Ferroelectric oxides are used widely in the microelectronic, ferroelectric, optoelectronic, and refrigerated cooling fields on account of their high dielectric constant, and large spontaneous polarization [1,2]. For several decades, many studies of ferroelectric materials have been conducted because of their excellent physical properties suitable for devices based on piezoresponse. For example, ferroelectric thin films and ceramics have a strong effect on a large number of technological applications [3,4].
Recently, novel opto-electronic systems have attracted considerable attention [5]. Among the many classes of materials, ferroelectric complex oxide materials are emerging as promising candidates for opto-electronic device applications, such as photovoltaic devices and photo detectors [6,7]. Complex oxides have excellent characteristics including ferroelectricity and excellent stability at high temperatures and high humidity. On the other hand, they have a much wider band gap than the materials available commercially for photovoltaic cells. Therefore, reducing the band gap without losing the useful characteristics is a key factor for achieving significant scientific and technological applications.
Among the range of complex oxides, Aurivillius phases are good alternative materials for solving the wide band gap problem. The Aurivillius family is a class of complex bismuth titanate-based oxides with a number of layered perovskite structures. These compounds, (Bi2O2)2+(An-1BnO3n+1)2−, are generally good insulators. In addition, unlike other classes of ferroelectric materials, the electrical and structural properties of Aurivillius phases have strong sustainability, even after extensive chemical doping because of their layered structure with alternating Bi2O2 and Bi2Ti3O10 layers [8,9].
To achieve the large bandgap tunability, W. S. Choi et al. suggested superlattice films by allying Bi4Ti3O12 and LaTMO3 (TM = Ti, V, Cr, Mn, Co, Ni, Al) [10]. They concluded that cobalt atoms might be the best candidate for reducing the band gap of BiT but they did not provide information on the substitution of Ti with Fe for tuning the bandgap in BiT.
A simple alloying method is used widely to reduce the bandgap of oxide materials. According to reference [11], lanthanum doping of BiFeO3 films induces a decrease in the band gap due to compositional changes in the host network. The substitution may also create localized states in the band gap, which will lead to a shift in the absorption edge towards a lower photon energy, and thus a decrease in the optical energy gap.
Recently, the optical bandgap of Bi4Ti3O12 (BiT) could be controlled using a simple alloying method without a superlattice structure [12]. This shows that the site specific substitution of layered ferroelectric BiT with La and Co could reduce the band gap of BiT considerably, while maintaining their original properties. Doping BiT with La enhances the ferroelectricity of samples, and cobalt atoms in BLT would contribute the most to modifying the electronic structure of BiT-based powder [13,14]. In this reference, the bandgaps (Eg) of Bi3.25La0.75CoTi2O12 (Co-BLT), Bi3.25La0.75Ti3O12 (BLT), and Bi4Ti3O12 (BiT) samples were estimated. The BLT sample did not show a remarkable change in the optical bandgap, whereas the Eg value of the Co-BLT sample decreased dramatically. On the other hand, the absorption edge of the optical band gap in cobalt-doped lanthanum bismuth titanate (Co-BLT) was not defined clearly in both alloying methods, superlattice and simple alloying method, even though cobalt doping induced band-gap narrowing and enhanced the visible light activity of bismuth titanate.
Iron atoms in the BLT structure could induce significant bandgap tunability like other 3d metals, but alloying has not been considered. Therefore, ferroelectric BLT powder was doped with Fe to identify more a useful candidate for accurately controlling the bandgap than doped cobalt.
In this study, pure BLT was synthesized and doped chemically using a simple solid reaction method. The structural and optical properties of the Fe or Co-doped lanthanum-modified bismuth titanate samples were examined. The synthesized powders were characterized structurally by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical properties of the synthesized powers were determined by ultraviolet–visible (UV–vis) absorption spectroscopy. As a result, the optical bandgap of iron-doped lanthanum bismuth titanate (Fe-BLT) powders were similar to Co-BLT powders without breaking the orthorhombic structure of bismuth titanate.