A Twist On Water Splitting

If water could be separated economically into oxygen and hydrogen, a clean-burning fuel, the world’s oceans would represent a free and virtually limitless feedstock for producing energy. A new strategy for designing light-activated catalysts that split water may help bring that alternative-energy goal a step closer to reality.

Researchers in China have found that crystals of the semiconductor Ga₂O₃ that are composed of a patchwork of structurally distinct nanosized domains can split water photocatalytically (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201207554). The study demonstrates that the interface between polymorphic crystal phases can play a key role in light-stimulated water splitting. The work may lead to photocatalysts that are more active than the relatively inefficient ones available today.

Photocatalysts split water by directing energy absorbed from light—often sunlight—to break water’s chemical bonds. The heart of the process is the light absorption event, which generates pairs of negatively charged electrons and positively charged holes (electron vacancies). The key to capitalizing on the energy absorbed from light is keeping the charges separated. Charge recombination can dissipate the absorbed energy before bonds are broken.

The standard strategy for maximizing charge separation in semiconductors calls for selectively doping the material to juxtapose positively (p-type) and negatively (n-type) charged regions. Researchers also make these types of interfaces, known as p-n junctions, by depositing two types of semiconductors side by side.

The new study, which was conducted by Xiang Wang, Can Li, and coworkers of the Dalian Institute of Chemical Physics, demonstrates an alternative way to keep charges separated. Rather than relying on p-n junctions to do the job, the Dalian team exploits the interfaces between structurally dissimilar nanosized domains of Ga₂O₃, which is known to crystallize in five polymorphic phases.

The researchers show that a simple heat treatment can be used to tailor the distribution of nanosized domains of Ga₂O₃’s so-called α and β phases. They further show that mixed α-β samples prepared by heating the starting material to roughly 600 °C are up to seven times more catalytically active in splitting water than samples composed of either phase alone.

The University of Tokyo’s Kazunari Domen comments that although in this proof-of-concept study the overall activity of the Ga₂O₃ photocatalysts is not especially high, the strategy described here represents a new approach to boosting catalytic water-splitting efficiency. He adds that the method’s applicability may be limited, however, as a result of the relatively small number of materials known to exhibit similar kinds of polymorphs.