Boron-Induced Electronic Modulation and Nanocrystal Fragmentation Synergistically Boost Photocatalytic Water Oxidation in Ionic Carbon Nitrides

Photocatalytic water oxidation is a critical half-reaction to realize overall water splitting, yet it remains challenging due to sluggish reaction kinetics and the need for efficient active centers. To overcome these limitations, we introduce a dual strategy combining boron doping with nanocrystal fragmentation to boost the photocatalytic oxygen evolution reaction (OER) performance of ionic carbon nitrides. The optimal catalyst (<i>0.25%B</i>-KPHI) achieves an apparent quantum efficiency of 4.6% at 420 nm with CoO _<i>x</i> as a cocatalyst for OER, outperforming most previously reported carbon nitride photocatalysts. Extensive experimental analyses revealed that boron incorporation induces the fragmentation of nanocrystalline domains within the potassium poly-(heptazine imide) (KPHI) matrix, resulting in extended visible-light absorption, improved hydrophilicity, more efficient charge separation, and accelerated water oxidation kinetics. Comprehensive density functional theory calculations further showed that boron preferentially localizes at the edges of heptazine units near structural defects, where it serves as a potential adsorption site for water and substantially lowers the energy barrier for the formation of the *O intermediate.