] S. Chakraborty, Soumya M., Siddarth M., J. Aus. Ceram. Soc. 51[1] (2015) 45 – 53. [8] S. Godara, Nidhi Sinha, Geeta Ray, Binay Kumar, J. Asian. Ceram. Soc., 2 (2014) 416 – 421. [9] S. Gupta, Monika T., V. Gupta, J. Applied Phy., 115 (2014) 234105(1 – 8). [10] J. Dhahri, M. Boudard, S. Zemni, H. Roussel, M. Ouemezzine, J. Solid State Chem, 181 (2008) 802 – 811. [11] S. Chakraborty, Soumya M., Siddartha M., Inter. J. of Semiconductor Sc. And Tech. (IJSST), 3[1] (2013)1 – 10. [12] A.K. Ghosh, H. Kevin, B. Chatterjee, S. State Comm., 152 (2012) 557 – 560. [13] Shengzhen Cai, Bismuth-containing multiferroics, Chalmers University of Technology, Sweden (2013) [14] A. Kumar, K.L. Yadav, J. Phy and Chem., 72, (2011) 1189 – 1194. [15] A. K. Ghosh, G.D. Dwivedi, B. Chatterjee et. al, S. State Comm., 166 (2013) 22 – 26. [16] H.B. Sharma, K. Nomita, S. Bobby Singh, J. Alloys and Compounds, 599 (2014) 32 – 39. [17] Soumya M., S. Chakraborty, Inter. J. Nano Dimen. 5[1] (2014) 41 – 46. [18] D. Varshney, A. Kumar, K. Verma, J. Alloys and Comm., 509 (2011) 8421 – 8426. [19] Mamata M., Sumitra P., Solid State Comm., 152 (2012) 320 – 323. [20] Ibetombi S., Sumitra P, Indian J. Phys., 83(3) (2009) 285 – 290. [21] K. Sarkar, Soumya M., Siddartha M., Process and Applic. of Ceram. 9[1] (2015) 53 – 60.
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