Ultrasonic‐Assisted Photocatalytic Conversion of Air to Nitric Acid Under Ambient Conditions

The production of nitric acid (HNO₃) holds significant importance in the chemical industry, and achieving direct air-to-HNO₃ conversion with high efficiency and stability under mild conditions remains promising but challenging. Here, we demonstrate an ultrasound-assisted photocatalytic method in which the generated hydroxyl (·OH) and superoxide (·O₂ ^-) radicals drive the direct synthesis of HNO₃ from air under ambient conditions. The synthesis rate of HNO₃ is up to 8594 µmol/h/g_cat, over 200 times higher than that of previous photocatalytic methods. Using multiple ultrasound sources causes an additional eight times rate enhancement with nearly 100% selectivity maintained over 700 h. The resulting product exhibits comparable fertilization efficacy to conventional fertilizers. By harnessing the UV portion of sunlight that plants do not use, this approach paves the way for full‑spectrum solar energy utilization in sustainable agriculture.