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.

Water oxidation is a rate-determining step in solar driven H₂ fuel synthesis and is technically challenging to promote. Despite decades of effort, only a few inorganic catalysts are effective and even fewer are effective under visible light. Recently, attention has been paid to synthetic semiconducting polymers, mainly on graphitic C₃N₄, with encouraging hydrogen evolution performance but lower activity for water oxidation. Here, a highly ordered covalent triazine-based framework, CTF-1 (C₈N₂H₄), is synthesised by a very mild microwave-assisted polymerisation approach. It demonstrates extremely high activity for oxygen evolution under visible light irradiation, leading to an apparent quantum efficiency (AQE) of nearly 4% at 420 nm. Furthermore, the polymer can also efficiently evolve H₂ from water. A high AQE of 6% at 420 nm for H₂ production has also been achieved. The polymer holds great potential for overall water splitting. This exceptional performance is attributed to its well-defined and ordered structure, low carbonisation, and superior band positions.

Oxidative coupling of methane (OCM) is considered one of the most promising catalytic technologies to upgrade methane. However, C₂ products (C₂H₆/C₂H₄) from conventional methane conversion have not been produced commercially owing to competition from overoxidation and carbon accumulation at high temperatures. Herein, we report the codeposition of Pt nanoparticles and CuO_x clusters on TiO₂ (PC‐50) and use of the resulting photocatalyst for OCM in a flow reactor operated at room temperature under atmospheric pressure for the first time. The optimized Cu_0.1Pt_0.5/PC‐50 sample showed a highest yield of C₂ product of 6.8 μmol h⁻¹ at a space velocity of 2400 h⁻¹, more than twice the sum of the activity of Pt/PC‐50 (1.07 μmol h⁻¹) and Cu/PC‐50 (1.9 μmol h⁻¹), it might also be the highest among photocatalytic methane conversions reported so far under atmospheric pressure. A high C₂ selectivity of 60 % is also comparable to that attainable by conventional high‐temperature (>943 K) thermal catalysis. It is proposed that Pt functions as an electron acceptor to facilitate charge separation, while holes could transfer to CuO_x to avoid deep dehydrogenation and the overoxidation of C₂ products.

The bandgap engineering of semiconductors, in particular low‐cost organic/polymeric photocatalysts could directly influence their behavior in visible photon harvesting. However, an effective and rational pathway to stepwise change of the bandgap of an organic/polymeric photocatalyst is still very challenging. An efficient strategy is demonstrated to tailor the bandgap from 2.7 eV to 1.9 eV of organic photocatalysts by carefully manipulating the linker/terminal atoms in the chains via innovatively designed polymerization. These polymers work in a stable and efficient manner for both H₂ and O₂ evolution at ambient conditions (420 nm < λ < 710 nm), exhibiting up to 18 times higher hydrogen evolution rate (HER) than a reference photocatalyst g‐C₃N₄ and leading to high apparent quantum yields (AQYs) of 8.6%/2.5% at 420/500 nm, respectively. For the oxygen evolution rate (OER), the optimal polymer shows 19 times higher activity compared to g‐C₃N₄ with excellent AQYs of 4.3%/1.0% at 420/500 nm. Both theoretical modeling and spectroscopic results indicate that such remarkable enhancement is due to the increased light harvesting and improved charge separation. This strategy thus paves a novel avenue to fabricate highly efficient organic/polymeric photocatalysts with precisely tunable operation windows and enhanced charge separation.