Charge-engineered TiO2-LDH composites for visible-light-driven degradation of phenol

• Novel Bi/N–TiO₂@Ni₄Al₁-LDH S-scheme heterojunction fabricated via eco-friendly hydrothermal route. • n-type conversion confirmed by Mott–Schottky analysis, indicating strong built-in electric field. • Enhanced charge separation: PL quenching and EIS show 22 Ω charge-transfer resistance. • High photocatalytic activity: 72 % phenol degradation in 4 h (kₐ pp = 0.25 min⁻¹) under visible light. • ROS profile dominated by h⁺ and • OH, retaining strong oxidation potential on Bi/N–TiO₂ side. Bi/N co-doped TiO₂ (Bi/N–TiO₂) was synthesized via a sol–gel method and coupled with NiAl-layered double hydroxide (NiAl-LDH) by hydrothermal treatment to fabricate Bi/N–TiO₂@NiAl-LDH heterojunction nanocomposites. Comprehensive structural and spectroscopic analyses (XRD, SEM-EDX, XPS) confirmed the coexistence of anatase TiO₂, Bi₂₀TiO₃₂, and LDH phases, while optical and electrochemical studies revealed narrowed band gaps, enhanced visible-light absorption, and suppressed photoluminescence. Mott–Schottky measurements showed that the composite exhibits n-type conversion despite its p-type components, indicating the formation of a built-in electric field and an S-scheme heterojunction rather than a conventional type-II or Z-scheme. This configuration preserves the strong oxidation ability of photogenerated holes on Bi/N–TiO₂ while efficiently transferring electrons to the LDH conduction band. The optimal Bi/N–TiO₂@Ni₄Al₁-LDH(1:2) composite achieved 72 % phenol degradation under visible light with the highest rate constant (0.25 min⁻¹). Reactive species trapping confirmed that h⁺ and • OH radicals dominate the degradation pathway. These results demonstrate that S-scheme charge transfer is key to maximizing light harvesting, carrier separation, and reactive oxygen species generation for efficient pollutant degradation.