Hydrogen is often advertised as a green and clean fuel of the future, but it already is an important chemical feedstock. Worldwide, 96% of hydrogen is used in the chemical industry, mostly in production of ammonia fertilisers and hydrocarbon refining. However, despite its status as the most abundant substance in the universe, it is very scarce in its most useful H 2 diatomic form on Earth, where it exists primarily in water, hydrocarbons and other organic matter. Hydrocarbons have been traditionally used in production of hydrogen, with coal gasification and steam methane reforming among the most popular methods. Both are highly energy-intensive and emit large quantities of COx gases (even when accounting for the proposed use of carbon capture), which is clearly unsustainable long-term. A clear alternative to hydrocarbon reforming lies in the other major source of hydrogen -water. In 1972, Honda and Fujishima discovered that hydrogen can be produced by water electrolysis using ultraviolet (UV) light shone on a TiO 2 electrode, popularising the idea of photocatalysis. TiO 2 has since become one of the most studied photocatalytic materials due to its high photocorrosive stability, ease of manufacture and nontoxicity. However, because of its wide band gap (3.2 eV) titania can only absorb UV light, which only comprises less than 5% of the entire incident light on Earth. To strike the balance between absorption of a wide light spectrum and satisfying the thermodynamic and kinetic demands of water splitting, 1.8-2.2 eV has been identified as the ideal band gap range for photocatalysis. While exceptionally stable in water, metal oxides suffer from low-lying O 2p states that push the valence band maximum down in energy, opening the band gap too wide. Sulphides, on the other hand, contain S 3p states that are higher in energy which close the band gap substantially without altering the conduction band edge. Unfortunately, sulphides tend to self-oxidise and photocorrode in a water medium, which makes them unsuitable as photocatalysts. However, the photocorrosion stability of oxides and favourable electronic structure of sulphides can be combined in mixed-anion oxysulphides.