1T phase boosted MoSe₂/pg-C₃N₄ with Z-scheme heterojunction for enhanced photocatalytic degradation of contaminants

Highlights

• Several MoSe₂ with different phase structure and morphology were synthesized by a NaBH₄-assisted hydrothermal method.

• A novel ordered 1T/2H MoSe₂/pg-C₃N₄ composites with 2D/2D hierarchical heterostructure was constructed.

• 1T/2H MoSe₂/pg-C₃N₄ composite showed superior photodegredation of tetracycline and rhodamine b.

• Ordered 1T/2H MoSe₂ was employed as co-catalyst for boosting the charge carriers transfer at the heterointerface.

• A 1T phase boosted Z-scheme MoSe₂-based heterojunction mechanism was proposed.

Abstract

An ordered 1T/2H MoSe₂ possessed few-layers nanosheets structure was synthesized by a NaBH₄-assisted hydrothermal method. Further, 1T/2H MoSe₂ was integrated with proton g-C₃N₄ via solvothermal method, forming a novel 2D/2D heterostructure composite (1T/2H MoSe₂/pg-C₃N₄) with Z-scheme heterojunction. 1T/2H MoSe₂/pg-C₃N₄ had narrower bandgap, abundant active sites, as well as better photocurrent response and conductivity, confirmed by spectroscopic and electrochemical characterizations. Consequently, the optimal 1T/2H MoSe₂/pg-C₃N₄ at 1 wt% MoSe₂ loading showed enhanced TC (RhB) photodegrading rate of 92% (99%), within 80 min (45 min) of simulated solar light irradiation, being 3.0 (3.4) and 1.2 (1.2) times than that of pg-C₃N₄ and 2H MoSe₂/pg-C₃N₄, respectively. Additionally, active species trapping experiments indicated that O₂⁻ and h⁺ were the main species of contaminants degradation over 1T/2H MoSe₂/pg-C₃N₄. Combining with DRS, Mott-Schottky and VB XPS analysis, a reasonable 1T phase boosted Z-scheme MoSe₂-based heterojunction mechanism was proposed.

Introduction

Solar light-driven semiconductor photocatalysis technology has been regarded as a potential strategy for relieving the energy shortage and environmental pollution in recent years [1], [2], [3]. Up to now, the conventional heterojunction (including type-I/II heterojunctions) photocatalyst has been proved for improving the photocatalytic performance, compared to single-component photocatalysts [4], [5]. But the redox activity of electrons and holes decreases because charge carriers transfer and separation in the typical heterojunctions [6], [7]. Thus, Z-scheme heterojunction has gained great attention for photocatalytic applications, since it could improve separation efficiency of charge carriers and retain the strong redox activity simultaneously [8], [9]. Based on the previous researches, Z-scheme heterojunction mainly includes following three typical modes: the charge carriers separate through a reversible redox mediator, an electron mediator or without any mediators [10], [11], [12], [13], [14].

Recently, layered transition-metal dichalcogenides (TMDs: MX₂, M = Mo or W, X = S or Se), with two-dimensional (2D) structure and ultrathin thickness, have been used for batteries, electrocatalysis and photocatalysis [15], [16]. However, bulk TMDs materials could not rival other superior catalysts, due to limited active sites and poor conductivity [17]. Various strategies, including phase engineering, edge engineering, defect engineering, doping and heterojunction constructing, have been studied to improve the photocatalytic performance of TMDs materials [18], [19]. Theoretical and experimental studies confirmed that the photocatalytic properties of TMDs are influenced by its electronic structure [20], [21]. For example, MoSe₂ materials can be prepared with semiconducting 2H phase (trigonal prismatic coordination) or metastable metallic 1T phase (octahedral coordination) [22]. 2H MoSe₂, as a noble-metal-free co-catalyst, has attracted many researchers’ attention for constructing heterojunctions towards photocatalytic hydrogen evolution [23], [24], [25], degrading organic contaminants [26], [27], [28], [29] and Cr (VI) reduction [30], [31]. Compared with 2H MoSe₂, 1T MoSe₂ with abundant active sites and improved conductivity has been considered as an emerging co-catalyst for photocatalysis [32]. Therefore, phase engineering for transferring 2H phase to 1T phase has been demonstrated as a useful pathway to improve photocatalytic activity. However, most preparation processes of 1T phase MoSe₂ are high-consuming and high-risk, including electrochemical lithium insertion, liquid-ammonia-assisted lithiation, chemical vapor deposition and CO₂-induced phase-transition [33], [34], [35], [36]. Until now, exploring facile methods of 1T MoSe₂-based materials with superior photocatalytic performance is still a great challenge. He et al. synthesized a Si-doped TiO₂ nanotube/1T-MoSe₂ hybrid, which showed the improved photocatalytic hydrogen evolution activities compared to nanotubes [37]. As the introduction of 1T MoSe₂ provided high conductivity, enhanced light absorbance as well as efficient interface-induced effect at the heterojunction. Tao et al. constructed a multiphasic 1T/2H MoSe₂ nanosheet integrated with CdS nanorods, which exhibited highly enhanced photocatalytic HER activities than 2H MoSe₂/CdS, CdS and Pt/CdS [38]. Since multiphasic 1T/2H MoSe₂ co-catalyst has suitable bandgap, dense active sites and improved conductivity than 2H MoSe₂ co-catalyst. Xu et al. prepared an interlayer-expanded 1T-MoSe₂ for maximizing the co-catalyst activity via optimizing surface activation capacity for photocatalytic HER at edge and basal sites [39]. While the interlayer-expanded 1T-MoSe₂ was integrated with 2D-C₃N₄, the hybrid showed an enhanced HER activity, which was greatly higher than that of 2H-MoSe₂ and most noble-metals. Particularly, phase engineering has been widely used as a pathway to enhance the co-catalyst activity for photocatalytic HER performance, but rarely was considered as a potential strategy for the construction of Z-scheme heterojunction towards photocatalytic degradation. As for conventional Z-scheme heterojunctions, most of them required electron mediators at the interface of semiconductors to form the Z-scheme photocatalytic system [10]. The noble-metals, metal materials and carbon-based materials, such as WO₃/Ag/CN [40], g-C₃N₄/Ag/MoS₂ [41], BiOCl-Bi-Bi₂O₃/rGO [42], BiVO₄/RGO/Ag₃PO₄ [43] and Bi₂WO₆/RGO/g-C₃N₄ [44], have been investigated as superior electron mediators due to their good conductivity. Similarly, metallic 1T phase MoSe₂ might be the promising noble-metal-free material for constructing Z-scheme photocatalytic system.

In this study, different MoSe₂ were prepared via a NaBH₄-assisted hydrothermal method, and corresponding MoSe₂-based composites with 2D proton g-C₃N₄ (pg-C₃N₄) were constructed through solvothermal method. Several MoSe₂ samples with different phase structure, morphology and light-harvest capacity were obtained by varying the reducing agent (NaBH₄) dosage. And the optimal MoSe₂ possessed an ordered 1T/2H phase structure, good conductivity and few-layers nanosheet structure with larger specific surface area. Besides, MoSe₂ could be employed as co-catalyst over pg-C₃N₄, forming a Z-scheme heterostructure for photocatalytic degradation. The photocatalytic performance of MoSe₂/pg-C₃N₄ composites was evaluated via degradation of contaminants (TC, RhB) under simulated solar light. And the characterizations demonstrated that the ordered 1T/2H phase MoSe₂ provided abundant reactive sites, good light-harvest capacity and efficient transfer as well as separation of charge carriers. Accordingly, the reasonable 1T phase boosted Z-scheme MoSe₂-based heterojunction photocatalytic mechanism was proposed.