Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe 3+, Co 2+, and Ni 2+ ) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H 2 ) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H 2 evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H 2 photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni 2+ used here). That is, the nickel precursor of Ni(OAc) 2 changes to Ni(H 2 O) 6 2+ in neutral H 2 O and eventually transforms to Ni(OH) 2 nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH) 2 to a defective structure [Ni x 0 /Ni 1– x (OH) 2 ], which acts as the real catalytic species of H 2 photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies ( V Ni ) on the Ni(OH) 2 nanosheets enhance the adsorption and dissociation of H 2 O molecules to enhance the local proton concentration, while the Ni 0 clusters behave as H 2 -evolution sites, thereby synergistically promoting the activity of H 2 photogeneration in alkaline media.
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Wang et al. (2020) studied this question.