Distinctive ternary CdS/Ni2P/g-C3N4 composite for overall water splitting: Ni2P accelerating separation of photocarriers

https://doi.org/10.1016/j.apcatb.2019.02.055Get rights and content

Highlights

  • Distinctive ternary CdS/Ni2P/g-C3N4 composite was constructed successfully.

  • CdS/Ni2P/g-C3N4 displayed excellent photocatalytic overall water splitting activity.

  • CdS/Ni2P/g-C3N4 composite had efficient separation efficiency of photocarriers.

  • Ni2P strengthened the electron transfer performance from g-C3N4 to CdS.

Abstract

Efficient overall water splitting over semiconductor photocatalyst is vital but difficult for resolving the energy and environmental crises. In this report, a novel transition metal phosphide Ni2P-based ternary CdS/Ni2P/g-C3N4 composite was constructed for achieving efficient overall water splitting activity under visible light (λ > 420 nm). The H2 and O2 evolution ratios are 15.56 and 7.75 μmol·g–1 h–1 over CdS/Ni2P/g-C3N4 with 3 wt% Ni2P, respectively, which is 4.02 times higher than that of binary type II CdS/g-C3N4. The separation efficiency of photocarriers of CdS/Ni2P/g-C3N4 is enhanced through greatly speeding up the transfer efficiency of electrons from the conduction band of g-C3N4 to that of CdS using Ni2P as electron-bridge, proved by the photoluminescence, transient photocurrent measurements and electrochemical impendence spectroscopy. The significant finding of this paper sheds light on the important role of transition metal phosphide as electron-bridge to connect the two conduction bands of type II semiconductor heterojuction for strengthening the separation efficiency of photocarriers.

Graphical abstract

Distinctive ternary CdS/Ni2P/g-C3N4 photocatalyst achieved outstanding photocatalytic activity for overall water splitting via the Ni2P electron-bridge to separate the photocarriers efficiently.

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Introduction

Solar overall water splitting for H2 and O2 evolution is a fascinating and potential strategy for resolving the energy and environmental crises [[1], [2], [3], [4]]. However, it is not easy to satisfy the potential standards for H2 and O2 generation simultaneously for a pure substance. Traditional electrochemistry method can successfully produce H2 and O2 [5,6], however, large amount of energy also be consumed at the same time. Therefore, solar overall water splitting on the basis of semiconductor photocatalysis technology is very attractive. Nevertheless, only a few photocatalysts are considered as potential research objects, thereinto, CdS and graphite-like carbon nitride (g-C3N4) are the representatives used for the overall water splitting under visible light [7,8]. It should be noted that, the biggest difficulty is still the fast recombination rate of photocarriers, which hinders the enhancement of photocatalytic activity.

As for the pure CdS and g-C3N4 photocatalysts, their photocarriers have low separation efficiency if only relying on the intrinsic inner electric field [9,10]. Thus, novel CdS/g-C3N4 heterojunction with typical type II energy band structure was designed on account of the interfacial effect for photocarriers separation [11,12]. It is well known that the interface resistance always exists during the photocarriers transfer process under light irradiation, which reduces the separation efficiency of photocarriers remarkably. As is reported, noble metals (Au, Ag, Pt) [[13], [14], [15]] with high electrical conductivity are commonly used as “electron-bridge” in an indirect Z-scheme composite system to lead the transfer of electrons from one component to combine with the holes on another component quickly, which reduces the interface resistance vastly between the two components. So, it is very potential to construct an efficient electron-bridge between the conduction bands of CdS/g-C3N4 composite.

Generally speaking, traditional excellent electron-bridge (Au, Ag, Pt) has the basic characteristics of high electrical conductivity. Because noble metals are high expensive, some alternatives including non-noble metals (Cu [16], W [17] and Cd [18]), reduced graphene oxide (RGO) [[19], [20], [21]], C quantum dots [22,23], even surface oxygen defects [24,25] have been developed as electron mediators to improve the separation of photocarriers. As is known to all, transition metal phosphides (TMPs) praised as quasi-platinum catalyst [26] can capture electrons intensively. In this case, TMPs can be applied as one of the potential electron-bridges because they are not only good conductors of electricity and heat, but also they are composed of earth-abundant elements and are inexpensive [27]. Therefore, TMPs have wide industrial applications, such as hydrodesulfurization [28], hydrodenitrogenation [29], electrocatalytic reaction [30] and photocatalytic reaction [31]. TMPs also have extensive potentials in magnetic, photonic, electronic and data storage devices due to their novel magnetic and semiconducting properties [32]. Considering the excellent electrical conductivity, TMPs have great advantages for applying as novel, low price and highly efficient electron-bridge in CdS/g-C3N4 heterojunction system. However, to the best of our knowledge, there are no reports on the application of TMPs as electron-bridge in an overall water splitting system.

In this paper, a distinctive ternary CdS/Ni2P/g-C3N4 composite was constructed, in which the Ni2P was stuck in the middle of CdS and g-C3N4. Under visible light (λ > 420 nm), CdS/Ni2P/g-C3N4 shows outstanding overall water splitting activity for H2 and O2 evolution compared to CdS/g-C3N4. The existence of Ni2P greatly reduced the recombination of photocarriers, proved by the photoluminescence, transient photocurrent measurements and electrochemical impendence spectroscopy. The significant finding of this paper opens a new window for TMPs as novel electron-bridge in type II heterojunction system for overall water splitting application.

Section snippets

Preparation of CdS/Ni2P/g-C3N4

All chemicals with analytical purity were obtained from Sinopharm Chemical Reagent Co., Ltd. and were used without further purification. Deionized water was employed in all experiments.

Ternary CdS/Ni2P/g-C3N4 composite was prepared according to the following 3 steps, as shown in Fig. 1.

Firstly, g-C3N4 was prepared on the basis of the previous work [33]. That is, the g-C3N4 substrate was prepared by directly heating melamine at 550 °C in a crucible with a cover in air for 4 h with a heating rate

XRD analysis

Fig. 2a shows the XRD patterns of the as-prepared samples. It is found that pure g-C3N4 has obvious diffraction peaks at 13.0° and 27.7°, which correspond to the (100) and (002) planes of g-C3N4 with the hexagonal phase structure (JCPDS file No. 87-1526) [36], respectively. Pure CdS also has strong diffraction peaks located at 25.0°, 26.8°, 28.4°, 43.8°, 48.1° and 52.0°, corresponding to the (100), (002), (101), (110), (103) and (112) planes of CdS with the hexagonal phase structure (JCPDS file

Conclusions

In this study, we successfully designed a distinctive ternary CdS/Ni2P/g-C3N4 composite to achieve highly efficient overall water splitting performance. The as-prepared CdS/Ni2P/g-C3N4 composite with 3 wt% Ni2P exhibited the highest H2 and O2 evolution rates with 15.56 and 7.75 μmol·g–1 h–1, respectively, which is 3.10 and 4.02 times higher than those of binary Ni2P/CdS and CdS/g-C3N4. The outstanding activity of CdS/Ni2P/g-C3N4 lies in the important interface electron transfer role of Ni2P as

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (51772118, 51472005), the Anhui Provincial Natural Science Foundation (1708085MB32), State Key Laboratory of Structural Chemistry (20160014) and Anhui Provincial Innovation Team of Design and Application of Advanced Energetic Materials (KJ2015TD003).

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