2D/2D Ti3C2 MXene/g-C3N4 nanosheets heterojunction for high efficient CO2 reduction photocatalyst: Dual effects of urea

•An ultrathin 2D/2D Ti3C2/g-C3N4 heterojunction is synthesized by a one-pot strategy.•Urea works both as the gas template for exfoliating Ti3C2 and the precursor of g-C3N4.•Ti3C2 stimulates the transfer of photo-generated electron-hole pairs, retarding the recombination.•2D/2D Ti3C2/g-C3N4 heterojunction boosts the photoreactivity of g-C3N4 for CO2 reduction.