Ammonia (NH 3 ), which is an indispensable chemical, is produced by the Haber−Bosch process using H 2 and N 2 under severe reaction conditions. Although photocatalytic N 2 fixation with water under ambient conditions is ideal, all previously reported catalysts show low efficiency. Here, we report that a metalfree organic semiconductor could provide a new basis for photocatalytic N 2 fixation. We show that phosphorus-doped carbon nitride containing surface nitrogen vacancies (PCN-V), prepared by simple thermal condensation of the precursors under H 2 , produces NH 3 from N 2 with water under visible light irradiation. The doped P atoms promote water oxidation by the photoformed valence-band holes, and the N vacancies promote N 2 reduction by the conduction-band electrons. These phenomena facilitate efficient N 2 fixation with a solar-to-chemical conversion (SCC) efficiency of 0.1%, which is comparable to the average solar-tobiomass conversion efficiency of natural photosynthesis by typical plants. Thus, this metal-free catalyst shows considerable potential as a new method of artificial photosynthesis.

Ammonia is an indispensable chemical. Photocatalytic NH 3 production via dinitrogen fixation using water by sunlight illumination under ambient conditions is a promising strategy, although previously reported catalysts show insufficient activity. Herein, we showed that ultraviolet light irradiation of a semiconductor, bismuth oxychloride with surface oxygen vacancies (BiOCl-OVs), in water containing chloride anions (Cl − ) under N 2 flow efficiently produces NH 3 . The surface OVs behave as the N 2 reduction sites by the photoformed conduction band electrons. The valence band holes are consumed by self-oxidation of interlayer Cl − on the catalyst. The hypochloric acid (HClO) formed absorbs ultraviolet light and undergoes photodecomposition into O 2 and Cl − . These consecutive photoreactions produce NH 3 with water as the electron donor. The Cl − in solution compensates for the removed interlayer Cl − and inhibits catalyst deactivation. Simulated sunlight illumination of the catalyst in seawater stably generates NH 3 with 0.05% solar-to-chemical conversion efficiency, thus exhibiting significant potential of the seawater system for artificial photosynthesis.

Photocatalytic N 2 reduction with water by sunlight irradiation is a challenging issue toward sustainable energy society, but previously reported photocatalysts had suffered from low stability and low activity. We prepared a boron-doped carbon nitride (BCN) semiconductor powder loaded with nickel phosphide particles (Ni 2 P) as cocatalysts. The Ni 2 P/BCN catalyst, when photoirradiated in pure water by simulated sunlight under N 2 flow, successfully produces NH 3 at room temperature. The B doping leads to a positive shift of the valence band level and enhances water oxidation by the photoformed holes. The Ni 2 P particles efficiently receive the conduction band electrons of BCN, leading to enhanced charge separation of the photoformed hole and electron pairs, and behave as N 2 reduction sites. Simulated sunlight irradiation of the catalyst in water stably generates NH 3 with 0.010% solar-to-chemical conversion efficiency. This noble-metal-free system therefore shows a significant potential for N 2 photofixation.