Synthesis, structures and properties of single phase BiFeO₃ and Bi₂Fe₄O₉ powders by hydrothermal method

Abstract

Single phase BiFeO₃ and Bi₂Fe₄O₉ powders have been synthesized via hydrothermal method by carefully controlling the reaction conditions. It is found that the BiFeO₃ shows dense microstructure with irregular, larger grains, the average grain size is ~5.0 μm, whereas the Bi₂Fe₄O₉ displays small, porous microstructure with the sheet shaped grains, the average thickness is ~55 nm and length is ~500 nm. X-ray photoemission spectra confirm that in both compositions, the Bi and Fe cations have the charge valence of +3. The magnetization–magnetic field (M–H) and magnetization–temperature (M–T) measurements reveal that the BiFeO₃ has weak ferromagnetism even at room temperature due to its canted antiferromagnetic interaction, whereas the Bi₂Fe₄O₉ shows antiferromagnetic nature in the temperature range of 10–300 K. Our results provide interesting supplements for controlling the morphology and understanding the structure–property relationship of BiFeO₃ and Bi₂Fe₄O₉.

1 Introduction

Multiferroics materials with at least two coexisted ferroic orders have attracted much attention due to their possible applications in novel devices like multiple-state memories and data-storage media as well as their fascinating fundamental physics like unique and strong coupling between magnetic, electric, and structural order parameters. BiFeO₃ is one of the most outstanding multiferroic materials with coexisted ferroelectric (Curie temperature T_c ~ 1103 K) and antiferromagnetic (Néel temperature T_N ~ 643 K) orders. BiFeO₃ has been investigated in many forms [1–3]. On the other hand, Bi₂Fe₄O₉ is generally easy to be formed as the second phase during the process of preparing BiFeO₃ powder or ceramics, making the phase purity of BiFeO₃ to be very processing-sensitive [4]. Actually, Bi₂Fe₄O₉ is also an excellent multiferroic material with ferroelectric (T_c ~ 250 K) and antiferromagnetic (T_N ~ 260 K) [5], but there are relatively less reports focusing on this material [5–7]. Though single-phase BiFeO₃ and Bi₂Fe₄O₉ are obtainable independently, their condition windows for fabricating are still critical and narrow, so it still remains a challenge to prepare single phase BiFeO₃ and Bi₂Fe₄O₉ easily. On the other hand, it is well known that BiFeO₃ has special magnetic structure, which makes the magnetic property is size-sensitive [1–3]. Based on these descriptions, further works are still necessary to study the relationship between preparing conditions and phase purity, morphologies, properties, etc. For this aim, it is necessary to prepare both materials using the same method so as to comparatively investigate their preparation condition, structures, and properties, etc.

Up to now, solid state reaction and many wet chemical methods, such as molten salt method, sol–gel, and co-precipitation, were used to prepare BiFeO₃ and Bi₂Fe₄O₉ powders. As well known, compared with other methods, the hydrothermal reaction route is a very important synthetic method to prepare metal-oxide materials with its advantages of low-cost, simple process and well-controllable processing [8–10].

Accordingly, in this paper we report a facial hydrothermal process for synthesizing single phase BiFeO₃ and Bi₂Fe₄O₉ powders. Their phase purity, morphology, cation’s charge valence, and magnetic property were comparatively investigated.

2 Experimental procedure

The BiFeO₃ and Bi₂Fe₄O₉ samples were synthesized by hydrothermal method, detailed preparation flowchart is presented in Fig. 1. All the chemicals used are analytic grade reagents without further purification. Iron chloride hexahydrate (FeCl₃·6H₂O) and bismuth nitrate [Bi(NO₃)₃·5H₂O] were used as the source materials, sodium hydroxide (NaOH) were added as a mineralizer, while polyethylene glycol (PEG) as additives. Deionized water was used to make all aqueous solutions. In the typical procedure of the BiFeO₃ powder, 5 mmol Bi(NO₃)₃·5H₂O and 5 mmol FeCl₃·6H₂O (1:1 in molar ratios) were dissolved in 50 ml acetone with stirring and ultrasound until completely dissolved. Then, 200 ml deionized water were added and the pH value of the above solution was controlled to be 10–11 by adding concentrated ammonia. The sediment was centrifuged out and washed with deionized water several times till the pH value was 7. Subsequently, 8 g NaOH and 40 ml deionized water were added with stirring for 30 min. The solution was transferred to a Teflon-lined stainless-steel autoclave and heated at 200 °C for 72 h, and then cooled to room temperature (RT) in air naturally. The products were filtered and washed several times with deionized water and absolute ethanol, and finally dried in air at 70 °C for 24 h. As for the preparation of Bi₂Fe₄O₉, 5 mmol Bi(NO₃)₃·5H₂O and 5 mmol FeCl₃·6H₂O in a stoichiometric ratio, were dissolved in 50 ml acetone with stirring and ultrasound until completely dissolved. Then 3.2 g NaOH and 0.8 g PEG (Mw = 4 × 10⁴ g/mol) were added with stirring for 30 min. The solution was transferred to a 50 ml Teflon-lined stainless-steel autoclave and heated at 180 °C for 72 h, and then cooled to RT in air naturally. The products were filtered and washed several times with deionized water and absolute ethanol, and finally dried in air at 70 °C for 24 h.

Fig. 1

The detailed preparation flowchart for the BiFeO₃ and Bi₂Fe₄O₉ powders

The crystal structures of the powder were characterized by powder XRD (Rigaku Ultima III) with Cu-Kα radiation operated at 40 kV and 40 mA with a step size of 0.03°. The microstructures were analyzed by scanning electron microscopy (SEM, ULTRA55, Zeiss). The chemical states of the Bi and Fe cations in both BiFeO₃ and Bi₂Fe₄O₉ samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha), which was performed at a base pressure of 5 × 10⁻⁷ mbar. Photoelectric peak of C1s located at 284.6 eV was used as the criterion to rectify binding energies of XPS spectra. The magnetization–magnetic field (M–H) curves and the zero-field-cooled (ZFC) and field-cooled (FC) magnetization–temperature (M–T) curves were measured by using a physical property measurement system (PPMS, Quantum Design).

3 Results and discussion

Figure 2a, b shows the XRD patterns of the as-prepared BiFeO₃ and Bi₂Fe₄O₉ samples, respectively. In generally, the XRD patterns reveal that the as-prepared BiFeO₃ and Bi₂Fe₄O₉ samples are single-phase without detectable second phases [2, 5, 7–9]. The diffraction peaks in Fig. 2a can be assigned to the BiFeO₃ (JCPDS No. 20-0169), indicating the BiFeO₃ powder has rhombohedral perovskite structure (space group: R3c), as reviewed in Ref. [2]. On the other hand, the XRD patterns of Bi₂Fe₄O₉ samples can be indexed to the orthorhombic structure, which are consistent with the standard data (JCPDS No. 25-0090) and other reports [5, 7].

Fig. 2

XRD patterns of the a BiFeO₃ and b Bi₂Fe₄O₉ powders

The micrographs of BiFeO₃ and Bi₂Fe₄O₉ are displayed in Fig. 3a, b, respectively. For the BiFeO₃ samples, dense microstructures with large, irregular grains are obtained, the average grain size is ~5.0 μm. However, the Bi₂Fe₄O₉ has relatively smaller, porous microstructure with sheet shaped grains with the average size of ~500 nm in length and ~55 nm in thickness. Actually, BiFeO₃ is easy to from relatively uniform particles [7], whereas Bi₂Fe₄O₉ has the tendency to form sheet shaped microstructure [11]. As well known, BiFeO₃ has rhombohedral structure with lattice of a = b = c ~5.6 Å and α ~ 60°, while Bi₂Fe₄O₉ has orthorhombic structure with a ~ 7.9 Å, b ~ 8.4 Å, c ~ 5.9 Å [7, 12]. Clearly Bi₂Fe₄O₉ has higher degree structural anisotropy than BiFeO₃, especially along c-axis direction, therefore, the growing rates may be different in different direction, leading to sheet-shaped morphology. However, this is not the case for BiFeO₃, thus relatively uniform grains are obtained.

Fig. 3

SEM micrograph of the a BiFeO₃ and b Bi₂Fe₄O₉ powders

The XPS was used to investigate the chemical states of Bi and Fe in both samples, typical results are shown in Fig. 4a, b, respectively. The XPS spectra show that the binding energies of Bi 4f_5/2, Bi 4f_7/2, Fe 2p _1/2 and Fe 2p _3/2 peaks in both samples are located at 158.5, 163.9, 724.3 and 711.5 eV, respectively, suggesting that Bi³⁺, Fe³⁺ existed in the pure BiFeO₃ and Bi₂Fe₄O₉ samples [13]. No peak corresponding to Fe²⁺ can be detected. Furthermore, the XPS spectra also indicate that the BiFeO₃ and Bi₂Fe₄O₉ powders are a single phase, which is in good agreement with the XRD measurements shown in Fig. 2.

Fig. 4

XPS spectra of the Bi and Fe cations in (a, b) BiFeO₃ and c, d Bi₂Fe₄O₉

The M–H curves of the BiFeO₃ and Bi₂Fe₄O₉ have been measured at 10, 50, 100 and 300 K under a maximum field of 20 kOe, as presented in Fig. 5a, b, respectively. BiFeO₃ is known to be antiferromagnetic with a G-type magnetic structure but has a residual magnetic moment due to a canted spin structure, consistent with the weak hysteresis (the inset of Fig. 5a), therefore, it is reasonable that BiFeO₃ shows weak ferromagnetism in the whole temperature range (10–300 K). At 10 K and 20 kOe external field, the maximum magnetization and remnant magnetization are about 0.13 and 0.004 emu/g, respectively, consistent with other report [13, 14]. It is noted the magnetization is relatively lower than nano-sized BiFeO₃ [15], which is due to the size effect: small BiFeO₃ can suppress the spiral spin structure with period of ~62 nm. Bi₂Fe₄O₉ has antiferromagnetic order, so it is responsible it shows almost linear M–H curves (Fig. 5b), the magnetization value at maximum field is consistent with other report [5]. For both samples, the magnetization decreases with increasing temperature due to their antiferromagnetic order.

Fig. 5

M–H curves of a BiFeO₃ and b Bi₂Fe₄O₉ measured at 10, 50, 100, and 300 K. The inset of a shows the partly enlarged curves

In order to further investigate the magnetic properties of the BiFeO₃ and Bi₂Fe₄O₉, FC and ZFC magnetization–temperature (M–T) curves are measured, as shown in Fig. 6a, b. In Fig. 6a, a remarkable separation between the FC and ZFC is observed, one can see the ZFC curve, a cusp which corresponds to the freezing temperature (T_f ~ 125 K) is observed. Within a temperature range of 300 K to T_f, the ZFC and FC magnetization values nearly superpose and increase with the decreasing temperature, indicating that the BiFeO₃ is antimagnetic. The ZFC and FC curves of BiFeO₃ start to diverge when the temperature is close to T_f = 125 K. The divagation of ZFC and FC magnetizations at low temperatures and cusps (T_f) are typical characteristics of spin-glass behavior. These results are consistent with the phenomenon observed in Fig. 6a. For the sheet shaped Bi₂Fe₄O₉ sample, the magnetization in both ZFC and FC M–T curves show a rapid increase at low temperatures (Fig. 6b). This may be due to the spin-glass-like behavior caused by the nanoscaled particles, and is consistent with the antiferroelectric nature observed in Fig. 5b. The FC and ZFC curves tend to diverge around 250 K, in good agreement with the reported Neel temperature T_N.

Fig. 6

M–T curves of a BiFeO₃ and b Bi₂Fe₄O₉

4 Conclusion

In summary, single phase BiFeO₃ and Bi₂Fe₄O₉ are synthesized by hydrothermal method. With the same bismuth and iron sources, two kinds of bismuth ferrite compounds with different morphologies can be synthesized by carefully controlling the hydrothermal reaction conditions. Different morphologies are observed and attributed to the different crystal structures. In addition, BiFeO₃ shows weak ferromagnetic behavior at low temperature range whereas Bi₂Fe₄O₉ is antiferromagnetic in the whole measuring temperature range. Our results may be helpful for optimizing the preparation conditions and furthering understanding the structure–property relationship of BiFeO₃ and Bi₂Fe₄O₉.