Annealing temperature mediated physical properties of bismuth ferrite (BiFeO₃) nanostructures synthesized by a novel wet chemical method

Highlights

• This study introduces a novel wet chemical method to synthesis bismuth ferrite nanostructures.

• Investigates the effect of annealing temperature on phase, morphology and physical properties.

• Bismuth ferrite flowers obtained for the first time in such our facile method.

• Reveals that the secondary phase and morphology of bismuth ferrite influence its optical property.

• Weak ferromagnetism obtained for the bismuth ferrite annealed at 550 °C which is attributed to oxygen vacancy defects.

Abstract

This study reports the synthesis of bismuth ferrite (BFO) by a facile wet chemical hydro-evaporation (HE) method and the effect of different annealing temperatures (450 °C, 550 °C and 650 °C) on the phase evolution, morphology, physical properties of the synthesized BFO. It is observed that the BFO phase is formed at all the three annealing temperature and the morphology is found to be flowers and spherical. Magnetization studies revealed that the BFO formed at 450 °C and 650 °C exhibits its typical anti-ferromagnetic nature while the BFO formed at 550 °C exhibits weak ferromagnetic nature. The observed weak ferromagnetism in BFO owes to the point defects associated with oxygen vacancy defects as well as the smaller size effect. The possible origin of the enhancement in magnetic property has been explained with a suitable model in terms of particle size and defects in the material.

Introduction

Multiferroics are a class of multifunctional materials which exhibit simultaneous effects of ferroelectricity, ferromagnetism, ferroelasticity, etc., as a result of the coupled electric, magnetic, mechanical and structural properties [1], [2]. In recent years, lot of efforts has been put in to design and synthesis of such multifunctional materials as they exhibit exotic properties at the nanoscale [3], [4], [5]. Perovskite bismuth ferrite (BiFeO₃ – BFO) belongs to the class of single phase multiferroic material which has a rhombohedrally distorted cell with polar R3c space group [6]. This perovskite BFO simultaneously exhibits spontaneous ferroelectric (T_C ~ 1103 K) and G type anti-ferromagnetic (T_N ~ 643 K) phenomena at room temperature [7]. Nevertheless, it also exhibits weak ferromagnetism at room temperature due to a magnetic cycloidal spin structure with a long periodicity of 62 nm [8]. BFO has become one of the appropriate materials to understand the physics of multiferroic materials from the view point that the stereo chemical activity of the Bi lone pair electrons give rise to ferroelectricity and the partially filled 3d orbital of the Fe³⁺ ions cause G-type antiferromagnetism [9].

It is known that synthesizing single phase BFO without any secondary or ternary phases such as Bi₂O₃, Fe₂O₃, Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ is still a difficult task due to challenges involved in the existing methods [10], [11], [13]. Also, BFO faces several issues in the primary ferroic properties such as small remenant polarization, large leakage current, high coercive field and weak magnetoelectric coupling effect [12]. Consequently more efforts have been put in to conquer these challenges and issues by synthesizing impurity free BFO, by controlling particle size, dimensions and modifying BFO with suitable dopants, etc. [13], [14], [15], [16]. Recently Silva et al. [13] reported various synthesis methods to prepare BiFeO₃ ceramics and nanomaterials. This includes the conventional solid state reaction, rapid liquid sintering and mechanical activation methods and the wet chemical methods include co-precipitation, conventional and microwave assisted hydro thermal methods, sonochemical, auto combustion and sol–gel methods such as the metal complex, modified Pechini, polymer complex solution, and glycol gel reaction methods. All these existing methods require subtle parameters such as processing temperature, pressure, time and various temperature treatment steps such as drying, calcinations and sintering. Similarly, these methods require different solvents and chelating agents, polymerizing agents, etc. which are to be effectively optimized to synthesis the single phase, impurity free BFO ceramics and nanopowders. Table 1 gives an overview of these existing methods for the synthesis of BFO materials [40], [8], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54].

Hence, it is essential to develop effective synthesis strategies to prepare BFO without any secondary or impurity phases as these unwanted phases screen the potential properties of BFO. In particular, the synthesis method should also be able to control the particle size and morphology as these two parameters play significant role in tuning the physical properties of BFO [17], [18]. Furthermore, the developed new synthesis strategies should also have the capability of reproducing BFO in large scale with enhanced physical properties. Thus we report here a simplistic process of synthesizing nanostructured BFO in large scale with controlled size and morphology that exhibits significant physical properties. This particular synthesis process generates phase pure nanostructured BFO, where the size and morphology can be effectively controlled by only varying the annealing temperatures. Authors would like to introduce this method exclusively as hydro-evaporation (HE) method, as the materials can be produced by evaporating the basic precursors which are dissolved in de-ionized water.

In short, this study introduces our facile hydro-evaporation (HE) method to synthesize the nanostructured BFO and investigates the phase and morphology evolution of BFO with respect to the different annealing temperature and collectively discusses their influence in the physical properties such as optical and magnetic properties of BFO.