Photoactive brownmillerite Ba2In2O5 for photocatalytic degradation of organic pollutants
Graphical abstract
Introduction
Existing water sources are under a lot of pressure due to current environmental conditions. At the same time release of industrial, pharmaceutical and municipal effluents to the fresh water sources without proper treatment cause major problems to the water quality [1]. Among the pharmaceutical effluents, persistent drugs like norfloxacin (NF), belonging to the class of fluoroquinolone antibiotics, is generally considered as a serious threat to public healthcare. Though it is widely used in treatment of many diseases in humans and veterinary, a major part of it is excreted into the environment because of its lower metabolism and poor biodegradability. Even a trace amount of NF in water may interfere with bacterial DNA replication that leads to the spread of antibiotic resistant bacteria [2]. Hence it is crucial to effectively remove these residuals from water using stable, efficient, low cost and visible-light active photocatalysts [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Research on some of these materials has focused on perovskites with semiconducting behavior [14] and few perovskite based oxide materials such as BaBiO3 [15], BaTiO3 [16], etc. have been put in use for photocatalytic degradation of these effluents. Light absorption by perovskites enables low energy-loss charge generation and collection [17]. However, most perovskite structured (ABO3) materials possess wider bandgap (>3 eV) attributed to the fundamental characteristics of the metal–oxygen, A–O and B–O, bonds in their typical structure [[18], [19], [20]]. This wider band gap can restrict their photoactivity in the ultraviolet (UV) region of the solar spectrum. Oxygen deficient perovskite structures, such as brownmillerites, possess lower band gap than their parent perovskite structure and are potential materials for environmental applications [21]. The material is represented by an empirical formula A2B2O5, where the A site atom corresponds to large atoms like Ca [22] or Ba [23] and B site atoms correspond to transition metals such as Fe, Al, Mn and Co [[24], [25], [26], [27]].
Present work is particularly focused on the oxygen deficient Ba2In2O5 (BIO) brownmillerite, synthesized through a solution combustion method. In BIO, the compensation of Ba2+ and In3+ cation charges lead to the removal of one sixth of oxygen atoms from parent perovskite structure, accounting for a higher ionic conductivity due to the increase in concentration of Oxygen vacancies [28]. BIO can exist in various space groups namely Ibm2, Pnmm and Icmm [29,30]. The density functional theory (DFT) calculations have revealed enhanced probability of occurrence of space group Ibm2 [31]. This structural uniqueness of BIO helps it to effectively take part in the oxygen reduction reaction and doped BIO samples have shown good performance when compared to the pristine samples [32]. Co-doping of Zr and La in In site enhances the stability, as reported by J.F. Shin et al. [33]. A major advantage with BIO is that its bandgap lies well within the visible band gap range (2.5 eV) and has the potential to utilize up to ∼43% of the entire solar energy spectrum. This may prove to be one of the major advantages for BIO, as a potential visible light photocatalyst for degradation of the effluents.
Section snippets
Experimental procedure
Simple, inexpensive and fast solution combustion method was adapted to prepare the samples under study. High purity precursors of Barium nitrate, Indium nitrate and Urea were taken in the molar ratio of 2:2:9. The precursors were dissolved in 10 ml of distilled water and stirred continuously with the magnetic stirrer, until it turned into a homogeneous and transparent suspension. Prior to this, the muffle furnace was preset to a temperature of 500 °C and the beaker containing the transparent
Results and discussion
XRD patterns were recorded for post combusted sample as well as after heat treatment at 700 °C and 900 °C (Fig. 1 (a)). XRD pattern of combusted sample shows diffraction peaks at 24°, 35°, 46° and 52°, pointing towards poor phase formation and lack of crystallinity (Fig. 1(a) (i)). Heat treatment at 700 °C for 12 h revealed additional diffraction peak at 37°. When the sample was further calcined at 900 °C, its XRD pattern corresponded to the reported JCPDS data of BIO (card number: 00-030-0068).
Conclusion
Solution combustion method facilitated the synthesis of brownmillerite BIO and it showed superior photocatalytic activity towards NF and MB degradation. A suitable bandgap to absorb visible light region of the natural solar irradiation marks BIO as a suitable candidate for photocatalytic applications. Along with a narrow bandgap, it also exhibits proper energy band positions comprising of O 2p and In 5s orbitals. MB degradation studies primarily reveal the potential of BIO for wastewater
Author statements
Raja Preethi.V- Conceptualization, Methodology, Writing - original draft, Formal analysis. Sangeeth John- Investigation, Reviewing and Editing. Gopalkrishna Bhalerao- Resources, Investigation. Bhavana Gupta- Resources, Investigation. Jaspreet Singh- Investigation and Resources. Shubra Singh- Idea, Supervision, Editing, Reviewing
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.
Acknowledgement
Dr. Shubra Singh would like to acknowledge University Grants Commission (UGC), DST/TMD-EWO/WTI/2K19/EWFH/2019/122, DST Solar Energy Harnessing Center- [Grant No. DST/TMD/SERI/HUB/1(C)], SERB [Grant No. EMR/2017/000794] and Mrs. Raja Preethi V would like to thank Indian Solar Energy Harnessing Center [Grant No. DST/TMD/SERI/HUB/1(C)] for the financial support.
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