Effect of defects on the band gap and photoluminescence emission of Bi and Li co-substituted barium strontium titanate ceramics
Introduction
The world's energy needs increase daily due to increased consumption, both from the living and industrial aspects. Without significant progress in alternative energy sources, the demand for fossil fuels for electricity production will increase, increasing the environmental pollution that threatens future life [1]. Clean energy such as solar energy is an alternative technique for fossil fuels utilized in energy production and storage with minimal environmental issues [[2], [3], [4]]. The solar cells with perovskite-type compounds have emerged as a third-generation solar cell, globally promoted as an alternative renewable technology that is economically and environmentally viable for solar cell technologies to meet global challenges in energy generation, security, and environmental impact [[5], [6], [7], [8], [9]]. Unfortunately, most of the perovskite materials are active only on the UV-spectra due to their large bandgap energies, which utilized only about 4% of the whole solar spectra [[10], [11], [12], [13]].
To induce a visible-light response in a wide range of wavelengths, donor and/or acceptor ions doping into a host lattice can be used [[14], [15], [16], [17], [18], [19]]. In the ABO3 structure, an important criterion for possible doping of A or B site ions by dopants is a comparable ionic radius of the host and dopant ions [[20], [21], [22]]. And therefore, incorporating donor and/or acceptor dopants into the perovskite lattice can lead to the formation of excess positive or negative charge carriers in the perovskite lattice [[23], [24], [25]]. This results in introducing an electronic state within the crystal bandgap by the donor and/or acceptor impurities near the conduction or valence bands, respectively [[26], [27], [28], [29]]. The location of these states within the bandgap depends on the impurities' chemical properties and the possible interactions with other defects [30,31]. In this work, the variation of band gap energy may relate to defects (oxygen vacancies) that are created due to the volatilization of Bi at high sintering temperatures. Based on the consideration of the charge balance, the doping with trivalent Bi3+ ions in Ba/Sr site requires other monovalent cations Li+ to maintain the charge neutrality condition, as we assume in this study. From this point of view, due to the electrical balance promoted by Bi3+ and Li+ in co-substituted (Ba0.60/Sr0.40)2+, no charge compensation should be expected in these compositions. However, the optical results exhibit strong absorption in the visible light region and a broad and intense photoluminescence emission spectrum, suggesting that these phenomena are strongly related to other kinds of defects, like oxygen vacancies. XPS and ESR results confirm the presence of defects. So, which is the mechanism that causes the oxygen vacancy formation? Chen et al. [32] pointed out that a high sintering temperature of >1350 °C for Bi-doped SrTiO3 could easily create the oxygen vacancies according to the following reaction [32]:
Ionization of the oxygen vacancy of the perovskite structure containing titanate to create a conducting electron can be represented as the following.
or these electrons may be bonded with Ti4+ in the form of:
The sintered samples exhibit a black color, and simultaneously, broad optical absorption and a broad, intense PL emission spectrum were observed in the visible region. This confirms a close correlation between the oxygen vacancies, optical absorption spectra, and color centre. This work's motivation lays in the possibility of using co-substituted samples as a source for green and yellow emission devices and possible laser rods in future practical applications. Therefore, it is essential to gain insight and a clear idea of how changed the optical properties in the present ceramics and their relationship with defects formation at high sintering temperatures. The XRD and Rietveld refinement, Raman Spectroscopy, XPS, ESR, UV–Vis absorption and photoluminescence (PL) measurements as well as analysis has been systematically carried out.
Section snippets
Experimental
Solid-state reaction technique was used to synthesis the (Ba0·60Sr0.40)(1-x)(Bi,Li)xTiO3 (BST6:BLx%);(0%≤x ≤ 8%) ceramics. The initial solid solution was formed using high purity powders of BaCO3, SrCO3, Li2CO3, Bi2O3 (Sigma-Aldrich, 99.99% purity), and TiO2 (Sigma-Aldrich, 99.98% purity) as starting materials. After weighing the contents of the starting materials, the high-energy ball milling was used for 4h in acetone media and zirconia balls at a rotation speed of 150 rpm to work on mixing
X-ray and rietveld refinement analyses
Fig. 1a shows the Rietveld refinement plot of the XRD patterns for sintered ceramics of (BST6:BLx%);(0%≤x ≤ 8%). As shown in Fig. 1, the XRD patterns indicate that the sintered samples are successfully crystallized and structurally ordered at a long-range. Furthermore, the XRD patterns of these compounds were verified using standard JCPDS card No. (file #34–0411) of the cubic type structure of BST [34]. To prove that the cubic phase is the right structure, the structural refinement was carried
Conclusion
In conclusion, Bi and Li co-substituted barium strontium titanate BST6:BLx%; (0%≤x ≤ 8%) ceramics have been successfully synthesized via conventional solid-state reaction technique to study the effect of defect formation due to the co-substitution and high sintering temperature on the structural and optical properties. The structural investigation was carried out using X-ray diffraction and micro-Raman spectroscopy. The defect behaviour in the surface of BST6:BLx% was studied by using X-ray
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the experimental facilities provided by UGC, DST at the University of Hyderabad and the experimental facilities provided by FAPESP, CNPq at the Departments of Physics/UFSCar. Dr Alkathy is greatly indebted to the Sao Paulo Research Foundation (FAPESP) (grant no# 2019/03110-8) for financial support.
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