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Enhanced visible light activated hydrogen evolution activity over cadmium sulfide nanorods by the synergetic effect of a thin carbon layer and noble metal-free nickel phosphide cocatalyst

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Abstract

Photocatalytic water splitting is considered to be a promising strategy for addressing the global energy crisis through the expanded use of solar energy. Herein, cadmium sulfide (CdS) nanorods modified with a thin conductive carbon layer and a nickel phosphide co-catalyst, referred to as cadmium sulfide coated with a carbon layer and nickel phosphide (CdS@C/Ni2P, where @ indicates a core–shell structure), were synthesized and applied as a novel composite photocatalyst for water splitting. The optimized CdS@C/Ni2P composite showed a high photocatalytic hydrogen generation rate of 32030 μmol h−1 g−1, which was approximately 19 times as high as that of pure CdS. We believed that the thin carbon layer acted as an electron acceptor to promote charge transfer and protect the CdS nanorods from photocorrosion. In addition, the surface loading of the nickel phosphide (Ni2P) cocatalyst was able to further draw photogenerated electrons from the cadmium sulfide coated with a carbon layer (CdS@C) heterojunction and provide active sites for hydrogen evolution. Thus, greatly enhanced hydrogen generation was achieved through a combination of carbon coating and surface cocatalyst loading. This development provides a new way to design composite photocatalysts with multiple junctions for efficient water splitting performance.

Graphical abstract

Cheap and nontoxic carbon and nickel phosphide were used to modification the CdS nonorods. The synthesized CdS@C-Ni2P shows excellent hydrogen evolution activity and stability.

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Introduction

The depletion of fossil resources and the deepening crisis of environmental pollution from their consumption have motivated considerable effort to develop sustainable technologies [1], [2], [3], [4], [5], [6]. Converting solar energy into hydrogen fuel is considered to be a promising way to generate clean and renewable energy [7], [8], [9], [10], [11], [12], [13], [14]. In particular, solar-driven photocatalytic water splitting has received considerable attention owing to the high product purity generated by this simple process [15], [16], [17]. To date, many different semiconductor based photocatalysts have been developed and studied for solar-driven hydrogen generation [18], [19], [20]. The design of inexpensive photocatalysts with good activity and stability is considered to be essential to the conversion process.

Visible light driven cadmium sulfide is regarded as a promising material for photocatalytic hydrogen evolution among various photocatalysts because of its superior characteristics, such as a suitable band gap (∼2.4 eV) and appropriate potential conduction band [21], [22], [23]. Unfortunately, bare CdS is not yet ready for practical applications owing to the relatively low efficiency of electron-hole separation and a tendency to undergo photocorrosion under irradiation [24], [25]. A thin layer coated on the host photocatalyst can often facilitate the separation of light-induced electron-hole pairs. For example, Tang et al. showed that the intimate coaxial interfacial contact between a MoS2 shell and CdS core facilitated the transfer of photoexcited electrons to MoS2 [26]. As a promising material widely applied in the energy conversion field, carbon can also serve as a coating layer owing to its unique features, such as high conductivity, low-toxicity, and good chemical stability [27], [28], [29]. Recently, it was reported that carbon coated cuprous oxide nanorods showed enhanced photocatalytic performance, attributed to the carbon layer facilitating charge transfer [30]. Considering the remarkable properties of carbonaceous materials and the limitations of CdS photocatalytic systems, coating CdS with a thin carbon layer might be a suitable strategy for constructing efficient photocatalytic systems.

Active sites play an important role in the process of photocatalytic hydrogen production. Currently, noble-metal-based co-catalysts have been widely applied in photocatalytic systems to provide active sites for catalytic hydrogen evolution [31], [32], [33]. However, noble metals are rare and expensive. Therefore, there is a need to find noble-metal free co-catalysts with high efficiency. Of non-precious earth-abundant co-catalysts, transition metal phosphides have attracted considerable attention and have been confirmed to be suitable co-catalysts which can provide active sites to promote the hydrogen evolution of semiconductors [34], [35]. For example, Xu et al. reported that Ni2P can act as a highly efficient co-catalyst when loaded on the surface of carbon nitride [36].

In order to combine all the advantages mentioned above and improve the photocatalytic performance of CdS, we designed and synthesized a highly efficient ternary photocatalyst system in which a carbon layer and Ni2P nanopaticles were used to modify CdS nanorods. The obtained hydrogen production yield reached 32030 μmol h−1 g−1, which was 19 times as high as that of pristine CdS. The carbon layer coated on the CdS nanorods featured high conductivity and good chemical stability, providing electron transport pathways and maintaining the stability of host photocatalyst. Furthermore, surface modification with Ni2P nanoparticles further improved the charge carrier separation and provided an abundance of active sites. Thus, excellent photocatalytic performance and stability were realized from the well-designed CdS@C/Ni2P semiconductor composites.

Section snippets

Materials

All chemicals were purchased and used without further purification. Cadmium chloride (CdCl2·2.5H2O, Sinopharm Chemical Reagent Co., Ltd, AR, ≥99.0%), thiourea (NH2CSNH2, Sinopharm Chemical Reagent Co., Ltd, AR, ≥99.0%), ethylenediamine (C2H8N2, Sinopharm Chemical Reagent Co., Ltd, AR, ≥99.0%), ascorbic acid (C6H8O6, Sinopharm Chemical Reagent Co., Ltd, AR, ≥99.7%), nickel nitrate hexahydrate [Ni(NO3)·6H2O, Sinopharm Chemical Reagent Co., Ltd, AR, ≥98.0%], sodium hydroxide (NaOH, Sinopharm

Results and discussion

Typical X-ray Diffraction (XRD) patterns recorded from CdS@C/Ni2P composites with different Ni2P contents along with CdS and CdS@C are shown in Fig. 1. The diffraction patterns of the pure CdS could be assigned to well-crystallized hexagonal wurtzite CdS (PDF# 65-3414) [39], [40]. The main peaks centered at 24.8°, 26.5°, 28.2°, and 43.8° corresponded to the (1 0 0), (0 0 2), (1 0 1), and (1 1 0) planes, respectively. The CdS@C exhibited an almost identical XRD pattern to that of CdS, likely

Conclusions

In summary, we successfully constructed a new noble metal free CdS@C/Ni2P photocatalyst for hydrogen production through water splitting. The ternary composites exhibited much higher photocatalytic performance for hydrogen generation than those of CdS. The photocatalytic hydrogen evolution rate of the optimized CdS@C/Ni2P composite was as high as 32030 μmol h−1 g−1. Based on the discussion in previous chapters, the improvement of the photocatalytic activity was also higher compared with other

Acknowledgements:

We are grateful for grants from National Science Funds for Creative Research Groups of China (No. 51421006), Natural Science Foundation of China (51679063), the Key Program of National Natural Science Foundation of China (No. 91647206), the National Science Foundation of China for Excellent Young Scholars (No. 51422902), the National Key Plan for Research and Development of China (2016YFC0502203), Fundamental Research Funds (No. 2016B43814), and PAPD.

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