Abstract
Covalently linking photosensitizers and catalysts in an inorganic-organic hybrid photocatalytic system is beneficial for efficient electron transfer between these components. However, general and straightforward methods to covalently attach molecular catalysts on the surface of inorganic semiconductors are rare. In this work, a classic rhenium bipyridine complex (Re catalyst) has been successfully covalently linked to the low toxicity CuInS2 quantum dots (QDs) by click reaction for photocatalytic CO2 reduction. Covalent bonding between the CuInS2 QDs and the Re catalyst in the QD-Re hybrid system is confirmed by UV-visible absorption spectroscopy, Fourier-transform infrared spectroscopy and energy-dispersive X-ray measurements. Time-correlated single photon counting and ultrafast time-resolved infrared spectroscopy provide evidence for rapid photo-induced electron transfer from the QDs to the Re catalyst. Upon photo-excitation of the QDs, the singly reduced Re catalyst is formed within 300 fs. Notably, the amount of reduced Re in the linked hybrid system is more than that in a sample where the QDs and the Re catalyst are simply mixed, suggesting that the covalent linkage between the CuInS2 QDs and the Re catalyst indeed facilitates electron transfer from the QDs to the Re catalyst. Such an ultrafast electron transfer in the covalently linked CuInS2 QD-Re hybrid system leads to enhanced photocatalytic activity for CO2 reduction, as compared to the conventional mixture of the QDs and the Re catalyst.
Highlights
The Re catalyst functionalized with an azide group, which can quickly bind to the linker molecules on the surface of CuInS2 quantum dots (QDs) through click reaction, was added to the reaction system, by forming the CuInS2 QD–Re hybrid system
TEM images (Fig. S2†) show that, compared to mercaptopropionic acid (MPA) capped CuInS2 QDs, there is no obvious change in size (7 ± 1 nm) or shape of the CuInS2 QDs after binding with the Re catalyst that could explain a better dispersibility
Similar time-resolved IR (TRIR) features were observed in CdSe QD– Re(CO)3Cl(dcbpy) complexes,[40] and these results suggested that the photoinduced electrons in conduction band of QDs could effectively transfer to the Re catalyst, generating the Re catalyst anion: QDs* þ Re ! QDsþ þ ReÀ: From Fig. 5A, we can find that the feature of the Re catalyst anion can be observed already at 300 fs after excitation, which means the electron transfer from the QDs to the Re catalyst must occur within less than 300 fs
Summary
As a part of artificial photosynthesis, photocatalytic reduction of CO2 has gained substantial interest, since it is a green method to convert solar energy and the greenhouse gas CO2 into chemical feedstock.[1,2,3,4] Until now, there were mainly three kinds of catalysts for CO2 photoreduction: semiconductors and metals,[5,6,7,8,9] synthetic molecular catalysts[10,11,12,13,14] and enzymes.[15,16,17,18] Though enzymes display high turnover efficiency and good selectivity, the range of possible operative conditions is restricted, which greatly limits their large-scale use.
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