Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called “Z-scheme” systems, which are inspired by the photosystem II–photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises two photocatalysts and generates two sets of charge carriers, splitting water into its constituent parts, hydrogen and oxygen, at separate locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.

We show that layered oxyhalides PbBiO2X (X = Cl, Br, I) with a Sillén-type structure possess band levels appropriate for visible-light-induced water splitting. Under visible light, PbBiO2Cl and PbBiO2Br with band gap (BG) of 2.51 and 2.48 eV, respectively, stably oxidized water to O2 in the presence of an Fe3+ electron acceptor. A comparison with structurally related SrBiO2Cl and BaBiO2Cl (BG = 3.55 and 3.54 eV) combined with DFT calculations revealed a significant interaction between O 2p and Pb 6s orbitals leading to the upward shift of the valence band maximum in PbBiO2X as compared with (Sr,Ba)­BiO2Cl. Z-scheme water splitting into H2 and O2 has been demonstrated using PbBiO2Cl as an O2-evolving photocatalyst, coupled with an appropriate H2-evolving photocatalyst in the presence of an Fe3+/Fe2+ redox mediator.

Oxyhalides Bi4MO8X are revealed to function as stable O2-evolving photocatalysts under visible light without suffering from self-oxidative deactivation due to the unique valence band structure.

An oxyhalide photocatalyst BiNbOCl has recently been proven to stably oxidize water under visible light, enabling the Z-scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of BiNbOCl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase BiNbOCl at as low as 650 °C. Thus, synthesized BiNbOCl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higher O evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO or Pt cocatalyst on BiNbOCl further enhanced the activity for O evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H-evolving photocatalyst Ru/SrTiO:Rh along with an Fe/Fe redox mediator, the RuO/BiNbOCl is an excellent O-evolving photocatalyst, exhibiting highly effective water splitting into H and O via the Z-scheme.