Hierarchical Macro-Mesoporous Polymeric Carbon Nitride Microspheres with Narrow Bandgap for Enhanced Photocatalytic Hydrogen Production

Polymeric carbon nitride (PCN) is emerging as promising visible-light photocatalyst but remains a challenge in greatly improving its photocatalytic activity. The superiority of multiscale porous nanostructures in mass transfer, active site exposure, solar energy harvesting, and rates of reactions are verified in achieving superior photocatalytic activity; however, hierarchical porous PCN microspheres with macropores and mesopores simultaneously are rarely reported in the literature to date. Herein hierarchical macroporous-mesoporous PCN microspheres (HM-PCNs) are presented for highly enhanced photocatalytic hydrogen production via an in situ precursor transformation technique. Utilizing the solubility difference between cyanamide and dicyanadiamide in aqueous solution, the in situ transformation of PCN precursor from cyanamide to dicyandiamide along the wall surfaces of the hierarchical porous SiO2 microspheres can produce robust HM-PCNs with both hierarchical macro-mesopores structure and narrow bandgap (1.88 eV). The obtained HM-PCNs present enhanced light absorption, promoted charge separation and transfer, and thus demonstrate highly enhanced hydrogen evolution activity (4635.8 mu mol h(-1) g(-1)) by a factor of 31.7 over the pristine PCN. Ab initio calculations indicate that C atoms at the outmost positions of the pores greatly reduce the bandgap and facilitate the electron-hole separation.