Abstract
A precious metal and Cd-free photocatalyst system for efficient CO2 reduction in water is reported. The hybrid assembly consists of ligand-free ZnSe quantum dots (QDs) as a visible-light photosensitiser combined with a phosphonic acid-functionalised Ni(cyclam) catalyst, NiCycP. This precious metal-free photocatalyst system shows a high activity for aqueous CO2 reduction to CO (Ni-based TONCO > 120), whereas an anchor-free catalyst, Ni(cyclam)Cl2, produced three times less CO. Additional ZnSe surface modification with 2-(dimethylamino)ethanethiol (MEDA) partially suppresses H2 generation and enhances the CO production allowing for a Ni-based TONCO of > 280 and more than 33% selectivity for CO2 reduction over H2 evolution, after 20 h visible light irradiation (λ > 400 nm, AM 1.5G, 1 sun). The external quantum efficiency of 3.4 ± 0.3% at 400 nm is comparable to state-of-the-art precious metal photocatalysts. Transient absorption spectroscopy showed that band-gap excitation of ZnSe QDs is followed by rapid hole scavenging and very fast electron trapping in ZnSe. The trapped electrons transfer to NiCycP on the ps timescale, explaining the high performance for photocatalytic CO2 reduction. With this work we introduce ZnSe QDs as an inexpensive and efficient visible light-absorber for solar fuel generation.
Highlights
Arti cial photosynthesis allows for the storage of solar energy through the conversion of water and carbon dioxide into chemical fuels and represents a promising strategy to overcome the global dependence on fossil energy sources.[1]
ZnSe-based photocathodes were recently reported for H2 evolution,[21] but unlike their cadmium analogues CdS22 and CdSe,[23] ZnSe quantum dots (QDs) have not been used in photocatalytic reduction of aqueous protons or CO2
At pH 5.5, the ZnSe conduction band (CB) is located at approximately À1.4 V vs. NHE,21a i.e. 400 mV more negative than the onset potential for electrocatalytic CO2 reduction at Ni(cyclam)Cl2 of approximately À1.0 V vs. NHE at pH 5.5 (E00CO2/CO 1⁄4 À0.43 V vs. NHE).14b While the energetics suggest that electron transfer from the ZnSe CB to Ni(cyclam)2+ is thermodynamically feasible, the electron transfer kinetics are crucial to efficiently drive CO2 reduction at Ni(cyclam)2+
Summary
Arti cial photosynthesis allows for the storage of solar energy through the conversion of water and carbon dioxide into chemical fuels and represents a promising strategy to overcome the global dependence on fossil energy sources.[1].
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