Single Atomically Anchored Cobalt on Carbon Quantum Dots as Efficient Photocatalysts for Visible Light-Promoted Oxidation Reactions

Xianjun Tu,Hongbo Liu,Miao Shu,Kai A I Zhang,Run Li,Calum T J Ferguson,Qin Wang,Jin Li,Rui Si

doi:10.1021/acs.chemmater.9b03708

Abstract

Generation of efficient light-induced charge separation inside the photocatalyst is an essential factor for a high catalytic efficiency. The usual immobilization of metal or metal oxide particles on semiconductor photocatalysts offers an uncontrolled assembly of active sites during the reaction. The introduction of single metal atoms on photocatalysts can lead to extremely high atomic utilization and precise active sites. However, this approach is limited because of the lack of suitable photosensitizers for single atom immobilization. Here, we have designed photocatalytic carbon quantum dots with anchoring sites for single cobalt atoms in a defined Co–N4 structure via facile pyrolysis of vitamin B12. Carbon dots functioned as both light-harvesting antenna and support for the cobalt atom with high atom loadings up to 3.27 wt %. This new photocatalytic material demonstrated enhanced visible light absorption, efficient charge separation, and reduced electrochemical impedance, while single Co atoms acted as the active site with strong oxidative ability. As a result, the photocatalysts showed excellent visible light-promoted photocatalytic efficiency with oxygen evolution rates up to 168 μmol h–1 g–1 via water oxidation, imine formation with high conversion (∼90%) and selectivity (>99%), and complete photodegradation of organic dyes.

Highlights

The efficiency of photocatalysts can be largely increased by enhancing the efficient light-induced charge separation inside the photocatalyst material
A vast number of catalyst materials and their modification methods have been explored.[1−3] Recently, carbon quantum dots (CQDs) have emerged as a promising photocatalyst candidate for solar-toenergy conversion, due in part to their ease of preparation, low cost, excellent stability, high quantum yield, nontoxicity, high aqueous affinity, and visible light absorption.[4−7] Recently, CQDs have been employed for energy-relevant applications as hydrogen evolution[8,9] or photocatalytic chemical transformations including epoxide ring opening;[10] reductive dehalogenation;[11] and the oxidation of amines,[12] benzyl alcohol[13] and 1,4dihydro-2,6-dimethylpyridine-3,5-dicarboxylate,[14] and so forth
The carbon precursor was obtained by pyrolysis of vitamin B12 (VB12) as a convenient approach to synthesize single cobalt atomdecorated carbon material similar to previous reports.[29,30]

Summary

■ INTRODUCTION

The efficiency of photocatalysts can be largely increased by enhancing the efficient light-induced charge separation inside the photocatalyst material. Single atom catalysts (SACs) have become a new active frontier in heterogeneous catalysis as they produce the highest possible per metal atom efficiency They can be utilized to produce desired products with excellent selectivity and conversion.[18−20] Unlike in classical metal catalysis, reducing the metal to single atoms enhances the number of available catalytic sites. Host materials mainly focus on g-C3N4.25−28 Single Fe, Pt, and Co atoms have been anchored onto g-C3N4, with improved photocatalytic efficiency for pollutant degradation,[25] hydrogen evolution,[26] water splitting,[27] and CO2 reduction[28] observed. To the best of our knowledge, this is the first case for CQD being employed as the host material for SACs. The obtained photocatalyst demonstrated enhanced visible light absorption and charge separation efficiency. The CQD-based photocatalysts anchored with single Co atoms showed high stability in all three oxidation reactions

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES