The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated device manufacturing. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.

Direct splitting of water using a particulate photocatalyst would be a good way to produce clean and recyclable hydrogen on a large scale, and in the past 30 years various photocatalysts have been found that function under visible light. Here we describe an advance in the catalysis of the overall splitting of water under visible light: the new catalyst is a solid solution of gallium and zinc nitrogen oxide, (Ga(1-x)Zn(x))(N(1-x)O(x)), modified with nanoparticles of a mixed oxide of rhodium and chromium. The mixture functions as a promising and efficient photocatalyst in promoting the evolution of hydrogen gas.

Water splitting to form hydrogen and oxygen using solar energy in the presence of semiconductor photocatalysts has long been studied as a potential means of clean, large-scale fuel production. In general, overall water splitting can be achieved when a photocatalyst is modified with a suitable cocatalyst. It is therefore important to develop both photocatalysts and cocatalysts. In the past five years, there has been significant progress in water splitting photocatalysis, especially in the development of cocatalysts and related physical and materials chemistry. This work describes the state of the art and future challenges in photocatalytic water splitting, with a focus on the recent progress of our own research.

Overall water splitting to form hydrogen and oxygen over a heterogeneous photocatalyst using solar energy is a promising process for clean and recyclable hydrogen production in large-scale. In recent years, numerous attempts have been made for the development of photocatalysts that work under visible-light irradiation to efficiently utilize solar energy. This article presents recent research progress in the development of visiblelight-driven photocatalysts, focusing on the refinement of non-oxide-type photocatalysts such as (oxy)nitrides and oxysulfides.

We investigated semiconductor characteristics for polymeric carbon nitride as a metal-free photocatalyst working with visible light and have shown that the efficiency of hydrogen production by photochemical water reduction can be improved by approximately 1 order of magnitude by introducing the right type of mesoporosity into polymeric C(3)N(4). We anticipate a wide rang of potential application of C(3)N(4) as energy transducers for artificial photosynthesis in general, especially with a 3D continuous nanoarchitecture. Moreover, the results of finding photoactivity for carbon nitride nanoparticles can enrich the discussion on prebiotic chemistry of the Earth, as HCN polymer clusters are unequivocal in the solar system.

Photocatalytic overall water splitting has been studied extensively from the viewpoint of solar energy conversion. Despite numerous attempts, none have yielded satisfactory results for the development of photocatalysts, which work under visible light irradiation to efficiently utilize solar energy. We report here the first example of visible-light-driven overall water splitting on a novel oxynitride photocatalyst, a solid solution of GaN and ZnO with a band gap of 2.58-2.76 eV, modified with RuO2 nanoparticles. In contrast to the conventional non-oxide photocatalysts, such as CdS, the solid solution is stable during the overall water splitting reaction. This is the first example of achieving overall water splitting by a photocatalyst with a band gap in the visible light region, which opens the possibility of new non-oxide-type photocatalysts for energy conversion.

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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.