Precursor-reforming protocol to synthesis of porous N-doped g-C3N4 for highly improved photocatalytic water treatments

•N-g-C3N4 was synthesized via precursor-reforming and thermal condensation method.•The band gap of g-C3N4 narrowed from 2.72 eV to 2.26 eV.•The specific surface area of porous N-g-C3N4 was improved to 105.28 m2 g−1.•RhB degradation efficiency of N-g-C3N4 is up to 93.5%, twice than bulk g-C3N4.