Elsevier

Applied Catalysis B: Environmental

Volume 230, 15 August 2018, Pages 210-219
Applied Catalysis B: Environmental

Cobalt phosphide nanowires as efficient co-catalyst for photocatalytic hydrogen evolution over Zn0.5Cd0.5S

https://doi.org/10.1016/j.apcatb.2018.02.043Get rights and content

Highlights

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    DFT is utilized to guide the application of CoP nanowires as a co-catalyst for photocatalytic hydrogen production.

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    CoP nanowires were prepared by a facile hydrothermal and calcination method.

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    The Zn0.5Cd0.5S/CoP NWs photocatalyst has excellent activity in water splitting with good stability.

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    Time-resolved PL decay spectra and photoelectrochemical methodology were used to study the effects of morphology.

Abstract

Previous studies have shown that co-catalysts play a pivotal role for improving both the activity and reliability of semiconductors in photocatalytic hydrogen production, however, designing highly efficient and cost-effective co-catalysts to replace expensive and rare metals is still a big challenge. In this work, DFT (density functional theory) is utilized to guide the application of CoP NWs (nanowires) as an earth-abundant co-catalyst for photocatalytic hydrogen production. Metallic 1D CoP NWs is rationally integrated with Zn0.5Cd0.5S solid solution semiconductor for the first time, to induce a remarkably improved photocatalytic hydrogen production activity of 12,175.8 μmol h−1 g−1, which is 22 times higher than that of the pristine Zn0.5Cd0.5S. This outstanding activity benefits from the collaborative advantages of excellent metallic conductivity and the rigid 1D nanostructure of CoP NWs. Moreover, the mechanism investigations demonstrate that this excellent activity arises from the strong electronic coupling, favourable band structure, highly efficient charge separation and migration based on the powerful characterizations, such as time-resolved PL decay spectra and photoelectrochemical methodology. This work brings new opportunities to employ 1D co-catalysts on photocatalysts for improving the catalytic activities in hydrogen production from water.

Introduction

Due to the zero carbon content, hydrogen has been considered to be a promising energy supply for solving global energy problems [1]. Especially, photocatalytic water splitting by using semiconductors has demonstrated great potential as a promising strategy for producing clean and carbon-neutral hydrogen fuel since the photoelectrochemical water splitting was discovered in 1972 by Honda and Fujishima [2]. To date, although kinds of impressive semiconductors have been developed [3], it still require great efforts to explore highly efficient, cost-effective and stable photocatalysts driven by natural light. Recent studies have shown that co-catalysts play a significant role in improving both the activity and stability of semiconductor photocatalysts. Currently, noble-metal-based co-catalysts including Ru [4], Rh [5], Pd [6], Pt [[7], [8], [9]], Au [10] and Ag [11] have been extensively investigated for photocatalytic hydrogen production. Unfortunately, the above noble-metal based co-catalysts are too rare and expensive, and the commercialization of current photocatalysts has been seriously restricted. Therefore, seeking noble-metal free and earth-abundant co-catalysts with high efficiency and low cost is of great significance for large-scale energy production.

To date, various earth-abundant co-catalysts have been employed to combine with photocatalysts, including transition metals, transition metal compounds (e.g. transition metal oxides, hydroxides, sulfides, carbides and phosphides) and nanocarbon-based co-catalysts [2,[12], [13], [14], [15], [16], [17], [18]]. However, most of the co-catalysts are 0D particles. Although the employment of 0D co-catalysts can actually enhance the photocatalytic performance, these particles are difficult to homogeneously disperse on the surface of photocatalysts, and the 0D nanoparticles have a tendency to agglomerate and some large clusters with irregular agglomerate are universal. In this context, targeted engineering of co-catalysts with different structure and morphology has obtained great attention, because the microscopic structural and morphology factors have synergistic impacts on the photocatalysts. Therefore, considering the basic mantra of “structure-dictates-function” in chemistry, it is urgent to design efficient co-catalysts with desirable architectural structure/morphology toward photocatalytic hydrogen production.

In view of high surface-to volume ratio and excellent electron transport property, one dimensional (1D) nanostructured materials are believed to play an important role in the next-generation building blocks for electronic devices, solar cells, photocatalysis and lithium-ion batteries. CoP NWs (nanowires), an important metal phosphides with good electrical conductivity, has shown great potential as electrocatalyst in hydrogen evolution reaction [18,19]. Notably, the distinguished characteristics of CoP NWs render them highly promising for photocatalytic hydrogen production: (i) the smaller overpotential of CoP NWs is beneficial to hydrogen production reactions on the surface of photocatalysts; (ii) the excellent metallic conductivity of CoP NWs assures efficient charge-carrier transfer; (iii) the acid-stability of CoP NWs is beneficial to hydrogen production in acid aqueous solution. Given the outstanding properties of CoP NWs, it can be predicted that CoP NWs will be a promising co-catalyst to be applied in photocatalysis. However, to the best of our knowledge, there is no exploring CoP NWs as a co-catalyst for photocatalytic hydrogen production.

Herein, we report a highly stable and inexpensive photocatalytic system with CoP NWs as the co-catalyst, Zn0.5Cd0.5S solid solution as the photocatalyst and ascorbic acid as the electron donor. And an excellent photocatalytic hydrogen production activity was obtained from CoP NWs system, whose hydrogen production performance was much higher than CoP NPs system, which was studied by both theoretical and experimental studies.

Section snippets

Preparation of Co(CO3)0.35Cl0.20(OH)1.10, Co3O4 and CoP nanowires

All chemicals were of analytical grade and were directly used as received without further purification. Co(CO3)0.35Cl0.20(OH)1.10 nanowires were prepared according to reported method with minor modifications [19]. Typically, 5 mmol CoCl2·6H2O and 5 mmol urea were dissolved in 40 mL distilled water under stirring. The solution was transferred to a 100 mL Teflon-lined autoclave and the autoclave was sealed and maintained at 100 °C for 12 h in an electric oven. After the autoclave cooled down

Theoretical and experimental exploration of CoP NWs as a co-catalyst

CoP NWs was obtained by a low-temperature phosphidation of Co3O4 NWs, which was acquired by calcinating a hydrothermally synthesized Co(CO3)0.35Cl0.20(OH)1.10, as illustrated in Scheme 1. Firstly, CoCl2 and the morphology inducer, CO(NH2)2, were dissolved in water and tightly coupled. After hydrothermal reaction at 100 °C, the purple cobalt precursor, Co(CO3)0.35Cl0.20(OH)1.10, formed. Then, the cobalt precursor was calcinated at 400 °C, and during this process, Co(CO3)0.35Cl0.20(OH)1.10 was

Conclusions

In summary, we have successfully prepared CoP nanowires through an organic solvent-free low-temperature phosphidation reaction as a high efficient co-catalyst to photocatalytic hydrogen production. On the basis of the theoretical and experimental results, 1D CoP nanowires have more outstanding features than bulk CoP nanoparticles, including larger BET specific surface area, excellent metallic conductivity and the rigid 1D nanostructure. Thus, we compounded Zn0.5Cd0.5S solid solution and CoP NWs

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21677080 and 21722702), the Ministry of Education, People’s Republic of China as an innovative team rolling project (IRT_17R58) and a 111 program (T2017002), special funds for basic scientific research services of central colleges and universities. Natural Science Foundation of Tianjin (Grant No. 15JCYBJC48400, 15JCZDJC41200, 16YFXTSF00440, 16ZXGTSF00020 and 16YFZCSF00300).

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