Phosphorus-Doped Graphitic Carbon Nitride Nanotubes with Amino-rich Surface for Efficient CO₂ Capture, Enhanced Photocatalytic Activity, and Product Selectivity

Phosphorus-doped graphitic carbon nitrides (P-g-C₃N₄) have recently emerged as promising visible-light photocatalysts for both hydrogen generation and clean environment applications because of fast charge carrier transfer and increased light absorption. However, their photocatalytic performances on CO₂ reduction have gained little attention. In this work, phosphorus-doped g-C₃N₄ nanotubes are synthesized through the one-step thermal reaction of melamine and sodium hypophosphite monohydrate (NaH₂PO₂·H₂O). The phosphine gas generated from the thermal decomposition of NaH₂PO₂·H₂O induces the formation of P-g-C₃N₄ nanotubes from g-C₃N₄ nanosheets, leads to an enlarged BET surface area and a unique mesoporous structure, and creates an amino-rich surface. The interstitial doping phosphorus also down shifts the conduction and valence band positions and narrows the band gap of g-C₃N₄. The photocatalytic activities are dramatically enhanced in the reduction both of CO₂ to produce CO and CH₄ and of water to produce H₂ because of the efficient suppression of the recombination of electrons and holes. The CO₂ adsorption capacity is improved to 3.14 times, and the production of CO and CH₄ from CO₂ increases to 3.10 and 13.92 times that on g-C₃N₄, respectively. The total evolution ratio of CO/CH₄ dramatically decreases to 1.30 from 6.02 for g-C₃N₄, indicating a higher selectivity of CH₄ product on P-g-C₃N₄, which is likely ascribed to the unique nanotubes structure and amino-rich surface.