MoS2/SnNb2O6 2D/2D nanosheet heterojunctions with enhanced interfacial charge separation for boosting photocatalytic hydrogen evolution

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Abstract

Semiconductor-based photocatalytic hydrogen evolution through water splitting is of great importance for tacking the ever-increasing energy concerns. Here, novel visible-light-driven MoS2/SnNb2O6 2D/2D nanosheet heterojunction for photocatalytic hydrogen evolution was designed and fabricated by a facile hydrothermal method. The as-prepared 2D/2D heterojunctions not only offer extended visible light absorption range but also accelerate the separation and transfer efficiency of charge carriers owing to the typical 2D interface. Therefore, the MoS2/SnNb2O6 2D/2D heterojunctions showed superior H2 evolution photocatalytic performance compared with bare MoS2 and SnNb2O6 nanosheet. The optimal H2 production rate of as-prepared MoS2/SnNb2O6 heterojunction was up to 4.3-fold and 5-fold as high as that of bare SnNb2O6 and MoS2. This works will provide a new perspective on the development of 2D/2D nanosheet heterojunction photocatalysts with high performance for water splitting.

Introduction

To mitigate the energy shortage crisis and environmental pollution issues, photocatalytic H2 production driven by renewable solar energy has been substantially investigated and offered as a green and efficient route to solve the above problems [1], [2], [3]. Up to now, various semiconductor photocatalysts have been exploited for H2 production from water splitting, whereas the photocatalytic performance of these photocatalysts is still limited by the dependency of ultraviolet light and rapid recombination rate of photoinduced electron-hole (e-h+) pairs [4], [5], [6], [7], [8]. Hence, it is urgent and indispensable to develop the novel photocatalysts to improve both the photochemical activity and stability. In recent years, the two-dimensional ultrathin nanosheets with a few atomic layer thickness and high-percent exposed active sites have drawn tremendous attention, owing to the diversity of its constituent elements and specific properties, such as large specific surface area, excellent physical, optical and electrochemical properties [9], [10]. Among various 2D nanosheets, layered niobate oxides (e.g., K4Nb6O17, HNbWO6, KCa2Nb3O10 and SnNb2O6) show excellent catalytic performance and have been extensively studied [11], [12], [13], [14], [15], [16]. In particular, as a typical layered niobate semiconductor material, SnNb2O6 is especially attractive in the applications of organic pollutants degradation and photocatalytic H2 evolution because of its unique foordite structure and appropriate energy band [17], [18], [19], [20]. Nevertheless, bare SnNb2O6 nanosheet suffers from an undesirably high photoinduced e/h+ pairs recombination rate and moderate photocatalytic efficiency. Therefore, to satisfy the practical application requirement, it is highly desirable to develop a reasonable approach to improve the photocatalytic activity of SnNb2O6 nanosheet.

Coupling SnNb2O6 with other materials including metal nanoparticles, carbon, and semiconductor materials to form heterojunction has been proved to be one of the most effective strategies to boost the photocatalytic performance of SnNb2O6 photocatalysts [21], [22], [23], [24]. In these heterogeneous photocatalysts, the photoinduced e/h+ pairs recombination rate was suppressed and the electron transport efficiency was enhanced, leading to an improved photocatalytic performance. For example, Xu and co-workers synthesized SnNb2O6-graphene heterojunction and found that the heterojunction showed enhanced photocatalytic activity for the RhB degradation [25]. Our group constructed SnNb2O6/g-C3N4 nanosheet 2D-2D heterojunction with strong interfacial reaction and significantly improved photocatalytic activity in the degradation of MB [26]. More recently, MoS2 as a 2D layered material with a narrow band-gap (∼1.75 eV) has sparked enormous interest in photocatalytic reaction due to the wide light-responsive range, proper band energy, and high chemical stability [27], [28], [29], [30]. Numerous researches have revealed that the interfacial charge transfer efficiency of few-layered MoS2 is greatly improved compared to that of bulk materials owing to the boosted catalytic active sites [31], [32], [33], [34]. Since the energy levels of MoS2 are well matched with those of SnNb2O6, construction of MoS2/SnNb2O6 nanosheet heterojunctions with 2D interface would be promising to obtain a high-efficient heterogeneous photocatalyst system for solar-to-H2 conversion.

In light of the above consideration, in the present work MoS2/SnNb2O6 (hereafter denoted as MoS2/SNO) 2D-2D nanosheet heterojunction was synthesized by a facile hydrothermal process. This typical MoS2/SNO 2D-2D heterojunction exhibited enhanced visible light absorption and improved photogenerated charge carrier separation and transportation efficiency due to the synergistic effect of MoS2 and SNO as well as the unique 2D interfacial structure with the enlarged contact area and abundant active sites. The optimal MoS2/SNO heterojunction showed photocatalytic H2 production rate of 12.9 μmol·h−1, which was up to 4.3-fold and 5-fold as high as that of bare SNO and MoS2.

Section snippets

Materials

Potassium hydroxide (KOH), niobium oxide (Nb2O5), hydrochloric acid (HCl), stannous chloride dihydrate (SnCl2·2H2O), sodium molybdate dihydrate (Na2MoO4·2H2O), thiourea (NH2CSNH2). Deionized water was used in all experiments and commercial chemicals in this work are all of the analytical grade and used without any further purification.

Synthesis of MoS2/SNO heterojunction

The synthesis of SNO nanosheet was based on the process reported previously [26]. The MoS2/SNO 2D-2D nanosheet heterojunctions with different MoS2 contents were

Morphological and structural information

The morphology and structure of the as-prepared samples were assessed by the TEM and SEM measurements. The pristine SNO sample exhibits typical a 2D nanosheet structure with a smooth surface (Fig. 1a), similar to the previously reported results. TEM and SEM images of bare MoS2 indicate a featured microsphere structure which is assembled from many tiny nanosheets (Fig. 1b and h). Fig. 1c–f shows the TEM images of MoS2/SNO heterojunctions with different MoS2 contents. These images show that MoS2

Conclusion

In summary, a MoS2/SNO 2D-2D nanojunction was constructed via a simple hydrothermal method. Compared with the bare MoS2 and SNO nanosheet, the obtained 2D-2D heterojunction exhibited significantly enhanced visible light H2 evolution photocatalytic performance. The hydrogen evolution rate over the optimal 10%-MoS2/SNO 2D-2D heterojunction was up to 4.3-fold and 5-fold as high as that of bare SNO and MoS2. This promoted photocatalytic activity can be mainly ascribed to the enhanced visible light

Acknowledgement

This work was supported by the financial supports of National Nature Science Foundation of China (21606111 and 21878130), Natural Science Foundation of Jiangsu Province (BK20150482), China Postdoctoral Science Foundation (2015 M570409 and 2017 T110453), and Research Foundation for Talented Scholars of Jiangsu University (15JDG054).

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