Photocatalytic CO₂ reduction to produce valuable chemicals is a promising strategy to address environmental issues and energy crisis. However, achieving high efficiency and selectivity for converting CO₂ into higher-energy CH₄ remains challenging due to the competitive two-electron reduction pathway producing CO. In this study, Cu₂O clusters were strongly anchored onto ultrathin TiO₂ nanosheets (Cu(I)-TiO₂) using a simple photo-deposition method. Compared to pure TiO₂, Cu(I)-TiO₂ samples exhibited a significantly enhanced photocatalytic activity and selectivity for CO₂-to-CH₄. The presence of Cu₂O can also enhance the photogenerated carrier separation and light absorption. By optimizing the amount of Cu₂O, the CH₄ production rate of 45.73 μmol·g⁻¹·h⁻¹ with selectivity up to 97.47% was achieved. Mechanistic investigations demonstrate that the presence of Cu₂O lowers the formation energy barrier of *COOH, a key intermediate for the photocatalytic CO₂ reduction. Moreover, Cu(I)-TiO₂ promotes the adsorption and hydrogenation of *CO to *CHO_x species, favoring CH₄ production over CO. This work provides valuable insights for designing highly efficient and selective photocatalyst for CO₂ reduction and deepens the understanding of reaction mechanism.

Constructing 2D/2D face-to-face heterojunctions is believed to be an effective strategy to enhance photocatalytic performance due to the enlarged contact interface and increased surface active sites. Herein, 2D porous NiCo oxyphosphide (NiCoOP) was synthesized for the first time and coupled with graphitic carbon nitride (g-C₃N₄) nanosheets to form 2D/2D heterojunctions via an in-situ phosphating method. The optimal 4 wt.% 2D/2D NiCoOP/g-C₃N₄ (OPCN) photocatalyst achieves a hydrogen evolution rate of 1.4 mmol·h⁻¹·g⁻¹, which is 33 times higher than that of pure g-C₃N₄. The greatly improved photocatalytic performance of the composite photocatalysts could be attributed to the formation of interfacial surface bonding states and sufficient charge transfer channels for accelerating carrier separation and transfer and the porous structure of NiCoOP nanosheets with abundant surface active sites for promoting surface reactions. Amazingly, the 2D/2D OPCN composite photocatalysts also exhibit superior stability during photocatalytic reactions. This study not only designs new noble-metal-free NiCoOP/g-C₃N₄ composite photocatalysts but also provides a new sight in fabricating face-to-face 2D/2D heterojunctions for their application in energy conversion areas.