During photosynthesis, plants harness solar radiation and convert it into energy. Most artificial photosynthesis systems try to mimic this natural process by exploiting light-absorbing dye molecules called chromophores to split water into hydrogen and oxygen. The hydrogen is produced in the reductive side of the reaction and the oxygen in the oxidative side. These so-called half-reactions are part of the process that converts light into energy, but the problem is that such technologies are inefficient and short-lived because the Sun's rays damage and destroy the light-absorbing dyes in just a few hours.

Now, a team of researchers led by Todd Krauss, Patrick Holland and Richard Eisenberg at the University of Rochester has developed a new photochemical hydrogen-generating system made of cadmium–selenide (CdSe) quantum dots, nickel salt catalysts and ascorbic acid (vitamin C). The system lasts for several weeks rather than just hours and, in water, has an quantum efficiency of 36% – for every 100 photons absorbed, 36 hydrogen molecules are produced. If the surrounding solution is a mix of water and ethanol, this efficiency increases to 66%. Such high values have never yet been observed for such all-solution-based systems. The only snag is that the vitamin C (which acts as an electron donor) gets used up and regularly needs to be replenished during each hydrogen production cycle.

How it works

The CdSe quantum dots absorb two photons of light and transfer two electrons to the nickel catalyst. The two remaining protons combine to produce a hydrogen molecule, explained Krauss. "Our work is different from most other previous research in that the catalyst is formed in situ from the quantum-dot ligands," he said. "Most other solution-based systems produce hydrogen for just hours, or at most a day, because the chromophores degrade, so our long-lived system is rather unusual."

The researchers said that their catalyst–nanocrystal pairs are better than previous artificial photosynthesis nanoparticle systems because they are more stable to sunlight, but admitted that they do not yet know why this is the case.

"This new system will also certainly help us better understand the reductive side of artificial photosynthesis – something that may one day help lead to more effective and efficient water splitting," added Krauss, "Our work is an important step in that direction."

Making ammonia

According to the team, such a clean source of hydrogen could not only find applications in green energy, but also in industry, for example in the Haber process for producing ammonia.

The Rochester team is now looking at other nanoparticle systems to try out. "We are also investigating other less-expensive catalysts and hope to find a way to replace the sacrificial vitamin C molecule with electrons, say from a circuit. Such experiments could be the next step towards a true artificial photosynthesis system, but we are still a far cry from that since we have only performed half of the full reaction," said Krauss.

The work is described in Science.