Ferroelectric BaTiO3 nanoparticles: Biosynthesis and characterization

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Abstract

A new low-cost, green and reproducible Lactobacillus sp. assisted biosynthesis of BaTiO3 nanoparticles is reported. X-ray and transmission electron microscopy analyses are performed to ascertain the formation of BaTiO3 nanoparticles. The apparent crystallite size and lattice strain are estimated from Williamson–Hall approach. XRD analysis of the compound indicated the formation of a single-phase tetragonal structure. Individual nanoparticles as well as a few aggregate having the size of 20–80 nm are found. A possible involved mechanism for the biosynthesis of nano-BaTiO3 has also been proposed in which ROS as well as partial pressure of gaseous hydrogen (rH2) of the culture solution seems to play an important role in the process. Remarkable enhancement in dielectric properties was observed in BaTiO3/polyvinylidene fluoride (PVDF) nanocomposite.

Introduction

Since the discovery of ferroelectricity in ternary perovskites, such as barium titanate (BaTiO3) in the 1940, it has been one of the most extensively studied systems due to its excellent dielectric, piezoelectric, pyroelectric and ferroelectric properties [1], [2], [3], [4], [5]. BaTiO3 is mainly used in multilayer ceramic capacitors (MLCCs), sensors and actuators, electro-optic devices, thermistors, integral capacitors in printed circuit boards (PCBs), temperature–humidity gas sensors, memory applications, etc. Traditionally, BaTiO3 is prepared using a high-temperature (>1100 °C) solid-state reaction between BaCO3 and TiO2 which yields large crystal grains (>3 μm) with a wide range of shape and size. However, it is difficult to develop a dielectric layer of less than 10 μm for increased capacitance, which is a major requirement for miniaturization of next generation of electronic/microelectronic devices and the MLCC industries [6]. To meet these requirements, the synthesis of BaTiO3 nanopowders having less than 100 nm particle size and a high dielectric constant is needed for further compactness.

Several efforts have been made towards the synthesis of BaTiO3 nanoparticles using various methods such as ball milling, solid-state reaction [4], solvothermal [7], hydrothermal [8], [9], [10], microemulsion [11] and different chemical routes [12], [13]. Each method has its own advantages and disadvantages like particle agglomeration, energy intensive, expensive chemicals and generation of toxic wastes. Thus, a low-cost, low temperature and eco-friendly synthesis of BaTiO3 on the nanometer scale remains a formidable challenge. In modern nano-science and technology, the interaction between inorganic molecules and biological structures are one of the most exciting areas of research. It is now well established that many organisms can produce inorganic materials either on intra- or extra-cellular level. Recently, fungus (Fusarium oxysporum) mediated synthesis of BaTiO3 nanoparticles has been reported [14]. In order to meet the requirements and exponentially growing technological demand, there is a need to develop an eco-friendly approach for nanomaterials synthesis that is devoid of using toxic chemicals in the synthesis protocol.

A simple yet highly adaptable molecular organization of microbes have gifted them a unique status in continuum of life. They have touched every walk of our lives whether as a prokaryote or eukaryote. Lactobacillus a commonly employed bacterium for the purpose of curdling of milk is highly beneficial to our system. It is non-pathogenic, partially oxygen-tolerant, Gram-positive, prokaryotic, anaerobic-mesophilic microbe. As such they are extremely energetic, adaptable and promising due to their potential metabolic fluxes. In our opinion, the capabilities of this benevolent microbe have not been taken into full use in terms of synthesizing the metallic and/or oxide nanoparticles. Though, taking use of Fusarium oxysporum, BaTiO3 nanoparticles of smaller dimension were synthesized [14], which is quite natural because the organism enjoys the advantage of being a eukaryote having much better organized cellular system compared to the Lactobacilli. In this effort, constituent chemicals in proper stoichiometry had been taken. This might pose the biological system with an additional liability of maintaining the stoichiometry especially in case of ternary or higher oxide systems. Keeping the above mentioned views in mind, we have made a novel effort to use Lactobacillus strain for the purpose of synthesizing BaTiO3 nanoparticles.

Accordingly, in the present effort, instead of taking Lactobacilli from the butter milk, the readily available pharmaceutical grade L. sporogens tablets were taken in order to assess its potential as putative candidate bacterial genera for the synthesis of BaTiO3 nanoparticles (abbreviated hereafter n-BT) by directly challenging it with the micro-scale BT instead of taking its constituent congeners separately. Furthermore, we have tried to explore and establish a cost-effective, eco-friendly and amenably reproducible approach for the purpose of scaling up and subsequent downstream processing. The BaTiO3 nanoparticles obtained were characterized by XRD and TEM studies. An effort has also been made to understand the putative mechanism of nano-transformation of accomplishing biosynthesis at the extra-cellular level. In order to study the dielectric properties of n-BT, nanocomposite of n-BT/polyvinylidene fluoride (n-BT/PVDF) was prepared.

Section snippets

Synthesis of BaTiO3 nanoparticles

Polycrystalline BaTiO3 powder was prepared from AR grade (99.9%+ pure, Merck) chemicals (BaCO3 and TiO2) using standard solid-state synthesis route in air atmosphere using the chemical reaction: BaCO3+TiO2ΔBaTiO3+CO2(g) at 1250 °C for 5 h under controlled heating and cooling cycles. The completion of reaction and the formation of desired compound were checked by X-ray diffraction technique (Fig. 1). Now, pharmaceutical grade Lactic acid Bacillus spore tablets (SporeLac DS, Sanyko

Results and discussion

Fig. 1 shows the X-ray diffraction profiles of BaTiO3 nanoparticles synthesized using Lactobacillus sp. The XRD analyses indicated the tetragonal unit cell. The least-squares regression fit to diffraction data yielded the lattice parameters: a = 3.9956 Å and c = 4.0310 Å, which is in excellent agreement with the literature report (JCPDS file no. 81-2203 and 89-1428). The volume of the unit cell calculated to be 64.3535 Å3. A linear least square fitting to η cos θ  sin θ data yielded the values of average

Conclusion

The present biosynthetic method is truly a novel, green cost-effective approach, capable of producing BaTiO3 nanoparticles. The protocol might open an avenue of opportunity for the synthesis of many other technological grade ternary or higher oxide systems and the hassle of maintaining proper stoichiometry is appreciably reduced. The synthesis of n-BT might have resulted due to stress-sensitive membrane bound oxido-reductases and carbon source dependent rH2 in the culture solution. Remarkable

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