The strategy on selecting the oxidation photocatalyst NiWO4 as a co-catalyst to enhance the performance of TiO2/rGO for H2 production under solar light

In this work, TiO₂/NiWO₄/rGO nanocomposite was prepared by a simple wet impregnation method. The experimental and characterization results revealed the significant contribution of NiWO₄ (NW) and rGO in enhancing the photocatalytic performance of TiO₂ for H₂ production. Though NW is an oxidation photocatalyst with more positive oxidation potential, its suitable band edge alignment with TiO₂ facilitated a Z-scheme electron pathway at the interface of TiO₂/NW with the staggered type II heterojunction. Additionally, Ni²⁺ ions substitution for Ti³⁺ and Ti⁴⁺ in the surface lattice of TiO₂ enhanced the formation of single electron oxygen vacancies, which trapped the photogenerated holes and enhanced the electrons’ participation in H⁺ ions reduction. Interestingly, the holes in the VB of NW with high positive oxidation potential favour the oxidation of water molecules into H⁺ ions. The Z-scheme electron pathway facilitated the excited electrons in the CB of TiO₂ for the H⁺ ions reduction without recombination. These excited electrons are quickly transported by another co-catalyst rGO, with high electron affinity and conductivity. Hence, the high interfacial charge transfer efficiency (IFCT) with reduced charge transfer resistance was observed for the synthesized nanocomposite material. Defect state energy levels and the overlapped p-orbitals of C and O reduced the band gap and enhanced the visible light absorption. Hence, the nanocomposite showed more than 3 times the photocatalytic efficiency of the pristine TiO₂ under direct solar light. This work highlights the necessity for profound insight in selecting the co-catalyst to reinforce the desired properties in a nanocomposite material to improve its performance.