The gradually dwindling resources of fossil fuels and the urgency to reduce greenhouse gas emissions portray a globally concerning image of our contemporary energy infrastructure with over reliance on coal, gas and oil. The transformation of the current fossil-fuel-based energy system to a more sustainable, renewable and cleaner alternative is desperately required to mitigate climate change. As a carbon-free and clean energy carrier, hydrogen has long been considered as a promising energy option. However, hydrogen does not exist naturally due to its inherently reactive features and has to be cost-effectively produced from hydrogen-containing compounds. Solar or electrochemically driven water splitting is an appealing vision for future sustainable hydrogen production. The state of the art water splitting technology via electro-catalysis is highly dependent on the efficiency of electrocatalysts to promote the oxygen evolution reaction (OER) or hydrogen evolution reaction (HER), a possible limiting step for the overall reaction. Electro-catalysts are currently dominated by these noble metals. To minimize the production cost, it is vital to develop noble-metal free catalysts for water splitting. In this context, metal nitrides have captured the imagination of academic researchers. This review summarizes the recent research progress made in these nanostructured metal nitrides as efficient and cheap catalysts for electrochemical and photo(electro)chemical water splitting. The various synthetic strategies for the fabrication and functionalization of these nanostructured metal nitrides are first presented followed by the introduction of their unique physical, chemical, and electronic properties and their respective applications in facilitating the HER, OER and the overall water splitting reaction. Some engineering perspectives to further enhance the performances of these nanostructured catalysts for water splitting are also discussed. Finally, opportunities for future development are briefly proposed.

Photocatalysis is a green, feasible and versatile technology that has been widely used for energy conversion and environmental applications. As photocatalysis bears a great potential for solar energy utilization, enormous investigations have been implemented in the past decades. The fundamental mechanism and some applications were well addressed in the last century. Currently, the major focus in photocatalysis research is the design and development of photocatalyst materials. This review firstly introduces the historic milestones in photocatalysis studies and then a comprehensive survey is conducted on the metal-based photocatalysts, including TiO₂-based photocatalysts, ZnO and other metal oxides, metal sulfides, metal nitrides, and plasmon photocatalysts. From a historical viewpoint, particular attention is paid to metal-free graphitic carbon nitride (g-C₃N₄), a novel visible-light photocatalyst. Various modification techniques for g-C₃N₄ are summarized and analyzed. In terms of its metal-free nature, the fabrication of a porous structure, shape-control synthesis and non-metal doping are discussed in detail. Photocatalytic studies on g-C₃N₄-based catalysts are introduced. Some emerging elemental photocatalysts are also introduced. Finally, perspectives on non-metal photocatalyst design and development are provided.