Enhanced photocatalytic degradation performance of BiVO4/BiOBr through combining Fermi level alteration and oxygen defect engineering
Graphical abstract
Introduction
The vigorous development of modern industrial technology brings environmental severe pollution problems [1], [2]. Oxytetracycline (OTC) is widely used as a preventive antibiotic due to its broad-spectrum activity and low cost. However, it is difficult to remove with conventional techniques, resulting in the accumulation of large amounts of OTC in water bodies and sediments. OTC exposure to the environment will inhibit the growth of microorganisms, induce the production of drug-resistant bacteria, and then bring risks to the ecosystem and human health [3], [4]. Fortunately, semiconductor photocatalytic technology has been recognized as an environmental-friendly pollutant treatment technology in the past few decades due to its non-toxic, low-cost and highly sustainable [5], [6]. However, the ultrafast recombination of photogenerated carriers has severely limited the performance of photocatalysts.
To cope with the above problem, many strategies have been explored to suppress the recombination of photogenerated carriers to improve photocatalytic performance, such as metal/non-metal doping, surface defect modification, morphology control, crystal-plane engineering and heterojunction construction [7], [8]. So far, the construction of heterojunctions (I-type, II-type and Z-scheme) can significantly improve carriers’ separation and transport and is considered to be an effective method to suppress carrier recombination [9]. Among them, inspired by natural photosynthesis, the researchers designed a Z-scheme heterojunction, which has been verified to achieve efficient charge separation and retain photogenerated holes and electrons with high oxidation/reduction potentials [10]. However, in the heterojunction system, if the energy level positions between the semiconductors are mismatched, it will lead to the formation of II-type heterojunction. Therefore, tuning the semiconductor energy level is crucial for constructing Z-scheme heterojunctions.
Oxygen vacancy (Ov), as a typical defect engineering, has been widely used to improve the separation and transport of photogenerated charges and then enhance photocatalytic activity [11]. At present, research on constructing oxygen vacancies to promote photocatalytic performance mainly focuses on enhanced light absorption, surface electron capture, energy band modulation, and molecular oxygen adsorption and activation [12], [13]. However, few studies have explained the effect of introducing oxygen vacancies on the electron transfer path at the heterojunction interface. It is well known that the introduction of additional donors (such as oxygen vacancies and non-metallic impurities) can shift the Fermi level in the direction of the conduction band, thereby reducing the semiconductor work function [14], [15]. Therefore, theoretically, the construction of oxygen vacancies can not only enhance light absorption, capture surface electrons, adsorb and activate molecular oxygen, etc., but also regulate the charge transfer path at the heterojunction interface. Sun et al. [16] realized the transition of the photogenerated charge transfer pathway of BiVO4/PCN from type-II to Z-scheme by regulating the oxygen vacancy concentration of BiVO4.
As visible-light-driven photocatalysts, the layered ternary oxides BiOX (X = Cl, Br and I) exhibited excellent performance in the field of photocatalytic. Among them, BiOBr has been extensively investigated for its chemical stability, nontoxicity, and suitable band gap [17], [18]. In addition, it has a unique structure that the [Bi2O2]2+ slabs were sandwiched between double Br- anion layers, which led to the easy breaking of Bi-O bonds to generate oxygen vacancies [19], [20]. Wang et al. [21] adopted a hydrothermal method to prepare BiOBr with surface oxygen defects, which exhibited much higher performance than BiOBr in photocatalytic superoxide radical generation and selective oxidative coupling reactions. Miao et al. [22] synthesized an S-scheme AgBr/BiOBr heterojunction with oxygen vacancies via a facile chemical method. The evolution rates for photoreduction of CO2 to CO and CH4 were 9.2 and 5.2 times higher than those of pure BiOBr, respectively. Recently, BiVO4 has attracted more and more attention as a Bi-based compound with excellent photocatalytic properties. However, due to the small specific surface area and extremely fast electron-hole pair recombination, it is difficult to further improve the photocatalytic performance of BiVO4 [23], [24]. Among various modification methods, researchers prefer to construct BiVO4-based heterojunctions to solve this problem, and multiple heterojunctions have been developed, such as Fe2O3/BiVO4, BiOCl/BiVO4/N-GQD, Ag3VO4/BiVO4, BiVO4/g-C3N4 [25], [26], [27], [28] and BiOBr/BiVO4, etc. Regrettably, although BiOBr/BiVO4 heterojunctions have been researched for several years, its photocatalytic degradation performance is still inferior [29], [30]. Therefore, it is necessary to explore some more detailed regulation strategies to enhance the photocatalytic activity of BiVO4/BiOBr.
Based on the above discussion, for the first time, through constructing the oxygen vacancies of BiOBr, we realized the transition of the BiOBr/BiVO4 interface charge transfer path from type-II to Z-scheme. The morphology, composition and structure of the as-prepared samples were comprehensively explored via analytical techniques. The OTC degradation efficiency of the 20% BVB-Ov photocatalyst was much higher than that of BiOBr-Ov and BiVO4, and also better than that of the 20% BiVO4/BiOBr composite. Furthermore, according to the liquid chromatography-mass spectrometry (LC-MS) analysis results, the possible photocatalytic pathway for the degradation of OTC was proposed. Photoelectrochemical, photoluminescence (PL) and time-resolved PL (TRPL) were employed to compare the photogenerated carrier separation and transport efficiencies of different samples. Finally, combined with Density functional theory (DFT) calculations, Kelvin probe force microscopy (KPFM) technology, energy band positions, trapping experiments, and electron paramagnetic resonance (EPR) studies, confirmed that the conversion of photogenerated carrier transfer path from type-II to Z-scheme.
Section snippets
Synthesis of BiOBr and OV-BiOBr with different OV concentrations
According to the previous research [31], the BiOBr-Ov was prepared using a solvothermal method. 4 mmol of bismuth nitrate was dispersed in a mixed solvent containing 6 mL HNO3 and 34 mL H2O, and stirred for 25 min. Subsequently, 6 mmol of ammonium bromide was dissolved in 20 mL H2O and stirred for 10 min, then added to the above solvent and stirred for 5 min. Afterwards, 8 mL, 12 mL or 16 mL of anhydrous ethanol was added to the mixed solution drop by drop under vigorously stirring. After
Structure and morphology
The crystal phases of samples were recognized by XRD spectra. As shown in Fig. 1a, the diffraction peaks of pure BiVO4 and BiOBr were well matched with monoclinic BiVO4 (JCPDS No. 14–0688) and tetragonal BiOBr (JCPDS No. 09–0393), respectively. It was worth noting that although the position of the diffraction peaks of BiOBr-Ov was consistent with that of BiOBr, the intensity was significantly weakened, and the weakening became more evident with ethanol content increasing (Fig. S1), presumably
Conclusion
In summary, the morphology, structure and composition of as-prepared samples were meticulously analyzed through various analytical characterization techniques, confirming that we prepared BVB-Ov Z-scheme photocatalysts via coupling BiOBr-Ov with BiVO4. Among them, 20% BVB-Ov exhibited the highest photocatalytic activity, and the degradation rate of OTC reached 91% within 60 min, which was much higher than that of 20% BVB (71%). Meantime, the 20% BVB-Ov photocatalyst possessed excellent
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.
Acknowledgments
This work was supported by the National Natural Science Foundation of China [grant numbers 52072180], and thanks to eceshi (www.eceshi.cn) for the LC-MS and Shiyanjia Lab (www.shiyanjia.com) for the XPS and EPR analysis.
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