Sequestering High-Energy Electrons to Facilitate Photocatalytic Hydrogen Generation in CdSe/CdS Nanocrystals

The photocatalytic H₂O splitting activities of CdSe and CdSe/CdS core/shell quantum dots are contrasted. CdSe/CdS core/shell quantum dots constructed from 4.0 nm CdSe quantum dots are shown to be strongly active for visible-light-driven photocatalytic H₂ evolution in 0.1 M Na₂S/Na₂SO₃ solution with a turnover number of 9.94 after 5 h at 103.9 μmol/h. CdSe quantum dots themselves are only marginally active in 0.1 M Na₂S/Na₂SO₃ solution with a turnover number of 1.10 after 5 h at 11.53 μmol/h, while CdSe quantum dots in pure H₂O are found to be completely inactive. Broad-band transient absorption spectroscopy is used to elucidate the mechanisms that facilitate the enhancement in the CdSe core/shell quantum dots, which is attributed to passivation of surface-deep trap states with energies lying below the reduction potential necessary for H₂O reduction. Thus, surface trapping dynamics and energetics can be manipulated to dictate the photocatalytic activities of novel CdSe quantum dot based photocatalytic materials

Red States versus Blue States in Colloidal Silicon Nanocrystals: Exciton Sequestration into Low-Density Traps

The ultrafast exciton photodynamics of red-emitting and blue-emitting colloidal Si nanocrystals are contrasted under low (1.5 mJ/cm²) and high (9.1 mJ/cm²) excitation powers with broadband transient absorption spectroscopy. While the low-power initiated transient signals differ strongly for the two samples, the high-power signals exhibit similar nonmonotonic kinetics, resulting in a new population formed on a 10 to 30-ps time scale with a sample independent spectrum and decay kinetics. This phenomenon is ascribed to the saturation of low-density red-emitting and blue-emitting traps via a state-filling mechanism to populate new meta-stable states at higher excitation powers. The states responsible for blue emission and high-power populations are ascribed to traps from low-density nitrogen and oxygen impurities, respectively, and share similar charge-transfer character with the silicon nanocrystal core