Elsevier

Catalysis Today

Volume 380, 15 November 2021, Pages 223-229
Catalysis Today

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

https://doi.org/10.1016/j.cattod.2020.12.037Get rights and content

Highlights

  • RuxP/PCN photocatalyst which completely phosphatized ultrafine ruthenium coupled with g-C3N4 nanosheets were prepared.

  • P+ centers were formed with the excessive doping P elements to accumulate photo-generated carriers.

  • Photo-generated charge carriers were separated and transferred through the chemical bridge between PCN and RuxP NPs.

  • The prepared sample showed a high photocatalytic activity for photocatalytic hydrogen evolution.

Abstract

Many researches have shown that phosphorus doping can affect the photocatalytic activity, mainly because the incorporation of P atoms significantly alters the electronic, surface chemical, and properties of different semiconductors. However, excessive phosphorous doping will form the accumulation and recombination center of photo-generated charge carrier, thereby limiting the activity of photocatalytic reactions. In this work, we successfully prepared excessive P-doping g-C3N4 with ruthenium phosphide nanoparticles (RuxP/PCN), which had excellent performance of hydrogen evolution (1.94 mmol g−1 h−1). Specifically, C was replaced by P in the melon units of PCN and positive charge center (P+) was introduced, reinforcing the chemical connection between PCN and RuxP. In fact, excessive P+ centers became the traps and stacking centers of photo-generated carriers. However, due to the introduction of RuxP NPs, the accumulated charges in the P+ centers through the chemical bridges between PCN and RuxP NPs migrated to surface and then separated on RuxP NPs. Our research illustrates the mechanism of the accumulated charges caused by excessive phosphorus doping migrate to the surface and separate on phosphides, which can prolong the lifetime of photo-generated carriers. This result promoted the significant increase of photocatalytic hydrogen production activity. Our research clarifies the mechanism of excessive phosphorus doping and phosphides act on this photocatalytic system, which will provide a new way to design a photocatalytic system with higher HER performance.

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).

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