The superior photocatalytic activity of Nb doped TiO2/g-C3N4 direct Z-scheme system for efficient conversion of CO2 into valuable fuels.

In this study, we firstly aimed to use Nb as dopant to dope into the TiO2 lattice in order to narrow band gap energy or enhance photocatalytic activity of the Nb-TiO2. Then, the prepared Nb-TiO2 was combined with g-C3N4 to establish Nb-TiO2/g-C3N4 direct Z-scheme system for superior reduction of CO2 into valuable fuels even under visible light. The obtained results indicated that the band gap energy of the Nb-TiO2 (2.91 eV) was lower than that of the TiO2 (3.2 eV). In the successfully established Nb-TiO2/g-C3N4 direct Z-scheme system, the photo-excited e- in the CB of the Nb-TiO2 combined with the photo-excited h+ in the VB of the g-C3N4 preserving the existence of e- in the CB of the g-C3N4 and h+ in the VB of Nb-TiO2, and thereby, the system produced numerous amount of available e-/h+ pairs for the reduction of CO2 into various valuable fuels. In addition, the produced e- of the Nb-TiO2/g-C3N4 existing in the CB of the g-C3N4, which the potential energy is approximately -1.2 V, would be strong enough for the reduction of CO2 to generate not only CH4 and CO but also HCOOH. Among established Nb-TiO2/g-C3N4 materials, the 50Nb-TiO2/50 g-C3N4 material was the best material for the CO2 reduction.