Synergy of P doping and crystallinity modulation in carbon nitride for enhancing photocatalytic uranyl reduction.

Doping modification is a useful way to promote the catalytic activity of carbon nitride (CN). However, most doped CNs have lower structural symmetry and several edge defects, which hinder the transfer of charge carriers. This work reports a P-doped crystalline carbon nitride (crystalline PCN) for the efficient photoreduction of uranyl. The thermal polymerization and salt post-treatment convert the amorphous PCN into crystalline PCN. Compared to the pristine CN, the crystalline PCN has over 1620 % higher activity for uranyl (U(VI)) reduction, reaching a 97.8 % reduction rate in 60 min. Furthermore, the 2-PCN shows excellent stability and a U(VI) removal efficiency >85.7 % in the pH range of 5-8. Characterization analysis reveal that both the P doping and crystalline modulation do not obviously change their morphology, light absorption property and energy band structure, but markedly promote the delocalization of electrons around the doped P atoms, thereby severely inhibit direct electron-hole recombination. Thus, the more efficient separation of charge carriers generates more reactive specials to participate in the photocatalytic uranyl reduction reaction. This study demonstrates a dual-modification strategy for the rational synthesis of highly active metal-free CN-based photocatalysts for uranyl reduction.