Low-Temperature Solid-State Synthesis of High-Purity BiFeO3

Sep 12, 2017 - The synthesis of high-purity BiFeO3 (BFO) ceramic by solid-state reaction is known to be very difficult due to inevitable formation of ...
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Low-Temperature Solid-State Synthesis of High-Purity BiFeO3 Ceramic for Ferroic Thin-Film Deposition Hyeon Han,† Ji Hyun Lee,† and Hyun Myung Jang* Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea S Supporting Information *

ABSTRACT: The synthesis of high-purity BiFeO3 (BFO) ceramic by solid-state reaction is known to be very difficult due to inevitable formation of the secondary phases, mostly mullite-type Bi2Fe4 O9 and sillenite-type Bi 25FeO 39. In particular, it is very difficult to completely remove the Bideficient Bi2Fe4O9 phase from sintered ceramic BFO targets. This problem consequently leads to the difficulty of fabricating high-quality BFO thin films using these sintered targets. Herein, we introduce a simple but effective low-temperature processing scheme for removing impurity phases in which optimized processing conditions are obtained by chemically correlating the first calcination step with the subsequent leaching and sintering steps. More specifically, we suitably avoid the formation of the high-temperature-stable Bi2Fe4O9 phase by performing the calcination at significantly low temperatures (between 650 and 675 °C) with Bi-excess starting powders. We have then fabricated epitaxially grown BFO thin films using these phase-pure ceramic targets and consequently achieved high-quality ferroelectricity and switchable photovoltaic responses. On the basis of the present experimental observations, we suggest that a low impurity concentration in the sintered BFO ceramic target, even with a low relative density, is advantageous for high-quality thin-film fabrication.

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INTRODUCTION Multiferroic materials exhibit more than one of the ferroic properties such as ferroelectricity, (anti)ferromagnetism, and ferroelasticity. Multiferroics have received a great deal of attention in the past two decades owing to their potential applications in data storage, sensors, filters, attenuators, and spintronic devices.1−7 Until now, BiFeO3, having a perovskite structure, is the most extensively investigated multiferroic material. The rhombohedrally distorted BiFeO3 exhibits both antiferromagnetic (AFM) behavior with a relatively high Néel temperature (TN ∼ 640 K) and ferroelectric responses with a high Curie temperature (TC ∼ 1100 K).8−10 In the case of BiFeO3, a thin-film form has attracted a great deal of interest since a remarkably enhanced polarization was reported in a thin-film heteroepitaxial structure.11 Enhanced properties in a thin-film form can be attributed, in general, to the fact that a large strain can be generated between the film and the underlying substrate and results in material properties that are significantly different from those obtained for the bulk structures. By suitably exploiting this, one can finely control cation/anion chemistries and defect structures, tailor the domain structure, and create artificial heterostructures engineered down to the unit cell, which enable new states of materials and phenomena.2,12−14 To produce such a highquality thin film, it is very important to fabricate a single-phase ceramic target having low impurity concentrations. © XXXX American Chemical Society

For the fabrication of BiFeO3 (BFO hereafter) ceramic target, solid-state reaction using Bi2O3 and Fe2O3 starting powders is the most commonly employed method. However, the solid-state synthesis of a phase-pure BFO ceramic is a challenging task as kinetics of phase formation in the Bi2O3− Fe2O3 pseudobinary system always lead to the formation of secondary phases such as mullite-type Bi2Fe4O9 and sillenitetype Bi25FeO39.15−21 Thus, it is extremely hard to synthesize a single-phase BFO even by extensive variations of sintering temperature, time, and atmosphere. The presence of impurity phases primarily results in high leakage current, low resistivity, and poor ferroelectricity. The observed high tendency toward the formation of secondary phases in BFO is attributed to various physicochemical factors. These include the following: presence of impurities,22 formation of nonstoichiometric solids,16 thermodynamic metastability,20 low peritetic decomposition temperature,23 and formation of secondary phases due to Bi2O3 evaporation.15 In order to remove these secondary phases by a solid-state reaction method, sintering in the N2 environment24 and leaching with nitric acid10 had been proposed. However, the problem is that only a part of the secondary phases can be removed. Accordingly, various alternative methods have been attempted to eliminate Received: July 25, 2017

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DOI: 10.1021/acs.inorgchem.7b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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RESULTS AND DISCUSSION Figure 1 shows theta−2theta (θ−2θ) X-ray diffraction (XRD) patterns of the BFO powders calcined at various temperatures

secondary phases in BFO. These include the following: rapid liquid-phase sintering,25,26 spark-plasma sintering (SPS),27,28 microwave-assisted sintering,29 wet chemical methods,30−34 and mechanochemical high-energy milling processing.35 However, all these alternative methods have some disadvantages such as a complicated manufacturing method, high cost, and difficulty in mass production, as compared with the conventional solid-state reaction method. In particular, it is very difficult to completely remove the Bi2Fe4O9 phase from sintered ceramic BFO targets. Moreover, this Bi-deficient phase is often observed by scanningelectron microscopy though more commonly used X-ray diffraction measurements do not detect its presence. Owing to this problem, it is extremely difficult to produce a highquality BFO thin film using solid-state sintered targets. Here, we introduce a simple but useful processing scheme for obtaining a high-purity BFO ceramic target for subsequent use in the fabrication of BFO thin-film heterostructures. More specifically, we suitably avoid the formation of the hightemperature-stable Bi-deficient Bi2Fe4O9 phase by performing the calcination at significantly low temperatures (between 650 and 675 °C) with Bi-excess starting powders. We have then fabricated epitaxially grown BFO thin films using these phasepure ceramic targets and consequently achieved high-quality ferroelectricity and switchable photovoltaic responses.

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Figure 1. X-ray diffraction (XRD) patterns of BiFeO3 powders calcined at (a) 500 °C, (b) 600 °C, (c) 650 °C, (d) 675 °C, (e) 700 °C, (f) 750 °C, and (g) 800 °C, respectively. The following abbreviations are used to denote various phases and species: BiFeO3 (B), Bi2Fe4O9 (*), Bi25FeO39 (#), Bi2O3 (+), Fe2O3 (−).

for 1.5 h. The XRD patterns show the phase evolution of crystalline phases with increasing temperature from 500 to 800 °C. Four distinct stages of the phase evolution can be deduced from these patterns. First, unreacted Bi2O3 (+) and Fe2O3 (−) exist separately at 500 °C, indicating that a higher temperature is required for the reaction between Bi2O3 and Fe2O3 to form BFO. With increasing temperature, both BFO (B) and Bi25FeO39 (#) appear at 600 °C, in addition to unreacted Bi2O3 (+) and Fe2O3 (−). In the third stage between 650 and 675 °C, the unreacted Bi2O3 and Fe2O3 completely disappear, and the corresponding power is composed of BFO (B) and Birich Bi25FeO39 (#) only. It is interesting to notice that BFO (B) and Bi25FeO39 (#) begin to appear almost simultaneously. The XRD patterns further indicate that the minimum temperature required for the disappearance of the remnant Bi2O3 (+) and Fe2O3 (−) to form BFO is ∼650 °C. At elevated temperatures between 700 and 800 °C, the Bi-deficient mullite-type Bi2Fe4O9 (*) phase begins to appear, in addition to BFO (B) and Bi25FeO39. Unlike the present result (Figure 1), it is generally known that three Bi-containing species coexist by the simultaneous generation of Bi25FeO39 and Bi2Fe4O9 secondary phases during the synthesis of BFO.15−21 In other words, BFO decomposes into Bi-deficient Bi2Fe4O9 and Bi-rich Bi25FeO39 according to the following reaction: 49BiFeO3 → 12Bi2Fe4O9 + Bi25FeO39. It is known that this three-phase coexisting system is thermodynamically stable during the solid-state reaction.20,21 The coexistence of these three distinct phases is explained by the diffusion of Bi ions into Fe2O3.36 More concretely, the nuclei of the Bi2Fe4O9 phase are formed in the Fe2O3 core, and the Bi25FeO39 phase is formed in the outer shell of Fe2O3. As the temperature increases, the diffusion of Bi ions leads to the formation of BFO. At the same time, the crystallization of Bideficient Bi2Fe4O9 in the core competitively proceeds, which presents as extremely stable behavior and thus tends to block the formation of BFO. In particular, the higher the processing temperature is, the faster the crystallization of Bi2Fe4O9 is, which explains why the fraction of the Bi2Fe4O9 phase increases with increasing calcination temperature (Figure 1). However, as described earlier, our XRD results show that only Bi-rich

EXPERIMENTAL SECTION

Fabrications of BiFeO3 Ceramics and Thin Films. Polycrystalline BiFeO3 (BFO) ceramic targets were prepared by solid-state reaction. High-purity Bi2O3 (99.99%, Aldrich) and Fe2O3 (99.99%, Alfa Aesar) powders were weighed with 10 mol % excess Bi and thoroughly mixed by ball milling for ∼15 h using high-purity isopropyl alcohol as a medium. The mixture was dried and calcined at various temperatures between 500 and 800 °C for 1.5 h in an alumina crucible. The calcined powders were leached under continuous stirring for 1 h with dilute nitric acid at three different concentrations, 0.1, 0.3, and 0.5 M. The leached residue was washed twice with a large volume of distilled water to neutralize the acidity. The mixture was then dried and pressed into pellets. The pellets were pressed again by a cold isostatic press (CIP) at 100 MPa and finally sintered in air at 730 °C for 1 h. BFO thin films were grown by pulsed laser deposition (PLD). The sintered BFO pellet prepared by the optimal processing conditions was used as the target for thin-film deposition. A conducting perovskite SrRuO3 electrode having 50 nm thickness was deposited on a SrTiO3 (100) substrate at 600 °C in an oxygen ambient of 100 mTorr, prior to the deposition of a 250 nm thin BFO layer at 600 °C in an oxygen ambient of 70 mTorr. The films were cooled with a cooling rate of 5 °C min−1 to 390 °C in an oxygen ambient of 1 atm, annealed for 1 h, and then cooled to room temperature. Characterizations of Ceramics and Thin Films. To obtain information on temperature-dependent phase evolution, powder X-ray diffraction (XRD) measurements (MAX-2500, Rigaku) were performed with Cu Kα radiation at 40 kV and 100 mA. XRD measurements of thin films were conducted by using a Bruker D8 DISCOVER diffractometer with Cu Kα radiation. Microstructural and chemical information on the calcined powder was obtained by using a field-emission scanning-electron microscope (FE-SEM) equipped with an energy-dispersive X-ray (EDX) spectrometer (JSM-7800F PRIME, JEOL Ltd.). For ferroelectric characterizations, P−E hysteresis loops were measured using a Precision LC system (Radiant Technologies, Inc.). The current density−voltage (J−V) characteristics were measured using a source meter (Compactstat, IVIUM tech.) under simulated AM 1.5G illumination (100 mW cm−2) provided by a solar simulator (Sun 3000, Abet tech.). The incident light intensity was calibrated with a Si solar cell (as a reference) equipped with an IRcutoff filter (KG-5, Schott). B

DOI: 10.1021/acs.inorgchem.7b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Bi25FeO39 begins to simultaneously form with BFO at 600 °C. We judge the main reason for obtaining this extraordinary result is that we used an excess amount of 10% in the original Bi2O3 powder, which induces a favorable thermodynamic condition for producing the Bi-rich Bi25FeO39 phase rather than the Bi-deficient Bi2Fe4O9 phase. On the contrary, Valant et al.22 reported that Bi2Fe4O9 is more readily generated when the amount of Bi is deficient. However, considering the difficulty associated with the subsequent leaching step, we conclude that a processing scheme of producing Bi-deficient starting powder is highly unfavorable and thus should be avoided. The leaching step (second step) is known to be effective for eliminating impurity phases from BiFeO3 powders.24 Figure 2

after the leaching with 0.3 or 0.5 M nitric acid. For the purpose of comparison, the powder calcined at 720 °C was leached with 0.3 M nitric acid. In this case, the Bi25FeO39 phase disappears upon leaching, but the peaks related to the Bi-deficient Bi2Fe4O9 phase are still observed in the corresponding XRD pattern (Figure 2d). Interestingly, a repeating leaching step (i.e., second leaching with the same 0.3 M nitric acid) even increases the fraction of the Bi2Fe4O9 (*) phase with a concomitant decrease in the fraction of the main R3c BFO phase (Figure 2e). This is probably due to (i) an unintended reaction with nitric acids or (ii) additional volatilization of Bi during the second leaching−drying process. Since the leaching−drying process has been carried out at a relatively low temperature, it is presumed that some other causes, rather than the Bi-volatilization, are responsible for this observation. In order to obtain a phase-pure BFO ceramic, thus, it is highly advantageous to carry out the calcination step at temperatures between 650 and 675 °C, which is characterized by the absence of the high-temperature-stable Bi-deficient Bi2Fe4O9 impurity phase. We performed FE-SEM analysis to microscopically examine impurity phases (Figure 3). Even though a single BFO phase is apparent according to the XRD results, additional analysis is necessary since SEM images often show secondary phases that are not detected by the XRD analysis.36 For doing this, backscattered electron-microscopic images were obtained for the powders calcined at 675, 720, and 800 °C (Figure 3a−c). The FE-SEM image and the corresponding EDX spectra were further examined for the powder calcined at 720 °C (Figure 3d,e). Before SEM analysis, the calcined powders were leached with 0.3 M nitric acid. The backscattered microscopic image of the BFO powder calcined at 675 °C (Figure 3a) does not show any evidence of impurity-phase segregation. However, the powder calcined at 720 °C (Figure 3b) reveals many internal dark-colored grains distributed sporadically, which is analogous to the previously reported images of secondary phases.22,36 Specifically, the presence of a small dark-colored particle inside each grain is explained by the proposed mechanism36 that the Bi2Fe4O9 particle is generated inside the core of the Fe2O3 grain as described previously. The contrast of the backscattered image is known to be highly sensitive to the atomic number.

Figure 2. XRD patterns of BiFeO3 powders after leaching with nitric acid at three different concentrations. Leaching with (a) 0.1 M, (b) 0.3 M, and (c) 0.5 M nitric acids for the powder calcined at 675 °C. (d) Leaching with 0.3 M nitric acid for the powder calcined at 720 °C and (e) leaching twice with 0.3 M nitric acid for the same powder calcined at 720 °C. The following abbreviations are used to denote phases involved: BiFeO3 (B), Bi2Fe4O9 (*), Bi25FeO39 (#).

shows the XRD patterns of BiFeO3 powders prepared using various conditions. The powder calcined at 675 °C was leached for 1 h with nitric acid at three different concentrations, 0.1, 0.3, and 0.5 M (Figure 2a−c). The Bi25FeO39 impurity phase remains unremoved even after the leaching with 0.1 M nitric acid (Figure 2a). However, this Bi-rich phase is neatly removed

Figure 3. Backscattered electron-microscopy images of BiFeO3 (BFO) powders calcined at (a) 675 °C, (b) 720 °C, and (c) 800 °C, respectively. (d) FE-SEM image and (e, f) the corresponding EDX line profiles of the BFO powder calcined at 720 °C. Herein, all the BFO powders were leached with 0.3 M nitric acid, regardless of the calcination temperature used before the leaching. C

DOI: 10.1021/acs.inorgchem.7b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

°C (b). The calcined powders were leached with 0.3 M nitric acid before the sintering at 730 °C for 1 h. Figure 4a reveals that the sintered BFO ceramic derived from the calcination at 675 °C is characterized by the absence of any impurity phase. On the contrary, comparing Figure 4b with Figure 2d reveals that, for the powder calcined at 720 °C, the fraction of impurity Bi2Fe4O9 phase increases significantly after the sintering at 730 °C. It is thus judged that the fraction of the Bi-deficient Bi2Fe4O9 impurity increases as the Bi-ion is further volatilized during the sintering at 730 °C. On the contrary, the relative density of the sintered BFO target derived from the calcination at 675 °C is 81% which is significantly lower than that of the BFO pellet (90%) derived from the calcination at 720 °C. This indicates that the sintered BFO target calcined at a higher temperature yields a higher density though the fraction of impurity phase tends to increase with the calcination temperature. We have further examined the suitability of our BFO ceramic targets for fabricating high-quality multiferroic thin films. For this purpose, we have grown 250 nm thick BFO thin films by adopting a pulsed laser deposition (PLD) method. To ensure heteroepitaxial growth of BFO, a conducting perovskite SrRuO3 electrode having 50 nm thickness was deposited on a SrTiO3 substrate prior to the deposition of a BFO layer. Figure 5a shows the effect of calcination temperature on the growth quality of the deposited BFO films obtained using the ceramic target sintered at 730 °C. According to the XRD patterns shown in Figure 5a, the BFO target prepared by the calcination at 675 °C (prior to the sintering at 730 °C) leads to a heteroepitaxial growth of the BFO layer along the crystalline direction of the SrTiO3 (001) substrate. On the contrary, the BFO film layer prepared by the BFO target calcined at 720 °C shows a very low peak-intensity of BFO, indicating that the film is not epitaxially grown and contains a substantial amount of

Thus, the internal dark-colored grains correspond to the Bi2Fe4O9 phase since the concentration of heavy Bi-ion in Bi2Fe4O9 is lower than that in BFO. For the powder calcined at 800 °C (Figure 3c), the crystalline growth of BFO proceeds remarkably. However, a dark-colored grain still exists inside the BFO grain. Additional information on the impurity phase was obtained by conducting FE-SEM and EDX measurements (Figure 3d−f) using the powder calcined at 720 °C. The FESEM image shows the presence of a small grain inside the BFO grain (Figure 3d). The corresponding EDX line profiles indicate that the internal grain is characterized by Fe-rich and Bi-deficient stoichiometry, which clearly supports the conclusion deduced from the backscattered images, namely, the Bi2Fe4O9 grain (Figure 3a−c). Figure 4 shows XRD patterns of the sintered BFO ceramic obtained using the powder calcined at 675 °C (a) and at 720

Figure 4. XRD patterns of BiFeO3 ceramics sintered at 730 °C using two different calcination temperatures: (a) 675 °C and (b) 720 °C. The following abbreviations are used to denote the two relevant phases: BiFeO3 (B), Bi2Fe4O9 (*).

Figure 5. (a) θ−2θ XRD patterns of two distinct BFO thin films fabricated using the ceramic target sintered at 730 °C. The upper profile (red) corresponds to the calcination at 720 °C before sintering of the BFO ceramic target at 730 °C. On the contrary, the lower profile (blue) corresponds to the calcination at 675 °C before sintering of the BFO ceramic target at the same temperature, 730 °C. (b) In-plane XRD φ-scan spectra of BFO, SrRuO3 layers, and SrTiO3 substrate, where the top BFO film was fabricated using the ceramic target calcined at 675 °C before sintering at 730 °C. Ferroelectric polarization hysteresis loops of two distinct BFO thin films prepared using the ceramic target calcined at 675 °C (c) and at 720 °C (d) before sintering at 730 °C. (e) Illuminated J−V characteristics and (f) zero-bias photocurrent density profiles of the BFO thin-film device under AM 1.5G illumination. Herein, the photosensitive BFO thin film was fabricated using the ceramic target calcined at 675 °C before sintering at 730 °C. D

DOI: 10.1021/acs.inorgchem.7b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the impurity phases (Figure 4b). The in-plane XRD φ-scan spectra, as obtained by keeping the Bragg angles at (202), confirm an epitaxial growth of the BFO film prepared using the ceramic target calcined at 675 °C (Figure 5b). The four peaks that are 90° apart from each other occur at the same azimuthal φ-angles for each of three distinct reflection planes, demonstrating cube-on-cube epitaxy of both the BFO and SrRuO3 layers on the SrTiO3 substrate. The calcination condition not only controls the growth quality (Figure 5a) but also strongly influences ferroelectric and photovoltaic responses of the deposited BFO film. The ferroelectric hysteresis loop of the 250 nm thick BFO thin film prepared by using the ceramic target calcined at 675 °C shows an ideal square-shaped P−E response with a remnant polarization (Pr) of ∼60 μC cm−2 (measured at 1 kHz; Figure 5c), which coincides with the reported P−E responses of highquality BFO thin films.8,11 In contrast, as shown in Figure 5d, the BFO thin film prepared by the ceramic target calcined at 720 °C (but using the same sintering temperature of 730 °C) shows a very poor P−E response, indicating high leakage currents due to the presence of impurity phases, mainly Bi2Fe4O9 (Figure 4b). We have further examined switchable photovoltaic responses of the BFO thin-film device measured under AM 1.5G illumination (Figure 5e). The BFO film was prepared by using the ceramic target calcined at 675 °C and subsequently sintered at 730 °C. Here, a transparent indium tin oxide (ITO) top-electrode layer was deposited by PLD. Two opposite electrical-poling directions were used to examine the switchable photovoltaic effect, where “upward poling” signifies the application of a positive voltage to the bottom electrode, and “downward poling” denotes the application of a negative voltage. To ensure a complete polarization switching by the poling, we applied an electric field of ∼300 kV cm−1, which is much stronger than the coercive field (Ec), 100 kV cm−1. The current direction is symmetrically switched as the polarization direction is changed. In the case of the upward poling, shortcircuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and the corresponding power conversion efficiency (PCE) are 0.01 mA cm−2, 0.34 V, 32%, and 0.001%, respectively. These are comparable to the previously reported results.37−39 Figure 5f shows the time-dependent photocurrent under a zero-bias voltage. The ON and OFF states are repeatable, which clearly indicates a stable photocurrent in the absence of a bias field. All these results suggest that a lowtemperature-sintered BFO target having an extremely low impurity concentration is advantageous for high-quality thinfilm fabrication though it tends to have a lower sintered density.

by the absence of any impurity phase, using this optimally calcined powder. We have fabricated heteroepitaxially grown BFO thin films using these high-purity ceramic targets and consequently achieved high-quality ferroelectricity and switchable photovoltaic responses. On the basis of all these experimental observations, we have concluded that a lowtemperature-sintered BFO target having an extremely low impurity concentration is advantageous for high-quality thinfilm fabrication though it tends to have a lower sintered density. The present study presents a simple practical way of synthesizing high-quality BFO ceramics and thin films without requiring sophisticated processing steps.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01893. XRD, SEM, and P−E hysteresis loop of BiFeO 3 fabricated by the traditional synthetic method (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hyeon Han: 0000-0002-2973-5225 Hyun Myung Jang: 0000-0002-1889-9515 Author Contributions †

H.H. and J.H.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) Grant funded by the Korean Government (MSIP Grant No. 2016R 1D1A1B 03933253) and by Pohang Steel Corporation (POSCO) through the Green Science Program (Project No. 2016Y038).

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REFERENCES

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CONCLUSIONS In summary, we propose a simple processing scheme for obtaining a high-purity BFO ceramic target by carefully examining temperature-dependent phase-formation characteristics. In the calcination step, we deliberately produce a Bi-rich minority phase (Bi25FeO39) which can be readily removed in the subsequent leaching step. In contrast, we suitably avoid the formation of a Bi-deficient phase, Bi2Fe4O9, which is very difficult to remove in the subsequent leaching step. To achieve this goal, we perform the calcination at a relatively lowtemperature region (between 650 and 675 °C) with Bi-excess starting powders. On the contrary, the presence of a Bideficient Bi2Fe4O9 phase was confirmed in the BFO powder calcined at a higher temperature (720 °C). We are able to obtain the sintered BFO ceramic target, which is characterized E

DOI: 10.1021/acs.inorgchem.7b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01893 Inorg. Chem. XXXX, XXX, XXX−XXX