Chemical synthesis and characterization of bismuth ferrite nanostructures

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Chemical synthesis and characterization of bismuth ferrite nanostructures


Author: Fei, Linfeng
Title: Chemical synthesis and characterization of bismuth ferrite nanostructures
Degree: M.Phil.
Year: 2011
Subject: Bismuth
Nanostructured materials
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: ix, 101 leaves : ill. ; 30 cm.
InnoPac Record:
Abstract: Motivated by the uniqueness of simultaneous antiferromagnetic and ferroelectric ordering at room temperature and hence, a fascinating vista of applications, research interest on perovskite bismuth ferrite (BiFeO₃) could be dated back to 1950s although it was almost impossible to produce pure crystals at that time. The discovery of large remnant polarization and strong ferromagnetism measured on BiFeO₃ thin films in 2003 has led to the revival of research in this area and many more studies have been conducted since then. On the other hand, the continuous evolution of nanoscience and nanotechnology have resulted in the fabrication of various bismuth ferrite nanostructures, including 0D nanoparticles, 1D nanowires, nanotubes and even 3D architectures. More importantly, these novel structures are proved to possess significantly different properties from their bulk or film counterparts because of the nanosized morphological characteristics. The ability to understand the size-dependent physical and chemical properties of these structures would help to ultimately integrate them as building blocks in future generation electronic devices, offering multiple controlling degrees of freedom. In this thesis, systematic experimental work has been carried out to synthesize and characterize bismuth ferrite nanostructures with a variety of sizes/morphologies, as outlined below.
First, a low-temperature wet chemical route was adopted to synthesize nanoparticles of BiFeO₃. In the process, bismuth nitrate and iron nitrate were employed as starting materials with excess tartaric acid and citric acid as chelating agents. It was found that the crystallization temperature of BiFeO₃ is dependent on the chelating agent. Under optimized processing conditions, BiFeO₃ nanoparticles with good quality of crystallization and sharp distribution of particle size could be obtained after a thermal treatment at ~350 °C, which is one of the lowest crystallization temperatures ever achieved. Structures and physical/chemical properties of BiFeO₃ nanoparticles with different sizes distributions were also characterized. Second, a sol-gel based electrospinning process was developed which enabled it to synthesize bismuth ferrite nanofibers at a relatively large scale. The annealing temperature after the electrospinning was significantly lowered by introducing appropriate chelating agent in the precursor solution. Nanofibers prepared under optimized conditions were found to be well crystallized, uniform in diameter and exhibit excellent ferroelectric properties when tested under piezoelectric force microscope. Third, uniform and phase-pure perovskite BiFeO₃ crystallites with different predominantly exposed facets were successfully synthesized via a facile one-pot hydrothermal approach at 200 °C in the presence of potassium hydroxide and polyethylene glycol. Pills and rods with dominant {111}c facets and cubes with {100}c exposed facets were obtained by adjusting the alkaline conditions of the precursor solution. Due to the distinct dominant facets of BiFeO₃ crystallites, {111}c dominant pills and rods were found to grant a significant enhanced visible light response, suggesting that the designed structures can give rise to better performance in future photovoltaic and photocatalytic applications of BiFeO₃. In summary, bismuth ferrite with controllable sizes and morphologies have been developed and characterized. The nanostructures are expected to be useful in the basic and applied research related to multiferroic nanomaterials.

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