Melem: A metal-free unit for photocatalytic hydrogen evolution

•Melem is found as a metal-free unit for photocatalytic hydrogen evolution.•Conjugating melem with dianhydride monomer greatly enhances visible-light absorbance and photocatalytic performance.•This work sheds light on developing novel metal-free photocatalysts based on melem unit.

Melem, the building unit of g-C3N4, is reported for the first time as a metal-free photocatalyst for hydrogen evolution from methanol aqueous solution. Melem is synthesized via a facile thermal condensation of melamine at 425 °C. Its chemical structure is verified by the combination of Fourier transform infrared spectroscopy (FTIR), solid-state 13C nuclear magnetic resonance (NMR) and elemental analysis. Density functional theory (DFT) calculations reveal that melem possesses frontier orbitals quite similar to those of g-C3N4. By conjugating melem with dianhydride monomer, an extended polyimide network with greatly enhanced visible-light absorption is formed. Consequently, the visible-light activity for hydrogen evolution is enhanced by six times and reaches to a rate of 13.1 μmol/h. This work may open up new opportunity to develop efficient metal-free photocatalysts based on melem unit.

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