Ultrathin porous graphitic carbon nitride from recrystallized precursor toward significantly enhanced photocatalytic water splitting

•Hydrothermal treatment induced biuret recrystallization to form biuret hydrate with regular morphology and large crystal size.•Biuret hydrate as precursor promoted the thermal polymerization of melem for the formation of graphitic carbon nitride (BCN-HT100) network.•The obtained BCN-HT100 possessed ultrathin nanosheet structure with high polymerization degree, large surface area and plenty of pores.•Photocatalytic overall water splitting was achieved with the AQY reaching 5.8% at 380 nm and 4.9% at 405 nm.