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MoS2/Zn3In2S6 composite photocatalysts for enhancement of visible light-driven hydrogen production from formic acid

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Abstract

Enhancing the separation efficiency of photogenerated carriers is propitious for the promotion of photocatalytic hydrogen production from formic acid decomposition. Herein, MoS2/Zn3In2S6 (MoS2/ZIS6) composite photocatalysts containing varying mass percentages of MoS2 were obtained by a straightforward synthetic method. The results confirmed that MoS2, as a cocatalyst, markedly promoted the photogenerated charge separation efficiency and visible light-driven hydrogen production activity of ZIS6 (λ > 400 nm). Specifically, the as-prepared 0.5% MoS2/ZIS6 photocatalyst exhibited the highest photocatalytic hydrogen production rate (74.25 µmol·h−1), which was approximately 4.3 times higher than that of ZIS6 (17.47 µmol·h−1). The excellent performance of the 0.5% MoS2/ZIS6 photocatalyst may be due to the fact that MoS2 has a low Fermi energy level and can thus enrich photogenerated electrons from ZIS6, and furthermore reduce H+ derived from formic acid, to form hydrogen. The structure and morphology of the MoS2/ZIS6 photocatalysts and the reactive species were determined by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy, among others; a plausible mechanistic rationale is discussed based on the results.

Graphical Abstract

A novel MoS2/Zn3In2S6 (MoS2/ZIS6) heterojunction photocatalyst was constructed with matching energy band potentials; it exhibited excellent performance in the excitation and transfer of photogenerated carriers and consequently, in visible-light-driven hydrogen production from formic acid.

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Introduction

Formic acid (FA) is rapidly becoming a potential hydrogen storage material due to its non-toxic, low cost, non-flammable characteristics, and high hydrogen content [1, 2]. As visible light accounts for approximately 43% of the solar spectrum, utilizing clean energy-visible light to drive hydrogen production from formic acid would be an effective approach for ameliorating the energy crisis [3, 4]. Therein, the development of efficient visible light catalysts is of crucial importance. Hitherto, numerous active photocatalysts, such as phosphides, nitrides, sulfides, metal oxides and so on [5, 6, 7, 8, 9, 10, 11, 12], have been developed and applied in the field of visible light photocatalysis for hydrogen production and other areas. However, the majority of these catalysts suffer from shortcomings such as poor light absorption, light corrosion, deactivation, or poor stability [13, 14, 15, 16, 17, 18]. In addition, the development of semiconductor photocatalysts faces significant challenges regarding optical absorption capacity, stability, low toxicity, and other factors [19]. Therefore, research into the development of efficient visible light-related semiconductor photocatalysts for FA photocatalytic hydrogen production is prolific and ongoing [20, 21, 22].

The key to achieving the above-mentioned objective is efficient photogenerated charge separation and transfer during the process of photocatalysis, which leads to an improvement in the catalyst’s photocatalytic performance. In recent years, ZIS6 has rapidly become a “photocatalyst semiconductor star”, due to characteristics such as strong visible light absorption, chemical stability, and low environmental impact [23, 24, 25, 26]. More importantly, the nanosheet structure of ZIS6 facilitates the formation of a heterojunction between it and other catalysts that can promote the separation of photo-generated carriers, thereby increasing its photocatalytic activity [27]. It is currently accepted that noble metals (such as Pt [28, 29], Au [30, 31], Rh [32], and Pd [33, 34]) can generally be applied as cocatalysts to enhance the separation efficiency of photoelectron-hole pairs. However, the limitation of noble metals is their high cost. Thus, the development of noble metal-free cocatalysts remains a considerable challenge for photocatalytic hydrogen production.

MoS2 has attracted considerable attention due to its excellent ability to promote photocatalytic hydrogen production, as well as its accessibility and high chemical stability. Numerous previous reports confirm that coupling MoS2 with a second semiconductor is an effective approach for enhancing the performance of semiconductor photocatalysts [35, 36, 37, 38]. For example, MoS2/ZnIn2S4 composite photocatalysts, synthesized by Li and co-workers, exhibited remarkable photocatalytic activity as a result of MoS2 loading. However, the synthesis of MoS2 involved numerous unfavorable procedures, such as the necessity for elevated temperatures, lengthy solvothermal treatment, and the use of toxic H2S gas, all of which limit the applicability of MoS2 [39].

In this study, the preparation of MoS2/ZIS6 samples was achieved by a straightforward hydrothermal method, and their photocatalytic activity was investigated under visible light irradiation. The MoS2/ZIS6 (0.5%) catalyst exhibited outstanding photocatalytic performance in the FA hydrogen production system. In addition, morphology effects and MoS2/ZIS6 photocatalyst properties were systematically studied, revealing key structural features of MoS2/ZIS6. On the basis of the obtained data, a reaction mechanism of the promoted photocatalytic hydrogen production is proposed. It is noteworthy that the MoS2 introduced into the ZIS6 photocatalyst can serve as an efficient electronic enrichment center, enhancing the separation efficiency of photogenerated charges, thereby promoting the photocatalytic performance of ZIS6 photocatalysts.

Section snippets

Sample preparation

The synthesis of MoS2/ZIS6 was performed according to a literature procedure [40], and adjusted accordingly. Typically, InCl3·4H2O (2 mmol), ZnSO4·7H2O (3 mmol), appropriate amounts of C2H5NS and (NH4)6Mo7O24·4H2O were placed into a 100 mL Teflon-lined autoclave filled with 70 mL deionized water as the solvent and stirred for 60 min. The autoclave was sealed and maintained at 160 °C for 12 h, and then allowed to cool to room temperature. The resultant precipitate was alternately and thoroughly

X-ray diffraction studies

XRD was applied to investigate the effect of MoS2 on the crystallographic structure and crystallinity of the as-prepared photocatalysts. The comparison results of pure MoS2, ZIS6, and a series of MoS2/ZIS6 composite photocatalysts are shown in Fig. 1. The diffraction patterns of all the composite photocatalysts were analogous, and indexed to a peak of ZIS6 (JCPDS card No. 65-4003). Although the MoS2/ZIS6 composite photocatalysts contain varying amounts of MoS2, no peak of MoS2 can be seen,

Conclusions

A straightforward one-pot hydrothermal method was successfully implemented for the preparation of a MoS2/ZIS6 composite photocatalyst, which was then applied to hydrogen production from FA, exhibiting excellent efficiency; the related charge transfer mechanism of this process was additionally discussed. Under visible light irradiation, the majority of synthesized MoS2/ZIS6 composite photocatalysts exhibited superior photocatalytic performance compared to pure ZIS6 and MoS2. The 0.5% MoS2/ZIS6

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    This work was supported by the National Natural Science Foundation of China (51972134, 51472005, 51772118), the Natural Science Foundation of Anhui Province for Distinguished Young Scholars (1808085J24), the Project of Anhui Province for Excellent Young Talents in Universities (gxyq2019029), and the Natural Science Foundation of Educational Committee of Anhui Province (KJ2019A0602, KJ2018A0387).

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