Bismuth-Rich Oxyhalides for Efficient Photocatalytic Nitrogen Fixation into Ammonia

Ammonia (NH 3 ) 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 (N 2 ) gas, photocatalytic N 2 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 N 2 fixation. A series of bismuth oxybromides with tunable Bi/Br ratios (BiOBr (1:1) to Bi 4 O 5 Br 2 (2:1), Bi 24 O 31 Br 10 (2.4:1), and Bi 3 O 4 Br (3:1)) were synthesized by a facile solvothermal route and evaluated for N 2 reduction in pure water under visible light without cocatalysts or sacrificial agents. Among them, Bi 3 O 4 Br achieved the highest NH 3 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 N 2 adsorption and bond cleavage. Selectivity is evidenced by NH 3 being the only detected nitrogen-containing reduction product in the postreaction solution, and concurrent water oxidation is evidenced by H 2 O 2 formation. These results establish a clear composition–activity relationship, highlighting bismuth-rich Bi 3 O 4 Br as a practical and effective photocatalyst for the fixation of aqueous N 2 into NH 3 .