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Ultrathin TiO 2 nanosheet with abundant Ti3+ is constructed as a photocatalyst for CO 2 conversion in the absence of any sacrificial reagents. The exposed Ti3+ as a Lewis base site may activate CO 2 to form bridging Ti···CO 2 −···Ti, resulting highly CH 4 yield via the synergistic effect of coordination-activation and photocatalysis. • Ultrathin TiO 2 nanosheets (NST) with abundant Ti3+ sites are constructed as a photocatalyst for conversion CO 2. • The catalyst (NST) exhibits a high yield (147.2 μmol g−1 h−1) and selectivity (96.8 %) for CH 4. • CO 2 would be selectively activated on the surface of NST via forming the bridging Ti···CO 2 −···Ti coordination species. • The synergistic effect about coordination-activation and photocatalysis can greatly enhance CH 4 yield efficiency. Ultrathin TiO 2 nanosheet (NST) with abundant Ti3+ sites is developed as a photocatalyst which exhibits a high yield (147.2 μmol g−1 h−1) and selectivity (96.8%) for CO 2 to CH 4. The results of in situ FTIR, in situ EPR and density functional theory (DFT) calculation suggest that CO 2 would be selectively activated on NST surface via forming the bridging Ti···CO 2 −···Ti coordination species which not only lowers the activation energy barrier, but also determines the selectivity of CH 4. Additionally, NST with the better separation and transfer ability for photogenerated electrons-holes can fleetly supply the electrons to CO 2 molecules for high efficiently producing CH 4 via the Ti-C coordination bond. Finally, a possible mechanism of the synergistic effect about coordination-activation and photocatalysis is discussed at a molecule level. This work would offer a perspective for designing an effective photocatalyst and tailoring the selectivity of products by coordination activation. [ABSTRACT FROM AUTHOR]