Fabrication of Z-scheme MoO₃/Bi₂O₄ heterojunction photocatalyst with enhanced photocatalytic performance under visible light irradiation

Abstract

Constructing Z-scheme heterojunction to improve the separation efficiency of photogenerated carriers of photocatalysts has gained extensive attention. In this work, we fabricated a novel Z-scheme MoO₃/Bi₂O₄ heterojunction photocatalyst by a hydrothermal method. XPS analysis results indicated that strong interaction between MoO₃ and Bi₂O₄ is generated, which contributes to charge transfer and separation of the photogenerated carriers. This was confirmed by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) tests. The photocatalytic performance of the as-synthesized photocatalysts was evaluated by degrading rhodamine B (RhB) in aqueous solution under visible light irradiation, showing that 15% MoO₃/Bi₂O₄ (15-MB) composite exhibited the highest photocatalytic activity, which is 2 times higher than that of Bi₂O₄. Besides, the heterojunction photocatalyst can keep good photocatalytic activity and stability after five recycles. Trapping experiments demonstrated that the dominant active radicals in photocatalytic reactions are superoxide radical (•O₂⁻) and holes (h⁺), indicating that the 15-MB composite is a Z-scheme photocatalyst. Finally, the mechanism of the Z-scheme MoO₃/Bi₂O₄ composite for photo-degrading RhB in aqueous solution is proposed. This work provides a promising strategy for designing Bi-based Z-scheme heterojunction photocatalysts for highly efficient removal of environmental pollutants.

Introduction

Organic pollutants are harmful to human health, such as dyes and volatile organic compounds (VOCs), which come from leather, textile, printing industries, and construction and decoration materials [1, 2, 3, 4]. Especially, organic dyes containing various refractory organic compounds were discharged into natural water without any treatment because of the waste in the production process, which affects the safety of water quality [5, 6]. Even though the concentration of organic dyes in the water is low, the water quality has been affected. The organic dyes in waste water are difficult to be degraded completely by traditional technologies, which poses a grave threat to human health and their production activities. Therefore, it is highly desirable to explore novel technologies for degradation of dye waste water. Advanced oxidation processes have been developed and applicated for organic pollutant degradation [7, 8]. Especially, semiconductor photocatalytic technology can remove and degrade organic pollutants by catalytic reaction driven by solar energy, which has been considered as a promising technology for environmental purification because of the advantages of its highly efficiency and environmental friendliness [9, 10]. The common photocatalysts, such as TiO₂ and ZnO, which can be excited by UV-light to generate electron-hole pairs and finally involved in the redox reaction, have been studied for solving problems of environmental pollution and energy shortage [11, 12, 13]. However, the visible-light excited photocatalysts can utilize solar energy more efficiently, which have wider application prospect. Therefore, it is desirable to explore novel visible-light excited photocatalysts.

In recent years, many visible light responsive Bi-based photocatalysts with highly hole mobility and fantastic optical properties have been reported. Typical Bi-based photocatalysts such as BiO_2–x [14, 15, 16], Bi₂WO₆ [17, 18, 19, 20], and BiOX (X = Cl, Br, I) [21, 22, 23, 24, 25, 26, 27, 28, 29] have been developed and applied for degradation of organic pollutants, hydrogen generation, CO₂ reduction, and NO_x removal. However, single-phase photocatalyst usually suffers from quick recombination of charge carriers, affecting their photocatalytic efficiency seriously [30, 31]. Especially, fabrication of hybrid semiconductor photocatalysts with staggered band alignments has been identified as one of the most promising ways for boosting their photocatalytic performance, benefiting from the faster interfacial charge transfer [32, 33, 34]. Recently, constructing Z-scheme photocatalysts to widen the range of light absorption and increase the redox ability of photocatalysts has drawn much attention [35, 36, 37]. Wang et al. [38] have successfully synthesized Bi₃TaO₇/g-C₃N₄ Z-scheme composites, and this Z-scheme photocatalyst shows higher visible light catalytic activity for degrading antibiotics. Nie et al. [39] have fabricated Z-scheme g-C₃N₄/ZnO composites, and the obtained composites showed higher photocatalytic performance for CO₂ reduction, benefiting from the high separation efficiency of charge. Therefore, developing novel Z-scheme photocatalysts for environment remediation is promising.

Bi₂O₄ is a novel visible light responsive photocatalyst with narrow band gap and wide absorption region. Wang et al. [40] synthesized Bi₂O₄ for application in bacterial inactivation and decomposition of organic pollutants. To enhance catalytic performance of single phase Bi₂O₄ photocatalyst, Xia et al. [41] fabricated Z-scheme C₃N₄/Bi₂O₄ heterojunction photocatalysts, which have better and stable photocatalytic performance. Besides, many studies on Bi₂O₄ have also been reported [42, 43, 44, 45, 46]. MoO₃ is a noticeable material for environmental treatment due to its chemical stability and non-toxicity. MoO₃ is a promising material to construct heterostructures with other semiconductors because of its energetically electrical properties [47]. He et al. [48] synthesized Z-scheme MoO₃/C₃N₄ composites by a mixed calcination method and evaluated their photocatalytic activities by degrading methyl orange (MO), and the obtained composites exhibited higher visible-light photocatalytic performance than C₃N₄. Therefore, it is meaningful to fabricate a novel MoO₃/Bi₂O₄ heterojunction photocatalyst for highly efficient photocatalysis.

Herein, we successfully synthesized MoO₃/Bi₂O₄ Z-scheme composites by a hydrothermal method. The structure, morphology, and surface chemical properties of the as-synthesized composites were studied by a series of characterizations. Subsequently, the photocatalytic activity of the composites was evaluated by using RhB as the target pollutant. Chemical trapping experiments were conducted to determine the participation of active radicals in photocatalytic reactions. Finally, the possible Z-scheme photocatalytic mechanism was proposed and discussed in detail.