Controlled synthesis and exceptional photoelectrocatalytic properties of Bi₂S₃/MoS₂/Bi₂MoO₆ ternary hetero-structured porous film

Abstract

In this work, Bi₂S₃/MoS₂/Bi₂MoO₆ hetero-structured porous films were fabricated via a facile anion exchange process using the as-prepared Bi₂MoO₆ nanoflake array film as substrate material. The formation of Bi₂S₃/MoS₂/Bi₂MoO₆ ternary hetero-structured porous film is both thermodynamically controllable and reaction time dependent. Systematic experiments were done to investigate the products at each reaction stage and disclose the relationships between the composite components and reaction temperature and time. The study showed that the energy barrier need to be overpassed when MoS₂ and Bi₂S₃ were simultaneously produced. The optimized Bi₂S₃/MoS₂/Bi₂MoO₆ photoelectrode exhibited significantly higher photoelectrocatalytic efficiency than Bi₂MoO₆, binary Bi₂S₃/Bi₂MoO₆ and Bi₂S₃/MoS₂ photoelectrodes. The remarkable degradation efficiency of the Bi₂S₃/MoS₂/Bi₂MoO₆ photoelectrode comes from the synergy of high quality assembly and heterostructure interfaces between the three components. The optimized film assembly and stepwise band alignment in the ternary heterostructure composite contribute to visible light utilization, transport and separation of charge carriers, mass transport, and accessibility of active sites. The generated active species such as superoxide anions (O₂⁻) and holes were detected to promote the decomposition of organic pollutants. The reasonable photoeletrocatalytic degradation mechanism was also proposed.

Introduction

In recent years, gradually increased organic contaminants demand a sustainable approach for wastewater treatment and reuse [1], [2]. Some conventional biological and chemical treatment technologies are able to degrade organic pollutants, but there still exist lots of challenges. More recently, photoelectrochemical catalysis (PEC) has been regarded as one of the promising technologies for wastewater purification because there exists synergy between photocatalysis and electrocatalysis [3], [4]. In the PEC system, photocatalyst films were immobilized on the conductive electrodes to decompose organics under certain bias potential. Such design can effectively avoid secondary contamination problems of powder photocatalysts happened in photocatalytic process [5], [6]. Meanwhile, the applied bias potential on the working electrode can further promote transfer and separation of photogenerated electron–hole pairs and increase PEC activity [7]. In the PEC system, wide band-gap semiconductors such as TiO₂ were considered to be promising photocatalysts because of their nontoxicity, high stability, and strong oxidation ability [8]. However, wide band-gap semiconductors can only absorb ultraviolet (UV) light (∼5% of solar spectrum). Meanwhile the separation efficiency of photogenerated electron−hole pairs is relatively low [9], [10], [11]. Therefore, it is desirable to develop efficient visible light-responsive photocatalysts.

In the past few years, bismuth-based semiconductors (e.g. Bi₆Mo₂O₁₅, BiVO₄, etc.) have attracted great attention in different application fields because of their excellent physical and chemical properties [12], [13], [14]. Notably, narrow band-gap Bi₂MoO₆ and Bi₂S₃ showed acceptable photocatalytic and photoelectrochemical performance [15], [16], [17], [18]. Even so, it is necessary to further improve their performance from the application perspective. In previous research, Bi₂MoO₆ and Bi₂S₃ have been reported to couple with other semiconductors to form heterostructure composites and exhibited significantly enhanced performance [19], [20], [21]. For example, Bi₂MoO₆/TiO₂, CdS/Bi₂MoO₆, Bi₂MoO₆/BiVO₄, Bi₂S₃/Bi₂WO₆, and Bi₂S₃@MoS₂ composites were successfully fabricated to improve photocatalytic or photoelectrochemical performance [22], [23], [24], [25]. Meanwhile, Bi₂MoO₆ and Bi₂S₃ have also coupled with each other to form Bi₂S₃/Bi₂MoO₆ heterostructure via a simple ion exchange method, and enhanced photocatalytic and photoelecrochemical performances were obtained [26], [27]. MoS₂, as a narrow bandgap material with a typical layered structure, has been proved to exhibit high photoelectricity induced catalytic activities [28], [29]. Therefore, MoS₂ was often used to couple with other semiconductors to improve photoelectronic properties through hydrothermal process using sulfide source and molybdenum salt as starting reactants at a relatively high temperature (≥180 °C) [30], [31], [32]. Since Bi₂MoO₆ can be used as starting material to prepare Bi₂S₃/Bi₂MoO₆ heterostructure through ion exchange under relatively low temperature, it is feasible to fabricate Bi₂S₃/MoS₂/Bi₂MoO₆ ternary heterostructure by simultaneously producing MoS₂ and Bi₂S₃ under a raised reaction temperature enough to produce MoS₂ using Bi₂MoO₆ as starting material, thus enhanced photoelectrocatalytic performance can be expected. But the related work has not been mentioned until now.

Inspired by these concerns, herein, Bi₂S₃/MoS₂/Bi₂MoO₆ composite films were controllably prepared via a facile hydrothermal method using the prepared Bi₂MoO₆ nanoflake array film as substrate material. The components of porous composite films can be tuned by changing reaction temperature and time (Scheme 1). The optimized Bi₂S₃/MoS₂/Bi₂MoO₆ porous composite films have exhibited significantly higher photoelectrocatalytic performance than pure Bi₂MoO₆ and binary composite films (Bi₂S₃/Bi₂MoO₆ and Bi₂S₃/MoS₂) for the degradation of organic pollutants. This high photoelectrocatalytic performance can be predominantly attributed to the synergistic effects of three components in the composite, robust visible light harvesting, and efficient transfer and separation of charge carriers.