In this review, we focus on analysing the fundamental electronic, optical and chemical properties of 2D phosphorene to assess its suitability as a metal-free water splitting photocatalyst.

This review focuses on the discussion of the latest progress and remaining challenges in selected metal-free photocatalysts for hydrogen production. The scope of this review is limited to the metal-free elemental photocatalysts (i.e. B, C, P, S, Si, Se etc.), binary photocatalysts (i.e. BC₃, B₄C, C_xN_y, h-BN etc.) and their heterojunction, ternary photocatalysts (i.e. BCN) and their heterojunction, and different types of organic photocatalysts (i.e. linear, covalent organic frameworks, microporous polymer, covalent triazine frameworks etc.) and their heterostructures. Following a succinct depiction of the latest progress in hydrogen evolution on these photocatalysts, discussion has been extended to the potential strategies that are deemed necessary to attain high quantum efficiency and high solar-to-hydrogen (STH) conversion efficiency. Issues with reproducibility and the disputes in reporting the hydrogen evolution rate have been also discussed with recommendations to overcome them. A few key factors are highlighted that may facilitate the scalability of the photocatalyst from microscale to macroscale production in meeting the targeted 10% STH. This review is concluded with additional perspectives regarding future research in fundamental materials aspects of high efficiency photocatalysts followed by six open questions that may need to be resolved by forming a global hydrogen taskforce in order to translate bench-top research into large-scale production of hydrogen.

Photocatalytic hydrogen production from water is a green and renewable path for solar fuel production. Hydrogen can be advantageously stored directly and burned without emission of deleterious CO and NO_x gases. Photocatalysis therefore shows significant promise as a part solution to a sustainable and affordable energy supply in an era post-fossil fuels. Influenced by the Fujishima–Honda effect, significant advances in photocatalytic hydrogen production have occurred at the laboratory-scale. For wide adoption however, the photocatalysts will need to be made from earth-abundant materials, be stable and scalable from laboratory-to-large-scale, and have high conversion efficiency. In this regard, metal-free photocatalysts show practical promise in meeting these requirements. To foster research in materials design, here we critically review recent significant developments in metal-free photocatalysts consisting of carbon, nitrogen and phosphorus, and discuss how future large-scale hydrogen production via overall water-splitting could be accomplished economically.