In a photocatalytic water splitting reaction, the catalyst, usually a semiconductor, captures photons. Electrons get excited and rise from the valence band to the conduction band. The electronic voids left behind in the valence band are regarded as positively charged “holes”. If the electrons and holes manage to migrate to the surface of the catalyst before the opposite charges recombine, they can be transferred to water molecules and used to reduce the water to make hydrogen or oxidize it to make oxygen.
New catalyst systems are constantly being researched and developed, but their efficiency has always been found to be lacking. Theoretically, catalysts based on tantalum nitride (Ta3N5) should be especially well-suited candidates for photocatalysis with visible light. However, two main problems have hindered their successful use in practice: First, on the surface of the catalyst, the resulting products, oxygen and hydrogen, immediately react to produce water. Second, the charge separation of the electrons and holes formed in the reaction doesn’t quite work correctly as they recombine too quickly.