Z-Scheme Water Splitting Using Two Different Semiconductor Photocatalysts

Water splitting on illuminated semiconductors has long been studied as a potential means of converting solar energy into chemical energy in the form of H2, a clean and renewable energy carrier. Photocatalytic water splitting through two-step photoexcitation using two different semiconductor powders and a reversible donor/acceptor pair (so-called shuttle redox mediator) is one of the possible forms of artificial photosynthesis. This system was inspired by natural photosynthesis in green plants and is called the "Z-scheme". The development of Z-scheme water splitting systems has relied on both finding a new semiconductor photocatalyst that efficiently works in the presence of a shuttle redox mediator and creating active sites to promote surface chemical reactions while suppressing backward reactions involving redox mediators. This review article describes the historical development of photocatalytic water splitting systems driven by the Z-scheme principle.