Surface stoichiometry manipulation enhances solar hydrogen evolution of CdSe quantum dots

Abstract

Surface stoichiometry is a sensitive parameter affecting the decay dynamics of photogenerated hole–electron pairs of QDs. However, the effect of this manipulation on artificial photocatalytic H₂ evolution is unclear. Here, we report that surface stoichiometry manipulation is a facile and feasible approach for enhancing H₂ photogeneration of QDs. In the absence of an external cocatalyst, a decrease in the surface Se ratio of CdSe QDs from ∼16.7% to ∼4.9% gives a more than 10-fold increase in solar H₂ evolution. Taking Ni(II) as an external cocatalyst, CdSe QDs with a surface Se ratio of ∼4.9% can produce ∼1600 ± 151 μmol H₂ gas during 27 h of visible-light irradiation, giving a total turnover number of (1.24 ± 0.12) × 10⁵ on CdSe QDs and an apparent quantum yield of 10.1%, which is about 8 times that of CdSe QDs with a surface Se ratio of ∼16.7% under the same conditions. Mechanistic insights obtained by a combination of steady-state and time-resolved spectroscopic techniques indicate that surface stoichiometry exerts a significant influence on the exciton kinetics of CdSe QDs: a higher ratio of surface Se would increase the possibility of exciton recombination through hole trapping, thus depressing the performance of solar H₂ evolution.