Photocatalytic conversion of solar energy to fuels, such as hydrogen, is attracting enormous interest, driven by the promise of addressing both energy supply and storage. Colloidal semiconductor nanocrystals have been at the forefront of these efforts owing to their favourable and tunable optical and electronic properties as well as advances in their synthesis. The efficiency of the photocatalysts is often limited by the slow transfer and subsequent reactions of the photoexcited holes and the ensuing high charge recombination rates. Here we propose that employing a hydroxyl anion/radical redox couple to efficiently relay the hole from the semiconductor to the scavenger leads to a marked increase in the H2 generation rate without using expensive noble metal co-catalysts. The apparent quantum yield and the formation rate under 447 nm laser illumination exceeded 53% and 63 mmol g(-1) h(-1), respectively. The fast hole transfer confers long-term photostability on the system and opens new pathways to improve the oxidation side of full water splitting.

Colloidal CdS nanorods have been decorated with extremely small, subnanometer sized Pt clusters and used for photocatalytic hydrogen production. We also show highly selective decoration of CdS nanorods with uniform, relatively large (4.8 nm mean size) Pt nanoparticles, with a remarkably high (90%) yield of samples decorated with exactly one Pt particle per rod. Samples with large Pt particles show no increase in hydrogen evolution rate compared to small Pt clusters, which implies that efficient hydrogen production utilizing CdS nanorods with reduced amounts of Pt is possible.

We use Pt-decorated CdS nanorods for photocatalytic hydrogen generation in the presence of sacrificial hole scavengers. Both the quantum efficiency for hydrogen generation and the stability of the colloidal nanocrystals in solution improve with increasing redox potential of the hole scavenger. The higher redox potential leads to faster hole scavenging, which increases quantum efficiency and stability since electron hole recombination and oxidation of the CdS become less important. The quantum efficiencies can be tuned over more than an order of magnitude. This finding is important for choosing hole scavengers and for comparing efficiencies and stabilities for different photocatalytic nanosystems.

Noble-metal-decorated colloidal semiconductor nanocrystals are currently receiving significant attention for photocatalytic hydrogen generation. A detailed knowledge of the charge-carrier dynamics in these hybrid systems under hydrogen generation conditions is crucial for improving their performance. Here, a transient absorption spectroscopy study is conducted on colloidal, Pt-decorated CdS nanorods addressing this issue. Surprisingly, under hydrogen generation conditions (i.e., in the presence of the hole-scavenger sodium sulfite), photoelectron transfer to the catalytically active Pt is slower than without the hole scavenger, where no significant hydrogen generation occurs. This unexpected behavior can be explained by different degrees of localization of the electron wavefunction in the presence and absence of holes on the nanorods, which modify the electron transfer rates to the Pt. The results show that solely optimizing charge transfer rates in photocatalytic nanosystems is no guarantee of improved performance. Instead, the collective Coulomb interaction-mediated electron-hole dynamics need to be considered.

This Feature Article offers an overview of hybrid colloidal heterostructures of anisotropic semiconductor nanocrystals decorated with metals; primarily gold and platinum. The nonspherical shapes of the semiconductor components create a great variety of metal-decorated hybrid nanostructures, whose synthesis and morphology are considered here. Due to the current interest in photocatalytic systems able to utilize solar energy for water-splitting, the use of Pt-decorated CdS-based nanorods for hydrogen generation is specifi cally addressed. Great fl exibility of the colloidal synthesis leading to well-defi ned hybrid semiconductor-metal nanostructures drastically increases the possibility of their integration into functional nanosystems with novel synergetic properties, making them promising candidates for a variety of photovoltaic, catalytic, and sensing applications.