The stems of water convolvulus were employed as biotemplates for the replication of their optimized 3D hierarchical architecture to synthesize porous MgO-modified TiO2 . The photocatalytic reduction of CO2 with H2O vapor into hydrocarbon fuel was studied with these MgO-TiO2 nanostructures as the photocatalysts with the benefits of improved CO2 adsorption and activation through incorporated MgO. Various factors involving CO2 adsorption capacity, migration of charge carriers to the surface, and the number of activity sites, which depend on the amount of added MgO, determine the photocatalytic conversion efficiency.