Facile construction of novel Bi2WO6/Ta3N5 Z-scheme heterojunction nanofibers for efficient degradation of harmful pharmaceutical pollutants
Graphical abstract
Introduction
The pollution of pharmaceutical wastewater has recently drawn increasing attention, since many pharmaceutical antibiotics that are difficult to be metabolized and poorly absorbable pose great threat to aquatic environment and human health [1], [2], [3]. Particularly, even low concentration (ng/L or μg/L) of these pollutants can cause the problems of antibiotic resistance, endocrine disruption, and chronic toxicity [3]. With the purpose of eliminating such refractory and toxic pollutants from pharmaceutical wastewater, a variety of techniques (e.g. adsorption, flocculation and biodegradation) have been developed [4], [5], [6], [7]. Unfortunately, their application is hampered owing to some shortcomings such as high cost, complicated procedures, long cycle, and incomplete removal. Nowadays, semiconductor-based photocatalytic technique, which utilizes abundant sunlight to effectively degrade and even mineralize these contaminants thoroughly under mild conditions, has been considered as one of the most promising technologies for antibiotics removal [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. To realize this goal, the vital issue is to develop outstanding photocatalysts with sufficient sunlight absorption and efficient charge separation.
Bismuth tungstate (Bi2WO6, Eg = 2.6–2.8 eV), a typical aurivillius phase compound made of alternating (Bi2O2)2+ and WO42− slices, has been widely reported as an active photocatalyst for environmental purification by virtue of its visible-light (VL) response, high activity, good stability, low cost, and nontoxicity [19]. However, its unsatisfactory photocatalytic performance severely limits its practical application, due to the weak sunlight harvesting ability and rapid recombination rate of photo-induced carriers [20]. To overcome these disadvantages, various strategies have been developed, including architecture tailoring [21], [22], [23], noble metal deposition [24], nonmetal doping [25], heterojunction construction [8], [9], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], and defect-introduction [39]. Among these, the fabrication of heterojunction photocatalysts is demonstrated to be a viable strategy to ameliorate the photocatalytic performance of Bi2WO6. Consequently, various Bi2WO6-based heterojunctions have been developed and present enhanced photocatalytic properties [8], [9], [26], [27], [28], [29], [30], [31], [32], [34], [35], [36], [37], [38]. But a critical defect for traditional heterojunction (e.g., n-n heterojunction, and p-n heterojunction) is the weakened redox ability of the photo-induced electrons and holes due to the migration process. By contrast, the Z-scheme heterojunction shows remarkable advantages in suppressing the recombination rate of photo-excited charge carriers as well as preserving the strong redox capability [9], [30], [31], [32], [33], [35], [36], [38], [40]. Though some progresses on the exploration of Z-scheme heterojunctions have been made, it is still of tremendous challenge and vital significance to further develop highly active, stable, and recyclable Bi2WO6-based Z-scheme heterojunction photocatalysts for environmental remediation.
Tantalum nitride (Ta3N5, Eg ≈ 2.1 eV), has become a “star” photocatalyst for contaminant removal and water splitting on account of its suitable band edge positions, broad sunlight absorption range (λ ≤ 600 nm), and high photochemical stability [41], [42], [43], [44], [45]. However, the photocatalytic property of Ta3N5 is markedly restrained by the fast recombination of charge carriers [46], [47], [48], [49], [50], [51]. Inspiringly, bismuth tungstate (Bi2WO6) has a well-matched band structure and special layered architecture that can be integrated with Ta3N5 to construct a Z-scheme heterojunction photocatalyst. A well-designed Z-scheme heterojunction can retain the superior reduction capability of the Ta3N5 with a more negative CB potential and promote the oxidation ability of the Bi2WO6 with a more positive VB potential. Additionally, these heterojunctions can be endowed with broadened sunlight absorption spectrum and excellent separation efficiency of charge carriers, thereby to have outstanding photocatalytic performance. Moreover, it is worth noting that fabricating heterojunction nanofibers is a viable route to endowing catalysts with easily recyclable characteristics [52], [53], [54]. As far as we know, there are no relevant reports about the fabrication of a Z-scheme Bi2WO6/Ta3N5 heterojunction photocatalyst for the photo-degradation of toxic pollutants. Therefore, it is meaningful to design and construct novel Bi2WO6/Ta3N5 Z-scheme heterojunction nanofibers for the removal of harmful pharmaceutical pollutants.
Herein, novel Z-scheme hetero-structures of Bi2WO6 nanosheets modified on Ta3N5 nanofibers are fabricated and applied to degrade harmful pharmaceutical pollutants (e.g., tetracycline hydrochloride (TC) and ciprofloxacin (CIP)) under visible-light illumination. The Z-scheme heterojunction between Bi2WO6 and Ta3N5 markedly ameliorates the light harvesting ability and prevents the fast recombination of photo-excited carriers. Photocatalytic experimental results demonstrate Bi2WO6/Ta3N5 has the remarkable photocatalytic property and robust stability in the degradation of toxic antibiotics under visible light, signifying its good prospect in the treatment of pharmaceutical pollutants.
Section snippets
Chemicals
Tantalum ethanolate (Ta(OEt)4, >99%), polyvinylpyrrolidone (PVP, M.W. ≈ 1,300,000), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, >99%), sodium tungstate dihydrate (Na2WO4, >99%), and other chemicals were bought and have not been further purified.
Synthesis of Ta3N5
Ta3N5 nanofibers were fabricated through an electrospinning-calcination-nitridation route reported in our previous research [54]. Briefly, 1 g polyvinylpyrrolidone (PVP, MW = 1,300,000) and 2 g Ta(OEt)4 were dissolved in 14 mL of ethanol and 4 mL of
Phase structure
Fig. 1 shows the schematic diagram of the synthesis of Bi2WO6/Ta3N5 Z-scheme heterojunction nanofibers. The Ta3N5 nanofibers were first fabricated by an electrospinning–calcination–nitridation procedure. Then the Bi2WO6/Ta3N5 Z-scheme heterojunctions were constructed by in-situ solvothermal growth of Bi2WO6 nanosheets on Ta3N5 nanofibers. For the XRD patterns shown in Fig. 2, the peaks in pristine Bi2WO6 and Ta3N5 are assigned to orthorhombic Bi2WO6 (JCPDS 73-1126) [27] and monoclinic Ta3N5
Conclusions
A novel Bi2WO6/Ta3N5 Z-Scheme heterojunction photocatalyst with strong interfacial interactions has been successfully constructed by in-situ solvothermal growth of Bi2WO6 nanosheets on the surface of Ta3N5 nanofibers. The charge transfer pathways in the unique Z-scheme Bi2WO6/Ta3N5 heterojunction and compact interfacial contact can substantially facilitate the separation of photo-generated carriers, leading to superior photocatalytic performance of Bi2WO6/Ta3N5 in degrading and mineralizing TC
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 research reported in this paper.
Acknowledgments
This work has been financially supported by the National Natural Science Foundation of China, China (51708504), the Fundamental Research Funds for Zhejiang Provincial Universities and Research Institutes (2019JZ00009), and the Natural Science Foundation of Zhejiang Province, China (LY20E080014). X. C. appreciates the support from the U.S. National Science Foundation (DMR-1609061), the School of Biological and Chemical Sciences and the College of Arts and Sciences, University of Missouri, United
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