Solar water splitting, in which water is separated into oxygen and hydrogen using sunlight, could be a clean and renewable way to produce energy, and researchers are busy looking for efficient photoactive materials for use in this process. Colloidal semiconductor nanocrystals could be ideal here because they absorb light over a wide spectrum of wavelengths thanks to the fact that their band gaps can be tuned over a large energy range by simply changing the size of the nanoparticles. They can also easily be synthesized in solution, which means that films of the particles can be deposited quickly and without fuss on a wide range of flexible or rigid substrates – just like paint or ink can.

When high-energy photons hit the nanocrystals, excited electrons and holes (charge carriers) are produced that have energies at least equal to or greater than the band gaps of the materials. Once the photoexcited charge carriers reach the surface of the crystals, they can take part in redox reactions with donor or acceptor molecules there.

Preventing recombination

There is a snag, however: the high surface area to volume ratio of nanoparticles results in bare surfaces that can became “traps” in which electrons invariably get stuck. This means that electrons and holes have time to recombine, something that inevitably reduces the efficiency of photocatalytic devices made from the nanocrystals. The main challenge here is to thus isolate the charge carriers before they have time to recombine.

There are two main ways to prevent recombination. The first involves confining holes to a part of the nanostructure that acts as a trap (for example a dot-in-a-rod structure) and which preferably is at some distance from where the electrons are. The second approach involves the holes being scavenged by a sacrificial agent.

Relaying holes

Now, a team of researchers led by Jochen Feldmann and Jacek Stolarczyk at the Ludwig-Maximilians University in Munich, has discovered that a hydroxyl anion/radical redox couple can efficiently relay holes from the semiconductor nanocrystal surface to the scavenger. The redox couple can thus increase the rate of hydrogen produced by the photocatalyst material without the need for expensive noble-metal co-catalysts. Indeed, the photocatalytic hydrogen generation has an external efficiency of more than 50% in a system containing dispersed CdS nanocrystals “decorated” with nickel in the presence of ethanol as a sacrificial electron donor, says team member Thomas Simon.

“The redox shuttle mechanism we have observed is efficient because one slow process in the photocatalyst is replaced by two fast processes,” explains Stolarczyk. “The slow process in question is direct oxidation of the hole scavenger by the photoexcited hole in the semiconductor and the two faster processes are oxidation of the hydroxyl anions by the photoexcited holes and subsequent oxidation of ethanol by the hydroxyl radicals.”

Hydroxyl anions and radicals, as very small molecules, can easily diffuse through the steric barrier of the ligand surrounding the nanocrystals, Feldmann tells “We know that the anions couple with the nanocrystal whereas the radicals oxidize hole scavengers, such as ethanol.”

And that is not all: because this oxidation reaction is thought to be a limiting factor in photocatalytic water splitting, by quickly removing the photoexcited holes, we reduce charge carrier recombination and allow the photoexcited electrons to efficiently reduce protons to hydrogen, he adds.

The team, which includes researchers from the City University of Hong Kong and the University of Liverpool, says that it will now be looking at other redox mediators for use in such photocatalytic reactions on semiconductor nanocrystals. “We would also like to investigate so-called full water splitting – that is producing not only hydrogen from the protons in water, but also oxygen via simultaneous water oxidation,” adds Stolarczyk.

The researchers describe their work in Nature Materials.