Photo-generated charges escape from P+ center through the chemical bridges between P-doped g-C3N4 and RuxP nanoparticles to enhance the photocatalytic hydrogen evolution
Graphical abstract
Introduction
Over the past couple of decades, seeking a green and renewable energy source has become a hot research topic to reduce the environmental pollution and climate change caused by the huge consumption of fossil fuel [1]. As a clean, renewable and sustainable energy, hydrogen provides an alternative to replace fossil fuels [[2], [3], [4]].In recent years, photocatalytic water splitting is recognized as one of the most effective strategies for hydrogen production [[5], [6], [7]]. Specifically, in photocatalysis, the photocatalysts absorb solar energy and then generate excited charges carriers. These charge carriers are used to split water to produce hydrogen [8].
A 2D polymer-like metal-free semiconductor, graphitic carbon nitride (g-C3N4) has exhibited its potential in the field of photocatalysts [9,10], due to its dramatic properties, such as narrow band gap (2.7 eV), tunable electronic structure, low cost, good physical-chemical stability and convenient preparation [11,12]. Although g-C3N4 has so many advantages, using pristine g-C3N4 as a photocatalyst to produce hydrogen is still a huge challenge in the field of photocatalysis [13]. The weak absorption of the visible light and fast recombination of photo-generated charge carriers seriously limit the photocatalytic performance of pure g-C3N4 [14].
To address these problems, metal phosphides have been reported as co-catalysts to improve the performance of the photocatalysts, such as Ni2P [15,16], CoP [17,18], FeP [19], NiFeP [20] and Ru2P [21], which resulted in the fast separation and transfer rate of photo-generated electrons. Recently, ruthenium phosphide nanoparticles gradually stand out in the field of hydrogen evolution [[22], [23], [24]]. It exhibited its potential photocatalytic promotion ability. In addition, many studies have shown that phosphorus doping can also affect the photocatalytic activity [25]. Specifically, the incorporation of P atoms significantly alters the electronic, surface chemical, and properties of different semiconductors [26]. However, excessive phosphorus doping can lead to the accumulation centers of photo-generated charges. Meanwhile, amounts of stacked photo-generated charges recombine at the charged center, which limits the performance of photocatalyst.
Based on the above considerations, herein, we designed a unique photocatalytic system (RuxP/PCN) which completely phosphatized ultrafine ruthenium coupled with g-C3N4 nanosheets. Compared with other samples, the prepared RuxP/PCN showed the highest rate of photocatalytic hydrogen evolution reaction (HER). Specifically, P+ centers were introduced by excessive P doping in the g-C3N4, which were the stacking and recombination centers of the photo-generated charge carriers. After the formation of RuxP nanoparticles (RuxP NPs), the chemical bridge was stablished between PCN and RuxP NPs. The stacking photo-generated charges in P+ centers were easier transferred to the surface and separated on RuxP NPs. In addition, this way could prolong the lifetime of the photo-generated carriers and boost the photocatalytic hydrogen evolution. From the further study of these changes of photo-generated charge carriers, we could explore how excessive phosphorus doping and phosphides act on this photocatalytic system. This provides a novel strategy and mechanism in the design of photocatalysts.
Section snippets
Synthesis of g-C3N4 nanosheets (CN)
Through heating the mixture of dicyandiamide and ammonium chloride (mass ratio of 1:5) at 530 ℃ for 3 h with a heating rate of 2 ℃ min−1 in a muffle furnace. g-C3N4 nanosheets were successfully fabricated [27].
Synthesis of Ru/CN
500 mg as-prepared CN was first suspended in 50 mL of ethylene glycol (EG), and then sonicated for 1 h to obtain uniform solution. 5 mL of 1 mg mL−1 RuCl3·3H2O was added into solution and the mixture was magnetically stirred for 3 h. Next, the solution temperature was increased to 120 ℃
Structural characterization for photocatalysts
The overall formation procedure of RuxP/PCN nanocomposites are schematically illustrated in Fig. 1. Firstly, the g-C3N4 nanosheets were prepared through heating the mixture of melamine and ammonium chloride. Then, Ru/CN was obtained by simultaneous wet chemical reduction of Ru3+ cations in the presence of sodium borohydride and g-C3N4 nanosheets. After that, the as-prepared Ru/CN were post-phosphatized in a tube furnace using sodium hypophosphite as the phosphorus source. Specifically, during
Conclusions
To sum up, a novel strategy combining P-doping and RuxP NPs to synthesize an efficient hybrid photocatalyst of RuxP/PCN has been proposed. By measuring photocatalytic hydrogen evolution efficiencies of the related samples, RuxP/PCN displayed a relatively high hydrogen evolution rate of 1.94 mmolg−1 h−1 under 300 W Xenon light fitted with AM 1.5 filter. To realize how excessive P-doping reacted in the photocatalytic system, we compared the hydrogen evolution performance of Ru/CN (1.42 mmol g−1 h
Declaration of Competing Interest
We declare that we have no conflict of interest.
Acknowledgements
This work was supported by National Natural Science Foundation of China (5171101651, 21811540394, 21972040), Shanghai Municipal Science and Technology Major Project (Grant No.2018SHZDZX03) and the Programme of Introducing Talents of Discipline to Universities (B20031, B16017).
References (47)
- et al.
Rational design of carbon-doped TiO2 modified g-C3N4 via in-situ heat treatment for drastically improved photocatalytic hydrogen with excellent photostability
Nano Energy
(2017) - et al.
g-C3N4/CoAl-LDH 2D/2D hybrid heterojunction for boosting photocatalytic hydrogen evolution
Int. J. Hydrogen Energy
(2020) - et al.
Graphitic carbon nitride (g-C3N4) nanocomposites: a new and exciting generation of visible light driven photocatalysts for environmental pollution remediation
Appl. Catal. B: Environ.
(2016) - et al.
Perovskite oxide ultrathin nanosheets/g-C3N4 2D-2D heterojunction photocatalysts with significantly enhanced photocatalytic activity towards the photodegradation of tetracycline
Appl. Catal. B: Environ.
(2017) - et al.
In-situ phosphating to synthesize Ni2P decorated NiO/g-C3N4 p-n junction for enhanced photocatalytic hydrogen production
Chem. Eng. J.
(2019) - et al.
Metalloid Ni2P and its behavior for boosting the photocatalytic hydrogen evolution of CaIn2S4
Int. J. Hydrogen Energy
(2018) - et al.
Dual enhancement of capturing photogenerated electrons by loading CoP nanoparticles on N-deficient graphitic carbon nitride for efficient photocatalytic degradation of tetracycline under visible light
Sep. Purif. Technol.
(2020) - et al.
Highly efficient photocatalytic hydrogen evolution from 0D/2D heterojunction of FeP nanoparticles/CdS nanosheets
Appl. Surf. Sci.
(2020) - et al.
Electron directed migration cooperated with thermodynamic regulation over bimetallic NiFeP/g-C3N4 for enhanced photocatalytic hydrogen evolution
Appl. Catal. B: Environ.
(2019) - et al.
Robust photocatalytic hydrogen evolution over amorphous ruthenium phosphide quantum dots modified g-C3N4 nanosheet
Appl. Catal. B: Environ.
(2018)