The significant role of Z-scheme band alignment at the heterojunction for the enhanced photocatalytic H2 production in TiO2/BiNbO4/rGO nanocomposite

An efficient ternary TiO₂/BiNbO₄/rGO nanocomposite was prepared by a simple wet impregnation process. PXRD confirmed the tetragonal and orthorhombic crystalline natures of titanium dioxide (TiO₂) and bismuth niobate (BiNbO₄) respectively. The loading of BiNbO₄ and reduced graphene oxide (rGO) shifted the light absorption of TiO₂ to a longer wavelength from 400 to 430 nm. The suitable conduction band position of BiNbO₄ facilitated a favourable band alignment to suppress the electron/hole (e⁻/h⁺) recombination in TiO₂ via the Z scheme mechanism at the heterojunction. The excited electrons at the CB of TiO₂ were easily transported via the fast electron accepting and conducting rGO matrix for the surface redox reactions with the improved charge transfer efficiency, which was confirmed by EIS and PL studies respectively. BiNbO₄ and rGO synergistically improved the specific surface area and solar light absorption. EPR analysis confirmed the presence of increased oxygen vacancies at the heterojunction. Hence, the ternary TiO₂/BiNbO₄/rGO nanocomposite demonstrated an enhanced hydrogen evolution (8910 μ mol h⁻¹ g_cat⁻¹) than bare TiO₂ (4820 μ mol h⁻¹ g_cat⁻¹), bare BiNbO₄ (950 μ mol h⁻¹ g_cat⁻¹) and TiO₂/BiNbO₄ (6730 μ mol h⁻¹ g_cat⁻¹) under direct solar light. The optimum of 1 wt % BiNbO₄ and rGO loaded TiO₂ nanocomposite produced twice the H₂ production than the bare TiO₂. This also further confirms that rGO played a major role during the photocatalytic process by highlighting the potential of metal oxides combined with carbon material as an efficient photocatalyst which prominently enhances the H₂ evolution.