Tuning Cu dopant of Zn0.5Cd0.5S nanocrystals enables high-performance photocatalytic H2 evolution from water splitting under visible-light irradiation

Cu-doping into Zn 1−x Cd x S can greatly enhance the photocatalytic H 2 evolution from water splitting under visible-light irradiation. However, it is still controversial for how the Cu-dopant improves this performance. Here, we report that appropriate Cu-doped Zn 0.5 Cd 0.5 S nanocrystals reach 21.4   mmol/h/g of H 2 evolution rate without cocatalyst in the visible-light region, which is also 2.8 times as high as that of the undoped counterpart, and the corresponding apparent quantum efficiency is 18.8% at 428   nm. It is firstly confirmed that the Cu 2+ changes into Cu + after being doped by soft X-ray absorption spectroscopy (sXAS). We theoretically propose that the transformation of 2Cu 2+ to 2Cu + results in one adjacent S 2 − vacancy (V S ) in host during the doping process, while the Cu + -dopant and V S attract the photoexcited holes and electrons, respectively. Accordingly, the photocatalytic activity is improved due to the enhanced separation of photoexcited carriers accompanied with the enhanced light absorption resulting from the Cu + -dopant and 2Cu + /V S complex as possible active site for photocatalytic H 2 evolution.