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Large-scale growth and shape evolution of bismuth ferrite particles with a hydrothermal method
Materials Chemistry and Physics 126 (2011) 560–567
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Large-scale growth and shape evolution of bismuth ferrite particles with a hydrothermal method Xian-Zhi Chen, Zhong-Cheng Qiu, Jian-Ping Zhou ∗ , Gangqiang Zhu, Xiao-Bing Bian, Peng Liu College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 June 2010 Received in revised form 21 October 2010 Accepted 10 January 2011 PACS: 61.46.Df 89.30.Ee Keywords: Ceramics Crystal growth SEM Crystal structure
a b s t r a c t Large-scale polyhedral bismuth ferrite (BiFeO3 ) particles were synthesized with a hydrothermal method under a series of experimental conditions. X-ray diffraction revealed that the BiFeO3 powders had a perovskite structure. Scanning electron microscopy images showed different BiFeO3 particles were formed, including sphere, octahedron, truncated octahedron, cubo-octahedron and truncated cube. The experimental results showed that the concentration of KOH, reaction time, heating and cooling rates had important impacts on the size and morphology of the BiFeO3 particles. The formation mechanism and change process of the large-scale polyhedral BiFeO3 particles were discussed in detail. The obtained BiFeO3 showed ferroelectric behavior and magnetic response, which approved the multiferroic property of the BiFeO3 crystallization. The optical behaviors of BiFeO3 particles revealed the band gap energy of about 2 eV, which is smaller than the BiFeO3 bulk due to the nano-crystallites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic materials, which possess a coexistence of ferroelectric and magnetic order parameters, have been one of the hot topics intensively studied due to their fascinating fundamental physical properties and their potential applications in multifunctional devices [1–6]. Bismuth ferrite (BiFeO3 ) is one of the well-known single-phase material possessing multiferroic behaviors. The perovskite BiFeO3 has a ferroelectric Curie temperature TC = 830 ◦ C and an antiferromagnetic Neel temperature TN = 370 ◦ C [6–8]. BiFeO3 has a rhombohedrally distorted perovskite structure belonging to the space group R3c with rhombohedral lattice parameters a = 0.563 nm, ˛ = 59.35◦ , or hexagonal parameters a = 0.558 nm, c = 1.388 nm. Though many efforts have been devoted to prepare BiFeO3 , it was difficult to avoid the impurities in the products such as Bi2 Fe4 O9 and Bi25 FeO39 [9]. Nitric acid leaching was used to eliminate the impure phases in the conventional solid-state reaction. But this would result in the coarse powders and poor reproducibility [8]. BiFeO3 perovskites could only stabilize within a narrow ranged temperature. Subsequently, other methods have been developed to prepare BiFeO3 , such as ferrioxalate precursor method [10], sol–gel process [11,12], co-precipitation [13], spray pyrolysis method [14], and hydrothermal method [15–19]. The
∗ Corresponding author. Tel.: +86 29 85303823; fax: +86 29 85303823. E-mail address: [email protected] (J.-P. Zhou). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.027
properties of BiFeO3 were improved by doped with La [20,21], Y [22], Ru [23], Gd [24], Co and Ni [25], etc. But the impurity phases cannot be avoided still. Recently, just a few works preparing BiFeO3 crystallites by the hydrothermal method have been reported. Pure BiFeO3 with the irregular shape was synthesized at different KOH concentrations and reaction temperatures [15]. BiFeO3 crystallites with spindle shape were prepared at pH 14 with adding a small amount of H2 O2 [16]. The cubic BiFeO3 crystallites were obtained by the hydrothermal treatment at 200 ◦ C for 9 h with 12 M KOH [17,18]. The pure BiFeO3 could be synthesized in wider experimental conditions with the help of NH4 Cl nonaqueous solvent [19]. But these works had not shown the changing process of the morphologies and the formation mechanism of the regular shapes has not been clearly understood yet. A fundamental understanding of the formation mechanism and key factors that determine the structures of these materials are crucial for the rational design and controlled synthesis of these materials in nanosize. On the other hand, BiFeO3 magnetic performance is greatly dependent on the particle shape and grain size [26–28]. In the present work, we prepared pure BiFeO3 powders in different hydrothermal conditions. We showed the changing process of BiFeO3 particles with different morphologies, which were obtained by controlling the hydrothermal conditions carefully. Then we presented a possible formation mechanism of the regular morphologies. Finally, magnetic, ferroelectric and optical behaviors were researched. The various morphologies are helpful to investigate the BiFeO3 foundation properties and photocatalytic behaviors [18].
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Fig. 1. XRD patterns of samples synthesized at 200 ◦ C for 12 h in the KOH range of 0.7–10 M.
2. Experimental The chemical reagents used to prepare BiFeO3 powder were Fe(NO)3 ·9H2 O, Bi(NO)3 ·5H2 O and KOH mineralizer. All chemicals were used as received without further purifications. The processes to prepare the precursor included the following steps: firstly, Fe(NO)3 ·9H2 O and Bi(NO)3 ·5H2 O were dissolved in distilled water under a mechanical stirring; then, the KOH solution was slowly added to the above solution to adjust different pH values. Finally, the brown suspension solution was transferred into a 30 mL Teflon vessel filled at 4/5 of its volume. The hydrothermal treatment was performed under different KOH concentrations for the different reaction times at 200 ◦ C because the most products are pure BiFeO3 phase at this reaction temperature after a series of experiments. The temperature was increased at a rate of 1 ◦ C/min. After cooling down to room temperature naturally, the products were washed several times with distilled water and then oven dried at 70 ◦ C in air. In the last experiment, the heating and cooling rates were designed as 2 ◦ C/min and 0.2 ◦ C/min, respectively. X-ray diffraction (XRD) analysis was performed by using a Rigsku D/Max 2550 ˚ source to determine the powdiffractometer system with a Cu K␣ ( = 1.5418 A) der phases. The particle size and morphology of the products were obtained with a scanning electron microscopy (SEM FEI-Quanta 200). The ferroelectric Curie temperature was determined by the thermal analysis systems (Q1000DSC+LNCS+FACS Q600SDT) at a heating rate of 10 ◦ C/min. Magnetic properties were characterized by an LDJ9600 type of Vibrating Sample Magnetometer (VSM). A Radiant Technologies’ Precision Premier II tester was used to measure the ferroelectric hysteresis loops. The UV–vis spectra of part BiFeO3 microparticles were performed in the diffused reflectance mode using a Perkin-Elmer UV–vis spectrometer (Lambd 950) of the USA.
3. Result and discussion 3.1. Effects of KOH concentration Fig. 1 shows the XRD patterns of samples synthesized at 200 ◦ C for 12 h with different KOH concentrations of 1, 2, 4, 6, 8, 9 and 10 M. The XRD patterns can be indexed as a pure BiFeO3 phase when
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the KOH concentrations are in the range of 1–9 M according to the JCPDS card nos. 71-2494 and 86-1518, which is consistent with the earlier report [17]. It is found that Bi2 Fe4 O9 and Fe2 O3 appeared at 10 M and Bi25 FeO39 at the KOH concentration lower than 1 M. Fig. 2 (a)–(f) shows SEM images corresponding to the samples synthesized with the KOH concentrations of 1, 2, 4, 6, 8, and 9 M, respectively. It is evident from the SEM images that the particle size increases from 10 m to 20 m approximately and the morphology changes from irregular agglomeration to regular polyhedron with increasing the KOH concentration. Various KOH concentrations can result in a series of BiFeO3 morphologies, such as agglomeration, octahedron and {1 0 0} truncated octahedron. As shown in Fig. 2(a), irregular plate-like BiFeO3 agglomerations are formed by small BiFeO3 crystallites at 1 M. In Fig. 2(b), small BiFeO3 crystallites gathered into large particles with irregular shape at 2 M. The sample synthesized at 4 M is agglomerated to spherical particles as shown in Fig. 2(c). The formation mechanism of these three kinds of particles belongs to random aggregation [29]. When the KOH concentration increases to 6 M, the BiFeO3 particles become homogenous octahedron of about 8 m in size as shown in Fig. 2(d). The inset in Fig. 2(d) shows an enlarged image of the clear octahedron. When the KOH concentration is 8 M, the typical shape of the particles transforms into {1 0 0} truncated octahedron about 18 m in size as shown in Fig. 2(e). The six vertices on the octahedral particles are not stable and dissolve into coarse square {1 0 0} surfaces. Meanwhile, eight trigonal {1 1 1} surfaces grow to smooth hexagons. With the further enhancement of the KOH concentration to 9 M, the coarse square {1 0 0} surfaces continually dissolve to big facets and the {1 1 1} surfaces grow to small trigonal facets as shown in Fig. 2(f). Thus, we believed that the BiFeO3 microparticles grew with a faster growth rate along 1 1 1 than along 1 0 0 in high KOH concentration. The insets of Fig. 2(e) and (f) present the magnified images of rough square facets of the truncated octahedrons. The slices in the reticulate {1 0 0} surfaces at 9 M are thinner than that at 8 M because the small particles are easily formed at high KOH concentration [15]. Meanwhile, the smooth surfaces grew more easily at high KOH concentration.
3.2. Effects of reaction time Based on the experimental results presented above, it is worth to change the reaction time to reveal the formation mechanism of the BiFeO3 polyhedrons. Fig. 3 shows the XRD patterns of samples synthesized at 200 ◦ C with 8 M KOH concentration for different reaction times of 30 min, 3, 6, 12 and 24 h, respectively. The foreign phases Bi25 FeO39 and Fe2 O3 appeared at reaction times of 30 min, and the pure BiFeO3 phase is formed with increasing the reaction time. The XRD patterns obtained at different times provide important information to probe the reaction mechanism although it may be complicated in the hydrothermal process. We propose that the BiFeO3 formation in the hydrothermal treatment involves three stages: (1) the nitrates transform into hydroxide precipitations after adding KOH; (2) Bi(OH)3 and Fe(OH)3 react under hydrothermal environment to form BiFeO3 as well as unstable Fe2 O3 and Bi25 FeO39 under the appropriate hydrothermal conditions. Fe2 O3 and Bi25 FeO39 crystallites are completed in cooling process and inspected by XRD; (3) but in the hydrothermal environment with high temperature and pressure, the mixture reacts to form the final BiFeO3 products under limited experimental conditions. The formation formula of BiFeO3 can be written as follows: Bi(NO3 )3 + Fe(NO3 )3 + 6KOH → Bi(OH)3 + Fe(OH)3 + 6NaNO3 (1) 25Bi(OH)3 + 3Fe(OH)3 → Bi25 FeO39 + Fe2 O3 + 42H2 O
(2)
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Fig. 2. SEM images of the samples synthesized at 200 ◦ C for 12 h with different KOH concentrations of (a) 1, (b) 2, (c) 4, (d) 6, (e) 8 and (f) 9 M, respectively.
Bi(OH)3 + Fe(OH)3 → BiFeO3 + 3H2 O Bi25 FeO39 + 12Fe2 O3 → 25BiFeO3
(3)
SEM images in Fig. 4(a)–(d) correspond to the samples synthesized for the reaction times of 3, 6, 12 and 24 h, respectively. The insets in Figs. 4(c) and (d) are the enlarged images of {1 0 0} facets. It is evident that the particle morphologies change from spherical agglomeration to polyhedron with the increase in the reaction time. The uniform spherical agglomerations synthesized at short reaction time of 3 h are composed of many small particles as shown in Fig. 4(a). Increasing the reaction time to 6 h, some polyhedrons appear as shown in Fig. 4(b). After reaction for 12 h, the aggrega-
tions become truncated octahedrons with 6 little rough squares and 8 large smooth hexagons. When the reaction time increases to 24 h, the rough squares dissolve to big squares in 1 0 0 directions and the hexagons grow to smooth triangles in 1 1 1 directions to form the final cubo-octahedron as shown in Fig. 4(d). In other words, the {1 0 0} surfaces dissolves due to their coarse appearance and the solute recrystallizes on the smooth {1 1 1} surfaces. 3.3. Effects of heating and cooling rates Then we chose the samples synthesized at 6 M and 8 M KOH for more careful study. The reagents were heated to 200 ◦ C at a rate of 2 ◦ C/min, reacted for 6 h and then cooled down to room temperature at a rate of −0.2 ◦ C/min. Fig. 5(a) shows the XRD patterns of
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Fig. 3. XRD patterns of samples synthesized at 200 ◦ C with 8 M KOH for different reaction times.
the two samples, which are indexed as pure BiFeO3 phase. Typical result of Rietveld’s refinement of the XRD pattern for BiFeO3 prepared at 8 M KOH is shown as an example in Fig. 5(b), where the observed and calculated XRD patterns are indicated by blank circle and line, respectively. XRD data were collected with Cu K␣ radiation by scanning step measurement (scanning step 0.01◦ , and counting time 2 s/step). Rietveld’s refinement was performed by
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using the program MAUD (Materials Analysis Using Diffraction), which combines the Rietveld method and a Fourier transform analysis. The perovskite structure with space group R3c (161) (JCPDS card no. 71-2494 and ICSD No. 15299) is used as a basic model. The background was described by a 5-order polynomial function. Anisotropic peak-broadening model is selected as Popa LB and sizestrain model is Popa rules. A 10-order harmonic correction is used for the preferred orientation in order to obtain smaller value of Rw. It can be seen that the fitted XRD profile agrees well with the experimental data. A typical result of Rietveld’s analysis gave the cell parameters a = 5.575(0) A˚ and c = 13.847(9) A˚ [reliability factors: Rw (%) = 11.2, sig = 1.13, Rwnb (%. nobkg) = 9.88, Rb (%) = 8.59, Rexp (%) = 9.94]. It revealed these BiFeO3 crystallites had a rhombohedrally distorted perovskite structure belonging to R3c (161) space group. The average crystallite size is about 195 nm. Fig. 6 shows the SEM images of the samples synthesized at 6 M and 8 M KOH concentration. Truncated cubic particles of BiFeO3 are obtained and the particles prepared at 8 M (about 30 m in size) are larger and more compact than that at 6 M (about 10 m in size). These particles are composed of many nano-crystallites for the average crystallite size is only about 195 nm from the XRD results. In comparison with the above experimental conditions, the BiFeO3 nano-crystallites have more chances to locate in suitable sites with low energy in a slow cooling rate. As a result, BiFeO3 crystallites grow more densely and regularly. On the other hand, ions slowly precipitate from the supersaturated solution in a slow cooling rate. The ions have less chance to nucleate and they precipitate on the crystallites. Therefore, uniform and large cubic BiFeO3 particles were successfully obtained through controlling the heating and cooling rates.
Fig. 4. SEM images of the samples synthesized at 200 ◦ C with 8 M KOH for reaction time of 3, 6, 12 and 24 h.
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agglomerate to isotropic spheres. At high KOH concentration, KOH could change the surface energy of the particle and an orientedattachment process dominates, which results in the octahedron formation as shown in Fig. 2(d). Penn and Banfield suggested that the dislocation formation operate during early growth [34]. So the two or more nano-crystallites gathered by the oriented attachment until it grows a small-scale particle. The surface of small-scale particle can catch the nano-crystallites by selective aggregation, which is dependent on the KOH concentration in the solution, until the nutrition was used up. With the further enhancement of the KOH concentration, dissolution-recrystallization predominates. The six vertices of the octahedron dissolve into the solution and the nanoparticles deposit on the {1 1 1} surfaces to form smooth facets. The higher KOH concentration results in the more dissolution of {1 0 0} surfaces to big squares and more growth of {1 1 1} surfaces to small facets, and finally to triangles as shown in Fig. 2(f). This supposition can be confirmed by changing the reaction time. At the initial stage, the concentration of solute reaches supersaturation, resulting in a lot of nucleations formed. To minimize the overall energy of the system, a great number of small seed parti-
Fig. 5. (a) XRD patterns of samples synthesized at 200 ◦ C for 6 h under KOH concentrations of 6 and 8 M with the slow cooling rate. (b) Typical Rietveld’s refinement of the XRD pattern for BiFeO3 powders prepared at 8 M KOH. Vertical bars show the positions of the Bragg reflections for BiFeO3 . The observed-fitted difference is depicted at the bottom of figure.
3.4. Growth mechanism BiFeO3 microparticles with various shapes could be synthesized via the simple precursor hydrothermal reaction. Although the detailed mechanism for the formation of different BiFeO3 architectures is still uncertain, the following growth mechanism can be proposed based on our experimental results with the Ostwald ripening mechanism [30]. Thermodynamic speciation can be a function of KOH concentration, reaction temperature, reaction time, and even the heating and cooling rates. Some excellent results have been reported about Cu2 O regular morphologies controlled by hydrothermal conditions [30–32]. But there were only a few reports about the regular BiFeO3 polyhedron [16,18]. In this paper, we provide more BiFeO3 polyhedrons controlled by the hydrothermal method. At the reaction proceeds, the polyhedral particles grow along different directions with different rates due to their different surface energies. In this system, the reaction temperature was kept at 200 ◦ C and KOH mineralizer was used as a solvent to dissolve the precursor, and the concentration of OH− ions also affected agglomerating rate, which can influence the final shapes. In the nucleus formation stage, the concentration of solute reaches the critical supersaturation to nucleate. Nucleation and growth of small crystallites simultaneously happen in hydrothermal condition, resulting in a lot of BiFeO3 nanoparticles formed and agglomerated. At low KOH concentration, KOH cannot change the surface energy of the particles, so the BiFeO3 nanoparticles
Fig. 6. SEM images of the samples synthesized at 200 ◦ C for 6 h under KOH concentrations of (a) 6 and (b) 8 M with the slow cooling rate.
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Fig. 7. Schematic illustration of the formation mechanism and the shape-evolution process of BiFeO3 microcrystals.
cles tend to aggregate together [34]. With enhancement of reaction time, the polyhedron microparticles formed. We believe that the regular particles should be attributed to the selective aggregation of the nanoparticles as discussed above. In hydrothermal condition, the surface of BiFeO3 particle is negatively charged [33]. Different KOH concentrations can result in different distribution of surface charge of the nanoparticles, which lead to the selective aggregation occurrence to form different polyhedrons. Oriented attachment involves spontaneous self-organization of adjacent particles. With the reaction time increase, the six vertices of the octahedral particles dissolved, meaning a rapid growth of the {1 1 1} surfaces. The enhanced growth rate on the eight equivalent {1 1 1} facets continues until the {1 1 1} surfaces shrink to triangles. This process was confirmed by the samples prepared with low cooling rate, in which the {1 0 0} squares become big and the {1 0 0} triangles shrink small. At last, compact truncated cubes were formed as shown in Fig. 6. The shape evolution of BiFeO3 microcrystals is illustrated in Fig. 7. 3.5. Thermal analysis Fig. 8 shows the thermal analysis curves of the sample synthesized at 200 ◦ C for 6 h under 8 M KOH concentration with the slow cooling rate. The curves (a), (b) and (c) represent the differential scanning calorimetry (DSC), thermogravimetry (TG) and deriva-
tive differential scanning calorimetry (DDSC) of BiFeO3 powders, respectively. The BiFeO3 weight keeps almost unchangeable over the measured temperature range as shown in curve (b), agreeing with the previous report [4]. It means the cubic BiFeO3 particles are quite stable, which may attribute to the effect of the compact crystallite. The endothermic peak in curve (a) around 930 ◦ C corresponds to BiFeO3 decomposing to Bi2 Fe4 O9 , which is higher than the transform temperature of 900 ◦ C observed by Arnold et al. [35]. It may be attributed to the different heating rates and different particle sizes. Curve (c) is the differential of curve (a) to show the weak thermal changes. The endothermic peak around 730 ◦ C results from the appearance of Bi2 Fe4 O9 and Bi25 FeO39 , which can react back to the perovskite phase at higher temperatures [9]. The endothermic peak around 830 ◦ C is due to the BiFeO3 transformation from ferroelectric phase to paraelectric phase [4,7,15]. 3.6. Magnetic and electric properties The magnetic measurements of the BiFeO3 microcrystals were performed at room temperature in order to investigate the magnetic ordering. Fig. 9(a) shows the hysteresis loops for part samples with different crystal morphologies. BiFeO3 is known to show a Gtype antiferromagnetic ordering with a residual magnetic moment caused by its canted spin structure. The direction of the small moment equally superimposes a spiral spin arrangement with a
Fig. 8. The thermal analysis curves of the sample synthesized at 200 ◦ C for 6 h under KOH concentration of 8 M with the slow cooling rate. (a–c) corresponds to the DSC, TG and DDSC curves, respectively.
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wavelength of 62 nm, thereby producing a helimagnetic order and a vanishing magnetization in the bulk [36]. But when the BiFeO3 crystal size is smaller than 62 nm, BiFeO3 shows a weak ferromagnetic behavior due to the incomplete rotation of the spins along the antiferromagnetic axis [26]. In this experiment, many noticeable changes can be observed in magnetization of different shape composing of different crystal sizes. It is clear that the cubic BiFeO3 shape with high compact microcrystals shows a weak ferromagnetic order while the less compact octahedrons show a relative higher saturation magnetization, agreeing with the size-dependent magnetic behavior in nano size [26]. Under consideration of BiFeO3 multiferroic properties, the sample synthesized at 200 ◦ C for 6 h under the KOH concentrations of 8 M with the slow cooling rate was chosen to study the ferroelectric behavior. The powders were pressured to a tablet with 0.61 mm in thickness and 5.87 mm in diameter. Then it was sintered at 650 ◦ C for 1 h. Fig. 9(b) shows the ferroelectric hysteresis loops measured at 1 kHz under different electric fields, which is an unambiguous proof of the BiFeO3 ferroelectricity. Fig. 10. UV–vis spectra for part BiFeO3 samples in the diffused reflectance mode.
3.7. Optical performance of BiFeO3 microparticles The optical performance is relevant to the electronic structure feature and the band gap of semiconductor. Fig. 10 gives the UV–vis spectra for three selected BiFeO3 samples in the diffused reflectance mode. We observed absorption bands in the range of
550–600 nm, which are consistent with previous results [18]. The UV–vis reflectance spectra were converted into absorption readings according to Kubelka–Munk’s function [37] ˛ = F(R∞ ) =
S(1 − R∞ )2 2R∞
(4)
where R∞ and S are diffuse reflection and scattering factor, respectively. Additionally, the optical absorption coefficient near the band edge follows the equation [38] ˛hv = A(hv − Eg )
n/2
(5)
where h, , Eg , and A are Planck’s constant, light frequency, band gap and a constant, respectively. The value of n is equal to 1 for direct band gap materials, including BiFeO3 . The plot of transformed Kubelka–Munk’s function versus the energy of light (inset in Fig. 10) affords band gap energies of 1.94, 1.99 and 2.07 eV for spherical particles, quasi-cubic particles and truncated cubic particles, respectively. These are lower than that in BiFeO3 bulk of 2.8 eV [39], possibly attributed to the nano-crystallite of BiFeO3 in the microparticles [4,18] because the crystal sizes of these three samples are about 200 nm. Accordingly, the absorption feature suggests that the photocatalysis can possibly be activated by visible light.
4. Conclusions
Fig. 9. Magnetic hysteresis loops for BiFeO3 samples with different morphologies and (b) P–E hysteresis loop measured at 1 kHz for the sample synthesized at 200 ◦ C for 6 h with KOH concentrations of 8 M with the slow cooling rate.
In conclusion, BiFeO3 microparticles with various morphologies including plates-like, spheres, octahedrons, truncated octahedrons, cubo-octahedrons and truncated cubes were obtained by carefully controlling the hydrothermal conditions. The shape evolution of BiFeO3 microparticles was observed through changing KOH concentration, reaction time, heating and cooling rates. The formation of the large-scale BiFeO3 polyhedron is attributed to the selective aggregation and dissolution-recrystallization. The nanoparticles attach together in selective directions to micron polyhedrons. The shape evolution from octahedron to cubo-octahedron and truncated cube is attributed to the dissolution-recrystallization process. The compact truncated cubes were obtained by reducing the cooling rate. The BiFeO3 powders obviously exhibit both magnetic and ferroelectric properties. The smaller band gap of the BiFeO3 nano-crystallites indicates a possible utilization of visible light for photocatalysis.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 50972088 and 50772065), Fundamental Research Funds for the Central Universities (Nos. GK200901018 and GK201001005) and Program for New Century Excellent Talents in University (No. NCET-10-0536). References [1] H. Hur, S. Park, P.A. Sharma, J.S. Ahn, S. Guha, S.W. Cheong, Nature 429 (2004) 392. [2] K.F. Wang, J.M. Liu, Z.F. Ren, Adv. Phys. 58 (2009) 321. [3] C.-W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [4] F. Gao, Y. Yuan, K.F. Wang, X.Y. Chen, F. Chen, J.-M. Liu, Z.F. Ren, Appl. Phys. Lett. 89 (2006) 102506. [5] J.-P. Zhou, Z.-C. Qiu, P. Liu, W.-C. Sun, H.-W. Zhang, J. Appl. Phys. 103 (2008) 103522. [6] G. Catalan, J.F. Scott, Adv. Mater. 21 (2009) 2463. [7] J. Wei, D.-S. Xue, Y. Xu, Scripta Mater. 58 (2008) 45. [8] M.M. Kumar, V.R. Palkar, K. Srinivas, S.V. Suryanarayana, Appl. Phys. Lett. 76 (2000) 2764. [9] S.M. Selbach, M.A. Einarsrud, T. Grande, Chem. Mater. 21 (2009) 169. [10] S. Ghosh, S. Dasgupta, A. Sen, H.S. Maiti, Mater. Res. Bull. 40 (2005) 2073. [11] J.K. Kim, S.S. Kim, W.J. Kim, Mater. Lett. 59 (2005) 4006. [12] J.-H. Xu, H. Ke, D.-C. Jia, W. Wang, Y. Zhou, J. Alloys Compd. 472 (2009) 473. [13] T.P. Comyn, D.F. Kanguwe, J. He, A.P. Brown, J. Eur. Ceram. Soc. 28 (2008) 2233. [14] T.P. Gujar, V.R. Shinde, C.D. Lokhande, Mater. Chem. Phys. 103 (2007) 142. [15] C. Chen, J. Cheng, S. Yu, L.J. Che, Z.Y. Meng, J. Cryst. Growth 291 (2006) 135. [16] J.T. Han, Y.H. Huang, X.J. Wu, C.L. Wu, W. Wei, B. Peng, W. Huang, J.B. Goodenough, Adv. Mater. 18 (2006) 2145.
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Large-scale growth and shape evolution of bismuth ferrite particles with a hydrothermal method Xian-Zhi Chen, Zhong-Cheng Qiu, Jian-Ping Zhou ∗ , Gangqiang Zhu, Xiao-Bing Bian, Peng Liu College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 June 2010 Received in revised form 21 October 2010 Accepted 10 January 2011 PACS: 61.46.Df 89.30.Ee Keywords: Ceramics Crystal growth SEM Crystal structure
a b s t r a c t Large-scale polyhedral bismuth ferrite (BiFeO3 ) particles were synthesized with a hydrothermal method under a series of experimental conditions. X-ray diffraction revealed that the BiFeO3 powders had a perovskite structure. Scanning electron microscopy images showed different BiFeO3 particles were formed, including sphere, octahedron, truncated octahedron, cubo-octahedron and truncated cube. The experimental results showed that the concentration of KOH, reaction time, heating and cooling rates had important impacts on the size and morphology of the BiFeO3 particles. The formation mechanism and change process of the large-scale polyhedral BiFeO3 particles were discussed in detail. The obtained BiFeO3 showed ferroelectric behavior and magnetic response, which approved the multiferroic property of the BiFeO3 crystallization. The optical behaviors of BiFeO3 particles revealed the band gap energy of about 2 eV, which is smaller than the BiFeO3 bulk due to the nano-crystallites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic materials, which possess a coexistence of ferroelectric and magnetic order parameters, have been one of the hot topics intensively studied due to their fascinating fundamental physical properties and their potential applications in multifunctional devices [1–6]. Bismuth ferrite (BiFeO3 ) is one of the well-known single-phase material possessing multiferroic behaviors. The perovskite BiFeO3 has a ferroelectric Curie temperature TC = 830 ◦ C and an antiferromagnetic Neel temperature TN = 370 ◦ C [6–8]. BiFeO3 has a rhombohedrally distorted perovskite structure belonging to the space group R3c with rhombohedral lattice parameters a = 0.563 nm, ˛ = 59.35◦ , or hexagonal parameters a = 0.558 nm, c = 1.388 nm. Though many efforts have been devoted to prepare BiFeO3 , it was difficult to avoid the impurities in the products such as Bi2 Fe4 O9 and Bi25 FeO39 [9]. Nitric acid leaching was used to eliminate the impure phases in the conventional solid-state reaction. But this would result in the coarse powders and poor reproducibility [8]. BiFeO3 perovskites could only stabilize within a narrow ranged temperature. Subsequently, other methods have been developed to prepare BiFeO3 , such as ferrioxalate precursor method [10], sol–gel process [11,12], co-precipitation [13], spray pyrolysis method [14], and hydrothermal method [15–19]. The
∗ Corresponding author. Tel.: +86 29 85303823; fax: +86 29 85303823. E-mail address: [email protected] (J.-P. Zhou). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.027
properties of BiFeO3 were improved by doped with La [20,21], Y [22], Ru [23], Gd [24], Co and Ni [25], etc. But the impurity phases cannot be avoided still. Recently, just a few works preparing BiFeO3 crystallites by the hydrothermal method have been reported. Pure BiFeO3 with the irregular shape was synthesized at different KOH concentrations and reaction temperatures [15]. BiFeO3 crystallites with spindle shape were prepared at pH 14 with adding a small amount of H2 O2 [16]. The cubic BiFeO3 crystallites were obtained by the hydrothermal treatment at 200 ◦ C for 9 h with 12 M KOH [17,18]. The pure BiFeO3 could be synthesized in wider experimental conditions with the help of NH4 Cl nonaqueous solvent [19]. But these works had not shown the changing process of the morphologies and the formation mechanism of the regular shapes has not been clearly understood yet. A fundamental understanding of the formation mechanism and key factors that determine the structures of these materials are crucial for the rational design and controlled synthesis of these materials in nanosize. On the other hand, BiFeO3 magnetic performance is greatly dependent on the particle shape and grain size [26–28]. In the present work, we prepared pure BiFeO3 powders in different hydrothermal conditions. We showed the changing process of BiFeO3 particles with different morphologies, which were obtained by controlling the hydrothermal conditions carefully. Then we presented a possible formation mechanism of the regular morphologies. Finally, magnetic, ferroelectric and optical behaviors were researched. The various morphologies are helpful to investigate the BiFeO3 foundation properties and photocatalytic behaviors [18].
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Fig. 1. XRD patterns of samples synthesized at 200 ◦ C for 12 h in the KOH range of 0.7–10 M.
2. Experimental The chemical reagents used to prepare BiFeO3 powder were Fe(NO)3 ·9H2 O, Bi(NO)3 ·5H2 O and KOH mineralizer. All chemicals were used as received without further purifications. The processes to prepare the precursor included the following steps: firstly, Fe(NO)3 ·9H2 O and Bi(NO)3 ·5H2 O were dissolved in distilled water under a mechanical stirring; then, the KOH solution was slowly added to the above solution to adjust different pH values. Finally, the brown suspension solution was transferred into a 30 mL Teflon vessel filled at 4/5 of its volume. The hydrothermal treatment was performed under different KOH concentrations for the different reaction times at 200 ◦ C because the most products are pure BiFeO3 phase at this reaction temperature after a series of experiments. The temperature was increased at a rate of 1 ◦ C/min. After cooling down to room temperature naturally, the products were washed several times with distilled water and then oven dried at 70 ◦ C in air. In the last experiment, the heating and cooling rates were designed as 2 ◦ C/min and 0.2 ◦ C/min, respectively. X-ray diffraction (XRD) analysis was performed by using a Rigsku D/Max 2550 ˚ source to determine the powdiffractometer system with a Cu K␣ ( = 1.5418 A) der phases. The particle size and morphology of the products were obtained with a scanning electron microscopy (SEM FEI-Quanta 200). The ferroelectric Curie temperature was determined by the thermal analysis systems (Q1000DSC+LNCS+FACS Q600SDT) at a heating rate of 10 ◦ C/min. Magnetic properties were characterized by an LDJ9600 type of Vibrating Sample Magnetometer (VSM). A Radiant Technologies’ Precision Premier II tester was used to measure the ferroelectric hysteresis loops. The UV–vis spectra of part BiFeO3 microparticles were performed in the diffused reflectance mode using a Perkin-Elmer UV–vis spectrometer (Lambd 950) of the USA.
3. Result and discussion 3.1. Effects of KOH concentration Fig. 1 shows the XRD patterns of samples synthesized at 200 ◦ C for 12 h with different KOH concentrations of 1, 2, 4, 6, 8, 9 and 10 M. The XRD patterns can be indexed as a pure BiFeO3 phase when
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the KOH concentrations are in the range of 1–9 M according to the JCPDS card nos. 71-2494 and 86-1518, which is consistent with the earlier report [17]. It is found that Bi2 Fe4 O9 and Fe2 O3 appeared at 10 M and Bi25 FeO39 at the KOH concentration lower than 1 M. Fig. 2 (a)–(f) shows SEM images corresponding to the samples synthesized with the KOH concentrations of 1, 2, 4, 6, 8, and 9 M, respectively. It is evident from the SEM images that the particle size increases from 10 m to 20 m approximately and the morphology changes from irregular agglomeration to regular polyhedron with increasing the KOH concentration. Various KOH concentrations can result in a series of BiFeO3 morphologies, such as agglomeration, octahedron and {1 0 0} truncated octahedron. As shown in Fig. 2(a), irregular plate-like BiFeO3 agglomerations are formed by small BiFeO3 crystallites at 1 M. In Fig. 2(b), small BiFeO3 crystallites gathered into large particles with irregular shape at 2 M. The sample synthesized at 4 M is agglomerated to spherical particles as shown in Fig. 2(c). The formation mechanism of these three kinds of particles belongs to random aggregation [29]. When the KOH concentration increases to 6 M, the BiFeO3 particles become homogenous octahedron of about 8 m in size as shown in Fig. 2(d). The inset in Fig. 2(d) shows an enlarged image of the clear octahedron. When the KOH concentration is 8 M, the typical shape of the particles transforms into {1 0 0} truncated octahedron about 18 m in size as shown in Fig. 2(e). The six vertices on the octahedral particles are not stable and dissolve into coarse square {1 0 0} surfaces. Meanwhile, eight trigonal {1 1 1} surfaces grow to smooth hexagons. With the further enhancement of the KOH concentration to 9 M, the coarse square {1 0 0} surfaces continually dissolve to big facets and the {1 1 1} surfaces grow to small trigonal facets as shown in Fig. 2(f). Thus, we believed that the BiFeO3 microparticles grew with a faster growth rate along 1 1 1 than along 1 0 0 in high KOH concentration. The insets of Fig. 2(e) and (f) present the magnified images of rough square facets of the truncated octahedrons. The slices in the reticulate {1 0 0} surfaces at 9 M are thinner than that at 8 M because the small particles are easily formed at high KOH concentration [15]. Meanwhile, the smooth surfaces grew more easily at high KOH concentration.
3.2. Effects of reaction time Based on the experimental results presented above, it is worth to change the reaction time to reveal the formation mechanism of the BiFeO3 polyhedrons. Fig. 3 shows the XRD patterns of samples synthesized at 200 ◦ C with 8 M KOH concentration for different reaction times of 30 min, 3, 6, 12 and 24 h, respectively. The foreign phases Bi25 FeO39 and Fe2 O3 appeared at reaction times of 30 min, and the pure BiFeO3 phase is formed with increasing the reaction time. The XRD patterns obtained at different times provide important information to probe the reaction mechanism although it may be complicated in the hydrothermal process. We propose that the BiFeO3 formation in the hydrothermal treatment involves three stages: (1) the nitrates transform into hydroxide precipitations after adding KOH; (2) Bi(OH)3 and Fe(OH)3 react under hydrothermal environment to form BiFeO3 as well as unstable Fe2 O3 and Bi25 FeO39 under the appropriate hydrothermal conditions. Fe2 O3 and Bi25 FeO39 crystallites are completed in cooling process and inspected by XRD; (3) but in the hydrothermal environment with high temperature and pressure, the mixture reacts to form the final BiFeO3 products under limited experimental conditions. The formation formula of BiFeO3 can be written as follows: Bi(NO3 )3 + Fe(NO3 )3 + 6KOH → Bi(OH)3 + Fe(OH)3 + 6NaNO3 (1) 25Bi(OH)3 + 3Fe(OH)3 → Bi25 FeO39 + Fe2 O3 + 42H2 O
(2)
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Fig. 2. SEM images of the samples synthesized at 200 ◦ C for 12 h with different KOH concentrations of (a) 1, (b) 2, (c) 4, (d) 6, (e) 8 and (f) 9 M, respectively.
Bi(OH)3 + Fe(OH)3 → BiFeO3 + 3H2 O Bi25 FeO39 + 12Fe2 O3 → 25BiFeO3
(3)
SEM images in Fig. 4(a)–(d) correspond to the samples synthesized for the reaction times of 3, 6, 12 and 24 h, respectively. The insets in Figs. 4(c) and (d) are the enlarged images of {1 0 0} facets. It is evident that the particle morphologies change from spherical agglomeration to polyhedron with the increase in the reaction time. The uniform spherical agglomerations synthesized at short reaction time of 3 h are composed of many small particles as shown in Fig. 4(a). Increasing the reaction time to 6 h, some polyhedrons appear as shown in Fig. 4(b). After reaction for 12 h, the aggrega-
tions become truncated octahedrons with 6 little rough squares and 8 large smooth hexagons. When the reaction time increases to 24 h, the rough squares dissolve to big squares in 1 0 0 directions and the hexagons grow to smooth triangles in 1 1 1 directions to form the final cubo-octahedron as shown in Fig. 4(d). In other words, the {1 0 0} surfaces dissolves due to their coarse appearance and the solute recrystallizes on the smooth {1 1 1} surfaces. 3.3. Effects of heating and cooling rates Then we chose the samples synthesized at 6 M and 8 M KOH for more careful study. The reagents were heated to 200 ◦ C at a rate of 2 ◦ C/min, reacted for 6 h and then cooled down to room temperature at a rate of −0.2 ◦ C/min. Fig. 5(a) shows the XRD patterns of
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Fig. 3. XRD patterns of samples synthesized at 200 ◦ C with 8 M KOH for different reaction times.
the two samples, which are indexed as pure BiFeO3 phase. Typical result of Rietveld’s refinement of the XRD pattern for BiFeO3 prepared at 8 M KOH is shown as an example in Fig. 5(b), where the observed and calculated XRD patterns are indicated by blank circle and line, respectively. XRD data were collected with Cu K␣ radiation by scanning step measurement (scanning step 0.01◦ , and counting time 2 s/step). Rietveld’s refinement was performed by
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using the program MAUD (Materials Analysis Using Diffraction), which combines the Rietveld method and a Fourier transform analysis. The perovskite structure with space group R3c (161) (JCPDS card no. 71-2494 and ICSD No. 15299) is used as a basic model. The background was described by a 5-order polynomial function. Anisotropic peak-broadening model is selected as Popa LB and sizestrain model is Popa rules. A 10-order harmonic correction is used for the preferred orientation in order to obtain smaller value of Rw. It can be seen that the fitted XRD profile agrees well with the experimental data. A typical result of Rietveld’s analysis gave the cell parameters a = 5.575(0) A˚ and c = 13.847(9) A˚ [reliability factors: Rw (%) = 11.2, sig = 1.13, Rwnb (%. nobkg) = 9.88, Rb (%) = 8.59, Rexp (%) = 9.94]. It revealed these BiFeO3 crystallites had a rhombohedrally distorted perovskite structure belonging to R3c (161) space group. The average crystallite size is about 195 nm. Fig. 6 shows the SEM images of the samples synthesized at 6 M and 8 M KOH concentration. Truncated cubic particles of BiFeO3 are obtained and the particles prepared at 8 M (about 30 m in size) are larger and more compact than that at 6 M (about 10 m in size). These particles are composed of many nano-crystallites for the average crystallite size is only about 195 nm from the XRD results. In comparison with the above experimental conditions, the BiFeO3 nano-crystallites have more chances to locate in suitable sites with low energy in a slow cooling rate. As a result, BiFeO3 crystallites grow more densely and regularly. On the other hand, ions slowly precipitate from the supersaturated solution in a slow cooling rate. The ions have less chance to nucleate and they precipitate on the crystallites. Therefore, uniform and large cubic BiFeO3 particles were successfully obtained through controlling the heating and cooling rates.
Fig. 4. SEM images of the samples synthesized at 200 ◦ C with 8 M KOH for reaction time of 3, 6, 12 and 24 h.
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agglomerate to isotropic spheres. At high KOH concentration, KOH could change the surface energy of the particle and an orientedattachment process dominates, which results in the octahedron formation as shown in Fig. 2(d). Penn and Banfield suggested that the dislocation formation operate during early growth [34]. So the two or more nano-crystallites gathered by the oriented attachment until it grows a small-scale particle. The surface of small-scale particle can catch the nano-crystallites by selective aggregation, which is dependent on the KOH concentration in the solution, until the nutrition was used up. With the further enhancement of the KOH concentration, dissolution-recrystallization predominates. The six vertices of the octahedron dissolve into the solution and the nanoparticles deposit on the {1 1 1} surfaces to form smooth facets. The higher KOH concentration results in the more dissolution of {1 0 0} surfaces to big squares and more growth of {1 1 1} surfaces to small facets, and finally to triangles as shown in Fig. 2(f). This supposition can be confirmed by changing the reaction time. At the initial stage, the concentration of solute reaches supersaturation, resulting in a lot of nucleations formed. To minimize the overall energy of the system, a great number of small seed parti-
Fig. 5. (a) XRD patterns of samples synthesized at 200 ◦ C for 6 h under KOH concentrations of 6 and 8 M with the slow cooling rate. (b) Typical Rietveld’s refinement of the XRD pattern for BiFeO3 powders prepared at 8 M KOH. Vertical bars show the positions of the Bragg reflections for BiFeO3 . The observed-fitted difference is depicted at the bottom of figure.
3.4. Growth mechanism BiFeO3 microparticles with various shapes could be synthesized via the simple precursor hydrothermal reaction. Although the detailed mechanism for the formation of different BiFeO3 architectures is still uncertain, the following growth mechanism can be proposed based on our experimental results with the Ostwald ripening mechanism [30]. Thermodynamic speciation can be a function of KOH concentration, reaction temperature, reaction time, and even the heating and cooling rates. Some excellent results have been reported about Cu2 O regular morphologies controlled by hydrothermal conditions [30–32]. But there were only a few reports about the regular BiFeO3 polyhedron [16,18]. In this paper, we provide more BiFeO3 polyhedrons controlled by the hydrothermal method. At the reaction proceeds, the polyhedral particles grow along different directions with different rates due to their different surface energies. In this system, the reaction temperature was kept at 200 ◦ C and KOH mineralizer was used as a solvent to dissolve the precursor, and the concentration of OH− ions also affected agglomerating rate, which can influence the final shapes. In the nucleus formation stage, the concentration of solute reaches the critical supersaturation to nucleate. Nucleation and growth of small crystallites simultaneously happen in hydrothermal condition, resulting in a lot of BiFeO3 nanoparticles formed and agglomerated. At low KOH concentration, KOH cannot change the surface energy of the particles, so the BiFeO3 nanoparticles
Fig. 6. SEM images of the samples synthesized at 200 ◦ C for 6 h under KOH concentrations of (a) 6 and (b) 8 M with the slow cooling rate.
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Fig. 7. Schematic illustration of the formation mechanism and the shape-evolution process of BiFeO3 microcrystals.
cles tend to aggregate together [34]. With enhancement of reaction time, the polyhedron microparticles formed. We believe that the regular particles should be attributed to the selective aggregation of the nanoparticles as discussed above. In hydrothermal condition, the surface of BiFeO3 particle is negatively charged [33]. Different KOH concentrations can result in different distribution of surface charge of the nanoparticles, which lead to the selective aggregation occurrence to form different polyhedrons. Oriented attachment involves spontaneous self-organization of adjacent particles. With the reaction time increase, the six vertices of the octahedral particles dissolved, meaning a rapid growth of the {1 1 1} surfaces. The enhanced growth rate on the eight equivalent {1 1 1} facets continues until the {1 1 1} surfaces shrink to triangles. This process was confirmed by the samples prepared with low cooling rate, in which the {1 0 0} squares become big and the {1 0 0} triangles shrink small. At last, compact truncated cubes were formed as shown in Fig. 6. The shape evolution of BiFeO3 microcrystals is illustrated in Fig. 7. 3.5. Thermal analysis Fig. 8 shows the thermal analysis curves of the sample synthesized at 200 ◦ C for 6 h under 8 M KOH concentration with the slow cooling rate. The curves (a), (b) and (c) represent the differential scanning calorimetry (DSC), thermogravimetry (TG) and deriva-
tive differential scanning calorimetry (DDSC) of BiFeO3 powders, respectively. The BiFeO3 weight keeps almost unchangeable over the measured temperature range as shown in curve (b), agreeing with the previous report [4]. It means the cubic BiFeO3 particles are quite stable, which may attribute to the effect of the compact crystallite. The endothermic peak in curve (a) around 930 ◦ C corresponds to BiFeO3 decomposing to Bi2 Fe4 O9 , which is higher than the transform temperature of 900 ◦ C observed by Arnold et al. [35]. It may be attributed to the different heating rates and different particle sizes. Curve (c) is the differential of curve (a) to show the weak thermal changes. The endothermic peak around 730 ◦ C results from the appearance of Bi2 Fe4 O9 and Bi25 FeO39 , which can react back to the perovskite phase at higher temperatures [9]. The endothermic peak around 830 ◦ C is due to the BiFeO3 transformation from ferroelectric phase to paraelectric phase [4,7,15]. 3.6. Magnetic and electric properties The magnetic measurements of the BiFeO3 microcrystals were performed at room temperature in order to investigate the magnetic ordering. Fig. 9(a) shows the hysteresis loops for part samples with different crystal morphologies. BiFeO3 is known to show a Gtype antiferromagnetic ordering with a residual magnetic moment caused by its canted spin structure. The direction of the small moment equally superimposes a spiral spin arrangement with a
Fig. 8. The thermal analysis curves of the sample synthesized at 200 ◦ C for 6 h under KOH concentration of 8 M with the slow cooling rate. (a–c) corresponds to the DSC, TG and DDSC curves, respectively.
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wavelength of 62 nm, thereby producing a helimagnetic order and a vanishing magnetization in the bulk [36]. But when the BiFeO3 crystal size is smaller than 62 nm, BiFeO3 shows a weak ferromagnetic behavior due to the incomplete rotation of the spins along the antiferromagnetic axis [26]. In this experiment, many noticeable changes can be observed in magnetization of different shape composing of different crystal sizes. It is clear that the cubic BiFeO3 shape with high compact microcrystals shows a weak ferromagnetic order while the less compact octahedrons show a relative higher saturation magnetization, agreeing with the size-dependent magnetic behavior in nano size [26]. Under consideration of BiFeO3 multiferroic properties, the sample synthesized at 200 ◦ C for 6 h under the KOH concentrations of 8 M with the slow cooling rate was chosen to study the ferroelectric behavior. The powders were pressured to a tablet with 0.61 mm in thickness and 5.87 mm in diameter. Then it was sintered at 650 ◦ C for 1 h. Fig. 9(b) shows the ferroelectric hysteresis loops measured at 1 kHz under different electric fields, which is an unambiguous proof of the BiFeO3 ferroelectricity. Fig. 10. UV–vis spectra for part BiFeO3 samples in the diffused reflectance mode.
3.7. Optical performance of BiFeO3 microparticles The optical performance is relevant to the electronic structure feature and the band gap of semiconductor. Fig. 10 gives the UV–vis spectra for three selected BiFeO3 samples in the diffused reflectance mode. We observed absorption bands in the range of
550–600 nm, which are consistent with previous results [18]. The UV–vis reflectance spectra were converted into absorption readings according to Kubelka–Munk’s function [37] ˛ = F(R∞ ) =
S(1 − R∞ )2 2R∞
(4)
where R∞ and S are diffuse reflection and scattering factor, respectively. Additionally, the optical absorption coefficient near the band edge follows the equation [38] ˛hv = A(hv − Eg )
n/2
(5)
where h, , Eg , and A are Planck’s constant, light frequency, band gap and a constant, respectively. The value of n is equal to 1 for direct band gap materials, including BiFeO3 . The plot of transformed Kubelka–Munk’s function versus the energy of light (inset in Fig. 10) affords band gap energies of 1.94, 1.99 and 2.07 eV for spherical particles, quasi-cubic particles and truncated cubic particles, respectively. These are lower than that in BiFeO3 bulk of 2.8 eV [39], possibly attributed to the nano-crystallite of BiFeO3 in the microparticles [4,18] because the crystal sizes of these three samples are about 200 nm. Accordingly, the absorption feature suggests that the photocatalysis can possibly be activated by visible light.
4. Conclusions
Fig. 9. Magnetic hysteresis loops for BiFeO3 samples with different morphologies and (b) P–E hysteresis loop measured at 1 kHz for the sample synthesized at 200 ◦ C for 6 h with KOH concentrations of 8 M with the slow cooling rate.
In conclusion, BiFeO3 microparticles with various morphologies including plates-like, spheres, octahedrons, truncated octahedrons, cubo-octahedrons and truncated cubes were obtained by carefully controlling the hydrothermal conditions. The shape evolution of BiFeO3 microparticles was observed through changing KOH concentration, reaction time, heating and cooling rates. The formation of the large-scale BiFeO3 polyhedron is attributed to the selective aggregation and dissolution-recrystallization. The nanoparticles attach together in selective directions to micron polyhedrons. The shape evolution from octahedron to cubo-octahedron and truncated cube is attributed to the dissolution-recrystallization process. The compact truncated cubes were obtained by reducing the cooling rate. The BiFeO3 powders obviously exhibit both magnetic and ferroelectric properties. The smaller band gap of the BiFeO3 nano-crystallites indicates a possible utilization of visible light for photocatalysis.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 50972088 and 50772065), Fundamental Research Funds for the Central Universities (Nos. GK200901018 and GK201001005) and Program for New Century Excellent Talents in University (No. NCET-10-0536). References [1] H. Hur, S. Park, P.A. Sharma, J.S. Ahn, S. Guha, S.W. Cheong, Nature 429 (2004) 392. [2] K.F. Wang, J.M. Liu, Z.F. Ren, Adv. Phys. 58 (2009) 321. [3] C.-W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [4] F. Gao, Y. Yuan, K.F. Wang, X.Y. Chen, F. Chen, J.-M. Liu, Z.F. Ren, Appl. Phys. Lett. 89 (2006) 102506. [5] J.-P. Zhou, Z.-C. Qiu, P. Liu, W.-C. Sun, H.-W. Zhang, J. Appl. Phys. 103 (2008) 103522. [6] G. Catalan, J.F. Scott, Adv. Mater. 21 (2009) 2463. [7] J. Wei, D.-S. Xue, Y. Xu, Scripta Mater. 58 (2008) 45. [8] M.M. Kumar, V.R. Palkar, K. Srinivas, S.V. Suryanarayana, Appl. Phys. Lett. 76 (2000) 2764. [9] S.M. Selbach, M.A. Einarsrud, T. Grande, Chem. Mater. 21 (2009) 169. [10] S. Ghosh, S. Dasgupta, A. Sen, H.S. Maiti, Mater. Res. Bull. 40 (2005) 2073. [11] J.K. Kim, S.S. Kim, W.J. Kim, Mater. Lett. 59 (2005) 4006. [12] J.-H. Xu, H. Ke, D.-C. Jia, W. Wang, Y. Zhou, J. Alloys Compd. 472 (2009) 473. [13] T.P. Comyn, D.F. Kanguwe, J. He, A.P. Brown, J. Eur. Ceram. Soc. 28 (2008) 2233. [14] T.P. Gujar, V.R. Shinde, C.D. Lokhande, Mater. Chem. Phys. 103 (2007) 142. [15] C. Chen, J. Cheng, S. Yu, L.J. Che, Z.Y. Meng, J. Cryst. Growth 291 (2006) 135. [16] J.T. Han, Y.H. Huang, X.J. Wu, C.L. Wu, W. Wei, B. Peng, W. Huang, J.B. Goodenough, Adv. Mater. 18 (2006) 2145.
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