Developing clean and renewable energy sources to replace fossil fuels is essential to meet the increasing human need in a sustainable manner. Photocatalytic water splitting is a promising strategy to produce storable and clean hydrogen fuel , and therefore should play an important role in relieving energy and environmental issues. The past decades have witnessed the fast development of effective and robust photocatalysts for water splitting. Inorganic materials including, but not limited to, transition metal oxides , nitrides , and sulfides have been widely studied as photocatalysts. However, these materials usually suffer low quantum efficiency (Q.E., see Glossary) because of the fast recombination of charge carriers . Besides, for some of them, toxicity, degradation problems, and high cost of transition metals have limited their wide application. Therefore, it remains to be important to develop stable and highly efficient metal-free photocatalysts for water splitting for potential applications at various scales.