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 center dot H2O). The phosphine gas generated from the thermal decomposition of NaH2PO2 center dot 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 H-2 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.