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Preparation of BiOCl0.9I0.1/β-Bi2O3 composite for degradation of tetracycline hydrochloride under simulated sunlight

https://doi.org/10.1016/S1872-2067(19)63486-8Get rights and content

Abstract

A novel and effective BiOCl0.9I0.1/x%β-Bi2O3 composite catalyst was synthesized through a precipitation method. The structure, morphology, and optical properties of the samples were certified by X-ray diffraction, UV-Vis diffuse reflectance, scanning electron microscopy, and X-ray photoelectron spectroscopic characterizations. Photocatalytic experiments demonstrated that the synthesized BiOCl0.9I0.1/x%β-Bi2O3 composite catalyst exhibited excellent photocatalytic performance toward the degradation of tetracycline hydrochloride (TCH) under simulated sunlight. Furthermore, the TCH degradation rate of BiOCl0.9I0.1/15%β-Bi2O3 increased by 27.6% and 61.4% compared with those of the pure BiOCl0.9I0.1 and pure β-Bi2O3, respectively. Due to the multiple vacancies and valence states possessed by BiOCl0.9I0.1/x%β-Bi2O3, namely Bi5+, Bi(3-x)+, Bi5+–O, Bi3+–O, I and I3, the charge separation in photocatalysis reactions can be effectively promoted. The Mott-Schottky measurements indicate that the conduction band (CB) level of BiOCl0.9I0.1/15%β-Bi2O3 becomes more negative relative to that of BiOCl0.9I0.1, guaranteeing an advantageous effect on the redox ability of the photocatalyst. This study provides a new bright spot for the construction of high-performance photocatalysts.

Graphical Abstract

β-Bi2O3 and element-doped BiOCl0.9I0.1 were combined to construct a BiOCl0.9I0.1/β-Bi2O3 composite catalyst; compared with those of the pure β-Bi2O3, BiOCl0.9I0.1 catalysts, the efficiency of the composite catalyst toward tetracycline hydrochloride degradation was significantly enhanced under simulated sunlight.

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Introduction

Along with the rapid development of domestic and foreign industries that engender impressive economic benefits, egregious environmental pollutants have been introduced to the planet, e.g., heavy metal ions, pesticides, and antibiotics. These pollutants not only affect the water and air quality but also seriously threaten human health [1, 2]. Within the past several decades, a large number of antibiotics have emerged and are still being accumulated globally. They are randomly discharged into natural environments, including surface water, groundwater, and sediments [2, 3]. Tetracycline hydrochloride (TCH) is one of them; it is common and widely used in pharmaceutical care and veterinary fields [4, 5]. The TCH residues in aquatic environment can induce the development of antibiotic-resistant pathogens and have a potential long-term detrimental effect on human health [6, 7, 8]. Moreover, TCH is difficult to eliminate [3, 9]; thus, an increasing number of studies is centered on its complete removal [10].

However, generally, the application of conventional physical adsorption and biodegradation methods to realize this objective is considered difficult owing to the cumbersome post-treatment challenges associated with adsorbents, high processing costs, and antibacterial properties [8]. In this case, advanced oxidation treatment has been considered in view of the possibility to generate superoxide free radicals (•O2) and hydroxyl radicals with strong oxidative abilities (•OH, +2.4 eV) to decompose many recalcitrant organic contaminants [8, 11].

•O2 + organic contaminants → Degradation products,

•OH + h+ + organic contaminants → Degradation products.

Thus far, conventionally, the degradation of TCH has been carried out using the Fenton reagent [8, 12] and photocatalytic oxidation [8, 13, 14, 15]. Photocatalytic technology is considered as a practical method for degrading TCH due to its high efficiency, simple operation, and low cost [8]. Most photocatalytic processes employ TiO2–based photocatalysts; however, TiO2 only responds to ultraviolet light, and its wide bandgap limits further practical application [13, 14, 16, 17]. Therefore, exploring new semiconductor catalysts with visible-light-harvesting ability, high efficiency, and easy recycling is an effective way of effecting the degradation of tetracycline [18]. In recent years, bismuth oxyhalide (BiOX, X = F, Cl, Br, I) has attracted wide attention from researchers due to its unique layered crystal structure and optical properties [19, 20, 21, 22]. BiOX (X = Cl, Br, and I) is a semiconductor with an indirect bandgap, and the recombination probability of the photogenerated carriers is relatively low [19, 20, 21, 23, 24]. These inherent structural advantages determine the intensive employment of BiOX in photocatalytic application [25, 26, 27, 28, 29, 30].

Conversely, β-Bi2O3 is also a common germanium-containing semiconductor. Similarly, BiOX (X = Cl, Br and I) exhibits good absorption of visible light; thus, it can significantly facilitate the production of superoxide and hydroxide species under exposure to sunlight [31]. Therefore, in this work, it is chosen for combination with BiOCl0.9I0.1, which has an indirect bandgap, to form a composite catalyst [32]. Due to the internal electrostatic field enrichment, BiOClxI1–x exhibits a relatively high photocatalytic activity [19]. After compounding with β-Bi2O3, the photocatalytic properties of the composite are evaluated by degrading 20 mg·L−1 of TCH under light irradiation. The relative valence band (VB) and conduction band (CB) edge positions and element valence states of the prepared samples were analyzed by analyzing the UV-vis diffuse reflectance spectra, Mott-Schottky plots, and XPS spectra. The possible mechanism of the photocatalytic activity of the BiOCl0.9I0.1/15%β-Bi2O3 complex in the degradation of TCH was discussed using energy band evaluation.

Section snippets

Materials

Bi(NO3)3·5H2O, KCl, KI, tetracycline hydrochloride, benzoquinone (BQ), triethanolamine (TEOA), isopropyl alcohol (IPA), and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. All the reagents used were of analytical grade and were used without further purification.

Preparation of β-Bi2O3

In a typical synthesis, the Bi(NO3)3·5H2O salt was first heated at 200 °C for 1 h to evaporate the water molecules contained therein. Subsequently, the temperature was increased to 300 °C for 2 h,

Results and discussion

Fig. 1(a) and Fig. 1(b) show the morphologies of the BiOCl0.9I0.1 and BiOCl0.9I0.1/15%β-Bi2O3 composites by SEM analysis, respectively. In Fig. 1(a), it is indicated that the pure BiOCl0.9I0.1 samples have a square nanosheet structure with a size of about 100 nm, which was regularly shaped and laminated. The basic morphology of the prepared BiOCl0.9I0.1/15%β-Bi2O3 composite was compared with those of the pure BiOCl0.9I0.1 and pure β-Bi2O3 catalysts, and the pure β-Bi2O3 exhibited a massive

Conclusions

In summary, the BiOCl0.9I0.1/15%β-Bi2O3 composite was successfully synthesized through a simple oil bath method at 90 °C. The TEM and XPS images could prove that BiOCl0.9I0.1 and β-Bi2O3 were successfully combined, and the (110) and (113) crystal lattice fringes of BiOCl0.9I0.1 and β-Bi2O3 were clearly observed in the HRTEM image. The BET analysis showed that the BiOCl0.9I0.1/15%β-Bi2O3 composite had a large specific surface area, providing more active sites for the reaction. The BiOCl0.9I0.1

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    Published 5 October 2020

    This work was supported by the National Natural Science Foundation of China (21663027, 21808189), the Science and Technology Support Project of Gansu Province (1504GKCA027), the Fundamental Research Funds for the Central Universities of Chang'an University (300102299304), the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (LYJ18205).

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