Metal–organic framework-templated synthesis of Ag/Ni-TiO2 for enhanced photocatalytic CO2 reduction

Nanosized semiconductors with hierarchical structures are attractive for use as catalysts. Development of high porous structures, to this end, has received intense interest. By using a well-defined titanium-based metal–organic framework, i.e., MIL-125(Ti), as a template, we report the synthesis of Ni-doped mesoporous TiO₂ nanocrystals with tablet morphologies. While the obtained Ni-TiO₂ composites could be used for photoreduction of CO₂ in the presence of water vapor, this photocatalytic property could be largely improved by further deposition of Ag nanoparticles. In principle, Ni species in the lattice matrix lead to the formation of impurity levels in the bandgap of TiO₂, promoting both light absorption and charge separation. On the other hand, Ag, as a cocatalyst, can trap electrons and simultaneously activate the C═O bonds from the adsorbed CO₂. Our results demonstrate that 1.0% Ag/0.5% Ni-TiO₂ photocatalyst exhibits the highest activity. The yields of CO and CH₄ reach 14.31 and 279.07 μmol / g, respectively, for a 3-h reaction. We believe the strategy based upon the synergy of Ni doping and Ag loading can be extended to other photocatalytic systems.