Bismuth-Rich Oxyhalides for Efficient Photocatalytic Nitrogen Fixation into Ammonia

Ammonia (NH₃) is essential for fertilizer production and is increasingly recognized as a promising carbon-free energy carrier. Given the abundant energy input from solar illumination and the widespread availability of relatively inexpensive atmospheric nitrogen (N₂) gas, photocatalytic N₂ fixation has attracted an increasing amount of interest. Here, we report a bismuth-rich design strategy within the Bi-O-Br system to enhance the photocatalytic N₂ fixation. A series of bismuth oxybromides with tunable Bi/Br ratios (BiOBr (1:1) to Bi₄O₅Br₂ (2:1), Bi₂₄O₃₁Br₁₀ (2.4:1), and Bi₃O₄Br (3:1)) were synthesized by a facile solvothermal route and evaluated for N₂ reduction in pure water under visible light without cocatalysts or sacrificial agents. Among them, Bi₃O₄Br achieved the highest NH₃ yield (790 μmol/g/h) and exhibited good structural stability, retaining more than 95% of its activity over three repeated cycles. The efficient activity is attributed to its stronger light absorption, more negative conduction band, efficient charge separation, and longer carrier lifetime. Spectroelectrochemical analysis confirmed its high reduction ability, while oxygen vacancies facilitated enhanced N₂ adsorption and bond cleavage. Selectivity is evidenced by NH₃ being the only detected nitrogen-containing reduction product in the postreaction solution, and concurrent water oxidation is evidenced by H₂O₂ formation. These results establish a clear composition-activity relationship, highlighting bismuth-rich Bi₃O₄Br as a practical and effective photocatalyst for the fixation of aqueous N₂ into NH₃.