Article
Rational design of ternary NiS/CQDs/ZnIn2S4 nanocomposites as efficient noble-metal-free photocatalyst for hydrogen evolution under visible light

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Abstract

The NiS/CQDs nanocomposite (CQDs represents carbon quantum dots), with a mass ratio of NiS/CQDs to be 1.19:1 based on the ICP result, was obtained via a facile hydrothermal method from a mixture of CQDs, Ni(OAc)2 and Na2S. The self-assembly of ZnIn2S4 microspheres on the surface of NiS/CQDs was realized under microwave conditions to obtain a ternary NiS/CQDs/ZnIn2S4 nanocomposite. The as-obtained NiS/CQDs/ZnIn2S4 nanocomposite was fully characterized, and its photocatalytic hydrogen evolution under visible light irradiation was investigated. The ternary NiS/CQDs/ZnIn2S4 nanocomposite showed superior photocatalytic activity for hydrogen evolution than ternary CQDs/NiS/ZnIn2S4, which was obtained by deposition of NiS in the preformed CQDs/ZnIn2S4. The superior photocatalytic performance of ternary NiS/CQDs/ZnIn2S4 is ascribed to the introduction of CQDs, which act as a bridge to promote the vectorial transfer of photo-generated electrons from ZnIn2S4 to NiS. This result suggests that the rational design and fabrication of ternary CQDs-based systems are important for the efficient photocatalytic hydrogen evolution. This study provides a strategy for developing highly efficient noble-metal-free photocatalysts for hydrogen evolution using CQDs as a bridge to promote the charge transfer in the nanocomposite.

Graphical abstract

CQDs act as a bridge to promote the charge transfer from ZnIn2S4 to NiS in the ternary NiS/CQDs/ZnIn2S4 nanocomposite.

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Introduction

Semiconductor-based photocatalytic hydrogen evolution can convert and store solar energy in the form of clean hydrogen and is believed to be one of the most promising strategies to tackle the global energy shortage and environmental pollution [1, 2]. Ever since the pioneer work by Fujishima and Honda [3] on a TiO2-based photoelectrochemical cell for hydrogen evolution, great efforts have been devoted to the development of semiconductors for photocatalytic hydrogen evolution. A variety of semiconductor-based photocatalysts have been developed for hydrogen evolution under visible light during the past two decades [4, 5, 6, 7, 8, 9]. Among them, metal sulfides are promising photocatalysts due to their lower band gaps, which fall in the visible light region [2, 10, 11, 12]. As a ternary chalcogenide, hexagonal ZnIn2S4 has attracted extensive interest in photocatalysis owing to its suitable band gap of ca. 2.4 eV and considerable stability [13, 14, 15, 16, 17]. However, the photocatalytic activity for hydrogen evolution over bare ZnIn2S4 is low because of the poor separation efficiency and low migration ability of the photoexcited charge carriers.

For semiconductor-based photocatalytic hydrogen evolution systems, cocatalysts are usually required because cocatalysts can provide catalytic active sites by lowering the overpotential for hydrogen evolution over the semiconductor photocatalysts [11, 18]. Although noble metals are usually used as cocatalysts for hydrogen evolution, their high price and scarcity restrict their practical applications [19]. Recently, using of inexpensive layered transition metal chalcogenides, like MoS2 [20], WS2 [21] and CoS [22], as noble-metal-free cocatalysts to promote the semiconductor-based photocatalytic hydrogen evolution has been extensively studied. As a p-type semiconductor, NiS is well known to be a good electrocatalyst for cathodic hydrogen evolution in water electrolysis [23]. The application of NiS as a cocatalyst to enhance the photocatalytic hydrogen evolution over CdS has been reported [24]. Our previous studies also revealed that NiS can be an appropriate cocatalyst for ZnIn2S4, and the hydrothermally prepared NiS/ZnIn2S4 shows superior photocatalytic activity for hydrogen evolution under visible light [25].

A fast electron transfer from the photo-excited semiconductor to the cocatalyst is important for the efficient photocatalytic hydrogen evolution, which depends not only on the relative band positions of the semiconductor and the cocatalyst, but also on their interface as well as the electronic conductivity of the nanocomposite [26, 27]. It is therefore anticipated that the introduction of a component with high electronic conductivity to promote the charge transfer in the semiconductor-cocatalyst system should enhance its photocatalytic activity for hydrogen evolution. Carbon quantum dots (CQDs), a novel class of nanocarbons, has shown many potentials in a variety of fields, ascribing to their fascinating properties [28, 29, 30, 31, 32]. Due to their good electronic conductivity, CQDs have been used in photocatalysis to fabricate binary semiconductor/CQDs nanocomposites to promote the charge separation in semiconductor-based photocatalysts [33, 34, 35]. It is therefore anticipated that the introduction of CQDs as a bridge into the binary NiS/ZnIn2S4 system would promote the charge transfer from ZnIn2S4 to NiS, which could result in a further improvement of its performance for photocatalytic hydrogen evolution.

Herein, we reported the rational design and fabrication of the ternary NiS/CQDs/ZnIn2S4 nanocomposite as a superior noble-metal-free photocatalyst for hydrogen evolution under visible light. The self-assembly of ZnIn2S4 microspheres in the presence of preformed NiS/CQDs resulted in the efficient ternary NiS/CQDs/ZnIn2S4 nanocomposite for hydrogen evolution (Scheme 1), in which CQDs act as a bridge to promote the electron transfer from photo-excited ZnIn2S4 to NiS. The photocatalytic performance over ternary NiS/CQDs/ZnIn2S4 is much higher than ternary CQDs/NiS/ZnIn2S4, which was obtained via deposition of NiS on the preformed CQDs/ZnIn2S4. This study provides a strategy to promote the photocatalytic hydrogen evolution over semiconductor-based photocatalysts by incorporating CQDs.

Section snippets

Catalyst preparation

All the reagents were commercially available and used without further purification. The CQDs was prepared by heating citric acid according to the literature [36]. For the preparation of NiS/CQDs nanocomposite, a mixture of CQDs (29 mg) and Ni(OAc)2 (56 mg, 0.32 mmol) was dispersed in a degassed N,N-dimethylformamide (DMF) (16 mL). Then 1.6 mL Na2S (0.2 mol/L) aqueous solution was added. Afterwards, the resultant solution was stirred at 90 °C in an oil bath for 4 h. To synthesize NiS/CQDs/ZnIn2S4

Results and discussion

The NiS/CQDs nanocomposite was obtained by a hydrothermal treatment in a mixture of CQDs, Ni(OAc)2 and Na2S. The XRD of the as-obtained product shows characteristic peaks at 30.1°, 34.7°, 46.0° and 53.5° assignable to (100), (101), (102) and (110) crystallographic planes of hexagonal NiS (JCPDS No. 75-0613), indicating the formation of NiS nanoparticles during the hydrothermal treatment (Fig. 1(a)) [37]. The FT-IR spectrum shows two peaks at 1393 and 1583 cm−1, corresponding to symmetric and

Conclusions

In summary, ternary NiS/CQDs/ZnIn2S4 nanocomposites which show superior photocatalytic hydrogen under visible light were obtained via self-assembly of ZnIn2S4 in the presence of preformed NiS/CQDs. The superior photocatalytic performance observed over ternary NiS/CQDs/ZnIn2S4 nanocomposite can be ascribed to the presence of CQDs acting as a bridge to promote the vectorial transfer of the photo-generated electrons from ZnIn2S4 to NiS. This study highlights the great potential of using CQDs for

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    This work was supported by the National Key Basic Research Program of China (973 Program, 2014CB239303), the National Natural Science Foundation of China (21872031, U1705251) and the Award Program for Minjiang Scholar Professorship.

    Published 5 March 2019

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