Topotactic engineering of high-entropy (oxy) hydroxide nanotubes for enhanced photocatalysis.

This study introduces a novel topotactic synthesis method for CeCoNiAlGa high-entropy oxyhydroxide (HE-OOH) nanotubes utilizing multiwalled carbon nanotubes (MWCNTs) as the parent crystal. This approach yields concentric nanotubes with high crystalline order and a fluorite-like structure supported by a distorted Ce-O framework. Neutral M-OH-M sheet stacking, resembling dehydrated brucite-like layers, governs the multiwalled configuration, with topotactic alignment between fluorite-like (111) and carbon (002) planes. Controlled heat treatment (80-600 °C) induces gradual dehydroxylation while preserving the multiwalled morphology and sole fluorite-like structure up to 500 °C, confirming CeCoNiAlGa HE-OOH as direct precursors of high-entropy oxides (HEOs). In CeCoNiAlGa nanotubes, the elevated number of oxygen vacancies compensates for the charge imbalance arising from multiple cations, and dehydroxylation enhances collective charge redistribution, thereby increasing the vacancy concentration. CeCoNiAlGa HE-OOH nanotubes obtained at 80 °C exhibit superior photocatalytic performance, achieving 96% ciprofloxacin (CIP) degradation in 45 min following pseudo-second-order kinetics (k = 0.71 L/mg∙min). Scavenger studies identify photogenerated holes as the dominant reactive species, while abundant surface hydroxyl groups facilitate interfacial charge transfer and reactive radical formation, enhancing photocatalytic efficiency. These findings establish CeCoNiAlGa HE-OOH as a structural precursor for enhanced photocatalytic performance through controlled topotactic and electronic-structure engineering.