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.