Regular ArticleIn situ construction bismuth oxycarbonate/bismuth oxybromide Z-scheme heterojunction for efficient photocatalytic removal of tetracycline and ciprofloxacin
Graphical abstract
Introduction
Currently, water contamination caused by abuse and substandard discharge of antibiotics has become a threat to human beings and the environment because it is difficult for the ecosystem to degrade accumulated antibiotics [1], [2]. Among the used antibiotics, tetracycline (TC) and ciprofloxacin (CIP) are frequently detected in domestic sewage. It is essential to develop an effective and sustainable approach to remove TC and CIP. Visible light-driven photocatalytic degradation is a kind of treatment process that uses a semiconductor, such as metal oxides, g-C3N4, and Ag- and Bi-based photocatalysts, and it has attracted much attention for removing pollutants [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Generally, this photocatalytic pollutant removal process involves the following: a photocatalyst adsorbs light and generates electrons and holes; superoxide (∙O2−) and/or hydroxyl (∙OH) radicals are produced via the reduction of absorbed O2; pollutants are transformed into intermediates or CO2 and H2O under the actions of radicals. Thus, an efficient pollutant removal process depends on the light response ability, photogenerated electron-hole separation efficiency, and the redox ability of electrons and holes of photocatalyst. New materials should be designed to have a low charge recombination rate and strong redox ability to boost pollutant degradation.
Bi2O2CO3 (BOC) is used to degrade pollutants because of its low toxicity and wide bandgap (Eg). However, BOC has poor visible-light adsorption and exhibits a fast recombination of photogenerated electrons and holes. The separation of electron-hole pairs of BOC could be accelerated by integrating BOC with other semiconductors, such as Bi5O7I, In2S3, and Co3O4 to construct a heterojunction [16], [17], [18], [19], [20], [21], [22]. In particular, it is desirable to build a BOC-based Z-scheme heterojunction, which can restrain the recombination of electron-hole pairs and also preserve the strong redox ability of electrons and holes. Wang et al. prepared a BOC/g-C3N4 Z-scheme photocatalyst that enhanced sulfamethazine (SMT) removal efficiency from 38.35% (g-C3N4) and 11.67% (Bi2O2CO3) to 90.31% (Bi2O2CO3/g-C3N4) [23]. Jia et al. prepared flower-like BOC-BiO2-x-graphene (BOC-BiO2-x-GR) Z-scheme microspheres for the photocatalytic removal of NO, increasing the NO removal efficiency from 27.8% (BOC) to 60.7% (BOC-BiO2-x(35 wt%)-GR) [24]. Other BOC-based Z-scheme photocatalysts, including CdS/BiOBr/BOC [25], WS2/BOC [26], BOC/Bi2WO6 [27], and graphene quantum dots [28], have also been shown to exhibit enhanced photocatalytic performance.
Generally, the charge carrier transport in type-II or Z-scheme heterojunction is affected by certain factors, such as the lattice matching degree and interfacial charge transfer resistance. It is important to finely engineer the morphology and regulate the hybrid mode of each component in a heterogeneous system to improve the charge separation efficiency. Three-dimensional (3D) hierarchical spheres assembled by nanosheets provide paths for transporting molecules and electrons [29]. Also, the construction of a heterojunction via an in situ self-growth strategy can preferentially achieve intimate contact with one component over another and achieve a uniform distribution in a heterojunction, further beneficial for carrier transport and separation [30].
BiOBr is an n-type semiconductor with a Eg of ca. 2.6–2.9 eV [31], and it is an attractive semiconductor for photocatalytic degradation of dyes, toxins, and pesticides because of its suitable band positions [32], [33]. It is often used to construct Z-scheme heterojunctions with other semiconductors to enhance the photocatalytic activity. Chen et al. prepared a BiO1−xBr/BOC Z-scheme photocatalyst using a one-step solvothermal method to degrade CIP, methylaminoantipyrine, and bisphenol-A, and it exhibited improved performance via the formation of oxygen vacancy [34]. Wu et al. synthesized a 3D hierarchical BiOBr/BiOIO3 Z-scheme heterojunction with rich oxygen vacancies via in situ growth of BiOBr on BiOIO3 and used it for removing heavy metals such as mercury [35]. The reported BiOIO3 has band edges (valence potential (EVB) = 3.18 eV, conductive potential (ECB) = 0.13 eV) similar to those of BOC. Zhou et al. added WS2 to a precursor solution of BOC and fabricated a flower-like WS2/BOC Z-scheme photocatalyst for the removal of Lanasol Red 5B and CIP [26]. In this regard, it is expected that the use of BOC microsphere as a matrix for in situ self-growth of BiOBr on its surface would result in an efficient Z-scheme photocatalyst for TC and CIP degradation, although it has not been reported.
Herein, we designed and synthesized a 3D BOC/BiOBr heterojunction via the preparation of BOC followed by in situ self-growth of BiOBr on the just generated BOC spheres. It was found that the photocatalytic performance of BOC/BiOBr for the degradation of TC and CIP could be significantly influenced by changing the time of addition of KBr to Bi(NO3)3 solution by regulating pH. The charge transfer routes and the degradation mechanism are proposed.
Section snippets
Synthesis of BOC/BiOBr heterojunction
The BOC/BiOBr heterojunction was synthesized using a previously reported hydrothermal method with some modification [24]. Scheme 1 shows the synthetic process of BOC/BiOBr heterojunction. Typically, 0.24 g of sodium citrate (C6H5Na3O7.·2H2O) and 1.164 g bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) were dissolved in 12 mL of 1 mol L–1 HNO3 solution, and then 1.6 mol L–1 sodium hydroxide (NaOH) was used to adjust the solution pH until a white solid appeared (pH = 2). Then, various amounts of
Structural characterization
Fig. 1 shows the powder X-ray diffraction (XRD) patterns of prepared BOC, BiOBr, and BiOBr/BOC-x samples. The diffraction peaks of BOC and BiOBr can be indexed to the standard cards for BOC (JCPDS# 41–1488) [24] and BiOBr (Fig. S1, JCPDS# 09–0393) [36], respectively. No new diffraction peak was observed after 0.4 and 0.8 mmol of KBr were introduced into BOC (Fig. 1b and c) compared to the XRD patterns of BOC sample. Also, the relative intensity of BOC peaks at 24.0°, 30.0°, 32.5°, 42.5°, 46.6°,
Conclusions
A flower-like BOC/BiOBr Z-scheme heterojunction with oxygen vacancies was synthesized via a new approach of in situ self-growth of BiOBr on just generated BOC. Two-dimensional (2D) BiOBr nanosheets were uniformly grown on 3D BOC microspheres. The 3D structure and oxygen vacancy induced electron injection of BOC/BiOBr improve the light absorption efficiency. A Z-scheme transport of charge carriers and intimate contact of in situ self-grown BiOBr on just generated BOC enhance the separation and
CRediT authorship contribution statement
Xuemei Yan: Methodology, Software, Validation, Data curation, Writing - original draft. Qingjie Ji: Software, Data curation. Chao Wang: Resources, Investigation. Jixiang Xu: Conceptualization, Supervision. Lei Wang: Investigation, Formal analysis.
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 would like to thank the National Natural Science Foundation of China (No. 21571112, 51772162) and Taishan Scholar Foundation.
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