Oxygen vacancies and octahedral distortion induced bandgap narrowing in [KNbO₃]_(1−x)–[Ba(Ni_0.1Zn_0.3Nb_0.6)O_3−δ]_x perovskites for visible-light photocatalysis: a combined experimental and theoretical study

Visible-light photocatalysis offers a sustainable approach to environmental remediation and renewable energy generation. However, traditional photocatalysts often require ultraviolet light, which limits their efficiency in utilizing sunlight. In this study we address this challenge by exploring the potential of [KNbO₃]_(1−x)–[Ba(Ni_0.1Zn_0.3Nb_0.6)O_3−δ]_x perovskites for visible-light photocatalysis. Incorporation of Ba(Ni_0.1Zn_0.3Nb_0.6)O_3−δ into KNbO₃ results in a structural phase transition from orthorhombic (at x = 0) to pseudo cubic (at x = 0.3) and reduces the bandgap from 3.14 eV to 1.1–2.0 eV, enhancing visible-light absorption. Theoretical models are investigated using the density functional theory (DFT) to provide the underlying physics, revealing that the incorporation of Zn²⁺/Ni²⁺ at the B-site introduces 3d states in the conduction band, and oxygen vacancies create impurity states near the valence band edge, which lower the bandgap. Additionally, octahedral distortion splits the degenerate Nb \(4d_{{z^{2} }}\) $4d_{z^2}$ and \(4d_{{x^{2} - y^{2} }}\) $4d_{x^2-y^2}$ orbitals, shifting them closer to the Fermi level and further contributing to the reduction of the bandgap. This combined experimental and theoretical approach provides valuable insights for designing visible-light-active ferroelectric perovskite oxides for enhanced photocatalytic applications.