B-site substitution-induced band edge shifts in perovskite-type Cu(Nb,Ta)O3 solid solutions for visible-light-driven hydrogen evolution

Abstract

The rational design of photocatalysts with precise bandgap and band-edge control is crucial for achieving the visible-light-driven conversion of solar to hydrogen. This study demonstrates a novel strategy for band-edge engineering through B-site cation substitution and symmetry evolution in Cu (3d¹⁰)-based perovskites. Substitution of Ta⁵⁺ (5d⁰) for Nb⁵⁺ (4d⁰) in CuNbO₃ induces sequential phase transitions (Pc–R3c–Rc), accompanied by systematic bandgap modulation from 1.57 to 2.23 eV. The Rc CuTaO₃ phase exhibits a much higher conduction band (−0.90 V vs. standard hydrogen electrode) than the H⁺/H₂ potential and a slightly lower valence band than the O₂/H₂O potential. Visible-light-driven hydrogen evolution occurs efficiently on CuTaO₃ with Ru cocatalyst, in the presence of S²⁻/SO₃²⁻ sacrificial agents. Our experiments and first-principles calculations reveal that the Ta-for-Nb-substitution widens the bandgap by lowering the valence band maximum via weakened Cu–O hybridization, while simultaneously elevating the conduction band minimum via Ta 5d orbital contributions. The inherent bandgap-narrowing tendency of structural evolution from polar Pc to centrosymmetric Rc symmetry is attenuated by Ta substitution-induced elongation of Cu–O bonds. The interplay between B-site cation engineering and symmetry-induced electronic degeneration establishes a materials design paradigm for visible-light-driven photocatalysis, demonstrating how to enable targeted bandgap optimization by coupling orbital-level modifications and phase transition.