A colloidal ZnTe quantum dot-based photocathode with a metal–insulator–semiconductor structure towards solar-driven CO2 reduction to tunable syngas

Producing solar fuels via photoelectrochemical (PEC) CO2 reduction is an energy-saving and environmentally benign technology. As a p-type semiconductor, ZnTe has drawn much attention because of its suitable band gap of 2.26 eV and very negative conduction-band-edge position. Herein, ZnTe quantum dots (QDs) were accurately synthesized and then applied to fabricate an innovative Ag3Cu/TiO2/ZnTe metal–insulator–semiconductor (MIS) photocathode. Owing to the unique MIS structure, fast interface charge-carrier migration, high catalytic selectivity, and excellent cell stability are integrated together. For the Ag3Cu/TiO2/ZnTe MIS photocathode, an extraordinary and highly stable photocurrent density of −5.10 mA cm−2 is achieved at −0.20 V versus the reversible hydrogen electrode (RHE) for syngas production with a ratio (CO : H2) as high as 6.8 under AM 1.5G illumination. Band energy analysis combined with transient absorption (TA) spectroscopy clarifies the migration mechanism of interfacial photo-generated charge-carriers in the MIS photocathode. Additionally, theoretical calculations reveal that the Ag3Cu co-catalyst significantly lowers the energy barrier in the step of key COOH intermediate formation for CO2 reduction. The MIS structure photocathode designed and fabricated here offers a model towards solar energy conversion using QD-based PEC systems.