Z-scheme plasmonic Ag decorated WO₃/Bi₂WO₆ hybrids for enhanced photocatalytic abatement of chlorinated-VOCs under solar light irradiation

Highlights

• A simple method for WO₃/Bi₂WO₆ Z-scheme heterojunction was developed.

• Z-scheme Ag/WO₃/Bi₂WO₆ has been constructed for enhanced degradation of Cl-VOCs.

• Ag/WO₃/Bi₂WO₆ achieved 80% conversion efficiency of chlorobenzene under sunlight.

• In situ FTIR and EPR were employed for investigation of the reaction mechanism.

• Surface plasmon resonance and the heterojunction contribute to the high activity.

Abstract

Ag decorated WO₃/Bi₂WO₆ hybrid heterojunction with a direct Z-scheme band structure has been synthesized by a novel method for efficient removal of gaseous chlorinated-volatile organic compounds (VOCs) under simulated sunlight irradiation. Bismuth atoms were inserted into the [WO₆] layers of tungstate acid to form [Bi₂O₂] layers toward the formation of a Bi₂WO₆ crystal phase, which results in Z-scheme WO₃/Bi₂WO₆ heterojunction. Ag nanoparticles (NPs) were further introduced and uniformly anchored on the surface of the WO₃/Bi₂WO₆ for improving visible-light absorption and adjusting behavior of photoinduced charge carriers in the heterostructure through surface plasmon resonance (SPR). In comparison with pristine Bi₂WO₆ and WO₃/Bi₂WO₆, Ag/WO₃/Bi₂WO₆ exhibits a better photocatalytic activity for removal of gaseous chlorobenzene under simulated sunlight irradiation. The conversion efficiency of 2% Ag/WO₃/Bi₂WO₆ heterojunction is 2.5 and 1.9 times higher than those of the pristine Bi₂WO₆ and WO₃/Bi₂WO₆ samples, respectively. The improved photocatalytic activity is mainly attributed to the formation of three-component heterojunction with the Z-scheme structure and SPR effect of Ag NPs, which could not only increase absorption of visible light, but also promote the separation efficiency of photogenerated electrons and holes in the hybrids.

Introduction

From the viewpoint of sustainable development, heterogeneous photocatalysis by utilizing solar energy for environmental remediation and water splitting is a promising approach to solve the increasing environmental issue and energy crisis. In general, the heterogeneous photocatalysis could involve in three steps, where photons are firstly harvested to produce electron-hole pairs on the surface and shallow atom layers of a solid photocatalyst [1,2]. Subsequently, the photogenerated charge carriers transport to the surface of the solid photocatalyst and react with H₂O or O₂ to produce reactive oxygen species (ROS), such as ·OH and ·O₂⁻ [3,4]. Finally, the adsorbed target substances, such as CO₂, volatile organic compounds (VOCs) and water, are oxidized or reduced by the ROS and electrons [[5], [6], [7]]. For example, TiO₂-based photocatalysts were reported for degradation of gaseous chlorobenzene under UV irradiation [8,9]. In addition, some groups studied the photocatalytic oxidation of monochlorobenzene over photocatalysts in aqueous phase, exhibiting great photocatalytic activities [10]. Therefore, the vital issue of this photocatalytic process is the development of novel photocatalysts with efficient separation, electron transfer and utilization of photoinduced charge carriers for excellent photocatalytic performance.

To achieve a better photocatalytic activity, intensive researches have been focused on developing novel ternary metal oxides as efficient photocatalysts, such as ZnFe₂O₄ [11], Bi₂MoO₆ [12,13], and CuWO₄ [14,15], due to their more flexible compositions and unique electronic structure. As a layered Aurivillius-related oxide semiconductor, bismuth tungstate (Bi₂WO₆) is composed of alternating corner-sharing [WO₆] octahedral layers and [Bi₂O₂] layers and is a promising photo-sensitive material due to its optical properties, for example, a suitable band gap of 2.75 eV and good photostability [[16], [17], [18]]. Moreover, density functional theory (DFT) calculation shows that the conduction band (CB) of Bi₂WO₆ is comprised of W 5d orbitals; whereas its valence band (VB) is mainly formed by hybridization of O 2p with Bi 6 s orbitals, which not only makes the VB largely dispersed, but also favors the mobility of photo-induced holes for specific oxidation reaction [19]. However, the photocatalytic activity of Bi₂WO₆ is still limited by a poor separation efficiency of the photoinduced charge carriers in the photocatalytic reactions. Many approaches, such as constructing heterojunction [20,21], doping [22] and loading noble metals [23], have been explored in the past decades. Therefore, coupling Bi₂WO₆ with other semiconductors for constructing a heterojunction system can be used to improve the photocatalytic performance of Bi₂WO₆. Among numerous semiconductors, tungsten trioxide (WO₃) possesses a suitable band gap (∼2.7 eV) and high valence band position ( + 3.4 eV vs. NHE) [24]. In fact, WO₃ is often used as an effective photocatalyst in photocatalytic or electrocatalytic oxygen evolution reaction due to its high valence band position and strong photostability [25,26], which may be a good candidate for improving charge carrier transportation by constructing a Z-scheme system with the ternary Bi₂WO₆ according to the principles of a direct Z-scheme photocatalytic system [27,28]. In general, a Z-scheme photocatalyst simultaneously has the strong redox ability for driving photocatalytic reactions and the spatially separated reductive and oxidative active sites [29]. Furthermore, it is revealed that noble metal nanoparticles (NPs) with a surface plasmon resonance (SPR) effect and strong interaction between the noble metal and semiconductor could remarkably enhance visible light absorption and inhibit the recombination of photo-generated electrons and holes [30,31].

Based on the above strategies, in this work, three-component Ag/WO₃/Bi₂WO₆ heterojunction with a SPR effect was synthesized by a special route for enhanced photocatalytic decontamination of monochlorobenzene (Cl-VOCs) under simulated sunlight irradiation. Firstly, two-component heterojunction between WO₃ and Bi₂WO₆ was constructed to form a Z-scheme band structure, which could be beneficial for improving separation efficiency of photoinduced electrons and holes. Secondly, Ag NPs were introduced to harvest low energetic photons in the range of visible light due to the SPR effect. As a result, the three-component Ag/WO₃/Bi₂WO₆ hybrids exhibit significantly enhanced visible light response and photocatalytic performance in comparison with pristine Bi₂WO₆ and two-component WO₃/Bi₂WO₆, which is ascribed to the more efficient separation of photoinduced electron-hole pairs and SPR effect. In addition, the process of the photocatalytic degradation of chlorobenzene was monitored by in-situ FTIR and the possible reaction pathways are further discussed.