Pr-doped BiFeO3 thin films growth on quartz using chemical solution deposition

Yofentina Iriani; Mercyurita Dewi Noviastuti; Risa Suryana; Dianisa Khoirum Sandi; Didier Fasquelle

doi:10.1515/eng-2022-0046

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Pr-doped BiFeO₃ thin films growth on quartz using chemical solution deposition

Yofentina Iriani , Mercyurita Dewi Noviastuti , Risa Suryana , Dianisa Khoirum Sandi and Didier Fasquelle

Published/Copyright: July 11, 2022

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From the journal Open Engineering Volume 12 Issue 1

Abstract

Bismuth ferrite (BiFeO₃) is an interesting multiferroic material due to its ferroelectric properties at room temperature. In this study, Bi_1−xPr_xFeO₃ and BiFeO₃ films were grown on quartz substrates by the chemical solution deposition method at 600^oC of annealing temperature. Variation in molar concentration in Bi_1−xPr_xFeO₃ was set (x = 0.03, 0.05, 0.1, and 0.2) to investigate their crystal structure and optical characteristics. BiFeO₃ and Bi_1−xPr_xFeO₃ films were examined using X-ray diffraction (XRD) and ultraviolet (UV)-vis spectrophotometer. The XRD results demonstrated that the addition of Pr in BiFeO₃ shifted the diffraction angle to smaller angles so that it reduced their lattice constant. Besides, the crystal size declined with more Pr numbers, while the lattice strain expanded. The UV-vis characteristics of the films were measured in the wavelength range of 200–800 nm. The transmittance values of the Pr-doped BiFeO₃ increased. Because of Pr doping, the refractive index of the Bi_1−xPr_xFeO₃ films decreased while the energy dispersion increased.

Keywords: Bismuth ferrite; praseodymium; crystal structure; optical properties; CSD method

1 Introduction

Solar energy is a clean energy source that can be converted directly into electricity by way of a photovoltaic (PV) effect [1]. PV ferroelectrics have gained much interest in the last decade due to their potential in energy conversion [2]. Bismuth ferrite (BiFeO₃) has gained special attention from various ferroelectric materials due to its strong ferroelectric properties and bandgap value of ∼2.7 eV [3,4]. This small bandgap makes BiFeO₃ suitable for PV applications [5,6].

Nevertheless, BiFeO₃ possesses drawbacks of high leakage currents, low dielectric constants, and high loss tangents that restrict their applications for device manufacture [7]. To overcome these drawbacks, doping with suitable materials such as rare earth elements at the Bi site, Fe site, or both sites of BiFeO₃ can be one of the best solutions. Besides, the doping of rare earth (RE) elements into the BiFeO₃ structure can advance its ferroelectric properties [8]. Much research has studied the introduction of RE elements into BiFeO₃, including Gd, Ho, Sm, Nd, Eu, and Pr [4,8–12]. BiFeO₃ with RE doping, especially Pr, still needs to be developed, primarily because not much work on the optical properties of Pr doped BiFeO₃ thin film has been reported.

Further, so far, chemical solution deposition (CSD) or sol-gel method has been frequently used to fabricate the thin film of BiFeO₃. It is because the method is simple that it allows better control of the composition of the material and produces a high degree of homogeneity [6,13,14]. Although several studies have reported several experimental works on the preparation of Pr-doped BiFeO₃ films, none have reported their growth on quartz substrates.

On these bases, this study focused on the synthesis of Bi_1−xPr_xFeO₃ thin film on the quartz substrate using the CSD method. The synthesis was done by varying mole concentration of Pr (x = 0.03, 0.05, 0.1, and 0.2). This study aimed to investigate the effects of various Pr concentration numbers of doping on the crystal structure and optical properties of BiFeO₃ and bismuth praseodymium ferrite (Bi_1−xPr_xFeO₃) films.

2 Materials and methods

2.1 Materials

The raw materials used in this research were the sources of Bi, Fe, and Pr, including bismuth(iii) nitrate ((BiNO₃)₃; Sigma Aldrich, ≥99%), ferrite nitrate (Fe(NO₃)₃; Sigma Aldrich, ≥99%), and praseodymium nitrate (Pr(NO₃)₃), and the solvents of acetic acid (CH₃CO₂H; Sigma Aldrich, ≥99.7%), 2-methoxyetanol (C₃H₈O₂; Sigma Aldrich, ≥99.8%), and acetylacetone (Sigma Aldrich, ≥99.3%).

2.2 Sample preparation

BiFeO₃ and Bi_1−xPr_xFeO₃ films were deposited on quartz substrates by the CSD method. The first step was making the solutions of BiFeO₃ and Bi_1−xPr_xFeO₃ (x = 0.03, 0.05, 0.1, and 0.2) as follows: the raw materials were weighed according to their respective chemical compositions, the source materials were placed in a beaker and then acetic acid and 2-methoxyethanol were poured, they were stirred continuously using a magnetic stirrer for 60 min, and finally, acetyl acetone was added and stirred for 45 min. The solutions BiFeO₃ and Bi_1−xPr_xFeO₃ (x = 0.03, 0.05, 0.1, and 0.2) were deposited on the quartz substrates using a spin coating with a rotational speed of 3,000 rpm. Moreover, the BiFeO₃ and Bi_1−xPr_xFeO₃ films were then annealed at a temperature of 600^oC. The samples were finally characterized using X-ray diffraction (XRD) and ultraviolet (UV)-vis spectrophotometer to determine the crystal structure and optical properties.

2.3 XRD characterization

Characterization by XRD was to determine the crystal structure of sample Bi_1−xPr_xFeO₃. The diffraction patterns were then utilized to compute crystal size, lattice constant, and lattice strain. The lattice constants were calculated using refinement of the General Structural Analysis System software with the Rietveld method and manual calculations. The lattice constant was calculated using equation (1) [15].

(1) 1 d 2 = 4 3 h 2 + h k + k 2 a 2 + l 2 c 2 .

where d is the interplanar distance, h, k, and l are Miller indices, and a, b, and c are the lattice constants. Meanwhile, the crystal size of the samples was estimated using linear regression by the Scherrer equation and Williamson-Hall methods. For the Scherrer equation, the plot of linear regression could be done by modifying the Scherrer equation as equation (4) [15], where D is the particle size (nm), λ is the wavelength (1.54056 Å), k is a constant equal to 0.94, β D is the full width at half-maximum intensity (FWHM), θ is the peak position , ε is lattice strain, and β h k l is the FWHM in different condition.

(2) D = k λ β D cos θ ,

(3) β D 2 = [ β measured 2 − β instrumental 2 ] ,

(4) cos θ = k λ D 1 β D .

Meanwhile, by using the W–H method, the crystal size could be obtained by the linear regression of the plot of ( β h k l cos θ ) vs ( 4 sin θ ) as equation (7) [15,16].

(5) β hkl = β D + β strain ,

(6) β h k l = k λ D cos θ + ( 4 ε tan θ ) ,

(7) β h k l cos θ = k λ D + ( 4 ε sin θ ) .

2.4 UV-vis spectrophotometer

Characterization by UV-vis spectrophotometer provides information on wavelength (λ)-dependent absorbance (A) and transmittance (T). From the transmittance data, the light dispersion and refractive index of the sample Bi_1−xPr_xFeO₃ could be found. The formula for determining the refractive index is shown in equation (8) [17]. In which, S is a transmission spectrum, N ₁ is a spectral region, T _S is the substrate transmittance, T _m is the sample transmittance, and n is the refractive index. Meanwhile, the light dispersion was determined by the plot of 1 / ( n 2 − 1 ) vs E 2 , where E is the energy determined by equation (11) [2].

(8) n = N 1 + ( N 1 2 − s 2 ) 1 2 1 2 ,

(9) N 1 = 2 s T m + s 2 + 1 2 ,

(10) S = 2 s T m + 1 T s 2 − 1 ,

(11) E 2 = ( h v ) 2 .

3 Results and discussion

Figure 1 shows XRD pattern of Bi_1−xPr_xFeO₃ with (x = 0, 0.03, 0.05, 0.1, and 0.2). The diffraction peaks match with BiFeO₃ (ICDD) database #861518. Figure 1a shows another phase formed in BiFeO₃ (x = 0) pattern that belongs to Bi₂FeO₉ with ICDD database #741098. This impurity is considered because of the immature crystal growth during annealing in the fabrication process. It is similar to the research conducted in ref. [18].

$Figure 1 (a) Diffraction patterns and (b) magnification of the main peaks of Bi1−x Pr x FeO3 films.$

Figure 1

(a) Diffraction patterns and (b) magnification of the main peaks of Bi_1−xPr_xFeO₃ films.

However, for all Bi_1−xPr_xFeO₃ samples, other phases were slightly reduced. It suggests that the addition of Pr dopants declines the formation of other phases during the fabrication [18]. Figure 1b shows the magnification of the main diffraction peaks showing that the addition of Pr dopant causes the angles to shift to bigger ones. It is because the atomic radius of Pr (1.13 Å) replacing the Bi atom (1.17 Å) is smaller [9], which leads to the change in the atomic distance so that the angle shifts toward the bigger theta [19].

Figure 2 exhibits the lattice constant of the BiFeO₃ and Bi_1−xPr_xFeO₃ films obtained from the GSAS refinement process using Rietveld analysis and manual calculation. The values from the two methods are not significantly different. Besides, based on Figure 2, the addition of Pr in BiFeO₃ decreases the lattice constant. It is because the atomic radius of Pr³⁺ (1.13 Å) replacing the Bi³⁺ atom (1.17 Å) is smaller in size [9]. The replacement of the Bi³⁺ atom by Pr^{3 +} causes the change in the atomic distance, which changes the unit cell volume. However, both using GSAS software and manual calculation, the lattice values are close to the ICDD, where a = b = 5.577 Å and c = 13.861 Å.

Figure 2

Lattice constants of BiFeO₃ and Bi_1−xPr_xFeO₃ films using (a) GSAS refinement and (b) manual calculation.

Figure 3 displays the crystal size of the BiFeO₃ and Bi_1−xPr_xFeO₃ films calculated using Debye Scherrer and William–Hall (W–H) methods. Based on Figure 3, using both methods, the crystal size in the Bi_1−xPr_xFeO₃ samples decreases with the increasing Pr doping in BiFeO₃. It is stated in [8] that replacing the Ba-site with a smaller ionic radius dopant induces a smaller crystal size. In addition, the replacement of the Bi atom by the Pr atoms results in a change in the unit cell volume, resulting in a small crystal size [34]. However, there is a significant difference in the crystal size values between W–H and Debye Scherrer. However, there is a significant difference in crystal size values between W–H and Debye Scherrer. This is because in the W–H plot method there is a correction to the FWHM value, so that the crystal size value using the W–H plot method is smaller than the crystal size value using the Debye Scherrer method. Figure 4 shows the lattice strain of BiFeO₃ and Bi_1−xPr_xFeO₃ films. It is seen from the figure that the lattice strain advances as the greater number of Pr concentrations. The increment is significantly from 0 to 5% Pr concentration. Here, the lattice strain increases due to the insertion of Pr dopants in the BiFeO₃ crystal structure.

Figure 3

Crystal size of BiFeO₃ and Bi_1−xPr_xFeO₃ films using William–Hall and Debye Scherrer methods.

Figure 4

Lattice strain of BiFeO₃ and Bi_1−xPr_xFeO₃ films.

Figure 5a shows the transmission values (T) of BiFeO₃ and Bi_1−xPr_xFeO₃ samples at the wavelengths of visible light in the range of 800–500 nm produce high transmittance (T), and the transmittance value decreases at UV wavelengths in the range of 400–200 nm. This indicates that most of the light energy is transmitted at visible light wavelengths. Meanwhile, Figure 5b presents the absorbance values of BiFeO₃ and Bi_1−xPr_xFeO₃ samples, which are inversely proportional to their transmittance values, where visible light wavelengths in the 400–200 nm range produce high absorbance values (A), and in the 800–500 nm range, the absorbance value decreases. On the other hand, the sample can absorb sunlight well at UV light wavelengths. With the addition of Pr doping on BiFeO₃, the transmittance value tends to increase. This indicates that the number of photons absorbed by the material is less. This confirms that the sample cannot absorb sunlight well at long wavelengths of visible light. This may be related to the lattice constant and the crystal size of the sample. As reported in other ferroelectric materials, decreasing the crystal size can reduce light scattering and increase transmittance [20].

Figure 5

(a) Transmittance (T) and (b) absorbance (A) spectra of BiFeO₃ and Bi_1−xPr_xFeO₃ films.

Figure 6a shows the relationship graph of 1/(n ² − 1) vs E ² (Ev) to determine the light dispersion of the BiFeO₃ and Bi_1−xPr_xFeO₃ samples. It reveals that the dispersion value is getting higher as the greater number of Pr dopants. The relationship between wavelength and refractive index of the BiFeO₃ and Bi_1−xPr_xFeO₃ samples is plotted in Figure 6b. At 400–800 nm wavelength, the refractive index values of the Bi_1−xPr_xFeO₃ samples are constant at around 1–1.5. The refractive index then increases in the wavelength range of 300–400 nm. Further, the refractive index (n) tends to decline with the increase in the mole number of Pr doping. The decrement is associated with the small crystal size [9]. The small crystal size leads to fewer electrons in the Bi_1−xPr_xFeO₃ samples so that the polarization produced is also reduced. Polarization is proportional to the refractive index based on the Lorentz–Lorentz equation [21]. Thus, as the higher Pr doping induces a smaller crystal size, it causes the reduction of the refractive index as well.

$Figure 6 (a) Dispersion energy and (b) refractive index of BiFeO3 and Bi1−x Pr x FeO3 films.$

Figure 6

(a) Dispersion energy and (b) refractive index of BiFeO₃ and Bi_1−xPr_xFeO₃ films.

4 Conclusion

The films of praseodymium (Pr) doped BiFeO₃ or Bi_1−xPr_xFeO₃ have been prepared using the CSD method with the mole number of x = 0.03, 0.05, 0.1, and 0.2. The XRD results show that the Pr addition in BiFeO₃ causes the diffraction angle to shift to smaller angles so that the lattice constant changes to smaller values. The crystal size also shrinks as the more Pr doping, while the lattice strain expands. Based on the UV-Vis spectrophotometer results, the transmittance spectra increase with the increasing Pr number, indicating that the absorbance spectra decrease. Moreover, the refractive index and light dispersion of the Bi_1−xPr_xFeO₃ samples increase with the increase of Pr doping.

Acknowledgments

The authors wish to thank Penelitian Unggulan Terapan 2020 PNBP UNS No. 425/UN27.21/PP/2020 for the financial support.

Funding information: The authors would like to thank Penelitian Unggulan Terapan 2020 PNBP UNS No. 452/UN27.21/PP/2020 for the financial support.
Author contributions: M.D.N.: performance of the experiment; Y.I., R.S., D.K.S.: data analysis and calculation; Y.I., M.D.N., D.K.S., R.S., and D.F.: report writing, translation, and revision. The financial was provided by Y. Iriani. The final report was committed by all contributors.
Conflict of interest: The authors declare that there is no conflict of interest.

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Received: 2021-10-26

Accepted: 2022-02-10

Published Online: 2022-07-11

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/eng-2022-0046

Keywords for this article

Bismuth ferrite; praseodymium; crystal structure; optical properties; CSD method

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