Efficient Extraction of Trapped Holes from Colloidal CdS Nanorods.

Cadmium Sulfide (CdS) nanostructures have been widely applied for solar driven H2 generations due to its suitable band gap and band edge energetics. For an efficient photoreduction reaction, hole scavenging from CdS needs to compete favorably with many recombination processes. Extensive spectroscopic studies show evidence for hole trapping in CdS nanostructures, which naturally leads the concern of extracting trapped holes from CdS in photocatalytic reactions. Here, we report a study of hole transfer dynamics from colloidal CdS nanorods (NRs) to adsorbed hole acceptor, phenothiazine (PTZ), using transient absorption spectroscopy. We show that >99% of the holes were trapped (with a time constant of 0.73 ps) in free CdS NRs to form a photoinduced transient absorption (PA) feature. In the presence of PTZ, we observed the decay of the PA feature and corresponding formation of oxidized PTZ(+) radicals, providing direct spectroscopic evidence for trapped hole transfer from CdS. The trapped holes were extracted by PTZ in 3.8 ± 1.7 ns (half-life) to form long-lived charge separated states (CdS(-)-PTZ(+)) with a half lifetime of 310 ± 50 ns. This hole transfer time is significantly faster than the slow conduction band electron-trapped hole recombination (half lifetime of 67 ± 1 ns) in free CdS NRs, leading to an extraction efficiency of 94.7 ± 9.0%. Our results show that despite rapid hole trapping in CdS NRs, efficient extraction of trapped holes by electron donors and slow recombination of the resulting charge-separated states can still be achieved to enable efficient photoreduction using CdS nanocrystals.