Solar Hydrogen Production Using Carbon Quantum Dots and a Molecular Nickel Catalyst

Benjamin C M Martindale,Erwin Reisner,Christine A Caputo,Georgina A M Hutton

doi:10.1021/jacs.5b01650

Benjamin C M Martindale, Erwin Reisner + Show 2 more

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https://doi.org/10.1021/jacs.5b01650

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Abstract

Carbon quantum dots (CQDs) are established as excellent photosensitizers in combination with a molecular catalyst for solar light driven hydrogen production in aqueous solution. The inexpensive CQDs can be prepared by straightforward thermolysis of citric acid in a simple one-pot, multigram synthesis and are therefore scalable. The CQDs produced reducing equivalents under solar irradiation in a homogeneous photocatalytic system with a Ni-bis(diphosphine) catalyst, giving an activity of 398 μmolH2 (gCQD)(-1) h(-1) and a "per Ni catalyst" turnover frequency of 41 h(-1). The CQDs displayed activity in the visible region beyond λ > 455 nm and maintained their full photocatalytic activity for at least 1 day under full solar spectrum irradiation. A high quantum efficiency of 1.4% was recorded for the noble- and toxic-metal free photocatalytic system. Thus, CQDs are shown to be a highly sustainable light-absorbing material for photocatalytic schemes, which are not limited by cost, toxicity, or lack of scalability. The photocatalytic hybrid system was limited by the lifetime of the molecular catalyst, and intriguingly, no photocatalytic activity was observed using the CQDs and 3d transition metal salts or platinum precursors. This observation highlights the advantage of using a molecular catalyst over commonly used heterogeneous catalysts in this photocatalytic system.

Highlights

Efficient and inexpensive photocatalytic water splitting is a major focus of research toward solar energy conversion and storage.[1−4] Photocatalytic H2 production from water requires the efficient coupling of light harvesting and charge transfer with catalytic processes
While exhibiting promising light-harvesting and electron transfer properties,[34,35] Carbon quantum dots (CQDs) have been underexplored in photocatalytic applications and have almost exclusively been used in conjunction with another photosensitizer and for photodegradation of organic dyes.[36−38] The first use of CQDs as a primary photosensitizer for fuel production was for the photoreduction of carbon dioxide to formic acid with polyethylene glycol-passivated CQDs modified with deposits of the noble metals Au or Pt.[39,40]
The exclusive use of cheap and abundant starting materials such as citric acid, a low-cost natural product and commodity chemical produced by fermentation on the megaton scale, as well as the low operating temperature makes the synthesis of CQDs highly scalable and environmentally sustainable

Summary

■ INTRODUCTION

Efficient and inexpensive photocatalytic water splitting is a major focus of research toward solar energy conversion and storage.[1−4] Photocatalytic H2 production from water requires the efficient coupling of light harvesting and charge transfer with catalytic processes. Perovskites based on lead have attracted recent attention as efficient light absorbers in solar cells and photoelectrochemical cells,[1] but they are toxic and highly unstable in water and so are currently unsuitable for aqueous systems.[14,15] Graphitic carbon nitride (CNx) has been put forward as a heterogeneous photosensitizer; visible light absorption and interfacial charge transfer limits its efficacy.[7,16,17] the hydrophobicity of these organic materials often restricts their application in aqueous solution and synthetic procedures require thermal condensation at high temperatures (400−600 °C) from a limited range of nitrogencontaining precursors such as cyanamide, dicyandiamide, or melamine.[16] Despite this recent progress, there remains a need to develop cheap, robust, and efficient photosensitizer materials for artificial photosynthetic systems. While exhibiting promising light-harvesting and electron transfer properties,[34,35] CQDs have been underexplored in photocatalytic applications and have almost exclusively been used in conjunction with another photosensitizer and for photodegradation of organic dyes.[36−38] The first use of CQDs as a primary photosensitizer for fuel production was for the photoreduction of carbon dioxide to formic acid with polyethylene glycol-passivated CQDs modified with deposits of the noble metals Au or Pt.[39,40] Here, we report a solar H2 production system using CQDs as the sole photosensitizer in combination with a molecular Ni catalyst − the first such use of CQDs in a homogeneous photocatalytic system in combination with a molecular catalyst (Figure 1)

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES