Although the construction of heterojunction photocatalysts is a promising way to achieve outstanding photocatalytic activities, a 2D heterojunction which possesses strong chemical bonding and appropriate interfacial contact toward efficient artificial photosynthesis is still a challenge. Herein, 2D/2D Nb2O5/g‐C3N4 S‐scheme heterojunction photocatalysts are successfully fabricated by a convenient in situ calcination route derived from niobic acid/urea precursor for the gas–solid CO2 reduction reaction. Under simulated solar irradiation, the total yield of C1 products (CH4 and CO) obtained on the optimized sample NOCN‐5 are 6.7 times and 5.3 times that over pristine Nb2O5 and g–C3N4 nanosheets, respectively, without sacrificial agent or cocatalysts. The enhanced performances of CO2 photoreduction might be attributed to the unique Nb─O─C chemical bonds induced charge transfer bridge, face‐to‐face contact, and the efficient S‐scheme transfer path of photoinduced electron–hole pairs, which is confirmed by in situ illuminated X‐ray photoelectron spectroscopy and density functional theory calculation. This work will provide a promising strategy for constructing S‐scheme heterojunction systems for efficient artificial photosynthesis reactions toward carbon neutrality.

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